WATER ABUNDANCE IN APATITE SOLID SOLUTIONS IN MARTIAN REGOLITH BRECCIAS AND POTENTIAL APPLICATIONS TO MARS 2020

Cl-rich apatite is the most abundant phosphate in Martian regolith breccias. It is widely distributed in the matrix and also contains hydroxy and fluorine in the structure to form apatite solid solutions (i.e., Ca₅ (PO₄)₃(F, Cl, OH), based on Electron Probe Micro Analyzer (EPMA) analysis). NWA 7034 has been confirmed as water-abundant Martian regolith breccia , and hydrous Fe oxide phases, phyllosilicates, Cl-rich apatite are three important water (H₂O or water-equivalent OH) carriers in the fine-grained matrix.

We have investigated eight apatite targets in Martian breccia NWA 8171 using Raman spectroscopy and three of them using EPMA. The accurate Raman peak positions and widths were determined by spectral deconvolution with WIRE 4.2 software. All the Raman spectra of apatite have a peak band at around 960 cm^-1, which is assigned as ν₁(PO₄), and there is little shift of this peak among these apatite phases. Chlorine and fluorine abundances were quantitively determined by EPMA, and the approximate OH contents in the three apatite grains are estimated based on stoichiometrical calculation. The atomic percentages of chlorine and fluorine are normalized to five calcium atoms per formula unit. All three apatites are water-abundant and hydration is spatially heterogeneous in each clast. Each apatite contains an average water-equivalent hydroxyl concentration of 11,000 ppm (1.10 wt %), 9,400 ppm (0.94 wt %), and 9800 ppm (0.98 wt %) in the structure, respectively. The collected Raman spectra of Cl-rich apatite will be a useful reference for future Raman payloads onboard future Martian rovers (e.g., Mars 2020) to identify apatite species with partial OH substitution for Cl or F on Mars’s surface. In the future, Rietveld refinement of powder X-ray diffraction (XRD) data will be done to determine apatite phase abundance quantitatively in Martian breccias. Total water contents, as constrained by apatite in a bulk sample of NWA 8171, will be estimated based on apatite abundance from Rietveld refinement as well as on average OH abundance in apatite as determined by EPMA.