CO2 fluxing and carbon assimilation by arc melts during magma–limestone interaction

PUBBLICATO: 30 Gennaio 2026

Chemical Geology, Deegan et al. 2026 - Figure 3 — **Figure 3** **(a)** SEM image of experiment (Exp.) with runtime = 0 s showing locations of FTIR analysis spots. **(b-e)** CO₂^mol, CO₃²⁻, bulk (total) CO₂, and H₂O glass profiles. **(f)** SEM image of experiment with runtime = 300 s showing locations of FTIR analysis spots. **(g-j)** CO₂^mol, CO₃²⁻, bulk (total) CO₂, and H₂O glass profiles. Asterisks in **(h)** and **(i)** indicate that the values are minima because the CO₃²⁻ vibrations in the Ca-rich glass were too strong to be resolved. Grey bars in panels **(d)** and **(i)** show *SOLWCAD* equilibrium dissolved CO₂ contents in the Ca-normal glass, which are similar to the estimated overall solubility of CO₂ in the starting material glass. *SOLWCAD* equilibrium dissolved CO₂ contents for the Ca-rich glass plot off-scale and range from 9300 to 40,944 μg/g CO₂, depending on the amount of assimilated limestone. Note that in the 300 s experiment, all CO₂ in the Ca-rich glass is dissolved as the carbonate anion while the co-existing fluid is comprised of CO₂ molecules. See text for further details. Abbreviations: Ca-intermed = Ca-intermediate; Equilib = equilibrium.

Deegan F.M., M. Capriolo, V.R. Troll, F.A. Weis, S. Callegaro, S. Colucci, C. Freda, V. Misiti, L.E. Aradi, H. Skogby, H. Darmawan, H. Geiger (2025).
Chemical Geology, 704. https://doi.org/10.1016/j.chemgeo.2026.123264

Abstract

Reworking of limestone (CaCO₃) by magma is an important source of carbon in volcanic arc emissions. However, while it is broadly understood that CO₂ is liberated during magma–limestone interaction, the degassing behaviour of calcite in silicate melts is less well constrained. In this study, we carried out microspectroscopic analysis of volatiles within fluid inclusions and glass (former melt) in the products of short-term experiments simulating limestone assimilation in mafic arc melt (T = 1200 °C, P = 0.5 GPa, runtimes of 0 to 300 s). The experimental products consist of partly to wholly assimilated limestone xenoliths enveloped by CaO-rich silicate glass (reacting melt) that grades into mafic glass (host melt). Micro- to milli-metric sized fluid-filled bubbles permeate the experimental products. This study reveals that limestone assimilation induces extremely fast apparent diffusivity of CO₂ (D_CO2 ≳ 10⁻⁷ m²/s) through both the reacting melt and the host melt. Volatile saturation is thus quickly reached, triggering nucleation of bubbles mainly containing CO₂ ± CO, CH₄, N₂, H₂, and H₂O. Crucially, we find that the host melt contains dissolved CO₂ from limestone, despite showing no other compositional evidence for limestone assimilation. Mafic melts in volcanic regions underlain by limestone may therefore mobilise and transport more carbon than previously thought, with implications for eruptive behaviour, volcanic CO₂ inventories, and long-term climate warming.

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