Microstructural and experimental constraints on the rheology of partially molten gabbro beneath oceanic spreading centers
Introduction
The rheology of the partially molten lower oceanic crust, particularly beneath fast-spreading ridges, may control such ridge-scale phenomena as axial bathymetry (Wang et al., 1996) and lateral melt migration and redistribution during crustal accretion (Macdonald, 1998), yet direct observation and quantification of the parameters that control the rheology (i.e. stress, strain rate, melt fraction) of partially molten gabbros beneath oceanic spreading centers remains elusive. Seismic reflection/refraction and tomography studies of the East Pacific Rise (EPR) indicate that a thin melt lens is present directly beneath the base of Layer 2 (presumed sheeted dikes). The melt lens overlies a ∼4-km-thick low-velocity zone (LVZ) interpreted to contain 0–50% melt (Detrick et al., 1987, Harding et al., 1989, Sinton and Detrick, 1992, Kent et al., 1994, Mainprice, 1997). Based on these observations, the LVZ has been suggested to be a zone of relatively low viscosity where large amounts of hypersolidus flow occurs to (a) accommodate along-strike crustal accretion away from the centers of ridge segments (Nicolas et al., 1996, Wang et al., 1996), and (b) advect crystallized magma down and away from the melt lens where crustal accretion occurs (Nicolas et al., 1993, Phipps-Morgan and Chen, 1993, Quick and Denlinger, 1993, Boudier et al., 1996, Macdonald, 1998).
One way of constraining the rheology of the partially molten lower crust is to extrapolate the results of rock deformation experiments to conditions appropriate for the lower oceanic crust. In this paper we summarize recent experimental rock deformation studies on gabbroic rocks and their constituent minerals and correlate the results of these experiments to microstructural observations from ophiolites and mid-ocean ridges to provide constraints on the rheology of the lower crust beneath a spreading ridge.
Section snippets
Conceptual model
Our analysis is motivated by the observation of olivine subgrain microstructures indicative of dislocation creep in rocks that show no evidence for dislocation creep of coexisting plagioclase or pyroxene. Such textures, an example of which is shown in Fig. 1, have been described for samples from the Oman ophiolite (Benn and Allard, 1989, Boudier and Nicolas, 1995), the Southwest Indian Ridge (Dick et al., 1999), and the Tortuga ophiolite, southern Chile (Yoshinobu et al., 2000). These
Application of experimental flow laws
We begin by assuming that the rheology of the partially molten gabbroic mush can be constrained by applying experimentally derived flow laws for gabbroic rocks and their constituent phases. In the end-member case that the material beneath the axis is completely solid, a flow law for high-temperature creep of dry diabase provides a constraint for the maximum effective viscosity (ηeff=stress/strain rate) at the ridge axis. We infer that the lower oceanic crust is primarily olivine gabbro, which
Timing of dislocation creep in olivine
Olivine single crystal flow laws can be used to constrain the rheology of partially molten gabbro if the olivine microstructures were acquired during formation of the magmatic foliation. Several microstructural observations indicate that olivine deforms by dislocation creep at hypersolidus conditions. First, samples that display a strong plagioclase shape and lattice preferred orientation (SPO and LPO), such as in Fig. 1a, commonly also have an olivine LPO suggesting that the olivine
Conclusions
Based on the analysis of experimental rock deformation results and microstructural observations from ophiolites and the South West Indian Ridge, we draw the following conclusions:
- 1.
Some oceanic gabbros exhibit dislocation creep microstructures in olivine crystals while coexisting plagioclase and pyroxene show no evidence for dislocation creep. These microstructures are interpreted to form at hypersolidus conditions.
- 2.
At temperatures above 1100°C and stresses less than 10 MPa, dislocation creep
Acknowledgements
We thank Ron Vernon for his career-long contribution to the understanding of microstructures and their application to unraveling tectonic problems. Scott Johnson and Mike Williams put together a fantastic symposium on microstructural processes in Ron's honor at the 15th Australian Geological Convention and we appreciate and acknowledge the opportunity to take part in this wonderful meeting. Yoshinobu thanks the second author for introducing him to the application of experimental rock
References (40)
- et al.
Magma chambers in the Oman ophiolite: fed from the top or from the bottom?
Earth and Planetary Science Letters
(1996) - et al.
A long in situ section of the lower ocean crust: results of ODP Leg 176 drilling at the Southwest Indian Ridge
Earth and Planetary Science Letters
(2000) - et al.
Grain boundary sliding in fine-grained ice I
Scr. Mater.
(1997) - et al.
Water in the oceanic upper mantle: implications for rheology, melt extraction and the evolution of the lithosphere
Earth and Planetary Science Letters
(1996) - et al.
Microstructures of olivine and stresses in the upper mantle beneath Eastern China
Tectonophysics
(1989) - et al.
Geochemistry of gabbro sills in the crust-mantle transition zone of the Oman Ophiolite: implications for the origin of the oceanic lower crust
Earth and Planetary Science Letters
(1997) Modeling the anisotropic seismic properties of partially molten rocks found at mid-ocean ridges
Tectonophysics
(1997)- et al.
Interpreting magmatic fabrics in plutons
Lithos
(1998) - et al.
High-temperature creep of olivine single crystals, 1. Mechanical results for buffered samples
Journal of Geophysical Research
(1991) - et al.
Preferred mineral orientations related to magmatic flow in ophiolite layered gabbros
Journal of Petrology
(1989)
Nature of the Moho transition zone in the Oman ophiolite
Journal of Petrology
Focused mantle upwelling below mid-ocean ridges due to feedback between viscosity and melting
Geophysical Research Letters
Numerical models of magma chambers in the Oman ophiolite
Journal of Geophysical Research
Multi-channel seismic imaging of a crustal magma chamber along the East Pacific Rise
Nature
High-temperature creep of partially molten plagioclase aggregates
Journal of Geophysical Research
Three-dimensional seismic structure and physical properties of the crust and shallow mantle beneath the East Pacific Rise
Journal of Geophysical Research
Plastic flow of oriented single crystals of olivine, 1. Mechanical data
J. Geophysical Research
Structure of young oceanic crust at 13°N on the East Pacific Rise from expanding spread profiles
Journal of Geophysical Research
Experimental constraints on the dynamics of the partially molten upper mantle: deformation in the diffusion creep regime
Journal of Geophysical Research
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2017, Journal of Asian Earth SciencesCitation Excerpt :Melting and melt transport are important for deformation processes occurring in the lower crust, upper mantle and during contact metamorphism (e.g., Koekpe et al., 2004; Acosta-Vigil et al., 2006; Kvassnes and Grove, 2008; Schulmann et al., 2008). Partial melting is considered to have a strong influence on the mechanical behavior of rocks (e.g., van der Molen and Paterson, 1979; Dell’Angelo et al., 1987; Rushmer, 1995; Rutter and Neumann, 1995; Yoshinobu and Hirth, 2002; Koekpe et al., 2004; Vanderhaeghe, 2009; Yoshino et al., 2009; Zhu et al., 2011). Weakening induced by partial melting may lead to strain localization during creep of rocks in the lower crust or upper mantle.
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