This interdisciplinary project examined the interactions between water content, deformation, and lower crustal rheology to constrain the stability of the deep crust in subduction zones.  Specifically, a type of rock called ?arclogite? is of interest. Arclogites are rich in garnet and clinopyroxene, and are thought to represent cumulates ? left-behind products ? of magma that passes from the mantle through the crust in subduction zones.  Owing to the high amount of garnet present, arclogites are very dense and have the potential to founder back into the mantle, which is lower density.  Removal of dense, magnesium-rich arclogite modifies the composition of the bulk continental crust over Earth?s history, gradually refining the crust to a low-density, silica-rich composition. 

However, important details about how arclogitic roots might become unstable and founder back into the mantle are lacking.  In particular, trace amounts of water in nominally anhydrous minerals (e.g., clinopyroxene and garnet) can have profound effect on physical properties of rocks; for instance, lowering the viscosity and thereby promoting deformation and foundering.  In addition, whether arclogite rocks show evidence of being deformed (e.g., have a fabric) is unclear. To address the water and fabric issues, two graduate students were directly involved and mentored throughout the entire lifetime of this award.  The students were trained in state-of-the-art geochemical techniques, particularly the use of secondary ion mass spectrometry (SIMS) to analyze trace amounts of water in nominally anhydrous minerals in the arclogites.  In addition, the students learned how to evaluate the degree of deformation of the arclogites by using electron backscatter diffraction, which determines the orientation of mineral grains in a sample and can help determine the type and extent of deformation that occurred. 

The arclogites examined included samples from northern Colorado, Arizona, and Kohistan, Pakistan ? all areas which were once active subduction zones with significant volcanic and magmatic activity.  Our water content analyses showed that despite previous assumptions that the lower crust is generally ?bone dry?, some minerals, particularly pyroxene, remarkably preserve moderate to high water contents, indicating that these arclogites were products left behind from relatively water-rich magmas, consistent with what we know about volatile-rich explosive arc magmatism at the surface.  In addition, EBSD data from the Arizona samples show variable fabric development as a function of depth, indicating the arc root preserved deformation as it was being removed.  Our findings have been published in two peer-reviewed articles to date (Chin et al., Journal of Geophysical Research Solid Earth, 2020; Urann et al., Nature Geoscience, 2022).  In addition, a paper is currently in review at Tectonics, and is first-authored by one of the graduate student mentees.

This award resulted in significant broader impacts.  PI Chin was a pre-tenure faculty member at Scripps Institution of Oceanography, and has since received tenure while carrying out the tasks outlined in the proposal.  Chin mentored two graduate students, one of whom has already qualified as a PhD candidate and will continue to work on aspects of this proposal until degree completion.  The students presented their work at national conferences.  The students gained expertise in laboratory techniques including SIMS and EBSD, both trained by Chin.  These  analytical skills will be important as they enter the job market, whether in academia or industry.        

Last Modified: 11/08/2022
Modified by: Emily J Chin