Experimentally-determined trace element characteristics of aqueous fluid from partially dehydrated mafic oceanic crust at 3.0 GPa, 650-700C

Abstract

Aqueous vapour, released from subducted dehydrated oceanic crust, has the potential to remove relatively large amounts of trace elements and to modify key trace element ratios in the overlying mantle wedge, an important source region for subduction zone magmatism. To experimentally assess this process an oceanic tholeiite (MORB) has been enriched (10–100 ppm) in 25 trace elements and crystallized at 3.0 GPa, 650 and 700°C for 2 weeks, with 30–50wt.% added water. At 650°C the fluid coexists with omphacite, lawsonite, chloritoid, coesite and accessory phengite, rutile, Ti-magnetite and talc. Changes at 700°C are that garnet is conspicuous, chloritoid absent and lawsonite, staurolite and talc are rare. LAM ICP-MS analyses establish patterns of trace element partitioning between minerals (data for omphacite, lawsonite, phengite, chloritoid and garnet) and fluid, but not absolute values. Similarly ratios of trace elements in the fluid can be well constrained. Relative to the bulk composition the fluid shows strong enrichment of LILE/HFSE and LILE/REE at both 650 and 700°C, but also shows significant decrease in Rb/Sr, Cs/Sr and Ba/Sr from 650 to 700°C, apparently controlled by residual lawsonite, and increase in La/Lu, clearly controlled by garnet. In addition, omphacite strongly fractionates both Zr/Nb and La/Lu, contributing to a decrease in Zr/Nb and an increase in La/Lu in the fluid. Residual rutile (analyzed with an electron microprobe) strongly takes up Nb and Ta, the latter more readily, causing an almost a 4-fold increase in the Nb/Ta ratio in the fluid relative to the bulk. Omphacite, garnet and chloritoid all have Nb/Ta less than and lawsonite greater than the starting composition. Omphacite and garnet strongly reject Sr, Ba, Rb and Cs; lawsonite favours all REE (HREE slightly more so), Sr, U and Th, whereas phengite favours Rb, Ba and Cu. Chloritoid most readily accommodates V, Cr, Ni and Zn. Involvement of a fluid derived by dehydration processes may increase LILE/HFSE and LILE/REE and decrease Zr/Nb in subduction-linked magmas, relative to MORB. Lowest Zr/Nb ratios point to the greatest relative involvement of clinopyroxene in the residue. Absence of major change to La/Lu restricts the role of garnet. Higher Rb/Sr, Cs/Sr and Ba/Sr may point to a low geothermal gradient in the subduction zone such that lawsonite remains stable during the dehydration process that produced the fluid. Residual rutile may explain Nb and Ta depletion and cause an increase in Nb/Ta of the coexisting fluid.