Compositional and Structural Mapping of Northwest Africa 15507 Angrite

Angrite meteorites represent interesting sampling of planetary crustal environments. Quench-textured angrites with strong crystal zoning originated from the shallow surface region, with evidence of reducing conditions during solidification. Plutonic angrites have more coarse-grained igneous and metamorphic textures with comparatively less zoning and are interpreted as having equilibrated at greater depth. Plutonic angrites contain the minerals magnetite Fe3O4 comprised of Fe2+ and Fe3+, and rhönite, where Fe3+ is required by inspection of mineral stoichiometry. NWA 15507 is a plutonic angrite with a microgabbroic texture (mean grainsize ∼1.4 mm) composed of zoned Al-Ti-augite, Ca-bearing olivine, anorthite, with accessory kirschsteinite, rhönite, hercynite, low-Ni kamacite, merrillite, Ti-free magnetite and troilite [1]. Rhönite analyzed by electron-probe microanalysis (EPMA) has the formula Ca2.04(Mg0.32Fe2+4.25Fe3+0.47Ti0.33Al0.61)(Si3.74Al2.26)O20, where Fe3+ was estimated by stoichiometric analysis. During preliminary cathodoluminescence (CL) analysis, complex subgrain and oscillatory zoning was observed in the anorthite. In this study we use a combined approach of electron probe microanalysis (EPMA), cathodoluminescence (CL), electron-backscatter diffraction (EBSD), and laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) to further investigate the anorthite and distribution of Fe3+ in NWA 15507. EPMA quantitative analysis and compositional mapping was performed at Washington University on a JEOL 8200 using 15 kV, 25-100 nA probe current, and a range of beam diameter and count times. Spectral CL data was collected with the xCLent system on a JEOL 8530F Plus electron microprobe operating at 20 kV, 30 nA probe current, 30 msec dwell, and 2, 5, or 10 um step size. LA-ICP-MS analyses were performed using a Resolution SE 155 laser system with an ArF excimer laser ablating with a 16 um beam at 5 Hz and 3.5 J/cm2 laser fluence for 30 seconds. The laser system was connected to an Agilent 8900 quadrupole ICP-MS that was tuned for maximum sensitivity while keeping ThO/Th <0.2%. Quantification of LA-ICP-MS results were made using a combination of USGS reference materials GSD-1g, BCR-2g and an in-house plagioclase (An66) standard depending on element and using 27Al as the internal standard element. Results were normalized to 100% assuming Fe as Fe3+. EBSD was collected with the Oxford Instruments Nordlys camera with 1 x 1 binning and 5 um step size on a FEI Quanta 450 FEG operated at 20 kV and spot 5. Data analysis was done with Aztec Crystal 2.2. Results of these analyses are shown in Fig. 1. The EPMA BSE images and quantitative compositional maps show phase distribution and lack of obvious chemical zoning in olivine and anorthite, with Al-Ti zoning present in augite. The observed zoning likely reflects primary igneous zoning as well as homogenization due to metamorphic equilibration. EPMA quantitative compositional maps may not document anorthite trace element chemistry as detection limits for EPMA mapping are approximately 1000 ppm, and anorthite from angrites is typically ∼ An100. Hyperspectral CL maps were acquired on the anorthite and deconvoluted into six peak intensity maps. Peak centroids at 1.73 eV, 2.58 eV, 2.84 eV, and 3.48 eV correspond to the Fe3+ emission center in feldspar group minerals [2,3,4], Ti4+ [5], intrinsic peak [6], and lattice defects [5], respectively. The remaining peaks are unassigned, however similarities between the Ce and Mg LA-ICP-MS maps with CL response is noted (Fig. 1). Inspection of the anorthite grains show complex microstructure with cores showing angular subgrain structure, rim overgrowth with oscillatory zoning, and primary twinning which extends from cores through rims. We interpret this zoning to be primary, but it may be that later processes are also involved. Late-stage overgrowths on anorthite are higher in Ti, Ce and lower in Na, Mg, and total Fe. Comparison of the 1.73 eV intensity map, a proxy for Fe3+, to the total Fe concentration map from LA-ICP-MS suggests that while the overall content of total Fe in the rims is lower, the relative proportion of Fe3+ has increased. These results suggest that comparatively higher oxygen fugacity was in effect during primary crystallization as evidenced by magnetite and rhönite, and further recorded during overgrowth of anorthite where combined CL and LA-ICP-MS data indicate the presence of Fe3+ and increasing incompatible elements including Ce and Ti [7]. Angrite NWA 15507. Backscattered electron image (BSE) showing anorthite (An), olivine (Ol), and clinopyroxene (Cpx) distribution. CaMgFe is a red, blue, green overlay of the calcium, magnesium, and iron x-ray intensity maps. Mg, Fe, Ti, and Ce concentration maps by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Cathodoluminescence peak intensity maps for the 2.58 eV, 2.84 eV, 3.48 eV, and 1.73 eV centroids which correspond to Ti4+, intrinsic, lattice defects, and Fe3+ are also shown. Field of view (FOV) is 5 millimeters. Magma look up table used for LA-ICP-MS and CL maps where dark purples are indicate low intensity signal and yellow indicates high intensity signal.