Chalcophile and highly siderophile element behaviour in subduction zones: implication for global recycling of crustal 186Os-187Os signatures.
Final Report Abstract
The DFG project had as main objective to improve, from both the natural paleo-subduction rocks and the experimental points of view, our understanding of the behavior and the fluxes of the chalcogens (Se and Te) and the highly siderophile elements (Os, Ir, Ru, Pt, Pd, Re) during the progressive dehydration and prograde metamorphism experienced by a down-going subducted oceanic slab. The main goals of the project were largely achieved, leading to interesting results and having already initiated new collaborative work with international researchers. When compared to their respective oceanic crust protoliths, the whole-rock metagabbros and metabasalts from the Greek island of Syros (Cycladic Blueschist Unit) suggest that Pt, Pd, Se and in a lesser extent Re behave as mobile elements during the dehydration and prograde metamorphism of a down-going oceanic crust, confirming the findings of Becker (2000) and Dale et al. (2007) on paleo-subduction rocks from worldwide locations. Additionally, mélange zone serpentinites, which likely derived from residual harzburgitic peridotite protolith, suggest that lithospheric mantle may lose 60-95 % of the chalcogens and HSE during subduction. Taken as a whole, these results demonstrate that the mantle wedge would be overprinted by Se, Te, and Re fluxes from the oceanic crust and peridotite-derived unradiogenic Os flux associated with Ir, Pt and Pd fluxes. The dehydrated subducting oceanic crust would then be characterized by lower Re/Os and Pt/Os than the bulk oceanic crust protolith, making the bulk assimilation of such a recycled component in intraplate magmas even more unrealistic to explain the 187Os-186Os radiogenic signatures of the latter. Investigations at the mineral scale (silicates, rutile, pyrite) in the metagabbros and the metabasalts reveal that while the chalcogens and HSE are predominantly hosted in Base Metal Sulfides (BMS, pentlandite, pyrrhotite, and chalcopyrite) in their oceanic crust protoliths, the modification of the sulfide mineral assemblage and the decrease of the BMS modal abundances contribute to their significant redistribution in silicates and platinumgroup minerals. These latter, observed during high resolution mineralogical investigations and suspected from the lack of reproducibility of whole-rock and mineral geochemical analyses, would likely stabilize in response to the decrease of BMS modal abundances. The experimental investigations of gabbro assemblages (silicates + sulfides) at temperatures similar to that of the Syros peak metamorphism (500 ºC) and pressures half of those estimated for Syros peak metamorphism (6.7-7.7 kbar vs. 14-15 kbar), and in presence of NaCl and S, reproduce the disappearance of pentlandite and the appearance of pyrite as described in the BMS assemblages of paleo-subduction rocks and also reveal re-equilibration and subsequent progressive replacement of the original pyrrhotite and chalcopyrite. The chalcogens and HSE mass balance between the starting and end bulk sulfides do not show any clear Os, Ir, Pd and Se losses despite the change of mineralogical assemblage but suggest that Pt, Re and Se are now partly hosted by phases others than the BMS or dissolved possibly in the fluid phase that was present in the experiments. The results of the experimental and natural paleo-subduction rocks investigations at the mineral scale and the contrasts between the findings of these two approaches call for subsequent geochemical, mineralogical (with high-resolution microscopy techniques), and experimental investigations in order to solve the newly-open questions and better understand the role of BMS, silicates, and platinum group minerals during subduction related dehydration and prograde metamorphism, and how this affects the behavior and fluxes of the chalcogens and the HSE at the mineral scales and ultimately at the whole-rock scale.
Publications
- 187Os-HSE-Se-Te systemarics of subducted oceanic crust. Short Course on Highly Siderophile Elements, SCRIPPS Institute, San Diego, December 2015
Lissner M., Luguet A., Nagel T., König S.
- 2015. 187Os-HSE-Se-Te systemarics of subducted oceanic crust. 2015 Goldschmidt Conference, Prag, Czech Republic
Lissner M., Luguet A., Nagel T., König S., Garbe-Schönberg D.
- Mineralogical insights on the HSE-Se-Te behaviour during subduction. International Workshop on Highly Siderophile Elements, Durham, July 2016
Lissner M., Landgraf F., Nagel T., König S., Luguet A.
