Determining the compositions of deep crustal fluids: in-situ sampling and cryo-LIBS-ICP-MS

Projet collaboratif du Geotop

2020-2022

Aqueous fluids are the dominant mechanism of element redistribution in the Earth’s crust. This results from the extreme differences in element solubility as the physical and chemical properties of the fluid vary (e.g. pH, T, salinity). As a result, fluids have an unequalled ability to selectively mobilise and deposit elements, culminating in the formation of ore deposits. Despite the recognized importance of fluids in mobilizing and transporting elements in the Earth, there is considerable uncertainty in the composition of deep fluids. Fluid inclusions, small aliquots of fluid trapped in minerals, are at present our best source of information, but their preservation, analysis and interpretation may be ambiguous, and they are not available for all settings of interest. A key concern is the role of mineral phases in fluid inclusions, which are variably interpreted as coentrapped solids or as post-entrapment precipitates. The former would suggest an important role for particulates in deep fluid element transport, whereas the latter would imply high metal contents. The rise of geothermal as a source of energy has opened a new avenue for investigating deep fluids in the Earth. Geothermal wells now tap fluids at multiple km depth, and the high flux in wells minimizes their reequilibration en route to the surface, thereby allowing for reliable samples of deep fluids to be collected at the well head. However, these fluids undergo phase separation at depth into a brine and vapour, in part initiated by the pressure drop resulting from surface fluid extraction. The wells invariably tap the vapour component of the deep geothermal fluid. This results in strong fractionation of the elements between vapour and brine (Heinrich et al. 1999), and well head fluids are therefore not a representative sample of the bulk deep fluid. Recently, in-situ deep sampling of geothermal fluids has become viable, thereby providing access to the brine, or even the one-phase pre-separation fluid. Interestingly, bulk metal contents in these solutions exceed their solubility as determined from solubility experiments and thermodynamic modelling. This has been attributed to the presence of colloids and/or particulates, to imprecise knowledge of the species in natural high-temperature solutions, or to result from analytical artefacts when analysing these highly complex and concentrated solutions. If these high concentrations can be shown to be real, much higher metal mobility than currently predicted exists in the crust, which has major implications for understanding and quantifying the cycling of elements in the Earth, and the formation of ore deposits. In this project, we aim to address the ambiguity in deep fluid compositions by: 1. Develop analytical methods for unambiguous analysis of these complex fluids; 2. Use secondary minerals to calculate the dissolved element load of the fluid; and 3. Develop a down-well sampler to obtain additional samples. colloids) are required to explain bulk fluid metal contents.