Physical properties of fresh or hydrothermalized volcanic rocks from the west coast of Basse-Terre and Terre-de-Haut (Guadeloupe archipelago)

Abstract

The physical rock properties (solid phase and bulk densities, porosity, permeability, thermal conductivity and P-wave velocity) of 64 samples from outcrops of the Guadeloupe archipelago and more especially from the Basal Complex (2.79 ± 0.04 Myr - 2.68 ± 0.04 Myr), the Vieux-Habitants-Bouillante area (1.023 ± 0.025 Myr - 0.435 ± 0.008 Myr) and Terre-de-Haut (2.98 ± 0.04 Myr to 2.00 ± 0.03 Myr) were measured and analyzed. Samples were classified according to their lithotypes and hydrothermal alteration, considering the Basal Complex exhibits proofs of hydrothermal activity and the central part of Terre-de-Haut as an exhumed geothermal palaeo-system. Lithotypes are lava flows and dikes, debris flows, volcanic breccias and pyroclastites including scoria, pumice and ashes.
Even if the solid phase density is relatively uniform for all lithotypes 2.57-2.75 g.cm-3 reflecting a constant mineralogical composition, physical properties obtained on these volcanic rocks display high variability. Porosity varies from 26 % to 42 % for volcaniclastic deposits and 7 % to 11 % for andesite lava. Permeability of lava is lower than 10-15 m2, while it is higher than 10-14 m2 and can reach 10-9 m2 in the volcaniclastics due to a well-connected porosity and open fractures. Both porosity and permeability increase with hydrothermal alteration in andesite lava. Average thermal conductivities are almost divided by two in volcaniclastic deposits 0.51-0.99 W.m-1.K-1 compare to lava’s values, 1.60 W.m-1.K-1. Thermal
conductivity slightly increases in hydrothermalized lava due to pyrite crystallization. P-wave velocities are often hardly measurable in pyroclastic rocks because of the low cohesion and fractures. However average Pwaves velocity measures display values twice lower in volcaniclastics 2045 m.s-1 than in lava 4122 m.s-1.