Layered Intrusion-Footwall Hydrothermal Interactions - An Analog For Subduction Zone Hydrothermal Sytems

Duke University, Durham NC

Investigators

Abstract

Subduction zones are formed where lithospheric plates that make up the rigid, outer shell of the Earth are recycled back into the Earth's mantle. When that happens, the rigid plate is heated and releases water and other volatiles into the overlying mantle. This induces melting in the mantle and expresses itself in explosive surface volcanism, such as those around the Pacific Ocean basin (the so-called 'Ring of Fire'). Volcanism associated with subduction zones is closely linked to the formation of economic mineral deposits. Unfortunately, the nature of fluid movement from the descending slab into the overlying mantle cannot be directly observed. This project will test if large igneous bodies that intruded into the shallow continental crust and dehydrated underlying rocks can serve as proxies to processes operation at depth in subduction zone hydrothermal systems. The study will be centered on the intrusion of the 2.7 billion year old Stillwater Complex in Montana, and which has since been exposed by erosion. Specifically, it is planned to investigate mechanisms (fractures, veins, etc.) by which fluids liberated from the underlying rocks and moved into the overlying igneous rocks and the degree that these were modified during this process. The study has broader applications to the study of the explosive volcanism that occurs at subduction zones as well as understanding the ultimate source of fluids involved in ore deposits in both the subduction zone environment and in unique magmatic intrusions that are the main sources of critical materials for our modern industry. Copper, chromium and the platinum-group elements are examples of critical resources with great importance for the national economy. The funding also will help support advanced training for a graduate student and senior thesis projects for undergraduate students. More specifically, the work entails mapping and characterization of hydrothermal features (high temperature veins and pegmatoidal bodies) of the lower part of the Stillwater Complex and its 2-3 km thick metamorphic aureole. This will include comparing the stable (particularly carbon) and radiogenic (Nd, Sr, and Pb) isotopic composition of veins pegmatoidal bodies in the Stillwater Complex with the surrounding host rock to test the degree country fluids were involved in their formation. The composition of these hydrothermal features will be compared with bulk rock major and trace element composition of the Stillwater Complex up to the Middle Banded series, for which there currently is no comprehensive coverage of the complex. These field data can be used to constrain theoretic models of fluid channelization in both subduction zone and layered intrusion settings. It can also lead to insights toward understanding the transition from magmatic to country fluid migration in three-phase crystal-liquid-gas mushes, and has consequences for ore deposit formation models. 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.