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Crystallization of the lunar magma ocean yielded a chemically unique liquid residuum named KREEP. This component is expressed as a large patch on the near side of the Moon and a possible smaller patch in the northwest portion of the Moon’s South Pole-Aitken basin on the far side. Thermal models estimate that the crystallization of the lunar magma ocean (LMO) could have spanned from 10 and 200 My, while studies of radioactive decay systems have yielded inconsistent ages for the completion of LMO crystallization covering over 160 My. Here, we show that the Moon achieved >99% crystallization at 4,429 ± 76 Ma, indicating a lunar formation age of ~4,450 Ma or possibly older. Using the¹⁷⁶Lu–¹⁷⁶Hf decay system (t_1/2= 37 Gy), we found that the initial¹⁷⁶Hf/¹⁷⁷Hf ratios of lunar zircons with varied U–Pb ages are consistent with their crystallization from a KREEP-rich reservoir with a consistently low¹⁷⁶Lu/¹⁷⁷Hf ratio of 0.0167 that emerged ~140 My after solar system formation. The previously proposed younger model age of ~4.33 Ga for the source of mare basalts (240 My after solar system formation) might reflect the timing of a large impact. Our results demonstrate that lunar magma ocean crystallization took place while the Moon was still battered by planetary embryos and planetesimals leftover from the main stage of planetary accretion. The study of Lu–Hf model ages for samples brought back from the South Pole-Aitken basin will help to assess the lateral continuity of KREEP and further understand its significance in the early history of the Moon. 

The Moon has had a complex history, with evidence of its primary crust formation obscured by later impacts. Existing U-Pb dates of >500 zircons from several locations on the lunar nearside reveal a pronounced age peak at 4.33 billion years (Ga), suggesting a major, potentially global magmatic event. However, the precision of existing geochronology is insufficient to determine whether this peak represents a brief event or a more protracted period of magmatism occurring over tens of millions of years. To improve the temporal resolution, we have analyzed Apollo 14, 15, and 17 zircons that were previously dated by ion microprobe at ~4.33 Ga using isotope dilution thermal ionization mass spectrometry. Concordant dates with sub-million-year uncertainty span ~4 million years from 4.338 to 4.334 Ga. Combined with Hf isotopic ratios and trace element concentrations, the data suggest zircon formation in a large impact melt sheet, possibly linked to the South Pole–Aitken basin. 

Abstract The thermal and compositional structure of arcs influence magmatic differentiation and lower-crustal foundering, two key processes impacting the evolution of the continental crust. Although many studies have proposed time scales of lithospheric recycling based on convective downwelling calculations, these models depend on the composition, density (ρ), and thermal structure of the lower crust and mantle, which are difficult to quantify in active continental arcs. Here, we constrained these properties for the Andean Northern Volcanic Zone using direct petrologic observations from a unique suite of lower-crust and mantle xenoliths from Mercaderes, Colombia. Chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb dates for zircons within the host tuff indicate the xenoliths erupted no earlier than 238 (±19) ka and thus capture a recent snapshot of the arc and subarc mantle. Equilibrium pressure-temperature (P-T) estimates for 81 xenoliths define three distinct thermal domains, interpreted as (1) a steep conductive geothermal gradient in the lower arc crust; (2) a convecting mantle wedge; and (3) cooled mantle in proximity to the subducting slab. Our results indicate the presence of an ~10–14-km-thick, high-density lithospheric root that is ~0.1 g/cm3 denser than the underlying mantle. Unlike records from exhumed paleoarcs, Rayleigh-Taylor instability calculations using our P-T-ρ constraints are unrealistically short for the northern Andes. We suggest the presence of partial melts in this hot arc root as a potential source of buoyancy preventing or significantly slowing down foundering. 

Abstract Silicic magmas within large igneous provinces (LIPs) are understudied relative to volumetrically dominant mafic magmas despite their prevalence and possible contribution to LIP-induced environmental degradation. In the 66 Ma Deccan LIP (India), evolved magmatism is documented, but its geographic distribution, duration, and significance remain poorly understood. Zircons deposited in weathered Deccan lava flow tops (“red boles”) offer a means of indirectly studying potentially widespread, silicic, explosive volcanism spanning the entire period of flood basalt eruptions. We explored this record through analysis of trace elements and Hf isotopes in zircon crystals previously dated by U–Pb geochronology. Our results show that zircon populations within individual red boles fingerprint distinct volcanic sources that likely developed in an intraplate setting on cratonic Indian lithosphere. However, our red bole zircon geochemical and isotopic characteristics do not match those from previously studied silicic magmatic centers, indicating that they must derive from yet undiscovered or understudied volcanic centers associated with the Deccan LIP. 

Abstract A tephra-rich cherty-clayey Famennian succession within the major Brzeźnica olistostrome in the Bardo Mountains, Central Sudetes, SW Poland, preserves a record of the lost ocean later incorporated into the Variscan orogenic belt. Fluctuating but mostly oligotrophic regimes and low primary production levels were influenced by weak up-welling below the perennial oxygen minimum zone, which controlled the interplay between biosiliceous and siliciclastic deposition in the oceanic basin, with episodic oxygen deficiency. The Hangenberg Black Shale has been identified in this oceanic setting based on its characteristics described worldwide (including mercury enrichments). A tectonic uplift of the sediment source area near the Devonian-Carboniferous boundary, recorded in the distinguishing provenance signal of old continental crust, was paired with a global transgression, anoxia, and volcanic episode in an interglacial interval. Assuming paleogeographic affinity with the Bavarian facies of the Saxothuringian terrane, we interpret the allochthonous sediments as part of an accretionary prism that was gravitationally redeposited into the late orogenic basin in front of advancing Variscan nappes. The oceanic basin parental to the Bardo pelagic succession is therefore thought to represent a tract of the waning Saxothuringian Ocean in the Peri-Gondwanan paleogeographic domain that was eventually subducted beneath the Brunovistulian margin of Laurussia. The sediments of the Bardo Ocean basin also include a distal record of Famennian explosive volcanic activity that was likely related to a continental magmatic arc whose remnants are preserved as the Vrbno Group of the East Sudetes. 

Abstract U-Pb geochronology by isotope dilution–thermal ionization mass spectrometry (ID-TIMS) has the potential to be the most precise and accurate of the deep time chronometers, especially when applied to high-U minerals such as zircon. Continued analytical improvements have made this technique capable of regularly achieving better than 0.1% precision and accuracy of dates from commonly occurring high-U minerals across a wide range of geological ages and settings. To help maximize the long-term utility of published results, we present and discuss some recommendations for reporting ID-TIMS U-Pb geochronological data and associated metadata in accordance with accepted principles of data management. Further, given that the accuracy of reported ages typically depends on the interpretation applied to a set of individual dates, we discuss strategies for data interpretation. We anticipate that this paper will serve as an instructive guide for geologists who are publishing ID-TIMS U-Pb data, for laboratories generating the data, the wider geoscience community who use such data, and also editors of journals who wish to be informed about community standards. Combined, our recommendations should increase the utility, veracity, versatility, and “half-life” of ID-TIMS U-Pb geochronological data.