Tungsten-182 evidence for an ancient kimberlite source

Abstract

Globally distributed kimberlites with broadly chondritic initial 143Nd-176Hf isotopic systematics may be derived from a chemically homogenous, relatively primitive mantle source that remained isolated from the convecting mantle for much of the Earth’s history. To assess whether this putative reservoir may have preserved remnants of an early Earth process, we report 182W/184W and 142Nd/144Nd data for “primitive” kimberlites from 10 localities worldwide, ranging in age from 1,153 to 89 Ma. Most are characterized by homogeneous μ182W and μ142Nd values averaging −5.9 ± 3.6 ppm (2SD, n = 13) and +2.7 ± 2.9 ppm (2SD, n = 6), respectively. The remarkably uniform yet modestly negative μ182W values, coupled with chondritic to slightly suprachondritic initial 143Nd/144Nd and 176Hf/177Hf ratios over a span of nearly 1,000 Mya, provides permissive evidence that these kimberlites were derived from one or more long-lived, early formed mantle reservoirs. Possible causes for negative μ182W values among these kimberlites include the transfer of W with low μ182W from the core to the mantle source reservoir(s), creation of the source reservoir(s) as a result of early silicate fractionation, or an overabundance of late-accreted materials in the source reservoir(s). By contrast, two younger kimberlites emplaced at 72 and 52 Ma and characterized by distinctly subchondritic initial 176Hf/177Hf and 143Nd/144Nd have μ182W values consistent with the modern upper mantle. These isotopic compositions may reflect contamination of the ancient kimberlite source by recycled crustal components with μ182W ≥ 0.

Perturbation of the deep-Earth carbon cycle in response to the Cambrian Explosion

“…While this process cannot be completely ruled out, it is not favored here because surface-derived, high-δ 13 C oxidized carbon can be efficiently recycled into the upper mantle as shown by some diamonds ( 83 ) and mantle xenoliths ( 84 ). Conversely, the transition zone provides an effective thermal barrier to the subduction of oxidized carbon into the lower mantle ( 43 ), from where most kimberlites are probably sourced ( 20 , 23 , 24 ).…”

“…The genetic link between kimberlites and deep mantle plumes is reinforced by the distribution of some kimberlite fields along age-progressive corridors corresponding to the continental portions of hot spot tracks ( 21 ). Furthermore, the relatively homogeneous Nd and Hf isotope composition and the occurrence of negative 182 W anomalies in kimberlites older than ~200 Ma requires tapping of a deep source largely isolated from mantle convection and associated recycled crustal components ( 22 , 23 ), and hence probably located in the lowermost mantle ( 24 ). This is supported by the entrainment of superdeep diamonds, which contain inclusions of minerals stable in the mantle transition zone and lower mantle [e.g., ringwoodite ( 25 ) and CaSi-perovskite ( 26 )], and the He and Ne isotope systematics of olivine in some kimberlites, which require deep-mantle plume contributions ( 27 ).…”

Perturbation of the deep-Earth carbon cycle in response to the Cambrian Explosion

“…While this process cannot be completely ruled out, it is not favored here because surface-derived, high- 13 C oxidized carbon can be efficiently recycled into the upper mantle as shown by some diamonds (83) and mantle xenoliths (84). Conversely, the transition zone provides an effective thermal barrier to the subduction of oxidized carbon into the lower mantle (43), from where most kimberlites are probably sourced (20,23,24).…”

“…The genetic link between kimberlites and deep mantle plumes is reinforced by the distribution of some kimberlite fields along ageprogressive corridors corresponding to the continental portions of hot spot tracks (21). Furthermore, the relatively homogeneous Nd and Hf isotope composition and the occurrence of negative 182 W anomalies in kimberlites older than ~200 Ma requires tapping of a deep source largely isolated from mantle convection and associated recycled crustal components (22,23), and hence probably located in the lowermost mantle (24). This is supported by the entrainment of superdeep diamonds, which contain inclusions of minerals stable in the mantle transition zone and lower mantle [e.g., ringwoodite (25) and CaSi-perovskite (26)], and the He and Ne isotope systematics of olivine in some kimberlites, which require deep-mantle plume contributions (27).…”

Geodynamic and Isotopic Constraints on the Genesis of Kimberlites, Lamproites and Related Magmas From the Finnish Segment of the Karelian Craton

“…Regardless of the exact depth of kimberlite genesis below the lithosphere‐asthenosphere boundary (LAB), the source region(s) is likely to represent a mix of multiple components. For example, some studies have indicated the presence of “ancient”, “refractory” or “primordial” components in the source of these rocks (e.g., Giuliani et al., 2021; Nakanishi et al., 2021; Tappe, Budde, et al., 2020; Woodhead et al., 2019), as well as the incorporation of “subducted/recycled” and “enriched” material into the kimberlite source region (e.g., Giuliani et al., 2022; Nowell et al., 2004; Tappe et al., 2013; Tovey et al., 2021; Woodhead et al., 2019).…”