Abstract
Online oxygen (δ18 O) and hydrogen (δ2 H) isotope analysis of fluid inclusion water entrapped in minerals is widely applied in paleo-fluid studies. In the state of the art of fluid inclusion isotope research, however, there is a scarcity of reported inter-technique comparisons to account for possible analytical offsets. Along with improving analytical precisions and sample size limitations, interlaboratory comparisons can lead to a more robust application of fluid inclusion isotope records. Mineral samples-including speleothem, travertine, and vein material-were analyzed on two newly setup systems for fluid inclusion isotope analysis to provide an inter-platform comparison. One setup uses a crusher unit connected online to a continuous-flow pyrolysis furnace and an isotope ratio mass spectrometry (IRMS) instrument. In the other setup, a crusher unit is lined up with a cavity ring-down spectroscopy (CRDS) system, and water samples are analyzed on a continuous standard water background to achieve precisions on water injections better than 0.1‰ for δ18 O values and 0.4‰ for δ2 H values for amounts down to 0.2 μL. Fluid inclusion isotope analyses on the IRMS setup have an average 1σ reproducibility of 0.4‰ and 2.0‰ for δ18 O and δ2 H values, respectively. The CRDS setup has a better 1σ reproducibility (0.3‰ for δ18 O values and 1.1‰ for δ2 H values) and also a more rapid sample throughput (<30 min per sample). Fluid inclusion isotope analyses are reproducible at these uncertainties for water amounts down to 0.1 μL on both setups. Fluid inclusion isotope data show no systematic offsets between the setups. The close match in fluid inclusion isotope results between the two setups demonstrates the high accuracy of the presented continuous-flow techniques for fluid inclusion isotope analysis. Ideally, experiments such as the one presented in this study will lead to further interlaboratory comparison efforts and the selection of suitable reference materials for fluid inclusion isotopes studies.
Highlights
Fluid inclusions in minerals constitute a unique archive of paleo-fluids in the geological record
The setup for fluid inclusion isotope analysis using cavity ring-down laser spectroscopy consists of a Picarro (Santa Clara, CA, USA) L2140-i analyzer coupled to an online preparation unit, broadly similar to the design presented by Affolter et al[40] and Dassié et al.[41]
Fluid inclusion isotope data were acquired on the cavity ring-down spectroscopy (CRDS) and IRMS analytical lines using the same mineral samples, and a direct comparison can be made between the setups
Summary
Fluid inclusions in minerals constitute a unique archive of paleo-fluids in the geological record. For a more robust application of fluid inclusion isotope techniques in paleo-fluid studies, the field, needs more thorough interlaboratory comparison experiments, ideally based on reliable fluid inclusion-bearing standard materials available to all laboratories that use the techniques This is valid for ongoing discussion on potential diagenetic exchange of oxygen between fluid inclusion water and host calcite in speleothems.[44,45] As long as the interlaboratory reproducibility of δ18Ofi data is underdefined, it remains difficult to distinguish between diagenetic effects and possible analytical artifacts. We can present a robust data comparison between the two main analytical techniques currently used in the field
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