Volatiles and lithophile elements in Taylor Creek Rhyolite: Constraints from glass inclusion analysis

Ansrnc.cr Glass-bearing inclusions in quartz phenocrysts from four eruptive units of the F-rich, Sn-mineralized Taylor Creek Rhyolite, New Mexico, have been analyzed by electron microprobe. Inclusions from two units contain F-rich glass and the daughter minerals fluorbiotite (>4 wto/o F), fluorite, and fluor-muscovite (>6 wto/o D; these fractions of magma contained extreme concentrations of F. Glass-rich inclusions from the other two units have also been analyzed by ion microprobe for H, Li, Rb, Cs, Ce, Be, Sr, Y, Nb, U, Th, Mo, W, Sn, and B. Close similarities in major-element composition between these glassrich inclusions and host whole rocks indicate that the inclusions are samples of melt from which the phenocrysts grew. F contents of these glass inclusions are variable and low relative to inclusions containing micas and fluorite. Concentrations of AlrOr, NarO, KrO, HrO, FeO, and CaO in glass-rich inclusions vary and correlate inversely with SiOr. This is interpreted to indicate that up to 15 wt0/0 SiO, crystallized within inclusions after trapping of melt, although secondarily precipitated quartz is not optically visible within inclusions. This postentrapment process does not measurably change trace-element concentrations in trapped glass relative to analytical uncertainty. Concentrations of minor and trace elements in glass inclusions vary by factors of 2 to > 10. These variations are significantly greater than those shown by whole rocks, indicate that melt was strongly heterogeneous for some trace elements, and express melt evolution as phenocrysts grew. Mean concentrations of Rb, Cs, and Cl (+ tl) in glass inclusions exceed those in glassy whole rocks; the concentrations of these and other relatively mobile elements (HrO and Li) in vitrophyres do not represent the composition of parent melt. Relatively low HrO and high Cl concentrations in melt imply that these fractions of melt were HrO undersaturated prior to trapping of inclusions. Preeruptive HrO content of melt, from which quartz phenocrysts grew, was =2.7 w0/0. Preeruptive abundances of other mobile constituents in this melt were 0.25 wto/o F, 0.26 wto/o CL 45 ppm B, l1 ppm U, 16 ppm Cs, and 15 ppm Sn. Mobile constituents were variably removed from Taylor Creek Rhyolite magma during or after eruption but after melt inclusions were entrapped in quartz. Mean concentrations of immobile trace elements in glass inclusions are equivalent to those in whole rocks.