Investigating ocean island mantle source heterogeneity with boron isotopes in melt inclusions

Recycling of the lithosphere via subduction drives the trace element and isotopic heterogeneity of the mantle, yet, the inventory of volatile elements in the diverse array of mantle reservoirs sampled at ocean islands remains uncertain. Boron is an ideal tracer of volatile recycling because it behaves similarly to volatiles during high-temperature geochemical reactions and carries a distinctive isotope signature into the mantle, but is subsequently little-influenced by degassing on return to the surface. Furthermore, B-rich recycled lithologies will have a strong influence on typical upper mantle compositions characterized by low B concentrations (<0.2 mu g/g and delta B-11 - 7.1 +/- 0.9 parts per thousand). Here, we present and compare the B abundances and isotope compositions, together with the volatile element contents (H2O, CO2, and Cl) of basaltic glasses and olivine-hosted melt inclusions from two different ocean island localities (La Palma, Canary Islands, and Piton de Caille, La Reunion Island). Our results suggest that olivine hosted melt inclusions are protected from contamination during ascent and provide more robust estimates of primary mantle source delta B-11 than previous bulk rock studies. We find that the delta B-11 of the La Reunion samples (-7.9 +/- 0.5 parts per thousand (2 sigma)) overlaps with the recently defined MORB datum, indicating that the depleted upper-mantle and 'primitive mantle' reservoirs are indistinguishable with respect to delta B-11, or that B concentrations are sufficiently low that they are diluted by partial melting in the uppermost mantle. In contrast, the La Palma samples, notable for their radiogenic Pb isotope ratios, are characterized by delta(11)Bvalues that are distinctly isotopically lighter (-10.5 +/- 0.7%o (2 sigma)) than La Reunion or MORB. We suggest these isotopically light values are derived from significantly dehydrated recycled materials preserved in the La Palma mantle source region, in keeping with their lower B/Zr and H2O/Ce. This work therefore provides strong new support for subduction zone processing as a mechanism for generating radiogenic Pb isotopic signatures and volatiles heterogeneities in the mantle. (C) 2018 Elsevier B.V. All rights reserved.