Granite: A Planetary Point of View

Abstract

The telluric planets and the asteroid belt display the same internal structure with a metallic inner core and a silicate outer shell. Experimental data and petrological evidence in silicate systems show that granite can be produced through all types of magmatic processes. On Moon, 4.4-3.9 Ga granite clasts displaying dry mineral assemblages correspond to discrete intrusive events. Large K/Ca enrichment and low REE abundances in granite relative to KREEP are consistent with silicate liquid immiscibility, a process observed in melt inclusions within olivine of Lunar basalts. On Mars, black-and-white rhythmic layers observed on the Tharsis rise along the flanks of Valles Marineris and the peripheral scarps of the Tharsis Montes giant volcanoes suggest the eruption of possibly felsic pyroclastites. Samples analysed in the Mars Pathfinder landing site yield basic to intermediate (45 to 62 wt.% SiO 2) compositions. Though no granites were found so far in the Martian SNC meteorites, a component close to terrestrial continental (granitic) crust is inferred from trace element and isotope systematics. Like the Martian northern hemisphere, Venus has suffered extensive volcanic resurfacing, whereas folded and faulted areas resemble terrestrial continents. The hypothesis of a granitic component is again “tantalizing”. Finally, extra-terrestrial granite is also found as enclaves within iron meteorites and ordinary chondrites. Granite, a major component of the Earth's crust, can be generated in all geodynamic settings. The low density of granite favours continental accretion. Thus, the occurrence or absence of granite and of associated silicic volcanism within the other telluric planets is not a trivial question. Granite is generally thought to be produced through “wet” processes. Lunar evidence shows that dry conditions may apply as well. In Venus, a large planet with high rates of magma production, it is speculated that significant volumes of granite can develop.