Fe3+/FeT Ratios in Amphiboles - A New Tool for Understanding the Redox State of Arc Magmas

California Institute Of Technology, Pasadena CA

Investigators

Abstract

Subduction zones, where one tectonic plate descends beneath another, are fundamental the formation of continental crust, the terra firma upon which humans live, and the generation of important ore deposits. A critical aspect of both of these processes is the exchange of material between the surface and deeper parts of the Earth. Magmas formed at subduction zones reflect this transfer of material through distinctive chemistry suggesting surface-derived material in their source region. One salient example of this is the elevated redox state of subduction zone volcanic rocks, as compared to mid-ocean ridge basalts, which is generally thought to arise from the subduction of oxidized surface material. Although the redox state of volcanic rocks can be assessed through more established methods, the redox state of subduction zone plutonic rocks can be more difficult to assess due to slower cooling rates and lack of appropriate mineral assemblages. This work will expand our knowledge of the redox state of arc magmas through development of a detailed understanding of Fe redox state in a ubiquitous mineral in arc magmas, amphibole, and application of this understanding to a suite of subduction zone plutonic rocks. This work will provide a new tool for the community to implement in the study of subduction zone redox. In addition to scientific contributions, this work will support the scientific training of a female post-doctoral scholar and summer undergraduate researcher at Caltech. Volcanic rocks erupted in subduction zone settings are generally more oxidized than those from mid-ocean ridges. The cause of elevated magmatic redox state in arc environments is controversial but commonly attributed to one or more of the following: (a) source processes: sub-arc mantle oxidation via melts/fluids carrying oxidized species from the slab; (b) crustal differentiation processes: assimilation and fractionation during storage in the crust; or (c) eruption and shallow level processes: degassing during ascent and eruption. Most studies on in arc rocks focus on volcanic rocks, which may have experienced some or all of the above processes. Consequently, volcanic rocks are difficult geological records from which to untangle the effects of various oxidizing mechanisms. To understand the processes responsible for the oxidized nature of arc magmas, this research focuses on temporally and genetically related arc plutonic rocks from different crustal depths using Fe valence state in amphibole. The research objectives are three-fold: (1) synthesis of amphibole grains under varying oxygen fugacities using high-pressure and temperature piston-cylinder experiments; (2) development of a new workflow to quantify both Fe valence state and accommodation mechanism of ferric iron in amphibole via in-situ, high-resolution synchrotron M?ssbauer spectroscopy (SMS); and (3) application of the results of part 1 and 2 to igneous rocks from different crustal depths in the accreted Talkeetna arc (Alaska). Although Fe speciation in amphibole has been characterized previously, primarily through bulk techniques (e.g., wet chemistry), SMS affords high spatial resolution and precision which is critical in determining amphibole Fe speciation at the sub-grain scale, as these minerals are often spatially zoned, can exhibit sub-solidus alteration along rims and fractures, and contain inclusions. 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.