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 143 Nd/ 144 Nd (≈0.5130), and small differences in 207 Pb/ 204 Pb (15.53–15.57), whereas 87 Sr/ 86 Sr 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‐ δ 11 B serpentinite fluid from serpentized mantle wedge dragged beneath the volcanic arc or from the subducting lithosphere and (2) a low‐ δ 11 B 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, 11 B‐enriched in the forearc by shallow fluids.