HR: 16:15h
AN: V34A-02    [Abstracts]
TI: The effect of iron on the chlorine concentration in felsic melts
AU: * Simon, A C
EM: asimon@jhu.edu
AF: Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218
AU: Candela, P
AF: Laboratory for Mineral Deposits Research, Department of Geology, University of Maryland, College Park, MD 20742
AU: Piccoli, P
AF: Laboratory for Mineral Deposits Research, Department of Geology, University of Maryland, College Park, MD 20742
AB: The concentration of Cl in hydrous magmas plays a role in a variety of processes including the timing of volatile exsolution and the mass transfer of metals from melt into an exsolved Cl-bearing volatile phase. Literature data indicate that the concentration of Cl in hydrous felsic melts increases with the activities of network modifying cations (Na+K+Fe+Ca) at the expense of Si (Metrich and Rutherford,GCA,1992). Cl in melt is correlated positively with Na, K, and Fe, negatively with Si and is independent of Al. Synthetic Fe-free peralkaline melts (Al/(Na+K)<1) can contain higher Cl concentrations than peraluminous melts (Al/(Na+K>1); Cl can increase four-fold with increasing melt peralkalinity (i.e., from 2000 to 8000 ppm as ASI decreases from 1 to 0.4). In natural Fe-bearing felsic melts, Cl in melt can be >2000 ppm higher than in Fe-free melts at an identical ASI suggesting the potential for Fe-Cl complexation in the melt. Additionally, one study has postulated a relationship between increasing Cl concentration and decreasing Fe3+/Fe2+ ratio in felsic melts (Webster and DeVivo, AmMin,2002). We have performed a series of experiments (n=20) in Mg- and Ca-free, Fe-bearing system (magnetite (mt)+haplogranite melt+vapor+brine) that definitively link increasing concentrations of Fe and Cl in felsic melts. Natural mt was reacted with synthetic haplogranite melt and an HCl-, NaCl- and KCl-bearing aqueous vapor ­O brine (initial molar K:Na=1 and K:H varied from 1 to 25) in Au and Pt capsules held inside traditional and rapid-quench cold-seal vessels at 800,aC, pressures of 100 to 145 MPa and bulk salinities ranging from 1.8 to 20 wt% NaCl eq. The oxygen fugacity of the experiments was fixed at either nickel-nickel oxide (NNO) or quartz-fayalite-magnetite (QFM). Varying the HCl concentration, imposed by varying the initial K:H of the fluid, allows us to control the final melt aluminosity, with ASI's ranging from 0.5 to 1.1. When melt concentration data for all cations are regressed against Cl, the positive relationship between Fe and Cl is significantly greater than that between Cl and all other cations. In slightly peralkaline melts (ASI=0.94),a negative correlation obtains between Cl and both Na and K. Runs of short duration, yielding heterogeneous melt compositions, display the most dramatic absence of a systematic relationship between ASI and Cl; in some runs there is a negative correlation between Cl and increasing melt peralkalinity. These data imply that Fe, and not ASI, is the fundamental control on Cl content in the melt. The concentration of Cl in melt is significantly higher in melts produced at QFM (up to 0.6 wt% Cl) relative to NNO (up to 0.3 wt% Cl) and this is directly correlated to the increase of Fe in melt at QFM (up to 4 wt% Fe) relative to NNO (up to 3 wt% Fe). The strong positive correlation between Fe and Cl concentrations at oxygen fugacities ranging from NNO to QFM indicates that the Fe content of water-saturated, peralkaline to peraluminous aluminosilicate melts plays a determinant role on Cl concent in the melt. These experimental results are broadly consistent with the work of Webster and De Vivo (2002) who suggest the timing of iron-bearing mineral crystallization in the melt plays a critical role in controlling the absolute Cl concentration in the melt. Our findings have important implications for magmatic degassing, hydrothermal alteration and the mass transfer of ore metals from melt to the volatile phase; the latter being critically dependent on the partitioning of Cl between melt and volatile phase.
DE: 8424 Hydrothermal systems (8135)
DE: 8135 Hydrothermal systems (8424)
DE: 3640 Igneous petrology
DE: 1030 Geochemical cycles (0330)
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting