"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)."
Intellectual Merit: Kimberlites are the most deeply derived terrestrial magmas and have brought diamonds to the surface that contain minerals believed to originate in the transition zone or lower mantle. The causes and sources of kimberlite magmatism remain hotly debated, as does the nature of interaction of protokimberlite melts with the lithosphere through which it erupts. Protokimberlite melts are those generated beneath or near the base of thick continental lithosphere, presumably in response to local or regional mantle upwelling, which, upon reaction with and assimilation of lithospheric peridotite, erupt as kimberlite. We propose to better characterize the sources and early evolution of protokimberlite magmas through the measurement of Pb-Sr-Nd-Hf isotopic and trace element compositions of kimberlite-hosted megacrysts from five localities in southern Africa. Megacrysts from kimberlite are thought to represent products of melt-rock reaction between ascending protokimberlite melts and the deep continental lithospheric mantle. As such, they provide information on both the deep source(s) of kimberlites and the nature of lithospheric mantle wallrock with which they react and assimilate. Isotope data that best constrain this interaction are rare and no megacrysts have yet been characterized for Sr, Nd, Pb and Hf isotope ratios. We will analyze megacrysts from five kimberlite localities, chosen on the basis of tectonic setting, kimberlite type, and megacryst compositional range representing localities on both Archean (Kaapvaal and Zimbabwe cratons) and Proterozoic lithosphere. Analysis will focus on clinopyroxene because it crystallizes across a wide range of megacryst formation conditions and contains significant quantities of Sr, Nd, Pb and Hf. We also intend to measure lithophile trace elements to aid in selecting samples for isotopic analysis. All trace element and isotope measurements and parent-daughter element concentration measurements (via isotope dilution) will be performed using SIMS and multicollector ICPMS facilities at Arizona State University. Through these analyses, we will compile the largest isotopic data set yet available for southern African megacrysts, which will be used to test quantitative models of protokimberlite melt evolution and melt-lithosphere interaction, resolve outstanding questions of megacryst-kimberlite relationships, and constrain the possible sources of protokimberlite melts.
Broader Impacts; Kimberlites are very important economically, especially to some developing countries, as the primary source of economic diamond deposits. Models of kimberlite-lithosphere interaction, and the interpretations of lithospheric structure derived from them can impact the search for such deposits. Scientifically, kimberlites are important for the unique direct information that their xenoliths provide on the composition of the upper mantle, for the clues to behavior of mantle volatiles that can impact global climate, and for their potential connection to large scale features in global mantle dynamics (e.g., the African "superplume"). The results generated in this proposal will improve our understanding of these connections, because they will be integrated in a broad collaborative international research program ("TopoAfrica" funded by ANR France) on the connection between deep mantle dynamics and the evolution of surface topography, sedimentary basins, climate and biodiversity on the African continent. It will also be integrated with our ongoing international research collaboration on connections between diamonds and early kimberlite evolution (funded by US-Israel BSF). At ASU the work will contribute to interaction between research laboratories, building capacity for interactive research and education. The results of this work will be disseminated to K-12 science educators as part of on-site (at ASU) and distance learning presentations on volcanism and the Earth's mantle, as well as a one-day field experience for science educators, run under the auspices of ASU's Mars Education program. The field trip will take 24 educators on a guided exploration of the world-class mantle xenolith locality at San Carlos, in southeastern Arizona.
