Matej Pec

Reaction infiltration instabilities in experiments on partially molten mantle rocks

To investigate channelization of a reactive melt in mantle rocks, we imposed a gradient in fluid pressure across a partially molten rock composed of olivine and clinopyroxene, sandwiched between a source of alkali basalt melt and a sink of porous alumina. We performed experiments at a confining pressure of 300 MPa and pore pressures of 0.1–300 MPa, resulting in fluid pressure gradients of 0–88 MPa/mm at temperatures of 1200–1250 °C. When the gradient in fluid pressure is zero, only a planar reaction layer composed of olivine + melt develops, in agreement with previous experiments. However, if the gradient in fluid pressure is greater than zero, in addition to the planar reaction layer, finger-like melt-rich channels that contain olivine + melt develop and propagate into the rock, significantly past the interface between the melt reservoir and the partially molten rock. Channelization of the melt results in a significant increase in permeability and hence in the flux of melt through the partially molten rock.

Semi-brittle flow of granitoid fault rocks in experiments

Field studies and seismic data show that semi-brittle flow of fault rocks probably is the dominant deformation mechanism at the base of the seismogenic zone at the so-called frictional - viscous transition. To understand the physical and chemical processes accommodating semi-brittle flow, we have performed an experimental study on synthetic granitoid fault rocks exploring a broad parameter space (temperature, T = 300, 400, 500 and 600°C, confining pressure, Pc ≈ 300, 500, 1000 and 1500 MPa, shear strain rate, γ ≈ 10-3, 10-4, 10-5 and 10-6 s-1, to finite shear strains, = 0 - 5). The experiments have been carried out using a granular material with grain size smaller than 200 μm with a little H2O added (0.2wt%). Only two experiments (performed at the fastest strain rates and lowest temperatures) have failed abruptly right after reaching peak strength ( ~ 1400 MPa). All other samples reach high shear stresses ( ~ 570 – 1600 MPa), then weaken slightly (by Δ ~ 10 – 190 MPa) and continue to deform at a more or less steady state stress level. Clear temperature dependence and a weak strain rate dependence of the peak as well as steady state stress levels is observed. In order to express this relationship, the strain rate – stress sensitivity has been fit with a stress exponent, assuming γ  n and yields high stress exponents (n ≈ 10 - 140), which decrease with increasing temperature. The microstructures show widespread comminution, strain partitioning and localization into slip zones containing, at first, nanocrystalline and partly amorphous material, later, during continued deformation, fully amorphous material in slip zones. Despite the mechanical steady state conditions, the fabrics in the slip zones and outside continue to evolve and do not reach a steady state microstructure below γ = 5. Within the slip zones, the fault rock material progressively transforms from a crystalline solid to an amorphous material. We present and interpret the experimental results both in terms of sliding friction and viscous flow, and we discuss the possible effect that the formation of nanocrystalline and amorphous layers may have on earthquake nucleation.

Reaction Infiltration Instabilities in Mantle Rocks: an Experimental Investigation

Melt extraction from partially molten regions of the mantle occurs along high-permeability pathways. Melt–rock reactions can lead to the formation of high-permeability channels due to a positive feedback between melt flow and reaction. To study this process, we performed a series of Darcytype experiments in which a cylinder of partially molten rock sandwiched between a melt source and a porous sink was annealed at high pressures (P1⁄4 300 MPa) and high temperatures (T1⁄4 1200 or 1250 C) under a controlled pressure gradient (oP/ox1⁄40–100 MPa mm) for up to 5 h. The partially molten rock was formed from 50:50 mixtures of olivine (Ol) and clinopyroxene (Cpx) plus 4, 10 or 20 vol. % of alkali basalt. The melt source was a disk of alkali basalt undersaturated in silica with respect to the partially molten rock, and the sink was a disk of porous alumina. During an experiment, melt from the source dissolved Cpx in the partially molten rock and precipitated Ol, thereby forming a Cpx-free reaction layer at the interface between the melt source and the partially molten rock. The melt fraction as well as the grain size in the reaction layer increased significantly compared with that present in the starting material, confirming that the reaction increased the local permeability of the partially molten rock, one of the prerequisites for the reaction infiltration instability process to operate. In experiments carried out under a small pressure gradient (and hence slow melt flow velocity), the reaction layer remained roughly planar and no channels developed. However, if the melt flow velocity by porous flow exceeded 0 1 mm s, the reaction layer locally protruded into the partially molten rock forming finger-like, melt-rich channels. The morphology and spacing of the channels depended on the initial melt fraction. In a partially molten rock with 20 vol. % melt, multiple, voluminous channels with an elliptical core of pure melt developed. At lower melt contents, fewer and thinner channels formed. Our experiments demonstrate that melt–rock reactions can lead to melt channelization in mantle lithologies, consistent with general predictions of the reaction infiltration instability theory.

Migration of Melt

Abstract Magma erupted on the Earths surface is generated by partial melting of rocks at depth. Melt, initially distributed along grain edges and in junctions, must segregate from the solid matrix. This segregation process has been modeled as two-phase or porous flow. The driving force for melt migration can be related to differences in interfacial energies, the buoyancy of melt relative to residual solids or to applied deviatoric stress. The rate of melt migration is controlled by the magnitude of the driving force, the permeability of the melt network, the viscosity of the melt, and the strength of the solid residuum. Modeling of the melt migration process is complicated by the interdependence of these physical parameters. Permeability is determined by melt network geometry and melt volume fraction. Melt distribution in the solid is affected by the deformation of the solid grains and any crystalline anisotropy that might develop as a result. Melt decreases the viscosity of the rock. Deviatoric stress and strain can result in a preferred alignment of the melt network in response, focusing melt into channels as strength contrasts within the partially molten region develop. Though not likely under mantle conditions, in crustal rocks, this melt preferred alignment in response to stress could lead to brittle failure of the solid matrix as the melt pressure exceeds the tensile strength of the rock. Chemical disequilibrium between melt and solid matrix could also lead to flow channelization through reaction-infiltration instability, further complicating melt migration models.