The nature of Earth's first crust and the processes that formed it are poorly constrained due to limited exposures of >3.7 billion-year-old (Ga) rocks. Here we report the discovery of a new Eoarchean terrane named the Muzidian gneiss complex in the Yangtze craton of central China where rocks as old as 3.81 Ga are preserved. In this study, we characterized six samples (including five TTGs and an amphibolite) through zircon U-Pb age, Hf isotope and bulk-rock 146,147Sm-142,143Nd isotope compositions. The subchondritic zircon initial Hf isotope compositions and the negative mu 142Nd values for the 3.81 - 3.65 Ga samples reveal that these rocks were most likely reworked from >4.3 Ga mafic crust. The 2.5 - 2.4 Ga rocks in the same complex are the youngest felsic rocks on Earth with deficits in 142Nd, highlighting the long-lived role of Hadean crustal components in the building of a stable continent. The 147Sm-143Nd isotopic systematics of these rocks are disturbed. The currently available data for global Eoarchean rocks suggest two distinct lineages for > 3.6 Ga Eoarchean crustal blocks, one produced by reworking of Hadean mafic crust with the isotopic signature of coupled negative epsilon Hf(t) and mu 142Nd, and the other by melting of incompatible-element-depleted mantle sources residual to Hadean crust formation characterized by positive mu 142Nd and near chondritic epsilon Hf(t). The Eoarchean samples of the Muzidian gneiss complex in the present study, along with >3.6 Ga rocks in the Acasta, Napier and Nuvvuagittuq regions imply a Hadean crustal source distinct from the products expected for magma ocean crystallization.(c) 2023 Elsevier B.V. All rights reserved.

The Tuli Basin represents one of the larger erosional remnants of the rift-related sequences of the Karoo continental flood basalts (CFB) of southern Africa. We present previously unpublished bulk-rock major and trace element analyses for 787 high-Ti picrites and basalts, including both surface and drill core samples, as well as SrNd-Hf-Pb-Os isotope compositions and rare earth element (REE) abundances for selected samples. Due to the identification of previously unrecognised basaltic sub-types, a new classification scheme for the high-Ti basalts and picrites of the Tuli Basin is proposed where they are divided into three types based on Zr/Nb and Ce/Y ratios: HZN-1, HZN-2, and LZN. The HZN-1 picrites/basalts occur at the base of the lava sequence and are overlain by the HZN-2 basalts, which, in turn, are overlain by the LZN basalts. We show that crustal assimilation is not the dominant process in influencing basalt and picrite petrogenesis. Using the Magma Chamber Simulator (MCS) program, the HZN-1 basalts are shown to have evolved over a range of pressure, whereas the HZN-2 and LZN basalts evolved at shallow depths. The HZN-1 and LZN picrites are highly enriched in LILE, HFSE, and LREE and are characterised by variable Zr/Y with high values at the base of the lava sequence that gradually decrease upward. This upwards decrease is shown to be related to increasing degrees of partial melting in the source with time. Compared to the picrites, the basalts have relatively low Zr/Y, indicating that evolution of basalt from a picritic parent began only after achieving higher degrees of partial melting in the source. The HZN-1 picrites and basalts have relatively radiogenic 87Sr/86Sri (0.7048 to 0.7059), low ENdi (-7.2 to -9.8), low EHfi (-8.9 to -14.2), and variable gamma Osi (-14.0 to +10.8), and are clearly distinguishable from HZN-2 and LZN basalts that have less radiogenic (87Sr/86Sr)i (-0.7045), higher ENdi (-1.2 to -4.5), EHfi (-0.1), and gamma Osi (-1.6 and +3.9). We show that the mantle source of southern African Karoo high-Ti basalts comprises at least three components with a dominant sub-continental lithospheric mantle (SCLM) signature. Each basalt type is shown to represent a temporally distinct episode of mantle melting and each of the three mantle components are progressively exhausted in the source with time. The possible presence of a sub-lithospheric component is only significantly reflected in the composition of the LZN basalts and picrites; the final phase of eruption.

Temporal variations in lava chemistry at active submarine volcanoes are difficult to decipher due to the challenges of dating their eruptions. Here, we use high-precision measurements of 226Ra-230Th disequilibria in basalts from Kama`ehuakanaloa (formerly LiVihi) to estimate model ages for recent eruptions of this submarine Hawaiian pre-shield volcano. The ages range from ca. 0 to 2300 yr (excluding two much older samples) with at least five eruptions in the past similar to 150 yr. Two snapshots of the magmatic evolution of Kama`ehuakanaloa (or "Kama`ehu") are revealed. First, a long-term transition from alkalic to tholeiitic volcanism was nearly complete by ca. 2 ka. Second, a systematic short-term fluctuation in ratios of incompatible elements (e.g., Th/Yb) for summit lavas occurred on a time scale of similar to 1200 yr. This is much longer than the similar to 200-yr-long historical cycle in lava chemistry at the neighboring subaerial volcano, Kilauea. The slower pace of the variation in lava chemistry at Kama`ehu is most likely controlled by sluggish mantle upwelling on the margin of the Hawaiian plume.

Isotope dilution W whole-rock data, coupled with data for other trace elements in 42 peridotite xenoliths from Tariat (Mongolia), Yangyuan and Hannuoba (China), and Letlhakane (Botswana) are used to constrain the W abundance and behavior in the continental lithospheric mantle. Tungsten concentrations are >11 ng g-1, even in the most depleted peridotites, and higher than the W abundance estimate of the bulk silicate Earth (BSE) in all but one sample. Combined W-Th-U systematics reveal fractionation of W from Th and U with increasing degree of melt depletion, while incompatible element-enriched samples display no such fractionation. Integrated melt reactions and degrees of melt depletion are estimated based on either residual Th or residual Yb abundances for each sample, relative to a BSE precursor. This allows modelling of different partial melting scenarios that consider maximized trace element retention (Th-based model; where measured Th contents are decoupled from Al2O3 contents) or correction for metasomatic trace element contributions (Yb-based model; where measured Yb contents are well-coupled with Al2O3 contents). Modelled residual W concentrations reveal W excesses of 11-19 ng g-1, even when correcting for metasomatic W contributions. Combined with available in situ trace element mineral data for the rock-forming silicates and spinel, which all display W abundances much higher than ex-pected, these findings suggest that either (i) W is less incompatible than previously thought, or that (ii) the peridotites experienced W metasomatism after the last melt depletion event with subsequent re-equilibration. The latter is unlikely, as similar geochemical patterns are observed for oceanic lithospheric mantle based on published data, requiring such W metasomatism to be a global phenomenon. We thus conclude that W is not as incompatible as Th and U during upper mantle melting and potential W-Th-U fractionation mechanisms are examined to explain near-constant W/Th and W/U in mantle melts. We propose revised W concentrations for the BSE of 27 +/- 16 ng g-1 to 31 +/- 14 ng g-1 and for the depleted mantle of 22 +/- 13 ng g- 1.

Tungsten and helium isotope ratios in lavas derived from deeply rooted mantle plumes are tracers of lower mantle compositional heterogeneity or core-mantle exchange. We measured the tungsten isotopic compositions of lavas with exceptionally high-He-3/He-4 ratios that erupted above the head of the Iceland plume on Baffin Island. These lavas have W-182/W-184 ratios that are indistinguishable from the convecting upper mantle, unlike younger lavas in Iceland that have lower W-182/W-184 ratios. This implies that only the Iceland plume tail was infused with low W-182/W-184 material, likely from the core. If high-He-3/He-4 helium also comes from the core, then diffusion across the core-mantle boundary may stratify plume-source mantle domains, with elevated He-3/He-4 travelling farther into the lower mantle than W-182/W-184 anomalies. Over Earth history, tungsten diffusion from the core can explain the decline of W-182/W-184 in the convecting mantle. We speculate that the uneven pace of this decline corresponds with evolving lower mantle dynamics.

Initial analyses of samples collected from two locations on the asteroid Ryugu indicated that the mineralogical, chemical, and isotopic characteristics of the Ryugu samples show similarities to carbonaceous chondrites, particularly the Ivuna-type (CI) group. In this study, we analysed a composite sample of four bulk Ryugu samples (A0106, A0106-A0107, C0107, and C0108) collected from both sampling locations that were combined in order to determine its mass independent Mo isotopic composition and reveal contributions from diverse nucleosynthetic sources. The epsilon Mo-94 and epsilon Mo-95 values for the Ryugu sample are characterised by the carbonaceous chondrite (CC)-type, which is consistent with the nucleosynthetic isotope compositions observed for other elements (Cr, Ti, Fe, and Zn). The Ryugu composite sample, however, is characterised by greater s-process depletion of Mo isotopes compared with any known bulk carbonaceous chondrite, even including CI chondrites. The observed Mo isotopic signature in the Ryugu composite was most likely caused by either incomplete digestion of s-process-rich presolar SiC, or biased sampling of materials enriched in aqueously-formed secondary minerals characterised by s-process-poor Mo isotopes, resulting from the physicochemical separation between s-process-rich presolar grains and a complementary s-process-poor aqueous fluid in the Ryugu parent body.

The Pampean flat slab in central Chile and Argentina is characterized by the inland migration and subsequent cessation of arc volcanism since the mid-Miocene. Slab flattening also affects the distribution and number of intermediate-depth earthquakes and the evolution of the overlying continental thermal structure. In this study, we combine thermal-mechanical models with petrological models to examine temporal changes in pressure, temperature, and composition during flat-slab subduction and estimate water carrying capacity, predicted melt distributions and predicted changes in melt composition. Model results indicate that the present-day flattened Nazca plate carries water to similar to 700 km inland from the trench and could cause flux melting if the material above the slab remains fertile. Observed slab seismicity matches areas where hydrated materials have similar to>3 wt% H2O in the oceanic crust and mantle lithosphere. Seismicity increases as slab water carrying capacity decreases (slab dehydration). As P-T conditions and compositions of the rock trapped above the slab change during slab flattening, flux melting switches from a peridotite-dominated early phase to a combined mid-ocean ridge basalt/eclogite and peridotite melting at similar to 8 Ma. The results provide broad consistency with known earthquake distributions, seismic velocities, and observed temporal and spatial changes in volcanic patterns above the Pampean flat slab and point toward the role of melt depletion in the decrease and ultimate cessation of arc volcanism in this region.