Late-stage oxidation of yamato 980459 shergottite: evidence from chromite + silica trails in olivine

Introduction: Melt characteristics preserved in shergottite olivine provide important information about the magmatic history of the Mars. For example, previous studies on trace element and hydrogen isotopic compositions of melt inclusions trapped in olivine have revealed possible interactions between the Martian mantle and crust [e.g., 1-2]. We previously reported chromite and melt inclusion trails in olivine megacrysts from Yamato 980459 (Y-98), a sample believed to represent a near-primary Martian melt [3]. Here we present additional microstructural and microchemical results, with the goal of reconstructing the magmatic and subsolidus redox history of Y-98. Methodology: Acquisition of backscattered electron (BSE) images and preparation of focused ion beam (FIB) sections were conducted using a FEI Quanta 3D FEG-SEM/FIB instrument. The extracted FIB sections were then characterized on a JEOL 2010F FASTEM field emission gun scanning transmission electron microscope (STEM/TEM) operated at 200 kV. Quantitative wavelength dispersive spectrometry (WDS) X-ray maps of minor elements were obtained on a JEOL JXA-8200 electron probe microanalyzer (EPMA) using a beam current of 500 nA, a spot size of <1 micron, an accelerating voltage of 15 kV, and a dwell time of 700 ms. Results: The chromite and melt inclusion trails are observed in the inner region of olivine megacrysts. Chromite is fine-grained (<1 μm) and has euhedral-tosubhedral shapes. BSE images show many of the chromite grains are associated with regions of silica that are <1 μm in size. Quantitative X-ray maps (Fig. 1) illustrate that the trails have enrichments in P and often crosscut the P-zoning pattern of olivine. To investigate the microstructures of the trails, we extracted a FIB section (Fig. 2) from the olivine grain shown in Fig. 1. Bright-field TEM (BF-TEM) images (Fig. 3a-b) show that chromite grains are typically ~500 nm in size and are closely associated with silica. Silica partially encloses chromite or occurs as thin layers (<50 nm in thickness) around chromite grains. Fractures that connect adjacent chromite + silica have been observed (Fig. 3b). High-resolution TEM (HRTEM) analysis demonstrates that silica is amorphous: Fast Fourier Transformation (FFT) analysis shows a diffuse ring pattern (Fig. 3c). We also investigated the crystallographic relationship between olivine and chromite, when the two phases are in direct contact; however, no orientation relationship is observed based on HRTEM images (Fig. 3d). The EDS analytical data show that silica contains ~97.4 wt% SiO2, ~1.1 wt% MgO, and ~1.0 wt% FeO. The CaO and Al2O3 contents are both below 0.5 wt%. The composition of olivine varies with the distance from the trail: elevated Cr2O3 contents (up to 1.2 wt%) and higher Mg# (up to 78.5) are observed within a ~300 nm distance from the trail, which drop abruptly to the bulk composition of olivine adjacent to the trail (~0.2 wt% Cr2O3 and ~74.3 Mg#). These EDS compositions of the bulk olivine are consistent with previous EPMA data [e.g., 4].