Paul R. Renne

Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating

The 40Ar/39Ar dating method depends on accurate intercalibration between samples, neutron fluence monitors, and primary 40Ar/40K (or other external) standards. The 40Ar/39Ar age equation may be expressed in terms of intercalibration factors that are simple functions of the relative ages of standards, or equivalently are equal to the ratio of radiogenic to nucleogenic K-derived argon (40Ar/39ArK) values for one standard or unknown relative to another. Intercalibration factors for McClure Mountain hornblende (MMhb-1), GHC-305 biotite, GA-1550 biotite, Taylor Creek sanidine (TCs) and Alder Creek sanidine (ACs), relative to Fish Canyon sanidine (FCs), were derived from 797 analyses involving 11 separate irradiations with well-constrained neutronfluence variations. Values of the intercalibration factors are RFCsMMhb-1 = 21.4876 ± 0.0079; RFCsGA-1550 = 3.5957 ± 0.0038; RFCsTCs = 1.0112 ± 0.0010; RFCsACs = 0.04229 ± 0.00006, based on the mean and standard error of the mean resulting from four or more spatially distinct co-irradiations of FCs with the other standars. Analysis of 35 grains of GHC-305 irradiated in a single irradiation yields RFCsGHC-305 = 3.8367 ± 0.0143. Results at these levels of precision essentially eliminate intercalibration as a significant source of error in 40Ar/39Ar dating. Data for GA-1550 (76 analyses, 5 fluence values), TCs (54 analyses, 4 fluence values), FCs (380 analyses, 40 fluence values) and ACs (86 analyses, 11 fluence values) yield MSWD values showing that the between-grain dispersion of 40Ar∗/39ArK values is consistent with analytical errors alone, whereas MMhb-1 (167 analyses, 4 irradiations) and GHC-305 (34 analyses, 1 fluence value) are heterogeneous and therefore unsuitable as standards for small sample analysis. New K measurements by isotope dilution for two primary standards, GA-1550 biotite (8 analyses averaging 7.626 ± 0.016 wt%) and intralaboratory standard GHC-305 (10 analyses averaging 7.570 ± 0.011 wt%), yield values slightly lower and more consistent than previous data obtained by flame photometry, with resulting 40Ar/40K ages of 98.79 ± 0.96 Ma and 105.6 ± 0.3 Ma for GA-1550 and GHC-305, respectively. Combining these data with the intercalibration approach described herein and using GA-1550 as the primary standard (1.343 × 10−9 mol/g of 40Ar∗; [McDougall, I., Roksandic, Z., 1974. Total fusion 40Ar/39Ar ages using HIFAR reactor. J. Geol. Soc. Aust. 21, 81–89.]) yields ages of 523.1 ± 4.6 Ma for MMhb-1, 105.2 ± 1.1 Ma for GHC-305, 98.79 ± 0.96 Ma for GA-1550, 28.34 ± 0.28 Ma for TCs, 28.02 ± 0.28 for FCs, and 1.194 ± 0.012 Ma for ACs (errors are full external errors, including uncertainty in decay constants). Neglecting error in the decay constants, these ages and uncertainties are: 523.1 ± 2.6 Ma for MMhb-1, 105.2 ± 0.7 Ma for GHC-305, 98.79 ± 0.54 for GA-1550, 28.34 ± 0.16 Ma for TCs, 28.02 ± 0.16 Ma for FCs, and 1.194 ± 0.007 Ma for ACs. Using GHC-305 as the primary standard (1.428 ± 0.004 × 10−9 mol/g of 40Ar∗), ages are 525.1 ± 2.3 Ma for MMhb-1, 105.6 ± 0.3 Ma for GHC-305, 99.17 ± 0.48 Ma for GA-1550, 28.46 ± 0.15 Ma for TCs, 28.15 ± 0.14 Ma for FCs, and 1.199 ± 0.007 Ma for ACs, neglecting decay constant uncertainties. The approach described herein facilitates error propagation that allows for straightforward inclusion of uncertainties in the ages of primary standards and decay constants, without which comparison of 40Ar/39Ar dates with data from independent geochronometers is invalid. Re-examination of 40K decay constants would be fruitful for improved accuracy.

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.

A test for systematic errors in 40Ar/39Ar geochronology through comparison with U/Pb analysis of a 1.1-Ga rhyolite

Important sources of systematic error in 40Ar/39Ar dating arise from uncertainties in the 40K decay constants and K/Ar isotopic data for neutron fluence monitors (standards). The activity data underlying the decay constants used in geochronology since 1977 are more dispersed than acknowledged by previous geochronologically oriented summaries, and compilations of essentially the same data in nuclear physics and chemistry literature since 1973 have consistently produced lower estimates (and larger assigned uncertainties) of the constants for 40K → 40Ar and 40K → 40Ca decay. Considering also uncertainties in 40K/K, and the questionable existence of a γ-less electron capture 40K → 40Ar decay direct to ground state, the total 40K decay constant is known to no better than ±2% at the 2σ level.40Ar∗/40K ratios for individual standards are known to better than ±2% in some cases, but interlaboratory discrepancies of more than 2% in the 40Ar/39Ar ages of secondary standards like the Fish Canyon sanidine (FCs) suggest larger uncertainties. The very precisely determined decay constants for 238U and 235U, and the existence of quantitative internal U/Pb reliability criteria, offer an alternative basis for evaluation of both the 40K decay constants and the ages of 40Ar/39Ar standards. High precision U/Pb (zircon) and 40Ar/39Ar (alkali feldspar) data from the 1.1-Ga Palisade Rhyolite provide a highly sensitive basis for comparison. Ten U/Pb analyses on abraded single zircons as well as one analysis of six fragmented and HF leached crystals yield a 207Pb/206Pb age of 1097.6 +5.2/−5.4 Ma (95% confidence, including decay constant errors). 40Ar/39Ar incremental CO2 laser heating of single alkali feldspar grains yields nine undisturbed age spectra with error-weighted plateau ages (based on conventional decay constants and an age of 28.02 Ma for FCs) from 1086.5 ± 4.8 Ma (RFCsF239 = 52.7740 ± 0.3062) to 1090.4 ± 4.3 Ma (RFCsF239 = 53.0281 ± 0.2746) (2σ, including analytical errors only), with MSWD = 0.95. The weighted mean RFCsF239 value of these plateau steps (52.9011 ± 0.2324), including irradiation-related errors, is inferred to reflect the eruption-age RFCsF239 value of the Palisade Rhyolite alkali feldspar. Reconciliation of the 40Ar/39Ar and U/Pb results suggests that either the age of the 40Ar/39Ar standard is older, or the 40K total decay constant is smaller, than values in current use by geochronologists. Comparison with constraints from an historic eruption indicates a total 40K decay constant of 5.37 × 10−10/yr and an age of 28.05 Ma for FCs. Further applications of this approach will provide more robust solutions and allow estimation of uncertainties.

Synchrony and causal relations between Permian-Triassic boundary crises and siberian flood volcanism

The Permian-Triassic boundary records the most severe mass extinctions in Earths history. Siberian flood volcanism, the most profuse known such subaerial event, produced 2 million to 3 million cubic kilometers of volcanic ejecta in approximately 1 million years or less. Analysis of 40Ar/39Ar data from two tuffs in southern China yielded a date of 250.0 � 0.2 million years ago for the Permian-Triassic boundary, which is comparable to the inception of main stage Siberian flood volcanism at 250.0 � 0.3 million years ago. Volcanogenic sulfate aerosols and the dynamic effects of the Siberian plume likely contributed to environmental extrema that led to the mass extinctions.

The age of parana flood volcanism, rifting of gondwanaland, and the jurassic-cretaceous boundary.

The Paran�-Etendeka flood volcanic event produced ∼1.5 x 106 cubic kilometers of volcanic rocks, ranging from basalts to rhyolites, before the separation of South America and Africa during the Cretaceous period. New 40Ar/39Ar data combined with earlier paleomagnetic results indicate that Paran� flood volcanism in southern Brazil began at 133 � 1 million years ago and lasted less than 1 million years. The implied mean eruption rate on the order of 1.5 cubic kilometers per year is consistent with a mantle plume origin for the event and is comparable to eruption rates determined for other well-documented continental flood volcanic events. Paran� flood volcanism occurred before the initiation of sea floor spreading in the South Atlantic and was probably precipitated by uplift and weakening of the lithosphere by the Tristan da Cunha plume. The Parana event postdates most current estimates for the age of the faunal mass extinction associated with the Jurassic-Cretaceous boundary.

Rapid Eruption of the Siberian Traps Flood Basalts at the Permo-Triassic Boundary

The Siberian Traps represent one of the most voluminous flood basalt provinces on Earth. Laser-heating 40Ar/39Ar data indicate that the bulk of these basalts was erupted over an extremely short time interval (900,000 � 800,000 years) beginning at about 248 million years ago at mean eruption rates of greater than 1.3 cubic kilometers per year. Such rates are consistent with a mantle plume origin. Magmatism was not associated with significant lithospheric rifting; thus, mantle decompression resulting from rifting was probably not the primary cause of widespread melting. Inception of Siberian Traps volcanism coincided (within uncertainty) with a profound faunal mass extinction at the Permo-Triassic boundary 249 � 4 million years ago; these data thus leave open the question of a genetic relation between the two events.

2.6-Million-year-old stone tools and associated bones from OGS-6 and OGS-7, Gona, Afar, Ethiopia

CRAFT Research Center, 419 N. Indiana Avenue, Indiana University, Bloomington, IN, 47405, USA Department of Anthropology, Southern Connecticut State University, 501 Crescent Street, New Haven, CT 06515-1355, USA Department of Geosciences, University of Arizona, Tucson, AZ, 85721, USA Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA Department of Earth and Planetary Science, University of California, Berkeley, CA 94709, USA Departmento de Prehistoria y Arquelogia, Facultad de Geografia, e Historia, Universidad Complutense de Madrid, Ciudad Universitaria 28040, Madrid, Spain Department of Anthropology and CRAFT Research Center, 419 N. Indiana Avenue, Indiana University, Bloomington, IN, 47405, USA Sterkfontein Research Unit, University of Witwatersrand, WITS 2050, Johannesburg, South Africa Department of Anatomy, Case Western Reserve University-School of Medicine, 10900 Euclid Avenue, Cleveland, OH, 44106-4930, USA Laboratory of Physical Anthropology, Cleveland Museum of Natural History, Cleveland, OH 44106, USA

Synchrony of the Central Atlantic magmatic province and the Triassic-Jurassic boundary climatic and biotic crisis

The evolution of life on Earth is marked by catastrophic extinction events, one of which occurred ca. 200 Ma at the transition from the Triassic Period to the Jurassic Period (Tr-J boundary), apparently contemporaneous with the eruption of the worlds largest known continental igneous province, the Central Atlantic magmatic province. The temporal relationship of the Tr-J boundary and the provinces volcanism is clarified by new multidisciplinary (stratigraphic, palynologic, geochronologic, paleomagnetic, geochemical) data that demonstrate that development of the Central Atlantic magmatic province straddled the Tr-J boundary and thus may have had a causal relationship with the climatic crisis and biotic turnover demarcating the boundary.

Intercalibration of astronomical and radioisotopic time

The 40Ar/39Ar radioisotopic dating technique is one of the most precise and versatile methods available for dating events in Earths history, but the accuracy of this method is limited by the accuracy with which the ages of neutron-fluence monitors (dating standards) are known. Calibrating the ages of standards by conventional means has proved difficult and contentious. The emerging astronomically calibrated geomagnetic polarity time scale (APTS) offers a means to calibrate the ages of 40Ar/39Ar dating standards that is independent of absolute isotopic abundance measurements. Seven published 40Ar/39Ar dates for polarity transitions, nominally ranging from 0.78 to 3.40 Ma, are based on the Fish Canyon sanidine standard and can be compared with APTS predictions. Solving the 40Ar/39Ar age equation for the age of the Fish Canyon sanidine that produces coincidence with the APTS age for each of these seven reversals yields mutually indistinguishable estimates ranging from 27.78 to 28.09 Ma, with an inverse variance-weighted mean of 27.95 ± 0.18 Ma. Normalized residuals are minimized at an age of 27.92 Ma, indicating the robustness of the solution.

Stratigraphic, chronological and behavioural contexts of Pleistocene Homo sapiens from Middle Awash, Ethiopia

Clarifying the geographic, environmental and behavioural contexts in which the emergence of anatomically modern Homo sapiens occurred has proved difficult, particularly because Africa lacked adequate geochronological, palaeontological and archaeological evidence. The discovery of anatomically modern Homo sapiens fossils at Herto, Ethiopia, changes this. Here we report on stratigraphically associated Late Middle Pleistocene artefacts and fossils from fluvial and lake margin sandstones of the Upper Herto Member of the Bouri Formation, Middle Awash, Afar Rift, Ethiopia. The fossils and artefacts are dated between 160,000 and 154,000 years ago by precise age determinations using the 40Ar/39Ar method. The archaeological assemblages contain elements of both Acheulean and Middle Stone Age technocomplexes. Associated faunal remains indicate repeated, systematic butchery of hippopotamus carcasses. Contemporary adult and juvenile Homo sapiens fossil crania manifest bone modifications indicative of deliberate mortuary practices.