Lead supervisor: Marian Holness, Earth Sciences
Co-supervisor: Jakob Keiding, Geological Survey of Denmark and Greenland
This is a CASE project with the Danish Geological Survey
Brief summary:
This project is focussed on the physical and chemical behaviour of unmixed immiscible Si-rich and Fe-rich liquids in a solidifying gabbro, using the Skaergaard intrusion of East Greenland as a natural laboratory
Importance of the area of research concerned:
Recent work has shown that basaltic liquids encounter a binode, resulting in unmixing to form a pair of immiscible conjugate liquids, one Fe-rich and the other Si-rich. Although continued fractionation results in an overall Si-enrichment and the cessation of immiscibility as the systems leaves the binode, while the liquid remains within the binode the very different physical properties of the two co-existing immiscible liquids drive fractionation, leading to intrusion-scale variations in bulk rock composition and the formation of economically viable concentrations of PGE. The behaviour of a porous medium containing two immiscible liquids with different wetting properties has been extensively investigated because of its relevance to the petrochemical industry, but solving the problem of immiscibility in a solidifying magma requires consideration of a poly-mineralic porous medium in which the grains are growing from the pore fluid emulsion. Understanding how PGE deposits are created in layered intrusions is dependent on gaining an understanding of this highly complex process.
Project summary :
The Paleogene Skaergaard intrusion of East Greenland formed by the progressive fractionation of a body of evolved ferro-basalt. It records abundant evidence of silicate liquid immiscibility which, during the earlier stages of fractionation, was confined to the interstitial liquid in the crystal mush, but during the later stages the remaining bulk magma also unmixed. The buoyant Si-rich conjugate collected near the roof of the intrusion, whereas the floor cumulates are correspondingly enriched in the dense Fe-rich conjugate. The intrusion hosts an economically important concentration of gold and PGE: the differential migration of the two immiscible conjugates was paramount in controlling its location and grade. The project is aimed at constraining the details of this differential migration, both the physical behaviour of the solidifying mush and the chemical effects of the migration.
What will the student do?:
The student will undertake detailed examination of the chemical and microstructural record of differential migration within the Skaergaard gabbros. The first stage involves demonstrating the origin of paired (mafic vs felsic) coarse-grained bodies that have been variably ascribed to volatile-mediated metasomatism and to the solidification of segregations of unmixed interstitial liquid. This will be extended to an investigation of large-scale localised replacement of gabbros on the chamber wall by more mafic compositions, testing the hypothesis that this represents a late-stage accumulation of Fe-rich interstitial liquid. The small-scale effects of differential migration will be investigated via detailed microstructural and chemical investigation of horizons in which it has been argued that significant reaction occurred between infiltrating Fe-rich liquid and less evolved primocrysts of the crystal mush.
Low-temperature analogue experiments will be used to investigate the behaviour of polymineralic mushes containing two immiscible liquids with varying wetting properties to investigate the effect of spatial mineralogic variations, such as modal grading, on differential migration.
References - references should provide further reading about the project:
Jakobsen, J.K., Veksler, I.V., Tegner, C. & Brooks, C.K. 2005. Immiscible iron- and silica-rich melts in basalt petrogenesis documented in the Skaergaard intrusion. Geology, vol. 33, pp. 885-888. doi: 10.1130/G21724.1
Holness, M.B., Stripp, G., Humphreys, M.C.S., Veksler, I.V., Nielsen, T.F.D. & Tegner, C. 2011. Silicate liquid immiscibility within the crystal mush: late-stage magmatic microstructures in the Skaergaard intrusion, East Greenland. Journal of Petrology, vol. 52, pp. 175-222. doi:10.1093/petrology/egq077
Applying
You can find out about applying for this project on the Department of Earth Sciences page.