The Dramatic Effects of C-S-O-H-Cl on the Melt-Fluid Partitioning of Cl and the Challenge of Accurately Modeling Cl Concentrations of Evolving Magmatic Fluids
Abstract
New experimental constraints on the distribution of Cl between Vesuvius phonolite melt and H2O-, CO2-, SO2-, and Cl-bearing vapor, vapor plus saline liquid, or saline liquid have been determined at 200 MPa and 900°C. Some experiments involve melts saturated in all four volatile components. The addition of small quantities of SO2, CO2, and SO2 plus CO2 to fluids dominated by water and alkali chlorides causes dramatic reductions in DXCl (mole fraction Cl in fluid(s)/mole fraction Cl in silicate melt). Experiments with XCO2fluid of 0.15 or XSO2fluid of 0.15 are characterized by values of DXCl that are an order of magnitude lower than those of S- and C-free runs. This observation has important consequences for open systems that exsolve CO2- and/or SO2-enriched fluids "early" in the chemical differentiation of magma. Extrapolation of our experimental results to other pressures and for other melt compositions indicates that CO2- and/or SO2-bearing fluids will not sequester significant abundances of Cl from magma. Thus, CO2- and/or SO2-enriched fluids that exsolve and escape "early" from magma will not dramatically alter the magmatic Cl content. This is consistent with the oft-quoted, general degassing order of: C (first), S, Cl, and H2O (last) associated with decreasing pressure. To apply these new partitioning data to models of the exsolution and chemical evolution of magmatic fluids, they must be integrated with experimental constraints on other parameters that also strongly influence the distribution of Cl between melts and fluids (e.g., pressure, temperature, melt composition, and the Cl content of the bulk system). Expressed as DXCl, published values for Cl partitioning range from 20 (with felsic melts) to ca. 0.4 (with mafic melts), but DXCl is a complex function of these parameters. For example, DXCl increases by an order of magnitude as temperature decreases from 1000° to 800°C with vapor-saturated felsic melts at 200 MPa. Conversely, DXCl decreases by an order of magnitude in experiments involving a pressure reduction of 280 to 80 MPa with phonolite melt saturated in Cl-bearing aqueous fluid(s), but other experiments show that DXCl increases with decreasing pressure. DXCl also varies positively with the Cl concentration of the bulk system. In addition, the chemical evolution of fluid-saturated magma from basaltic to rhyolitic melt compositions also involves an order of magnitude increase in DXCl with all other parameters equal. In summary, as magmas ascend through the crust, cool, and differentiate, values of DXCl (and hence the Cl content of coexisting fluids) tend to increase due to the attendant reduction in temperature, because of the evolution of magma to increasingly felsic compositions, and due to the potential loss of CO2 and/or SO2 from magma to fluids that exsolve and escape "early". Conversely, values of DXCl (and the Cl content of coexisting fluids) may also tend to decrease because of the reduction in pressure during ascent. Moreover, the crystallization of Cl-free and Cl-poor minerals will tend to increase the Cl content of the magma and increase values of DXCl; whereas, the loss of Cl to C- and S-poor aqueous fluid(s) in open magmatic systems will reduce the Cl content of the system and tend to reduce DXCl during late-stage fluid-melt interactions.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.V53E..08W
- Keywords:
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- 8410 Geochemical modeling (1009;
- 3610);
- 8412 Reactions and phase equilibria (1012;
- 3612);
- 8425 Effusive volcanism;
- 8430 Volcanic gases