Summary Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (⩽1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (⩽2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by the recycling of the enriched oceanic lithosphere back into the mantle.

Chemical and isotopic systematics of oceanic basalt : implications for mantle composition and processes

Chemical and isotopic systematics of oceanic basalts. Implications for Mantle Composition and Processes

Sub-calcic garnets encapsulated by diamonds from relatively young (90 Myr) kimberlites in southern Africa, yield ancient Sm–Nd and Rb–Sr model ages (3,200–3,300 Myr). The chemistry and distribution of these and associated sub-calcic garnets from kimberlite concentrate, indicate diamonds formed following enrichment of residual sub-cratonic mantle such as that remaining after widespread extraction of 3,500-Myr komatiitic lavas.

Origin of diamonds in old enriched mantle

Previously reported initial 87Sr/86Sr and 143Nd/144Nd ratios of non-micaceous southern African Cretaceous kimberlites indicate that their mantle source rocks are undifferentiated to slightly depleted relative to the bulk Earth1–6. Anomalously radiogenic Pb isotopic ratios of these kimberlites, on the other hand, are smilar to those of oceanic island basalts and indicate that the source regions have had high U/Pb ratios7. Some micaceous kimberlites have a different Sr and Pb isotopic character, reflecting different source regions characterized by higher Rb/Sr, but lower U/Pb ratios1,2,7. Nd isotopic analyses have not been reported for the micaceous kimberlites. I present here new whole-rock Pb, Sr and Nd isotopic analyses of examples of both types of kimberlite which, in conjunction with previously published data, confirm the existence of both depleted and enriched upper mantle sources for southern African Cretaceous kimberlites.

Pb, Sr and Nd isotopic evidence for sources of southern African Cretaceous kimberlites

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On and Off the North China Craton: Where is the Archaean Keel?

Kimberlites are characterized by a high concentration of incompatible elements, including light rare earths. However, Nd isotopic evidence indicates that their source regions do not have a long history of enrichment. If kimberlites can be generated from non-enriched mantle sources by simple partial melting processes, this implies that crystal–liquid partition coefficients for some trace elements are lower for kimberlitic liquids than for basalts or andesites. From a comparison of clinopyroxene megacrysts (regarded as equilibrated with kimberlite at depth) with existing kimberlite data we argue here that low crystal–liquid partition coefficients for the rare earths are plausible.

F. R. Boyd

Papers

Can Kimberlites be generated from an ordinary mantle