Isotopic and petrographic evidence for young Martian basalts

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Geochimica et Cosmochimica Acta 72 (2008) 5819–5837

Isotopic and petrographic evidence for young Martian basalts

Erin L. Walton a,*, Simon P. Kelley b, Christopher D.K. Herd a

a Department of Earth & Atmospheric Sciences, 1-26 Earth Sciences Building, University of Alberta, Edmonton, AB, T6G 2E3 Canadab Department of Earth Sciences, The Open University, Milton Keynes, UK

Received 27 February 2008; accepted in revised form 2 September 2008; available online 18 September 2008

Abstract

Radiometric age data for shergottites yield ages of 4.0 Ga and 180–575 Ma; the interpretation of these ages has been, andremains, a subject of debate. Here, we present new 39Ar–40Ar laser probe data on lherzolitic shergottites Allan Hills (ALH)77005 and Northwest Africa (NWA) 1950. These two meteorites are genetically related, but display very different degrees ofshock damage. On a plot of 40Ar/36Ar versus 39Ar/36Ar, the more strongly shocked ALH 77005 (45–55 GPa) does not yield anarray of values indicating an isochron, but the data are highly scattered with the shock melts yielding 40Ar/36Ar ratios of1600–2026. Apparent ages calculated from these extractions range from 374–8183 Ma, with 50% of the data, particularly fromthe shock melts, yielding impossibly old ages (>4.567 Ga). On the same plot, extractions from igneous minerals in the lessshocked NWA 1950 (30–44 GPa) yield a fitted age of 382 ± 36 Ma. Argon extractions from the shock melts are well distin-guished from minerals, with the melts exhibiting the highest 40Ar/36Ar ratios (1260–1488) and the oldest apparent ages. Laserstep heating was also performed on maskelynite separates from NWA 1950 yielding ages of 1000 Ma at the lowest releasetemperatures, and ages of 360 and 362 Ma at higher temperature steps. Stepped heating data from previous studies haveyielded ages of 500 and 700 Ma to 1.7 Ga for ALH 77005 maskelynite separates. If the ages obtained from igneous mineralsrepresent undegassed argon from an ancient (4.0 Ga) rock, then the ages are expected to anticorrelate with the degree of shockheating. The data do not support this inference. Our data support young crystallization ages for minerals and Martian atmo-sphere as the origin of excess 40Ar in the shock melts.

The shock features of shergottites are also reviewed in the context of what is known of the geologic history of the Martiansurface through remote observation. The oldest, most heavily cratered surfaces of Mars are thought to be P4.0 Ga; we con-tend that ancient rocks from Mars (Noachian >3.5 Ga) are likely to record multiple impact events reflecting megaregolithformation and the cumulative effects of erosion and aqueous alteration occurring during or since that era. Young rocks (LateAmazonian, <0.6 Ga) should record a relatively simple history of emplacement and ejection from the near surface. We showthat although shergottites are strongly shocked, they are relatively pristine crystalline igneous rocks and not pervasivelyaltered breccias. The petrography of shergottites is at odds with an ancient age interpretation. A model in which young coher-ent rocks are preferentially sampled by hypervelocity impact because of material strength is considered highly plausible.� 2008 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

Martian meteorites are volcanic, subvolcanic and cumu-late igneous rocks, traditionally divided into shergottites,nakhlites and chassignites (SNCs), a classification schemesubsequently expanded to include one orthopyroxenite,

0016-7037/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.gca.2008.09.005

* Corresponding author. Fax: +1 780 492 2030.E-mail address: [email protected] (E.L. Walton).

Allan Hills (ALH) 84001. Of the �43 Martian meteoritesrecovered to date, 33 are shergottites; basaltic igneous rockswith variable amounts of cumulus crystals. In the absence ofa sample return mission, Martian meteorites provide the onlydirect access to materials from this planet and are of unsur-passed value to research in the field of planetary sciences.This is attested to by the sheer volume of literature generatedon their study (e.g., the comprehensive bibliography ofMeyer, 2008). Much of this research has focused on the po-tential for the meteorites to provide ground truth to remote

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5820 E.L. Walton et al. / Geochimica et Cosmochimica Acta 72 (2008) 5819–5837

observations, which has stemmed from the notion that obser-vations of the meteorites can be generalized to become prob-able observations of Mars. Martian meteorites give preciseradiometric crystallization ages for rocks that were derivedfrom a small number (5–8) of unidentified impact sites onMars, determined from the distribution of ejection ages (Ny-quist et al., 2001); an absolute chronology awaits precise iso-topic measurement of returned samples for which the fieldcontext is known. At present, the chronology of Martian geo-logic history is constrained by two independent data sets:radiometric dating of Martian meteorites and Mars’ craterretention ages. The purpose of this study is to examine argonisotopes in different phases of two lherzolitic shergottites; theages derived from these isotopes are then assessed in the con-text of impact (shock) history and degree of shock damage.We review the petrography of shergottites to consider ancientformation ages for these rocks. Our results and observationshave implications for shergottite crystallization ages and theage of the Martian surface.

It is uncontested that one Martian meteorite is ancient(�4.5 Ga; ALH 84001), and that crystallization ages of nakh-lites and chassignites are concordant at �1.3 Ga (Nyquistet al., 2001). However, the radiometric age data for the sher-gottites have been, and remain, a subject of debate (e.g., Shihet al., 1982; Chen and Wasserburg, 1986; Jagoutz andWanke, 1986; Jones, 1986; Nyquist et al., 1998; Borg et al.,2005). The main source of contention is the U–Pb system,as exemplified by recent mineral separate Pb isotopic investi-gations combined with Sm–Nd and Rb–Sr (Borg et al., 2005)or Sm–Nd and Lu–Hf (Bouvier et al., 2005) on the same sep-arates. Borg et al. (2005) interpret Sm–Nd (166 ± 12 Ma) andRb–Sr (166 ± 6 Ma) ages of Zagami as crystallization ages,and the U–Pb system as disturbed. Despite a wide accep-tance, the young shergottite age scenario is not without prob-lems. The number of young Martian meteorites in the world’scollection seems inconsistent with age estimates of the Mar-tian surface based on crater counting; the proportion of large,sparsely cratered, and hence young, terrain on Mars is small(Hartmann and Berman, 2000). Although it has been shownthat several volcanic units of quite young age do exist onMars (Basilevsky et al., 2006), it is the proportion of youngsurface units relative to older terrains that presents a prob-lem. Models of the distribution of surface unit ages on Marsshow that most of the surface should be older than 1300 Ma(Tanaka et al., 1992; Nyquist et al., 1998; Hartmann and Ber-man, 2000); from such models it can be predicted that thereshould be more impacts, and presumably more Martianmeteorite-forming events, in terrain as old or older than thatof the nakhlites and chassignites (i.e., >1.3 Ga). However,just the opposite is observed; any workable model for sher-gottite ages of 180–575 Ma must accommodate the preferen-tial delivery of young rocks from Mars. In addition, thepresence of 142Nd and 182W anomalies in SNC meteorites(Foley et al., 2005; Debaille et al., 2007) argues against astrongly convective mantle over most of Mars’ history. Thelatter is difficult to reconcile with magmatism persisting to180 Ma.

Bouvier et al. (2005, 2008) propose that shergottites havean ancient formation age of�4.0 Ga and that Martian acidicweathering has affected minerals such as phosphates. In their

2005 and 2008 papers, Bouvier and coworkers argue thatsince phosphates are important components to the mineralisochrons in U–Th–Pb radiometric systems, their metaso-matic alteration has caused the ages to be reset. Young ages(�180 and 330–575 Ma) are thus interpreted to representthe last re-equilibration of phosphate with Martian ground-water. Although the interpretation of shergottites as ancientrocks reconciles the age of the Martian surface with that ofthe current inventory of Martian meteorites, as well as allow-ing for active convection of the Martian mantle, strong argu-ments against this concept have been made (Jones, 1986;Borg et al., 2005; Nyquist et al., 2006; Fritz et al., 2007; Gaff-ney et al., 2007; Jones, 2007). The proposal that shergottitesare 4.0 Ga has received considerable attention, because ifproven correct, it would have the important implication thatthe Martian surface is older than previously thought.

In this study, we use spatially resolved argon isotopemeasurements combined with petrographic observationsto provide a test for the ancient age hypothesis by address-ing the following issues:

1) If the shergottites formed >4.0 Ga, but were subse-quently shock-heated to variable degrees withoutthe implantation of gas from the atmosphere, thenwe would expect a more strongly shocked and heatedshergottite to have lost more radiogenic 40Ar and tohave a lower age. To test this inference, we comparethe apparent 40Ar ages of heavily shock-heated ALH77005 with those of less shock-heated NWA 1950.

2) If the shergottites formed >4.0 Ga ago and remainedwithin a few tens of meters of the Martian surface,then we would expect them to show evidence for sucha protracted near surface history including breccia-tion and pervasive alteration. This is because impactcratering has been shown to be an important geologicprocess shaping the solid surface of Mars since theplanet’s formation, and ancient Martian terrains(>3.5 Ga) are intensely faulted and densely crateredrocks, comprising largely impact breccias (Tanaka,1986; Tanaka et al., 1992; Hartmann and Neukum,2001). Evidence from remotely sensed data, includingmineralogical observations (e.g., Bibring et al., 2005;Poulet et al., 2005) and geomorphology (e.g., Bakeret al., 1991; Baker, 2001) indicate that early Marshad a dynamic hydrological cycle, with the influenceof liquid water decreasing throughout its history(e.g., Solomon et al., 2005). Persistent or episodic flu-ids interacting with near surface rocks would leavetheir trace as alteration assemblages (Bridges et al.,2001). All Martian meteorites resided near the sur-face of Mars before ejection and delivery to Earth(e.g., Nishiizumi et al., 1986b; Nishiizumi and Caffee,1996; Nishiizumi et al., 1999; Nishiizumi et al., 2000).Thus, any rock from Mars, whether ancient orrecent, should contain petrographically observablefeatures that are consistent with what is known ofthe nature of the Martian surface of that generalage. To test for a protracted history, we review thepetrography of Martian meteorites, expecting thatancient Martian rocks should be altered breccias.

Evidence for young Martian basalts 5821

2. PETROGRAPHY OF THE STUDIED METEORITES

ALH 77005 is one of the most strongly shocked Martianmeteorites in the current collection, recording shock pres-sure in the range 45–55 GPa with a post-shock temperatureincrease of 1000 �C (Fritz et al., 2005). NWA 1950 was lessstrongly shocked, in the range 30–44 GPa (Walton andHerd, 2007a), a range that corresponds to a post-shocktemperature increase of �200 �C (Fritz et al., 2005). Thesedifferences in shock pressure and temperature are inferredfrom the shock effects recorded in these two meteorites,as tabulated in Table 1.

The two primary shock features that set NWA 1950 andALH 77005 apart are the response of plagioclase to shockand the degree of shock melting. In NWA 1950, plagioclasehas been completely transformed to diaplectic glass (mask-elynite), which retains the crystal shape and compositionalzoning of precursor plagioclase, but lacks flow features andvesicles. In contrast, rather than maskelynite, ALH 77005contains abundant vesiculated plagioclase glass (or normalthermal glass) with a birefringent plagioclase rim. Maskely-nite forms in the pressure range 14–45 GPa (Stoffler andHornemann, 1972; Dodd and Jarosewich, 1979; Stoffleret al., 1991). At pressures >45 GPa, the magnitude ofpost-shock temperature is high enough to keep the shockedmaterial in a liquid state after pressures release so that ‘nor-mal glass’ quenched from a low pressure liquid is formed(Stoffler, 2000). Thus, the observation of normal glass inALH 77005 indicates a shock pressure higher than45 GPa (Fritz et al., 2005).

Shock melts occur as veins and pockets of localizedglassy material, distributed heterogeneously throughoutthe more strongly shocked Martian meteorites. Shock meltsare a minor component of NWA 1950 (1.8 vol%) comparedto the relative abundance in ALH 77005 (up to 29 vol%)(Table 1). The shock melts in polished sections of ALH77005 and NWA 1950 exhibit similar characteristics; both

Table 1Shock effects recorded in NWA 1950 and ALH 77005

NWA 1950 ALH 77005

Plagioclase Maskelynite Plagioclase glass withflow lines and vesicles

Olivine & Pyroxene Irregular andplanar fracturesmosaicism

Irregular and planarfractures mosaicismdeformation bands(olivine only)

Shock 1.8 vol% 12.9–29 vol%b,c

Melt productsa

Shock Pd <45 Gpa(35–44 GPa)e

>45 Gpa (45–55 GPa)e

Post Shock Tf 200–560 kg 1000 kg

a The abundance of shock melt products are reported as modalabundances (in vol%) of shock veins and melt pockets.

b Treiman (2005).c Walton and Herd, 2007.d Shock pressure; equilibration shock pressure.e Fritz et al. (2005).f Stoffler et al. (1991).g Post shock T = post shock temperature increase.

reside within a poikilitic portion of the host rock, consistingof cumulate olivine and chromite enclosed by low-Capyroxene. The olivine-rich nature of the region of shockmelting is reflected in the bulk composition of the melts,which are olivine-saturated. One large melt pocket(�0.5 � 1.4 mm size) observed in NWA 1950 is self-con-tained and is surrounded (within the 2 dimensions of thethin section) by granular olivine cut across by veins of meltmaterial emanating from the main melt pocket. At�250 lm from the shock melt, the texture is igneous show-ing no signs of local (grain boundary) melting. The re-sponse to shock is manifest as complete transformation ofplagioclase to maskelynite and mechanical deformation inolivine, pyroxene and oxides. In ALH 77005, the shockmelt is not observed as a distinct self-contained pocket,but forms an extensive network of interconnected veinsand pockets of melt material surrounded by a broad zoneof granular olivine cut across by veins of melt material. Thismelt-intruded zone extends to �200–300 lm distance,wherein, as in NWA 1950, the response to shock is largelymechanical (olivine/pyroxene/oxides) with plagioclaseglass.

3. ANALYTICAL METHODS

The analyzed component of each meteorite sample wasprepared as a 0.25 mm thick, �1 cm � 1 cm doubly pol-ished thick section (or tile), which was then photographedto provide a map of the distribution of igneous mineralsand shock melts. Prior to irradiation, both the ALH77005 and NWA 1950 tiles were documented using fieldemission scanning electron microscopy (FE-SEM) back-scattered electron (BSE) and secondary electron (SE) imag-ing to characterize microtextures. After documentation, thetiles were cleaned in Analar methanol and deionized water,and irradiated in the McMaster Nuclear Reactor, Hamilton(Canada), along with the international biotite standardGA1550, which has a calibrated age of 98.8 ± 0.5 Ma (Re-nne et al., 1998). The amounts of 38Ar derived from Cl wereminimized by use of Cd shielding in the reactor to minimizethe slow neutron flux. The irradiation parameter, J, wasdetermined to be 0.00700 (NWA 1950 and ALH 77005),with estimated errors of 0.5%. On return, the tiles and chipswere placed on an Al platen and loaded into a UHV laserchamber with a Kovar window and baked to 120 �C over-night to remove adsorbed terrestrial atmospheric Ar fromthe samples and chamber walls. Samples remained in thevacuum system for several days prior to analysis to mini-mize terrestrial atmospheric contamination.

Argon isotopic data for laser probe and stepped heatingare given in the Appendix. For the laser probe measure-ments, a total of 49 extractions were obtained using anSPI 25W 1090 nm fibre laser focused through a Leica petro-logical microscope. The beam, employing an output powerof 15 W, resulted in �70–150 lm diameter melt pits on thesurface of the sample. Each extraction consisted of two orthree melt pits collected in 100 ls pulses. The depth of thepit is estimated to be less than half of its diameter (i.e.,35–75 lm). Following analysis, each tile was turned overto ensure that the laser did not penetrate the sample; neither


showed any evidence of melting on the opposite side. Thelaser stepped heating extractions used the same setup asthe probe extractions, except argon was released by heatingfor 1 min in the high vacuum.

Active gases were removed by two SAES AP 10 getters,one operating at 400 �C and one at room temperature. Theremaining noble gases were then admitted into the spec-trometer and analyzed isotopically for argon only by aMAP 215-50 noble gas spectrometer. Argon isotope peakintensities (36Ar, 37Ar, 38Ar, 39Ar, 40Ar) were measuredten times in sequence over a period of �15 min. The blankswere determined by employing the normal extraction proce-dures, but without firing the laser. Blanks for 40Ar, 39Ar,38Ar, 37Ar and 36Ar for the laser stepped heating and laserspot extractions were 4.7, 0.015, 0.015, 0.2, 0.043, and 2.5,0.005, 0.016, 0.5, and 0.06, respectively. The analyzed ALH77005 tile with a corresponding laser extraction map is illus-trated as a sketch of igneous minerals and shock melt net-works (Fig. 1).

Corrections for mass spectrometer discrimination andirradiation interferences were applied to all argon data fol-lowing extraction. 36Ar concentrations have also been cor-rected for a cosmogenic component as in Walton et al.(2007), using a 38Ar/36Ar production ratio for cosmic-rayproduced noble gases in meteorites of 1.544 and a38Ar/36Ar ratio = 0.244 for the Martian atmosphere. Thecoarse size (mm) of igneous minerals (referred to also as‘host rock’ extractions; Fig. 1) and melt pockets in the stud-ied meteorites enabled clean argon extractions (i.e., fusion

200 m400 m

a

b

c

a

c b

shock melt

recrystallized olivine

clinopyroxene

plagioclase glass

olivine

Fig. 1. Sketches of the dated ALH 77005 thick section (tile). The sketch oright shows the location of laser probe melt pits. Solid black circles denotextractions from host rock minerals and shock-melt intruded olivine (see leconsist of one, two or three melt pits, shown as a series of circles with a sinis � 70 lm diameter in cross section). (a) Host rock portion of the tile, wiplagioclase glass (dark phase containing vesicles). (b) Texture of the shoglass with minor chromite (white). (c) Granular olivine, cut cross by vein

of a single phase) for these phases. Areas adjacent to thepockets comprise minerals + invaded stringers of melt anddo not represent ‘clean’ analyses for this phase only(Fig. 1c). Additional details for the sample preparation, ar-gon extraction and measurement, and corrections to thedata can be found in Walton et al. (2007).

In this study, we present ages, referred to as ‘apparentages’, calculated for individual laser spot extractions andindividual heating steps, using the standard Ar–Ar ageequation (McDougall and Harrison, 1999). The apparentage is calculated assuming all 40Ar is radiogenic; the datahave not been corrected for any trapped or excess argon.A ‘fitted’ age is also reported for both meteorites by fittingthe host rock data on a plot of 40Ar/36Ar versus 39Ar/36Ar.

The method for CRE age determination is outlined inWalton et al. (2007) using the general equation for cosmicray exposure:

ð38Ar=36ArÞm ¼ ð38Ar=36Arc�38Ar=36ArTÞkð37Ar=36ArÞmþ38Ar=36ArT

where subscript T = trapped, c = cosmogenic, k =P(38Ar) � Texp/a in cc Ma�1 for the composition of theshergottite, and 37Ar = aCa. This analysis has the effect ofdiscriminating against the trapped and solar wind compo-nents which have the same 38Ar/36Ar ratio but includesassumptions concerning the composition of the fragmentanalyzed, the ratio of trapped Ar and the effect of chlorineand terrestrial weathering (Walton et al., 2007). As such the

200 m

5

14

2

7

1210

15

6

13

4

3

8

17

11

9

16a

19

1

400 m

16b

19

n the left shows the location of BSE images (a–c). The sketch on thee argon extractions from shock melts and solid white circles denotegend). A single circle marks an individual melt pit. Extractions maygle number reference. Melt pits are shown roughly to scale (each pitth chromite (white), clinopyroxene (grey), olivine (lighter grey), andck melt, dominated by strongly zoned olivine microphenocrysts ins of melt, are labeled ‘recrystallized olivine’ in the legend.


cosmic ray exposure ages produced tend to be minimumages.

4. RESULTS

4.1. Laser probe results

Data obtained by laser probe measurement of individualphases in each meteorite are shown on a plot of 40Ar/36Arversus 39Ar/36Ar (Fig. 2). The 36Ar has been corrected forcosmogenic production under the assumptions outlined inSection 3, concerning cosmogenic and trapped isotope ra-tios. Argon extractions are denoted as host rock, shockmelt or melt-intruded olivine (Fig. 1). Those labeled ‘hostrock’ are extractions from igneous minerals (pyroxene, oliv-ine) and maskelynite (NWA 1950) or plagioclase glass(ALH 77005). Argon extractions from the main shock melt(pocket or network) are denoted ‘shock melts’. Argonextractions labeled ‘melt-intruded olivine’ are from granu-lar olivine adjacent to the main shock melt pocket(Fig. 1c). The melt-intruded olivine was analyzed fromthe ALH 77005 section only because of the large size ofthe melt area in this sample. Although this texture was ob-served in the NWA 1950 section, only one melt pocket waspresent in the analyzed tile which meant that after chipswere removed for stepped heating, only a small portion ofthe shock melt remained, limiting the number of extractionsthat could be made on this phase compared to ALH 77005.

The data obtained from NWA 1950 indicate apparentages ranging from 552–2246 Ma for igneous minerals, andare lower than those obtained from the shock melt (4643–6920 Ma). All three melt pocket analyses yielded impossiblyold apparent ages (older than 4.567 Ga), while the majorityof apparent ages for the host rock are less than 1.0 Ga. Ona plot of 40Ar/36Ar versus 39Ar/36Ar (Fig. 2), argon extrac-

1400

2200

1800

1000

600

200

4036

Ar/

Ar

0 10 20 30 4039 36Ar/ Ar

NWA 1950 shock melthost rock minerals

host rock mineralsnot included in the fit

included in the fit

Fig. 2. 40Ar/36Ar versus 39Ar/36Ar isochron diagram for ALH 77005 and36Ar (see text for details). Extraction 14 (NWA 1950) is not plotted becaus1r. On the NWA 1950 plot, 11 extractions fall along a correlation line vedata are shown on the plot.

tions from the shock melt pocket are well distinguishedfrom those obtained from igneous minerals; 40Ar/36Ar ra-tios lie in the range 1260–1488, while the same ratios forhost rock minerals plot at lower values (249–1365). Threeextractions from maskelynite yielded high 40Ar/36Ar ratios(1063–1565). These anomalous extractions are not corre-lated with high 39Ar/36Ar ratios, as would be expected forradiogenic 40Ar, and are more correctly attributed to excessargon in this phase (see discussion). One feature of the dataarray (Fig. 3), for NWA 1950 is a linear trend of host rockdata defining a slope, interpreted as a maximum age (sinceit may still contain some homogeneously distributed excessargon). The points included in the linefit were selected bycompiling all the data points, sorted by increasing40Ar/39Ar ratios. The data are then plotted on a 40Ar/36Arversus 39Ar/36Ar diagram, rejecting data points that fallmore than 3-sigma off the line, resulting in an array of 11points. While this procedure does not constitute an iso-chron, it does yield the fitted age defined by the data (seeWalton et al., 2007). The slope of the fitted line correspondsto an age of 382 ± 36 Ma for NWA 1950, although the lineindicates an anomalously high 40Ar/36Ar intercept value of277 ± 16 which appears to be slightly higher than previousvalues determined by laser spot dating (Walton et al., 2007).This fitted age is approximately twice that of the Rb–Sr(185 ± 11 Ma; Borg et al., 2002) age obtained for a geneti-cally related meteorite ALH 77005, the interpretation ofwhich is discussed in Section 5.3.2.

The data obtained from ALH 77005 are shown as a plotof 40Ar/36Ar versus 39Ar/36Ar in Fig. 2. The data are farmore scattered than NWA 1950 extractions, and tend tolie closer to the 40Ar/36Ar axis. This indicates higher relativeconcentrations of non-radiogenic argon. In general, meltpockets have the highest 40Ar/36Ar ratios (1600–2026) andlie closest to the 40Ar/36Ar axis, while unmelted igneous

0 10 4020 3039 36Ar/ Ar

ALH 77005 shock melthost rock mineralsmelt-intruded olivinehost rock mineralsincluded in the fit

NWA 1950. Data have been corrected for cosmogenic production ofe the data point is dominated by cosmogenic 36Ar. Error crosses arersus 3 extractions for ALH 77005, shown in Fig. 3. All ALH 77005

Slope = 33.7 ± 3.5 (2σ)Inter = 277.0 ± 16MSWD = 1.10

host rock minerals

500

12 24 28

1300

1100

900

700

300

1000 4 8 16

39 36Ar/ Ar

4036

Ar/

ArNWA 1950

20

Slope = 16.17 ± 3.5 (2 σ)Inter = 488.0 ± 97MSWD = 2.10

host rock minerals

500

30

1300

1100

900

700

300

1000 10 20 40

39 36Ar/ Ar

ALH 77005

50

Fig. 3. 40Ar/36Ar versus 39Ar/36Ar plot of NWA 1950 and ALH 77005 data used in the linefit. All extractions are from host rock minerals(NWA 1950 spots 1, 2, 3, 4, 5, 8, 9, 12, 13, 17 and ALH 77005 spots 3, 4, 6; see Appendix). For NWA 1950, an age of 382 ± 36 Ma is obtainedfrom the slope of the line fitted to the data (40Ar/39Ar ratio) and the measured J value (0.007) (see text for details). The fitted age obtained forALH 77005 using the same method is 194 ± 77 Ma.


minerals possess lower 40Ar/36Ar ratios (�300). Extractionsfrom granular olivine cut across by melt vein networksform a mixing array between the two (40Ar/36Ar = 940–1509). Apparent ages for individual laser spot extractionsrange from 373–8108 Ma, with the highest ages obtainedfrom melt pocket extractions. Almost 50% of the data, par-ticularly the data from the melt pocket (Fig. 1), yield impos-sibly old ages, and there is no concentration of ages around4.0 Ga. On applying the same procedure to ALH 77005host rock data, we find that only the first three data points(those with the lowest 40Ar/39Ar ratios) can be taken before

1.1

0.3

100 300 500 800

1.5

1.3

0.9

0.7

0.5

0.10 200 400 600 700

37 36Ar/ Ar

3836

Ar/

Ar

NWA 1950

Fig. 4. 38Ar/36Ar versus 37Ar/36Ar isochron diagram for NWA 1950 aincluded in the fits and open symbols are used for extractions not included36Ar (see text for details). For NWA 1950, extraction number 14 is not sh36Ar; extraction numbers 8 and 10 are not included in the fit. All ALH 770included in the fit. Error crosses are 1r.

the scatter becomes large, resulting in an imprecise fittedage of 194 ± 77 Ma (Fig. 3). The large error reflects thesmall sample size. Although this age is within error of theRb–Sr age for ALH 77005, it is clearly very different fromthe majority of analyses from this sample (apparent agesfrom individual laser probe extractions), which yield agesolder than those from NWA 1950.

CRE ages were determined from plots of 38Ar/36Ar ver-sus 37Ar/36Ar, as shown in Fig. 4. Using cosmogenic 38Ar(38Arc), the CRE ages for NWA 1950 and ALH 77005are 1.5 ± 0.3 Ma and 2.1 ± 0.4 Ma, respectively.

037 36Ar/ Ar

ALH 77005

100 300 500 800200 400 600 700

nd ALH 77005. For both plots, solid symbols denote extractionsin the fits. Data have been corrected for cosmogenic production of

own on the plot because the data point is dominated by cosmogenic05 data are shown on the plot, however, extraction number 3 is not


4.2. Stepped heating results

Two separates of maskelynite from NWA 1950 were la-ser step heated using a similar setup to the spot dates ex-cept individual grains were heated for �1 min. Theremaining melt pocket proved to be too small to obtainmeaningful results. Both experiments yielded similar re-sults with initial steps over 1.0 Ga, falling in a decreasingstaircase to values of 360 and 362 Ma (Fig. 5a, b). In bothcases, complete melting of the sample at higher tempera-tures released large amounts of gas and the second halfof the release pattern therefore has low resolution. Hadthey been extracted in smaller steps, it is conceivable thatthe ages might have continued to decline, although bothreached very similar values to the isochron obtained fromthe laser spot data.

No sample of ALH 77005 was step heated since severalother analyses of this meteorite exist in the literature (seeSection 5.1). The release pattern obtained by Bogard andGarrison (1999) is shown in Fig. 5c for comparison.

NWA 1950 maskelynite 1

Cumulat

0

2000

4000

6000

8000

0.0 0.2 0.4

ALH 77005 mas

3940

Ar-

Ar A

ge (M

a)

a

c

3940

Ar-

Ar A

ge (M

a)

39Cumulative Ar fraction

0

1000

2000

3000

4000

5000

0.0 0.2 0.4 0.6 0.8 1.0

360 Ma

Fig. 5. Apparent 39Ar–40Ar ages (boxes) as a function of fractional relea1950 maskelynite, and (c) ALH 77005 maskelynite separates from Bogarsteps are shown as filled boxes and rejected steps are open.

5. DISCUSSION

The purpose of analyzing two lherzolitic shergottitesexhibiting contrasting degrees of shock damage was tocompare their Ar–Ar characteristics in order to shed lighton the debate over their antiquity, and the debate overthe meaning of apparently old ages obtained from some re-gions of the samples (melt pockets). If these two meteoritescrystallized >4.0 Ga, then the more strongly shock heatedmeteorite (ALH 77005) should have lost more radiogenic40Ar and should yield lower ages.

5.1. Previous Ar–Ar work on ALH 77005 and NWA 1950

data from this study

Spatially resolved (laser probe) argon isotopic analyseswere first applied to a Martian meteorite by Jessbergeret al. (1981). By combining laser extractions on individualmaskelynite grains with whole rock stepped heating ofALH 77005, Jessberger et al. (1981) found ‘extra’ 40Ar

200

NWA 1950 maskelynite 2

39ive Ar fraction0.6 0.8 1.0

kelynite (Bogard and Garrison, 1999)

b

39Cumulative Ar fraction

0

400

600

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0.0 0.2 0.4 0.6 0.8 1.0

362 Ma

se of 39Ar for stepwise temperature extractions in (a and b) NWAd and Garrison (1999). The width of the age boxes are 2 r. Plateau


released simultaneously with in situ radiogenic 40Ar fromseveral individual maskelynite grains, with varying amountsdistributed inhomogeneously in the grains. Three ages ob-tained from one grain yielded an age of 500 Ma, however,the ages within two other grains varied from 700 Ma to1.7 Ga. Stepped heating release patterns for ALH 77005are also available in Bogard and Garrison (1999). Likemany of the laser spot analyses for ALH 77005 in the pres-ent study, the final 30% of stepped gas release yieldedimpossibly old ages, greater than 4.567 Ga (Fig. 5c). The re-lease patterns exhibit scattered rising ages with mean agesover 1.0 Ga in both studies, though the ages reported byBogard and Garrison (1999) are significantly older.

5.2. Cosmic-ray exposure ages

The CRE ages for NWA 1950 and ALH 77005, deter-mined in this study using cosmogenic 38Ar (38Arc), are1.5 ± 0.3 Ma and 2.1 ± 0.4 Ma, respectively (Fig. 4). TheCRE age for ALH 77005 is within the range of ages re-ported for this meteorite by various chronometers (10Be2.5 ± 0.3 Ma, Nishiizumi et al. (1986a); 10Be2.8 ± 0.6 Ma, Pal et al. (1986); 38Ar � 2.6 Ma, Bogardet al. (1984); Nyquist et al. (2001) calculate an averageexposure age of 2.87 ± 0.2 Ma). NWA 1950, however,yielded a slightly younger apparent age than previously

9000

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4000

3000

2000

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00 500 1000 1500

Age

(Ma)

40A

Fig. 6. 39Ar–40Ar ages uncorrected for the atmospheric component versuNWA 1950 (this study) and data from Walton et al. (2007) from shergottextractions from ALH 77005 and NWA 1950 shock melts exceed 4.567 Gobtained for host rock minerals. Although scattered, data from Walton eminerals and melt pockets are distinguished from one another with theprevious work plot at, or in excess of, 4.567 Ga.

determined by 3He (5.3 ± 3.0 Ma), 21Ne (3.5 ± 0.8 Ma)and 38Ar (2.3 ± 1.0 Ma) (Gillet et al., 2005). As noted inSection 3, 1.5 Ma represents a minimum age because ofthe method used for CRE age determination.

5.3. Argon isotope composition of ALH 77005 and NWA

1950

5.3.1. Ar–Ar ‘ages’ of shock melts

In both meteorites, the apparent ages derived fromshock melt extractions are oldest (4.6–6.9 Ga, NWA 1950;2.7–8.2 Ga, ALH 77005). The observed range in granularolivine 40Ar/36Ar ratios in ALH 77005 is interpreted to re-sult from the presence of shock melt in each extraction;those samples containing a higher proportion of shock melthave higher 40Ar/36Ar ratios compared to extractions con-sisting predominantly of olivine with minute stringers ofshock melt. The extreme heterogeneity in texture and abun-dance of shock melt in this meteorite has been documentedby Walton and Herd (2007a). The most striking observa-tion is that all of NWA 1950 shock melt data, and morethan half of the ages derived from ALH 77005 shock melts,are impossibly ancient, older than the Solar System itself(4.567 Ga; Fig. 6). Moreover, ancient ages (>4.567 Ga)from shock melts are known in meteorites, in particularthe Peace River L6 chondrite, studied by Ar–Ar stepped

2000 2500 3000 350036r/ Ar

This study, host rockThis study, melt pocketWalton et al. 2007, host rockWalton et al. 2007, melt pocket

s 40Ar/36Ar ratios of shock melt extractions from ALH 77005 andites Los Angeles, Zagami, NWA 1068 and DaG 476. Nearly all the

a (grey box), and are clearly distinguished from the younger agest al. (2007) show that the ages and 40Ar/36Ar ratios from host rockoldest ages from shock melts. A few of the data points from our


heating and localized outgassing by a laser probe (McCon-ville et al., 1988). McConville and co-workers demonstratedthat the groundmass revealed an age for the reheatingevent, whereas extractions from the shock melt gave vari-able and meaningless ages, in some cases older than the So-lar System. This experiment indicates that rapidly formedshock melt veins do not reliably yield Ar–Ar age data andare unlikely to record meaningful geochronological infor-mation. Although we note that some shock melts haveyielded impact ages (e.g., Buchanan et al., 2005), this isnot the case for Martian meteorites where the impossiblyancient ages derived from ALH 77005 and NWA 1950shock melts in this study are consistent with the results ofMcConville et al. (1988)—that shock melt ages are mean-ingless in terms of any real event. For Martian meteorites,it is clear that an event is not being dated, but may reflect anaveraged trapped gas which did not escape during suddenheating and melting (e.g., McConville et al., 1988) or morelikely incorporation of ‘radiogenic’ Martian atmosphere,the latter based on a number of well-founded lines of evi-dence as reviewed in the following paragraphs.

The high 40Ar/36Ar ratios obtained from NWA 1950and ALH 77005 melt pockets overlap with that that ofthe Martian atmosphere inferred from other shergottites(�1600–2400; Becker and Pepin, 1984; Bogard et al.,1984; Swindle et al., 1986; Bogard and Garrison, 1999; Wal-ton et al., 2007). A Martian atmospheric origin for trappedgases is supported by the discovery of other components inthe melt pockets, notably unique xenon and nitrogen signa-tures (see review of volatiles in Bogard et al., 2001), but alsoN2 and CO2 (Becker and Pepin, 1984; Carr et al., 1985).This argument is bolstered by the success of laboratoryshock recovery experiments in implanting a sample ofambient gas into basalt by shock in relatively large amountsand at moderately low shock pressure without mass frac-tionation of heavy noble gases and nitrogen (Bogardet al., 1986; Wiens and Pepin, 1988).

We also find that a trapped Martian atmospheric com-ponent is consistent with models for shock melt pocket for-mation in shergottites. Heterogeneity in rocks has beenshown to profoundly affect the way they respond to shockwaves (Stoffler et al., 1991; Heider and Kenkmann, 2003;Beck et al., 2007; Stewart et al., 2007). A void collapsemechanism for large melt pockets in shergottites (i.e, thosefound to exhibit the highest 40Ar/36Ar ratios; Walton et al.,2007), is strongly favored by physical models (Beck et al.,2007), shock recovery experiments (Heider and Kenkmann,2003; Stewart et al., 2007), and petrography, compositionand redox state of the pockets (Walton and Herd,2007a,b). Thus, shock melts formed by void collapse wouldbe expected to contain a sample of the Martian atmosphere,because open space in the rock would be in equilibriumwith the Martian atmosphere particularly in rocks thathave crystallized near to the surface (e.g., Zagami; McCoyet al., 1992; Marti et al., 1995).

On Mars, the 40Ar/36Ar ratio of the atmosphere (1600–2400) is larger than that of the mantle (�500). This isopposite to the situation on Earth, where atmospheric40Ar/36Ar = 295.5 and the upper mantle 40Ar/36Ar >40,000. The terrestrial example has been used to argue that

the 40Ar/36Ar ratio of the Martian mantle could not begreater than that of the Martian interior and that the excessAr in shergottites is indigenous and radiogenic rather thantrapped (see box model of Bouvier et al., 2008). However,this excludes one of the most important aspects of the Mar-tian atmosphere—the fact that it is not conservative. Theatmospheric composition of the terrestrial system is dic-tated by irreversible degassing of the planetary interior;once it reaches the atmosphere it is retained. There is anextensive and detailed literature discussing the evolutionof the Martian atmosphere that takes into account the sig-nificant loss of volatiles by sputtering (Luhmann et al.,1992; Jakosky et al., 1994; Hutchins and Jakosky, 1996;Jakosky and Phillips, 2001; Catling, 2004). For example,the Martian atmosphere is estimated to have lost as muchas 99% of its nitrogen (cf. Fox and Hac, 1997; McElroyet al., 1977). Models of the Ar concentration and isotopecomposition of the Martian atmosphere are reviewed byBogard et al. (2001).

It is thus concluded that the old (4.0 Ga) Ar–Ar ages re-ported in this and previous Ar–Ar studies of shergottites,do not represent any meaningful age data and do not sup-port ancient crystallization ages for shergottites. However,we do note that spatially resolved argon data might be usedto shed light on the petrology of shergottite magmas (seefollowing section).

5.3.2. Ar–Ar systematics for igneous minerals

In order to interpret the Ar–Ar system of igneous min-erals in our studied samples, we briefly revisit the degreeof shock damage recorded in the investigated meteorites.The equilibration shock pressure of NWA 1950 is less thanthat of ALH 77005, as indicated by petrographic observa-tions (Table 1) calibrated from shock-recovery experiments(e.g., Dodd and Jarosewich, 1979; Stoffler et al., 1991). Themagnitude of post-shock temperature increase is correlatedwith shock pressure; if two rocks, similar in terms of poros-ity, mineralogy, grain size and ambient temperature, expe-rience different degrees of shock metamorphism, the rockexperiencing greater shock pressure will be more stronglyheated (e.g., Stoffler et al., 1991). Quantitative determina-tion of DT has been described by Artemieva and Ivanov(2004) and reviewed by Fritz et al. (2005). At shock pres-sures in the range of 20–30 GPa, the post-shock tempera-ture elevations vary between 50 and 100 �C, from 30 to35 GPa they increase up to 200 �C, and at 45 GPa a post-shock temperature of �600 �C is reached; for shock com-pression of 55 GPa, a post-shock temperature increase of�1000 �C is calculated (Fritz et al., 2005). Thus, ALH77005 has experienced a greater post-shock temperature in-crease (1000 �C) compared to NWA 1950 (200 �C). In addi-tion, a general relationship between shock melt abundanceand equilibration shock pressure has been established: asshock pressure increases, so does the modal abundance ofshock melt. In this study, the more strongly shockedALH 77005 contains up to 29 vol% shock melt comparedto 1.8 vol% in NWA 1950. A correlation between meltabundance and shock pressure has been observed in anindependent study on shergottite EET 79001 (Fritz et al.,2005).


The shock features of both samples (Table 1) confirmthat ALH 77005 has been more strongly shocked andheated compared to NWA 1950. As stated in Section 1,we use this to test the ancient age hypothesis. On a plotof 40Ar/36Ar versus 39Ar/36Ar, extractions from host rockminerals in NWA 1950 show a correlation line (Fig. 3) thatyields a fitted age of 382 ± 36 Ma, and the lowest ages inthe step heating release were �360 Ma. These are not in factequivalent since the fitted age involves the removal of aninitial contaminating component with a 40Ar/36Ar ratio of277 that is most likely attributed to terrestrial contamina-tion (40Ar/36Ar = 296). Nevertheless, both the age and con-taminant values clearly are not ancient (>4.0 Ga) but areolder than their formation ages determined by Sm–Ndand Rb–Sr geochronometers (obtained from genetically re-lated ALH 77005). The igneous minerals therefore also con-tain excess 40Ar of which there are three conceivableorigins: (1) shock-implanted Martian atmospheric Ar (seeSection 5.3.1), (2) 40Ar arising from in situ decay that wasnot completely degassed, or (3) excess 40Ar acquired bythe parent magma. We rule out excess 40Ar in igneous min-erals from the Martian atmosphere based on the followinglines of evidence. The Martian atmospheric signature hasbeen shown to be specifically sited with shock melts (Martiet al., 1995; Bogard and Garrison, 1999). The two thick sec-tions analyzed for argon were investigated in detail usingSEM and EM. Although a shock melt intruded zone wasdocumented near to the pockets of shock melt, areas>200–300 lm outside this zone are igneous with no signsof melting (grain boundary etc.). Shock melt and thereforeMartian atmospheric argon contamination in the igneousminerals is not likely. Also, in the stepped heating measure-ment, ages were obtained on pure maskelynite separatesand because of the clear (transparent) nature of thesegrains, any contaminating shock melt would be easily visi-ble. Furthermore, the excess argon component in igneousminerals has a 40Ar/36Ar ratio that is distinct from that of

12

10

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8

6

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Age (Ma)

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num

ber

Fig. 7. Histograms for NWA 1950 and ALH 77005 showing the frequenextractions assuming all 40Ar is radiogenic. The ages have been calculatintruded olivine (ALH only) (see Appendix A). For NWA 1950, extractio1950 are younger and more reproducible compared to the older and mo

the shock melts (i.e., it is not Martian atmospheric inorigin).

To further consider an ancient age for shergottites, wenote that to be reconciled with a hypothetical 4.0 Ga age,than the much younger ages that we measure must be ex-plained by the loss of most of the radiogenic 40Ar. Themeteorite suffering the stronger thermal event(s) shouldyield lower ages. Following this line of reasoning, the morestrongly shock heated sample (ALH 77005), should yieldlower ages because of degassing effects. Instead, althoughALH 77005 data on a plot of 40Ar/36Ar versus 39Ar/36Arare scattered and yield an imprecise fitted age, apparentages calculated from individual host rock extractions rangefrom �373–2900 Ma. These ages are within the range ofthose from NWA 1950 (�500–3300 Ma), and the NWA1950 apparent ages are, in general, younger and morereproducible compare to those of ALH 77005 (Fig. 7).Additional stepped heating measurements of pure maskely-nite separates in ALH 77005 (Jessberger et al., 1981; Bo-gard and Garrison, 1999) yield ages of 500 Ma and700 Ma–1.7 Ga. It is considered significant that all of thestepped heating ages from igneous minerals for NWA1950 and ALH 77005 are less than �2.0 Ga. In addition,the Ar–Ar ages are not anti-correlated with shock, as mightbe expected and thus argue against the 40Ar being residualfrom incomplete shock degassing. Ruling out Martianatmospheric and undegassed radiogenic argon as the sourceof excess 40Ar in igneous minerals, we favor the conclusionsof Bogard and Park (in press) and Park et al. (2008); thatexcess 40Ar in igneous minerals is inherited from the parentmagma. By compiling a database of all analyzed shergot-tites these authors demonstrate a strong relationship be-tween K content and excess Ar concentration, indicatinga similar concentration of excess argon dissolved in all ofthe meteorites. This is strong evidence for a trapped compo-nent from the Martian mantle. This interpretation is alsoconsistent with results from our previous Ar–Ar laser probe

0 1000 700050003000

Age (Ma)

ALH 77005

9000

cy (number) of apparent ages, calculated for individual laser probeed for host rock minerals, shock melts (NWA and LH) and melt-n number 14 is not plotted (negative). In general, ages from NWA

re scattered age data obtained for ALH 77005.


study (Walton et al., 2007), wherein trapped 40Ar wasfound to occur in individual igneous minerals of Los Ange-les, NWA 1068 and DaG 476 and the fitted ages of theseminerals were found to be older than ages determined fromSm–Nd or Rb–Sr geochronometers.

5.4. Cratering chronology of Mars and impact history of

Martian meteorites

Here, we compare the petrography of Martian meteor-ites with what is known of the nature of the Martian surfacefrom remote observation. The impact history is consideredan important parameter; it is a relative means of determin-ing age that is independent of isotopic arguments. A 4.0 Gaage would correspond to the Noachian Period (>3.5 Ga;Tanaka, 1986) while young ages (180–575 Ma) correspondto the late Amazonian Period of Martian history (<0.3–0.6 Ga; Hartmann and Neukum, 2001). We first considerwhat Mars would have been like during these Periods andthe processes that rocks residing near the Martian surfaceduring these Periods would likely record. We consider sur-ficial processes because some Martian meteorites, fine-grained basalts and glass-bearing basalts, are well con-strained to have crystallized as lava flows of variable thick-ness based on the suppression of plagioclase nucleation andhence rapid cooling (Yamato 980459; Greshake et al.,2004), flow alignment of pyroxenes (e.g., McSween, 1994),and cooling rates derived from diffusion profiles in olivine(0.3–3 �C/h; Mikouchi et al., 2001; Mikouchi and Miyam-oto, 2002). The petrography of ALH 84001 is used as aproxy for an ancient Martian rock, because an ancientage for this meteorite, 4.51 ± 0.11 Ga, is widely accepted(cf. Nyquist et al., 2001).

It is well established that the cratering flux has changedthrough time (Strom et al., 1992; Ivanov, 2001; Neukumet al., 2001; Stoffler and Ryder, 2001) and before 3.5 Ga,the cratering rate was higher (Hartmann and Neukum,2001). By definition, surfaces of Noachian age approachthe saturation equilibrium density (the empirical upper lim-it for crater density; Hartmann and Neukum, 2001) and theoldest, most heavily cratered surfaces are thought to beabout 4.0 Ga old (Hartmann and Neukum, 2001; Tanaka,1986). During the Noachian Period, it is most likely thatthe Martian highlands were gardened to depths on the or-der of a kilometer or more (see calculations by Headet al., 2002 and references therein). These regions are an-cient, intensely faulted and densely cratered, largely com-prising impact breccias formed early in the planet’shistory when impact rates were high (e.g., Tanaka, 1986).These breccias were subsequently weathered pervasivelyand loosely consolidated by evaporites (Ming et al., 2006;Squyres et al., 2006a; Squyres et al., 2006b). Remotelysensed data has demonstrated the presence of widespreadphyllosilicates in Noachian rocks, supporting an early ac-tive hydrologic system on Mars (Poulet et al., 2005). Thework of Hartmann (1999), Hartmann and Berman (2000),Hartmann and Neukum (2001) and Basilevsky et al.(2006) supports previous Mariner 9 era suggestions ofyouthful Martian volcanism, as evidenced by observationsof Arsia Mons, Elysium Planitia, Amazonis Planitia and

Olympus Mons. The higher resolution imagery from MarsGlobal Surveyor (MGS) has shown the presence of thin,nearly craterless lava flows on areas that appeared moreheavily cratered at Viking resolution, significantly extend-ing the Martian surface covered by young basalts (Hart-mann and Berman, 2000; Hartmann and Neukum, 2001).Observation of the Martian surface by MGS has estab-lished that the contemporary surface of Mars continues torecord processes of impact cratering, with twenty new im-pact craters, 2–150 m diameter, created between May1999 and March 2006 (Malin et al., 2006). These recentobservations confirm that impact cratering is an ongoingprocess shaping the Martian surface, that surfaces on Marsdevoid of craters are truly young and that possible Martianmeteorite-forming events continue to present day.

We therefore approach the following discussion,expecting that ancient rocks should testify to processesof impact bombardment and megaregolith formation, withsubsequent weathering and cementing, while young rocksshould record a relatively simple history of emplacement,minor alteration or weathering, possible impact relocationduring their residence near the Martian surface, and ejec-tion in an impact event. We first consider the case for theonly undisputed ancient rock from Mars, ALH 84001, andlater discuss chassignites, nakhlites and shergottitesseparately.

5.4.1. Allan Hills 84001

Among Martian meteorites, there is only one monomictbreccia: orthopyroxenite ALH 84001. Petrologic character-istics indicate that brecciation developed on Mars beforethe ejection event (e.g., Treiman, 1998). ALH 84001 con-tains fracture zones, glass of orthopyroxene composition,and lacks shock melts (veins and pockets) and associatedhigh-pressure, high-temperature minerals such as stishovite(Mittlefehldt, 1994). Carbonate rosettes (6250 lm across)comprise �1 vol% of the rock and are zoned in terms ofCa-, Fe- and Mg from core to rim, with magnetite andFe-sulfides associated with Fe-rich domains (Mittlefehldt,1994; Treiman, 1995; Harvey and McSween, 1996; McKayet al., 1996). Minor phyllosilicates and silica veins have alsobeen reported (Mittlefehldt, 1994). The fracture zones rep-resent the earliest deformational event recorded by the rockand have been interpreted to predate the carbonates (e.g.,Scott et al., 1998); the latter formed at 3.9–4.0 Ga (Borget al., 1999). Multiple generations of these fractures havebeen observed, leading to debate over the details of theseprocesses, in particular on the number of shock events re-quired and the temperature of the carbonate forming fluids(e.g., Treiman, 1995; Kirschvink et al., 1997; Treiman,1998; Barber and Scott, 2006). Regardless of the numberof impact events, it is clear that ALH 84001 has experienceda complex geological history with the effects of shock, meta-morphism and alteration superimposed on an original igne-ous texture and mineralogy. All studies indicate at least twoimpacts recorded in ALH 84001; the first related to the ma-jor (early) cratering event on Mars (�3800–4050 Ma; Ashet al., 1996; Turner et al., 1997; Borg et al., 1999; Barberand Scott, 2006), and the second responsible for ejectioninto space at �15 Ma, indicated by the CRE age (Miura


et al., 1995; Swindle et al., 1995). Shock effects observed inALH 84001, thus corroborate the idea that an ancient Mar-tian rock should be brecciated with the effects of alterationoverprinting these features.

5.4.2. Chassignites

Chassignites are represented by two meteorites: Chas-signy and NWA 2737. Both meteorites show evidence forstrong shock (>26 GPa; Langenhorst and Greshake, 1999;Fritz et al., 2005; Beck et al., 2006; Treiman et al., 2007) in-ferred from shock features such as the complete (NWA2737) or partial (Chassigny) transformation of plagioclaseto maskelynite, and olivine to wadsleyite (NWA 2737), pla-nar deformation features and mosaicism in olivine, and localshock melting (Melosh et al., 1983; Fritz et al., 2005; Becket al., 2006; Treiman et al., 2007). Detailed observations ofNWA 2737 suggest a two-stage shock history for this meteor-ite (Treiman et al., 2007), consistent with microstructural evi-dence suggesting Chassigny has experienced two shockevents (Langenhorst and Greshake, 1999), as indicated bythe presence of discontinuous fractures in olivine, whichcoexist with planar fractures. Both chassignites show evi-dence for very minor low-temperature alteration observedas calcite, Mg-carbonate and gypsum in veins within silicates(Misawa et al., 2005; Treiman et al., 2007). There is no evi-dence for brecciation in chassignites.

5.4.3. Nakhlites

Nakhlites are the least shocked of all Martian meteorites(5–20 GPa; Fritz et al., 2005), recorded as mechanicaldeformation of igneous pyroxene and olivine, includingmechanical twinning (pyroxene), undulatory extinction(pyroxene and olivine), and planar fractures (olivine) (Fritzet al., 2005; Treiman, 2005). Alteration mineral assemblagesin nakhlites reach up to 1–2 vol% and have been the subjectof much investigation (see Treiman, 2005 for a review);olivine and mesostasis glass have been partially replacedby patches and veinlets of iddingsite. Rare salt assemblageshave also been reported (halite, siderite and anhydrite/gyp-sum). The timing of nakhlite alteration is thought to bewithin the last several hundred Ma (Swindle et al., 2000).There is no evidence for brecciation or multiple shockevents in nakhlites.

5.4.4. Shergottites

The shock melts (veins and pockets) in shergottites exhibita cross cutting relationship with primary igneous minerals(Fig. 8a–f), as well as with expansion fractures associatedwith maskelynite (Fig. 8e) and planar/irregular fractures inpyroxene, olivine and chromite (Fig. 8a, b, e and f). Shockmelts postdate late-stage igneous crystallization assemblagesincluding shearing and displacement of felsic mesostasis(Fig. 8c) and melting of pyroxferroite breakdown material(Fig. 8d). Clasts of plagioclase, entrained within the shockmelt, display birefringence (Fig. 8b, e); however, adjacentto the shock melt, plagioclase is transformed to maskelynite.The petrographic relationships illustrated in Fig. 8 show thatthe melt products record the last major event before the mete-orite’s transit through the Earth’s atmosphere and terrestrialalteration. Some shergottites have been found to contain

granular zones and veins (Ikeda, 1997); however crush zonesare minor and local in the meteorites. These veins are identi-cal to those found in some chondrites (see images in Coleman,1977; Buchanan et al., 2005), which are an extension of thethin shock veins in the meteorites (Langenhorst and Poirier,2000).

Some Martian basalts have a single stage impact historyassociated with ejection (Yamato 980459; Greshake et al.,2004). We review these briefly here. Yamato 980459 is theonly shergottite that does not contain plagioclase/maskely-nite and has thus had an unusual magmatic history. Calcula-tions by McKay and and Mikouchi (2003) show thatplagioclase nucleation was suppressed because of rapid cool-ing, suggesting the basalt was erupted quickly as a thin lavaflow onto the surface of Mars, consistent with the observedflow alignment (Greshake et al., 2004). Yamato 980459 istherefore the most shallowly emplaced shergottite recoveredto date; it is ideal for considering near surface processes onMars. Despite its near surface residence, Yamato 980459 isthe least shocked of all shergottites (20–25 GPa; Greshakeet al., 2004; Fritz et al., 2005). The glassy residual matrix ofthis meteorite, most susceptible to aqueous alteration, hasbeen shown by TEM studies to be pristine (Greshake et al.,2004). Shock effects in constituent igneous minerals, olivineand pyroxene, in agreement with previous studies of Sher-gotty, demonstrate that the type and homogeneity of shockdamage of constituent igneous minerals indicates a singleshock event associated with ejection (Stoffler et al., 1986;Muller, 1993; Greshake et al., 2004). A shock pressure of20–25 GPa (Greshake et al., 2004) for Yamato 980459 is ingood agreement with the spallation model, proposed by Me-losh (1985) for ejection of meteorites. In this case, the mostshallowly emplaced rocks record the weakest shock pressure.Finally, the trace element distribution between coexistinghigh-pressure minerals in shock veins of Zagami has beenstudied to measure the duration of the pressure pulse duringimpact, and so to infer the size of the crater from impact scal-ing laws (Beck et al., 2005). The results show that the veinsformed in an impact event having a shock duration of�10 ms generating a crater 1.5–5.0 km, consistent with thecrater size predicted from models of shergottite ejection(�3 km; Head et al., 2002). This relationship ties formationof the veins to the ejection event.

The evidence used to establish a Martian origin for thesecondary mineral assemblages in nakhlites and ALH84001 has not been as rigorously demonstrated for shergot-tites (e.g., radiometric dating, isotopic studies, cross cuttingrelationships; Bridges et al., 2001) and some contain terres-trial salts along with Martian assemblages. Trace amountsof Ca- and Fe-sulfates, calcite and S-rich aluminosilicategrains have been identified in shergottites (Gooding et al.,1988). We see no evidence for brecciation or alteration inshergottites and some basalts show evidence for a single stageimpact history. Shergottites are thus pristine crystalline igne-ous rocks that record conditions of strong shock >20–55GPa.

5.4.5. Summary of shock in ALH 84001 and SNCs

The oldest rock from Mars, ALH 84001 is highlyshocked (32 ± 1 GPa) and brecciated, containing ancient

Fig. 8. BSE images of shock melts in basaltic (a–d) and lherzolitic (e and f) shergottites. (a) The melt has ‘healed’ irregular fractures in Zagamipyroxene and maskelynite. The white box shows the location for (b). (b) Igneous exsolution lamellae in pyroxene are cut across by the shockmelt. The arrow indicates the stringer of plagioclase glass. (c) A felsic, late stage igneous crystallization product in Los Angeles has beensheared and offset. Maskelynite shows no evidence for brittle deformation and the shock melt (containing stishovite) has partially melted thefelsic portion at the contact. (d) Late stage pyroxferroite breakdown material (PBM), has been partially melted in direct contact with shockmelt in Los Angeles. (e) Irregular fractures in the NWA 4468 are postdated by the shock melt. Expansion fractures, typically observedemanating from the margins of maskelynite grains into neighboring pyroxene, are also healed. (f) Small, isolated shock melt postdatesfractures in pyroxene and chromite (NWA 4468). In all photos (a–f) it is noted that irregular fractures do not cut across the shock melts.


carbonate alteration (3.9–4.0 Ga), testifying to its longMartian near surface history. Nakhlites and chassigniteshave concordant formation ages at �1.3 Ga—all are crys-talline rocks that have been shocked to varying degrees withalteration assemblages in the nakhlites formed within thelast several hundred Ma. Shergottites are all pristine crys-

talline igneous rocks with alteration products of an un-known age occurring in trace amounts. The alterationproducts in shergottites are distinctly different from thosein ALH 84001 (Bibring et al., 2001); the sulfate assemblagesin shergottites are, in general, consistent with SO4 accumu-lations on younger (Hesperian and Amazonian) Martian


surfaces, compared to phyllosilicate-rich landscapes thatcharacterize the Noachian (Bibring et al., 2005; Pouletet al., 2005).

Shock features described for shergottites are from thestrongest impact event (20–55 GPa), although whether thiswas the event which ejected the rocks remains to be re-solved; whereas the ejection ages are measured by CREages, ages of shock features have not been directly dated.Petrographic observations indicate that this event occurredafter the late-stage igneous products (symplectic inter-growth and felsic interstitial products: Fig. 8), and thatshergottites have not been fractured or mechanically de-formed to any great extent following the formation ofshock melts. We suggest two possibilities for shergottiteshock history: the rocks were shocked in a strong impactevent (22–55 GPa) and remained in situ on Mars and werethen later ejected with minimal overprinting of earliershock effects, or the rocks were strongly shocked duringejection from Mars in a single shock event. If the meltsformed in situ [on Mars], then the shock pressure associ-ated with the ejection event, which in this scenario wouldpostdate the shock melts, could not have been greaterthan �10 GPa—otherwise the shock melts would also bemechanically deformed. Although we cannot unequivo-cally rule out a two-stage shock history for shergottites,we contend that a one-stage impact history associatedwith ejection of the rocks is the most straightforwardinterpretation that is consistent with petrographic observa-tions of some Martian basalts. What is clear from study ofshergottites is that they are all pristine crystalline igneousrocks that have been heavily shocked but not brecciated.

We consider these two observations—lack of brecciatedand pervasively altered rocks—as the greatest petrographiccounter-argument to an old (>4.0 Ga) age interpretationfor shergottites. Their petrography and shock history areconsistent with their crystallization as lava flows and shal-low igneous intrusions <1.3 Ga ago.

5.5. Transfer of material between Earth and Mars;

implications for the age of shergottites

In the introduction, we pose the question: If only smalltracts of Martian terrain are a few hundred million yearsold, why do we have so many young rocks from Mars inthe world’s meteorite collection? The overrepresentation ofyoung Martian meteorites presumably means that eitherthe estimates of the ages of terrains on Mars are wrongor that there is an inherent bias towards ejection of youngrocks by the impact cratering process. Based on the previ-ous arguments, we contend that the latter scenario is con-sistent with the petrography of the shergottites; thelikelihood of a rock becoming a meteorite depends onthe age of the terrain because the ejection process is moreefficient for younger, more consolidated and less alteredrocks (Jones, 1989), as discussed by Warren (1994), Headand Melosh (1999), Head et al. (2002) and Fritz et al.(2007). The highly brecciated material from the ancientMartian highlands is likely discriminated against in thelaunch process, because the near surface environment ofMars has been subject to active weathering, via aqueous

alteration or wind erosion, or a combination (Minget al., 2006; Squyres et al., 2006a; Squyres et al., 2006b),in addition to potential effects of glacial weathering (e.g.,Shean et al., 2007). The impact glasses of ancient rockswould be particularly susceptible to this breakdown. Thisfriable and fine-grained sediment is unlikely to be ejectedintact and by mantling the Martian basement, it decreasesthe likelihood of large fragments escaping. This is becausecoherent, homogeneous material is required for build-upof maximal pressure gradients (Head et al., 2002). Addi-tional conditions contributing to favorable launch ofyoung rocks from Mars lie in the source regions for theserocks: Tharsis, Elysium and Amazonis Planitia, all ofwhich are partially covered by young lava flows. Aspointed out by Gladman (1997) and Fritz et al. (2005,2007), these regions are at low latitudes (30�N to 30�S)and young basaltic terrains are commonly at high eleva-tions. The equatorial position of these terrains means thattheir angular velocity (24 km/s at the equator) may addenergy to the launch process depending on direction. Also,the high elevation (up to 5 km above datum) correspondsto a shorter trajectory length for rocks traversing theatmosphere because atmospheric density at such elevationis 2.3 times lower.

6. CONCLUSION

Petrography is a strong tool in interpreting isotope sys-tems of samples possessing complex geologic histories. Theoldest, most heavily cratered surfaces on Mars are thoughtto be 4.0 Ga old; rocks on Mars dating to the NoachianPeriod (>3.5 Ga) should record multiple impacts reflectingmegaregolith formation and the cumulative effects of ero-sion and aqueous alteration occurring during or since thatera. The only proven ancient rock from Mars, ALH84001, records a history of multiple impact bombardmentincluding breccia formation and subsequent alteration(carbonate formation at 3.9–4.0 Ga). In contrast, all mete-orites dated by various methods to be 1.3 Ga old (chassig-nites and nakhlites) are crystalline igneous rocks, and notbreccias. What is clear from study of the shergottites isthat they are crystalline igneous rocks that record minimalweathering or alteration; their petrography is consistentwith crystallization <1.3 Ga ago than at a much earliertime.

Our observation that laser probe Ar–Ar ages from igne-ous minerals do not correlate with the degree of shock heat-ing is consistent with the interpretation of our youngisochron (382 Ma) and stepped heating ages (360 Ma) asyoung crystallization ages with minor amounts of excess40Ar inherited from the magma. Our Ar–Ar results forshock melts—ages in >4.567 Ga and 40Ar/36Ar ratios thatoverlap with previous measurements of the Martian atmo-sphere—indicate that shock melt ‘ages’ are meaningless interms of any real event. Our results support the view thatshergottites have ages <1.3 Ga. A plausible explanationfor the overrepresentation of young meteorites is thatyoung Martian rocks are less likely to have been brecciatedand/or altered than old rocks and therefore better able towithstand the stresses of transit to Earth.


ACKNOWLEDGMENTS

This work has been funded by the Natural Sciences and Engi-neering Research Council of Canada (NSERC), the CanadianSpace Agency and the Alberta Ingenuity Fund through grantsawarded to E.L.W., NSERC Discovery Grant 261740-03 toC.D.K.H. and the Leverhulme grant awarded to S.P.K. Thanksto John Spray at the New Brunswick for preparing the samples

of both meteorites into tiles for the argon work. The authors grate-fully acknowledge the Antarctic Meteorite Working Group forallocation of the ALH 77005 chip used in this study. The NWA1950 sample was purchased for the University of Alberta usingfunds from the Department of Earth and Atmospheric Sciences.Thanks to Don Bogard, Yukio Ikeda and an anonymous reviewer,whose comments, along with those of the associate editor (GregoryHerzog), improved an earlier draft of this manuscript.

APPENDIX A

Isotopic data for irradiated Martian meteorites ALH 77005 and NWA 1950 analyzed by stepped heating and laser spotextraction

Sample
40Ar ± 39Ar ± 38Ar ± 37Ar ± 36Ar ± 40Ar/39Ar ± Age (Ma) ±
Stepped heating NWA 1950

A step 1
21.60 0.20 0.0395 0.0072 0.003 0.012 15.46 0.13 0.0600 0.0113 546.69 99.92 2840.0 261.5 A step 2 5.30 0.19 0.0644 0.0072 0.040 0.013 7.92 0.13 0.0240 0.0120 82.32 9.68 821.0 77.6 A step 3 30.34 0.19 0.3129 0.0072 0.040 0.012 9.81 0.13 0.0440 0.0113 96.98 2.32 934.7 17.8 A step 4 22.15 0.19 0.2880 0.0072 0.036 0.013 21.14 0.13 0.0200 0.0120 76.92 2.04 777.2 17.0 A step 5 29.87 0.19 0.4785 0.0072 0.064 0.013 7.18 0.13 0.0400 0.0113 62.42 1.02 654.0 9.4 A step 6 17.79 0.19 0.3791 0.0072 0.023 0.013 9.82 0.13 0.0240 0.0120 46.92 1.02 512.4 9.9 A step 7 68.89 0.23 1.6754 0.0138 0.240 0.013 50.64 0.13 0.1800 0.0120 41.12 0.36 456.4 4.1 A step 8 68.37 0.21 2.1683 0.0072 0.171 0.013 41.23 0.13 0.1080 0.0113 31.53 0.14 359.8 2.2 B step 1 4.58 0.19 0.0000 0.0059 0.007 0.013 7.19 0.13 N/A N/A N/A N/A N/A N/A B step 2 11.08 0.20 0.0934 0.0072 0.036 0.013 46.95 0.13 0.0480 0.0120 118.72 9.41 1091.3 65.0 B step 3 27.49 0.19 0.3501 0.0072 0.027 0.013 24.24 0.13 0.0320 0.0120 78.52 1.70 790.2 14.2 B step 4 44.53 0.23 0.6318 0.0072 0.077 0.013 33.72 0.13 0.0800 0.0120 70.49 0.89 723.6 8.1 B step 5 44.07 0.22 0.8016 0.0072 0.142 0.013 35.63 0.14 0.0960 0.0120 54.98 0.57 587.4 5.7 B step 6 119.42 0.20 2.8765 0.0102 0.379 0.013 58.39 0.14 0.2880 0.0120 41.52 0.16 460.2 2.6 B step 7 129.52 0.26 4.0817 0.0138 0.322 0.012 50.83 0.14 0.2360 0.0113 31.73 0.12 361.9 2.1 B step 8 18.84 0.19 0.4371 0.0072 0.040 0.012 5.31 0.14 0.0160 0.0113 43.09 0.83 475.6 8.3
Laser spot data for NWA 1950

spot 1
40.45 0.07 0.1265 0.0041 0.0327 0.0041 8.88 0.01 0.1576 0.0000 319.74 10.46 2120 41 spot 2 42.67 0.31 0.4612 0.0083 0.0818 0.0000 21.70 0.01 0.1262 0.0040 92.51 1.79 901 14 spot 3 61.07 0.06 0.7355 0.0083 0.0859 0.0041 25.85 0.01 0.1652 0.0040 83.04 0.94 827 8 spot 4 91.06 0.24 1.1372 0.0083 0.1349 0.0041 37.26 0.01 0.2581 0.0040 80.07 0.62 803 6 spot 5 176.19 0.30 2.0690 0.0207 0.3516 0.0058 148.99 0.01 0.5485 0.0040 85.15 0.86 844 8 spot 6 145.71 0.25 1.1171 0.0099 0.1554 0.0082 44.79 0.18 0.2161 0.0204 130.44 1.17 1170 9 spot 7 180.01 0.33 2.1782 0.0135 0.2780 0.0116 97.47 0.18 0.3302 0.0204 82.65 0.53 824 5 spot 8 224.68 0.30 4.3456 0.0174 0.3557 0.0091 50.50 0.18 0.4466 0.0204 51.70 0.22 557 3 spot 9 180.59 0.31 3.1627 0.0099 0.2984 0.0082 102.54 0.18 0.3648 0.0204 57.10 0.20 607 3 spot 10 185.05 0.40 3.6151 0.0083 0.4804 0.0046 120.66 0.01 0.4260 0.0100 51.19 0.16 553 3 spot 11 304.56 0.71 4.2725 0.0124 0.3945 0.0124 120.36 0.01 0.5341 0.0108 71.28 0.27 730 4 spot 12 238.94 1.56 3.3760 0.0124 0.3986 0.0046 100.70 0.01 0.6633 0.0108 70.78 0.53 726 5 spot 13 262.40 0.77 0.5326 0.0046 0.4518 0.0074 124.96 0.15 1.0249 0.0146 492.70 4.53 2693 15 spot 14 245.83 25.52 �0.0716 0.1943 0.4988 0.0370 129.15 0.01 0.9778 0.1321 N/A N/A N/A N/A spot 15 220.24 1.04 1.2255 0.0083 0.4988 0.0058 187.65 0.01 0.4423 0.0057 179.72 1.48 1469 10 spot 16 237.86 0.50 0.6734 0.0083 0.2617 0.0169 32.36 0.06 0.6054 0.0184 353.23 4.40 2246 17 spot 17 90.65 0.11 0.4943 0.0083 0.0899 0.0183 8.92 0.06 0.2876 0.0184 183.39 3.08 1490 18 spot 18 207.77 0.28 1.5004 0.0083 0.3352 0.0169 101.78 0.06 0.3590 0.0180 138.48 0.79 1223 7 spot 19 111.80 0.14 0.5324 0.0041 0.2044 0.0169 39.34 0.06 0.3116 0.0184 209.97 1.65 1631 10 spot 20 204.01 0.25 0.2726 0.0056 0.0998 0.0078 16.31 0.07 0.3725 0.0129 748.33 15.43 3303 32 spot 21 914.16 1.83 0.1412 0.0056 0.2061 0.0078 8.72 0.07 0.7225 0.0146 6476.37 257.70 6920 71 spot 22 1015.08 0.90 0.1747 0.0056 0.2101 0.0078 7.96 0.07 0.7107 0.0146 5810.09 186.62 6729 57 spot 23 557.06 0.61 0.3219 0.0056 0.1202 0.0078 10.44 0.07 0.4500 0.0146 1730.51 30.22 4643 30
Laser spot data for ALH77005

spot 1
2290.03 2.80 0.4584 0.0053 0.5581 0.0217 18.44 0.04 2.3031 0.0165 4995.77 58.02 6464 22 spot 2 1104.47 1.12 0.2682 0.0053 0.2841 0.0149 12.17 0.04 0.9408 0.0179 4118.09 81.39 6126 36 spot 3 32.20 0.11 0.1386 0.0068 0.2044 0.0129 106.60 0.00 0.1818 0.0082 232.31 11.41 1742 55 spot 4 36.28 0.11 0.7616 0.0068 0.1145 0.0129 38.38 0.00 0.1238 0.0045 47.64 0.45 519 5 spot 5 403.56 0.37 0.4232 0.0099 0.0899 0.0123 12.22 0.00 0.2788 0.0045 953.66 22.24 3677 37 spot 6 50.28 0.21 1.5301 0.0099 0.1267 0.0129 32.54 0.00 0.1214 0.0020 32.86 0.25 373 3
(continued on next page)


Appendix A (continued)

Sample
40Ar ± 39Ar ± 38Ar ± 37Ar ± 36Ar ± 40Ar/39Ar ± Age (Ma) ±
spot 7
186.70 0.48 1.0538 0.0068 0.1104 0.0129 34.06 0.00 0.2010 0.0045 177.18 1.23 1455 9 spot 8 1098.72 2.56 0.8328 0.0099 0.2657 0.0129 17.87 0.00 0.7853 0.0045 1319.26 15.92 4196 21 spot 9 114.83 0.15 1.5531 0.0135 0.1922 0.0129 60.69 0.00 0.2139 0.0020 73.93 0.65 752 6 spot 10 390.49 0.78 0.7307 0.0068 0.2412 0.0129 28.73 0.00 0.6864 0.0082 534.40 5.07 2808 15 spot 11 856.67 0.65 0.1993 0.0068 0.2657 0.0129 6.95 0.00 1.1522 0.0082 4299.29 146.31 6201 60 spot 12 698.52 1.60 1.2426 0.0084 0.5696 0.0056 48.17 0.02 2.3632 0.0069 562.14 3.99 2880 12 spot 13 152.07 0.44 0.3077 0.0043 0.0913 0.0056 11.14 0.02 0.2970 0.0069 494.19 7.04 2697 21 spot 14 548.71 2.08 0.4587 0.0043 0.1567 0.0056 14.21 0.02 0.3842 0.0069 1196.34 12.09 4038 18 spot 15 1576.74 1.72 0.3685 0.0083 0.3570 0.0090 12.97 0.02 1.1806 0.0098 4278.41 97.04 6192 41 spot 16 602.80 0.81 1.2240 0.0083 0.4184 0.0056 32.22 0.02 1.9115 0.0069 492.47 3.42 2692 12 spot 17 200.20 0.25 0.0282 0.0043 0.0586 0.0056 2.41 0.02 0.2114 0.0069 7100.39 1081.75 7083 270 spot 18 260.53 0.28 0.1163 0.0043 0.0750 0.0039 6.70 0.02 0.2462 0.0069 2239.34 82.72 5077 63 spot 19 326.88 0.28 0.6241 0.0083 0.1363 0.0056 14.12 0.02 0.2243 0.0069 523.79 7.02 2779 20 spot 20 76.26 0.28 0.1543 0.0043 0.0422 0.0039 5.52 0.02 0.0785 0.0057 494.22 13.87 2697 40 spot 21 642.66 0.94 0.2862 0.0041 0.1615 0.0046 11.24 0.01 0.4690 0.0089 2245.74 32.61 5081 26 spot 22 358.46 0.81 0.7271 0.0041 0.1778 0.0046 32.40 0.01 0.3834 0.0089 493.02 3.02 2694 11 spot 23 4190.37 3.97 0.3183 0.0041 0.9873 0.0084 12.64 0.01 4.3926 0.0113 13164.02 171.40 8180 25 spot 24 1239.02 2.01 0.4735 0.0041 0.4354 0.0046 15.56 0.01 1.9239 0.0089 2616.76 23.24 5342 17 spot 25 1772.99 2.34 0.2298 0.0041 0.5458 0.0046 9.00 0.01 2.9616 0.0113 7716.83 139.20 7230 33 spot 26 664.83 1.39 0.3022 0.0041 0.1574 0.0046 11.96 0.01 0.4288 0.0089 2199.74 30.44 5046 25
Mean Blank Levels

Step heating
4.70 0.02 0.0145 0.0015 0.0156 0.0018 0.208 0.005 0.043 0.002 Laser spot 2.46 0.03 0.0054 0.0001 0.0161 0.0008 0.529 0.006 0.058 0.002 Amounts of argon are expressed as 10-12 ccSTP
REFERENCES

Artemieva N. and Ivanov B. (2004) Launch of martian meteoritesin oblique impacts. Icarus 171, 84–101.

Ash R. D., Knott S. F. and Turner G. (1996) A 4-Gyr shock agefor a Martian meteorite and implications for the crateringhistory of Mars. Nature 380, 57–59.

Baker V. R. (2001) Water and the martian landscape. Nature 412,

228–236.

Baker V. R., Strom R. G., Gulick V. C., Kargel J. S., Komatsu G.and Kale V. S. (1991) Ancient oceans, ice sheets and thehydrological cycle on Mars. Nature 352, 589–594.

Barber D. J. and Scott E. R. D. (2006) Shock and thermal historyof Martian meteorite Allan Hills 84001 from transmissionelectron microscopy. Meteorit. Planet. Sci. 41, 643–662.

Basilevsky A. T., Werner S. C., Neukum G., Head J. W., vanGasselt S., Gwinner K. and Ivanov B. A. (2006) Geologicallyrecent tectonic, volcanic and fluvial activity on the eastern flankof the Olympus Mons volcano, Mars. Geophys. Res. Lett. 33.

doi:10.1029/2006gl026396.

Becker R. H. and Pepin R. O. (1984) The case for a Martian originof the shergottites—nitrogen and noble-gases in EETA 79001.Earth Planet. Sci. Lett. 69, 225–242.

Beck P., Gillet P., El Goresy A. and Mostefaoui S. (2005)Timescales of shock processes in chondritic and martianmeteorites. Nature 435, 1071–1074.

Beck P., Barrat J. A., Gillet P., Wadhwa M., Franchi I. A.,Greenwood R. C., Bohn M., Cotten J., de Moortele B. V. andReynard B. (2006) Petrography and geochemistry of thechassignite Northwest Africa 2737 (NWA 2737). Geochim.

Cosmochim. Acta 70, 2127–2139.

Beck P., Ferroir T. and Gillet P. (2007) Shock-induced compaction,melting, and entrapment of atmospheric gases in Martianmeteorites. Geophys. Res. Lett. 34, L01203. Art. No..

Bibring J. P., Langevin Y., Gendrin A., Gondet B., Poulet F.,Berthe M., Soufflot A., Arvidson R., Mangold N., Mustard J.

and Drossart P.The OMEGA Team (2005) Mars surfacediversity as revealed by the OMEGA/Mars Express observa-tions. Science 307, 1576–1581.

Bogard D. D. and Park J. (in press) 39Ar–40Ar dating of theZagami shergottite and implications for magma origin of excess40Ar. Meteorit. Planet. Sci..

Bogard D. D. and Garrison D. L. H. (1999) Argon-39-argon-40ages” and trapped argon in Martian shergottites, Chassigny,and Allan Hills 84001. Meteorit. Planet. Sci. 34, 451–473.

Bogard D. D., Nyquist L. E. and Johnson P. (1984) Noble-gascontents of shergottites and implications for the martianorigin of SNC meteorites. Geochim. Cosmochim. Acta 48,

1723–1739.

Bogard D. D., Horz F. and Johnson P. H. (1986) Shock-implantednoble-gases—an experimental-study with implications for theorigin of Martian gases in Shergottite meteorites. J. Geophys.

Res. Solid Earth Planets 91, E99–E114.

Bogard D. D., Clayton R. N., Marti K., Owen T. and Turner G.(2001) Martian volatiles: isotopic composition, origin, andevolution. Space Sci. Rev. 96, 425–458.

Borg L. E., Connelly J. N., Nyquist L. E., Shih C. Y., WiesmannH. and Reese Y. (1999) The age of the carbonates in martianmeteorite ALH84001. Science 286, 90–94.

Borg L. E., Nyquist L. E., Wiesmann H. and Reese Y. (2002)Constraints on the petrogenesis of Martian meteorites from theRb–Sr and Sm–Nd isotopic systematics of the lherzoliticshergottites ALH77005 and LEW88516. Geochim. Cosmochim.

Acta 66, 2037–2053.

Borg L. E., Edmunson J. E. and Asmerom Y. (2005) Constraintson the U–Pb isotopic systematics of Mars inferred from acombined U–Pb, Rb–Sr, and Sm–Nd isotopic study of theMartian meteorite Zagami. Geochim. Cosmochim. Acta 69,

5819–5830.

Bouvier A., Blichert-Toft J., Vervoort J. D. and Albarede F. (2005)The age of SNC meteorites and the antiquity of the Martiansurface. Earth Planet. Sci. Lett. 240, 221–233.

http://dx.doi.org/10.1029/2006gl026396


Bouvier A., Blichert-Toft J., Vervoort J. D., Gillet P. and AlbaredeF. (2008) The case for old basaltic shergottites. Earth Planet.

Sci. Lett. 266, 105–124.

Bridges J. C., Catling D. C., Saxton J. M., Swindle T. D., Lyon I.C. and Grady M. M. (2001) Alteration assemblages in Martianmeteorites: implications for near-surface processes. Space Sci.

Rev. 96, 365–392.

Buchanan P. C., Noguchi T., Bogard D. D., Ebihara M. andKatayama I. (2005) Glass veins in the unequilibrated eucriteYamato 82202. Geochim. Cosmochim. Acta 69, 1883–1898.

Carr R. H., Grady M. M., Wright I. P. and Pillinger C. T. (1985)Martian atmospheric carbon-dioxide and weathering productsin SNC meteorites. Nature 314, 248–250.

Catling D. (2004) Atmospheric evolution of Mars. In Encyclopedia

of Paleoclimatology and Ancient Environments. 1–16.Chen J. H. and Wasserburg G. J. (1986) Formation ages

and evolution of Shergotty and its parent planet fromU–Th–Pb systematics. Geochim. Cosmochim. Acta 50, 955–

968.

Coleman L. C. (1977) Ringwoodite and majorite in the Cather-wood meteorite. Can. Mineral. 15, 97–101.

Debaille V., Brandon A. D., Yin Q. Z. and Jacobsen B. (2007)Coupled 142Nd–143Nd evidence for a protracted magma oceanin Mars. Nature 450, 525–528.

Dodd R. T. and Jarosewich E. (1979) Incipient melting in andshock classification of L-group chondrites. Earth Planet. Sci.

Lett. 44, 335–340.

Foley C. N., Wadhwa M., Borg L. E., Janney P. E., Hines R. andGrove T. L. (2005) The early differentiation history of Marsfrom W-182-Nd-142 isotope systematics in the SNC meteorites.Geochim. Cosmochim. Acta 69, 4557–4571.

Fox J. L. and Hac A. (1997) The 15N/14N isotope fractionation indissociative recombination of N2+. J. Geophys. Res. Planets

102, 9191–9204.

Fritz J., Artemieva N. and Greshake A. (2005) Ejection of Martianmeteorites. Meteorit. Planet. Sci. 40, 1393–1411.

Fritz J., Greshake A. and Stoffler D. (2007) The Martian meteoriteparadox: climatic influence on impact ejection from Mars?Earth Planet. Sci. Lett. 256, 55–60.

Gaffney A. M., Borg L. E. and Asmerom Y. (2007) Disturbance ofSm–Nd, Rb–Sr and U–Pb isochrons during shock and thermalmetamorphism—an experimental approach. Lunar Planet. Sci.

XXXVIII, 1424 (Abstract).

Gillet P., Barrat J. A., Beck P., Marty B., Greenwood R. C.,Franchi I. A., Bohn M. and Cotten J. (2005) Petrology,geochemistry, and cosmic-ray exposure age of lherzoliticshergottite Northwest Africa 1950. Meteorit. Planet. Sci. 40,

1175–1184.

Gladman B. (1997) Destination: Earth. Martian meteorite delivery.Icarus 130, 228–246.

Gooding J. L., Wentworth S. J. and Zolensky M. E. (1988)Calcium carbonate and sulfate of possible extraterrestrial originin the EET 79001 meteorite. Geochim. Cosmochim. Acta 52,

909–916.

Greshake A., Fritz J. and Stoffler D. (2004) Petrology and shockmetamorphism of the olivine-phyric shergottite Yamato980459: evidence for a two-stage cooling and a single-stageejection history. Geochim. Cosmochim. Acta 68, 2359–2377.

Hartmann W. K. (1999) Martian cratering VI: crater countisochrons and evidence for recent volcanism from Mars GlobalSurveyor. Meteorit. Planet. Sci. 34, 167–177.

Hartmann W. K. and Berman D. C. (2000) Elysium Planitia lavaflows: crater count chronology and geological implications. J.

Geophys. Res. Planets 105, 15011–15025.

Hartmann W. K. and Neukum G. (2001) Cratering chronology andthe evolution of Mars. Space Sci. Rev. 96, 165–194.

Harvey R. P. and McSween H. Y. (1996) A possible high-temperature origin for the carbonates in the Martian meteoriteALH84001. Nature 382, 49–51.

Head J. N. and Melosh H. J. (1999) Effects of layering on spallvelocity: numerical simulations. Lunar Planet. Sci. XXX, 1761

(Abstract).

Head J. N., Melosh H. J. and Ivanov B. A. (2002) Martianmeteorite launch: high-speed ejecta from small craters. Science

298, 1752–1756.

Heider N. and Kenkmann T. (2003) Numerical simulation oftemperature effects at fissures due to shock loading. Meteorit.

Planet. Sci. 38, 1451–1460.

Hutchins K. S. and Jakosky B. M. (1996) Evolution of Martianatmospheric argon: implications for sources of volatiles. J.

Geophys. Res. Planets 101, 14933–14949.

Ikeda Y. (1997) Petrology and mineralogy of the Y-793605Martian meteorite. Antarctic Meteorit. Res. 10, 13–40.

Ivanov B. A. (2001) Mars/Moon cratering rate ratio estimates.Space Sci. Rev. 96, 87–104.

Jagoutz E. and Wanke H. (1986) Sr and Nd isotopic systematics ofshergotty meteorite. Geochim. Cosmochim. Acta 50, 939–953.

Jakosky B. M. and Phillips R. J. (2001) Mars’ volatile and climatehistory. Nature 412, 237–244.

Jakosky B. M., Pepin R. O., Johnson R. E. and Fox J. L. (1994)Mars atmospheric loss and isotopic fractionation by solar-wind-induced sputtering and photochemical escape. Icarus 111,

271–288.

Jessberger E. K., Schaeffer O. A., Warasila R., Walker R. andLabotka T. (1981) Unmasking ‘‘Extra” 40Ar in ALH77005 bythe laser extraction technique. Meteoritics 16, 331–332.

Jones J. H. (1986) A discussion of isotopic systematics and mineralzoning in the shergottites: evidence for a 180 m.y. igneouscrystallization age. Geochim. Cosmochim. Acta 50, 969–977.

Jones J. H. (1989) Isotopic relationships among the shergottites,the nakhlites and Chassigny. Proc. 19th Lunar Planet. Sci.

Conf., 465–474.Jones J. H. (2007) The shergottites are young. Meteorit. Planet. Sci.

42(Suppl.), A77.

Kirschvink J. L., Maine A. T. and Vali H. (1997) Paleomagneticevidence of a low-temperature origin of carbonate in theMartian meteorite ALH84001. Science 275, 1629–1633.

Langenhorst F. and Greshake A. (1999) A transmission electronmicroscope study of Chassigny: evidence for strong shockmetamorphism. Meteorit. Planet. Sci. 34, 43–48.

Langenhorst F. and Poirier J. P. (2000) Anatomy of black veins inZagami: clues to the formation of high-pressure phases. Earth

Planet. Sci. Lett. 184, 37–55.

Luhmann J. G., Johnson R. E. and Zhang M. H. G. (1992)Evolutionary impact of sputtering of the martian atmosphereby O+ pickup ions. Geophys. Res. Lett. 19, 2151–2154.

Malin M. C., Edgett K. S., Posiolova L. V., McColley S.M. and Dobrea E. Z. N. (2006) Present-day impactcratering rate and contemporary gully activity on Mars.Science 314, 1573–1577.

Marti K., Kim J. S., Thakur A. N., McCoy T. J. and Keil K. (1995)Signatures of the Martian atmosphere in glass of the Zagamimeteorite. Science 267, 1981–1984.

McConville P., Kelley S. and Turner G. (1988) Laser probe40Ar–39Ar studies of the peace river shocked L6 chondrite.Geochim. Cosmochim. Acta 52, 2487–2499.

McCoy T. J., Taylor G. J. and Keil K. (1992) Zagami—product ofa two-stage magmatic history. Geochim. Cosmochim. Acta 56,

3571–3582.

McDougall I. and Harrison T. M. (1999) Geochronology and

thermochronology by the 40Ar–39Ar method. Oxford UniversityPress, New York.


McElroy M. B., Kong T. Y. and Yung Y. L. (1977) Photochem-istry and evolution of Mars’ atmosphere; a Viking perspective.J. Geophys. Res. 82, 4379–4388.

McKay G. and Mikouchi T. (2003) Crystallization of Antarcticshergottite Yamato 980459. International Symposium on the

Evolution of Solar System materials: A New Perspective from

Antarctic Meteorites, NIPR, Tokyo.McKay D. S., Gibson E. K., ThomasKeprta K. L., Vali H.,

Romanek C. S., Clemett S. J., Chillier X. D. F., Maechling C.R. and Zare R. N. (1996) Search for past life on Mars: possiblerelic biogenic activity in Martian meteorite ALH84001. Science

273, 924–930.

McSween H. Y. (1994) What we have learned about Mars fromSNC meteorites. Meteoritics 29, 757–779.

Melosh H. J. (1985) Ejection of rock fragments from planetarybodies. Geology 13, 144–148.

Melosh H. J., Treiman A. H. and Grieve R. A. F. (1983) Olivinecomposition glass in the Chassigny meteorite: implications forshock history. EOS, Trans. AGU 64, 254.

Meyer C. (2008) The Mars meteorite compendium, http://www-curator.jsc.nasa.gov/curator/.%5Cantmet/mmc/index.cfm. Ast-

romaterials Research and Exploration Science, Lyndon B.Johnson Space Center, Houston.

Mikouchi T. and Miyamoto M. (2002) Olivine cooling rate of theNorthwest Africa 1068 shergottite. Lunar Planet. Sci. XXXIV,

1346 (Abstract).

Mikouchi T., Miyamoto M. and McKay G. A. (2001) Mineralogyand petrology of the Dar al Gani 476 martian meteorite:implications for its cooling history and relationship to othershergottites. Meteorit. Planet. Sci. 36, 531–548.

Ming D. W., Mittlefehldt D. W., Morris R. V., Golden D. C.,Gellert R., Yen A., Clark B. C., Squyres S. W., Farrand W. H.,Ruff S. W., Arvidson R. E., Klingelhofer G., McSween H. Y.,Rodionov D. S., Schroder C., de Souza P. A. and Wang A.(2006) Geochemical and mineralogical indicators for aqueousprocesses in the Columbia Hills of Gusev crater, Mars. J.

Geophys. Res. Planets, 111.Misawa K., Shih C.-Y., Reese Y., Nyquist L. E. and Barrat J. A.

(2005) Rb–Sr and Sm–Nd isotopic systematics of NWA 2737chassignite. Meteorit. Planet. Sci. 40, A104 (Abstract).

Mittlefehldt D. W. (1994) ALH84001, a cumulate ortho-pyroxenite member of the Martian meteorite clan.Meteoritics 29, 214–221.

Miura Y. N., Nagao K., Sugiura N., Sagawa H. and Matsubara K.(1995) Orthopyroxenite ALH84001 and shergottiteALH77005—additional evidence for a Martian origin fromnoble-gases. Geochim. Cosmochim. Acta 59, 2105–2113.

Muller W. F. (1993) Thermal and deformation history of theshergotty meteorite deduced from clinopyroxene microstruc-ture. Geochim. Cosmochim. Acta 57, 4311–4322.

Neukum G., Ivanov B. A. and Hartmann W. K. (2001) Crateringrecords in the inner solar system in relation to the lunarreference system. Space Sci. Rev. 96, 55–86.

Nishiizumi K. and Caffee M. W. (1996) Exposure history ofshergottite Queen Alexandra Range 94201. Lunar Planet. Sci.

XXVII, 961–962.

Nishiizumi K., Arnold J. R., Goswami N., Klein J. and MiddletonR. (1986a) Solar cosmic-ray effects in Allan Hills 77005.Meteoritics 21, 472–473.

Nishiizumi K., Klein J., Middleton R., Elmore D., Kubik P. W.and Arnold J. R. (1986b) Exposure history of shergottites.Geochim. Cosmochim. Acta 50, 1017–1021.

Nishiizumi K., Masarik J., Welton K. C., Caffee M. W., Jull A. J.T. and Klandrud S. E. (1999) Exposure history of the newMartian meteorite Dar al Gani 476. Lunar Planet. Sci. XXX,

1566 (Abstract).

Nishiizumi K., Caffee M. W. and Masarik J. (2000) Cosmogenicradionuclides in the Los Angeles Martian meteorite. Meteorit.

Planet. Sci. 35, A120 (Abstract).

Nyquist L. E., Borg L. E. and Shih C. Y. (1998) The Shergottiteage paradox and the relative probabilities for Martian meteor-ites of differing ages. J. Geophys. Res. Planets 103, 31445–31455.

Nyquist L. E., Bogard D. D., Shih C. Y., Greshake A., Stoffler D.and Eugster O. (2001) Ages and geologic histories of Martianmeteorites. Space Sci. Rev. 96, 105–164.

Nyquist L. E., Shih C. Y., Reese Y. D. and Irving A. J. (2006)Concordant Rb–Sr and Sm–Nd ages for NWA 1460: A 340 Maold basaltic shergottite related to lherzolitic shergottites. Lunar

Planet. Sci. XXXVII, 723 (Abstract).

Pal D. K., Tuniz C., Moniot R. K., Savin W., Kruse T. and HerzogG. F. (1986) Be-10 contents of shergottites, nakhlites, andChassigny. Geochim. Cosmochim. Acta 50, 2405–2409.

Park J., Bogard D. D. and Garrison D. H. (2008) 39Ar–40Ar datingof Martian shergottite, DaG 476. Lunar Planet. Sci. XXXIX,

1204 (Abstract).

Poulet F., Bibring J. P., Mustard J. F., Gendrin A., Mangold N.,Langevin Y., Arvidson R. E., Gondet B., Gomez C. and TeamO. (2005) Phyllosilicates on Mars and implications for earlymartian climate. Nature 438, 623–627.

Renne P. R., Swisher C. C., Deino A. L., Karner D. B., Owens T.L. and DePaolo D. J. (1998) Intercalibration of standards,absolute ages and uncertainties in Ar-40/Ar-39 dating. Chem.

Geol. 145, 117–152.

Scott E. R. D., Krot A. N. and Yamaguchi A. (1998)Carbonates in fractures of Martian meteorite AllanHills 84001: petrologic evidence for impact origin.Meteorit. Planet. Sci. 33, 709–719.

Shean D. E., Head J. W., Fastook J. L. and Marchant D. R. (2007)Recent glaciation at high elevations on Arsia Mons, Mars:implications for the formation and evolution of large tropicalmountain glaciers. J. Geophys. Res. Planets 112, E03004.

Shih C. Y., Nyquist L. E., Bogard D. D., McKay G. A., Wooden J.L., Bansal B. M. and Wiesmann H. (1982) Chronology andpetrogenesis of young achondrites, shergotty, Zagami, andAlha77005—late magmatism on a geologically active planet.Geochim. Cosmochim. Acta 46, 2323–2344.

Solomon S. C., Aharonson O., Aurnou J. M., Banerdt W. B., CarrM. H., Dombard A. J., Frey H. V., Golombek M. P., Hauck S.A., Head J. W., Jakosky B. M., Johnson C. L., McGovern P. J.,Neumann G. A., Phillips R. J., Smith D. E. and Zuber M. T.(2005) New perspectives on ancient Mars. Science 307, 1214–

1220.

Squyres S. W., Arvidson R. E., Blaney D. L., Clark B. C.,Crumpler L., Farrand W. H., Gorevan S., Herkenhoff K. E.,Hurowitz J., Kusack A., McSween H. Y., Ming D. W., MorrisR. V., Ruff S. W., Wang A. and Yen A. (2006a) Rocks of theColumbia Hills. J. Geophys. Res. Planets, 111.

Squyres S. W., Arvidson R. E., Bollen D., Bell J. F., Bruckner J.,Cabrol N. A., Calvin W. M., Carr M. H., Christensen P. R.,Clark B. C., Crumpler L., Des Marais D. J., d’Uston C.,Economou T., Farmer J., Farrand W. H., Folkner W., GellertR., Glotch T. D., Golombek M., Gorevan S., Grant J. A.,Greeley R., Grotzinger J., Herkenhoff K. E., Hviid S., JohnsonJ. R., Klingelhofer G., Knoll A. H., Landis G., Lemmon M., LiR., Madsen M. B., Malin M. C., McLennan S. M., McSweenH. Y., Ming D. W., Moersch J., Morris R. V., Parker T., Rice J.W., Richter L., Rieder R., Schroder C., Sims M., Smith M.,Smith P., Soderblom L. A., Sullivan R., Tosca N. J., Wanke H.,Wdowiak T., Wolff M. and Yen A. (2006b) Overview of theopportunity Mars exploration rover mission to meridianiplanum: eagle crater to purgatory ripple. J. Geophys. Res.

Planets, 111.

http://www-curator.jsc.nasa.gov/curator/.%5Cantmet/mmc/index.cfm

http://www-curator.jsc.nasa.gov/curator/.%5Cantmet/mmc/index.cfm


Stewart S. T., Seifter A., Kennedy G. B., Furlanetto M. R. andObst A. W. (2007) Measurements of emission temperaturesfrom shocked basalt: hot spots in meteorites. Lunar Planet. Sci.

XXXVIII, 2413 (Abstract).

Stoffler D. (2000) Maskelynite confirmed as diaplectic glass:indication for peak shock pressures below 45 GPa in allMartian meteorites. Lunar Planet. Sci. XXXI, 1170 (Abstract).

Stoffler D. and Hornemann U. (1972) Quartz and feldspar glassesproduced by natural and experimental shock. Meteoritics 7,

371–394.

Stoffler D. and Ryder G. (2001) Stratigraphy and isotope ages oflunar geologic units: chronological standard for the inner solarsystem. Space Sci. Rev. 96, 9–54.

Stoffler D., Ostertag R., Jammes C., Pfannschmidt G., Gupta P. R.S., Simon S. B., Papike J. J. and Beauchamp R. H. (1986)Shock metamorphism and petrography of the Shergottyachondrite. Geochim. Cosmochim. Acta 50, 889–903.

Stoffler D., Keil K. and Scott E. R. D. (1991) Shock metamorphismof ordinary chondrites. Geochim. Cosmochim. Acta 55, 3845–

3867.

Strom R. G., Croft S. and Barlow N. (1992) The Martian impactcratering record (eds. H. H. Kieffer, B. M. Jakosky, C. W.Snyder and M. S. Mathews). Mars. University of ArizonaPress, Tucson.

Swindle T. D., Caffee M. W. and Hohenberg C. M. (1986) Xenonand other noble-gases in shergottites. Geochim. Cosmochim.

Acta 50, 1001–1015.

Swindle T. D., Grier J. A. and Burkland M. K. (1995) Noble-gasesin orthopyroxenite ALH84001—a different kind of Martianmeteorite with an atmospheric signature. Geochim. Cosmochim.

Acta 59, 793–801.

Swindle T. D., Treiman A. H., Lindstrom D. J., Burkland M. K.,Cohen B. A., Grier J. A., Li B. and Olson E. K. (2000) Noblegases in iddingsite from the Lafayette meteorite: evidence forliquid water on Mars in the last few hundred million years.Meteorit. Planet. Sci. 35, 107–115.

Tanaka K. L. (1986) The stratigraphy of Mars. J. Geophys. Res.

Solid Earth Planets 91, E139–E158.

Tanaka K. L., Scott C. H. and Greeley R. (1992) Globalstratigraphy (eds. H. H. Kieffer, B. M. Jakosky, C. W. Snyderand M. S. Mathews). Mars. University of Arizona Press,Tucson.

Treiman A. H. (1995) A petrographic history of Martian meteoriteALH84001—2 shocks and an ancient age. Meteoritics 30, 294–

302.

Treiman A. H. (1998) The history of Allan Hills 84001 revised:multiple shock events. Meteorit. Planet. Sci. 33, 753–764.

Treiman A. H. (2005) The nakhlite meteorites: augite-rich igneousrocks from Mars. Chem Erde-Geochem. 65, 203–270.

Treiman A. H., Dyar M. D., McCanta M., Noble S. K. and PietersC. M. (2007) Martian dunite NWA 2737: petrographicconstraints on geological history, shock events, and olivinecolor. J. Geophys. Res. Planets 112, E04002.

Turner G., Knott S. F., Ash R. D. and Gilmour J. D. (1997) Ar–Archronology of the Martian meteorite ALH84001: evidence forthe timing of the early bombardment of Mars. Geochim.


Walton E. L. and Herd C. D. K. (2007a) Localized shock meltingin lherzolitic shergottite Northwest Africa 1950: comparisonwith Allan Hills 77005. Meteorit. Planet. Sci. 42, 63–80.

Walton E. L. and Herd C. D. K. (2007b) Dynamic crystallizationof shock melts in Allan Hills 77005: implications for meltpocket formation in Martian meteorites. Geochim. Cosmochim.

Acta 71, 5267–5285.

Walton E. L., Kelley S. P. and Spray J. G. (2007) Shockimplantation of Martian atmospheric argon in four basalticshergottites: a laser probe Ar-40/Ar-39 investigation. Geochim.


Warren P. H. (1994) Lunar and Martian meteorite deliveryservices. Icarus 111, 338–363.

Wiens R. C. and Pepin R. O. (1988) Laboratory shock emplace-ment of noble-gases, nitrogen, and carbon-dioxide into basalt,and implications for trapped gases in Shergottie Eeta-79001.Geochim. Cosmochim. Acta 52, 295–307.

Associate editor: Gregory F. Herzog

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Isotopic and petrographic evidence for young Martian basalts