The fractionation of Highly Siderophile Elements (HSE) during partial melting, and its implication for the HSE composition of the Primitive Upper Mantle
Zusammenfassung der Projektergebnisse
The project had as its main objective to understand how the so‐called highly siderophile elements (HSE) behaved in response to magmatic processes, such as partial melting, crystallization and terrestrial differentiation. Project work focussed on different aspects of HSE geochemistry, namely which phases control these elements and host them, how such phases form, and what factors affect their stability. Moreover, it was the aim of this project to assess how variations in redox conditions (i.e. fO2 and fS2) led to changes in the behaviour of the HSE during partial melting and crystallization. Finally, one important goal of the proposal was to investigate how major as well as subtle changes in sulphide and silicate melt composition can affect the behaviour of the HSE and the stability of the phases that host them. Results obtained within the scope of this proposal have shown that residual noble metal alloys, are very important hosts to the HSE in the Earth’s mantle. When present, alloys were shown to reproduce the observed Re/Os fractionation found in mid‐ocean ridge basalts, despite the fact that their modal abundances are immeasurably small. Alloy formation and their compositions was also shown to be strongly redox controlled, with variations in fS2 leading to the reproduction of global compositional trends found in natural magmatic alloys. A model for alloy formation was also proposed, which involves the HSE becoming concentrated initially in residual sulphide melt and becoming oversaturated during desulphurization. This process is found to lead to alloy saturation from a HSE‐rich residual sulphide liquid. Results of different studies carried out within the context of this project have clearly shown that compositional variations, both in sulphide and silicate liquids, may greatly affect the behaviour of the HSE during partial melting and crystallization. For example, the use of FeO‐bearing silicate melts was shown to produce differences in the solubility of Pd in basalt, especially at fO2 relevant of basalt formation. Moreover, the presence of S2‐dissolved in silicate melt was shown to enhance the solubility of Ru in silicate melt, while Pd, for example, becomes less soluble if S is present. Such results show that past experimental work carried out using simple silicate melt compositions may be inadequate to simulate mantle melting as it does not include the purported compositional effect of S2‐ and FeO. Studies that focussed on sulphide systems have also borne fruit. Ligands like As, Te, Se etc. have been shown to strongly affect HSE behaviours. For example, Pt was shown to strongly associate with As to the extent that it directly impacted on the partitioning behaviour of Pt. Enticingly, nano‐scale Pt‐As associations were identified in sulphide melt even when the system was grossly under‐saturated in a PtAs macroscopic liquidus phase, showing that Pt bonds with As at the molecular level in a sulphide liquid. The presence of such ligands was also shown to impact on which HSE‐hosting phases crystallize from a sulphide liquid during cooling, affecting the liquid line of descent of HSE‐bearing phases. Another aspect that was explored in the scope of this DFG project was the role of oxygen dissolved in sulphide liquid and how it affects both the composition of the sulphide as well as the behaviour of the HSE. Results show that if oxygen is present, the HSE will become less soluble in sulphide melt becoming considerably less chalcophile as a result. Moreover, more oxidized systems will tend to favour the formation of Cu‐rich sulphides at the expense of Fe‐ and Ni‐rich varieties. This phenomenon is shown to lead to the fractionation of the HSE, as Pt and Pd will favour Cu‐rich sulphides and Os, Ir and Ru favour Fe‐ and Ni‐rich phases. The contribution to the highly siderophile element scientific community has been positive as it furthered our understanding of how these elements behave in high temperature systems. In particular we now have a better grasp of the processes, phases and conditions that determine HSE behaviour during partial melting and crystallization. The work carried out within the scope of this project has also opened further avenues of research in the future.
Projektbezogene Publikationen (Auswahl)
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(2010). Partitioning of Se, As, Sb, Te and Bi between monosulphide solid solution and sulphide melt – Application to magmatic sulphide deposits. Geochimica et Cosmochimica Acta 74, 6174‐ 6179
Helmy, H. M., Ballhaus, C., Wohlgemuth‐Ueberwasser, C., Fonseca, R. O. C., and Laurenz, V.
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(2011). Solubility of Os and Ir in sulphide melt: Implications for Re/Os fractionation during mantle melting. Earth and Planetary Science Letters 311, 339‐350
Fonseca, R. O. C., Mallmann, G., O’Neill, H. St. C., Campbell, I. H. and Laurenz, V.
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(2012). New constraints on the genesis and long‐term stability of Os‐rich alloys in the Earth’s mantle. Geochimica et Cosmochimica Acta 87, 227‐242
Fonseca, R. O. C., Laurenz, V., Mallmann, G., Luguet, A. Hoehne, N., and Jochum, K. P.
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(2013). Fractionation of platinum, palladium, nickel, and copper in sulphide–arsenide systems at magmatic temperature. Contributions to Mineralogy and Petrology 166, 1725‐1737
Helmy, H. M., Ballhaus, C., Fonseca, R. O. C., and Nagel, T. J.
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(2013). Sulphide oxidation as a process for the formation of copper‐rich magmatic sulphides. Mineralium Deposita 48, 115‐127.
Wohlgemuth‐Ueberwasser, C. C., Fonseca, R. O. C., Ballhaus, C. and Berndt, J. (
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(2013). The solubility of palladium and ruthenium in picritic melts: 2. The effect of sulphur. Geochimica et Cosmochimica Acta 108, 227‐242
Laurenz, V., Fonseca, R. O. C., Ballhaus, C., Jochum, K. P., Heuser, A., and Sylvester, P. J.
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(2014). Redox controls on tungsten and uranium crystal/silicate melt partitioning and implications for the U/W and Th/W ratio of the lunar mantle. Earth and Planetary Science Letters 404, 1‐13
Fonseca, R. O. C., Mallmann, G., Sprung, P., Sommer, J. E., Heuser, A., Speelmanns, I. M., and Blanchard, H.
