Although oceanic lithosphere covers more than 60% of the earth's surface, models for its formation at mid-ocean ridges have been hampered by a lack of geochronological constraints on intrusive rocks that underlie volcanic rocks erupted on the seafloor. Recent advances in the U-Pb dating of zircons, refractory minerals that are found in gabbors and other intrusive rocks in ocean crust, now allow dates of intrusions to be measured. This research pursues this objective using a combined secondary ion mass spectrometry (SIMS) and thermal ionization mass spectrometry (TIMS) approach. Samples of intrusive gabbros from Hole 735B on the Southwest Indian Ridge from the Ocean Drilling Program will be analyzed. SIMS, which is a high resolution but large uncertainty technique that can measure geochemical differences on a micron scale, will be used to determine the trace element composition of zoning in zircons. High precision isotope dilution TIMS dating of the zircons will be used to determine the magmatic evolution of the samples. Uncertainties in the ages measured are expected to be 12,000 years or less. This precision will provide unprecedented insight into how long it takes for ocean crust to grow at slow spreading ridges. Broader impacts of the work include postdoctoral training and mentoring. It also fosters inter-institutional collaboration and will support two PIs, one of whom is from a gender under-represented in the sciences, from an EPSCoR state. Outreach to high schools in the Cambridge MA area will be carried out and educational materials for them will be created on geochronology. Results of the project will be archived in NSF-funded publicly accessible databases.
The goal of this project was to understand how oceanic crust grows at slow spreading mid-ocean ridges. Oceanic crust covers ~60% of the Earth’s surface. At mid-ocean ridges, new crust forms when two tectonic plates are pulled apart, and magmas from the mantle rise and crystallize. Most geochronology for the oceanic crust comes from lavas exposed on the seafloor, which erupt from deeper magma chambers. These deeper magma chambers also crystallize in-situ within the crust and the resulting plutonic rocks make up a majority of the crust in many areas. It is necessary to understand the temporal and spatial distribution of magmatism during formation of this plutonic crust, to better understand the formation of the oceanic crust. In this study, we used high precision U-Pb geochronology and geochemistry to study the formation of the plutonic oceanic crust along the slow spreading Southwest Indian Ridge. The studied samples come from the deepest drill core into the lower oceanic crust from the Ocean Drilling Program. The drilling penetrated 1.5 km into plutonic rocks that are exposed on the seafloor as a result of faulting. We dated zircon crystals from a total of 21 rock samples from depths in the core of 25–1430 meters below the seafloor. The dates range from 12.71–11.76 million years ago (Ma), with errors as low as ± 14,000 years. The ages of the rocks are progressively younger deeper in the core. Studies of seismic wave propagation below active slow spreading mid-ocean ridges suggest that during times of crustal growth the ridge is underlain by a mush zone, composed of mostly solid, hot rock, and a small volumes of magma. After formation, the new crust is moved away from the ridge axis by movement of the tectonic plates. The U-Pb dates from this study likely reflect crystallization of magmas along the margin of this mush zone. If true, the dates, combined with estimates for the rate of plate motions, suggest that the mush zone where the crust forms is ~3 km wide. The combined geochemistry and U-Pb dates of the zircon crystals constrain how quickly magmas crystallized and cooled as the crust was forming.
