Main supervisor: Olivier Bachmann (ETH Zurich)
Co-supervisors and collaborators: Prof. Chris Huber (Brown University), Maria Schönbächler (ETH Zurich), Amir Khan (ETH Zurich), Tony Irving (University of Washington)
Silicic igneous rocks (enriched in SiO2 and quartzo-feldspathic components like granitoids and rhyolites), due to their low densities, play a fundamental role in many processes on Earth. The fact that they “float” on the mantle, forming stable lids (cratons) standing tall above ocean floors, is at the origin of the complex tectonic processes that govern the dynamics of our planet and have allowed the richness of environments to develop on Earth since the Archean. Despite their abundance (> 85% of the upper crust is granodioritic in composition; e.g., Wedepohl, 1995; Bonin et al., 2002), these silicic rocks did not form on the Moon, are rare or absent on Mars, and only appear as clasts in a few meteorites (Bonin et al., 2002). It has been suggested that the presence of abundant water on Earth is key to the generation of granites (e.g., Campbell and Taylor, 1983), but these rocks form in every tectonic setting (even in the driest hot spot conditions), and could be abundant on ‘dry’ Venus (Bonin et al., 2002), Hence, understanding the necessary conditions for the construction of low-density crust in telluric planets warrants further study.
Mars appears to play a unique role in this discussion. Despite detailed assessment of its petrological diversity over the past decades (using spectroscopic methods, direct measurements with rovers, and the study of meteoritic samples; Nimmo and Tanaka, 2004) and evidence that mafic magmas hold some water (McSween et al., 2001), large bodies of silicic magmas have not yet been discovered on the red planet. The most common rock types that are directly analyzable from meteorites belong to the SNC meteorite group (Shergottites, Nakhlites, Chassignites; McSween, 1994), and they range from dunites (Chassignites) to peridotites, pyroxenites, and mafic to intermediate volcanic units (basalts, basaltic andesites; Nakhlites and Shergottites). Of course, those meteorites may not cover most of the surface of Mars, nor do they represent the full time span of its magmatic processes. These meteorites likely come from the northern lowlands (Hesperian and Amazonian material), which covers the older Noachian rocks (formed ~ 4.5 Gyr B.P. and forming much of the ~ 50 km thick Martian crust; Nimmo and Tanaka, 2005), and crystallization ages appear fairly young (clustering between 1300-900 Ma - with some as low as 150-200 Ma; McSween, 1994). The InSight Mars Lander will provide more opportunities to explore previously unknown corners of Mars, but it is likely that the amount of silicic lithologies present on the Red Planet will remain much lower than on Earth.
A key aspect that controls the physical and chemical evolution of a magma reservoir is the depth at which magmas are trapped. As depth (i.e., pressure) increases, the background temperature rises, and the mineral phases that are crystallizing from any given initial magma composition varies significantly (hence controlling the composition of the residual melt). On Earth, many subvolcanic reservoirs seem to be located between ~5 and 10 km depth (~1.5 to 2.5 kb; Huber et al., 2019). Even considering earth-like primordial melts, the difference in thermal gradient between Earth and Mars and lower gravity on Mars will impact the optimal pooling depth of magmas and the longevity of magma reservoirs on Mars (Figure 1). We propose to test this hypothesis with an update to a recently developed thermo-mechanical model (Degruyter and Huber, 2014). This model (shown schematically in Figure 2) allows us to directly couple the thermal and mechanical evolution of the magma reservoir, assuming a system that grows with periodic magma injection, loses mass through magma withdrawal, and accounts for gas exsolution and crystallization from the melt (3-phase model). The magma reservoir is allowed to lose heat to its surroundings (crystalline mush transitioning to crustal wall-rocks), which responds visco-elastically to volume changes within the reservoir. Modifications of the model to convert it to Mars conditions include the determination and definition of relevant melting curves, volatile solubility and geotherms (under Mars-like conditions).
