Boron isotope fractionation in magma via crustal carbonate dissolution.

Frances M Deegan,Carmela Freda,Ester M Jolis,Valentin R Troll,Martin J Whitehouse

doi:10.1038/srep30774

Frances M Deegan, Carmela Freda + Show 3 more

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https://doi.org/10.1038/srep30774

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Abstract

Carbon dioxide released by arc volcanoes is widely considered to originate from the mantle and from subducted sediments. Fluids released from upper arc carbonates, however, have recently been proposed to help modulate arc CO2 fluxes. Here we use boron as a tracer, which substitutes for carbon in limestone, to further investigate crustal carbonate degassing in volcanic arcs. We performed laboratory experiments replicating limestone assimilation into magma at crustal pressure-temperature conditions and analysed boron isotope ratios in the resulting experimental glasses. Limestone dissolution and assimilation generates CaO-enriched glass near the reaction site and a CO2-dominated vapour phase. The CaO-rich glasses have extremely low δ11B values down to −41.5‰, reflecting preferential partitioning of 10B into the assimilating melt. Loss of 11B from the reaction site occurs via the CO2 vapour phase generated during carbonate dissolution, which transports 11B away from the reaction site as a boron-rich fluid phase. Our results demonstrate the efficacy of boron isotope fractionation during crustal carbonate assimilation and suggest that low δ11B melt values in arc magmas could flag shallow-level additions to the subduction cycle.

Highlights

Carbon is transferred from the Earth’s interior to the surface mainly by CO2 degassing at arc volcanoes[1,2]
Boron is a fluid-mobile trace element and its isotope ratios have been used to evaluate the involvement of dehydrated slab materials in arc magma-genesis for the past ca. 25 years
It is envisaged that the tendency for many arc magmas to display boron concentrations higher than typical Mid-Ocean Ridge Basalt (MORB) mantle is due to input of slab-derived boron and that isotopically diverse fluids produced during subduction can react with the mantle wedge to generate the wide range of δ11B values recorded in global arc suites

Summary

Introduction

Carbon is transferred from the Earth’s interior to the surface mainly by CO2 degassing at arc volcanoes[1,2]. Carbon dioxide outgassing has varied considerably throughout Earth history‒during the warmer Cretaceous period, for example, it has been calculated that there was as much as 220 to 550% CO2 outgassing relative to present day rates[8,9] If this calculation is correct, two important implications arise; (i) that intensified global continental arc activity may have led to excess degassing at paleo-arcs to explain paleo-climate fluctuations and (ii) that decarbonation reactions in the Earth’s crust may be an important, albeit time-variable, factor in modulating the carbon cycle (cf refs 3, 4, 8–10). We offer high temperature-high pressure experimental simulations of carbonate assimilation at conditions corresponding to the over-riding plate and provide spatially controlled boron isotope analyses of the experimental products by Secondary Ionisation Mass Spectrometry (SIMS)

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