high and ultrahigh pressures.

Transcription

1 Stability of hydrous phases in serpentinites to high and ultrahigh pressures. Study of fluid and multiphase solid inclusions in high, very high pressure rocks and metamorphic veins

2 The crust to mantle transfer of fluids is controlled by the stability of a number of key hydrous phases in the various rock systems, acting as water carriers to variable depths Pressure (kbar) Partially molten region limited fluid flow large-scale fluid flow Depth (km) Serpentinites play a key role in the water and element cycle at subduction zones because: (1) They can be widespread at the ocean floors; (2) prolonged stability of hydrous phases (antigorite, chlorite, Ticlinohumite) ensures water transport to considerable depths.

3 (1) Serpentine formation at fast spreading ridges. In other ridge types (slow and ultraslow) mantle rocks floor large areas and are more easily serpentinized Kerrick, 2002, Science 289,

4 (2) Seismic evidence for hydrated regions in the subducting oceanic lithosphere and in the mantle wedge The seismic structure of the mantle wedge is characterized by an extensive low-velocity zone connecting the slab-wedge interface at about Km depth with the volcanic front and with the back-arc basin. Seismic layer inside the slab The Moho tends to disappear at the tip of the wedge. Hydration and serpentinization of both slab and mantke wedge Van Keken, 2003, EPSL, 215,

5 Prograde olivine vein in serpentinite. Stage 1 The field-based studies of serpentinites in Liguria and in the Betic Cordillera show that the subduction story of these rocks involves two main dehydration steps at increasing pressure and temperature. The first stage causes partial deserpentinization due to breakdown of serpentine and brucite. The second stage drives to full antigorite breakdown and to massive discharge of fluid and elements at sub-arc depths. Vein system development and entrapment of fluids in HP minerals are achieved at both stages, enabling to monitor the evolution of rocks and fluids with increasing subduction depth

6 seafloor hydration First stage of serpentinite dehydration: first olivine formation ec logite-fac ies dehydration and veining 25 ol + ilm + H2O Ticl 20 ol + tlc + H2O 15 atg 10 Pressure ( kbar) ol + H2O atg + brc atg + brc 5 ctl T emperature ( C) Primary saline inclusion in vein

7 Total homogenization C Halite saturation curve wt% NaCl Brines in high-pressure serpentinites imply the recycling of oceanic components in the subduction fluid (see part on element cycling) Scambelluri et al., EPSL, 1997, 148,

8 Second stage of serpentinite dehydration: full antigorite breakdown Chlorite metamorphic harzburgites are produced by the reaction Antigorite = ol + opx + water 35 P Kbar Antigorite serpentinite 1 Atg Atg Ol + opx+ fl Ol + tlc+ fl 2 Spinifex harzburgite T C

9 Primary inclusions in opx and ol Wavenumber (cm-1) Wavenumber (cm-1) Presence of solute-rich aqueous inclusions. Their trace element composition is strongly enriched in incompatible element (see part on element cycling) Scambelluri et al., EPSL, 2001, 192,

10 Stability of serpentinites in the Alpine HP and UHP ophiolites provides buoyant media for the exhumation of eclogites to the surface. The antigorite breakdown reaction represents a boundary of no-return of ultramafic rocks to the surface. This reaction re-transforms serpentinites into mantle rocks of much higher density. After the antigorite breakdown ultramafites are exhumed only if associated with continental crust Pressure Gpa = Low grade oceanic serpentinite (N. Apennine and Erro-Tobbio) 2 = High-pressure antigorite serpentinite (Erro-Tobbio, ET; Betic Cordillera, BC) 3 = Metamorphic harzburgite (Betic Cordillera) ET 2BC Temperature C Hermann et al., 2000, Tectonophysics, 327,