Elsevier

Journal of Structural Geology

Volume 24, Issues 6–7, June–July 2002, Pages 1101-1107
Journal of Structural Geology

Microstructural and experimental constraints on the rheology of partially molten gabbro beneath oceanic spreading centers

https://doi.org/10.1016/S0191-8141(01)00094-3Get rights and content

Abstract

Flow laws for high-temperature creep of olivine, plagioclase, and diabase are used to place constraints on the rheology of partially molten lower oceanic crust. This analysis is motivated by the observation of olivine lattice preferred orientations and subgrain microstructures in oceanic gabbros that lack evidence for dislocation creep in coexisting plagioclase and pyroxene. Extrapolation of experimental flow laws indicates that at temperatures above 1100°C and stresses less than 10 MPa, olivine may be the weakest phase in rocks with gabbroic composition. By accounting for variations in the melt fraction (0–10%) and grain size of partially molten plagioclase aggregates we can constrain the rheological conditions where olivine deforms by dislocation creep while plagioclase deforms by diffusion creep. Calculated effective viscosities range from 1015 to 1019 Pa s; based on observations of the geometry of the partially molten zone beneath the East Pacific Rise and the microstructural and experimental constraints we favor a value of ∼1018 Pa s. This value approaches estimates for the viscosity of the upper mantle beneath ridge axes, but is significantly higher than previously suggested for the partially molten lower crust. Such high viscosities are inconsistent with ridge evolution models that require large amounts of lower crustal flow to accommodate melt redistribution. However, the results are compatible with recent models that favor local magma replenishment from the mantle at closely spaced intervals along the spreading center axis in a 2D, ‘sheet-like’ fashion.

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

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