Experimental petrology

 

The stability of hydrous minerals and carbonates in subduction zones

A knowledge of the stability of hydrous phases and carbonates at high pressure and low to moderate temperature is necessary to understand the role that H2O and CO2 play in subduction zone processes. Some hydrous phases transport H2O from shallow depths to the source of arc magmatism, e.g. chlorite and antigorite; others only form as the lower-pressure minerals break down, and these may return H2O deep into the Earth’s mantle, e.g. 10-Å phase. Carbonates are stable over a wide P-T range, with double carbonates stable at shallower depths breaking down to single carbonates during subduction. We have used phase-equilibrium experiments to study the stabilities of these and other subduction zone minerals.

  • Bromiley, G.D., and Pawley, A.R. (2003) The stability of antigorite in the systems MgO-SiO2-H2O (MSH) and MgO-A12O3-SiO2-H2O (MASH): The effects of Al3+ substitution on high-pressure stability. American Mineralogist, 88, 99-108.
  • Pawley, A.R. (2003) Chlorite stability in mantle peridotite: the reaction clinochlore + enstatite = forsterite + pyrope + H2O. Contributions to Mineralogy and Petrology, 144, 449-456.
  • Morlidge, M.F., Pawley, A.R., and Droop, G.T.R. (2006) Double carbonate breakdown reactions at high pressures: an experimental study in the system CaO–MgO–FeO–MnO–CO2. Contributions to Mineralogy and Petrology, 152, 365-373.

For more information contact: Alison Pawley

Equation of state measurements using synchrotron radiation

Complementing the phase-equilibrium studies are in-situ measurements at high pressure and temperature using synchrotron radiation. We have measured the equations of state of a number of phases using the on-line multi-anvil press on station 16.4 at Daresbury Laboratory, which has provided useful data for calculations of the P-T conditions of mantle reactions. We are also using a diamond-anvil cell on station 9.5HPT to perform similar measurements on 10-Å phase.

  • Pawley, A.R., Clark, S.M., and Chinnery, N.J. (2002) Equation of state measurements of chlorite, pyrophyllite, and talc. American Mineralogist, 87, 1172-1182.

For more information contact: Alison Pawley

10-Å phase synthesised at 6.5 GPa, 600°C, 400 hrs (Crystal is 150 µm diameter)

Crystal chemistry of high-pressure phases

A knowledge of the crystal chemistry of high-P phases under ambient conditions and at high pressures and temperatures is essential for understanding the mechanisms for their stabilisation in the Earth’s mantle. Detailed characterisation of the 10-Å phase has shown it to have a much more complex composition and structure than previously thought. Diamond-anvil cell experiments are being undertaken to investigate its structural response to increased pressure.

  • Welch, M.D., Pawley, A.R., Ashbrook, S.E., Mason, H.E., and Phillips, B.L. (2006) Si vacancies in the 10-Å phase. American Mineralogist, 91, 1707-1710.

For more information contact: Alison Pawley

High-pressure infrared spectroscopy

Infrared spectroscopy is very useful for investigating the structures of minerals that contain H2O, particularly as hydrogen is not visible using X-rays. Synchrotron IR has the advantage over conventional sources of a much greater flux, thus providing much better spatial resolution. This means it can be used to study the vibrational behaviour of small samples compressed in a diamond-anvil cell. In this way we have studied talc and 10-Å phase; 10-Å phase has a composition and structure related to talc, but with interlayer H2O that is involved in stabilising it to much higher pressures. The IR study has shown the complexity of the high-P interactions.

  • Parry, S.A., Pawley, A.R., Jones, R.L., and Clark, S.M. (2007) An infrared spectroscopic study of the OH stretching frequencies of talc and 10-Å phase to 10 GPa. American Mineralogist, in press.

For more information contact: Alison Pawley

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