Pressure-Temperature Stability Studies of Talc and 10-Å phase using x-ray diffraction. Arianna E. Gleason 1, Martin Kunz 1, Stephen Parry 2, Alison Pawley 2, and Simon M. Clark 1,2. 1Advanced Light Source, Lawrence-Berkeley National Laboratory, MS 4R 0230, 1 Cyclotron Rd, Berkeley CA 94720 2School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK. Abstract Experimental Setup -ALS Beamlines 12.2.2 and 11.3.1; The pressure-temperature phase boundary for talc plus water to 10-Å probed with energies 20 to 30 keV phase conversion has been determined. Our data suggest that 10-Å phase may form at room temperature while at elevated pressures. X-ray -Diffraction patterns acquired using MAR345 image plate and Bruker CCD diffraction measurements of natural talc plus water at combined temperatures of 27oC – 450oC and pressures of 0-15 GPa were made at -Resistively heated, membrane driven the Advanced Light Source beamlines 12.2.2 and 11.3.1. Elevated DAC was used to reach P and T temperatures and pressures were achieved with a resistively heated, -contained in gasket: natural talc + membrane driven diamond-anvil cell. The data were modeled using a water + gold (pressure calibrant) Diamond-anvil cell [DAC] cylinder with heater and gasket monoclinic unit cell. Our data and proposed phase diagram are presented. Data Analysis Isotherms: we note the pressure range where the 002 peak discontinuity occurs. Lattice parameters for each phase (talc and gold) „Up‟ is compression of the DAC and „down‟ is decompression. The „down‟ points were determined using Le Bail whole pattern are not necessarily considered due to hysteresis. Straight lines drawn are only a Introduction refinement with the GSAS program. o guide for the eye. o 200 C: talc 002 vs. pressure 100 C: talc 002 vs. pressure 9.5 9.5 9.4 9.4 Talc, Mg3Si4O10(OH)2, is a sheet silicate commonly Basal spacing talc 9.3 9.3 9.2 peaks: 002 and 006 9.2 002 positions (Å) formed as an alteration product from ultramafic rock. talc 002 (Å) 9.1 9.1 9 9 200deg up At elevated pressures and temperatures, H2O can fit Diffraction pattern 8.9 100deg down 8.9 100deg up 8.8 200deg down taken at ALS BL12.2.2 8.8 between the TOT (tetrahedra-octaheadra-tetrahedra) with Bruker CCD at 30 8.7 8.6 8.7 8.6 8.5 layers of talc to form 10-Å phase. Volatile recycling, keV 8.5 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 pressure (GPa) 12 14 16 18 20 pressure (GPa) via oceanic slab subduction, plays a major geodynamic o 300 C: talc002 vs. pressure o 400 C: talc 002 vs pressure role triggering mass transfer, melting and volcanism. Unit Cell of Talc. 9.5 9.4 9.5 9.4 9.3 9.3 9.2 9.2 talc 002 (Å) talc 002 (Å) 9.1 9.1 The pressure-temperture [P-T] 9 8.9 300 deg up 9 8.9 400deg up stability field for 10-Å phase, as a 8.8 8.7 8.8 8.7 potential water carrier within the 8.6 8.5 8.6 8.5 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 slab, is important to shed light on pressure (GPa) pressure (GPa) the Earth‟s water budget. When comparing our room temperature isotherm 9.8 o 27 C: talc 002 vs. pressure 27 deg upstroke (27oC) data to compression data collected by S. Parry 9.6 Parry 2004 Talc talc 002 position (Å) 9.4 for both 10-Å phase and talc, we note a conversion of 9.2 Parry 2004 10-A Talc Compression; Parry talc to 10-Å phase with only the increase of pressure. 9 Poly. (Parry 2004 10-A) Schematic Diagram extracted from Gill 1981 Based on the transition pressures seen in the above 8.8 8.6 isotherm data, we can plot a more accurate phase 0 2 4 pressure (GPa) 6 8 10 Method boundary between talc and 10-Å phase and generate P-T diagram. To determine the phase boundary between P-T Plot for Talc and 10Å Talc; Gleason talc and 10-Å phase we can collect 14 10-A; Gleason isothermal data across the zone where we 10-Å Enstatite + Stishovite +V Talc; Pawley 12 expect a “change”. This change is noted En+Stish;Pawley by monitoring the basal spacing of talc 10 En+Cs; Pawley Pressure (GPa) stishovite during x-ray diffraction. We should be 8 coesite able to see a discontinuity in the 002 6 Enstatite + Coesite + V spacing with pressure when water starts 4 fitting into the talc structure for each Talc 2 Plot extracted from: Parry S, Pawley A, Clark S, Jones R. 2004. In-situ high isotherm. pressure synchrotron and X-ray diffraction study of talc and 10-Angstrom phase. 0 0 100 200 300 400 500 600 700 800 900 Temperature (oC) 10 References Discussion and Conclusions - Parry S, Pawley A, Clark S, Jones R. 2004. In-situ high 9.35 pressure synchrotron and X-ray diffraction study of talc and 10-Angstrom phase. COMPRES Annual Meeting 2004. Prior to this study, the stability region of 10-Å phase within a Extracted from Poli and Schmidt 2002 - Pawley, 2004 unpublished personal communication; pdf subducting slab was thought to be here. However, this new received Dec.3, 2004. - Pawley A, Redfern S, Wood B. 1995. Thermal In Talc stability version of the phase boundary would indicate that 10-Å phase expansivities and compressibilities of hydrous phases in the is also stable at lower pressures. The boundary‟s decrease in system MgO-SiO2-H2O: talc, phase A and 10-Å phase. In 10-Å phase Contributions to Mineral Petrology: Vol 122: 301- 307. stability slope from 150 – 500oC could mean that 10-Å phase is less - Shim S, Duffy T, Kenichi T. 2002. Equation of state of gold and its application to the phase boundaries near 660 We need to see sensitive to temperature increases. 10-Å phase would reside km depth in Earth‟s mantle. Earth and Planetary Science this jump. longer inside the slab‟s center (where it is cooler) and could Letters: Vol. 203: 729-739. carry more water in the slab as it subducts.
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