Lead strontium, and osmium isotopic data have been obtained for whole rocks and mineral separates (olivine, orthopyroxene, clinopyroxene, and spinel) for spinel lherzolite xenoliths hosted by lavas of the Quaternary Alligator Lake volcanic centre, southern Yukon. Whole-rock xenolith samples display a large variation of lead concentrations, from 16 ppb for a harzburgite to up to 400 ppb for a lherzolite. However, their lead isotope ratios are relatively homogeneous with 206Pb/204Pb of 19.07 ± 0.08, 207Pb/204Pb of 15.65 ± 0.07, and 208Pb/204Pb of 38.67 ± 0.17 (n = 7). However, the 238U/204Pb ratios display a large variation, from 12.2 to 46.5, and do not correlate with indices of fertility such as calcium or aluminum content. Mineral separates yield even larger variations in lead isotopic composition and lead and uranium concentrations. Some olivine fractions have both the lowest radiogenic compositions (206Pb/204Pb = 18.75) and the lowest 238U/204Pb ratios (∼3.1). Clinopyroxenes (cpx) display the highest lead and uranium concentrations (up to 1277 ppb and 195 ppb, respectivelly) and generally similar or more radiogenic lead isotopic composition and higher 238U/204Pb ratios than their whole-rock compositions. Orthopyroxene and spinel fractions yield intermediate compositions between olivine and cpx. Although whole rocks and cpx for individual samples yield almost identical 87Sr/86Sr, the xenoliths (n = 5) display a large variation of strontium isotopic compositions (87Sr/86Sr from 0.7033 to 0.7050), and again no correlation is observed with the fertility of the samples. The positive correlation between lead and strontium isotope ratios of cpx and the distribution of the data in a U-Pb isochron diagram suggest that the subcontinental lithosphere under the Yukon was affected by a recent (< ∼30 Ma) enrichment in uranium, lead, and strontium. The metasomatic fluid/magma might have had an isotopic composition close to that of some sediments in the northern Pacific Ocean. When compared to Kd values reported in the literature, olivine is enriched in lead relative to coexisting cpx and, depending on the Kd values used for the calculations, is either enriched or depleted in uranium compared to cpx. Mantle cpx from the subcontinental lithosphere that has suffered metasomatism is U- and Pb-rich when compared to "unmetasomatised" cpx suggesting that cpx scavanges these elements from metasomatic fluids. Fractionation of uranium and lead between 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 187Os/188Os corresponding to γOs of -0.3, +0.4, and +3.0. In contrast to the lead and strontium data, the 187Os/188Os and 187Re/188Os ratios are positively correlated with calcium and aluminum contents of the xenoliths. Their 187Re/188Os ratios are slightly higher than the mean chondritic values, 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 lherzolite and/or re-fertilization with rhenium enrichment of the mantle lithosphere. The protolithic lithosphere would have had a γOs typical of depleted MORB (ca. -1.4) at 650 Ma.
ASJC Scopus subject areas
- Geochemistry and Petrology