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High-pressure studies of subduction zone related mineral phases

  • Author(s): O'Bannon, Earl F.
  • Advisor(s): Williams, Quentin
  • et al.
Abstract

Subduction zones play a fundamental role in the cycling of minerals from the surface of the Earth into the deep Earth. Minerals that are subducted experience high-pressure and temperature as they are transported to greater and greater depths. The work presented here takes advantage of pressure as a thermodynamic variable in order to gain insight into the physics and chemistry of subduction zone related mineral phases.

The experiments conducted here use the diamond anvil cell at pressures up to ~65 GPa in conjunction with synchrotron based single-crystal x-ray diffraction as well as Raman, infrared, and luminescence spectroscopy. The work is divided into three major parts (1) water bearing minerals found in subduction zones such as lawsonite [CaAl2Si2O7(OH)2· H2O] and clinochlore [Mg5Al(AlSi3O10)(OH)8], (2) B and F bearing minerals found in subduction zones such as topaz [Al2SiO4(F,OH)2], and dravite variety tourmaline [Na(Mg3)Al6(Si6O18(BO3)3(OH)3(OH)] and (3) mineral phases that are closely related to subduction zone minerals but not likely found in abundance in subduction zones such as beryl [Be3Al2Si6O18], and spodumene [LiAlSi2O6].

The high-pressure crystal structure of lawsonite was solved for the first time using synchrotron based single-crystal x-ray diffraction. The H-bonding environment of the high-pressure monoclinic phase of lawsonite was also determined. High-pressure luminescence spectra of clinochlore up to ~25 GPa show that the electronic properties of this sheet silicate are complex under high-pressure conditions. High-pressure luminescence spectra of topaz up to ~55 GPa show that the deformation mechanism of topaz likely changes above ~38 GPa. A new high-pressure phase of tourmaline that is stable above ~15 GPa at room temperature was discovered and solved using synchrotron based single-crystal x-ray diffraction. Luminescence spectra suggest this phase is stable to at least ~65 GPa.

A new high-pressure phase of beryl that is stable above ~14 GPa at room temperature was discovered using luminescence and Raman spectroscopy. Experimentally our results confirm the previous theoretically predicted high-pressure soft-mode transition. Beryl is structurally related to the high-temperature phase of cordierite [(Mg,Fe)2Al3(AlSi5O18)], a subduction zone mineral phase that shows extensive high-pressure polymorphism. Spodumene is certainly not abundant in the Earth's crust and upper mantle, however understanding high-pressure phase transitions in C2/c pyroxenes is important for understanding the high-pressure behavior of mantle relevant pyroxenes (e.g. hedenbergite [CaFeSi2O6] and diopside [CaMgSi2O6]). Luminescence spectra suggest that only one transition at 3.2 GPa occurs under compression from room pressure to ~15 GPa which is in contrast to previous work that suggests a second transition near 7.7 GPa.

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