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_Journal of Structural Geology 32 (2010) 1476-1484_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com/locate/jsg_ _THERMAL DECOMPOSITION OF SERPENTINE DURING COSEISMIC FAULTING: NANOSTRUCTURES AND MINERAL REACTIONS_ _Cecilia Viti a,*, Takehiro Hirose b_ _a Earth Science Department, Siena University, Via Laterina 8, 53100 Siena, Italy_ _b Kochi Institute for Core Sample Research, JAMSTEC, 200 Monobe-otsu Nankoku, Kochi 783-8502, Japan_ _article info_ _Article history: Received 29 June 2010; Received in revised form 16 September 2010; Accepted 17 September 2010; Available online 25 September 2010_ _Keywords: Serpentine Dehydration Frictional heating Coseismic faulting TEM_ _abstract_ _THIS PAPER REPORTS A DETAIL CHARACTERIZATION OF AN ANTIGORITE-BEARING SERPENTINITE, DEFORMED AT SEISMIC SLIP-RATE (1.1 m s) IN A HIGH-VELOCITY FRICTION APPARATUS. MICRO-NANOSTRUCTURAL INVESTIGATION OF THE SLIP ZONE (200 mm THICK) REVEALED A ZONAL ARRANGEMENT, WITH A CLOSE JUXTAPOSITION OF HORZONS WITH SIGNIFICANTLY DIFFERENT STRENGTH, RESPECTIVELY CONSISTING OF AMORPHOUS TO POORLY-CRYSTALLINE PHASES (WITH BULK ANHYDRIOUS COMPOSITION CLOSE TO STARTING ANTIGORITE) AND OF HIGHLY-CRYSTALLINE ASSEMBLAGES OF FORSTERITE AND DISORDERED ENSTATITE (200 NM IN SIZE AND IN POLYGONAL-LIKE NANOTEXTURES). THE SLIP ZONE ALSO HOSTS MICRO-NANOMETRE-SIZED CR-MAGNETITE GRAINS, ALIGNED AT LOW ANGLE WITH RESPECT TO THE SLIPPING SURFACE AND INHERITED FROM THE HOST SERPENTINITE._ _OVERALL OBSERVATIONS SUGGEST THAT FRICTIONAL HEATING AT ASPERITIES ON THE SLIPPING SURFACE INDUCED A TEMPERATURE INCREASE UP TO 820-1200 °C (IN AGREEMENT WITH FLASH TEMPERATURE THEORY), RESPONSIBLE FOR SERPENTINE COMPLETE DEHYDRATION AND AMORPHIZATION, FOLLOWED BY CRYSTALLIZATION OF FORSTERITE AND ENSTATITE (UNDER POST-DEFORMATION, STATIC CONDITIONS). THE RESULTS OF THIS STUDY MAY PROVIDE IMPORTANT KEYS FOR THE FULL COMPREHENSION OF THE MECHANICAL BEHAVIOUR AND OF THE POSSIBLE GEODYNAMICAL ROLE OF SERPENTINITE-HOSTED FAULTS THROUGH THE SEISMIC CYCLE._ _© 2010 Elsevier Ltd. All rights reserved._ _1. INTRODUCTION_ _SERPENTINITES, PREDOMINANTLY FORMED BY CHRYSOTILE OR CHRYSOHITE ASSOCIATIONS IN PSEUDOMORPHIC OR INTER-PENETRATING TEXTURES (E.G., WICKS AND WHITTAKER, 1977; VITI AND MELLINI, 1998; MEVEL, 2003), PLAY A KEY ROLE IN OCEANIC LITHOSPHERE DYNAMICS, PARTICULARLY IN SUBDUCTION AND SHEAR FAULT ZONES._ _DUE TO THEIR RELATIVELY LOW FRICTION COEFFICIENT (DOWN TO 0.2 FOR CHRYSOTILE; E.G., REINEN ET AL., 1991; MOORE ET AL., 1996; MORROW ET AL., 2000), DEFORMATION IN THE OCEANIC LITHOSPHERE MAY BE PREFERENTIALLY ACCOMMODATED BY SERPENTINITIC ROCKS AND SERPENTINE-BEARING GOUGES, POTENTIALLY RESULTING IN ASEISMIC FAULT CREEP, AT LEAST AT SHALLOW CONDITIONS (REINEN ET AL., 1991; MOORE ET AL., 1997; ESCARTIN ET AL., 2001; ANDREANI ET AL., 2005)._ _HOWEVER, THE MECHANICAL BEHAVIOUR OF SERPENTINITES IS STRONGLY DEPENDENT ON P-T CONDITIONS. IN THIS REGARD, THE MOST IMPORTANT PROCESS IS SERPENTINE DEHYDRATION, OCCURRING UNDER ELEVATED TEMPERATURE CONDITIONS. ON ONE HAND, SERPENTINE DEHYDRATION IS RESPONSIBLE FOR SIGNIFICANT FLUID RELEASE IN SUBDUCTION ZONES, WITH MAJOR CONSEQUENCES FOR PARTIAL MELTING AND SUBDUCTION-RELATED MAGMATISM (E.G., ULMER AND TROMMSDORFF, 1995; ULMER, 2001). ON THE OTHER HAND, SERPENTINE DEHYDRATION MAY RESULT IN A PORE-PRESSURE INCREASE (POSSIBLY ATTAINING LITHOSTATIC PRESSURE) AND ultimately IN THE FORMATION OF STRONGER ANHYDRIOUS ASSEMBLAGES. AS A CONSEQUENCE, SERPENTINE DEHYDRATION IS RESPONSIBLE FOR THE OBSERVED TEMPERATURE-DEPENDENT TRANSITION FROM DUCTILE TO BRITTLE ("DEHYDRATION EMBRITTLEMENT"; RALEIGH AND PATERSON, 1965), POSSIBLY LEADING TO INTERMEDIATE-DEPTH SEISMICITY (E.G., PEACOCK, 2001; JUNG AND GREEN, 2004). THE MECHANICAL EFFECTS OF DEFORMATION AND DEHYDRATION ON SERPENTINITES HAVE BEEN WIDELY INVESTIGATED BOTH IN NATURAL CONTEXT (E.G., WICKS, 1984a,b; HOOGERDUIJN STRATING AND VISSER, 1994; HERMANN ET AL., 2000; REINEN, 2000; ANDREANI ET AL., 2005; AUZENDE ET AL., 2006) AND UNDER EXPERIMENTAL CONDITIONS (E.G., RALEIGH AND PATERSON, 1965; RUSSERT AND BRODIE, 1988; IRIFUNE ET AL., 1996; ESCARTIN ET AL., 1997; JUNG AND GREEN, 2004; HIROSE ET AL., 2006; RUSSERT ET AL., 2009; VITI AND HIROSE, 2009)._ _SERPENTINE DEHYDRATION CAN ALSO BE TRIGGERED BY FRICTIONAL HEATING DURING COSEISMIC FAULTING, WITH POSSIBLE CONSEQUENCES IN FAULT BEHAVIOUR. TO HELP UNDERSTANDING THE MECHANICAL BEHAVIOUR OF SERPENTINITES DURING THE GENERATION OF EARTHQUAKES, HIROSE AND BYSTRICKY (2007) PERFORMED A HIGH-VELOCITY FRICTION EXPERIMENT ON A MASSIVE SERPENTINITE, AT CONDITIONS REPRODUCING NATURAL EARTHQUAKE CONDITIONS, BOTH IN TERMS OF SLIP VELOCITY AND DISPLACEMENT._ _C. VITI, T. HIROSE Journal of Structural Geology 32 (2010) 1476-1484_ _1477_ _THIS PAPER REPORTS A DETAIL MINERALOGICAL AND MICRO-NANOSTRUCTURAL INVESTIGATION OF THE SLIP ZONE FORMED DURING THE EXPERIMENT BY HIROSE AND BYSTRICKY (2007), FOCUSING ON SERPENTINITE DEHYDRATION MECHANISM AND ON RESULTING ANHYDROUS PRODUCTS. THE PAPER AIMS TO CONTRIBUTE TO AN INCREASED KNOWLEDGE OF DEFORMATION-INDUCED PROCESSES WITHIN SERPENTINITE-HOSTED FAULTS._ _2. EXPERIMENTAL AND STARTING SAMPLE_ _THE STARTING SAMPLE WAS A NATURAL SERPENTINITE FROM TAIWAN, CHARACTERIZED BY HIGH CRYSTALLINITY AND PREDOMINANTLY CONSISTING OF ANTIGORITE LAMELLAE UP TO 80 mm IN SIZE. IN ORDER TO SIMULATE A SEISMIC SLIP, THE SAMPLE WAS SHEARED BY A HIGH-VELOCITY FRICTION APPARATUS (HIROSE AND BYSTRICKY, 2007), BASICALLY CONSISTING OF A PAIR OF CYLINDRICAL SPECIMENS OF MASSIVE SERPENTINITE PRESSED TOGETHER WITH ONE SPECIMEN KEPT STATIONARY WHILE THE OTHER ONE ROTATED AT HIGH SPEED (FURTHER DETAILS IN HIROSE AND SHIMAMOTO, 2005). THE EXPERIMENT WAS PERFORMED AT SLIP VELOCITY OF 1.1 m/s AND NORMAL STRESS OF 24.5 MPa WITH DISPLACEMENTS OF 3.6 m UNDER NOMINALLY-DRained CONDITIONS AND ROOM TEMPERATURE (RUN HVR694). Thus, the experimental conditions are representative of natural earthquake conditions, especially in terms of slip velocity and displacement._ _HUMIDITY MEASUREMENTS REVEALED THAT FRICTIONAL HEATING PRODUCED A FLUID LOSS, SOON AFTER RAPID SLIP INITIATED (RUN HVR694 IN HIROSE AND BYSTRICKY, 2007)._ _THE SHEARED SERPENTINITE WAS OBSERVED BY SCANNING ELECTRON MICROSCOPY (SEM) AND TRANSMISSION ELECTRON MICROSCOPY (TEM). SEM WAS PERFORMED USING A PHILIPS XL30, OPERATING AT 20 kV AND EQUIPPED WITH AN EDAX-DX4 ENERGY DISPERSE SPECTROMETER (EDS), PROVIDING CHEMICAL ANALYSIS FOR ATOMS HEAVIER THAN C. COUNTING RATE WAS KEPT CLOSE TO 2200-2300 COUNTS PER SECOND OVER THE WHOLE ENERGY SPECTRUM. ANALYTICAL PRECISION, CHECKED BY REPEATED ANALYSES, WAS BETTER THAN 0.5 wt.% FOR MAJOR ELEMENTS (ON THE ABSOLUTE VALUE) AND BETTER THAN 20% RELATIVE FOR MINOR ELEMENTS (I.E., THOSE WITH CONTENTS RANGING FROM 0.3 wt.% UP TO 3-5 wt.%). BACK-SCATTERED ELECTRONS (BSE) WERE USED FOR IMAGE FORMATION._ _THE TEM INVESTIGATION WAS PERFORMED USING A JEOL 2010 MICROSCOPE, WORKING AT 200 kV, WITH ULTRA-HIGH RESOLUTION (UHR) POLE PIECE AND POINT-TO-POINT RESOLUTION OF 0.19 nm. A TV-RATE CHARGE-COUPLED DEVICE CAMERA (LHERITIER S.A. LH 74 LL) WITH IMAGE AMPLIFIER WAS USED FOR FOCUS AND ASTIGMATISM CORRECTIONS. THE MICROSCOPE IS EQUIPPED WITH SEMI-STEM CONTROL AND ULTRA-THIN WINDOW ENERGY DISPERSE SPECTROMETER (EDS-EISIS OXFORD). TEM GRIDS WERE PREPARED FROM A THIN SECTION OF SAMPLE HVR694, SAMPLING A) ANTIGORITE CRYSTALS FAR AWAY FROM THE SLIP ZONE, B) THE BOUNDARY BETWEEN HOST ANTIGORITES AND SLIP ZONE, AND C) THE SLIP ZONE. SELECTED GRIDS WERE THINNENED BY AR+ ION MILLING (GATAN DUAL ION MILL), FROM 5 TO 1.5 kV AND IMPINGING ANGLE FROM 20 TO 12°._ _3. SEM OBSERVATIONS_ _FAR AWAY FROM THE SLIPPING ZONE, THE STARTING SERPENTINITE PRESERVES ITS MINERALOGY AND TEXTURAL CHARACTERISTICS, SHOWING INTER-PENETRATING UNDEFORMED ANTIGORITE LAMELLAE, W80 mm LONG, WITH AN AVERAGE SEM EDS COMPOSITION Mg2.64 Fe0.11 Cr0.01 Al0.11 Si2.00 (ATOMS PER FORMULA UNIT, a.p.f.u., ON THE BASIS OF 7 OXYGENS). ANTIGORITE LAMELLAE ARE ASSOCIATED WITH MINOR CR-RICH MAGNETITE GRAINS AND ISOLATED PODS OF BRUCITE. MAGNETITE, UP TO MILLIMETRE IN SIZE, HAS HIGHLY VARIABLE Cr CONTENTS (FROM 1.4 TO 14.8 wt.% Cr2O3), WITH (FeO + Cr2O3) FeO RATIOS VARYING FROM 81 UP TO 96 (AVERAGE VALUE OF 91)._ _FIG. 1a SHOWS A REPRESENTATIVE SEM BSE IMAGE OF THE SAMPLE AT THE SLIP ZONE, I.E., THE LIGHT-GREY SUB-HORIZONTAL VEIN (DOUBLE-ARROWED), WITH ULTRA-FINE GRAIN SIZE. THE LIGHT-GREY BSE COLOUR SUGGESTS THE OCCURRENCE OF A MINERAL ASSOCIATION WITH AVERAGE ATOMIC WEIGHT HIGHER THAN STARTING SERPENTINITE. THE SLIP ZONE, HERE CUTTING A LARGE MAGNETITE CRYSTAL (BRIGHT IN BSE IMAGES), IS CHARACTERIZED BY THE OCCURRENCE OF BRIGHT ELONGATED FEATURES ALIGNED AT LOW ANGLE WITH RESPECT TO VEIN WALLS. THE MAIN DISCONTINUITY (BROAD ARROW IN FIG. 1a) CORRESPONDS TO THE SLIPPING SURFACE AND SEPARATES THE ROTATORY SPECIMEN (LOWER SIDE IN FIG. 1a) FROM THE TRUE SLIP ZONE. THE SERPENTINITE OCCURRING WITHIN THE ROTATORY SIDE APPEARS TO BE UNAFFECTED BY DEFORMATION-INDUCED TRANSFORMATIONS, AS TESTIFIED BY ITS HOMOGENEOUS DARK-GREY BSE CONTRAST EVEN AT THE BOUNDARY WITH THE SLIPPING SURFACE._ _THE SLIP ZONE HAS A RELATIVELY CONSTANT THICKNESS OF 200-250 mm AND OFTEN DISPLAYS AN ASYMMETRIC INNER CONFIGURATION (FIG. 1b and c), WITH A GRADUAL TRANSITION FROM THE SLIPPING SURFACE TO THE HOST SERPENTINITE (STATIONARY SIDE). AS SHOWN IN FIG. 1b, FOUR MAIN HORZONS CAN BE DISTINGUISHED. HORIZON 1 CONSISTS OF HOMOGENEOUS AND UNDEFORMED ANTIGORITE LAMELLAE THAT APPEAR TO BE UNAFFECTED BY POSSIBLE DEFORMATION-INDUCED PROCESSES. IN HORIZON 2, ANTIGORITE CRYSTALS ARE ASSOCIATED WITH A LIGHT-GREY MATERIAL OCCURRING IN THIN FILMS AT GRAIN BOUNDARIES OR PARALLEL TO (001)ATG CLEAVAGE PLANES (E.G., ARROW IN FIG. 1d)_ Ключевые слова: e, r, o