Weathered profiles in tropical volcanic islands by combined geochemical and geophysical approaches

Tropical volcanic islands commonly have pronounced relief and high runoff rates, and consist of easily weathered volcanic material. Intense mechanical and chemical weathering in volcanic terrains has been recognized as being an important component in the transport of the global dissolved load to the oceans [1]. High chemical weathering rates are mainly due to the impact of hydrothermalism inputs with subsurface water circulations [2]. A helicopter-borne TDEM (Time Domain ElectroMagnestism) and magnetic survey was conducted by BRGM (French Geological Survey) over 3 islands of Guadeloupe, Martinique and Reunion in 2012 and 2013 for a total of 20,000 km of flightlines. TDEM method uses the diffusion of a transient EM field to determine the electrical resistivity versus depth. Erosion timescales were calculated from U-series analyses of river sediments. Our results show a broad range: 0 to150 ka in Martinique, 0 to 60 ka in Guadeloupe and 55 to 90 ky for Piton de la Fournaise (Réunion). At watershed scale, the estimated weathered profile (WP) depth obtain by using U-series method are consistent with TDEM helicopter-borne geophysical imagery method, ranging from 0 to 70 m. WP are locally impacted by hydrothermal circulations with associated secondary minerals (halloysite, tridymite…). Among the combined impact of all parameters (climate, runoff, slopes, hydrothermalism inputs, vegetation etc.), basin age seems to be the key control parameter: the younger the basin, the higher the weathering rate is.