TY - JOUR

T1 - The effect of alkalis and polymerization on the solubility of H2O and CO2 in alkali-rich silicate melts

AU - Vetere, Francesco

AU - Holtz, Francois

AU - Behrens, Harald

AU - Botcharnikov, Roman E.

AU - Fanara, Sara

N1 - Funding information: We thank the editor Prof. G. Moore for handling the paper and Dr. Masotta and two anonymous reviewers for constructive comments and suggestions that greatly improved the manuscript. A. Husen is acknowledge for the Fe redox state determination and J. Feige for EPMA and IR samples preparation. This research has been supported by Marie Curie Fellowship IEF_SolVoM #297880 to F.VETERE.

PY - 2014/5/11

Y1 - 2014/5/11

N2 - The effect of alkalis on the solubility of H2O and CO2 in alkali-rich silicate melts was investigated at 500 MPa and 1,250 °C in the systems with H2O/(H2O + CO2) ratio varying from 0 to 1. Using a synthetic analog of phonotephritic magma from Alban Hills (AH1) as a base composition, the Na/(Na + K) ratio was varied from 0.28 (AH1) to 0.60 (AH2) and 0.85 (AH3) at roughly constant total alkali content. The obtained results were compared with the data for shoshonitic and latitic melts having similar total alkali content but different structural characteristics, e.g., NBO/T parameter (the ratio of non-bridging oxygens over tetrahedrally coordinated cations), as those of the AH compositions. Little variation was observed in H2O solubility (melt equilibrated with pure H2O fluid) for the whole compositional range in this study with values ranging between 9.7 and 10.2 wt. As previously shown, the maximum CO2 content in melts equilibrated with CO2-rich fluids increases strongly with the NBO/T from 0.29 wt % for latite (NBO/T = 0.17) to 0.45 wt % for shoshonite (NBO/T = 0.38) to 0.90 wt % for AH2 (NBO/T = 0.55). The highest CO2 contents determined for AH3 and AH1 are 1.18 ± 0.05 wt % and 0.86 ± 0.12 wt %, respectively, indicating that Na is promoting carbonate incorporation stronger than potassium. At near constant NBO/T, CO2 solubility increases from 0.86 ± 0.12 wt % in AH1 [Na/(Na + K)] = 0.28, to 1.18 ± 0.05 wt % in AH3 [Na/(Na + K)] = 0.85, suggesting that Na favors CO2 solubility on an equimolar basis. An empirical equation is proposed to predict the maximum CO2 solubility at 500 MPa and 1,100-1,300 °C in various silicate melts as a function of the NBO/T, (Na + K)/∑cations and Na/(Na + K) parameters: (Formula presented.) This model is valid for melt compositions with NBO/T between 0.0 and 0.6, (Na + K)/∑cation between 0.08 and 0.36 and Na/(Na + K) ratio from 0.25 to 0.95 at oxygen fugacities around the quartz-fayalite-magnetite buffer and above.

AB - The effect of alkalis on the solubility of H2O and CO2 in alkali-rich silicate melts was investigated at 500 MPa and 1,250 °C in the systems with H2O/(H2O + CO2) ratio varying from 0 to 1. Using a synthetic analog of phonotephritic magma from Alban Hills (AH1) as a base composition, the Na/(Na + K) ratio was varied from 0.28 (AH1) to 0.60 (AH2) and 0.85 (AH3) at roughly constant total alkali content. The obtained results were compared with the data for shoshonitic and latitic melts having similar total alkali content but different structural characteristics, e.g., NBO/T parameter (the ratio of non-bridging oxygens over tetrahedrally coordinated cations), as those of the AH compositions. Little variation was observed in H2O solubility (melt equilibrated with pure H2O fluid) for the whole compositional range in this study with values ranging between 9.7 and 10.2 wt. As previously shown, the maximum CO2 content in melts equilibrated with CO2-rich fluids increases strongly with the NBO/T from 0.29 wt % for latite (NBO/T = 0.17) to 0.45 wt % for shoshonite (NBO/T = 0.38) to 0.90 wt % for AH2 (NBO/T = 0.55). The highest CO2 contents determined for AH3 and AH1 are 1.18 ± 0.05 wt % and 0.86 ± 0.12 wt %, respectively, indicating that Na is promoting carbonate incorporation stronger than potassium. At near constant NBO/T, CO2 solubility increases from 0.86 ± 0.12 wt % in AH1 [Na/(Na + K)] = 0.28, to 1.18 ± 0.05 wt % in AH3 [Na/(Na + K)] = 0.85, suggesting that Na favors CO2 solubility on an equimolar basis. An empirical equation is proposed to predict the maximum CO2 solubility at 500 MPa and 1,100-1,300 °C in various silicate melts as a function of the NBO/T, (Na + K)/∑cations and Na/(Na + K) parameters: (Formula presented.) This model is valid for melt compositions with NBO/T between 0.0 and 0.6, (Na + K)/∑cation between 0.08 and 0.36 and Na/(Na + K) ratio from 0.25 to 0.95 at oxygen fugacities around the quartz-fayalite-magnetite buffer and above.

KW - Alkali

KW - Infrared spectroscopy

KW - NBO/T

KW - Silicate melt

KW - Solubility

UR - http://www.scopus.com/inward/record.url?scp=85008260118&partnerID=8YFLogxK

U2 - 10.1007/s00410-014-1014-6

DO - 10.1007/s00410-014-1014-6

M3 - Article

AN - SCOPUS:85008260118

VL - 167

SP - 1

EP - 17

JO - Contributions to Mineralogy and Petrology

JF - Contributions to Mineralogy and Petrology

SN - 0010-7999

IS - 5

M1 - 1014

ER -