Dolores H. Hill

Petrography and bulk chemistry of Martian orthopyroxenite ALH84001: Implications for the origin of secondary carbonates

New petrologic and bulk geochemical data for the SNC-related (Martian) meteorite ALH84001 suggest a relatively simple igneous history overprinted by complex shock and hydrothermal processes. ALH84001 is an igneous orthopyroxene cumulate containing penetrative shock deformation textures and a few percent secondary extraterrestrial carbonates. Rare earth element (REE) patterns for several splits of the meteorite reveal substantial heterogeneity in REE abundances and significant fractionation of the REEs between crushed and uncrushed domains within the meteorite. Complex zoning in carbonates indicates nonequilibrium processes were involved in their formation, suggesting that CO2-rich fluids of variable composition infiltrated the rock while on Mars. We interpret petrographic textures to be consistent with an inorganic origin for the carbonate involving dissolution-replacement reactions between CO2-charged fluids and feldspathic glass in the meteorite. Carbonate formation clearly postdated processes that last redistributed the REE in the meteorite.

Petrography and bulk chemistry of Martian lherzolite LEW88516

The meteorite Lewis Cliffs 88516 is a gabbroic lherzolite with geochemical and petrologic characteristics typical of the shergottites, a distinct subgroup of basaltic achondrites belonging to the SNC (Martian) meteorite clan. We report new INAA and microprobe data for LEW88516, noting small but significant differences between our results and those previously published for this meteorite, These discrepancies are mainly attributed to heterogeneities in the bulk rock powder which was distributed to several laboratories for geochemical studies. Other discrepancies are attributed to interlaboratory bias. We emphasize that even minor variations between datasets may have the potential to significantly affect geochemical models for Martian basalts. Small differences in bulk trace element chemistry between LEW88516 and Martian lherzolite ALH77005 may indicate that they crystallized from different magmas. Copyright

Chemical, isotopic and mineralogical evidence for the origin of matrix in ordinary chondrites

Abstract We report the first combined chemical, isotopic and mineralogical study of fine-grained opaque matrix material from a type 3 ordinary chondrite, Allan Hills A77299 (H3.7). Electron microprobe and instrumental neutron activation analysis of a large matrix lump show that it has a major element composition typical of matrix material in type 3 chondrites. Unlike chondrules, it shows no siderophile element depletions and is remarkably unfractionated relative to CI chondrites, suggesting that it is primitive solar system material. The matrix lump has an unique oxygen isotopic composition which lies below the terrestrial fractionation line and differs markedly from the composition of fine-grained matrix lumps and chondrule rims from Semarkona ( Grossman et al ., 1987). Transmission electron microscope (TEM) studies of microtomed samples of the fine-grained fraction ( μm ) of the matrix lump show that it consists of rare, angular, clastic grains (>1 μm ) of pyroxene and olivine, embedded within a groundmass of rounded, fine-grained ( μm ) olivines. The mineralogy and textures of the matrix lump are typical of other occurrences of fine-grained matrix studied by TEM in other type 3 ordinary chondrites. We conclude that the matrix lump in ALHA 77299 consists largely of material that formed by annealing of amorphous presolar dust or nebular condensates and could not have been derived from chondrules by any reasonable mechanism. However, the observed compositional differences between the matrix lump and chondrules in ordinary chondrites are consistent with the idea that some chondrules formed by melting of matrix-like material, accompanied by loss of siderophile and volatile elements.

Trace elements in rims and interiors of Chainpur chondrules

Trace elements were measured in the rims and interiors of nine chondrules separated from the Chainpur LL-3 chondrite. Whole rock samples of Chainpur and samples of separated rims were also measured. Chondrule rims are moderately enriched in siderophile and volatile elements relative to the chondrule interiors. The enriched volatile elements include the lithophilic volatile element Zn. The moderate enrichment of volatiles in chondrule rims and the lack of severe depletion in chondrules can account for the complete volatile inventory in Chainpur. These results support a three-component model of chondrite formation in which metal plus sulfide, chondrules plus rims and matrix silicates are mixed to form chondrites.