SMALL-SCALE OXYGEN ISOTOPE VARIATIONS AND PETROCHEMISTRY OF ULTRADEEP (>300 KM) AND TRANSITION ZONE XENOLITHS

Abstract

The oxygen isotopic composition of 6 xenoliths for which an origin between 310 to 450 km has been deduced was analyzed. An eclogite from Koidu, Sierra Leone, is composed of garnet with δ 18Ogt = 5.67‰ vs SMOW (standard mean ocean water) ± 0.06‰ SEM (standard error of the mean) and clinopyroxene with δ 18Ocpx = 5.52 ± 0.18. Results for xenoliths from Jagersfontein, South Africa, are; harzburgite: garnet δ 18Ogt = 5.78 ± 0.16, clinopyroxene δ 18Ocpx = 5.24 ± 0.27, orthopyroxene δ 18Oopx = 4.39 ± 0.21, olivine δ 18Ool = 5.69 ± 0.08; lherzolite: δ 18Ogt = 5.63 ± 0.07, δ 18Ocpx = 4.3, δ 18Oopx = 5.62 ± 0.27; a bimineralic gt/cpx xenolith with minor opx: δ 18Ogt = 5.08 ± 0.13, δ 18Ocpx = 4.32 ± 0.08, δ 18Oopx = 4.21 ± 0.31. Two single crystal garnets with minor associated pyroxenes and in one case, olivine, gave the following results: #1 δ 18Ogt = 5.62 ± 0.05, δ 18Ocpx = 5.09 ± 0.07, δ 18Oopx = 5.62 ± 0.12, δ 18Ool = 5.59; #2 δ 18Ogt = 5.62 ± 0.09. While evidence for isotopic equilibrium is observed between some gt/ol, gt/opx and cpx/opx pairs, the 18O/16O fractionation between garnet and clinopyroxene tends to be out of equilibrium. Based on the observed oxygen isotope variability, a diffusion model for the data, and published radiogenic isotope data for the samples, it is concluded that these xenoliths experienced a series of metasomatic alterations, including an event which lowered the 18O content of the pyroxenes a few Ma prior to the eruption of the kimberlite.