Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.424r&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 424
Mathematics
Logic
1
Acfer 217, Carbonaceous Chondrites, Carlisle Lakes, Chondrite Compositions, Chondrite Groups, Enstatite Chondrites, Ordinary Chondrites
Scientific paper
Although chondrites are all solarlike in their abundances of nonvolatile elements, there are appreciable differences among chondrite groups in texture and mineralogical, chemical, and O-isotopic composition. There are now 12 chondrite groups, each containing at least five members: CI, CR, CM, CO, CV, and CK carbonaceous chondrites; H, L, and LL ordinary chondrites; EH and EL enstatite chondrites; and Carlisle Lakes chondrites. Eight Carlisle Lakes chondrites have been identified: Carlisle Lakes, ALH 85151, Y 75302, Y 793575, Y 82002, Acfer 217, PCA 91002, and PCA 91241; the latter two may be paired. The primary petrographic characteristics of the group include abundant matrix (42 +/- 11 vol%) and chondrules averaging 400 micrometers in apparent diameter. Secondary petrographic characteristics include moderate metamorphic recrystallization and, in most members, extensive brecciation. As a family, carbonaceous chondrite groups (excepting CI in some cases) are characterized by (1) group/CI mean-refractory-lithophile/Si abundance ratios of 1.00-1.35, (2) a moderate to high degree of Fe oxidation, (3) high fine- grained-matrix/chondrule modal abundance ratios (0.5-7), (4) an appreciable abundance of refractory inclusions (~0.5-5 vol%), (5) whole-rock O-isotopic compositions significantly below the terrestrial fractionation (TF) line, (6) siderophile and chalcophile abundance patterns that decrease monotonically with increasing volatility (with low to moderate CI-normalized Se/Sb concentration ratios, 0.6-0.9), (7) relatively abundant opaque-mineral-rich porphyritic chondrules, and (8) where present, plagioclase with high molar An. Ordinary chondrites (OC) are characterized by (1) group/CI mean-refractory- lithophile/Si abundance ratios of 0.77-0.82, (2) a low to moderate degree of Fe oxidation, (3) low fine-grained-matrix/chondrule modal abundance ratios (~0.3, excluding metamorphosed OC), (4) a negligible abundance of refractory inclusions, (5) whole-rock O-isotopic compositions above the TF line, (6) moderate CI-normalized Se/Sb concentration ratios (0.9-1.1), (7) relatively few opaque-mineral-rich porphyritic chondrules, and (8) where present, plagioclase with low molar An. Enstatite chondrites resemble OC in many respects. They are characterized by (1) group/CI mean-refractory-lithophile/Si abundance ratios of ~0.60, (2) a very low degree of Fe oxidation, (3) essentially no fine-grained matrix, (4) a negligible abundance of refractory inclusions, (5) whole-rock O-isotopic compositions on the TF line, (6) moderate CI-normalized Se/Sb concentration ratios (1.0-1.2), (7) a low to moderate abundance of opaque-mineral-rich porphyritic chondrules, and (8) plagioclase with very low molar An. Carlisle Lakes chondrites are characterized by (1) a CI-normalized mean- refractory-lithophile abundance ratio of 0.85, (2) a high degree of Fe oxidation (as indicated by a molar ratio of [FeO/(FeO + MgO)] x 100 of 37 and an atomic ratio of metallic-Fe/Si of 10^-4), (3) a high matrix/chondrule modal abundance ratio (1.6 +/- 0.9), (4) essentially no refractory inclusions, (5) whole-rock O-isotopic compositions appreciably above the TF line (e.g., delta ^17O = 5.12 per mil), (6) a high mean CI-normalized Se/Sb concentration ratio (1.5 +/- 0.2), (7) few opaque-mineral-rich porphyritic chondrules, and (8) plagioclase with low molar An. Our previous INAA data for Carlisle Lakes and ALH 85151 [1] showed high Ca and low Au, which we attributed to weathering and nebular processing respectively. However, new data for PCA 91002 reveal only a small Ca enhancement and no Au depletion. We now suspect that the low Au abundances in Carlisle Lakes and ALH 85151 are also weathering artifacts resulting from oxidation of the Au carrier in the matrix and transport to new sites. The carbonaceous chondrite family comprises three clans (CI-CR?, CM-CO, and CV-CK). These chondrites are characterized by a generally high oxidation state and, except in CI chondrites, the presence of abundant, isotopically heterogeneous refractory inclusions; it seems likely that this family formed relatively far from the Sun because higher temperatures at small heliocentric distances are typically associated with reduction and isotopic homogeneity. The other major family of chondrites, here dubbed the inner solar system family, also comprises three clans (H-L-LL, EH-EL, and Carlisle Lakes). In both chondrite families, individual groups within each clan differ in mean- refractory-lithophile/Si abundance ratios by <=8%, indicating that they must have agglomerated from similar precursor components at similar times and similar heliocentric distances. Within each family, interclan agglomeration locations were more widely separated in space and time but were still within the same general region of the solar nebula. Reference: [1] Rubin A. E. and Kallemeyn G. W. (1989) GCA, 53, 3035-3044.
Kallemeyn Gregory W.
Rubin Alan E.
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