Continental mantle lithosphere, and shallow level enrichment processes in the Earth's mantle

Abstract

The continental mantle lithosphere may be a significant reservoir for incompatible elements, and it remains a key natural laboratory in which to investigate element fractionation processes in the upper mantle. Major, trace element and isotope data on mantle xenoliths, lamproites and kimberlites, and on selected continental flood basalts are integrated to develop a model for the continental mantle lithosphere. It is argued that the composition of such mantle lithosphere, and hence its density, thickness and capacity to generate basalt, varies with age. Archaean mantle lithosphere is characterised by relatively low FeO abundances, which are attributed to komatiite extraction, and thus it is intrinsically less dense than the surrounding asthenosphere. In contrast the post-Archaean mantle lithosphere may be compositionally similar to that sampled recently as spinel peridotite inclusions in alkali basalts. It is therefore sufficiently fertile to contribute in the generation of continental flood basalts, and dense enough to be more readily delaminated and incorporated into the asthenosphere source regions of ocean basalts. Combining the available data on mantle xenoliths and continental flood basalts suggests that the continental mantle lithosphere contains less than 10% of the K, and 3.5% of the Sr and Nd in the crust/mantle system. Many continental mafic rocks have distinctive isotope ratios with low ε Nd , variable ε Sr and often low 206Pb/ 204Pb . In particular the combination of slightly elevated 87Sr/ 86Sr at low 206Pb/ 204Pb is increasingly regarded as a feature of the continental mantle lithosphere. Elemental data on lamproites, kimberlites, melilitites and oceanic basalts demonstrate that potassic phase(s) strongly influence(s) U/Pb fractionation in the upper mantle. Locally, as in the western U.S., there is evidence for amphibole control, but in most cases the potassic phase appears to be phlogopite, consistent with the negative Rb/Sr U/Pb arrays inferred from Sr- and Pb-isotopes. The significance of such arguments is that both amphibole and phlogopite are restricted to relatively shallow levels ( < 250 km), and thus they have a key role in distinguishing shallow from deep level enrichment processes in the Earth's mantle.