Frictional strength and healing behaviour of natural carbonated serpentinites at hydrothermal conditions

Ophicalcites (carbonated ultramafic rocks) are commonly found when serpentinized mantle rocks are exposed to fluid-rock interaction in settings such as slip and damage zone of mid ocean ridges, transform faults and subduction zones. In this study, we analyze the frictional strength and healing properties of ophicalcites under hydrothermal conditions because of their possible role in the nucleation of “fast” (i.e., earthquakes) and slow slip events.

The ophicarbonates used in this study belong to the exhumed ophiolitic unit of Eastern Elba Island (Italy). The hand samples are black, red, and purple ophicarbonates and include veined breccias and cataclasites. The mineral assemblages, determined through optical microscopy and quantitative (Rietveld) X-ray powder diffraction, consist of serpentine (lizardite and chrysotile with rare relicts of pyroxene and olivine), talc and calcite veins. Hematite is only found in the red ophicalcite.

We conducted slide-hold-slide experiments with a rotary-shear apparatus coupled with a hydrothermal vessel (ROSA-HYDROS, Padua University, Italy) on a synthetic fault gouge derived from a red natural ophicalcite cataclasite with a composition of 26.9% lizardite and chrysotile, 12.3% talc, 57.6% calcite, 2.9% hematite and traces of smectite. The experiments were performed at an effective normal stress (σ_n^eff = σ_n - P_p) of 20 MPa, a fluid pressure (P_p) of 6 MPa, constant slip velocity of 10 µm/s between the holds (which lasted from 10 to 10.000 s) and temperatures ranging from 20 to 400°C. Our results show that, the “serpentinite” component dominates the bulk friction and the healing behaviour. For T≤ 200°C and in the presence of water in liquid state, the friction coefficient (µ = shear stress/σ_n^eff) is relatively low (µ = 0.35-0.45), poorly sensitive to fluid temperature and we observe creep.  Instead, for T ≥ 300°C and in the presence of water in vapor state, the µ is higher (= 0.80-0.90) and we observe stick-slip behaviour.

The experimental approach of this study aims to understand the nucleation of earthquakes or of slow slip events along mid ocean ridges and transform faults and to contribute to the assessment of seismic hazard associated with CO₂ storage by carbonation of serpentinite in deep reservoirs.