Kimberlites reveal 2.5-billion-year evolution of a deep, isolated mantle reservoir

Abstract

In the widely accepted paradigm of Earth's geochemical evolution, successive extraction of melts from the mantle over the past ~4.5 billion-years formed the continental crust and produced at least one complementary, melt-depleted reservoir (Depleted MORB Mantle-DMM), now recognised as the upper mantle source of Mid Ocean Ridge Basalts (MORB) 1. However, geochemical modelling, and the occurrence of high 3 He/ 4 He (primordial) signatures in some volcanic rocks, suggest that volumes of relatively undifferentiated mantle may reside in deeper, isolated regions 2. Some basalts from Large Igneous Provinces (LIPs) may provide temporally-restricted glimpses of the most primitive parts of the mantle 3,4 but key questions over the longevity of such sources on planetary timescales, and whether any actually survive today, remain unresolved. Kimberlites-small volume, volcanic rocks that are the source of most diamonds offer rare insights into aspects of Earth's deep mantle composition. Radiogenic isotope ratios measured in kimberlites of different ages allow us to map the evolution of this domain through time. Here we show that globally-distributed kimberlites originate from a single homogeneous reservoir with an isotopic composition indicative of a uniform and pristine mantle source that evolved in isolation over at least 2.5 billion years of Earth historythe only such reservoir yet identified. Subsequently, around 200 million years ago, extensive volumes of the same source were perturbed dramatically, probably due to contamination by exogenic material. The distribution of affected kimberlites suggests that this event may be related to subduction along the margin of the Pangaea Supercontinent. These results reveal a hitherto unrecognised, long-lived, globally extensive, mantle reservoir that underwent subsequent disruption, possibly heralding a significant change to large-scale mantle mixing regimes. These processes may explain why uncontaminated primordial mantle is so difficult to identify in recent mantlederived melts. Kimberlites, a group of volatile-rich, silica-poor magmas, represent the only melts known to sample the Earth's deep mantle. Diamonds brought to the surface in some kimberlite eruptions contain inclusions of minerals that can only derive from great depthfor example, majoritic garnet and ringwoodite from the transition zone 5 , and Mg-and Caperovskite from at least the upper/lower mantle boundary and possibly up to 800 km deep 6. Consequently some, if not all, kimberlites likely originate from at least this depth 7. Importantly, kimberlitic magmas have been erupting on Earth for at least 2.85 billion years 8 and thus offer a unique window into the chemical evolution of deep mantle regions across much of Earth history. This view is unavailable from Ocean Island Basalts (OIB), which are frequently used as probes of present-day mantle compositions, owing to the relative youth of the ocean basins in which they occur, and also from LIPs which are temporally and spatially restricted. We have compiled new and exist...

Kimberlite genesis from a common carbonate-rich primary melt modified by lithospheric mantle assimilation

“…In conclusion, bulk rock incompatible trace element and olivine compositions are both consistent with primary kimberlite melts being generated from similar convective mantle sources since at least ~1635 Ma. This interpretation agrees with the temporal uniformity of radiogenic isotope compositions in pre-Mesozoic kimberlites (39) and implies that the composition of kimberlite sources and their conditions of melting have not undergone major changes over time. Alternatively, all kimberlites have equilibrated with similar asthenospheric mantle before assimilation of lithospheric material.…”

Effects of Heat‐Producing Elements on the Stability of Deep Mantle Thermochemical Piles

“…Primordial material could result from either compositional layering during magma ocean solidification (e.g., Deschamps et al, ; Labrosse et al, ) or subducted Hadean crust (e.g., Tolstikhin et al, ), and studies of noble gases from deep mantle sources have shown that lower mantle heterogeneity has been stable and isolated since the first 100 Myr of Earth history (Mukhopadhyay, ; Pető et al, ). Recent examination of kimberlites also suggests an isolated primordial reservoir that has persisted since at least 2.5 Ga (Woodhead et al, ). Alternatively, recycled oceanic crust leftover from decomposed subducted slabs could also form LLSVPs (Christensen & Hofmann, ; Coltice & Ricard, ; Hofmann, ; Mulyukova et al, ; Tackley, ).…”

Enigmatic origin of diamond-bearing rocks revealed

“…These peculiar characteristics raise questions about the nature of the kimberlite source and its location in the mantle. On page 578, Woodhead et al 1 suggest that all kimberlites originate from a single deep reservoir that has survived for most of Earth's history.…”

“…A study of this source's evolution over two billion years provides valuable information about its properties. See Letter p.578 1 suggest that volcanic rocks called kimberlites originate from a reservoir that has survived deep in Earth's mantle for most of the planet's history. This image, which was made using polarized light, shows the wide range and complex structure of minerals (such as diamonds, garnets and zircons) in these rocks.…”

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