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

Author(s)

Igor S. Puchtel, Andrea Mundl-Petermeier, Mary Horan, Eero J. Hanski, Janne Blichert-Toft, Richard Walker

Abstract

Rhenium-Os, Pt–Os, Sm–Nd, Lu–Hf, and Hf–W 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 Re–Os isochron with an age of 2049 ± 13 Ma and an initial γ

¹⁸⁷Os = −0.2 ± 0.2 (2SE), indicating long-term chondritic Re/Os in the mantle source. By contrast, Pt–Os data for a set of Jeesiörova chromite separates define an average initial μ

¹⁸⁶Os = +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

¹⁴⁷Sm–

¹⁴³Nd and

¹⁷⁶Lu–

¹⁷⁶Hf data yield isochron ages and initial ratios of, respectively, 2046 ± 22 Ma with ε

¹⁴³Nd = +3.7 ± 0.3, and 2072 ± 20 Ma with ε

¹⁷⁶Hf = +8.7 ± 0.4 (2SE), indicating a long-term history of depletions of Nd relative to Sm, and Hf relative to Lu. The measured μ

¹⁸²W = +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, assuming the contaminant was similar in composition to the calculated Fennoscandian Tonalite Average (FTA). This level of contamination would have also significantly modified the Nd, Hf, and W isotope compositions of the original komatiitic magma, but not the Os isotope compositions or HSE abundances. The calculated original komatiite magma, corrected for the effects of crustal contamination, would have had initial ε

¹⁴³Nd ~ +4.9, ε

¹⁷⁶Hf ~ +10.2, and μ

¹⁸²W ~ −10. Our modeling indicates that the initial

^186,187Os/

¹⁸⁸Os isotopic compositions and suprachondritic HSE abundances, coupled with the projected negative μ

¹⁸²W, are best explained by either (1) derivation from a mantle domain characterized by an excess of late accreted, differentiated planetesimal core metal, i.e., “grainy” late accretion, or (2) addition of chemically fractionated terrestrial core metal to the mantle source domain of the komatiites. The presence of these characteristics in the Jeesiörova-Kevitsa komatiite mantle source provides further evidence for the early creation and long-term survival of chemically diverse domains within the mantle.

Organisation(s)

Department of Lithospheric Research

External organisation(s)

University of Maryland, College Park, University of Oulu, Carnegie Institution for Science, Université de Lyon

Journal

Chemical Geology

Volume

554

No. of pages

23

ISSN

0009-2541

DOI

https://doi.org/10.1016/j.chemgeo.2020.119801

Publication date

11-2020

Peer reviewed

Yes

Austrian Fields of Science 2012

105105 Geochemistry

Keywords

ASJC Scopus subject areas

Geochemistry and Petrology, Geology

Portal url

https://ucrisportal.univie.ac.at/en/publications/5a3582ff-1202-4604-a446-5bdd9205d1d0