Experimental Constraints on the Melting Behavior of an Mg-Rich Exoplanetary Mantle

Outside of our solar system, there have been over 350 terrestrial planets confirmed to date, in addition to over 790 Super-Earths. Stellar compositions of the stars these planets orbit suggest these exoplanets likely have Mg:Si ratios varying from 0.6-2 (Roger and Seager 2010; Hinkel et al., 2014; Dorn et al. 2016; Unterborn et al., 2017). In this study, we focus on conducting experiments on an anhydrous, silicate mantle with an Mg:Si of 1.42, higher than any terrestrial planet in our solar system, to explore the melting behavior of a family of likely exoplanet compositions. Experiments were performed at Arizona State University in the Experimental Petrology and Igneous processes Center (EPIC) from 1-2GPa at 1000-1500°C for 24 hours to determine the phase equilibria and location of the exoplanetary mantle solidus. An Au80Pd20 capsule was used to limit Fe loss. Experimental results were analyzed via electron microprobe with a Fe-loss tolerance of 0-5%. The experiments contain an Earth mantle-like assemblage of olivine, clinopyroxene, orthropyroxene, spinel, and varying degrees of melt. The composition of the FeMg-bearing phases deviates from the Earth's mantle as expected for a higher Mg:Si ratio, with subsolidus olivine Mg #'s of 92-96 and pyroxene Mg #'s of 85-90. The solidus for this composition is located between 1300 and 1400°C at 2 GPa. Future work will focus on constraining the melting reaction and composition of the melt as a function of temperature and pressure to constrain the composition of likely exoplanetary volcanism, as well as bracket the solidus at lower pressures.