Chemical layering in the upper mantle of Mars: Evidence from olivine‐hosted melt inclusions in Tissint

Abstract

Melting of Martian mantle, formation, and evolution of primary magma from the depleted mantle were previously modeled from experimental petrology and geochemical studies of Martian meteorites. Based on in situ major and trace element study of a range of olivine‐hosted melt inclusions in various stages of crystallization of Tissint, a depleted olivine–phyric shergottite, we further constrain different stages of depletion and enrichment in the depleted mantle source of the shergottite suite. Two types of melt inclusions were petrographically recognized. Type I melt inclusions occur in the megacrystic olivine core (Fo76‐70), while type II melt inclusions are hosted by the outer mantle of the olivine (Fo66‐55). REE‐plot indicates type I melt inclusions, which are unique because they represent the most depleted trace element data from the parent magmas of all the depleted shergottites, are an order of magnitude depleted compared to the type II melt inclusions. The absolute REE content of type II displays parallel trend but somewhat lower value than the Tissint whole‐rock. Model calculations indicate two‐stage mantle melting events followed by enrichment through mixing with a hypothetical residual melt from solidifying magma ocean. This resulted in ~10 times enrichment of incompatible trace elements from parent magma stage to the remaining melt after 45% crystallization, simulating the whole‐rock of Tissint. We rule out any assimilation due to crustal recycling into the upper mantle, as proposed by a recent study. Rather, we propose the presence of Al, Ca, Na, P, and REE‐rich layer at the shallower upper mantle above the depleted mantle source region during the geologic evolution of Mars.

What Martian Meteorites Reveal About the Interior and Surface of Mars

“…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).…”

Consolidated Chemical Provinces on Mars: Implications for Geologic Interpretations

“…Moreover, the non‐systematic trend of the abundances of LILEs and other secular chemical trends may necessitate complex melting processes in a highly differentiated mantle source on a regional to global scale (cf. Basu Sarbadhikari et al., 2016, 2017; Udry et al., 2020) that existed from the early‐stage evolution of Mars.…”

Convective activity in a Martian magma chamber recorded by P‐zoning in Tissint olivine

“…3). The high concentration of P 2 O 5 in the outer olivine rims is likely related to cooling during lava flow emplacement(Basu Sarbadhikari et al 2017).…”