GRANITIC COMPOSITIONS IN GABBROIC MARTIAN METEORITE NWA 6963: EXTREME FRACTIONAL CRYSTALLIZATION OF A HYDROUS MAGMA?

Felsic rocks such as granite and the volcanic equivalent, rhyolite, are common on Earth where they crystallize from highly evolved siliceous melts formed from magmatic processes associated with plate tectonics. These siliceous melts can be produced from a parent magma through extensive fractional crystallization [1,2]. There is little evidence that Mars had plate tectonic and the crust is dominated by basaltic rocks that have experienced little magmatic evolution [3]. Mars Science Laboratory (MSL) Curiosity Rover analyzed felsic rocks and soils near Gale Crater [4]. However, until recently felsic/granitic compositions were missing from the meteorite collection. Martian meteorite breccia Northwest Africa (NWA) 7034 (and pairs) is the first sample that contains more felsic rock types such as trachyte [5]. Gabbroic shergottite NWA 6963 contains micrographic intergrowths of quartz and feldspar that has a late-stage granitic melt composition.

The present lack of plate tectonics on Mars suggests that granites formed in terrestrial intraplate settings may be more analogous to Mars than granite formed in subduction zones. The finding of small pockets of granitic-like melt composition in NWA 6963 is consistent with extreme fractional crystallization within the martian crust. Pressures could have been midcrustal due to the low total alkali content and high K₂O content compared to terrestrial granites [1,2]. However, experimental fractional crystallization results simulating intraplate magmatism of terrestrial basalts at ~4 kbar, have shown that a moderate bulk water content of ~>0.3 wt% is required in order to produce potassic-granitic compositions, similar to the ones in NWA 6963 [6]. This suggests that the parental melt of NWA 6963 could have contained water contents similar to that of terrestrial intraplate ‘hotspot’-type basalts.

References: [1] Spulber and Rutherford (1983) J. Petrol., 24, 1-25. [2] Whitaker et al. (2007) J. Petrol., 48, 365-393. [3] Horgan (2013) Nat. GeoSci., 6, 991-992. [4] Sautter et al (2015) Nat. Geosci., 8, 605-609. [5] Agree (2014) Elements 168. [6] Whitaker et al. (2005) LSPC #1440.