[P-PS04] Mercury Science and Exploration

[PPS04-P04] Mercurian magma ocean crystallization as a function of oxygen fugacity and sulfur content

*Jiejun Jing^1,2、Jurrien S. Knibbe²、Paul ten Dam²、 Yong Wu³、Qing-Han Yuan⁴、Ben-Xun Su⁴、Wim van Westrenen² (1.Geodynamics Research Center, Ehime University、2.Departement of Earth Sciences, Vrije Universiteit Amsterdam、3.Beijing Research Institute of Uranium Geology、4.Institute of Geology and Geophysics, Chinese Academy of Sciences)

キーワード：Mercury, Sulfur, Magma Ocean, Chondrite, Redox state

We investigated the effects of oxygen fugacity and sulfur content on silicate magma crystallization through high-pressure and high-temperature experiments conducted under conditions (1600-1750 ^oC and 5 GPa) simulating the core-mantle boundary of Mercury. The experimental results indicate that sulfur bonds with Mg and Ca under highly reducing conditions (oxygen fugacities -5.5 to -9.0 log units below the oxygen fugacity of the iron-wüstite (IW) buffer, in equilibrium with a metallic Fe-Si phase), consistent with previous experiments at lower pressures. For a bulk composition with lithophile element ratios of carbonaceous (CB) chondrites, olivine is the liquidus phase under highly reducing conditions (IW-8.6 to -9.0), whereas crystallization of garnet occurs if 2.1 wt% sulfur is added to the starting composition. At less reducing conditions (IW-5.5 to -6.7), the ratio of crystallized orthopyroxene/olivine increases with increasing bulk sulfur contents until orthopyroxene becomes the liquidus phase when ~7.7 wt% sulfur is added. Based on these experimental findings, we modeled the crystallization of the Mercurian magma ocean (MeMO) using CB chondrites and EH chondrites as analogues for Mercury. We estimated the MeMO silicate compositions through geochemical modeling of metal-silicate differentiation, and assumed a range of sulfur contents in these initial MeMO. Our modeling demonstrates that under highly reducing conditions, MeMO sulfur contents significantly influence the occurrence and timing of orthopyroxene and plagioclase crystallization, as well as their corresponding total abundance in Mercurian crust. Assuming that plagioclase-bearing cumulates from MeMO crystallization represent the primary Mercurian crust, an EH-chondrite-like bulk composition for Mercury with 3-7 wt% sulfur in the silicate mantle, would yield a crustal thickness of 47-57 km. This is just within the 35±18 km thickness inferred from gravity and topography MESSENGER data. Beneath the plagioclase-clinopyroxene crust, a stratified Mercurian mantle, composed of dunite and harzburgite would form from the bottom upward.