ABSTRACT

Ocean crust, which makes up about 65% of the volume of Earth's surface, is created at mid-ocean ridges, one of the most dynamic geological environments on the planet. Vast quantities of heat are dissipated along these features, facilitating the generation of large, potentially economic, mineral deposits and hydrothermal vents that host unique chemosynthetic ecosystems that live without the need for sunlight. At mid-ocean ridges, melts from the upwelling mantle rise toward the surface and either erupt as lavas on the seafloor or freeze on their way up, resulting in ocean crust that has a thick coarsely crystalline base made of gabbroic and peridotitic rock covered by a veneer of basaltic lava. With the exception of the surface lavas, the lower portions of crust can only be sampled and studied from deep rips and tears in the seafloor caused by structural features or tectonic processes. One of the places where the gabbroic layer is exposed is a place called the Pito Deep in the eastern Pacific Ocean. This research involves a major multidisciplinary oceanographic expedition to the Pito Deep to both sample and study the lower crustal gabbros of fast spread ocean crust and their relation to overlying basalts and underlying crustal components. On the expedition, samples will also be taken to determine the ancient thermal structure at the ridge axis. Broader impacts of the work include support of an institution and faculty in an EPSCoR state (Wyoming); good student training at the college, community college, and K-12 level; and the inclusion of a K-12 teacher on the cruise to produce educational materials for K-12 and communicate with students back on land.

The science being done in this project has the potential to resolve long standing debates about how gabbroic lower crust is accreted at fast spread mid-ocean ridges and will shed light on unresolved questions regarding the depth of hydrothermal circulation in the seafloor, the width and temporal evolution of the axial magma chamber of mid-ocean ridges, and the processes of melt transport and crystallization between the mantle and the seafloor. Samples will be collected using a remotely operated vehicle, with sampling taking place along a ~20-km long flow line at Pito Deep. The nearly ridge-perpendicular exposures at Pito Deep span multiple geomagnetic polarity intervals, providing a unique opportunity to document the shape of the fossil 580°C isotherm in the lower crust and thus to differentiate between the different thermal predictions of lower crustal accretion models. These end-member models also make predictions of the variation of geochemical, petrologic and microstructural parameters with depth. Major and trace element geochemistry combined with petrofabrics from electron backscattered diffraction will be examined for a significant vertical portion (>1km) of the upper gabbroic layer to further discriminate between these accretion models. While no single crustal exposure is likely to definitively resolve the processes by which the gabbroic lower crust is accreted, the exposures at Pito Deep offer the possibility of addressing outstanding questions regarding the depth of hydrothermal heat extraction, the width and temporal evolution of the axial magma chamber, and the processes of melt transport and crystallization.

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