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

Rupture fronts can cause fault displacement, reaching speeds up to several ms−1 within a few milliseconds, at any distance away from the earthquake nucleation area. In the case of silicate-bearing rocks the abrupt slip acceleration results in melting at asperity contacts causing a large reduction in fault frictional strength (i.e., flash weakening). Flash weakening is also observed in experiments performed in carbonate-bearing rocks but evidence for melting is lacking. To unravel the micro-physical mechanisms associated with flash weakening in carbonates, experiments were conducted on pre-cut Carrara marble cylinders using a rotary shear apparatus at conditions relevant to earthquakes propagation. In the first 5 mm of slip the shear stress was reduced up to 30% and CO2 was released. Focused ion beam, scanning and transmission electron microscopy investigations of the slipping zones reveal the presence of calcite nanograins and amorphous carbon. We interpret the CO2 release, the formation of nanograins and amorphous carbon to be the result of a shock-like stress release associated with the migration of fast-moving dislocations. Amorphous carbon, given its low friction coefficient, is responsible for flash weakening and promotes the propagation of the seismic rupture in carbonate-bearing fault patches.

9The application of clumped isotopes ( 47 ) in carbonate minerals as a sensitive temperature proxy in 10 paleo-environments depends on a well-constrained clumped isotope fractionation for the necessary 11 step of phosphoric acid digestion of the carbonate mineral to produce CO 2 . Published estimates for 12 clumped isotope fractionations vary, and the effect of different carbonate mineralogies is still under 13 debate. Differences in the sample preparation design and sample digestion temperatures are 14 potential sources for varying acid fractionations and could be a source for discrepant  47 -15 temperature calibrations observed in different laboratories. To evaluate the clumped isotope acid 16 fractionation at 70 °C and simultaneously account for a potential cation effect we analyzed a set of 17 eight carbonate minerals (calcite, aragonite, dolomite and magnesite) that were driven to a 18 stochastic isotope distribution by heating them to temperatures of 1000 °C. Our study reveals 19 significant cation-and mineral-specific differences for the  47 acid fractionation of carbonate 20 minerals digested at 70 °C or 100 °C. The  47 acid fractionation at 70 °C for calcite is 0.197±0.002 ‰, 21 for aragonite 0.172±0.003 ‰, whereas dolomite has a significantly larger acid fractionation of 22 0.226±0.002 ‰. For magnesite digested at 100 °C we observed a  47 acid fractionation of 23 0.218±0.020 ‰. Projected to an acid digestion at 25 °C, our acid fractionation for calcite of 0.260 ‰ 24 is statistically indistinguishable from existing studies. We further show that the  47 of the calcite 25 standards ETH-1 and ETH-2 of 0.265 ‰ and 0.267 ‰, respectively, are in the range of the 26 determined acid fractionation projected to 25 °C suggesting that they have an identical and near 27 stochastic isotope distribution. The observed differences in the  47 acid fractionation between calcite 28 and aragonite ( 47 = -0.025 ‰) and between calcite and dolomite ( 47 = -0.029 ‰) does not 29 correlate with the phosphoric acid fractionation of oxygen isotopes, but rather depends on the radius 30 of the cation as well as on the mineral structure. Our results reveal that the acid fractionation of 31 dolomite at 70 °C is significantly distinct from the one of calcite, but at 90 °C the two are within error 32 © 2016. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ with M being the metal cation of the corresponding carbonate mineral (e.g. calcite and aragonite 52 (CaCO 3 ), dolomite (CaMg(CO 3 ) 2 ), magnesite (MgCO 3 ), siderite (FeCO 3 ), witherite (BaCO 3 )). Having 53 attained equilibrium this exchange reaction (Eq. 1) is thermodynamically controlled and produces an 54 excess in the doubly substituted isotopologue relative to a stochastic isotope distribution (0.5 ‰ at 0 55 °C), which decreases with increasing temperature and approaches a stochastic isotope composition 56 at 1000 °C (Schauble et al., 2006). Ghosh et al. (2006a) demonstrated the feasibility...

[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 À5 s À1 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 Q GB = 59 AE 15 KJ/mol and Q GFB = 456 AE 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 AE 100 C, in agreement with the Icelandic seismogenic zone.Citation: Violay, M., B. Gibert, D. Mainprice, B. Evans, J.-M. Dautria, P. Azais, and P. Pezard (2012), An experimental study of the brittle-ductile transition of basalt at oceanic crust pressure and temperature conditions,