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

The isotopic composition of speleothems is a useful palaeoclimatic indicato r, but its value would be enhanced if information on the composition of the parent dripwaters could be recovered from fluid inclusions in the speleoth em calcite. To develop a robust method for extracting and measuring oxygen and hydrogen isotopic composition of fluid inclusions we have used anhydrou s Iceland Spar and microlitre glass capillaries of water as an analogue sys tem. Crushing the capillary and calcite together in a high vacuum cell we h ave investigated the adsorbtive and isotopic behaviour of water when expose d to clean fracture surfaces. Significant water adsorption occurs at room t emperature, accompanied by large negative isotopic shifts of both oxygen an d 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 deple tion is inversely related to the water/calcite ratio, fractionation of hydr ogen isotopes exceeding -20%, and oxygen isotopes -10%, at ratios typically observed in natural speleothems. Heating the crushed calcite at 150<degree s>C for 60 min, totally desorbs the water and allows retrieval of the corre ct isotopic composition. Application of these methods to a British Late Hol ocene speleothem yields delta O-18 and delta H-2 compositions for the inclu sion water which are closely comparable with the modern cave dripwaters and local precipitation. The results show that isotopic compositions can be re covered from inclusion samples of <1<mu>L (equivalent to approximately Ig o f 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 wi ll require replicate analysis for each speleothem growth increment. Copyrig ht (C) 2001 Elsevier Science Ltd.