Silicate Earth Research Articles

The non-carbonaceous nature of Earth’s late-stage accretion

Constraining the origin of Earth’s building blocks requires knowledge of the chemical and isotopic characteristics of the source region(s) where these materials accreted. The siderophile elements Mo and Ru are well suited to investigating the mass-independent nucleosynthetic (i.e., “genetic”) signatures of material that contributed to the latter stages of Earth’s formation. Studies contrasting the Mo and Ru isotopic compositions of the bulk silicate Earth (BSE) to genetic signatures of meteorites, however, have reported conflicting estimates of the proportions of the non-carbonaceous type or NC (presumptive inner Solar System origin) and carbonaceous chondrite type or CC (presumptive outer Solar System origin) materials delivered to Earth during late-stage accretion (likely including the Moon-forming event and onwards). The present study reports new mass-independent isotopic data for Mo, which are presumed to reflect the composition of the BSE. A comparison of the new estimate for the BSE composition with new data for a select suite of NC iron meteorites is used to constrain the genetic characteristics of materials accreted to Earth during late-stage accretion. Results indicate that the final 10 to 20 wt% of Earth’s accretion was dominated by NC materials that were likely sourced from the inner Solar System, although the addition of minor proportions of CC materials, as has been suggested to occur during accretion of the final 0.5 to 1 wt% of Earth’s mass, remains possible. If this interpretation is correct, it brings estimates of the genetic signatures of Mo and Ru during the final 10 to 20 wt% of Earth accretion into concordance.

Ferric iron stabilization at deep magma ocean conditions.

Fe2O3 produced in a deep magma ocean in equilibrium with core-destined alloy sets the early redox budget and atmospheric composition of terrestrial planets. Previous experiments (≤28 gigapascals) and first-principles calculations indicate that a deep terrestrial magma ocean produces appreciable Fe3+ but predict Fe3+/ΣFe ratios that conflict by an order of magnitude. We present Fe3+/ΣFe of glasses quenched from melts equilibrated with Fe alloy at 38 to 71 gigapascals, 3600 to 4400 kelvin, analyzed by synchrotron Mössbauer spectroscopy. These indicate Fe3+/ΣFe of 0.056 to 0.112 in a terrestrial magma ocean with mean alloy-silicate equilibration pressures of 28 to 53 gigapascals, producing sufficient Fe2O3 to account for the modern bulk silicate Earth redox budget and surficial conditions near or more oxidizing than the iron-wüstite buffer, which would stabilize a primitive CO- and H2O-rich atmosphere.

The behavior of nickel isotopes during mantle melting

Mantle-derived mafic rocks show relatively large variations in nickel (Ni) isotopes and mostly isotopically light compared to the bulk silicate Earth (BSE). Whether this signature is due to the source heterogeneity or controlled by melting processes has been a debatable issue. Here, we analyzed Ni isotopic compositions of 18 intraplate basalts from the Sabzevar region, northern Iran and 16 serpentinized peridotites from Cyprus and South China. The Sabzevar basalts were likely sourced from a sulfur-free/barren mantle domain with recycled pyroxenites involved, manifested by their high Cu contents (127–265 ppm), FeT/Mn (58.2–77.4) and Zn/FeT ratios (11.9–16.4). The Ni isotopic compositions of the Sabzevar basalts (average δ60/58Ni = +0.15 ± 0.08 ‰; 2SD) are slightly heavier than the BSE value (average δ60/58Ni = +0.11 ± 0.06 ‰; 2SD), consistent with equilibrium Ni isotope fractionation during mantle silicate melting as predicted by ionic model calculations. Our new data of serpentinized peridotites (average δ60/58Ni = +0.16 ± 0.06 ‰; 2SD), together with previously reported data for oceanic sediments and metabasalts, suggest that recycled lithologies have mantle-like or relatively heavy Ni isotopic compositions. Thus, the isotopically light Ni in mafic rocks is unlikely to be caused by crustal recycling-induced mantle heterogeneity. Rather, the light Ni isotopic signature is caused by the dissolution of recycled sulfides into the mantle melts. We suggest two sulfide dissolution models (“dissolve and go” and “dissolve and equilibrium”) to describe the δ60/58Ni and Cu systematics in terrestrial basalts. In both models, the initial sulfide content (Sinitial) and oxygen fugacity (fO2) exert a major control on Ni isotopic compositions of resulting melts. These two parameters vary among different geological settings. Basalts derived from mantle sources with high Sinitial and fO2 are characterized by light Ni isotopic compositions, whereas basalts sourced from sulfur-free/barren and reduced mantle domains are likely characterized by heavy Ni isotopic compositions, aligning with the characteristic observed in natural samples.

The Effect of Nitrogen on the Dihedral Angle Between Fe−Ni Melt and Ringwoodite: Implications for the Nitrogen Deficit in the Bulk Silicate Earth

AbstractNitrogen (N) is extremely depleted in the bulk silicate Earth (BSE). However, whether the silicate magma ocean was as N‐poor as the present‐day BSE is unknown. We performed multi‐anvil experiments at 20 GPa and 1,673−2,073 K to determine the dihedral angle of Fe−Ni−N alloy melt in ringwoodite matrix to investigate whether percolation of Fe‐rich alloy melt in the solid mantle can explain N depletion in the BSE. The dihedral angles ranged from 112° to 137°, surpassing the wetting boundary. Our experiments suggest that N removal from the mantle by percolation of Fe‐rich alloy melt to the Earth's core is unlikely. Therefore, besides N loss to space during planetesimal and planetary differentiation, as well as its segregation into the Earth core, the stranded Fe‐rich metal in the deep mantle could be a hidden N reservoir, contributing to the anomalous depletion of N in the observable BSE.

Trace element partitioning in a deep magma ocean and the origin of the Hf-Nd mantle array.

Crystallization in Earth's deep magma ocean could have caused trace element fractionation in the lower mantle that might be inherited to the isotopic compositions of the present-day mantle. However, the trace element partitioning has been experimentally investigated only up to the uppermost lower-mantle pressures. Here, we determined the bridgmanite/melt partition coefficients D of La, Nd, Sm, Lu, and Hf from 24 to 115 gigapascals, covering the wide pressure range of the lower mantle. Results demonstrate substantial reductions in DLu and DHf from >1 to ≪1 with increasing pressure to 91 gigapascals. We also found DLu/DHf > 1 and DSm/DNd < 1 under deep lower-mantle conditions, evolving melts toward low Lu/Hf and high Sm/Nd ratios by crystallizing bridgmanite. If residual melts form a dense hidden reservoir in the lowermost mantle, the complementary accessible mantle has the Hf and Nd isotopic compositions matching the observed terrestrial mantle array that deviates from the bulk silicate Earth reference.

Rb-Sr isotope systematics of the oldest crustal rock in the solar system

The RbSr isotopic system is an important cosmochemical tool for constraining the evolution of moderately volatile (Rb) and refractory (Sr) elements in the earliest Solar System. Presented here are RbSr isotopic systematics of twenty-two fractions of the evolved achondrite Erg Chech 002, the oldest crustal rock yet identified in the Solar System. Although the data show a large amount of scatter, likely due to terrestrial alteration, an errorchron yields an age within uncertainties of published PbPb ages. Initial 87Sr/86Sr ratios of both the errorchron and of individual mineral fractions show that Erg Chech 002 formed from materials evolving with a typical CI-chondrite nebular Rb/Sr ratio and that any Rb depletion on its parent body must have occurred almost simultaneously with the crystallization of the meteorite. Additionally, LuHf systematics of bulk Erg Chech 002 overlap with the chondritic range. Other early-formed parent bodies similarly show initial 87Sr/86Sr ratios close to a CI-chondrite evolution line. Despite nucleosynthetic variations of 84Sr/86Sr being evident between inner and outer Solar System materials, no variations are detectable in 87Sr/86Sr for Erg Chech 002 or other early-formed parent bodies. Although the initial 87Sr/86Sr of Erg Chech 002 overlaps with that of all other early-accreted bodies, the initial 87Sr/86Sr of asteroid 4 Vesta (BABI) remains the most precise and relevant initial value for the bulk silicate Earth.

Contribution of the Moon-forming Impactor to the Volatile Inventory in the Bulk Silicate Earth

The timing and mechanism by which the present-day inventory of life-essential volatiles hydrogen–carbon–nitrogen–sulfur (H–C–N–S) in the bulk silicate Earth (BSE) was established are debated. In this study we have modeled the equilibrium partitioning of H–C–N–S between core, magma ocean (MO), and atmosphere to determine whether the Moon-forming impactor (MFI) was the primary source of volatiles in the BSE. Our findings suggest that the MFI’s core and MO-degassed atmosphere were its primary H–C–N–S reservoirs. Since the MFI likely lost its MO-degassed atmosphere before the giant impact, most of the BSE’s volatiles must come from the small fraction of the MFI’s core which reequilibrated with Earth’s post-impact MO. This implies a high H–C–N–S inventory in the MFI (up to 50% of volatile-rich carbonaceous chondrites) to establish the BSE’s volatile budget. Although isotopic compositions of nonvolatile elements do not rule out the possibility of substantial volatile-rich, carbonaceous material accretion, MFI’s collisional growth from thermally metamorphosed/differentiated planetesimals makes it improbable that it contained ∼50% carbonaceous chondrite equivalent of H–C–N during its differentiation. Therefore, the MFI was unlikely the primary source of volatiles in the BSE. A significant portion of the BSE’s volatile inventory (especially H and C) likely predates the Moon-forming event. To prevent loss to space and segregation into Earth’s core, volatile-bearing materials must be delivered during the final accretion event(s) preceding the Moon-forming event. The substantial size of the proto-Earth at this stage, combined with limited metal–silicate equilibration during the Moon-forming event, facilitated the retention of these volatiles within the BSE.

Cosmic pears from the Havelland (Germany): Ribbeck, the twelfth recorded aubrite fall in history

AbstractIn 1889 the German poet and novelist Theodor Fontane wrote the popular literary ballad “Herr von Ribbeck auf Ribbeck im Havelland.” The Squire von Ribbeck is described as a gentle and generous person, who often gives away pears from his pear trees to children passing by and continued donating pears after his death. Now, 135 years later the rock called Ribbeck is giving us insight into processes that happened 4.5 billion years ago. The meteorite Ribbeck (official find location: 52°37′15″N, 12°45′40″E) fell January 21, 2024, and has been classified as a brecciated aubrite. This meteoroid actually entered the Earth's atmosphere at 00:32:38 UTC over Brandenburg, west of Berlin, and the corresponding fireball was recorded by professional all sky and video cameras. More than 200 pieces (two proved by radionuclide analysis to belong to this fresh fall) were recovered totaling about 1.8 kg. Long‐lived radionuclide and noble gas data are consistent with long cosmic ray exposure (55–62 Ma) and a preatmospheric radius of Ribbeck between 20 and 30 cm. The heavily brecciated aubrite consists of major (76 ± 3 vol%) coarse‐grained FeO‐free enstatite (En99.1Fs&lt;0.04Wo0.9), with a significant abundance (15.0 ± 2.5 vol%) of albitic plagioclase (Ab95.3 An2.0Or2.7), minor forsterite (5.5 ± 1.5 vol%; Fo99.9) and 3.5 ± 1.0 vol% of opaque phases (mainly sulfides and metals) with traces of nearly FeO‐free diopside (En53.2Wo46.8) and K‐feldspar (Ab4.6Or95.4). The rock has a shock degree of S3 (U‐S3), and terrestrial weathering has affected metals and sulfides, resulting in the brownish appearance of rock pieces and the partial destruction of certain sulfides already within days after the fall. The bulk chemical data confirm the feldspar‐bearing aubritic composition. Ribbeck is closely related to the aubrite Bishopville. Ribbeck does not contain solar wind implanted gases and is a fragmental breccia. Concerning the Ti‐ and O‐isotope compositions, the data are similar to those of other aubrites. They are also similar to E chondrites and fall close to the data point for the bulk silicate Earth (BSE). Before the Ribbeck meteoroid entered Earth's atmosphere, it was observed in space as asteroid 2024 BX1. The aphelion distance of 2024 BX1's orbit lies in the innermost region of the asteroid belt, which is populated by the Hungaria family of minor planets characterized by their E/X‐type taxonomy and considered as the likely source of aubrites. The spectral comparison of an average large‐scale emission spectrum of Mercury converted into reflectance and of the Ribbeck meteorite spectrum does not show any meaningful similarities.

The Composition of Earth's Lower Mantle

Determining the composition of Earth's lower mantle, which constitutes almost half of its total volume, has been a central goal in the Earth sciences for more than a century given the constraints it places on Earth's origin and evolution. However, whether the major element chemistry of the lower mantle, in the form of, e.g., Mg/Si ratio, is similar to or different from the upper mantle remains debated. Here we use a multidisciplinary approach to address the question of the composition of Earth's lower mantle and, in turn, that of bulk silicate Earth (crust and mantle) by considering the evidence provided by geochemistry, geophysics, mineral physics, and geodynamics. Geochemical and geodynamical evidence largely agrees, indicating a lower-mantle molar Mg/Si of ≥1.12 (≥1.15 for bulk silicate Earth), consistent with the rock record and accumulating evidence for whole-mantle stirring. However, mineral physics–informed profiles of seismic properties, based on a lower mantle made of bridgmanite and ferropericlase, point to Mg/Si ∼ 0.9–1.0 when compared with radial seismic reference models. This highlights the importance of considering the presence of additional minerals (e.g., calcium-perovskite and stishovite) and possibly suggests a lower mantle varying compositionally with depth. In closing, we discuss how we can improve our understanding of lower-mantle and bulk silicate Earth composition, including its impact on the light element budget of the core. ▪The chemical composition of Earth's lower mantle is indispensable for understanding its origin and evolution.▪Earth's lower-mantle composition is reviewed from an integrated mineral physics, geophysical, geochemical, and geodynamical perspective.▪A lower-mantle molar Mg/Si of ≥1.12 is favored but not unique.▪New experiments investigating compositional effects of bridgmanite and ferropericlase elasticity are needed to further our insight.

Experimental determination of Si, Mg, and Ca isotope fractionation during enstatite melt evaporation

Abstract Evaporation of silicate materials from Earth or its precursors may be important in shaping their primordial compositions represented by undifferentiated meteorites, e.g., enstatite chondrites; however, the conditions under which evaporation occurs and the extent of evaporation-induced elemental and isotope fractionation remain uncertain. Here, we experimentally determine the volatility and isotope fractionation of Si, Mg, Ca, Nb, and Ta during enstatite melt evaporation at 2423–2623 K using a high-temperature conical nozzle levitator. Homogenous glasses are recovered after experiments; then we use EPMA and LA-ICP-MS to measure the elemental compositions, MC-ICP-MS to measure the Si and Mg isotopes, and TIMS to measure the Ca isotopes. Our results show that the evaporation rates of Si are larger than Mg, and the mean vapor/melt isotope fractionation factors (α = Rvapor/Rmelt; R = isotope ratio) are 0.99585 ± 0.00002 for 29Si/28Si and 0.98942 ± 0.00130 for 25Mg/24Mg. However, neither evaporative loss of Ca, Nb, and Ta nor Ca isotope fractionation was observed within analytical uncertainty. In conjunction with previous studies, we find that in an evaporation experiment the saturation degree (partial vapor pressure/equilibrium vapor pressure) of Si (SSi) is larger than SMg when Si is more volatile than Mg, and vice versa. If the Mg/Ca and Si/Ca ratios and isotopes in the bulk silicate Earth are attributed to the evaporation of enstatite chondrite-like precursors, evaporation temperatures &amp;gt;5000 K and SSi &amp;lt; SMg are required.

Degassing of CO2 triggers large-scale loss of helium from magma oceans

The moon-forming impact caused widespread melting and vaporization of the proto-Earth, leading to the formation of a protolunar disk1,2. Volatile elements trapped in the inner parts of the disk condensed together with heavier elements into a global magma ocean3,4. By subsequent cooling and degassing, volatiles contributed to the formation of the Earth’s secondary atmosphere5. The thermodynamic conditions at the magma ocean-atmosphere interface profoundly impacted the degree of degassing that occurred. Today, our understanding of these conditions remains unclear. Here, we simulate the degassing of carbon and helium from a magma ocean with a bulk silicate Earth composition6 at presumed typical conditions for the early Earth5,7. We employ first-principles molecular dynamics calculations. We find that volatile loss is determined by pressure, temperature, and melt composition. Carbon and helium devolatilize more easily when both elements are present in the melt, leading to pronounced loss from the magma ocean. Our findings suggest that the early Earth atmosphere was carbon-rich, with a high fraction of helium and other noble gases, and thicker and hotter than previously thought. We evaluate the implications for Earth’s composition and thermal evolution, as well as other planets that may undergo similar processes with different chemistries.

The Genesis of A‐Type Granites in Lower Yangtze River Belt: Evidence From Calcium Isotopes

AbstractA‐type granite is a favorable rock for the study of crustal evolution, crust‐mantle interaction and metallogenesis. However, the origin of A‐type granite remains controversial. In this study, we report high‐precision Ca isotopic compositions of a co‐genetic suite of A‐type granites from the Lower Yangtze River Belt (LYRB) in eastern China. Our results show that the δ44/40Ca of the A‐type granites ranges from 0.55 ± 0.11‰ to 1.44 ± 0.06‰, and is negatively correlated with CaO, Sr and Ba contents and Eu/Eu*, indicating that the Ca isotope fractionation of A‐type granites is dominated by plagioclase and potassium feldspar. There is kinetic fractionation of Ca isotopes during A‐type granite magma evolution. Sample BSL‐7, the least evolved sample, revealed that it had a low δ44/40Ca of 0.59‰, which is significantly lower than that of the upper continental crust (δ44/40Ca = 0.71–0.72‰) and the bulk silicate earth (δ44/40Ca = 0.94‰). The low δ44/40Ca can be best explained by partial melting of the enriched mantle metasomatized by subduction material. Combined with the geodynamic background of the LYRB, we propose that the formation of A‐type granites in the LYRB originates from partial melting of the enriched mantle, triggered by early Cretaceous ridge subduction of the Pacific and Izanagi plates.

Nitrogen sequestration in the core at megabar pressure and implications for terrestrial accretion

Nitrogen (N) is the most abundant element in Earth’s atmosphere, but is extremely depleted in the silicate Earth. However, it is not clear whether core sequestration or early atmospheric loss was responsible for this depletion. Here we study the effect of core formation on the inventory of nitrogen using laser-heated diamond anvil cells. We find that, due to the simultaneous dissolution of oxygen in the metal, N becomes much less siderophile (iron-loving) at pressures and temperatures up to 104 GPa and 5000 K, a thermodynamic condition relevant to the bottom of the magma ocean in the aftermath of the moon-forming giant impact. Using a core–mantle–atmosphere coevolution model, we show that the impact-induced processes (core formation and/or atmospheric loss) are unlikely to account for the observed N anomaly, which is instead best explained by the accretion of mainly N-poor impactors. The terrestrial volatile pattern requires severe N depletion on precursor bodies, prior to their accretion to the proto-Earth.

Early planetesimal differentiation and late accretion shaped Earth's nitrogen budget.

The relative roles of protoplanetary differentiation versus late accretion in establishing Earth's life-essential volatile element inventory are being hotly debated. To address this issue, we employ first-principles calculations to investigate nitrogen (N) isotope fractionation during Earth's accretion and differentiation. We find that segregation of an iron core would enrich heavy N isotopes in the residual silicate, while evaporation within a H2-dominated nebular gas produces an enrichment of light N isotope in the planetesimals. The combined effect of early planetesimal evaporation followed by core formation enriches the bulk silicate Earth in light N isotopes. If Earth is comprised primarily of enstatite-chondrite-like material, as indicated by other isotope systems, then late accretion of carbonaceous-chondrite-like material must contribute ~ 30-100% of the N budget in present-day bulk silicate Earth. However, mass balance using N isotope constraints shows that the late veneer contributes only a limited amount of other volatile elements (e.g., H, S, and C) to Earth.

Calcium isotopic composition of the bulk silicate Earth: A komatiite perspective

Calcium (Ca) isotopic composition of the mantle has been previously estimated as δ44/40Ca. However, δ44/40Ca of the mantle records both its stable isotopic composition and excess/deficit in radiogenic 40Ca that can be produced by the decay of 40K. Here, we present the high-precision Ca isotopic compositions of 16 Archean komatiites from five representative localities (Barberton, Munro, Abitibi, Yilgarn, and Taishan). Due to the high degrees of partial melting that led to their formation, these komatiites are used here to simultaneously estimate ε40/44Ca, δ44/40Ca, and δ44/42Ca of the bulk silicate Earth. The komatiites exhibit a resolvable negative ε40/44Ca relative to SRM915a, with an average of −0.6 ± 0.1 (2SE, 95 c.i.% if not specified). Given the lack of significant differences among the samples and the non-zero value of ε40/44Ca, we suggest using δ44/42Ca to report the stable Ca isotopic composition in this study. Excluding one sample (SD6/400), which may have been affected by alteration, the remaining komatiites display relatively homogeneous δ44/42Ca, ranging from 0.39 ± 0.02‰ (2SE, N = 16) to 0.49 ± 0.02‰ (2SE, N = 10), defining a mean of 0.44 ± 0.02‰ (2SE, N = 15). The absence of correlations between δ44/42Ca and LOI, Sr/Nd, (Dy/Yb)N, and (La/Sm)N among the komatiites indicates that post-eruption alteration, the influence of residual garnet, as well as prior melt extraction from the mantle source have had a negligible influence on their stable Ca isotopic compositions. Based on available experimental petrological data and isotope fractionation factors, it is demonstrated that komatiites were insignificantly fractionated from their mantle sources (i.e., <0.02% in δ44/42Ca). Their δ44/42Ca represents the ambient mantle at the time of segregation. Accordingly, we suggest that the average δ44/42Ca and ε40/44Ca of the komatiites measured here are representative of the bulk silicate Earth. Our new estimate can serve as a reference point for Ca isotopes as a two-dimensional tracer for sources and geological processes.

Copper and zinc isotope compositions of pristine eucrites as analogues for differentiated planetary feedstocks

Preferential enrichment of the heavier isotopes of moderately volatile elements (MVE) in samples from asteroids and the Moon have been attributed to volatile loss during the formation and differentiation of their parent bodies. Analogs for planetary feedstocks include the howardite-eucrite-diogenite meteorites, which originate from a differentiated planetesimal or planetesimals, likely including (4) Vesta. Complications arise in the interpretation of volatile depletion in these meteorites, however, due to post-crystallization processes including metamorphism and later impacts that acted upon them. We present new coupled Cu and Zn isotope data for a suite of eucrites that, when combined with published data, show significant ranges (δ65Cu = -1.6 to +0.9‰; δ66Zn = -7.8 to +13.5‰). Exclusion of eucrites that have been affected by metamorphism, impact contamination or surface condensation of isotopically light Zn and Cu leads to a range of ‘pristine’ compositions (δ66Zn = +1.1 ± 2.3‰; δ65Cu = +0.5 ± 0.5‰; 2 St. Dev.), implying inherent MVE variability within the eucrite parent body. As low-mass differentiated bodies, Vesta and the Moon represent endmembers in planet evolution. For the Moon, extensive volatile loss can be explained by a cataclysmic giant impact origin and later magma ocean crystallization. In contrast the parent body of eucrite meteorites likely heterogeneously lost volatile elements and compounds during differentiation. Vesta as the potential source of eucrite meteorites offers an important endmember composition for likely feedstocks to planets, representing the remaining vestige of what was likely to have been a larger population of differentiated objects in the inner Solar System shortly after nebula accretion. Mixing contributions of non-carbonaceous and carbonaceous chondrites constrained by nucleosynthetic Zn isotope anomalies suggests a significant fraction of Earth's accretion could have come from volatile-poor and differentiated planetary feedstocks that would have had limited effects on the bulk silicate Earth (BSE) Zn isotope composition. Furthermore, volatile-poor feedstocks cannot explain the BSE Cu isotope composition, which instead may have been modified by terrestrial core formation. Pristine eucrites offer key insights into early planetesimal differentiation and the role of volatile loss on small mass bodies within nascent solar systems.

The thermal equation of state of the magma Ocean

As the protolunar disk formed by the giant impact with the incoming asteroid Theia started to cool, the Earth condensed in its center as a molten planet. The outer layer, rich in lithophile oxides, formed the magma ocean, whose dynamical behavior heavily influenced the entire evolution of the early Earth. Here, we employ first-principles molecular dynamics simulations to characterize the magma ocean in its initial stage. We compute the compressibility and the viscosity of the molten pyrolite, which best approximates the bulk silicate Earth composition. With an extensive set of calculations, we span the entire relevant pressure-temperature range. We provide a detailed analysis of the athermal and thermal equations of state in various formulations. We obtain a linear temperature dependence for the bulk modulus using the 4th order Birch-Murnaghan equation of state. We show that the liquid silicate accommodates compression by the collapse of the second Si-O coordination sphere, whose corresponding coordination number passes from about 24 at the surface of the magma ocean to about 40 at its deepest point. We found that pyrolite melts have a very low viscosity, about one order of magnitude lower than that of molten basalt. We also propose a revised position of the critical point of the bulk silicate Earth, which lies around 6000 K and 1.1–1.2 kbars.

The role of calcium phosphates and silicates in the storage and transport of phosphorus at the upper-to-lower mantle transition: An experimental study to 25 GPa in a model peridotitic bulk composition

Calcium (Ca) phosphates are major hosts for phosphorus, halogens and incompatible trace elements in the Earth’s crust and mantle and, by supplying almost all phosphorus to the biosphere, are indispensable for sustaining life on Earth. To better understand the role of Ca-phosphates in the deep silicate Earth portion of the global phosphorus cycle at depths of the transition zone, we performed experiments in the P-T range of 15–25 GPa and 1600–2000 °C using a moderately fertile peridotite doped with 3 % synthetic β-Ca3(PO4)2 and 1 % of a trace element mix containing a range of HFSE, LILE, REEs, Cl and Br. Tuite is stable to at least 25 GPa at which pressure its upper T-stability limit is between 1700 and 1750 °C. At 15 GPa, by comparison, the upper T-stability limit is located between 1750 and 1800 °C, indicating a negative slope of the tuite-out reaction(s). Thus, tuite is stable to at least 700-km depth with a maximum T-stability close to or slightly above the average current mantle adiabat (ACMA) at 20–25 GPa and significantly above the ACMA at 15 GPa. Beyond the P-T stability limit of majoritic garnet, tuite is the principal subsolidus phosphorus carrier in the uppermost lower mantle. Beyond the P-T stability limit of tuite, phosphorus is likely to either (i) enter a Ca3(PO4)2 polymorph with an even higher density, (ii) form an entirely new phosphate phase or (iii) be accommodated in lower mantle silicates in six-fold coordination. Within the P-T range of our experiments, the phases encountered show the following trend of phosphorus contents:Pmelt>Pmajoritic garnet > Polivine ≅ Pringwoodite > Pdavemaoite ≅ Pwadsleyite > Pclinoenstatite ≅ Pbridgmanite ≅ Pferropericlase. Even when coexisting with tuite, bridgmanite and ferropericlase show phosphorus contents < 100 µg/g, and davemaoite has phosphorus contents that do not exceed 390 µg/g, testifying to the very limited phosphorus storage capacity of the lower mantle. Model calculations based on the phosphorus contents of ocean island basalts (OIBs) compared with those of mid ocean ridge basalts (MORBs) indicate that the OIB source is significantly richer in phosphorus than the MORB source and, for a wide range of OIB phosphorus contents, is likely to contain modal apatite. The relative importance of Ca-phosphates and silicates as phosphorus carriers in the peridotitic upper and uppermost lower mantle is strongly dependent upon pressure and the bulk phosphorus content. With increasing P, phosphorus is progressively transferred from Ca-phosphates to silicates with an ensuing decrease in the modal amount of the Ca-phosphates. Dependent upon the bulk phosphorus content, this may cause the Ca-phosphates to disappear before the upper-to-lower mantle boundary is reached, leaving majoritic garnet as the major phosphorus host. Breakdown of garnet will subsequently cause tuite to re-appear in the uppermost lower mantle and to resume its role of the principal subsolidus phosphorus carrier. For sufficiently high bulk phosphorus contents, tuite will remain stable throughout the transition zone, thereby maintaining phosphorus saturation in the coexisting silicates, and will eventually be joined by the tuite newly generated by garnet breakdown.

Iron alloys of volatile elements in the deep Earth's interior.

Investigations into the compositional model of the Earth, particularly the atypical concentrations of volatile elements within the silicate portion of the early Earth, have attracted significant interest due to their pivotal role in elucidating the planet's evolution and dynamics. To understand the behavior of such volatile elements, an established 'volatility trend' has been used to explain the observed depletion of certain volatile elements. However, elements such as Se and Br remain notably over-depleted in the silicate Earth. Here we show the results from first-principles simulations that explore the potential for these elements to integrate into hcp-Fe through the formation of substitutional alloys, long presumed to be predominant constituents of the Earth's core. Based on our findings, the thermodynamic stability of these alloys suggests that these volatile elements might indeed be partially sequestered within the Earth's core. We suggest potential reservoirs for volatile elements within the deep Earth, augmenting our understanding of the deep Earth's composition.

Insights on the origin of oldhamite in enstatite meteorites from Ca stable isotopes

In order to understand the origin of oldhamite (CaS) in enstatite meteorites, we report Ca isotopic compositions (δ44/40Ca) of oldhamite (obtained from water leachate of bulk chondrites and aubrites and mineral separates from the Norton County aubrite) and silicate minerals from different types of enstatite chondrites and aubrite. The δ44/40Ca of the bulk enstatite chondrites range from 1.05 ‰ to 1.24 ‰, with an average of 1.13 ± 0.12 ‰, higher than that of the estimate of the bulk silicate earth (∼0.94 ‰). Major and trace element analyses show that the water leachates of enstatite chondrites are mainly composed of oldhamite, and they take over 20.6–68.5 % Ca of the bulk meteorite Ca budget. The Ca isotope fractionation between oldhamite and residual silicate (Δ44/40Caoldhamite-silicate) for the studied enstatite chondrites is minimum (−0.44 ‰) for Abee (impact-melt breccia) and maximum (+0.16) for St.Marks (EH5). The Ca isotope fractionation between oldhamite (individual mineral grains and leachate) and silicates in Norton County varies from −0.47 ‰ to −0.31 ‰ with an average of −0.41 ‰. These Δ44/40Caoldhamite-silicate correlate well with previous theoretical calculation and suggests that the oldhamites in Norton County are in isotopic equilibrium with co-existing silicates, and therefore were formed during magmatic processes. However, in enstatite chondrites, the large variation on Δ44/40Caoldhamite-silicate and its negative correlation with metamorphic temperature reflects the redistribution and equilibration of Ca isotopes during metamorphism. The variable Δ44/40Caoldhamite-silicate found in unequilibrated chondrites reflect kinetic Ca isotope fractionation between oldhamite and nebular gas and therefore is evidence for the formation of oldhamite by condensation in the solar nebula.