Isosymmetric pressure-induced bonding increase changes compression behavior of clinopyroxenes across jadeite-aegirine solid solution in subduction zones

Jingui Xu, Dongzhou Zhang, Dawei Fan, Robert T. Downs, Yi Hu, Przemyslaw K. Dera

Research output: Contribution to journalArticle

7 Scopus citations

Abstract

Pyroxenes are among the most important minerals of Earth's crust and upper mantle and play significant role in controlling subduction at convergent margins. In this study, synchrotron-based single-crystal X-ray diffraction experiments were carried out on a natural aegirine [NaFe3+Si2O6] sample at ambient temperature and high pressures to 60 GPa, simulating conditions within the coldest part of a subduction zone consisting of old lithosphere. The diffraction data reveal no obvious sign of structural phase transition in aegirine within this pressure range; however, several relevant structural parameter trends change noticeably at approximately 24 GPa, indicating the presence of the previously predicted isosymmetric bonding change, related to increase of coordination number of Na+ at M2 site. The pressure-volume data, fit with third-order Birch-Murnaghan (BM3) equation of state over the whole pressure range, yields KT0 = 126(2) GPa and K′T0 = 3.3(1), while separate BM3 fits performed for the 0–24.0 GPa and 29.9–60.4 GPa pressure ranges give KT0 = 118(3) GPa, K′T0 = 4.2(3) and KT0 = 133(2) GPa, K′T0 = 3.0(1), suggesting that the structure stiffens as a result of the new bond formation. Aegirine exhibits strong anisotropic compression with unit strain axial ratios ε123 = 1.00:2.44:1.64. Structural refinements reveal that NaO8 polyhedron is the most compressible and SiO4 tetrahedron has the lowest compressibility. The consequence of bonding transition is that the compressional behavior of aegirine below ~24 GPa and above that pressure is quite different, with likely consequences for relevant thermodynamic parameters and ion diffusion coefficients.

Original languageEnglish (US)
Pages (from-to)142-157
Number of pages16
JournalJournal of Geophysical Research: Solid Earth
Volume122
Issue number1
DOIs
StatePublished - Jan 1 2017

Keywords

  • bonding transition
  • high pressure
  • mantle
  • pyroxenes
  • subduction zone
  • synchrotron single-crystal X-ray diffraction

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

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