Focused melt flow and localized deformation in the upper mantle: Juxtaposition of replacive dunite and ductile shear zones in the Josephine peridotite, SW Oregon

Abstract

Results of a field study demonstrate that melt flow in the shallow mantle was focused along actively deforming ductile shear zones in the Josephine peridotite in SW Oregon. Intergranular flow of ascending liquids dissolved pyroxene and precipitated olivine, forming zones of dunite replacing harzburgite. Syntectonic formation of dunite was focused along near‐vertical shear zones, with both vertical and dextral displacement, in the shallow mantle at less than 30 km depth and temperatures between 950 and 1100°C. The migrating magmas which formed dunite and related pyroxenite included high‐Mg, calc‐alkaline andesites characteristic of subduction‐related magmatic arcs. Explanations for focusing of melt flow along shear zones include the presence of a pressure gradient between strong and weak material during active deformation (Stevenson, 1989) and rapid infiltration of liquid when grain boundaries are aligned in mantle tectonites (Waff and Faul, 1992). Focused flow of melt may, in turn, lead to localized deformation and the development of shear zones. It is likely that melt‐lubricated shear zones provide an important locus for both melt extraction and strain in the shallow mantle beneath spreading ridges and in subduction‐related magmatic arcs.

Composition of the Oceanic Crust

“…The second observation is the diversity of melt inclusion compositions mentioned above. A third line of evidence is the abundant dunite veins and channels observed in mantle sections of ophiolites, apparently produced as melt-rock reaction results in dissolution of orthopyroxene and precipitation of olivine (e.g., Kelemen and Dick, 1995;. At depths greater than 20 km, these melt channels may be harzburgite or lherzolite (Liang et al, 2010).…”

Generation of permeability barriers during melt extraction at mid‐ocean ridges

“…Even though solid flow lines are subhorizontal, melt continues to migrate vertically because permeability is high enough for melt velocities to exceed solid mantle velocities [McKenzie, 1985;Spiegelman and Elliott, 1993;Lundstrom et al, 1995;Sims et al, 2002] and buoyancy contrast dominates over flow in inducing pressure gradients for reasonable mantle viscosities [Spiegelman and McKenzie, 1987;Phipps Morgan, 1987]. Dissolution instabilities further facilitate vertical melt extraction [Kelemen and Dick, 1995], but we do not consider the chemical effects of forming a channelized melt extraction network.…”

“…Ultraslow spreading ridges (less than ∼10 mm/yr half rate) are characterized by discontinuous magmatism at localized volcanic centers separated by amagmatic segments [Michael et al, 2003;Dick et al, 2003] indicating a more complex focusing mechanism through a thick thermal boundary. [3] Proposed mechanisms for focused ridge magmatism include (1) large pressure gradients that focus the flow of melt [Phipps Morgan, 1987;Spiegelman and McKenzie, 1987;Ribe, 1988], (2) buoyancy-driven convection due to lateral variations in melt content [Rabinowicz et al, 1984;Buck and Su, 1989;Scott and Stevenson, 1989], (3) hydrofracturing [Sleep, 1988;Nicolas, 1990], (4) development of a stress-induced anisotropic permeability [Phipps Morgan, 1987;Katz et al, 2006], (5) reactioninfiltration instability [Aharonov et al, 1995;Kelemen and Dick, 1995;Kelemen et al, 1995aKelemen et al, , 1995b, leading to (6) a fractal melt extraction tree [Hart, 1993] and (7) development of a high-porosity channel along the base of the sloping lithosphere, allowing melt to flow toward the ridge axis (Figure 1) [Sparks and Parmentier, 1991;Spiegelman, 1993aSpiegelman, , 1993bGhods and Arkani-Hamed, 2000;Katz, 2008]. Mechanisms 1 and 2 are unlikely to play an important role as they require higher viscosities or lower porosities than currently estimated beneath MOR spreading centers and mechanisms 3-6 remain to be fully tested.…”

Multi-stage melt–rock interaction in the Mt. Maggiore (Corsica, France) ophiolitic peridotites: microstructural and geochemical evidence

“…As a consequence, they are not in equilibrium with the country peridotite and interact with it, dissolving pyroxenes and precipitating olivine (Quick 1981;Kelemen 1990;Kelemen et al 1992Kelemen et al , 1997Kelemen and Dick 1995). This process causes modal and bulk-chemical depletion in the lithospheric mantle, originating ''reactive harzbugites'' and replacive dunites.…”