Ridge magmatism is by volume the most important chemical differentiation process on the planet. Mapping the chemical and mineralogical heterogeneities in rocks representative of the mantle (i.e., peridotites) and linking them to the lavas erupted on the seafloor (i.e., basalts) provides essential information on Earth's heat budget and helps to constrain mantle convection models. This research carries out geochemical studies of peridotites collected from the Gakkel mid-ocean ridge spreading center on the Arctic seafloor by the 2001 AMORE expedition. This expedition returned a treasure trove of peridotites that is unsurpassed in its extent and preservation. With these rocks, questions fundamental to our understanding of mid-ocean ridge processes can now be answered. Goals of this research include determining compositional variations of the peridotites because this tells us about the structure and heterogeneity of the mantle, examining if melt depletions observed on a local scale are related to the current melting process or represent previous melting events, and determining the age distribution of the sub-ridge mantle that enters the melting zone. Major and trace element analyses, petrography, and neodymium and osmium isotope systematics will be used to help answer these questions. Broader impacts of the work include training students and including middle and high school students and their teachers in the research.
This project was to evaluate the isotopic composition of mantle rocks that represent the residues of partial melting of the mantle to create mid-ocean ridge basalt, the most abundant magma type on earth. The overarching goal of the project is to determine if the primitive mantle source for mid-ocean ridge basalt has a uniform composition, or whether it varies significantly in composition beneath the Earth's crust, and if it does, at what scales does this occur? Our work shows that the composition of the upper mantle upwelling at mid-ocean ridges during seafloor spreading is not uniform, but is very heterogeneous, locally at the scale of kilometers, and regionally over the scale of thousands of kilometers around the world. A surprising result is that the composition of mid-ocean ridge basalt does not reflect the full compositional variability of the underlying mantle, which in terms of the element neodynium extends to isotopic compositions radiogenic than seen in the basalts erupted from the mantle. This means that mixing of magmas produced by mantle melting of different regions, dilutes out the extreme isotopic signatures. This work then shows that the mantle is more complex in its composition, reflecting a broader range of events in Earth history than scienists previously suspected. Our work is basic research into the planetary evolution of the Earth since its formation over four billion years ago. And thus contributes to our fundamental understanding of the universe in which mankind exists. In terms of a practical application, beyond understanding how our planet evolved and formed, this work will play a role in evaluating Earth resources, such as the nature and distribution of seafloor ore deposits, and the chemical exchanges that occur between the Earth's oceans, crust, and deep interior. As the mantle is the source of magmas coming out of the Earth, and as the latter are the source for metals that are concentrated by hydrothermal activy in the oceans and continents that created many ore deposits, understanding how the mantle varies in composition, will allow geologists to better evaluate the potential of different regions of the Earth for such deposits - particularly in such poorly explored places as the ocean crust, which extends over some 3/5ths of the planet. In the past, the ocean crust and the mantle were viewed as very uniform in composition, and preliminary research suggested that there was little potential for economically significant ore deposits. Now after decades of research such as that funded by this project, scientists realize that this is not the case. The ocean crust is highly variable in composition, and thickness, and the source mantle itself is similarly variable. As it turns out, there are potential ore deposits in the oceans, but these vary considerably in their content of valuable minerals. Thus, understanding how the crust and mantle vary with tectonic setting along the global ocean ridge system is of considerable interest to many countries, with China, India, Russia and Japan beginning to stake out areas in the oceans where there may be potentially valuable ore deposits and resources.
