A calcite reference material for LA-ICP-MS U-Pb geochronology

U-Pb dating of calcite is an emerging but rapidly growing field of application in geochronology with great potential to inform problems in landscape, basin and mountain belt evolution, through age determination of diagenetic cements, vein mineralisation and geological formations difficult to date otherwise. In this brief, we present isotope dilution U-Pb isotope measurements on a sample of calcite (WC-1) that has been and will continue to be used as a reference material for in-situ U-Pb Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) dating, and which is suitable to be distributed to the geochronological community. We present in-situ measurements using LA-ICP-MS to demonstrate the suitability of WC-1 for use as a U-Pb dating reference material, in spite of it not being isotopically homogeneous. The WC-1 calcite sample is 254.4 ± 6.4 Ma old, and comprised of 85 to 98% radiogenic lead. It presents a suitable reference material that can facilitate dating of calcite ranging in age from Precambrian to late Neogene age

Formation of Archean continental crust constrained by boron isotopes

The continental crust grew and matured compositionally during the Palaeo- to Neoarchean through the addition of juvenile tonalite-trondhjemite-granodiorite (TTG) crust. This change has been linked to the start of global plate tectonics, following the general interpretation that TTGs represent ancient analogues of arc magmas. To test this, we analysed B concentrations and isotope compositions in 3.8-2.8 Ga TTGs from different Archean terranes. The 11B/10B values and B concentrations of the TTGs, and their correlation with Zr/Hf, indicate differentiation from a common B-poor mafic source that did not undergo addition of B from seawater or seawater-altered rocks. The TTGs thus do not resemble magmatic rocks from active margins, which clearly reflect such B addition to their source. The B- and 11B-poor nature of TTGs indicates that modern style subduction may not have been a dominant process in the formation of juvenile continental crust before 2.8 Ga

Deep anoxic aquifers could act as sinks for uranium through microbial-assisted mineral trapping

Uptake of uranium (U) by secondary minerals, such as carbonates and iron (Fe)-sulfides, that occur ubiquitously on Earth, may be substantial in deep anoxic environments compared to surficial settings due to different environment-specific conditions. Yet, knowledge of U reductive removal pathways and related fractionation between 238U and 235U isotopes in deep anoxic groundwater systems remain elusive. Here we show bacteria-driven degradation of organic constituents that influences formation of sulfidic species facilitating reduction of geochemically mobile U(VI) with subsequent trapping of U(IV) by calcite and Fe-sulfides. The isotopic signatures recorded for U and Ca in fracture water and calcite samples provide additional insights on U(VI) reduction behaviour and calcite growth rate. The removal efficiency of U from groundwater reaching 75% in borehole sections in fractured granite, and selective U accumulation in secondary minerals in exceedingly U-deficient groundwater shows the potential of these widespread mineralogical sinks for U in deep anoxic environments

Japan Trench event stratigraphy: First results from IODP giant piston coring in a deep-sea trench to advance subduction zone paleoseismology

International audienceThe International Ocean Discovery Program (IODP) Expedition 386, Japan Trench Paleoseismology, represents the first utilization of giant piston coring (GPC) within scientific ocean research drilling. This allowed for a Mission Specific Platform (MSP) multi-site, multi-hole, shallow subsurface coring in an ultra-deep water subduction zone trench. The primary objective of the expedition was to investigate the concept of submarine paleoseismology in the Japan Trench, which involves studying long-term records of deposits in the deep sea that can provide insights into past earthquake events. In this paper, we compile and interpret initial shipboard data and results to (1) establish first-order event stratigraphic correlation of thick event beds (> 50 cm in thickness) between sites, (2) test previously published event-stratigraphic predictions of earthquake-related event deposits as proposed based on high-resolution hydro-acoustic subbottom profiler (SBP) data, and (3) derive SBP-scale event deposits age estimates to (4) discuss the advantages and limitations of giant piston coring for scientific drilling operations and the potential of new event stratigraphy results for advancing submarine paleoseismology.The findings of the study identified a total of 77 SBP-scale event beds across 15 sites along a trench-parallel transect spanning over 600 km. These event beds exhibit clear expressions in SBP data, with approximately 49 % matching precisely with SBP units previously identified by Kioka et al. (2019a). For the remaining 51 % of SBP-scale event beds, thin, acoustically-transparent bodies were observed between high-amplitude horizons, for which SBP-based seismic interpretation alone would not be definitive. Consequently, the study concluded that the SBP-scale event-stratigraphy observed in IODP 386 cores validates the event-bed mapping conducted by Kioka et al. (2019a) and improves SBP interpretation for event beds in the 0.5 to 1 m thickness range.The initial age constraints obtained from shipboard radiolarian biostratigraphy enable us to provide rough estimates of event ages by linearly interpolating between previously dated events occurring less than 2000 years ago and a datum around 11,000 years ago reported in four boreholes from trench basins in the Southern, Central, and Northern Japan Trench. Inter-site stratigraphic correlation reveals distinct SBP-scale event stratigraphies for the trench segments located to the north and south of the structurally complex “boundary area” at approximately 39.3–39.4°N, which is hypothesized to potentially act as a persistent rupture barrier for megathrust earthquakes. We observe more frequent but thinner event deposits in the Southern and Central Japan Trench, and fewer but thicker event beds in the Northern Japan Trench. This spatial variation may be related to the different seismogenic behavior of the various asperities along the Japan Trench megathrust and/or to differences in the response of slope sediments to earthquake shaking. However, here-presented investigations at the SBP-scale level are deemed too simplistic for robust application of the “submarine paleoseismology” approach. The extensive and high-quality dataset from IODP GPC, coupled with the encouraging initial correlation results presented here, leads us to hypothesize that further detailed studies can identify and characterize event deposition dynamics at the micro-facies level, refine sediment provenance, and constrain precise event ages necessary for evaluating synchronicity in paleoseismological interpretation. These studies will also enable robust exploration of along-strike correlations or variations, facilitating the extraction of paleo-earthquake signals from Japan Trench event stratigraphies