Intellectual Merit: For over a quarter century the ocean crust has been though to be a simple layered structure comprised of lavas overlying "sheeted dikes", the frozen remains of magma transported along the cracks that form beneath the lavas as the earth tears apart, and finally a thick layer of coarse gabbro rock crystallized in magma chambers beneath the seafloor volcanoes. The goals of this research are to integrate data from detailed rock descriptions of rocks from a new class of mid-ocean ridge spreading center with available local bathymetry from side scan sonar and seafloor video as well as with sea surface magnetics and gravity for three densely sampled regions of the Gakkel and Southwest Indian Ridges (SWIR). The resulting synthesis will draw together, for the first time, a picture of the geology and crustal architecture of representative tectonic end-members for ultraslow spreading centers ridges. The work is important because this relatively recently discovered class of mid-ocean ridge spreading centers, whose geology is presently poorly known, comprise about 30% of the global ridge system.
Broader Impacts: The broader impacts of this work will increase the infrastructure for seafloor science by generating a well-documented readily accessible database for samples from ultraslow spreading ridges that will enable additional investigators to identify and obtain key material for new research. It will also train undergraduate geology majors in the identification of seafloor rocks and data from these will form the basis of their senior theses. An innovative laboratory for graduate students in rock identification, description, and integration of seafloor samples into marine geophysical studies will also be developed. The research also forms the basis for public talks on the ocean crust at local schools and at the Woods Hole Oceanographic Institution's public lecture series "Science Made Public" at its exhibit center.
This project investigated the relationship between plate tectonics and the composition of the ocean crust and mantle. It involved detailed examination of the composition of upper mantle and lower crustal rocks exposed tectonically at the Mid-Atlantic Ridge and the Southwest Indian ridge, and their relationship to seafloor spreading rate, and tectonic environment. Two main projects were undertaken and the results published in major scientific journals. The first was an investigation of lower crustal and mantle rocks exposed by faulting in the western rift mountains of the Mid-Atlantic Ridge over 150 km of the seafloor at 23°N. These represent the plutonic foundation of the ocean crust where the uppermost crust of lava flows and dikes were tectonically removed by faulting. The work added significantly to our understanding of the composition and architecture of the ocean crust. This is important societally, as ocean crust formed at the mid-ocean ridges represents three fifths of the the crust, and understanding its formation and composition is needed in order to understand such fundamental issues as carbon sequestration, potential Earth resources, and geologic hazards. We found that the lower ocean crust at 23°N does not consist of a uniform layer of basaltic rock sitting on top of lower crust made of coarse crystalline gabbroic rock (the remains of magma chambers). Instead, at the center of the field area, the crust once consisted only of lava flows and dikes erupted directly over the mantle, while flanking magmatic centers formed on the flanks of the ancient ridge segment near the a fracture zone to the north, and the end of the paleo-ridge segment to the south. Later, magmatism shifted more to the segment center. This proved for the first time that while melt flowing out of the mantle focuses to a point to create local magmatic centers and volcanoes, these points are not fixed in time, but change every few hundred thousand years. Our work also showed that magmas coming out of the mantle pool interstially at the base of the crust, producing a variety of new rock types by reaction, and that these rocks, consisting of magmatically altered mantle, are later tectonically incorporated into the crust. It had been long assummed that the composition of the ocean crust was that of the basaltic magma erupting out of the mantle. Our work shows that instead, the crust is a mixture of such magma,and hybridized mantle rock. The second major project undertaken under this grant was investigation of mantle rocks exposed along the full length of the SW Indian Ridge. This work is submitted for publication to a major scientific journal. It shows that when the ancient Gondwana Supercontinent broke up some 185 million years ago, that the asthenosphere from beneath southern Africa was advected to form the shallow mantle beneath the southern oceans, where today much of it is tectonically exposed along the SW Indian Ridge. As a consequence of the shallow mantle having been depleted during the formation of the Karoo basalts 185 million years ago, remelting it beneath the modern SW Indian Ridge produces little new magma to erupt along the ridge as the seafloor spreads. Thus the crust is very thin and sporadic, reflecting an ancient mantle event beneath southern Africa and the formation of a ancient large igneous province - the remnants of which can be found scattered across southern Africa, and Australia an Antarctic which were once joined. The large oceanic rise, where from around 67°E to north of Marion Island at 33E where the SW Indian Ridge rises from ~5 km depth to less than a kilometer, then appears to be largely an isostatic response the formation of highly depleted light mantle originally emplaced beneath southern Africa during the Karoo event. Beyond a better understanding of the nature of the ocean crust and mantle beneath the oceans, our work included education impacts. These included mentoring students and younger faculty working on the ocean cust and mantle, as well as educating middle shool students by giving presentations at local schools. In addition, a graduate student from Italy worked with the P.I. on lower crustal and mantle rocks from a back-arc basom o tje Philippine Sea south of Japan. The student, on a 4 month visit to Woods Hole Oceanographic Institution investigated melt-rock reaction processes that have affected the lower crust. His work showed that melt-rock reaction is a major process that is produced by the interaction of melts passing through the shallow mantle and lower ocean crust that shapes the composition of mid-ocean ridge in unexpected ways. This work constituted a major chapter in his PhD Thesis, and according to him, has strongly influenced him to work in mantle geodynamics and marine geiology as a focus of his future career.
