Nanostructural domains in martian apatites that record primary subsolidus exsolution of halogens: Insights into nakhlite petrogenesis

Abstract

The microstructures of selected F-, Cl-, OH-bearing martian apatite grains, two in Northwest Africa (NWA) 998 (cumulus apatites, embedded in pyroxene) and a set of four in Nakhla This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

Epitaxial fluorapatite vein in Northwest Africa 998 host apatite: Clues on the geochemistry of late hydrothermal fluids on Mars

“…TEM work reveals that the apatite grain sampled by the FIB section is a single crystal, except for a small (2 μm-sized square region), compositionally distinct polycrystalline region (dashed rectangle in Figure 2a in Martıńez et al, 2023b). The apatite contains an ∼3 μm-sized melt inclusion, which is described in Martıńez et al (2023b), and a fragmented vein, 100-200 nm wide, crosscutting the apatite FIB section (Figure 5b in Martıńez et al, 2023a).…”

“…A completed BSE mosaic of the studied thin section of NWA 998 and the petrographic characteristics of the apatite grains are described in Martıńez et al (2023a). Northwest Africa 998 is the only nakhlite known to contain cumulus apatite, and thus, apatites in this sample are the largest among the nakhlites, ranging from ∼50 to ∼200 μm in size (Figure 1).…”

“…They mainly occur embedded within pigeonite grains or between augite and titanomagnetite crystals. The cumulus apatite grain selected for FIB/TEM studies is closely intergrown with titanomagnetite and Ca-rich pyroxene and the FIB foil was cut perpendicular to the elongation direction of the crystal (Figure 1b in Martıńez et al, 2023a). The region contains five other apatite grains, ranging from ∼10 to ∼60 μm in size (Figure 2c in Martıńez et al, 2023b).…”

“…The nakhlites are cumulus igneous rocks from Mars that crystallized from basaltic magmas at relatively low pressures ∼1.3 Ga ago (Beck et al, 2006;Harvey & McSween, 1992;Mikouchi et al, 2012;Nyquist et al, 2001;Swindle & Olson, 2004;Treiman, 2005). The chemical and isotopic characteristics of the nakhlites show evidence of mixing with a volatile-rich crustal component (i.e., a fluid) that assimilated into the nakhlite melt (e.g., Martıńez et al, 2023a;McCubbin et al, 2013;Shearer et al, 2018). The nakhlites are clinopyroxenites with cumulus augite and less abundant cumulus olivine, with variable amounts of mesostasis, interstitial regions to cumulus grains that contain minor phases, including silicates (e.g., plagioclase, alkali feldspar, and silica), oxides (titanomagnetite, ilmenite), sulfides (troilite, pyrrhotite, chalcopyrite), and phosphates (apatite), among others (e.g., Harvey & McSween, 1992;Treiman, 2005;Wadhwa & Crozaz, 1995).…”

“…Specifically, apatite in NWA 998 reflects a complex record of both magmatic and subsolidus processes and extracting petrogenetic and volatile records are difficult and fraught with interpretive issues. For these reasons, examining apatite at the nanometer scale provides valuable insights into the complex thermal, geochemical, and fluid evolution history (Martıńez et al, 2023a(Martıńez et al, , 2023b.…”