Behaviour of Chalcogens and Highly Siderophile Elements in subduction zones: The mantle wedge perspective
Final Report Abstract
The objectives of this project were to constrain the origin of the highly siderophile elements (HSE) and Re-Os isotopic signatures recorded by mantle wedge xenoliths and understand the role of subduction zones in contributing to the evolution of Earth’s mantle composition. This study focused on the mantle wedge xenoliths from Avacha, Kamchatka Peninsula (KP) and from Alligator Lake and Llangorse Mountains in the Northern Canadian Cordillera (NCC). In each locality, the least metasomatically-overprinted samples (based on trace elements systematics and petrographic observations) and thus the closest to residues of partial melting are systematically the HSE-poorest harzburgites, with below-PUM (Primitive Upper Mantle) HSE abundances, variable Ru and Pt anomalies and are devoid of Base Metal Sulfides (BMS). These signatures cannot be purely explained by the incongruent melting of BMS and the partitioning of the HSE between Mss and Cu-Ni rich sulfide during anhydrous partial melting but further require the removal of the residual BMS formed during partial melting. In Avacha, the high fO2 conditions, established by the slab component influx, likely allow S to be stable as sulfate, possibly leading to the destabilisation of the BMS (Mss) and the extraction of Os, Ir and Ru into the partial melts. The signatures of the NCC harzburgites rather result from the percolation of S-undersaturated carbonatitic/silicate melts, which are likely linked to the opening of the slab window (and the cessation of the arc) since the early Miocene. Both the destabilisation of the BMS in oxidizing conditions (KP) and the BMS reaction with S-undersaturated melts (NCC) may also allow for the stabilisation of discrete Pt and Ir phases, unless these formed during the initial anhydrous partial melting of the mantle wedge peridotites, which likely occurred in a mid-oceanic ridge setting. The most metasomatised KP and NCC mantle wedge peridotites are comparatively enriched in Pd and Re, or possibly in all HSE and exhibit often PUM-like HSE patterns. These along with the observation of metasomatic BMS in these samples demonstrate that they represent residual harzburgitic peridotites having experienced 70-250 ppm S addition as metasomatic BMS during percolation of small volumes of S-saturated carbonatitic/picritic/alkaline silicate melts. The radiogenic composition of these melts, inferred from the BMS addition modelling, hint, namely in Avacha, at the fluxing of Os from the crustal portion of the subducting slab. Finally, the survival of Proterozoic Re-Os TRD model ages in the most residual mantle wedge peridotites confirm the recycling of “old” lithospheric mantle into younger mantle wedges and the robustness of the Re-Os geochronometer to complex petrogenetic histories.
Publications
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HSE and 187Os/188Os isotopic systematics in mantle wedge peridotites xenoliths from the Canadian Cordillera. Goldschmidt abstract 3234. (2019)
Strack R., Luguet A., Minarik W. & Nowell G.
