Abstract
Geochemical signatures of arc basalts can be explained by addition of aqueous fluids, melts, and/or supercritical fluids from the subducting slab to the sub-arc mantle. Partitioning of large-ion lithophile elements between aqueous fluids and melts is crucial as these two liquid phases are present in the sub-arc pressure-temperature conditions. Using a micro-focused synchrotron X-ray beam, in situ X-ray fluorescence (XRF) spectra were obtained from aqueous fluids and haplogranite or jadeite melts at 0.3 to 1.3 GPa and 730°C to 830°C under varied concentrations of (Na, K)Cl (0 to 25 wt.%). Partition coefficients between the aqueous fluids and melts were calculated for Pb, Rb, and Sr ( D Pb , Rb , Sr fluid / melt ). There was a positive correlation between D Pb , Rb , Sr fluid / melt values and pressure, as well as D Pb , Rb , Sr fluid / melt values and salinity. As compared to the saline fluids with 25 wt.% (Na, K)Cl, the Cl-free aqueous fluids can only dissolve one tenth (Pb, Rb) to one fifth (Sr) of the amount of large-ion lithophile elements when they coexist with the melts. In the systems with 13 to 25 wt.% (Na, K)Cl, D Pb , Rb fluid / melt values were greater than unity, which is indicative of the capacity of such highly saline fluids to effectively transfer Pb and Rb. Enrichment of large-ion lithophile elements such as Pb and Rb in arc basalts relative to mid-oceanic ridge basalts (MORB) has been attributed to mantle source fertilization by aqueous fluids from dehydrating oceanic plates. Such aqueous fluids are likely to contain Cl, although the amount remains to be quantified.
Highlights
H2O-rich fluids play significant roles in magma genesis and elemental recycling via subduction processes (Kushiro 1972; Green 1973; Perfit et al 1980; Tatsumi and Eggins 1995; Keppler 1996; Bureau and Keppler 1999; Kawamoto 2006; Mibe et al 2007; Kawamoto et al 2012)
Their chemical compositions (Table 1) were characterized by lower concentrations of Na and K and a higher concentration of Al than the starting materials, resulting in a higher norm corundum and higher alumina saturation index (ASI) than the starting materials. This indicates that Na and K were selectively dissolved into aqueous fluids while the silicates were melted
In order to determine the effect of pressure, partition coefficients are plotted as a function of pressure in the Cl-free water and haplogranite or jadeite systems (Figure 4A,B,C)
Summary
H2O-rich fluids play significant roles in magma genesis and elemental recycling via subduction processes (Kushiro 1972; Green 1973; Perfit et al 1980; Tatsumi and Eggins 1995; Keppler 1996; Bureau and Keppler 1999; Kawamoto 2006; Mibe et al 2007; Kawamoto et al 2012). While aqueous fluids dissolve silicate components under HTHP conditions (Anderson and Burnham 1965; Nakamura and Kushiro 1974), the solutes are not quenched into a single phase and can be recovered as a mixture of glass globules, quench crystals, and other materials (Schneider and Eggler 1986; Keppler 1994; Brenan et al 1995). Such materials quenched from aqueous fluids under HTHP conditions can be analyzed after recovery through various methods including (1) leaching with water (Cullers et al 1970; Flynn and Burnham 1978; Borchert et al 2010a, b), (2) leaching with. Because of the differences in estimating fluid compositions, the reported values of partition coefficients between melts and aqueous fluids have varied to some extent (Ayers and Eggler 1995; Keppler 1996)
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