Experimental Studies On Rheological Properties Of Clinopyroxene And Felsic Mylonite Under High Temperature And High Pressure

In this paper,we have carried out a series of high temperature and high pressure experimental studies on the rheological properties of clinopyroxene and felsic mylonite respectively.We will briefly summarize each of them in the following:1.Experimental study on clinopyroxeneThe dynamics process of the earth's interior is closely related to the plastic rheological behavior of its constituent minerals under high temperature and pressure.Clinopyroxene is one of the main mineral in lithosphere,although its less content than that of olivine and enstatite,its relative rheological strength and distribution characteristics in rocks also affect the plastic deformation behavior of the rocks in the lithosphere to a great extent.Therefore,using high-T and-P experiments to investigate the rheology behavior of clinopyroxene under different conditions can provide accurate experimental constraints for exploring the material composition of the earth's interior.Previous studies on the rheological properties of clinopyroxene mainly focused on clinopyroxene from the lower crust,but rarely on clinopyroxene from the upper mantle.However,the Fe content of clinopyroxene from lower crust([Fe]?16%)is higher than that of clinopyroxene from upper mantle([Fe]?3%).previous studies of olivine and garnet shown that there is a positive correlation between Fe content and creep rate.For clinopyroxene which are both silicate minerals,if the difference of Fe content has the same effect on their rheological properties,then the rheology determined by previous scholars using clinopyroxenes from the crust are not suitable for the upper mantle environment.Therefore,it is necessary to study the rheological properties of mantle derived clinopyroxene.In this study,we used the mantle derived clinopyroxene,and conducted a series of dislocation creep experiments on the hot-pressed clinopyroxene aggregates by using the Paterson apparatus.The rheological parameters of clinopyroxene under different water conditions were determined.Beside,we carried out deformation experiments on dry clinopyroxene under different oxygen fugacity(Fe/FeO and Ni/NiO)to explore the effect of oxygen fugacity on the rheology of clinopyroxene.Water content and microstructure characteristics of the samples before and after deformation was calculated and analyzed by FTIR and micrographs,respectively.The results of this study are compared and discussed with previous studies on dislocation creep properties of clinopyroxenes from lower crust.The results of this study fill the gap in the study of the rheological properties of mantle derived clinopyroxene,and help us to accurately explain the rheological behavior of the upper mantle clinopyroxene and the constraints on the rheological properties of the upper mantle.In the following,we will summarize the results obtained in this study from three aspects:the influence of water,the influence of oxygen fugacity and the influence of Fe content:<1>Water effectThe pre-test results show that the oxygen fugacity condition is negatively correlated with the water content of clinopyroxene.Therefore,when discussing the influence of water,we will ensure that the oxygen fugacity condition is fixed.(?)When the oxygen fugacity is Ni/NiO,the creep rate of water saturated clinopyroxene is about 45 times faster than that anhydrous clinopyroxene.The stress exponent and activation energy of dislocation creep of clinopyroxene under anhydrous and water saturated conditions are:n_dry=5.1±0.4,Q_dry=443±12k J/mol;n_wet=3.0±0.1,Q_wet=396±30 k J/mol,respectively.Accordingly,water has no significant effect on the activation energy,but is negatively correlated with the stress exponent,indicating that the existence of water can affect the deformation mechanism of the clinopyroxene.(?)When the oxygen fugacity is Fe/FeO,the strength of once drying,secondary drying and nominal anhydrous clinopyroxene with different trace amount of water were compared.The results showed that even a bit of water can weaken clinopyroxene obviously,and the weakening degree increases with the increase of water content.H₂O mainly increases the point defects in the crystal in the form of H⁺,which accelerates the dislocation climbing rate and then weakens the rheological strength.In addition,there is a weak negative correlation between the stress exponent of the three groups of clinopyroxene,but there is no significant difference in the activation energy.Therefore,the existence of water can affect the deformation mechanism of clinopyroxene.(?)We quantified the effect of water on the creep behavior of clinopyroxene according to the creep strength of once drying,secondary drying and nominal anhydrous clinopyroxene(Fe/FeO)with different water contents.Taking the stress exponent as a fixed value of 4.3,we determined the water content index r'=0.5±0.1and the activation energy Q=301±30 k J/mol,in the dislocation creep equation of unsaturated clinopyroxene.(?)The microstructure of clinopyroxene aggregates before and after deformation shows that,for the water saturated condition,the clinopyroxene grains are slightly oriented,and the grain boundaries are serrated and highly stitched,indicating that the fluidity is high;while for the anhydrous condition,the clinopyroxene grains exhibit obvious preferred orientation perpendicular to the compression direction.The grain boundaries are flat or slightly curved,indicating relatively low fluidity.<2>Iron effect(?)The creep rate of mantle derived clinopyroxene([Fe]?2 at.%)is about 4times slower than that of crust derived clinopyroxene([Fe]?11 at.%)studied by Bystricky and Mackwell(2001)under the same conditions,indicating that there is a positive correlation between Fe content and creep rate(??[Fe]).(?)The rheological parameters of clinopyroxene obtained in this study and Bystricky and Mackwell(2001)are:n_{this study}=4.4?5.1,Q_{this study}=389?443k J/mol;n _B&M=4.7±0.2,Q _B&M=760±40 k J/mol,respectively.The Fe content in clinopyroxene has no obvious effect on the stress exponent,but is positively correlated with the activation energy,indicating there is no obvious effect on the dominant deformation mechanism.But the specific effect of Fe content on the creep activation energy of clinopyroxene is worthy of further experimental exploration.(?)The degree of water-induced weakening of clinopyroxenes is positively dependent on their Fe content.This may be due to the influence of Fe content on the creep properties of clinopyroxene,which makes the water induced weakening effect of the apparent upper crust clinopyroxene more significant.However,the effect of Fe content on the rheological properties of clinopyroxene needs further experimental study.(?)In addition,Fe as a variable valence element in clinopyroxene will be affected by oxygen fugacity,which will affect the rheological properties of clinopyroxene.The higher the Fe content is,the more significant the effect of oxygen fugacity is.<3>Oxygen fugacity effect(?)Under the same condition,the strain rate of clinopyroxene at Ni/NiO condition is about 1.5 times faster than that at Fe/FeO.By re-drawing the mechanical data obtained by Bystricky and Mackwell(2001),it was found that the creep rate under Ni/NiO oxygen fugacity is about 3 times faster than that under Fe/FeO oxygen fugacity,which indicates that the condition oxygen fugacity was positively correlated with creep rate.(?)The influence of oxygen fugacity in this study is lower than that in Bystricky and Mackwell(2001).The reason is that the Fe content in clinopyroxene for our study is lower than that in their samples,showing that oxygen fugacity can change the valence state of variable valence element Fe,change the characteristics of point defects in clinopyroxene crystal,and then affect the dislocation creep mechanism,and finally lead to the change of rheological properties.Therefore,the higher the iron content in clinopyroxene,the more obvious the effect.(?)The activation energy and stress exponent of clinopyroxene creep at Ni/NiO condition are larger than that at Fe/FeO condition.In addition,under Fe/FeO condition,the activation energy of clinopyroxene is basically consistent with that of Si self-diffusion in clinopyroxene,and the stress exponent of 4.4 is closer to the theoretical value of 3 when dislocation climb dominates the deformation,indicating that dislocation climb dominates the deformation of clinopyroxene with Fe/FeO oxygen fugacity,while the form of dislocation creep for clinopyroxene under Ni/NiO condition is complex,involved in the deformation of clinopyroxene other creep forms of dislocations(such as kink,slip,etc.).The higher the oxygen fugacity,the faster the creep rate of clinopyroxene and the more complex the dislocation creep mechanism.2.Experimental study of felsic mylonite rockGeological and geophysical evidence indicate that the continental crust are mainly composed of felsic rocks,these rocks generally exhibit inhomogeneous and anisotropic on a wide variety of scales with the presence of sedimentary layering,metamorphic shape-preferred orientation.The existence of these textures may affect the deformation behaviors of these rocks,and then most probably affect the formation of detachment faults and the evolution of continental orogenic belt.Therefore,knowing the deformation behaviors of anisotropy felsic rocks has a direct indication for our understanding crustal behaviors in different tectonic settings.So far,extensive experimental studies have been conducted to investigate the original foliation effect on felsic rocks,and all those results yield that the compressive fracture or ductile strengths of rocks shortened in direction perpendicular to foliation were consistently stronger than those compression parallel to foliation.However,quite rare studies have focused on the influence of lineation which may actually furnish information on the direction tectonic transport.Besides,the past deformational experiments were typically preformed on solid media apparatuses,which yield a problem that the difficulty of achieving steady state flow,and they provided only limited data about plastic flow and the development of textures for anisotropy crustal rocks.In this study,we have carried out a series of deformational experiments on high-temperature steady state creep of felsic mylonite by particular compression directions with respect to foliation and lineation,at a constant confining pressure of300 MPa,temperatures from 850 to 975°C,and strain rates from 1×10^-4 s^-1 to2.5×10^-6 s^-1,to separately investigate the effect of pre-existing foliation and lineation on the rheological behaviors of felsic rocks and to develop accurate constitutive laws that can be applied to natural deformations.The experimental results showed that:(?)Over the range of experimental conditions,samples deformed in dislocation creep regime,with consistent rheological parameters for all compression directions,the average stress exponent ^an=3.0±0.5,and the average activation energy ^aQ=500±52 k J/mol.(?)The mechanical results demonstrate that:for individual S-effect,the samples perpendicular to S are stronger than those parallel to S,while for individual L-effect,the strength of samples parallel to L are stronger than those perpendicular to L.(?)Microstructures and CPOs characteristics displayed that all the constituent minerals especially feldspar and quartz in samples which were parallelly compressed to the initial foliation or lineation,were significantly transformed into shape preferred orientations and crystallographic preferred orientations with progressing strains,which generally accompanied by new fabrics or a ductile shear zone with an angle of?30°to the older ones.While both the CPOs patterns and microstructures of samples in direction perpendicular to foliation and lineation retained unchanged or became strengthened.In this study,the influence of lineation and foliation fabric on the strength of felsic rocks was considered respectively,enriche the database of felsic rock mechanics,and provide a more favorable experimental basis for inferring the change of rock strength in detachment ductile shear zone and the scale and occurrence of detachment fault in the future.