Experimental determination of ferric iron partitioning between pyroxene and melt at 100 kPa

Avishek Rudra, Elizabeth Cottrell, Marc M. Hirschmann

Research output: Contribution to journalArticlepeer-review

Abstract

Pyroxene is the principal host of Fe3+ in basalt source regions, hosting 79 and 81% of the Fe3+ in spinel and garnet lherzolite, respectively. In spinel peridotite, orthopyroxene (opx) and clinopyroxene (cpx) host 48% and 31%, respectively, of the total Fe3+. Yet the relationship between mantle mineralogy, pyroxene chemistry, and the oxygen fugacity (fO2) recorded by mantle-derived basalts remains unclear. To better understand partitioning of Fe3+ between pyroxene and melt we conducted experiments at 100 kPa with fO2 controlled by CO-CO2 gas mixes between ∆QFM −1.19 to +2.06 in a system containing andesitic melt saturated with opx or cpx only. To produce large (100–150 μm), homogeneous pyroxenes, we employed a dynamic cooling technique with a 5–10 °C/h cooling rate, and initial and final dwell temperatures 5–10 °C and 20–30 °C super and sub-liquidus, respectively. Resulting pyroxene crystals have absolute variation in Al2O3 and TiO2 < 0.05 wt% and < 0.02 wt%, respectively. Fe3+/FeT in pyroxenes and quenched glass were measured by Fe K-edge XANES. We used a newly developed XANES calibration for cpx and opx by selecting spectra with X-rays vibrating on the optic axial plane at 45 ± 5° to the crystallographic c axis. Values of DFe3+cpx/melt increase from 0.03 to 0.53 as fO2 increases from ∆QFM −0.44 to +2.06, while DFe3+opx/melt remains unchanged at 0.26 between ∆QFM −1.19 to +1.37. In comparison to natural peridotitic pyroxenes, Fe3+/FeT in pyroxenes crystallized in this study are lower at similar fO2, presumably owing to lower Al3+ contents. Comparison to thermodynamic models implemented in pMELTS and Perple_X suggest that these over-predict the stability of Fe3+ in pyroxenes, causing these models to underpredict the fO2 of spinel peridotites under conditions of basalt genesis.

Original languageEnglish (US)
Article number120532
JournalChemical Geology
Volume584
DOIs
StatePublished - Dec 5 2021

Bibliographical note

Funding Information:
AR thanks Jed Mosenfelder and Amanda Dillman for assistance with gas mixing furnace experiments; Anette von der Handt for helping with electron microprobe analysis; Nick Seaton for assistance with EBSD data collection and Zachary Michels for helping with processing the EBSD data. AR is thankful to Tony Lanzirotti and Matt Newville for assistance with beamline data collection and processing. AR gratefully thanks Alan Woodland and Dante Canil for providing pyroxene samples to be used as XANES standards. AR is grateful to Peat Solheid for collecting the M?ssbauer spectra, which is a facility at the Institute of Rock Magnetism supported by grants from the Instrument and Facilities Program, Division of Earth Science, National Science Foundation. We gratefully acknowledge support from National Science Foundation grants EAR1426772 and EAR2016215 (to M.H.) and OCE 1433212 to E.C. The XANES data collection was performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation ? Earth Sciences (EAR ? 1634415) and Department of Energy - GeoSciences (DE-FG02-94ER14466). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. We thank Antony Burnham and two anonymous reviewers for constructive comments and Donald Dingwell for efficient editorial handling.

Funding Information:
AR thanks Jed Mosenfelder and Amanda Dillman for assistance with gas mixing furnace experiments; Anette von der Handt for helping with electron microprobe analysis; Nick Seaton for assistance with EBSD data collection and Zachary Michels for helping with processing the EBSD data. AR is thankful to Tony Lanzirotti and Matt Newville for assistance with beamline data collection and processing. AR gratefully thanks Alan Woodland and Dante Canil for providing pyroxene samples to be used as XANES standards. AR is grateful to Peat Solheid for collecting the Mössbauer spectra, which is a facility at the Institute of Rock Magnetism supported by grants from the Instrument and Facilities Program, Division of Earth Science, National Science Foundation . We gratefully acknowledge support from National Science Foundation grants EAR1426772 and EAR2016215 (to M.H.) and OCE 1433212 to E.C. The XANES data collection was performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation – Earth Sciences ( EAR – 1634415 ) and Department of Energy - GeoSciences ( DE-FG02-94ER14466 ). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357 . We thank Antony Burnham and two anonymous reviewers for constructive comments and Donald Dingwell for efficient editorial handling.

Publisher Copyright:
© 2021 Elsevier B.V.

Keywords

  • Oxidation state
  • Pyroxene
  • XANES

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