Sulfur isotopic fractionation during planetary differentiation
Abstract:Early equilibration of the Earth’s mantle with the metallic core accounts for the depletion of siderophile elements in the mantle compared to a chondritic reference. The mantle is highly depleted in sulfur, implying partition into the core. Furthermore, the sulfur isotopic composition of the Earth’s mantle deviates significanlty from chondrites, with a 34S/32S shift of ~1.3‰. On the other hand, the martian mantle is chondritic for S isotopes. Here, we experimentally determine whether the sulfur incorporated in the core could have left such isotopic signatures on planetary mantles.
Experiments were conducted at 1 and 1.5 GPa at 1650°C using Geophysical Laboratory’s piston-cylinders. Times series were performed to prove that equilibrium has been reached, with durations chosen between 10 and 180 minutes. Starting materials consisted of basalt, Fe and FeS. Isotope analysis on metallic and silicate fractions was subsequently performed using the University of Maryland MAT 253 gas source mass spectrometer.
The influence of the capsule on S behavior has been adressed by running 10 experiments in boron-nitride and 9 in graphite. The role of silicate melt structure to control S isotopic fractionation has also been tested by imposing variations of aluminium, boron and silicon oxide content in the silicate melt.
Reproducible δ34S values for metal and silicate fractions for times > 25 minutes indicate that at the high temperature conditions of our experiments, isotopic equilibrium was reached. There is a resolvable 34S/32S isotopic fractionation that consistently showed the silicate fraction to be depleted in 34S , consistent with the observed mantle value. We found the isotope fractionation between the two studied phases, 1000lnamet-sil, to be between 0.2±0.1‰ and 1.4±0.2‰ at 1650ºC, positively correlated to the abundance of aluminium and boron, but not silicon oxides.
Isotope fractionations are hence related to charge compensation effects, and consistent with variation of coordination environment. Isotope fractionation will therefore increase with compaction, predicting non-trivial fractionation at high-pressure core-mantle equilibrium, hence offering an explanation for the S isotope composition of both the terrestrial and martian mantles.
1 Labidi et al. 2013
2 Franz et al. 2014