FELSIC MELTS ON MARS FROM EXTREME FRACTIONAL CRYSTALLIZATION OF HYDROUS MAGMAS AS EVIDENT FROM METEORITE NORTH WEST AFICA 6963 (Invited Presentation)

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. In addition, siliceous melts can also be produced from a magma that underwent extensive fractional crystallization. There is little evidence that Mars had plate tectonic and it seems to be dominated by basaltic rock that has experienced little magmatic evolution. Nevertheless, felsic rocks from several locations on the martian surface have been reported: from orbit through spectral analyses from CRISM onboard the Mars Reconnaissance Orbiter; at Gale Crater Mars Science Laboratory (MSL) Curiosity Rover discovered felsic rocks and soils; and even limited felsic compositions such as trachyte were reported as clasts in the martian breccia Northwest Africa (NWA) 7034 and its pairs. Gabbroic shergottite NWA 6963 contains small pockets of quartz-feldspar intergrowths (some that are Na-rich, others K-rich) that have late-stage granitic melt compositions. Graphic intergrowths of quartz and feldspar are common in terrestrial plutonic rocks; though these intergrowths are rare in volcanic rocks they can form from late-stage fractionated melt. Similarly, the intergrowths in NWA 6963 formed from a late-stage simultaneous eutectic crystallization of quartz and feldspar at the end of extreme fractional crystallization within the martian crust of the melt parental to NWA 6963. Experimental fractional crystallization results of terrestrial basalts have shown that a moderate bulk water content of ~>0.3 wt% is required to produce K-granitic-like compositions like those in NWA 6963. This suggests that the parental melt of NWA 6963 contained water contents like that of primitive terrestrial basalts, consistent with NWA 6963’s apatite chemistry. These results are also consistent with models of various martian starting compositions producing felsic igneous compositions similar to those analyzed at Gale crater. Further, such a process is comparable to fractionated terrestrial intraplate volcanic rock and thus, most likely represent localized products of the fractionation of mildly alkaline to alkaline basaltic magmas that may have been more common in early Mars history.