Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
Garnet growth in high-pressure, mafic garnet granulites formed by dehydration melting of hornblende-gabbronorite protoliths in the Jijal complex (Kohistan palaeo-island arc complex, north Pakistan) was investigated through a microstructural EBSD-SEM and HRTEM study. Composite samples preserve a sharp transition in which the low-pressure precursor is replaced by garnet through a millimetre-sized reaction front. A magmatic foliation in the gabbronorite is defined by mafic-rich layering, with an associated magmatic lineation defined by the shape-preferred orientation (SPO) of mafic clusters composed of orthopyroxene (Opx), clinopyroxene (Cpx), amphibole (Amp) and oxides. The shape of the reaction front is convoluted and oblique to the magmatic layering. Opx, Amp and, to a lesser extent, Cpx show a strong lattice-preferred orientation (LPO) characterized by an alignment of [001] axes parallel to the magmatic lineation in the precursor hornblende-gabbronorite. Product garnet (Grt) also displays a strong LPO. Two of the four < 111 > axes are within the magmatic foliation plane and the density maximum is subparallel to the precursor magmatic lineation. The crystallographic relationship < 111 >(Grt)//[001](Opx,Cpx,Amp) deduced from the LPO was confirmed by TEM observations. The sharp and discontinuous modal and compositional variations observed at the reaction front attest to the kinetic inhibition of prograde solid-state reactions predicted by equilibrium-phase diagrams. The P-T field for the equilibration of Jijal garnet granulites shows that the reaction affinities are 5-10 kJ mol.(-1) for the Grt-in reaction and 0-5 kJ mol.(-1) for the Opx-out reaction. Petrographic and textural observations indicate that garnet first nucleated on amphibole at the rims of mafic clusters; this topotactic replacement resulted in a strong LPO of garnet. Once the amphibole was consumed in the reaction, the parallelism of [001] axes of the mafic-phase reactants favoured the growth of garnet crystals with similar orientations over a pyroxene substrate. These aggregates eventually sintered into single-crystal garnet. In the absence of deformation, the orientation of mafic precursor phases conditioned the nucleation site and the crystallographic orientation of garnet because of topotaxial transformation reactions and homoepitaxial growth of garnet during the formation of high-pressure, mafic garnet-granulite after low-pressure mafic protoliths.
ISSN: 0263-4929

hal-00411323
https://hal.archives-ouvertes.fr/hal-00411323
DOI : 10.1111/J.1525-1314.2008.00789.x

Éditeur(s) : HAL CCSD Springer Verlag (Germany) Springer Verlag
Résumé : Dehydration reactions result in minerals’ replacement and a transient fluid-filled porosity. These reactions involve interface-coupled dissolution–precipitation and might therefore lead to fixed crystallographic orientation relations between reactant (protolith) and product phases (i.e. topotaxy). We investigate these two phenomena in the dehydration of a foliated antigorite (atg) serpentinite by comparing the crystallographic preferred orientation (CPO) developed by olivine (ol), orthopyroxene (opx) and chlorite (chl) during high-pressure antigorite and chlorite dehydration in piston-cylinder experiments and in natural samples recording the dehydration of antigorite (Cerro del Almirez, Betic Cordillera, Spain). Experiments were performed under undrained conditions resulting in fluid-filled porosity and in strong CPO of the prograde minerals, controlled by the pre-existing antigorite CPO in the reactant foliated serpentinite. The orientation of a ol,opx and c∗chl is parallel to c∗atg from the protolith. The Cerro del Almirez samples show similar, locally well-developed topotactic relations between orthopyroxene, chlorite and antigorite, but the product CPOs are weaker and more complex at the thin section scale. In contrast to the experiments, olivine from natural samples shows a weak correlation between b ol and the former c∗atg. We relate the strengthening of local topotactic relations and the weakening of the inherited CPO at a larger scale in natural samples to compaction and associated fluid migration. Microstructural features that might be related to compaction in the natural samples include: (1) smooth bending of the former foliation, (2) gradual crystallographic misorientation (up to 16°) of prismatic orthopyroxene due to buckling by dislocation creep, (3) inversion of enstatite to low clinoenstatite (P21/c) along lamellae and (4) brittle fracturing of prismatic orthopyroxene enclosed by plastically deformed chlorite. The coexistence of orthopyroxene buckling and clinoenstatite lamellae enables estimating the local strain rates and shear stresses generated during compaction. An lower bound for the strain rates in the order of 10−12 to 10−13 s−1 and shear stresses of 60–70 MPa are estimated based on creep data. Lower shear stresses (20–40 MPa) are retrieved using a theoretical approach. These data point to slow compaction (and fluid extraction) in nature if the system is not perturbed by external forces, with rates only marginally higher than the viscoplastic deformation of the solid matrix.
ISSN: 0010-7999

hal-01174158
https://hal.archives-ouvertes.fr/hal-01174158
DOI : 10.1007/s00410-015-1129-4

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
We report on microstructural data obtained by optical microscopy and transmission electron microscopy concerning the crystallographic relationships of serpentine minerals with their host olivine in two contrasting situations. In the first case, mesh-textured lizardite (liz) is developed in a standard 60% serpentinized oceanic harzburgite from the Oman ophiolite where olivine converts to columnar lizardite. The joined columns are perpendicular to the basal plane (001)(liz), corresponding to the pseudofibres observed optically. The plane (001)(liz) is locally parallel to the narrow boundary ol-liz; thus column orientations register the interface of serpentinization. The ol-liz relationships are not strictly topotactic, but reflect preferred cracking orientations in olivine, parallel to (010)(ol). In the second case, antigorite (atg) develops in a rare sample of antigorite schist in a kimberlite from Moses Rock (Colorado Plateau), representative of a suprasubduction-zone mantle wedge. High-resolution transmission electron microscopy (HRTEM) images along [010](atg) show domains of very regular modulation with a 43 center dot 5 A wavelength (m = 17, where m is the number of silicate tetrahedra along the wave), with few defects, indicative of HP-HT antigorite, and also heavily kinked regions as fingerprints of strong tectonic shear. TEM imaging and electron diffraction patterns reveal two topotactic relationships between antigorite and olivine: [100](atg)//[010](ol) and < 100 >(atg)//< 100 >(ol); the planes in contact are (001)(atg)//(100)(ol) and (001)(atg)//(010)(ol), respectively. The [010](atg)//[001](ol) and antigorite lamellae are parallel to the forsterite b-axis. In both cases, the topography of olivine-serpentine interfaces is controlled by open fluid pathways along microcracks oriented according to the anisotropy of the olivine aggregate. In the cases studied, the serpentine aggregate exhibits a preferred orientation inherited from that of the peridotite. These results have some relevance to the seismic anisotropy of serpentinized mantle. Anisotropy of propagation of seismic waves as a result of the olivine fabric is maintained and reinforced with the development of lizardite. Conversely, the development of antigorite produces a trench-parallel fast S-wave polarization and an anisotropy that is lowered at low degrees of serpentinization and then increased with increasing serpentinization.
ISSN: 0022-3530

hal-00456061
https://hal.archives-ouvertes.fr/hal-00456061
DOI : 10.1093/petrology/egp049