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  1. Abstract

    Detrital zircons from the Jack Hills are the dominant source of Hadean (pre-4000 Ma) terrestrial material available for study today. Values of δ18O in many of these zircons (6.0 to 7.5‰ are above the mantle-equilibrated value. For two decades, these mildly elevated values have been the primary evidence that protoliths of the zircon-forming magmas interacted at low temperature with liquid water before burial and melting, implying that the surface of Earth cooled quickly after core and moon formation, and that habitable conditions for life existed within 250 Myr of the formation of Earth, over 800 Myr before the oldest generally accepted microfossils. These conclusions are based on oxygen isotope analyses of zircon domains with well-defined growth zoning and nearly concordant U-Pb ages within zircon grains with low magnetic susceptibility, which are further inferred to be unaltered by various tests. However, no studies of Jack Hills zircons have directly correlated oxygen isotope ratios and radiation damage, which facilitates alteration in zircon. Several previous studies have selected zircons that show radiation damaged, discordant and/or hydrous domains, and have shown that such altered material is not reliable as a record of igneous composition. In contrast, this study targeted zircons that are interpreted to pristine and not altered, and demonstrates the importance of testing zircons for radiation damage and alteration as part of any geochemical study, regardless of age. This study expands on existing data, and presents the first comprehensive evaluation of δ18O, OH/O, CL imaging, U-Pb concordance and radiation-damage state within Jack Hills zircons. A total of 115 Hadean zircon grains in this study have water contents similar to nominally anhydrous standard reference zircons and are interpreted as pristine. In situ Raman data for band broadening correlated with δ18O analyses document low levels of radiation damage, indicating significant annealing. The present-day effective doses (Deff) are uniformly less than the first percolation point (dose where damage domains, that are isolated at lower damage state, overlap to form a continuous pathway through the crystal, ~2×1015 α-decays/mg, Ewing et al., 2003) and most zircons have Deff<1×1015 α-decays/mg. Modeling of representative alpha-recoil damage and annealing histories indicates that most zircons in this study have remained below the Deff of the first percolation point throughout their history. The δ18O values for these primary zircons include many that are higher than would be equilibrated with the mantle at magmatic temperatures and average 6.32 ± 1.3‰ in the Hadean and 6.26 ± 1.6‰ in the Archean. There is no correlation in our suite of pristine Hadean zircons between δ18O and OH/O, Deff, age, or U-Pb age-concordance. These carefully documented Hadean-age zircons possess low amounts of radiation damage in domains sampled by δ18O analysis, are water-poor. The mildly elevated δ18O values are a primary-magmatic geochemical signature. These results strengthen the conclusion that mildly elevated-δ18O magmas existed during the Hadean, supporting the hypothesis that oceans and a habitable Earth existed before 4300 Ma.

     
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  5. Abstract A better characterization of subsurface processes in hydrothermal systems is key to a deeper understanding of fluid-rock interaction and ore-forming mechanisms. Vent systems in oceanic crust close to subduction zones, like at Brothers volcano and in the eastern Manus basin, are known to be especially ore rich. We measured B concentrations and isotope ratios of unaltered and altered lava that were recovered from drilling sites at Brothers volcano and Snowcap (eastern Manus basin) to test their sensitivity for changing alteration conditions with depth. In addition, for Brothers volcano, quartz-water oxygen isotope thermometry was used to constrain variations in alteration temperature with depth. All altered rocks are depleted in B compared to unaltered rocks and point to interaction with a high-temperature (>150°C) hydrothermal fluid. The δ11B values of altered rocks are variable, from slightly lower to significantly higher than those of unaltered rocks. For Brothers volcano, at the Upper Cone, we suggest a gradual evolution from a fluid- to a more rock-dominated system with increasing depth. In contrast, the downhole variations of δ11B at Snowcap as well as δ11B and δ18O variations at the NW Caldera (Site U1530) of Brothers volcano are suggested to indicate changes in water-rock ratios and, in the latter case, also temperature, with depth due to permeability contrasts between different lithology and alteration type boundaries. Furthermore, δ11B values from the NW Caldera (Site U1527) might point to a structural impact on the fluid pathway. These differences in the subseafloor fluid flow regime, which ranges from more pervasive and fluid-controlled to stronger and controlled by lithological and structural features, have significant influence on alteration conditions and may also impact metal precipitation within the sea floor. 
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  6. Abstract Subannual climate reconstructions of the Holocene are rare despite the ability of such records to provide a better understanding of the underlying factors that drive subannual climate variability. We used specialized confocal laser fluorescent microscope imaging and automated secondary ion mass spectrometry microanalysis to resolve a seasonal oxygen isotope (δ18O) record of a late Holocene–aged (2.7–2.1 ka) speleothem from mid-continental North America. We did this by measuring intra-band δ18O variability (Δ18O) within 117 annual bands over a 600 yr span of the late Holocene. We interpret a change in Δ18O values after 2.4 ± 0.1 ka to reflect an increase in the amount of winter precipitation. Our study produced direct measurements of past seasonality, offers new insights into shifting seasonal precipitation patterns that occurred during the late Holocene in central North America, and adds a new tool for understanding the complex precipitation and temperature histories of this region. 
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  7. The disappearance of mass-independent sulfur isotope fractionation (S-MIF) within the c. 2.3-billion-year-old (Ga) Rooihoogte Formation has been heralded as a chemostratigraphic marker of permanent atmospheric oxygenation. Reports of younger S-MIF, however, question this narrative, leaving significant uncertainties surrounding the timing, tempo, and trajectory of Earth’s oxygenation. Leveraging a new bulk quadruple S-isotope record, we return to the South African Transvaal Basin in search of support for supposed oscillations in atmospheric oxygen beyond 2.3 Ga. Here, as expected, within the Rooihoogte Formation, our data capture a collapse in Δ 3× S values and a shift from Archean-like Δ 36 S/Δ 33 S slopes to their mass-dependent counterparts. Importantly, the interrogation of a Δ 33 S-exotic grain reveals extreme spatial variability, whereby atypically large Δ 33 S values are separated from more typical Paleoproterozoic values by a subtle grain-housed siderophile-enriched band. This isotopic juxtaposition signals the coexistence of two sulfur pools that were able to escape diagenetic homogenization. These large Δ 33 S values require an active photochemical sulfur source, fingerprinting atmospheric S-MIF production after its documented cessation elsewhere at ∼2.4 Ga. By contrast, the Δ 33 S monotony observed in overlying Timeball Hill Formation, with muted Δ 33 S values (<0.3‰) and predominantly mass-dependent Δ 36 S/Δ 33 S systematics, remains in stark contrast to recent reports of pronounced S-MIF within proximal formational equivalents. If reflective of atmospheric processes, these observed kilometer-scale discrepancies disclose heterogenous S-MIF delivery to the Transvaal Basin and/or poorly resolved fleeting returns to S-MIF production. Rigorous bulk and grain-scale analytical campaigns remain paramount to refine our understanding of Earth’s oxygenation and substantiate claims of post-2.3 Ga oscillations in atmospheric oxygen. 
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