11. SEISMIC VELOCITIES AND ELASTIC PROPERTIES OF OCEANIC GABBROIC ROCKS FROM HOLE 735B1
11. SEISMIC VELOCITIES AND ELASTIC PROPERTIES OF OCEANIC GABBROIC ROCKS FROM HOLE 735B1
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11 735B1 孔大洋辉长岩的地震速度和弹性特性
DOI：
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发表时间：
1991
期刊：
影响因子：
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通讯作者：
M. Salisbury
中科院分区：
文献类型：
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作者：
G. Iturrino;N. Christensen;S. Kirby;M. Salisbury
在7358孔，特别是在该剖面底部附近，几乎连续地恢复了0.5公里的一般新鲜的第3层辉长岩，为科学家提供了一个不寻常的机会来研究下洋壳的详细弹性特性。扩展压缩波和密度船在室内压力下的测量，Vp和Vs的测量压力从20至200 MPa使用脉冲传输方法。所有的岩石都表现出显着的速度增加，随着压力增加到约150 MPa，微裂纹孔隙度的关闭归因于一个功能。测得的速度反映了7358孔构造历史期间获得的矿物组成和显微结构。大部分未变形和未蚀变辉长岩约65：35斜长石/单斜辉石岩石加橄榄石或氧化物矿物，和所观察到的密度和速度是完全一致的Voigt-Reuss-Hill（弗赫）平均值的组分矿物和它们的比例。根据其橄榄石含量，主要的橄榄石辉长岩在200 MPa下的平均V = 7.1：t 0.2 km/s，Vs = 3.9：t 0.1 km/s，颗粒密度为2.95：t 0.5 g/cm 3。含量较少的铁-钛（Fe-Ti）氧化辉长岩平均Vp = 6.75：t 0.15 km/s，Vs = 3.70：t 0.1 kmls，颗粒密度为3.22：t 0.05 g/cm 3，反映了氧化物矿物的密度高于橄榄石，速度较低。大约30%的核是塑性变形的，这些剪切带构造岩的密度和定向平均速度一般与辉长岩，它们的原岩。三组观测结果表明，剪切带变辉长岩具有弹性各向异性：（1）垂直和水平方向以及面理和线理的Vp'方向变化：（2）水平岩芯的Vp值与组分矿物的弗赫平均值及其矿物比例的差异，表明各向异性矿物的择优晶体取向;（3）Vs的变化可达7%，偏振方向平行于叶理或垂直于叶理。光学检查薄切片相同的样品表明，斜长石，单斜辉石，和角闪石通常显示晶体学的优选取向，这一点，加上这些矿物的弹性各向异性，这表明，优选取向是负责大部分观察到的各向异性，特别是在高压。蚀变倾向于局限于脆性断层和角砾化带，典型的蚀变矿物是角闪石和次生斜长石，它们不会显著改变速度-密度关系。
The nearly continuous recovery of 0.5 km of generally fresh, layer 3 gabbroic rocks at Hole 7358, especially near the bottom of the section, presents scientists an unusual opportunity to study the detailed elastic properties of the lower oceanic crust. Extending compressional-wave and density shipboard measurements at room pressure, Vp and Vs were measured at pressures from 20 to 200 MPa using the pulse transmission method. All of the rocks exhibit significant increases in velocity with increasing pressure up to about 150 MPa, a feature attributed to the closing of microcrack porosity. Measured velocities reflect the mineralogical makeup and microstructures acquired during the tectonic history of Hole 7358. Most ofthe undeformed and unaltered gabbros are approximately 65:35 plagioclase/ clinopyroxene rocks plus olivine or oxide minerals, and the observed densities and velocities are fully consistent with the Voigt-Reuss-Hill (VRH) averages of the component minerals and their proportions. Depending on their olivine content, the predominant olivine gabbros at 200 MPa have average V = 7.1 :t 0.2 kmls, Vs = 3.9 :t 0.1 km/s, and grain densities of 2.95 :t 0.5 g/cm3. The less abundant iron-titanium (Fe-Ti) oxide gabbros average Vp = 6.75 :t 0.15 km/s, Vs = 3.70 :t 0.1 kmls, and grain densities of 3.22 :t 0.05 g/cm3, reflecting the higher densities and lower velocities of oxide minerals compared to olivine. About 30% of the core is plastically deformed, and the densities and directionally averaged velocities of these shear-zone tectonites are generally consistent with those of the gabbros, their protoliths. Three sets of observations indicate that the shear-zone metagabbros are elastically anisotropic: (I) directional variations in Vp' both vertical and horizontal and with respect to foliation and lineation; (2) discrepancies among Vp values for the horizontal cores and the VRH averages of the component minerals and their mineral proportions, suggesting preferred crystallographic orientations of anisotropic minerals; and (3) variations of Vs of up to 7%, with polarization directions parallel and perpendicular to foliation. Optical inspection of thin sections of the same samples indicates that plagioclase feldspar, clinopyroxene, and amphibole typically display crystallographic-preferred orientations, and this, plus the elastic anisotropy of these minerals, suggests that preferred orientations are responsible for much of the observed anisotropy, particularly at high pressure. Alteration tends to be localized to brittle faults and brecciated zones, and typical alteration minerals are amphibole and secondary plagioclase, which do not significantly change the velocity-density relationships.