The Effect Of Water On The Rheology Of Clinopyroxene And Plagioclase Under High Temperature And High Pressure

A trace amount of hydrogen as defects in the lattice of nominally anhydrous minerals can significantly influence the physical and chemical properties of rocks and minerals,such as the weakening effect on the strength.The constitutive relation between structurally bound hydroxyl and rheological strength in minerals and rocks is a hot and difficult issue in current rheological studies.Clinopyroxene and plagioclase are important constituent minerals of the lower crust,whose rheological strength,especially under hydrous conditions,are of great significance to constrain the strength of the lower crust and the strength of the lithosphere.In this dissertation,systemic axial compression experiments under high temperature and high pressure are performed to investigate the water weakening effect on the mechanical strength of clinopyroxene and plagioclase,together with water content and microstructural analyses of clinopyroxene and plagioclase in layered gabbros from the Panxi region.The new discoveries concluded in this dissertation are as follows:1.The water weakening effect on the rheology of clinopyroxene has been quantitatively determined in terms of the water content exponent?r=1.6?by high temperature and high pressure experiments.Systematic axial compression experiments have been performed on clinopyroxene?augite?aggregates using modified Griggs-type deformation apparatus under hydrous conditions at P=1 GPa,T=1000°C and a strain rate of 5×10^-5 s^-1.The strength of clinopyroxene aggregates decreases with increasing water content:the strength decreases by 4 times when the water content increases from 84 ppm to 662 ppm.Compared with the hot-pressed samples,clinopyroxene grains in deformed samples are significantly elongated along the foliation plane,forming a shape preferred orientation?SPO?with strong intracrystalline deformation.The lattice preferred orientation?LPO?are strong and characterized by the[001]axis forming girdles perpendicular to the compression direction and the?100?/?010?plane parallel to the foliation plane.These microstructures indicate clinopyroxene aggregates are deformed in the dislocation creep regime.The water weakening mechanism is inferred as:Hydrogen enters clinopyroxene lattice,and forms?3H?/?4H?defects combined with Si vacancies,which accelerates the rate of Si diffusion,and results in weakening of the strength of clinopyroxene.Assuming n=3 for a dislocation creep,the water content exponent?r?is 1.6,larger than that of olivine?r=1.0?and smaller than that of garnet?r=2.4?,respectively.The strength of clinopyroxene aggregates is one order of magnitude weaker than that of peridotite under hydrous conditions.2.The flow law of plagioclase under hydrous conditions can be expressed as ???=10^-3.2?^2.5C_OH^2.4exp?-378?kJ/mol?/RT?.Systematic axial compression experiments on plagioclase?labradorite?aggregates have been conducted under the conditions of high temperature and high pressure.FTIR spectra show a broad absorption band at?3400cm^-1,indicative of water in the form of structural IIb OH in plagioclase.The strength of plagioclase decreases with increasing water content:the strength decreases by 5times when the water content increases from 181 ppm to 588 ppm.The stress exponent n=2.4,associated with the weak fabric and the intracrystalline deformation is interpreted as the result of dislocation creep dominant deformation accompanied by grain boundary sliding.The water weakening mechanism is inferred as:Hydrogen enters plagioclase lattice in the substitution of Si⁴⁺=4H⁺,which accelerates the rate of Si diffusion that results in the weakening of plagioclase strength.When extrapolated to natural conditions,plagioclase with 100-500 ppm water is slightly weaker than wet quartz with 0.6 wt.%?6000 ppmw?water under low temperature conditions?lower than 600°C?,and an order of magnitude weaker than olivine with10-50 ppm water,respectively.Combined with the water weakening effect on clinopyroxene,our results provide quantitative evidence that implies a“weak”lower crustal in the lithosphere and supports the strong-weak-strong"Jelly sandwich"continental lithosphere strength profile.3.The water contents and fabrics of clinopyroxene and plagioclase in layered gabbros from the Panxi region are reported.The water contents of eight gabbro samples from the Panzhihua and Hongge layered intrusions are measured.The typical absorption bands of clinopyroxene appear at 3635-3600 cm^-1,3550-3510 cm^-1,3470-3445 cm^-1,with an average water content of 534±108 ppm.The typical absorption bands of plagioclase appear at 3630-3610 cm^-1,3580-3560 cm^-1,3430-3410 cm^-1,with an average water content of 480±95 ppm.Clinopyroxene has more water than plagioclase.The water partitioning coefficient between the clinopyroxene and plagioclase is 1.1±0.23,lower than previous reports,which may be attributed to different geological settings.The average bulk water content is 366±53 ppm,which represents only the lower limit of the bulk water content.Our new results indicate that the newborn lower crust could be“wet”,but not“dry”as previously thought.These water contents are lower than those in the Precambrian lower crust,implying either a low water content in the initial magma or later degassing of the magma.The fabrics of plagioclase are strong and characterized by the[100]axes nearly parallel to the lineation and the?010?planes parallel to the foliation.In contrast,the fabrics of clinopyroxene are weak and characterized by the[001]axes nearly parallel to the lineation and the?100?planes forming girdles parallel to the foliation.The fabrics of ilmenite are weak and featured with the?0001?planes forming girdles parallel to the foliation.Combining with their intracrystalline deformation features,it is suggested that clinopyroxene,plagioclase and ilmenite have experienced plastic deformation in the dislocation creep regime.The stronger fabrics of plagioclase indicates the strength of plagioclase is probably weaker than that of clinopyroxene under the conditions of similar water fugacity.Such a conclusion is also consistent with previous experimental results.Magnetite shows random fabric without significant intracrystalline deformation,and the extensive quadruples among magnetite grains are indicative of superplasticity in the regime of grain boundary sliding.Microstructures and the significant strength contrast between the silicates and the Ti-Fe oxides indicate that the Ti-Fe oxides have crystalized later than silicates.Compared with the Manjinggou granulites from the Huai'an region,North China,the fabric strengths of clinopyroxene and plagioclase are reversed.Because the water contents in clinopyroxene and plagioclase from the Manjinggou granulite are 1094±122 ppm,272±10 ppm,respectively.The water content reversal in the two minerals could be the reason for the fabric strength reversal between the two regions.Such a conclusion is also consistent with the experimental results of water weakening effect on the strength of clinopyroxene and plagioclase under high temperature and pressure.4.The effects of mineral fraction,fabric and layering on the seismic properties of layered gabbro have been evaluated quantitatively.Seismic property calculations show that the seismic velocities of gabbro are mainly controlled by the proportions of magnetite and silicates while the seismic anisotropies of gabbro depend on the plagioclase component.With increasing plagioclase proportions,the seismic velocities decrease,and the seismic anisotropies increase.Once the volume proportion of plagioclase reaches 20%in layered gabbro,the seismic anisotropy pattern of layered gabbro is similar to that of 100%plagioclase,indicating that plagioclase LPO plays a dominant role in determining the seismic anisotropy pattern.The anisotropies of Vp and Vs calculated with the Voigt-Reuss-Hill method are 2.6%and 2.63%respectively if neglecting the effects of layering.In contrast,combining the effects of both LPOs and the layering,the AVp increased from 2.6%to 5.1%and the AVs increases from 2.63%to 5.52%,respectively,using the AEH-FE method.However,the seismic velocities do not vary significantly.The maximum Vp increases slightly from 7.15 to 7.18 km/s,indicating layering has a limited effect on whole-rock seismic velocities,but a significant effect on seismic anisotropies.These results show that the AEH-FE method can explicitly estimate the effects of morphology,distribution and fabric on seismic properties for rocks with a layering structure.