The recovery and isotopic measurement of water from fluid inclusions in speleothems

摘要

The isotopic composition of speleothems is a useful palaeoclimatic indicator, but its value would be enhanced if information on the composition of the parent dripwaters could be recovered from fluid inclusions in the speleothem calcite. To develop a robust method for extracting and measuring oxygen and hydrogen isotopic composition of fluid inclusions we have used anhydrous Iceland Spar and microlitre glass capillaries of water as an analogue system. Crushing the capillary and calcite together in a high vacuum cell we have investigated the adsorbtive and isotopic behaviour of water when exposed to clean fracture surfaces. Significant water adsorption occurs at room temperature, accompanied by large negative isotopic shifts of both oxygen and hydrogen in the recovered foe water at H2O/CaCO3 mass ratios < 10 mg g(-1). Cryogenic pumping fails to achieve total desorption. The degree of depletion is inversely related to the water/calcite ratio, fractionation of hydrogen isotopes exceeding -20%, and oxygen isotopes -10%, at ratios typically observed in natural speleothems. Heating the crushed calcite at 150C for 60 min, totally desorbs the water and allows retrieval of the correct isotopic composition. Application of these methods to a British Late Holocene speleothem yields delta O-18 and delta H-2 compositions for the inclusion water which are closely comparable with the modern cave dripwaters and local precipitation. The results show that isotopic compositions can be recovered from inclusion samples of <1L (equivalent to approximately Ig of calcite) with precisions that are useful for palaeoclimatic research, +/-0.4% for delta O-18 and +/-3% for delta H-2. Greater precision than this will require replicate analysis for each speleothem growth increment. Copyright (C) 2001 Elsevier Science Ltd. [References: 46]