The Importance of the Size and Distribution of Mantle Heterogeneities to the Interpretation of Isotope Ratios and Trace Element Abundances in Basalts

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Oceanic islands such as Hawaii are formed by volcanic eruptions. Magmas that feed the volcanos are produced by partial melting in the interior of the Earth. Oceanic basalts, which are sampled at oceanic islands and along mid-ocean ridges on the seafloor, provide important clues to understanding the composition of the inaccessible Earth and the processes and time scales of magma generation and transport in the mantle. A fundamental observation in geochemistry is that the source regions of oceanic basalts in the mantle are heterogeneous and consist of chemically and lithologically distinct domains of variable size and distribution. Partial melting of such heterogeneous mantle sources gives rise to a diverse range of basaltic magmas erupted on the surface. The size and distribution of mantle heterogeneities are unknown, but may have played an important role in controlling chemical variations observed in basalts. One of the long-standing problems of geochemistry is the mapping of heterogeneities observed in basalts into the chemical and lithological heterogeneities in the mantle source. Such a mapping is nonlinear and its mathematical expression is unknown. The thesis of this project is to investigate how the size and distribution of mantle heterogeneities affect the isotope ratios and trace element abundances in oceanic basalts. Results from this project will enable geochemists to establish the nonlinear mapping between basalts and their mantle sources. The project would advance STEM education by providing research opportunities for undergraduate students and by developing of a competitive STEM workforce through training of graduate students and postdoc scholar. The primary objectives of this proposed study are (1) to find the functional forms for the nonlinear mapping between basalts and their mantle sources; (2) to develop a framework for understanding and interpreting variations in isotope ratios and trace element abundances in spatially and temporally associated oceanic basalts; and (3) to test the new models through selected case studies where temporal or spatial geochemical data are available. To establish the nonlinear mapping, one needs to know the nature of heterogeneity (chemical vs. lithological, size and distribution), the style of melting (batch vs. fractional), the process of melt migration (channelized vs. grain-scale porous flow), and the extent of chemical disequilibrium between residual minerals and the percolating melt. Time and spatial variables will be added to the widely used melting models in the geochemical literature. These time-dependent melting models will enable geochemists to systematically study the roles of the size and distribution of heterogeneities in the mantle source in controlling the variations of isotope ratios and incompatible trace element abundances in extracted melts. Two-dimensional models will be constructed using the one-dimensional time-dependent melting models. These physically more realistic models will be used to examine variations of Sr-Nd-Hf-Pb isotope and selected incompatible trace element ratios in mid-ocean ridge basalts and ocean island basalts. An important outcome of the proposed work is a set of new interpretation tools to petrologists and geochemists in their studies of spatially and temporally correlated mantle and mantle-derived samples.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.