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

K. J. Walowski*, L. A. Kirstein, J. C.M. De Hoog, T. Elliott, I. P. Savov, R. E. Jones, EIMF

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

Abstract

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 Réunion, 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 Réunion) 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 δ11B (−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 (∼−9 to +20‰). Importantly, some trace element enriched OIB endmembers display lighter δ11B 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.

Original languageEnglish
Pages (from-to)83-100
Number of pages18
JournalGeochimica et Cosmochimica Acta
Volume302
DOIs
Publication statusPublished - 1 Jun 2021

Bibliographical note

Funding Information:
For analytical assistance we thank J. Craven (SIMS), R. Hinton (SIMS), and C. Hayward (EPMA). We thank C. Devey for access to samples in the GEOMAR collection. Funding was provided by National Environmental Research Council grant NE/M000443/1 and EIMF proposals IMF573/1015 and IMF590/0516. IPS appreciates the support and discussions with staff from Carnegie Institution (Shirey, Tera, Foustoukos, Hauri) and CNR-IGG-Pisa (Tonarini, Agostini), where the initial steps of revisiting the B isotope systematics of OIBs were made (on whole rocks and experimental glasses). We also thank A. Simonetti, M. Kendrick, and an anonymous reviewer for their insightful comments and S. Huang for editorial handling.

Funding Information:
For analytical assistance we thank J. Craven (SIMS), R. Hinton (SIMS), and C. Hayward (EPMA). We thank C. Devey for access to samples in the GEOMAR collection. Funding was provided by National Environmental Research Council grant NE/M000443/1 and EIMF proposals IMF573/1015 and IMF590/0516. IPS appreciates the support and discussions with staff from Carnegie Institution (Shirey, Tera, Foustoukos, Hauri) and CNR-IGG-Pisa (Tonarini, Agostini), where the initial steps of revisiting the B isotope systematics of OIBs were made (on whole rocks and experimental glasses). We also thank A. Simonetti, M. Kendrick, and an anonymous reviewer for their insightful comments and S. Huang for editorial handling.

Publisher Copyright:
© 2021 Elsevier Ltd

Keywords

  • Boron isotopes
  • Mantle geochemistry
  • Melt inclusions
  • Ocean island basalts

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