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A) C) B) Diopside Albite enclosed in Olivine? Orthopyroxene Albite enclosed in Pyroxene? Albite Olivine 50 µm 50 µm Conclusions Compositionally uniform albite is the only K bearing mineral, and accounts for all K in the meteorite
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Albite enclosed in Olivine?
Albite enclosed in Pyroxene?
Abundance and Composition of K and Ca Bearing Minerals in Ordinary Chondrites and their Application to Ar/Ar Dating
J. Weirich and T. Swindle
Lunar and Planetary Laboratory, University of Arizona
Extraction of Ar from ordinary chondrites suggests two sources of K, but only one source of K (albitic feldspar) has been found. What gives?
Two release patterns
Table 1. (Left) K and Ca mineral wt% and modal abundance for an ideal H6 chondrite. *From . All other data from this work. (Right, bold) Elemental abundances taken from .
Fig. 1. (Left) K/Ca for a relict clast OCA1 in Orvinio, an H impact melt . (Right) K/Ca for Kernouve, an H6 S1, 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.
Compare with Previous Work
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.
E for K-feldspar is ~45 kcal/mol 
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 
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 
Fig 4. Data from Fig. 4 plotted as K/Ca. K/Ca from this study has also been plotted.
References:  Grier J. A. et al. (2004) Meteoritics & Planet. Sci., 39, 1475–1493.  Turner G. et al. (1978) Proc. LPSC IX, 989–1025.  McDougall I. and Harrison T. M. (1999) Geochronology and Thermochronology by the 40Ar/39Ar Method.  Dodd R. T. (1981) Meteorites.  Lodders K. and Fegley B. (1998) The Planetary Scientist’s Companion.  Van Schmus W. R. et al. (1968) Geochim. Cosmochim. Acta, 32, 1327–1342.