MINERAL-AQUEOUS FLUID PARTITIONING OF TRACE-ELEMENTS AT 900-DEGREES-CAND 2.0 GPA - CONSTRAINTS ON THE TRACE-ELEMENT CHEMISTRY OF MANTLE AND DEEP-CRUSTAL FLUIDS

To constrain the trace element composition of aqueous fluids in the de ep crust and upper mantle, mineral-aqueous fluid partition coefficient s (D-min/fluid) for U, Th, pb, Nb, Ba, and Sr have been measured for c linopyroxene, garnet, amphibole, and olivine in experiments at 2.0 GPa and 900 degrees C. Clinopyroxene-and garnet-fluid partition coefficie nts are similar for Nb (0.01-0.7) and Bat(similar to 10(-4)-10(-5)), w hereas values of D-epx/fluid for Sr (0.5-3), Th (0.6-9), and Pb (0.04- 0.09) are similar to 10x (Th, Pb) to similar to 1000x (Sr) higher than D-garnet/fluid. At the same fO(2) (FMQ + 1), garnet-fluid partition c oefficients for U are similar to 10x higher than those for clinopyroxe ne. Amphibole-fluid partition coefficients are uniformly high (similar to 1) for all elements studied, and, with the exception of Ba, intere lement fractionations are similar to clinopyroxene. The olivine-fluid partition coefficient for Nb is similar to values measured for the oth er silicates, whereas D-olivine/fluid for U, Th, Pb, Sr, and Ba are si gnificantly lower. Clinopyroxene and garnet partition coefficients fol low Henry's Law up to similar to 300 ppm of either Ba, ph, or Sr in th e fluid. Both the major-element chemistry of clinopyroxene and fluid h ave some influence on partitioning, with the magnitude of these effect s varying according to element type. Although clinopyroxene concentrat ions of Pb, Ba, and Sr were found to be homogeneous, core-to-rim decre ases in wt% Al2O3 were found to correlate with reductions in the conce ntrations of Nb, U and Th, and hence D-epx/fluid. Both increases in so lute content and the addition of NaCl to fluids lower Be measured part ition coefficients. A decrease in experiment fO(2) reduces D-Th/D-U fo r clinopyroxene, which is consistent with the compatibility of U4+ rel ative to U6+ in the clinopyroxene structure. Comparison of mineral/flu id partition coefficients with mineral/basaltic melt values from the l iterature reveal notable distinctions in partitioning behavior for flu ids vs. melts. Mineral-melt and mineral-fluid partitioning for element s such as Ba, Pb, and Sr are similar, but in contrast, U, Th, and Nb a re more strongly partitioned into silicate melts than aqueous fluids. Such differences may provide a means of discerning the products of mel t- vs. fluid-mediated metasomatism. Bulk eclogite- and Iherzolite-aque ous fluid partition coefficients, calculated from mineral/aqueous flui d values, are used to illustrate how partitioning data can constrain ( 1) the trace element composition of fluids that may be a product of de hydration of basaltic oceanic crust and (2) the effect of the subarc m antle on trace element fractionation processes. The silicate assemblag e produced during basalt dehydration (garnet + cpx +/- amphibole) does not selectively deplete the coexisting fluid in Nb relative to the ot her elements studied, nor is Nh preferentially withdrawn from the flui d by passage through an amphibole Iherzolite mantle. Results, therefor e, reaffirm the notion that residual rutile is necessary to selectivel y deplete slab-derived fluids in high held strength elements. Calculat ions also indicate that fluids with excess [U-238] relative to [Th-230 ] may be produced during dehydration of basaltic oceanic crust, and su ch excesses are retained or enhanced during transit through the mantle wedge, provided that mildly oxidizing conditions prevail. Slab-derive d fluids can therefore produce the requisite low ratios of high-field- strength/large-ion-lithophile elements (such as Nb/Th) and [U-238]/[U- 238] > 1 in the source regions of island are basalts by metasomatism o f the mantle wedge. In addition to constraints on the composition of t he fluid liberated during slab dehydration, our data allow us to estim ate the trace element composition of the material returned to the deep mantle during subduction. Calculations indicate that, following dehyd ration, the U/Pb ratio in basaltic crust is increased and Rb/Sr is lik ely to be dramatically reduced. Subduction and prolonged aging of this material produces an isotopic reservoir with the characteristics of t he HIMU component sampled by some oceanic island basalts.