Jan R. Wijbrans

Synchronizing rock clocks of Earth history.

Calibration of the geological time scale is achieved by independent radioisotopic and astronomical dating, but these techniques yield discrepancies of ∼1.0% or more, limiting our ability to reconstruct Earth history. To overcome this fundamental setback, we compared astronomical and 40Ar/39Ar ages of tephras in marine deposits in Morocco to calibrate the age of Fish Canyon sanidine, the most widely used standard in 40Ar/39Ar geochronology. This calibration results in a more precise older age of 28.201 ± 0.046 million years ago (Ma) and reduces the 40Ar/39Ar methods absolute uncertainty from ∼2.5 to 0.25%. In addition, this calibration provides tight constraints for the astronomical tuning of pre-Neogene successions, resulting in a mutually consistent age of ∼65.95 Ma for the Cretaceous/Tertiary boundary.

Short-lived and discontinuous intraplate volcanism in the South Pacific: Hot spots or extensional volcanism?

[1] South Pacific intraplate volcanoes have been active since the Early Cretaceous. Their HIMU-EMIEMII mantle sources can be traced back into the West Pacific Seamount Province (WPSP) using plate tectonic reconstructions, implying that these distinctive components are enduring features within the Earth’s mantle for, at least, the last 120 Myr. These correlations are eminent on the scale of the WPSP and the South Pacific Thermal and Isotopic Anomaly (SOPITA), but the evolution of single hot spots emerges notably more complicated. Hot spots in the WPSP and SOPITA mantle regions typically display intermittent volcanic activity, longevities shorter than 40 Myr, superposition of hot spot volcanism, and motion relative to other hot spots. In this review, we use 40 Ar/ 39 Ar seamount ages and Sr-Nd-Pb isotopic signatures to map out Cretaceous volcanism in the WPSP and to characterize its evolution with respect to the currently active hot spots in the SOPITA region. Our plate tectonic reconstructions indicate cessation of volcanism during the Cretaceous for the Typhoon and Japanese hot spots; whereas the currently active Samoan, Society, Pitcairn and Marquesas hot spots lack long-lived counterparts in the WPSP. These hot spots may have become active during the last 20 Myr only. The other WPSP seamount trails can be only ‘‘indirectly’’ reconciled with hot spots in the SOPITA region. Complex age distributions in the Magellan, Anewetak, Ralik and Ratak seamount trails would necessitate the superposition of multiple volcanic trails generated by the Macdonald, Rurutu and Rarotonga hot spots during the Cretaceous; whereas HIMU-type seamounts in the Southern Wake seamount trail would require 350–500 km of hot spot motion over the last 100 Myr following its origination along the Mangaia-Rurutu ‘‘hotline’’ in the Cook-Austral Islands. These observations, however, violate all assumptions of the classical Wilson-Morgan hot spot hypothesis, indicating that long-lived, deep and fixed mantle plumes cannot explain the intraplate volcanism of the South Pacific region. We argue that the observed short-lived and discontinuous intraplate volcanism has been produced by another type of hot spot-related volcanism, as opposed to the strong and continuous Hawaiian-type hot spots. Our results also indicate that other geological processes (plate tension, hotlines, faulting, wetspots, self-propagating volcanoes) may act in conjunction with hot spot volcanism in the South Pacific. In all these scenarios, intraplate volcanism has to be controlled by ‘‘broad-scale’’ events giving rise to multiple closely-spaced mantle plumelets, each with a distinct isotopic signature, but only briefly active and stable over geological time. It seems most likely that these plumelets originate and dissipate at very shallow mantle depths, where they may shoot off as thin plumes from the top of a ‘‘superplume’’ that is present in the South Pacific mantle. The absence of clear age progressions in most

Laser 40Ar/39Ar dating of single detrital muscovite grains from early foreland-basin sedimentary deposits in India: Implications for early Himalayan evolution

In India, the Dagshai and overlying Kasauli Formations represent the oldest exposed continental foredeep sediments eroded from the Himalayan orogen. 40Ar/39Ar dating of individual detrital white micas from these sedimentary units has provided maximum depositional ages of <28 Ma for the Dagshai Formation at one locality and <25 Ma at a second locality, whereas deposition of the Kasauli Formation occurred after 28 Ma at two localities and after 22 Ma at a third locality. This timing suggests that, in India, the start of substantial exhumation and erosion from the rising Himalayan orogen was delayed until 28 Ma.

New 40Ar/39Ar age of the Bishop Tuff from multiple sites and sediment rate calibration for the Matuyama-Brunhes boundary

Precise dating of sanidine from proximal ash flow Bishop Tuff and air fall Bishop pumice and ash, California, can be used to derive an absolute age of the Matuyama Reversed-Brunhes Normal (M-B) paleomagnetic transition, identified stratigraphically close beneath the Bishop Tuff and ash at many sites in the western United States. An average age of 758.9±1.8 ka, standard error of the mean (SEM), was obtained for individual sanidine crystals or groups of several crystals, determined from ∼70 individual analyses of sanidine separates from 11 sample groups obtained at five localities. The basal air fall pumice (757.7±1.8 ka) and overlying ash flow tuff (762.2±4.7 ka) from near the source yield essentially the same dates within errors of analysis, suggesting that the two units were emplaced close in time. A date on distal Bishop air fall ash bed at Priant, California, ∼100 km to the west of the source area, is younger, 750.1±4.3 ka, but not significantly different within analytical error (±1 standard deviation). Previous dates of the Bishop Tuff, obtained by others using conventional K-Ar and the fission track method on zircons, ranged from ∼650 ka to ∼1.0 Ma. The most recent, generally accepted date by the K-Ar method on sanidine was 738±3 ka. We infer, as others before, that many K-Ar dates on sanidine feldspar are too young owing to incomplete degassing of radiogenic Ar during fusion in the K-Ar technique and that many older K-Ar dates are too old owing to detrital or xenocrystic contamination in the larger samples that are necessary for the technique. The new dates are similar to recent 40Ar/39Ar ages of the Bishop Tuff determined on individual samples by others but are derived from a larger proximal sample population and from multiple analysis of each sample. The results provide a definitive and precise age calibration of this widespread chronostratigraphic marker in the western United States and northeastern Pacific Ocean. We calculated the age of the M-B transition at five sites, assuming constant sedimentation rates, the age of the Bishop ash bed and one or more well-dated chronostratigraphic horizons above and below the Bishop Tuff ash bed and M-B transition, and stratigraphic separations between these datum levels. The age of the M-B transition is 774.2±2.8 ka, based on the average of eight such calculations, close to other recent determinations, and similar to that determined from the astronomically tuned polarity timescale. Our approach provides an alternative and surprisingly precise method for determining the age of the M-B and other chronostratigraphic levels. The above dates, calculated using U.S. Geological Survey values of 27.92 Ma for the Taylor Creek (TC) sanidine can be recalculated to other widely used values for these monitors. For example, using recently published values of 28.34 Ma (TC) and 523.1 Ma (McLure Mountain hornblende, MMhb-1), the resulting ages are ∼774 ka for the Bishop Tuff and ash bed and ∼789 ka for the M-B transition.

40Ar/39Ar ages and paleomagnetism of São Miguel lavas, Azores

We present new 40 Ar= 39 Ar ages and paleomagnetic data for Sao Miguel island, Azores. Paleomagnetic samples were obtained for 34 flows and one dike; successful mean paleomagnetic directions were obtained for 28 of these 35 sites. 40 Ar= 39 Ar age determinations on 12 flows from the Nordeste complex were attempted successfully: ages obtained are between 0.78 Ma and 0.88 Ma, in contrast to published K-Ar ages of 1 Ma to 4 Ma. Our radiometric ages are consistent with the reverse polarity paleomagnetic field directions, and indicate that the entire exposed part of the Nordeste complex is of a late Matuyama age. The duration of volcanism across Sao Miguel is significantly less than previously believed, which has important implications for regional melt generation processes, and temporal sampling of the geomagnetic field. Observed stable isotope and trace element trends across the island can be explained, at least in part, by communication between different magma source regions at depth. The 40 Ar= 39 Ar ages indicate that our normal polarity paleomagnetic data sample at least 0.1 Myr (0-0.1 Ma) and up to 0.78 Myr (0-0.78 Ma) of paleosecular variation and our reverse polarity data sample approximately 0.1 Myr (0.78-0.88 Ma) of paleosecular variation. Our results demonstrate that precise radiometric dating of numerous flows sampled is essential to accurate inferences of long-term geomagnetic field behavior. Negative inclination anomalies are observed for both the normal and reverse polarity time-averaged field. Within the data uncertainties, normal and reverse polarity field directions are antipodal, but the reverse polarity field shows a significant deviation from a geocentric axial dipole direction.

Dating crystalline groundmass separates of altered Cretaceous seamount basalts by the Ar-40/Ar-39 incremental heating technique

Abstract Alteration of submarine basalts compromises geochronology using conventional K/Ar and 40 Ar/ 39 Ar techniques. To help overcome these problems, we re-evaluated the potential of groundmass dating techniques. Incremental heating on acid-leached groundmass samples, following an overnight bakeout at 200°C and using high-resolution heating schedules, eliminated most of the low temperature alteration effects in the submarine basalts studied. More than 75% of the groundmass analyses (n=32) display accurate age plateaus consisting of 30–70% of the total amount of 39 Ar K released. More than 50% of the analyses have plateau ages concordant with their total fusion ages implying minor or proportional loss of radiogenic 40 Ar * and 39 Ar K . Overall, we could show a high degree of coherence between ages of groundmass separates and comagmatic phenocrysts. This suggests that the dating of aphyric basalts, which previously has proven problematic, can be accomplished with increasing confidence as well. Adding these rock types to the list of datable submarine basalts significantly enhances our ability to understand the eruptive history of linear submarine volcanic chains.

Middle-Late Alpine thermotectonic evolution of the southern Rhodope Massif, Greece

AbstractThe transition from the Alpine tectonic assembly to the exhumation of the units in the Rhodope metamorphic province in northernmost Greece has been refined by 40Ar/39Ar laserprobe mica analyses. Preservation of pre-Alpine (~ 280 Ma and 145 Ma) muscovite cooling ages at the western margin of the Rhodope indicate that subsequent events failed to reset the argon system thermally in white mica in the outcropping basement of this region. The central and eastern Rhodope are characterized by white mica cooling ages of 40–35 Ma with ages gradually decreasing to ca. 15 Ma near the eastern margin of the Strymon Valley. The Eo-Oligocene ages reflect the regional exhumation of the metamorphosed units to shallow crustal levels, with corresponding temperatures below ca. 350 °C, by 40–35 Ma. The younger cooling ages are attributed to the initiation and subsequent operation of the Strymon-Thasos detachment system since ca. 30 Ma. This study provides a crucial contribution to future regional tectonic models for th...

Long-term cosmogenic 3He production rates (152 ka - 1.35 Ma) from 40Ar/39Ar dated basalt flows at 29ºN latitude

A set of time integrated cosmogenic 3He production rates in olivines for the last 1.35 Ma are presented. We investigated three basaltic lava flow tops from Lanzarote, Canary Islands, Spain. The 40Ar/39Ar ages determined for those basalt flows by incremental laser heating of leached groundmass samples are 152±26 ka, 281±19 ka and 1.35±0.01 Ma (±2σ). Three or four different olivine phenocryst samples have been analyzed from each flow for their cosmogenic 3He abundances. The resulting 3He production rates in olivine at sea level at 29° latitude are 82±14 and 82±8 atoms g−1 a−1, as obtained from the 152 ka and 281 ka old flows, respectively. Considering effects of erosion on the 1.35 Ma old flow we find that the production rate of 82±8 atoms g−1 a−1 is consistent with the cosmogenic 3He production during the last 1.35 Ma. There appears to be a 14% discrepancy between previously published production rates derived at higher latitudes and altitudes if the scaling factors of Lal are used to compare results. This discrepancy is greatly reduced, however, if the revised scaling factors of Dunai (this issue) are applied. Using the new scaling factors we derive a production rate for cosmogenic 3He in olivine at sea level and high latitudes (>60°) of 118±11 atoms g−1 a−1 (±2σ). The correspondingly revised value of Cerling and Craig, and Ackert et al.) is 123±6 atoms g−1 a−1 (±2σ). The mean value of these two calibrations is 121±6 atoms g−1 a−1 (±2σ). We suggest that the production rate of 121±6 atoms g−1 a−1 at sea level and high latitudes may be applied to the complete time range where paleomagnetic data indicate that there was no long-term averaged intensity variation in the Earth’s magnetic field, i.e. over the last 10 Ma.

Episodic exhumation in the Western Alps

Oligocene to Miocene clastic sediments of the Tertiary Piedmont Basin (northwest Italy) were derived from erosion of Western Alps source rocks. Detrital white micas from dif- ferent stratigraphic units and from sands of three present-day rivers draining the internal Western Alps have been analyzed by 40 Ar/ 39 Ar geochronology. Our data suggest a wide- spread, fast cooling and exhumation event prior to ca. 38 Ma followed by a .30 m.y. period of slower cooling and exhumation combined with erosion of crustal rocks with uniform 40 Ar/ 39 Ar signatures. These processes have resulted in a pattern of regularly increasing lag time up section.

40Ar/39Ar laserprobe direct dating of discrete deformational events: continuous record of Early Alpine tectonics in the Pelagonian Zone, NW Aegean area, Greece.

Abstract The Pelagonian Zone of the Hellenides, which occurs in the northwestern Aegean region of the Eastern Mediterranean, contains remnants of a high-pressure metamorphic belt which subsequently underwent localized reworking during low-temperature deformational events. Formation of Alpine deformational structures in the Ossa Massif (northern Pelagonian Zone) occurred at metamorphic temperatures which failed to reset most pre-Alpine 40Ar/39Ar cooling ages in muscovite porphyroclasts. The low metamorphic temperatures at Alpine times have preserved the argon isotopic signature in white mica bearing deformational structures, which allows direct dating of these structures and refinement of the Alpine tectonic history in the region. After separation of carefully selected and characterized mica generations, application of sensitive 40Ar/39Ar laserprobe dating has resulted in a continuous record of the early Alpine tectonic history of the Ossa Massif from the Early Cretaceous well into the Eocene. The early Alpine history of the Ossa Massif records cooling of basement thrust slices, which are mostly Hercynian granites and metamorphics, below ca. 350°C by 100 Ma, while lower tectonostratigraphic levels of the basement had already cooled below these temperatures at around ca. 285 Ma. Blueschist facies mylonite fabrics, related to a top-to-ENE direction of tectonic transport, yield ages as old as 84.5±3.3 Ma. Exhumation of the blueschist facies sequences was initiated at ca. 54 Ma and involved tectonic activity along blueschist facies and greenschist facies mylonites zones. Final WSW-ward transport of the metamorphic sequence across the structurally lowest, and supposed, autochthonous, series occurred at around 45 Ma, and resulted in termination of the ductile deformation history in the studied area. This study confines the onset of high-pressure metamorphism in the Pelagonian Zone to an interval between 100 Ma and 85 Ma and has shown that high-pressure metamorphism had terminated by ca. 54 Ma. The recognition of an early to middle Alpine cycle, lasting over 30 million years and involving crustal shortening, high-pressure metamorphism, exhumation and subsequent crustal shortening is a key contribution to a better understanding of the early Alpine tectonic history of the Aegean region and eastern Mediterranean.