High-pressure crystal structure and equation of state of ferromagnesian jeffbenite: implications for stability in the transition zone and uppermost lower mantle

Fei Wang*, Elizabeth C. Thompson, Dongzhou Zhang, Ercan E. Alp, Jiyong Zhao, Joseph R. Smyth, Steven D. Jacobsen

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

2 Scopus citations

Abstract

Jeffbenite, ideally Mg3Al2Si3O12, has been identified as inclusions in super-deep diamonds originating from depths that exceed 300 km. Although Mg-end member jeffbenite has limited stability at upper-mantle conditions, iron-bearing jeffbenite may have broader P–T stability that extends to the transition zone or uppermost lower mantle, incorporating significant amounts of ferric iron. Using synchrotron-based, single-crystal X-ray diffraction (XRD) and synchrotron Mössbauer spectroscopy (SMS) at pressures up to 29 GPa, we report the crystal structure, compressibility, and likely spin transition of iron in ferromagnesian jeffbenite (Mg2.32Al0.03Fe2+1.28Fe3+1.77Si2.85O12). High-pressure structure refinements reveal that Fe3+ substitution for Si in the T2 site, which shares edges with the M2 octahedron, likely stabilizes jeffbenite at high pressure, because it increases the cation-to-cation distance between these sites. Although ferromagnesian jeffbenite does not undergo a structural phase transition below 30 GPa, SMS hyperfine parameters suggest the onset of an electronic spin transition of iron from high-spin (HS) to low-spin (LS) at around 22 GPa, which may increase its stability at high pressures. Pressure–volume data were fit to a third order Birch–Murnaghan equation of state, resulting in V0 = 816.54(9), KT0 = 181.54(1.39), and KT0′ = 2.76(14). These equation of state parameters are applicable to evaluating the encapsulation pressures of super-deep diamonds. The density and bulk modulus of ferromagnesian jeffbenite are similar to or higher than pyrope–almandine, pyrope–majorite, and skiagite–majorite solid solution garnets, further suggesting that jeffbenite may be an important ferric–iron silicate in the deeper parts of the mantle transition zone and uppermost lower mantle. However, future studies on the influence of temperature and oxidation state on the stability and equations of state of iron-bearing jeffbennite are still needed to determine what role, if any, jeffbenite plays in transition-zone mineralogy.

Original languageEnglish (US)
Article number93
JournalContributions to Mineralogy and Petrology
Volume176
Issue number11
DOIs
StatePublished - Nov 2021

Funding

This research was supported by grants from National Science Foundation (NSF) EAR-1853521 to S.D. Jacobsen and EAR-1725673 to E.C. Thompson. This work was performed at Sector 3 and Sector 13 of the Advanced Photon Source (APS) at Argonne National Laboratory. GeoSoilEnviroCARS (Sector 13) 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. Use of the COMPRES-GSECARS gas loading system was supported by COMPRES under NSF Cooperative Agreement EAR-1606856. Single-crystal diffraction experiments on beamline 13-BM-C were supported in part by the Partnership for Extreme Crystallography (PX^2) under NSF EAR-1661511. We thank Sergey Tkachev for help with gas loading. We would like to thank two anonymous reviews for their helpful comments and Dante Canil for handling this paper.

Keywords

  • Equation of state
  • Ferric iron
  • High pressure
  • Jeffbenite
  • Mössbauer
  • Transition zone

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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