1

Isotopes in Meteorites

16Oxygen!17Oxygen!18Oxygen!

2

CAI & Chondrule Formation

3

ANRV309-EA35-19 ARI 20 March 2007 17:9

Figure 2Equilibrium diagram showing which minerals are stable between 900 and 1800 K in a nebulaof solar composition at 10−3 bar (after Davis & Richter 2003). At 900 K, half the atoms (0.55)in a CI chondrite are in minerals; S and other volatile elements are in the gas. Minerals stableabove 1400 K are found in refractory inclusions; minerals stable below 1400 K predominate inchondrules and matrix material. Only three minerals condense entirely from the gas oncooling—corundum (Al2O3), forsterite (Mg2SiO4), and Fe,Ni metal—the remainder form byreaction between solids and gas. Liquids are unstable unless the total pressure or the dust/gasratio is increased 10–100 × .

probably formed in under a few hundred thousand years (see below) in a uniquely 16O-rich environment during the most energetic phase of disk evolution (Wood 2004).They commonly have outermost rims of forsterite with minor refractory minerals thatclosely match the mineralogy of AOAs, suggesting that AOAs and forsterite rims onCAIs are coeval and postdate CAIs. Preliminary Al-Mg dates suggest that AOAs maybe ∼0.5 Myr younger (Itoh et al. 2002). Both types of refractory inclusions formedin a highly reducing (solar), 16O-rich environment before chondrules. Roughly equalproportions of AOAs and CAIs are present in nearly all chondrite groups, but theirtotal volume varies enormously from 0.01 to 10% (Table 1). CAI sizes in each chon-drite group are very roughly correlated with chondrule size, although the range in agiven chondrite may be very large. CAIs show mass-independent isotopic anomaliesthat are larger than those in chondrules; however, they are much smaller than those in

www.annualreviews.org • Chondrites and the Protoplanetary Disk 587

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Equilibrium diagram showing which minerals are stable between 900 and 1800 K in a nebula of solar composition at 10−3 bar (after Davis & Richter 2003). At 900 K, half the atoms (0.55) in a CI chondrite are in minerals; S and other volatile elements are in the gas. Minerals stable above 1400 K are found in refractory inclusions; minerals stable below 1400 K predominate in chondrules and matrix material. Only three minerals condense entirely from the gas on cooling—corundum (Al2O3), forsterite (Mg2SiO4), and Fe,Ni metal—the remainder form by reaction between solids and gas. Liquids are unstable unless the total pressure or the dust/gas ratio is increased 10–100× .

CAI & Chondrule Formation

4[from Meteorites & the Early Solar System II]

Oxygen, Oxygen, Oxygen!5

Below silicate condensation temperature, but above condensation temperature of ices, ~25% of oxygen was likely incorporated into solid phases.

Therefore, oxygen isotopes in solids provide a window into the range in oxygen compositions in the solar nebula.

What are the variations in solar system oxygen and what do they tell us about geologic processes?

Isotope “Delta” Notation6

It is easier to measure ratios of isotopes than it is to measure absolute concentrations of a given isotope.

The ratio for a sample is measured and compared against the isotope ratio for a known standard material (Standard Mean Ocean Water, SMOW, typically used for oxygen).

The ratio for a sample is measured and compared against the isotope ratio for a known reference material (Standard Mean Ocean Water, SMOW, typically used for oxygen).

Negative values mean the ratio in the sample is lower than in the reference material, positive values mean the sample ratio is higher.

Oxygen, Oxygen, Oxygen!7

Clayton, Grossman, & Mayeda (1973)

Oxygen, Oxygen, Oxygen!8

fragments of asteroids into the inner SolarSystem. There are two fundamental kindsof asteroidal meteorites: chondrites, whichare, by far, the most common and areaggregates of solar nebular dust grains, andplanetary meteorites, mainly achondritesand irons, which are derived from largerasteroids that have undergone partial tocomplete planetary differentiation pro-cesses such as core formation and crustalevolution. Not all meteorites come fromasteroids; a small fraction are known to belunar in origin (by comparison with Apolloand Luna samples), and some likelycome from Mars. Micrometeorites (cosmicdust particles or interplanetary dust) aresimilar to but not identical with meteoritesand partly derive from comets. WithApollo and Luna, we received extrater-restrial materials from manned and roboticspacecraft sent specifically to return sam-ples to Earth. Such sample return missionsare expensive and therefore, rare, butthey provide one kind of information thatcannot be obtained from meteorites orcosmic dust: context. We know exactlyfrom where the samples come. Roboticmissions have delivered samples from theMoon (Luna; USSR), the solar wind(Genesis; United States), a comet (Star-dust; United States), and an asteroid(Hayabusa; Japan).Laboratory studies of extraterrestrial

materials over the past 50 y have led toa number of truly remarkable discoveries.

Origin and Evolution of the MoonChemical data from the Apollo samplesshowed a remarkable depletion in water,sodium, and other volatile componentscompared with Earth rocks. However,the oxygen isotopes of the lunar samplesare indistinguishable from those isotopeson Earth, although the Solar System as awhole has a huge diversity of isotopiccompositions. Based on these observations,it is now generally thought that Earth’sMoon likely formed as a result of giantcollision between Earth and a Mars-sizedbody only a few tens of millions of years

after Earth’s formation (Fig. 4). After itsformation, the Moon may have beenlargely to completely molten, and theflotation of the mineral feldspar (plagio-clase) on top of this magma ocean gaverise to the lunar highlands. Hence, thehighlands are light-colored anorthosite,and the low-lying plains (Mare) are filledwith dark volcanic basalt. Even beyond thehighland–Mare dichotomy, the Moon is

highly heterogeneous in another and moresubtle way: most of the near side Lunarsurface, centered on the Procellerumbasin, is enriched potassium, rare earthelements, and phosphorous (given theacronym KREEP) relative to the re-mainder of the Moon’s surface (far side).

Meteorites from MarsMost meteorites are on the order of 4.5 Gain age. One small subset of meteoritesstands out in this respect, being in somecases as young as ∼200 my. These mete-orites are all igneous rocks, and some areeven volcanic basalts. They clearly origi-nated on a differentiated planetary bodythat, until very recently, was volcanicallyactive. The most likely culprit was Mars(5). This thought was confirmed in 1983(6) when trapped gases contained in therocks were found to be identical to theMartian atmosphere as measured by theViking spacecraft in 1976. It was proposedthat one Martian meteorite, ALH84001(found in the Antarctic), containedevidence for fossil Martian life (7). Thishighly controversial (but if true, spectacu-lar) idea generated a huge amount of

Fig. 2. Artist’s rendition of the fall of the Allende meteorite over the northern Mexico desert, February8, 1969. Smithsonian Institution image. Don Davis, artist. (Reproduced with permission.)

Fig. 3. Calcium–aluminum-rich inclusions in the Allende meteorite.

MacPherson and Thiemens PNAS | November 29, 2011 | vol. 108 | no. 48 | 19131

SPECIA

LFEATURE:IN

TRODUCTIO

NMacPherson & Thiemens (2011)

δ17O & δ18O values in CAIs are nothing like those in materials from Earth.

The values also do not follow a mass dependent fractionation line.

The CAIs also differ from other components in chondrites.

Range in CAI O-isotope values in a given chondrite can be surprisingly large.

Oxygen in the Solar System9

MacPherson & Thiemens (2011)

Oxygen in the Solar System10

Mass Independent Fractionation11

What is the source of MIF?

Mass Independent Fractionation12

Photochemical MIF & CO Self-Shielding

CO is broken down by photodissociation induced by far-UV radiation (91-110 nm)

CO absorption of UV is in proportion to column densities of the isotopologues.....and 16O is much more prevalent than 17/18O.

Leads to ‘interior’ C16O being self-shielded (no more UV to absorb, it was all absorbed by outer portions rich in C16O).

‘Deeper’ in molecular cloud, C17O & C18O are still broken down and the heavier O combines with H to make H217O or H218O.

In warmer interior of disk, these heavy ices can convert to gas.

[see Yurimoto &Kuramoto (2004)]

CAI & Chondrule Formation

13[Itoh & Yurimoto (2003) Nature]

How do we get material to disk interior?14

Genesis Mission15

How do we test this model?How do we determine what the ‘real’ composition of the Sun is?

Sample Return Mission!

Genesis Mission16

The landing was not as smooth as predicted, but the collected material was still preserved and analyses were carried out.

Oxygen in the Solar System

17

scale of thermal lattice QCD computations viahadronic observables. Furthermore, this gives ascale for temperatures that is compatible with theresonance gas model, as shown in Fig. 3. As wediscussed in the introduction, this closes a circleof inferences that shows that phenomena ob-tained in heavy ion collisions are fully compat-ible with hadron phenomenology and provides afirst check in bulk hot and dense matter for thestandard model of particle physics.

Conclusions and outlook.Wehave performeda direct comparison between experimental datafrom high-energy heavy ion collisions on net-proton number distributions and lattice QCD cal-culations of net-baryon number susceptibilities.The agreement between experimental data, latticecalculations, and a hadron resonance gas modelindicates that the system produced in heavy ioncollisions attained thermalization during its evo-lution. The comparison further enables us to setthe scale for nonperturbative, high-temperaturelattice QCD by determining the critical tem-perature for the QCD phase transition to be175þ1

"7 MeV.This work reveals the rich possibilities that

exist for a comparative study between theory andexperiment of QCD thermodynamics and phasestructure. In particular, the current work can beextended to the search for a critical point. In athermal system, the correlation length (x) divergesat the critical point. x is related to variousmomentsof the distributions of conserved quantities, suchas net-baryons, net-charge, and net-strangeness.Finite size and dynamical effects in heavy ioncollisions put constraints on the values of x (34).The lattice calculations discussed here and sev-eral QCD-based models have shown that mo-ments of net-baryon distributions are related tobaryon number susceptibilities and that the ratio

of cumulants m2 = ks2, which is related to theratio of fourth-order to second-order susceptibi-lities, shows a large deviation from unity nearthe critical point. Experimentally, ks2 can be mea-sured as a function of

ffiffiffiffiffiffiffisNN

p(or T and mB) in

heavy ion collisions. A nonmonotonic variationof ks2 as a function of

ffiffiffiffiffiffiffisNN

pwould indicate

that the system has evolved in the vicinity of thecritical point and thus could be taken as evidencefor the existence of a critical point in the QCDphase diagram.

References and Notes1. F. Wilczek, Phys. Today 52N11, 11 (1999).2. F. Wilczek, Phys. Today 53N1, 13 (2000).3. G. Sterman et al., Rev. Mod. Phys. 67, 157 (1995).4. M. Peardon, V. Crede, P. Eugenio, A. Ostrovidov,

AIP Conf. Proc. 1257, 19 (2010).5. I. Arsene et al., Nucl. Phys. A 757, 1 (2005).6. B. B. Back et al., Nucl. Phys. A 757, 28 (2005).7. J. Adams et al., Nucl. Phys. A 757, 102 (2005).8. K. Adcox et al., Nucl. Phys. A 757, 184 (2005).9. B. I. Abelev et al., Phys. Rev. C 79, 034909 (2009).10. S. Borsanyi et al., J. High Energy Phys. 1009, 073

(2009).11. Y. Aoki, Z. Fodor, S. Katz, K. Szabo, Phys. Lett. B 643, 46

(2006).12. M. Cheng et al., Phys. Rev. D 74, 054507 (2006).13. C. Schmidt, paper presented at Quark Confinement and

the Hadron Spectrum IX, 30 August 30 to 3 September2010, Madrid, Spain; available at http://arxiv.org/abs/1012.2230v1.

14. F. R. Brown, N. H. Christ, Y. F. Deng, M. S. Gao,T. J. Woch, Phys. Rev. Lett. 61, 2058 (1988).

15. Y. Aoki, G. Endrodi, Z. Fodor, S. D. Katz, K. K. Szabó,Nature 443, 675 (2006).

16. P. Braun-Munzinger, J. Wambach, Rev. Mod. Phys. 81,1031 (2009).

17. J. Cleymans, K. Redlich, Phys. Rev. Lett. 81, 5284(1998).

18. R. V. Gavai, S. Gupta, Phys. Lett. B 696, 459 (2011).19. M. M. Aggarwal et al., Phys. Rev. Lett. 105, 022302

(2010).20. M. A. Stephanov, K. Rajagopal, E. V. Shuryak, Phys.

Rev. D 60, 114028 (1999).

21. R. D. Pisarski, F. Wilczek, Phys. Rev. D 20, 338(1984).

22. S. Gupta, PoS LATTICE2010, 007 (2010).23. B. I. Abelev et al., Phys. Rev. C 81, 024911 (2010).24. B. Mohanty, Nucl. Phys. A. 830, 899c (2009).25. mB of bulk nuclear matter is quoted in the usual

convention adopted in relativistic treatments becauseanti-baryon production also needs to be accounted for.An alternative (nonrelativistic) definition that takesnucleon number to be fixed would give much smallerchemical potential. However, this does not correspond tothe physics of baryon number fluctuations that we examine.

26. R. V. Gavai, S. Gupta, Phys. Rev. D 68, 034506 (2003).27. M. Asakawa, U. Heinz, B. Muller, Phys. Rev. Lett. 85,

2072 (2000).28. S. Jeon, V. Koch V, Phys. Rev. Lett. 85, 2076

(2000).29. Y. Hatta, M. A. Stephanov, Phys. Rev. Lett. 91, 102003

(2003).30. S. Gupta, PoS CPOD2009, 25 (2009).31. The impact parameter is determined through a Glauber

Monte Carlo procedure. The selected events correspond tothe most central 0 to 5% and 0 to 10% events of thetotal hadronic cross section in Au+Au collisions forffiffiffiffiffiffiffisNN

p= 200, 62.4, and 19.6 GeV, respectively.

32. The change in the radius of convergence in going frommp /mr = 0.33 to 0.2 is likely to be between 10 and 15%(22). The corresponding effect on m3 is about 2% or lessat the two highest energies and less than 20% at anenergy of 19.6 GeV.

33. F. Karsch, K. Redlich, Phys. Lett. B 695, 136 (2011).34. B. Berdnikov, K. Rajagopal, Phys. Rev. D Part. Fields 61,

105017 (2000).35. M. G. Alford, K. Rajagopal, F. Wilczek, Phys. Lett. B 422,

247 (1998).Acknowledgments: We thank Z. Fodor, R. V. Gavai,

F. Karsch, D. Keane, V. Koch, B. Mueller, K. Rajagopal,K. Redlich, H. Satz, and M. Stephanov for enlighteningdiscussions. We acknowledge the Indian LatticeGauge Theory Initiative for computational support,the Department of Atomic Energy–Board of Researchin Nuclear Sciences through the project sanction2010/21/15-BRNS/2026, the U.S. Department of Energyunder contract DE-AC03-76SF00098, and the ChineseMinistry of Education.

21 February 2011; accepted 4 May 201110.1126/science.1204621

The Oxygen Isotopic Composition ofthe Sun Inferred from CapturedSolar WindK. D. McKeegan,1* A. P. A. Kallio,1 V. S. Heber,1 G. Jarzebinski,1 P. H. Mao,1,2 C. D. Coath,1,3

T. Kunihiro,1,4 R. C. Wiens,5 J. E. Nordholt,5 R. W. Moses Jr.,5 D. B. Reisenfeld,6

A. J. G. Jurewicz,7 D. S. Burnett8

All planetary materials sampled thus far vary in their relative abundance of the major isotopeof oxygen, 16O, such that it has not been possible to define a primordial solar system composition.We measured the oxygen isotopic composition of solar wind captured and returned to Earth byNASA’s Genesis mission. Our results demonstrate that the Sun is highly enriched in 16O relativeto the Earth, Moon, Mars, and bulk meteorites. Because the solar photosphere preserves the averageisotopic composition of the solar system for elements heavier than lithium, we conclude that essentiallyall rocky materials in the inner solar system were enriched in 17O and 18O, relative to 16O, by~7%, probably via non–mass-dependent chemistry before accretion of the first planetesimals.

The gravitational collapse of a molecularcloud fragment 4.57 billion years ago ledto an accretion disc of gas and dust, the

solar nebula, from which the Sun and planetsformed. This nebula was approximately homo-geneous with respect to isotopic abundances,

which, given that isotope ratios from variousstellar nucleosynthetic processes vary widely,points to efficient mixing either in interstellarspace or in the solar nebula. Thus, the discovery(1) that high-temperature minerals in carbona-ceous chondrite meteorites are enriched prefer-entially in 16O compared to 17O and 18O relativeto the abundances in terrestrial samples was con-sidered evidence for the presence of exotic ma-terial that escaped thorough mixing and thereby

1Department of Earth and Space Sciences, University ofCalifornia–Los Angeles (UCLA), Los Angeles, CA 90095–1567,USA. 2Division of Physics, Math, and Astronomy, CaliforniaInstitute of Technology (Caltech), Pasadena, CA 91125, USA.3School of Earth Sciences, University of Bristol, Bristol BS8 1RJ,UK. 4Institute for Study of the Earth’s Interior, Okayama Uni-versity, Misasa, Tottori 682-0193, Japan. 5Los Alamos NationalLaboratory, Los Alamos, NM 87545, USA. 6Department of Phys-ics and Astronomy, University of Montana, Missoula, MT 59812,USA. 7Center for Meteorite Studies, Arizona State University,Tempe, AZ 85287, USA. 8Division of Geological and PlanetaryScience, Caltech, Pasadena, CA 91125, USA.

*To whom correspondence should be addressed. E-mail:mckeegan@ess.ucla.edu

24 JUNE 2011 VOL 332 SCIENCE www.sciencemag.org1528

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Science [2011]

studies, we thus take as the most plausible com-position of the Sun the intersection of the CAImixing line with a mass-dependent fractionationline passing through the SW L1 point, whichyields d18O = −58.5‰, d17O = −59.1‰ (Fig. 4).The total fractionation from SW to the Sun im-plied is ~22‰/amu, in accordance with the modelpredictions.

The 16O-enriched SW composition measuredin the Genesis sample is in broad agreement witha component found from oxygen isotope mea-surements in the surface layers of some lunarmetal grains (28) and of metal in a carbonaceouschondrite that shows evidence for SW exposureearly in solar system history (29), although inboth cases the data show a large degree of mass-dependent fractionation from the L1 SW value.Other analyses (30) of oxygen isotopes in a lunarregolith sample that indicate a 16O-depletedcomposition (D17O > 0) probably reflect otherextralunar sources, such as water brought by im-pacting interplanetary dust or cometary ices (31).Our measured SW composition is in markedcontrast to the strongly 17O- and 18O-enrichedvalues inferred from observations of rovibration-al bands of CO in the solar atmosphere (32); weattribute this discrepancy to systematic uncertain-ties in the thermal profile models that underlie theabundance calculations (11). Our solar values of16O/18O = 530 and 16O/17O = 2798 also disagreewith other marginally 18O-enriched values deter-mined spectroscopically (33), although the dataoverlap within 2s uncertainties.

The composition of the photosphere isthought to be representative of the convectingenvelope of the Sun, representing ~2.5% of its

mass, perhaps modified slightly by gravitationalsettling of heavier elements [see (11)]. Althoughnot directly determined, such settling could po-tentially lead to a small mass-dependent fraction-ation favoring retention of heavy isotopes deeperin the Sun, i.e., the same sense of fractionation asthat caused by inefficient Coulomb drag in theSW (34). Other changes to the original isotopiccomposition of heavy elements in the photo-sphere are unlikely given that there is no mixingof nuclear processed matter into the convectivezone (11). Fractionation mechanisms hypothe-sized as potentially operating in the solar atmo-sphere, e.g., mass-independent effects inducedduring dissociation of CO molecules in a coollayer of the chromosphere (35), are unlikely tolead to quantitative changes of the photospheric(or SW) oxygen isotopic abundances given thehigh temperatures (>3000 K) involved and ra-pidity of isotope exchange back-reactions andreservoir mixing in this dynamic environment.Thus, the large oxygen isotopic differences ob-served between the Sun and meteoritic or terres-trial samples are not caused by changes in solarmatter, but instead reflect processes acting toinduce mass-independent shifts in the oxygenisotopic compositions of planetary materials.

Implications for the solar nebula. The onlyknown materials with oxygen isotopic composi-tions close to those of the Sun are the CAIs andother refractory phases of chondritic meteorites(Fig. 4), interplanetary dust (36), and at least onecomet (37). CAIs are thought to be the earliestsolar system condensates, and most crystallizedwith spallogenic beryllium and lithium, probablyfrom proton bombardment in the magnetically

active environment near the still-accreting proto-Sun (38, 39). That their oxygen isotopes are dom-inated by a solar, rather than planetary, componentreinforces their status as xenoliths (2, 40, 41) inthe asteroid belt. However, most CAIs, includingultrarefractory hibonite grains (Fig. 4), are notquite as 16O-enriched as the Sun, implying somemixing with isotopically heavier oxygen fromother solar system reservoirs.

Our results suggest that essentially all plane-tary objects in the inner solar system (<5 AU)have oxygen isotopic compositions distinct fromthe average of the solar nebula from which theyformed, having been enriched by ~70‰ in both18O/16O and 17O/16O by one or more non–mass-dependent fractionation processes before accre-tion. Considering that oxygen is by far the mostabundant element in the terrestrial planets, thispoints to efficient, planetary-scale processes that,if based on molecular speciation, must involvethe dominant O-bearing molecules in the solarnebula: CO, H2O, and/or silicate dust (SiO,MgO,FeO, and others in combination). A leading hy-pothesis, which predicted our results (6), invokesisotope-selective self-shielding during ultraviolet(UV) photolysis of CO. Because of their rela-tively low abundances, the C17O and C18O iso-topomers continue to be dissociated after all thephotons capable of dissociating C16O have beenabsorbed; the liberated 17O and 18O atoms arethen rapidly sequestered into H2O ice and even-tually are incorporated into oxide and silicategrains (7, 10). The places and times where thisresults in a slope 1 fractionation trajectory on theoxygen three-isotope plot (i.e, in pure enrichmentor depletion of 16O) are constrained by gas column

Fig. 4. Oxygen three-isotope plot showing rep-resentative compositions of major primary compo-nents of solar system matter, the solar wind (SW),and our preferred value for the Sun. All data fallpredominantly on a single mixing line characterizedby excesses (lower left) or depletions (upper right)of 16O relative to all samples of the Earth andMoon. Plotted are the most 16O-enriched solar systemsamples: an unusual chondrule (47); individual platyhibonite grains (55), which are ultrarefractory oxidesfrom carbonaceous chondrites (CC); water inferredto have oxidized metal to magnetite (56) in ordi-nary chondrites (OC); very 16O-depleted water fromthe CC Acfer 094 (3), and whole CAIs from CC (19);and chondrules from CC and OC (19), bulk Earth(mantle), and Mars (SNC meteorites). The mass-dependent fractionation trajectory of primary min-erals in FUN inclusions and the pure 16O (slope 1.0)line (57) are also shown.

Earth and Moon

CAI

Sun

SW

δ18

O (‰)

δ17O

(‰

)

FUN

SW fractionation

-100 -80 -60 -40 -20 0 20 40

-80

-60

-40

-20

0

20

H O (Semarkona)2

16O rich chondrulechondrules (CC)

bulk Earth, MoonMars

chondrules (OC)

Refractory inclusions

HiboniteCAI

H O (Acfer 094)2

Heavy water

Sun

slope 1 lin

e

CAIEarth

-100 0 100 200

0

100

Heavy water

www.sciencemag.org SCIENCE VOL 332 24 JUNE 2011 1531

RESEARCH ARTICLES

Fig. 4. Oxygen three-isotope plot showing rep- resentative compositions of major primary compo- nents of solar system matter, the solar wind (SW), and our preferred value for the Sun. All data fall predominantly on a single mixing line characterized by excesses (lower left) or depletions (upper right) of 16O relative to all samples of the Earth and Moon. Plotted are the most 16O-enriched solar system samples: an unusual chondrule (47); individual platy hibonite grains (55), which are ultrarefractory oxides from carbonaceous chondrites (CC); water inferred to have oxidized metal to magnetite (56) in ordi- nary chondrites (OC); very 16O-depleted water from the CC Acfer 094 (3), and whole CAIs from CC (19); and chondrules from CC and OC (19), bulk Earth (mantle), and Mars (SNC meteorites). The mass- dependent fractionation trajectory of primary min- erals in FUN inclusions and the pure 16O (slope 1.0) line (57) are also shown.

The Sun is enriched in 16O, consistent with the self-shielding model for the solar nebula...but does that mean it is correct??

To Shield or Not to Shield...That is the Question18

GENESIS mission confirmed the Sun is enriched in 16O!

Krot et al. (2010) But......

CO self-shielding models assume that primordial dust and solar nebula gas have identical 16O-enriched compositions.

O isotope values within different minerals in single CAIs follow condensation sequence, and no dust grains with ‘primordial’ 16O values have been found.

So perhaps the jury is still out....what is the photochemical process responsible for the MIF? Or is it due to Galactic Evolution (non-chemical)?

Oxygen in the Solar System

19

“Alternative hypotheses to isotope-selective photochemical self-shielding are based on laboratory experiments (48) and observations (49) of gas-phase (9) or surface (8) chemistry wherein non–mass-dependent isotopic effects are manifestations of molecular symmetry–induced degeneracy effects on isotope reaction rates. So-called non-RRKM effects may also play a role during growth of silicate condensates via surface reactions (8, 50); however, definitive experimental evidence for such effects on oxygen isotope abundances is currently lacking.”

scale of thermal lattice QCD computations viahadronic observables. Furthermore, this gives ascale for temperatures that is compatible with theresonance gas model, as shown in Fig. 3. As wediscussed in the introduction, this closes a circleof inferences that shows that phenomena ob-tained in heavy ion collisions are fully compat-ible with hadron phenomenology and provides afirst check in bulk hot and dense matter for thestandard model of particle physics.

Conclusions and outlook.Wehave performeda direct comparison between experimental datafrom high-energy heavy ion collisions on net-proton number distributions and lattice QCD cal-culations of net-baryon number susceptibilities.The agreement between experimental data, latticecalculations, and a hadron resonance gas modelindicates that the system produced in heavy ioncollisions attained thermalization during its evo-lution. The comparison further enables us to setthe scale for nonperturbative, high-temperaturelattice QCD by determining the critical tem-perature for the QCD phase transition to be175þ1

"7 MeV.This work reveals the rich possibilities that

exist for a comparative study between theory andexperiment of QCD thermodynamics and phasestructure. In particular, the current work can beextended to the search for a critical point. In athermal system, the correlation length (x) divergesat the critical point. x is related to variousmomentsof the distributions of conserved quantities, suchas net-baryons, net-charge, and net-strangeness.Finite size and dynamical effects in heavy ioncollisions put constraints on the values of x (34).The lattice calculations discussed here and sev-eral QCD-based models have shown that mo-ments of net-baryon distributions are related tobaryon number susceptibilities and that the ratio

of cumulants m2 = ks2, which is related to theratio of fourth-order to second-order susceptibi-lities, shows a large deviation from unity nearthe critical point. Experimentally, ks2 can be mea-sured as a function of

ffiffiffiffiffiffiffisNN

p(or T and mB) in

heavy ion collisions. A nonmonotonic variationof ks2 as a function of

ffiffiffiffiffiffiffisNN

pwould indicate

that the system has evolved in the vicinity of thecritical point and thus could be taken as evidencefor the existence of a critical point in the QCDphase diagram.

References and Notes1. F. Wilczek, Phys. Today 52N11, 11 (1999).2. F. Wilczek, Phys. Today 53N1, 13 (2000).3. G. Sterman et al., Rev. Mod. Phys. 67, 157 (1995).4. M. Peardon, V. Crede, P. Eugenio, A. Ostrovidov,

AIP Conf. Proc. 1257, 19 (2010).5. I. Arsene et al., Nucl. Phys. A 757, 1 (2005).6. B. B. Back et al., Nucl. Phys. A 757, 28 (2005).7. J. Adams et al., Nucl. Phys. A 757, 102 (2005).8. K. Adcox et al., Nucl. Phys. A 757, 184 (2005).9. B. I. Abelev et al., Phys. Rev. C 79, 034909 (2009).10. S. Borsanyi et al., J. High Energy Phys. 1009, 073

(2009).11. Y. Aoki, Z. Fodor, S. Katz, K. Szabo, Phys. Lett. B 643, 46

(2006).12. M. Cheng et al., Phys. Rev. D 74, 054507 (2006).13. C. Schmidt, paper presented at Quark Confinement and

the Hadron Spectrum IX, 30 August 30 to 3 September2010, Madrid, Spain; available at http://arxiv.org/abs/1012.2230v1.

14. F. R. Brown, N. H. Christ, Y. F. Deng, M. S. Gao,T. J. Woch, Phys. Rev. Lett. 61, 2058 (1988).

15. Y. Aoki, G. Endrodi, Z. Fodor, S. D. Katz, K. K. Szabó,Nature 443, 675 (2006).

16. P. Braun-Munzinger, J. Wambach, Rev. Mod. Phys. 81,1031 (2009).

17. J. Cleymans, K. Redlich, Phys. Rev. Lett. 81, 5284(1998).

18. R. V. Gavai, S. Gupta, Phys. Lett. B 696, 459 (2011).19. M. M. Aggarwal et al., Phys. Rev. Lett. 105, 022302

(2010).20. M. A. Stephanov, K. Rajagopal, E. V. Shuryak, Phys.

Rev. D 60, 114028 (1999).

21. R. D. Pisarski, F. Wilczek, Phys. Rev. D 20, 338(1984).

22. S. Gupta, PoS LATTICE2010, 007 (2010).23. B. I. Abelev et al., Phys. Rev. C 81, 024911 (2010).24. B. Mohanty, Nucl. Phys. A. 830, 899c (2009).25. mB of bulk nuclear matter is quoted in the usual

convention adopted in relativistic treatments becauseanti-baryon production also needs to be accounted for.An alternative (nonrelativistic) definition that takesnucleon number to be fixed would give much smallerchemical potential. However, this does not correspond tothe physics of baryon number fluctuations that we examine.

26. R. V. Gavai, S. Gupta, Phys. Rev. D 68, 034506 (2003).27. M. Asakawa, U. Heinz, B. Muller, Phys. Rev. Lett. 85,

2072 (2000).28. S. Jeon, V. Koch V, Phys. Rev. Lett. 85, 2076

(2000).29. Y. Hatta, M. A. Stephanov, Phys. Rev. Lett. 91, 102003

(2003).30. S. Gupta, PoS CPOD2009, 25 (2009).31. The impact parameter is determined through a Glauber

Monte Carlo procedure. The selected events correspond tothe most central 0 to 5% and 0 to 10% events of thetotal hadronic cross section in Au+Au collisions forffiffiffiffiffiffiffisNN

p= 200, 62.4, and 19.6 GeV, respectively.

32. The change in the radius of convergence in going frommp /mr = 0.33 to 0.2 is likely to be between 10 and 15%(22). The corresponding effect on m3 is about 2% or lessat the two highest energies and less than 20% at anenergy of 19.6 GeV.

33. F. Karsch, K. Redlich, Phys. Lett. B 695, 136 (2011).34. B. Berdnikov, K. Rajagopal, Phys. Rev. D Part. Fields 61,

105017 (2000).35. M. G. Alford, K. Rajagopal, F. Wilczek, Phys. Lett. B 422,

247 (1998).Acknowledgments: We thank Z. Fodor, R. V. Gavai,

F. Karsch, D. Keane, V. Koch, B. Mueller, K. Rajagopal,K. Redlich, H. Satz, and M. Stephanov for enlighteningdiscussions. We acknowledge the Indian LatticeGauge Theory Initiative for computational support,the Department of Atomic Energy–Board of Researchin Nuclear Sciences through the project sanction2010/21/15-BRNS/2026, the U.S. Department of Energyunder contract DE-AC03-76SF00098, and the ChineseMinistry of Education.

21 February 2011; accepted 4 May 201110.1126/science.1204621

The Oxygen Isotopic Composition ofthe Sun Inferred from CapturedSolar WindK. D. McKeegan,1* A. P. A. Kallio,1 V. S. Heber,1 G. Jarzebinski,1 P. H. Mao,1,2 C. D. Coath,1,3

T. Kunihiro,1,4 R. C. Wiens,5 J. E. Nordholt,5 R. W. Moses Jr.,5 D. B. Reisenfeld,6

A. J. G. Jurewicz,7 D. S. Burnett8

All planetary materials sampled thus far vary in their relative abundance of the major isotopeof oxygen, 16O, such that it has not been possible to define a primordial solar system composition.We measured the oxygen isotopic composition of solar wind captured and returned to Earth byNASA’s Genesis mission. Our results demonstrate that the Sun is highly enriched in 16O relativeto the Earth, Moon, Mars, and bulk meteorites. Because the solar photosphere preserves the averageisotopic composition of the solar system for elements heavier than lithium, we conclude that essentiallyall rocky materials in the inner solar system were enriched in 17O and 18O, relative to 16O, by~7%, probably via non–mass-dependent chemistry before accretion of the first planetesimals.

The gravitational collapse of a molecularcloud fragment 4.57 billion years ago ledto an accretion disc of gas and dust, the

solar nebula, from which the Sun and planetsformed. This nebula was approximately homo-geneous with respect to isotopic abundances,

which, given that isotope ratios from variousstellar nucleosynthetic processes vary widely,points to efficient mixing either in interstellarspace or in the solar nebula. Thus, the discovery(1) that high-temperature minerals in carbona-ceous chondrite meteorites are enriched prefer-entially in 16O compared to 17O and 18O relativeto the abundances in terrestrial samples was con-sidered evidence for the presence of exotic ma-terial that escaped thorough mixing and thereby

1Department of Earth and Space Sciences, University ofCalifornia–Los Angeles (UCLA), Los Angeles, CA 90095–1567,USA. 2Division of Physics, Math, and Astronomy, CaliforniaInstitute of Technology (Caltech), Pasadena, CA 91125, USA.3School of Earth Sciences, University of Bristol, Bristol BS8 1RJ,UK. 4Institute for Study of the Earth’s Interior, Okayama Uni-versity, Misasa, Tottori 682-0193, Japan. 5Los Alamos NationalLaboratory, Los Alamos, NM 87545, USA. 6Department of Phys-ics and Astronomy, University of Montana, Missoula, MT 59812,USA. 7Center for Meteorite Studies, Arizona State University,Tempe, AZ 85287, USA. 8Division of Geological and PlanetaryScience, Caltech, Pasadena, CA 91125, USA.

*To whom correspondence should be addressed. E-mail:mckeegan@ess.ucla.edu

24 JUNE 2011 VOL 332 SCIENCE www.sciencemag.org1528

RESEARCH ARTICLES

Science [2011]

Oxygen in the Solar System

20

aqueous tubes cladded with the nanoparticle sur-factants were smaller in diameter and exception-ally long, reminiscent of glass wool (Fig. 3G)except that the structures are fully liquid. The 3Dnature of the morphology, the continuity of thetubular structures, and the cladding of the inter-faces by the nanoparticle surfactant are evident.Thus, simply by stirring, a bicontinuous jammedsystem—a “bijel” type ofmorphology—is achievedwithout the need to modify and tune the surfacechemistry of the nanoparticle to achieve neutralwetting, which is necessary to confine the nano-particles to the liquid/liquid interface and impartlong-term stability to the morphology (13, 14).The carboxylate-amine interactions between thenanoparticles and functionalized silicone oil areself-regulating, maximizing the reduction in theinterfacial energy, overcoming thermal energiesand stabilizing the nanoparticle surfactants at theinterface.

When themonofunctional end-capped siliconeoil was replacedwith a difunctional PDMS, cappedon both ends with primary amines, the nano-particle surfactant assemblieswere stabilized evenfurther, as the PDMS chains bridge adjacent nano-particles, effectively cross-linking the jammednanoparticle assembly (Fig. 4A). Shown in Fig.4, B to E, are two drops of water with polystyrenenanoparticles that were suspended in silicone oilcontaining the difunctional PDMS. The length oftime that the drop at the bottom was in contactwith the silicone oil is 2 min, whereas the con-tact time of the upper drop is 30 s. A 4.6-kV/cmelectric field is applied, and the drop at the bot-tom does not deform, whereas the drop at the topdeforms. Even though the interfacial energy ofthe drop at the bottom is lower, due to the in-crease in the number of nanoparticle surfactantsformed at the interface, the cross-linking of thenanoparticle surfactants is greater, preventingthe deformation of the drop (movies S6 and S7).

The very strong resistance of the drop to defor-mation demonstrates an alternate route by whichdrops of different shapes can be stabilized.

We have demonstrated a simple route to produceand stabilize fluid drops having shapes far re-moved from their equilibrium spherical shape,using the in situ formation of nanoparticle sur-factants. The increased interfacial activity of thenanoparticle surfactants stabilized the assembliesagainst desorption, allowing them to jam at theinterface, arresting change in the drop shape, andimparting long-term stability to drops with un-usual shapes. The sequential application of ex-ternal fields in different directions leads to localunjamming and jamming of the assemblies, en-abling the shape of the drop to be tailored into awide range of unusual shapes. We are currentlyinvestigating the possibility of stabilizing asym-metric shapes to determine the importance on thejamming process. Both electric and shear fieldswere used to deform the drops, although otherfields, like magnetic and ultrasonic fields, are alsobeing investigated. Cross-linking the nanoparti-cle surfactant assemblies at the interface is shownto increase the stability of the drop shape. Thesestabilized assemblies provide easy routes for en-capsulation, bicontinuous flow (microfluidic de-vices) or separationsmedia, delivery vehicles, andreaction platforms.

References and Notes1. R. Hong et al., J. Am. Chem. Soc. 128, 1078–1079 (2006).2. Y. Q. Shi, F. Li, Y. W. Chen, New J. Chem. 37, 236–244

(2013).3. K. Bramhaiah, N. S. John, RSC Adv. 3, 7765 (2013).4. D. Lee, D. A. Weitz, Adv. Mater. 20, 3498–3503 (2008).5. C. N. R. Rao, K. P. Kalyanikutty, Acc. Chem. Res. 41,

489–499 (2008).6. A. D. Dinsmore et al., Science 298, 1006–1009 (2002).7. Y. Lin, H. Skaff, T. Emrick, A. D. Dinsmore, T. P. Russell,

Science 299, 226–229 (2003).8. A. B. Subramaniam, M. Abkarian, L. Mahadevan,

H. A. Stone, Nature 438, 930 (2005).

9. M. Abkarian et al., Phys. Rev. Lett. 99, 188301(2007).

10. A. B. Pawar, M. Caggioni, R. Ergun, R. W. Hartel,P. T. Spicer, Soft Matter 7, 7710 (2011).

11. K. Stratford, R. Adhikari, I. Pagonabarraga, J. C. Desplat,M. E. Cates, Science 309, 2198–2201 (2005).

12. E. M. Herzig, K. A. White, A. B. Schofield, W. C. K. Poon,P. S. Clegg, Nat. Mater. 6, 966–971 (2007).

13. B. P. Binks, S. O. Lumsdon, Langmuir 16, 8622–8631(2000).

14. B. P. Binks, T. S. Horozov, Colloidal Particles at LiquidInterfaces (Cambridge Univ. Press, Cambridge, 2006).

15. L. Li et al., Nano Lett. 11, 1997–2003 (2011).16. C. Zeng, F. Brau, B. Davidovitch, A. D. Dinsmore,

Soft Matter 8, 8582 (2012).17. P. S. Clegg et al., J. Phys. Condens. Matter 17, S3433

(2005).18. P. S. Clegg et al., Langmuir 23, 5984–5994 (2007).19. H. L. Cheng, S. S. Velankar, Langmuir 25, 4412–4420

(2009).20. J. W. Tavacoli, J. H. J. Thijssen, A. B. Schofield,

P. S. Clegg, Adv. Funct. Mater. 21, 2020–2027 (2011).21. S. A. F. Bon, S. D. Mookhoek, P. J. Colver, H. R. Fischer,

S. van der Zwaag, Eur. Polym. J. 43, 4839–4842 (2007).22. D. Orsi, L. Cristofolini, G. Baldi, A. Madsen, Phys. Rev. Lett.

108, 105701 (2012).23. Y. Lin et al., Langmuir 21, 191–194 (2005).24. A. J. Liu, S. R. Nagel, Nature 396, 21–22 (1998).25. G. Taylor, Proc. R. Soc. London A Math. Phys. Sci. 280,

383–397 (1964).26. O. Vizika, D. A. Saville, J. Fluid Mech. 239, 1 (1992).27. D. A. Saville, Annu. Rev. Fluid Mech. 29, 27–64 (1997).28. H. A. Stone, J. R. Lister, M. P. Brenner, Proc. R. Soc. London A

455, 329–347 (1999).29. J. W. Ha, S. M. Yang, J. Fluid Mech. 405, 131–156

(2000).30. J. Q. Feng, T. C. Scott, J. Fluid Mech. 311, 289 (1996).

Acknowledgments: This work was supported by the U.S.Department of Energy Office of Basic Energy Science throughcontract DE-FG02-04ER46126. There are no conflicts ofinterest.

Supplementary Materialswww.sciencemag.org/content/342/6157/460/suppl/DC1Materials and MethodsFigs. S1 to S4Movies S1 to S7

8 July 2013; accepted 25 September 201310.1126/science.1242852

Mass-Independent Oxygen IsotopicPartitioning During Gas-PhaseSiO2 FormationSubrata Chakraborty,* Petia Yanchulova, Mark H. Thiemens

Meteorites contain a wide range of oxygen isotopic compositions that are interpreted as heterogeneityin solar nebula. The anomalous oxygen isotopic compositions of refractory mineral phases may reflecta chemical fractionation process in the nebula, but there are no experiments to demonstrate thisisotope effect during particle formation through gas-phase reactions. We report experimental results ofgas-to-particle conversion during oxidation of silicon monoxide that define a mass-independent line(slope one) in oxygen three-isotope space of 18O/16O versus 17O/16O. This mass-independent chemicalreaction is a potentially initiating step in nebular meteorite formation, which would be capable ofproducing silicate reservoirs with anomalous oxygen isotopic compositions.

The oxygen isotopic composition of high-temperature mineral phases in the firstcondensates in the protoplanetary disk,

calcium-aluminum-rich inclusions (CAIs), are dis-tributed along a slope one line in an oxygen three-isotope plot (18O/16O versus 17O/16O) with a large

and equal depletion in 17O and 18O [~50 permil (‰)with respect to the terrestrial composition] (1).Most chondrules (glassy globular condensates)formed shortly (<1 million years) after CAIs (2)display an oxygen isotopic distribution along anapproximate slope 1 line, with about equal 17Oand 18O enrichments over CAIs (3, 4). The oxy-gen isotopic distributions are notable because oftheir departure from the normal terrestrial mass-dependent (MD) fractionation line observed forequilibrium and kinetic fractionation processesof slope one-half (5, 6). Defining the source ofthe anomalous isotopic distribution of oxygenis critical in the elucidation of the overall for-mation and evolutionary events in the early solarsystem.

After the failure to find meteoritic supernovadebris signatures, the initially proposed nuclear

Department of Chemistry and Biochemistry, University ofCalifornia, San Diego, La Jolla, CA 92093–0356, USA.

*Corresponding author. E-mail: subrata@ucsd.edu

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Science [2013]theory (1) was largely abandoned, and chemi-cal, particularly photochemical, theories tookprecedence as the favored explanation for theobserved oxygen isotopic distribution. Photo-chemical isotopic self-shielding of CO (7) hasbeen proposed on the basis of the predissociativenature (isotopologue-specific absorption lines)in the vacuum ultraviolet spectral region of CO, acommon phenomenon in molecular clouds (8).Ozone formation was the first demonstration ofa mass-independent (MI) fractionation processin a chemical reaction (9). Since this discovery,the potential for producing a CAI-like isotopiccompositional trend in a gas-phase symmetry-driven recombination reaction has been suggested(9–11).Marcus (12) introduced a symmetry-basedgrain-surface assisted theoretical treatment of re-combination of adsorbed species, e.g., O and SiO,to explain the compositions of CAIs. At present,there are no experiments that determine the isotopicfractionation induced by the conversion of gas-phase nebular oxygen species to solid species,which is a key step in the early evolutionary stageof the solar system.

Here, we present results of the measure-ment of oxygen isotopic compositions of SiO2

solids generated via gas-phase reactions undercontrolled experimental conditions. In these ex-periments, ultrahigh purity (UHP) SiO nuggetswere lasedwith an excimer laser [Lambda PhysikCompex 110 (Coherent Incorporated, Santa Clara,California), KrF, 248 nm] inside a vacuum cham-ber at two different initial conditions: set I, in thepresence of UHP O2, and set II, in mixtures ofUHP O2 and H2 of differing proportions. Lasingof SiO generates a plume of neutral SiO gas(see supplementary materials), which reacts withthe gases inside the chamber to form silicondioxide particles throughout the chamber. In setIII experiments, the product SiO2 is formed viaboth mechanisms of set I and set II (as subsets)experiments, and the products were collectedfor analysis. Scanning electron microscopicanalysis provided the stoichiometry of SiO2 forthe product solids for all cases (fig. S2).

The measured oxygen isotopic composi-tions of SiO2 formed in set I–type experiments(without H2) show MD fractionation, whereasthose produced in set II–type experiments (withH2) reflect MI fractionation (Fig. 1 and table S1).The corresponding isotopic compositions of theresidual oxygen reservoirs are fractionated inMD andMI fashion, respectively, for set I and setII experiments (fig. S3 and tables S2 and S3). Theextent of the MI component in SiO2, measuredby D17O (=d17O – 0.516 × d18O), increases withincreasing initial H2/O2 ratio, and the maximumD17O value was ~1.7‰, measured at an H2/O2

ratio of 25.6. The higher ambient gas pressure sup-presses the expansion of the laser plume and alsorapidly cools the plume by collisional deactivationlimiting the extent of oxidation in the higher ratio andpressure experiments (see supplementary text).

Comparing the expected oxidation reactions1 to 4 and 1 to 10 for set I and set II experiments

(Table 1), respectively, and their correspondingexperimental results (Fig. 1, fig. S3, and tablesS1 to S3), we suggest that the observed MI com-position in the residual oxygen and product SiO2

may not be due to ozone formation [known togenerate a MI composition (9)]. Set I–type ex-

periments are the most oxidative and kinetical-ly favor ozone formation compared with set IItypes. The results from set I experiments are strict-ly MD (Fig. 1 and fig. S3) and cannot involveozone formation. Analyzing the entire data set,we observed a MI composition only when H2 is

Fig. 1. Measured oxygen iso-topic compositions in three-isotope plot. The compositions ofproduct SiO2 from set I– and set II–types experiments in set III are shownin blue squares and circles, respec-tively, with respect to the initial SiOcomposition. Compositionsof the start-ing SiO and oxygen and product SiO2from set I–type experiments all lieon a line with a MD slope of 0.516.The SiO2 formed in set II–type experi-ments show MI compositions witha slope value of 0.6. The calculatedisotopic compositions of SiO2 formedby group 2 reactions (OH dominated,see supplementary text) lie on aregression line with a slope value of1.09 T 0.1. The standard deviationof data presented here are T 0.2‰ (much smaller than the size of the symbols).

Table 1. List of relevant reactions. Relevant reactions for set I and set II experiments.

Set I–type experiments Set II–type experiments Reaction number

Group 1SiO + O2 → SiO2 + O SiO + O2 → SiO2 + O 1

SiO + O → SiO2 SiO + O → SiO2 2O2 + hn → O + O O2 + hn → O + O 3

O + O2 + M → O3 + M O + O2 + M → O3 + M 4Group 2

O + H2 + M → OH + H + M 5OH + H2 → H2O + H 6SiO + OH → SiO2 + H 7

SiO + H2O → SiO2 + OH 8H + O2 + M → HO2 + M 9SiO + HO2 → SiO2 + OH 10

Fig. 2. Results of kineticmodel simulation. Time-dependent change in con-centrations of SiO2 formedvia group 1 and group 2 re-actions and the concentra-tion of O2 for the simulationrun with H2/O2 ratio of 30 isshown for three different tem-peratures. The SiO2 amountmeasured in the experimentsbest matched the simulationrun of effective chamber tem-perature of 50°C. Althoughthe temperature of the laserplume is high (>1000°C), theestimated volume of the laserplume is only ~1/100 of the chamber volume, and, therefore, the match at lower temperature (50°C) with theexperiment is reasonable. The result shows that, in set II–type experiments, group 1 reactions are the dominantsource of SiO2 formation, whereas group 2 reactions contribute only 11 to 18% of the total SiO2.

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theory (1) was largely abandoned, and chemi-cal, particularly photochemical, theories tookprecedence as the favored explanation for theobserved oxygen isotopic distribution. Photo-chemical isotopic self-shielding of CO (7) hasbeen proposed on the basis of the predissociativenature (isotopologue-specific absorption lines)in the vacuum ultraviolet spectral region of CO, acommon phenomenon in molecular clouds (8).Ozone formation was the first demonstration ofa mass-independent (MI) fractionation processin a chemical reaction (9). Since this discovery,the potential for producing a CAI-like isotopiccompositional trend in a gas-phase symmetry-driven recombination reaction has been suggested(9–11).Marcus (12) introduced a symmetry-basedgrain-surface assisted theoretical treatment of re-combination of adsorbed species, e.g., O and SiO,to explain the compositions of CAIs. At present,there are no experiments that determine the isotopicfractionation induced by the conversion of gas-phase nebular oxygen species to solid species,which is a key step in the early evolutionary stageof the solar system.

Here, we present results of the measure-ment of oxygen isotopic compositions of SiO2

solids generated via gas-phase reactions undercontrolled experimental conditions. In these ex-periments, ultrahigh purity (UHP) SiO nuggetswere lasedwith an excimer laser [Lambda PhysikCompex 110 (Coherent Incorporated, Santa Clara,California), KrF, 248 nm] inside a vacuum cham-ber at two different initial conditions: set I, in thepresence of UHP O2, and set II, in mixtures ofUHP O2 and H2 of differing proportions. Lasingof SiO generates a plume of neutral SiO gas(see supplementary materials), which reacts withthe gases inside the chamber to form silicondioxide particles throughout the chamber. In setIII experiments, the product SiO2 is formed viaboth mechanisms of set I and set II (as subsets)experiments, and the products were collectedfor analysis. Scanning electron microscopicanalysis provided the stoichiometry of SiO2 forthe product solids for all cases (fig. S2).

The measured oxygen isotopic composi-tions of SiO2 formed in set I–type experiments(without H2) show MD fractionation, whereasthose produced in set II–type experiments (withH2) reflect MI fractionation (Fig. 1 and table S1).The corresponding isotopic compositions of theresidual oxygen reservoirs are fractionated inMD andMI fashion, respectively, for set I and setII experiments (fig. S3 and tables S2 and S3). Theextent of the MI component in SiO2, measuredby D17O (=d17O – 0.516 × d18O), increases withincreasing initial H2/O2 ratio, and the maximumD17O value was ~1.7‰, measured at an H2/O2

ratio of 25.6. The higher ambient gas pressure sup-presses the expansion of the laser plume and alsorapidly cools the plume by collisional deactivationlimiting the extent of oxidation in the higher ratio andpressure experiments (see supplementary text).

Comparing the expected oxidation reactions1 to 4 and 1 to 10 for set I and set II experiments

(Table 1), respectively, and their correspondingexperimental results (Fig. 1, fig. S3, and tablesS1 to S3), we suggest that the observed MI com-position in the residual oxygen and product SiO2

may not be due to ozone formation [known togenerate a MI composition (9)]. Set I–type ex-

periments are the most oxidative and kinetical-ly favor ozone formation compared with set IItypes. The results from set I experiments are strict-ly MD (Fig. 1 and fig. S3) and cannot involveozone formation. Analyzing the entire data set,we observed a MI composition only when H2 is

Fig. 1. Measured oxygen iso-topic compositions in three-isotope plot. The compositions ofproduct SiO2 from set I– and set II–types experiments in set III are shownin blue squares and circles, respec-tively, with respect to the initial SiOcomposition. Compositionsof the start-ing SiO and oxygen and product SiO2from set I–type experiments all lieon a line with a MD slope of 0.516.The SiO2 formed in set II–type experi-ments show MI compositions witha slope value of 0.6. The calculatedisotopic compositions of SiO2 formedby group 2 reactions (OH dominated,see supplementary text) lie on aregression line with a slope value of1.09 T 0.1. The standard deviationof data presented here are T 0.2‰ (much smaller than the size of the symbols).

Table 1. List of relevant reactions. Relevant reactions for set I and set II experiments.

Set I–type experiments Set II–type experiments Reaction number

Group 1SiO + O2 → SiO2 + O SiO + O2 → SiO2 + O 1

SiO + O → SiO2 SiO + O → SiO2 2O2 + hn → O + O O2 + hn → O + O 3

O + O2 + M → O3 + M O + O2 + M → O3 + M 4Group 2

O + H2 + M → OH + H + M 5OH + H2 → H2O + H 6SiO + OH → SiO2 + H 7

SiO + H2O → SiO2 + OH 8H + O2 + M → HO2 + M 9SiO + HO2 → SiO2 + OH 10

Fig. 2. Results of kineticmodel simulation. Time-dependent change in con-centrations of SiO2 formedvia group 1 and group 2 re-actions and the concentra-tion of O2 for the simulationrun with H2/O2 ratio of 30 isshown for three different tem-peratures. The SiO2 amountmeasured in the experimentsbest matched the simulationrun of effective chamber tem-perature of 50°C. Althoughthe temperature of the laserplume is high (>1000°C), theestimated volume of the laserplume is only ~1/100 of the chamber volume, and, therefore, the match at lower temperature (50°C) with theexperiment is reasonable. The result shows that, in set II–type experiments, group 1 reactions are the dominantsource of SiO2 formation, whereas group 2 reactions contribute only 11 to 18% of the total SiO2.

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Mass Independent (MI) Fractionation is

produced during formation of SiO2 (gas - solid exchange

that is relevant to solar nebula!)

?

Other Isotope Fractionations21

Extreme fractionations can be observed in 13C/12C, 15N/14N, 34S/32S

Such enrichments cannot be tied to equilibrium partitioning, they are due to kinetic effects.

Some extreme enrichments are associated with organic material in meteorites and IDPs (Interplanetary Dust Particles).

Molecules on grain surfaces in the coldness of the molecular cloud do not have enough energy to overcome the activation energy to promote reactions.

However, the molecules can be hit with high-energy ions, and this will preferentially remove lighter isotopes and ultimately ‘enrich’ the molecules in heavier isotopes.

Other Isotope Fractionations22

C-chondrites and IDPs are some of the most primitive materials in the Solar System and contain organic matter.

~2 wt.% C in C-chondrites~35 wt.% C in IDPs!!

[C in organic matter]

Much of the C is insoluble organic matter (macromolecular) (IOM).

H & N isotopes suggest the IOM could be interstellar or from cold outer regions of the solar protoplanetary disk.

Previous work suggested largest enrichments in D and 15N were in IDPs, possibly ones that are representative of comets.

In contrast, meteorites tended to show smaller isotopic anomalies.This was believed to be due to more processing (alteration?) on parent bodies.

Organics & Isotopes23

isotope anomalies and exchanged them withisotopically normal matter. The detection ofisotope anomalies indicates that the pristinecharacter of the IOM has not been entirelylost.

Until now, the most extreme enrichmentsin D (8) and 15N (9) have been found in so-called hotspots (regions that are extremelyisotopically enriched relative to the sur-rounding matter) in anhydrous cluster IDPs,which may originate from comets. In contrast,IOM from meteorites, whose parent bodies arein the asteroid belt, showed bulk isotopeanomalies that were relatively small relativeto those in IDP hotspots (6, 10). This dif-ference was assumed to be the result of themore severe parent body alteration and pos-sibly nebular processing Ee.g., (11)^ ex-perienced by meteorites. However, very fewanalyses Ee.g., (12)^ on meteorites have beencarried out on the same spatial scales as theIDP studies.

Here we report D and 15N hotspots in me-teoritic IOM that are comparable to, or evenexceed, those reported in IDPs. Thus, organicmatter that is as primitive as that found inIDPs survives in some meteorites (Table 1),despite the more extensive alteration experi-enced by the meteorites on their parent bodies.This means that large samples of primitiveorganic matter can be prepared frommeteoritesfor studies that would not be possible withIDPs, which typically have masses on the orderof 10j12 g.

We analyzed matrix fragments from twocarbonaceous chondrites (Al Rais and TagishLake) and IOM separates from five carbona-ceous chondrites EGrosvenor Mountains (GRO)95577, Elephant Moraine (EET) 92042, AlRais, Murchison, and Bells^ (Table 1) byimaging secondary ion mass spectrometry(13). All samples exhibited large isotopicheterogeneities EdD È1700 to 19,400 per mil(°), d15N È400 to 3200°; the d notationgives measured isotopic ratios as deviationsfrom terrestrial standards^ on scales compara-

ble to the spatial resolutions of the instruments(Table 1) (13). The most extreme D/H valueswere found in pure IOM separates. Becausethe hotspots survive the chemical separationprocedure and exhibit a range of composi-tions, the hotspots appear to be robust unitsthat formed in a range of environments.Figure 1A is a D/H map of an IOM samplefrom EET 92042 (a Renazzo-type, or CR2,

chondrite recovered in Antarctica) that con-tains two large D hotspots and several smallerones. The dD values for one of these (16,300 T2100°) and for a similar hotspot in GRO95577 (19,400 T 4600°) are the largest everreported for meteoritic material. In total, Dhotspots in EET 92042 IOM made up È1.5%of the area analyzed (Table 1). Note that thebulk IOM has a dD value of È3000° (14),

Table 1. dD and d15N in carbonaceous chondrites, as measured by SIMSand NanoSIMS (13) (n.m., not measured). The hotspots are manuallydefined regions of Q1.3 mm (dD) and Q500 nm (d15N), respectively.‘‘Heterogeneity’’ has been parameterized with the fraction of automatically

created regions of interest [ROIs (13)] that are isotopically anomalous. Weadded up all ROIs with kdDROI – dDaveragek 93 ! sROI and sROI G25%. Notethat all hotspot values are lower limits because their sizes are comparable tothe spatial resolution of the imaging techniques.

dD d15N

Meteorite ClassMaximum,hotspot

Bulk IOM(14)

Analyzedarea (mm2)

Heterogeneity(area %)

Maximum,hotspot

Bulk IOM(14)

Analyzedarea (mm2)

Heterogeneity(area %)

IOMGRO 95577 CR1 19,400 T 4,600 2973 11,780 0.6 1510 T 240 233.2 1440 0.04EET 92042 CR2 16,300 T 2,100 3004 13,112 2.4 1770 T 280 185.5 1937 1.0Al Rais CR2 14,300 T 3,900 2658 6,261 0.3 1740 T 350 146.3 3480 0.005Murchison CM2 1,740 T 280 712 738 4.3 n.m. n.m. n.m. n.m.Bells Anomalous CM2 9,700 T 2,100 3283 5,702 0.3 3200 T 700 415.3 2844 0.11

MatrixAl Rais CR2 6,200 T 650 867 6.2 2000 T 200 637 0.03Tagish Lake Ungrouped C2 8,600 T 1,000 3,963 2.9 410 T 130 1234 0.10

Fig. 1. Maps of (A) dD and (B) d15Nin a sample of IOM from the CR2chondrite EET 92042. Most D and15N hotspots in EET 92042 (dD up to16,300% and d15N up to 1770°)are not spatially associated.

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New high-resolution methods have revealed a more complex (and interesting!) story.

Enrichments in D & N in C-chondrites can be as high or higher than in IDPs.

They are highly localized.

Suggests some IOM in chondrites may be as primitive as in IDPs, despite alteration histories.

Larger fragments in chondrites (as compared to IDPs) can allow different studies of early solar system processes.

Organics & Isotopes24

and therefore these hotspots make only asmall contribution to the bulk composition.This is true of all analyzed IOM. Regions thatare highly D-enriched have also been found inmatrix fragments of Al Rais and Tagish Lake(Fig. 2).

The meteoritic IOM and matrix fragmentsalso exhibit substantial spatial heterogeneity intheir N isotopic compositions (Fig. 1B). EET92042 has a bulk d15N of 185° (14) but hasnumerous regions with higher values up tod15N 0 1770 T 280°. Bells IOM shows evenlarger enrichments in 15N than does EET92042, both in bulk (415°) and in severalhotspots with extreme d15N values between2000 and 3200°. These values are the highestever reported for extraterrestrial material, ex-cept in presolar circumstellar grains (15). Notethat the d15N values are relative to terrestrialatmospheric N, but the Sun has isotopicallylighter N Ed15N e –240°, e.g., (16)^. The en-richments reported here are therefore even largerrelative to the solar value (2100 to 5400°). ThedDvalues given here are relative to oceanwater,which is also isotopically much heavier than wasthe initial solar H EdD , –870° (17)^.

There is no general spatial correlation be-tween H and N isotopes in any of the measuredsamples (Fig. 1). Although some D hotspots arerelatively 15N-enriched, the largest 15N enrich-ments of 91000° are not spatially related to Dhotspots; this indicates that the most extremeanomalies are generally in different molecularcarriers and probably formed through differentchemical pathways.

Our data show that highly anomalous mat-ter survived essentially unaltered in the parentbodies of primitive meteorites. dD values ofup to È19,000° and d15N values above3000° indicate that a complete homogeniza-tion of the pristine IOM did not occur. Denrichments comparable to those found in theIOM of the CR chondrites (Table 1) were pre-viously observed only in two fragments of acluster IDP (8, 18). Also, the highest observedd15N hotspot values (È2000 to 3200° inBells, 1770° in EET 92042) far exceed thehighest value of 1270 T 25° found in IDPs(9, 19). The parent bodies of the cluster IDPs(possibly Kuiper Belt comets) have been as-sumed to contain the most primitive matter insolar system objects (8). The new results

imply that the parent bodies of both meteoritesand IDPs acquired a comparably primitiveassemblage of organic matter that survives inmeteorites despite the more extensive pro-cessing that they experienced.

The largest D enrichment previously re-ported in a meteorite (dD È8000°) wasfound by ion microprobe imaging of a matrixfragment of the CR2 chondrite Renazzo (12).We found comparable D enrichments in AlRais (CR2) matrix, and even higher dD values(914,000°) in IOM separates from three CRchondrites. These observations support theview, based on N isotopes in bulk samples,that CR chondrites are the carbonaceouschondrite group that preserved the mostprimitive organic matter (6). Bells IOM iseven more isotopically anomalous than that ofthe CR chondrites, but Bells appears to beunique among the CM chondrites. The pres-ence of D and 15N hotspots in the matrix ofthe ungrouped C2 chondrite Tagish Lake(Table 1) shows that primitive organics havesurvived in this meteorite, even though nu-clear magnetic resonance studies (20) haverevealed that bulk Tagish Lake IOM has beensubstantially altered by oxidation and is lessprimitive than the CR2 IOM. Microscopicanalyses are necessary to fully understand thesurvival and alteration of pristine organics inmeteorites; our micro-scale isotope examina-tion of meteoritic components allows for thelocalization of these primitive organic compo-nents for further investigation.

The isotopic anomalies observed here musthave originated either in cold interstellarclouds, where large dD values have beenobserved and large d15N values have beenpredicted (2–6), or in the outer regions of theprotoplanetary disk (7), where large D enrich-ments have been predicted for gas-phasemolecules. Viable mechanisms for producinglarge dD and d15N values in either environmentare low-temperature (È10 K) ion-moleculereactions in the gas phase and catalytic processeson dust grains. An interstellar origin is supportedby the similarity of the IOM infrared andultraviolet (UV) spectra to interstellar mediumfeatures of refractory organics (21, 22). More-over, the presence of circumstellar grains inmeteorites and IDPs shows that interstellar

Fig. 2. (A) Scanning electron micro-graph (secondary image) of a matrixfragment of Tagish Lake. (B) Theoverlaid dD map shows two D hot-spots. (C) The overlaid 15N/14N mapshows hotspots with d15N values up toÈ400° (arrows). The largest of these[at upper left, arrow in (B)] is alsoD-rich and is spatially related to around carbonaceous region discernablein (A). These hotspots likely correspondto the ‘‘nano-globules’’ observed inthis meteorite (13, 30).

Fig. 3. dD and C/H(atomic) in the IOM ofEET 92042. The mostD-rich regions (‘‘hot-spots,’’ solid circles) exhib-it dD values between4500 and 16,300°.These values exceed thoseof suggested end mem-bers in the organic matterof IDPs (stars, OM1 toOM3) (28) and reach thedD value of cometaryHCN ice (31). The aver-age of automaticallydefined image subregions2 mm in diameter (graydots) (13) is 2613°,close to 3004° givenfor EET 92042 bulk IOM(open circle) (14), which indicates that sputtering equilibrium is reached and terrestrial contamination was notimportant for the EET 92042 measurements. Data from bulk IOM analyses of the same meteorites that areanalyzed here are given for comparison (open circles) (14). Thermal alteration results in higher C/H values andultimately homogeneous and low dD values.

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and therefore these hotspots make only asmall contribution to the bulk composition.This is true of all analyzed IOM. Regions thatare highly D-enriched have also been found inmatrix fragments of Al Rais and Tagish Lake(Fig. 2).

The meteoritic IOM and matrix fragmentsalso exhibit substantial spatial heterogeneity intheir N isotopic compositions (Fig. 1B). EET92042 has a bulk d15N of 185° (14) but hasnumerous regions with higher values up tod15N 0 1770 T 280°. Bells IOM shows evenlarger enrichments in 15N than does EET92042, both in bulk (415°) and in severalhotspots with extreme d15N values between2000 and 3200°. These values are the highestever reported for extraterrestrial material, ex-cept in presolar circumstellar grains (15). Notethat the d15N values are relative to terrestrialatmospheric N, but the Sun has isotopicallylighter N Ed15N e –240°, e.g., (16)^. The en-richments reported here are therefore even largerrelative to the solar value (2100 to 5400°). ThedDvalues given here are relative to oceanwater,which is also isotopically much heavier than wasthe initial solar H EdD , –870° (17)^.

There is no general spatial correlation be-tween H and N isotopes in any of the measuredsamples (Fig. 1). Although some D hotspots arerelatively 15N-enriched, the largest 15N enrich-ments of 91000° are not spatially related to Dhotspots; this indicates that the most extremeanomalies are generally in different molecularcarriers and probably formed through differentchemical pathways.

Our data show that highly anomalous mat-ter survived essentially unaltered in the parentbodies of primitive meteorites. dD values ofup to È19,000° and d15N values above3000° indicate that a complete homogeniza-tion of the pristine IOM did not occur. Denrichments comparable to those found in theIOM of the CR chondrites (Table 1) were pre-viously observed only in two fragments of acluster IDP (8, 18). Also, the highest observedd15N hotspot values (È2000 to 3200° inBells, 1770° in EET 92042) far exceed thehighest value of 1270 T 25° found in IDPs(9, 19). The parent bodies of the cluster IDPs(possibly Kuiper Belt comets) have been as-sumed to contain the most primitive matter insolar system objects (8). The new results

imply that the parent bodies of both meteoritesand IDPs acquired a comparably primitiveassemblage of organic matter that survives inmeteorites despite the more extensive pro-cessing that they experienced.

The largest D enrichment previously re-ported in a meteorite (dD È8000°) wasfound by ion microprobe imaging of a matrixfragment of the CR2 chondrite Renazzo (12).We found comparable D enrichments in AlRais (CR2) matrix, and even higher dD values(914,000°) in IOM separates from three CRchondrites. These observations support theview, based on N isotopes in bulk samples,that CR chondrites are the carbonaceouschondrite group that preserved the mostprimitive organic matter (6). Bells IOM iseven more isotopically anomalous than that ofthe CR chondrites, but Bells appears to beunique among the CM chondrites. The pres-ence of D and 15N hotspots in the matrix ofthe ungrouped C2 chondrite Tagish Lake(Table 1) shows that primitive organics havesurvived in this meteorite, even though nu-clear magnetic resonance studies (20) haverevealed that bulk Tagish Lake IOM has beensubstantially altered by oxidation and is lessprimitive than the CR2 IOM. Microscopicanalyses are necessary to fully understand thesurvival and alteration of pristine organics inmeteorites; our micro-scale isotope examina-tion of meteoritic components allows for thelocalization of these primitive organic compo-nents for further investigation.

The isotopic anomalies observed here musthave originated either in cold interstellarclouds, where large dD values have beenobserved and large d15N values have beenpredicted (2–6), or in the outer regions of theprotoplanetary disk (7), where large D enrich-ments have been predicted for gas-phasemolecules. Viable mechanisms for producinglarge dD and d15N values in either environmentare low-temperature (È10 K) ion-moleculereactions in the gas phase and catalytic processeson dust grains. An interstellar origin is supportedby the similarity of the IOM infrared andultraviolet (UV) spectra to interstellar mediumfeatures of refractory organics (21, 22). More-over, the presence of circumstellar grains inmeteorites and IDPs shows that interstellar

Fig. 2. (A) Scanning electron micro-graph (secondary image) of a matrixfragment of Tagish Lake. (B) Theoverlaid dD map shows two D hot-spots. (C) The overlaid 15N/14N mapshows hotspots with d15N values up toÈ400° (arrows). The largest of these[at upper left, arrow in (B)] is alsoD-rich and is spatially related to around carbonaceous region discernablein (A). These hotspots likely correspondto the ‘‘nano-globules’’ observed inthis meteorite (13, 30).

Fig. 3. dD and C/H(atomic) in the IOM ofEET 92042. The mostD-rich regions (‘‘hot-spots,’’ solid circles) exhib-it dD values between4500 and 16,300°.These values exceed thoseof suggested end mem-bers in the organic matterof IDPs (stars, OM1 toOM3) (28) and reach thedD value of cometaryHCN ice (31). The aver-age of automaticallydefined image subregions2 mm in diameter (graydots) (13) is 2613°,close to 3004° givenfor EET 92042 bulk IOM(open circle) (14), which indicates that sputtering equilibrium is reached and terrestrial contamination was notimportant for the EET 92042 measurements. Data from bulk IOM analyses of the same meteorites that areanalyzed here are given for comparison (open circles) (14). Thermal alteration results in higher C/H values andultimately homogeneous and low dD values.

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Chart of the Nuclides25

N (# neutrons)

Z (a

tom

ic #

)45°

Chart of the Nuclides26

Note that for heavier elements to be stable (black squares), more neutrons are needed.

This is because the repulsive charges of the protons must be overcome by increasing the distance between them.

Cosmogenic Nuclides/Isotopes27

24Mg

n

24Mg 21Ne

35Cl 36Cl

n

pp

High energy neutron (“cosmic ray”)

excited nucleus

Spallation

Neutron Capture

36Ar

β decay

Cosmogenic Nuclides/Isotopes28Cosmic rays (and secondary neutrons) dissipate within ~ 2-3 meters of planet’s surface

(schematic only – shapes may differ and relative spallation and n-capture production rates are notional )

A

B Example Interpretation:

Assume production rate=1 atom/g/yr at surface

Measure 106 atoms/g

- implies 1 million years exposure at surface

- or, say, 4 million years at 1 m depth

- or ~10 million years at 2 m depth

- or….

Convention: “surface exposure age” is the time required to build up the measured concentration at the surface

Schematic only - shapes and relative rates are notional

A conventional “surface exposure age” is the time required to accumulate the measured concentrations if it were at the surface.

What do such “ages” mean for meteorites?