Absence of silicic volcanism on Mars: Implications for crustal composition and volatile abundance

Abstract

On Earth, eruptions of silicic magmas with volumes of 1–100 km3 are generally associated with stratovolcanoes or composite cones, whereas larger (100–1000 km3) ignimbrite deposits are erupted from resurgent calderas and related structures. Small volumes of silicic rocks are generated by differentiation within basic magma chambers associated with stratovolcanoes, but large silicic deposits are largely confined to areas where the granitic crust is thicker than 25 km. As mantle‐derived magmas rise through the thick granitic crust, fractional crystallization and assimilation of hydrous crustal rocks concentrate volatiles and SiO2 in the magma, yielding explosive silicic eruptions. On Mars, under similar chemical conditions, ignimbrite eruptions could take place over a broader range of physical conditions than on earth, but no large volume ignimbrite deposits can be identified positively. Explosive basic to ultrabasic eruptions may be responsible for small, cinder conelike features on Mars, and perhaps for the ancient, flank‐scoured paterae. Such eruptions may have been driven by near‐surface water/ice‐magma interactions or volatile rich magmas. The paterae represent a unique style of volcanism that stopped∼2 b.y. ago. We suggest that the volatiles associated with these features were derived from mantle sources, and that the cessation of patera formation may coincide with the termination of the period of maximum planetary degassing. Volcanism since∼2 b.y. ago has been dominantly effusive, except for minor explosive ash events such as those which blanketed part of the summit of Hecates Tholus. We propose that such eruptions were driven by volatiles generated by differentiation of a magma chamber, similar to Icelandic explosive eruptions. The apparent lack of large scale silicic volcanism on Mars is circumstantial evidence that Mars is deficient in volatiles and lacks a granitic crust.