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Laboratory measurements of ‘porosity‐free’ intrinsic V p and V s in an olivine gabbro of the O man ophiolite: Implication for interpretation of the seismic structure of lower oceanic crust
Author(s) -
Saito Satoshi,
Ishikawa Masahiro,
Arima Makoto,
Tatsumi Yoshiyuki
Publication year - 2015
Publication title -
island arc
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.554
H-Index - 58
eISSN - 1440-1738
pISSN - 1038-4871
DOI - 10.1111/iar.12092
Subject(s) - olivine , geology , gabbro , ophiolite , mineralogy , porosity , crust , analytical chemistry (journal) , seismology , geophysics , igneous rock , geochemistry , chemistry , geotechnical engineering , tectonics , chromatography
In order to determine ‘porosity‐free’ intrinsic ultrasonic compressional ( V p ) and shear wave ( V s ) velocities and V p / V s of an olivine gabbro from the O man ophiolite, we developed a new experimental system using a piston‐cylinder type high‐pressure apparatus. The new system allowed us to measure velocities at pressures ranging from 0.20 to 1.00 GP a and at temperatures up to 300°C for V p and 400°C for V s . At room temperature, the V p and V p / V s increase rapidly with pressure up to 0.40 GP a, while between 0.45 and 1.00 GP a the increase is more gradual. The change in increasing rate is attributed to closure of porosity at pressures above 0.45 GP a. Based on the linear regression of data obtained at higher pressures (0.45–1.00 GP a) and extrapolation to the lower pressures, combined with temperature derivatives of velocities of the sample measured at 1.00 GP a, we determined the intrinsic V p and V s of the sample as a function of pressure ( P , in GP a) and temperature ( T , in ° C ). The intrinsic velocities can be expressed as V p (km/s) = 7.004 + 0.096 × P − 0.00015 × T , and V s (km/s) = 3.827 + 0.007 × P − 0.00008 × T . We evaluated the intrinsic V p and V s of the olivine gabbro at oceanic crustal conditions and compared them with a velocity depth‐profile of the borehole seismic observatory WP‐2 area in the northwestern Pacific Basin. Although the intrinsic V p (∼7.0 km/s) and V s (∼3.8 km/s) for the olivine gabbro studied are comparable to those of seismic layer 3 in the WP ‐2 area, the estimated vertical gradients of intrinsic velocities are significantly smaller than those reported from layer 3. These results suggest that velocity profiles of layer 3 in the WP ‐2 area may reflect the presence of a minor porosity in lower oceanic crust, which closes with increasing depth and/or continuous changes in mineralogy of layer 3 rocks.