ON THE RECENT ENRICHMENT OF SUBCONTINENTAL LITHOSPHERE - A DETAILED U-PB STUDY OF SPINEL LHERZOLITE XENOLITHS, YUKON, CANADA

Lead strontium, and osmium isotopic data have been obtained for whole rocks and mineral separates (olivine, orthopyroxene, clinopyroxene, an d spinel) for spinel Iherzolite xenoliths hosted by lavas of the Quate rnary Alligator Lake volcanic centre, southern Yukon. Whole-rock xenol ith samples display a large variation of lead concentrations, from 16 ppb for a harzburgite to up to 400 ppb for a Iherzolite. However, thei r lead isotope ratios are relatively homogeneous with Pb-206/Pb-204 of 19.07 +/- 0.08, Pb-207/Pb-204 Of 15.65 +/- 0.07, and Pb-208/Pb-204 of 38.67 +/- 0.17 (n = 7). However, the U-238/Pb-204 ratios display a la rge variation, from 12.2 to 46.5, and do not correlate with indices of fertility such as calcium or aluminum content. Mineral separates yiel d even larger variations in lead isotopic composition and lead and ura nium concentrations. Some olivine fractions have both the lowest radio genic compositions (Pb-206/Pb-204 = 18.75) and the lowest U-238/Pb-204 ratios (similar to 3.1). Clinopyroxenes (cpx) display the highest lea d and uranium concentrations (up to 1277 ppb and 195 ppb, respectivell y) and generally similar or more radiogenic lead isotopic composition and higher U-238/Pb-204 ratios than their whole-rock compositions. Ort hopyroxene and spinel fractions yield intermediate compositions betwee n olivine and cpx. Although whole rocks and cpx for individual samples yield almost identical Sr-87/Sr-86, th, xenoliths (n = 5) display a l arge variation of strontium isotopic compositions (Sr-87/Sr-86 from 0. 7033 to 0.7050), and again no correlation is observed with the fertili ty of the samples. The positive correlation between lead and strontium isotope ratios of cpx and the distribution of the data in a U-Pb isoc hron diagram suggest that the subcontinental Lithosphere under the Yuk on was affected by a recent (< similar to 30 Ma) enrichment in uranium , lead, and strontium. The metasomatic fluid/magma might have had an i sotopic composition close to that of some sediments in the northern Pa cific Ocean. When compared to K-d values reported in the literature, o livine is enriched in lead relative to coexisting cpx and, depending o n the K-d values used for the calculations, is either enriched or depl eted in uranium compared to cpx. Mantle cpx from the subcontinental li thosphere that has suffered metasomatism is U- and Pb-rich when compar ed to ''unmetasomatised'' cpx suggesting that cpx scavanges these elem ents from metasomatic fluids. Fractionation of uranium and lead betwee n fluids and cpx is not easy to constrain, but the data for Alligator Lake suggest a possible uranium enrichment over lead in metasomatised cpx. Three whole rock samples have variable Os-187/Os-188 correspondin g to gamma Os of -0.3, +0.4, and +3.0. In contrast to the lead and str ontium data, the Os-187/Os-188 and Re-187/Os-188 ratios are positively correlated with calcium and aluminum contents of the xenoliths. Their Re-187/Os-188 ratios are slightly higher than the mean chondritic val ues, and an age of 650 Ma is obtained when the data are regressed in a Re-Os isochron diagram. This age may date partial melting of the Iher zolite and/or re-fertilization with rhenium enrichment of the mantle l ithosphere. The protolithic lithosphere would have had a gamma Os typi cal of depleted MORE (ca. -1.4) at 650 Ma.