Collaborative Research: Redox Ratios in Amphiboles as Proxies for Volatile Budgets in Igneous Systems

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Iron is the only element with multiple valence states (iron metal or Fe0, ferrous iron or Fe2+, and ferric iron or Fe3+) that is a major constituent in rock-forming minerals. It has long been a goal of analytical geochemists to develop methods to analyze the amounts of each of these individually, because their relative abundances record how oxygen evolved in the systems from which the minerals formed. Hydrogen is also an important element in understanding magma evolution. This project will develop methodology to make such measurements on one important mineral group: amphiboles using two types of spectroscopy. X-ray absorption spectroscopy will measure iron valence state, and Raman spectroscopy will measure both iron valence state and the amount of hydrogen present. This work is important because it will enable geochemists to trace how hydrogen and the different valence states of iron behave in magmas as amphiboles crystallize. It will also support undergraduate and graduate student researchers by providing hands-on laboratory training, contributing to workforce development and graduate school preparation.This project will undertake four interrelated tasks aimed at creating and applying a calibration for hydrogen, ferric, and ferrous iron in amphibole minerals using Raman and x-ray absorption spectroscopies.1. The team will create an amphibole calibration for microanalysis of ferric iron using x-ray absorption spectroscopy (XAS). Development of techniques for microanalysis of Fe3+/Fe2+ remains a high priority for in situ analyses of geological samples in standard thin sections. The need is particularly acute for amphiboles, as it is a dominant silicate host for ferric iron in igneous and metamorphic rocks. Calibration of this technique requires access to dozens of amphibole samples with known Fe3+ and H contents and time-consuming analyses of oriented single crystals. 2. The team will create a Raman spectral library of the same well-characterized samples for use in interpreting and potentially deriving Fe3+ and H contents. Recent work by a group at the University of Hamburg suggests that both Fe3+ and H may be determined from Raman spectra of amphiboles. Testing this work and establishing robust Raman calibrations will enable the use of Raman scattering as a way to probe both the ferric iron and hydrogen content of amphiboles, and could be applied to an extremely diverse set of amphibole data collected both in the lab and in the field. It will also fill in the sparse amphibole single-crystal data in the existing RRUFF database with powder data increasing the viability of the database. 3. The team will characterize the partitioning of Fe3+ and H between amphibole and melt in controlled experimental conditions. Measuring the ferric iron and H contents in amphiboles will provide immense geologic value only if they can account for the intensive and extensive variables that control the geochemical partitioning between melt and crystal, and the dehydrogenation of amphibole. Amphibole synthesis experiments at controlled P, T, XH2O and fO2 will be conducted. The synthetic amphiboles, glasses, and associated minerals will be analyzed for hydrogen and iron partitioning behavior. By conducting experiments with a range of starting compositions and oxygen fugacities, they will build a database that can be applied to natural amphiboles. 4. The team will explore the effect of Fe3+ on partitioning and geobarometers involving amphibole using the Shiveluch volcano super-hydrous magmas as a case study. Using the calibrations from the above three tasks, they will be able to better constrain the P-T- fO2 evolution of amphiboles from Shiveluch volcano, the most explosive volcano in the world during the Holocene.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.