ECLOGITES WITH OCEANIC CRUSTAL AND MANTLE SIGNATURES FROM THE BELLSBANK KIMBERLITE, SOUTH-AFRICA, .2. SR, ND, AND O ISOTOPE GEOCHEMISTRY

The Sr, Nd, and O isotopic compositions of garnet and clinopyroxene mineral separates from nine eclogite xenoliths from the Bellsbank kimberlite (erupted at 120 Ma) define three groups. Group A eclogites, considered to be mantle cumulates, are characterized by δ18O, and 87Sr 86Sr values typical of mantle-derived materials (+5.1 to +5.6‰ and 0.7042-0.7046, respectively), and very low Sm/Nd ratios, (apparent) Rb/Sr ratios and ε{lunate}Nd[120] values (0.057-0.078, 0.00005-0.00136 and -14 to -16 respectively). The REE and isotopic data for these eclogites can be modelled in terms of crystallization from a Group II kimberlite magma at ∼ 1-1.5 Ga. Group B and C eclogites, believed to be the metamorphosed products of ancient subducted oceanic crust, are characterized by low δ18O (+2.9 to +4.7), extremely high ε{lunate}Nd[120] (∼ +40 to +219), and radiogenic 87Sr/86Sr ratios (0.708-0.710). The Sm/Nd ratios of the Group B eclogites are very high (up to 1.6). The data for Group B and C eclogites define a linear correlation on Sm/Nd and 1/Nd vs. ε{lunate}Nd[120] diagrams. These relationships are consistent with mixing of the Bellsbank kimberlite (ε{lunate}Nd[120] = -10; Sm/Nd = 0.10) with a depleted eclogite end-member (ε{lunate}Nd[120] +219; Sm/Nd = 1.6) during a cryptic metasomatic event. The Sr isotopic variations in Group B and C eclogites cannot be generated by simple two-component mixing. The Sr, Nd, and O isotope data for Group B and C eclogites probably reflect a complex sequence of depletion and enrichment events, in both crust and mantle settings. Enrichments which possibly affected the Group B and C eclogites include seawater-alteration of a MORB-like protolith, which lowered the δ18O and raised the 87Sr 86Sr ratio, but left the Nd isotopic compositions unchanged, and cryptic metasomatism by the magmatism that produced the Bellsbank kimberlite. The high Sm/Nd ratio of the depleted eclogite end-member cannot be generated by extraction of a melt from a modern MORB composition. Rather, it is argued that such high Sm/Nd ratios are produced as a result of partitioning during the recrystallization of a MORB-like component to eclogite during subduction. Nd model ages suggest that this process occurred ∼ 2.3-2.4 Ga. © 1990.