Carl C. Swisher

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.

Cretaceous age for the feathered dinosaurs of Liaoning, China

The ancient lake beds of the lower part of the Yixian Formation, Liaoning Province, northeastern China, have yielded a wide rangeof well-preserved fossils: the ‘feathered’ dinosaurs Sinosauropteryx, Protarchaeopteryx and Caudipteryx, the primitive birds Confuciusornis and Liaoningornis, the mammal Zhangheotherium and the reportedly oldest flowering plant, Archaefructus. Equally well preserved in the lake beds are a wide range of fossil plants, insects, bivalves, conchostracans, ostracods, gastropods, fish, salamanders, turtles, lizards, the frog Callobatrachus and the pterosaur Eosipterus,. This uniquely preserved assemblage of fossils is providing newinsight into long-lived controversies over bird–dinosaur relationships,, the early diversification of birds,, and the origin and evolution of flowering plants. Despite the importance of this fossil assemblage, estimates of its geological age have varied widely from the Late Jurassic to the Early Cretaceous. Here we present the first 40Ar/39Ar dates unambiguously associated with the main fossil horizons of the lower part of the Yixian Formation, and thus, for the first time, provide accurate age calibration of this important fauna. The results of this dating study indicate that the lower Yixian fossil horizons are not Jurassic but rather are at least 20 Myr younger, placing them within middle Early Cretaceous time.

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.

Latest Homo erectus of Java: Potential Contemporaneity with Homo sapiens in Southeast Asia

Hominid fossils from Ngandong and Sambungmacan, Central Java, are considered the most morphologically advanced representatives of Homo erectus. Electron spin resonance (ESR) and mass spectrometric U-series dating of fossil bovid teeth collected from the hominid-bearing levels at these sites gave mean ages of 27 ± 2 to 53.3 ± 4 thousand years ago; the range in ages reflects uncertainties in uranium migration histories. These ages are 20,000 to 400,000 years younger than previous age estimates for these hominids and indicate that H. erectus may have survived on Java at least 250,000 years longer than on the Asian mainland, and perhaps 1 million years longer than in Africa. The new ages raise the possibility that H. erectus overlapped in time with anatomically modern humans (H. sapiens) in Southeast Asia.

Coeval 40Ar/39Ar Ages of 65.0 Million Years Ago from Chicxulub Crater Melt Rock and Cretaceous-Tertiary Boundary Tektites

40Ar/39Ar dating of drill core samples of a glassy melt rock recovered from beneath a massive impact breccia contained within the 180-kilometer subsurface Chicxulub crater in Yucat�n, Mexico, has yielded well-behaved incremental heating spectra with a mean plateau age of 64.98 � 0.05 million years ago (Ma). The glassy melt rock of andesitic composition was obtained from core 9 (1390 to 1393 meters) in the Chicxulub 1 well. The age of the melt rock is virtually indistinguishable from 40Ar/39Ar ages obtained on tektite glass from Beloc, Haiti, and Arroyo el Mimbral, northeastern Mexico, of 65.01 � 0.08 Ma (mean plateau age for Beloc) and 65.07 � 0.10 Ma (mean total fusion age for both sites). The 40Ar/39Ar ages, in conjunction with geochemical and petrological similarities, strengthen the recent suggestion that the Chicxulub structure is the source for the Haitian and Mexican tektites and is a viable candidate for the Cretaceous-Tertiary boundary impact site.

Stratigraphy and chronology of Upper Cretaceous–lower Paleogene strata in Bolivia and northwest Argentina

Integration of sequence stratigraphy, magnetostratigraphy, Ar/Ar dating, and paleontology considerably advances knowledge of the Late Cretaceous–early Paleogene chronostratigraphy and tectonic evolution of Bolivia and adjacent areas. The partly restricted marine El Molino Formation spans the Maastrichtian and Danian (°73–60.0 Ma). Deposition of the alluvial to lacustrine Santa Lucia Formation occurred between 60.0 and 58.2 Ma. The widespread erosional unconformity at the base of the Cayara Formation is 58.2 Ma. This unconformity separates the Upper Puca and Corocoro supersequences in Bolivia, and is thus coeval with the Zuni-Tejas sequence boundary of North America. The thick overlying Potoco and Camargo formations represent a late Paleocene–Oligocene foreland fill. The onset of shortening along the Pacific margin at °89 Ma initially produced rifting in the distal foreland. Santonian–Campanian eastward-onlapping deposits indicate subsequent waning of tectonic activity along the margin. Significant tectonism and magmatism resumed along the margin at °73 Ma and produced an abrupt increase in subsidence rate and other related phenomena in the basin. Subsidence was maximum between °71 and °66 Ma. Due to the early Maastrichtian global sea-level high, marine waters ingressed from the northwest into this underfilled basin. Subsidence decreased during the Late Maastrichtian and was low during the Danian. It increased again in the latest Danian, for which a slight transgression is recorded, and peaked in the early Selandian. Tectonism between 59.5 and 58.2 Ma produced a variety of deformational and sedimentary effects in the basin and correlates with the end of emplacement of the Coastal batholith. The subsequent 58.2 Ma major unconformity marks the onset of continental foreland basin development, which extended into Andean Bolivia during the late Paleocene–Oligocene interval. This basin underwent internal deformation as early as Eocene time in the Altiplano and Cordillera Oriental. These early structures, previously assigned to the late Oligocene–early Miocene orogeny, probably accommodated observed tectonic rotations in the Eocene–Oligocene.

The Ischigualasto Tetrapod Assemblage (Late Triassic, Argentina) and 40Ar/39Ar Dating of Dinosaur Origins

40Ar/39Ar dating of sanidine from a bentonite interbedded in the Ischigualasto Formation of northwestern Argentina yielded a plateau age of 227.8 � 0.3 million years ago. This middle Carnian age is a direct calibration of the Ischigualasto tetrapod assemblage, which includes some of the best known early dinosaurs. This age shifts last appearances of Ischigualasto taxa back into the middle Carnian, diminishing the magnitude of the proposed late Carnian tetrapod extinction event. By 228 million years ago, the major dinosaurian lineages were established, and theropods were already important constituents of the carnivorous tetrapod guild in the Ischigualasto—Villa Uni�n Basin. Dinosaurs as a whole remained minor components of tetrapod faunas for at least another 10 million years.

Late Cenozoic Antarctic paleoclimate reconstructed from volcanic ashes in the Dry Valleys region of southern Victoria Land

We report the discovery of numerous in situ Miocene and Pliocene airfall volcanic ashes that occur within the hyperarid Dry Valleys region of the Transantarctic Mountains in southern Victoria Land, Antarctica. Ashes that occur above 1000 m elevation rest at the ground surface, covered only by a thin ventifact pavement 1 to 2 cm thick. The ash deposits are loose and unconsolidated and show no signs of chemical weathering. Laser-fusion 40 Ar/ 39 Ar analyses of volcanic crystals and glass shards indicate that the ashes range from 4.33 Ma to 15.15 Ma in age. The Arena Valley ash (4.33 ± 0.07 Ma) rests on the surface of a well-developed desert pavement and ultraxerous soil profile at 1410 m elevation. Lack of geomorphic evidence of liquid water on surficial sediments coeval and older than the Arena Valley ash, together with the pristine condition of volcanic crystals and lack of authigenic clay formation, indicates a cold desert at and since 4.33 Ma. The Beacon Valley ash (10.66 ± 0.29 Ma), the Koenig Valley ash (13.65 ± 0.06 Ma), and the Nibelungen Valley ash (15.15 ± 0.02 Ma) fill the upper half of relict sand-wedge troughs that form only in cold-desert conditions. The lack of authigenic clay-sized minerals in these ash deposits, along with preservation of sharp lateral contacts with surrounding sand-and-gravel deposits, suggests that frozen conditions (without rain or well-developed active layers during summer months) have persisted in Beacon, Koenig, and Nibelungen Valleys since ash deposition. Ash-avalanche deposits that rest on rectilinear slopes contain matrix ash dated to 7.42 ± 0.31 Ma in upper Arena Valley and 11.28 ± 0.05 Ma in lower Arena Valley. Little slope development has occurred since emplacement of these ash-avalanche deposits. Such slope stability is consistent with cold-desert conditions well below 0 °C. Taken together, these ash deposits point to persistent polar conditions similar to the present at elevations above 1000 m in the western Dry Valleys region during at least the last 15.0 m.y. This conclusion contradicts the view that, during part of the Pliocene epoch, East Antarctica was largely free of glacier ice and that scrub vegetation (Nothofagus, Southern Beech) survived along the Transantarctic Mountain front in the Dry Valleys region and to at least lat 86°S (Webb and Harwood, 1993). Instead, it supports marine and geomorphological evidence that calls for a stable Antarctic cryosphere, much the same as today, since middle Miocene time.

Element fluxes from the volcanic front of Nicaragua and Costa Rica

10 10 kg/m/Myr) and central Costa Rica (2.4 � 10 10 kg/m/Myr) is greatly reduced from previous estimates and now within the range of error estimates. We estimate the subducted component of flux for Cs, Rb, Ba, Th, U, K, La, Pb, and Sr by subtracting estimated mantle-derived contributions from the total element flux. An incompatible element-rich OIB source for the Cordillera Central segment in Costa Rica makes the subducted element flux there highly sensitive to small changes in the modeled mantle-derived contribution. For the other three segments studied, the estimated errors in concentrations of highly enriched, subductionderived elements (Cs, Ba, K, and Pb) are less than 26%. Averaged over the time of the current episode of volcanism, the subduction-derived fluxes of Cs, Ba, K, Pb, and Sr are not significantly different among the four segments of the Central American volcanic front in Nicaragua and Costa Rica. The subductionderived fluxes of Th and La appear to increase to the SE across Nicaragua and Costa Rica, but the estimated errors in their subduction-derived concentrations are very high, making this variation questionable. The lack of change in the fluxes of Cs, Ba, K, Pb, and Sr argues that the well-defined regional variation in Ba/La is the result of changes in the mode or mechanics of fluid delivery into the mantle wedge, not the total amounts of fluids released from the slab. Concentrated or focused fluids in Nicaragua lead to high degrees of melting. Diffuse fluids in Costa Rica cause lower degrees of melting. Components: 12,742 words, 11 figures, 5 tables.

Miocene Glacial Stratigraphy and Landscape Evolution of the Western Asgard Range, Antarctica

40Ar/39Ar dated in-situ volcanic ashfall deposits indicate that the surficial stratigraphy of the western Asgard Range in the Dry Valleys region extends back at least to 15.0 Ma. The preservation o...