Ca-rich carbonate melts; a regular-solution model, with application to carbonatite magma + vapor equilibria and carbonate lavas on Venus

Abstract

Other| February 01, 1995 Ca-rich carbonate melts: A regular-solution model, with application to carbonatite magma + vapor equilibria and carbonate lavas on Venus Allan H. Treiman Allan H. Treiman Lunar and Planetary Institute, Houston, TX, United States Search for other works by this author on: GSW Google Scholar American Mineralogist (1995) 80 (1-2): 115–130. https://doi.org/10.2138/am-1995-1-212 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Allan H. Treiman; Ca-rich carbonate melts: A regular-solution model, with application to carbonatite magma + vapor equilibria and carbonate lavas on Venus. American Mineralogist 1995;; 80 (1-2): 115–130. doi: https://doi.org/10.2138/am-1995-1-212 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract A thermochemical model of the activities of species in carbonate-rich melts would be useful in quantifying chemical equilibria between carbonatite magmas and vapors and in extrapolating liquidus equilibria to unexplored PTX. A regular-solution model of Ca-rich carbonate melts is developed here, using the fact that they are ionic liquids, and can be treated (to a first approximation) as interpenetrating regular solutions of cations and of anions. Thermochemical data on systems of alkali metal cations with carbonate and other anions are drawn from the literature; data on systems with alkaline earth (and other) cations and carbonate (and other) anions are derived here from liquidus phase equilibria. The model is validated in that all available data (at 1 kbar) are consistent with single values for the melting temperature and heat of fusion for calcite, and all liquidi are consistent with the liquids acting as regular solutions.At 1 kbar, the metastable congruent melting temperature of calcite (CaCO3) is inferred to be 1596 K, with ΔHfus(calcite) = 31.5 ± 1 kj/mol. Regular solution interaction parameters (W) for Ca2+ and alkali metal cations are in the range −3 to −12 kj/mol2; W for Ca2+-Ba2+ is approximately −11 kj/mol2; W for Ca2+-Mg2+ is approximately −40 kj/ mol2, and W for Ca2+-La3+ is approximately +85 kj/mol2. Solutions of carbonate and most anions (including OH−, F−, and SO42−⁠) are nearly ideal, with W between 0 (ideal) and −2.5 kj/mol2. The interaction of carbonate and phosphate ions is strongly nonideal, which is consistent with the suggestion of carbonate-phosphate liquid immiscibility. Interaction of carbonate and sulfide ions is also nonideal and suggestive of carbonate-sulfide liquid immiscibility. Solution of H2O, for all but the most H2O-rich compositions, can be modeled as a disproportionation to hydronium (H3O+) and hydroxyl (OH ) ions with W for Ca2+-H3O+ ≈ 33 kj/mol2.The regular-solution model of carbonate melts can be applied to problems of carbonatite magma + vapor equilibria and of extrapolating liquidus equilibria to unstudied systems. Calculations on one carbonatite (the Husereau dike, Oka complex, Quebec, Canada) show that the anion solution of its magma contained an OH− mole fraction of ~0.07, although the vapor in equilibrium with the magma had P(H2O) = 8.5 × P(CO2). F in carbonatite systems is calculated to be strongly partitioned into the magma (as F−) relative to coexisting vapor. In the Husereau carbonatite magma, the anion solution contained an F− mole fraction of ~6 × 10−5.Calcite and anhydrite may be present on the surface of Venus, but they would not be molten at ambient surface temperature (660–760 K) because the minimum melt temperature (eutectic) for the calcite + anhydrite system is calculated to be 1250 K. The Venus atmosphere contains 5 ppb HF, which implies that the anion solution of a carbonate-rich magma in equilibrium with the atmosphere would contain a F− mole fraction of ~7 × 10−3, or about 0.1 wt%. Although this proportion of F is much enriched compared with the atmosphere, it would have little effect on phase relations of the carbonatite. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.