Radiometric Ages Research Articles

The Lac au Renard Tephra Cluster: a record of Lochkovian (Lower Devonian) volcanism in the Indian Point Formation, Gaspe Peninsula, Quebec, Canada

Approximately twenty tephra beds, comprising the Lac au Renard Tephra Cluster (new), occur in the Rosebush Cove and Petit Portage members of the Indian Point Formation (Chaleurs Group) on the Gaspe Peninsula of Quebec, Canada. The tephra beds range in thickness from <1.0 cm to 82 cm and occur in a mudrock-dominated sequence with coarser tempestite interbeds. Mineralogically and texturally graded accumulations of phenocrysts mark the bases of the thickest tephra beds. Early diagenetic concretions in one composite tephra preserve pre-compaction fabrics and original hypocrystalline textures with microphenocrysts and devitrified glass shards. The presence of plagioclase, quartz, K-feldspar, biotite, apatite, and zircon suggest a rhyolitic source. The coarseness of the microphenocrysts in the basal accumulations, along with the abundance and thickness of the tephras, suggest that deposition in the Gaspe area was in a proximal position relative to the volcanic source. The zonal graptolites Monograptus praehercynicus and Monograptus aequabilis ssp. from the Petit Portage Member indicate a middle Lochkovian age for the Lac au Renard Tephra Cluster of the Indian Point Formation. Correlation with the tephra cluster that includes the Judds Falls Bentonite in the New Scotland Formation and other possible tephras in the Kalkberg Formation (Helderberg Group) of New York and the Corriganville and Mandata formations of Pennsylvania is likely. The graptolite fauna of the Indian Point and probable correlations to New York may provide additional biostratigraphic constraints on a U–Pb zircon radiometric age determination of 417.6 Ma from New York that has been used to estimate the age of the Silurian–Devonian boundary.

U–Th disequilibrium, (U–Th)/He and 40Ar/39Ar geochronology of distal Nisyros Kyra tephra deposits on Datça peninsula (SW Anatolia)

Abstract Kyra is an important tephra unit for Nisyros pre-caldera stage due to a number of subunits with various compositions (basaltic andesite to dacite) and distal dispersion on surrounding Greek islands (Tilos and Chalki) and Datca peninsula (Turkey). As for the most Nisyros tephra units, the available radiometric ages of Kyra tephra are not compatible with the relatively well-established Nisyros tephrostratigraphy. This beclouds the characterization of both proximal and distal Kyra tephra and also the understanding of Nisyros pre-caldera volcanism. In order to provide new insight into Nisyros tephrochronology, we here present the first (U–Th)/He and 40Ar/39Ar ages of Kyra distal andesitic pumice previously reported on Datca peninsula. (U–Th)/He dating of apatite (DZS pumice, Sindikoy) yielded weighted mean ages of 134.8 ± 19.8 ka (2σ, single-crystal aliquots, n = 12, MSWD = 0.21) and 139.6 ± 15.0 ka (2σ, multi-crystal aliquots, n = 5, MSWD = 0.78). Combined U–Th disequilibrium and (U–Th)/He dating of zircon (DZS pumice) yielded an age of 131.1 ± 8.6 ka (2σ, single-crystal aliquots, n = 8, MSWD = 0.58). The inverse isochron 40Ar/39Ar age of DAH (Hizirsah) pumice (plagioclase, 135.7 ± 28.2 ka, 2σ, MSWD = 1.70) is almost comparable with the (U–Th)/He ages, while the plateau ages of DAB (Belen) pumice are relatively older (plagioclase, 144.8 ± 26.6 ka, 2σ, n = 17, MSWD = 0.61; hornblende, 151.5 ± 19.5 ka, 2σ, n = 27, MSWD = 1.41). DAB pumice has also more evolved glass composition (mainly dacitic) compared to andesitic DAH and DZS pumice. Therefore, DAB tephra is considered as a different Kyra subunit. All new ages on distal Kyra tephra are in accordance with the Nisyros tephrostratigraphy and indicate a long-lasting eruptive history (~20 ka) of the tuff cone that formed Kyra sequence. Considering the available ages for the Kos Plateau Tuff (KPT) eruption (161 or 165 ka) and the presence of stratigraphically older Nisyros tephra units than Kyra, there should be no significant time gap between the KPT eruption and the onset of Nisyros subaerial volcanism. In addition, the recent findings on distal Kyra tephra will probably provide new opportunities for correlating the undefined tephra units that have comparable composition and age reported in both marine and terrestrial settings. Thus, our study will indirectly contribute to paleoenvironmental/climatological and tephrochronology studies around the Mediterranean region.

Correlation of carbon isotope stratigraphy and paleoenvironmental conditions in the Cretaceous Jehol Group, northeastern China

ABSTRACTThe Jehol Biota represents an important phase in the history of biological evolution on Earth. Two sites with Aptian Jehol Biota, located in western Liaoning Province and in northern Hebei Province, northeastern China, have generated a major debate about the relationships between their fossil-bearing layers, resulting in confusions about their stratigraphic correlation and biogenetic sequence. We have applied carbon isotope analysis of total organic carbon to provide new data relevant to this debate and have re-examined the connection between volcanism and carbon cycle perturbations, with consideration of the paleoenvironmental conditions. The rare earth element data show that the provenances of the Jehol Group in western Liaoning and northern Hebei were similar and derived mainly from the underlying andesite. Constrained by the radiometric age, the δ13Corg curve suggests that the Dadianzi Formation in northern Hebei is comparable to the three lacustrine layers of the Yixian Formation (ranging from the Jianshangou Bed to the Jingangshan Bed) in western Liaoning. These layers are highly correlated with the early Aptian marine sequence in Oceanic Anoxic Event 1a (OAE 1a), implying that a possible teleconnection (possibly related to a large igneous province) existed between oceanic and terrestrial realms. We also propose that the Xiguayuan Formation above the Dadianzi Formation in northern Hebei is equivalent to the Huanghuashan Breccia Bed at the top of the Yixian Formation in western Liaoning. Moreover, the δ13Corg and element geochemical data suggest that the carbon cycle perturbation and the paleolimnological environments were distinctly influenced by the effects of volcanic CO2 during the early Aptian.

Time of re-emergence of Christmas Island and its biogeographical significance

Christmas Island in the eastern Indian Ocean forms the pinnacle of a once-drowned, coral atoll. Exactly when the seamount re-emerged above the seas is, unfortunately, imprecisely understood. One of the consequences of this uncertainty is that it can hamper studies dealing with its land-locked faunal assemblage. We attempt to address this issue using geological and geophysical information. A first constraint is provided by sub-aerially erupted volcanic rocks that accumulated on a karstified land-surface. Radiometric ages for five associated basalt samples yield a mean of 4.39 ± 0.08 Ma (2σ). These data, together with the geological observations, indicate that the island existed in the mid-Early Pliocene. A second constraint is provided by the seamount's postulated uplift path as it ascended the outer-trench high on its journey towards the subduction zone south of Java. Here, the edifice is estimated to have become sub-aerial between 5.66 and 4.49 Ma (5.03 + 0.63/−0.54 Ma). In terms of biogeography, landmass emergence in the latest Miocene-Early Pliocene is largely concordant with the island's native land-locked reptile and mammal species (eight of the nine extant and recently extirpated forms are endemic) as most show only small to negligible levels of genetic differentiation with respect to their nearest off-island relatives. Significantly, though, molecular-clock data for at least two, possibly three, of the lizard species indicate that they split from their nearest living congeners (incidentally, all from lands to the east of the Wallace Line >1100 km away) prior to Christmas Island's emergence, that is in the Late Miocene or earlier (specifically at c. 7 Ma, c. 13 Ma and c. 26 Ma). Assuming the ages are reliable, these dichotomies could be explained if there are/were intermediate relatives that exist elsewhere/are now-extinct as this would facilitate the ancestors of the Christmas Island species colonizing the landmass sometime in the last five million years or so.

Results, problems and future tasks of palaeostress and fault-slip analyses in the Pannonian Basin: the Hungarian contribution

In the Pannonian Basin fault-slip analysis and palaeostress determinations started in 1982–1984 by the work of Françoise Bergerat, Jacques Geyssant and Claude Lepvrier. After F. Bergerat introduced some Hungarian researchers to field and laboratory techniques, fault-slip analysis expanded into the Pannonian Basin in Hungary and in some adjacent areas, and is still going on. This paper reviews the results achieved during this ~35 year time span from a historical perspective, by the compilation of the determined Cenozoic stress fields, their temporal boundaries, and uncertainties. The model for the evolution of the stress field became more complicated, and more and more details in the deformation history were realised, both in the Alcapa and Tisza–Dacia units of the Pannonian Basin. The integration of palaeomagnetic and stress data indicated that some changes in stress axes (and in fault orientation) are due to vertical-axis block rotation, and are not real rotation of the external stress field itself. With the development of new approaches, like tilt test, combination of surface and subsurface fault data, radiometric age determination of deformed rocks, fine separation of fracture sets can be achieved and the multiple recurrences of stress fields with identical principal axes were demonstrated. The refined methods permitted the demonstration of basin-wide inhomogeneity of the stress field for some phases. However, the precise timing, the number of phases, separation of events with similar stress axes, and the variability of the maximal stress axes of a given stress field remain the major questions to be solved in the future. Rheological conditions of the deforming media and physical parameters of the fracturing will be the other future steps in research, to be achieved by studies of deformation bands, fluid inclusions, isotopes, in order to better understand the connection of deformation, subsidence, diagenesis, and fluid flow.

The first radiometric age by isochron 26Al/10Be burial dating for the Early Pleistocene Yuanmou hominin site, southern China

Abstract The Yuanmou H. erectus site is generally considered to be one of the oldest hominin sites in southern China. The chronology of this site has been repeatedly investigated using magnetostratigraphy since its first discovery in the 1960s. However, there are multiple interpretations of the magnetostratigraphy due to the complexity of the sedimentary environment, resulting in a one million year age discrepancy by different studies. Because Yuanmou's age has yet to be verified radio-isotopically, we apply isochron 26Al/10Be burial dating to constrain the existing magnetostratigraphy. Six samples from a gravel bed 38 m above the fossil layer were collected, yielding an age of 1.54 ± 0.06 (0.11) My. Based on the estimated sedimentation rate, we interpolate an age of 1.72 ± 0.03 My for the fossil layer. This result is consistent with the most recent paleomagnetic stratigraphy of 1.7 My for the Yuanmou H. erectus fossils. Combined with other recent chronological studies of paleoanthropological sites across Asia, this study supports a model for earlier hominin dispersals out of Africa.

U-Th dating of a Paleolithic site in Guanyindong Cave, Guizhou Province, southwestern China

Guanyindong Cave in southwestern China has received considerable attention in the investigation of Paleolithic hominin origins and evolution in China, due to its rich archaeological finds composed of thousands of stone artifacts and a unique fauna with 23 species of mammalian fossils found. However, despite earlier extensive excavations and descriptive studies, debates still centre around the conflict between previous radiometric age data and evidence from biostratigraphic correlations. In this study, we carried out detailed field investigations and sampling, and obtained 35 U-Th dates on flowstone layers and other datable materials from the cave. The age results from materials in stratigraphic context provide a robust chronological framework of the cave. The data suggest that the deposition of Group B sediments and fossil assemblages widely distributed within the cave should have occurred after ~370 ka but before ~70 ka, with the bulk of the sediments and associated fossils laid down during 200–140 ka. Our new U-Th dates of in situ flowstone layers intercalated with one rhinoceros tooth and several other fossil fragments near the Hall at the centre of the cave constrain the deposition ages of these mammalian fossils to the period between 469 ± 37 and 336 ± 7 ka. Combined our U-Th data with recent OSL dates of Hu et al. (2019), we suggest that Group A sediments and associated fossils were likely deposited episodically from ca. 90 ka to <40 ka. Overall, our data indicate that the cave development started >469 ± 37 ka ago, whilst the cave system framework took shape as we see today >340 ± 10 ka ago. Subsequently, the cave might have experienced several flooding and washout events, resulting in recycling and mixing of older sediments and fossils into younger sequences, a hypothesis consistent with tight clustering of both U-Th ages of speleothems in this study and the OSL dates of clastic sediments (Hu et al., 2019). This would reconcile the contradiction between the great antiquity of mammalian fossils inferred from biostratigraphic correlation and the much younger radiometric dates of materials in stratigraphic context, and explain the lack of technological advance despite an apparently long presence of the “Guanyindong culture”, as well as the presence of the Levallois technologies. In this regard, our U/Th age data, combined with other recent studies, have resulted in an improved understanding of Paleolithic hominid evolution and stone technologies in south China.

Paleomagnetism and rock magnetism of East and West Clearwater Lake impact structures

The East and West Cleawater Lake impact structures (Wiyâshâkimî Lake, Québec), ∼26 and 32 km in diameter, respectively, have been proposed to represent an impact doublet. We investigated their paleomagnetism to contribute to this debate. The paleomagnetic directions of the impact melt rocks and impact melt-bearing breccias from the West Clearwater structure are compatible with the radiometric age of 280–290 Ma previously determined for this structure and indicate that the impact occurred during a reverse polarity interval of the geomagnetic field. A similar remagnetization direction is found in the basement within 10 km of the structure center, whereas basement farther away from the center has escaped remagnetization by the impact. Samples for the East Clearwater structure come from two holes drilled in 1963 and 1964. Unfortunately, the drill hole through the melt rocks is tilted by 30° from the vertical with an unknown azimuth. The paleomagnetic inclination of these melt rocks cannot be constrained to better than between −28° and +32°. This is, however, distinct from the inclination of the melt rocks of the West Clearwater Lake impact structure (−27.8° ± 3.7°), suggesting that the two structures do not represent an impact doublet, in agreement with recent radiometric dating. The basement rocks and the melt rocks within 10 km of the center of the West Clearwater Lake impact structure show a magnetic signature of titanohematite that crystallized during postimpact hydrothermal activity under oxidizing conditions. This is not observed in the basement or the melt rocks from the East Clearwater Lake impact structure.

三浦半島第四系宮田層中の不整合と挟在する“船久保タフ(Fn）”のFt, U-Pb, K-Ar 年代

三浦半島第四系宮田層中の不整合と挟在する“船久保タフ(Fn）”のFt, U-Pb, K-Ar 年代

Integrating palaeocaves into palaeolandscapes: An analysis of cave levels and karstification history across the Gauteng Malmani dolomite, South Africa

The Drimolen Palaeocave System in the ‘Fossil Hominid Sites of South Africa’ UNESCO World Heritage Site is well known for numerous remains of early hominins such as Paranthropus robustus and early Homo. These hominin fossils, along with bone tools and notably diverse accumulation of non-hominin primates and fauna, have all been excavated from the 'Main Quarry' area of the site where extensive lime-mining took place. Here we report the first radiometric age of 1.712 ± 0.269 Ma for hominin bearing deposits associated with the DNH7 Paranthropus robustus cranium in the Main Quarry area of the site, which is consistent with recent biochronological estimates. This age is similar to recent estimates for Swartkrans Member 1 Hanging Remnant (somewhere between 2.3 and 1.8 Ma) which also contains Paranthropus and early Homo. Simultaneously, we integrate the newly radiometrically dated Main Quarry deposits with a new fossil deposit, the Drimolen Makondo, discovered in 2013, that is situated some 50 m up the hill to the west from the Main Quarry. It has experienced only limited disturbance from mining but much more extensive erosion. Preliminary excavations and analysis have revealed that the Makondo infill is older than the Main Quarry, dating to 2.706 ± 0.428 Ma. Its greater age is confirmed by biochronology. The Makondo thus overlap with the suggested end of deposition of Australopithecus bearing Sterkfontein deposits, although it is yet to yield any hominin remains. These new dates for the two Drimolen Palaeocave System deposits indicates that, contrary to prior age estimates, the Drimolen site as a whole records the critical hominin and faunal turnover in South African palaeocommunities that occurred around 2.3–1.7 Ma. Finally, as the Drimolen Makondo represents a rare example of a pre-2 Ma fossil bearing deposit in the Gauteng exposures of the Malmani dolomite, we also integrate our results into the greater South African record of palaeodeposit formation (most of which occur between ∼2.0 and 1.0 Ma). An analysis of the age of palaeocave infillings across the Malmani dolomite suggests that, as is classically the case with karst, the height within the dolomite is broadly correlated to their age, although with some notable exceptions that are likely related to localised geological features. Our analysis also indicates that most caves have undergone some form of secondary karstification related to a younger phase of cave formation, contrasting with models that suggest the cavities all formed at the same time and that infill is related to erosion and the opening up of cave passages. As such, the reason that few pre-2 Ma deposits have been identified in the Gauteng exposures of the Malmani dolomite is probably because these older caves have been eroded away. Identifying such early caves is critical in understanding whether earlier hominins may have once existed in South Africa or if erosion of older deposits (or an absence of speleogenesis at this time) has made such early periods absent from the geological record.

Mineralogical and Geochemical Characteristics of the Utanobori Gold Deposit in Northern Hokkaido, Japan

AbstractThe Utanobori gold deposit is a low‐sulfidation, epithermal vein‐type deposit located in northern Hokkaido, Japan. The deposit is hosted by conglomerate, sandstone, and tuff of the Middle to Late Miocene Esashi Formation. These rocks were hydrothermally altered. Silica sinters and quartz‐adularia veins are common in the deposit. The quartz‐adularia veins either contain a ginguro band, which corresponds to the main gold‐bearing vein (Type 1 Veins), or do not contain a ginguro band but contain minor adularia (Type 2 Veins). Type 1 Veins are divided into three stages with 12–14 substages. Ore minerals identified include electrum, naumannite, chlorargyrite, bromargyrite, an unidentified Fe‐Sb mineral, and an Fe‐(Sb)‐As mineral. These ore minerals formed in the main mineralization stages I (bands I‐b and I‐d) and II (band II‐a). Scanning electron microscopy with cathodoluminescence images show that cathodoluminescence‐dark microcrystalline quartz exhibiting colloform (ghost‐sphere) texture is closely associated with ore minerals in the Type 1 Vein and Type 2 Vein, and the Al and K contents of such quartz are commonly &gt;1000 ppm. This indicates that the ore minerals were crystallized from alkaline, silica‐saturated fluids at temperatures &lt;200°C, which initially deposited amorphous silica that was recrystallized to microcrystalline quartz. The average Au content of electrum is 52.5 at% Au (n = 10), 65.7 at% Au (n = 20), and 55.5 at% Au (n = 5) in bands I‐b, I‐d, and II‐a, respectively, of Type 1 Veins. The δ34SCDT values of two fine‐grained disseminated pyrites in the altered conglomerate and bedded tuff in the argillic altered zone are −4.3 and −4.2‰. Ar‐Ar dating on adularia yielded 13.6 ± 0.06 Ma, 13.6 ± 0.07 Ma, and 13.6 ± 0.06 Ma for the stages I, II, and III of the Type 1 Vein, respectively. K‐Ar ages determined on adularia in the silica sinter and on whole‐rock of glassy rhyolite of the Esashi Formation are 15.0 ± 0.4 Ma and 14.6 ± 0.4 Ma, respectively. These radiometric ages indicate that silica sinter associated with the rhyolitic volcanic rocks formed prior to the main gold mineralization.

A productivity collapse to end Earth’s Great Oxidation

It has been hypothesized that the overall size of-or efficiency of carbon export from-the biosphere decreased at the end of the Great Oxidation Event (GOE) (ca. 2,400 to 2,050 Ma). However, the timing, tempo, and trigger for this decrease remain poorly constrained. Here we test this hypothesis by studying the isotope geochemistry of sulfate minerals from the Belcher Group, in subarctic Canada. Using insights from sulfur and barium isotope measurements, combined with radiometric ages from bracketing strata, we infer that the sulfate minerals studied here record ambient sulfate in the immediate aftermath of the GOE (ca. 2,018 Ma). These sulfate minerals captured negative triple-oxygen isotope anomalies as low as ∼ -0.8‰. Such negative values occurring shortly after the GOE require a rapid reduction in primary productivity of >80%, although even larger reductions are plausible. Given that these data imply a collapse in primary productivity rather than export efficiency, the trigger for this shift in the Earth system must reflect a change in the availability of nutrients, such as phosphorus. Cumulatively, these data highlight that Earth's GOE is a tale of feast and famine: A geologically unprecedented reduction in the size of the biosphere occurred across the end-GOE transition.

Refined chronology of prehistoric cultures and its implication for re-evaluating human-environment relations in the Hexi Corridor, northwest China

The reconstruction of high-resolution chronologies for prehistoric cultures is a prerequisite for understanding the history of human evolution and its relationship with environmental change, and is valuable for exploring the trajectory of transcontinental cultural exchanges in prehistoric time. The Hexi Corridor of northwest China was one of the earliest centers for long-distance culture exchange in the prehistoric world. The timing and sequence of cultural changes in this area remains poorly understood resulting from the lack of radiometric age control. This paper presents a refined radiocarbon (14C) chronology to resolve the timing of human occupation and cultural evolution in the Hexi Corridor. Radiocarbon dating of crop remains, which have an annual life cycle, has the advantage of eliminating problems such as the carbon stored in ‘old wood’. As a result, 14C dates from crop remains are used to test the validity of the 14C dates derived from charcoal, bone and plant remains. The resultant database of ages is modelled using the Bayesian approach. The updated chronology presented in this paper shows generally good agreement with the original dates used to constrain cultural evolution in the Hexi Corridor. However, the occupation of the corridor by the Shajing and Shanma Late Bronze Age-Early Iron Age cultures appears to have taken place ~300 years later than previously thought. On the basis of comparing this updated chronology of cultural evolution with palaeoclimatic and historical records, it is proposed that the collapse of the Shajing and Shanma cultures occurred as a result of geopolitical impact rather than climate change.

Onset of Giant Planet Migration before 4480 Million Years Ago

Abstract Soon after their formation, the terrestrial planets experienced intense impact bombardment by comets, leftover planetesimals from primary accretion, and asteroids. This temporal interval in solar system evolution, termed late accretion, thermally and chemically modified solid planetary surfaces and may have impeded life's emergence on the Hadean (pre-3850 Ma) Earth. The sources and tempo of bombardment, however, remain obscure. Here we present a timeline that relates variably retentive radiometric ages documented from asteroidal meteorites to new dynamical models that invoke an early episode of planetesimal-driven giant planet migration after the dispersal of the protoplanetary disk. Reconciliation of geochronological data with dynamical models shows that such giant planet migration should lead to an intense ∼30 Myr influx of comets to the entire solar system manifested in radiometric age data. The absence of wholesale crustal reset ages after ∼4450 Ma for the most resilient chronometers from Earth, Moon, Mars, 4 Vesta, and various meteorite parent bodies confines the onset of giant planet migration to ca. 4480 Ma. Waning impacts continue to strike the inner planets through a protracted monotonic decline in impactor flux, in agreement with predictions from crater chronology. New global 3D thermal analytical bombardment models derived from our revised impact mass-production functions show also that persistent niches for prebiotic chemistry leading to the emergence of life on the early Hadean Earth could endure late accretion since at least about 4400 million years ago.

Enhanced upwelling and phosphorite formation in the northeastern Pacific during the late Oligocene: Depositional mechanisms, environmental conditions, and the impact of glacio-eustacy

AbstractThe late Oligocene–early Miocene interval records a discernable episode of phosphorite formation, which is considered as the first of four main phosphogenic episodes during the late early and late Cenozoic. In order to better constrain the processes leading to widespread phosphorite formation we present new radiometric, geochemical, palynological, and sedimentological data from a drill core of the Roca Fosfórica Mexicana phosphorite mine at San Juan de la Costa, Baja California Sur (Mexico). In this region, phosphogenesis was enabled by the combination of high productivity and low sediment-accumulation rates due to enhanced upwelling and low detrital input related to regionally dry climate conditions. Phosphatic particles were formed in a shallow and well-oxygenated setting, subsequently concentrated by winnowing, and transported by gravity currents, which were mostly triggered by seismic activity. Following their deposition in a deeper and less well oxygenated setting pervasive phosphogenesis contributed to cementing the accumulated phosphatic grains.Correlation with global paleoclimate records suggests that this phosphogenic episode was linked to the expansion of the Antarctic ice sheet. Glacial weathering and the establishment of large-amplitude glacio-eustatic variations enhanced phosphorus supply on a global scale. Both glacial and interglacial phases participated in enhancing primary productivity in oceans, increasing the phosphorus flux into sediments, and favoring phosphogenesis, with glaciation being the prime cause.In addition, radiometric ages obtained in this study (28.62, 28.1, 27.19, 27.08, and 26.94 Ma) indicate that the onset of the late Oligocene–early Miocene phosphogenic episode was diachronous on a global scale with 2–3 m.y. older ages in the eastern Pacific in comparison to the Mediterranean and central Atlantic. This delay is explained by regional differences in paleoenvironmental and paleoceanographic conditions.

Plagioclase-hornblende geothermobarometry of metamafites from the Arroio Grande Ophiolite, Dom Feliciano Belt, southernmost Brazil

This paper reports the first geothermobarometry data for the Brazilian sector of the Punta del Este Terrane (southeasternmost Dom Feliciano Belt), based on petrographic features (optical and electron microscopy) and mineral chemistry (electron probe microanalysis) of metamafites from the Arroio Grande Ophiolite, located near the Brazil/Uruguay border. Microtextures found in the studied amphibolite and metagabbro samples suggest that the southern portion of the ophiolite underwent two main metamorphic events, named M1 and M2. Based on field relations and radiometric ages from the literature, the minimum age for the M1 is constrained at ca. 640 Ma (migmatization event in the northern portion of the ophiolite). The M2 is constrained at ca. 610-585 Ma (syn-kinematic granite intrusion and formation of blackwall reaction zones in its southern portion). The P-T conditions of both events are estimated from plagioclase-hornblende geothermobarometry, constraining the M1 between 706 and 750 ± 40 °C and 5.5–6.3 ± 1.5 kbar, and the M2 between 595 and 698 ± 40 °C and 3.4–5.3 ± 1.5 kbar. It suggests that, as early as 640 Ma, fragments of the ophiolite were buried at depths of ca. 20–23 ± 5 km and, at around 610-585 Ma, these rocks were affected by a shear zone system at depths of ca. 15–19 ± 5 km. The P-T estimates presented in this paper, together with the suggested ages for the M1 and M2 events, are in good agreement with data reported for the southern (Cerro Olivo Complex) and northern (Paso del Dragón Complex) sectors of the Punta del Este Terrane in Uruguay. Thus, the present contribution further demonstrates that high-T/low-P conditions, up to the lower-granulite facies, together with partial melting events, are ubiquitous in the southeasternmost sector of the Dom Feliciano Belt during the Late Cryogenian events related to the Brasiliano/Pan-African orogenic cycle and the amalgamation of the Southwest Gondwana paleocontinent.

40Ar-39Ar geochronology of the active phonolitic Cadamosto Seamount, Cape Verde

Cadamosto Seamount is located in the SW of the Cape Verde Archipelago in the central Atlantic Ocean off the west coast of Africa. Many radiometric dates exist for the islands in the archipelago, however, no geochronological information has been obtained from the numerous seamounts. The timescales for igneous processes in the submarine realm are thus poorly understood.In this study, we investigated five lavas that were sampled by dredging and ROV (remotely operated vehicle) from the flanks and summit areas of the largely phonolitic Cadamosto Seamount during two different research cruises. Chemical zonation patterns of minerals were determined by electron microprobe, and radiometric ages were obtained from single-crystal total-fusion and single−/multi-grain step-heating 40Ar-39Ar analyses of sanidine, nepheline and sodalite-group minerals.Our 40Ar-39Ar results reveal young sanidine eruption ages (all <100 ka) at Cadamosto Seamount: (1) Three western flank/summit lavas have a relatively simple petrology dominated by phenocrysts, and overlap with mean sanidine ages of 20.98 ± 0.87 ka, 21.44 ± 0.80 ka and 22.3 ± 2.0 ka, with a combined mean age of 21.04 ± 0.62 ka from the three samples (all uncertainties are quoted at 2σ). (2) The remaining two samples from the summit/NE flank are dominated by complex zoned sanidines with resorbed antecrystic cores and phenocrystic rims. These samples yield older sanidine ages of 51.8 ± 2.4 ka and 97 ± 14 ka, which are interpreted to be maximum eruption ages. This is due to the dominance of antecrysts in these two samples and the possibility that the analyzed sanidine grains may be a mixture of older antecrystic cores and younger phenocrystic rims.The older 40Ar-39Ar ages of many sanidine and nepheline antecrysts also give us clues regarding older magmatic events at Cadamosto Seamount, despite these grains having undergone resorption and phenocrystic rim overgrowths, resulting in some radiogenic 40Ar loss during entrainment in the subsequent magmas. The antecrysts minimum ages extend back to 1.5215 ± 0.0083 Ma, which supports the age progression of magmatism observed in the southern islands chain of the Cape Verde Archipelago.The youngest volcanic eruption period (21.14 ± 0.62 ka) occurred during the Last Glacial Maximum, a period of global sea level lowstands. We suggest that the comparatively rapid crust unloading leading up to the lowstand may have reduced pressure conditions within the Cadamosto Seamount magma plumbing system, and thus led to enhanced submarine eruption activity.

Geological mapping and chronology of lunar landing sites: Apollo 11

Crater size-frequency distribution (CSFDs) measurements allow the derivation of absolute model ages (AMAs) for geological units across various terrestrial bodies in the Solar System based on body-specific adjustments to the lunar chronology (e.g., Hartmann, 1970; Neukum et al., 1975, 1983, 2001; Stöffler and Ryder, 2001; Stöffler et al., 2006; Hiesinger et al., 2012; Robbins, 2014). Thus, it is important to revisit and test the accuracy of the lunar chronology using data from recent lunar missions (e.g., Hiesinger et al., 2000, 2012, 2015; Rajmon and Spudis, 2004; Stöffler et al., 2006), as well as newer analyses of lunar samples (e.g., Gaffney et al., 2011, Meyer, 2012; Snape et al., 2016; Welsh et al., 2018). We generated a new detailed geological map of the Apollo 11 landing region based on spectral characteristics, topography, and albedo maps, which shows several mare units adjacent to the lunar module. Lunar Reconnaissance Orbiter Camera (LROC) images were used to measure new CSFDs and derive the cumulative number of craters with diameters ≥1 km or N(1) for the Apollo 11 landing site. The newly derived N(1) values are consistent with the presence of only one surficial unit at the landing site: the Group A, High-K (high potassium) “young” mare basalt as described in Meyer (2012). We reviewed the radiometric ages for Apollo 11 samples that have been determined since the calibration of the lunar cratering chronology, used our new geological map to reinterpret their provenance, and correlated them with the new N(1) values. These are plotted and compared with the lunar chronology of Neukum et al. (1983). Our calibration point for the Apollo 11 landing site is consistent with the earlier values, thus, confirming Neukum's 1983 and Neukum and Ivanov's 2001 lunar chronology curves.

Establishment of Upper Cretaceous bio- and carbon isotope stratigraphy in the northwest Pacific Ocean and radiometric ages around the Albian/Cenomanian, Coniacian/Santonian and Santonian/Campanian boundaries

The Yezo Group, exposed in Hokkaido, northern Japan, is one of the few strata that recorded Cretaceous paleo-oceanographic changes in the Pacific Ocean. Although the group yields abundant marine macro- and microfossils, sporadic occurrences of age-diagnostic species in several stratigraphic intervals has prevented high-resolution international biostratigraphic correlation. The proposed integrated bio- and carbon isotope stratigraphy of the Yezo Group, which is exposed in northwest Hokkaido, enables high-resolution international stratigraphic correlation spanning from Upper Albian to Lower Campanian. The correlation of the present study identifies the detailed stratigraphic horizons of the Albian/Cenomanian, Cenomanian/Turonian, Turonian/Coniacian, Coniacian/Santonian, and Santonian/Campanian boundaries, as well as the Late Cretaceous paleo-environmental events in the Yezo Group. The Yezo Group is also important in order to complement the chronometric age of the Cretaceous geologic time scale because the group includes numerous felsic tuffs. The U-Pb zircon ages of some of these tuffs, which are interbedded near the Albian/Cenomanian, Coniacian/Santonian and the Santonian/Campanian boundaries in the Yezo Group, are dated at 99.7±0.3 Ma (Quidelleur et al. 2011), 86.87±0.60/0.67 (internal/total error) Ma and 84.7±0.7/1.8 (internal/total error) Ma, respectively. These radiometric ages are consistent with the latest age model of the Cretaceous time scale.

Optimizing subsampling strategies for U/Th dating and geochemical proxies in carbonate speleothems

Since they provide the chronological frame on which paleoclimate and paleoenvironmental reconstructions are built, radiometric ages of carbonate speleothems, especially U/Th dating, are of fundamental importance to speleothem and paleoclimate sciences. Based on 17 aragonite-calcite speleothems from the Pont-de-Ratz Cave (S. France), we propose an integrated petrographical and geochemical approach for optimizing subsampling in speleothems. Portable energy-dispersive X-ray fluorescence was used as a fast and non-destructive in-situ trace-element screening of the speleothems. These geochemical results are combined with petrographical and mineralogical information to produce distribution maps showing the growth discontinuities and the identified primary and secondary (diagenetic) products on the speleothem slabs. Because they provide a clear and precise spatial distribution of primary and secondary carbonate phases at the sample scale, these maps constitute a valuable tool for selecting the position of subsamples for radiometric dating or environmental proxy analysis. The efficiency and reliability of our integrative approach is convincingly demonstrated by the remarkable coherence of U/Th ages both within and between speleothem samples.