Strontium isotope evidence for mantle events in the continental lithosphere

Following current interpretation of isotope data from oceanic basalts, the case for inheritance of Rb/Sr and 87Sr/86Sr ratios in continental igneous rocks from long-lived heterogeneities in the mantle is reviewed, both from the point of view of necessary conditions and the evidence of published 'mantle isochrons'. Such inheritance seems unlikely for magmas undergoing plagioclase fractionation and can hardly be claimed for rocks where crustal contamination (especially selective contamination with trace elements or 87Sr alone) is a feasible alternative. Nevertheless, some unfractionated transitional or undersaturated basic volcanics do show remarkable co-variance of Rb/Sr and initial 87Sr/86Sr ratios, indicating ages far in excess of extrusion. These and other arguments suggest that heterogeneous mantle, both enriched and depleted in lithophile trace elements, is incorporated into the continental lithosphere during major crust-mantle differentiation events. Re-distribution of isotopes and incompatible elements in fluid phases or small partial melts may be an important mechanism, although others have suggested addition of enriched material from below the asthenosphere.