Figure 3
From: Boron isotope fractionation in magma via crustal carbonate dissolution
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.
