The Fate of Sulfur during Decompression Melting of Peridotite and Crystallization of Basalts – Implications for Sulfur Geochemistry of MORB and the Earth’s Upper Mantle

Abstract:

Magmatism in mid-ocean ridges is the main pathway of sulfur (S) from the Earth’s mantle to the surficial reservoir. MORB is generally considered sulfide saturated due to the positive correlation between S and FeO_T concentration (e.g., [1]). However, most MORBs are differentiated, and both S content and sulfur concentration at sulfide saturation (SCSS) change with P, T, and magma composition (e.g., [2]). Therefore, it remains uncertain, from the MORB chemistry alone, whether mantle melts parental to MORB are sulfide saturated.

In this study, we modeled the behavior of S during isentropic partial melting of a fertile peridotite using pMELTS [3] and an SCSS parameterization [4]. Our results show that during decompression melting, at a fixed mantle potential temperature, T_P (e.g., 1300 °C), SCSS of aggregate melt first slightly increases then decreases at shallower depth with total variation <200 ppm. However, an increase of T_P results in a significant increase of SCSS of primitive melts. Our model shows that at 15% melting (F), sulfide in the residue is exhausted for a mantle with <200 ppm S. The resulted sulfide-undersaturated partial melts contain <1000 ppm S and are 4-6 times enriched in Cu compared to the source. In order to compare our modeled results directly to the differentiated basalts, isobaric crystallization calculation was performed on 5, 10, and 15% aggregate melts. SCSS changes along liquid line of descent with a decrease in T and increase in FeO_T. Comparison of S contents between the model results and MORB glasses [5] reveals that many MORBs derive from sulfide undersaturated melts. Further, for a T_P of 1300-1350 °C and F of 10-15 wt.%, reproduction of self-consistent S, and Cu budget of many MORB glasses requires that S of their mantle source be ~25-200 ppm.

We will discuss the interplay of T_P, average F, and the conditions of differentiation to bracket the S geochemistry of MORB and MORB source mantle and develop similar systematics for OIBs and OIB source.

References: [1] Le Roux et al. (2006) EPSL, 251, 209-231. [2] Baker and Moritti (2011) Rev. in Mineral. Geochem, 73, 167-213. [3] Ghiorso et al. (2002) Geochem. Geophy. Geosy. 3, 5. [4] Li and Ripley (2009) Econ. Geol. 104, 405-412. [5] Jenner and O’Neill (2012) Geochem. Geophy. Geosy. 13, 1.