HIGH PRESSURE-TEMPERATURE PHASE EQUILIBRIUM STUDIES ON MARTIAN BASALTS: IMPLICATIONS FOR THE FAILURE OF PLATE TECTONICS ON MARS

Mars lacks ongoing tectonic activities such as volcanism and mountain building processes. Modern plate tectonic movements on the Earth’s surface are driven primarily by the descent of the subducting slabs into the mantle. Slab crusts, made of the dense eclogite metamorphosed from the Mid-Ocean Ridge Basalt, provide one of the most important driving forces for slab subduction. Thus, mantle convection inside Mars can be hindered if the density contrast between Martian slab crusts and the ambient Martian mantle is sufficiently smaller than that of Earth. To evaluate this hypothesis, we carried out high pressure-temperature phase equilibrium experiments on 3 different Martian basalts: Yamato 980459, NWA 8159, and GUSEV basalt (Humphrey). The GUSEV basalt and NWA 8159 undergo partial or complete melting along the Martian areotherm due to the high Fe content, suggesting that both compositions are geochemically evolved. Yamato 980459, the nearly primitive Martian basalt, on the other hand, would transform to a low-density eclogite at a depth of ~250 km. The density contrast between a Martian crustal slab made of Yamato 980459, and the ambient Martian mantle is much smaller than that of the Earth. Calculated slab sinking torques and velocities further suggest that sustained buoyancy-driven subduction of thin slabs on early Mars is difficult. Future experiments exploring wider composition and pressure-temperature ranges can help us to understand the possible consequences of Martian mantle compositions and cooling history for the plate tectonic history of Mars.