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. 2016 Aug 4:6:30774.
doi: 10.1038/srep30774.

Boron isotope fractionation in magma via crustal carbonate dissolution

Frances M Deegan  1   2 , Valentin R Troll  1   3 , Martin J Whitehouse  2 , Ester M Jolis  1 , Carmela Freda  3
Affiliations

Boron isotope fractionation in magma via crustal carbonate dissolution

Frances M Deegan et al. Sci Rep. .

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 δ(11)B values down to -41.5‰, reflecting preferential partitioning of (10)B into the assimilating melt. Loss of (11)B from the reaction site occurs via the CO2 vapour phase generated during carbonate dissolution, which transports (11)B 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 δ(11)B melt values in arc magmas could flag shallow-level additions to the subduction cycle.

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Figures

Figure 1
Figure 1. Overview of existing boron data and new data from this study.
(a) Literature data for various geochemical reservoirs compared to our experiments. The full range of Mid-Ocean Ridge Basalt (MORB) and Ocean Island Basalt (OIB) literature data is shown and the δ11B values suggested to represent the primary mantle are highlighted (after refs and 42). (b) Literature δ11B data for arc volcanoes. (c–e) A four component arc system comprising mantle, slab lithologies, slab fluids, and upper crustal carbonates is shown. Process arrows indicate the effects that shallow slab fluids versus assimilation of degassing crust would have on δ11B values and B concentration of erupted products. The plotted experimental data correspond to limestone-bearing experiments except for panel e) which shows dolostone experimental data. Error bars (1σ) are smaller than symbol size. Full data sources are provided in Supplementary Table S-5. Abbreviations: TIMS, thermal ionisation mass spectrometry; SIMS, secondary ionisation mass spectrometry.
Figure 2
Figure 2. Experimental data.
Back scattered electron (BSE) images and δ11B profiles for representative (a,b) Vesuvius and (c,d) Merapi experiments. The solid red line on the BSE images (a,c) represents the SIMS traverse and the red symbols indicate analysis spots. In (b) and (d), average δ11B values for different glass compositional domains are represented by red horizontal bars and the full range of starting material values measured is indicated by black dashed horizontal lines. The CaO-rich glasses have significantly lower δ11B values than both the starting material and the CaO-normal glass due to transport of 11B away from the reaction site in the CO2 vapour. Error as in Fig. 1.
Figure 3
Figure 3. Conceptual model.
Tetrahedrally coordinated boron is present in carbonate and silicate melt and decarbonation at the onset of assimilation triggers boron isotope fractionation as follows: CaCO3(BOH)4 (carbonate) +SiO2(BOH)4 (silicate melt) → CaO-rich silicate melt +10B(OH)4 (in melt) + CO2 (fluid) +11B(OH)3 (in CO2-rich fluid). In other words, assimilation of carbonate into the melt gives rise to Ca-rich melt and a co-existing CO2 phase that mingles with CaO-normal melt. Transport of trigonally coordinated 11B in CO2 bubbles away from the reaction site and subsequent partial reabsorption in CO2-undersaturated melt at the distal parts of the capsule gives rise to relatively high δ11B values in portions of the CaO-normal glass.
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