Tungsten-182 heterogeneity in modern ocean island basalts

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.

A dry ancient plume mantle from noble gas isotopes

Primordial volatiles were delivered to terrestrial reservoirs during Earth's accretion, and the mantle plume source is thought to have retained a greater proportion of primordial volatiles compared with the upper mantle. This study shows that mantle He, Ne, and Xe isotopes require that the plume mantle had low concentrations of volatiles like Xe and H2O at the end of accretion compared with the upper mantle. A lower extent of mantle processing alone is not sufficient to explain plume noble gas signatures. Ratios of primordial isotopes are used to determine proportions of solar, chondritic, and regassed atmospheric volatiles in the plume mantle and upper mantle. The regassed Ne flux exceeds the regassed Xe flux but has a small impact on the mantle Ne budget. Pairing primordial isotopes with radiogenic systems gives an absolute concentration of 130Xe in the plume source of ∼1.5 × 107 atoms 130Xe/g at the end of accretion, ∼4 times less than that determined for the ancient upper mantle. A record of limited accretion of volatile-rich solids thus survives in the He-Ne-Xe signatures of mantle rocks today. A primordial viscosity contrast originating from a factor of ∼4 to ∼250 times lower H2O concentration in the plume mantle compared with the upper mantle may explain (a) why giant impacts that triggered whole mantle magma oceans did not homogenize the growing planet, (b) why the plume mantle has experienced less processing by partial melting over Earth's history, and (c) how early-formed isotopic heterogeneities may have survived ∼4.5 Gy of solid-state mantle convection.

Primitive noble gases sampled from ocean island basalts cannot be from the Earth's core

Noble gas isotopes in plumes require a source of primitive volatiles largely isolated in the Earth for 4.5 Gyrs. Among the proposed reservoirs, the core is gaining interest in the absence of robust geochemical and geophysical evidence for a mantle source. This is supported by partitioning data showing that sufficient He and Ne could have been incorporated into the core to source plumes today. Here we perform ab initio calculations on the partitioning of He, Ne, Ar, Kr and Xe between liquid iron and silicate melt under core forming conditions. For He our results are consistent with previous studies allowing for substantial amounts of He in the core. In contrast, the partition coefficient for Ne is three orders of magnitude lower than He. This very low partition coefficient would result in a ³He/²²Ne ratio of ~10³ in the core, far higher than observed in ocean island basalts (OIBs). We conclude that the core is not the source of noble gases in OIBs.

Kilometer-scale structure on the core-mantle boundary near Hawaii

The lowermost mantle right above the core-mantle boundary is highly heterogeneous containing multiple poorly understood seismic features. The smallest but most extreme heterogeneities yet observed are 'Ultra-Low Velocity Zones' (ULVZ). We exploit seismic shear waves that diffract along the core-mantle boundary to provide new insight into these enigmatic structures. We measure a rare core-diffracted signal refracted by a ULVZ at the base of the Hawaiian mantle plume at unprecedentedly high frequencies. This signal shows remarkably longer time delays at higher compared to lower frequencies, indicating a pronounced internal variability inside the ULVZ. Utilizing the latest computational advances in 3D waveform modeling, here we show that we are able to model this high-frequency signal and constrain high-resolution ULVZ structure on the scale of kilometers, for the first time. This new observation suggests a chemically distinct ULVZ with increasing iron content towards the core-mantle boundary, which has implications for Earth's early evolutionary history and core-mantle interaction.

Earth’s geodynamic evolution constrained by 182W in Archean seawater

Radiogenic isotope systems are important geochemical tools to unravel geodynamic processes on Earth. Applied to ancient marine chemical sediments such as banded iron formations, the short-lived 182Hf-182W isotope system can serve as key instrument to decipher Earth’s geodynamic evolution. Here we show high-precision 182W isotope data of the 2.7 Ga old banded iron formation from the Temagami Greenstone Belt, NE Canada, that reveal distinct 182W differences in alternating Si-rich (7.9 ppm enrichment) and Fe-rich (5.3 ppm enrichment) bands reflecting variable flux of W from continental and hydrothermal mantle sources into ambient seawater, respectively. Greater 182W excesses in Si-rich layers relative to associated shales (5.9 ppm enrichment), representing regional upper continental crust composition, suggest that the Si-rich bands record the global rather than the local seawater 182W signature. The distinct intra-band differences highlight the potential of 182W isotope signatures in banded iron formations to simultaneously track the evolution of crust and upper mantle through deep time.

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.

Low-spin ferric iron in primordial bridgmanite crystallized from a deep magma ocean

The crystallization of the magma ocean resulted in the present layered structure of the Earth's mantle. An open question is the electronic spin state of iron in bridgmanite (the most abundant mineral on Earth) crystallized from a deep magma ocean, which has been neglected in the crystallization history of the entire magma ocean. Here, we performed energy-domain synchrotron Mössbauer spectroscopy measurements on two bridgmanite samples synthesized at different pressures using the same starting material (Mg_0.78Fe_0.13Al_0.11Si_0.94O₃). The obtained Mössbauer spectra showed no evidence of low-spin ferric iron (Fe³⁺) from the bridgmanite sample synthesized at relatively low pressure of 25 gigapascals, while that directly synthesized at a higher pressure of 80 gigapascals contained a relatively large amount. This difference ought to derive from the large kinetic barrier of Fe³⁺ rearranging from pseudo-dodecahedral to octahedral sites with the high-spin to low-spin transition in experiments. Our results indicate a certain amount of low-spin Fe³⁺ in the lower mantle bridgmanite crystallized from an ancient magma ocean. We therefore conclude that primordial bridgmanite with low-spin Fe³⁺ dominated the deeper part of an ancient lower mantle, which would contribute to lower mantle heterogeneity preservation and call for modification of the terrestrial mantle thermal evolution scenarios.

Investigating Magma Ocean Solidification on Earth Through Laser-Heated Diamond Anvil Cell Experiments

We carried out a series of silicate fractional crystallization experiments at lower mantle pressures using the laser-heated diamond anvil cell. Phase relations and the compositional evolution of the cotectic melt and equilibrium solids along the liquid line of descent were determined and used to assemble the melting phase diagram. In a pyrolitic magma ocean, the first mineral to crystallize in the deep mantle is iron-depleted calcium-bearing bridgmanite. From the phase diagram, we estimate that the initial 33%-36% of the magma ocean will crystallize to form such a buoyant bridgmanite. Substantial calcium solubility in bridgmanite is observed up to 129 GPa, and significantly delays the crystallization of the calcium silicate perovskite phase during magma ocean solidification. Residual melts are strongly iron-enriched as crystallization proceeds, making them denser than any of the coexisting solids at deep mantle conditions, thus supporting the terrestrial basal magma ocean hypothesis (Labrosse et al., 2007).

Tungsten-182 evidence for an ancient kimberlite source

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.

Iron isotopes trace primordial magma ocean cumulates melting in Earth's upper mantle

The differentiation of Earth ~4.5 billion years (Ga) ago is believed to have culminated in magma ocean crystallization, crystal-liquid separation, and the formation of mineralogically distinct mantle reservoirs. However, the magma ocean model remains difficult to validate because of the scarcity of geochemical tracers of lower mantle mineralogy. The Fe isotope compositions (δ⁵⁷Fe) of ancient mafic rocks can be used to reconstruct the mineralogy of their mantle source regions. We present Fe isotope data for 3.7-Ga metabasalts from the Isua Supracrustal Belt (Greenland). The δ⁵⁷Fe signatures of these samples extend to values elevated relative to modern equivalents and define strong correlations with fluid-immobile trace elements and tungsten isotope anomalies (μ¹⁸²W). Phase equilibria models demonstrate that these features can be explained by melting of a magma ocean cumulate component in the upper mantle. Similar processes may operate today, as evidenced by the δ⁵⁷Fe and μ¹⁸²W heterogeneity of modern oceanic basalts.

Convective isolation of Hadean mantle reservoirs through Archean time

Although Earth has a convecting mantle, ancient mantle reservoirs that formed within the first 100 Ma of Earth's history (Hadean Eon) appear to have been preserved through geologic time. Evidence for this is based on small anomalies of isotopes such as ¹⁸²W, ¹⁴²Nd, and ¹²⁹Xe that are decay products of short-lived nuclide systems. Studies of such short-lived isotopes have typically focused on geological units with a limited age range and therefore only provide snapshots of regional mantle heterogeneities. Here we present a dataset for short-lived ¹⁸²Hf-¹⁸²W (half-life 9 Ma) in a comprehensive rock suite from the Pilbara Craton, Western Australia. The samples analyzed preserve a unique geological archive covering 800 Ma of Archean history. Pristine ¹⁸²W signatures that directly reflect the W isotopic composition of parental sources are only preserved in unaltered mafic samples with near canonical W/Th (0.07 to 0.26). Early Paleoarchean, mafic igneous rocks from the East Pilbara Terrane display a uniform pristine µ¹⁸²W excess of 12.6 ± 1.4 ppm. From ca 3.3Ga onward, the pristine ¹⁸²W signatures progressively vanish and are only preserved in younger rocks of the craton that tap stabilized ancient lithosphere. Given that the anomalous ¹⁸²W signature must have formed by ca 4.5 Ga, the mantle domain that was tapped by magmatism in the Pilbara Craton must have been convectively isolated for nearly 1.2 Ga. This finding puts lower bounds on timescale estimates for localized convective homogenization in early Earth's interior and on the widespread emergence of plate tectonics that are both important input parameters in many physical models.

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.

A Chromatographic Method for Separation of Tungsten (W) from Silicate Samples for High-Precision Isotope Analysis Using Negative Thermal Ionization Mass Spectrometry

A new chromatographic method for isolation of W from large masses of silicate samples (>1 g) for ultrahigh precision isotopic analysis was developed. The purification of W was achieved through two stages of rapid chromatographic separations. In the first step, Ti, Zr, Hf, and W were separated collectively from the sample matrix through an AG1-X8 (100-200 mesh) column with a 10 mL resin volume. Subsequently, W was rapidly separated from Ti and Zr-Hf with high purity by a two-step extraction chromatographic method using 0.6 and 0.3 mL TODGA resin columns (50-100 μm particle size), respectively. The total yield of W, including the anion exchange and the TODGA chromatographic separation steps, is greater than 90%. The procedure was employed to isolate W from rock reference materials GSJ JB-3 and USGS BHVO-2; the separated W was analyzed by TRITON Plus TIMS, yielding a ¹⁸²W/¹⁸⁴W of 0.864898 ± 0.000005 (n = 8, 2 SD) for JB-3 and ¹⁸²W/¹⁸⁴W of 0.864896 ± 0.000006 (n = 5, 2 SD) for BHVO-2, which are in agreement with previously reported values within analytical errors.

Paired EMI-HIMU hotspots in the South Atlantic-Starting plume heads trigger compositionally distinct secondary plumes?

Age-progressive volcanism is generally accepted as the surface expression of deep-rooted mantle plumes, which are enigmatically linked with the African and Pacific large low-shear velocity provinces (LLSVPs). We present geochemical and geochronological data collected from the oldest portions of the age-progressive enriched mantle one (EMI)-type Tristan-Gough track. They are part of a 30- to 40-million year younger age-progressive hotspot track with St. Helena HIMU (high time-integrated 238U/204Pb) composition, which is also observed at the EMI-type Shona hotspot track in the southernmost Atlantic. Whereas the primary EMI-type hotspots overlie the margin of the African LLSVP, the HIMU-type hotspots are located above a central portion of the African LLSVP, reflecting a large-scale geochemical zonation. We propose that extraction of large volumes of EMI-type mantle from the margin of the LLSVP by primary plume heads triggered upwelling of HIMU material from a more internal domain of the LLSVP, forming secondary plumes.

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.

Identification of chondritic krypton and xenon in Yellowstone gases and the timing of terrestrial volatile accretion

Volatile elements play a critical role in the evolution of Earth. Nevertheless, the mechanism(s) by which Earth acquired, and was able to preserve its volatile budget throughout its violent accretionary history, remains uncertain. In this study, we analyzed noble gas isotopes in volcanic gases from the Yellowstone mantle plume, thought to sample the deep primordial mantle, to determine the origin of volatiles on Earth. We find that Kr and Xe isotopes within the deep mantle have a similar chondritic origin to those found previously in the upper mantle. This suggests that the Earth has retained chondritic volatiles throughout the accretion and, therefore, terrestrial volatiles cannot not solely be the result of late additions following the Moon-forming impact.

Identifying the origin of noble gases in Earth’s mantle can provide crucial constraints on the source and timing of volatile (C, N, H₂O, noble gases, etc.) delivery to Earth. It remains unclear whether the early Earth was able to directly capture and retain volatiles throughout accretion or whether it accreted anhydrously and subsequently acquired volatiles through later additions of chondritic material. Here, we report high-precision noble gas isotopic data from volcanic gases emanating from, in and around, the Yellowstone caldera (Wyoming, United States). We show that the He and Ne isotopic and elemental signatures of the Yellowstone gas requires an input from an undegassed mantle plume. Coupled with the distinct ratio of ¹²⁹Xe to primordial Xe isotopes in Yellowstone compared with mid-ocean ridge basalt (MORB) samples, this confirms that the deep plume and shallow MORB mantles have remained distinct from one another for the majority of Earth’s history. Krypton and xenon isotopes in the Yellowstone mantle plume are found to be chondritic in origin, similar to the MORB source mantle. This is in contrast with the origin of neon in the mantle, which exhibits an isotopic dichotomy between solar plume and chondritic MORB mantle sources. The co-occurrence of solar and chondritic noble gases in the deep mantle is thought to reflect the heterogeneous nature of Earth’s volatile accretion during the lifetime of the protosolar nebula. It notably implies that the Earth was able to retain its chondritic volatiles since its earliest stages of accretion, and not only through late additions.

Sequencing seismograms: A panoptic view of scattering in the core-mantle boundary region

Scattering of seismic waves can reveal subsurface structures but usually in a piecemeal way focused on specific target areas. We used a manifold learning algorithm called "the Sequencer" to simultaneously analyze thousands of seismograms of waves diffracting along the core-mantle boundary and obtain a panoptic view of scattering across the Pacific region. In nearly half of the diffracting waveforms, we detected seismic waves scattered by three-dimensional structures near the core-mantle boundary. The prevalence of these scattered arrivals shows that the region hosts pervasive lateral heterogeneity. Our analysis revealed loud signals due to a plume root beneath Hawaii and a previously unrecognized ultralow-velocity zone beneath the Marquesas Islands. These observations illustrate how approaches flexible enough to detect robust patterns with little to no user supervision can reveal distinctive insights into the deep Earth.

Ruthenium isotope vestige of Earth's pre-late-veneer mantle preserved in Archaean rocks

The accretion of volatile-rich material from the outer solar system represents a crucial prerequisite for Earth developing oceans and becoming a habitable planet^1–4. However, the timing of this accretion remains controversial^5–8. It was proposed that volatile elements were added to Earth by late accretion of a late veneer consisting of carbonaceous chondrite-like material after core formation had ceased^6,9,10. This view, however, could not be reconciled with the distinct ruthenium (Ru) isotope composition of carbonaceous chondrites^5,11 compared to the modern mantle¹², and in fact also not with any known meteorite group⁵. As a possible solution, Earth’s pre-late veneer mantle could already have contained a significant amount of Ru that was not fully extracted by core formation¹³. The presence of such pre-late veneer Ru could only be proven if its isotope composition would be distinct from that of the modern mantle. Here we report the first high-precision mass-independent Ru isotope compositions for Eoarchean ultramafic rocks from SW Greenland, which display a relative ¹⁰⁰Ru excess of +22 parts per million compared to the modern mantle value. This ¹⁰⁰Ru excess indicates that the source of the Eoarchean rocks already contained a significant fraction of Ru prior to the late veneer. By 3.7 Gyr the mantle beneath the SW Greenland rocks had not yet fully equilibrated with late accreted material. Otherwise, no Ru isotopic difference relative to the modern mantle would be observed. By considering constraints from other highly siderophile elements beyond Ru¹⁴, the composition of the modern mantle can only be reconciled if the late veneer contained significant portions of carbonaceous chondrite-like materials with their characteristic ¹⁰⁰Ru deficits. These data therefore relax previous constraints on the late veneer and now permit that volatile-rich material from the outer solar system was delivered to Earth during late accretion.

A compositionally heterogeneous martian mantle due to late accretion

The approximately chondritic estimated relative abundances of highly siderophile elements (HSE) in the bulk martian mantle suggest that these elements were added after Mars' core formed. The shergottite-nakhlite-chassigny (SNC) meteorites imply an average mantle Pt abundance of ≈3 to 5 parts per billion, which requires the addition of 1.6 × 10²¹ kilograms of chondritic material, or 0.25% martian masses, to the silicate Mars. Here, we present smoothed particle hydro-dynamics impact simulations that show that Mars' HSE abundances imply one to three late collisions by large differentiated projectiles. We show that these collisions would produce a compositionally heterogeneous martian mantle. Based mainly on W isotopes, it has been argued that Mars grew rapidly in only about 2 to 4 million years (Ma). However, we find that impact generation of mantle domains with variably fractionated Hf/W and diverse ¹⁸²W could imply a Mars formation time scale up to 15 Ma.

Early Moon formation inferred from Hafnium-Tungsten systematics

The date of the Moon-forming impact places an important constraint on Earth's origin. Lunar age estimates range from about 30 Myr to 200 Myr after solar system formation. Central to this age debate is the greater abundance of 182W inferred for the silicate Moon than for the bulk silicate Earth. This compositional difference has been explained as a vestige of less late accretion to the Moon than the Earth, following core formation. Here we present high-precision trace element composition data from inductively coupled plasma mass spectrometry for a wide range of lunar samples. Our measurements show that the Hf/W ratio of the silicate Moon is higher than that of the bulk silicate Earth. By combining these data with experimentally derived partition coefficients, we find that the 182W excess in lunar samples can be explained by the decay of now extinct 182Hf to 182W. 182Hf was only extant for the first 60 Myr after solar system formation. We conclude that the Moon formed early, approximately 50 Myr after the solar system, and that the excess 182W of the silicate Moon is unrelated to late accretion.

LIP formation and protracted lower mantle upwelling induced by rifting and delamination

Large Igneous Provinces (LIPs) are commonly attributed to mantle plumes, hot upwellings from the deep lower mantle, apparently unrelated to plate motions. However, LIPs often form in association with rifting and breakup. Using numerical modelling, we introduce a novel idea that explains plume-like mantle upwelling by plate tectonic processes. Our model indicates that rifting-induced delamination of orogenic lithosphere can perturb the thermochemical mantle stratification and induce lower mantle upwelling which causes syn-rift LIP formation followed by protracted and enhanced mid ocean ridge basalt (MORB) generation. Our model provides an explanation for the geographical correlation between the Caledonian suture, the North Atlantic Igneous Province (NAIP) and present-day Icelandic magmatism.

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'.

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

Active volcanic hotspots can tap into domains in Earth's deep interior that were formed more than two billion years ago. High-precision data on variability in tungsten isotopes have shown that some of these domains resulted from differentiation events that occurred within the first fifty million years of Earth history. However, it has not proved easy to resolve analogous variability in neodymium isotope compositions that would track regions of Earth's interior whose composition was established by events occurring within roughly the first five hundred million years of Earth history. Here we report ¹⁴²Nd/¹⁴⁴Nd ratios for Réunion Island igneous rocks, some of which are resolvably either higher or lower than the ratios in modern upper-mantle domains. We also find that Réunion ¹⁴²Nd/¹⁴⁴Nd ratios correlate with helium-isotope ratios (³He/⁴He), suggesting parallel behaviour of these isotopic systems during very early silicate differentiation, perhaps as early as 4.39 billion years ago. The range of ¹⁴²Nd/¹⁴⁴Nd ratios in Réunion basalts is inconsistent with a single-stage differentiation process, and instead requires mixing of a conjugate melt and residue formed in at least one melting event during the Hadean eon, 4.56 billion to 4 billion years ago. Efficient post-Hadean mixing nearly erased the ancient, anomalous ¹⁴²Nd/¹⁴⁴Nd signatures, and produced the relatively homogeneous ¹⁴³Nd/¹⁴⁴Nd composition that is characteristic of Réunion basalts. Our results show that Réunion magmas tap into a particularly ancient, primitive source compared with other volcanic hotspots, offering insight into the formation and preservation of ancient heterogeneities in Earth's interior.

New insights into Mo and Ru isotope variation in the nebula and terrestrial planet accretionary genetics

When corrected for the effects of cosmic ray exposure, Mo and Ru nucleosynthetic isotope anomalies in iron meteorites from at least nine different parent bodies are strongly correlated in a manner consistent with variable depletion in s-process nucleosynthetic components. In contrast to prior studies, the new results show no significant deviations from a single correlation trend. In the refined Mo-Ru cosmic correlation, a distinction between the non-carbonaceous (NC) group and carbonaceous chondrite (CC) group is evident. Members of the NC group are characterized by isotope compositions reflective of variable s-process depletion. Members of the CC group analyzed here plot in a tight cluster and have the most s-process depleted Mo and Ru isotopic compositions, with Mo isotopes also slightly enriched in r- and possibly p-process contributions. This indicates that the nebular feeding zone of the NC group parent bodies was characterized by Mo and Ru with variable s-process contributions, but with the two elements always mixed in the same proportions. The CC parent bodies sampled here, by contrast, were derived from a nebular feeding zone that had been mixed to a uniform s-process depleted Mo-Ru isotopic composition. Six molybdenite samples, four glacial diamictites, and two ocean island basalts were analyzed to provide a preliminary constraint on the average Mo isotope composition of the bulk silicate Earth (BSE). Combined results yield an average μ 97Mo value of +3 ± 6. This value, coupled with a previously reported μ 100Ru value of +1 ± 7 for the BSE, indicates that the isotopic composition of the BSE falls precisely on the refined Mo-Ru cosmic correlation. The overlap of the BSE with the correlation implies that there was homogeneous accretion of siderophile elements for the final accretion of 10 to 20 wt% of Earth's mass. The only known cosmochemical materials with an isotopic match to the BSE, with regard to Mo and Ru, are some members of the IAB iron meteorite complex and enstatite chondrites.

Seismic evidence for partial melting at the root of major hot spot plumes

Ultralow-velocity zones are localized regions of extreme material properties detected seismologically at the base of Earth's mantle. Their nature and role in mantle dynamics are poorly understood. We used shear waves diffracted at the core-mantle boundary to illuminate the root of the Iceland plume from different directions. Through waveform modeling, we detected a large ultralow-velocity zone and constrained its shape to be axisymmetric to a very good first order. We thus attribute it to partial melting of a locally thickened, denser- and hotter-than-average layer, reflecting dynamics and elevated temperatures within the plume root. Such structures are few and far apart, and they may be characteristic of the roots of some of the broad mantle plumes tomographically imaged within the large low-shear-velocity provinces in the lower mantle.

Heterogeneous delivery of silicate and metal to the Earth by large planetesimals

After the Moon's formation, Earth experienced a protracted bombardment by leftover planetesimals. The mass delivered during this stage of late accretion has been estimated to be approximately 0.5% of Earth's present mass, based on highly siderophile element concentrations in the Earth's mantle and the assumption that all highly siderophile elements delivered by impacts were retained in the mantle. However, late accretion may have involved mostly large (≥ 1,500 km in diameter)-and therefore differentiated-projectiles in which highly siderophile elements were sequestered primarily in metallic cores. Here we present smoothed-particle hydrodynamics impact simulations that show that substantial portions of a large planetesimal's core may descend to the Earth's core or escape accretion entirely. Both outcomes reduce the delivery of highly siderophile elements to the Earth's mantle and imply a late accretion mass that may be two to five times greater than previously thought. Further, we demonstrate that projectile material can be concentrated within localized domains of Earth's mantle, producing both positive and negative ¹⁸²W isotopic anomalies of the order of 10 to 100 ppm. In this scenario, some isotopic anomalies observed in terrestrial rocks can be explained as products of collisions after Moon formation.

The ruthenium isotopic composition of the oceanic mantle

The approximately chondritic relative, and comparatively high absolute m antle abundances of the highly siderophile elem ents (HSE), suggest that their concentrations in the bulk silicate Earth were primarily established during a final ~0.5 to 1% of "late accretion" to the mantle, following the cessation of core segregation. Consequently, the isotopic composition of the HSE Ru in the mantle reflects an amalgamation of the isotopic compositions of late accretionary contributions to the silicate portion of the Earth. Among cosm ochem ical materials, Ru is characterized by considerable mass-independent isotopic variability, making it a powerful genetic tracer of Earth's late accretionary building blocks. To define the Ru isotopic composition of the oceanic mantle, the largest portion of the accessible mantle, we report Ru isotopic data for materials from one Archean and seven Phanerozoic oceanic m antle domains. A sample from a continental lithospheric mantle domain is also examined. All samples have identical Ru isotopic compositions, within analytical uncertainties, indicating that Ru isotopes are well mixed in the oceanic mantle, defining a μ ¹⁰⁰Ru value of 1.2 ± 7.2 (2SD). The only known meteorites with the same Ru isotopic composition are enstatite chondrites and, when corrected for the effects of cosmic ray exposure, mem bers of the Main Group and sLL subgroup of the lAB iron meteorite complex which have a collective CRE corrected μ ¹⁰⁰Ru value of 0.9 ± 3.0. This suggests that materials from the region(s) of the solar nebula sampled by these m eteorites likely contributed the dominant portion of late accreted materials to Earth's mantle.