Benoit Gibert

An experimental study of the brittle-ductile transition of basalt at oceanic crust pressure and temperature conditions

Received 21 September 2011; revised 10 January 2012; accepted 1 February 2012; published 23 March 2012. [1] The brittle to ductile transition (BDT) in rocks may strongly influence their transport properties (i.e., permeability, porosity topology…) and the maximum depth and temperature where hydrothermal fluids may circulate. To examine this transition in the context of Icelandic crust, we conducted deformation experiments on a glassy basalt (GB) and a glass-free basalt (GFB) under oceanic crust conditions. Mechanical and micro-structural observations at a constant strain rate of 10 � 5 s � 1 and at confining pressure of 100–300 MPa indicate that the rocks are brittle and dilatant up to 700–800 � C. At higher temperatures and effective pressures the deformation mode becomes macroscopically ductile, i.e., deformation is distributed throughout the sample and no localized shear rupture plane develops. The presence of glass is a key component reducing the sample strength and lowering the pressure of the BDT. In the brittle field, strength is consistent with a Mohr-Coulomb failure criterion with an internal coefficient of friction of 0.42 for both samples. In the ductile field, strength is strain rate- and temperature-dependent and both samples were characterized by the same stress exponent in the range 3 < n < 4.2 but by very different activation energy QGB =5 9� 15 KJ/mol and QGFB = 456 � 4 KJ/mol. Extrapolation of these results to the Iceland oceanic crust conditions predicts a BDT at � 100 � C for a glassy basalt, whereas the BDT might occur in non-glassy basalts at deeper conditions, i.e., temperatures higher than 550 � 100 � C, in agreement with the Icelandic

Effect of water on the frictional behavior of cohesive rocks during earthquakes

Fluid-rock interactions can control earthquake nucleation and the evolution of earthquake sequences. Experimental studies of fault frictional properties in the presence of fluid can provide unique insights into these interactions. We report the first results from experiments performed on cohesive silicate-bearing rocks (microgabbro) in the presence of pressurized pore fluids (H2O, drained conditions) at realistic seismic deformation conditions. The experimental data are compared with those recently obtained from carbonate-bearing rocks (Carrara marble). Contrary to theoretical arguments, and consistent with the interpretation of some field observations, we show that frictional melting of a microgabbro develops in the presence of water. In microgabbro, the initial weakening mechanism (flash melting of the asperities) is delayed in the presence of water; conversely, in calcite marble the weakening mechanism (brittle failure of the asperities) is favored. This opposite behavior highlights the importance of host-rock composition in controlling dynamic (frictional) weakening in the presence of water: cohesive carbonate-bearing rocks are more prone to slip in the presence of water, whereas the presence of water might delay or inhibit the rupture nucleation and propagation in cohesive silicate-bearing rocks.

Brittle versus ductile deformation as the main control of the deep fluid circulation in oceanic crust

The brittle to ductile transition may strongly influence hydraulic properties of rocks at the depth and temperature ranges that hydrothermal fluids circulate. To examine this transition in the context of the oceanic crust, we conducted a series of deformation experiments on a natural basalt sample at in situ oceanic crust conditions. Dilatancy was measured during deformation. The method consisted in monitoring the volume of pore fluid that flows into or out of the sample at constant pore pressure. Mechanical and microstructural observations at experimental constant strain rate of 10−5 s−1 indicated that the basalt was brittle and dilatant up to 800°C. At higher temperature, the deformation mode became macroscopically ductile and samples compacted. These observations have important implications on heat transfer and fluid migration in oceanic crust.

Thermal diffusivity of olivine single‐crystals and polycrystalline aggregates at ambient conditions—a comparison

[1] Heat transfer is a key process for the mantle dynamics. However, analysis of experimental data on thermal transport properties of upper mantle materials highlights a large scatter of absolute values of thermal diffusivity. In particular, conduction of heat in single crystals is systematically higher than in polycrystalline samples. Here we present new thermal diffusivity measurements on San Carlos olivine single crystals and mantle rocks at 300K. Measured components of the olivine thermal diffusivity tensor along the [100], [010], and [001] directions are 2.73, 1.70, and 2.49 mm 2 . s -1 , respectively. Measurements of thermal diffusivity of peridotites as a function of structural orientation show absolute values and anisotropy in good agreement with those predicted by petrophysical models based on the crystal preferred orientation of olivine and the above thermal diffusivity tensor. This suggests that the upper mantle thermal diffusivity is up to 50% higher than indicated by previous measurements on mantle rocks.

Effect of glass on the frictional behavior of basalts at seismic slip rates.

We performed 31 friction experiments on glassy basalts (GB) and glass-free basalts (GFB) at slip rates up to 6.5 m s−1 and normal stress up to 40 MPa (seismic conditions). Frictional weakening was associated to bulk frictional melting and lubrication. The weakening distance (Dw) was about 3 times shorter in GB than in GFB, but the steady state friction was systematically higher in GB than in GFB. The shorter Dw in GB may be explained by the thermal softening occurring at the glass transition temperature (Tg ~500°C), which is lower than the bulk melting temperature (Tm ~1250°C) of GFB. Postexperiment microanalyses suggest that the larger crystal fraction measured in GB melts results in the higher steady state friction value compared to the GFB melts. The effect of interstitial glass is to facilitate frictional instability and rupture propagation in GB with respect to GFB.