Masters Theses

Date of Award

5-2003

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Larry Taylor

Abstract

The petrogenesis of four lunar highlands meteorites, Dhofar 025 (Dh-025), Dhofar 081 (Dh-081 ), Dar al Gani 262 (DaG 262), and Dar al Gani 400 (DaG 400) have been studied. For Dh-025, measured oxygen isotopic values (0180 = +5.47%0 and 017 0 = +2.81 %0) and Fe-Mn ratios for mafic minerals (Px-49.7; 01-86) provide corroboratory evidence that it originated on the Moon. Similarly, Fe-Mn ratios in the mafic minerals (Px-52.3; 01-87 .8) of Dh-081 also indicate its lunar origin. Whole-rock Sc and Al2O3 concentrations of Dh-025 (10.2 ppm and 26.1 wt%, resp.), Dh-081 (5.4 ppm and 30.5 wt%, resp.), DaG 262 (7.85 ppm and 27.5 wt%), and DaG 400 (5.40 ppm and 26.2 wt%, resp.; Bischoff et al., 1998) are also indicative of anorthositic lunar highlands material. Each of the four lunar samples is a breccia, with texturally pristine and nonpristine lithologies. Lithologies in Dh-025 and Dh-081 include igneous rocklets, partially recrystallized impact melt rocklets, granulites, and mineral fragments. Some of these lithologies have plagioclase AN and coexisting mafic mineral Mg# that plot within the "gap" separating ferroan anorthosite suite (FAN) and high-magnesium suite (HMS) rocks. These atypical compositions are more common in recent collections of lunar highlands meteorites, and may indicate the FAN field should be expanded. This is consistent with whole-rock Ti-Sm ratios for Dh-025 (1.5 ppm), Dh-081 (0.63 ppm), and DaG 262 (1.15), which are also intermediate compared to FAN and HMS lithologies. Compared to other lunar highlands meteorites, whole-rock compositions also show that Dh-025 is enriched in Sc and Mg# (70), whereas Dh-081 has the highest A12O3 content measured to date. Systematic depletions (by a factor of 10) in whole-rock Sm concentrations are apparent in Dh-025 (1.5 ppm), Dh-081 (0.63 ppm), and DaG 262 (1.15 ppm), as compared to Apollo 14 and 16 highland regolith breccias. This may indicate that these samples have been derived from an entirely different terrain of highlands regolith, perhaps on the lunar far-side. Ion-microprobe analyses were also performed to evaluate trace-element (including rare earth elements-REE) variations in Dh-025, Dh-081, DaG 262, and DaG 400 clasts and minerals. These analyses show far stronger FAN affinities for these meteorites than whole-rock data suggest, although most clasts indicate admixture of a <=12% HMS component. Ratios of Sc-Sm in all clasts are typical of low-incompatible trace-element abundances in FAN lithologies (> 12). Several analyses of Dh-025 plagioclases indicate redistribution of HREEs due to shock metamorphism. However, the majority of mineral analyses in all these meteorites are typical of FAN lithologies. Coexisting plagioclase-pyroxene REE concentration ratios in several clasts were compared to experimentally determined plagioclasepyroxene REE distribution coefficient ratios. Two Dh-025 clasts have concordant plagioclase-pyroxene profiles indicating that the equilibrium between these minerals has not been modified by shock metamorphism. These clasts plot within the FAN-HMS gap and possibly represent new pristine highland compositions. Since these compositions have not been sampled on the lunar near-side, it is possible they represent far-side rocks and suggest that the FAN or HMS fields should be expanded.

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