Silicate Earth Research Articles

Noble gas insights into early impact delivery and volcanic outgassing to Earth's atmosphere: A limited role for the continental crust

Earth's atmosphere, crust and mantle have evolved together through continuous geochemical exchange throughout Earth's history. Constraints on the transport of volatile elements and compounds between these reservoirs are crucial for understanding how Earth could have stayed habitable for extended periods of time. Here we present a new forward model of He, Ne and Ar in the mantle, crust and atmosphere. We explore concentrations of noble gases at the end of accretion, bulk silicate Earth K/U ratios, crustal growth scenarios, and upper mantle processing rates throughout Earth's history. We search for parameter combinations that simultaneously satisfy observational constraints on present-day mantle 4He/3He and 40Ar/36Ar (sensitive to mantle outgassing and continental crust growth, which depletes the mantle of U, Th and K), atmospheric 20Ne/22Ne (which tracks the mix of outgassed vs. delivered Ne in the atmosphere), and atmospheric 40Ar/36Ar in the past and today (sensitive to volatile delivery, mantle outgassing and crustal growth). Leveraging this intertwined set of noble gas abundances and isotopic compositions yields new constraints on initial noble gas abundances in the mantle and on the proportions of atmospheric volatiles originating from delivery by impact degassing, mantle outgassing, and degassing of the continental crust. We find that atmospheric Ar isotopic evolution is primarily sensitive to the mantle processing rate history; the atmospheric Ar isotopic record should therefore not be used to reconstruct continental crust growth, but instead provides valuable insights into mantle processing rates. Our model predicts a measurably low 20Ne/22Ne ratio of ∼9.7 in Archean atmospheric samples. Most of atmospheric primordial 36Ar was directly delivered by chondritic bodies and not transferred to the atmosphere during an intense early episode of mantle outgassing. Nitrogen delivered by impact degassing could account for the present-day atmospheric nitrogen inventory.

No magma ocean surface after giant impacts between rocky planets

Vaporization is a major outcome of giant impacts during planet formation. The last giant impact marked a major stage in the early history of our planet, with the formation of a highly vaporized protolunar disk, that condenses onto the final Earth and Moon. The thermodynamic state of the disk and its condensation path are still uncertain, as most impact simulations have not used accurate material models. In this study, we compute the critical point and liquid spinodal of the bulk silicate Earth composition. We find that the thermal profiles through portions of the protolunar disk and the post-impact Earth exceeded the mantle critical point of 80-130 MPa kbars and 6500-7000 K. We find that Earth, and most rocky planets, will traverse a temporary state that lacks a surface defined by a magma ocean-atmosphere boundary. Furthermore, the atomic structure of the silicate fluid varies with the radius within the disk due to strong pressure and temperature gradients. Fluffy short-lived chemical species dominate the outer parts of the disk, and long-lasting dense polymers abound in the deeper parts. During cooling, the silicate vapor condenses and the composition of the post-impact atmosphere is dominated by species along the mantle vapor curve.

Earth Mantle's Isotopic Record of Progressive Chemical Depletion

AbstractMid ocean ridge basalts are partial melts of mantle rocks that have become incompatible element depleted prior to recent sub‐ridge melting. This prior extent of melt extraction and concomitant incompatible element depletion is often inferred from the radiogenic isotope ratios of ridge basalts, such as hafnium and neodymium. However, hafnium isotope ratios by far exceeding those in ridge basalts are observed sporadically in exposed mantle rocks, abyssal peridotites. At least locally, therefore, Earth's mantle has evolved with much larger extents of incompatible element depletion over geologic time periods (108–109 years) than inferred from ridge basalts. Here we show that such “ultra‐depleted” hafnium isotope signatures also characterize peridotites exposed at the Doldrums Fracture Zone in the equatorial Mid Atlantic. These peridotites are closely associated with peridotites characterized by hafnium‐neodymium isotope ratios similar to ridge basalts, but extensive incompatible element depletion. Our results demonstrate that Earth mantle's isotopic record of prior melt extraction is dampened by ubiquitous past and recent reaction with migrating melts. Hence, a better understanding of chemically reactive flow is required for deducing the mantle's rate of incompatible element depletion by recurring processing through melting regions in the shallow mantle, which is the driving force for silicate Earth evolution. Ultimately, better constraining the rate of incompatible element depletion from Earth's mantle is therefore of first‐order importance for understanding our planet's basic mode of operation.

Triple silicon isotope insights into the formation of Precambrian cherts

Cherts are pervasive in the Precambrian and their O and Si isotopic compositions have been used extensively for reconstructing seawater paleo-temperatures. However, these reconstructions are obscured by the fact that cherts exhibit, at a given age, a large isotopic (O and Si) heterogeneity at bulk and micrometer scales. This is understood as reflecting the fact that cherts are an assemblage of silica precursors having different origins, and that diagenesis and metamorphism have modified the isotopic compositions acquired during the last equilibration with seawater. To test the contributions of the different processes that control the Si isotopic composition of cherts, we developed high-precision triple Si isotope measurements by Multicollector-Inductively Coupled-Plasma Mass-Spectrometer (MC-ICP-MS). This approach allows to identify deviations from the equilibrium Si isotopic fractionation at the level of a few ppm. Triple Si isotope data combined with trace elements and O isotope data in Precambrian cherts demonstrate that three different sources of silica were involved in the formation of cherts, and that both equilibrium and kinetic isotopic fractionation took place for Si. Archean seawater is predicted to have a triple Si isotopic composition kinetically fractionated relative to bulk silicate Earth. This requires a large kinetic isotopic fractionation during hydrothermalism of the oceanic crust and appears consistent with warm oceans in the Precambrian. However more work is required on the triple isotope systematics of Si inputs and output to seawater by rivers and authigenic clays formation. Triple Si isotope variations can thus be used as a new tool to unravel the origin of cherts and of their O and Si isotopic compositions. The present evidence of kinetic isotopic fractionations for Si isotopes also implies that Si isotopes should be associated to triple O isotope for paleotemperature reconstructions.

Temperature dependence of nitrogen solubility in bridgmanite and evolution of nitrogen storage capacity in the lower mantle

Relative nitrogen abundance normalized by carbonaceous chondrites in the bulk silicate Earth appears to be depleted compared to other volatile elements. Especially, nitrogen behavior in the deep part of the Earth such as the lower mantle is not clearly understood. Here, we experimentally investigated the temperature dependence of nitrogen solubility in bridgmanite which occupies 75 wt.% of the lower mantle. The experimental temperature ranged from 1400 to 1700 °C at 28 GPa in the redox state corresponding to the shallow lower mantle. The maximum nitrogen solubility in bridgmanite (MgSiO3) increased from 1.8 ± 0.4 to 5.7 ± 0.8 ppm with increasing temperature from 1400 to 1700 °C. The nitrogen storage capacity of Mg-endmember bridgmanite under the current temperature conditions is 3.4 PAN (PAN: mass of present atmospheric nitrogen). Furthermore, the nitrogen solubility of bridgmanite increased with increasing temperature, in contrast to the nitrogen solubility of metallic iron. Thus, the nitrogen storage capacity of bridgmanite can be larger than that of metallic iron during the solidification of the magma ocean. Such a “hidden” nitrogen reservoir formed by bridgmanite in the lower mantle may have depleted the apparent nitrogen abundance ratio in the bulk silicate Earth.

Earth’s volatile depletion trend is consistent with a high-energy Moon-forming impact

The abundance of volatile elements in the silicate Earth relative to primitive chondrites provides an important constraint on the thermochemical evolution of the planet. However, an overabundance of indium relative to elements with similar nebular condensation temperatures is a source of debate. Here we use ab initio molecular dynamics simulations to explore the vaporization behavior of indium from pyrolite melt at conditions of the early magma ocean just after the Moon-forming impact. We then compare this to the vaporization behavior of other minor elements. When considering the volatility of the elements from the magma ocean in the absence of the solar nebula gas, we find that there is no overabundance of indium. On the contrary, there is a slight deficit in the abundance of indium, which is consistent with its moderately siderophile nature. Thus, we propose that a high-energy Moon-forming impact may have had a more significant contribution to volatile depletion than previously believed.

Cerium-Nd isotope evidence for an incompatible element depleted Moon

The Moon is a key example of a planetary body that originated from a giant impact collisional event. By better understanding its bulk composition, we gain critical constraints on the building blocks of the Earth-Moon system. Combined measurements of long-lived 147Sm-143Nd and short-lived 146Sm-142Nd isotope compositions of Earth and Moon have lead to controversial interpretations in the past and it remains ambiguous, whether or not the Moon is similar to primitive chondrites in its refractory lithophile element composition. We investigated coupled 138La-138Ce and 147Sm-143Nd isotope and trace element compositions across a wide range of lunar rock types to provide an independent assessment of the bulk Moon composition. All measured lunar rocks define a tight array in 138Ce-143Nd space, intersecting initial εNd=0 at an initial εCe =−0.26±0.04, significantly lower than the currently accepted chondritic 138Ce reference value. The results of combined modeling of 138Ce-143Nd-176Hf isotope and trace element behavior during lunar magma ocean (LMO) crystallization are in good agreement with the bulk silicate Moon having a slight depletion in its highly incompatible trace element inventory. Our calculated composition of the silicate Moon evolves towards εCe=−0.26 and εNd =+1.4 at 3.30 Ga, the approximate age of most lunar samples investigated here. This proposed lunar isotope composition at 3.30 Ga agrees well with the intersection of the 3.30±0.25 Ga lunar array and the terrestrial array defined by rocks from the Archean Pilbara and the Kaapvaal Cratons. We take this as evidence that accessible silicate Earth and the Moon may share a common reservoir slightly depleted in highly incompatible trace elements, named here Slightly Depleted Earth-Moon reservoir (SDEM). The SDEM reservoir proposed here is generally in line with previous models claiming a depleted composition of the accessible silicate Earth, but the degree of depletion is significantly smaller than previously proposed.

Nitrogen and carbon fractionation in planetary magma oceans and origin of the superchondritic C/N ratio in the bulk silicate Earth

Volatiles are important for planetary geodynamics, climate, and habitability. The C/N ratio of the bulk silicate Earth (BSE) is superchondritic, which provides a useful tool for understanding the origin of Earth's volatiles. The Earth accreted largely from differentiated planetesimals and embryos, and the fate of nitrogen and carbon in magma oceans (MOs) of such rocky bodies is key in shaping the BSE's C/N ratio. Here we present experiments performed at 0.3–3 GPa and 1400–1600°C to determine both the solubility and partitioning of nitrogen and carbon between Fe-rich metallic and silicate melts at graphite-saturation and the saturation of N2-rich gas. The quenched samples were analyzed by electron microprobe, secondary ion mass spectrometry, and Raman spectroscopy for their N–C–H–O contents and speciation. Our results show that the C/N solubility ratios of the silicate melts (ppm/ppm by wt.) are a multi-function of pressure, temperature, silicate melt composition, and mainly oxygen fugacity (fO2), and increase from 0.01 to 1.6 with increasing fO2 from IW-3.7 to IW+0.4 (IW refers to the iron–wüstite buffer). Raman spectra and theoretical considerations reveal that the main species in silicate melts are N2, N3−, and N–H in the case of nitrogen, and CO, CO2−3, and C–H in the case of carbon. Nitrogen and carbon may also form complex species, which, however, could not be identified presently. The metal/silicate partition coefficients of nitrogen and carbon DN,Cmetal/silicate are 1–114 and 34–3050, respectively. The DCmetal/silicate/DNmetal/silicate ratios are 1.5–1100, which decrease with increasing pressure, fO2, and the water content in silicate melts. Our results imply that N–C fractionation could occur during core-formation and silicate MO degassing. For a rocky body starting with a chondritic C/N ratio, core-formation would result in a superchondritic C/N ratio in its core if that rocky body is S- and Si-poor. However, a superchondritic C/N ratio can also be achieved in the silicate mantle through C-saturation coupled with preferential nitrogen degassing and loss into space, if the rocky body is oxidized and has a S-rich core, or is reduced and has a Si-rich core. Both Earth's accretion of planetesimals and embryos with cores as the major nitrogen and carbon reservoirs, and Earth's disequilibrium accretion of C-saturated embryos through core–core merging, could have helped establish the BSE's superchondritic C/N ratio. During Earth's accretion of the last few giant impactors, multiple episodes of MO degassing and erosion-induced atmospheric loss would have also favored the formation of a superchondritic C/N ratio in the BSE, due to the oxidized nature of Earth's surface MO (fO2 > IW) and the preferential loss of nitrogen into space. Finally, we emphasize that oxidization of emulsified planetesimal cores in Earth's upper mantle during its final accretion stages could have further helped establish the BSE's superchondritic C/N ratio. Accordingly, the BSE's superchondritic C/N ratio may be an outcome of combined processes operating both on the accreted planetesimals and embryos and on the proto-Earth itself.

The Phosphorus Budget of the Silicate Earth Based on an Updated Estimate of the P/Nd Ratio

AbstractCalculating the phosphorus (P) budget of the bulk silicate Earth (BSE) is necessary to understand the global P cycle and the origin and evolution of life. Covariations of P and neodymium (Nd) contents suggest that P is 1.1–1.4 times more incompatible than Nd during mantle melting and magmatic differentiation. As such, the P/Nd ratios of mantle sources can be estimated using inverse modeling, and the P budget of the BSE can be calculated from the P/Nd ratios of the continental crust, mid‐ocean ridge basalts (MORBs), and ocean island basalts (OIBs). The P/Nd ratios for the MORB and OIB mantle sources and continental crust are 62.5 ± 0.8 (2σm), 57.0 ± 0.5 (2σm), and 28.4 ± 12.0 (2σm), respectively. These reservoirs have different P/Nd ratios due to subduction, magmatic differentiation, and mineralogy. Subduction can significantly affect the global P and Nd cycles. Magmatic differentiation, especially in the continental crust, may redistribute P by changing the contents of garnet, apatite, and other minerals. The fractional contributions of these global reservoirs suggest the modern BSE has a P/Nd ratio of 57.7 ± 3.0 (2σm), equating to ∼3.1 × 1020 kg of P in the BSE.

Origin of the superchondritic carbon/nitrogen ratio of the bulk silicate Earth – an outlook from iron meteorites

Disagreement regarding the origin of the bulk silicate Earth’s (BSE) superchondritic carbon/nitrogen (C/N) ratio is due, in part, to the unknown C/N ratios of differentiated planetesimals – the building blocks of Earth-like rocky planets. In this study we report solid–liquid metal partitioning experiments for C and N that allow us to reconstruct, from the C and N contents of iron meteorites, the C/N ratios of the cores of the earliest formed planetesimals. Due to their siderophile character, most of the C and N retained in these bodies after differentiation resides in their cores. Therefore, estimates of the bulk C and N contents and C/N ratios of the cores yield confident estimates of these quantities in the complete parent bodies of iron meteorites. Our experimental data, at 1 GPa and 1200–1400 °C, show that C and N are incompatible in solid metal relative to S-poor liquids but compatible in solid metal relative to S-rich liquids. Crucially, N is approximately an order of magnitude more compatible than C in S-rich systems. S itself is incompatible in solid metal and so the late-crystallizing liquids persisting at the end of core freezing were S-rich for most cores. Although these late-crystallizing liquids are unsampled by iron meteorites, we infer that their N contents and C/N ratios were generally lower and higher, respectively, than those in iron meteorites. Depending upon the fraction of unsampled late-crystallizing liquids as well as their S contents, the C/N ratios of the bulk cores and complete parent bodies are either similar to or higher than those measured in iron meteorites. The reconstructed C/N ratios of most of the parent bodies of iron meteorites are chondritic, except that the volatile-rich IC and IIC groups have superchondritic C/N ratios. Importantly, the C/N ratio of the parent body of the IC iron meteorite group lies within the estimated range of the BSE, whereas the C/N ratios of all other groups are lower. Correlated depletion of moderately volatile elements like Ge and Ga with C and N, variations in metallographic cooling rates, and Pd-Ag isotope systematics suggest that the parent cores of the volatile-depleted iron meteorite groups were likely affected by volatile degassing. If volatile-rich iron meteorite groups like the IC better capture the C and N inventories of the parent cores of the earliest formed planetesimals, then delivery of C and N via such planetesimals makes the superchondritic C/N ratio of the BSE a natural consequence of the Earth’s accretion history. Otherwise, poorly constrained processes like atmospheric erosion or C and N delivery by exotic materials are required to explain the superchondritic C/N ratio of the BSE.

Can Zn isotopes in sediments record past eutrophication of freshwater lakes? A pilot study at Lake Baldegg (Switzerland)

In this study, the speciation and isotopic composition of Zn were traced across the sediments of a freshwater lake that experienced one hundred years of strong eutrophication, in order to assess the potential of sedimentary Zn isotopes to record such an environmental disturbance. The results indicate that the sedimentary Zn isotope signal varied with the change from pre-eutrophic to eutrophic conditions in the investigated lake. The average δ66ZnJMC value of the dominantly allochthonous lithogenic sediments deposited during the pre-eutrophic period was +0.27‰ ±0.05‰ (i.e. similar to the δ66ZnJMC value of +0.28‰ ±0.05‰ proposed for Bulk Silicate Earth), while enhanced autochthonous biochemical sedimentation during the eutrophic period resulted in significantly lower δ66ZnJMC values down to +0.04‰ ±0.06‰. Synchrotron-based X-ray absorption spectroscopy data revealed a concomitant change in Zn speciation from a dominant fraction of Zn in clay minerals during the pre-eutrophic period to a major fraction of Zn in ZnS during the eutrophic period. A linear regression relating the sedimentary Zn isotope signal to the fraction of Zn in ZnS indicated δ66ZnJMC values of +0.27‰ ±0.06‰ and 0.00‰ ±0.08‰ for Zn in clay minerals and in ZnS, respectively. The enrichment of light Zn in ZnS in the eutrophic sediments is tentatively attributed to enhanced biological uptake of light Zn in the water column, which resulted in an enhanced flux of organic-bound Zn towards the sediments and further transformation of organic Zn into ZnS upon biomass mineralization during early diagenesis. This hypothesis is in agreement with the fractionation towards lighter Zn reported for both biological uptake of Zn and ZnS precipitation. The results of this study emphasize the potential of sedimentary Zn isotopes to register past eutrophic periods in freshwater lakes, and thus to serve as a probe of paleo-environmental conditions and/or past land use at the catchment scale.

A new compositional estimate for refractory lower continental crust with implications for the first terrestrial Pb-isotope paradox

The lower continental crust, representing up to 50% of the continental mass, is largely inaccessible, making its composition difficult to constrain. Previous composite models based on geophysical evidence and geochemical data of granulite terrains and xenoliths have proposed varying results, from a mafic, relatively refractory lower crust to an intermediate-felsic, more enriched composition. Here, we investigated the mineralogy and geochemistry of predominantly mafic granulite xenoliths from eastern Australia and the Kola Peninsula, Russia, using an in situ analytical approach that minimises host magma contamination. The resulting xenolith compositions are variably and often strongly depleted in most highly incompatible trace elements, including the heat-producing elements. These xenoliths represent an extremely refractory component of the lower continental crust, likely formed after high degrees of partial melting or crystallisation from a depleted source. A lower crust composed solely of this refractory endmember would be too exhausted in heat-producing elements to satisfy heat-flow constraints. However, a volumetrically significant component of the lower crust is this mafic and refractory material, combined with undifferentiated material and a felsic or metapelitic portion. Using geophysical constraints on proportions of refractory (55%), undepleted (38%) and enriched (7%) components, a new estimate for average lower continental crust that satisfies heat flow limits was calculated, including for elements such as Be, B, Cs, W and Tl, where previous estimates relied on very few data. Finally, we show that because much of the lower continental crust is so refractory and depleted in incompatible elements, it is unlikely to be a reservoir that can balance radiogenic isotope (unradiogenic Pb) and trace element ratios (e.g. Rb/Cs, Nb/Ta) for which bulk silicate Earth departs from chondritic ratios.

Cadmium isotope fractionation during metal-silicate partitioning – Results and implications for Earth's volatile accretion

Metal-silicate partitioning experiments were carried out at 1.5 GPa and 1508 to 1843 K to constrain Cd partitioning and isotope fractionation during core formation. At the studied conditions, there was no significant stable isotope fractionation during Cd partitioning between metal and silicate phases with a mean ∆114Cdmet-sil = −0.02 ± 0.09‰ (2SD, n = 7). Two experiments that investigated sulphide-silicate partitioning of Cd yielded fractionation factors of −0.04 ± 0.06‰ and − 0.23 ± 0.07‰ (2SE), whereby the latter result was obtained for a short run that may not represent full equilibrium. In summary, the findings suggest that Cd isotope fractionation during segregation of Earth's core was either absent or very minor. The Cd partitioning data of this and previous investigations were combined in multiple linear regression analyses to better constrain Cd metal-silicate partitioning during core formation. In accord with earlier work, the analyses reveal that Cd metal-silicate partitioning is not significantly impacted by temperature and pressure but affected by the S content of the metal phase. In addition, it is shown that the presence of C and Si in the metal reduce the siderophile character of Cd. Based on estimates for the composition of Earth's core, the data suggest a metal-silicate partition coefficient DCd of about 0.4 for a single-stage core formation event. However, given uncertainties about the light element composition of Earth's core, DCd values larger than 1 cannot be ruled out at present for core formation. The results of this study and data on the composition of the bulk silicate Earth and chondritic meteorites were applied in mass balance calculations to constrain the Cd signature of Earth's main stage accretion material prior to delivery of the late veneer. The modelling indicates Earth's main stage of accretion involved material with an average Cd isotope composition that was lighter than that of known carbonaceous and enstatite chondrites. Most likely, this reflects either poor characterisation of these meteorites by the few precise data currently available or that a significant fraction of the terrestrial volatile inventory was acquired from material not directly related to carbonaceous and enstatite chondrites. Furthermore, terrestrial accretion most likely did not encompass the addition of a volatile-rich late veneer exceeding 2% of Earth's mass, in accord with accretion models, which invoke that volatile delivery occurred primarily during main stage accretion, alongside core formation.

Dissipation of Tungsten-182 Anomalies in the Archean Upper Mantle: Evidence from the Black Hills, South Dakota, USA

Most Eoarchean rocks are characterized by positive μ182W anomalies averaging ∼ +13 ppm (where μ182W values are the part per million difference in 182W/184W between a sample and a laboratory reference material presumed to be representative of the bulk silicate Earth, BSE). Prior studies have concluded that the positive 182W anomalies in the upper mantle disappeared by the end of the Archean, yet the timing, nature, and causes of the inferred transition remain poorly understood. In this study, we obtained SmNd mantle extraction model ages (TDM) and μ182W values for Neoarchean and Paleoproterozoic granitic and metasedimentary rocks from the Black Hills, South Dakota, USA. The rocks examined have TDM model ages ranging between ∼3.6 Ga and 2.3 Ga, permitting the tracing of the evolution of 182W in the upper mantle precursors to these rocks, during the purported period of isotopic transition. Of these crustal rocks, the 2.55 Ga Little Elk Granite, with an average TDM age of ∼3.2 Ga, is characterized by a well resolved positive anomaly (μ182W = +8.2 ± 3.1, 2SE). This observation is consistent with a modest diminution in the upper mantle μ182W value relative to the Eoarchean average. By contrast, the 2.60 Ga Bear Mountain Granite, with a slightly younger average TDM age of ∼3.0 Ga, exhibits no resolved anomaly. Most other individual rocks examined also lack resolved anomalies, although the group average μ182W value of +3.3 ± 1.3 (2SE) for the 1.72 Ga Harney Peak Granite, with the majority of TDM model ages ranging from ∼2.7 to 2.3 Ga, may indicate a small positive anomaly for their Neoarchean to Paleoproterozoic upper mantle precursors.The Black Hills rocks provide new evidence for the uneven dissipation during the Mesoarchean through Paleoproterozoic of the positive μ182W value that typified the Eoarchean upper mantle. When the new data are combined with data from prior studies, it becomes evident that the transition to a “modern” BSE W isotopic composition was not the result of a smooth linear decrease, but rather an irregular trend. Nevertheless, the collective data indicate that the positive anomalies in the upper mantle were nearly eliminated by the beginning of the Proterozoic, presumably by mantle mixing processes. The diminution in the scale of 182W anomalies during the Archean is similar to that of 142Nd, and requires a global-scale process that most likely involved vertical and/or lateral mixing within the mantle. The reasons for the delay in the initiation of this mixing process until the end of the Eoarchean, and the completion of mantle mixing by ∼2.4 Ga, awaits exploration via geodynamical modeling of different mixing scenarios.

The effect of collisional erosion on the composition of Earth-analog planets in Grand Tack models: Implications for the formation of the Earth

Impact-induced erosion of the Earth’s early crust during accretion of terrestrial bodies can significantly modify the primordial chemical composition of the Bulk Silicate Earth (BSE, that is, the composition of the crust added to the present-day mantle). In particular, it can be particularly efficient in altering the abundances of elements having a strong affinity for silicate melts (i.e. incompatible elements) as the early differentiated crust was preferentially enriched in those. Here, we further develop an erosion model (EROD) to quantify the effects of collisional erosion on the final composition of the BSE. Results are compared to the present-day BSE composition models and constraints on Earth’s accretion processes are provided. The evolution of the BSE chemical composition resulting from crustal stripping is computed for entire accretion histories of about 50 Earth analogs in the context of the Grand Tack model. The chosen chemical elements span a wide range of incompatibility degrees. We find that a maximum loss of 40wt% can be expected for the most incompatible lithophile elements such as Rb, Th or U in the BSE when the crust is formed from low partial melting rates. Accordingly, depending on both the exact nature of the crust-forming processes during accretion and the accretion history itself, Refractory Lithophile Elements (RLE) may not be in chondritic relative proportions in the BSE. In that case, current BSE estimates may need to be corrected as a function of the geochemical incompatibility of these elements. Alternatively, if RLE are indeed in chondritic relative proportions in the BSE, accretion scenarios that are efficient in affecting the BSE chemical composition should be questioned.

Defining Earth's elusive thermal budget in the presence of a hidden reservoir

A hidden geochemical reservoir has long been a popular concept in the solid Earth sciences, often invoked to explain geochemical paradoxes. Easily overlooked, however, is that such a hidden reservoir invalidates traditional estimates of bulk silicate Earth (BSE) composition, including BSE heat production. Unconstrained BSE heat production threatens our ability to model Earth's thermal evolution. We present a novel method for estimating BSE heat production in the presence of a basal magma ocean—a putative hidden reservoir that has been linked to deep mantle seismic anomalies known as ultralow velocity zones. Our model tracks the thermal evolution of a fully coupled mantle, basal magma ocean, and core. Monte Carlo sampling, together with constraints on upper mantle cooling history and a physical bound on the degree of fractional crystallization, quantifies the major characteristics of Earth's thermal evolution. Chief among these constraints is the previously undefined present-day BSE heat production, which we estimate to be 19±3 TW. Our approach resolves the fundamental difficulty of self-consistent thermal modeling in the presence of a hidden geochemical reservoir, while simultaneously exploring the relevant parameter space. The mantle budget of non-heat-producing elements may require substantial revisions.

Zinc isotope fractionation in mantle rocks and minerals, and a revised δ66Zn value for the Bulk Silicate Earth

The isotope composition of zinc (Zn) can be an important tracer of planetary formation and differentiation, but estimates of the Bulk Silicate Earth (BSE) and the extent and origin of Zn isotope variations in the mantle continue to be debated. We report 153 high-precision Zn isotope analyses of whole-rock samples and minerals of well-characterized, mantle-derived spinel and garnet peridotites from five sites in Siberia, Mongolia and China: 38 xenoliths in basaltic rocks (fertile to strongly melt-depleted as well as variably metasomatized or veined peridotites) and 3 orogenic peridotites. Data quality was rigorously tested by twenty-two duplicated analyses using several dissolution techniques. Spinel (Sp) contains 760–1460 μg/g Zn and hosts up to 58 % of whole-rock Zn budget in the peridotites followed by olivine (Ol, 30–66 % of whole-rock Zn budget), orthopyroxene (Opx), clinopyroxene (Cpx) and garnet, with abundance ratios of [Zn]Sp/[Zn]Ol = 15–47, [Zn]Ol/[Zn]Opx = ∼1.5, [Zn]Ol/[Zn]Cpx = ∼6. Spinel has higher δ66Zn values than coexisting silicates (∼0.12 ‰ higher than in Ol). Minerals in orogenic peridotites show disequilibrium Zn element and isotope partitioning due to slow inter-grain Zn diffusion at retrograde metamorphism.Fertile peridotites (≥3 wt% Al2O3) contain 52–60 μg/g Zn, usually higher than melt-depleted (<3 wt% Al2O3) rocks (40–53 μg/g). The δ66Zn values in the majority of spinel and garnet peridotites range from 0.11 to 0.26 ‰ and show no systematic co-variations with melt extraction indices (e.g., Al2O3, Mg#), suggesting, unlike some previous studies, that partial melting does not appreciably affect Zn isotope composition of residual mantle. Garnet lherzolites yield an average δ66Zn of 0.22 ± 0.09 ‰ (2SD), similar to that (0.18 ± 0.07 ‰, 2SD) for shallower spinel peridotites, indicating uniform Zn isotope composition with depth. Four fertile lherzolites from earlier work, duplicated in this study to assess inter-laboratory bias, fall in the same δ66Zn range, suggesting that previously reported higher δ66Zn for these samples could be due to analytical problems. We thus estimate the δ66Zn of the BSE at 0.19 ± 0.08 ‰ (2SD) based on suitable fertile peridotites from several localities in this study and the literature.The δ66Zn values (−0.10 to 0.04 ‰) are much lower in Fe-rich (9.3–13.1 wt% FeOT) and veined spinel peridotites as well as in spinel pyroxenite veins from Mongolia (−0.20 to 0.11 ‰). The Fe-rich peridotites and pyroxenites are rich in Zn (up to 113 μg/g) and were previously found to have anomalously low δ56Fe (−0.33 to − 0.27 ‰) and δ53Cr (−1.36 to − 0.05 ‰). We attribute these features to kinetic isotope fractionation during reaction with mafic melts migrating in conduits in the lithospheric mantle or intrusion of Fe-Zn-rich melts with light Fe-Zn isotope compositions. Kinetic Zn isotope fractionation, rather than equilibrium partial melting, may be a major cause of Zn isotope heterogeneity in the mantle.

The stable chromium isotope composition of different mantle reservoirs

The stable chromium isotope composition of Earth’s mantle has been suggested to be heterogeneous, with mantle-derived chromitites slightly enriched in heavier Cr isotopes than mantle-derived silicate rocks. To better understand such heterogeneity and to search for a potentially complementary light Cr isotope reservoir in the mantle, we analyzed the stable Cr isotope compositions (δ53Cr expressed as deviation of sample 53Cr/52Cr from NIST SRM 979) of whole-rock pyroxenite and peridotite xenoliths from the North China Craton using a double spike technique. The δ53Cr values of the peridotites vary from −0.21‰ to −0.07‰ with an average value of −0.14 ± 0.10‰ (2SD, N = 5), which overlaps with the range of the Bulk Silicate Earth (−0.12 ± 0.10‰). In contrast, pyroxenites in this study yield lower δ53Cr values (−0.30‰ to −0.14‰ with an average value of −0.21 ± 0.08‰, 2SD, N = 20). The variation in the δ53Cr values of the pyroxenites can be explained by a Rayleigh distillation model during partial melting of the primitive mantle. The model provides minimum Cr isotope fractionation factors during partial melting ranging from 0.01‰ to 0.12‰, with light isotopes enriched in the melt. Combined with literature data, our results suggest that both peridotites and basalts are indistinguishable from the Bulk Silicate Earth in terms of δ53Cr. However, pyroxenites and chromitites are isotopically lighter and heavier than the Bulk Silicate Earth, respectively. Future work is needed to investigate the petrogenetic relationship between chromitites and pyroxenites in order to understand why they seem to complement each other in δ53Cr.

Episodic alteration within a gold-bearing Archean shear zone revealed by in situ biotite Rb–Sr dating

Many economic gold deposits formed in the Neoarchean under greenschist facies conditions in association with ductile–brittle shear zones that were conduits for sulfur-bearing hydrothermal fluids. Although the large-scale architecture and formation history of Neoarchean structurally controlled gold deposits has been the subject of many studies, the timing, rates, and durations of mineralization-related alteration remain challenging to deconvolve owing to a lack of event-specific geochronometers and the potential multitude of overprinting processes. In an effort to better understand the alteration history along the mineralized Zuleika Shear Zone in the Eastern Goldfields Superterrane of the Yilgarn Craton in Western Australia, we present the results of biotite Rb–Sr from 13 drill core samples (seven with apatite Sr isotopes) from the Kundana gold camp and the Carbine area. The Kundana district is a lode gold deposit that represents an ideal case study area to test the longevity of mineralization-associated alteration systems. We use in situ laser ablation collision cell triple quadrupole mass spectrometry on biotite from a range of characteristic alteration assemblages with well-defined relative growth ages. Biotite Rb–Sr ages constrain the timing of four different alteration stages between ∼2700 and 2550 Ma. The correlation between relative alteration chronology and Rb–Sr age and the lack of any relationship between grain size and age supports an interpretation of Rb–Sr ratios reflecting biotite growth during hydrothermal alteration with subsequent rapid cooling. Initial 87Sr/86Sr ratios of biotite and apatite are similar and indicate compositions that are more radiogenic than chondritic values and generally slightly more radiogenic than modelled Neoarchean Bulk Silicate Earth values, consistent with a mixture of mantle and crustal fluid sources. Ti-in-biotite thermometry yields temperatures between 540 and 615 °C, peaking towards the main gold mineralization phase. Although it is not possible to link individual Rb–Sr ages to specific faulting events, the absolute age dating for the various alteration assemblages indicates a prolonged, multiphase alteration system lasting ∼150 Ma. Long-term permeability networks with episodic short-lived fluid pulses that outlast terrane- or camp-scale deformation may not only apply to Neoarchean lode gold deposits, but to any tectonically driven zone of weakness in the continental crust.

The effect of high-temperature alteration of oceanic crust on the potassium isotopic composition of seawater

High-temperature hydrothermal alteration of oceanic crust is one of the two main sources of potassium (K) to the oceans, with modern flux estimates ranging roughly from ∼8 to 30 % of the K flux from rivers. Despite the role of high-temperature hydrothermal fluids in the global seawater K budget, little is known about its effect on the K isotopic composition of the oceans. Here we present stable K isotope measurements (δ41KSRM3141a) of globally distributed high-temperature hydrothermal fluids from three mid-ocean ridge systems: the East Pacific Rise (n = 21), the Juan de Fuca Ridge (n = 34), and the Mid-Atlantic Ridge (n = 18). We find a strong correlation between δ41K and Mg/K ratios, consistent with conservative mixing between a high-temperature hydrothermal fluid (i.e., Mg = 0) and seawater as fluids ascend to the seafloor and/or due to seawater entrainment during sampling. The δ41K of end-member hydrothermal fluids is found to range between −0.80 ‰ and 0.07 ‰, with an average value of −0.36 ± 0.30 ‰ (2σ, n = 38). Most (∼76 %) of the variability in end-member fluid δ41K compositions observed here can be explained by high-temperature fluid-rock K exchange, with little (∼0.2 ‰) to no K isotope fractionation between hydrothermal fluid and altered crust. Larger deviations from the average end-member hydrothermal fluid value are likely to result from processes other than high-temperature fluid-rock exchange, such as (1) low-temperature hydrothermal reactions during fluid recharge, (2) reaction of fluids with local sedimentary sources, and (3) phase separation. The K contents of end-member fluids vary considerably, from ∼1 to 38 mM, thus a K-weighted average of −0.37 ± 0.24 ‰ (2σ, n = 38) is estimated to represent the δ41K composition of the global hydrothermal K flux. Our results suggest that K sourced from axial hydrothermal alteration does not contribute to the elevated 41K/39K of seawater compared to bulk silicate Earth (BSE). In addition, subduction of oceanic crust altered under high-temperature conditions is unlikely to be a significant source of K isotopic heterogeneity to Earth’s mantle.