Geochemical fractionation during melting of sulfide and sulfate bearing mantle lithologies
Doctor of Philosophy
Earth’s surficial biogeochemical sulfur (S) cycle is connected to the deep sulfur cycle at convergent margins, where, crust and oceanic lithospheric mantle subducts beneath an overriding plate and transporting sulfur and other materials from the surface to the deep Earth. A part of the subducted sulfur gets released from the subducting slab to the surrounding mantle at sub-arc depths while the rest continues deeper and gets recycled into the mantle. As S is a multi-valent element with high redox potential, presence of different S species in a system will help us discern the redox conditions of the system. Also, S bearing phases in deep mantle being the primary host of chalcophile and Highly Siderophile Elements (HSEs) plays an important role in economic mineral deposits of these elements. In spite of having huge significance in planet scale geochemistry, S cycling at subduction zones and deep Earth and how different S species control redox environments and behavior of other elements are inadequately understood. This thesis investigates sulfur carrying capacity of mantle derived partial melts in presence of S phases and partitioning of trace elements (including chalcophiles and HSEs) between relevant S bearing phases both at subduction zones and Earth’s deep upper mantle with the aid of high pressure-temperature laboratory experiments. Sulfur solubility of a primary mantle wedge magma beneath arc volcanoes in the presence of oxidized S species are measured in Chapter 2. The high P-T experimental measurements are then modelled and compared with natural olivine hosted melt-inclusions from different arcs to understand the relevant sulfur speciation. At subduction zones, parental basaltic melt will extract similar amount of S via mantle melting irrespective of sulfide or sulfate at sub-solidus conditions, but there is possible contribution of oxidized S species seen at various arcs. The mantle wedge magmas are likely to be sulfate undersaturated allowing these undersaturated magmas to assimilate crustal sulfur during its ascent. To understand S contribution from the subducting slab and the oxygen fugacity of the subducting crust, partitioning experiments of trace elements between anhydrite (sulfate mineral) and hydrous slab melt were performed (Chapter 3). As Light Rare Earth Elements (LREEs) partitions into sulfate phase and chalcophile elements (ChE) partitions into sulfide phase, LREE/ChE ratios (Ce/Cu and Ce/Mo) were used to determine the saturating S phase in the downgoing subducting crust. This ratio in turn was used as a redox proxy by the virtue of S transforming from sulfide to sulfate over a narrow oxygen fugacity range. Using Ce/Cu and Ce/Mo ratio of the subducting crust and comparing them with natural arc lava data suggests that oxygen fugacity of subducting crusts is likely to be variable from one subduction zone to another. As S gets recycled beyond subduction zones and gets incorporated in the deep mantle where vast portions of the mantle are affected by CO2±H2O induced melting, understanding S solubility of these carbonated melts in presence of sulfide (reduced sulfur) and partitioning of ChEs and HSEs between sulfide and carbonated melts becomes crucial. Chapter 4 measures solubility of S in the presence of sulfide for carbonated melts. Solubility model of S for these low-degree carbonated melts beneath mid-ocean ridges show 5-15% extraction of S by these melts, whereas, at sub-continental lithospheric mantle (SCLM) these melts evolve to mildly enrich the shallow mantle leading to supposed sulfide metasomatism. Model comparison with natural carbonated kimberlites show that most of the natural kimberlites are products of sulfide present carbonated melting. Presence of sulfides in deep upper mantle, thus, exerts control on mobility and behavior of ChEs and HSEs. Chapter 5 shows measurements of partition coefficients for ChEs and HSEs between sulfide melt and carbonated melts. The partition coefficients of these elements are lower when compared to basaltic silicate melts and hence low-degree carbonated melts are enriched in ChEs and HSEs in comparison to silicate melts. HSE pattern for mantle derived carbonated melts is similar to ocean island basalts and alkali basalts, linking their melting to carbonated sources. Using mass balance calculations based on Ru content of mantle derived low degree carbonated melts and comparison with natural kimberlite HSE content show presence of xenolith detritus in these rocks from SCLM. The model estimates are comparable to calculations by previous studies using various other proxies.