Evidences for Bimodal volcanism on 4-Vesta Asteroid from impact-melt clast

Silicic / acid volcanism has not been widely described either on Moon, Mars or in Asteroid 4 Vesta. The occurrence of sialic crustal rocks on the lunar surface is extremely limited. Reports on silicic (non-mare) volcanic rocks on Moon is found to be associated in Compton-Belkovich volcanic complex, Hansteen Alpha volcanic crater, Lassell massif, Gruithuisen domes and ejecta of Aristarchus crater (Clegg-Watkins et al., 2017). The occurrence of several volcanic constructs (e.g. collapse features, domes) and volatile-rich pyroclastics in association with silicic rocks further emphasize existence of viscous magmas on Moon. A localized occurrence of silicic volcanism on Mars is also envisaged by the presence of tridymite in mudstone of Gale crater (Morris et al., 2016). However, the exact formation mechanism of silicic volcanism on Moon, Mars or even in 4-Vesta has been largely hindered due to lack of silicic meteorite samples or mission-returned samples.

The HED (Howardite, Eucrite, Diogenite) meteorites is considered to have originated from a common parent body Asteroid 4-Vesta. Recent Dawn mission also attempts to validate its geologic context and formulate a possible HED-Vesta connection (McSween et al., 2013). Based on Dawn findings, Vesta’s surface appears to be similar to a mixture of basaltic eucrite and diogenite resembling a more complex breccia howardite (De Sanctis et al., 2012; Prettyman et al., 2012). A variety of clasts are apparently common in howardite. Here, we report the petrography and major element geochemistry of a new impact-melt clast from Lohawat howardite. Our results show that the clast composition is unique and unlikely to be explained by typical impact melting of HED mafic lithologies. One of the impact melts (~20µ across) hosted in ferroaugite (Wo₄₂En_2.7Fs_55.3) clast substantially differ in composition from the other impact-melt (~50µ across) hosted in ilmenite clast, specially in terms of SiO₂ wt%, CaO wt%, K₂O wt% and K₂O / (K₂O + Na₂O) ratio. Moreover, one appears nearly homogeneous in contrast to evolved nature with limited heterogeneity as compared to other. Both the melts are oblong-shaped, smooth textured with sharp outline and embedded in the host monomict mineral clast of different composition belonging to possible parent of cumulate eucrite.

The average bulk composition of Lohawat is consistent with basaltic crusts (SiO₂ ~50.3-51.8 wt%, Al₂O₃ ~3.5-8.2 wt%, total iron-magnesia ~31.2-38.0 wt%, CaO ~2.2-7.6 wt%) (Chattopadhyay et al. 1998; Sisodia et al. 2001; Ghosh, 2011). Supplement to basaltic volcanism, we report for the first time the incipient acid volcanism in a HED meteorite based on two impact melt inclusions of nearly rhyolitic composition (SiO₂ ~76-79.5 wt%, Al₂O₃ ~11.4 - 12.8 wt%, total alkali ~3 - 8 wt% with K₂O/ (Na₂O + K₂O) ~0.21-0.95, CaO ~ 0.8 - 4.67wt% and low total iron-magnesia ~1-2 wt%). Our study thus reinforces to conceive the idea that some rhyolitic crusts formed due to differentiation of mafic magma were exposed on Vesta and heterogeneity of Vestan surface is definitely different from one as previously thought.

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