Boron recycling in the mantle: Evidence from a global comparison of ocean island basalts

Radiogenic and noble gas isotopes have been integral for demonstrating the existence, source, and age of (geo) chemical reservoirs in the mantle, yet, the volatile element composition of the Earth's interior remains poorly characterized. Boron isotopes have the potential to constrain the processes that generate distinct mantle reservoirs, as they fractionate strongly at the surface of the Earth and during subduction but are little perturbed during high-temperature mantle processes, and so can inform our understanding of mantle volatile cycling. Here, we present the largest, internally consistent, high-precision B isotope dataset from ocean island basalt (OIB) glasses and olivine-hosted melt inclusions measured by Secondary Ion Mass Spectrometry (SIMS) to date, including new data derived from the Pitcairn Islands, Tristan da Cunha, St. Helena, Ascension Island, the MacDonald (Ra) Seamount, and Fogo (Cape Verde Islands) in addition to previously published data from La Reunion, La Palma (Canary Islands), Iceland, and Hawai'i. This dataset allows a comparison of ocean island basalts that contain heterogeneous recycled components (e.g., Pitcairn Islands) to those with primordial components (e.g., La Re ' union) in their sources. We focus on basaltic glass and melt inclusions (>6 wt% MgO) as they are least affected by shallow differentiation and assimilation processes. We find that our new OIB data show a limited spread in average delta B-11 (-5.9 +/- 3.0% to - 10.8 +/- 0.7%), which is smaller compared to previous OIB studies. These data generally overlap with mid-ocean ridge basalts (MORB; 7.1 +/- 0.9%) and display lighter values when compared to mafic arc magmas (similar to 9 to +20%). Importantly, some trace element enriched OIB endmembers display lighter delta B-11 values and lower B/P and B/Zr, indicative of a source with lower B concentrations relative to the primordial mantle. This suggests that the deeper mantle is becoming increasingly B-depleted over time because boron is effectively stripped from recycled lithologies during subduction and slab dehydration. In addition, the results highlight the decoupling of B isotopes from radiogenic (Sr, Pb) isotopes providing a new perspective on volatile recycling. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The study presents the largest and most precise boron isotope dataset from ocean island basalt glasses and olivine-hosted melt inclusions, revealing the characteristics of volatile elements in the mantle and the processes generating different mantle reservoirs. It suggests that the deep mantle is becoming increasingly depleted in boron over time and highlights the decoupling of boron isotopes from radiogenic isotopes, providing a new perspective on volatile recycling.