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

(40)Ar/(39)Ar 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 Yucatán, 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 (40)Ar/(39)Ar 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 (40)Ar/(39)Ar 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.     

The Manson Impact Structure: 40Ar/39Ar Age and Its Distal Impact Ejecta in the Pierre Shale in Southeastern South Dakota

The (40)Ar/(39)Ar ages of a sanidine clast from a melt-matrix breccia of the Manson, Iowa, impact structure (MIS) indicate that the MIS formed 73.8 +/- 0.3 million years ago (Ma) and is not coincident with the Cretaceous-Tertiary boundary (64.43 +/- 0.05 Ma). The MIS sanidine is 9 million years older than (40)Ar/(39)Ar age spectra of MIS shock-metamorphosed microcline and melt-matrix breccia interpreted earlier to be 64 to 65 Ma. Grains of shock-metamorphosed quartz, feldspar, and zircon were found in the Crow Creek Member (upper Campanian) at a biostratigraphic level constrained by radiometric ages in the Pierre Shale of South Dakota that are consistent with the (40)Ar/(39)Ar age of 73.8 +/- 0.3 Ma for MIS reported herein.     

40Ar-39Ar Dating of the Manson Impact Structure: A Cretaceous-Tertiary Boundary Crater Candidate

The mineralogy of shocked mineral and lithic grains in the Cretaceous-Tertiary (K-T) boundary claystone worldwide is most consistent with a bolide impact on a continent. Both the concentrations and sizes of these shocked grains are greatest in the western interior of North America. These data suggest that the Manson impact structure in north-central Iowa is a viable candidate for the K-T boundary impact event. Argon-40-argon-39 age spectrum dating of shocked microcline from the crystalline central uplift of the Manson impact structure indicates that there was severe argon-40 loss at 65.7 +/- 1.0 million years ago, an age that is indistinguishable from that of the K-T boundary, within the limits of analytical precision.     

Age and Duration of Weathering by 40K-40Ar and 40Ar/39Ar Analysis of Potassium-Manganese Oxides

Supergene cryptomelane [K(1-2)(Mn(3+)Mn(4+))(8)O(16). chiH(2)O] samples from deeply weathered pegmatites in southeastern Brazil subjected to (40)K-(40)Ar and (40)Ar/(39)Ar analysis yielded (40)K-(40)Ar dates ranging from 10.1 +/- 0.5 to 5.6 +/- 0.2 Ma (million years ago). Laser-probe (40)Ar/(39)Ar step-heating of the two most disparate samples yielded plateau dates of 9.94 +/- 0.05 and 5.59 +/- 0.10 Ma, corresponding, within 2 sigma, to the (40)K-(40)Ar dates. The results imply that deep weathering profiles along the eastern Brazilian margin do not reflect present climatic conditions but are the result of a long-term process that was already advanced by the late Miocene. Weathering ages predate pulses of continental sedimentation along the eastern Brazilian margin and suggest that there was a time lag between weathering and erosion processes and sedimentation processes.     

Paleocene-Eocene thermal maximum and the opening of the Northeast Atlantic

The Paleocene-Eocene thermal maximum (PETM) has been attributed to a sudden release of carbon dioxide and/or methane. 40Ar/39Ar age determinations show that the Danish Ash-17 deposit, which overlies the PETM by about 450,000 years in the Atlantic, and the Skraenterne Formation Tuff, representing the end of 1 +/- 0.5 million years of massive volcanism in East Greenland, are coeval. The relative age of Danish Ash-17 thus places the PETM onset after the beginning of massive flood basalt volcanism at 56.1 +/- 0.4 million years ago but within error of the estimated continental breakup time of 55.5 +/- 0.3 million years ago, marked by the eruption of mid-ocean ridge basalt-like flows. These correlations support the view that the PETM was triggered by greenhouse gas release during magma interaction with basin-filling carbon-rich sedimentary rocks proximal to the embryonic plate boundary between Greenland and Europe.     

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

(40)Ar/(39)Ar 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 Unión Basin. Dinosaurs as a whole remained minor components of tetrapod faunas for at least another 10 million years.     

Synchrony and causal relations between permian-triassic boundary crises and siberian flood volcanism

The Permian-Triassic boundary records the most severe mass extinctions in Earth's 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 (40)Ar/(39)Ar 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.     

Lunar impact history from (40)Ar/(39)Ar dating of glass spherules

Lunar spherules are small glass beads that are formed mainly as a result of small impacts on the lunar surface; the ages of these impacts can be determined by the (40)Ar/(39)Ar isochron technique. Here, 155 spherules separated from 1 gram of Apollo 14 soil were analyzed using this technique. The data show that over the last approximately 3.5 billion years, the cratering rate decreased by a factor of 2 to 3 to a low about 500 to 600 million years ago, then increased by a factor of 3.7 +/- 1.2 in the last 400 million years. This latter period coincided with rapid biotic evolutionary radiation on Earth.     

40Ar/39Ar Age of the Lathrop Wells Volcanic Center, Yucca Mountain, Nevada

Paleomagnetic and (40)Ar/(39)Ar analyses from the Lathrop Wells volcanic center, Nevada, indicate that two eruptive events have occurred there. The ages (136 +/- 8 and 141 +/- 9 thousand years ago) for these two events are analytically indistinguishable. The small angular difference (4.7 degrees ) between the paleomagnetic directions from these two events suggests they differ in age by only about 100 years. These ages are consistent with the chronology of the surficial geological units in the Yucca Mountain area. These results contradict earlier interpretations of the cinder-cone geomorphology and soil-profile data that suggest that at least five temporally discrete eruptive events occurred at Lathrop Wells approximately 20,000 years ago.     

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

The Paraná-Etendeka flood volcanic event produced approximately 1.5 x 10(6) cubic kilometers of volcanic rocks, ranging from basalts to rhyolites, before the separation of South America and Africa during the Cretaceous period. New (40)Ar/(39)Ar data combined with earlier paleomagnetic results indicate that Paraná 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. Paraná 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.     

Early and Late Alkali Igneous Pulses and a High-3He Plume Origin for the Deccan Flood Basalts

Several alkalic igneous complexes of nephelinite-carbonatite affinities occur in extensional zones around a region of high heat flow and positive gravity anomaly within the continental flood basalt (CFB) province of Deccan, India. Biotites from two of the complexes yield (40)Ar/(39)Ar dates of 68.53 +/- 0.16 and 68.57 +/- 0.08 million years. Biotite from a third complex, which intrudes the flood basalts, yields an (40)Ar/(39)Ar date of 64.96 +/- 0.1 1 million years. The complexes thus represent early and late magmatism with respect to the main pulse of CFB volcanism 65 million years ago. Rocks from the older complexes show a (3)He/(4)He ratio of 14.0 times the air ratio, an initial (87)Sr/(86)Sr ratio of 0.70483, and other geochemical characteristics similar to ocean island basalts; the later alkalic pulse shows isotopic evidence of crustal contamination. The data document 3.5 million years of incubation of a primitive, high-(3)He mantle plume before the rapid eruption of the Deccan CFB.     

Single-Crystal 40Ar/39Ar Dating of the Eocene-Oligocene Transition in North America

Explanations for the causes of climatic changes and associated faunal and floral extinctions at the close of the Eocene Epoch have long been controversial because of, in part, uncertainties in correlation and dating of global events. New single-crystal laser fusion (SCLF) (40)Ar/(39)Ar dates on tephra from key magnetostratigraphic and fossilbearing sections necessitate significant revision in North American late Paleogene chronology. The Chadronian-Orellan North American Land Mammal "Age" boundary, as a result, is shifted from 32.4 to 34.0 Ma (million years ago), the Orellan-Whitneyan boundary is shifted from 30.8 to 32.0 Ma, and the Whitneyan-Arikareean boundary is now approximately 29.0 Ma. The new dates shift the correlation of Chron C12R from the Chadronian to within the Orellan-Whitneyan interval, the Chadronian becomes late Eocene in age, and the North American Oligocene is restricted to the Orellan, Whitneyan, and early Arikareean. The Eocene-Oligocene boundary, and its associated climate change and extinction events, as a result, correlates with the Chadronian-Orellan boundary, not the Duchesnean-Chadronian 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 (40)Ar/(39)Ar 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.     

High-3He Plume Origin and Temporal-Spatial Evolution of the Siberian Flood Basalts

An olivine nephelinite from the lower part of a thick alkalic ultrabasic and mafic sequence of volcanic rocks of the northeastern part of the Siberian flood basalt province (SFBP) yielded a (40)Ar/(39)Ar plateau age of 253.3 +/- 2.6 million years, distinctly older than the main tholeiitic pulse of the SFBP at 250.0 million years. Olivine phenocrysts of this rock showed (3)He/(4)He ratios up to 12.7 times the atmospheric ratio; these values suggest a lower mantle plume origin. The neodymium and strontium isotopes, rare earth element concentration patterns, and cerium/lead ratios of the associated rocks were also consistent with their derivation from a near-chondritic, primitive plume. Geochemical data from the 250-million-year-old volcanic rocks higher up in the sequence indicate interaction of this high-(3)He SFBP plume with a suboceanic-type upper mantle beneath Siberia.     

A tropical rainforest in Colorado 1.4 million years after the Cretaceous-Tertiary boundary

An extremely diverse lower Paleocene (64.1 million years ago) fossil leaf site from Castle Rock, Colorado, contains fossil litter that is similar to the litter of extant equatorial rainforests. The presence of a high-diversity tropical rainforest is unexpected, because other Paleocene floras are species-poor, a feature generally attributed to the Cretaceous-Tertiary (K-T) extinction. The site occurs on the margin of the Denver Basin in synorogenic sedimentary rocks associated with the rise of the Laramide Front Range. Orographic conditions caused by local topography, combined with equable climate, appear to have allowed for the establishment of rainforests within 1.4 million years of the K-T boundary.     

40Ar/39Ar dates from the West Siberian Basin: Siberian flood basalt province doubled

Widespread basaltic volcanism occurred in the region of the West Siberian Basin in central Russia during Permo-Triassic times. New 40Ar/39Ar age determinations on plagioclase grains from deep boreholes in the basin reveal that the basalts were erupted 249.4 +/- 0.5 million years ago. This is synchronous with the bulk of the Siberian Traps, erupted further east on the Siberian Platform. The age and geochemical data confirm that the West Siberian Basin basalts are part of the Siberian Traps and at least double the confirmed area of the volcanic province as a whole. The larger area of volcanism strengthens the link between the volcanism and the end-Permian mass extinction.     

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 approximately 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 method's absolute uncertainty from approximately 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 approximately 65.95 Ma for the Cretaceous/Tertiary boundary.     

Late cretaceous polar wander of the pacific plate: evidence of a rapid true polar wander event

We reexamined the Late Cretaceous-early Tertiary apparent polar wander path for the Pacific plate using 27 paleomagnetic poles from seamounts dated by (40)Ar/(39)Ar geochronology. The path shows little motion from 120 to 90 million years ago (Ma), northward motion from 79 to 39 Ma, and two groups of poles separated by 16 to 21 degrees with indistinguishable mean ages of 84 +/- 2 Ma. The latter phenomenon may represent a rapid polar wander episode (3 to 10 degrees per million years) whose timing is not adequately resolved with existing data. Similar features in other polar wander paths imply that the event was a rapid shift of the spin axis relative to the mantle (true polar wander), which may have been related to global changes in plate motion, large igneous province eruptions, and a shift in magnetic field polarity state.     

40Ar/39Ar Dating of the Brunhes-Matuyama Geomagnetic Field Reversal

Magnetostratigraphic studies are widely used in conjunction with the geomagnetic polarity time scale (GPTS) to date events in the range 0 to 5 million years ago. A critical tie point on the GPTS is the potassium-argon age of the most recent (Brunhes-Matuyama) geomagnetic field reversal. Astronomical values for the forcing frequencies observed in the oxygen isotope record in Ocean Drilling Project site 677 suggest that the age of this last reversal is 780 ka (thousand years ago), whereas the potassium-argon-based estimate is 730 ka. Results from 4039; Ar incremental heating studies on a series of lavas from Maui that straddle the Brunhes-Matuyama reversal give an age of 783 + 11 ka, in agreement with the astronomically derived value. The astronomically based technique appears to be a viable tool for dating young sedimentary sequences.     

An Archean Geomagnetic Reversal in the Kaap Valley Pluton, South Africa

The Kaap Valley pluton in South Africa is a tonalite intrusion associated with the Archean Barberton Greenstone Belt. Antipodal paleomagnetic directions determined from the central and marginal parts of the pluton record a geomagnetic reversal that occurred as the pluton cooled. The age of the reversal is constrained by an 40Ar/39Ar plateau age from hornblende at 3214 +/- 4 million years, making it the oldest known reversal. The data presented here suggest that Earth has had a reversing, perhaps dipolar, magnetic field since at least 3.2 billion years ago.