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In situ apatite U-Pb petrochronology and Sr-Nd isotope geochemistry requires well-characterized and matrix-matched references materials (RMs), yet only a few suitable apatite RMs are currently available. To ameliorate this issue, we determined the U-Pb, Sm-Nd, and Sr isotopic and elemental compositions of a suite of prospective apatite RMs using isotope dilution (ID) TIMS and laser ablation (LA) ICP-MS. The two RMs, from Morocco (MRC-1) and Brazil (BRZ-1), are cm-sized and available in significant quantities. The U-Pb ID-TIMS data yield an isochron age of 153.3 ± 0.2 Ma for MRC-1. This age is consistent with laser ablation split stream ICP-MS (LASS) analyses that produce an isochron age of 152.7 ± 0.6 Ma. The weighted mean of ID-TIMS analyses for 143Nd/144Nd analyses is 0.512677 ± 3, for 147Sm/144Nd is 0.10923 ± 9, and for 87Sr/86Sr is 0.707691 ± 2. The range and mean of TIMS Sm-Nd isotopic data are reproducible by LA-ICP-MS, but laser ablation Sr data are consistently offset towards more radiogenic values. For BRZ-1 apatite, ID-TIMS U-Pb analyses are dispersed, but a subset of the data yields a coherent age intercept of 2078 ± 13 Ma. The vast majority of LASS spot transects across the apatite produce an isochron that define a younger age of 2038 ± 14 Ma. We interpret this as incorporation of cryptic, younger altered domains within BRZ-1. Discordant U-Pb spot analyses are associated with chemically distinct cracks, likely a result of fluid infiltration. The weighted means of ID-TIMS analyses of BRZ-1 yield 143Nd/144Nd = 0.510989 ± 5, 147Sm/144Nd = 0.10152 ± 8, and 87Sr/86Sr = 0.709188 ± 3. The distribution of Nd isotopic compositions of this RM measured by LA-MC-ICP-MS analyses are comparable to TIMS analyses. By contrast, 87Sr/86Sr measurements by LA-ICP-MS are inaccurate and exhibit large uncertainties, but this RM can be useful for empirically correcting in situ 87Sr/86Sr measurements. The data indicate that MRC-1 apatite may serve well as a U-Pb, Sm-Nd, and Sr RM, whereas BRZ-1 apatite has the most potential as a Sm-Nd RM. These potential RMs provide new benchmarks for in situ apatite chemical analyses and inter-laboratory calibrations. 

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. 

Abstract The bimodal Wichita igneous province (WIP) represents the only exposed Ediacaran to Cambrian anorogenic magmatic assemblage present along the buried southern margin of Laurentia and was emplaced during rifting in the Southern Oklahoma Aulacogen prior to Cambrian opening of the southern Iapetus Ocean. Here, we establish the first high-precision U-Pb zircon geochronological framework for the province. Weighted mean 206Pb/238U dates from mafic and felsic rocks in the Wichita Mountains indicate emplacement in a narrow time frame from 532.49 ± 0.12 Ma to 530.23 ± 0.14 Ma. Rhyolite lavas in the Arbuckle Mountains farther east yield weighted mean 206Pb/238U dates of 539.20 ± 0.15 Ma and 539.46 ± 0.13 Ma. These dates for the WIP indicate that magmatism in the Southern Oklahoma Aulacogen postdated the ca. 540 Ma rift-drift transition along the Appalachian margin to the east. Whole-rock trace-element and isotopic geochemistry, supplemented by trace elements in zircon, tracks the evolution of magma sources during WIP petrogenesis. These data indicate that initial melting and assimilation of subcontinental mantle lithosphere by an uprising mantle plume were followed by increasing involvement of asthenospheric melts with time. We suggest that upwelling of this plume in the area of the Southern Oklahoma Aulacogen triggered an inboard jump of the spreading center active along the eastern margin of Laurentia, which led to separation of the Precordillera terrane (now located in Argentina) from the Ouachita embayment present in the southern Laurentian margin. 

Abstract We present chemostratigraphy, biostratigraphy, and geochronology from a succession that spans the Ediacaran-Cambrian boundary in Sonora, Mexico. A sandy hematite-rich dolostone bed, which occurs 20 m above carbonates that record the nadir of the basal Cambrian carbon isotope excursion within the La Ciénega Formation, yielded a maximum depositional age of 539.40 ± 0.23 Ma using U-Pb chemical abrasion–isotope dilution–thermal ionization mass spectrometry on a population of sharply faceted volcanic zircon crystals. This bed, interpreted to contain reworked tuffaceous material, is above the last occurrences of late Ediacaran body fossils and below the first occurrence of the Cambrian trace fossil Treptichnus pedum, and so the age calibrates key markers of the Ediacaran-Cambrian boundary. The temporal coincidence of rift-related flood basalt volcanism in southern Laurentia (>250,000 km3 of basalt), a negative carbon isotope excursion, and biological turnover is consistent with a mechanistic link between the eruption of a large igneous province and end-Ediacaran extinction.