vsg - Minsoc '97
D. G. Pearson (firstname.lastname@example.org)
Department of Geological Sciences, Durham University, South Road, Durham, DH1 3LE, U.K.
Sr-Nd-Pb isotope systems have been highly successful in studying crust-mantle evolution and constraining magma source regions. When applied to ultramafic xenoliths, these systems have revealed the complex, ancient histories of continental lithospheric mantle (CLM) roots. Diamond inclusions have been used as more robust, closed system recorders of lithospheric chronology although recent studies have suggest a re-evaluation of the information provided by sub-calcic garnet inclusions. With the possible exception of Pb, the incompatible element isotope systems appear to record enrichment events that post-dated lithosphere formation. These processes generate Sr-Nd-Pb characteristics in much of the CLM that overlap those of the crust, limiting our ability to assess the role of CLM in magma genesis.
The Re-Os isotope system offers a means of determining lithosphere formation ages due the large parent-daughter isotope fractionation induced during mantle melting and its lower vulnerability to metasomatic disturbance. In addition, distinct differences between the Os isotope composition of CLM and crustal reservoirs makes it a powerful tracer of magma source regions.
Os data for peridotite xenoliths from the Kaapvaal, Siberian, Tanzanian and Wyoming cratons require their evolution as low Re/Os reservoirs from the Mid- to Late-Archaean. The oldest ages coincide with the main crust-building episodes of the cratonic cores. Diamondiferous peridotite and eclogite xenoliths from the base of the Siberian CLM are of Mid-Archaean age and indicate the early establishment of a thick keel beneath this craton. Peridotites from the margin of the Superior craton have minimum ages of 2.7 Ga, congruent with the oldest crust in the Abitibi area. Xenoliths erupted through the Namibian Early Proterozoic crust surrounding the Kaapvaal crust have Re depletion model ages of 2.1 Ga, synchronous with the age of the oldest basement. This data shows long term crust mantle coupling on and around cratons, despite the impact of major plumes. The impetus for CLM delamination beneath parts of some cratons is not obvious but perhaps related to large-scale magma infiltration.
The CLM has been implicated as the source for Group II kimberlites and lamproites on the basis of isotope and trace element data. Os isotopic data offer a different perspective. When "contamination" by dissaggregated lithospheric xenoliths is accounted for, the Os isotope characteristics of Group I and Group II kimberlites show substantial overlap and their sources cannot easily be distinguished from those of OIB's. These characteristics are difficult to reconcile with the notion that kimberlites are direct melts of depleted lithospheric mantle unless their xenolith inventory is not representative. Parental kimberlitic magmas of Group I and Group II may originate from the convecting mantle, and acquire their varying, distinct lithospheric inputs whilst traversing the lithosphere. Other alternatives include a source within metasomatised, subducted oceanic lithosphere beneath the craton. In contrast, certain lamproites contain a component with high time-integrated Re/Os that may originate from melted phlogopite-pyroxenite veins within the CLM.
Os isotopic characterisation of the CLM allows assessment of mass balance models predicting the mass flux associated with lithospheric delamination, if it is invoked to explain the isotope systematics of the OIB reservoir. Such models require large mass fluxes of CLM to be recycled over the history of the Earth. The observed longevity of much of the isotopically extreme CLM (the most desirable material for creating isotopic heterogeneity in the OIB source) is not in agreement with such models.
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