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In order to further evaluate the timing and possible mechanisms responsible for the transition from both positive and negative to no 142Nd and 182W anomalies in the Archean mantle, we obtained 142,143Nd, 176Hf, 186,187Os, 182W isotope and lithophile trace and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundance data for mantle plume-derived ~2.0 Ga picrites and an associated differentiated mafic–ultramafic sill from the Onega Basin on the Fennoscandian Shield. The Onega Basin picrites share striking chemical similarities with the modern Kilauea picrites, featuring enrichments in light rare earth elements (LREE; La/SmN = 1.5 ± 0.2), depletions in heavy rare earth elements (HREE; Gd/YbN = 2.2 ± 0.1), positive high field strength element (HFSE) anomalies (Hf/Hf* = 1.2 ± 0.1, Nb/Nb* = 1.6 ± 0.1), and modern bulk silicate Earth (BSE)-like average W/Th = 0.20 ± 0.08 (2SD). Model calculations indicate that the parental picritic magmas were likely derived from 3 % equilibrium batch melting of a LREE-depleted garnet lherzolite PREMA-type mantle source containing a component of recycled oceanic crust. The 147Sm-143Nd, 176Lu-176Hf, and 187Re-187Os mineral-whole-rock isochron ages constrain precisely the timing of the Onega Basin lava emplacement at ~1974 Ma. The corresponding initial ε143Nd = +3.0 ± 0.5 and ε176Hf = +2.8 ± 1.2 values indicate evolution of the Onega mantle source with time-integrated suprachondritic Sm/Nd and Lu/Hf ratios. The lower ε176Hf relative to ε143Nd further implies decoupling of the two lithophile element isotope systems in the source. The initial μ186Os and γ187Os values are suprachondritic at +4.9 ± 2.1 and +2.9 ± 0.4, respectively, indicating evolution of the Onega mantle plume source with time-integrated slightly suprachondritic Pt/Os and Re/Os ratios. The μ142Nd = –1.1 ± 3.3 (2SD) and μ182W = 0.0 ± 4.8 (2SD) obtained for the Onega picritic magmas are unresolvable from the modern BSE values, implying that their mantle source had 142Nd and 182W compositions similar to those of the BSE. When considered together, the trace element systematics, suprachondritic Pt/Os and Re/Os ratios, and Hf-Nd isotopic decoupling are best explained in terms of incorporation into the Onega mantle plume source of 10–20 % recycled komatiite-basalt crust aged in the mantle for 1 to 2 Ga. These results provide new evidence that 142Nd and 182W anomalies that were common in the Archean mantle were effectively homogenized by 2.0 Ga ago on the scale of the mantle domains sampled by the Onega Basin magmas. This may have occurred due to the enhanced convective whole-mantle mass and heat transfer facilitated by processes of oceanic crust recycling that may have operated since at least the early Archean.
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This content will become publicly available on August 1, 2026
Osmium and Tungsten Isotopes Reveal Earth’s Youthful Exuberance
The siderophile elements, which include Re, Pt, Os, and W, directly constrain the accretionary history of Earth. The largely chondritic 186,187Os/188Os ratios of Earth’s mantle, coupled with excesses in siderophile element abundances, provide nearly incontrovertible evidence that some meteoritic addition continued after core formation was complete. Osmium and W isotope systematics of plume-derived mafic-ultramafic rocks reveal the complex chemical evolution of their deep mantle sources. In the upper mantle, Re-Os dating of whole-rock xenoliths and sulfide inclusions in diamonds hosted by kimberlites indicate both ancient melt depletion and subsequent modification of the mantle lithosphere beneath the earliest continents, with Re-Os ages of eclogitic diamonds possibly recording the transition to a sustained plate tectonic regime on Earth.
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- Award ID(s):
- 2220936
- PAR ID:
- 10647988
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Elements
- Volume:
- 21
- Issue:
- 4
- ISSN:
- 1811-5209
- Page Range / eLocation ID:
- 277 to 282
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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