Two suites of well-characterized granulite-facies xenoliths, from Chudleigh and McBride provinces (200 km separation), exhibit an extremely large range in 187Os/186O isotopic ratios (1.437-15.071) and Os concentrations (4 ppt -1.04 ppb). This variation in Os isotopic composition and concentration clearly illustrates the heterogeneity of the present lower-crust and shows that isotopic homogenization through fluid flow or melting is not an important process in these areas.
Xenoliths from Chudleigh were carried in <2Ma basalts erupted through 3 vents cross-cutting Paleozoic rocks of the Tasman Fold Belt. All the xenoliths are mafic (SiO2<51.2wt%), five are plagioclase-rich and three are pyroxene-rich or transitional. In this suite, 187Os/186O ranges from 1.57 to 15.071 with Os concentrations from 50 to 4 ppt respectively. Geochemically, 187Os/186O and 1/Os correlate positively with 18O, 87Sr/86Sr and incompatible trace element abundances like K, Rb and Nb, and negatively with Mg# and 143Nd/144Nd. These correlations are consistent with the interpretation that the Chudleigh xenolith are genetically-related cumulates from basaltic magma undergoing simultaneous assimilation and fractional crystallization in the deep crust (Rudnick et al., 1986). Importantly, these trends demostrate that the Os isotopic composition of a basaltic magma is affected by crustal assimilation, and indicate the presence of material with relatively high Os content and radiogenic Os isotopic composition in the lower crust.
Xenoliths from McBride were carried in <3Ma basalts erupted through a single vent cross-cutting the Proterozoic Georgetown Inlier. This suite represents extreme lithologic diversity ranging from mafic to felsic orthogneisses and paragneisses, in contrast to the Chudleigh xenoliths which are exclusively mafic. The 187Os/186O of eight samples ranges from 1.437 to 14.622, and the Os concentration ranges from 47 ppt to 1.043 ppb. Geochemically, the lack of correlations between 187Os/186O and trace element abundances, and 18O, Sr, Nd and Pb isotopic ratios are indicative of the genetic diversity of this xenolith suite. Nevertheless, 187Os/186O correlates negatively with Mg#, Cr, Ni content. These tendencies also suggest a mixing trend between mafic melts and crustal rocks.
The lower crustal xenoliths analyzed here have an average Os concentration of 105 ppt with a median of 49 ppt. The difference between average and median is produced by an outlier sample with high Os concentration of 1.04 ppb and low 187Os/186O of 1.44. Two mean 187Os/186O ratios were estimated for the lower crust 3.44 to 6.71 (weighted by Os concentration) including and excluding the outlier sample. We can infer from these data that the lower crust could have up to twice as much Os concentration as the mean upper continental crust (40 ppt), and has an Os isotopic ratio about one half of the mean upper continental crustal value of 10.5 (Esser and Turekian, 1993). The Os concentration and isotopic composition of the lower crust is important in evaluating: a) the processes responsible for the formation of the lower crust; b) crustal contamination in lavas erupted through the continental crust; c) the average Os isotopic composition and concentration of the continental crust; and d) mantle heterogeneity caused by delamination and recycling of the lower crust.
Esser, B. K. & Turekian, K. K., Geochim. Cosmochim. Acta 57, 3093-3104 (1993).
Rudnick, R. L., McDonough, W. F., McCulloch, M. T. & Taylor, S. R., Geochim. Cosmochim. Acta 50, 1099-1115 (1986).