C. R. Neal

Eclogites with oceanic crustal and mantle signatures from the Bellsbank kimberlite, South Africa, part 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 6180, and 87Sr/Srsr values typical of mantle-derived materials (+5.1 to +5.6%o and 0.7042-0.7046, respectively), and very low Sm/Nd ratios, (apparent) Rb/Sr ratios and ~NaLlZ01 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 3180 (+2.9 to +4.7), extremely high ~Nd[a20] (-- +40 to +219), and radiogenic 87Sr/S6Sr 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. ~Nal~201 diagrams. These relationships are consistent with mixing of the Bellsbank kimberlite (CNO [1201 = --10; Sm/Nd = 0.10) with a depleted eclogite end-member (eNd[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 St, 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 6180 and raised the SVSr/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.