Pb-Pb Ages Research Articles

MAGMATIC SOURCES OF THE EVOLVING CONTINENTAL TECTONOSPHERE IN INDIA: GENERATION OF ALKALINE IGNEOUS COMPLEXES WITH CARBONATITES IN THE SAMALPATTI (SOUTHERN INDIA) AND AMBA DONGAR (WESTERN INDIA) MASSIFS

Neoproterozoic Samalpatti and Cretaceous-Paleogene Amba Dongar massifs are considered to evaluate the change of sources of carbonatite magmatism in evolving tectonosphere of India. The inherited character of Ba and Sr distribution components is traced in heterogeneous carbonatites of India, whereas the independent trends in Ba and Sr enrichment are traced in carbonatites of North Asia. It is inferred that in the process of activity in the tectonosphere of the Indian subcontinent ~800 Ma ago, carbonatite melts were generated in the mantle whose photolith has an age of 4.26 Ga. This protomantle reservoir was remarkably different from the primordial mantle reservoir in the solidified magma ocean in terms of low initial μ and high initial Nd isotope ratio. After the Indian subcontinent had separated from Gondwana 130–100 Ma ago and began to join Asia ~66 Ma ago, ELMU source was activiated in the tectonosphere. The Amba Dongar carbonatites might have been derived from the 4.40 Ga mantle protoliths. Silicate melts of the Deccan LIP were simultaneously generated from the protoliths with Pb-Pb age of ~2 Ga.

Rb-Sr isotope systematics of the oldest crustal rock in the solar system

The RbSr isotopic system is an important cosmochemical tool for constraining the evolution of moderately volatile (Rb) and refractory (Sr) elements in the earliest Solar System. Presented here are RbSr isotopic systematics of twenty-two fractions of the evolved achondrite Erg Chech 002, the oldest crustal rock yet identified in the Solar System. Although the data show a large amount of scatter, likely due to terrestrial alteration, an errorchron yields an age within uncertainties of published PbPb ages. Initial 87Sr/86Sr ratios of both the errorchron and of individual mineral fractions show that Erg Chech 002 formed from materials evolving with a typical CI-chondrite nebular Rb/Sr ratio and that any Rb depletion on its parent body must have occurred almost simultaneously with the crystallization of the meteorite. Additionally, LuHf systematics of bulk Erg Chech 002 overlap with the chondritic range. Other early-formed parent bodies similarly show initial 87Sr/86Sr ratios close to a CI-chondrite evolution line. Despite nucleosynthetic variations of 84Sr/86Sr being evident between inner and outer Solar System materials, no variations are detectable in 87Sr/86Sr for Erg Chech 002 or other early-formed parent bodies. Although the initial 87Sr/86Sr of Erg Chech 002 overlaps with that of all other early-accreted bodies, the initial 87Sr/86Sr of asteroid 4 Vesta (BABI) remains the most precise and relevant initial value for the bulk silicate Earth.

A 4,565-My-old record of the solar nebula field

Magnetic fields in protoplanetary disks are thought to play a prominent role in the formation of planetary bodies. Acting upon turbulence and angular momentum transport, they may influence the motion of solids and accretion onto the central star. By searching for the record of the solar nebula field preserved in meteorites, we aim to characterize the strength of a disk field with a spatial and temporal resolution far superior to observations of extrasolar disks. Here, we present a rock magnetic and paleomagnetic study of the andesite meteorite Erg Chech 002 (EC002). This meteorite contains submicron iron grains, expected to be very reliable magnetic recorders, and carries a stable, high-coercivity magnetization. After ruling out potential sources of magnetic contamination, we show that EC002 most likely carries an ancient thermoremanent magnetization acquired upon cooling on its parent body. Using the U-corrected Pb-Pb age of the meteorite's pyroxene as a proxy for the timing of magnetization acquisition, we estimate that EC002 recorded a field of 60 ± 18 µT at a distance of ~2 to 3 astronomical units, 2.0 ± 0.3 My after the formation of calcium-aluminum-rich inclusions. This record can only be explained if EC002 was magnetized by the field prevalent in the solar nebula. This makes EC002's record, particularly well resolved in time and space, one of the two earliest records of the solar nebula field. Such a field intensity is consistent with stellar accretion rates observed in extrasolar protoplanetary disks.

Disparate Pb-isotopic ages of silicate and phosphate minerals in the diabasic angrite Northwest Africa 12320

Pb-isotopic dating of the recently discovered diabasic angrite Northwest Africa (NWA) 12320 yielded ages of 4564.17 ± 0.50 Ma and 4562.82 ± 0.37 Ma for acid-insoluble and soluble minerals respectively. These ages are calculated using measured 238U/235U ratios of soluble and insoluble minerals, thereby excluding the possibility that the apparent age difference is caused by internal U-isotopic variations. The age of the insoluble minerals is indistinguishable from published Pb isotopic ages of other medium-grained angrites SAH 99555 and D'Orbigny. The age of acid-soluble minerals is distinctly younger, with a 1.35 ± 0.62 Ma age gap between the PbPb ages of the two mineral fractions. The mineral hosts of U, Th and radiogenic Pb in the meteorite were identified through in-situ SIMS measurements of these elements in all minerals. The principal carriers of U, Th and radiogenic Pb are acid insoluble Al-Ti-rich augite and soluble Si-rich phosphate. Closure temperatures for diffusion of radiogenic Pb in these minerals are calculated to be respectively 820 ± 90 °C and 450 ± 80 °C using the observed range of crystal dimensions and the published experimental data for Pb diffusion in augite and apatite. The age difference between the pyroxene and phosphates has two potential explanations: (1) a late reheating event >450 °C which may have re-set the UPb systematics of the phosphates but not the pyroxenes or (2) slow cooling of the rock from ∼450–820 °C. If slow cooling is the reason for the age difference, we can estimate a model cooling rate of 270 ± 150 °C/Ma over the estimated closure temperature range between pyroxenes and phosphates. This rate is ∼11 orders of magnitude slower than petrologic cooling rates of quenched angrites at temperatures above 1000 °C determined experimentally using the composition of Asuka 881371 groundmass (Keil, 2012), which is similar to NWA 12320 groundmass. If there was no late reheating event, the discrepancy between fast cooling rates at high temperatures near the solidus, followed by much slower cooling at lower temperatures, might have been caused by impact melting. It could also be attributed to the initial eruption of the source magma of NWA 12320 near the surface of the parent body, followed by burial due to subsequent eruptions.

Submicron spatial resolution Pb-Pb dating for the formation age of Chang'e-5 basalt

Chang'e-5 (CE-5) basalt, the youngest lunar basalt sample collected so far, is crucial for unraveling the mechanism of the prolonged volcanic activity on the Moon. To better understand the petrogenesis of this basalt, it is necessary to determine its eruption duration accurately. Previous Pb-Pb age measurements have yielded a wide range from 2040 Ma to 1963 Ma. This large range may be attributed to 80 million-year multi-eruption, analytical or interpretational biases from different laboratories, or disturbances in the U-Pb system. Here, we conducted sub-micron Pb-Pb dating on 25 Zr-rich mineral grains with various sizes (∼2 μm to 20 μm) from six CE-5 basalt fragments by a Cameca NanoSIMS 50L. After excluding anomalies caused by Pb enrichment, we obtained a weighted mean Pb-Pb age of 2059 ± 21 Ma and an isochron age of 2046 ± 29 Ma. By integrating all reported age data of the CE-5 basalt, we found inconsistencies in previous studies, probably due to analytical biases of insufficient spatial resolution, leading to the undetected presence of terrestrial common Pb. Isochron regression with a filtering process yields an age of 2031.1 ± 3.8 Ma, which we recommend as the best estimate of the formation age of the CE-5 basalt. This result suggests that all analyzed CE-5 basalt fragments have nearly the same crystallization age, indicating that the lava erupted in a very short period. Combined with previous studies on the mineralogy and geochemistry of the CE-5 basalt, this result contributes new insights into the volcanic history of the Moon.

Pb diffusion in magnetite: Dating magnetite crystallization and the timing of remanent magnetization in banded iron formation

The ferrimagnetic mineral magnetite (Fe3O4) is abundant in banded iron formation (BIFs), and has the potential to provide UPb or PbPb age information on these rocks because it incorporates small amounts of U during growth. Combined with age measurements, paleomagnetic studies of BIF magnetites may also yield insight into the history of Earth's magnetic field and its relationship to early evolution of Earth's interior and atmosphere.Reliable magnetite ages utilizing Pb isotopes require knowledge of Pb diffusion in the magnetite structure. For this reason, we undertook an experimental investigation of Pb diffusion in magnetite by diffusing Pb2+ ions into pre-polished slabs of natural magnetite oriented parallel to {001} or {111}. A mixture of PbSO4 and Fe2O3 was used as a surface powder source to supply Pb2+ diffusant at the sample surface and at the same time buffer the oxygen fugacity of the system at magnetite-hematite (MH)—a typical fO2 for banded iron formations (BIFs) due to the common presence of both iron oxides (and where Pb2+ is stable relative to other Pb valence states). Diffusion experiments spanned temperatures of 500–675 °C and durations of 75 to 2035 h. Following each experiment, in-diffused Pb was depth-profiled using Rutherford backscattering spectroscopy (RBS) and Pb diffusivities were calculated from the profiles using an infinite half-space diffusion model. The following diffusion law for Pb2+ in magnetite is based upon 12 independent diffusivity measurements:DPb (m2·s−1) = (9 × 10−17 m2·s−1) exp.(−98,000 J·mol−1)/RT)where the uncertainties in the pre-exponential constant and activation energy are ±6% and ± 15%, respectively.Pb diffusion in magnetite over the temperature range of our study is orders of magnitude slower than projected for other divalent cations based on down-temperature extrapolation of previously measured diffusion laws (e.g., for Mn2+, Fe2+, Co2+, and Ni2+). This finding is encouraging in terms of the potential suitability of magnetite for UPb age determinations of BIFs and other magnetite-bearing rocks. Indeed, classical Dodson closure temperatures well above 500 °C are not unrealistic in cases where magnetite crystals having large diffusion domains (e.g., >100 μm in radius) are cooled relatively rapidly (e.g., at 100 °C/MYr). This is of particular significance for paleomagnetic studies, since the Curie temperature of magnetite is 580 °C and therefore the age of magnetization in magnetite-bearing rocks may be directly dated. However, slow cooling of magnetites having small diffusion domains can lead to Pb loss at temperatures of 200 °C or lower. Pb mobilization is evaluated for various time-temperature scenarios that involve both heating and cooling as well as “closed-loop” time-temperature paths. We conclude that UPb or PbPb age determinations of BIF magnetites are potentially reliable, but isotopic results should be assessed in concert with knowledge of the thermal history of the host rock and the effective grain size of the magnetites.

Pb-Pb age of the Gotan Limestone, Marwar Supergroup: Implications for Ediacaran-Cambrian transition events in the peninsular India

The rocks of the Marwar Supergroup of western India are known to contain important clues for the evolution of complex multicellular life, changes in chemistry of ocean water, and changes in global climate during the Proterozoic-Cambrian transition. However, the timings of these event markers remain tentative due to insufficient radiometric ages. Here, we present dating results of the only limestone formation of the supergroup, the Gotan Limestone of the Bilara Group, by Pb-Pb method and use the existing chemo/chrono stratigraphic information to provide constraints on the age of deposition of the supergroup. The 207Pb/204Pb-206Pb/204Pb isochron yielded an age of 537 ± 19 (2σ) Ma, which is in agreement with the age estimate suggested by 87Sr/86Sr stratigraphy. Therefore, the negative excursions observed in the δ13C record of the Bilara Group should now be assigned terminal Ediacaran and early Cambrian ages, respectively. This new Pb-Pb age considered together with δ13C and 87Sr/86Sr stratigraphic records suggests that the Ediacaran-Cambrian transition in the Marwar Supergroup is likely contained within the Bilara Group, and it is the first such report from the peninsular India.

Sources of Metals for the Rudny Altai VMS Deposits: Results of High-Precision MC-ICP-MS Lead Isotope Study

The Rudny Altai metallogenic province, which is concordant to the structures of the eponymous terrane and located in the Central Asian orogenic belt (CAOB), is one of the largest volcanic massive sulfide (VMS) provinces in the world. Lead isotopic composition of galena (61 samples in total) was measured for the first time with high accuracy (±0.02%, SD) for 20 sulfide base metal deposits representing the dominant Kuroko type in the Rudny Altai. They are enclosed in the Early–Middle Devonian volcano-sedimentary sequence in association with volcanic rocks of the bimodal basalt-rhyolite series. We studied the large and superlarge deposits of this type: Ridder-Sokol’noe, Tishinskoe, Novo-Leninogorskoe, Zyryanovskoe, Zmeinogorskoe, and Korbalikha. In the province, the isotope ratios 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb vary within narrow limits. At average values of 206Pb/204Pb = 17.820, 207Pb/204Pb = 15.517 and 208Pb/204Pb = 37.669, the root mean square variations (variation coefficient, %) are 0.22, 0.038 and 0.063% respectively. Even more homogeneous composition is observed within ore districts of the province (0.054, 0.012 and 0.020%) and especially within deposits (0.025, 0.010 and 0.013%), where the scale of variations of lead isotope ratios reaches their measurement error (±0.02%). The lead isotopic composition in the province shows no isotopic “signatures” of juvenile (asthenospheric) origin. The evolutionary characteristics of the lead source (its depletion in uranium, the old model Pb-Pb age, and moderate values of the µ2 parameter) together with the persistent isotopic composition allow us to regard the lithospheric mantle consisting of metasomatized and recycled rocks as its source. This source was of regional significance, chemically (U-Th-Pb) and isotopically (Pb-Pb) homogeneous, and common for all deposits. Among other CAOB terranes, including the Chinese Altai, the 206Pb, 207Pb and 208Pb isotopes in the ore lead of the Rudny Altai terrane have the least radiogenic composition. The previously noted (Chiaradia et al., 2006) systematic decrease of radiogenic isotope content in the lead of ores and rocks of the CAOB terranes from the southwest to the northeast correlates with a decrease in the lower crustal contribution in the same direction in their composition, where fragments and blocks of Precambrian crust are also involved. The peculiarity of the Pb isotopic composition of the Rudny Altai terrane is largely determined by the absence of Precambrian crustal blocks in its composition.

Statistical chronometry of meteorites. I. A Test of 26Al homogeneity and the Pb-Pb age of the solar system’s [formula omitted

We use rapidly cooled achondrites to test the assumption of 26Al homogeneity in the solar nebula, by checking if there is a single value of tSS, the absolute “Pb-Pb” age of the Solar System’s t=0, that makes concordant their ages from the Al-Mg and Pb-Pb systems. We find that values tSS=4568.42±0.24 Myr do make these ages concordant, and therefore the hypothesis of homogeneous 26Al is not falsified. This age, defined to be when the solar nebula had (26Al/27Al)=5.23×10−5, is significantly older than the ≈4567.3 Myr inferred from direct measurements of Pb-Pb ages in CAIs. Discrepancies between the Al-Mg and Pb-Pb chronometers in chondrules and CAIs have previously been interpreted as arising from heterogeneities in 26Al, under the presumption that the Al-Mg and Pb-Pb systems in CAIs closed simultaneously. We examine this assumption and show that resetting is to be expected in CAIs. In particular, we quantitatively demonstrate that it is plausible that Pb-Pb ages of CAIs were reset at late times, without resetting the earlier Al-Mg ages, if they were transiently heated in the same manner as chondrules. We critically examine Pb-Pb isochrons, refining data and suggesting best practices for their calculation and reporting. We advocate reporting chronometry as times of formation after t=0 rather than absolute ages, as only the former is useful for astrophysical models of the solar nebula. We advocate averaging of multiple samples, rather than anchoring to individual meteorites, to improve precision.

Magnetic field hyperactivity during the early Neoproterozoic: A paleomagnetic and cyclostratigraphic study of the Katav Formation, southern Urals, Russia

We present a detailed magnetostratigraphic and cyclostratigraphic profile through the Riphean (Tonian) Katav Formation in the southern Urals. The study confirms the primary nature of the magnetization in these rocks. The cyclostratigraphic study identified several orbital periods including the 405 ka long eccentricity. This allows us to quantify the reversal frequency in the Katav and our estimates range of 7–12 reversals per million years. Based on our study, we identify an interval of magnetic field reversal hyperactivity in the Neoproterozoic interval. Age estimates for the Katav are contentious and range somewhere between 800 Ma and 900 Ma based on carbonate Pb-Pb ages and stable isotope correlations. The paleomagnetic poles obtained in this study of the Katav (and overlying Inzer) Formation do not fit anywhere on the Baltica apparent polar wander path between 1100 Ma and 900 Ma. Furthermore, they lie 90° away from the 900 Ma segment of the path. We tentatively estimate their age to be closer to 800 Ma and perhaps confirm a previously hypothesized pulse of rapid true polar wander between 825 Ma and 790 Ma.

A unified chronology of dust formation in the early solar system

The chronology of dust formation in the early solar system remains controversial. Chondrules are the most abundant high-temperature objects formed during the evolution of the circumsolar disk. Considering chondrule formation, absolute lead‑lead (PbPb) ages and aluminum‑magnesium (26Al26Mg) ages relative to calcium‑aluminum-rich inclusions (CAIs) provide inconsistent chronologies, with PbPb ages showing early and protracted chondrule formation episodes whereas 26Al26Mg ages suggest that chondrule production was delayed by >1.5 Ma. Here, we develop a new method to precisely determine in situ26Al26Mg ages of spinel-bearing chondrules, which are not affected by secondary asteroidal processes. Our data demonstrate that 26Al26Mg chondrule formation ages are actually 1 Ma older than previously thought and extend over the entire lifetime of the disk. This shift in chondrule formation ages relative to CAIs, however, is not sufficient to reconcile the PbPb and 26Al26Mg chronologies of chondrule and achondrite formation. Thus, either chondrules' PbPb ages and volcanic achondrites' 26Al26Mg ages are incorrect or the age of CAIs should be reevaluated at 4,568.7 Ma to ensure consistency between chronometers. We favor the second hypothesis, given that (i) the canonical age of CAIs was determined using only 4 specimens and (ii) older ages of 4,568.2 Ma have also been measured. We show that the adoption of 4,568.7 Ma as the new canonical age of CAIs and the use of our new spinel-derived 26Al26Mg ages enable reconciling the PbPb and 26Al26Mg ages of chondrules and achondrites. This new chronology implies the existence of a 0.7–1 Ma gap between the formation of refractory inclusions and chondrules, and supports the homogeneous distribution of 26Al in the circumsolar disk.

Al-Mg and U-Pb chronological records of Erg Chech 002 ungrouped achondrite meteorite

Achondrite meteorites are remnants of the earliest planetary differentiation processes in the Solar System. They have been used to anchor short-lived radiochronometers to absolute ages determined from long-lived radiochronometers. More specifically, when comparing the isotopic systematics of the short-lived 26Al-26Mg chronometer anchored to absolute U-corrected Pb-Pb ages, inferences about the distribution of 26Al (half-life of ∼717 000 yr) in the protoplanetary disk can be evaluated. The ungrouped achondrite Erg Chech (EC) 002 has a distinct mineralogy and more evolved elemental composition compared to basaltic achondrites. In situ and solution 26Al-26Mg chronometry and 53Mn-53Cr chronometry suggest that EC 002 formed within ∼0.7 to 2.2 Ma after the formation of Ca-Al-rich inclusions (CAIs), making it the oldest known sample of igneous crust in the Solar System (Barrat et al., 2021; Anand et al., 2022; Zhu et al., 2022; Fang et al., 2022). Here we present the U-corrected Pb-Pb age and 26Al-26Mg age obtained by MC-ICPMS solution analysis of the same mineral separate samples of EC 002. In addition, six merrillite grains were analyzed by in-situ SIMS to determine their Pb-Pb individual ages.The U isotope composition of EC 002 exhibits internal heterogeneities between leached pyroxene (238U/235U = 137.766 ± 0.027) and the bulk rock (238U/235U = 137.8190 ± 0.0074). The Pb isotope composition of progressively leached pyroxenes are characterized by radiogenic 206Pb/204Pb ratios (ranging from 41 to 23487). Using the U isotope composition of the leached pyroxenes, the resulting age of the 207Pb/206Pb-204Pb/206Pb isochron is 4565.87 ± 0.30 Ma (2σ). The weighted mean of the Pb-Pb ages of seven SIMS analyses of merrillites are 4564.3 ± 5.2 Ma (2σ). These similar ages (within uncertainty) indicate rapid cooling and the absence of significant thermal events after ∼4559 Ma on the parent body of EC 002. The 26Al-26Mg isochron through a bulk rock, pyroxene, fine-grained and four plagioclase fractions defines an initial 26Al/27Al ratio of [8.89 ± 0.79] × 10−6 corresponding to a formation age of 1.83 ± 0.12 Ma after CAIs ([5.23 ± 0.13] × 10−5; Jacobsen et al., 2008). The initial 26Al abundance is consistent with previous MC-ICP-MS 26Al-26Mg reported systematics for EC 002 (Fang et al., 2022), but 0.46 ± 0.13 Myr older than the in situ SIMS 26Al-26Mg age previously reported by Barrat et al. (2021).When anchored to the absolute Pb-Pb age of CV3 CAIs (4567.30 ± 0.16 Ma; Connelly et al., 2012), the Al-Mg model age of EC 002 is 4565.47 ± 0.20 Ma, slightly younger than its U-corrected Pb-Pb age.The concordance of the Pb-Pb and 26Al-26Mg ages of ungrouped CC achondrites when anchored to EC 002 suggest that 26Al was homogeneously distributed between the NC and CC reservoirs at the time of their parent body accretion. Furthermore, the presence of internal U isotope heterogeneities found between mineral and whole-rock samples of EC 002 supports the need of U isotope analysis of meteoritic samples dated using the Pb-Pb chronometer.

ISOTOPE-GEOCHEMISTRY EVIDENCE FOR GLOBAL CHANGES IN THE HYDROSPHERE COMPOSITION IN PALEOPROTEROZOIC OF SARMATIA

In the carbonate rocks of the Kursk block of Sarmatia, evidence of global perturbations in the atmosphere and hydrosphere of the Earth has been recorded. Dolomites of the Rogovskaya Formation accumulated on the passive continental margin and are characterized by a chondrite Y/Ho ratio, the absence of Ce* anomalies, and δ 13 C and δ 18 O values typical of Neoarchean and Early Paleoproterozoic marine carbonates. Therefore, their accumulation took place before the GOE . Limestones of the Tim Formation are contrastingly distinguished by negative Ce* anomalies, superchondrite Y/Ho ratio, and anomalously high values of δ 13 C = 10.4 to 12.1 ‰ V-PDB. It follows from this that carbonates of the Tim Formation were formed after the GOE during the Yatuli-Lomagundi event. Their Pb-Pb age of 2233 ± 8 Ma is close to its lower boundary.

Age of Uranium Mineralisation from North Delhi Fold Belt (NDFB): Evidences from U-Pb, Pb-Pb and Sm-Nd Systematics

Abstract Results of multi-method geochronological study (U-Pb, Pb-Pb and Sm-Nd), on uranium mineralized rocks along the albitite line in the North Delhi Fold Belt (NDFB, Rajasthan, India) are presented. The Pb-Pb ages obtained, on uranium mineralized core samples, from Rohil uranium deposit, Sikar district, Rajasthan (south western part of NDFB), are 836 ± 5 Ma (Pb Sequential Leaching-PbSL, weighted average of 11 isochron ages), 826 ± 9 Ma (Pb-Pb isochron age on uraninite), 842 ± 18 Ma (U-Pb concordia upper intercept age, on uraninite). Similar ages of uraninites were also obtained from areas close to Rohil uranium deposit i.e. Jahaz (823 ± 10 Ma, U-Pb and 826 ± 14 Ma, Pb-Pb) and Ghateshwar areas (834 ± 14 Ma, U-Pb concordia upper intercept age and 826 ± 25 Ma, Pb-Pb isochron age). The ages of uranium mineralisation (PbSL) further in northeastern part of NDFB i.e. in Gorir area, Jhunjhunu district, Rajasthan, are 827 ± 7 Ma, 818 ± 24 Ma (PbSL) and in Dhancholi area, Mahendragarh Dist., Haryana, are 824 ± 12 Ma (U-Pb) and 824 ± 6.5 Ma (Pb-Pb). The above age data in general indicates age of uranium mineralization as 830 Ma in the NDFB. These ages are consistent with age of uranium mineralization in albitites from Brazil.

SIMS Pb-Pb dating of phosphates in the Proterozoic strata of SE North China Craton: Constraints on eukaryote evolution

Meso-Neoproterozoic sedimentary successions in the southeastern margin of North China Craton contain a wealth of fossil and chemostratigraphic information, which aids in our understanding of early multicellular life and environment coevolution. However, a lack of reliable age constraints means the timing of these fossil occurrences and the corresponding evolutionary tempo of multicellular life remain disputed. This study aims to provide new age constraints by means of authigenic xenotime and monazite geochronology, coupled with morphological observation and mineral chemical analyses. Morphological observation and mineral chemistry allows differentiation of authigenic xenotime and monazite from detrital origins. Authigenic xenotime from the Shijia Formation, upper part of Meso-Neoproterozoic sedimentary successions, gave a SIMS Pb-Pb age of 919 ± 23 Ma, whereas authigenic monazite from the Xinxing and Liulaobei formations, lower part of Meso-Neoproterozoic sedimentary successions, produced SIMS Pb-Pb ages of 1086 ± 17 Ma and 1086 ± 120 Ma, respectively, all of which are interpreted as minimum depositional ages. The new ages suggest the crown group multicellular eukaryotes have highly evolved and diversified prior to 1.0 Ga, and confirm the use of Trachyhystrichosphaera aimika as an index fossil for the Mesoproterozoic-Neoproterozoic transitional period.

Impact-related crystallization and modification of small zircons in Apollo 15 and 16 impactites at 4.2 Ga

Because of their robustness against resetting, in situ U-Pb ages of zircons in lunar impactites have the potential to provide constraints on the lunar bombardment history that may complement the more common K-Ar ages. Most previous work has focused on relatively large zircons that show growth zoning and ages were mostly interpreted as early igneous crystallization ages. Here we combine high-resolution mineralogical imaging and in situ U-Pb dating by ion microprobe to identify, characterize and date <20 μm size zircons in thin sections of lunar impact breccias. Several tens of grains of zircons of this size range were identified in thin sections of impactites from the Apollo 15 and 16 landing sites. Small zircons are more abundant in both noritic and evolved clinopyroxene, SiO2 or K-feldspar bearing lithologies compared to anorthositic bulk compositions. Both granular zircon aggregates and overgrowth on existing zircon or baddeleyite (in breccias 15455 and 67915) are interpreted to reflect high-temperature recrystallization of zircons or its high-temperature-pressure precursor phases, following shock heating events by impact. In contrast, conchoidal or poikilitic zircons <10 μm in Fe-Ni metal bearing noritic clasts or matrix (67915, 67955) crystallized in situ from impact melt. Most U-Pb ages of the 24 analyzed grains are either concordant or reverse discordant with 207Pb-206Pb ages ranging from 4.15 to 4.25 Ga. The small age range, combined with a large textural spectrum and the frequent presence of Fe-Ni metal suggest zircon crystallization from impact melt and recrystallization of pre-existing zirconium-bearing minerals by impact heating. Such ‘impact’ zircons with 4.2 Ga ages have now been reported from most Apollo landing sites, suggesting widespread formation and modification of zircons by basin-forming impacts at this time. The contrast between U-Pb zircon (predominantly 4.2 Ga) and K-Ar feldspar ages (predominantly 3.9 Ga) likely reflects resetting of the latter chronometer by impact heating.

Baddeleyite Pb[sbnd]Pb geochronology and paleomagnetic poles for ~1.89–1.86 Ga mafic intrusions from the Dharwar craton, India, and their paleogeographic implications

We present new key paleomagnetic pole at 13°S, 152°E (k = 21, A95 = 7.8°) for recently identified 1864.4 ± 2.7 Ma (weighted mean age of four PbPb ages) mafic magmatic event, based on a detailed paleomagnetic study of dolerite dykes and sills intruding Archean basement rocks and Tadipatri formation of the Cuddapah basin, Dharwar craton respectively. The PbPb baddeleyite geochronology yields a crystallisation age of 1867.1 ± 1.0 Ma (MSWD = 1.02) for N77°E trending dyke in the southern region to Cuddapah basin. This new age obtained, confirms the presence of ~1864 Ma magmatic episode with a spatial extent of ~400 km in the Eastern Dharwar craton, within the brief period of ~5 Ma. The paleomagnetic results in these dykes revealed reverse polarity magnetisation direction with mean D = 107°, I = 24° (N = 13 sites, α95 = 10°). Here, we also update the normal polarity magnetic directions on ~1.89 Ga swarm, and the corresponding paleopole situated at 21°N, 336°E (N = 79 sites, A95 = 3.6°). The paleoposition of India is constrained around the equator during ~1.89–1.86 Ga time. The paleogeographic reconstructions were also been attempted at ~1.89 Ga and ~ 1.86 Ga with available key poles from other cratons, indicates the possibility of single plume acting as a source for two distinguishable radial emplacement of mafic dyke swarms across India (Dharwar and Bastar craton) and Western Australia (Yilgarn craton) within a time span of ~35 Ma. The individual movement of India, Baltica and Siberia with a drift rate of ~5.55 cm/yr towards the south, whereas Amazonia craton has moved rapidly to the north (~24.9 cm/yr), do not suggest the amalgamation of a supercontinent (Columbia/ Nuna) during ~1.88–1.86 Ga time.

The fall, recovery, classification, and initial characterization of the Hamburg, Michigan H4 chondrite.

The Hamburg meteorite fell on January 16, 2018, near Hamburg, Michigan, after a fireball event widely observed in the U.S. Midwest and in Ontario, Canada. Several fragments fell onto frozen surfaces of lakes and, thanks to weather radar data, were recovered days after the fall. The studied rock fragments show no or little signs of terrestrial weathering. Here, we present the initial results from an international consortium study to describe the fall, characterize the meteorite, and probe the collision history of Hamburg. About 1 kg of recovered meteorites was initially reported. Petrology, mineral chemistry, trace element and organic chemistry, and O and Cr isotopic compositions are characteristic of H4 chondrites. Cosmic ray exposure ages based on cosmogenic 3He, 21Ne, and 38Ar are ~12 Ma, and roughly agree with each other. Noble gas data as well as the cosmogenic 10Be concentration point to a small 40–60 cm diameter meteoroid. An 40Ar‐39Ar age of 4532 ± 24 Ma indicates no major impact event occurring later in its evolutionary history, consistent with data of other H4 chondrites. Microanalyses of phosphates with LA‐ICPMS give an average Pb‐Pb age of 4549 ± 36 Ma. This is in good agreement with the average SIMS Pb‐Pb phosphate age of 4535.3 ± 9.5 Ma and U‐Pb Concordia age of 4535 ± 10 Ma. The weighted average age of 4541.6 ± 9.5 Ma reflects the metamorphic phosphate crystallization age after parent body formation in the early solar system.

Timing and duration of Archean orogenic gold deposits in the Bourlamaque pluton, Val d’Or mining camp, Abitibi, Canada

In the Val-d’Or mining district of the Archean Superior Province, the relationship between regional metamorphism/deformation and the timing of orogenic vein-type gold mineralization is debated; some authors arguing that gold mineralization was coeval with the regional D2 deformation event and broadly synchronous with metamorphism, and others suggesting that it is 60 to 100 Ma younger than peak metamorphism. Using the 40Ar/39Ar method and the compilation of U-Pb or Pb-Pb ages of ore-related minerals, this contribution presents a geochronological study on the timing and the inferred duration of the formation of vein-type gold deposits hosted by the Bourlamaque pluton in the Val d’Or mining district. We present an extensive set (49 samples) of 40Ar/39Ar CO2 laser probe data acquired on amphibole and white mica single grains from three major gold deposits, the Lac-Herbin, Beaufor and Beacon-2 mines. In each deposit, gold mineralization is host by shear and extensional quartz veins associated to steeply south-dipping networks of ductile shear zones. Barren north-dipping faults also occur at each deposit and are attributed to post-mineralization structural event(s). Amphiboles 40Ar/39Ar analyses yield magmatic crystallization ages clustering around 2704 Ma and preserve evidences for at least two subsequent disturbances linked to probable hydrothermal circulation events at c. 2667 Ma and c. 2652 Ma. White micas 40Ar/39Ar age spectra from auriferous quartz veins and hosting mylonitic shear zones are typical of long-lasting and sequential dynamic recrystallization and suggest that Au-bearing hydrothermal circulation was establish, at least, around c. 2650 Ma, and followed by a succession of hydrothermal pulses at c. 2597, 2575, 2551, 2500 and 2452 Ma. 40Ar/39Ar ages distribution among the different deposits as well as between the auriferous quartz veins and hosting shear zones are consistent with fluids production and circulation along an active network of seismically active fault zones.

Xenon systematics of individual lunar zircons, a new window on the history of the lunar surface

We demonstrate a new way of investigating the processing of the lunar surface (and other planetary regoliths) that combines XeS-XeN ages (based on uranium fission) in individual zircons with their xenon isotopic record of solar wind and cosmic ray exposure. We report the first xenon isotopic analyses of individual lunar zircons (from Apollo 14 soil and breccias samples). Parallel analyses of a suite of zircons from the Vredefort impact structure in South Africa revealed XeS-XeN ages that agree well with U-Pb systematics, suggesting that the diffusion kinetics of xenon and lead in zircon are similar in the pressure-temperature environment of sub-basin floors. In contrast, all Apollo 14 zircons examined exhibit XeS-XeN ages markedly younger than the associated U-Pb and 207Pb-206Pb ages, and soil zircons with 207Pb-206Pb ages greater than 3900 Ma produced an abundance of XeS-XeN ages <1000 Ma. The young ages cannot be explained by thermal neutron irradiation on the lunar surface, and diurnal heating is unlikely to cause preferential loss of xenon. As such these young soil zircon ages likely record regolith gardening processes. The breccia zircons typically record older ages, >2400 Ma, suggesting that these samples may be useful for investigating ancient events and regolith processing at an earlier epoch. However, none of the zircons contain xenon from now-extinct 244Pu implying that either the samples have completely degassed since ∼3900 Ma or that the initial Pu/U ratio of the Moon is lower than that on Earth. We also describe a methodology for conducting component deconvolution that can be applied to multi-isotopes systems beyond xenon. We have also determined new xenon isotopic yields from rare earth element spallation in the lunar environment and high precision yields for neutron induced fission of 235U in geologic samples.