GCA: Sulfur extraction via carbonated melts from sulfide-bearing mantle lithologies – Implications for deep sulfur cycle and mantle redox
Proteek Chowdhury and Rajdeep Dasgupta
Transport of sulfur via mantle-derived partial melts from deep Earth to the surface reservoirs is a critical step in the deep
global sulfur cycle. Given that sulfur is stored mostly in sulfide phases in mantle lithologies, the critical parameter is sulfur
concentration at sulfide saturation (SCSS) of mantle-derived magmas. CO2 and H2O-induced melting beneath oceanic and
continental mantle produces incipient CO2-rich melts. Although, SCSS of silicate melts of a variety of compositions is extensively
studied, the SCSS of carbonatitic and carbonated silicate melts have not received much attention. Here we present
experiments in graphite capsules at pressures (P) of 2.5–6.0 GPa and temperatures (T) of 1350–1650 C investigating the SCSS
of carbonatitic and carbonated silicate melts. All experiments produced quenched Fe +- Ni-sulfide melt blobs + carbonated
melt matrix +- ol +- cpx +- opx +- gt, with melt composition on a CO2-free basis varying from 7 to 40 wt.% SiO2, 0.5 to 7
wt.% Al2O3, and 9 to 17 wt.% FeO* (total FeO). SCSS measured using EPMA increases with SiO2 and T but is not affected
by P; the effect of composition being more pronounced than P-T. The composition of sulfide melt phase also affects SCSS.
With increasing Ni in the molten sulfide phase, the SCSS changes from 2000 to 4000 ppm (Ni-free) to is 800–3000 ppm (33 wt.
% Ni). Comparison of our measured SCSS with the existing SCSS models for nominally CO2-free silicate melts and with one
study for carbonated melts show that these parameterizations fail to capture the sulfide saturation values in CO2-rich melts
from our study. Using our new SCSS data and previous SCSS data for melt compositions that span the range from carbonatite
to basalts via carbonated silicate melts, we develop a new empirical SCSS parameterization. Unlike a previous model,
which suggested SCSS of carbonated melt is only affected by melt FeO* (other than P-T) and did not constrain how SCSS
evolves from low-silica carbonatitic melt to low-CO2 basaltic melt, our new parameterization captured complex effects of
many melt compositional parameters, including silica on SCSS. Using our new SCSS model, we constrained the efficiency
of S extraction from the mantle beneath mid-oceanic ridges and continents via low-degree carbonated melts. Deep carbonated
melts beneath ridges are expected to mobilize 5–15% of the initial sulfur before nominally-volatile-free peridotite melting
begins. In continental mantle, deep kimberlitic melt can act as an agent to mildly enrich the shallow mantle in sulfide as it
evolves to a carbonatitic melt upon reactive cooling. Application of our data to subduction zones suggests that low degree
carbonatitic melt is not an efficient agent to extract residual sulfide from the subducting oceanic crust.
Geochimica et Cosmochimica Acta: https://doi.org/10.1016/j.gca.2019.11.002
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