Isotope analysis of fluid inclusion water in non-marine carbonates

Non-marine carbonates have turned into a hot topic in the geosciences over the past few decades, which is due to several developments in this field of science. First, there is the remarkable increase of studies of speleothem... [ view full abstract ]

Non-marine carbonates have turned into a hot topic in the geosciences over the past few decades, which is due to several developments in this field of science. First, there is the remarkable increase of studies of speleothem calcite as a climate archive. Owing to the great advances in precision of Uranium series dating of such calcites, speleothem records now form the well-dated backbone of terrestrial climate archives. Their dominantly calcitic mineralogy allows many climate proxy records to be applied that were already developed for marine carbonate studies.
Another reason why non-marine carbonates have moved to the center of attention is that the large oil reserves in the South Atlantic "pre-salt" reservoirs are largely developed in non-marine carbonates (travertines and tufas). Initially, the genesis and architecture of such deposits were poorly understood by, traditionally marine-based, oil company carbonate sedimentologists, which has initiated considerable industry funding for academic groups working on this topic over the past years.

One of the most important limitations for the application of (isotope)-geochemical proxy records on non-marine carbonates is that the chemical composition of the fluid from which the carbonate forms is much more variable than that of marine carbonates. Since most geochemical proxy records depend on reasonably precise knowledge of the composition of the fluid from which the carbonate formed, it is of pivotal importance to beter characterize the fluids (and conditions) in which non-marine carbonates grow. Many researchers have done so by studying and monitoring modern non-marine carbonate deposits, like caves and travertine/tufa systems, carefully linking carbonate structure and composition to fluid chemistry and growth conditions.

Another approach to get information regarding the formation fluids, is the study of water trapped in fluid inclusions in non marine carbonates. In several labs worldwide, analytical procedures have been developed and refined over the past 15 years for the stable isotope (δ18O and δ2H) analysis of fluid inclusion waters in speleothems and other non-marine carbonates. The lab at VU-University Amsterdam has been one of the pioneering labs in this field.

In speleothem studies, fluid inclusion isotope values provide key paleoclimate parameters. Such data are a direct proxy for the isotopic composition of paleo-rainfall, because cave drip water captured in the fluid inclusions isotopically approximates rainwater recharging the cave. Besides direct paleorainfall isotope records, this technique also allows for the calculation of changing formation temperatures through time, by combining the oxygen isotope data of fluid inclusion water and host calcite.
This ability to measure direct fluid composition and calculate isotope equilibrium temperatures is also of good use when this technique is applied to higher temperature systems, like subsurface reservoir diagenetic systems, or minerals formed at hydrothermal springs.

In this presentation some recent work done in our lab in Amsterdam is lined up to demonstrate the potential and limitations of the fluid inclusion stable isotope technique for the study of non-marine carbonates.