Small-scale oxygen isotope variations and petrochemistry of ultradeep (> 300 km) and transition zone xenoliths

摘要

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 delta O-18(gt) = 5.67 parts per thousand vs SMOW (standard mean ocean water) +/- 0.06 parts per thousand SEM (standard error of the mean) and clinopyroxene with delta O-18(cpx) = 5.52 +/- 0.18. Results for xenoliths from Jagersfontein, South Africa, are; harzburgite: garnet delta O-18(gt) = 5.78 +/- 0.16, clinopyroxene delta O-18(cpx) = 5.24 +/- 0.27, orthopyroxene delta O-18(opx) = 4.39 +/- 0.21, olivine delta O-18(ol) = 5.69 +/- 0.08; Iherzolite: delta O-18(gt) = 5.63 +/- 0.07, delta O-18(cpx) = 4.3, delta O-18(opx) = 5.62 +/- 0.27; a bimineralic gt/cpx xenolith with minor opx: delta O-18(gt) = 5.08 +/- 0.13, delta O-18(cpx) = 4.32 +/- 0.08, delta O-18(opx) = 4.21 +/- 0.31. Two single crystal garnets with minor associated pyroxenes and in one case, olivine, gave the following results: #1 delta O-18(gt) = 5.62 +/- 0.05, delta O-18(cpx) = 5.09 +/- 0.07, delta O-18(opx) = 5.62 + 0.12, delta O-18(ol) = 5.59; #2 delta O-18(gt) = 5.62 +/- 0.09. While evidence for isotopic equilibrium is observed between some gt/ol, gt/opx and cpx/opx pairs, the O-18/O-16 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 O-18 content of the pyroxenes a few Ma prior to the eruption of the kimberlite. Copyright (C) 2000 Elsevier Science Ltd. [References: 42]