New insights from 182W anomalies in Eoarchean rocks from northern Labrador, Canada

Abstract:

High-precision (< ±5 ppm 2σ SD on ¹⁸²W/¹⁸⁴W) measurements of W isotopic ratios have boosted recent exploration of early Earth processes [1, 2, 3]. Tungsten isotope anomalies have been reported for ~3.8 Ga rocks from Isua, Greenland [1], 2.8 Ga komatiites from Kostomuksha, Russia [2] and ≥3.8 Ga rocks from Nuvvuagittuq, Quebec [3]. All of these rocks are characterized by ¹⁸²W enrichments of as much as 15 ppm. These compositions are within the estimated mantle composition prior to addition of ~0.5 wt.% of late accreted materials to Earth [4]. However, the calculated higly siderophile element (HSE) contents of the mantle source for the Kostomuksha komatiites and Nuvvuagittug rocks are similar to modern mantle, inconsistent with a source dominated by a pre-late accretionary mantle reservoir. Thus, their mantle source must have incorporated an ancient (while ¹⁸²Hf was still extant; i.e., <50 Ma after Solar System formation), high Hf/W component [2].

We have determined the W isotopic compositions of >3.8 Ga ultramafic rocks (including mantle peridotites and meta-komatiites), plus tonalitic to mafic Uivak gneisses from northern Labrador, Canada. Within the peridotitic suite, 9 samples show well-resolved, variable ¹⁸²W enrichments ranging from +9 to +22 ppm, while three meta-komatiites have uniform ~+9 ppm enrichments. The four gneisses also show +8 to +13 ppm enrichments. As with the Kostomuksha and Nuvvuagittuq suites, the relative and absolute HSE abundances in both peridotitic and komatiitic suites are similar to their late Archean equivalents, indicating the delivery of late-accreted materials prior to 3.8 Ga. Prominent W enrichments, relative to elements with similar incompatibilities in peridotitic rocks suggest that W in these rocks may not be representative of their mantle sources, but instead could reflect redistribution of W in the crust by metamorphic processes. The ¹⁸²W enrichment of this component could be inherited from a pre-late accretionary or early depleted parental mantle reservoir, or late-accreted materials that were preferentially enriched in ¹⁸²W relative to most such materials.

[1] Willbold et al. (2011) Nature 477, 195-198. [2] Touboul et al. (2012) Science 335, 1065-1069. [3] Touboul et al. (2014) Chem. Geol. 383, 63-75. [4] Walker (2009) Chem. der Erde 69, 101-125.