Vapour phase transport in Earth’s surface environment

Overview

Fluids in Earth’s surface environment are commonly present as an aqueous-vapour phase, and evaporation, boiling and liquid-vapour separation play a key role in element transport, from the evaporation of seawater, to the formation of metal ores, from porphyry to epithermal [1] Conventionally, it has been assumed that metals concentrate in the liquid phase and that boiling promotes the partitioning of volatile elements alone. However, over recent years this conventional understanding has been questioned and there is increasing evidence from experimental studies that a number of metal species are stable in aqueous vapour, and that in all cases, metal solubility may be orders of magnitude higher than predicted from volatility data alone [2]. This is consistent with modelling [3] observations of metals in the vapour phase of fluid inclusions [4] and volcanic condensates [5]. Consequently, the vapour phase is now considered as being capable of transporting substantial quantities of many metals in its own right [6].

If metal transport via the vapour phase is simply controlled by volatile behaviour, then in principle, isotope fractionation should occur, with lighter isotopes being lost more readily to the vapour phase than heavier isotopes. However, contrary to this expected behaviour our preliminary data for Fe stable isotopes in the vapour phase of fluids from the Reykjanes hydrothermal system in Iceland show preferential enrichment of the heavy isotopes of Fe. At this stage it is not clear whether this fractionation results from the precipitation of sulfide or other secondary phases during boiling, or metal speciation in the vapour phase.

This project will involve the measurement of metal stable isotopes in the liquid and vapour phase from first, samples taken from a range of high-temperature hydrothermal systems hosted by basalt, gneiss and sediments in Iceland, Italy and Germany, combined with thermodynamic modelling of the dominant elemental species present, second experimental gas-liquid samples, involving evaporation and boiling experiments on a range of surface water compositions. These results will allow us to determine the role of volatile behaviour, metal speciation, and secondary phase precipitation in the segregation and transport of metals in the vapour phase.

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Image Captions

Vapour phase segregation at Borgarhraun, Iceland.

Methodology

This project will use state-of-the-art chemical and analytical techniques for the measurement of metal stable isotopes in liquid-vapour hydrothermal samples, host rocks and secondary minerals, and liquid-vapour experiments, to achieve the research aims outlined above.

Hydrothermal (liquid-vapour) samples, rocks and minerals will be collected in Iceland, Italy and Germany, and the project will also have access to unique sample archives of hydrothermal waters collected previously.

Project Timeline

Year 1

Training in the chemical and analytical procedures for the measurement of metal stable isotopes, including isotope analysis by MC-ICP-MS and TIMS; field sampling in Iceland, Italy and Germany; write/ defend Research Proposal;

Year 2

Sample and data processing; liquid-vapour experiments including modelling of water chemistry, develop writing and presentation skills, involving manuscript preparation and conference presentation.

Year 3

Synthesise isotope datasets and model results; attend international conferences; publication and thesis writing

Year 3.5

Complete and submit thesis; finalise manuscripts for publication.

Training
& Skills

Training in the measurement of metal stable isotopes using high precision MC-ICP-MS and TIMS techniques at Durham, and liquid-gas phase characterisation.

Fieldwork in Iceland (basalt) and Italy (genissic rock types) and Germany (sediments).

Liquid-vapour experiments in Iceland (University of Iceland)

Interpretation and modelling of isotope and elemental data to understand the controls on metal partitioning between liquid and vapour phase.

Presentation of research at both national and international geochemistry conferences.

References & further reading

[1] Pokrovski, G.S., Borisova, A.Y. & Harrichoury, J.-C. 2008. The effect of sulfur on vapor liquid fractionation of metals in hydrothermal systems

[2] Barnes, H.L. 2015. Hydrothermal processes: the development of geochemical concepts in the latter half of the twentieth century. Geochemical Perspectives, 4, 1-87.

[3] Hurtig, N.C & Williams-Jones, A.E. 2015. Porphyry-epithermal Au-Ag-Mo ore formation by vapor-like fluids: New insights from geochemical modeling. Geology, G36685. 1.

[4] Williams-Jones, A.E. & Heinrich, C. A. 2005. 100th Anniversary special paper: vapor transport of metals and the formation of magmatic-hydrothermal ore deposits. Economic Geology, 100, 1287-1312.

[4] Heinrich et al., 1999. Metal fractionation between magmatic brine and vapor, determined by microanalysis of fluid inclusions. Geology, 27, 755-758.

Further Information

For further information please contact Kevin Burton (kevin.burton@durham.ac.uk)

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