Collaborative Research: The four-dimensional distribution of magmatism during the growth of lower oceanic crust: High precision U-Pb dating of IODP Hole U1473A, Atlantis Bank, SWIR

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

Although ocean basins make up over 70% of the Earth's surface, our knowledge of how the lower portions of the underlying ocean crust evolves over time, thickening and cooling after originating as erupting lavas in mid-ocean ridges or via thrust faulting and roll over in oceanic core complexes is poorly known. Because this material makes up the bulk of the down-going portion of the crust consumed into the mantle in subduction zones, which are the loci of the vast majority of the world's earthquakes and volcanoes, understanding the structure, composition, and evolution of this part of the crustal lithosphere is important for both scientific knowledge and disaster risk analysis. To learn more about this part of the ocean crust, a major community-driven effort to drill deep bore holes into magmatic rocks on the seafloor by the International Ocean Discovery Program, an effort involving more than 30 scientists from around the world, was carried out in 2015-2016 on the Southwest Indian Ridge at a location called the Atlantis Bank. Goals of this research are to determine the size and shape and timing of the intrusive bodies that make up the lower ocean crust, their spatial sequence, and their dimensions. Samples come from the core drilled on the recent ocean drilling leg. Samples from a previously drilled core in the general vicinity will also be analyzed and compared to those from the new hole to get an idea of the spatial continuity of the igneous intrusive bodies. To accomplish research goals, high-precision Uranium-Lead (U-Pb) dating of zircons, a mineral resistant to alteration once it is formed, will be used to provide insights into the time and space distribution of magmatism during lower crustal accretion in the ocean basins. As shown by other recent studies, these data permit the recognition of individual intrusive events that can provide constraints on the constructional dimensions and history of the lower oceanic crust and indicate the cooling rates of the intrusive bodies. Over 90 samples from the 789 meter-long core drilled into the Atlantis Bank will be analyzed using isotope-dilution thermal ionization mass spectrometry. Additional isotopic work on Hafnium in the zircons will be carried out at a sister institution in Idaho (Boise State University). Broader impacts of the work are multi-faceted and include support of two faculty and one researcher at institutions (University of Wyoming and Boise State University) in two EPSCoR states (Wyoming and Idaho, respectively) where EPSCoR indicates a state in which there is no significant federal spending compared to other states. Additional impacts include graduate student training, one of whom is an exchange student from China and involved at no cost to this award, middle and high school outreach in both Wyoming and California, and the development of new software for the integration of U-PB and trace element data. The study is also part of the large, multi-national Ocean Drilling Program and involves significant international collaboration.