Ca Isotope Variations in Oceanic Basalts

Harvard University, Cambridge MA

Investigators

Abstract

Ca Isotope Variations in Oceanic Basalts Intellectual Merit. Preliminary work demonstrates that measurable mass dependent 44Ca/40Ca isotopic variations Hawaiian basaltic lavas can be attributed to the presence of recycled ancient carbonates in the underlying mantle. It is proposed to extend this work to other oceanic islands and other tectonic settings to determine if Hawaii is a unique case or if recycling of ancient carbonates is a more general process in generating mantle heterogeneity? It is predicted that such a process would significantly affect the Ca and Sr, as well as C, budgets of the Earth?s mantle. Radiogenic isotope geochemistry (Rb­Sr, Sm-Nd, U-Th­Pb, Lu­Hf, and noble gas systems) provides compelling evidence that widespread mantle heterogeneity exists, and can be described in terms of apparent isotopic end­member compositions (e.g., EM1, EM2, HIMU and DMM). It is proposed to determine Ca isotope compositions of representative oceanic basalts to see how this parameter varies with the other isotopically determined end­member variants, and to further constrain the importance of carbonate recycling. Specifically, the new measurements will be used to [1] test current hypothesis that mass dependent 44Ca/40Ca variations are due to carbonate recycling, [2] investigate if non-mass dependent 44Ca/40Ca variations, due to 40K decay, are present in samples with very high 87Sr/86Sr ratios and [3] quantify fluxes of recycled materials that may have caused the variations. Processes that could account for mass dependent 44Ca/40Ca variations incluse: [1] carbonate recycling, [2] altered oceanic crust subduction, [3] recycling of plagioclase­rich gabbroic crust, and [4] igneous fractionation (diffusion, partial meting and fractional crystallization). A research plan is designed to help resolve the following questions: [1] Is carbonate recycling a common feature in oceanic basalts with high 87Sr/86Sr or is it limited to rare and unique sites like Hawaii? [2] Are Ca isotope data consistent with upper crustal recycling proposed to explain very high 87Sr/86Sr ratios in Samoan lavas? This research aims to establish a Ca isotope constraint on the magnitude of upper crustal recycling in mantle sources of basalts and also to test the influence of garnet and plagioclase fractionation on isotopic variations in oceanic basalts. Broader Impacts. T the proposed research facilitates for the first time extensive application of Ca isotopes to the field of mantle geochemistry. This proposal involves an early career research scientist (Huang) at Harvard University, and will educate and train graduate and undergraduate students in application of state­of­the-art analytical techniques to geosciences. Part of the proposed study will evolve into the senior thesis of an undergraduate student. The proposed research will leverage previous NSF investments by using samples recovered from previous NSF funded programs, such as ODP. The scientific results from this research will be presented in an educational website, which will be used in undergraduate and graduate education. As part of the proposed study, a new silicate standard will be prepared for intercalibration of future Ca isotopic work. Finally, results of the proposed study will be made available for outreach education via a project web page and an exhibition setup.