The Role of Decarbonation in Seismicity: A Study of the Mechanical Behaviour of Portlandite

The impact of heat, readily provided by magma, circulating hot fluids, and rapid fault slip, in carbonate substrata, is one of the most important factors in determining the development of geothermal fields, flank stability of volcanoes, and mechanical behaviour of faults in areas where carbonate lithologies are prevalent. In fact, thermally-induced decarbonation (CaCO3 → CaO + CO2) and microcracking (due to calcite thermal expansion) are likely to be primary mechanisms in controlling the mechanical and hydrologic properties of carbonate rocks. In addition, the process and products of decarbonation will likely exert significant influence on the behaviour of faults at both geologic and earthquake time scales by causing changes in; 1) the effective normal stress on the fault and 2) the frictional behaviour of material within it. Due to the paucity of scientific information on the effects of decarbonation and thermal microcracking on the mechanical properties of carbonate fault rocks and the identification of decarbonation processes in natural fault rock, we present results from experiments performed on Carrara Marble (calcite >98 wt.%) and portlandite. Portlandite, a main decarbonation reaction product, was produced through the thermal-decomposition of Carrara Marble at 950 °C and atmospheric humidity, and is a common mineral phase of metasomatic skarn rocks formed during fluid-rock interaction in magmatic and geothermal environments. We sheared gouge layers of the water-reacted, decarbonation product (portlandite >90 wt.%), under saturated conditions at room temperature. These tests were designed to evaluate the frictional strength, stability, and healing behaviour of portlandite-bearing rocks to better understand how its presence affects fault mechanics. Post-experiment samples were collected for microstructural analysis. Preliminary results indicate that the conversion of calcite (μ = ~0.65) to portlandite (μ = ~0.45), at 5 MPa normal stress, would result in a distinct weakening of the fault. Under the same conditions and at low shearing rates (0.1 and 0.3 μm/s), portlandite fails through stick-slip motion whereas calcite slides stably. Preliminary SEM analyses indicate that active solution-transfer processes take place within the observed area of intense deformation. Our data suggest that faults patches where decarbonation has occurred are: 1) frictionally weaker, 2) more frictionally unstable, and 3) likely to regain their frictional strength more quickly, than patches in pure carbonate rocks. Therefore, fault patches affected by decarbonation are more prone to generate or experience induced seismicity. This outcome has significant implications for fault behaviour where decarbonation has occurred on entire faults, due to magmatic or geothermal processes, and fault patches, due to seismic slip. Continued study of these thermally-activated weakening processes is vital to shed light on the complex interplay between fault systems and geothermal fluids.