Stress-driven Melt Segregation in Partially Molten Olivine-rich Rocks Deformed in Torsion

King, Daniel S.; Zimmerman, M. E.; Kohlstedt, D. L.

doi:10.1093/petrology/egp062

Cited by 65 publications

(108 citation statements)

References 28 publications

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“…This mode of segregation was predicted with two-phase flow theory (1) and subsequently discovered in experiments (2,3). It has been proposed that meltenriched bands, if present in the mantle of Earth, would permit rapid extraction of melt (4), produce significant anisotropy in seismic wave propagation (5), and provide a mechanism for the seismic discontinuity that is, in some places, associated with the lithosphere-asthenosphere boundary (6).…”

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confidence: 81%

Experimental test of the viscous anisotropy hypothesis for partially molten rocks

Kohlstedt

Katz

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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Chemical differentiation of rocky planets occurs by melt segregation away from the region of melting. The mechanics of this process, however, are complex and incompletely understood. In partially molten rocks undergoing shear deformation, melt pockets between grains align coherently in the stress field; it has been hypothesized that this anisotropy in microstructure creates an anisotropy in the viscosity of the aggregate. With the inclusion of anisotropic viscosity, continuum, two-phase-flow models reproduce the emergence and angle of melt-enriched bands that form in laboratory experiments. In the same theoretical context, these models also predict sample-scale melt migration due to a gradient in shear stress. Under torsional deformation, melt is expected to segregate radially inward. Here we present torsional deformation experiments on partially molten rocks that test this prediction. Microstructural analyses of the distribution of melt and solid reveal a radial gradient in melt fraction, with more melt toward the center of the cylinder. The extent of this radial melt segregation grows with progressive strain, consistent with theory. The agreement between theoretical prediction and experimental observation provides a validation of this theory.viscous anisotropy | melt segregation | partial melts | basalt | olivine S hear deformation of partially molten rocks gives rise to melt segregation into sheets (bands in cross-section) that emerge at a low angle to the shear plane. This mode of segregation was predicted with two-phase flow theory (1) and subsequently discovered in experiments (2, 3). It has been proposed that meltenriched bands, if present in the mantle of Earth, would permit rapid extraction of melt (4), produce significant anisotropy in seismic wave propagation (5), and provide a mechanism for the seismic discontinuity that is, in some places, associated with the lithosphere-asthenosphere boundary (6). The emergence (7) and low angle (8) of melt-enriched bands under simple-shear deformation can be reproduced using two-phase flow theory with a non-Newtonian, isotropic viscosity. This theory describes the flow of a low-viscosity liquid (melt) through a permeable and viscously deformable solid matrix (grains) (9). However, an unrealistically strong stress dependence of viscosity was required to match the low angle of bands observed in experiments (8).This disagreement between models and experiments found a possible resolution by the incorporation of anisotropic viscosity arising from coherent alignment of melt pockets between grains [i.e., melt-preferred orientation (MPO)] in response to a deviatoric stress (10-13).Crucially, with the inclusion of viscous anisotropy, two-phase flow theory also predicts a simultaneous but distinct mode of melt segregation driven by large-scale gradients in shear stress. This mode is termed base-state melt segregation (13-15). Base-state melt segregation is not predicted if viscosity is isotropic; thus, its occurrence in experiments represents a test of the hypothesis that M...

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mentioning

confidence: 81%

Experimental test of the viscous anisotropy hypothesis for partially molten rocks

Kohlstedt

Katz

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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“…Experimental [e.g., Holtzman et al, 2003;King et al, 2010] and theoretical [e.g., Stevenson, 1989;Spiegelman, 2003;Katz et al, 2006] studies have shown that partially molten mantle materials subject to shear can develop bands of locally increased porosity and deformation. These rheologically weak bands tend to emerge at a particular angle to the shear plane under simple shear deformation.…”

Section: On the Absence Of Porosity/shear Bandsmentioning

confidence: 99%

Porosity‐driven convection and asymmetry beneath mid‐ocean ridges

Katz

2010

Geochem Geophys Geosyst

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[1] Seismic tomography of the asthenosphere beneath mid-ocean ridges has produced images of wave speed and anisotropy that are asymmetric across the ridge axis. These features have been interpreted as resulting from an asymmetric distribution of upwelling and melting. Using computational models of coupled magma/mantle dynamics beneath mid-ocean ridges, I show that such asymmetry should be expected if buoyancy forces contribute to mantle upwelling beneath ridges. The sole source of buoyancy considered here is the dynamic retention of less dense magma within the pores of the mantle matrix. Through a scaling analysis and comparison with a suite of simulations, I derive a quantitative prediction of the contribution of such buoyancy to upwelling; this prediction of convective vigor is based on parameters that, for the Earth, can be constrained through natural observations and experiments. I show how the width of the melting region and the crustal thickness, as well as the susceptibility to asymmetric upwelling, are related to convective vigor. I consider three causes of symmetry breaking: gradients in mantle potential temperature and composition and ridge migration. I also report that in numerical experiments performed for this study, the fluid dynamical instability associated with porosity/shear band formation is not observed to occur.

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“…Compared to homogeneous two phase mixtures, layered polyphase can be, however, substantially stronger if compressed loaded normal to layering (Ji et al, 2000). Localized deformation may also result from formation of amorphous material at low temperature and high stresses (Pec et al, 2012) and by stress-driven segregation of melt at high temperatures (Holtzman et al, 2012;Holtzman and Kohlstedt, 2007;King et al, 2010;Kohlstedt et al, 2010;Misra et al, 2009). …”

Section: Introductionmentioning

confidence: 99%

Strain localization during high temperature creep of marble: The effect of inclusions

et al. 2014

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The deformation of rocks in the Earth's middle and lower crust is often localized in ductile shear zones. To better understand the initiation and propagation of high-temperature shear zones induced by the presence of structural and material heterogeneities, we performed deformation experiments in the dislocation creep regime on Carrara marble samples containing weak (limestone) or strong (novaculite) second phase inclusions. The samples were mostly deformed in torsion at a bulk shear strain rate of ≈ 1.9x10 -4 s -1 to bulk shear strains γ between 0.02 and 2.9 using a Paterson-type gas deformation apparatus at 900°C temperature and 400 MPa confining pressure. At low strain, twisted specimens with weak inclusions show minor strain hardening that is replaced by strain weakening at γ > 0.1 -0.2.Peak shear stress at the imposed conditions is about 20 MPa, which is ≈8% lower than the strength of intact samples. Strain progressively localized within the matrix with increasing bulk strain, but decayed rapidly with increasing distance from the inclusion tip.Microstructural analysis shows twinning and recrystallization within this process zone, with a strong crystallographic preferred orientation, dominated by {r} and (c) slip in .Recrystallization-induced weakening starts at local shear strain of about 1 in the process zone, corresponding to a bulk shear strain of about 0.1. In contrast, torsion of a sample containing strong inclusions deformed at similar stress as inclusion-free samples but do not show localization. The experiments demonstrate that the presence of weak heterogeneities initiates localized creep at local stress concentrations around the inclusion tips. Recrystallizationinduced grain size reduction may only locally promote grain boundary diffusion creep. Accordingly, the bulk strength of the twisted aggregate is close to or slightly below the lower (isostress) strength bound, determined from the flow strength and volume fraction of matrix and inclusions.

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Stress-driven Melt Segregation in Partially Molten Olivine-rich Rocks Deformed in Torsion

Cited by 65 publications

References 28 publications

Experimental test of the viscous anisotropy hypothesis for partially molten rocks

Experimental test of the viscous anisotropy hypothesis for partially molten rocks

Porosity‐driven convection and asymmetry beneath mid‐ocean ridges

Strain localization during high temperature creep of marble: The effect of inclusions

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