Masters Theses

Date of Award

12-2000

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Harry Y. McSween Jr.

Committee Members

Larry A. Taylor

Abstract

For several ordinary chondrite mineral phases, CIPW normative abundances are noticeably higher than modal abundances The discrepancy may result in part from the failure of the CIPW calculation to account for the A1 and Ca enriched mesostasis phase found in ordinary chondrites.

The discrepancy is relatively uniform among the analyzed chondrites and may not significantly affect certain petrologie trends that were uncovered using normative calculations. Specifically, the norm trend suggesting oxidation with increasing metamorphism (McSween and Labotka, 1993) is supported by Fe enrichment in H chondrite olivine and low-Ca pyroxene, and an increase in modal H chondrite olivine abundance.

The chemical and mineralogical data for H5 and H6 values commonly overlap, possibly a result of our small sample size or the misclassification of one or more chondrites. However,overlap also appears in L and LL chondrite data (e.g., Gastineau, 2000; McSween and Labotka, 1993) and may actually be the result of the gradational nature of ordinary chondrite metamorphism. Petrologie trends may be more apparent from analyses of a wider range of petrologic types, as opposed to observations of adjacent petrologie types.

Plagioclase abundance did not appear to increase from type 5 to type 6 in our meteorites,suggesting that plagioclase had crystallized completely by type 5. While mesostasis remains in type 6 chondrites, the plagioclase component appears to have devitrified during type 5 metamorphism. Geothermometers that rely on plagioclase crystallizing during type 6 metamorphism may not be valid However, the leading alternative — the pyroxene geothermometer — has its own pitfalls when applied to ordinary chondrites, including the possibility that high-Ca pyroxene may not have been completely equilibrated.

The high-Fe chondrite Bumwell, while unique among the known ordinary chondrites, may not represent a specimen from a previously unsampled parent body. Many of the trends in our study indicate that Bumwell petrologic characteristics lie outside of the present range for Hchondrites. However, in several trends, the gap between H chondrite and Bumwell values is smaller than the gaps between other ordinary chondrite petrologic groups. Hopefully, additional low-FeO falls will be collected, and classifications can be determined in light of petrologic types other than type 4.

Because normative and modal data are not identical, previous applications of normative data to asteroid spectroscopy should be revisited While trends and overall conclusions probably will not need adjustment, the spectral data sets may need to be re-evaluated Since overall conclusions should persist in the light of new modal data, we still believe that asteroid 6 Hebe represents the best candidate for an H chondrite parent body.

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