Abundance and Composition of K and Ca Bearing Minerals in Ordinary Chondrites and their Application to Ar/Ar Dating

J. Weirich and T. SwindleLunar and Planetary Laboratory, University of Arizona

0

0.2

0.4

0.6

0.8

0 20 40 60 80 100

Cumulative %39Ar released

K/C

a

Idealized Meteorite

Kernouve

Extraction of Ar from ordinary chondrites suggests two sources of K, but only one source of K (albitic feldspar) has been found. What gives?

Idealized Meteorite (Avg. of Wagon Mound and Payson)

Elemental Abund. in H

Mineral K wt%Modal Abund%

Total wt% K wt%

Feldspar 0.83 10.4 0.086 0.078

Mineral Ca wt%ModalAbund%

Totalwt%

Diopside 15.70 *4.00 0.628

Apatite 39.00 *0.60 0.234

Feldspar 1.51 10.4 0.157

Orthopyroxene 0.46 *24.5 0.113

Olivine 0.07 *36.2 0.025 Ca wt%

Total Ca wt% 1.157 1.22

Introduction• Ar extraction of ordinary chondrites via diffusion (heating) has revealed two release patterns, one at low temperature with a K/Ca of 0.2-0.6, and one at high temperature with a K/Ca of about 0.05

• Typically, this has been interpreted as two separate mineral sources of K

• Mass balances of K and Ca have been performed to identify these two sources

• For this study, we have analyzed Payson and Wagon Mound, both of which are H6 S2

Fig. 1. (Left) K/Ca for a relict clast OCA1 in Orvinio, an H impact melt [1]. (Right) K/Ca for Kernouve, an H6 S1[2], and the Idealized Meteorite from Table 1. The Idealized Meteorite assumes 45% of the Ar in feldspar is released at same time as Ar in all other minerals. Note: Since irradiation parameters were not known for Orvinio, total K and Ca was scaled to match the idealized meteorite.

Fig. 2. Arrhenius plot for Orvinio, clast OCA3. Open circles represent calculation assuming a single diffusion domain. Closed circles represent calculation assuming two diffusion domains, one from 500-675°C and the other from 800-1400°C. From [1]

E for K-feldspar is ~45 kcal/mol [3]

Two release patterns

Microprobe Work• Weight percent of all K and Ca bearing minerals was measured by a Cameca SX50 at the University of Arizona

• Compositionally uniform albite was found to be the only mineral containing measurable amounts of K

• Modal abundance of feldspar was determined from elemental maps by using Photoshop

• Results are consistent with previous work

• Summary of results shown in Table 1

Table 1. (Left) K and Ca mineral wt% and modal abundance for an ideal H6 chondrite. *From [4]. All other data from this work. (Right, bold) Elemental abundances taken from [5].

Fig. 3. Ternary plot of uniform chondritic feldspar compositions: Open circles, LL; solid circles, L; squares, H; triangle, E. All are type 6, of varying shock degrees. From [6]

Compare with Previous Work

K/Ca of Feldspar in ordinary chondrites

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

H L Thisstudy

K/C

a

Fig 4. Data from Fig. 4 plotted as K/Ca. K/Ca from this study has also been plotted.

Implications• Low temperature release pattern due solely to albite

• High temperature release pattern is a combination of albite and all other Ca bearing minerals (See Fig. 1)

Why two releases of K?• Shock effects disrupt some of the feldspar, changing diffusion parameters

• Unlikely since low shock meteorites still have two releases, and high temperature release has an activation energy higher than K-feldspar

• Albite completely enclosed in pyroxene or olivine, and hence forced to outgas simultaneously

• This may be possible, as seen below in elemental maps. More promising, but hard to positively confirm from microprobe work

• Could ~50% of the albite really be enclosed?

Conclusions• Compositionally uniform albite is the only K bearing mineral, and accounts for all K in the meteorite

• Albite is the source of the low temperature release, while albite plus other Ca bearing minerals is the source of the high temperature release

• One possibility for this is shock effects change the diffusion parameters

• Another possible explanation for this is some of the albite is trapped in either pyroxene or olivine and the Ar cannot escape at lower temperatures

• To help test this idea, the activation energy of pyroxene and olivine will be measured

0

0.05

0.1

0.15

0.2

0.25

0 20 40 60 80 100

%39Ar

K/C

a

References: [1] Grier J. A. et al. (2004) Meteoritics & Planet. Sci., 39, 1475–1493. [2] Turner G. et al. (1978) Proc. LPSC IX, 989–1025. [3] McDougall I. and Harrison T. M. (1999) Geochronology and Thermochronology by the 40Ar/39Ar Method. [4] Dodd R. T. (1981) Meteorites. [5] Lodders K. and Fegley B. (1998) The Planetary Scientist’s Companion. [6] Van Schmus W. R. et al. (1968) Geochim. Cosmochim. Acta, 32, 1327–1342.

Fig. 5. A) BSE image of Wagon Mound with major minerals labeled. Microprobe element map of B) Wagon Mound showing albite surrounded by pyroxene and C) Cat Mountain showing albite surrounded by olivine. Color schemes between the two are identical. Note: Due to limitations of probe, colors at the upper right and lower left are darkened.

50 µm

Albite enclosed in Albite enclosed in Pyroxene?Pyroxene?

B)B)

Albite enclosed in Albite enclosed in Olivine?Olivine?

50 µm

C)

DiopsideDiopside

OrthopyroxeneOrthopyroxene

AlbiteAlbiteOlivineOlivine

A)A)

Wagon Mound

Cat Mountain