Influence of alteration on physical properties of volcanic rocks

Abstract

Physical properties of some weathered/altered volcanic rocks and their variation with the degree of alteration are described in detail. A series of tests was performed to identify and quantify the progressive degradation of the properties: 1) petrographycal and chemical studies; 2) effective (ηe) and total porosity (ηt) measurements and 3D pore reconstruction 3) ultrasonic pulse velocity and spatial attenuation (αs) measurements.Four different volcanic lithologies have been tested: i) trachytic lava with abundant crystals; ii) pyroclastic deposits, with lava clasts and pumice elements; iii) green tuff, made prevalently of pumice clasts; and iv) non-welded ignimbrite deposits. Chemical indices of weathering (CIW) reveal large differences (42.73<Chemical Index of Alteration [CIA]<69.24) not only between lithotypes, but also between samples. These differences are reflected by physical properties, in particular ηt (6.0<ηt from X-ray tomography images<49.8%), ηe (11.0<ηe from mercury porosimeter<65.0%) and shear wave velocity values (0.50<Vs<2.90km/s). Pore network evolution with alteration for each lithology is well documented by fractal dimension (D) and αs. Mean values of porosity are strictly related to P and S wave velocity (Vp and Vs) and the degree of alteration.Values of CIA are well correlated with the degradation trend exhibited by measured physical properties. The combination of techniques to measure the ηt and ηe provides a good estimate of grain size and pore size distribution and rock structure. Defects and particular characteristics in the rock sample (e.g. micro-fractures, voids, cavities and orientation and sizes of certain minerals and clasts) are revealed by αs values: the smaller the αs the more homogeneous and less altered is the sample.