The behaviour of the extended HFSE group (Nb, Ta, Zr, Hf, W, Mo) during the petrogenesis of mafic K-rich lavas: The Eastern Mediterranean case

The marked enrichment of all HFSE in the lavas is controlled by the composition of the subducted sediments as shown by low 176Lu/177Hf (0.008630-0.02433) and Zr/Nb (11.3-29.4), combined with variable εHf (−3 to +11) and elevated W contents (up to 2.45 ppm) in the lavas. Nevertheless, the lavas display unfractionated ratios of Nb/Ta and Zr/Hf of 12.3-16.5 and 34.4-38.6, respectively, with respect to MORB. This feature may be explained by the Nb/Ta in the sediments that are on average higher (15.2 ± 2) compared to typical upper crustal estimates (12.5 ± 1.8), indicating a strong regional control on the HFSE ratios in Eastern Mediterranean sediments. The lavas exhibit low Ta/W (<1.33), but do not show elevated W/Th, as expected for island-arcs entirely dominated by subduction zone fluids. Ratios of Mo/W in all lavas are low (<1.79), which, according to experimental evidence, can be explained by low fluid salinities at moderately oxidised conditions. Collectively, our data therefore confirm the mobility of W in subduction zones, but the selective mobility of W relative to the similar incompatible Th and Mo depends on the proportion of melt-like components, the composition of the material being subducted and ambient redox conditions in the subducted material.