On The Deformation Of Hydrous Minerals (Anitgorite And Chlorite) And Their Seismic Anisotropies In Ocean Subduction Zones

Material cycling in subduction zones are hot topics in geodynamics because subduction zones are where sediments,oceanic crust,and mantle lithosphere return to and reequilibrate with Earth's mantle,as they act as the geodynamic factories in Earth material cycling.The plate and sediments are subducted into Earth's interior,while they can be recycled to the Earth's surface through arc magmatism caused by fluid or melt,originated from dehydration or melting of the subducting slab.However,the outcrops of HP/UHP metamorphic rocks along the paleo-subduction shear zone indicate that there are also materials recycled directly along the shear zone.Hydrous minerals with low density and low rheological strength are suggested to be the main reason for inducing exhumation.Meanwhile,the studies of mantle wedge seismic anisotropy show that the polarization direction of fast shear wave is parallel to trench in for-arc regions,in contradiction with the classic corner flow model.Under a cold and hydrated conditions,the deformation geometry or mechanisms for mantle could be different from back-arc to for-arc regions.One of the keys to decipher dynamic processes in subduction channels and mantle wedges is the deformation of hydrous minerals.This dissertation aims to decipher the deformation and deformation mechanisms of hydrous minerals?antigorite and chlorite?,through a systematic microstructural study on the deformation of the Val Malenco serpentinized peridotite from the paleo-subduction mantle wedge,as well as experimental investigations at high pressure and high temperature conditions using a 5GPa Griggs-type deformation apparatus.The main conclusions are:?1?The complicated multi-stage metamorphic/metasomatic deformation processes for the Val Malenco serpentinized peridotite during the alpine orogen subduction and exhumation are elaborated.The minerals can be divided into four groups:rare porphyroblasts with strong intra-crystalline deformation?Ol^P,Cpx^P?,recrystallized grains?Ol^R and core of Cpx^C?,metamorphism?Ol^C,Atg^C?and metasomatism?rim of Cpx^C,Di,Atg^F?,respectively.The Val Malenco serpentinized peridotite experienced a stealth metasomatism,high temperature plastic deformation and then recrystallization during the exhumation of mantle,as evidenced by:1)Trace elements contents of the Cpx porphyroblast are low but enriched in fluid mobile elements and low olivine Mg#?86-87?,implying a metasomatism process.2)Strong intracrystalline deformation in olivine and Cpx porphyroblasts with developed subgrains and the slip system activated are active only at temperature of 800-1000^?.3)The topotaxy relationship between the exsolved Mag and Cpx is?100?_Cpx//?111?_Mag,?010?_Cpx//[-110]_Mag,[001]_Cpx//[-1-12]_Mag,[101]Cpx//[112]_Mag,?-101?_Cpx//?-1-11?_Mag,indicating a formation temperature around800-900^?.4)The characteristic of grain boundaries movement and inherited fabrics from single crystals suggest a recrystallization process.After the exhumation,oceanic serpentinization occurred and the subsequent subduction result in the formation of granoblastic texture composed of coarse olivine and antigorite during prograde metamorphism:1)Well developed triple junction,non-plastic deformation,extremely low dislocation density supported the idea of metamorphic olivine.2)Metamorphic temperature?430-460^??,as calculated by using the Al content of coarse antigorite,is consistent with former study.3)Coarse antigorite is enriched in As,Sb and B,which is suggested to be the characteristic of oceanic serpentine.Dissolution-Precipitation took place and was responsible for the formation of antigorite-rich layer and diopside aggregate pockets during the exhumation of serpentinized peridotite:1)The zigzag fine grained olivine grain boundaries with voids in antigorite-rich layer is the classic dissolution texture.2)The temperature?300-370^??estimated by the Al content of fine grained antigorite indicate the antigorite-rich layer were developed after the granobalstic texture in olivine-rich layer,suggesting exhumation of subducted terrace.3)Metasomatic microstructures,such as embayment structure,olivine inclusions,diopside growth zoning,olivine single crystal separated by fine diopside aggregates,curved boundaries between diopside aggregate pockets and olivine,illustrate that diopside grains were formed by consuming the olivine,which support dissolution-precipitation creep took place during the exhumation of Val Malenco serpentinized peridotite.?2?We present here for the first time that dissolution precipitation creep as a new mechanism for the development of strong B type olivine fabric during serpentinization in a low temperature and fluid enriched mantle wedge.Olivine in the serpentine matrix develops a pronounced B-type fabric and strong SPO,while olivine in the olivine-rich layer displays nearly random fabrics.At the same time,the chemical composition and non-dislocation of fine grained olivine are the same with coarse olivine.The temperature estimated by the Al content of fine grained antigorites is 300-370^?.We suggest that anisotropic dissolution resulted in anisotropic grain shape,which induced strong CPO by rotation during syntectonic deformation.The long axis with slowest dissolution rate is pralallel to[001]axis,while the fastest dissolution rate is along the[010]that is the short axis and normal to foliation.The B type fabric developed with the help of rotation and grain boundary sliding.These results document for the first time the B-type olivine CPO formed by dissolution creep at low temperatures during serpentinization and provide a mechanism to reconcile petrofabric observations with geophysical observations of trench parallel fast S-wave seismic anisotropy in fore-arc mantle wedge regions.?3?Antigorite developed strong[010]?001?fabric in antigorite matrix,and strong[010]?h0k?fabrics were developed in olivine-rich layer,which is supposed to be a high stress,low strain environment.As presented above,the antigorite-rich layer was formed by deformation concomitant with serpentinization,and the contradiction between developed[010]?001?fabric and[010]rotation axis exclude the dislocation creep mechanism,but suggest a precipitation creep.Grain growth process should be also important for coarse antigorite grains in olivine-rich layer since growth zoning can be observed.The strain-controlled anisotropic growth result in anisotropic grain shape with long and short axis parallel with[010]and[001],respectively.The stress-controlled anisotropic growth result in anisotropic grain shape with only long axis parallel to the[010].Thus,[010]?001?and[010]?h0k?fabrics are developed in strain-controlled antigorite matrix and high stress-controlled olivine-rich layer.As antigorite are formed at low temperature and fluid enriched environment,it can be envisaged that DCP is an effective deformation mechanism to accommodate the strain in mantle wedge and subduction channel during serpentinization.?4?DCP resulted in the development of strong clinopyroxene[001]?100?fabric and enrichment of clinopyroxene bands in olivine-rich layer,suggesting a strain localization process.The clinopyroxene enriched bands and connected clinopyroxene pockets are strain controlled,and developed[001]?100?fabric,which is correlated with macroscopic lineation.However,high concentration of the[001]axes is not related to macroscopic lineation in some sealed pockets,indicating the critical role played by stress.Since DCP is the main process during deformation,we suggest that growth rate along[001]and[100]are the fastest and slowest,respectively,resulting in the development of low index boundaries parallel with the[001]direction.Thus,clinopyroxene will develop[001]?100?fabrics or concentration of axis correlated with strain or stress,respectively.Our results indicate that[001]?100?is not a reliable indication of activation of low temperature slip system and the development of stronger clinopyroxene fabrics with weaker olivine fabrics can be explained by DCP in orogenic peridotite.?5?The rheological law of chlorite is constructed for the first time through systematic experiments conducted at 1-2.5 GPa,500-700^? using a 5 GPa Griggs apparatus(?=e^-27.2?^4.1exp[-??48830+11.51*P?/RT?]).The deformation behavior of antigorite is semi-brittle and the yield strength is much larger than chlorite under the same deformation conditions.The stress exponent of the flow law and the developed kink bands are consistent,indicating a dominant dislocation creep.Chlorite could result in strain localization and the decoupling of plate-wedge in subduction zone,based on the weaker rheological strength in comparison with olivine aggregate and eclogite.Antigorite can't accommodate large strain by dislocation creep,at least under the laboratory strain rate conditions.?6?We have conducted a systematic shear deformation experiments on antigorite and chlorite aggregates and illustrated their fabric development,which provide constraints on the explanation of seismic anisotropies in subdcution zones.As the experiment results showed,both of them develop strong fabrics with?001?plane aligned in foliation.The[010]axes of antigorite concentrate along the lineation as temperature increases,while the[100]and[010]axes of chlorite form a girdle.The chlorite intra-crystalline deformation analyses suggest the deformation are accommodated by twist boundares with?001?rotation axis and tilt boundaries associated with[100]?001?,[010]?001?and[hk0]?001?slip systems.We suggest that[010]?001?is the easiest slip system for antigorite at high temperatures,while the contributions of[100]?001?,[010]?001?and[hk0]?001?slip systems are more or less the same for chlorite deformation.The inter layer slip contributes significantly to shear plastic deformation.The development of strong[hk0]?001?can explain the large shear wave delay time observed in subduction zone with dip angle.