"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)."
Intellectual Merit: Several lines of evidence suggest that melt generation and segregation regions in the mantle are heterogeneous and consist of chemically (e.g., enriched vs. depleted) and lithologically (e.g., peridotite vs. pyroxenite) distinct domains of variable size and dimension. Partial melting of such regions give rises to a diverse range of basaltic magma compositions. To preserve geochemical signatures developed at depth, basalts must rise to the surface without extensive re-equilibration with their surrounding mantle wallrocks. This can be accomplished by focused melt flow through high-porosity melt channels. To form high-porosity channels, pyroxenes in the peridotite matrix must dissolve in the percolating melt. The matrix dissolution rate and melt suction rate are important in determining the spatial distribution of the channel and chemical compositions of the melt extracted at the top of the channel and the matrix. During grain-scale porous flow, re-equilibration between crystals and their interstitial melt may be incomplete. Models for trace element fractionation and distribution during melting and melt extraction in the mantle must include these fundamental observations or constraints as part of the formulation. The primary objective of this project is to develop self-consistent models for trace element and isotope fractionation and distribution during concurrent melting, melt migration, and melt-rock interaction in an upwelling, heterogeneous, two-porosity double lithology mantle column. Both steady-state models and time-dependent models will be developed. The steady-state models will be used to constrain the relative melt suction rate using selected incompatible trace element abundances in diopside in abyssal peridotites and peridotites from Oman ophiolite. Laboratory dissolution experiments will be conducted at selected temperatures and pressures to determine the rates of harzburgite and lherzolite dissolution and the depth of high-porosity melt channel initiation along two mantle adiabats: one for melt migration beneath the mid-ocean ridge and the other for melt migration in a mantle plume. Results from these laboratory and geochemical studies will provide critical inputs to the general time-dependent melting models that will allow geochemists to map chemical heterogeneities observed in erupted basalts into their mantle sources, and to infer the processes of melt segregation and melt-rock interaction from trace element abundances in residual peridotites, mid-ocean ridge basalts (MORB) and ocean island basalts (OIB).
Broader Impacts: The origin and distribution of mantle heterogeneity are of central importance to a broad range of Earth and planetary scientists. The dynamic parameters obtained using the new melting models will likely stimulate geodynamists to develop more sophisticated 2-D and 3-D models for melt migration beneath mid-ocean ridges and in mantle plumes. The distribution of high-porosity melt channels in the mantle as inferred from this proposed study will motivate structure geologists to conduct more detailed mapping in ophiolites, geophysicists and seismologists to design new experiments to image the mantle beneath mid-ocean ridges. Results from this study will also be disseminated to a broader audience through public lectures, and undergraduate and graduate courses. And finally, the proposed project will provide hands on experience for undergraduates, research opportunities for senior thesis, experimental, computational, and educational experience for graduate students.
