Smectite quantification in hydrothermally altered volcanic rocks

Léa Lévy (1, 2, 3) , Thráinn Fridriksson (3) , Nathaniel Findling (4) , Bruno Lanson (4) , Bernard Fraisse (5) , Nicolas Marino (6) , Benoit Gibert (6)

1 LGENS - Laboratoire de géologie de l'ENS
2 Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, 101 Reykjavík, Iceland
3 ÍSOR - Iceland GeoSurvey, Reykjavík, Iceland
4 ISTerre - Institut des Sciences de la Terre
5 ICGM - Institut Charles Gerhardt Montpellier - Institut de Chimie Moléculaire et des Matériaux de Montpellier
6 Géosciences Montpellier

In volcanic environments, the presence of smectite may indicate recent hydrothermal circulations. Smectite is also responsible for enhanced rock electrical conductivity, as well as mechanical weakening. Therefore, 1 quantifying smectite is important in geothermal exploration. Smectite identification requires X-ray diffraction (XRD) but quantification based on XRD is time-consuming and not always accurate. In the present study, we investigate the use of an optimized unbuffered Cation Exchange Capacity (CEC) measurement, by back-titration of the Coppertriethylenetetramine(II) "Cu-trien" molecule, to quantify the smectite content of altered volcanic rock samples. We establish that a satisfying trade-off between the instrument uncertainty and an independant systematic error is theoretically reached for a fraction of reactants consumed of about 30% at the end of the exchange reaction. We suggest a modification to classical protocols to fall in that range. Finally, we show that optimized CEC measurements by Cu-trien are a direct measure of the smectite weight fraction in altered volcanic samples, with an average CEC of pure smectite of 90 ± 5 meq/100g.

Domains: Continental interfaces, environment

Léa Lévy, Thráinn Fridriksson, Nathaniel Findling, Bruno Lanson, Bernard Fraisse, et al.. Smectite quantification in hydrothermally altered volcanic rocks. Geothermics, 2020, 85, pp.101748. ⟨10.1016/j.geothermics.2019.101748⟩. ⟨insu-03043134⟩