Melt topology and seismic anisotropy in mantle peridotites of the Oman ophiolite

This paper presents shape measurements of plagioclase and clinopyroxene inclusions, assumed to reflect melt topology, in peridotites of the uppermost mantle section of the Oman ophiolite. The plagioclase and clinopyroxene grains are devoid of any intracrystalline deformation in all samples. In contrast, the olivine in these rocks has recorded a high temperature plastic deformation, with different strengths of the crystallographic preferred orientation (CPO) of the olivine grains. Individual ‘melt pockets’ are first described by ellipses in two dimensions. They are more elongated when they have a larger area, and they are preferentially oriented parallel to the lineation (the X structural axis) of the sample, with a better defined preferred orientation for samples that have a stronger CPO. In a second step, an average melt phase shape is defined in three dimensions for each sample, using the image autocorrelation technique. The average shape is nearly spherical for the samples with weak CPOs, and it is ellipsoidal, with a long axis parallel to X and the short axis parallel to Z (normal to the foliation) for samples with strong CPOs. The long axis of the ellipsoid is 3 times as long as the short axis for the sample with the strongest CPO. We use an anisotropic differential effective medium method to estimate the seismic properties of partially molten upper mantle peridotites. The melt pockets were modelled as basalt filled inclusions with the average shape and orientation given by the image analysis. The CPO of the olivine crystals was used to calculate the elastic properties of the anisotropic background medium. The calculated P-wave seismic anisotropies ranged from 5 to 15% with the anisotropy increasing with the CPO strength and melt fraction. The maximum P-wave velocities are found along X with velocities above 8 km=s at 0% melt and an average 0.5 km=s reduction for 10% of melt. The minimum P-wave velocities are found along Z with velocities generally below 7.5 km=s at 0% melt and an average reduction of 0.8 km=s for 10% of melt. © 1998 Elsevier Science B.V. All rights reserved.

[1]  J. Francheteau,et al.  Plagioclase-wehrlites and peridotites on the East Pacific Rise (Hess Deep) and the Mid-Atlantic Ridge (DSDP Site 334): evidence for magma percolation in the oceanic upper mantle , 1993 .

[2]  A. Nicolas,et al.  Crystalline plasticity and solid state flow in metamorphic rocks , 1976 .

[3]  Mineo Kumazawa,et al.  Elastic moduli, pressure derivatives, and temperature derivatives of single‐crystal olivine and single‐crystal forsterite , 1969 .

[4]  H. Bunge.  Texture Analysis in Materials Science , 1982 .

[5]  G. Bussod,et al.  Textural Development and Melt Topology in Spinel Lherzolite Experimentally Deformed at Hypersolidus Conditions , 1991 .

[6]  F. Boudier,et al.  Mapping oceanic ridge segments in Oman ophiolite , 1995 .

[7]  J. Korringa,et al.  On the Dependence of the Elastic Properties of a Porous Rock on the Compressibility of the Pore Fluid , 1975 .

[8]  F. Boudier,et al.  Detailed mapping of a mantle diapir below a paleo-spreading center in the Oman ophiolite , 1998 .

[9]  D. Bideau,et al.  Petrology of the East Pacific Rise crust and upper mantle exposed in Hess deep (Eastern Equatorial Pacific) , 1993 .

[10]  F. Gaßmann.  Uber die Elastizitat poroser Medien. , 1961 .

[11]  Renée Heilbronner,et al.  The autocorrelation function: an image processing tool for fabric analysis , 1992 .

[12]  Gerald M. Mavko,et al.  Velocity and attenuation in partially molten rocks , 1980 .

[13]  D. Kohlstedt,et al.  Influence of deformation on melt topology in peridotites , 1997 .

[14]  A. Nicolas,et al.  Textures and Fabrics of Upper-Mantle Peridotites as Illustrated by Xenoliths from Basalts , 1975 .

[15]  Nikolaus von Bargen,et al.  Permeabilities, interfacial areas and curvatures of partially molten systems: Results of numerical computations of equilibrium microstructures , 1986 .

[16]  D. Mainprice,et al.  Methods of calculating petrophysical properties from lattice preferred orientation data , 1994 .

[17]  Amos Nur,et al.  Melt squirt in the asthenosphere , 1975 .

[18]  Anil K. Jain,et al.  Texture Analysis , 2018, Handbook of Image Processing and Computer Vision.

[19]  T. Murase,et al.  The use of laboratory velocity data for estimating temperature and partial melt fraction in the low‐velocity zone: Comparison with heat flow and electrical conductivity studies , 1989 .

[20]  H. Green,et al.  Melt topology in partially molten mantle peridotite during ductile deformation , 1994, Nature.

[21]  F. Boudier,et al.  Mantle flow patterns at an oceanic spreading centre: The Oman peridotites record , 1988 .

[22]  D. Toomey,et al.  Intergranular basaltic melt is distributed in thin, elogated inclusions , 1994 .

[23]  D. Bideau,et al.  Plastic deformation and magmatic impregnation in serpentinized ultramafic rocks from the Garrett transform fault (East Pacific Rise) , 1990 .

[24]  A. McBirney,et al.  Properties of some common igneous rocks and their melts at high temperatures , 1973 .

[25]  D. Toomey,et al.  Seismological evidence for three-dimensionalmelt migrationbeneath the EastPacificRise , 2022 .

[26]  J. A. Tyburczy,et al.  Mechanical and thermodynamic constraints on fluid distribution in partial melts , 1979 .

[27]  U. Faul,et al.  Effects of Crystalline Anisotropy on Fluid Distribution in Ultramafic Partial Melts , 1992 .

[28]  S. Solomon,et al.  THE STRUCTURE OF MID-OCEAN RIDGES , 1992 .

[29]  A. Nicolas,et al.  CLASSIFICATION OF TEXTURES AND FABRICS OF PERIDOTITE XENOLITHS FROM SOUTH AFRICAN KIMBERLITES , 1975 .

[30]  F. Boudier,et al.  Nature of the Moho Transition Zone in the Oman Ophiolite , 1995 .

[31]  H. Schmeling,et al.  Numerical models on the influence of partial melt on elastic, anelastic and electric properties of rocks. Part I: elasticity and anelasticity , 1985 .

[32]  D. Mainprice.  Modelling the anisotropic seismic properties of partially molten rocks found at mid-ocean ridges , 1997 .

[33]  J. Bodinier.  Geochemistry and petrogenesis of the Lanzo peridotite body, western Alps , 1988 .

[34]  P. Silver,et al.  Interpretation of SKS-waves using samples from the subcontinental lithosphere , 1993 .