Remnants of early Earth differentiation in the deepest mantle-derived lavas

Abstract

The noble gas isotope systematics of ocean island basalts suggest the existence of primordial mantle signatures in the deep mantle. Yet, the isotopic compositions of lithophile elements (Sr, Nd, Hf) in these lavas require derivation from a mantle source that is geochemically depleted by melt extraction rather than primitive. Here, this apparent contradiction is resolved by employing a compilation of the Sr, Nd, and Hf isotope composition of kimberlites—volcanic rocks that originate at great depth beneath continents. This compilation includes kimberlites as old as 2.06 billion years and shows that kimberlites do not derive from a primitive mantle source but sample the same geochemically depleted component (where geochemical depletion refers to ancient melt extraction) common to most oceanic island basalts, previously called PREMA (prevalent mantle) or FOZO (focal zone). Extrapolation of the Nd and Hf isotopic compositions of the kimberlite source to the age of Earth formation yields a143Nd/144Nd-176Hf/177Hf composition within error of chondrite meteorites, which include the likely parent bodies of Earth. This supports a hypothesis where the source of kimberlites and ocean island basalts contains a long-lived component that formed by melt extraction from a domain with chondritic143Nd/144Nd and176Hf/177Hf shortly after Earth accretion. The geographic distribution of kimberlites containing the PREMA component suggests that these remnants of early Earth differentiation are located in large seismically anomalous regions corresponding to thermochemical piles above the core–mantle boundary. PREMA could have been stored in these structures for most of Earth’s history, partially shielded from convective homogenization.

The Mercury Isotopic Composition of Earth's Mantle and the Use of Mass Independently Fractionated Hg to Test for Recycled Crust

“…Rizo et al, 2019) -this mantle reservoir is most likely to preserve signatures associated with terrestrial accretion. Indeed, arguments have been made that the high-3 He/ 4 He reservoir is the oldest domain that has survived in Earth's mantle(Giuliani et al, 2020;Jackson et al, 2010) but alternative views exist (e.g.,Mukhopadhyay & Parai, 2019). Therefore, the absence of Hg-MIF in the high-…”

Sulfur Isotope Constraints on the Petrogenesis of the Kimberley Kimberlites

“…Further support for this idea can be found in previously published Sr isotopic data for perovskite in the Kimberley kimberlites (Woodhead et al, 2009), which have more radiogenic initial 87 Sr/ 86 Sr ratios (∼0.7045) than coeval kimberlites, such as those in Botswana (0.703-0.704), or in HIMU-like initial 206 Pb/ 204 Pb ratios in the Kimberley kimberlites (Collerson et al, 2010;Kramers, 1977;Smith, 1983). This change in composition in young kimberlites might support recent speculation by Woodhead et al (2019) of abrupt incorporation of subducted material from a slab "graveyard" into the PREMA-like mantle source of kimberlites globally (see Giuliani et al, 2021). Alternatively, interaction between mantle plumes and subducted material in the mantle transition zone (Nowell et al, 2004;Pearson et al, 2019;Tappe et al, 2013) may also explain the compositional features of the Kimberley kimberlites.…”

Tungsten-182 evidence for an ancient kimberlite source

“…The chondritic to slightly suprachondritic initial 143 Nd/ 144 Nd and 176 Hf/ 177 Hf ratios of the primitive kimberlite reservoir could have been created by the mixing of depleted mantle and enriched crustal materials. However, such a mixing scenario is unlikely to have generated a source with comparatively uniform isotopic evolution through at least 2,000 Mya, given likely changes in the compositions of subducted crustal materials and the physical conditions present in subduction zones through time (9).…”

“…Two recent studies examined the long-lived 147 Sm- 143 Nd and 176 Lu- 176 Hf isotopic systems in a global suite of fresh kimberlites, with formation ages ranging from ∼2,050 Ma to present day (8,9). Most of the kimberlites that are older than ∼200 Ma are characterized by initial 143 Nd/ 144 Nd slightly above the range of chondritic evolution and corresponding initial 176 Hf/ 177 Hf ratios within the range of chondritic evolution (e.g., ref.…”

“…The collective 143 Nd-176 Hf isotopic growth trajectories of these samples, which Woodhead et al (8) termed "primitive" kimberlites, are much more restricted than and well resolved from those of midocean ridge basalts (MORB), which provide representative samples of the convective upper mantle. The Nd and Hf isotopic growth trajectories of the primitive kimberlites require derivation from one or more isolated, chemically homogeneous sources that maintained relatively constant and chondritic to slightly suprachondritic Sm/Nd and Lu/Hf ratios for at least the ∼2,000-Ma range of sample ages that define the trend and likely extending back to the earliest stages of Earth history (8,9).…”