三斜晶系的长石是地壳表层——地壳中含量最多的矿物（51 vol%），其流变学性质控制整个地壳的变形。如何通过地震学方法了解地壳深处的应变状态与变形构造呢？ Plagioclase preferred orientation and induced seismic anisotropy in mafic igneous rocks Shaocheng Ji, Tongbin Shao, Matthew H. Salisbury, Shengsi Sun, Katsuyoshi Michibayashi, Weihua Zhao, Changxing Long, Fenghua Liang, and Takako Satsukawa Journal of Geophysical Research (2014): DOI: 10.1002/2014JB011352 Abstract （250 words） Fractional crystallization and crystal segregation controlled by settling or floating of minerals during the cooling of magma can lead to layered structures in maficand ultramafic intrusions in continental and oceanic settings in the lowercrust. Thus the seismic properties and fabrics of layered intrusions must be calibrated to gain insight into the origin of seismic reflections andanisotropy in the deep crust. To this end, we have measured P- and S-wave velocities and anisotropy in 17 plagioclase-rich mafic igneous rocks such as anorthosite and gabbro at hydrostatic pressures up to 650 MPa. Anorthosites and gabbroic anorthosites containing 80 vol% plagioclase and gabbros consisting of nearly equal modal contents of plagioclase and pyroxene display distinctiveseismic anisotropy patterns: V p (Z)/ V p (Y)≥1 and V p (Z)/ V p (X)≥1 for anorthosites while 0.8 V p (Z)/ V p (Y)≤1 and 0.8V p (Z)/V p (X)≤1 for gabbros. Amphibolites lie in the same domain as gabbros, but show a significantly stronger tendency of V p (X) V p (Y) than the gabbros. Laminated anorthosites with V p (X)≈ V p (Y) ≪ V p (Z) display a strong crystal preferred orientation (CPO)of plagioclase whose (010) planes and and directions parallel to the foliation. For the gabbros and amphibolites characterized by V p (X)≈ V p (Y) V p (Z)and V p (X) V p (Y) V p (Z), respectively, pyroxene and amphibole play a dominantrole over plagioclase in the formation of seismic anisotropy. T he Poisson's ratio calculated using the average P- and S-wave velocities from the three principal propagation-polarization directions (X, Y and Z) of a highly anisotropic anorthosite cannot represent the value of a true isotropic equivalent. The CPO-induced anisotropy enhances and decreases the foliation-normal incidence reflectivity at gabbro-peridotite and anorthosite-peridotite interfaces, respectively. Keywords: Mafic intrusion, Plagioclase, Seismic wave velocities, Anisotropy, Fabric, Continental and oceanic crust 参考文献： References A.S.M.E. (1953), Viscosity and density ofover forty lubricating fluids of known composition at pressure to 150 000p.s.i. and temperatures to 425 °F, New York: Amer. Soc. Mech. Engrs . Ague, D. M., and H.-R. 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Seismic velocities, anisotropy and shear-wave splitting of antigorite serpentinites and tectonic implications for subduction zones Shaocheng Ji, Awei Li, Qian Wang, Changxing Long, Hongcai Wang, Denis Marcotte, and Matthew Salisbury Journal of Geophysical Research: Solid Earth Volume 118, Issue 3, pages 1015–1037, 2013, DOI: 10.1002/jgrb.50110 Abstract Antigorite, the high temperature form of serpentinite, is believed to play a criticalrole in various geological processes of subduction zones. We have measured P- and S-wave velocities (V p and V s ), anisotropy and shear-wave splitting of 17 serpentinite samples containing 90% antigoriteat pressures up to 650 MPa. The new results, combined with data forlow temperature lizardite and/or chrysolite, reveal distinct effects of low and high temperature (LT and HT) serpentinization on the seismic properties of mantlerocks. At 600 MPa, V p =5.10 and 6.68 km/s, V s =2.32 and 3.67 km/s, and V p /V s =2.15 and 1.81 for pure LT and HT serpentinites, respectively. Above the crack-closure pressure (~150 MPa), thevelocity ratio of antigorite serpentinites displays little dependence onpressure or temperature. Serpentine contents within subduction zones and forearc mantle wedges where temperature is 300 °Cshould be at least twice that of previous estimates based on LT serpentinization. The presence of seismic anisotropy, high-pressure fluids or partial melt is also needed to interpret HT serpentinized mantle with V p 6.68 km/s, V s 3.67km/s and V p /V s 1.81. The intrinsic anisotropy of the serpentinites (3.8-16.9% with an average value of 10.5% for V p , and3.6-18.3% with an average value of 10.4% for V s ) is caused by dislocation creep-induced lattice-preferred orientation (LPO) of antigorite. Three distinct patterns of seismic anisotropy correspond to threetypes of antigorite fabrics (S-, L-, and LS-tectonites) formed by threecategories of strain geometry (i.e., coaxial flattening, coaxial constriction,and simple shear), respectively. Our resultsarethought to provide a new explanation for various anisotropic patterns ofsubduction systems observed worldwide. http://onlinelibrary.wiley.com/doi/10.1002/jgrb.50110/abstract
标签: 各向异性
