Collaborative Research: Thin Crust Over the Marion Rise: Remelting the Gondwanan Mantle

Florida State University, Tallahassee FL

Investigators

Abstract

It has long been assumed that the Earth consists of a thin, outer, silica-rich, hardened crust overlying a thick layer of silica-poor, magnesium-rich, mantle rock known as peridotite and an inner, nickel-iron core. Compared to Earth's ~40 kilometer thick continental crust, ocean crust is generally considered to be relatively thin (i.e., 6-7 km thick). One of the most exciting discoveries in ocean sciences over the last 15 years has been the discovery that parts of the seafloor do not have normal ocean crust, but rather Earth's mantle is exposed directly on the seafloor over large regions of the Arctic, Indian, and Atlantic Oceans. Just how much of the seafloor is exposed mantle not presently known, although estimates have been made that predict up to 25%. This research comprises the US portion of a two-ship, US-German, collaborative project designed to map, collect gravity and magnetic geophysical data, and sample a large areas of the seafloor along the Southwest Indian Ridge in the western Indian Ocean to determine how much mantle rock is exposed there. Geophysical data and geochemical analysis of major and trace elements and various isotopes will be carried out post-cruise at shore-based laboratories. The results of this work, combined with ongoing French studies on the eastern portion of the southwest Indian Ridge will allow, for the first time, an accurate estimate of how much mantle is exposed along an entire mid-ocean ridge. More than 80% of this region has never been mapped and sampling has been largely restricted to a few sections of a narrow 3-mile-wide rift valley that forms the southwest Indian Ridge mid-ocean ridge spreading center. This work is important because mantle rock is very unstable at the Earth's surface, particularly on the seafloor where it is exposed to and extensively reacts with seawater. These reactions produce hydrogen and methane, which, in turn, provide energy for bacterial life in the deep sea and support what could be an extensive biomass below the seafloor that is presently not accounted for in the inventory of life supported by our planet. The reactions between seawater and mantle rock also potentially sequester carbon in the form of carbonate minerals that form by the removal of CO2 from seawater, thereby affecting global and atmospheric carbon budgets at different time scales. Broader impacts of the work include training of graduate students at three institutions, support of an early career scientist from a gender under-represented in the sciences, outreach to elementary and high schools, and public outreach. This project takes to sea a high school teacher and a scientific blogger, who will conduct live interaction sessions with students during the oceanographic expedition and who will prepare age-appropriate educational materials and radio documentaries of the research and oceanographic cruise. There is also a significant component of international collaboration with German, Italian, and Chinese scientists some of whom will participate in the expedition and interact closely with the students and US faculty, further building international relations with these scientific communities. This research consists of an oceanographic expedition to the crest of the Marion Rise on the Southwest Indian Ridge to test the hypothesis that that the Marion Rise is supported by lateral mantle heterogeneity produced by the recycling different Gondwanan mantle provinces beneath the modern ocean ridge, as opposed to a thermal anomaly due to a mantle plume. This is the first leg of a two-leg US-German-Chinese international program to study the Marion platform, a little studied part of the seafloor, and its origin. The cruise will use multibeam sonar to map the seafloor in the area, will dredge rocks for later shore-based laboratory analysis, collect gravity and magnetic geophysical data, and carry out geochemical analyses of select samples. Sea surface magnetics will be used to locate central magnetic anomalies to identify spreading centers and determine spreading rate asymmetries. Regions of a amagmatic seafloor spreading will be determined by their weak magnetic signal and lack of easily definable magnetic lineations. Gravity surveys will allow calculation of residual mantle Bouguer anomalies from which magmatically robust regions can be identified on the basis of their characteristic negative lows. In addition to on-axis work, the expedition will include extensive off-axis dredging to help delineate mantle domains and major magmatic centers. Post-cruise research includes petrographic and geochemical analysis of collected seafloor basalts and peridotites to determine the nature and origin of the mantle source in the Marion Rise area. Chinese collaborators will conduct major and trace element analyses of dredged rocks and US laboratories will analyze the isotopes and isotopic ratios of Hf, and Os as well as radiogenic isotopes of Sr, Nd, and Pb. Calculation of the mantle using geochemically determined mantle density gradients will estimate the extent of serpentinization in the exposed mantle sections. This, together with processed magnetization data, lithologic analysis of collected rock samples, and multi-channel sonar bathymetry will be used to construct a geologic map of the area to determine the tectonic evolution and crustal architecture of the Platform. This work fills a significant sampling gap on mid-ocean ridges and will enhance significantly our understanding of global mantle variability as well as provide new insights into the nature of shallow mantle convection and on the origin of mantle hotspots. 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.