Sm-Nd and Rb-Sr Systematics in Volcanics and Ultramafic Xenoliths From Malaita, Solomon Islands, and the Nature of the Ontong Java Plateau

Abstract

A Rb-Sr and Sm-Nd study was carried out on five ultrabasic inclusions, three alnoite host rocks, and one sample of basalt from Malaita in an effort to determine the isotopic stratigraphy of the mantle in this area. The results of this study show the presence of relatively undepleted mantle segments underlying relatively older depleted upper mantle. The alnoite samples, three megacrysts and one Iherzolite inclusion, and the basalt are relatively uniform isotopically and yielded ε_(Nd) (T) ≈ +3.5 and ε_(Sr) (T) ≈ −6 to +17 (T = 100 m.y. for the basalt and Iherzolite and 34 m.y. for the rest of the samples). It is suggested that a mantle segment of that isotopic composition exists at a depth of more than 100 km. This mantle segment is relatively undepleted. A Iherzolite inclusion with ε_(Nd) (T) = 6.6 is included in the alnoite and indicates the location of a light rare earth element depleted layer overlying a less depleted mantle at a depth of approximately 100 km. The mantle source for the alnoite and basalt cannot be old, depleted oceanic mantle or subducted old continental crust but may come from a relatively young, slightly depleted mantle segment. The isotopic results are compatible with a mixture of about 99% depleted oceanic mantle homogenized with 1% continental crust possibly by metasomatic processes. Such a mixture is only slightly fractionated and has ƒ^(Sm/Nd) ≈ 0.13 and ƒ^(Rb/Sr) ≈ 0. Assuming that the basalt is characteristic of the Ontong Java Plateau, a volume greater than 108 km3 of mantle source with ε_(Nd) ∼ 3.5, ε_(Sr) ∼ 0 was involved. This corresponds to a block of continental crust of ∼ 10^6 km^3 which had to reach a depth greater than 100 km to produce the proposed nearly homogeneous isotopic mixture. Alternatively, the source of the Ontong Java Plateau may be a segment of a type of young continental crust with the appropriate isotopic signature which was subducted below 100 km. This would not require homogenization with a large volume of depleted mantle.

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