HR: 15:25h
AN: V53E-08    [Abstracts]
TI: 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
AU: * Webster, J D
EM: jdw@amnh.org
AF: American Museum of Natural History, Dept. Earth and Planetary Sciences, Central Park West 79th St., New York, NY 10024, United States
AU: Sintoni, M F
EM: sintoni@amnh.org
AF: Universita di Napoli Federico II, Dipartimento di Scienze della Terra, Via Mezzocannone 8, Napoli, 80134, Italy
AU: De Vivo, B
EM: bdevivo@unina.it
AF: Universita di Napoli Federico II, Dipartimento di Scienze della Terra, Via Mezzocannone 8, Napoli, 80134, Italy
AB: 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 D_XCl (mole fraction Cl in fluid(s)/mole fraction Cl in silicate melt). Experiments with X_CO2^fluid of 0.15 or X_SO2^fluid of 0.15 are characterized by values of D_XCl 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 D_XCl, published values for Cl partitioning range from 20 (with felsic melts) to ca. 0.4 (with mafic melts), but D_XCl is a complex function of these parameters. For example, D_XCl increases by an order of magnitude as temperature decreases from 1000° to 800°C with vapor-saturated felsic melts at 200 MPa. Conversely, D_XCl 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 D_XCl increases with decreasing pressure. D_XCl 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 D_XCl with all other parameters equal. In summary, as magmas ascend through the crust, cool, and differentiate, values of D_XCl (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 D_XCl (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 D_XCl; 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 D_XCl during late-stage fluid-melt interactions.
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8412 Reactions and phase equilibria (1012, 3612)
DE: 8425 Effusive volcanism
DE: 8430 Volcanic gases
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting