Hadean silicate differentiation preserved by anomalous 142Nd/144Nd ratios in the Réunion hotspot source

Earth's evolving geodynamic regime recorded by titanium isotopes

Earth's mantle has a two-layered structure, with the upper and lower mantle domains separated by a seismic discontinuity at about 660 km (refs. 1,2). The extent of mass transfer between these mantle domains throughout Earth's history is, however, poorly understood. Continental crust extraction results in Ti-stable isotopic fractionation, producing isotopically light melting residues3-7. Mantle recycling of these components can impart Ti isotope variability that is trackable in deep time. We report ultrahigh-precision 49Ti/47Ti ratios for chondrites, ancient terrestrial mantle-derived lavas ranging from 3.8 to 2.0 billion years ago (Ga) and modern ocean island basalts (OIBs). Our new Ti bulk silicate Earth (BSE) estimate based on chondrites is 0.052 ± 0.006‰ heavier than the modern upper mantle sampled by normal mid-ocean ridge basalts (N-MORBs). The 49Ti/47Ti ratio of Earth's upper mantle was chondritic before 3.5 Ga and evolved to a N-MORB-like composition between approximately 3.5 and 2.7 Ga, establishing that more continental crust was extracted during this epoch. The +0.052 ± 0.006‰ offset between BSE and N-MORBs requires that <30% of Earth's mantle equilibrated with recycled crustal material, implying limited mass exchange between the upper and lower mantle and, therefore, preservation of a primordial lower-mantle reservoir for most of Earth's geologic history. Modern OIBs record variable 49Ti/47Ti ratios ranging from chondritic to N-MORBs compositions, indicating continuing disruption of Earth's primordial mantle. Thus, modern-style plate tectonics with high mass transfer between the upper and lower mantle only represents a recent feature of Earth's history.

Variation in bridgmanite grain size accounts for the mid-mantle viscosity jump

A viscosity jump of one to two orders of magnitude in the lower mantle of Earth at 800-1,200-km depth is inferred from geoid inversions and slab-subducting speeds. This jump is known as the mid-mantle viscosity jump1,2. The mid-mantle viscosity jump is a key component of lower-mantle dynamics and evolution because it decelerates slab subduction3, accelerates plume ascent4 and inhibits chemical mixing5. However, because phase transitions of the main lower-mantle minerals do not occur at this depth, the origin of the viscosity jump remains unknown. Here we show that bridgmanite-enriched rocks in the deep lower mantle have a grain size that is more than one order of magnitude larger and a viscosity that is at least one order of magnitude higher than those of the overlying pyrolitic rocks. This contrast is sufficient to explain the mid-mantle viscosity jump1,2. The rapid growth in bridgmanite-enriched rocks at the early stage of the history of Earth and the resulting high viscosity account for their preservation against mantle convection5-7. The high Mg:Si ratio of the upper mantle relative to chondrites8, the anomalous 142Nd:144Nd, 182W:184W and 3He:4He isotopic ratios in hot-spot magmas9,10, the plume deflection4 and slab stagnation in the mid-mantle3 as well as the sparse observations of seismic anisotropy11,12 can be explained by the long-term preservation of bridgmanite-enriched rocks in the deep lower mantle as promoted by their fast grain growth.

I/Pu reveals Earth mainly accreted from volatile-poor differentiated planetesimals

The observation that mid-ocean ridge basalts had ~3× higher iodine/plutonium ratios (inferred from xenon isotopes) compared to ocean island basalts holds critical insights into Earth's accretion. Understanding whether this difference stems from core formation alone or heterogeneous accretion is, however, hindered by the unknown geochemical behavior of plutonium during core formation. Here, we use first-principles molecular dynamics to quantify the metal-silicate partition coefficients of iodine and plutonium during core formation and find that both iodine and plutonium partly partition into metal liquid. Using multistage core formation modeling, we show that core formation alone is unlikely to explain the iodine/plutonium difference between mantle reservoirs. Instead, our results reveal a heterogeneous accretion history, whereby predominant accretion of volatile-poor differentiated planetesimals was followed by a secondary phase of accretion of volatile-rich undifferentiated meteorites. This implies that Earth inherited part of its volatiles, including its water, from late accretion of chondrites, with a notable carbonaceous chondrite contribution.

Nd isotope variation between the Earth-Moon system and enstatite chondrites

Reconstructing the building blocks that made Earth and the Moon is critical to constrain their formation and compositional evolution to the present. Neodymium (Nd) isotopes identify these building blocks by fingerprinting nucleosynthetic components. In addition, the ¹⁴⁶Sm-¹⁴²Nd and ¹⁴⁷Sm-¹⁴³Nd decay systems, with half-lives of 103 million years and 108 billion years, respectively, track potential differences in their samarium (Sm)/Nd ratios. The difference in Earth's present-day ¹⁴²Nd/¹⁴⁴Nd ratio compared with chondrites^1,2, and in particular enstatite chondrites, is interpreted as nucleosynthetic isotope variation in the protoplanetary disk. This necessitates that chondrite parent bodies have the same Sm/Nd ratio as Earth's precursor materials². Here we show that Earth and the Moon instead had a Sm/Nd ratio approximately 2.4 ± 0.5 per cent higher than the average for chondrites and that the initial ¹⁴²Nd/¹⁴⁴Nd ratio of Earth's precursor materials is more similar to that of enstatite chondrites than previously proposed^1,2. The difference in the Sm/Nd ratio between Earth and chondrites probably reflects the mineralogical distribution owing to mixing processes within the inner protoplanetary disk. This observation simplifies lunar differentiation to a single stage from formation to solidification of a lunar magma ocean³. This also indicates that no Sm/Nd fractionation occurred between the materials that made Earth and the Moon in the Moon-forming giant impact.

Long-term preservation of Hadean protocrust in Earth's mantle

Due to active plate tectonics, there are no direct rock archives covering the first ca. 500 million y of Earth’s history. Therefore, insights into Hadean geodynamics rely on indirect observations from geochemistry. We present a high-precision ¹⁸²W dataset for rocks from the Kaapvaal Craton, southern Africa, revealing the presence of Hadean protocrustal remnants in Earth’s mantle. This has broad implications for geochemists, geophysicists, and modelers, as it bridges contrasting ¹⁸²W isotope patterns in Archean and modern mantle-derived rocks. The data reveal the origin of seismically and isotopically anomalous domains in the deep mantle and also provide firm evidence for the operation of silicate differentiation processes during the first 60 million y of Earth’s history.

With plate tectonics operating on Earth, the preservation potential for mantle reservoirs from the Hadean Eon (>4.0 Ga) has been regarded as very small. The quest for such early remnants has been spurred by the observation that many Archean rocks exhibit excesses of ¹⁸²W, the decay product of short-lived ¹⁸²Hf. However, it remains speculative whether Archean ¹⁸²W anomalies and also ¹⁸²W deficits found in many young ocean island basalts (OIBs) mirror primordial Hadean mantle differentiation or merely variable contributions from older meteorite building blocks delivered to the growing Earth. Here, we present a high-precision ¹⁸²W isotope dataset for 3.22- to 3.55-Ga-old rocks from the Kaapvaal Craton, southern Africa. In expanding previous work, our study reveals widespread ¹⁸²W deficits in different rock units from the Kaapvaal Craton and also the discovery of a negative covariation between short-lived ¹⁸²W and long-lived ¹⁷⁶Hf–¹⁴³Nd–¹³⁸Ce patterns, a trend of global significance. Among different models, these distinct patterns can be best explained by the presence of recycled mafic restites from Hadean protocrust in the ancient mantle beneath the Kaapvaal Craton. Further, the data provide unambiguous evidence for the operation of silicate differentiation processes on Earth during the lifetime of ¹⁸²Hf, that is, the first 60 million y after solar system formation. The striking isotopic similarity between recycled protocrust and the low-¹⁸²W endmember of modern OIBs might also constitute the missing link bridging ¹⁸²W isotope systematics in Archean and young mantle-derived rocks.

Ancient helium and tungsten isotopic signatures preserved in mantle domains least modified by crustal recycling

Rare high-³He/⁴He signatures in ocean island basalts (OIB) erupted at volcanic hotspots derive from deep-seated domains preserved in Earth's interior. Only high-³He/⁴He OIB exhibit anomalous ¹⁸²W-an isotopic signature inherited during the earliest history of Earth-supporting an ancient origin of high ³He/⁴He. However, it is not understood why some OIB host anomalous ¹⁸²W while others do not. We provide geochemical data for the highest-³He/⁴He lavas from Iceland (up to 42.9 times atmospheric) with anomalous ¹⁸²W and examine how Sr-Nd-Hf-Pb isotopic variations-useful for tracing subducted, recycled crust-relate to high ³He/⁴He and anomalous ¹⁸²W. These data, together with data on global OIB, show that the highest-³He/⁴He and the largest-magnitude ¹⁸²W anomalies are found only in geochemically depleted mantle domains-with high ¹⁴³Nd/¹⁴⁴Nd and low ²⁰⁶Pb/²⁰⁴Pb-lacking strong signatures of recycled materials. In contrast, OIB with the strongest signatures associated with recycled materials have low ³He/⁴He and lack anomalous ¹⁸²W. These observations provide important clues regarding the survival of the ancient He and W signatures in Earth's mantle. We show that high-³He/⁴He mantle domains with anomalous ¹⁸²W have low W and ⁴He concentrations compared to recycled materials and are therefore highly susceptible to being overprinted with low ³He/⁴He and normal (not anomalous) ¹⁸²W characteristic of subducted crust. Thus, high ³He/⁴He and anomalous ¹⁸²W are preserved exclusively in mantle domains least modified by recycled crust. This model places the long-term preservation of ancient high ³He/⁴He and anomalous ¹⁸²W in the geodynamic context of crustal subduction and recycling and informs on survival of other early-formed heterogeneities in Earth's interior.

The 142Nd/144Nd variations in mantle-derived rocks provide constraints on the stirring rate of the mantle from the Hadean to the present

Early silicate differentiation events for the terrestrial planets can be traced with the short-lived 146Sm-142Nd system (∼100-My half-life). Resulting early Earth-produced 142Nd/144Nd variations are an excellent tracer of the rate of mantle mixing and thus a potential tracer of plate tectonics through time. Evidence for early silicate differentiation in the Hadean (4.6 to 4.0 Ga) has been provided by 142Nd/144Nd measurements of rocks that show both higher and lower (±20 ppm) values than the present-day mantle, demonstrating major silicate Earth differentiation within the first 100 My of solar system formation. We have obtained an external 2σ uncertainty at 1.7 ppm for 142Nd/144Nd measurements to constrain its homogeneity/heterogeneity in the mantle for the last 2 Ga. We report that most modern-day mid-ocean ridge basalt and ocean island basalt samples as well as continental crustal rocks going back to 2 Ga are within 1.7 ppm of the average Earth 142Nd/144Nd value. Considering mafic and ultramafic compositions, we use a mantle-mixing model to show that this trend is consistent with a mantle stirring time of about 400 My since the early Hadean. Such a fast mantle stirring rate supports the notion that Earth's thermal and chemical evolution is likely to have been largely regulated by plate tectonics for most of its history. Some young rocks have 142Nd/144Nd signatures marginally resolved (∼3 ppm), suggesting that the entire mantle is not equally well homogenized and that some silicate mantle signatures from an early differentiated mantle (>4.1 Ga ago) are preserved in the modern mantle.

Paroxysmal eruptions tracked by variations of helium isotopes: inferences from Piton de la Fournaise (La Réunion island)

Helium (He) with its isotopes (³He, ⁴He) is a key tracer enabling the Earth's mantle and dynamics to be characterized. Enrichment in primordial helium (³He) has been detected in volcanic gases of numerous magmatic systems in different geodynamic settings. Despite past use to monitor volcano-tectonic unrest, temporal ³He/⁴He variability in volcanic emissions is still poorly constrained. Here, we investigate noble gas chemistry of Piton de la Fournaise hotspot volcano, where temporal fluctuations of ³He/⁴He in response to the eruptive activity have never been studied. We compare the ³He/⁴He signature of volcanic gases and fluid inclusions and we highlight analogous evolution of the ³He/⁴He signature in both during the last decades of eruptive activity (1990-2017), even during the same eruption. We show that the maximum enrichment in ³He is found in magmatic fluids that fed the most voluminous eruptions which culminated in caldera collapse events. We argue that this enrichment in ³He mostly reflects a greater contribution of magmatic fluids from a primitive component of the mantle plume. These results emphasize that He isotopes may provide warnings of increases in deep magmatic contributions that potentially herald paroxysmal eruptions, as documented here at Piton de la Fournaise (2007) and also at Kilauea (2018).

The dependence of planetary tectonics on mantle thermal state: applications to early Earth evolution

For plate tectonics to operate on a planet, mantle convective forces must be capable of forming weak, localized shear zones in the lithosphere that act as plate boundaries. Otherwise, a planet's mantle will convect in a stagnant lid regime, where subduction and plate motions are absent. Thus, when and how plate tectonics initiated on the Earth is intrinsically tied to the ability of mantle convection to form plate boundaries; however, the physics behind this process are still uncertain. Most mantle convection models have employed a simple pseudoplastic model of the lithosphere, where the lithosphere 'fails' and develops a mobile lid when stresses in the lithosphere reach the prescribed yield stress. With pseudoplasticity high mantle temperatures and high rates of internal heating, conditions relevant for the early Earth, impede plate boundary formation by decreasing lithospheric stresses, and hence favour a stagnant lid for the early Earth. However, when a model for shear zone formation based on grain size reduction is used, early Earth thermal conditions do not favour a stagnant lid. While lithosphere stress drops with increasing mantle temperature or heat production rate, the deformational work, which drives grain size reduction, increases. Thus, the ability of convection to form weak plate boundaries is not impeded by early Earth thermal conditions. However, mantle thermal state does change the style of subduction and lithosphere mobility; high mantle temperatures lead to a more sluggish, drip-like style of subduction. This 'sluggish lid' convection may be able to explain many of the key observations of early Earth crust formation processes preserved in the geologic record. Moreover, this work highlights the importance of understanding the microphysics of plate boundary formation for assessing early Earth tectonics, as different plate boundary formation mechanisms are influenced by mantle thermal state in fundamentally different ways.This article is part of a discussion meeting issue 'Earth dynamics and the development of plate tectonics'.