Bulk silicate Earth-like 142Nd and 182W mantle component sampled by 2.0 Ga Onega Basin picrites, Fennoscandia

Abstract

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/Sm_N = 1.5 ± 0.2), depletions in heavy rare earth elements (HREE; Gd/Yb_N = 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.