Felix Oberli

Timing of the Permian–Triassic biotic crisis: implications from new zircon U/Pb age data (and their limitations)

Abstract The most profound biotic crisis in the Earth’s history, causing the near extinction of both terrestrial and marine life, occurred at the end of the Permian period about 253 Myr ago and marks the Paleozoic–Mesozoic era boundary. The cause of this event is still a matter of vigorous debate, with both brief and catastrophic as well as gradual mechanisms having been proposed. Similar to a recent landmark study, this study uses the U–Pb method on zircons from the uppermost Permian/lowermost Triassic ash fall deposits at Meishan (Zhejiang Province, SE China) in order to examine time and rate constraints for these events. The results of both this study and previous work show that for these ash layers, the effects of Pb loss are combined with varying amounts and sources of inheritance, resulting in an age scatter which prohibits the extraction of a statistically robust age in many cases. Though the effects of Pb loss on the zircons analyzed in this study were reduced by leaching the grains in hydrofluoric acid (as opposed to commonly applied air abrasion) prior to analysis, the presence within a single ash layer of multiple generations of older xenocrysts (in many cases only slightly older than the depositional age) has made quantitative interpretation even more difficult. When these combined phenomena bias individual zircon ages by less than a percent, they are extremely difficult to deconvolute, and, if multi-grain analyses are used, can become impossible to recognize (because of the resulting age averaging). Monte Carlo simulations using actual measurements of individual zircon crystals show that age excursions due to Pb loss and xenocrystic contamination for the Meishan bentonites are easily homogenized to the point of undetectability when replicate analyses of multi-grain zircon samples are compared. Thus this study uses only high-precision analyses of single crystals, whether from our work or that of previous studies. Three main conclusions have emerged. First, our data require a significant increase in the age of the Permian–Triassic boundary by more than 2 myr compared to the previous study, which shifts the age to a value older than 253 Ma. Second, neither our data nor those from previous work can confirm or negate the possibility of a very abrupt biotic crisis. Third, even large suites of very high-quality, single-zircon U–Pb analyses for these tuffs cannot, in most cases, yield objective, reliable, and robust dates with accuracies at the sub-myr level – though the temptation to perform arbitrary selection of subsets of the analyses for that purpose is almost irresistible. The last conclusion is not an indictment of zircon U/Pb dating in general (other rocks and other zircon populations can – and do – behave very differently), and further technical advances will likely improve our ability to prepare grains or sub-grains of adequately enhanced quality for analysis. Consequently, the results of the present study strongly suggest that for problems requiring time-scale accuracy, inferences from zircon U–Pb dating must be based on sufficiently large suites of single-crystal or crystal domain, high-precision analyses (

High resolution UPb dating of Middle Triassic volcaniclastics: Time-scale calibration and verification of tuning parameters for carbonate sedimentation

Abstract We report high-resolution single-zircon UPb age data for Middle Triassic volcaniclastic intercalations in biostratigraphically calibrated pelagic successions of the Southern Alps. The results require a redefinition of the chronometric scale for the Middle Triassic. Moreover, they do not support current models relating cyclic sedimentation in platform carbonates of the Dolomites to orbital tuning. Tight concordant age clusters were obtained for five volcaniclastic layers in three ammonoid biozones of late Anisian to early Ladinian age. Two layers in the (Nevadites) Secedensis Zone yielded identical mean 206 Pb 238 U ages of 241.2 + 0.8/−0.8 Ma and 241.2 + 0.8/−0.6 Ma (errors given at the 95% confidence level). A layer in the Gredleri Zone is dated at 238.8 + 0.5/−0.2 Ma, and two horizons in the Archelaus Zone yield similar ages of 237.9 + 1.0/−0.7 Ma and 238.0 + 0.4/−0.7 Ma. These results are significantly older than the age values of 233–235 Ma assigned to the Anisian/Ladinian boundary by several current time scales [1–4]. Moreover, our estimate of 240.7–241.3 Ma (depending on biostratigraphic collocation) for the Anisian/Ladinian boundary casts doubts on the reliability of age values of 245–250 Ma proposed by most time scales for the Permian/Triassic boundary. The occurrence of pelagic fossils in basinal sediments as well as in age-equivalent shallow marine Middle Triassic platform carbonates in the Dolomites allows the sedimentary sequences of both environments to be correlated. The 800 m thick Latemar platform (western Dolomites) is characterized by cyclic stacking patterns, which have been interpreted as results of Milankovitch-type high-frequency/low-amplitude sea-level fluctuations. The 12 m.y. interval of platform growth postulated from the assignment of orbital periodicities to the platform carbonate cycles [5,6] is in conflict with a maximum time span of 4.7 m.y. allowed by the present zircon data.

Timing of normal faulting along the Indus Suture in Pakistan Himalaya and a case of major 231Pa/235U initial disequilibrium in zircon

We report age data by TIMS U–Pb, LA-ICP-MS and 39Ar–40Ar techniques for main magmatic events in the Lower Swat region of Pakistan, in order to constrain the tectonic evolution of the northwestern Himalaya. The pre-Himalayan history of the Indian continent is documented by single-zircon U–Pb results from the peraluminous Choga granite gneiss, which yielded a 468±5 Ma lower concordia intercept interpreted to approximate the time of magmatic emplacement. The presence of a well-defined 870±7 Ma inherited component (upper intercept) suggests a plutonic or volcanic protolith residing at unexposed levels of the Indian crust. Zircon data for the Swat granite gneiss from the northern part of the Loe Sar dome give an emplacement age of 267+6/−3 Ma, which is at variance with earlier correlations favoring an early Paleozoic origin. Subsequent to metamorphic overprint by the Himalayan orogeny, the Swat granite was intruded by late kinematic alkali-granite dykes. Single zircons from one of these dykes show reproducible 206Pb/238U data giving a precise mean age of 29.26±0.12 Ma, whereas the 207Pb/235U ages scatter between 34 and 81 Ma, pointing to huge and variable enrichment in 207Pb. The unsupported 207Pb can be explained by incorporation of 231Pa, an intermediate long-lived daughter nuclide in the 235U decay chain, in excess of the secular equilibrium ratio. LA-ICP-MS measurements confirm the presence of unsupported 207Pb, but do not show any correlation between the latter and other selected trace element concentrations in these zircons. A concordant Ar–Ar muscovite age of 28.4±1.1 Ma obtained for the same dyke postdates regional mica ‘cooling’ ages and indicates a lack of younger regional events capable of resetting the K–Ar system in muscovite. Because the dyke is pre- to synkinematic relative to normal faulting and related north-vergent folds, its emplacement age provides a maximum age for this event or reflects already ongoing local extension. A lower limit of 15 Ma has previously been established by apatite fission track analysis. The 29–15 Ma age bracket for normal faulting is coeval with extension along the South Tibetan Detachment System (STDS) on the north side of the High Himalaya. This suggests that the Indus Suture in Pakistan has acted as a western continuation of the STDS and that related faulting was roughly contemporaneous over most of the Himalaya.

The early evolution of the Southwest Swedish Gneiss Province: geochronological and isotopic evidence from southernmost Sweden

Abstract The continental crust in southwestern Sweden has a complicated history of formation, apparently commencing ∼ 1.75 Ga ago and continuing until the end of the Sveconorwegian orogeny ∼ 0.9 Ga ago. Most of the rocks are polymetamorphic gneisses, but massifs of intrusive rocks can commonly be recognized. This study reports new UPb zircon ages from the province of Skane in the southernmost part of the region. Two gneiss samples and four samples of granitoid rocks were dated by the UPb zircon method. In the two gneiss samples, selected single zircon crystals were analyzed in addition to the standard multi-grain size fractions. The multigrain zircon analyses yielded an upper intercept age of 1613 ± 6 Ma for a grey gneiss from the central part of the province of Skane, and 1557−27+32 Ma for a red aplitic gneiss from southern Skane. Individual zircon crystals from these samples gave 207Pb 206Pb -ages between 1335 and 1669 Ma. For the grey gneiss, the individual zircons define a separate discordia with an upper-intercept age of 1640±16 Ma; in the red gneiss sample they scatter around a line with an upper intercept at 1675±25 Ma. Core-bearing zircons from a red gneissic granite in central Skane yielded an age of 1575−61+77 Ma, while similar zircons from a strongly foliated red granite from the Kullaberg horst in northwestern Skane gave an age of 1497−34+47 Ma. Core-free zircons from the Beden granodiorite in southern Skane have an age of 1449−11+23 Ma, and core-bearing zircons from a gneissic charnockite near Orkelljunga in northern Skane yielded an age of 1452−50+350 Ma. Although the latter age is poorly defined, it allows for similar intrusion ages of the Orkelljunga charnockite and the Beden granodiorite. SmNd isotopic analyses yield TDM ages of 1.84 o 1.99 Ga, and TCHUR ages of 1.48 to 1.60 Ga for these rocks, while RbSr analyses give TUR ages between 1.46 and 1.71 Ga. The combined evidence suggests that the main crust-forming episode in this part of southwestern Sweden occurred 1.6 to 1.7 Ga ago, with presumably subduction-related magmatism forming the protoliths of the red and grey gneisses. Around 1.5 to 1.6 Ga ago red anatectic granites were generated. The Beden granodiorite and the Orkelljunga charnockite, as well as the Varberg charnockite farther north, were all formed ∼ 1.45 Ga ago, possibly by mantle-generated anorogenic magmatism. Some 0.9 Ga ago, after the Sveconorwegian crustal thickening and high-pressure metamorphism, uplift within the Southwest Swedish Gneiss Province exposed lower- to mid-crustal granulitic and charnockitic rocks in its southern part.

Re-assessment of silicon isotope reference materials using high-resolution multi-collector ICP-MS

Silicon isotope ratios can now be measured to very high precision using high-resolution multi-collector ICP-MS. Based on this technique we report that the Si isotope composition of IRMM-018 is significantly lighter than the NBS28 standard, in direct contrast to previously published results. Our data are also inconsistent with recently published absolute Si isotope abundances for these standards by Valkiers et al. (2005) and Ding et al. (2005). Instead, our results are coherent with the certified values for NIST standard SRM990 that was used to determine the atomic weight of Si, with a 30Si/29Si ratio that is over 6 permil lower for the same atomic weight. In order to avoid problems with future assessments of stable Si isotope variations, the NBS28 silica sand standard (RM8546) should remain the zero point. Therefore, an inter-laboratory calibration of NBS28 and other references materials is recommended to solve the observed discrepancies and establish a reliable scale for reporting Si isotopes.

The influence of high U-Th inclusions on the U-Th-Pb systematics of almandine-pyrope garnet: results of a combined bulk dissolution, stepwise-leaching, and SEM study

Abstract The isotope systematics of garnet are increasingly being used to define prograde thermal histories in metamorphic belts. This approach to geochronology has the distinct advantage that time information is directly related to the pressure-temperature-deformation history recorded by the same mineral. One potential problem with the technique, however, is that the inventories of radiogenic isotopes apparently measured in garnet separates may be located in inclusions rather than the garnet lattice itself. Inclusions of old, refractory minerals such as zircon may bias the ages to the extent that they are rendered meaningless in terms of dating garnet growth, establishing PT histories, and constraining tectonic mechanisms. In this contribution we present U-Th-Pb data obtained on bulk dissolutions of garnet from the Himalaya as well as the results of a detailed SEM and U-Th-Pb stepwise-leaching study in order to further clarify these issues. Our experiment has the advantage that the age difference between the garnets and potential inclusions is very large so that the influence of the latter is highlighted. The U-Th-Pb data from the bulk dissolutions are highly complex and yield apparent ages ranging from 1000 Ma to 30 Ma. The stepwise-leaching study demonstrates that this complexity results from small inclusions of allanite and zircon detected with the SEM. The bulk of the radiogenic 206Pb in these samples was released only by treatment at high pressure in a bomb. In addition, the 208Pb/206Pb ratio of the radiogenic Pb released in earlier leach steps is inconsistent with the available constraints on the Th/U ratio of garnet lattices as measured by SIMS. In this case, the U-Pb ages of these garnets is dependent purely on the age of the included high U-Th phases and the degree to which they have isotopically re-equilibrated with the whole rock during the subsequent metamorphism that grew garnet. Simple mass-balance arguments show that, in this particular instance, very small amounts of these inclusions are required, amounts that escape detection during conventional sample preparation. These data also have implications for current thinking on the closure temperature for the Sm-Nd and U-Pb systems in garnet.

Quantification of transient signals in multiple collector inductively coupled plasma mass spectrometry: accurate lead isotope ratio determination by laser ablation of individual fluid inclusions

This work establishes the analytical protocol for accurate Pb isotopic analysis of fast transient signals by multiple-collector ICP-MS instruments. Individual synthetic fluid inclusions of known Pb and Tl isotopic compositions (dissolved SRM 981 with or without SRM 997 from NIST, enclosed in quartz by a hydrothermal crack annealing technique) were liberated by 193 nm UV laser ablation (LA). Data were recorded on Faraday detectors, for which correction schemes for bias in amplifier response (“tau correction”) are presented and evaluated. tau-Corrected Pb isotope data reveal LA-induced isotope fractionation amounting to ∼0.5% amu−1 for Pb isotopes over the course of an entire fluid inclusion ablation. Instrumental mass bias correction was effected within-run using Tl provided by the fluid inclusion itself or admixed to the ablation aerosol via desolvated nebulization. Isotope ratios derived from the transient signals were either based on individual readings or on bulk signal integration, of which the latter produces significantly more accurate data. The external precision achieved by ablating SRM 610 glass with a 60 µm beam is ±0.011% (2 SD, relative) for 208Pb/206Pb and 207Pb/206Pb ratios and ±0.032% for Pb isotope ratios normalized to mass 204 (n = 18). Inclusion-to-inclusion reproducibilities (n = 11; ∼0.1 ng Pb per inclusion) are ±0.05% (2 SD; 208Pb/206Pb and 207Pb/206Pb) and ±0.13% (20xPb/204Pb), respectively; inclusions containing as little as 0.005 ng Pb returned ±0.1% and ±0.8%. These results are accurate as demonstrated by analysis of synthetic fluid inclusions containing SRM 981 Pb. The analytical protocol presented here for measuring isotope ratios on minute analyte quantities by multiple-collector ICP-MS in fast transient signal mode has great potential for applications to geochemical, archaeological, environmental and possibly biochemical problems.

Climate and vegetation history of western Portugal inferred from Albian near-shore deposits (Galé Formation, Lusitanian Basin)

The late Early Cretaceous greenhouse climate has been studied intensively based on proxy data derived essentially from open marine archives. In contrast, information on continental climatic conditions and on the accompanying response of vegetation is relatively scarce, most notably owing to the stratigraphic uncertainties associated with many Lower Cretaceous terrestrial deposits. Here, we present a palynological record from Albian near-shore deposits of the Lusitanian Basin of W Portugal, which have been independently dated using Sr-isotope signals derived from low-Mg oyster shell calcite. 87 Sr/ 86 Sr values fluctuate between 0.707373 ± 0.00002 and 0.707456 ± 0.00003; absolute values and the overall stratigraphic trend match well with the global open marine seawater signature during Albian times. Based on the new Sr-isotope data, existing biostratigraphic assignments of the succession are corroborated and partly revised. Spore-pollen data provide information on the vegetation community structure and are flanked by sedimentological and clay mineralogical data used to infer the overall climatic conditions prevailing on the adjacent continent. Variations in the distribution of climate-sensitive pollen and spores indicate distinct changes in moisture availability across the studied succession with a pronounced increase in hygrophilous spores in late Early Albian times. Comparison with time-equivalent palynofloras from the Algarve Basin of southern Portugal shows pronounced differences in the xerophyte/hygrophyte ratio, interpreted to reflect the effect of a broad arid climate belt covering southern and southeastern Iberia during Early Albian times.

Quantitative extraction and high precision isotope measurements of nickel by MC-ICPMS

This paper presents a new technique for the precise and accurate determination of Ni isotopic composition in geological samples, involving a two-stage ion-exchange procedure for nickel extraction and the use of a large-geometry high-resolution MC-ICPMS instrument. The key step of the chemical separation is an ion-exchange procedure based on the complexation of Ni with dimethylglyoxime which replaces the previous liquid–liquid extraction technique. The new method allows for quantitative recovery of Ni even for silicate samples as opposed to the previous technique. Measurements of an Ni standard doped with Fe or Zn show that isobaric interferences of these elements can be accurately corrected as long as Fe/Ni < 0.1 and Zn/Ni < 0.0006. Replicate measurements of a standard solution yield long-term external reproducibilities (2σ) of ±50 ppm for 60Ni/58Ni and ±120 ppm for 61Ni/58Ni, when ratios are normalized to 62Ni/58Ni.

The half-life of 126Sn refined by thermal ionization mass spectrometry measurements

Abstract Recently, Haas et al. [Nucl. Instrum. Methods Phys. Res. B 114 (1996) 131] have determined the half-life of 126 Sn by combining measurements of activity concentration (by gamma counting) and isotope abundance [by accelerator mass spectrometry (AMS)]. A new determination of the isotope abundance of 126 Sn in the same material by thermal ionization mass spectrometry (TIMS) is reported, which minimizes the error contribution to the overall uncertainty of the decay constant. The greatly improved value for the half-life of 126 Sn (2.345 ± 0.071) × 10 5 a (1σ), is consistent with the less precise determination partly based on AMS, and with an independent measurement by Zhang et al. [J. Radioanal. Nucl. Chem. 212 (1996) 93] partly based on a fission yield estimate. A value of (5.2849 ± 0.0017) × 10 −5 (95% confidence level) is reported for 234 U/ 238 U in the National Bureau of Standards Standard Reference Material (SRM) 960/New Brunswick Laboratory Certified Reference Material (CRM) 112a uranium standard.