Decarbonation and thermal microcracking under magmatic P-T-fCO2 conditions: the role of skarn substrata in promoting volcanic instability

We present a systematic study on the influence of pressure (0.1–600 MPa), temperature (750–
1200 ◦C), carbon dioxide fugacity (logfCO2 = −4.41 to 3.60) and time (2–12 hr) on the
chemical and physical properties of carbonate rock. Our experiments aim to reproduce the
conditions at the periphery of magma chamber where carbonate host rock is influenced by,
but not readily assimilated by, magma. This permits the investigation of the natural conditions
at which circulating fluids/gases promote infiltration reactions typical of metasomatic skarns
that can involve large volumes of subvolcanic carbonate basements. Results show that, providing
that carbon dioxide is retained in the pore space, decarbonation does not proceed at
any magmatic pressure and temperature. However, when the carbon dioxide is free to escape,
decarbonation can occur rapidly and is not hindered by a low initial porosity or permeability.
Together with carbon dioxide and lime, portlandite, a mineral commonly found in voluminous
metasomatic skarns, readily forms during carbonate decomposition. Post-experimental analyses
highlight that thermal microcracking, a result of the highly anisotropic thermal expansion
of calcite, exerts a greater influence on rock physical properties (porosity, ultrasonic wave
velocities and elastic moduli) than decarbonation. Our data suggest that this will be especially
true at the margins of dykes or magma bodies, where temperatures can reach up to 1200 ◦C.
However, rock compressive strength is significantly reduced by both thermal cracking and decarbonation,
explained by the relative weakness of lime + portlandite compared to calcite, and
an increase in grain size with increasing temperature. Metasomatic skarns, whose petrogenetic
reactions may involve a few tens of cubic kilometres, could therefore represent an important
source of volcanic instability.