Plagioclase preferred orientation and induced seismic anisotropy in mafic igneous rocks. Issue 11 (13th November 2014)

• Record Type: Journal Article
• Authors: Ji, Shaocheng; Shao, Tongbin; Salisbury, Matthew H.; Sun, Shengsi; Michibayashi, Katsuyoshi; Zhao, Weihua; Long, Changxing; Liang, Fenghua; Satsukawa, Takako
• Is Part Of: Journal of geophysical research. Volume 119:Issue 11(2014:Nov.)
• Page Start: 8064
• Page End: 8088
• Publication Date: 2014-11-13
• Subjects: Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1002/2014JB011352
• Languages: English
• ISSNs: 2169-9313

Abstract

<abstract abstract-type="main"> <title>Abstract</title> <p>Fractional crystallization and crystal segregation controlled by settling or floating of minerals during the cooling of magma can lead to layered structures in mafic and ultramafic intrusions in continental and oceanic settings in the lower crust. Thus, the seismic properties and fabrics of layered intrusions must be calibrated to gain insight into the origin of seismic reflections and anisotropy in the deep crust. To this end, we have measured <italic>P</italic> and <italic>S</italic> 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 distinctive seismic anisotropy patterns: <italic>V</italic><sub>p</sub>(Z)/<italic>V</italic><sub>p</sub>(Y) ≥ 1 and <italic>V</italic><sub>p</sub>(Z)/<italic>V</italic><sub>p</sub>(X) ≥ 1 for anorthosites while 0.8 < <italic>V</italic><sub>p</sub>(Z)/<italic>V</italic><sub>p</sub>(Y) ≤ 1 and 0.8 < V<sub>p</sub>(Z)/V<sub>p</sub>(X) ≤ 1 for gabbros. Amphibolites lie in the same domain as gabbros, but show a significantly stronger tendency of <italic>V</italic><sub>p</sub>(X) > <italic>V</italic><sub>p</sub>(Y) than the gabbros. Laminated anorthosites with <italic>V</italic><sub>p</sub>(X) ≈ <italic>V</italic><sub>p</sub>(Y) ≪ <italic>V</italic><sub>p</sub>(Z) display a strong crystal preferred orientation (CPO) of plagioclase whose (010) planes and [100] and [001] directions parallel to the foliation. For the gabbros and amphibolites characterized by <italic>V</italic><sub>p</sub>(X) ≈ <italic>V</italic><sub>p</sub>(Y) > <italic>V</italic><sub>p</sub>(Z) and <italic>V</italic><sub>p</sub>(X) > <italic>V</italic><sub>p</sub>(Y) > <italic>V</italic><sub>p</sub>(Z), respectively, pyroxene and amphibole play a dominant role over plagioclase in the formation of seismic anisotropy. The Poisson's ratio calculated using the average <italic>P</italic> and <italic>S</italic> 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.</p> </abstract>