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 overall objective of this project was to better understand how lower oceanic crust is created at slow spreading mid-ocean ridges. Oceanic crust forms ~60% of the Earth’s surface and slow spreading ridges account for ~50% of the world’s mid-ocean ridges. At mid-ocean ridges tectonic plates are pulled apart, leading to upwelling of the mantle and the generation of melts which rise towards the Earth’s surface and crystallize to form new ocean crust. Most geochronology for the oceanic crust comes from lavas exposed on the seafloor, which erupt from deeper magma chambers. These magma chambers crystallize in-situ within the crust and the resulting plutonic rocks make up a majority (75%) of the crust. However, the details of how the melt is distributed and cools in these magma chambers are poorly understood. Questions like: "what is the shape of the magma chamber and how does it vary with time?" remain unanswered. To answer this question, it is necessary to understand the temporal history and spatial distribution of magmatism during formation of this plutonic lower oceanic crust. In this study, we used high precision U-Pb geochronology and geochemistry to study the crystallization of the plutonic lower oceanic crust at the slow/ultra-slow spreading Southwest Indian Ridge. The studied samples come from two drill cores, one of which (ODP 735B) is the deepest drill core into the lower oceanic crust collected by the Ocean Drilling Program. ODP Hole 735B penetrated 1508m into plutonic rocks that are exposed on the seafloor as a result of denudation by a major detachment fault. We dated zircon crystals from 18 rock samples from 25–1430 meters below the seafloor in ODP Hole 735B and from 3 rock samples from 37m to 157m in ODP Hole 1105A. 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 from ODP Hole 735B, but show no age difference in the less deep ODP Hole 1105A. These dates correspond to crystallization temperatures of 750-900°C and thus represent solidification ages rather than intrusive ages. 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 volume 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 motion, indicate significant lateral and/or temporal variation in the shape of this mush zone and hence of the shape of the magmas chamber. The younging ages with depth in ODP Hole 735B suggest that the mush zone rapidly narrows upward by ~3km over the upper 500m of the plutonic crust, whereas the lack of age variation with depth in ODP Hole 1105A suggests that the mush zone did not narrow upwards when the rocks of this Hole crystallized. Together these results place constraints on the 3-D geometry of the magma chamber that generated the plutonic lower oceanic crust. In addition to dating, we performed a comprehensive textural/geochemical study of zircon in 25 samples from the two ODP Holes. The degree of intra-sample variability in zircon chemistry indicates a complicated magmatic history. Some samples show little chemical variability which may reflect discrete sampling of melt during fractionation, or rapid cooling and crystallization of magma. Other samples show a larger range in zircon chemistry and complex zircon textures (resorbed cores/rims or high uranium rims) indicating more complicated growth histories that could include interaction with multiple melts and/or re-melting. In addition, combining the U-Pb dates with crystallization temperatures determined from the geochemistry places some constraints on the cooling rate of the crust. The project formed part of post-doctoral associate Matt Rioux’s career development and provided training for Masters student Lauren Colwell. Importantly it established both important comparisons and the synergy between the secondary ion mass spectrometry (SIMS) and isotope dilution-thermal ionization mass spectrometry (ID-TIMS) methods. Outreach activities were undertaken at local middle schools.
