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We have performed in situ analyses of Li-Be-B and Al-Mg isotope systematics, and abundances of rare earth elements (REEs) in two Ca-Al-rich inclusions (CAIs) from the Ornans-like carbonaceous chondrite Yamato-81020 (CO3.05). The present CO CAIs are depleted in ultra-refractory heavy REEs (group II REE pattern), suggesting condensation of these CAIs or their precursors from the solar nebula. Initial 26Al/27Al ratios, (26Al/27Al)0, of these CO CAIs are found to be (4.8 ± 0.5) × 10–5 and (4.9 ± 0.3) × 10–5 (95% confidence), indicating their contemporaneous formation with a majority of CAIs from Vigarano-like carbonaceous (CV) chondrites. Melilite grains in the present CO CAIs show clear excesses in 10B, ranging from ∼370 to ∼4300‰ relative to the chondritic B isotopic composition, which are correlated well with 9Be/11B ratios. The correlation indicates in situ decay of 10Be in the present CO CAIs and yields initial 10Be/9Be ratios, (10Be/9Be)0, for the individual CAIs of (2.9 ± 0.6) × 10–3 and (2.2 ± 1.0) × 10–3 (95% confidence), which are significantly greater than the average (10Be/9Be)0 = ∼0.7 × 10–3 recorded in CAIs from CV chondrites. The apparent variation in (10Be/9Be)0 between the CO and CV CAIs, despite having indistinguishable (26Al/27Al)0 of ∼5 × 10–5, provides evidence for heterogeneous distribution of 10Be in the CAI forming-regions at the very beginning of the Solar System. The elevated (10Be/9Be)0 and group II REE patterns in the CO CAIs may reflect that compared with the CV CAIs having unfractionated REEs the present CO CAIs have formed closer to the Sun where 10Be was produced more efficiently through solar cosmic ray irradiation caused by solar flares. Alternatively, if the present CO CAIs and CV CAIs formed in the same region, and 26Al was distributed homogeneously at the CAI-forming region, our results indicate that solar cosmic ray fluxes at the forming region have fluctuated by a factor of six within a short duration (∼0.2 million years) inferred from the Al-Mg chronology.

Ophiolite pulses, which are periods of enhanced ophiolite generation and emplacement, are thought to have a relevance to highly active superplumes (superplume model). However, the Cambrian-Ordovician pulse has two critical geological features that cannot be explained by such a superplume model: predominance of subduction-related ophiolites and scarcity of plume-related magma activities. We addressed this issue by estimating the mechanism and condition of magma generation, including mantle potential temperature (MPT), from a ~500 Ma subduction-related ophiolite, the Hayachine-Miyamori ophiolite. We developed a novel method to overcome difficulties in global MPT estimation from an arc environment by using porphyritic ultramafic dikes showing flow differentiation, which have records of the chemical composition of the primitive magma, including its water content, because of their high pressure (~0.6 GPa) intrusion and rapid solidification. The solidus conditions for the primary magmas are estimated to be ~1450 °C, ~5.3 GPa. Geochemical data of the dikes show passive upwelling of a depleted mantle source in the garnet stability field without a strong influence of slab-derived fluids. These results, combined with the extensive fluxed melting of the mantle wedge prior to the dike formation, indicate sudden changes of the melting environment, its mechanism, and the mantle source from extensive fluxed melting of the mantle wedge to decompressional melting of the sub-slab mantle, which has been most plausibly triggered by a slab breakoff. The estimated MPT of the sub-slab mantle is ~1350 °C, which is very close to that of the current upper mantle and may reflect the global value of the upper mantle at ~500 Ma if small-scale convection maintained the shallow sub-slab mantle at a steady thermal state. We, therefore, conclude that the Cambrian-Ordovician ophiolite pulse is not attributable to the high temperature of the upper mantle. Frequent occurrence of slab breakoff, which is suggested by our geochemical compilation of Cambrian-Ordovician ophiolites, and subduction termination, which is probably related to the assembly of the Gondwana supercontinent, may be responsible for the ophiolite pulse.

Acid leaching has been widely used in Pb–Pb chronology especially on meteorite samples, as it enhances the precision and accuracy of the chronology by separating radiogenic Pb from non-radiogenic Pb in the samples. Yet, the detailed mechanism of the radiogenic Pb separation remains elusive. Here we report scanning electron microscopy observations of acid-leached minerals from three achondrites that were previously analyzed for pyroxene Pb Pb dating (D'Orbigny, Ibitira and Northwest Africa 6704), and one stony‑iron meteorite (Brenham). Moreover, we present chemical analysis of acid leachates and residues of pyroxene and whole-rock fractions from the three achondrites using inductively coupled plasma mass spectrometry. The combined results reveal the resistance of minerals to acid treatments with HNO3, HCl, and HF and, in turn, allow us to explore the prospects and limitations of acid leaching for Pb Pb chronology. In particular, we show that (i) washing with dilute acids can efficiently liberate contaminant terrestrial Pb adsorbed on the mineral surface; (ii) hot and more concentrated (~6 M) HNO3 and HCl can separate pyroxenes from sulfides and anorthitic plagioclase that are highly enriched in initial non-radiogenic Pb; (iii) albitic plagioclase and pyroxenes show limited dissolution during the HNO3 and HCl treatments, but are progressively leached by hot 1 M HF, making it difficult to separate them from each other; and (iv) within single pyroxene grains having exsolved lamellar textures, high-Ca lamellae are more efficiently leached by the HF treatment than low-Ca ones. In the context of a two-stage Pb isotopic evolution model for pyroxene lamellae, we demonstrate that heterogeneous and incomplete dissolution of pyroxene lamellae with dilute HF can result in inaccurate Pb Pb age estimates. This may account for previously observed scattered Pb isotopic data for meteorite samples that were taken as evidence for isotope fractionation in a radiogenic Pb component during HF treatments.

Northwest Africa (NWA) 6704 is a unique achondrite characterized by a near-chondritic major element composition with a remarkably intact igneous texture. To investigate the origin of this unique achondrite, we have conducted a combined petrologic, chemical, and (187)Re–(187)Os, O, and Ti isotopic study. The meteorite consists of orthopyroxene megacrysts (En(55–57)Wo(3–4)Fs(40–42); Fe/Mn = 1.4) up to 1.7 cm in length with finer interstices of olivine (Fa(50–53); Fe/Mn = 1.1–2.1), chromite (Cr# ~ 0.94), awaruite, sulfides, plagioclase (Ab(92)An(5)Or(3)) and merrillite. The results of morphology, lattice orientation analysis, and mineral chemistry indicate that orthopyroxene megacrysts were originally hollow dendrites that most likely crystallized under high super-saturation and super-cooling conditions (1–10(2) °C/h), whereas the other phases crystallized between branches of the dendrites in the order of awaruite, chromite → olivine → merrillite → plagioclase. In spite of the inferred high supersaturation, the remarkably large size of orthopyroxene can be explained as a result of crystallization from a melt containing a limited number of nuclei that are preserved as orthopyroxene megacryst cores having high Mg# or including vermicular olivine. The Re–Os isotope data for bulk and metal fractions yield an isochron age of 4576 ± 250 Ma, consistent with only limited open system behavior of highly siderophile elements (HSE) since formation. The bulk chemical composition is characterized by broadly chondritic absolute abundances and only weakly fractionated chondrite-normalized patterns for HSE and rare earth elements (REE), together with substantial depletion of highly volatile elements relative to chondrites. The HSE and REE characteristics indicate that the parental melt and its protolith had not undergone significant segregation of metals, sulfides, or silicate minerals. These combined results suggest that a chondritic precursor to NWA 6704 was heated well above its liquidus temperature so that highly volatile elements were lost and the generated melt initially contained few nuclei of relict orthopyroxene, but the melting and subsequent crystallization took place on a timescale too short to allow magmatic differentiation. Such rapid melting and crystallization might occur as a result of impact on an undifferentiated asteroid. The O–Ti isotope systematics (Δ(17)O = −1.052 ± 0.004, 2 SD; ε(50)Ti = 2.28 ± 0.23, 2 SD) indicate that the NWA 6704 parent body sampled the same isotopic reservoirs in the solar nebula as the carbonaceous chondrite parent bodies. This is consistent with carbonaceous chondrite-like refractory element abundances and oxygen fugacity (FMQ = −2.6) in NWA 6704. Yet, the Si/Mg ratio of NWA 6704 is remarkably higher than those of carbonaceous chondrites, suggesting significant nebular fractionation of forsterite in its provenance.

Monazite [(Ce, La, Th) PO4], a common accessory mineral in low-Ca felsic rocks and a wide range of metamorphic rocks, is suitable for in situ U–Pb dating, rare earth element (REE) geochemistry, and Nd isotope tracer studies. To link monazite composition with igneous petrogenesis and differentiation, we carried out comprehensive LA-ICP-MS measurements of REE-Th-U abundances and Sm-Nd isotope ratios in monazites from magnetite-series (oxidized type) and ilmenite-series (reduced type) granitic rocks across the Japan arc. The data revealed systematic differences in monazite composition between magnetite-series and ilmenite-series samples, and between pegmatites and granites. In ilmenite-series granitic rocks, monazites from pegmatites showed larger negative Eu anomalies, lower light-REE/middle-REE ratios, and higher middle-REE/heavy-REE ratios than those from granites. These geochemical variations were attributed to significant fractional crystallization of feldspars, monazite, xenotime, and garnet during differentiation in relatively reduced peraluminous granitic magmas. In contrast, there was no remarkable difference in the REE fractionation pattern between magnetite-series pegmatites and granites. Furthermore, the magnitudes of the negative Eu anomalies in the magnetite-series monazites were smaller than those observed in the ilmenite-series samples. These features were interpreted to reflect the suppression of monazite, xenotime, and garnet fractionation and limited Eu incorporation into fractionating feldspars in relatively oxidized and non-peraluminous magmas. In addition, our data demonstrated intra-grain variations in REE-Th-U compositions in a monazite from the Masaki ilmenite-series pegmatite, despite its homogenous Nd isotope composition. These variations suggest that the core and rim of the grain crystallized from distinct magmas that were generated from similar crustal rocks, but were differentiated under different conditions. A comparison of the present data with previously reported data indicates that igneous monazites from granitic rocks are distinct from a wide range of metamorphic monazites in REE-Th-U composition, especially due to their larger negative Eu anomalies. This finding highlights the potential utility of monazite REE-Th-U systematics as a provenance indicator for detrital monazites.

Abundant porphyritic granites, including Grt-bearing and Bt-bearing porphyritic granites, and flesh-red porphyritic granites, as various scale batholiths are widely distributed within the Jiao-Liao-Ji belt (JLJB), which extends for a distance of at least 1000 km, rather than being limited to sporadic outcrops in southern Liaoning and Jilin as stated in previous studies. On the basis of morphology, CL and BSE zoning patterns, and mineral inclusions of dated zircon and monazite grains from samples of various porphyritic granites, the grains exhibit evident magmatic zones with distinct cores and rims and preserve felsic mineral inclusions of Qtz + Kfs + Pl + Qtz ± Ap in both domains. The magmatic zircon cores and rims from 18 dated samples yielded two distinct weighted mean age groups, from 1872 ± 9 to 1866 ± 7 Ma and 1851 ± 14 to 1842 ± 10 Ma, with age peaks at 1869 ± 1.7 and 1848 ± 2.4 Ma, respectively. Similarly, the magmatic cores and rims of monazites from 10 dated samples also showed two distinct weighted mean age groups, from 1870 ± 3 to 1864 ± 2 Ma and 1851 ± 6 to 1842 ± 12 Ma, with age peaks at 1868 ± 0.8 and 1847 ± 1.2 Ma, respectively. The numerous U-Pb age data of both zircons and monazites revealed that the JLJB experienced a major episode of granitic magmatism in the Paleoproterozoic with initial emplacement timing of melts at ∼1.87 Ga and crystallisation end timing of melts at ∼1.84 Ga as represented by the magmatic cores and rims of the zircons and monazites, respectively. A combined study of whole-rock geochemistry and zircon Lu-Hf isotopic analyses revealed that the three types of porphyritic rocks are enriched in HREE, Zr, Nb, Ga, and Y and are depleted in V, Cr, and Ni, and that most have high TFeO/MgO ratios and Zr-saturation temperatures of ∼850 °C, which is suggestive of an affinity with A-type granite. Moreover, the dominant e Hf (t) values of the magmatic zircons range from −4.8 to +3.3, which is consistent with those of the 2.2–2.1 Ga calc-alkaline meta-granites in the JLJB. Considering the high contents of garnet, Al-rich paragneiss enclaves, and ∼1.92 Ga inherited metamorphic zircons in the granites, these porphyritic rocks were most likely derived from the partial melting of the 2.2–2.1 Ga calc-alkaline meta-granites with abundant involvement of Al-rich pelitic granulites from the basement of the JLJB. As the result of a newly combined study of traditional geothermobarometers and phase equilibria modelling together with geochronological U-Pb dating, a precise clockwise P-T-t path with a subsequent near isothermal decompression process has been well determined in the HP-MP granulite facies metamorphic rocks in the JLJB. Thus, the following sequence of events can be inferred. Abundant ∼2.2–2.1 Ga granitic rocks and voluminous Paleoproterozoic sedimentary rocks were subducted to lower crustal depths and underwent a peak HP-MP granulite facies metamorphism at 1950–1900 Ma. These granulite facies rocks were exhumed to middle crustal levels and experienced a post-peak MP-LP granulite facies metamorphism at 1875–1840 Ma. The retrograde rocks continued to be exhumed to shallow crustal levels at 1830–1800 Ma and were finally overprinted by late amphibolite facies retrogression. The numerous magmatic zircons and monazites from the various porphyritic granites record a distinct Paleoproterozoic granitic magmatic event with two evident age peaks of ∼1.87 Ga and ∼1.84 Ga, which are completely consistent with the post-peak MP-LP granulite facies metamorphism (1875–1840 Ma), representing an anorogenic magmatic association formed in an extensional setting possibly resulting from mantle upwelling and lithospheric thinning and resulting in the violent partial melting of ∼2.2–2.1 Ga meta-granites along with abundant Al-rich pelitic granulites. Considering these data in combination with previous whole-rock geochemistry and age data, it can be stated that similar porphyritic granites are not only limited to the entire JLJB but are also widely distributed in North and South Korea. Thus, it can be concluded that the ∼1.87–1.84 Ga porphyritic granites, together with massive rapakivi granites, Kfs-megacrysts, and porphyritic monzogranites, as products of an anorogenic granitic magmatic event, may have widely formed in both the JLJB and Korean Peninsula in an extensional and thinning tectonic setting.

The Earth was born in violence. Many giant collisions of protoplanets are thought to have occurred during the terrestrial planet formation. Here we investigated the giant impact stage by using a hybrid code that consistently deals with the orbital evolution of protoplanets around the Sun and the details of processes during giant impacts between two protoplanets. A significant amount of materials (up to several tens of percent of the total mass of the protoplanets) is ejected by giant impacts. We call these ejected fragments the giant-impact fragments (GIFs). In some of the erosive hit-and-run and high-velocity collisions, metallic iron is also ejected, which comes from the colliding protoplanets' cores. From ten numerical simulations for the giant impact stage, we found that the mass fraction of metallic iron in GIFs ranges from ~ 1wt% to ~ 25wt%. We also discussed the effects of the GIFs on the dynamical and geochemical characteristics of formed terrestrial planets. We found that the GIFs have the potential to solve the following dynamical and geochemical conflicts: (1) The Earth, currently in a near circular orbit, is likely to have had a highly eccentric orbit during the giant impact stage. The GIFs are large enough in total mass to lower the eccentricity of the Earth to its current value via their dynamical friction. (2) The concentrations of highly siderophile elements (HSEs) in the Earth's mantle are greater than what was predicted experimentally. Re-accretion of the iron-bearing GIFs onto the Earth can contribute to the excess of HSEs. In addition, the estimated amount of iron-bearing GIFs provides significant reducing agent that could transform primitive CO2-H2O atmosphere and ocean into more reducing H2-bearing atmosphere. Thus, GIFs are important for the origin of Earth's life and its early evolution., Accepted for publication in Earth and Planetary Science Letters, 33 pages, 7 figures, 1 table

The similar, but not identical chemical properties of W compared with Mo suggest that the stable isotope system of W could be a novel proxy to explore the modern and ancient ocean as is the case in the well-established utility of Mo isotopes. We experimentally investigated the isotopic fractionation of W during adsorption on Fe and Mn (oxyhydr)oxides (ferrihydrite and δ-MnO 2 ), a key process in the global ocean budget of this element. Our adsorption experiments confirmed that W isotopes fractionate substantially on both ferrihydrite and δ-MnO 2 : lighter W isotopes are preferentially adsorbed on both oxides as a result of equilibrium isotopic exchange between dissolved and adsorbed species, and the obtained values of Δ 186/183 W liquid–solid ( = δ 186 W dissolved − δ 186 W adsorbed ) are 0.76 ± 0.09‰ for ferrihydrite and 0.88 ± 0.21‰ for δ-MnO 2 (2 σ , n = 6). Compared with the case of Mo isotopes, fractionation of W isotopes is (i) of comparable magnitude between ferrihydrite and δ-MnO 2 , and (ii) much smaller than that of Mo on δ-MnO 2 . Our previous XAFS observations and newly-performed DFT calculations both indicate that the observed W isotopic fractionations are caused by the symmetry change from Td (tetrahedral) WO 4 2− to distorted Oh (octahedral) monomeric W species via formation of inner-sphere complexes on both ferrihydrite and δ-MnO 2 . The similar isotopic fractionations between the two oxides relate to the strong tendency for W to form inner-sphere complexes, which causes the symmetry change, in contrast to the outer-sphere complex of Mo on ferrihydrite. The smaller isotopic fractionation of W compared with Mo on δ-MnO 2 despite their similar molecular symmetry seems to be due to their different degrees of distortion of Oh species. Our findings imply that the isotopic composition of W in modern oxic seawater is likely to become heavier relative to the input by removal of lighter W isotopes via adsorption on ferromanganese oxides in analogy with the Mo isotope budget. In contrast, the isotopic composition of W in ancient seawater should have evolved in response to the extent of deposition of both Fe and Mn oxides; this is likely to be different compared with that of the Mo isotopes, which is strongly associated with the occurrence of Mn oxides relative to Fe oxides.

The 176 Lu– 176 Hf radioactive decay system has been widely used to study planetary crust–mantle differentiation. Of considerable utility in this regard is zircon, a resistant mineral that can be precisely dated by the U–Pb chronometer and record its initial Hf isotope composition due to having low Lu/Hf. Here we review zircon U–Pb age and Hf isotopic data mainly obtained over the last two decades and discuss their contributions to our current understanding of crust–mantle evolution, with emphasis on the Lu–Hf isotope composition of the bulk silicate Earth (BSE), early differentiation of the silicate Earth, and the evolution of the continental crust over geologic history. Meteorite zircon encapsulates the most primitive Hf isotope composition of our solar system, which was used to identify chondritic meteorites best representative of the BSE ( 176 Hf/ 177 Hf = 0.282793 ± 0.000011; 176 Lu/ 177 Hf = 0.0338 ± 0.0001). Hadean–Eoarchean detrital zircons yield highly unradiogenic Hf isotope compositions relative to the BSE, providing evidence for the development of a geochemically enriched silicate reservoir as early as 4.5 Ga. By combining the Hf and O isotope systematics, we propose that the early enriched silicate reservoir has resided at depth within the Earth rather than near the surface and may represent a fractionated residuum of a magma ocean underlying the proto-crust, like urKREEP beneath the anorthositic crust on the Moon. Detrital zircons from world major rivers potentially provide the most robust Hf isotope record of the preserved granitoid crust on a continental scale, whereas mafic rocks with various emplacement ages offer an opportunity to trace the Hf isotope evolution of juvenile continental crust (from eHf [4.5 Ga] = 0 to eHf [present] = + 13). The river zircon data as compared to the juvenile crust composition highlight that the supercontinent cycle has controlled the evolution of the continental crust by regulating the rates of crustal generation and intra-crustal reworking processes and the preservation potential of granitoid crust. We use the data to explore the timing of generation of the preserved continental crust. Taking into account the crustal residence times of continental crust recycled back into the mantle, we further propose a model of net continental growth that stable continental crust was firstly established in the Paleo- and Mesoarchean and significantly grew in the Paleoproterozoic.

Slab-melting is considered to have played an important role in the formation of continental crust. The combination of oxygen (O) and hafnium (Hf) isotope signatures can provide key information relating to the melting components during slab-melting. To reveal the melting components, we used SIMS and LA-ICP-MS to determine O isotope ratios at 220 spots and Hf isotope ratios at 61 spots in zircons from five plutons in the Taitao Peninsula, where slab-melting occurred at ca. 4–5 Ma. In addition, we measured whole-rock Hf O isotope ratios in 11 rocks that were intruded by the granitoids. The zircon δ18O values of the Seno Hoppner pluton (5.37 ± 0.44‰) are identical to those of mantle-equilibrated zircons, whereas those of the other four plutons are relatively high (6.09–6.53‰). The ɛHf(t) values of zircons in the granitoids are negatively correlated with δ18O, and they fell along the mixing line between a juvenile component and sedimentary rocks. Therefore, the Hf O profiles can be attributed to mixing of juvenile granitoid magma and the sedimentary rocks, the amount of which was likely minimal in the Seno Hoppner pluton. As such, the δ18O values of the Seno Hoppner pluton could best reflect the juvenile component. In view of the δ18O variation in whole-rock samples of the Taitao ophiolite, the melting of hydrothermally altered basalt and dolerite is considered responsible for the juvenile magma generation. This further implies that juvenile granitoid magmas can be generated by the melting of the upper half of subducted oceanic crusts.

The late heavy bombardment (LHB) hypothesis, wherein the terrestrial planets are thought to have suffered intense collisions ca. 3.9 billion years ago, is under debate. Coupled with new dynamical calculations, re-examination of geochronological data seem to support an earlier solar system instability and a smooth monotonic decline in impacts, as opposed to a “cataclysm.” To better understand this collisional history, records from the asteroidal meteorites are required. Here, we report a uranium–lead (U–Pb) chronological dataset for eucrite meteorites thought to originate from the asteroid 4 Vesta; this dataset indicates to a continuous history of collisions prior to 4.15 Ga. Our 207Pb⁎/206Pb⁎ model ages of apatite [Ca5(PO4)3(F,Cl,OH)] and merrillite [Ca9NaMg(PO4)7] from three brecciated basaltic eucrites—Juvinas (4150.3 ± 11.6 million years ago (Ma); merrillite only), Camel Donga (disturbed around 4570–4370 Ma), and Stannern (4143.0 ± 12.5 Ma)—record multiple thermal metamorphic events during the period of ∼4.4–4.15 Ga. We interpret this to mean that Vesta or the vestoid cluster underwent multiple impacts and moderate high-temperature reheating during this time. The ages of ∼4.4–4.15 Ga are distinctly younger than the initial magmatic process on Vesta (>4.5 Ga) but are significantly older than a later “impact peak” based on some interpretations of 40Ar–39Ar chronologies (∼3.9–3.5 Ga). The intense collisions prior to 4.15 Ga on Vesta are at odds with the conventional LHB hypothesis but not inconsistent with the much earlier bombardment and monotonic decline scenario. Different radiometric chronologies of the asteroid likely represent the different stages of a continual collisional process. Conversely, the model 207Pb⁎/206Pb⁎ ages of apatite in the unbrecciated basaltic eucrite, Agoult, returned an age of 4524.8 ± 9.6 Ma. This may represent slow cooling from an earlier global reheating of the crust on Vesta at 4.55 Ga, as documented by other radiometric chronologies. The apatite in Juvinas recorded a coincident timing of 4516.9 ± 10.4 Ma, which could be due to either slow crustal cooling or impact.

Geoneutrino observations, first achieved by KamLAND in 2005 and followed by Borexino in 2010, have accumulated statistics and improved sensitivity for more than ten years. The uncertainty of the geoneutrino flux at the surface is now reduced to a level small enough to set useful constraints on U and Th abundances in the bulk silicate earth (BSE). However, in order to make inferences on earth's compositional model, the contributions from the local crust need to be understood within a similar uncertainty. Here we develop a new method to construct a stochastic crustal composition model utilizing Bayesian inference. While the methodology has general applicability, it incorporates all the local uniqueness in its probabilistic framework. Unlike common approaches for this type of problem, our method does not depend on crustal segmentation into upper, (middle) and lower, whose classification and boundaries are not always well defined. We also develop a new modeling method to infer rock composition distributions that conserve mass balance and therefore do not bias the results. Combined with a new vast collection of geochemical data for rock samples in the Japan arc, we apply this method to geoneutrino observation at Kamioka, Japan. Currently a difficulty remains in the handling of correlations in the flux integration; we conservatively assume maximum correlation, which leads to large flux estimation errors of 60~70%. Despite the large errors, this is the first local crustal model for geoneutrino flux prediction with probabilistic error estimation in a reproducible way., Comment: 26 pages, 11 figures, with 9 figures in Appendix

Iron isotope ratios (δ56/54Fe) of various anthropogenic materials produced by combustion processes were analyzed. It was revealed that particles emitted through combustion processes, such as fine a...

Detrital monazite geochronology has been used in provenance studies. However, there are complexities in the interpretation of age spectra due to their wide occurrence in both igneous and metamorphic rocks. We use the multinomial logistic regression (MLR) and cross-validation (CV) techniques to establish a geochemical discrimination of monazite source rocks. The elemental abundance-based geochemical discrimination was tested by selecting 16 elements from granitic and metamorphic rocks. The MLR technique revealed that light rare earth elements (REEs), Eu, and some heavy REEs are important discriminators that reflect elemental fractionation during magmatism and/or metamorphism. The best model yielded a discrimination rate of ~97%, and the CV method validated this approach. We applied the discrimination model to detrital monazites from African rivers. The detrital monazites were mostly classified as granitic and of garnet-bearing metamorphic origins, however, their proportion of metamorphic origin was smaller than the proportion that was obtained by using the elemental-ratio-based discrimination proposed by Itano et al. in Chemical Geology (2018). Considering the occurrence of metamorphic rocks in the hinterlands and the different age spectra between monazite and zircon in the same rivers, a ratio-based discrimination would be more reliable. Nevertheless, our study demonstrates the advantages of machine-learning-based approaches for the quantitative discrimination of monazite.

Knowledge of planetary differentiation is crucial for understanding the chemical and thermal evolution of terrestrial planets. The (176)Lu-(176)Hf radioactive decay system has been widely used to constrain the timescales and mechanisms of silicate differentiation on Earth, but the data interpretation requires accurate estimation of Hf isotope evolution of the bulk Earth. Because both Lu and Hf are refractory lithophile elements, the isotope evolution can be potentially extrapolated from the present-day (176)Hf/(177)Hf and (176)Lu/(177)Hf in undifferentiated chondrite meteorites. However, these ratios in chondrites are highly variable due to the metamorphic redistribution of Lu and Hf, making it difficult to ascertain the correct reference values for the bulk Earth. In addition, it has been proposed that chondrites contain excess (176)Hf due to the accelerated decay of (176)Lu resulting from photoexcitation to a short-lived isomer. If so, the paradigm of a chondritic Earth would be invalid for the Lu-Hf system. Herein we report the first, to our knowledge, high-precision Lu-Hf isotope analysis of meteorite crystalline zircon, a mineral that is resistant to metamorphism and has low Lu/Hf. We use the meteorite zircon data to define the Solar System initial (176)Hf/(177)Hf (0.279781 ± 0.000018) and further to identify pristine chondrites that contain no excess (176)Hf and accurately represent the Lu-Hf system of the bulk Earth ((176)Hf/(177)Hf = 0.282793 ± 0.000011; (176)Lu/(177)Hf = 0.0338 ± 0.0001). Our results provide firm evidence that the most primitive Hf in terrestrial zircon reflects the development of a chemically enriched silicate reservoir on Earth as far back as 4.5 billion years ago.

We report U-Pb, 87Rb-87Sr, 40Ar-39Ar, and 238U/235U isotopic data for paired ungrouped achondrites NWA 6704 and NWA 6693 that were derived from a highly oxidised parent body with broadly chondritic composition (Warren et al., 2013; Hibiya et al., 2019). Pb-isotopic ages derived from isochrons for multiple acid-leached pyroxene fractions are 4562.76 + 0.22/−0.30 Ma for NWA 6704 and 4562.63 + 0.29/−0.21 Ma for NWA 6693, calculated using 238U/235U ratio of 137.7784 ± 0.0097 measured in NWA 6704. The Rb-Sr mineral isochron age of 4543 ± 46 Ma (initial 87Sr/86Sr = 0.699013 ± 0.000055) is consistent with the Pb-isotopic age. Together with 187Re-187Os isochron age of 4576 ± 250 Ma for NWA 6704 (Hibiya et al., 2019), and 26Al-26Mg and 53Mn-53Cr ages calculated using the rapidly crystallized angrite D’Orbigny as a time anchor are also consistent with the Pb-isotopic age ( Sanborn et al., 2019 ), these data indicate that the parent rocks of NWA 6693 and NWA 6704 remained closed to migration of both lithophile and siderophile elements since crystallisation and initial cooling. The whole rock 40Ar-39Ar age of 4199 ± 32 Ma suggests a complete resetting of the K-Ar system approximately 360 Ma after crystallisation. A later event at ≤2.12 Ga partially reset the K-Ar system as shown by the low temperature heating steps. Both meteorites have high 87Rb/86Sr ratios (up to 7.0 in NWA 6693 pyroxene) and very radiogenic 87Sr/86Sr up to 1.15. Together with the absence of secondary disturbance in the Rb-Sr and U-Pb systems, this makes them suitable for cross-calibration of the isotopic chronometers. These meteorites are also promising candidates to serve as age reference samples for the early Solar System chronology, as an alternative or complement to angrites of the early generation (D’Orbigny, Sahara 99555) that are currently used for this purpose. Plagioclase in NWA 6704 has a sufficiently low Rb/Sr ratio to define precise initial 87Sr/86Sr of 0.698997 ± 0.000027, which corresponds to the time of separation of the parent body precursor material from the solar nebula of 1.5 ± 2.1 Ma. This value suggests that the parent asteroid accreted within 3.6 Ma after CAI formation, or before 4563.7 Ma using the CAI age of 4567.3 Ma (Connelly et al., 2012).

The African continent consists of several cratons rimmed by orogenic belts, which were mainly formed by collisions of the cratons during the Gondwana supercontinent assembly. Understanding of the timing and nature of the Pan-African Orogeny is essential to decipher the development of the African continent and Gondwana supercontinent. We carried out U–Pb dating and geochemical analyses of approximately 500 detrital monazites from the Nile, Niger, Congo, Zambezi and Orange Rivers. The U–Pb dating defined age peaks at ∼600 Ma for the Nile, ∼580 Ma for the Niger, ∼630, ∼610 and ∼550 Ma for the Congo, ∼500 Ma for the Zambezi and 1200–1000 Ma for the Orange. All but the Orange age peaks correspond to the period of the Pan-African Orogeny. In the Niger, Congo and Zambezi, the Pan-African age peaks are 20–40 Myr younger than those defined by the detrital zircons from the same rivers. In the Nile, however, the detrital monazite and zircon age peaks are contemporaneous. The geochemical analyses revealed significant variations in Eu/Eu∗, Gd/Lu and Th/U ratios among the detrital monazites, which can be attributed to elemental partitioning with the co-existing feldspars, garnet, and zircon/rutile, respectively. Considering that typical abundances of these minerals vary among igneous and metamorphic rocks, these geochemical data can be potentially linked to the genesis of the detrital monazites. The geochemical systematics, together with the discrepancy between the detrital monazite and zircon ages, suggest that the Pan-African monazite age peaks essentially reflect the timing of syn- to post-collisional metamorphic events in the source terranes, whereas the detrital zircons provide a more representative record of magmatic events. The results imply that the Pan-African Orogeny was promoted by pre- to syn-collisional felsic magmatism during the early stages and, 20–40 Myr later, it was dominated by syn- and/or post-collisional metamorphism.