Shock-Induced Geochemical Variations in the Keplerite-Bearing Assemblages of Tissint and Intergrown Apatite-Merrillite Assemblages of ALH 84001,146



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Shock metamorphism describes changes (physical, chemical) that occur to materials (structures, rocks, minerals) as a result of shock wave deformation. All Martian meteorites have been subjected to shock, though the degree of shock, and subsequent changes to the physical and chemical characteristics of these rocks, is highly variable. Extensive chemical and structural heterogeneities may occur as a result of shock metamorphism; however, the way in which phosphate minerals respond to shock-metamorphism is not well constrained. Here we present new evidence of shock-induced chemical variations in the keplerite-bearing assemblages of Tissint and the apatite-merrillite assemblages of ALH 84001. Geochemical and structural studies of phosphate phases were investigated using Electron Probe MicroAnalyzer (EMPA), Raman Spectroscopy, Scanning Electron Microscopy (SEM), and Electron Backscatter Diffraction (EBSD) analytical techniques. Phosphates identified included Low-Na (Na# = 2.3) keplerite in Tissint and intergrown Cl-rich-apatite with High-Na merrillite (Na# = 8.3) in ALH 84001. Na-number is the calculated atomic ratio of sodium to the sum of sodium and calcium ([Naatomic/(Naatomic+Caatmoic)]*100). Presence of keplerite (a newly defined, high-temperature, Na-deficient, phosphate) in Tissint suggests it may be common and occur in a broader range of environments (i.e., meteorite groups) than initially established. ALH 84001 apatite was found to be heterogeneously enriched in halogens with Cl between 3.85 and 5.05 wt% and F between 0.30 and 0.92 wt%. Assuming F + Cl + OH = 1.0 structural formula unit (sfu); average atoms per formula unit (apfu) of F = 0.16, while apfu of Cl = 0.63. Therefore, water content of apatite in ALH 84001 is estimated at 0.21 sfu or, ~21% hydroxyl apatite component. Thus, indicating that parental magmas were Cl-rich and OH-poor. Intergrown apatite-merrillite phases were observed as well, with textures suggesting the replacement of magmatic apatite by merrillite in the solid-state following a shock-metamorphic event.



Keplerite, merrillite, apatite, intergrown, Tissint, ALH 84001, shock metamorphism, Mars, MicroProbe Analysis, EMPA, Raman, EBSD