Portable instrument and analytical method using laser-induced breakdown spectrometry for in situ characterization of speleothems in karstic caves

Abstract

A portable laser-induced plasma spectrometer has been developed and tested in the determination of thickness and semi-quantitative composition of alteration layers on the surface of speleothems. The portable instrument consists of a 50 mJ pulse ⁻¹ Q-switched Nd:YAG laser operating at 1064 nm and a high resolution spectrometer equipped with a 2048-element linear silicon CCD array detector. Its weight and size allow in situ analysis in difficult environments or relatively inaccesible places such as caves, avoiding sample collection or extraction. The instrument was evaluated in the interior Nerja Cave (Spain) over soils, rocks and speleothems. The geochemical degradation of these materials has been studied by the portable instrument. Laser-induced plasma analysis reveals the remarkable presence of several elements in the surface of the speleothems, such as Fe, Si, Al or Mn which are absent in non-degraded regions of the speleothem. Composition depth profiles of these elements have been measured in order to evaluate alteration processes in the Nerja Cave.