TY - JOUR A1 - Wunder, Bernd A1 - Kutzschbach, Martin A1 - Hosse, Luisa A1 - Wilke, Franziska Daniela Helena A1 - Schertl, Hans-Peter A1 - Chopin, Christian T1 - Synthetic B-[4]-bearing dumortierite and natural B-[4]-free magnesiodumortierite from the Dora-Maira Massif BT - differences in boron coordination in response to ultrahigh pressure JF - European journal of mineralogy N2 - Dumortierite was synthesized in piston-cylinder experiments at 2.5-4.0 GPa, 650-700 degrees C in the Al2O3 -B2O3-SiO2-H2O (ABSH) system. Electron-microprobe (EMP) analyses reveal significant boron-excess (up to 0.26 B-[4] per formula unit, pfu) and silicon-deficiency relative to the ideal anhydrous dumortierite stoichiometry Al7BSi3O18 . The EMP data in conjunction with results from single-crystal Raman spectroscopy and powder X-ray diffraction provide evidence that silicon at the tetrahedral site is replaced by excess boron via the substitution Si-[4] <--> B-[4] + H. The Raman spectrum of synthetic dumortierite in the frequency region 2000 4000 cm(-1) comprises eight bands, of which six are located at frequencies below 3400 cm(-1). This points to strong hydrogen bonding, most likely O2-H center dot center dot center dot O7 and O7-H center dot center dot center dot O2, arising from a high number of octahedral vacancies at the All site and substitution of trivalent Al3+ and B3+ for Si4+ at Si1 and Si2 sites, causing decreasing acceptor-donor distances and lower incident valence at the acceptor oxygen. Contrary to the synthetic high-pressure ABSH-dumortierite, magnesiodumortierite from the Dora-Maira Massif, which is assumed to have formed at similar conditions (2.5-3.0 GPa, 700 degrees C), does not show any B-excess. Tourmaline shows an analogous behaviour in that magnesium-rich (e.g., dravitic) tourmaline formed at high pressure shows no or only minor amounts of tetrahedral boron, whereas natural aluminum-rich tourmaline and synthetic olenitic tourmaline formed at high pressures can incorporate significant amounts of tetrahedral boron. Two mechanisms might account for this discrepancy: (i) Structural avoidance of Mg-[6]-(OR3+)-R-[4] configurations in magnesiodumortierite due to charge deficieny at the oxygens O2 and O7 and strong local distortion of M1 due to decreased O2-O7 bond length, and/or (ii) decreasing fluid mobility of boron in Al-rich systems at high pressures. KW - dumortierite KW - magnesiodumortierite KW - Dora-Maira KW - ultrahigh-pressure (UHP) KW - ABSH-system KW - synthesis KW - tetrahedral boron KW - Raman spectroscopy Y1 - 2018 U6 - https://doi.org/10.1127/ejm/2018/0030-2742 SN - 0935-1221 SN - 1617-4011 VL - 30 IS - 3 SP - 471 EP - 483 PB - Schweizerbart CY - Stuttgart ER - TY - JOUR A1 - Bousquet, Romain A1 - Goffe, B. A1 - Le Pichon, X. A1 - de Capitani, Christian A1 - Chopin, C. A1 - Henry, P. T1 - Subduction factory : 1. Theoretical mineralogy, densities, seismic wave speeds, and H2O contents Y1 - 2005 ER - TY - JOUR A1 - Plunder, Alexis A1 - Agard, Philippe A1 - Chopin, Christian A1 - Pourteau, Amaury A1 - Okay, Aral I. T1 - Accretion, underplating and exhumation along a subduction interface: From subduction initiation to continental subduction (Tavsanli zone, W. Turkey) JF - Lithos : an international journal of mineralogy, petrology, and geochemistry N2 - We herein reappraise the pressure-temperature (PT) evolution of the high-pressure and low-temperature (HP-LT) Tavsanli zone (western Turkey) in order to (i) better characterize rock units exhumed along a cooling subduction interface, from birth to steady state and (ii) constrain exhumation and detachment dynamics, as well as mechanical coupling between plates. Based on PT estimates and field observations three oceanic complexes are recognized between the HP-LT continental margin and the obducted ophiolite, with PT estimates ranging from incipient metamorphism to blueschist-fades conditions. PT conditions for the continental unit are reappraised to 24 kbar and similar to 500 degrees C on the basis of pseudosection modelling and Raman spectroscopy on carbonaceous material. A tentative reconstruction of the subduction zone evolution is proposed using available radiometric and palaeogeographic data and recent thermomechanical modelling. Both PT conditions and field observations point out to the slicing of km-sized units at different preferred depths along the subduction interface, thus providing constraints on the dynamics of accretion and underplating. In particular, the comparison of PT estimates for the Tavsanli zone and for other broadly similar fossil subduction settings (i.e., Oman, Corsica, New Caledonia, Franciscan, Schistes Lustres) suggests that units are detached preferentially from the slab at specific depths of 30-40 km (i.e., downdip of the seismogenic zone) and similar to 80 km. We propose that these depths are controlled by major changes in mechanical coupling along the plate interface, whereas exhumation through time would rather be controlled by large-scale geodynamic boundary conditions. (C) 2015 Elsevier B.V. All rights reserved. KW - Subduction interface KW - PT estimates KW - HP-LT rocks KW - Accretion, underplating and exhumation processes Y1 - 2015 U6 - https://doi.org/10.1016/j.lithos.2015.01.007 SN - 0024-4937 SN - 1872-6143 VL - 226 SP - 233 EP - 254 PB - Elsevier CY - Amsterdam ER -