Boron isotope evidence for shallow fluid transfer across subduction zones by serpentinized mantle
Marco Scambelluri, Dipartimento per lo Studio del Territorio e delle sue Risorse, University of Genova, Corso Europa 26, 16132 Genoa, Italy; and Sonia Tonarini. Posted online 1 August 2012; doi: 10.1130/G33233.1.
The rock reservoirs enabling deep transport of ocean waters at subduction zones, together with fluid-transfer mechanisms to the mantle sources of arc magmas, are highly debated issues. Does fluid transfer occur by subducting-plate dehydration beneath arc fronts or by hydration of fore-arc mantle and subsequent subduction of the hydrated mantle? Historically, the deep slab dehydration hypothesis had strong support, but the hydrated mantle wedge idea is gaining ground. Defining subduction-fluid sources and dehydration mechanisms using appropriate isotopic tracers is therefore timely. Marco Scambelluri and Sonia Tonarini analyzed serpentinized (hydrated) mantle rocks recording deep subduction dehydration. Serpentinites are volatile and fluid-mobile element reservoirs for arc magmatism, though direct proof of their dominance in the subduction-zone volatile cycles has been elusive. Boron (B) isotopes are markers of fluid-mediated mass transfer during subduction. Altered oceanic crust and sediments release 11B-depleted fluids in the sub-arc mantle, which cannot explain 11B enrichment of many arcs. In contrast, Scambelluri and Tonarini document high 11B values retained in serpentinites. No 11B fractionation occurs in these rocks with subduction metamorphism: the released 11B-ric
|Contact: Kea Giles|
Geological Society of America