Petrophysical properties of hydrothermally-altered pyroclastic deposits from Deception Island (Bransfield Strait, Antarctica)

Abstract

Deception Island is a composite volcano located in the Bransfield Strait, between the South Shetland Islands and the Antarctic Peninsula. The volcano is defined by a caldera-forming event, and the rocks forming the island—which range in composition from basalts to trachydacites-rhyolites—are typically classified as either pre-, syn-, or post-caldera. Here, we provide petrophysical properties (bulk density, porosity, P-wave velocity, permeability, thermal properties, Young's modulus, and uniaxial compressive strength) for representative lapilli tuffs (pyroclastic density current and fall deposits) from the pre-, syn-, and post-caldera volcanic activity. We find that dry bulk density varies from ∼982 to ∼1813 kg·m⁻³, connected porosity varies from 0.30 to 0.62, P-wave velocity varies from ∼0.3 to ∼1 km·s⁻¹, permeability varies from ∼10⁻¹⁵ to ∼10⁻¹¹ m², thermal conductivity varies from ∼0.3 to ∼0.65 W·m⁻¹·K⁻¹, thermal diffusivity varies from ∼0.35 to ∼0.25 mm²·s⁻¹, specific heat capacity varies from ∼0.8 to ∼1.3 J·kg⁻¹·K⁻¹, Young's modulus varies from ∼1 to ∼9 GPa, and that uniaxial compressive strength varies from ∼1 to ∼25 MPa. Our data show that P-wave velocity, Young's modulus, uniaxial compressive strength, thermal conductivity, and thermal diffusivity decrease, permeability increases, and specific heat capacity does not change systematically as a function of increasing porosity. We also find that the fall tuffs are more porous than the pyroclastic density current tuffs, and therefore have a lower P-wave velocity, Young's modulus, uniaxial compressive strength, thermal conductivity, and thermal diffusivity, but a higher permeability. Our data expose the heterogeneity of the petrophysical properties of the lapilli tuffs at Deception Island, and are in good agreement with those for similar lapilli tuffs from Surtsey volcano (Iceland) and Cracked Mountain (Canada). Microscale models for permeability, thermal conductivity, and uniaxial compressive strength provide insight into the microscale factors controlling the petrophysical properties of the lapilli tuffs, and can be used to help predict their petrophysical properties when data are absent or laboratory experiments are not possible. Large-scale hydromechanical models that use our laboratory data provide the range of expected surface displacement at Deception Island following fluid injection at depth, and highlight the importance of choosing appropriate rock property input parameters for volcano modelling.