Petrology and geochemistry of olivine‐phyric shergottites LAR 12095 and LAR 12240: Implications for their petrogenetic history on Mars

Abstract

Larkman Nunatak (LAR) 12095 and LAR 12240 are recent olivine‐phyric shergottite finds. We report the results of petrographic and chemical analyses of these two samples to understand their petrogenesis on Mars. Based on our analyses, we suggest that these samples are likely paired and are most similar to other depleted olivine‐phyric shergottites, particularly Dar al Gani (DaG) 476 and Sayh al Uhaymir (SaU) 005 (and samples paired with those). The olivine megacryst cores in LAR 12095 and LAR 12240 are not in equilibrium with the groundmass olivines. We infer that these megacrysts are phenocrysts and their major element compositions have been homogenized by diffusion (the cores of the olivine megacrysts have Mg# ~70, whereas megacryst rims and groundmass olivines typically have Mg# ~58–60). The rare earth element (REE) microdistributions in the various phases (olivine, low‐ and high‐Ca pyroxene, maskelynite, and merrillite) in both samples are similar and support the likelihood that these two shergottites are indeed paired. The calculated parent melt (i.e., in equilibrium with the low‐Ca pyroxene, which is one of the earliest formed REE‐bearing minerals) has an REE pattern parallel to that of melt in equilibrium with merrillite (i.e., one of the last‐formed minerals). This suggests that the LAR 12095/12240 paired shergottites represent the product of closed‐system fractional crystallization following magma emplacement and crystal accumulation. Utilizing the europium oxybarometer, we estimate that the magmatic oxygen fugacity early in the crystallization sequence was ~IW. Finally, petrographic evidence indicates that LAR 12095/12240 experienced extensive shock prior to being ejected from Mars.

What Martian Meteorites Reveal About the Interior and Surface of Mars

“…Pyroxene Ti/Al can help to constrain a range of pressures of crystallization (not an exact pressure, as it is not fully calibrated for Mars; Filiberto et al., 2010; Nekvasil et al., 2007). The application of this geobarometer to various shergottites and chassignites suggests that formation of staging chambers at the crust/mantle boundary may be widespread on Mars, possibly leading to the formation of the various shergottite lithologies (Combs et al., 2019; Dunham et al., 2019; Filiberto 2017; Howarth et al., 2018; Nekvasil et al., 2004; Rahib et al., 2019; Udry et al., 2017). Minor element compositions in pyroxene in nakhlites also suggest that they could have formed at the bottom of the martian crust (McCubbin et al., 2013; Udry & Day, 2018).…”

“…First are the basaltic shergottites, which mostly contain pyroxene (average lengths of 0.3 mm, up to 1 mm) and maskelynite and are characterized by the absence of olivine phenocrysts or megacrysts (Figure 3a; e.g., He et al., 2015; Howarth et al., 2018; McSween et al., 1996; Rubin et al., 2000). Second in abundance are the olivine‐phyric shergottites, which are porphyritic and contain olivine phenocrysts (sometimes megacrystic with sizes up to 2.5 mm) with later‐crystallizing olivine, pyroxene, and maskelynite (Figure 3b; grains in the groundmass of ∼0.25 mm; e.g., Balta et al., 2015; Basu Sarbadhikari et al., 2009; Basu Sarbadhikari et al., 2016; Chen et al., 2015; Dunham et al., 2019; Goodrich, 2002; Liu et al. 2016a).…”

“…First are the basaltic shergottites, which mostly contain pyroxene (average lengths of 0.3 mm, up to 1 mm) and maskelynite and are characterized by the absence of olivine phenocrysts or megacrysts (Figure 3a; e.g., He et al, 2015;Howarth et al, 2018;McSween et al, 1996;Rubin et al, 2000). Second in abundance are the olivine-phyric shergottites, which are porphyritic and contain olivine phenocrysts (sometimes megacrystic with sizes up to 2.5 mm) with later-crystallizing olivine, pyroxene, and maskelynite ( Figure 3b; grains in the groundmass of ∼0.25 mm; e.g., Balta et al, 2015;Basu Sarbadhikari et al, 2009;Basu Sarbadhikari et al, 2016;Chen et al, 2015;Dunham et al, 2019;Goodrich, 2002;Liu et al, 2016a). Third are the poikilitic shergottites that contain olivine chadacrysts (up to 1.8 mm) enclosed by large pyroxene oikocrysts (from 3 to 10 mm in length), with later-crystallizing olivine, pyroxene, and maskelynite ( Figure 3c; Combs et al, 2019;Howarth et al, 2014;Kizovski et al, 2019;Rahib et al, 2019;Walton et al, 2012).…”

Heavy halogen geochemistry of martian shergottite meteorites and implications for the halogen composition of the depleted shergottite mantle source

“…LAR 12095 and LAR 12240 were recovered from the Larkman Nunataks region of the Transantarctic mountains, East Antarctica in 2012 (McBride et al 2013). They have been described in detail by several previous workers (Howarth et al 2014;Mikouchi and Takenouchi, 2014;Balta et al 2015a;Funk, 2015, Funk et al 2016Dunham et al 2019) suggesting that on the basis of petrography, mineral-chemistry and whole rock major and trace element geochemistry they are paired stones from the same meteorite fall (Dunham et al 2019). Both are olivine-pyhric shergottites containing olivine phenocrysts up to 3 mm in size, with rare pyroxene macrocrysts up to 2 mm in size.…”

“…Both are olivine-pyhric shergottites containing olivine phenocrysts up to 3 mm in size, with rare pyroxene macrocrysts up to 2 mm in size. Dunham et al (2019) characterized the modal mineralogy of LAR 12095/12240 and found the groundmass predominantly comprises olivine (~16-17 vol %), pyroxene (61)(62) vol %) and maskelynite (21-23 vol %). Minor phosphates, oxides (~1 vol %) and sulfides (~1 vol %) are also present.…”