The provenance of fertile off-craton lithospheric mantle: Sr-Nd isotope and chemical composition of garnet and spinel peridotite xenoliths from Vitim, Siberia

Nd and Sr isotope compositions were obtained for pure garnet and pyroxene separates from coarse peridotite xenoliths from the Vitim volcanic province east of Lake Baikal. Internal Sm-Nd isochrons give ages of 24-42 Ma. These dates are older than the eruption age for the host picrite tuff (∼16 Ma) indicating that garnet and clinopyroxene may not have been in isotopic equilibrium in the mantle (at 19-23 kbar, 990-1070 °C). In spite of their fertile major element chemistry, many peridotites have extremely depleted Nd and Sr isotope compositions (calculated from mineral analyses: ɛNdT=+14 to +27; 87Sr/86Sr=0.7017-0.7028) and therefore plot outside the MORB field in the Sr-Nd isotope diagram. Sm-Nd and Rb-Sr model ages for unmetasomatised peridotites are concordant for each sample within ±0.4 Ga and suggest a depletion event of a primitive mantle ∼2 Gy ago, also consistent with Re-Os data on the same xenolith suite [Pearson, D.G., Irvine, G.J., Ionov, D.A., Boyd, F.R., Dreibus, G.E., 2004. Re-Os isotope systematics and Platinum Group Element fractionation during mantle melt extraction: a study of peridotite xenoliths from N. Lesotho and S. Namibian kimberlites, the Vitim volcanic field and massif peridotites from Beni Bousera. Chem. Geol. 208, 29-59]. This suggests that the mantle depletion event was roughly matched with formation of the mature continental crust in the region (∼2.0 Ga according to U-Pb and Sm-Nd dating) and implies that the mantle peridotites have been attached to the continental lithosphere since that time and survived several major tectono-thermal events. The Paleoproterozoic formation age for the fairly fertile Vitim mantle domain indicates that average lithospheric mantle compositions cannot be used to assess lithospheric formation ages in continental off-craton mantle.