CO2 solubility and speciation in rhyolitic sediment partial melts at 1.5–3.0GPa – Implications for carbon flux in subduction zones

• Published in: Geochimica et cosmochimica acta Vol. 124; pp. 328 - 347
• Main Authors: Duncan, Megan S., Dasgupta, Rajdeep
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.01.2014
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2013.09.026

Partial melts of subducting sediments are thought to be critical agents in carrying trace elements and water to arc basalt source regions. Sediment partial melts may also act as a carrier of CO2. However, the CO2 carrying capacity of natural rhyolitic melts that derive from partial fusion of downgoing sediment at sub-arc depths remains unconstrained. We conducted CO2-solubility experiments on a rhyolitic composition similar to average, low-degree experimental partial melt of pelitic sediments between 1.5 and 3.0GPa at 1300°C and containing variable water content. Concentrations of water and carbon dioxide were measured using FTIR. Molecular CO2(CO2mol.) and carbonate anions (CO32-) both appear as equilibrium species in our experimental melts. Estimated total CO2 concentrations (CO2mol.+CO32-) increased with increasing pressure and water content. At 3.0GPa, the bulk CO2 solubility are in the range of ∼1–2.5wt.%, for melts with H2O contents between 0.5 and 3.5wt.%. For melts with low H2O content (∼0.5wt.%), CO2mol. is the dominant carbon species, while in more H2O-rich melts CO32- becomes dominant. The experimentally determined, speciation-specific CO2 solubilities yielded thermodynamic parameters that control dissolution of CO2 vapor both as CO2mol. and as CO32- in silicate melt for each of our compositions with different water content; CO2vapor↔CO2melt:lnK0=-15to-18, ΔV0=29 to 14cm3mol−1 and CO2vapor+O2-melt→CO32-melt:lnK0=-20to-14, ΔV0=9 to 27cm3mol−1, with ΔV0 of reaction being larger for formation of CO2mol. in water-poor melts and for formation of CO32- in water-rich melts. Our bulk CO2 solubility data, [CO2] (in wt.%) can be fitted as a function of pressure, P (in GPa) and melt water content, [H2O] (in wt.%) with the following function: [CO2](wt.%)=(-0.01108[H2O]+0.03969)P2+(0.10328[H2O]+0.41165)P. This parameterization suggests that over the range of sub-arc depths of 72–173km, water-rich sediment partial melt may carry as much as 2.6–5.5wt.% CO2 to the sub-arc mantle source regions. At saturation, 1.6–3.3wt.% sediment partial melt relative to the mantle wedge is therefore sufficient to bring up the carbon budget of the mantle wedge to produce primary arc basalts with 0.3wt.% CO2.