Southern African Kimberlites and Their Megacrysts: Pb-Sr-Nd-Hf Isotopes Define the Link
Abstract
New Pb-Sr-Nd-Hf isotope data for Cr-poor and Cr-rich clinopyroxene megacrysts from southern African kimberlites strongly support a genetic link between megacrysts and kimberlites and further indicate that the isotopic differences between the two are largely explained by assimilation-fractional crystallization processes that occur as sub-lithospheric melts ascend into the base of the continental lithospheric mantle. Although much of the compositional variation of Cr-poor megacrysts can be explained purely by fractional crystallization, most megacryst evolution trends are at some point deflected toward relatively Cr- and Mg-rich compositions, most likely as a result of assimilation of lithospheric peridotite. Isotopic compositions of samples spaced along these evolution trends show fairly regular variations that indicate mixing between sublithospheric melts with a moderate to strong HIMU affinity and lithospheric peridotite assimilant having relatively EM-type isotope characteristics. It is likely that the isotopic compositions of the more primitive Cr-poor megacrysts closely resemble those of the sublithospheric melts prior to significant interaction with the lithosphere, whereas those of the more Cr-rich megacryst varieties, as well as kimberlite magmas (prior to bulk mixing with lithospheric mantle and crust) represent melts that have undergone a high degree of lithospheric assimilation. The prevalence of the HIMU signature in Cr-poor megacrysts, as in other African alkaline magmas, is consistent with a common source for their parental sublithospheric melts (e.g., the African superplume).
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.T33D2441J
- Keywords:
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- 1025 GEOCHEMISTRY / Composition of the mantle;
- 1040 GEOCHEMISTRY / Radiogenic isotope geochemistry;
- 8415 VOLCANOLOGY / Intra-plate processes;
- 9305 GEOGRAPHIC LOCATION / Africa