Eclogites with oceanic crustal and mantle signatures from the Bellsbank kimberlite, South Africa, part 2: Sr, Nd, and O isotope geochemistry
Neal, Clive R.; Taylor, Lawrence A.; Davidson, Jon P.; Holden, Peter; Halliday, Alexander N.; Nixon, Peter H.; Paces, James B.; Clayton, Robert N.; Mayeda, Toshiko K.
1990-09
Citation
Neal, Clive R., Taylor, Lawrence A., Davidson, Jon P., Holden, Peter, Halliday, Alex N., Nixon, Peter H., Paces, James B., Clayton, Robert N., Mayeda, Toshiko K. (1990/09)."Eclogites with oceanic crustal and mantle signatures from the Bellsbank kimberlite, South Africa, part 2: Sr, Nd, and O isotope geochemistry." Earth and Planetary Science Letters 99(4): 362-379. <http://hdl.handle.net/2027.42/28415>
Abstract
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 [delta]18O, and 87Sr/86Sr values typical of mantle-derived materials (+5.1 to +5.6[per mille sign] and 0.7042-0.7046, respectively), and very low Sm/Nd ratios, (apparent) Rb/Sr ratios and [epsilon]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 [delta]18O (+2.9 to +4.7), extremely high [epsilon]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. [epsilon]Nd[120] diagrams. These relationships are consistent with mixing of the Bellsbank kimberlite ([epsilon]Nd[120] = -10; Sm/Nd = 0.10) with a depleted eclogite end-member ([epsilon]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 [delta]18O 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.Publisher
Elsevier
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Article
URI
http://www.sciencedirect.com/science/article/B6V61-47261F4-7V/2/529a113e56f408df6086a5ea0f3d7ce6Metadata
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