Evidence for serpentinite fluid in convergent margin systems: The example of El Salvador (Central America) arc lavas

A comprehensive geochemical study, including B, Pb, Sr, and Nd isotopes, has been carried out on El Salvador subduction-related lavas. The rocks have arc-type incompatible element distributions with high LILE/HFSE ratios, nearly constant Nd-143/Nd-144 (approximate to 0.5130), and small differences in Pb-207/Pb-204 (15.53-15.57), whereas Sr-87/Sr-86 ranges from 0.7035 to 0.7039. Boron isotopic composition varies widely, between -2.7% and +6.3%. The boron isotope signature points to involvement of fluid inputs from ( 1) a high-delta B-11 serpentinite fluid from serpentized mantle wedge dragged beneath the volcanic arc or from the subducting lithosphere and ( 2) a low-delta B-11 fluid from the progressive dehydration of subducted altered basaltic crust and/or sediments. The observed sample variability is explained with a model in which different proportions of serpentinite-derived (10-50%) and slab-derived fluids are added to an enriched-DMM source, triggering its partial melting. We suggest a model in which tectonic erosion, i.e., dragging down of slivers of serpentinized upper plate mantle, was responsible for the occurrence of serpentinite reservoir, B-11-enriched in the forearc by shallow fluids.