Extreme enriched-mantle (EM) compositions recorded by the Sr-Nd-Hf isotopes of global cratonic lamproites

Abstract

Cratonic lamproites are rare ultrapotassic rocks that occur in every continent and were emplaced over the last 2 billion years. Owing to their highly radiogenic Sr and unradiogenic Nd-Hf isotopic compositions along with enrichment in K and incompatible trace elements (e.g., Ba, Sr, Ti, Zr), lamproites are believed to be formed by melting geochemically enriched regions in the sub-continental lithospheric mantle (SCLM). In contrast, olivine compositions indistinguishable from those of kimberlites and the occurrence of diamonds containing inclusions of majorite-bearing garnet in some cratonic lamproites, suggest that primary lamproitic (or proto-lamproitic) melts may originate in the convecting mantle. Here we re-evaluate the different source component(s) responsible for the genesis of cratonic lamproites worldwide using major-, trace-element and radiogenic isotope geochemistry.

We report new major-, trace-element and Sr, Nd and Hf isotope compositions for 61 cratonic lamproite samples from sixteen cratons, with an emphasis on localities lacking Hf isotope data. These data, combined with published results, reveal three discernible end-member compositions in Sr-Nd-Hf isotope space. The first end-member includes lamproites with the least geochemically-enriched isotopic signatures (i.e. less radiogenic Sr, less negative and locally positive ɛNd and ɛHf values) (e.g., Wajrakarur and Bunder in India, Melville in Canada), similar to the Bulk Silicate Earth (BSE), which overlap with those of global ocean island basalts (OIBs) and archetypal kimberlites. The second end-member is represented by the Cenozoic West Kimberley lamproites (Australia), which exhibit the highest ɛSr values with low ɛNd and ɛHf compositions and resemble EM II OIBs from Samoa. The third end-member, defined by lamproites from Leucite Hills and Smoky Butte (USA), exhibits strongly negative ɛNd and ɛHf values, associated with moderately negative ɛSr compositions similar to, although more extreme than, EM I OIBs. The remaining lamproites included in this study have intermediate isotopic compositions that fall between these three end-members. The similar Sr-Nd-Hf compositions of the least geochemically enriched lamproites, OIBs and kimberlites suggest a predominant input from the convective mantle for these lamproites. Conversely, the lamproites with more geochemically enriched Sr-Nd-Hf isotopic compositions (i.e. more radiogenic Sr, more negative ɛNd and ɛHf values) necessitate extreme source compositions that, locally, cannot be accounted for by commonly observed mantle components, such as lithospheric mantle xenoliths dominated by mica (e.g., MARID) or subducted crust/sediment. Theoretical components generated by long-term enrichment of the lithospheric mantle mediated by subduction-related fluids appear to be required to produce mica-bearing peridotites that, with time, develop isotopic compositions similar to those observed in the most geochemically enriched lamproites. The spread in isotopic compositions observed in lamproites from some regions (e.g., Cenozoic West Kimberley) is best explained via assimilation of anciently metasomatised lithospheric mantle peridotites by melts derived from moderately depleted convective mantle sources rather than exclusive melting of lithospheric mantle sources. Differences in the proportions of various end-members, combined with the diverse isotopic compositions of metasomatized SCLM across different regions and at different times are crucial in creating the unique mineralogical and isotopic characteristics documented in lamproites from each province. Thus, cratonic lamproites offer insights into the cratonic lithosphere and its metasomatic evolution.