Trace element parameterisation for slab fluid composition from K-free oceanic crust

Abstract

The geodynamical conditions of subduction zones make this tectonic environment essential for global geochemical cycles and for the heterogeneity of the Earth’s mantle. A complex series of processes such as slab dehydration, melt/fluid percolation, sediment/mantle wedge melting and fractional crystallisation, is responsible for continental crust generation and the particular chemical signature of island arc basalts. Moreover, trace element signatures can be used as a proxy for the influence of each of these processes during the formation of island arc basalts. To quantify these signatures, it is necessary to use mineral–melt or mineral–fluid partition coefficients. While mineral–melt partition coefficients are well studied and used to identify different magmatic processes, slab–fluid interactions at different P-T-X conditions are poorly due to a lack of sufficient, integrated data on this subject.
An extensive compilation of experimental partition coefficient (Dimin/fluid) data for trace elements between a range of relevant K-free minerals and aqueous fluids was compiled in order to parameterise the partition coefficients at different temperatures. Most mineral–fluid partition coefficients have a negative correlation with temperature, where fluids become more enriched at higher P-T conditions in the subduction zone. The exception to this is zoisite, for which most of the trace element partition coefficients increase with increasing temperature, meaning larger concentrations of trace element are retained in zoisite.
The integration of partition coefficients in a combined geodynamical-thermodynamical subduction model allows prediction of the fluid trace element composition during mineral dehydration in a subduction process. This method was used to simulate the fluid composition resulting from K-free basaltic crustal dehydration in natural examples of a cold (Marianas) and a hot (Vanuatu) subduction zone. Generally, the fluids calculated in this work show enrichment in fluid-mobile (FM) elements and LILEs in comparison with REEs and HFSEs. Moreover, fluids released at shallow depth will be less enriched in trace elements in comparison with those released at greater depth.
Comparison of Ba/Yb and Ba/Th ratios of the fluids calculated here with arc basalts from the two natural subduction zones show similarities, which illustrates that fluids with a similar composition as those calculated in this dissertation were likely involved in the origin of the primitive IAB from these examples.