Production and Preservation of Sulfide Layering in Mercury's Mantle

Abstract

A striking feature of Mercury's volcanic surface is its high S and low FeO contents, which is thought to be produced by very reducing conditions compared to other terrestrial bodies. Experiments show that S solubility in silicate melts increases to % wt levels for oxygen fugacities lower than three log units below the iron‐wustite (IW) buffer. During magma ocean solidification, large amounts of sulfide could potentially precipitate. This work investigates the effects of primordial sulfide layering on the first 750 Myr of Mercury's mantle dynamics. It is proposed that sulfide layering could have been produced by fractional solidification in the highly reduced Mercury magma ocean (MMO). Such chemical layering implies mantle sources with variable sulfur contents that might have played an important role in early Mercurian magmatism. Our models investigate the production of sulfide‐rich layers and their preservation during post‐MO solid‐state mantle dynamics. An intriguing question is the role of these sulfide‐rich layers on mantle dynamics as they are expected to incorporate a substantial amount of heat‐producing elements (U, Th, and K). We use experimentally determined sulfur solubility in silicate melts to predict the depth at which sulfides precipitate in the MMO. The model produces primordial sulfide layers whose thickness and locations depend upon the oxygen fugacity (fO2) and initial sulfur content (Sinit) of the MMO. Several geodynamic regimes have been identified in the fO2‐Sinit space. This study shows that oxygen fugacity, bulk sulfur content, and sulfide segregation are key for the early thermochemical evolution of Mercury.

No FeS layer in Mercury? Evidence from Ti/Al measured by MESSENGER

“…A last scenario that we considered consists of the late precipitation of (Mg, Fe, Ca)S sulfides during fractional crystallization of an initially sulfur-undersaturated magma ocean (Boukaré et al, 2019).…”

“…In this case no FeS melt would have exsolved during core formation, and the precipitation of sulfides would occur only when the sulfur content of the residual silicate melt of the magma ocean would have reached sulfur solubility. In this context, Boukaré et al (2019) modeled the crystallization of a magma ocean initially containing 5 wt.% S and with a fixed f O 2 of IW-6 which would start to precipitate sulfides at around half of the CMB depth. Applying the same mass balance calculation as for the previous scenario, surface lavas would display a Ti/Al ratio of 0.02.…”

“…Applying the same mass balance calculation as for the previous scenario, surface lavas would display a Ti/Al ratio of 0.02. For a magma ocean initially containing 1 wt.% S, sulfides would start precipitating at very shallow depth (1/4 of the CMB depth, Boukaré et al, 2019) and surface lavas would display a Ti/Al ratio of 0.04. We therefore suggest that the precipitation of (Mg, Fe, Ca)S during cooling of the magma ocean does not fractionate the Ti/Al ratio of the magma very strongly and certainly not as much as to core formation.…”

Mercury's Interior Structure Constrained by Density and P‐Wave Velocity Measurements of Liquid Fe‐Si‐C Alloys

“…If interior structure models of Mercury with the lower I are preferred, the density of the mantle must be lowered relative to such Mg‐rich mineral assemblages. The mantle density can be lowered by the presence of graphite or calcium (Ca)‐rich or Mg‐rich sulfides in Mercury's mantle (Boukaré et al., 2019; Malavergne et al., 2014; Vander Kaaden & McCubbin, 2015; Weider et al., 2012). Graphite, CaS and MgS have densities of ∼2.1 g∙cm −3 , ∼2.6 g∙cm −3 , and ∼2.7 g∙cm −3 , respectively (Peiris et al., 1994; Vander Kaaden & McCubbin, 2015).…”

“…If the interior structure models with the large I are preferred, the density of the mantle must be higher than the density of Mg‐rich olivine and/or pyroxene minerals. This can be achieved by Fe‐rich material mixed in the mantle or by the presence of solid FeS in the outer solid shell (Boukaré et al., 2019; Hauck et al., 2013; Smith et al., 2012). Solid FeS has a density of ∼4.5 g∙cm −3 at the pressure range of Mercury's mantle (Urakawa et al., 2004).…”