First coupled Sr and Pb isotopic measurements in volcanic gas condensates and groundwaters of Vulcano Island (Italy)

Abstract

For the first time, coupled Sr-Pb concentrations and isotopic compositions have been measured on four crater fumaroles and six groundwaters sampled between 1999 and 2001 at Vulcano Island, southern Italy. Sr concentrations and isotopic compositions of groundwaters show that the Sr budget results mainly from the leaching of volcanic rock by groundwaters. Leaching intensity is controlled by the variable dissolution of volcanic gases into shallow aquifers. The data do not reveal any influence of seawater except at one coastal site. Low Pb concentrations together with calculated saturation indexes with respect to mineral phases indicate that Pb content in groundwaters is strongly influenced by adsorption and/or precipitation processes. Pb isotopic ratios of two steam-heated waters display a clear anthropized meteoric water signature, suggesting gas dissolution or wall rock alteration to be a negligible process for this element in this particular context. Low (≈0.5) and high (10 2-103) enrichment factors were calculated for Sr and Pb, respectively, indicating their refractory and volatile behavior in volcanic gases. Most of the gas samples display Sr and Pb isotopic ratios similar to parent rocks. These data suggest that Sr is mainly derived from wall rock or particle contamination during gas ascent, whereas Pb results mainly from magma degassing. However, two gas condensates collected from two different fumarolic vents and at different periods show a dramatic Pb and Sr isotopic shift with a clear atmospheric/anthropogenic signature. Episodic permeability variations of the volcanic structure, probably arising from microseismicity swarms, changes of local stress field, and migration of self-sealing processes lead to changing interactions between freshly infiltrated meteoric water and volcanic gases that temporarily modify the trace metal signature of the fumarolic gas. Copyright 2005 by the American Geophysical Union.