Paleo-deviatoric stress magnitudes from calcite twins and related structural permeability evolution in minor faults: Example from the toarcian shale of the French Causses Basin, Aveyron, France

The relationship between deformation and so-called fluid paleotransfers in minor faults has been analysed in an argillaceous formation located in the Causses Basin in France. The fluid paleotransfers are related to the fault activity to a large extent. We attempt to estimate the intensity of paleo-deviatoric stress magnitudes under which the fault activity may have occurred and consequently, the change in the structural fault permeability. The paleo-deviatoric stress magnitudes were calculated with the inverse method of Etchecopar applied to calcite twinning. The measured crystals are contained within the core zone of minor faults and this study is based on a previous complete microtectonic and microstructural analysis of the faults. In this paper, analysis of calcite twinning has been applied for the first time to vein fillings associated small faults in a context of relatively weak deformation, a condition ensured by the tectonic and structural analysis. Calculation and discussion of the paleo-deviatoric stress tensors in relation to the evolution of the structural fault permeability and to the hydraulic behaviour of the faults are the aim of this paper. The analysed faults, created and active during the same tectonic event, were permeable under a (σ₁–σ₃) mean value of 40–50 MPa. On the other hand, the reactivation of faults during a second tectonic event implies mean (σ₁–σ₃) value higher than 40–50 MPa, especially for the faults that are poorly oriented with respect to the principal tectonic stress directions. The core zone of these faults remained sealed and impermeable or became permeable by development of microcracks inside the pre-existing fillings.     

Trace Element Data for Gold, Iridium and Silver in Seventy Geochemical Reference Materials

New concentrations for Au, Ir and Ag obtained by instrumental neutron activation analysis are presented for seventy geochemical reference materials. Results in agreement with literature values for Au and Ir down to concentrations of a few ng g⁻¹ were obtained. For Au and Ir concentrations above 10 ng g⁻¹, the repeatability of replicate analyses of reference materials was mostly better than 10%. For concentrations between 1 and 10 ng g⁻¹ the RSD for Ir was 10–30%, whereas for Au it was higher and more variable (20–50%). In addition, concentrations for Cd and Hg are presented for some of the same reference materials. The high RSD at relatively high concentrations seen in gold for some RMs (e.g., WMG-1, WMS-1) did not exist for Ir and suggests homogeneity for this platinum-group element at the sub-sample size used in this study. For the following eight RMs, mostly ultramafic rocks (CHR-Pt+, OREAS-13P, OREAS-14P, PCC-1, UMT-1, WMG-1, WMS-1, WPR-1), Ir measurements agreed within ± 10% of mostly certified or recommended concentrations, which ranged from 2 ng g⁻¹ to 6 μg g⁻¹. For the reference material UB-N, iridium concentration compared favourably to published results obtained by isotope dilution ICP-MS methods and a previously unrecognised heterogeneity is inferred for Au, Hg and Sb, but not for the other measured elements.     

Gabbroic intrusions and magmatic metasomatism in harzburgites from the Garrett transform fault: implications for the nature of the mantle–crust transition at fast-spreading ridges

 Mafic and ultramafic rocks sampled in the Garrett transform fault at 13°28′S on the East Pacific Rise (EPR) provide insight on magmatic processes occurring under a fast-spreading ridge system. Serpentinized harzburgite from Garrett have modal, mineral and bulk chemical compositions consistent with being mantle residue of a high degree of partial melting. Along with other EPR localities (Terevaka transform fault and Hess Deep), these harzburgites are among the most residual and depleted in magmatophile elements of the entire mid-ocean ridge system. Geothermometric calculations using olivine-spinel pairs indicate a mean temperature of 759 ± 25 °C for Garrett residual harzburgite similar to the average of 755 °C for tectonite peridotites from slow-spreading ridges. Results of this study show that mid-ocean ridge peridotites are subject to both fractional melting and metasomatic processes. Evidence for mantle metasomatism is ubiquitous in harzburgite and is likely widespread in the entire Garrett peridotite massif. Magma-harzburgite interactions are very well preserved as pyroxenite lenses, plagioclase dunite pockets or dunitic wall rock to intrusive gabbros. Abundant gabbroic rocks are found as intrusive pockets and dikes in harzburgite and have been injected in the following sequence: olivine-gabbro, gabbro, gabbronorite, and ferrogabbro. The wide variety of magmas that crystallized into gabbros contrast sharply with present-day intratransform basalts, which have a highly primitive composition. Ferrogabbro dikes have been intruded at the ridge-transform intersection and as they represent the last event of a succession of gabbros intrusive into the peridotite, they likely constrain the origin of the entire peridotite massif to the same location. In peridotite massifs from Pacific transform faults (Garrett and Terevaka), primitive to fractionated basaltic magmas have flowed and crystallized variable amounts of dunite (±plagioclase) and minor pyroxenite, followed by a succession of cumulate gabbroic dikes which have extensively intruded and modified the host harzburgitic rocks. The lithosphere and style of magmatic activity within a fast-slipping transform fault (outcrops of ultramafic massif, discontinuous gabbro pockets intrusive in peridotite, magnesian and phyric basalts) are more analogous to slow-spreading Mid-Atlantic Ridge type than the East Pacific Rise. Received: 13 October 1997 / Accepted: 5 February 1999     

Process of serpentinization in the ultramafic massif of Beni Bousera (internal Rif,Morocco)

The Beni Bousera massif forms part of the Sebtide units in the internal Rif Mountain (Morocco). It is mainly composed of mantle peridotites surrounded by crustal metamorphic rocks (kinzigites, micaschists, and schists). The serpentinization affects all of peridotite massif to various degrees. Serpentinization is concentrated at the top of the peridotites, along the mylonitized zone, and in the NE part of the massif. It is manifested by the formation of mesh and hourglass textures along the tectonic foliation in the highly serpentinized peridotites; and brecciated texture in the least serpentinized peridotites. Pyroxene minerals are still intact hosting few serpentine veins. These petrographic features are consistent with the geochemical data, marked by the increasing of LOI and decreasing of MgO and FeO toward the top of the massif and Aaraben fault. The Raman characterization of serpentine with the brecciated mesh and hourglass textures correspond to lizardite type whereas the serpentine with the vein texture is formed by lizardite + chrysotile.