Elastic behaviour of zoisites and their geological implications

The relationships between arc volcanism-earthquakes and subduction zones have been systematically observed and investigated around the globe ([1] and references therein). Several authors ([1-3]) have argued that the correct model for explaining the generation of many volcanic arc and intermediate earthquakes in the subduction zones involves the progressive dehydration of the subducting slab through a series of reactions that release H2O into the mantle wedge. These fluids released from subducting slabs could trigger hydration-driven partial melting reactions, inducing partial melting of the mantle wedge above the slab, which in turn is considered responsible for the intermediate earthquakes ([2] and [3]), as well as for arc volcanism ([1]) in the subduction zones. A key role in these dehydration reactions is played by the hydrous mineral phases that are mainly contained on the subducting slab. Therefore a detailed study of the properties of the hydrous phases, and especially the thermo-elastic behaviour, will be an important requirement for understanding mantle properties and processes within the subduction zones, in particular for constraining their stability and the position of the related dehydration reactions in which they are involved as a function of pressure and temperature ([4] and [5]). In order to evaluate the effect of these dehydration reactions at the structural scale, we focused our work on a natural zoisite sample (Fe-free end member), from Merelani Hills in the Arusha Region, United Republic of Tanzania. It is a hydrous mineral containing 2 wt% of water, which belongs to epidote group of minerals, with ideal formula Ca2Al2Si3O12OH. Zoisite occurs in high and ultrahigh-pressure metamorphic rocks from a wide variety of geological settings, including continental collisions and subduction zones ([4], [5] and [6]). Minerals of the epidotes group are part of numerous phase equilibria, which need to be accurately evaluated in order to understand several geological processes. Many authors have investigated minerals of the epidotes group at high pressures with different techniques obtaining different results ([5], [7]). In order to clarify the discrepancies we investigated the sample at high-pressure (up to 6.5 GPa) by means of single crystal x-ray diffraction, We determined the unit-cell parameters in order to calculate the Equation of State and measured intensity data to follow the evolution of the structure with pressure. Preliminary results using a Birch-Murnhagan EoS yield a Bulk modulus (KT0) of 122(1) GPa with a K` = 6.9(4) and V0 = 903.43(7) Å3, not in very good agreement with data by [7] on a Fe-rich sample (0.1 apfu), probably because of their limited pressure range, but in very good agreement with the more recent Brillouin data [5] on a Fe-free sample. As shown by the Ff plot and confirmed by the evolution of some geometrical parameters there is a change in the structural evolution at about 2 GPa. In order to fully understand the cause of this change, which probably involves the OH group, further data collection and analysis is under way and new spectroscopic measurements will be required. References. [1] Forneris, J.F., Holloway, J.R. (2003): Earth and Planetary Science Letters 214, 187; [2] Peacock, S.M. (2001): Geology 29, 299; [3] Kirby, S.H., Stein, S., Okal, E.A., Rubie, D.C. (1996): Reviews in Geophysics, 34, 261; [4] Hacker, B.R., Peacock, S. M., Abers, G.A., Holloway, S.D. (2003): Journal of Geophysical Research 108, 2030: [5] Mao, Z., Jiang, F., Duffy, T.S. (2007): American Mineralogist 92, 570; [6] Enami, M., Mizukami, T. & Yokoyama, K. (2004): Journal of Metamorphic Geology 22, 1-15; [7] Comodi, P. & Zanazzi, P.F. (1997) American Mineralogist 82, 61-68. This work was supported in part by NSF grant EAR 0738692 to NL Ross and RJ Angel