Experimental Determination of Rocky Exoplanet Crust Compositions

Over 4000 exoplanets have been confirmed, presenting the scientific community with a myriad of planetary geology questions including, "What are the surface compositions of the >1000 exoplanets that are at least partially composed of rock?" While most investigations of planets from outside our solar system have primarily utilized tools from the fields of astronomy and geophysics, we apply methods from the field of igneous petrology to determine the effect of planetary radius, core mass fraction, and bulk silicate composition on the likely silicate crust compositions formed by melting in the upper crusts of these bodies.

A host star's composition can be used as a first order proxy to an exoplanet's composition, as demonstrated by the relationship between Earth and the Sun. We exploit this relationship to examine the range of likely bulk silicate compositions for exoplanets. To determine the effect of the variation in rock-forming elements on exoplanet mantle solidi, we conducted piston-cylinder experiments over 1-2 GPa and 1100-1475°C on two non-Earth silicate exoplanet compositions. The first exoplanet bulk composition explores the effect of a higher Mg/Si relative to Earth (1.42 vs. 1.06) and represents the high Mg end member of exoplanets. The second composition has an Mg/Si (0.93) similar to that of Earth, but a higher Ca/Al (1.81 vs. 1.07) to represent a system where clinopyroxene is favored to crystallize over garnet.

Our initial results indicate that anhydrous exoplanet mantle phase proportions and melt compositions do deviate from that of Earth. Using our experimentally-derived solidi, as well as calculated exoplanetary adiabatic gradients and mantle potential temperatures, we present the range of melting depths and melt compositions that are possible given the range of known stellar compositions in Mg/Si compositional space. These melt compositions can be used to predict differences in crust compositions of exoplanets with similar bulk compositions to those explored here.