Program > Browse abstracts by speaker > Hlede Matej

The association between porphyry deposits and subduction zones is well established. However, the ultimate primary source of metals such as Cu, Mo, Zn and W is still poorly constrained. One potential source may be the subducted marine sediment. The NaCl content of the fluids released from the dehydrating slab may help to mobilize the metals by chloride complexing. In this project, we studied the partitioning of Cu, Mo, Zn and W between fluids of variable salinity (0–15 wt.% NaCl) and subducted sediments between 600–700°C and 2.5–3.5 GPa. The diamond trap method was used for experiments in end-loaded piston cylinder presses. The trace element concentration in both fluids and minerals was determined by laser ablation ICP-MS. We found that Cu and Zn remain virtually immobile in the absence of Cl. Experiments with 0 wt.% NaCl show fluid/mineral partition coefficients below 1. However, with increasing salinity both elements start partitioning into the fluid phase (Dfluid/mineral=1–80). Contrarily, Mo and W show consistently high solubility with little dependence on salinity (Dfluid/mineral=3-96). This is probably because both metals are hard acids, and they are more likely to form hydroxy complexes rather then chloride complexes. Notably, only rutile, which was very rare or absent in the run products, has the capacity to strongly retain Mo and W (Dfluid/rutile<0.1). Generally, our data show that the mobilization of Mo from a subducted sediment by aqueous fluid is very efficient, in particular compared to a MORB source, which would contain more rutile. Since Mo may be greatly enriched in some marine sediments formed under reducing conditions, a link between subducted Mo-rich black shales and porphyry molybdenum deposits may be plausible. Cu and Zn mobilized by saline fluids from a subducted sediment may also contribute to metal enrichment, but likely to a lesser extent.