Plagioclase Fe X-ray Absorption Spectroscopy: Potential for a New Magmatic Oxybarometer

Session: Advancing Mineralogy and Spectroscopy Across the Solar System in Honor of MSA Roebling Medalist M. Darby Dyar

Presenting Author:

Molly McCanta

Authors:

McCanta, Molly¹, Dyar, M. Darby², Ytsma, Cai³, Haupt, Cordula⁴, Koch, Lennart⁵, Leuthold, Julien⁶, Marxer, Felix⁷, Ridolfi, Filippo⁸, Pimenta Silva, Manuel⁹, Heckmann, Paul¹⁰, Beaudry, Patrick¹¹, Hammer, Julia E.¹², Grove, Timothy L.¹³, Iacono-Marziano, Giada¹⁴

(1) Earth, Environmental, and Planetary Sciences, University of Tennessee, Knoxville, TN, USA, (2) Planetary Science Institute, Freeport, ME, USA, (3) Cai Consulting, Glasgow, United Kingdom; Institute of Health Informatics, University College London, London, United Kingdom, (4) CNRS, University of Orléans, Orléans, France, (5) Leibniz University, Institute of Earth System Sciences, Hannover, Germany, (6) Dpeartment of Earth Sciences, ETH Zürich, Zürich, Switzerland; School of Earth Sciences, University of Bristol, Bristol, United Kingdom, (7) Institute of Earth System Sciences, Leibniz University, Hannover, Germany, (8) Institute of Earth System Sciences, Leibniz University, Hannover, Germany, (9) Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, USA, (10) Department of Geosciences, The Arctic University of Norway, Tromsø, Norway, (11) Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA; Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA, (12) Dept. Earth Sciences, POST701, Honolulu, HI, USA, (13) Dept Earth & Planetary Science, Massachusetts Institute Technology, Cambridge, MA, USA, (14) Institut des Sciences de la Terre d’Orléans, Orléans, France,

Abstract:

Plagioclase is the most common rock-forming mineral in the Earth’s crust and constitutes a significant component by volume of magmas spanning compositions from basaltic to rhyolitic, thereby making it an important repository of information on both pre-eruptive and eruptive conditions. For magmas, it is of great interest to constrain equilibration oxidation state (oxygen fugacity [f_O2]), which plays an essential role in regulating volatile behavior within the melt and thus has direct implications for the eruptibility and explosivity of a volcanic system. Prior work investigated the use of the trace element Eu in plagioclase as an oxybarometer. Here we assess the applicability of Fe in plagioclase as an oxybarometer during crystallization. Fe is a trace element in feldspar and may substitute as Fe³⁺ or Fe²⁺, depending on f_O2.

Plagioclase-bearing experimental charges covering a wide range of starting compositions and f_O2 (relative to the iron-wüstite buffer [ΔIW-2 to +6.5]) were sourced from multiple labs. Samples were analyzed using Fe X-ray Absorption Spectroscopy (XAS) at the Advanced Photon Source (APS) and the National Synchrotron Light Source II (NSLS II) using a ~1um beam diameter. Multiple samples were analyzed at both facilities to determine instrument coherence.

Machine learning (ML) was used to directly predict f_O2 from the collected Fe XAS spectra because plagioclase Fe²⁺/Fe³⁺ is difficult to measure independently. Slight orientation effects were noted when XAS spectra were collected at different angles. As with orientation effects in other minerals such as pyroxene and amphibole, ML algorithms appear able to account for them. Equilibration f_O2 was predicted with an RMSE-CV of better than ΔIW± 1.6 and is expected to improve with additional analyses. Uncertainty is also decreased when the calibration data is centered on the compositional space where Fe²⁺/Fe³⁺ ratios vary most significantly, f_O2 ≥ IW. When only f_O2 >ΔIW=0 data is considered, RMSE-CV improves to ΔIW< ±1.0. ML methods have difficulty accurately predicting f_O2 in feldspars crystallized in experiments below IW because Fe³⁺ concentrations asymptotically approach zero. Overall, the Fe oxidation state in plagioclase provides an accurate record of magmatic f_O2 and this new oxybarometer calibration will allow for redox characterization in a wide range of terrestrial and planetary igneous materials.