Abstract

Experiments have been conducted with H 2 O and H 2 O + CO 2 -bearing fluids and melts of a Cl-, F-, and lithophile trace element-enriched vitrophyre associated with topaz rhyolite from Spor Mountain, Utah. The starting material is mildly peraluminous and contains 1.25 wt percent F and 1,500 ppm Cl. The experiments were conducted with f (sub O 2 ) controlled near Ni-NiO, at pressures of 0.5 to 4 kbars, and at temperatures of 770 degrees to 950 degrees C. Fluid/melt trace element distribution coefficients (D i ) were calculated for Li, Be, B, Rb, Sr, Y, Zr, Nb, Cs, Ba, Ce, Th, and U after ion microprobe analysis of starting material and run product glasses. The concentration of element i in the fluid was calculated by mass balance and the fluid/melt distribution coefficient, D i , was calculated as (ppm by weight of i in fluid/ppm by weight of i in melt).The partitioning of Li, Rb, Sr, Y, Nb, Cs, and Ce between aqueous fluids and melts of topaz rhyolite composition depends strongly on temperature and pressure. At 0.5 kbars Li, Rb, Sr, Y, Nb, Cs, and Ce concentrate in the fluid at temperatures > or = 900 degrees C; at 2 kbars this same group of lithophile elements, except for Sr, concentrate in the fluid at T > or = 900 degrees C. Be, B, and Ba partition in favor of the melt at 0.5 and 2 kbars and at all temperatures of this study.In the presence of CO 2 -free aqueous fluids at 4 kbars and at temperatures of 770 degrees to 950 degrees C, all lithophile elements partition in favor of the melt. This behavior for lithophile elements mimics that of published data on the partitioning of Pb and Zn between rhyolitic melts and Cl-bearing fluids. Previous studies have demonstrated an equivalent effect of pressure on the distribution of chalcophile trace elements between granitic melts and Cl-bearing aqueous fluids.Fluid/melt distribution coefficients for the lithophile trace elements are significantly smaller for topaz rhyolite melts in equilibrium with mixed CO 2 + H 2 O fluids than for topaz rhyolite melts plus CO 2 -free aqueous fluids. There is no evidence for CO 2 complexing with the lithophile trace elements at pressures < or = 4 kbars.For equilibrium between topaz rhyolite melt and aqueous fluids at 2 kbars and 800 degrees C, the distribution coefficients of Li, Rb, Sr, Y, Nb, Cs, Ba, and Ce increase as the Cl concentration of the fluid increases from [asymp]0.2 m to >6 m. The data imply that the alkali elements (M) dissolve into the fluid as MCl 0 complexes and that Ce dissolves into the fluid as a CeCl 03 complex.Water-saturated magmas of topaz rhyolite composition may become differentiated with respect to Li, Cs, and Rb (+ or - Ce, Y, and Nb) if an aqueous fluid phase is evolved and moves through the magma under the influence of a temperature gradient that has a maximum temperature of at least 850 degrees to 900 degrees C.

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