Ultra-depleted 2.05 Ga komatiites of Finnish Lapland: Products of grainy late accretion or core-mantle interaction?

• Main Authors: Puchtel, Igor, Mundl-Petermeier, Andrea, Horan, Mary, Hanski, Eero, Blichert-Toft, Janne, Walker, Richard
• Other Authors: Department of Geology College Park, University of Maryland College Park, University of Maryland System-University of Maryland System, Universität Wien, Carnegie Institution for Science, University of Oulu, Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement Lyon (LGL-TPE), École normale supérieure - Lyon (ENS Lyon)-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Université de Lyon-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2020
• Subjects: grainy late accretion; Paleoproterozoic komatiites; highly siderophile elements; Re-Os and Pt-Os isotope systems; Lu-Hf; Sm-Nd; Hf-W; core-mantle interaction; Finnish Lapland; [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry; Fennoscandia; Lapland
• Online Access: https://hal.archives-ouvertes.fr/hal-02991119; https://hal.archives-ouvertes.fr/hal-02991119/document; https://hal.archives-ouvertes.fr/hal-02991119/file/CHEMGE13066R1.pdf

International audience Rhenium-Os, Ptsingle bondOs, Smsingle bondNd, Lusingle bondHf, and Hfsingle bondW isotope data, together with lithophile trace element and highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, and Pd) abundances, are reported for 2.05 Ga Jeesiörova and Kevitsa komatiites from the Central Lapland Greenstone Belt, Fennoscandia, Finland. Both komatiites are closely genetically related, with the Kevitsa dikes having served as feeding magma conduits to the Jeesiörova pillowed and massive lavas. The parental komatiite magma is estimated to have contained ~25 wt% MgO and was, thus, derived from a mantle source at least as hot as those of some of its late Archean counterparts. A suite of Jeesiörova and Kevitsa whole-rock komatiite samples and olivine and chromite separates define an internal Resingle bondOs isochron with an age of 2049 ± 13 Ma and an initial γ187Os = −0.2 ± 0.2 (2SE), indicating long-term chondritic Re/Os in the mantle source. By contrast, Ptsingle bondOs data for a set of Jeesiörova chromite separates define an average initial μ186Os = +29 ± 2 (2SE), indicating a long-term history of suprachondritic Pt/Os in the mantle source. The absolute HSE abundances in the mantle source of the Jeesiörova-Kevitsa komatiite system are estimated to have been 120 ± 5% of the present-day Bulk Silicate Earth (BSE). This is the first komatiite system for which excess HSE in the mantle source, relative to modern BSE, has been documented.The 147Smsingle bond143Nd and 176Lusingle bond176Hf data yield isochron ages and initial ratios of, respectively, 2046 ± 22 Ma with ε143Nd = +3.7 ± 0.3, and 2072 ± 20 Ma with ε176Hf = +8.7 ± 0.4 (2SE), indicating a long-term history of depletions of Nd relative to Sm, and Hf relative to Lu. The measured μ182W = +1.5 ± 3.3 is indistinguishable from the modern mantle value. Despite being strongly depleted in highly incompatible lithophile trace elements, the Th-Nb-La systematics of the komatiites indicate ~1% crustal contamination of the original komatiite magma, ...