Boron isotope evidence for devolatilized and rehydrated recycled materials in the Icelandic mantle source

Enriched mantle heterogeneities are widely considered to be generated through subduction, but the connections between specific subducted materials and the chemical signatures of mantle heterogeneities are not clearly defined. Boron is strongly isotopically fractionated at the surface and traces slab devolatilization, making it a potent tracer of previously subducted and recycled materials. Here, we present high-precision SIMS boron concentrations and isotope ratios on a comprehensive suite of quenched basaltic glasses from all neovolcanic zones in Iceland, two rhyolite glasses, and a set of primitive melt inclusions from central Iceland. Boron isotope ratios (delta B-11) in Icelandic basalts and melt inclusions range from -11.6 parts per thousand to -1.0 parts per thousand, averaging -4.9 parts per thousand, which is higher than mid-ocean ridge basalt (MORB; delta B-11 =-7.1 parts per thousand). Because the delta B-11 value of the Icelandic crust is low, the high delta B-11 compositions of the Icelandic lavas are not easily explained through crustal assimilation processes. Icelandic basalt glass and melt inclusion B/Ce and delta B-11 values correlate with trace element ratio indicators of the degree of mantle partial melting and mantle heterogeneity (e.g. Nb/Zr, La/Yb, Sm/Yb), which indicate that the boron systematics of basalts are controlled by mantle heterogeneity. Additionally, basalts with low B/Ce have high Pb-206/Pb-204, further indicating mantle source control. These correlations can be used to deduce the boron systematics of the individual Icelandic mantle components. The enriched endmember within the Iceland mantle source has a high delta B-11 value and low B/Ce, consistent with the composition of "rehydrated" recycled oceanic crust. The depleted endmember comprises multiple distinct components with variable B/Ce, likely consisting of depleted MORB mantle and/or high He-3/He-4 mantle and two more minor depleted components that are consistent with recycled metasomatized mantle wedge and recycled slab gabbro. The compositions of these components place constraints on the devolatilization history of recycled oceanic crust. The high delta B-11 value and low B/Ce composition of the enriched component within the Iceland mantle source is inconsistent with a simple devolatilization process and suggests that the recycled oceanic crust component may have been isotopically overprinted by B-rich fluids derived from the underlying hydrated slab lithospheric mantle (i.e. "rehydration"). Further, the B/Ce and delta B-11 systematics of other OIBs can be used to constrain the devolatilization histories of recycled components on a global scale. Globally, most OIB B/Ce compositions suggest that recycled components have lost >99% of their boron, and their delta B-11 values suggest that rehydration may be a sporadic process, and not ubiquitous. (C) 2021 Elsevier B.V. All rights reserved.