Quantification of mineral proportions in rocks and soils by Raman spectroscopy on a planetary surface is best done by taking many narrow‐beam spectra from different locations on the rock or soil, with each spectrum yielding peaks from only one or two minerals. The proportion of each mineral in the rock or soil can then be determined from the fraction of the spectra that contain its peaks, in analogy with the standard petrographic technique of point counting. The method can also be used for nondestructive laboratory characterization of rock samples. Although Raman peaks for different minerals seldom overlap each other, it is impractical to obtain proportions of constituent minerals by Raman spectroscopy through analysis of peak intensities in a spectrum obtained by broad‐beam sensing of a representative area of the target material. That is because the Raman signal strength produced by a mineral in a rock or soil is not related in a simple way through the Raman scattering cross section of that mineral to its proportion in the rock, and the signal‐to‐noise ratio of a Raman spectrum is poor when a sample is stimulated by a low‐power laser beam of broad diameter. Results obtained by the Raman point‐count method are demonstrated for a lunar thin section (14161,7062) and a rock fragment (15273,7039). Major minerals (plagioclase and pyroxene), minor minerals (cristobalite and K‐feldspar), and accessory minerals (whitlockite, apatite, and baddeleyite) were easily identified. Identification of the rock types, KREEP basalt or melt rock, from the 100‐location spectra was straightforward.
Abstract Magnesium‐rich spinel assemblages occur in the two lunar vitric breccia meteorites—Dhofar (Dho) 1528 and Graves Nunataks (GRA) 06157. Dho 1528 contains up to ~0.7 mm cumulate Mg‐rich spinel crystals associated with Mg‐rich olivine, Mg‐ and Al‐rich pyroxene, plagioclase, and rare cordierite. Using thermodynamic calculations of these mineral assemblages, we constrain equilibration depths and discuss an origin of these lithologies in the upper mantle of the Moon. In contrast, small, 10 to 20 μm spinel phenocryst assemblages in glassy melt rock clasts in Dho 1528 and GRA 06157 formed from the impact melting of Mg‐rich rocks. Some of these spinel phenocrysts match compositional constraints for spinel associated with “pink spinel anorthosites” inferred from remote sensing data. However, such spinel phenocrysts in meteorites and Apollo samples are typically associated with significant amounts of olivine ± pyroxene that exceed the compositional constraints for pink spinel anorthosites. We conclude that the remotely sensed “pink spinel anorthosites” have not been observed in the collections of lunar rocks. Moreover, we discuss impact‐excavation scenarios for the spinel‐bearing assemblages in Dhofar 1528 and compare the bulk rock composition of Dho 1528 to strikingly similar compositions of Luna 20 samples that contain ejecta from the Crisium impact basin.
Rock and soil samples from the planet Mars are due to be returned to Earth within a decade. Martian samples initially will be tested for evidence of life and biological hazard under strict biological containment. Wider distribution of samples for organic and inorganic analysis may occur only if neither evidence of life nor hazard is detected, or if the samples are first sterilized. We subjected a range of Mars analog rocks and minerals to high doses of gamma radiation in order to determine the effects of gamma sterilization on the samples' isotopic, chemical, and physical properties. Gamma photons from 60 Co (1.17 and 1.33 MeV) in doses as high as 3×10 7 rads did not induce radioactivity in the samples and produced no measurable changes in their isotopic and chemical compositions. This level of irradiation also produced no measurable changes in the crystallographic structure of any sample, the surface areas of soil analogs, or the fluid inclusion homogenization temperature of quartz. The only detectable effects of irradiation were dose‐dependent changes in the visible and near‐infrared spectral region (e.g., discoloration and darkening of quartz and halite and an increase in albedo of carbonates) and increases in the thermoluminescence of quartz and plagioclase. If samples returned from Mars require biological sterilization, gamma irradiation provides a feasible option.
Abstract The Mars rover Opportunity has collected in situ compositional data with the Alpha Particle X‐ray Spectrometer at almost 500 sites. To analyze these data, hierarchical clustering analysis and an error‐weighted similarity index are applied to a subset of 57 APXS target compositions and selected Martian meteorites. Hierarchical clustering provides a rapid first approximation of compositional relationships, whereas the error‐weighted similarity index provides an in‐depth and quantifiable comparison of individual composition pairs. These analyses are combined into a statistical grouping model that provides insight into lithologic relationships and is critically informed by examination of Panoramic Camera and Microscopic Imager images. Major lithologies are (1) the Burns formation sulfate sandstones; (2) Shoemaker impact breccias (Endeavour crater ejecta/rim deposits); (3) the morphologically distinct Grasberg formation, associated with Endeavour crater rim deposits; (4) the Matijevic formation, an exposure interpreted to be Endeavour crater target rocks; and (5) erratics or other rocks that do not cluster with groups 1–4. The Grasberg formation is more similar to the Shoemaker formation than any other formation, and thus likely incorporated eroded Shoemaker material. The lowest Shoemaker member (Copper Cliff breccia) may contain material from the preimpact Matijevic formation. The Matijevic formation is the most chemically distinct formation and is most similar to the volcanic erratic rock “Marquette Island.” Clustering and similarity index values also show that regolith breccia Martian meteorites (represented by the NWA 7475/7034 paired meteorites) are similar in bulk composition to Mars surface materials at Meridiani, especially the Matijevic formation.
Abstract The Northwest Africa ( NWA ) 7475 meteorite is one of the several stones of paired regolith breccias from Mars based on petrography, oxygen isotope, mineral compositions, and bulk rock compositions. Its inventory of lithic clasts is dominated by vitrophyre impact melts that were emplaced while they were still molten. Other clast types include crystallized impact melt rocks, evolved plutonic rocks, possible basalts, contact metamorphosed rocks, and siltstones. Impact spherules and vitrophyre shards record airborne transport, and accreted dust rims were sintered on most clasts, presumably during residence in an ejecta plume. The clast assemblage records at least three impact events, one that formed an impact melt sheet on Mars ≤4.4 Ga ago, a second that assembled NWA 7475 from impactites associated with the impact melt sheet at 1.7–1.4 Ga, and a third that launched NWA 7475 from Mars ~5 Ma ago. Mildly shocked pyroxene and plagioclase constrain shock metamorphic conditions during launch to >5 and <15 GPa. The mild postshock‐heating that resulted from these shock pressures would have been insufficient to sterilize this water‐bearing lithology during launch. Magnetite, maghemite, and pyrite are likely products of secondary alteration on Mars. Textural relationships suggest that calcium‐carbonate and goethite are probably of terrestrial origin, yet trace element chemistry indicates relatively low terrestrial alteration. Comparison of Mars Odyssey gamma‐ray spectrometer data with the Fe and Th abundances of NWA 7475 points to a provenance in the ancient southern highlands of Mars. Gratteri crater, with an age of ~5 Ma and an apparent diameter of 6.9 km, marks one possible launch site of NWA 7475.
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