Weathering regime associated with subsurface circulation on volcanic islands

Volcanic islands, being characterized by highly porous basaltic/andesitic lava flows and pyroclastic deposits, are subject to important chemical weathering by subsurface waters. Moreover, such subsurface weathering is impacted by hydrothermal springs in both active and non-active volcanic areas, thus increasing dissolved load concentrations. Here, we focus on the subsurface water chemistry in the volcanic islands of the Lesser Antilles and Réunion and on the origin of these subsurface flows. We are able, through the use of various isotopic tools (C, Sr, U–Th), to identify hydrothermal influences in river water. For example, Li concentrations show a positive correlation with temperature of hot and cold springs and also a relationship with δ¹³C; from this, we can show that several sources of hydrothermal activity influence the rivers of the Lesser Antilles and that some rivers also reveal an important organic influence. As much as 20% of the subsurface hydrothermal springs go to feed the rivers. The increasing temperatures result in more dissolved elements being mobilized and an increase in chemical weathering rates. In addition, using the (²³⁰Th/²³⁸U) isochron for the well and river dissolved loads in Martinique, Guadeloupe and Réunion, we can evaluate residence times in the river water, i.e. the average residence time in the water along the circulation path to the sampling point. Alteration takes longer when the water circulates through thick soil, for example, 400–5,000 years when circulating under an ash profile and 1,200–15,000 years when circulating through a collapse zone. It would appear that waters circulation is globally three times longer for subsurface water than for surficial water. The weathering regime in tropical volcanic environments seems to be controlled mainly by such subsurface circulation with high chemical concentration from hydrothermal inputs. The origin of these compositions is varied and not controlled by a single hydrothermal spring. Consequently, it is subsurface circulation that determines the weathering regime in tropical volcanic islands with the main controlling parameters being temperature and residence time.