Chemical characteristics of fluid phase released from a subducted lithosphere and origin of arc magmas: Evidence from high-pressure experiments and natural rocks

Tatsumi, Y., Hamilton, D.L. and Nesbitt, R.W. (1986) Chemical characteristics of fluid phase released from a subducted lithosphere and origin of arc magmas: Evidence from high-pressure experiments and natural rocks. Journal of Volcanology and Geothermal Research, 29 (1-4), 293-309. (doi:10.1016/0377-0273(86)90049-1).

Abstract

Dehydration experiments on synthetic serpentine spiked with 11 trace elements (Cs, Rb, K, Ba, Sr, La, Sm, Tb, Y, Yb and Nb) have been carried out at 12 kbar and 850°C to examine the nature of element migration processes in the upper mantle beneath volcanic arcs. The results indicate that an element with a larger ionic radius is more readily transported by the aqueous fluid phase through the dehydration process.

The geochemistry of arc magnesian andesite and basalt, which are part of the compositional spread included in the range of arc primary magmas, has shown that source mantle materials of subduction zone magmas are enriched in incompatible elements with larger ionic radii compared to the source mantle of oceanic magmas; the enrichment should be caused by the transfer of elements by the fluid phase from the downgoing lithosphere.

The downgoing slab is anhydrous beneath volcanic arcs, and the relatively shallow breakdown of hydrous phases means that the mantle wedge peridotite is infiltrated by the slab-derived fluid phase beneath the fore-arc regions. Drag of the metasomatized peridotite wedge downwards with the subducted lithosphere transports a larger-ion-element-enriched source to the regions of arc-magma generation.

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