The 40Ar/39Ar and K/Ar dating of lavas from the Hilo 1-km core hole, Hawaii Scientific Drilling Project

USGS Publications Warehouse

Sharp, W.D.; Turrin, B.D.; Renne, P.R.; Lanphere, M.A.

1996-01-01

Mauna Kea lava flows cored in the HilIo hole range in age from <200 ka to about 400 ka based on 40Ar/39Ar incremental heating and K-Ar analyses of 16 groundmass samples and one coexisting plagioclase. The lavas, all subaerially deposited, include a lower section consisting only of tholeiitic basalts and an upper section of interbedded alkalic, transitional tholeiitic, and tholeiitic basalts. The lower section has yielded predominantly complex, discordant 40Ar/39Ar age spectra that result from mobility of 40Ar and perhaps K, the presence of excess 40Ar, and redistribution of 39Ar by recoil. Comparison of K-Ar ages with 40Ar/39Ar integrated ages indicates that some of these samples have also lost 39Ar. Nevertheless, two plateau ages of 391 ?? 40 and 400 ?? 26 ka from deep in the hole, combined with data from the upper section, show that the tholeiitic section accumulated at an average rate of about 7 to 8 m/kyr and has an mean recurrence interval of 0.5 kyr/flow unit. Samples from the upper section yield relatively precise 40Ar/39Ar plateau and isotope correlation ages of 326 ?? 23, 241 ?? 5, 232 ?? 4, and 199 ?? 9 ka for depths of -415.7 m to -299.2 m. Within their uncertainty, these ages define a linear relationship with depth, with an average accumulation rate of 0.9 m/kyr and an average recurrence interval of 4.8 kyr/flow unit. The top of the Mauna Kea sequence at -280 m must be older than the plateau age of 132 ?? 32 ka, obtained for the basal Mauna Loa flow in the corehole. The upward decrease in lava accumulation rate is a consequence of the decreasing magma supply available to Mauna Kea as it rode the Pacific plate away from its magma source, the Hawaiian mantle plume. The age-depth relation in the core hole may be used to test and refine models that relate the growth of Mauna Kea to the thermal and compositional structure of the mantle plume.

K-Ar dating of young volcanic rocks

DOE Office of Scientific and Technical Information (OSTI.GOV)

Damon, P.E.; Shafiqullah, M.

1991-01-31

Potassium-Argon (K-Ar) age dates were determined for forty-two young geologic samples by the Laboratory of Isotope Geochemistry, Department of Geosciences, in the period February 1, 1986 to June 30, 1989. Under the terms of Department of Energy Grant No. FG07-86ID12622, The University of Arizona was to provide state-of-the-art K-Ar age dating services, including sample preparation, analytical procedures, and computations, for forty-two young geologic samples submitted by DOE geothermal researchers. We billed only for forty samples. Age dates were determined for geologic samples from five regions with geothermal potential: the Cascade Mountains (Oregon); the Cascade Mountains (Washington); Ascension Island, South Atlanticmore » Ocean; Cerro Prieto, Mexico; and Las Azufres, Mexico. The ages determined varied from 5.92 m.a. to 0.62 m.a. The integration of K-Ar dates with geologic data and the interpretation in terms of geologic and geothermal significance has been reported separately by the various DOE geothermal researchers. Table 1 presents a detailed listing of all samples dated, general sample location, researcher, researcher's organization, rock type, age, and probable error (1 standard deviation). Additional details regarding the geologic samples may be obtained from the respective geothermal researcher. 1 tab.« less

30Ar-40Ar Ages of Silicates from IIE Iron Meteorites

NASA Astrophysics Data System (ADS)

Garrison, D. H.; Bogard, D. D.

1995-09-01

Several IIE iron meteorites contain small silicate inclusions, dispersed within metal, which suggest formation by a common process involving different degrees of heating and silicate fractionation from a chondrite-like parent (see discussion and references in McCoy [1]). The isotope chronology of IIE meteorites addresses two major questions concerning their origin. How many formation events are required, and do the isotopic ages also represent the times of silicate differentiation in some meteorites, or do they represent later impact heating events? We have determined ^39Ar-^40Ar ages of whole silicate samples of Watson, Techado, and Miles [1]. Although each meteorite gives a complex Ar age spectrum, each spectrum gives a well-defined age plateau over a significant (55-65%) portion of the total ^39Ar release. The ^39Ar-^40Ar degassing ages derived are 3.656 +/-0.005 Ga for Watson, 4.482 +/-0.025 Ga for Techado, and 4.408 +/-0.011 Ga for Miles (one-sigma errors). Absolute ages have an additional ^-0.5% uncertainty arising from the hornblende age monitor used. None of our Ar-Ar spectra show any significant evidence for an age older than those given, and only Miles shows modest evidence for recent diffusive loss of ^40Ar (affecting ^-10% of the ^39Ar release). Previous studies of Kodaikanal gave these ages: Rb-Sr = 3.7 +/-0.1 Ga [2], Pb-Pb = 3.676 +/-0.003 Ga [3], and K-^40Ar = 3.5 Ga [4]. Netschaevo gave a ^39Ar-^40Ar age of 3.74 Ga +/-0.03 Ga [5], and Watson gave a K-^40Ar age of 3.5 Ga [6]. (Some ages have been adjusted for changes in decay and irradiation constants.) All three meteorites suggest a common formation age of ^-3.70 +/-0.05 Ga. The ^39Ar-^40Ar age for Techado is identical to a ^39Ar-^40Ar age of 4.49 +/-0.03 Ga reported for Weekeroo Station [5] and to a Rb-Sr age of 4.51 Ga for Colomera [7]. These ages resemble ^39Ar-^40Ar ages of unshocked ordinary chondrites, and suggest that metal-silicate mixing and cooling to closure for Ar diffusion occurred

Cosmic Ray Exposure Ages, Ar-Ar Ages, and the Origin and History of Eucrites

NASA Technical Reports Server (NTRS)

Wakefield, Kelli; Bogard, Donald; Garrison, Daniel

2004-01-01

HED meteorites likely formed at different depths on the large asteroid 4-Vesta, but passed through Vesta-derived, km-sized intermediary bodies (Vestoids), before arriving at Earth. Most eucrites and diogenites (and all howardites) are brecciated, and impact heating disturbed or reset the K-Ar ages (and some Rb-Sr ages) of most eucrites in the time period of approx. 3.4 - 4.1 Gyr ago. Some basaltic eucrites and most cumulate eucrites, however, are not brecciated. We recently showed that the Ar-39 - Ar-40 ages for several of these eucrites tightly cluster about a value of 4.48 +/- 0.02 Gyr, and we argue that this time likely represents a single large impact event on Vesta, which ejected these objects from depth and quenched their temperatures. A different parent body has been suggested for cumulate eucrites, although the Ar-Ar ages argue for a common parent. Similarities in the cosmic-ray (space) exposure ages for basaltic eucrites and diogenites also have been used to infer a common parent body for some HEDs. Here we present CRE ages of several cumulate and unbrecciated basaltic (UB) eucrites and compare these with CRE ages of other HEDs. This comparison also has some interesting implications for the relative locations of various HED types on Vesta and/or the Vestoids.

Molecular modeling of the effects of 40Ar recoil in illite particles on their K-Ar isotope dating

NASA Astrophysics Data System (ADS)

Szczerba, Marek; Derkowski, Arkadiusz; Kalinichev, Andrey G.; Środoń, Jan

2015-06-01

The radioactive decay of 40K to 40Ar is the basis of isotope age determination of micaceous clay minerals formed during diagenesis. The difference in K-Ar ages between fine and coarse grained illite particles has been interpreted using detrital-authigenic components system, its crystallization history or post-crystallization diffusion. Yet another mechanism should also be considered: natural 40Ar recoil. Whether this recoil mechanism can result in a significant enough loss of 40Ar to provide observable decrease of K-Ar age of the finest illite crystallites at diagenetic temperatures - is the primary objective of this study which is based on molecular dynamics (MD) computer simulations. All the simulations were performed for the same kinetic energy (initial velocity) of the 40Ar atom, but for varying recoil angles that cover the entire range of their possible values. The results show that 40Ar recoil can lead to various deformations of the illite structure, often accompanied by the displacement of OH groups or breaking of the Si-O bonds. Depending on the recoil angle, there are four possible final positions of the 40Ar atom with respect to the 2:1 layer at the end of the simulation: it can remain in the interlayer space or end up in the closest tetrahedral, octahedral or the opposite tetrahedral sheet. No simulation angles were found for which the 40Ar atom after recoil passes completely through the 2:1 layer. The energy barrier for 40Ar passing through the hexagonal cavity from the tetrahedral sheet into the interlayer was calculated to be 17 kcal/mol. This reaction is strongly exothermic, therefore there is almost no possibility for 40Ar to remain in the tetrahedral sheet of the 2:1 layer over geological time periods. It will either leave the crystal, if close enough to the edge, or return to the interlayer space. On the other hand, if 40Ar ends up in the octahedral sheet after recoil, a substantially higher energy barrier of 55 kcal/mol prevents it from leaving

40Ar/39Ar and K-Ar data bearing on the metamorphic and tectonic history of western New England.

USGS Publications Warehouse

Sutter, J.F.; Ratcliffe, N.M.; Mukasa, S.B.

1985-01-01

40Ar/39Ar ages of coexisting biotite and hornblende from Proterozoic Y gneisses of the Berkshire and Green Mt massifs, as well as 40Ar/39Ar and K/Ar mineral and whole-rock ages from Palaeozoic metamorphic rocks, suggest that the thermal peaks for the dominant metamorphic recrystallization in western New England occurred 465 + or - 5 m.y. (Taconian). 40Ar/39Ar age data from a poorly-defined terrain along the eastern strip of the area suggests that the area has been retrograded during a metamorphism that peaked at least 376 + or - 5 m.y. (Acadian). Available age and petrological data from western New England indicate the presence of at least three separate metamorphic-structure domains of Taconic age: 1) a small area of relict high-P and low-T metamorphism, 2) a broad area of normal Barrovian metamorphism from chlorite to garnet grade characterized by a gentle metamorphic gradient and, 3) a rather narrow belt of steep-gradient, Barrovian series metamorphic rocks. Areas of maximum metamorphic intensity within the last domain coincide with areas of maximum crustal thickening in the later stage of Taconic orogeny. -L.di H

The Plio-Quaternary Volcanic Evolution of Gran Canaria Based on new Unspiked K-Ar ages and Magnetostratigraphy

NASA Astrophysics Data System (ADS)

Guillou, H.; Carracedo, J.; Perez Torrado, F.

2003-12-01

The combined use of radioisotopic dating, magnetostratigraphy and field geology is a powerful tool to provide reliable chronological frameworks of volcanic edifices. This approach has been used to investigate the last two stages of the volcanic evolution of Gran Canaria. Fifty samples were dated using the unspiked K-Ar method and had their magnetic polarity measured both in the field and in laboratory. Ages were compared to their stratigraphic positions and magnetic polarities before accepting their validity. The unspiked K-Ar chronology constrains the timing of lateral collapses, eruption rates and the contemporaneity of different volcano-magmatic stages at Gran Canaria. Our new data set modifies significantly the previous chronological framework of Gran Canaria, especially between 4 and 2.8 Ma. Based on these new ages, we can bracket the age of the multiple lateral collapses of the Roque Nublo stratovolcano flanks between 3.5 and 3.1 Ma .This time interval corresponds to a main period of volcanic quiescence. Calculated eruptive rates during the stratovolcano edification are about 0.1 km3/kyr which is significantly lower than the published estimates. The dating also reveals that the two main last stages are not separated by a major time gap, but that the early stages of the rift forming eruption and the vanishing activity of the Roque Nublo strato-volcano were contemporaneous for at least 600 kyrs. These results support that our combined approach provides a rapid first-pass and reliable geochronology. Nevertheless, this chronology can be amplified and made more precise where necessary through detailed Ar-Ar incremental-heating methods. Samples which should be investigated using this method are the oldest and youngest K-Ar dated flows of each volcanic stage, and samples from stratigraphic sections that hold potential to study the behaviour of the earth's magnetic field during reversals (Gauss-Gilbert transition, Olduvai and Reunion events).

Application of K-Ar Dating to the Chronology of Young Volcanic Centers

NASA Astrophysics Data System (ADS)

Lanphere, M. A.

2003-12-01

K-Ar dating and a derivative technique, 40Ar/39Ar dating, are methods of high-precision chronology applicable to young volcanic centers. Cascade volcanoes studied in detail by several USGS volcanologists, Duane Champion paleomagetist, and me include Mt. Baker, WA; Mt. Rainier, WA; Mt. Adams, WA; Mt. Hood, OR; Crater Lake, OR; and Medicine Lake, CA. For Mt. Adams using detailed geologic mapping by Hildreth and Fierstein and 74 K-Ar ages for 63 mapped units, Hildreth and Lanphere established a detailed chronology for the stratovolcano. Good agreement has been achieved for K-Ar ages and 40Ar/39Ar ages of rocks from Mt. Adams as young as 36 ka. A similar detailed chronology has been established for other Cascade volcanoes using andesites, in particular. These chronologies often take 10 years or more to develop. Major advantages of the 40Ar/39Ar technique are the ability to work with small sample sizes and the possibility to push the technique to very young ages. The Campanian Ignimbrite erupted from the Campi Flegrei crater near Naples, Italy is an example of the use of small samples. Nine incremental-heating ages were determined on samples of sanidine ranging in size from 47 mg to 67 mg. These samples yielded ages for the Campanian Ignimbrite ranging from 37.1 +/- 0.75 ka to 39.5 +/- 0.62 ka and averaging 38.1 +/- 0.8 ka. Other workers have proposed 40Ar/39Ar ages for the Campanian Ignimbrite of 37.1 +/- 0.4 ka and 39.3 +/- 0.1 ka. An example of the use of 40Ar/39Ar dating of very young samples is the Christian Era (CE) age of the Vesuvius eruption of year 79. Eight packets of sanidine weighing 213-296 mg from two localities, Casti Amanti in Pompeii and Villa Poppea in nearby Oplontis, yielded a weighted-mean incremental-heating age of 1924 +/- 66 years. The known age for the CE 79 eruption of Vesuvius is 1924 years. Earlier studies of Vesuvius by other workers yielded an 40Ar/39Ar age for the Villa Poppea locality of 1922 +/- 72 years.

K-Ar age of the late Pleistocene eruption of Toba, north Sumatra

USGS Publications Warehouse

Ninkovich, D.; Shackleton, N.J.; Abdel-Monem, A. A.; Obradovich, J.D.; Izett, G.

1978-01-01

The late Pleistocene eruption of Toba is the largest magnitude explosive eruption documented from the Quaternary. K-Ar dating of the uppermost unit of the Toba Tuff gives an age of [~amp]sim; 75,000 yr. A chemically and petrographically equivalent ash layer in deep-sea cores helps calibrate the Stage 4-5 boundary of the standard oxygen isotope stratigraphy. A similar ash in Malaya that overlies finds of Tampan Palaeolithic tools indicates that they are older than 75,000 yr. ?? 1978 Nature Publishing Group.

Ar-39-Ar-40 Ages of Euerites and the Thermal History of Asteroid 4-Vesta

NASA Technical Reports Server (NTRS)

Bogard, Donald D.; Garrison, Daniel H.

2002-01-01

Eucrite meteorites are igneous rocks that derive from a large asteroid, probably 4 Vesta. Prior studies have shown that after eucrites formed, most were subsequently metamorphosed to temperatures up to equal to or greater than 800 C, and much later many were brecciated and heated by large impacts into the parent body surface. The uncommon basaltic, unbrecciated eucrites also formed near the surface but presumably escaped later brecciation, whereas the cumulate eucrites formed at depth where metamorphism may have persisted for a considerable period. To further understand the complex HED parent body thermal history, we determined new Ar-39-Ar-40 ages for nine eucrites classified as basaltic but unbrecciated, six eucrites classified as cumulate, and several basaltic-brecciated eucrites. Relatively precise Ar-Ar ages of two cumulate eucrites (Moama and EET87520) and four unbrecciated eucrites give a tight cluster at 4.48 +/1 0.01 Gyr. Ar-Ar ages of six additional unbrecciated eucrites are consistent with this age, within their larger age uncertainties. In contrast, available literature data on Pb-Pb isochron ages of four cumulate eucrites and one unbrecciated eucrite vary over 4.4-4.515 Gyr, and Sm-147 - Nd-143 isochron ages of four cumulate and three unbrecciated eucrites vary over 4.41-4.55 Gyr. Similar Ar-Ar ages for cumulate and unbrecciated eucrites imply that cumulate eucrites do not have a younger formation age than basaltic eucrites, as previously proposed. Rather, we suggest that these cumulate and unbrecciated eucrites resided at depth where parent body temperatures were sufficiently high to cause the K-Ar and some other chronometers to remain open diffusion systems. From the strong clustering of Ar-Ar ages at approximately 4.48 Gyr, we propose that these meteorites were excavated from depth in a single large impact event approximately 4.48 Gyr ago, which quickly cooled the samples and started the K-Ar chronometer. A large (approximately 460 km) crater

Ar-Ar and I-Xe Ages and the Thermal History of IAB Meteorites

NASA Technical Reports Server (NTRS)

Bogard, Donald D.; Garrison, Daniel H.; Takeda, Hiroshi

2005-01-01

high closure temperatures and were not reset during metamorphism approx.4.53 Gyr ago; b) a bias exists in the 40K decay constants which makes these Ar-Ar ages approx.30 Myr too young; c) the reported Sm-Nd and Rb-Sr ages for Caddo are in error by amounts equal to or exceeding their reported 2-sigma uncertainties; and d) about 30 Myr after the initial heating that produced differentiation of Caddo silicate and mixing of silicate and metal, a mild metamorphism of the IAB parent body reset the Ar-Ar ages.

Geochemical characteristics and K-Ar ages of rare-metal bearing pegmatites from the Birimian of southeastern Ghana

USGS Publications Warehouse

Chalokwu, C.I.; Ghazi, M.A.; Foord, E.E.

1997-01-01

The pegmatite-aplite rocks at Mankwadzi (Ejisimanku Hills) in southeastern Ghana are part of the pegmatite district that extends from Cape Coast to Winneba along the Atlantic coastline. The pegmatites are associated with the Cape Coast granite complex and were intruded during the waning phase of the Eburnian Orogeny (???2.0 Ga). Three muscovite separates from pegmatite give K-Ar retention ages of 1909 ?? 13 Ma, 1965 ?? 13 Ma and 2019 ?? 14 Ma. A biotite separate from granite yields a K-Ar age of 1907 ?? 13 Ma. These ages are similar to K-Ar dates previously reported for the Cape Coast granites, indicating that the granites and pegmatites are coeval and probably genetically linked. The pegmatites are enriched in Li, Be, Nb and Sn and considerably impoverished in Rb, Th, Y and REEs. Microscopic examination of quartz from the pegmatites shows a large number of low salinity fluid inclusions that can be divided into two types: (1) one-phase liquid or gas-filled inclusions; and (2) two-phase liquid-vapour inclusions, with the vapour occupying 2-5% of the volume. The homogenisation temperature of the fluid inclusions clusters between 129 and 144??C. These homogenisation temperatures lead to an inferred entrapment temperature of ???300??C at a pressure of ???2.5 kbar, which is estimated for the metamorphism of host hornblende schists. The pegmatite fluid inclusions are interpreted as being secondary to the quartz hosts. ?? 1997 Elsevier Science Limited.

40Ar/39Ar Ages of Carbonaceous Xenoliths in 2 HED Meteorites

NASA Technical Reports Server (NTRS)

Turrin, B.; Lindsay, F. N.; Park, J.; Herzog, G. F.; Delaney, J. S.; Swisher, C. C., III; Johnson, J.; Zolensky, M.

2016-01-01

The generally young K/Ar and 40Ar/39Ar ages of CM chondrites made us wonder whether carbonaceous xenoliths (CMX) entombed in Howardite–Eucrite–Diogenite (HED) meteorites might retain more radiogenic 40Ar than do ‘free-range’ CM-chondrites. To find out, we selected two HED breccias with carbonaceous inclusions in order to compare the 40Ar/39Ar release patterns and ages of the inclusions with those of nearby HED material. Carbonaceous inclusions (CMXs) in two HED meteorites lost a greater fraction of radiogenic 40Ar than did surrounding host material, but a smaller fraction of it than did free-range CM-chondrites such as Murchison or more heavily altered ones. Importantly, however, the siting of the CMXs in HED matrix did not prevent the 40Ar loss of about 40 percent of the radiogenic 40Ar, even from phases that degas at high laboratory temperatures. We infer that carbonaceous asteroids with perihelia of 1 astronomical unit probably experience losses of at least this size. The usefulness of 40Ar/39Ar dating for samples returned from C-type asteroids may hinge, therefore, on identifying and analyzing separately small quantities of the most retentive phases of carbonaceous chondrites.

Mineralogy and Ar-Ar Age of the Tarahumara IIE Iron, with Reference to the Origin of Alkali-Rich Materials

NASA Technical Reports Server (NTRS)

Takeda, Hiroshi; Bogard, Donald D.; Otsuki, Mayumi; Ishii, Teruaki

2003-01-01

Silicate inclusions in nine known IIE irons show diversity in mineralogy, and Colomera, Kodaikanal, Elga and Miles contain alkali-rich silicate inclusions. Bogard et al. showed evidence of a complex parent body evolution for IIE irons based on Ar-39-Ar-40 ages. Colomera contained a sanidine-rich surface inclusion and the K-enrichment trends in the Na-rich inclusions are different from those of other IIEs. To elucidate the origin of K-rich materials, we studied the mineralogy and Ar-Ar age of silicate inclusions from the Tarahumara IIE iron meteorite.

40Ar/39Ar laser probe evidence concerning the age and associated hazards of the Lake Nyos Maar, Cameroon

USGS Publications Warehouse

Dalrymple, G.B.; Lockwood, J.P.

1990-01-01

The waters of Lake Nyos are impounded by a fragile natural dam composed of pyroclastic rocks ejected during the formation of the lake crater (maar). Lateral erosion of this dam has reduced its width from over 500 m to only 45 m. Published whole-rock K-Ar ages of about 100 ka on juvenile basalt from the dam suggests that erosion has been slow and that the dam poses no imminent threat. New apparent 40Ar/39Ar ages of 1.4 to 232 Ma on xenocrystic K-feldspar contained in the basalt show that the xenocrysts, whose source is the 528-Ma crystalline basement, are carriers of inherited radiogenic 40Ar and would cause the whole-rock K-Ar ages to be too old. The best estimate for the age of the maar is provided by a 14C age of 400 ?? 100 yr BP on charcoal from the base of the dam. This young age indicates that the dam is eroding at a relatively rapid rate; its failure, perhaps within a few decades, would result in a major flood and imperil thousands of people living downstream in Cameroon and eastern Nigeria. ?? 1990 Kluwer Academic Publishers.

40Ar/ 39Ar, K-Ar and 230Th- 238U dating of the Laschamp excursion: A radioisotopic tie-point for ice core and climate chronologies

NASA Astrophysics Data System (ADS)

Singer, Brad S.; Guillou, Hervé; Jicha, Brian R.; Laj, Carlo; Kissel, Catherine; Beard, Brian L.; Johnson, Clark M.

2009-08-01

A brief period of enhanced 10Be flux that straddles the interstadial warm period known as Dansgaard-Oeschger event 10 in Greenland and its counterpart in Antarctica, the Antarctic Isotope Maximum 10 is but one consequence of the weakening of Earth's magnetic field associated with the Laschamp excursion. This 10Be peak measured in the GRIP ice core is dated at 41,250 y b2k (= before year 2000 AD) in the most recent GICC05 age model obtained from the NorthGRIP core via multi-parameter counting of annual layers. Uncertainty in the age of the 10Be peak is, however, no better than ± 1630 y at the 95% confidence level, reflecting accumulated error in identifying annual layers. The age of the Laschamp excursion [Guillou, H., Singer, B.S., Laj, C., Kissel, C., Scaillet, S., Jicha, B., 2004. On the age of the Laschamp geomagnetic excursion. Earth Planet. Sci. Lett. 227, 331-343.] is revised on the basis of new 40Ar/ 39Ar, unspiked K-Ar and 238U- 230Th data from three lava flows in the Massif Central, France, together with the 40Ar/ 39Ar age of a transitionally magnetized lava flow at Auckland, New Zealand. Combined, these data yield an age of 40,700 ± 950 y b2k, where the uncertainty includes both analytical and systematic ( 40K and 230Th decay constant) errors. Taking the radioisotopic age as a calibration tie point suggests that the layer-counting chronologies for the NorthGRIP and GISP2 ice cores are more accurate and precise than previously thought at depths corresponding to the Laschamp excursion.

Discordant K-Ar and Young Exposure Dates for the Windjana Sandstone, Kimberley, Gale Crater, Mars

NASA Technical Reports Server (NTRS)

Vasconcelos, P. M.; Farley, K. A.; Malespin, C. A.; Mahaffy, P.; Ming, D.; McLennan, S. M.; Hurowitz, J. A.; Rice, Melissa S.

2016-01-01

K-Ar and noble gas surface exposure age measurements were carried out on the Windjana sandstone, Kimberley region, Gale Crater, Mars, by using the Sample Analysis at Mars instrument on the Curiosity rover. The sandstone is unusually rich in sanidine, as determined by CheMin X-ray diffraction, contributing to the high K2O concentration of 3.09 +/- 0.20 wt % measured by Alpha-Particle X-ray Spectrometer analysis. A sandstone aliquot heated to approximately 915 C yielded a K-Ar age of 627 +/- 50 Ma. Reheating this aliquot yielded no additional Ar. A second aliquot heated in the same way yielded a much higher K-Ar age of 1710 +/- 110 Ma. These data suggest incomplete Ar extraction from a rock with a K-Ar age older than 1710 Ma. Incomplete extraction at approximately 900 C is not surprising for a rock with a large fraction of K carried by Ar-retentive K-feldspar. Likely, variability in the exact temperature achieved by the sample from run to run, uncertainties in sample mass estimation, and possible mineral fractionation during transport and storage prior to analysis may contribute to these discrepant data. Cosmic ray exposure ages from He-3 and Ne-21 in the two aliquots are minimum values given the possibility of incomplete extraction. However, the general similarity between the He-3 (57 +/- 49 and 18 +/- 32 Ma, mean 30 Ma) and Ne-21 (2 +/- 32 and 83 +/- 24 Ma, mean 54 Ma) exposure ages provides no evidence for underextraction. The implied erosion rate at the Kimberley location is similar to that reported at the nearby Yellowknife Bay outcrop.

Direct Measurement of Recoil Effects on Ar-Ar Standards

NASA Astrophysics Data System (ADS)

Hall, C. M.

2011-12-01

Advances in the precision possible with the Ar-Ar method using new techniques and equipment have led to considerable effort to improve the accuracy of the calibration of interlaboratory standards. However, ultimately the accuracy of the method relies on the measurement of 40Ar*/39ArK ratios on primary standards that have been calibrated with the K-Ar method and, in turn, on secondary standards that are calibrated against primary standards. It is usually assumed that an Ar-Ar total gas age is equivalent to a K-Ar age, but this assumes that there is zero loss of Ar due to recoil. Instead, traditional Ar-Ar total gas ages are in fact Ar retention ages [1] and not, strictly speaking, comparable to K-Ar ages. There have been efforts to estimate the importance of this effect on standards along with prescriptions for minimizing recoil effects [2,3], but these studies have relied on indirect evidence for 39Ar recoil. We report direct measurements of 39Ar recoil for a set of primary and secondary standards using the vacuum encapsulation techniques of [1] and show that significant adjustments to ages assigned to some standards may be needed. The fraction f of 39Ar lost due to recoil for primary standards MMhb-1 hornblende and GA-1550 biotite are 0.00367 and 0.00314 respectively. It is possible to modify the assumed K-Ar ages of these standards so that when using their measured Ar retention 40Ar*/39ArK ratios, one obtains a correct K-Ar age for an unknown, assuming that the unknown sample has zero loss of 39Ar due to recoil. Assuming a primary K-Ar age for MMhb-1 of 520.4 Ma, the modified age would be 522.1 Ma and assuming a primary K-Ar age for GA-1550 of 98.79 Ma [4] yields a modified effective age of 99.09 Ma. Measured f values for secondary standards FCT-3 biotite, FCT-2 sanidine and TCR-2 sanidine are 0.00932, 0.00182 and 0.00039 respectively. Using an R value for FCT-3 biotite relative to MMhb-1 [5], the K-Ar age for this standard would be 27.83 Ma and using R values

Preliminary Ar-40/Ar-39 age spectrum and laser probe dating of the M1 core of the Manson Impact Structure, Iowa: A K-T boundary crater candidate

NASA Technical Reports Server (NTRS)

Kunk, M. J.; Snee, L. W.; French, B. M.; Harlan, S. S.; Mcgee, J. J.

1993-01-01

Preliminary Ar-40/Ar-39 age spectrum and laser probe dating results from new drill core from the 35-km-diameter Manson Impact Structure (MIS), Iowa indicates a reasonable possibility that the MIS is a Cretaceous-Tertiary (K-T) boundary impact event. Several different types of samples from a melt-matrix breccia, a unit of apparent crater fill intersected by the M1 core, were analyzed. Ar-40/Ar-39 results from these samples indicate a maximum age for the MIS of about 65.4 plus or minus 0.4(2 sigma) Ma. Petrographic analyses of the samples indicate a high probability that all the dated samples from the melt-matrix breccia contain relict grains that were not entirely melted or degassed at the time of impact, suggesting that the actual age of the MIS could be somewhat younger than our preliminary results indicate. The results are consistent with a previously published age estimate of shocked microcline from the MIS central uplift of 65.7 plus or minus 1.0 Ma.

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

NASA Astrophysics Data System (ADS)

Johnson, Catherine L.; Wijbrans, Jan R.; Constable, Catherine G.; Gee, Jeff; Staudigel, Hubert; Tauxe, Lisa; Forjaz, Victor-H.; Salgueiro, Mário

1998-08-01

We present new 40Ar/ 39Ar ages and paleomagnetic data for São 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. 40Ar/ 39Ar 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 São 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 40Ar/ 39Ar 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.

Ar-Ar and I-XE Ages and the Thermal History of IAB Meteorites

NASA Technical Reports Server (NTRS)

Bogard, Donald D.; Garrison, Daniel H.; Takeda, Hiroshi

2006-01-01

Studies of several samples of the large Caddo County IAB iron meteorite reveal andesitic material, enriched in Si, Na, Al and Ca which is essentially unique among meteorites. This material is believed to have formed from a chondritic source by partial melting and to have further segregated by grain coarsening. Such an origin implies extended metamorphism of the IAB parent body. New Ar-39- Ar-40 ages for silicate from three different Caddo samples are consistent with a common age of 4.50- 4.51 Gyr ago. Less well defined Ar-Ar degassing ages for inclusions from two other IABs, EET8333 and Udei Station, are approx. 4.32 Gyr, whereas the age for Campo del Cielo varies considerably over approx. 3.23-4.56 Gyr. New I-129-Xe-129 ges for Caddo County and EET8333 are 4561.9 plus or minus 0.1 Myr and 4560-4563 Myr, respectively, relative to an age of 4566 Myr for Shallowater. Considering all reported Ar-Ar ages for IABs and related winonaites, the range is approx. 4.32-4.53 Gyr, but several IABs give similar Ar ages of 4.50-4.52 Gyr. We interpret these older ages to represent cooling after the time of last significant metamorphism on the parent body, and the younger ages to represent later 40Ar diffusion loss. These older Ar-Ar ages are similar to Sm-Nd and Rb-Sr isochron ages reported in the literature for Caddo County. Considering the possibility that IAB parent body formation was followed by impact disruption, reassembly, and metamorphism (e.g., Benedix et al. 2000), the time of the postassembly metamorphism may have been as late as approx. 4.53 Gyr ago. However, precise I-Xe ages reported for some IABs define a range of ages of approx. 4560 to approx. 4576 Myr. The older I-Xe ages exceed the oldest precise radiometric ages of meteorites, appear unrealistic, and suggest a bias in the calibration of all I-Xe ages. But even with such a bias, the I-Xe ages of IABs cannot easily be reconciled with the much younger Ar-Ar and Sm-Nd ages and with cooling rates deduced from Ni

K-Ar dating of lunar fines - Apollo 12, Apollo 14, and Luna 16.

NASA Technical Reports Server (NTRS)

Pepin, R. O.; Bradley, J. G.; Dragon, J. C.; Nyquist, L. E.

1972-01-01

K-Ar ages were determined on a 6-in. double-focus mass spectrometer in fines of less than 1 mm from Apollo 14 and 16, and Luna 16 lunar soil samples. Age estimates of about 2.8 AE and about 4.0 AE are suggested for the two low-K components whose presence in the samples must be assumed to accommodate the age data. An average value of 0.1849 plus or minus 0.0008 was obtained for the Ar-18/Ar-36 ratio in the solar wind from ordinate intercept correlations for the Apollo 14 and Luna 16 samples. Cosmic ray exposure ages were close to 440 m.y. for both Apollo 14 samples and close to 840 m.y. for both Luna 16 samples.

Palaeomagnetism and K-Ar and 40Ar/39Ar ages in the Ali Sabieh area (Republic of Djibouti and Ethiopia): constraints on the mechanism of Aden ridge propagation into southeastern Afar during the last 10 Myr

NASA Astrophysics Data System (ADS)

Audin, L.; Quidelleur, X.; Coulié, E.; Courtillot, V.; Gilder, S.; Manighetti, I.; Gillot, P.-Y.; Tapponnier, P.; Kidane, T.

2004-07-01

A new detailed palaeomagnetic study of Tertiary volcanics, including extensive K-Ar and 40Ar/39Ar dating, helps constrain the deformation mechanisms related to the opening processes of the Afar depression (Ethiopia and Djibouti). Much of the Afar depression is bounded by 30 Myr old flood basalts and floored by the ca 2 Myr old Stratoid basalts, and evidence for pre-2 Ma deformation processes is accessible only on its borders. K-Ar and 40Ar/39Ar dating of several mineral phases from rhyolitic samples from the Ali Sabieh block shows indistinguishable ages around 20 Myr. These ages can be linked to separation of this block in relation to continental breakup. Different amounts of rotation are found to the north and south of the Holhol fault zone, which cuts across the northern part of the Ali Sabieh block. The southern domain did not record any rotation for the last 8 Myr, whereas the northern domain experienced approximately 12 +/- 9° of clockwise rotation. We propose to link this rotation to the counter-clockwise rotation observed in the Danakil block since 7 Ma. This provides new constraints on the early phases of rifting and opening of the southern Afar depression in connection with the propagation of the Aden ridge. A kinematic model of propagation and transfer of extension within southern Afar is proposed, with particular emphasis on the previously poorly-known period from 10 to 4 Ma.

K-Ar chronology of the Luohe iron district, Anhui Province, China

USGS Publications Warehouse

McKee, E.H.

1988-01-01

Twelve samples of rock from the four mapped units or cycles and one of the major intrusive bodies were collected and evaluated for K-Ar age determination. These include specimens from outcrop and from drill core. Biotite from two outcrop and two core samples and hornblende from one outcrop sample were separated from the sample and dated; a sixth sample was dated using crushed, sieved, and acid-treated whole rock. The ages and analytical data to support them are compatible with the observed relationships in the field or from the drill holes. The percent of K2O in all samples is typical of fresh unaltered mineral phases and the percent of radiogenetic 40Ar relative to total 40Ar is high (88.8 to 63.8%) yielding relatively low analytical errors. -from Authors

Characterizing Cretaceous Glaciation Events: K-Ar Ages of Southern Ocean Sediments

NASA Astrophysics Data System (ADS)

Wright, M. A.; Hemming, S. R.; Barbeau, D. L.; Torfstein, A.; Pierce, E. L.; Williams, T.; McManus, J. F.; Gombiner, J.

2012-12-01

Evidence from paleosols and carbonate weathering models suggest that the Late Cretaceous had a supergreenhouse climate due to atmospheric CO2 concentrations two to four times greater than modern levels, tropical sea surface temperatures exceeding 35°C, and high-latitude temperatures exceeding 20°C. Despite this warmth, the Late Cretaceous was apparently punctuated by large (>25 m) and rapid (<<1 million year) sea-level changes, as recorded by marginal marine stratigraphic architectures and pelagic stable isotope compositions. The magnitude and tempo of these changes suggest a glacio-eustatic control, presumably from the growth and decay of continental ice sheets on Antarctica. Because continental glaciation tends to increase the weathering of bedrock and production of sediment delivered to the oceans, circum-Antarctic marine sediment flux would be expected to increase during periods of glaciation. In order to identify a Late Cretaceous glaciation signal from such marine records, we must first constrain the compositional signal of continental detritus in marine sediments. Here we report the results of downcore K-Ar analysis of the terrigenous sediments of Quaternary Weddell Sea cores PS1170-1 and PS1388-3 in order to identify the compositional signature of continent-derived detritus deposited in the Weddell Sea during a known glacial period. Further, we use our K-Ar analyses of circum-Antarctic Quaternary sediment cores to pinpoint potential sediment source areas. Having constrained this glaciation signal, we also present preliminary K-Ar and Sm-Nd analysis of the Campanian-Maastrictian boundary event (69 Ma) at Ocean Drilling Project site 690C to assess the controversial hypothesis of Late Cretaceous glaciation of Antarctica.

The tectonic significance of K/Ar illite fine-fraction ages from the San Luis Formation (Eastern Sierras Pampeanas, Argentina)

NASA Astrophysics Data System (ADS)

Wemmer, Klaus; Steenken, André; Müller, Stefan; de Luchi, Mónica G. López; Siegesmund, Siegfried

2011-04-01

The Sierra de San Luis forms the southern tip of the Eastern Sierras Pampeanas in central Argentina. Two narrow belts of low-grade phyllites and quartz arenites, i.e. the San Luis Formation, have accommodated part of the strain-related differential exhumation of the medium- to high-grade metamorphic domains that constitute to the basement complex of the sierra. Eleven phyllite samples were subjected to the K/Ar fine-fraction dating technique. Results are interpreted in relation to the Kübler index of the illites, which indicate epimetamorphic conditions for the majority of the samples. Obtained ages between 330 and 290 Ma cover a period of compressional tectonics in the late Mississippian (Visean/Serpukhovian boundary) followed by the subsidence during the formation of the Paganzo Basin in the provinces of La Rioja and San Luis. These tectonic movements are coincident with the Toco orogeny in northern Chile and southern Bolivia. This suggests that the older K/Ar ages document the compressional stage and that younger ages record the cooling of the basement during the subsequent extensional uplift of the basement.

40Ar/39Ar Dating of Volcanic Glass

NASA Astrophysics Data System (ADS)

Morgan, L. E.; Renne, P. R.; Watkins, J. M.

2007-12-01

Application of the 40Ar/39Ar method to volcanic glasses has been somewhat stigmatized following several studies demonstrating secondary mobility of K and Ar. Much of the stigma is unwarranted, however, since most studies only impugned the reliability of the K-Ar and 40Ar/39Ar techniques when applied to glass shards rather than obsidian clasts with low surface area to volume ratios. We provide further evidence for problematic K loss and/or 39Ar recoil ejection from glass shards in 40Ar/39Ar step heating results for comagmatic feldspars and shards. In an extreme case, the plateau age of the feldspars (0.17 ± 0.03 Ma at 2σ) is significantly younger than the plateau age of the glass (0.85 ± 0.05 Ma at 2σ). If the feldspar age is reasonably interpreted as the eruption age of the ash, it is likely that the glass shards experienced K and/or 39Ar loss. Electron microprobe analyses of the glass shards have low totals (~93%) and no systematic lateral variability (i.e., diffusion gradients) in K, suggesting that the lengthscale of the glass shards is smaller than the lengthscale of K diffusion. Obsidian clasts should not be as susceptible to K loss since any hydrated (K-depleted) volume represents a small fraction of the total material and can often be physically removed prior to analysis. Samples described here are detrital obsidian clasts from the Afar region of Ethiopia. Evidence from Fourier Transform Infrared Spectroscopy (FTIR), and previous work by Anovitz (1999), confirm that the scale of water and potassium mobility are often small in comparison to the size of obsidian clasts but large enough to effect the bulk composition of glass shards. This expectation is confirmed in another tuff wherein comagmatic obsidian clasts and sanidine phenocrysts yield indistinguishable 40Ar/39Ar ages of 4.4 Ma High abundances of non-radiogenic 40Ar, and kinetic fractionation of Ar isotopes during quenching and/or laboratory degassing resulting in incomplete equilibration between

Robust 24 ± 6 ka 40Ar/39Ar age of a low-potassium tholeiitic basalt in the Lassen region of NE California

USGS Publications Warehouse

Turrin, Brent D.; Muffler, L. J. Patrick; Clynne, Michael A.; Champion, Duane E.

2007-01-01

40Ar/39Ar ages on the Hat Creek Basalt (HCB) and stratigraphically related lava flows show that latest Pleistocene tholeiitic basalt with very low K2O can be dated reliably. The HCB underlies ∼ 15 ka glacial gravel and overlies four andesite and basaltic andesite lava flows that yield 40Ar/39Ar ages of 38 ± 7 ka (Cinder Butte; 1.65% K2O), 46 ± 7 ka (Sugarloaf Peak; 1.85% K2O), 67 ± 4 ka (Little Potato Butte; 1.42% K2O) and 77 ± 11 ka (Potato Butte; 1.62% K2O). Given these firm age brackets, we then dated the HCB directly. One sample (0.19% K2O) clearly failed the criteria for plateau-age interpretation, but the inverse isochron age of 26 ± 6 ka is seductively appealing. A second sample (0.17% K2O) yielded concordant plateau, integrated (total fusion), and inverse isochron ages of 26 ± 18, 30 ± 20 and 24 ± 6 ka, all within the time bracket determined by stratigraphic relations; the inverse isochron age of 24 ± 6 ka is preferred. As with all isotopically determined ages, confidence in the results is significantly enhanced when additional constraints imposed by other isotopic ages within a stratigraphic context are taken into account.

In situ dating on Mars: A new approach to the K-Ar method utilizing cosmogenic argon

NASA Astrophysics Data System (ADS)

Cassata, William S.

2014-01-01

Cosmogenic argon isotopes are produced in feldspars via nuclear reactions between cosmic rays and Ca and K atoms within the lattice. These cosmogenic isotopes can be used as proxies for K and Ca, much like nuclear reactor-derived 39Ar and 37Ar are used as proxies for K and Ca, respectively, in 40Ar/39Ar geochronology. If Ca and K are uniformly distributed, then the ratio of radiogenic 40Ar (40Ar*) to cosmogenic 38Ar or 36Ar (38Arcos or 36Arcos) is proportional to the difference between the radioisotopic and exposure ages, as well as the K/Ca ratio of the degassing phase. Thus cosmogenic, radiogenic, and trapped Ar isotopes, all of which can be measured remotely and are stable over geologic time, are sufficient to generate an isochron-like diagram from which the isotopic composition of the trapped component may be inferred. Such data also provide a means to assess the extent to which the system has remained closed with respect to 40Ar*, thereby mitigating otherwise unquantifiable uncertainties that complicate the conventional K-Ar dating method.

Temporal evolution of the Western and Central volcanism of the Aeolian Island Arc (Italy, southern Tyrhhenian Sea)

NASA Astrophysics Data System (ADS)

Leocat, E.; Gillot, P.-Y.; Peccerillo, A.

2009-04-01

The Aeolian Archipelago is a volcanic arc in the Southern Tyrrhenian Sea located on the continental margin of the Calabro-Peloritan basement. The Aeolian volcanism occurs in a very complex geodynamic setting linked to the convergence of the European and African plates. For that reason, it is strongly related to regional tectonic lineaments, such as the NW-SE trending Tindari-Letojani (TL) fault. The archipelago consists of seven main islands and several seamounts, which extend around the Marsili Basin, forming a ring-like shape, typical for an island arc. While the seamounts began their activities around 1 Ma , the emerged part is active since about 400 ka. The magmatic products of the whole arc range from typical island arc calc-alkaline (CA) and shoshonitic series, to slightly silica undersaturated potassic alkaline series that are typical of post-collisional settings. Furthermore, the TL fault, along which the Lipari and Vulcano islands are developed, separates a calc-alkaline western sector (Alicudi, Filicudi and Salina islands) from the calc-alkaline to potassic eastern system (Panarea and Stromboli islands) (Peccerillo,1999). This makes of the Aeolian Islands a complex volcanism, with a still controversial origin. In this context, the aim of this work is to constrain the sources and spatio-temporal evolution of this magmatism. We present here new K-Ar ages based on the accurate Cassignol-Gillot technique devoted to the dating of very young rocks (Gillot et Cornette, 1986). These geochronological data were used together with new geochemical data on the same samples. In this study, we attempt to understand the origin of those magmatic events and the relationship between the deep processes and the shallow structures. Our results allow us to define specific periods of very quick geomechemical changes. In the case of Filicudi island, the first rocks range in composition from CA basalts to andesites. This period ended with the edification of the Mte Guardia at 189�

K/Ar dating of lunar soils. IV - Orange glass from 74220 and agglutinates from 14259 and 14163

NASA Technical Reports Server (NTRS)

Alexander, E. C., Jr.; Coscio, M. R., Jr.; Dragon, J. C.; Saito, K.

1980-01-01

Total fusion Ar-40 - A-39 analyses of orange glass from lunar soil 74220 combined with the sums of earlier stepwise heating data by other workers have yielded a precise K/Ar isochron with a slope corresponding to an age of 3.66 + or - 0.03 G.y. for the orange glass. The result is in marginal agreement with Huneke's (1978) age of 3.60 + or - 0.04 G.y. for 74220 glass. The Ar systematics in the agglutinates from 14259 and 14163 are dominated by volume correlated argon. Step-wise heating analyses yield data which define experimentally reproducible linear arrays in Ar-40/Ar-36 vs. K-40/Ar-36 diagrams. The slopes of these arrays correspond formally to very old ages, but it is not clear, however, that such ages have any physical significance.

K-Ar constraints on fluid-rock interaction and dissolution-precipitation events within the actively creeping shear zones from SAFOD cores

NASA Astrophysics Data System (ADS)

Ali, S.; Hemming, S. R.; Torgersen, T.; Fleisher, M. Q.; Cox, S. E.; Stute, M.

2009-12-01

The San Andreas Fault Observatory at Depth (SAFOD) was drilled to study the physical and chemical processes responsible for faulting and earthquake generation along an active, plate-bounding fault at depth. SAFOD drill cores show multiple zones of alteration and deformation due to fluid-rock interaction in the fault rocks(Schleicher et al. 2008). In context of fluid studies in the SAFZ, noble gas and potassium measurements were performed on solid samples of sedimentary rocks obtained from drill cores across the fault (3050-4000m-MD). We used a combination of 40Ar/39Ar and K-Ar methods on crushed samples of mudrock with variable amounts of visible slickensides to constrain the degree of resetting of the K-Ar system across the San Andreas Fault zone. 40Ar/39Ar was analyzed from small fragments (sand sized grains) while K-Ar was measured in crushed bulk rock samples (100-250 mg for Ar, and 5-10 mg for K analyses). The apparent 40Ar/39Ar ages based on single step laser fusion of small fragments corresponding to the detrital component in the coarse fraction, show varying ages ranging from the provenance age to <13Ma. Although more data are needed to make detailed comparisons, the apparent K-Ar ages of bulk samples in the fault zone are biased toward authigenic materials contained in the fine fraction, similar to the 40Ar/39Ar ages reported for mineralogical separates from very fine size fractions of samples obtained from 3065.98m-MD and 3294.89m-MD (Schleicher et al., submitted to Geology). The small samples measured for 40Ar/39Ar show scatter in the apparent ages, generally bracketing the bulk ages. However they are picked from sieved portions of the samples, and it is likely that there may be a loss of the younger (finer) material. Detrital provenance ages appear to be 50-60Ma in the Pacific Plate, and 100Ma in the North American Plate. 40Ar/39Ar ages within the SAFZ, as defined by geophysical logs (3200-3400m MD), are dominated by apparent detrital ages of ˜100Ma

An in-situ K-Ar isochron dating method for planetary landers using a spot-by-spot laser-ablation technique

NASA Astrophysics Data System (ADS)

Cho, Yuichiro; Sugita, Seiji; Miura, Yayoi N.; Okazaki, Ryuji; Iwata, Naoyoshi; Morota, Tomokatsu; Kameda, Shingo

2016-09-01

Age is essential information for interpreting the geologic record on planetary surfaces. Although crater counting has been widely used to estimate the planetary surface ages, crater chronology in the inner solar system is largely built on radiometric age data from limited sites on the Moon. This has resulted in major uncertainty in planetary chronology. Because opportunities for sample-return missions are limited, in-situ geochronology measurements from one-way lander/rover missions are extremely valuable. Here we developed an in-situ isochron-based dating method using the K-Ar system, with K and Ar in a single rock sample extracted locally by laser ablation and measured using laser-induced breakdown spectroscopy (LIBS) and a quadrupole mass spectrometer (QMS), respectively. We built an experimental system combining flight-equivalent instruments and measured K-Ar ages for mineral samples with known ages (~1.8 Ga) and K contents (1-8 wt%); we achieved precision of 20% except for a mineral with low mechanical strength. Furthermore, validation measurements with two natural rocks (gneiss slabs) obtained K-Ar isochron ages and initial 40Ar consistent with known values for both cases. This result supports that our LIBS-MS approach can derive both isochron ages and contributions of non-in situ radiogenic 40Ar from natural rocks. Error assessments suggest that the absolute ages of key geologic events including the Noachian/Hesperian- and the Hesperian/Amazonian-transition can be dated with 10-20% errors for a rock containing ~1 wt% K2O, greatly reducing the uncertainty of current crater chronology models on Mars.

A new approach to cosmogenic corrections in 40Ar/ 39Ar chronometry: Implications for the ages of Martian meteorites

DOE PAGES

Cassata, W. S.; Borg, L. E.

2016-05-04

Anomalously old 40Ar/ 39Ar ages are commonly obtained from Shergottites and are generally attributed to uncertainties regarding the isotopic composition of the trapped component and/or the presence of excess 40Ar. Old ages can also be obtained if inaccurate corrections for cosmogenic 36Ar are applied. Current methods for making the cosmogenic correction require simplifying assumptions regarding the spatial homogeneity of target elements for cosmogenic production and the distribution of cosmogenic nuclides relative to trapped and reactor-derived Ar isotopes. To mitigate uncertainties arising from these assumptions, a new cosmogenic correction approach utilizing the exposure age determined on an un-irradiated aliquot and step-wisemore » production rate estimates that account for spatial variations in Ca and K is described. Data obtained from NWA 4468 and an unofficial pairing of NWA 2975, which yield anomalously old ages when corrected for cosmogenic 36Ar using conventional techniques, are used to illustrate the efficacy of this new approach. For these samples, anomalous age determinations are rectified solely by the improved cosmogenic correction technique described herein. Ages of 188 ± 17 and 184 ± 17 Ma are obtained for NWA 4468 and NWA 2975, respectively, both of which are indistinguishable from ages obtained by other radioisotopic systems. For other Shergottites that have multiple trapped components, have experienced diffusive loss of Ar, or contain excess Ar, more accurate cosmogenic corrections may aid in the interpretation of anomalous ages. In conclusion, the trapped 40Ar/ 36Ar ratios inferred from inverse isochron diagrams obtained from NWA 4468 and NWA 2975 are significantly lower than the Martian atmospheric value, and may represent upper mantle or crustal components.« less

Ar-Ar_Redux: rigorous error propagation of 40Ar/39Ar data, including covariances

NASA Astrophysics Data System (ADS)

Vermeesch, P.

2015-12-01

Rigorous data reduction and error propagation algorithms are needed to realise Earthtime's objective to improve the interlaboratory accuracy of 40Ar/39Ar dating to better than 1% and thereby facilitate the comparison and combination of the K-Ar and U-Pb chronometers. Ar-Ar_Redux is a new data reduction protocol and software program for 40Ar/39Ar geochronology which takes into account two previously underappreciated aspects of the method: 1. 40Ar/39Ar measurements are compositional dataIn its simplest form, the 40Ar/39Ar age equation can be written as: t = log(1+J [40Ar/39Ar-298.5636Ar/39Ar])/λ = log(1 + JR)/λ Where λ is the 40K decay constant and J is the irradiation parameter. The age t does not depend on the absolute abundances of the three argon isotopes but only on their relative ratios. Thus, the 36Ar, 39Ar and 40Ar abundances can be normalised to unity and plotted on a ternary diagram or 'simplex'. Argon isotopic data are therefore subject to the peculiar mathematics of 'compositional data', sensu Aitchison (1986, The Statistical Analysis of Compositional Data, Chapman & Hall). 2. Correlated errors are pervasive throughout the 40Ar/39Ar methodCurrent data reduction protocols for 40Ar/39Ar geochronology propagate the age uncertainty as follows: σ2(t) = [J2 σ2(R) + R2 σ2(J)] / [λ2 (1 + R J)], which implies zero covariance between R and J. In reality, however, significant error correlations are found in every step of the 40Ar/39Ar data acquisition and processing, in both single and multi collector instruments, during blank, interference and decay corrections, age calculation etc. Ar-Ar_Redux revisits every aspect of the 40Ar/39Ar method by casting the raw mass spectrometer data into a contingency table of logratios, which automatically keeps track of all covariances in a compositional context. Application of the method to real data reveals strong correlations (r2 of up to 0.9) between age measurements within a single irradiation batch. Propertly taking

AR-39-AR-40 "Age" of Basaltic Shergottite NWA-3171

NASA Technical Reports Server (NTRS)

Bogard, Donald D.; Park, Jisun

2007-01-01

North-West-Africa 3171 is a 506 g, relatively fresh appearing, basaltic shergottite with similarities to Zagami and Shergotty, but not obviously paired with any of the other known African basaltic shergottites. Its exposure age has the range of 2.5-3.1 Myr , similar to those of Zagami and Shergotty. We made AR-39-AR-40 analyses of a "plagioclase" (now shock-converted to maskelynite) separate and of a glass hand-picked from a vein connected to shock melt pockets.. Plagioclase was separated using its low magnetic susceptibility and then heavy liquid with density of <2.85 g/cm(exp 3). The AR-39-AR-40 age spectrum of NWA-317 1 plag displays a rise in age over 20-100% of the 39Ar release, from 0.24 Gyr to 0.27 Gy.

Constraining the alteration history of a Late Cretaceous Patagonian volcaniclastic bentonite-ash-mudstone sequence using K-Ar and 40Ar/39Ar isotopes

NASA Astrophysics Data System (ADS)

Warr, L. N.; Hofmann, H.; van der Pluijm, B. A.

2017-01-01

Smectite is typically considered unsuitable for radiometric dating, as argon (40Ar) produced from decay of exchangeable potassium (40K) located in the interlayer sites can be lost during fluid-rock interaction and/or during wet sample preparation in the laboratory. However, age analysis of Late Cretaceous Argentinian bentonites and associated volcaniclastic rocks from Lago Pellegrini, Northern Patagonia, indicates that, in the case of these very low-permeability rocks, the radioactive 40Ar was retained and thus can provide information on smectite age and the timing of rock alteration. This study presents isotopic results that indicate the ash-to-bentonite conversion and alteration of the overlying tuffaceous mudstones in Northern Patagonia was complete 13-17 my after middle Campanian sedimentation when the system isotopically closed. The general absence of illite in these smectite-rich lithologies reflects the low activity of K and the low temperature (<60 °C) of the formation waters that altered the parent ash.

Ar-40/Ar-39 age determinations for the Rotoiti eruption, New Zealand

NASA Astrophysics Data System (ADS)

Flude, S.; Storey, M.

2013-12-01

The contemporaneous Rotoiti and Earthquake Flat ignimbrites, erupted from the Taupo Volcanic zone, New Zealand, form a distinctive tephrostratigraphic horizon in the Southern Pacific. Radioisotopic dating results for these eruptions remain controversial, with published ages ranging from 35.1 × 2.8 ka [1] to 71 × 6 ka [2], with 61.0 × 1.5 ka [3] often being cited as the most widely accepted age. These eruptions are difficult to date as their age is near the limit for various radiometric dating techniques, which are complicated by a large proportion of inherited material (xenocrysts) and a lack of phases suitable for dating. Glass-bearing plutonic blocks erupted with the Rotoiti and Earthquake Flat ignimbrites have previously been interpreted as deriving from a slowly cooled and incompletely solidified magma body that was sampled by the eruptions. They contain large vugs lined with euhedral quartz, sanidine and biotite crystals, indicating that these crystals grew in a gas or aqueous fluid rich environment and are interpreted to have formed shortly before or during eruption. Here we will present Ar-40/Ar-39 ages for sanidines and biotites extracted from vugs in lithic blocks erupted as part of the Earthquake Flat ignimbrite. We show that, even for vug-lining material, inherited ages remain a problem and are the likely source of the wide variation in published radiometric ages. Nevertheless, many of the Ar-40/Ar-39 ages are much younger than the 61 ka age [3] and are more consistent with the recent stratigraphic, C-14 and U-238/Th-230+(U-Th)/He ages that have been suggested (e.g. [4,5]). 1. Whitehead, N. & Ditchburn, R. New Zealand Journal of Geology and Geophysics 37, 381-383 (1994). 2. Ota, Y., Omura, A. & Iwata, H. New Zealand Journal of Geology and Geophysics 32, 327-331 (1989). 3. Wilson, C. J. N. et al. Quaternary Science Reviews 26, 1861-1870 (2007). 4. Molloy, C., Shane, P. & Augustinus, P. Geological Society of America Bulletin 121, 1666-1677 (2009). 5

40Ar/39Ar technique of KAr dating: a comparison with the conventional technique

USGS Publications Warehouse

Brent, Dalrymple G.; Lanphere, M.A.

1971-01-01

K-Ar ages have been determined by the 40Ar/39Ar total fusion technique on 19 terrestrial samples whose conventional K-Ar ages range from 3.4 my to nearly 1700 my. Sample materials included biotite, muscovite, sanidine, adularia, plagioclase, hornblende, actinolite, alunite, dacite, and basalt. For 18 samples there are no significant differences at the 95% confidence level between the KAr ages obtained by these two techniques; for one sample the difference is 4.3% and is statistically significant. For the neutron doses used in these experiments (???4 ?? 1018 nvt) it appears that corrections for interfering Ca- and K-derived Ar isotopes can be made without significant loss of precision for samples with K/Ca > 1 as young as about 5 ?? 105 yr, and for samples with K/Ca < 1 as young as about 107 yr. For younger samples the combination of large atmospheric Ar corrections and large corrections for Ca- and K-derived Ar may make the precision of the 40Ar/39Ar technique less than that of the conventional technique unless the irradiation parameters are adjusted to minimize these corrections. ?? 1971.

Ar-39-Ar-40 Ages of Two Nakhlites, MIL03346 and Y000593: A Detailed Analysis

NASA Technical Reports Server (NTRS)

Park, Jisun; Garrison, Daniel; Bogard, Donald

2007-01-01

Radiometric dating of martian nakhlites by several techniques have given similar ages of approx.1.2-1.4 Ga [e.g. 1, 2]. Unlike the case with shergottites, where the presence of martian atmosphere and inherited radiogenic Ar-40 produce apparent Ar-39-Ar-40 ages older than other radiometric ages, Ar-Ar ages of nakhlites are similar to ages derived by other techniques. However, even in some nakhlites the presence of trapped martian Ar produces some uncertainty in the Ar-Ar age. We present here an analysis of such Ar-Ar ages from the MIL03346 and Y000593 nakhlites.

Geochemical and K Ar age constraints on the Late Neoproterozoic (?) gneisses at Um Tenassib area, north Eastern Desert, Egypt

NASA Astrophysics Data System (ADS)

Eliwa, Hassan A.

2007-05-01

Wadi Um Tenassib metamorphic rocks consist mainly of biotite gneiss and biotite-hornblende gneiss with subordinate intercalations of amphibolite, migmatitic gneiss, and aplitic granite. Biotite-hornblende gneiss, biotite gneiss, and aplitic granite are geochemically characterized and their cooling ages are determined by using the K-Ar method on biotite. The Um Tenassib gneisses (UTG) range in composition from quartz diorite/monzodiorite to granodiorite. They were derived from igneous rocks that pertain to calc alkaline and metaluminous to weakly peraluminous affinities and were generated in continental volcanic arc setting. REE patterns of the UTG are moderately fractionated (La N/Lu N = 5.9-7.5) relative to those of the aplitic granite (La N/Lu N = 33). The similarity in the geochemical characteristics and REE patterns of both gneiss types indicate their magmatic consanguinity. Amphiboles of the UTG biotite-hornblende gneisses are mainly hornblende, together with few paragasitic hornblende and edenite. Plagioclase composition is oligoclase to andesine (An 21-46) in the biotite-hornblende gneiss, and oligoclase (An 11-26) in the biotite gneiss. Mineral chemistry of amphibole and plagioclase indicate that the gneisses were metamorphosed under low- to medium-pressure of 2.6-6.4 kbar and at medium to high temperatures of 660-755 °C. The K-Ar biotite cooling ages (seven samples) range from 585 ± 12 Ma to 598 ± 12 Ma for the UTG, except one biotite-hornblende gneiss sample gives age of 577 ± 11 Ma. These ages suggest a latest metamorphic cooling event at ca. 585-600 Ma time span, which is consistent with the proposed cooling ages of ˜600 Ma for the Elat metamorphic rocks [Cosca, M.A., Shimron, A., Caby, R., 1999. Late Precambrian metamorphism and cooling in the Arabian-Nubian Shield: petrology and 40Ar/ 39Ar geochronology of metamorphic rocks of the Elat area (southern Israel). Precamb. Res. 98, 107-127]. It may indicate that the metamorphism of the UTG might have

A geochronological 40Ar/39Ar and 87Rb/81Sr study of K-Mn oxides from the weathering sequence of Azul, Brazil

NASA Astrophysics Data System (ADS)

Ruffet, G.; Innocent, C.; Michard, A.; Féraud, G.; Beauvais, A.; Nahon, D.; Hamelin, B.

1996-06-01

KMn oxides of hollandite group minerals such as cryptomelane (K 1-2(Mn 3+, Mn 4+) 8O 16nH 2O) are often precipitated authigenically in weathering profiles. The presence of structural K allows these minerals to be dated by the KAr and 40Ar/ 39Ar methods, making it possible to study the progression of oxidation fronts during weathering processes. Within the context of a recent 40Ar/ 39Ar study of cryptomelane from the Azul Mn deposit in the Carajàs region (Amazônia, Brazil), Vasconcelos et al. (1994) defined three age clusters (65-69, 51-56, and 40-43 Ma) and proposed that they correspond to the episodic precipitation of the three generations of Mn oxide that have been identified in the deposit (Beauvais et al., 1987). We performed a laser probe 40Ar/ 39Ar and 87Rb/ 87Sr study on new samples from the same Mn deposit. Our 40Ar/ 39Ar data confirm that cryptomelane is a suitable mineral for 40Ar/ 39Ar dating, although in some cases we clearly identify the existence of 39Ar recoil effects. Although the corresponding age spectra are generally strongly disturbed, our results also confirm that the earliest cryptomelane generation is of Upper Cretaceous-Paleocene age. We obtained good plateau ages from veins and concretions of the second cryptomelane generation. Some of these results allow definition of a well-constrained age cluster at 46.7-48.1 Ma not observed by Vasconcelos et al. (1994). A petrographic study confirms that none of the samples analyzed in the present study contained material associated with the third generation of cryptomelane. We propose that these new results support the idea of a more or less continuous crystallization of KMn oxides, mainly constrained by local factors, rather than the model advanced by Vasconcelos et al. (1994), which suggests that each cryptomelane generation corresponds to distinct weathering events related to global climatic changes. 87Sr/ 86Sr data show large variations, clearly inherited from the 2.1 Ga parent rock of

Magnetostratigraphy of the Naude's Nek Section (Karoo Traps): News Constraints on the Eruptive Sequence

NASA Astrophysics Data System (ADS)

Moulin, M.; Jay, A. E.; Fluteau, F.; Marsh, J. S.; Courtillot, V.; Gerard, M.; Quidelleur, X.

2008-12-01

A causal relationship between large igneous provinces (LIP) and mass extinctions (ME) or oceanic anoxia events (OAE) is supported by an increasing data base of age determinations. Although the impact of recent (much smaller) volcanic eruptions on climate is understood as being largely due to injection of SO2 into the stratosphere, the environmental effects of LIP eruptions remain ill understood. Successful climate modelling requires high-resolution timing of volcanism, i.e. number, volume and duration of peak episodes. Chenet et al (2008, and in revision) have studied the Deccan traps, which correlate with the major KT crisis, combining geochronology (K-Ar), paleomagnetism (secular variation), volcanology (flow types) and analysis of alteration levels between flows (red boles); they have shown that emplacement occurred as a small number of discrete, very large and short-lived pulses (see Courtillot et al, same session). But mass extinctions are not all alike, in pattern or intensity. Some large igneous provinces are correlated with only minor crises, e.g. the Karoo traps formed near the Pliensbachian-Toarcian boundary (~180 Ma). Remnants of these traps outcrop over >2.5×106 km2. Jourdan et al. (2008) find that the Karoo traps were emplaced over some 4 Myr, significantly longer than the Deccan. However, the Karoo province does seem to comprise brief (<1-2 Myr), spatially and temporally distinct magmatic events. We report here our results from the Naude's Nek section (the lower 700 m of the traps), located in South Africa near the southern border of Lesotho. Our first determinations (40K-40Ar Cassignol-Gillot technique) yield ages of 180±1.8 Ma, in good agreement with previous studies. Detailed flow by flow magnetostratigraphy (site-mean directions based on thermal demagnetization) shows that the eruptive sequence can be divided into several volcanic pulses having likely lasted less than ~100 years. We have obtained a particularly detailed record of the only

Significance of the cosmogenic argon correction in deciphering the 40Ar/39Ar ages of the Nakhlite (Martian) meteorites

NASA Astrophysics Data System (ADS)

Cohen, B. E.; Cassata, W.; Mark, D. F.; Tomkinson, T.; Lee, M. R.; Smith, C. L.

2015-12-01

All meteorites contain variable amounts of cosmogenic 38Ar and 36Ar produced during extraterrestrial exposure, and in order to calculate reliable 40Ar/39Ar ages this cosmogenic Ar must be removed from the total Ar budget. The amount of cosmogenic Ar has usually been calculated from the step-wise 38Ar/36Ar, minimum 36Ar/37Ar, or average 38Arcosmogenic/37Ar from the irradiated meteorite fragment. However, if Cl is present in the meteorite, then these values will be disturbed by Ar produced during laboratory neutron irradiation of Cl. Chlorine is likely to be a particular issue for the Nakhlite group of Martian meteorites, which can contain over 1000 ppm Cl [1]. An alternative method for the cosmogenic Ar correction uses the meteorite's exposure age as calculated from an un-irradiated fragment and step-wise production rates based on the measured Ca/K [2]. This calculation is independent of the Cl concentration. We applied this correction method to seven Nakhlites, analyzed in duplicate or triplicate. Selected samples were analyzed at both Lawrence Livermore National Laboratory and SUERC to ensure inter-laboratory reproducibility. We find that the cosmogenic argon correction of [2] has a significant influence on the ages calculated for individual steps, particularly for those at lower temperatures (i.e., differences of several tens of million years for some steps). The lower-temperature steps are more influenced by the alternate cosmogenic correction method of [2], as these analyses yielded higher concentrations of Cl-derived 38Ar. As a result, the Nakhlite data corrected using [2] yields step-heating spectra that are flat or nearly so across >70% of the release spectra (in contrast to downward-stepping spectra often reported for Nakhlite samples), allowing for the calculation of precise emplacement ages for these meteorites. [1] Cartwright J. A. et al. (2013) GCA, 105, 255-293. [2] Cassata W. S., and Borg L. E. (2015) 46th LPSC, Abstract #2742.

40Ar/39Ar mica ages from marble mylonites: a cautionary tale

NASA Astrophysics Data System (ADS)

Rogowitz, Anna; Huet, Benjamin; Schneider, David; Grasemann, Bernhard

2014-05-01

40Ar/39Ar geochronology on white mica is a popular method to date deformation under moderate (brittle-ductile) temperatures. In particular, deformation events preserved in greenschist facies shear zones have been successfully dated with this method. A consequence of strain localization in many tectonic settings that bear calcitic marbles is the formation of marble mylonites and ultramylonites. Little is known, however, about the behaviour of the K/Ar systems and the influence of deformation on the ages in such rocks. We studied an extremely localized shear zone (2 cm thick) in marble from Syros (Cyclades, Greece) and performed microstructural, chemical and isotopic analysis on samples from the host rock and the shear zone. The host rock is composed of coarse-grained (300 µm) calcite with only minor undulatory extinction and slightly curved grain boundaries. This initial large grain size is likely to have formed during the Eocene high-pressure - low-temperature event that is well documented in the Cyclades. In contrast, the marble within the shear zone shows evidence of strong intracrystalline deformation and recrystallization resulting in grain size reduction and the formation of an ultramylonite. Both microstructures and kinematics are consistent with the low grade evolution described on Syros. White mica (100's microns in size) are preferentially orientated parallel to the foliation. In both samples there is no clear evidence for crystal plastic deformation of the mica grains. Bigger grains behave brittle resulting in grain size reduction. A deformation mechanism map for calcite at 300 °C indicates that the host rock deformed at strain rates of around 10-12.5 s-1 whereas within the shear zone strain rates of up to 10-9.5 s-1 are attained. We performed laser-heating 40Ar/39Ar analysis on white mica located in the host rock and the shear zone. The low-strain host rock yielded a ca. 40 Ma age, and the shear zone recorded a ca. 37 Ma age; both ages are

A closer look at 40Ar/39Ar systematics of illite, recoil, retention ages, total gas ages, and a new correction method

NASA Astrophysics Data System (ADS)

Fitz-Diaz, E.; Hall, C. M.; van der Pluijm, B.

2013-12-01

One of the fundamentals of 40Ar-39Ar systematics of illite considers the effects of 39Ar recoil (ejection of 39Ar from tiny illite crystallites during the nuclear reaction 39K(n,p)39Ar), for which sample vacuum encapsulation prior to irradiation has been used since the 1990's. This technique separately measures the fraction of recoiled 39Ar and the Ar (39Ar and 40Ar) retained within illite crystals as they degas during step heating in vacuum. Total-gas ages (TGA) are calculated by using both recoiled and retained argon, while retention ages (RA) only involve retained Ar. Observations in numerous natural examples have shown that TGA fit stratigraphic constraints of geological processes when the average illite crystallite thickness (ICT) is smaller than 10nm, and that RA better matches these constrains for larger ICTs. Illite crystals with ICT >50nm show total gas and retention ages within a few My and they are identical, within analytical error, when ICT exceeds 150nm. We propose a new age correction that takes into account the average ICT and corresponding recoil for a sample , with such corrected ages (XCA) lying between the TGA and RA end-member ages. We apply this correction to samples containing one generation of illite and it particularly affects illite populations formed in the anchizone, with typical ICT values between 10-40nm. We analyzed bentonitic samples (S1, S2 and S3) from sites in Cretaceous carbonates in the front of the Monterrey salient in northern Mexico. Four size fractions (<0.05, 0.05-0.2, 0.2-1 & 1-2 μm) were separated, analyzed with XRD and dated by Ar-Ar. XRD analysis provides mineralogic characterization, illite polytype quantification, and illite crystallite thickness (ICT) determination using half-height peak width (illite crystallinity) and the Scherrer equation. All samples contain illite as the main mineral phase, ICT values between 8-27nm, from fine to coarser grain size fractions. Ages show a range in TGA among the different size

Re-Evaluation of Ar-39 - Ar-40 Ages for Apollo Lunar Rocks 15415 and 60015

NASA Technical Reports Server (NTRS)

Park, J.; Nyquist, L. E.; Bogard, D. D.; Garrison, D. H.; Shih, C.-Y.

2010-01-01

We re-analyzed 39Ar-40Ar ages of Apollo lunar highland samples 15415 and 60015, two ferroan anorthosites analyzed previously in the 1970 s, with a more detailed approach and with revised decay constants. From these samples we carefully prepared 100-200 mesh mineral separates for analysis at the Noble Gas Laboratory at NASA-Johnson Space Center. The Ar-39-Ar-40 age spectra for 15415 yielded an age of 3851 +/- 38 Ma with 33-99% of Ar39 release, roughly in agreement with previously reported Ar-Ar ages. For 60015, we obtained an age of 3584 +/- 152 Ma in 23-98% of Ar39 release, also in agreement with previously reported Ar-Ar ages of approximately 3.5 Ga. Highland anorthosites like these are believed by many to be the original crust of the moon, formed by plagioclase floatation atop a magma ocean, however the Ar-Ar ages of 15415 and 60015 are considerably younger than lunar crust formation. By contrast, recently recovered lunar anorthosites such as Dhofar 489, Dhofar 908, and Yamato 86032 yield older Ar-Ar ages, up to 4.35 Ga, much closer to time of formation of the lunar crust. It follows that the Ar-Ar ages of the Apollo samples must have been reset by secondary heating, and that this heating affected highland anorthosites at both the Apollo 15 and Apollo 16 landing sites but did not affect lunar highland meteorites. One obvious consideration is that while the Apollo samples were collected from the near side of the moon, these lunar meteorites are thought to have originated from the lunar far side

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

USGS Publications Warehouse

Sarna-Wojcicki, A. M.; Pringle, M.S.; Wijbrans, J.

2000-01-01

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 Friant, 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

Geology and 40Ar/39Ar geochronology of the medium- to high-K Tanaga volcanic cluster, western Aleutians

USGS Publications Warehouse

Jicha, Brian R.; Coombs, Michelle L.; Calvert, Andrew T.; Singer, Brad S.

2012-01-01

We used geologic mapping and geochemical data augmented by 40Ar/39Ar dating to establish an eruptive chronology for the Tanaga volcanic cluster in the western Aleutian arc. The Tanaga volcanic cluster is unique in comparison to other central and western Aleutian volcanoes in that it consists of three closely spaced, active, volumetrically significant edifices (Sajaka, Tanaga, and Takawangha), the eruptive products of which have unusually high K2O contents. Thirty-five new 40Ar/39Ar ages obtained in two different laboratories constrain the duration of Pleistocene–Holocene subaerial volcanism to younger than 295 ka. The eruptive activity has been mostly continuous for the last 150 k.y., unlike most other well-characterized arc volcanoes, which tend to grow in discrete pulses. More than half of the analyzed Tanaga volcanic cluster lavas are basalts that have erupted throughout the lifetime of the cluster, although a considerable amount of basaltic andesite and basaltic trachyandesite has also been produced since 200 ka. Major- and trace-element variations suggest that magmas from Sajaka and Tanaga volcanoes are likely to have crystallized pyroxene and/or amphibole at greater depths than the older Takawangha magmas, which experienced a larger percentage of plagioclase-dominated fractionation at shallower depths. Magma output from Takawangha has declined over the last 86 k.y. At ca. 19 ka, the focus of magma flux shifted to the west beneath Tanaga and Sajaka volcanoes, where hotter, more mafic magma erupted.

Interpretation of K-Ar dates of illitic clays from sedimentary rocks aided by modeling

USGS Publications Warehouse

Srodon, J.; Clauer, Norbert; Eberl, D.D.D.

2002-01-01

K-Ar dates of illitic clays from sedimentary rocks may contain "mixed ages," i.e., may have ages that are intermediate between the ages of end-member events. Two phenomena that may cause mixed ages are: (1) long-lasting reaction during the burial illitization of smectite: and (2) physical mixing of detrital and diagenetic components. The first phenomenon was investigated by simulation of illitization reactions using a nucleation and growth mechanism. These calculations indicate that values for mixed ages are related to burial history: for an equivalent length of reaction time, fast burial followed by slow burial produces much older mixed ages than slow burial followed by fast. The type of reaction that occured in a rock can be determined from the distribution of ages with respect to the thickness of illite crystals. Dating of artificial mixtures confirms a non-linear relation between mixed ages and the proportions of the components. Vertical variation of K-Ar age dates from Gulf Coast shales can be modeled by assuming diagenetic illitization that overprints a subtle vertical trend (presumably of sedimentary origin) in detrital mineral content.

Potassium Isotopic Compositions of NIST Potassium Standards and 40Ar/39Ar Mineral Standards

NASA Technical Reports Server (NTRS)

Morgan, Leah; Tappa, Mike; Ellam, Rob; Mark, Darren; Higgins, John; Simon, Justin I.

2013-01-01

Knowledge of the isotopic ratios of standards, spikes, and reference materials is fundamental to the accuracy of many geochronological methods. For example, the 238U/235U ratio relevant to U-Pb geochronology was recently re-determined [1] and shown to differ significantly from the previously accepted value employed during age determinations. These underlying values are fundamental to accurate age calculations in many isotopic systems, and uncertainty in these values can represent a significant (and often unrecognized) portion of the uncertainty budget for determined ages. The potassium isotopic composition of mineral standards, or neutron flux monitors, is a critical, but often overlooked component in the calculation of K-Ar and 40Ar/39Ar ages. It is currently assumed that all terrestrial materials have abundances indistinguishable from that of NIST SRM 985 [2]; this is apparently a reasonable assumption at the 0.25per mille level (1s) [3]. The 40Ar/39Ar method further relies on the assumption that standards and samples (including primary and secondary standards) have indistinguishable 40K/39K values. We will present data establishing the potassium isotopic compositions of NIST isotopic K SRM 985, elemental K SRM 999b, and 40Ar/39Ar biotite mineral standard GA1550 (sample MD-2). Stable isotopic compositions (41K/39K) were measured by the peak shoulder method with high resolution MC-ICP-MS (Thermo Scientific NEPTUNE Plus), using the accepted value of NIST isotopic SRM 985 [2] for fractionation [4] corrections [5]. 40K abundances were measured by TIMS (Thermo Scientific TRITON), using 41K/39K values from ICP-MS measurements (or, for SRM 985, values from [2]) for internal fractionation corrections. Collectively these data represent an important step towards a metrologically traceable calibration of 40K concentrations in primary 40Ar/39Ar mineral standards and improve uncertainties by ca. an order of magnitude in the potassium isotopic compositions of standards.

NWA 7034 Martian breccia: Ar/Ar ages of ca. 1.2 to 1.4 Ga

NASA Astrophysics Data System (ADS)

Cohen, B. E.; Mark, D. F.; Cassata, W.; Lee, M. R.; Smith, C. L.

2015-12-01

NWA 7034 and its paired stones are some of the oldest and most diverse of the Martian meteorites. They are complex polymict breccias of impact, igneous, and sedimentary clasts set in a dark grey matrix [1; 2]. The rock also contains angular mineral fragments, including K-feldspar, plagioclase feldspar, and pyroxene [1; 2]. Mineral fragments are often > 1 mm wide, and clasts can be > 1 cm. This diverse breccia assemblage indicates formation via repeated impact events, supported by Rb-Sr, Sm-Nd and U-Pb ages ranging from 1.3 to 4.4 Ga [1, 2, and references therein]. In this study we investigate the distribution of ages yielded by Ar/Ar, with nine aliquots analyzed to date, and additional analyses planned. In order to analyze only single phases, chips of matrix/clasts were restricted to visibly monomict fragments < 1 mm diameter, while mineral separates were analyzed as single crystals. Cosmogenic Ar corrections are from [3]. Analyses were undertaken at SUERC and Lawrence Livermore National Laboratory, and the results pooled. The bulk of aliquots (n = 8) yielded ages of ca. 1.2-1.4 Ga indicating a major thermal event occurred at around the same time as crystallization of the Nakhlite group of meteorites. Select step ages are considerably older (> 2 Ga), supporting results of other chronometers that much older material is present in this sample. These results also demonstrate that some older fragments retained Ar during breccia formation. [1] Wittmann A. et al. (2015) Meteoritics & Planet. Sci., 50, 326-352. [2] Santos A. R. et al. (2015) GCA, 157, 56-85. [3] Cassata W. S., and Borg L. E. (2015) 46th LPSC, Abstract #2742.

Testing Astronomical and 40Ar/39Ar Timescales for the K/Pg Boundary Interval Using High-Resolution Magnetostratigraphy and U-Pb Geochronology in the Denver Basin of Colorado

NASA Astrophysics Data System (ADS)

Clyde, W.; Bowring, S. A.; Johnson, K. R.; Ramezani, J.; Jones, M. M.

2015-12-01

Accurate and precise calibration of the Geomagnetic Polarity Timescale (GPTS) in absolute time is critical for resolving rates of geological and biological processes which in turn help constrain the underlying causes of those processes. Numerical calibration of the GPTS was traditionally carried out by interpolation between a limited number of 40Ar/39Ar dated volcanic ash deposits from superpositional sequences with well-defined magnetostratigraphies. More recently, the Neogene part of the GPTS has been calibrated using high-resolution astrochronological methods, however the application of these approaches to pre-Neogene parts of the timescale is controversial given the uncertainties in relevant orbital parameters this far back in time and differing interpretations of local cyclostratigraphic records. The Cretaceous-Paleogene (K/Pg) boundary interval is a good example, where various astronomical and 40Ar/39Ar calibrations have been proposed with varying degrees of agreement. The Denver Basin (Colorado, USA) contains one of the most complete stratigraphic sequences across the K/Pg boundary in the world, preserving evidence of bolide impact as well as biotic extinction and recovery in a thick stratigraphic package that is accessible by both core and outcrop. We present a series of high-precision U-Pb age determinations from interbedded volcanic ash deposits within a tightly constrained magnetobiostratigraphic framework across the K/Pg boundary in the Denver Basin. This new timeline provides a precise absolute age for the K/Pg boundary, constrains the ages of magnetic polarity Chrons C28 to C30, and provides a direct and independent test of early Paleogene astronomical and 40Ar/39Ar based timescales. Temporal calibration of fossil pollen evidence of the "fern spike" in the Denver Basin shows that plant extinctions peaked within ~50-500 years of the bolide impact and primary productivity recovered ~500-5000 years after the impact.

Legacy K/Ar and 40Ar/39Ar geochronologic data from the Alaska-Aleutian Range batholith of south-central Alaska

USGS Publications Warehouse

Koeneman, Lisa L.; Wilson, Frederic H.

2018-04-06

Sample descriptions and analytical data for more than 200 K/Ar and 40Ar/39Ar analyses from rocks of the Alaska-Aleutian Range batholith of south-central Alaska are reported here. Samples were collected over a period of 20 years by Bruce R. Reed and Marvin A. Lanphere (both U.S. Geological Survey) as part of their studies of the batholith.

Ar-40 to Ar-39 ages of the large impact structures Kara and Manicouagan and their relevance to the Cretaceous-Tertiary and the Triassic-Jurassic boundary

NASA Technical Reports Server (NTRS)

Trieloff, M.; Jessberger, E. K.

1992-01-01

Since the discovery of the Ir enrichment in Cretaceous-Tertiary boundary clays in 1980, the effects of a 10-km asteroid impacting on the Earth 65 Ma ago have been discussed as the possible reason for the mass extinction--including the extinction of the dinosaurs--at the end of the Cretaceous. But up to now no crater of this age that is large enough (ca. 200 km in diameter) has been found. One candidate is the Kara Crater in northern Siberia. Kolesnikov et al. determined a K-Ar isochron of 65.6 +/- 0.5 Ma, indistinguishable from the age of the K-T boundary and interpreted this as confirmation of earlier proposals that the Kara bolide would have been at least one of the K-T impactors. Koeberl et al. determined Ar-40 to Ar-39 ages ranging from 70 to 82 Ma and suggested an association to the Campanian-Maastrichtian boundary, another important extinction horizon 73 Ma ago. We dated four impact melts, KA2-306, KA2-305, SA1-302, and AN9-182. Results from the investigation are discussed.

Ar-40 to Ar-39 ages of the large impact structures Kara and Manicouagan and their relevance to the Cretaceous-Tertiary and the Triassic-Jurassic boundary

NASA Astrophysics Data System (ADS)

Trieloff, M.; Jessberger, E. K.

Since the discovery of the Ir enrichment in Cretaceous-Tertiary boundary clays in 1980, the effects of a 10-km asteroid impacting on the Earth 65 Ma ago have been discussed as the possible reason for the mass extinction--including the extinction of the dinosaurs--at the end of the Cretaceous. But up to now no crater of this age that is large enough (ca. 200 km in diameter) has been found. One candidate is the Kara Crater in northern Siberia. Kolesnikov et al. determined a K-Ar isochron of 65.6 +/- 0.5 Ma, indistinguishable from the age of the K-T boundary and interpreted this as confirmation of earlier proposals that the Kara bolide would have been at least one of the K-T impactors. Koeberl et al. determined Ar-40 to Ar-39 ages ranging from 70 to 82 Ma and suggested an association to the Campanian-Maastrichtian boundary, another important extinction horizon 73 Ma ago. We dated four impact melts, KA2-306, KA2-305, SA1-302, and AN9-182. Results from the investigation are discussed.

New 40Ar/39Ar eruption ages for the western Aeolian Arc, Southern Italy

NASA Astrophysics Data System (ADS)

Creamer, J. B.; Calvert, A. T.; Spera, F. J.

2011-12-01

New 40Ar/39Ar ages are presented for the western Aeolian Arc, southern Italy which help constrain eruption frequency and revise the local uplift history through bracketing the timing of marine terrace formation. The islands of Alicudi, Filicudi, and Salina are late-Quaternary composite stratovolcanoes which feature basaltic to dacitic calc-alkaline and high-K series lavas and pyroclastics. These islands have been the focus of several geochronologic studies in the past but many open questions remain due to recently revised mapping, conflicting ages, and incomplete coverage of eruptive sequences. On Salina, a single new age constrains the previously undated Corvo volcano to 128.8 +/- 5.3 ka. On Filicudi, three new dates constrain the oldest exposed episodes of volcanism to ~200-205 ka, in good agreement with previous K/Ar ages. The age of an intervening marine terrace is constrained by these oldest units to c. 200 ka. Additionally there is one new date of 80.4 ± 9.4 ka on the youngest effusive eruption on Filicudi, a dacite dome that overlies the youngest marine terrace on the island. On Alicudi, five new dates constrain the oldest exposed units (Paleo Alicudi Synthem) to 167.4 ± 26.1 ka, the Scoglio Galera to Spano synthems between 64.1 ± 4.1 ka and 59.3 ± 1.3 ka, and confirm the youngest eruptions (Fillo dell'Arpa Fm) at 30.0 ± 0.7 ka. Importantly, an undated formation that overlies the 59.3 ka unit is cut by a marine terrace that was previously interpreted to be ~80 ka (MIS 5a highstand). This result calls into question the archipelago-wide age correlation of marine terraces attempted by Lucchi et al. 2008 since the youngest terrace on Alicudi is younger than eruptive units that overly all terraces on Filicudi. Previous work suggesting that marine terraces on Filicudi were formed during Marine Isotope Stages (MIS) 5a,c, and e is permitted by our new ages, but the oldest terrace ('Paleoshoreline B') on Filicudi was possibly formed during MIS 7 rather than MIS

Petrology and K-Ar ages of rift-related basaltic rocks, offshore northern Brazil, 3/sup 0/N

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fodor, R.V.; McKee, E.H.

1986-07-01

Tholeiitic basaltic rock in three cores from Petrobras drill site APS-21, 1960-2480 m depths, Amapa basin, offshore Brazil is compositionally similar to rift-related basaltic rock associated with the opening of both the North and South Atlantic Oceans (SiO/sub 2/ 52-54 wt %; K/sub 2/O 0.7-1.3%; TiO/sub 2/ 1.3-2%). Whole-rock K-Ar ages are 185.4, 183.2, and 126.5 m.y. If these represent crystallization ages, then the older samples correspond to North Atlantic tectonism (as represented by the Liberian dike system) and the younger correlates with South Atlantic rift-related magmatism (of which Serra Geral flood basalts are the best example). Trace- and REE-elementsmore » identify T-type mantle source-areas (La/Sm/sub (n)/ approx. 2; Zr/Nb 8-11) that feasibly were mixes of N-type and P-type components (metasomatized or veined upper mantle). These Amapa basin mafic rocks document the southernmost magmatism related to North Atlantic rifting, as well as early Mesozoic mantle source-areas and processes beneath Gondwanaland such as those identified with basalts in the South Atlantic basin.« less

40Ar/39Ar ages of the AD 79 eruption of Vesuvius, Italy

USGS Publications Warehouse

Lanphere, Marvin A.; Champion, Duane E.; Melluso, Leone; Morra, Vincenzo; Perrotta, Annamaria; Scarpati, Claudio; Tedesco, Dario; Calvert, Andrew T.

2007-01-01

The Italian volcano, Vesuvius, erupted explosively in AD 79. Sanidine from pumice collected at Casti Amanti in Pompeii and Villa Poppea in Oplontis yielded a weighted-mean 40Ar/39Ar age of 1925±66 years in 2004 (1σ uncertainty) from incremental-heating experiments of eight aliquants of sanidine. This is the calendar age of the eruption. Our results together with the work of Renne et al. (1997) and Renne and Min (1998) demonstrate the validity of the 40Ar/39Ar method to reconstruct the recent eruptive history of young, active volcanoes.

K-Ar geochronology of basement rocks on the northern flank of the Huancabama deflection, Ecuador

USGS Publications Warehouse

Feininger, Tomas; Silberman, M.L.

1982-01-01

The Huancabamba deflection, a major Andean orocline located at the Ecuador-Peru border, constitutes an important geologic boundary on the Pacific coast of South America. Crust to the north of the deflection is oceanic and the basement is composed of basic igneous rocks of Cretaceous age, whereas crust to the south is continental and felsic rocks of Precambrian to Cretaceous age make up the basement. The northern flank of the Huancabamba Deflection in El Oro Province, Ecuador, is underlain by Precambrian polymetamorphic basic rocks of the Piedras Group; shale, siltstone, sandstone, and their metamorphosed equivalents in the Tahuin Group (in part of Devonian age); concordant syntectonic granitic rocks; quartz diorite and alaskite of the Maroabeli pluton; a protrusion of serpentinized harzburgite that contains a large inclusion of blueschist-facies metamorphic rocks, the Raspas Formation, and metamorphic rocks north of the La Palma fault. Biotite from gneiss of the Tahuin Group yields a Late Triassic K-Ar age (210 ? 8 m.y.). This is interpreted as an uplift age and is consistent with a regional metamorphism of Paleozoic age. A nearby sample from the Piedras Group that yielded a hornblende K-Ar age of 196 ? 8 m.y. was affected by the same metamorphic event. Biotite from quartz diorite of the mesozonal Maroabeli pluton yields a Late Triassic age (214 ? 6 m.y.) which is interpreted as an uplift age which may be only slightly younger than the age of magmatic crystallization. Emplacement of the pluton may postdate regional metamorphism of the Tahuin Group. Phengite from politic schist of the Raspas Formation yields an Early Cretaceous K-Ar age (132 ? 5 m.y.). This age is believed to date the isostatic rise of the encasing serpentinized harzburgite as movement along a subjacent subduction zone ceased, and it is synchronous with the age of the youngest lavas of a coeval volcanic arc in eastern Ecuador. A Late Cretaceous K-Ar age (74.4 ? 1.1 m.y.) from hornblende in

40Ar/39Ar ages of the AD 79 eruption of Vesuvius, Italy

NASA Astrophysics Data System (ADS)

Lanphere, Marvin; Champion, Duane; Melluso, Leone; Morra, Vincenzo; Perrotta, Annamaria; Scarpati, Claudio; Tedesco, Dario; Calvert, Andrew

2007-01-01

The Italian volcano, Vesuvius, erupted explosively in AD 79. Sanidine from pumice collected at Casti Amanti in Pompeii and Villa Poppea in Oplontis yielded a weighted-mean 40Ar/39Ar age of 1925±66 years in 2004 (1σ uncertainty) from incremental-heating experiments of eight aliquants of sanidine. This is the calendar age of the eruption. Our results together with the work of Renne et al. (1997) and Renne and Min (1998) demonstrate the validity of the 40Ar/39Ar method to reconstruct the recent eruptive history of young, active volcanoes.

Astronomical calibration of 40Ar/39Ar reference minerals using high-precision, multi-collector (ARGUSVI) mass spectrometry

NASA Astrophysics Data System (ADS)

Phillips, D.; Matchan, E. L.; Honda, M.; Kuiper, K. F.

2017-01-01

The new generation of multi-collector mass spectrometers (e.g. ARGUSVI) permit ultra-high precision (<0.1%) 40Ar/39Ar geochronology of rocks and minerals. At the same time, the 40Ar/39Ar method is limited by relatively large uncertainties (>1%) in 40K decay constants and the ages of natural reference minerals that form the basis of the technique. For example, reported ages for widely used 40Ar/39Ar reference materials, such as the ca. 28 Ma Fish Canyon Tuff sanidine (FCTs) and the ca. 1.2 Ma Alder Creek Rhyolite sanidine (ACRs), vary by >1%. Recent attempts to independently calibrate these reference minerals have focused on K-Ar analyses of the same minerals and inter-comparisons with astronomically tuned tephras in sedimentary sequences and U-Pb zircon ages from volcanic rocks. Most of these studies used older generation (effectively single-collector) mass spectrometers that employed peak-jumping analytical methods to acquire 40Ar/39Ar data. In this study, we reassess the inter-calibration and ages of commonly used 40Ar/39Ar reference minerals Fish Canyon Tuff sanidine (FCTs), Alder Creek Rhyolite sanidine (ACRs) and Mount Dromedary biotite (MD2b; equivalent to GA-1550 biotite), relative to the astronomically tuned age of A1 Tephra sanidine (A1Ts), Faneromeni section, Crete (Rivera et al., 2011), using a multi-collector ARGUSVI mass spectrometer. These analyses confirm the exceptional precision capability (<0.1%) of this system, compared to most previous studies. All sanidine samples (FCTs, ACRs and A1Ts) exhibit discordant 40Ar/39Ar step-heating spectra, with generally monotonically increasing ages (∼1% gradients). The similarity in these patterns, mass-dependent fractionation modeling, and results from step-crushing experiments on FCTs, which yield younger apparent ages, suggest that the discordance may be due to a combination of recoil loss and redistribution of 39ArK and isotope mass fractionation. In contrast to our previous inferences, these results imply

The effect of SEM imaging on the Ar/Ar system in feldspars

NASA Astrophysics Data System (ADS)

Flude, S.; Sherlock, S.; Lee, M.; Kelley, S. P.

2010-12-01

Complex microtextures form in K-feldspar crystals as they cool and are affected by deuteric alteration. This complex structure is the cause of variable closure temperatures for Ar-Ar, a phenomenon which has been utilized in multi domain diffusion (MDD) modelling to recover thermal histories [1]. However, there has been substantial controversy regarding the precise interaction between feldspar microtextures and Ar-diffusion [2,3]. A number of studies have addressed this issue using coupled SEM imaging and Ar/Ar UV laser ablation microprobe (UV-LAMP) analysis on the same sample, to enable direct comparison of microtextures with Ar/Ar age data [4]. Here we have tested the idea that SEM work may affect Ar/Ar ages, leading to inaccurate results in subsequent Ar/Ar analyses. Three splits of alkali feldspar from the Dartmoor Granite in SW England were selected for Ar/Ar UV-LAMP analysis. Split 1 (“control”) was prepared as a polished thick section for Ar/Ar analysis. Split 2 (“SEM”) was prepared as a polished thick section, was chemically-mechanically polished with colloidal silica and underwent SEM imaging (uncoated) and focussed ion beam (FIB) milling (gold coated); electron beam damage in the SEM was maximised by leaving the sample at high magnification for eight minutes. Split 3 (“Etch”) is a cleavage fragment that was etched with HF vapour and underwent low to moderate magnification SEM imaging. The control split gave a range of laser-spot ages consistent with the expected cooling age of the granite and high yields of radiogenic 40Ar* (>90%). The area of the “SEM” split that experienced significant electron beam damage gave younger than expected ages and 40Ar* yields as low as 57%. These are interpreted as a combination of implantation of atmospheric Ar and local redistribution of K within the sample. The area of “SEM” that underwent FIB milling gave ages and 40Ar* yields comparable to the control split, suggesting that the Au-coat minimises FIB

Early Pleistocene 40Ar/39Ar ages for Bapang Formation hominins, Central Jawa, Indonesia

PubMed Central

Larick, Roy; Ciochon, Russell L.; Zaim, Yahdi; Sudijono; Suminto; Rizal, Yan; Aziz, Fachroel; Reagan, Mark; Heizler, Matthew

2001-01-01

The Sangiran dome is the primary stratigraphic window for the Plio-Pleistocene deposits of the Solo basin of Central Jawa. The dome has yielded nearly 80 Homo erectus fossils, around 50 of which have known findspots. With a hornblende 40Ar/39Ar plateau age of 1.66 ± 0.04 mega-annum (Ma) reportedly associated with two fossils [Swisher, C.C., III, Curtis, G. H., Jacob, T., Getty, A. G., Suprijo, A. & Widiasmoro (1994) Science 263, 1118–1121), the dome offers evidence that early Homo dispersed to East Asia during the earliest Pleistocene. Unfortunately, the hornblende pumice was sampled at Jokotingkir Hill, a central locality with complex lithostratigraphic deformation and dubious specimen provenance. To address the antiquity of Sangiran H. erectus more systematically, we investigate the sedimentary framework and hornblende 40Ar/39Ar age for volcanic deposits in the southeast quadrant of the dome. In this sector, Bapang (Kabuh) sediments have their largest exposure, least deformation, and most complete tephrostratigraphy. At five locations, we identify a sequence of sedimentary cycles in which H. erectus fossils are associated with epiclastic pumice. From sampled pumice, eight hornblende separates produced 40Ar/39Ar plateau ages ranging from 1.51 ± 0.08 Ma at the Bapang/Sangiran Formation contact, to 1.02 ± 0.06 Ma, at a point above the hominin-bearing sequence. The chronological sequence of 40Ar/39Ar ages follows stratigraphic order across the southeast quadrant. An intermediate level yielding four nearly complete crania has an age of about 1.25 Ma. PMID:11309488

Early Pleistocene 40Ar/39Ar ages for Bapang Formation hominins, Central Jawa, Indonesia.

PubMed

Larick, R; Ciochon, R L; Zaim, Y; Sudijono; Suminto; Rizal, Y; Aziz, F; Reagan, M; Heizler, M

2001-04-24

The Sangiran dome is the primary stratigraphic window for the Plio-Pleistocene deposits of the Solo basin of Central Jawa. The dome has yielded nearly 80 Homo erectus fossils, around 50 of which have known findspots. With a hornblende (40)Ar/(39)Ar plateau age of 1.66 +/- 0.04 mega-annum (Ma) reportedly associated with two fossils [Swisher, C.C., III, Curtis, G. H., Jacob, T., Getty, A. G., Suprijo, A. & Widiasmoro (1994) Science 263, 1118-1121), the dome offers evidence that early Homo dispersed to East Asia during the earliest Pleistocene. Unfortunately, the hornblende pumice was sampled at Jokotingkir Hill, a central locality with complex lithostratigraphic deformation and dubious specimen provenance. To address the antiquity of Sangiran H. erectus more systematically, we investigate the sedimentary framework and hornblende (40)Ar/(39)Ar age for volcanic deposits in the southeast quadrant of the dome. In this sector, Bapang (Kabuh) sediments have their largest exposure, least deformation, and most complete tephrostratigraphy. At five locations, we identify a sequence of sedimentary cycles in which H. erectus fossils are associated with epiclastic pumice. From sampled pumice, eight hornblende separates produced (40)Ar/(39)Ar plateau ages ranging from 1.51 +/- 0.08 Ma at the Bapang/Sangiran Formation contact, to 1.02 +/- 0.06 Ma, at a point above the hominin-bearing sequence. The chronological sequence of (40)Ar/(39)Ar ages follows stratigraphic order across the southeast quadrant. An intermediate level yielding four nearly complete crania has an age of about 1.25 Ma.

A double-spike method for K-Ar measurement: A technique for high precision in situ dating on Mars and other planetary surfaces

NASA Astrophysics Data System (ADS)

Farley, K. A.; Hurowitz, J. A.; Asimow, P. D.; Jacobson, N. S.; Cartwright, J. A.

2013-06-01

A new method for K-Ar dating using a double isotope dilution technique is proposed and demonstrated. The method is designed to eliminate known difficulties facing in situ dating on planetary surfaces, especially instrument complexity and power availability. It may also have applicability in some terrestrial dating applications. Key to the method is the use of a solid tracer spike enriched in both 39Ar and 41K. When mixed with lithium borate flux in a Knudsen effusion cell, this tracer spike and a sample to be dated can be successfully fused and degassed of Ar at <1000 °C. The evolved 40Ar∗/39Ar ratio can be measured to high precision using noble gas mass spectrometry. After argon measurement the sample melt is heated to a slightly higher temperature (˜1030 °C) to volatilize potassium, and the evolved 39K/41K ratio measured by Knudsen effusion mass spectrometry. Combined with the known composition of the tracer spike, these two ratios define the K-Ar age using a single sample aliquot and without the need for extreme temperature or a mass determination. In principle the method can be implemented using a single mass spectrometer. Experiments indicate that quantitative extraction of argon from a basalt sample occurs at a sufficiently low temperature that potassium loss in this step is unimportant. Similarly, potassium isotope ratios measured in the Knudsen apparatus indicate good sample-spike equilibration and acceptably small isotopic fractionation. When applied to a flood basalt from the Viluy Traps, Siberia, a K-Ar age of 351 ± 19 Ma was obtained, a result within 1% of the independently known age. For practical reasons this measurement was made on two separate mass spectrometers, but a scheme for combining the measurements in a single analytical instrument is described. Because both parent and daughter are determined by isotope dilution, the precision on K-Ar ages obtained by the double isotope dilution method should routinely approach that of a pair of

Ar-Ar and Rb-Sr Ages of the Tissint Olivine-phyric Martian Shergottite

NASA Technical Reports Server (NTRS)

Park, J.; Herzog, G. F.; Nyquist, L. E.; Shih, C.-Y.; Turin, B.; Lindsay, F. N.; Delaney, J. S.; Swisher, C. C., III; Agee, C.

2013-01-01

The fifth martian meteorite fall, Tissint, is an olivine-phyric shergottite that contains olivine macrocrysts (approximately 1.5 mm) [1]. [2] reported the Sm-Nd age of Tissint as 596 plus or minus 23 Ma along with Rb-Sr data that defined no isochron. [3] reported Lu-Hf and Sm-Nd ages of 583 plus or minus 86 Ma and 616 plus or minus 67 Ma, respectively. The cosmic-ray exposure ages of Tissint are 1.10 plus or minus 0.15 Ma based on 10Be [4], and 1.0-1.1 Ma, based on 3He, 21Ne, and 38Ar [5,6].We report Ar-Ar ages and Rb-Sr data.

Single-crystal 40Ar/39Ar incremental heating reveals bimodal sanidine ages in the Bishop Tuff

NASA Astrophysics Data System (ADS)

Andersen, N. L.; Jicha, B. R.; Singer, B. S.

2015-12-01

The 650 km3 Bishop Tuff (BT) is among the most studied volcanic deposits because it is an extensive marker bed deposited just after the Matuyama-Brunhes boundary. Reconstructions of the vast BT magma reservoir from which high-silica rhyolite erupted have long influenced thinking about how large silicic magma systems are assembled, crystallized, and mixed. Yet, the longevity of the high silica rhyolitic melt and exact timing of the eruption remain controversial due to recent conflicting 40Ar/39Ar sanidine vs. SIMS and ID-TIMS U-Pb zircon dates. We have undertaken 21 40Ar/39Ar incremental heating ages on 2 mm BT sanidine crystals from pumice in 3 widely separated outcrops of early-erupted fall and flow units. Plateau ages yield a bimodal distribution: a younger group has a mean of 766 ka and an older group gives a range between 772 and 782 ka. The younger population is concordant with the youngest ID-TIMS and SIMS U-Pb zircon ages recently published, as well as the astronomical age of BT in marine sediment. Of 21 crystals, 17 yield older, non-plateau, steps likely affected by excess Ar that would bias traditional 40Ar/39Ar total crystal fusion ages. The small spread in older sanidine ages, together with 25+ kyr of pre-eruptive zircon growth, suggest that the older sanidines are not partially outgassed xenocrysts. A bimodal 40Ar/39Ar age distribution implies that some fraction of rhyolitic melt cooled below the Ar closure temperature at least 10 ky prior to eruption. We propose that rapid "thawing" of a crystalline mush layer released older crystals into rhyolitic melt from which sanidine also nucleated and grew immediately prior to the eruption. High precision 40Ar/39Ar dating can thus provide essential information on thermo-physical processes at the millenial time scale that are critical to interpreting U-Pb zircon age distributions that are complicated by large uncertainties associated with zircon-melt U-Th systematics.

Ar-39-Ar-40 Evidence for Early Impact Events on the LL Parent Body

NASA Technical Reports Server (NTRS)

Dixon, E. T.; Bogard, D. D.; Garrison, D. H.; Rubin, A. E.

2006-01-01

We determined Ar-39-Ar-40 ages of eight LL chondrites, and one igneous inclusion from an LL chondrite, with the object of understanding the thermal history of the LL-chondrite parent body. The meteorites in this study have a range of petrographic types from LL3.3 to LL6, and shock stages from S1 to S4. These meteorites reveal a range of K-Ar ages from 23.66 to 24.50 Ga, and peak ages from 23.74 to 24.55 Ga. Significantly, three of the eight chondrites (LL4, 5, 6) have K-Ar ages of -4.27 Ga. One of these (MIL99301) preserves an Ar-39-Ar-40 age of 4.23 +/- 0.03 Ga from low-temperature extractions, and an older age of 4.52 +/- 0.08 Ga from the highest temperature extractions. In addition, an igneous-textured impact melt DOM85505,22 has a peak Ar-39-Ar-40 age of >= 4.27 Ga. We interpret these results as evidence for impact events that occurred at about 4.27 Ga on the LL parent body that produced local impact melts, reset the Ar-39-Ar-40 ages of some meteorites, and exhumed (or interred) others, resulting in a range of cooling ages. The somewhat younger peak age of 3.74 Ga from GR095658 (LL3.3) suggests an additional impact event close to timing of impact-reset ages of some other ordinary chondrites between 3.6-3.8 Ga. The results from MIL99301 suggest that some apparently unshocked (Sl) chondrites may have substantially reset Ar-39-Ar-40 ages. A previous petrographic investigation of MIL99301 suggested that reheating to temperatures less than or equal to type 4 petrographic conditions (600C) caused fractures in olivine to anneal, resulting in a low apparent shock stage of S1 (unshocked). The Ar-39-Ar-40 age spectrum of MIL99301 is consistent with this interpretation. Older ages from high-T extractions may date an earlier impact event at 4.52 +/- 0.08 Ga, whereas younger ages from lower-T extractions date a later impact event at 4.23 Ar-39-Ar-40 0.03 Ga that may have caused annealing of feldspar and olivine

Recalibration of the Palaeocene-Eocene boundary (P-E) using high precision U-Pb and Ar-Ar isotopic dating

NASA Astrophysics Data System (ADS)

Chambers, L.; Pringle, M.; Fitton, G.; Larsen, L. M.; Pedersen, A. K.; Parrish, R.

2003-04-01

In the current time scales (Cande and Kent, 95; Berggren et al, 95) the P-E Boundary is positioned at 55 Ma based primarily on the age of the -17 ash layer in Denmark. In the absence of a global stratigraphic section and point the boundary is an interval of 1 m.y. from 55.5 to 54.5 Ma that includes all of the different means of calibrating the boundary tie point, including the NP9/NP10 calcareous nannofossil zonal boundary, the planktonic foraminiferal P5/P6a zonal boundary, preliminary ages for the -17 and +19 ash layers (unpub.), the base of the London Clay Formation, and the δ13C spike. Here we present new Ar-Ar ages for the -17 and +19 ash layers in Denmark and combine this study with a calibration of the Ar-Ar with the U-Pb method. As Ar-Ar ages are relative to the known age of a standard or monitor, U-Pb ages on zircons from the same rocks from the British Tertiary Igneous Province provide an absolute age calibration for all of our Ar-Ar ages (including the monitors). An additional complication arises because the time scale is currently being revised (J. Ogg, Pers. Comm.). In the new time scale the P-E boundary will stay at 55 Ma and the K-T boundary will move by 0.5 m.y. to 65.5 Ma. Our results have a direct impact on the positioning of the P-E Boundary relative to the K-T boundary as definitive K-T tektite is used as one of our Ar-Ar standards. Ar-Ar ages and U-Pb ages for the same sample from the BTIP are indistinguishable when the ages used for the Ar-Ar monitor minerals are those recommended in Renne et al (98). This means that the K-T tektite is 65.78 ± 0.03 Ma, the -17 ash is 54.52 ± 0.05Ma, and the +19 ash is 54.04 ± 0.14 Ma. If the P-E boundary is taken to be between the -17 and +19 ash layers, as in DSDP Hole 550 (the ashes bracket the planktonic foraminiferal P5/P6a zonal boundary) then the current position at 55 Ma is too old. We therefore suggest that if the K-T boundary moves to 65.5 Ma, then the P-E boundary should not stay at 55 Ma, but

Age and thermal history of the Geysers plutonic complex (felsite unit), Geysers geothermal field, California: A 40Ar/39Ar and U-Pb study

USGS Publications Warehouse

Dalrymple, G.B.; Grove, M.; Lovera, O.M.; Harrison, T.M.; Hulen, J.B.; Lanphere, M.A.

1999-01-01

Sixty-nine ion microprobe spot analyses of zircons from four granite samples from the plutonic complex that underlies the Geysers geothermal field yield 207Pb/206Pb vs. 238U/206Pb concordia ages ranging from 1.13 ?? 0.04 Ma to 1.25 ?? 0.04 (1??) Ma. The weighted mean of the U/Pb model ages is 1.18 ?? 0.03 Ma. The U-Pb ages coincide closely with 40Ar/39Ar age spectrum plateau and 'terminal' ages from coexisting K-feldspars and with the eruption ages of overlying volcanic rocks. The data indicate that the granite crystallized at 1.18 Ma and had cooled below 350??C by ~0.9-1.0 Ma. Interpretation of the feldspar 40Ar/39Ar age data using multi-diffusion domain theory indicates that post-emplacement rapid cooling was succeeded either by slower cooling from 350??to 300??C between 1.0 and 0.4 Ma or transitory reheating to 300-350??C at about 0.4-0.6 Ma. Subsequent rapid cooling to below 260??C between 0.4 and 0.2 Ma is in agreement with previous proposals that vapor-dominated conditions were initiated within the hydrothermal system at this time. Heat flow calculations constrained with K-feldspar thermal histories and the present elevated regional heat flow anomaly demonstrate that appreciable heat input from sources external to the known Geysers plutonic complex is required to maintain the geothermal system. This requirement is satisfied by either a large, underlying, convecting magma chamber (now solidified) emplaced at 1.2 Ma or episodic intrusion of smaller bodies from 1.2 to 0.6 Ma.

Ar-40/Ar-39 Studies of Martian Meteorite RBT 04262 and Terrestrial Standards

NASA Technical Reports Server (NTRS)

Park, J.; Herzog, G. F.; Turrin, B.; Lindsay, F. N.; Delaney, J. S.; Swisher, C. C., III; Nagao, K.; Nyquist, L. E.

2014-01-01

Park et al. recently presented an Ar-40/Ar-39 dating study of maskelynite separated from the Martian meteorite RBT 04262. Here we report an additional study of Ar-40/Ar-39 patterns for smaller samples, each consisting of only a few maskelynite grains. Considered as a material for Ar-40/Ar-39 dating, the shock-produced glass maskelynite has both an important strength (relatively high K concentration compared to other mineral phases) and some potentially problematic weaknesses. At Rutgers, we have been analyzing small grains consisting of a single phase to explore local effects that might be averaged and remain hidden in larger samples. Thus, to assess the homogeneity of the RBT maskelynite and for comparison with the results of, we analyzed six approx. 30 microgram samples of the same maskelynite separate they studied. Furthermore, because most Ar-40/Ar-39 are calculated relative to the age of a standard, we present new Ar-40/Ar-39 age data for six standards. Among the most widely used standards are sanidine from Fish Canyon (FCs) and various hornblendes (hb3gr, MMhb-1, NL- 25), which are taken as primary standards because their ages have been determined by independent, direct measurements of K and A-40.

Dating of the youngest volcanoes of Ardeche (Massif Central, France) using 40Ar/39Ar and unspiked K/Ar

NASA Astrophysics Data System (ADS)

Nomade, Sebastien; Sasco, Romain; Guillou, Herve; Scao, Vincent; Kissel, Catherine; Genty, Dominique

2014-05-01

Since the first description in 1778 of the relationship between prismatic basaltic flow and volcano in the high valleys of the Ardèche (Faujas Saint-Font, 1778), "L'Ardèche", a small region at the south-west of Massif Central, became worldwide famous among volcanologists. This volcanism is found dispersed over an area of more than 20 km2 and is made of strombolian cones and prismatic flows filling NS to NW-SE valleys. This volcanism has then been considered as one of the most recent one in the entire Massif Central (40 ka to 170 ka, TL ages, Guérin et al., 2007). Unfortunately and despite several attempts over the last 25 years this volcanism has never been dated using radio-isotopic methods. The two main reasons usually advocated to explain this lack of success were the young age of the volcanism itself and the large amounts of mantle and lower crust xenoliths in the lavas (Guérin et al., 2007). In this contribution, we will present combined 40Ar/39Ar ages and unspiked K/Ar results obtained on five lava flows. The obtained ages range from 26 ± 5.5 ka to 55 ± 6.0 ka (1s, full propagated uncertainty relative to ACS-2 at 1.194Ma, Nomade et al., 2005). The ages from three of the investigated lava flows coming from distinct cones, are clustered between 26 ± 5.5 ka and 34 ± 4 ka. These cones are found stretched along a NW-SE tectonic accident. These first radio-isotopic constraints prove that the volcanic activity occurred during the last glacial period and is as young as "la chaîne de Puys" located in the northern part of the Massif Central. Incidentally, the volcanic activity is contemporaneous with the first Aurignacian occupation and related art found in the Chauvet cave (37-29 ka, Valladas et al., 2005) localized only 35 km SE. Based on both the spatial and chronological coincidences reported above we suggest that the Aurignacian population(s) that lived in this area have witnessed one or several of these eruptions.

Ar-40/Ar-39 Age of Hornblende-bearing R Chondrite LAP 04840

NASA Technical Reports Server (NTRS)

Righter, K.; Cosca, M.

2014-01-01

Chondrites have a complex chronology due to several variables affecting and operating on chondritic parent bodies such as radiogenic heating, pressure and temperature variation with depth, aqueous alteration, and shock or impact heating [1]. Unbrecciated chondrites can record ages from 4.56 to 4.4 Ga that represent cooling in small parent bodies. Some brecciated chondrites exhibit younger ages (<<4 to 4.4 Ga) that may reflect the age of brecciation, disturbance, or shock and impact events (<< 4 Ga). A unique R chondrite was recently found in the LaPaz Icefield of Antarctica - LAP 04840 [2]. This chondrite contains approx.15% hornblende and trace amounts of biotite, making it the first of its kind. Studies have revealed an equigranular texture, mineral equilibria yielding equilibration near 650-700 C and 250-500 bars, hornblende that is dominantly OH-bearing (very little Cl or F), and high D/H ratios [8,9,10]. To help gain a better understanding of the origin of this unique sample, we have measured the Ar-40/Ar-39 age. Age of 4.290 +/- 0.030 Ga is younger than one would expect for a sample that has cooled within a small body [4], and one might instead attribute the age to a younger shock event, On the other hand, there is no evidence for extensive shock in this meteorite (shock stage S2; [3]), so this sample may have been reannealed after the shock event. This age is similar to Ar-Ar ages determined for some other R chondrites

Ar and K partitioning between clinopyroxene and silicate melt to 8 GPa

NASA Astrophysics Data System (ADS)

Chamorro, E. M.; Brooker, R. A.; Wartho, J.-A.; Wood, B. J.; Kelley, S. P.; Blundy, J. D.

2002-02-01

The relative incompatibility of Ar and K are fundamental parameters in understanding the degassing history of the mantle. Clinopyroxene is the main host for K in most of the upper mantle, playing an important role in controlling the K/Ar ratio of residual mantle and the subsequent time-integrated evolution of 40Ar/36Ar ratios. Clinopyroxene also contributes to the bulk Ar partition coefficient that controls the Ar degassing rate during mantle melting. The partitioning of Ar and K between clinopyroxene and quenched silicate melt has been experimentally determined from 1 to 8 GPa for the bulk compositions Ab80Di20 (80 mol% albite-20 mol% diopside) and Ab20Di80 with an ultraviolet laser ablation microprobe (UVLAMP) technique for Ar analysis and the ion microprobe for K. Data for Kr (UVLAMP) and Rb (ion probe) have also been determined to evaluate the role of crystal lattice sites in controlling partitioning. By excluding crystal analyses that show evidence of glass contamination, we find relatively constant Ar partition coefficients (DAr) of 2.6 × 10-4 to 3.9 × 10-4 for the Ab80Di20 system at pressures from 2 to 8 GPa. In the Ab20Di80 system, DAr shows similar low values of 7.0 × 10-5 and 3.0 × 10-4 at 1 to 3 GPa. All these values are several orders of magnitude lower than previous measurements on separated crystal-glass pairs. DK is 10 to 50 times greater than DRb for all experiments, and both elements follow parallel trends with increasing pressure, although these trends are significantly different in each system studied. The DK values for clinopyroxene are at least an order of magnitude greater than DAr under all conditions investigated here, but DAr appears to show more consistent behavior between the two systems than K or Rb. The partitioning behavior of K and Rb can be explained in terms of combined pressure, temperature, and crystal chemistry effects that result in changes for the size of the clinopyroxene M2 site. In the Ab20Di80 system, where

Laser probe /sup 40/Ar//sup 39/Ar and conventional K/Ar dating of illites associated with the McClean Uranium deposits, N. Saskatchewan, Canada

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bray, C.J.; Spooner, E.T.C.; Hall, C.M.

Mineralization at McClean occurs in elongate pods at the unconformity between the Athabasca sandstone sequence and the underlying Archean/Aphebian basement. Laser probe /sup 40/Ar//sup 39/Ar and conventional K/Ar dates were obtained on illites from the following environments (i) alteration halo around high grade U(-Ni-Co) mineralization, (ii) recrystallized sedimentary layers within the sandstone sequence, (iii) interstitial to quartz grains in sandstone, and (iv) regolith. The alteration halo illites gave ages between 1321+/-44 and 1002+/-33 Ma (n=20) which are in good agreement with published U-Pb dates on other U deposits in the area. One sample was analyzed twice, and gave a significantmore » age difference of 20 Ma. Hence the age range is interpreted as being either a reflection of variable argon retentivity or due to continued hydrothermal activity (i.e. U mineralization) or a combination of both. Dates obtained on (ii), (iii) and (iv) were 1438-1038 (n=8), 1459-1113 (n=3) and 1482-1262 (n=6) Ma respectively. The older ages are in good agreement with a published Rb-Sr whole rock age of 1470 Ma on tuffaceous sediments from the Athabasca group. The spread to younger ages is interpreted as due to resetting by the hot (approx. 200/sup 0/C) formation water which transported the U, and variable argon retentivity. Diffusion studies on a single sample of illite from the sedimentary sequence indicate a low blocking temperature of about 140/sup 0/C and imply that there have been very low temperatures in the area for the last 1000 Ma.« less

XRD-based 40Ar/39Ar age correction for fine-grained illite, with application to folded carbonates in the Monterrey Salient (northern Mexico)

NASA Astrophysics Data System (ADS)

Fitz-Díaz, Elisa; Hall, Chris M.; van der Pluijm, Ben A.

2016-05-01

Due to their minute size, 40Ar/39Ar analysis of illite faces significant analytical challenges, including mineral characterization and, especially, effects of grain size and crystallography on 39Ar recoil. Quantifying the effects of 39Ar recoil requires the use of sample vacuum encapsulation during irradiation, which permits the measurement of the fraction of recoiled 39Ar as well as the 39Ar and 40Ar∗ retained within illite crystals that are released during step heating. Total-Gas Ages (TGA) are calculated by using both recoiled and retained argon, which is functionally equivalent to K-Ar ages, while Retention Ages (RA) only involve retained Ar in the crystal. Natural applications have shown that TGA fits stratigraphic constraints of geological processes when the average illite crystallite thickness (ICT) is smaller than 10 nm, and that RA matches these constraints for ICTs larger than 50 nm. We propose a new age correction method that takes into account the average ICT and corresponding recoiled 39Ar for a sample, with X-ray Corrected Ages (XCA) lying between Total-Gas and Retention Ages depending on ICT. This correction is particularly useful in samples containing authigenic illite formed in the anchizone, with typical ICT values between 10 and 50 nm. In three samples containing authigenic illite from Cretaceous carbonates in the Monterrey Salient in northern Mexico, there is a range in TGAs among the different size-fractions of 46-49, 36-43 and 40-52 Ma, while RAs range from 54-64, 47-52 and 53-54 Ma, respectively. XCA calculations produce tighter age ranges for these samples of 52.5-56, 45.5-48.5 and 49-52.5 Ma, respectively. In an apparent age vs ICT or %2M 1illite plot, authigenic illite grains show a slope that is in general slightly positive for TGA, slightly negative for RA, but close to zero for XCA, with thinner crystallites showing more dispersion than thicker ones. In order to test if dispersion is due to a different formation history or the result

40Ar-39Ar age clustering in the active phonolitic Cadamosto Seamount (Cape Verdes): Indications for periodic magmatic activity

NASA Astrophysics Data System (ADS)

Samrock, L. K.; Hansteen, T. H.; Wartho, J. A.

2017-12-01

The Cape Verde archipelago is situated 400-800 km off the west coast of Africa and is comprised of a northern and southern chain of islands and seamounts. Morphological observations and previous radiometric dating of the islands indicate a slow age progression, over 22 Ma, from east to west (Holm et al. 2008). We present the first radiometric ages for Cadamosto Seamount, which is composed of complex evolved volcanics and is situated at the southwestern tip of the Cape Verde archipelago (e.g. Barker et al. 2012). We analyzed five different submarine phonolites that were sampled by remotely operated vehicles (ROV) Kiel 6000 and dredging during the RV Meteor (M80/3) and RV Poseidon (POS320/2) cruises. Fresh sanidine, nepheline, and biotite grains were selected and carefully prepared for 40Ar-39Ar single grain total fusion analysis. Sanidine single grain 40Ar-39Ar ages from 5 samples range from 11.5 ± 6.5 ka to 349.0 ± 20.4 ka (2σ errors), and cluster in several age groupings (using the decay constant and atmospheric air ratio of Steiger & Jäger (1977), and age standard TCS2 (27.87 ± 0.04 Ma; 1σ; M.A. Lanphere, pers. comm.)). Three age groups can be identified within the youngest (0-170 ka) sanidines, which are separated by periods of 52-54 ka. Nepheline grains from one sample yielded much older ages of 169.5 ± 16.5 ka to 1521.5 ± 8.3 ka (2σ). Our data suggests young ages for the Cadamosto Seamount, which is in accordance with recorded seismic activity (Grevemeyer et al. 2010), and its position adjacent to the recently active islands of Fogo (last eruption in 2014/2015) and Brava (recent seismic activity). The different sub-groups of sanidine 40Ar-39Ar ages can be used to identify different activity maxima corresponding to cycles of magmatic productivity in a long-lived magmatic system. Ongoing petrologic investigations will be used to identify the relative importance of processes such as mantle melting rates, magma replenishment and magma chamber processes

40Ar/39Ar ages and Sr-Nd-Pb isotopic compositions of alkaline and tholeiitic rocks from the northern Deccan Traps

NASA Astrophysics Data System (ADS)

Marzoli, A.; Parisio, L.; Jourdan, F.; Melluso, L.; Sethna, S. F.; Bellieni, G.

2015-12-01

The Deccan large igneous province in India was emplaced close to the Cretaceous-Paleogene boundary (K-Pg; 66.0 Ma) and is formed by tholeiitic and alkaline rocks. Definition of the origin of Deccan magmatism and of its environmental impact relies on precise and accurate geochronological analyses. We present new 40Ar/39Ar ages from the northern sector of the province. In this area, tholeiitic and alkaline rocks were contemporaneously emplaced at 66.60±0.35 to 65.25±0.29 Ma in the Phenai Mata area, while rocks from Rajpipla and Mt. Pavagadh yielded ages ranging from 66.40±2.80 to 64.90±0.80 Ma. Indistinguishable ages for alkaline and tholeiitic magmatism, coupled with distinct major and trace element and Sr-Nd-Pb isotopic compositions suggest that distinct mantle sources, necessary for the two magmatic series were synchronously active. The new ages are compared with previous ages, which were carefully screened and filtered and then recalculated in order to be comparable. The entire data set of geochronological data does not support a time-related migration of the magmatism related to the northward Indian Plate movement relative to the Reunion mantle plume. The main phase of magmatism, including the newly dated rocks from the Northern Deccan occurred across the K-Pg boundary, confirming a causal link between the emplacement of the province and the K-Pg mass extinction.

40Ar-39Ar Ages of the Large Impact Structures Kara and Manicouagan and their Relevance to the Cretaceous-Tertiary and the Triassic-Jurassic Boundary

NASA Astrophysics Data System (ADS)

Trieloff, M.; Jessberger, E. K.

1992-07-01

Since the discovery of the iridium enrichment in Cretaceous- Tertiary boundary clays by Alvarez et al. (1980) the search for the crater of the K/T impactor is in progress. Petrographic evidence at the K/T boundary material points towards an impact into an ocean as well as onto the continental crust, multiple K/T impacts are now being considered (Alvarez and Asaro, 1990). One candidate is the Kara crater in northern Siberia of which Kolesnikov et al. (1988) determined a K-Ar isochrone age of 65.6 +- 0.5 Ma, regarding this as indicating that the Kara bolide is at least one of the K/T impactors. Koeberl et al. (1990) determined ^40Ar-^39Ar ages of six impact melts ranging from 70 to 82 Ma and suggested rather an association to the Campanian- Maastrichtian boundary, another important extinction horizon 73 Ma ago (Harland et al., 1982). We dated with the ^40Ar-^39Ar technique four impact melts, KA2- 306, KA2-305, SA1-302 and AN9-182. The spectra have rather well- defined plateaus, shown with highly extended age scales (Fig. 1). The plateau ages range from 69.3 to 71.7 Ma. Our data do not support an association either with the Cretaceous-Tertiary or with the Campanian-Maastrichtian boundary. We deduce an age of 69-71 Ma for the Kara impact structure. Nazarov et al. (1991) have demonstrated by isotopic hydrogen studies that the Kara bolide impacted on dry land, while the last regression at the target area before the end of the Cretaceous occurred 69-70 Ma ago. Our data are consistent with an impact shortly after the regression. We further dated impact metamorphic anorthosite samples (10BD5 and 10BD3C) of the Manicouagan crater, Canada, which may be related to the Triassic-Jurassic boundary (McLaren and Goodfellow, 1990). The samples consist of two different phases, one degassing at low temperatures yielding a plateau age of 212 Ma and another phase which was degassed during the cratering event to varying degrees with apparent ages increasing up to 950 Ma, the age of the

Direct 40Ar/39Ar age determination of fluid inclusions using in-vacuo¬ stepwise crushing - Example of garnet from the Cycladic Blueschist Unit on Syros

NASA Astrophysics Data System (ADS)

Uunk, Bertram; Postma, Onno; Wijbrans, Jan; Brouwer, Fraukje

2017-04-01

Metamorphic minerals and veins commonly trap attending hydrous fluids in fluid inclusions, which yield a wealth of information on the history of the hosting metamorphic system. When these fluids are sufficiently saline, the KCl in the inclusions can be used as a K/Ar geochronologic system, potentially dating inclusion incorporation. Whilst primary fluid inclusions (PFIs) can date fluid incorporation during mineral or vein growth, secondary fluid inclusion trails (SFIs) can provide age constraints on later fluid flow events. At VU Amsterdam, a new in-vacuo crushing apparatus has been designed to extract fluid inclusions from minerals for 40Ar/39Ar analysis. Separates are crushed inside a crusher tube connected to a purification line and a quadrupole mass spectrometer. In-vacuo crushing is achieved by lifting and dropping a steel pestle using an externally controlled magnetic field. As the gas can be analyzed between different crushing steps, the setup permits stepwise crushing experiments. Additionally, crushed powder can be heated by inserting the crusher tube in an externally controlled furnace. Dating by 40Ar/39Ar stepwise crushing has the added advantage that, during neutron irradiation to produce 39Ar from 39K, 38Ar and 37Ar are also produced from 38Cl and 40Ca, respectively. Simultaneous analysis of these argon isotopes permits constraining the chemistry of the argon source sampled during the experiment. This allows a distinction between different fluid or crystal lattice sources. Garnet from three samples of the HP metamorphic Cycladic Blueschist Unit on Syros, Greece was stepwise crushed to obtain fluid inclusion ages. Initial steps for all three experiments yield significant components of excess argon, which are interpreted to originate from grain boundary fluids and secondary fluid inclusions trails. During subsequent steps, age results stabilize to a plateau age. One garnet from North Syros yields an unusually old 80 Ma plateau age. However, isochrons

In-situ Ar isotope, 40Ar/39Ar analysis and mineral chemistry of nosean in the phonolite from Olbrück volcano, East Eifel volcanic field, Germany: Implication for the source of excess 40Ar

NASA Astrophysics Data System (ADS)

Sudo, Masafumi; Altenberger, Uwe; Günter, Christina

2014-05-01

Since the report by Lippolt et al. (1990), hauyne and nosean phenocrysts in certain phonolites from the northwest in the Quaternary East Eifel volcanic field in Germany were known to contain significant amounts of excess 40Ar, thus, show apparent older ages than the other minerals. However, its petrographic meaning have not been well known. Meanwhile, Sumino et al. (2008) has identified the source of the excess 40Ar in the plagioclase phenocrysts from the historic Unzen dacite lava as the melt inclusions in the zones parallely developed to the plagioclase rim by in-situ laser Ar isotope analysis. In order to obtain eruption ages of very young volcanoes as like Quaternary Eifel volcanic field by the K-Ar system, it is quite essential to know about the location of excess 40Ar in volcanic rocks. We have collected phonolites from the Olbrück volcano in East Eifel and investigated its petrography and mineral chemistry and also performed in-situ Ar isotope analyses of unirradiated rock section sample and also in-situ 40Ar/39Ar analysis of neutron irradiated section sample with the UV pulse laser (wavelength 266 nm) and 40Ar/39Ar analytical system of the University of Potsdam. Petrographically, nosean contained fine melt and/or gas inclusions of less than 5 micrometer, which mostly distribute linearly and are relatively enriched in chlorine than the areas without inclusions. Solid inclusions of similar sizes contain CaO and fluorine. In nosean, typically around 5 wt% of sulfur is contained. The 40Ar/39Ar dating was also performed to leucite, sanidine and groundmass in the same section for comparison of those ages with that of nosean. In each analysis, 200 micrometer of beam size was used for making a pit with depth of up to 300 micrometer by laser ablation. As our 40Ar/39Ar analyses were conducted one and half year after the neutron irradiation, thus, short lived 37Ar derived from Ca had decayed very much, we measured Ca and K contents in nosean by SEM-EDS then applied

Comparative K-Ar and Ar/Ar dating of Ethiopian and Yemenite Oligocene volcanism: implications for timing and duration of the Ethiopian traps

NASA Astrophysics Data System (ADS)

Coulié, E.; Quidelleur, X.; Gillot, P.-Y.; Courtillot, V.; Lefèvre, J.-C.; Chiesa, S.

2003-02-01

It is now generally accepted that continental flood basalt (CFB) volcanism bears a strong relationship with continental breakup. The Ethiopian Afar plume has been linked to the opening of the Afar depression. Propagation of the Red Sea and Gulf of Aden rifts within the depression, still an ongoing process, has rifted away the Ethiopian and Yemenite trap sequences. They are in some locations more than 2 km thick and comprise a wide range of volcanic products, from tholeiitic basalts, in the lower part, to more acidic material in the upper part. Recent studies have established that the bulk of trap volcanism erupted about 30 Ma ago over a period of 1 Myr in the Ethiopian sections, while ages obtained on the Yemenite sections seem more distributed through time. Here, for the first time in a single study, we present geochronological results obtained for basalts and more evolved products for both Ethiopian and Yemenite traps. This approach eliminates inter-laboratory biases and discrepancies in the ages of standards, and imposes better constraints on the eruptive chronology of this CFB province. In addition, both the K-Ar and 40Ar/ 39Ar techniques have been applied simultaneously, in order to demonstrate that similar ages are indeed obtained for undisturbed samples. The two dating techniques used here yield concordant ages for most samples. On both sides of the Afar depression, our results support that the onset of basaltic volcanism is coeval, with undistinguishable ages of 30.6±0.4 and 30.2±0.4 Ma obtained from Ethiopia and Yemen, respectively. Most of the basaltic lava pile has been erupted in less than 1 Myr, but acidic volcanism seems more spread out through time. It is coeval with basalts in northern Ethiopia but extends to about 26 Ma in Yemen, as already recognized. A younger rhyolitic episode, probably related to the major 20 Ma phase of opening of the Red Sea and Gulf of Aden, as expressed in the Afar depression, is also observed in Yemen and central

LA-ICP-MS zircon U-Pb and muscovite K-Ar ages of basement rocks from the south arm of Sulawesi, Indonesia

NASA Astrophysics Data System (ADS)

Jaya, Asri; Nishikawa, Osamu; Hayasaka, Yasutaka

2017-11-01

The zircon U-Pb and muscovite K-Ar age from the Bantimala, Barru and Biru basement complexes in the South Arm of Sulawesi, Indonesia provide new information regarding the timing of magmatism, metamorphism and sedimentation in this region and have implications for the origin and evolution of the study area. The study area is at the juncture between the southeast margin of Sundaland and Bird's Head-Australia. The age of both the zircon U-Pb of detrital materials in the Bantimala Complex and the muscovite K-Ar of amphibolite in the Biru Complex fall in the Late Early Cretaceous (between 109 and 115 Ma), which is a similar age range to previous data for both the sedimentary and metamorphic rocks. The youngest detrital zircon in the schist samples from the Barru Complex fall into the Triassic in age (between 243 and 247 Ma). These age data indicate that the protolith of all three basement complexes were involved in the subduction system and metamorphosed in the late Early Cretaceous, but there are several differences in their deposition environment under and out of the influence of the late Early Cretaceous magmatism in the Bantimala and Barru Complexes, respectively. Felsic igneous activities are confirmed in the Late Cretaceous and the Eocene by the zircon U-Pb age of igneous rocks intruding or included as detrital fragments in three basement complexes. These dates are similar to those reported from the Meratus Complex of South Kalimantan. The detrital zircon age distributions of the basement rocks in the South Arm of Sulawesi display predominant Mesozoic (Cretaceous and Triassic) and Paleozoic populations with a small population of Proterozoic ages supporting the hypothesis that the West Sulawesi block originated from the region of the circum Bird's Head-Australian, namely the Inner Banda block. The absence of Jurassic zircon age population in the South Arm of Sulawesi suggests the division of the South Arm of Sulawesi from the Inner Banda block in early stage of

Studies of K-Ar dating and xenon from extinct radioactivities in breccia 14318; implications for early lunar history

NASA Technical Reports Server (NTRS)

Reynolds, J. H.; Alexander, E. C., Jr.; Davis, P. K.; Srinivasan, B.

1974-01-01

The lunar breccia 14318 is one of three Apollo-14 breccias containing substantial amounts of parentless xenon from the spontaneous fission of extinct Pu-244. The argon and xenon contained in this breccia were studied by stepwise heating of pristine and neutron-irradiated samples. The isotopic composition of xenon from fission, determined by an improved method, is shown to be from Pu-244. Concentrations of this fissiogenic xenon are in substantial excess (15-fold) of what could be produced by spontaneous fission of U-238. The breccia is found to contain abundant trapped argon with an Ar-40/Ar-36 ratio of roughly 14. Otherwise, the argon is radiogenic and gives a convincing K-Ar age of 3.69 plus or minus 0.09 b.y. by the stepwise Ar-40/Ar-39 method, nearly in agreement with ages for other Apollo-14 breccias.

Chemical and physical erosion rhythms of the West African Cenozoic morphogenesis: The 39Ar-40Ar dating of supergene K-Mn oxides

NASA Astrophysics Data System (ADS)

Beauvais, Anicet; Ruffet, Gilles; HéNocque, Olivier; Colin, Fabrice

2008-12-01

Chemical weathering and mechanical erosion are first-order processes of long-term tropical morphogenesis, which is still poorly deciphered for lack of time constraints. We address this issue by laser probe 39Ar-40Ar dating of generations of cryptomelane [K1-2Mn8O16, nH2O] from the manganese ore deposit of Tambao in northern Burkina Faso. This Mn deposit results from the supergene weathering of carbonate and silicate Mn protores underneath lateritic palaeolandsurfaces. It consists of an upper cryptomelane-rich domain and a lower domain where pyrolusite (β-MnO2) is the dominant Mn oxide. The oldest 39Ar-40Ar ages (59-45 Ma) are obtained on surface outcrops while the youngest ones characterize deep oxidation fronts (3.4-2.9 Ma). Apparent correlations of 39Ar-40Ar age groups with δ18O and eustatic curves allow definition of the different stages of morphogenesis. Paleocene-Eocene ages (59-45 Ma) bracket a greenhouse period propitious to bauxitic weathering. The lack of significant ages between ˜45 and 29 Ma characterizes a period dominated by mechanical erosion, during which detrital sediments, including lateritic materials, were accumulated in intracratonic basins allowing the exhumation of a new lateritic landsurface. Two major weathering periods separated by a second erosion episode (24-18 Ma) are also depicted at the end of Oligocene (29-24 Ma) and lower to mid-Miocene (18-11.5 Ma) in the upper domain, during which newly shaped land surfaces conspicuously weathered. The shorter-weathering and erosion episodes recorded in the lower domain from ˜18 to ˜2.9 Ma led to the final geomorphic changes that were conducive to the formation of glacis. The preservation of old cryptomelane (59-45 Ma) in the upper part of the ore deposit indicates a Cenozoic denudation limited to the erosion of previous bauxites, and partly, of ferricretes.

New K Ar age determinations of Kilimanjaro volcano in the North Tanzanian diverging rift, East Africa

NASA Astrophysics Data System (ADS)

Nonnotte, Philippe; Guillou, Hervé; Le Gall, Bernard; Benoit, Mathieu; Cotten, Joseph; Scaillet, Stéphane

2008-06-01

The Kilimanjaro is the African highest mountain and culminates at 5895 m high. This huge volcanic edifice is composed of three main centres along a N110°E-striking axis (Shira, Kibo and Mawenzi from W to E), and emplaced in a key area where a major N80°E-oriented volcanic lineament intersects a first-order NW-SE basement fault-like discontinuity. Seventeen K-Ar ages (on microcrystalline groundmass) acquired on lavas and intrusive facies from the three eruptive centres confirm that the Plio-Quaternary volcanicity of Kilimanjaro is clearly polyphased. The oldest phases of volcanic activity begun at ~ 2.5 Ma in the Shira vent and our data suggest that the latest important phases occurred around 1.9 Ma, just before the collapse of the Northern part of the edifice. Magmatic activity then shifted eastwards in the Mawenzi and Kibo twin centres where initial volcanism is dated at ~ 1 Ma. Two K-Ar ages obtained for the most recent Mawenzi rocks from the Neumann Tower-Mawenzi group (492 ka) and Mawenzi eruptive centre (448 ka), near the present summit, are linked to the final stage of edification for this centre. Whereas the eruptive activity ceased in the Mawenzi, it still continued on Kibo since sub-actual time. The oldest dated rocks from Kibo (482 ka) is obtained on a dyke from the Lava Tower group cropping out at 4600 m high. The main phase of magmatism on Kibo is recorded by two lava formations with a great spatial extension - the Rhomb Porphyry group and the Lent group - that have been emplaced in a short time interval at ~ 460-360 ka (including two erosive stages) and 359-337 ka, respectively. Based on the dating of Caldera rim group lavas, it is shown that the edification of the present cone was accomplished in a period ranging from 274 to 170 ka. The new ages obtained for the main episodes of volcanic activity on Kibo appear to roughly coincide with the oldest known Quaternary glaciations. The interaction between eruptive phenomena and the ice cover is assumed to

Cratering on 4 Vesta - Comparison of Crater Retention Ages and Ar-Ar Ages of HED Meteorites.

NASA Astrophysics Data System (ADS)

Schmedemann, N.; Kneissl, T.; Michael, G.; Neukum, G.; Nathues, A.; Sierks, H.; Wagner, R.; Krohn, K.; Reddy, V.; Hiesinger, H.; Jaumann, R.; Raymond, C. A.; Russell, C. T.

2012-04-01

In July 2011 the Dawn spacecraft entered orbit around the Main Belt asteroid 4 Vesta utilizing three different instruments to map the asteroid [1]. The Main Belt is the source region of most impactors in the inner solar system [2]. We compare the obtained crater size-frequency distribution (CSFD) of Vesta with that of the Moon and other Main Belt asteroids such as 951 Gaspra, 243 Ida, and 21 Lutetia. We also compare our results of crater counting on Vesta with K/Ar-Ar reset ages of HED meteorites, which most likely originated from Vesta [3]. To properly compare the lunar CSFD with that of the asteroids we applied scaling laws [4] to account for various impact velocities, surface gravities as well as material properties between the investigated bodies. We found well defined lunar-like CSFDs of impact craters on all four asteroids. The CSFD of Vesta and Lutetia had to be constructed from several individual measurements following [5]. We were able to derive lunar-like chronologies for each asteroid utilizing intrinsic collision probabilities [6], lunar-like CSFDs and the ground truth-derived lunar chronology. Since the Moon and the Main Belt asteroids share the same main impactor source, it is straightforward to also assume a very similar time-dependent impact rate over the solar system history. Alternative chronologies, which are based on computer models of the LHB [7], result in surface ages >4.5 Ga, which are highly unlikely for Vesta. Our lunar-like chronology for Vesta is able to match three out of four peaks in age probabilities of HED meteorites [3]. We measured the age of the Rheasilvia basin with 3.70 +/-0.02 Ga, which coincides with a wide-spread resurfacing age on Vesta. An underlying older basin, partially obliterated by the Rheasilvia impact was formed 3.81 +/-0.05 Ga ago. Finally the large basins and craters >150 km in diameter and the densest cratered areas on Vesta indicate a global resurfacing event 4.00 +/-0.02 Ga ago. This represents the oldest age

White mica K-Ar geochronology of HP-UHP units in the Lago di Cignana area, western Alps, Italy: Tectonic implications for exhumation

NASA Astrophysics Data System (ADS)

Gouzu, Chitaro; Yagi, Koshi; Thanh, Ngo Xuan; Itaya, Tetsumaru; Compagnoni, Roberto

2016-04-01

High-pressure and ultra-high pressure (HP-UHP) blueschist- and eclogite-facies metabasaltic and metasedimentary rocks occur in four different tectonic units near Lago di Cignana, western Alps. We have determined K-Ar ages for white micas (matrix phengite and paragonite) from the Lago di Cignana UHP unit (LCU; 39-41 Ma); the lower and upper units of the Zermatt-Saas meta-ophiolite (LU and UU; 37-38 Ma and 38-41 Ma respectively), and the Combin unit (CU; 36-40 Ma). These K-Ar ages overlap with single-grain Ar-Ar plateau ages (36-42 Ma) previously determined for phengites from LCU metasediments. Matrix white micas have been severely deformed during exhumation, and their chemistries differ from those of micas included in garnet. Although individual mica grains in the matrix could have experienced different degrees of deformation which have reset their K-Ar systems, "bulk" white mica separates provide the average age of all the individual grains in the separate. The similarity of ages determined for white micas from the LCU, LU, UU and CU units, regardless of rock type and mineral species, suggests that these four units were metamorphosed together as part of a single metamorphic sequence in the Piemonte-Liguria paleosubduction zone and were subsequently exhumed together. However, present-day structural relationship among those units and the limited occurrence of UHP minerals in LCU suggests that the exhumation of LCU was more rapid than that for LU, UU and CU. The age gaps between the youngest value of white mica K-Ar ages in each unit and the inferred timing of the metamorphic peak (U-Pb age: 44 Ma) is 5, 7, 6 and 8 Myr for LCU, LU, UU and CU, respectively. These intervals are considerably shorter than that determined for the Sanbagawa HP metamorphic belt of Southwest Japan (> 31 Myr). The short interval observed for the Lago di Cignana units that we have studied is consistent with the model of rapid exhumation of the UHP-bearing metamorphic domain, suggesting the

K-Ar age and geochemistry of the SW Japan Paleogene cauldron cluster: Implications for Eocene-Oligocene thermo-tectonic reactivation

NASA Astrophysics Data System (ADS)

Imaoka, T.; Kiminami, K.; Nishida, K.; Takemoto, M.; Ikawa, T.; Itaya, T.; Kagami, H.; Iizumi, S.

2011-01-01

Systematic K-Ar dating and geochemical analyses of Paleogene cauldrons in the Sanin Belt of SW Japan have been made to explore the relationship between the timing of their formation and the Paleogene subduction history of SW Japan documented in the Shimanto accretionary complex. We also examine the magma sources and tectonics beneath the backarc region of SW Japan at the eastern plate boundary of Eurasia. Fifty-eight new K-Ar ages and 19 previously reported radiometric age data show that the cauldrons formed during Middle Eocene to Early Oligocene time (43-30 Ma), following a period of magmatic hiatus from 52 to 43 Ma. The hiatus coincides with absence of an accretionary prism in the Shimanto Belt. Resumption of the magmatism that formed the cauldron cluster in the backarc was concurrent with voluminous influx of terrigenous detritus to the trench, as a common tectono-thermal event within a subduction system. The cauldrons are composed of medium-K calc-alkaline basalts to rhyolites and their plutonic equivalents. These rocks are characterized by lower concentrations of large ion lithophile elements (LILE) including K 2O, Ba, Rb, Th, U and Li, lower (La/Yb) n ratios, lower initial Sr isotopic ratios (0.7037-0.7052) and higher ɛNd( T) values (-0.5 to +3.5) relative to Late Cretaceous to Early Paleogene equivalents. There are clear trends from enriched to depleted signatures with decreasing age, from the Late Cretaceous to the Paleogene. The same isotopic shift is also confirmed in lower crust-derived xenoliths, and is interpreted as mobilization of pre-existing enriched lithospheric mantle by upwelling depleted asthenosphere. Relatively elevated geothermal gradients are presumed to have prevailed over wide areas of the backarc and forearc of the SW Japan arc-trench system during the Eocene to Oligocene. Newly identified Late Eocene low silica adakites and high-Mg andesites in the Sanin Belt and Early Eocene A-type granites in the SW Korea Peninsula probably formed

An Approximately 4.35 Ga Ar-Ar Age for GRA 8 and the Complex Chronology of its Parent Body

NASA Technical Reports Server (NTRS)

Park, J.; Nyquist, Laurence E.; Bogard, D. D.; Garrison, D. H.; Shih, C.-Y.; Reese, Y. D.

2010-01-01

GRA06128 and GRA06129 (hereafter GRA 8 and GRA 9) are partial melts of a parent body of approximately chondritic composition. We reported a conventional Sm-147-Nd-143 isochron age of 4.559+/-0.096 Ga and a 146 Sm-142Nd model age of 4.549+/-0.036 for combined data for the two rocks. Plagioclase plus whole rock and leachate (approx.phosphate) samples gave a secondary Sm-147-Nd-143 age of 3.4+/-0.4 Ga. An Ar-39-Ar-40 age of 4.460+/-0.028 Ga was interpreted as dating metamorphism in GRA 9. We report Ar-39-Ar-40 ages in the range approx.4344-4366 Ma for GRA 8, establishing similar but different Ar-39-Ar-40 ages for the two rocks, consistent with their different Sr-isotopic systematics, and discuss these ages in the context of the complex sequence of events that affected these samples.

An Approximately 4.35 Ga Ar-Ar Age for GRA 8 and the Complex Chronology of its Parent Body

NASA Technical Reports Server (NTRS)

Park, J.; Nyquist, L. E.; Bogard, D. D.; Garrison, D. H.; Shih, C.-Y.; Reese, Y. D.

2010-01-01

GRA06128 and GRA06129 (hereafter GRA 8 and GRA 9) are partial melts of a parent body of approximately chondritic composition. We reported a conventional SM-147Sm-ND_143 isochron age of 4.559 +/-.096 Ga and a SM-146-142Nd model age of 4.549 +/- 0.036 for combined data for the two rocks. Plagioclase plus whole rock and leachate (approximately phosphate) samples gave a secondary SM-147-ND-143 age of 3.4 +/-0.4 Ga. An Ar-39-Ar-40 age of 4.460+/-0.028 Ga was interpreted by as dating metamorphism in GRA 9. We report Ar-39-Ar-40 ages in the range approximately 4344-4366 Ma for GRA 8, establishing similar but different Ar-39-Ar-40 ages for the two rocks, consistent with their different Sr-istopic systematics, and discuss these ages in the context of the complex sequence of events that affected these samples

The effects of acid leaching on 40Ar/39Ar age dating results using samples from the Walvis Ridge hotspot trail

NASA Astrophysics Data System (ADS)

Klath, J. F.; Koppers, A. A.; Heaton, D. E.; Schnur, S.

2013-12-01

of alteration within the groundmass and how it is changing as acid leaching strength and duration progresses. We will discuss bulk composition comparisons as well as how depletions and enrichments vary amongst major and trace elements. Lastly, we will report 40Ar/39Ar step heating results, in particular K/Ca ratios and age spectra, to better understand the effect of leaching strength on the effective removal of groundmass alteration during 40Ar/39Ar age dating.

Ar-Ar and U-Pb ages of marble-hosted ruby deposits from Central and South-east Asia

NASA Astrophysics Data System (ADS)

Garnier, V.; Giuliani, G.; Maluski, H.; Ohnenstetter, D.; Deloule, E.

2003-04-01

Marble-hosted ruby deposits represent the first source of gemstones in Asia. The deposits from Jegdalek (Afghanistan), Hunza Valley (Pakistan), Nangimali (Azad-Kashmir), Chumar, Ruyil (Nepal), Mogok (Myanmar), Luc Yen, Yen Bai and Quy Chau (Vietnam) were dated using the 40Ar-39Ar laser stepwise heating technique on syngenetic micas. The following ages were obtained : 24.7 ± 0.3 Ma at Jegdalek ; 10.8 ± 0.3 to 5.4 ± 0.3 Ma at Hunza ; 17.2 ± 0.2 to 15.3 ± 0.1 Ma at Nangimali ; 4.6 ± 0.1 Ma at Ruyil ; 5.6 ± 0.4 Ma at Chumar ; 18.7 ± 0.2 to 17.1 ± 0.2 Ma at Mogok ; 33.8 ± 0.4 to 30.8 ± 0.8 Ma at Luc Yen ; 24.4 ± 0.4 to 23.2 ± 0.6 Ma at Yen Bai, 22.1 ± 0.6 to 21.6 ± 0.7 Ma at Quy Chau. These ages represent cooling ages and thus minimum ages for ruby formation. The ages obtained for Nangimali are close to the Ar-Ar cooling age of 19 Ma recorded in the Chichi granite, North to the ruby deposit. However, (C,O)-isotopic studies of the ruby-bearing marbles show no genetic relation between granite emplacement and ruby deposition in this area. The age found at Jegdalek is similar to the K-Ar ages obtained on the Sairobi pegmatitic dykes (20-26 Ma) and of the Jalalabad pluton (25 Ma), located close to the ruby deposit. At Mogok, the ruby deposits yield ages close to those obtained on high grade metamorphic and foliated intrusive regional rocks (15.8 ± 0.7 - 19.5 ± 1.0 Ma). The ages obtained at Chumar and Ruyil agree with those of the Lesser Himalaya Formation (12 - 6 Ma). Those found at Quy Chau agree with those found for the shear zone activity. Furthermore, U-Pb dating was done on zircons included in a ruby from Luc Yen and spinels in marble from Luc Yen and Hunza. The wide range of 238U-206Pb ages obtained for Luc Yen (266 - 45 Ma) evidences a complex metamorphic history. Ruby crystallised at 45 Ma during ductile activity of the Red River shear zone. At Hunza, an 238U-206Pb age of 94.0 ± 2.1 Ma obtained on inherited zircons confirms the U-Pb age obtained on

Laser-fusion 40Ar/39Ar Ages of Darwin Impact Glass

NASA Astrophysics Data System (ADS)

Lo, Ching-Hua; Howard, Kieren T.; Chung, Sun-Lin; Meffre, Sebastien

2002-11-01

Three samples of Darwin Glass, an impact glass found in Tasmania, Australia at the edge of the Australasian tektite strewn field were dated using the 40Ar/39Ar single-grain laser fusion technique, yielding isochron ages of 796-815 ka with an overall weighted mean of 816 ± 7 ka. These data are statistically indistinguishable from those recently reported for the Australasian tektites from Southeast Asia and Australia (761-816 ka; with a mean weighted age of 803 ± 3 ka). However, considering the compositional and textural differences and the disparity from the presumed impact crater area for Australasian tektites, Darwin Glass is more likely to have resulted from a distinct impact during the same period of time.

Newly combined 40Ar/39Ar and U-Pb ages of the Upper Cretaceous timescale from Hokkaido, Japan

NASA Astrophysics Data System (ADS)

Gaylor, J. R.; Heredia, B. D.; Quidelleur, X.; Takashima, R.; Nishi, H.; Mezger, K.

2011-12-01

The main targets for GTS next project (www.gtsnext.eu) are to develop highly refined geological time scales, including the Upper Cretaceous. The Cretaceous period is characterised by numerous global anoxic events in the marine realm, rich ammonitic fossil assemblages and specialised foraminifera. However, lack of age diagnostic macro and micro fossils in the North Pacific sections has made it difficult to link these with global sections such as the Western Interior Basin (North America). Using advances with terrestrial C-isotope and planktic foraminifera records within Central Hokkaido we are able to correlate these sections globally. The Cretaceous Yezo group in Central Hokkaido comprises deep marine mudstones and turbidite sandstones interbedded with acidic volcanic tuffs. Using various sections within the Yezo group, we radiometrically dated tuffs at the main stage boundaries in the Upper Cretaceous. The samples derive from the Kotanbetsu, Shumarinai, Tiomiuchi and the Hakkin river sections, spanning the time from the Albian-Cenomanian up until the Campanian-Santonian boundaries, and were dated using 40Ar/39Ar, K/Ar and U-Pb techniques. Recent age constraints in the Hokkaido counterparts (Kotanbetsu sections) show good coherence between radiometric chronometers on the various Upper Cretaceous stage boundaries. These additional ages together with our isotope ages from the different sections around the Hokkaido basin are well linked by the various faunal assemblages and C-isotope curves. The combined radio isotope ages contribute to previous attempts (such as those focused in the Western Interior Basin) supporting the synchronicity of events such as global oceanic anoxic events. Finally, the ages obtained here also compliment the previous C-isotope and planktic foraminifera records allowing for a more precise climatic history of the Northwest Pacific during the Cretaceous. The research within the GTSnext project is funded by the European Community's Seventh

Time constraints on post-rift evolution of the Southwest Indian passive margin from ^{40}Ar-^{39Ar dating of supergene K-Mn oxides

NASA Astrophysics Data System (ADS)

Bonnet, Nicolas; Arnaud, Nicolas; Beauvais, Anicet; Chardon, Dominique

2013-04-01

The high-elevation passive margin of Southwest India is marked by the Western Ghats escarpment, which separates the coastal domain from the low-relief East-dipping Mysore plateau. The escarpment has evolved from the Seychelles rifting at ~ 63 Ma following the Deccan traps volcanic event at ~ 65-63 Ma. This escarpment results from differential erosion processes across the passive margin, the rate and timing of which depend upon whether the margin has evolved according to a model of downwarped or rising flank topography. We explore the post-rift evolution of the South Indian passive margin through the characterisation of stepped relicts of lateritic paleosurfaces across that margin, and notably by 40Ar-39Ar dating of in-situ formed K-Mn oxides in supergene Mn-ore deposits carried by these paleosurfaces. The genesis and maturation of Mn-ore deposits are generally linked to progressive weathering processes of the paleosurfaces, which expose them. Dating of K-Mn oxides thus document the timing of these processes [1], and potentially the ages of the altered paleosurface. Moreover, the elevation differences between successive lateritic paleosurfaces of different ages may provide denudation rates for the considered time spans. Previous work (e.g., [2]) and our own field investigations, allow identifying three main lateritic paleosurfaces on the plateau at altitude ranges of 1000-900 m (S2), 900-800 m (S3) and 800-700 m (S3d), and a lower paleosurface in the coastal domain at 150-50 m (S4). K-Mn oxides (cryptomelane) were sampled in Mn ore deposits from different paleosurfaces, particularly in the coastal area around Goa on S4 and in Sandur and Shimoga Mn-ore deposits exposed on S2 and S3. The 40Ar-39Ar ages obtained from carefully characterised mineralogical assemblages range from ~ 26 to ~ 36 Ma in the Sandur Mn-ore deposit indicating intense lateritic weathering processes at the Eocene-Oligocene transition underneath paleosurface S2. Similar ages of ~ 24 and ~ 32 Ma are

Ar-Ar dating techniques for terrestrial meteorite impacts

NASA Astrophysics Data System (ADS)

Kelley, S. P.

2003-04-01

The ages of the largest (>100 km) known impacts on Earth are now well characterised. However the ages of many intermediate sized craters (20-100 km) are still poorly known, often the only constraints are stratigraphic - the difference between the target rock age and the age of crater filling sediments. The largest impacts result in significant melt bodies which cool to form igneous rocks and can be dated using conventional radiometric techniques. Smaller impacts give rise to thin bands of melted rock or melt clasts intimately mixed with country rock clasts in breccia deposits, and present much more of a challenge to dating. The Ar-Ar dating technique can address a wide variety of complex and heterogeneous samples associated with meteorite impacts and obtain reasonable ages. Ar-Ar results will be presented from a series of terrestrial meteorite impact craters including Boltysh (65.17±0.64 Ma, Strangways (646±42 Ma), and St Martin (220±32 Ma) and a Late Triassic spherule bed, possibly representing distal deposits from Manicouagan (214±1 Ma) crater. Samples from the Boltysh and Strangways craters demonstrate the importance of rapid cooling upon the retention of old ages in glassy impact rocks. A Late Triassic spherule bed in SW England is cemented by both carbonate and K-feldspar cements allowing Ar-Ar dating of fine grained cement to place a mimimum age upon the age of the associated impact. An age of 214.7±2.5 Ma places the deposit with errors of the age of the Manicouagan impact, raising the possibility that it may represent a distal deposit (the deposit lay around 2000 km away from the site of the Manicouagan crater during the Late Triassic). Finally the limits of the technique will be demonstrated using an attempt to date melt rocks from the St Martin Crater in Canada.

First-principles calibration of 40Ar/39Ar mineral standards and complete extraction of 40Ar* from sanidine

NASA Astrophysics Data System (ADS)

Morgan, L. E.; Kuiper, K.; Mark, D.; Postma, O.; Villa, I. M.; Wijbrans, J. R.

2010-12-01

40Ar/39Ar geochronology relies on comparing argon isotopic data for unknowns to those for knowns. Mineral standards used as neutron fluence monitors must be dated by the K-Ar method (or at least referenced to a mineral of known K-Ar age). The commonly used age of 28.02 ± 0.28 Ma for the Fish Canyon sanidine (FCs) (Renne et al., 1998) is based upon measurements of radiogenic 40Ar in GA1550 biotite (McDougall and Roksandic, 1974), but underlying full data were not published (these measurements were never intended for use as an international standard), so uncertainties are difficult to assess. Recent developments by Kuiper et al. (2008) and Renne et al. (2010) are limited by their reliance on the accuracy of other systems. Modern technology should allow for more precise and accurate calibration of primary K-Ar and 40Ar/39Ar standards. From the ideal gas law, the number of moles of 40Ar in a system can be calculated from measurements of pressure, volume, and temperature. Thus we have designed and are proceeding to build a pipette system to introduce well-determined amounts of 40Ar into noble gas extraction lines and mass spectrometers. This system relies on components with calibrations traceable to SI unit prototypes, including a diaphragm pressure gauge (MKS Instruments), thermocouples, and a “slug” of an accurately determined volume to be inserted into the reservoir for volume determinations of the reservoir and pipette. The system will be renewable, with a lifetime of ca. 1 month for gas in the reservoir, and portable, to permit interlaboratory calibrations. The quantitative extraction of 40Ar* from the mineral standard is of highest importance; for sanidine standards this is complicated by high melt viscosity during heating. Experiments adding basaltic “zero age glass” (ZAG) to decrease melt viscosity are underway. This has previously been explored by McDowell (1983) with a resistance furnace, but has not been quantitatively addressed with laser heating

K-Ar dating and delta O-18-delta D characterization of nanometric illite from Ordovician K-bentonites of the Appalachians: illitization and the Acadian-Alleghenian tectonic activity

USGS Publications Warehouse

Clauer, Norbert; Fallick, Anthony E.; Eberl, Dennis D.; Honty, Miroslav; Huff, Warren D.; Auberti, Amelie

2013-01-01

Nanometric (2 diagram that illitization occurred in all fractions by simultaneous nucleation and crystal growth, except for one sample. In that sample, a period of growth without nucleation was detected on top of the nucleation and growth episode. The K-Ar ages organize into two isochrons, the first at 319.9 ± 2.0 Ma with an initial 40Ar/36Ar ratio of 271 ± 66 Ma, and the second at 284.9 ± 1.2 Ma with an initial 40Ar/36Ar ratio of 310 ± 44. One data point above the older isochron and three between the two isochrons suggest a detrital contamination for the former separate and a possible further generation of nanoparticles for the three others. The samples with the older crystallization age consist of illite and illite-rich mixed-layers, and those with the younger age contain smectite-rich mixed-layers without illite, or illite-enriched illite-smectite mixed-layers. The K-Ar ages fit the age trends published previously for similar K-bentonites with regional age patterns between 240 and 270 Ma in the southwestern region, between 270 and 300 Ma in the central zone and the southern Appalachians, and between 315 and 370 Ma in the northernmost. Each of the two generations of illite crystals yields very consistent δ18O (V-SMOW) values at 17 ± 1‰ for the older and at 21 ± 1‰ for the younger. If crystallization temperatures of the nanometric illite were between 100 and 200 °C, as suggested by microthermometric determinations, the hydrothermal fluids had δ18O values of 4 ± 1‰ in the Dalton district and of 8 ± 1‰ in the Lafayette, Trenton, and Dirtseller districts at 100 °C, and of 11 ± 1 and 15 ± 1‰ in the same locations at 200 °C, probably because the water-rock isotope exchanges at elevated temperature occurred in rock-dominated systems. The δ18O of the fluids remained unchanged during local crystal growth, but varied depending on the geographic location of the samples and timing of illitization. The δD (V-SMOW) values of the different size

High-precision 40Ar/39Ar age for the Jehol Biota

NASA Astrophysics Data System (ADS)

Chang, S.; Zhang, H.; Renne, P. R.; Fang, Y.

2008-12-01

Abundant fossils of the terrestrial Jehol Biota, including plants, insects, dinosaurs, birds, mammals and freshwater invertebrates, were discovered from the Yixian Formation and the overlying Jiufotang Formation in Inner Mongolia, Hebei Province and Liaoning Province, northeastern China. Because of the exceptional preservation of fossils, the Jehol Biota is one of the most important Mesozoic fossil outcrops and referred to as a "Mesozoic Pompeii". The Jehol Biota has provided a rare opportunity to address questions about the origin of birds, the evolution of feathers and flight, the early diversification of angiosperms and the timing of the radiation of placental mammals. The Tuchengzi Formation, which lies unconformably just below the Yixian Formation and consists mainly of variegated sandstones, is less fossiliferous than the two overlying formations. However, dinosaur tracks, silicified wood and compressed plants are found in this formation. A systematic 40Ar/39Ar dating of the Yixian and the Jiufotang formations was undertaken to provide a framework for understanding the timing and duration of the Jehol Biota and evolutionary events represented within it. Furthermore, determining the absolute age of the Tuchengzi Formation provides information to interpret abundant dinosaur tracks within and provide better age constrains for the beginning of the Jehol Biota. Here we present robust high-precision 40Ar/39Ar data for six tuff samples and two basalt samples collected from the Tuchengzi, the Yixian and the Jiufotang formations near the classic outcrops in western Liaoning, NE China. We obtain an age of 139.5 ± 1.0 Ma for the uppermost Tuchengzi Formation, an age of 129.7 ± 0.5 Ma for a basaltic lava from the bottom of the Yixian Formation and an age of 122.1 ± 0.3 Ma for a tuff from the base of the overlying Jiufotang Formation. Our data indicate that the Yixian Formation was deposited during the Early Cretaceous, the Barremian to early Aptian, within a time span

In situ 40K-40Ca ‘double-plus’ SIMS dating resolves Klokken feldspar 40K-40Ar paradox

NASA Astrophysics Data System (ADS)

Harrison, T. Mark; Heizler, Matthew T.; McKeegan, Kevin D.; Schmitt, Axel K.

2010-11-01

The 40K- 40Ca decay system has not been widely utilized as a geochronometer because quantification of radiogenic daughter is difficult except in old, extremely high K/Ca domains. Even these environments have not heretofore been exploited by ion microprobe analysis due to the very high mass resolving power (MRP) of 25,000 required to separate 40K + from 40Ca +. We introduce a method that utilizes doubly-charged K and Ca species which permits isotopic measurements to be made at relatively low MRP (~ 5000). We used this K-Ca 'double-plus' approach to address an enduring controversy in 40Ar/ 39Ar thermochronology revolving around exsolved alkali feldspars from the 1166 Ma Klokken syenite (southern Greenland). Ion microprobe 40K- 40Ca analysis of Klokken samples reveal both isochron and pseudoisochron behaviors that reflect episodic isotopic and chemical exchange of coarsely exsolved perthites and a near end-member K-feldspar until ≤ 719 Ma, and perhaps as late at ~ 400 Ma. Feldspar microtextures in the Klokken syenite evolved over a protracted interval by non-thermal processes (fluid-assisted recrystallization) and thus this sample makes a poor model from which to address the general validity of 40Ar/ 39Ar thermochronological methodologies.

40Ar/39Ar geochronology of submarine Mauna Loa volcano, Hawaii

NASA Astrophysics Data System (ADS)

Jicha, B.; Rhodes, J. M.; Singer, B. S.; Vollinger, M. J.; Garcia, M. O.

2009-12-01

A major impediment to our understanding of the nature and structure of the Hawaiian plume, and evaluating the competing plume models has been a lack of thick stratigraphic sections from which to obtain long temporal records of magmatic history. The Hawaii Scientific Drilling Project (HSDP) made a significant advance towards solving this problem by documenting the long-term magmatic evolution of Mauna Kea volcano on the Kea side of the plume. To evaluate comparable long-term magmatic history on the Loa side of the plume we collected a stratigraphically controlled sample suite using Jason and Pisces dives from three vertical transects of the 1.6 km high Kae Lae landslide scarp cut into Mauna Loa’s submarine southwest rift zone (SWR). We have undertaken an 40Ar/39Ar investigation of Mauna Loa’s growth history to integrate new geochronologic constraints with geochemical, and isotopic data, illuminating temporal trends within the Hawaiian plume. Obtaining precise 40Ar/39Ar ages from tholeiitic lavas younger than 500 ka containing only 0.2-0.6 wt.% K2O is challenging due to the extremely low radiogenic 40Ar contents. Furnace incremental heating experiments of groundmass separated from 15 submarine lavas have yielded four new age determinations (a 27% success rate). These four lavas give concordant age spectra with plateau and isochron ages that agree with stratigraphy. We also analyzed two previously-dated subaerial Mauna Kea tholeiites from the HSDP-2 drill core, to assess inter-laboratory reproducibility and calibrate our results to those obtained for the core. Two experiments on sample SR413-4.0 and one experiment from SR781-21.2 gave weighted mean plateau ages of 364 ± 95 ka and 473 ± 109, respectively, which are indistinguishable from the published 40Ar/39Ar ages of 390 ± 70 ka and 482 ± 67. Although Sharp and Renne (2005) preferred isochron ages for the submarine Mauna Kea tholeiites recovered from HSDP, we find that submarine Mauna Loa lavas contain

Neogene geomorphic and climatic evolution of the central San Juan Mountains, Colorado: K/Ar age and stable isotope data on supergene alunite and jarosite from the Creede mining district

USGS Publications Warehouse

Rye, Robert O.; Bethke, Philip M.; Lanphere, Marvin A.; Steven, Thomas A.

2000-01-01

K/Ar age determinations or supergene alunite and jarosite, formed during Neogene weathering of the epithermal silver and base-metal ores of the Creede mining district, have been combined with geologic evidence to estimate the timing of regional uplift of the southern Rocky Mountains and related canyon cutting. In addition, oxygen and hydrogen isotopic studies suggest climate changes in the central San Juan Mountains during the past 5 m.y. Alunite [ideally (K,Na)Al3(SO4)2(OH)6] and jarosite [ideally KFe3(SO4)2(OH)6] can be dated by K/Ar or 40Ar/39Ar techniques and both contain OH and SO4 sites that enable four stable isotope analyses (δD, δ18OOH, and δ34S) to be made. This supergene alunite and jarosite formed by weathering of sulfide-rich ore bodies may record the evolution of the chemical and hydrologic processes affecting ancient oxidized acid ground water, as well as details of climate history and geomorphic evolution. Fine-grained (1-10 μm) supergene alunite and jarosite occur in minor fractures in the upper, oxidized parts of the 25 Ma sulfide-bearing veins of the Creede mining district, and jarosite also occurs in adjacent oxidized Ag-bearing clastic sediments. K/Ar ages for alunite range from 4.8 to 3.1 Ma, and for jarosite range from 2.6 to 0.9 Ma. The δD values for alunite and jarosite show opposite correlations with elevation, and values for jarosite correlate with age. Calculated δDH2O values of alunite fluids approach but are larger than those of present-day meteoric water. Calculated δDH2O values for jarosite fluids are more variable; the values of the youngest jarosites are lowest and are similar to those of present-day meteoric water in the district. The narrow δD-δ18OSO4 values of alunites reflects oxidation of sulfide below the water table. The greater range in these values for jarosites reflects oxidation of sulfide under vadose conditions. The ages of alunite mark the position of the paleo-water table at the end of a period of moderate

K-Ar geochronology of the Survey Pass, Ambler River and Eastern Baird Mountains quadrangles, southwestern Brooks Range, Alaska

USGS Publications Warehouse

Turner, Donald L.; Forbes, R.B.; Mayfield, C.F.

1978-01-01

We report 76 previously unpublished K-Ar mineral ages from 47 metamorphic and igneous rocks in the southwestern Brooks Range. The pattern of radiometric ages is complex, reflecting the complex geologic history of this area. Local and regional radiometric evidence suggests that the southern Brooks Range schist belt has, at least in part, undergone a late Precambrian metamorphism and that the parent sedimentary and igneous rocks for the metamorphic rocks dated as late Precambrian are at least this old (Precambrian Z). This schist terrane experienced a major thermal event in mid-Cretaceous time, causing widespread resetting of nearly all K-Ar mica ages. A series of apparent ages intermediate between late Precambrian and mid-Cretaceous are interpreted as indicating varying amounts of partial argon loss from older rocks during the Cretaceous event. The schist belt is characterized by dominant metasediments and subordinate metabasites and metafelsites. Blueschists occur within the schist belt from the Chandalar quadrangle westward to the Baird Mountains quadrangle, but geologic evidence does not support the existence of a fossil subduction zone.

The 40Ar/39Ar dating technique applied to planetary sciences

NASA Astrophysics Data System (ADS)

Jourdan, F.

2012-12-01

The 40Ar/39Ar technique is a powerful geochronological method that can help to unravel the evolution of the solar system. The 40Ar/39Ar system can not only record the timing of volcanic and metamorphic processes on asteroids and planets, it finds domain of predilection in dating impact events throughout the solar system. However, the 40Ar/39Ar method is a robust analytical technique if, and only if, the events to be dated are well understood and data are not over interpreted. Yet, too many 'ages' reported in the literature are still based on over-interpretation of perturbed age spectra which tends to blur the big picture. This presentation is centred on the most recent applications of the 40Ar/39Ar technique applied to planetary material and through several examples, will attempt to demonstrate the benefit of focusing on statistically robust data. For example, 40Ar/39Ar dating of volcanic events on the Moon suggests that volcanism was mostly concentrated between ca. 3.8 and 3.1 Ga but statistical filtering of the data allow identifying a few well-defined eruptive events. The study of lunar volcanism would also benefit from dating of volcanic spherules. Rigorous filtering of the 40Ar/39Ar age database of lunar melt breccias yielded concordant and ages with high precision for two major basins (i.e. Imbrium & Serenitatis) of the Moon. 40Ar/39Ar dating of lunar impact spherules recovered from four different sites and with high- and low-K compositions shows an increase of ages younger than 400 Ma suggesting a recent increase in the impact flux. The impact history of the LL parent body (bodies?) has yet to be well constrained but may mimic the LHB observed on the Moon, which would indicate that the LL parent body was quite large. 40Ar/39Ar dating (in progress) of grains from the asteroid Itokawa recovered by the japanese Hayabusa mission have the potential to constrain the formation history and exposure age of Itokawa and will allow us to compare the results with the

40Ar/39Ar age of the Manson impact structure, Iowa, and correlative impact ejecta in the Crow Creek member of the Pierre Shale (Upper Cretaceous), South Dakota and Nebraska

USGS Publications Warehouse

Izett, G.A.; Cobban, W.A.; Dalrymple, G.B.; Obradovich, J.D.

1998-01-01

A set of 34 laser total-fusion 40Ar/39Ar analyses of sanidine from a melt layer in crater-fill deposits of the Manson impact structure in Iowa has a weighted-mean age of 74.1 ?? 0.1 Ma. This age is about 9.0 m.y. older than 40Ar/39Ar ages of shocked microcline from the Manson impact structure reported previously by others. The 74.1 Ma age of the sanidine, which is a melt product of Precambrian microcline clasts, indicates that the Manson impact structure played no part in the Cretaceous-Tertiary (K-T) mass extinction at 64.5 Ma. Moreover, incremental-heating 40Ar/39Ar ages of the sanidine show that it is essentially free of excess 40Ar and has not been influenced by postcrystallization heating or alteration. An age spectrum of the matrix of the melt layer shows effects of 39Ar recoil, including older ages in the low-temperature increments and younger ages in the high-temperature increments. At 17 places in eastern South Dakota and Nebraska, shocked quartz and feldspar grains are concentrated in the lower part of the Crow Creek Member of the Pierre Shale (Upper Cretaceous). The grains are largest (3.2 mm) in southeastern South Dakota and decrease in size (0.45 mm) to the northwest, consistent with the idea that the Manson impact structure was their source. The ubiquitous presence of shocked grains concentrated in a thin calcarenite at the base of the Crow Creek Member suggests it is an event bed recording an instant of geologic time. Ammonites below and above the Crow Creek Member limit its age to the zone of Didymoceras nebrascense of earliest late Campanian age. Plagioclase from a bentonite bed in this zone in Colorado has a 40Ar/39Ar age of 74.1 ?? 0.1 Ma commensurate with our sanidine age of 74.1 Ma for the Manson impact structure. 40Ar/39Ar ages of bentonite beds below and above the Crow Creek are consistent with our 74.1 ?? 0.1 Ma age for the Manson impact structure and limit its age to the interval ?? 74.5 0.1 to 73.8 ?? 0.1 Ma. Recently, two origins for the

Noble Gas Analysis for Mars Robotic Missions: Evaluating K-Ar Age Dating for Mars Rock Analogs and Martian Shergottites

NASA Technical Reports Server (NTRS)

Park, J.; Ming, D. W.; Garrison, D. H.; Jones, J. H.; Bogard, D. D.; Nagao, K.

2009-01-01

The purpose of this noble gas investigation was to evaluate the possibility of measuring noble gases in martian rocks and air by future robotic missions such as the Mars Science Laboratory (MSL). The MSL mission has, as part of its payload, the Sample Analysis at Mars (SAM) instrument, which consists of a pyrolysis oven integrated with a GCMS. The MSL SAM instrument has the capability to measure noble gas compositions of martian rocks and atmosphere. Here we suggest the possibility of K-Ar age dating based on noble gas release of martian rocks by conducting laboratory simulation experiments on terrestrial basalts and martian meteorites. We provide requirements for the SAM instrument to obtain adequate noble gas abundances and compositions within the current SAM instrumental operating conditions, especially, a power limit that prevents heating the furnace above approx.1100 C. In addition, Martian meteorite analyses from NASA-JSC will be used as ground truth to evaluate the feasibility of robotic experiments to constrain the ages of martian surface rocks.

40Ar 39Ar Ages and tectonic setting of ophiolite from the Neyriz area, southeast Zagros Range, Iran

USGS Publications Warehouse

Lanphere, M.A.; Pamic, J.

1983-01-01

An ophiolite, considered to be an allochthonous fragment of Tethyan oceanic crust and mantle, crops out near Neyriz in the Zagros Range, Iran. 40Ar 39Ar ages ranging from 76.8 ?? 23.8 Ma to 105 ?? 23.3 Ma were measured on hornblende from five samples of plagiogranite and diabase from the ophiolite. The most precise ages are 85.9 ?? 3.8 Ma for a diabase and 83.6 ?? 8.4 Ma for a plagiogranite. The weighted mean age of hornblende from the five samples is 87.5 ?? 7.2 Ma which indicates that the igneous part of the Neyriz ophiolite formed during the early part of the Late Cretaceous. Pargasite from amphibolite below peridotite of the Neyriz ophiolite has a 40Ar 39Ar age of 94.9 ?? 7.6 Ma. The pargasite age agrees within analytical uncertainty with the ages measured on diabase and plagiogranite. Comparable ages have been measured on igneous rocks from the Samail ophiolite of Oman and on amphibolite below peridotite of the Samail ophiolite. ?? 1983.

Evolution on a volcanic conveyor belt: using phylogeographic reconstructions and K-Ar-based ages of the Hawaiian Islands to estimate molecular evolutionary rates.

PubMed

Fleischer, R C; McIntosh, C E; Tarr, C L

1998-04-01

The Hawaiian Islands form as the Pacific Plate moves over a 'hot spot' in the earth's mantle where magma extrudes through the crust to build huge shield volcanos. The islands subside and erode as the plate carries them to the north-west, eventually to become coral atolls and seamounts. Thus islands are ordered linearly by age, with the oldest islands in the north-west (e.g. Kauai at 5.1 Ma) and the youngest in the south-east (e.g. Hawaii at 0.43 Ma). K-Ar estimates of the date of an island's formation provide a maximum age for the taxa inhabiting the island. These ages can be used to calibrate rates of molecular change under the following assumptions: (i) K-Ar dates are accurate; (ii) tree topologies show that derivation of taxa parallels the timing of island formation; (iii) populations do not colonize long after island emergence; (iv) the coalescent point for sister taxa does not greatly predate the formation of the colonized younger island; (v) saturation effects and (vi) among-lineage rate variation are minimal or correctable; and (vii) unbiased standard errors of distances and regressions can be estimated from multiple pairwise comparisons. We use the approach to obtain overall corrected rate calibrations for: (i) part of the mitochondrial cytochrome b gene in Hawaiian drepanidines (0.016 sequence divergence/Myr); (ii) the Yp1 gene in Hawaiian Drosophila (0.019/Myr Kambysellis et al. 1995); and (iii) parts of the mitochondrial 12S and 16S rRNA and tRNAval in Laupala crickets (0.024-0.102/Myr, Shaw 1996). We discuss the reliability of the estimates given the assumptions (i-vii) above and contrast the results with previous calibrations of Adh in Hawaiian Drosophila and chloroplast DNA in lobeliods.

Quantitative Potassium Measurements with Laser-Induced Breakdown Spectroscopy Using Low-Energy Lasers: Application to In Situ K-Ar Geochronology for Planetary Exploration.

PubMed

Cho, Yuichiro; Horiuchi, Misa; Shibasaki, Kazuo; Kameda, Shingo; Sugita, Seiji

2017-08-01

In situ radiogenic isotope measurements to obtain the absolute age of geologic events on planets are of great scientific value. In particular, K-Ar isochrons are useful because of their relatively high technical readiness and high accuracy. Because this isochron method involves spot-by-spot K measurements using laser-induced breakdown spectroscopy (LIBS) and simultaneous Ar measurements with mass spectrometry, LIBS measurements are conducted under a high vacuum condition in which emission intensity decreases significantly. Furthermore, using a laser power used in previous planetary missions is preferable to examine the technical feasibility of this approach. However, there have been few LIBS measurements for K under such conditions. In this study, we measured K contents in rock samples using 30 mJ and 15 mJ energy lasers under a vacuum condition (10 -3  Pa) to assess the feasibility of in situ K-Ar dating with lasers comparable to those used in NASA's Curiosity and Mars 2020 missions. We obtained various calibration curves for K using internal normalization with the oxygen line at 777 nm and continuum emission from the laser-induced plasma. Experimental results indicate that when K 2 O < 1.1 wt%, a calibration curve using the intensity of the K emission line at 769 nm normalized with that of the oxygen line yields the best results for the 30 mJ laser energy, with a detection limit of 88 ppm and 20% of error at 2400 ppm of K 2 O. Futhermore, the calibration curve based on the K 769 nm line intensity normalized with continuum emission yielded the best result for the 15 mJ laser, giving a detection limit of 140 ppm and 20% error at 3400 ppm K 2 O. Error assessments using obtained calibration models indicate that a 4 Ga rock with 3000 ppm K 2 O would be measured with 8% (30 mJ) and 10% (15 mJ) of precision in age when combined with mass spectrometry of 40 Ar with 10% of uncertainty. These results strongly suggest that high precision

Using 40Ar/39Ar and Lu-Hf isotopic dates to further understand the deformational history of an ancient subduction zone in Alaska

NASA Astrophysics Data System (ADS)

Wildgoose, M.; Roeske, S.; Vervoort, J. D.

2011-12-01

Although muscovite is widely accepted as a reliable geochronometer, 40Ar/39Ar dates of K-white mica from blueschist facies rocks have proven difficult to interpret because of chemically mixed populations. We plan to use the in-situ 40Ar/39Ar UV laser ablation technique to address whether single crystals of chemically heterogeneous K-white mica can provide reliable ages of metamorphic events. This in-situ method allows for more detailed study of K-white mica than bulk separates, as individual crystals can be analyzed on a microstructural basis. Zoned K-white mica commonly occurs in blueschist rocks, but can be difficult to link to a P-T-t path as both isothermal decompression and isobaric heating could result in the formation of phengite cores and muscovite rims. Garnet-glaucophane-phengite-epidote blueschists from the Ruby Terrane, west-central Alaska, have zoned amphibole and K-white mica, with the latter containing phengite cores with more muscovitic rims. In-situ 40Ar/39Ar UV laser ablation of single K-white mica crystals from this area, in conjunction with Lu/Hf dating of co-occurring garnets, will allow a delineation of P-T paths by comparing garnet ages, which record the highest temperature on a P-T path, to the ages of the K-white mica cores and rims. Results will be presented at the meeting in December. Results from these dating techniques will be presented at the meeting in December.

Tracking the timing of subduction and exhumation using 40Ar/39Ar phengite ages in blueschist- and eclogite-facies rocks (Sivrihisar, Turkey)

NASA Astrophysics Data System (ADS)

Fornash, Katherine F.; Cosca, Michael A.; Whitney, Donna L.

2016-07-01

Geochronologic studies of high-pressure/low-temperature rocks can be used to determine the timing and rates of burial and exhumation in subduction zones by dating different stages of the pressure-temperature history. In this study, we present new in situ UV laser ablation 40Ar/39Ar phengite ages from a suite of lawsonite blueschist- and eclogite-facies rocks representing different protoliths (metabasalt, metasediment), different structural levels (within and outside of a high-strain zone), and different textural positions (eclogite pod core vs. margin) to understand the timing of these events in an exhumed Neo-Tethyan subduction zone (Sivrihisar Massif, Tavşanlı Zone, Turkey). Weighted mean in situ 40Ar/39Ar ages of phengite from the cores of lawsonite eclogite pods (90-93 Ma) are distinctly older than phengite from retrogressed, epidote eclogite (82 ± 2 Ma). These ages are interpreted as the age of peak and retrograde metamorphism, respectively. Eclogite records the narrowest range of ages (10-14 m.y.) of any rock type analyzed. Transitional eclogite- and blueschist-facies assemblages and glaucophane-rimmed lawsonite + garnet + phengite veins from eclogite pod margins record a much wider age range of 40Ar/39Ar ages (~20 m.y.) with weighted mean ages of ~91 Ma. Blueschists and quartzites record more variable 40Ar/39Ar ages that may in part be related to structural position: samples within a high-strain zone at the tectonic contact of the HP rocks with a meta-ultramafic unit have in situ UV laser ablation 40Ar/39Ar ages of 84.0 ± 1.3-103.7 ± 3.1 Ma, whereas samples outside this zone range to older ages (84.6 ± 2.4-116.7 ± 2.7 Ma) and record a greater age range (22-38 m.y.). The phengite ages can be correlated with the preservation of HP mineral assemblages and fabrics as well as the effects of deformation. Collectively, these results show that high-spatial resolution UV laser ablation 40Ar/39Ar phengite data, when considered in a petrologic and structural

An extremely low U Pb source in the Moon: UThPb, SmNd, RbSr, and 40Ar 39Ar isotopic systematics and age of lunar meteorite Asuka 881757

USGS Publications Warehouse

Misawa, K.; Tatsumoto, M.; Dalrymple, G.B.; Yanai, K.

1993-01-01

We have undertaken UThPb, SmNd, RbSr, and 40Ar 39Ar isotopic studies on Asuka 881757, a coarse-grained basaltic lunar meteorite whose chemical composition is close to low-Ti and very low-Ti (VLT) mare basalts. The PbPb internal isochron obtained for acid leached residues of separated mineral fractions yields an age of 3940 ?? 28 Ma, which is similar to the U-Pb (3850 ?? 150 Ma) and Th-Pb (3820 ?? 290 Ma) internal isochron ages. The Sm-Nd data for the mineral separates yield an internal isochron age of 3871 ?? 57 Ma and an initial 143Nd 144Nd value of 0.50797 ?? 10. The Rb-Sr data yield an internal isochron age of 3840 ?? 32 Ma (??(87Rb) = 1.42 ?? 10-11 yr-1) and a low initial 87Sr 86Sr ratio of 0.69910 ?? 2. The 40Ar 39Ar age spectra for a glass fragment and a maskelynitized plagioclase are relatively flat and give a weighted mean plateau age of 3798 ?? 12 Ma. We interpret these ages to indicate that the basalt crystallized from a melt 3.87 Ga ago (the Sm-Nd age) and an impact event disturbed the Rb-Sr system and completely reset the K-Ar system at 3.80 Ga. The slightly higher Pb-Pb age compared to the Sm-Nd age could be due to the secondary Pb (from terrestrial and/or lunar surface Pb contamination) that remained in the residues after acid leaching. Alternatively, the following interpretation is also possible; the meteorite crystallized at 3.94 Ga (the Pb-Pb age) and the Sm-Nd, Rb-Sr, and K-Ar systems were disturbed by an impact event at 3.80 Ga. The crystallization age obtained here is older than those reported for low-Ti basalts (3.2-3.5 Ga) and for VLT basalts (3.4 Ga), but similar to ages of some mare basalts, indicating that the basalt may have formed from a magma related to a basin-forming event (Imbrium?). The age span for VLT basalts from different sampling sites suggest that they were erupted over a wide area during an interval of at least ~500 million years. The impact event that thermally reset the K-Ar system of Asuka 881757 must have been post

Mineralogical, chemical and K-Ar isotopic changes in Kreyenhagen Shale whole rocks and <2 μm clay fractions during natural burial and hydrous-pyrolysis experimental maturation

NASA Astrophysics Data System (ADS)

Clauer, N.; Lewan, M. D.; Dolan, M. P.; Chaudhuri, S.; Curtis, J. B.

2014-04-01

Progressive maturation of the Eocene Kreyenhagen Shale from the San Joaquin Basin of California was studied by combining mineralogical and chemical analyses with K-Ar dating of whole rocks and <2 μm clay fractions from naturally buried samples and laboratory induced maturation by hydrous pyrolysis of an immature outcrop sample. The K-Ar age decreases from 89.9 ± 3.9 and 72.4 ± 4.2 Ma for the outcrop whole rock and its <2 μm fraction, respectively, to 29.7 ± 1.5 and 21.0 ± 0.7 Ma for the equivalent materials buried to 5167 m. The natural maturation does not produce K-Ar ages in the historical sense, but rather K/Ar ratios of relative K and radiogenic 40Ar amounts resulting from a combined crystallization of authigenic and alteration of initial detrital K-bearing minerals of the rocks. The Al/K ratio of the naturally matured rocks is essentially constant for the entire depth sequence, indicating that there is no detectable variation in the crystallo-chemical organization of the K-bearing alumino-silicates with depth. No supply of K from outside of the rock volumes occurred, which indicates a closed-system behavior for it. Conversely, the content of the total organic carbon (TOC) content decreases significantly with burial, based on the progressive increasing Al/TOC ratio of the whole rocks. The initial varied mineralogy and chemistry of the rocks and their <2 μm fractions resulting from differences in detrital sources and depositional settings give scattered results that homogenize progressively during burial due to increased authigenesis, and concomitant increased alteration of the detrital material. Hydrous pyrolysis was intended to alleviate the problem of mineral and chemical variations in initially deposited rocks of naturally matured sequences. However, experiments on aliquots from thermally immature Kreyenhagen Shale outcrop sample did not mimic the results from naturally buried samples. Experiments conducted for 72 h at temperatures from 270 to 365 �

Revised error propagation of 40Ar/39Ar data, including covariances

NASA Astrophysics Data System (ADS)

Vermeesch, Pieter

2015-12-01

The main advantage of the 40Ar/39Ar method over conventional K-Ar dating is that it does not depend on any absolute abundance or concentration measurements, but only uses the relative ratios between five isotopes of the same element -argon- which can be measured with great precision on a noble gas mass spectrometer. The relative abundances of the argon isotopes are subject to a constant sum constraint, which imposes a covariant structure on the data: the relative amount of any of the five isotopes can always be obtained from that of the other four. Thus, the 40Ar/39Ar method is a classic example of a 'compositional data problem'. In addition to the constant sum constraint, covariances are introduced by a host of other processes, including data acquisition, blank correction, detector calibration, mass fractionation, decay correction, interference correction, atmospheric argon correction, interpolation of the irradiation parameter, and age calculation. The myriad of correlated errors arising during the data reduction are best handled by casting the 40Ar/39Ar data reduction protocol in a matrix form. The completely revised workflow presented in this paper is implemented in a new software platform, Ar-Ar_Redux, which takes raw mass spectrometer data as input and generates accurate 40Ar/39Ar ages and their (co-)variances as output. Ar-Ar_Redux accounts for all sources of analytical uncertainty, including those associated with decay constants and the air ratio. Knowing the covariance matrix of the ages removes the need to consider 'internal' and 'external' uncertainties separately when calculating (weighted) mean ages. Ar-Ar_Redux is built on the same principles as its sibling program in the U-Pb community (U-Pb_Redux), thus improving the intercomparability of the two methods with tangible benefits to the accuracy of the geologic time scale. The program can be downloaded free of charge from http://redux.london-geochron.com.

Variation of illite/muscovite 40Ar/39Ar age spectra during progressive low-grade metamorphism: an example from the US Cordillera

NASA Astrophysics Data System (ADS)

Verdel, Charles; van der Pluijm, Ben A.; Niemi, Nathan

2012-09-01

40Ar/39Ar step-heating data were collected from micron to submicron grain-sizes of correlative illite- and muscovite-rich Cambrian pelitic rocks from the western United States that range in metamorphic grade from the shallow diagenetic zone (zeolite facies) to the epizone (greenschist facies). With increasing metamorphic grade, maximum ages from 40Ar/39Ar release spectra decrease, as do total gas ages and retention ages. Previous studies have explained similar results as arising dominantly or entirely from the dissolution of detrital muscovite and precipitation/recrystallization of neo-formed illite. While recognizing the importance of these processes in evaluating our results, we suggest that the inverse correlation between apparent age and metamorphic grade is controlled, primarily, by thermally activated volume diffusion, analogous to the decrease in apparent ages with depth observed for many thermochronometers in borehole experiments. Our results suggest that complete resetting of the illite/muscovite Ar thermochronometer occurs between the high anchizone and epizone, or at roughly 300 °C. This empirical result is in agreement with previous calculations based on muscovite diffusion parameters, which indicate that muscovite grains with radii of 0.05-2 μm should have closure temperatures between 250 and 350 °C. At high anchizone conditions, we observe a reversal in the age/grain-size relationship (the finest grain-size produces the oldest apparent age), which may mark the stage in prograde subgreenschist facies metamorphism of pelitic rocks at which neo-formed illite/muscovite crystallites typically surpass the size of detrital muscovite grains. It is also approximately the stage at which neo-formed illite/muscovite crystallites develop sufficient Ar retentivity to produce geologically meaningful 40Ar/39Ar ages. Results from our sampling transect of Cambrian strata establish a framework for interpreting illite/muscovite 40Ar/39Ar age spectra at different

(40)Ar/(39)Ar Age of Hornblende-Bearing R Chondrite LAP 04840

NASA Technical Reports Server (NTRS)

Righter, K.; Cosca, M.

2015-01-01

Chondrites have a complex chronology due to several variables affecting and operating on chondritic parent bodies such as radiogenic heating, pressure and temperature variation with depth, aqueous alteration, and shock or impact heating. Unbrecciated chondrites can record ages from 4.56 to 4.4 Ga that represent cooling in small parent bodies. Some brecciated chondrites exhibit younger ages (much less than 4 to 4.4 Ga) that may reflect the age of brecciation, disturbance, or shock and impact events (much less than 4 Ga). A unique R chondrite was recently found in the LaPaz Icefield of Antarctica - LAP 04840. This chondrite contains approximately 15% hornblende and trace amounts of biotite, making it the first of its kind. Studies have revealed an equigranular texture, mineral equilibria yielding equilibration near 650-700 C and 250-500 bars, hornblende that is dominantly OH-bearing (very little Cl or F), and high D/H ratios. To help gain a better understanding of the origin of this unique sample, we have measured the (40)Ar/(39)Ar age (LAP 04840 split 39).

Ar-Ar Dating of Martian Meteorite, Dhofar 378: An Early Shock Event?

NASA Technical Reports Server (NTRS)

Park, J.; Bogard, D. D.

2006-01-01

Martian meteorite, Dhofar 378 (Dho378) is a basaltic shergottite from Oman, weighing 15 g, and possessing a black fusion crust. Chemical similarities between Dho378 and the Los Angeles 001 shergottite suggests that they might have derived from the same Mars locale. The plagioclase in other shergottites has been converted to maskelenite by shock, but Dho378 apparently experienced even more intense shock heating, estimated at 55-75 GPa. Dho378 feldspar (approximately 43 modal %) melted, partially flowed and vesiculated, and then partially recrystallized. Areas of feldspathic glass are appreciably enriched in K, whereas individual plagioclases show a range in the Or/An ratio of approximately 0.18-0.017. Radiometric dating of martian shergottites indicate variable formation times of 160-475 Myr, whereas cosmic ray exposure (CRE) ages of shergottites indicate most were ejected from Mars within the past few Myr. Most determined Ar-39-Ar-40 ages of shergottites appear older than other radiometric ages because of the presence of large amounts of martian atmosphere or interior Ar-40. Among all types of meteorites and returned lunar rocks, the impact event that initiated the CRE age very rarely reset the Ar-Ar age. This is because a minimum time and temperature is required to facilitate Ar diffusion loss. It is generally assumed that the shock-texture characteristics in martian meteorites were produced by the impact events that ejected the rocks from Mars, although the time of these shock events (as opposed to CRE ages) are not directly dated. Here we report Ar-39-Ar-40 dating of Dho378 plagioclase. We suggest that the determined age dates the intense shock heating event this meteorite experienced, but that it was not the impact that initiated the CRE age.

In-situ 40Ar/39Ar Laser Probe Dating of Micas from Mae Ping Shear Zone, Northern Thailand

NASA Astrophysics Data System (ADS)

Lin, Y. L.; Yeh, M. W.; Lo, C. H.; Lee, T. Y.; Charusiri, P.

2012-04-01

The Mae Ping Shear Zone (MPSZ, also known as Wang Chao Fault Zone), which trends NW-SE from Myanmar to central Thailand, was considered as the southern boundary of the SE extrusion of Indochina and Sibumasu block during the Cenozoic escape tectonic event of SE asia. Many analyses of 40Ar/39Ar dating on biotite and K-feldspar, K/Ar dating on biotite and illite, zircon fission-track and apatite fission-track dating had been accomplished to constrain the shearing period. Nevertheless, it is hard to convince that the ages could represent the end of the shearing since none of the dated minerals have been proved to be crystallized syn-tectonically. Meta-granitoid and gneiss from the MPSZ were analyzed in this study by applying in-situ 40Ar/39Ar laser probe dating with combination of petrology and micro-structural analysis in the purpose to decipher the geological significance of the dates. Plagioclase was replacing K-feldspar for K-feldspar was cut and embayed by plagioclase observed by SEM + EDS. Muscovite in the granitoid own fish shapes of sinistral sense of shearing, and are always in contact with plagioclase and quartz, which suggests that the muscovite crystallized from the dissolving K-feldspar under amphibolite facies condition. 117 spots on 12 muscovite fishes yield ages from 44 Ma to 35 Ma and have a mean age of 40 Ma. Since the growth condition of the muscovite is higher than the closure temperature, thus we can interpret these muscovite ages as cooling ages. Hence left-lateral shearing of the MPSZ can be deduced as syn- to post-muscovite growth and uplifted the crystalline rocks within the shear zone. The ages of matrix biotite in gneiss has a mean age of 35 Ma, which is consistent with the cooling path reconstructed from previous studies. While the ages of inclusion biotite in the K-feldspar phenocryst scatter from 40 to 50 Ma due to the isotopes were not totally re-equilibrated during the shearing. Consequently, the left-lateral shearing of the MPSZ was

Coeval Ar-40/Ar-39 ages of 65.0 million years ago from Chicxulub crater melt rock and Cretaceous-Tertiary boundary tektites

NASA Technical Reports Server (NTRS)

Swisher, Carl C., III; Grajales-Nishimura, Jose M.; Montanari, Alessandro; Margolis, Stanley V.; Claeys, Philippe; Alvarez, Walter; Renne, Paul; Cedillo-Pardo, Esteban; Maurrasse, Florentin J.-M. R.; Curtis, Garniss H.

1992-01-01

Ar-40/Ar-39 dating of drill-core samples of a glassy melt rock recovered from beneath a massive impact breccia contained with the 180-kilometer subsurface Chicxulub crater yields 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 Ar-40/Ar-39 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 Ar-40/Ar-39 ages, in conjunction with geochemical and petrological similarities, strengthen the 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.

Tracking the timing of subduction and exhumation using 40Ar/39Ar phengite ages in blueschist- and eclogite-facies rocks (Sivrihisar, Turkey)

USGS Publications Warehouse

Fornash, Katherine F.; Cosca, Michael A.; Whitney, Donna L.

2016-01-01

Geochronologic studies of high-pressure/low-temperature rocks can be used to determine the timing and rates of burial and exhumation in subduction zones by dating different stages of the pressure–temperature history. In this study, we present new in situ UV laser ablation 40Ar/39Ar phengite ages from a suite of lawsonite blueschist- and eclogite-facies rocks representing different protoliths (metabasalt, metasediment), different structural levels (within and outside of a high-strain zone), and different textural positions (eclogite pod core vs. margin) to understand the timing of these events in an exhumed Neo-Tethyan subduction zone (Sivrihisar Massif, Tavşanlı Zone, Turkey). Weighted mean in situ 40Ar/39Ar ages of phengite from the cores of lawsonite eclogite pods (90–93 Ma) are distinctly older than phengite from retrogressed, epidote eclogite (82 ± 2 Ma). These ages are interpreted as the age of peak and retrograde metamorphism, respectively. Eclogite records the narrowest range of ages (10–14 m.y.) of any rock type analyzed. Transitional eclogite- and blueschist-facies assemblages and glaucophane-rimmed lawsonite + garnet + phengite veins from eclogite pod margins record a much wider age range of 40Ar/39Ar ages (~20 m.y.) with weighted mean ages of ~91 Ma. Blueschists and quartzites record more variable 40Ar/39Ar ages that may in part be related to structural position: samples within a high-strain zone at the tectonic contact of the HP rocks with a meta-ultramafic unit have in situ UV laser ablation 40Ar/39Ar ages of 84.0 ± 1.3–103.7 ± 3.1 Ma, whereas samples outside this zone range to older ages (84.6 ± 2.4–116.7 ± 2.7 Ma) and record a greater age range (22–38 m.y.). The phengite ages can be correlated with the preservation of HP mineral assemblages and fabrics as well as the effects of deformation. Collectively, these results show that high-spatial resolution UV laser ablation 40Ar/39Ar phengite data, when considered

A natural laboratory for 40Ar/39Ar geochronology: ICDP cores from Lake Van, Turkey

NASA Astrophysics Data System (ADS)

Engelhardt, Jonathan; Sudo, Masafumi; Oberhänsli, Roland

2015-04-01

Pore water samples from ICDP Paleovan cores indicate a limited pore water exchange within Quaternary lake sediments. The core's volcaniclastic sections bear unaltered K-rich ternary feldspar and fresh to altered glass shards of predominantly rhyolitic composition. Whereas applying the 40Ar/39Ar method on feldspars resulted in ages timing a late-stage crystallization, glass shards had the potential to date the eruption. Volcanic glass is prone to modifications such as hydrous alteration (palagonitization) and devitrification (Cerling et al., 1985). These modifications affect the glass' chemistry and challenge the application of the 40Ar/39Ar method. Gaining precise radiometric ages from two phases has the potential to strengthen a climate-stratigraphic age-model (Stockhecke et al., 2014), and to significantly increase the temporal resolution on the deposition of the lake sediments. Vice versa the core's previous age model has the ability to question the reliability of 40Ar/39Ar eruption ages derived from ternary feldspars and glass shards. Multi- and single-grain total fusion on alkali feldspars from six volcaniclastic deposits resulted in Pleistocene ages that are in good agreement with the predicted age model. Feldspar phenocrysts from three ashes in the core's youngest section yielded consistent isochron ages that are significantly older than the model's prediction. Several distinct stratigraphic and paleomagnetic time markers of similar stratigraphic positions contradict to the older radiometric dates (Stockhecke et al., 2014). Partial resorption features of inherited feldspar domains and the involvement of excess 40Ar indicate incomplete degassing of older domains. To evaluate the magmatic history of the different domains EMPA mappings of trace elements that could be interpreted as Ar diffusion couples are currently conducted. Geochronology on Paleovan cores offers unique opportunities to monitor the effect of alteration on the Ar-systematics of volcanic glass

New K-Ar ages for calculating end-of-shield extrusion rates at West Maui volcano, Hawaiian island chain

USGS Publications Warehouse

Sherrod, D.R.; Murai, T.; Tagami, Takahiro

2007-01-01

Thirty-seven new K-Ar ages from West Maui volcano, Hawai'i, are used to define the waning stages of shield growth and a brief episode of postshield volcanism. All but two samples from shield-stage strata have reversed polarity magnetization, so conceivably the exposed shield is not much older than the Olduvai Normal-Polarity subchron, or about 1.8 Ma. The oldest ages obtained are in the range 1.9-2.1 Ma but have large analytical error. Shield volcanism ended about 1.35 Ma, and postshield volcanism followed soon thereafter, persisting until about 1.2 Ma. Exposed shield-stage strata were emplaced at a rate of about 0.001 km3 per year, a rate smaller than historic Hawaiian magmatic rates by a factor of 100. Stratigraphic accumulation rates are similar to those measured previously at Wai'anae volcano (O'ahu) or the upper part of the Mauna Kea shield sequence (Hilo drill core, Hawai'i). These rates diminish sharply during the final 0.3-0.5 m.y. of the shield stage. Hawaiian shield volcanoes begin waning well before their last 0.5 m.y. of life, then end quickly, geologically speaking, if West Maui is representative. ?? Springer-Verlag 2006.

New K Ar ages for calculating end-of-shield extrusion rates at West Maui volcano, Hawaiian island chain

NASA Astrophysics Data System (ADS)

Sherrod, David R.; Murai, Takashi; Tagami, Takahiro

2007-04-01

Thirty-seven new K Ar ages from West Maui volcano, Hawai‘i, are used to define the waning stages of shield growth and a brief episode of postshield volcanism. All but two samples from shield-stage strata have reversed polarity magnetization, so conceivably the exposed shield is not much older than the Olduvai Normal-Polarity subchron, or about 1.8 Ma. The oldest ages obtained are in the range 1.9 2.1 Ma but have large analytical error. Shield volcanism ended about 1.35 Ma, and postshield volcanism followed soon thereafter, persisting until about 1.2 Ma. Exposed shield-stage strata were emplaced at a rate of about 0.001 km3 per year, a rate smaller than historic Hawaiian magmatic rates by a factor of 100. Stratigraphic accumulation rates are similar to those measured previously at Wai‘anae volcano (O‘ahu) or the upper part of the Mauna Kea shield sequence (Hilo drill core, Hawai‘i). These rates diminish sharply during the final 0.3 0.5 m.y. of the shield stage. Hawaiian shield volcanoes begin waning well before their last 0.5 m.y. of life, then end quickly, geologically speaking, if West Maui is representative.

Astronomically calibrated 40Ar/39Ar age for the Toba supereruption and global synchronization of late Quaternary records

PubMed Central

Storey, Michael; Roberts, Richard G.; Saidin, Mokhtar

2012-01-01

The Toba supereruption in Sumatra, ∼74 thousand years (ka) ago, was the largest terrestrial volcanic event of the Quaternary. Ash and sulfate aerosols were deposited in both hemispheres, forming a time-marker horizon that can be used to synchronize late Quaternary records globally. A precise numerical age for this event has proved elusive, with dating uncertainties larger than the millennial-scale climate cycles that characterized this period. We report an astronomically calibrated 40Ar/39Ar age of 73.88 ± 0.32 ka (1σ, full external errors) for sanidine crystals extracted from Toba deposits in the Lenggong Valley, Malaysia, 350 km from the eruption source and 6 km from an archaeological site with stone artifacts buried by ash. If these artifacts were made by Homo sapiens, as has been suggested, then our age indicates that modern humans had reached Southeast Asia by ∼74 ka ago. Our 40Ar/39Ar age is an order-of-magnitude more precise than previous estimates, resolving the timing of the eruption to the middle of the cold interval between Dansgaard–Oeschger events 20 and 19, when a peak in sulfate concentration occurred as registered by Greenland ice cores. This peak is followed by a ∼10 °C drop in the Greenland surface temperature over ∼150 y, revealing the possible climatic impact of the eruption. Our 40Ar/39Ar age also provides a high-precision calibration point for other ice, marine, and terrestrial archives containing Toba sulfates and ash, facilitating their global synchronization at unprecedented resolution for a critical period in Earth and human history beyond the range of 14C dating. PMID:23112159

Astronomically calibrated 40Ar/39Ar age for the Toba supereruption and global synchronization of late Quaternary records

NASA Astrophysics Data System (ADS)

Storey, Michael; Roberts, Richard G.; Saidin, Mokhtar

2012-11-01

The Toba supereruption in Sumatra, ∼74 thousand years (ka) ago, was the largest terrestrial volcanic event of the Quaternary. Ash and sulfate aerosols were deposited in both hemispheres, forming a time-marker horizon that can be used to synchronize late Quaternary records globally. A precise numerical age for this event has proved elusive, with dating uncertainties larger than the millennial-scale climate cycles that characterized this period. We report an astronomically calibrated 40Ar/39Ar age of 73.88 ± 0.32 ka (1σ, full external errors) for sanidine crystals extracted from Toba deposits in the Lenggong Valley, Malaysia, 350 km from the eruption source and 6 km from an archaeological site with stone artifacts buried by ash. If these artifacts were made by Homo sapiens, as has been suggested, then our age indicates that modern humans had reached Southeast Asia by ∼74 ka ago. Our 40Ar/39Ar age is an order-of-magnitude more precise than previous estimates, resolving the timing of the eruption to the middle of the cold interval between Dansgaard-Oeschger events 20 and 19, when a peak in sulfate concentration occurred as registered by Greenland ice cores. This peak is followed by a ∼10 °C drop in the Greenland surface temperature over ∼150 y, revealing the possible climatic impact of the eruption. Our 40Ar/39Ar age also provides a high-precision calibration point for other ice, marine, and terrestrial archives containing Toba sulfates and ash, facilitating their global synchronization at unprecedented resolution for a critical period in Earth and human history beyond the range of 14C dating.

Astronomically calibrated 40Ar/39Ar age for the Toba supereruption and global synchronization of late Quaternary records.

PubMed

Storey, Michael; Roberts, Richard G; Saidin, Mokhtar

2012-11-13

The Toba supereruption in Sumatra, ∼74 thousand years (ka) ago, was the largest terrestrial volcanic event of the Quaternary. Ash and sulfate aerosols were deposited in both hemispheres, forming a time-marker horizon that can be used to synchronize late Quaternary records globally. A precise numerical age for this event has proved elusive, with dating uncertainties larger than the millennial-scale climate cycles that characterized this period. We report an astronomically calibrated (40)Ar/(39)Ar age of 73.88 ± 0.32 ka (1σ, full external errors) for sanidine crystals extracted from Toba deposits in the Lenggong Valley, Malaysia, 350 km from the eruption source and 6 km from an archaeological site with stone artifacts buried by ash. If these artifacts were made by Homo sapiens, as has been suggested, then our age indicates that modern humans had reached Southeast Asia by ∼74 ka ago. Our (40)Ar/(39)Ar age is an order-of-magnitude more precise than previous estimates, resolving the timing of the eruption to the middle of the cold interval between Dansgaard-Oeschger events 20 and 19, when a peak in sulfate concentration occurred as registered by Greenland ice cores. This peak is followed by a ∼10 °C drop in the Greenland surface temperature over ∼150 y, revealing the possible climatic impact of the eruption. Our (40)Ar/(39)Ar age also provides a high-precision calibration point for other ice, marine, and terrestrial archives containing Toba sulfates and ash, facilitating their global synchronization at unprecedented resolution for a critical period in Earth and human history beyond the range of (14)C dating.

40Ar/ 39Ar dating of micas from granites of NE Kibaran Belt (Karagwe-Ankolean), NW Tanzania

NASA Astrophysics Data System (ADS)

Ikingura, J. R.; Reynolds, P. H.; Watkinson, D. H.; Bell, K.

1992-11-01

40Ar/ 39Ar total gas ages of muscovites and biotites from granites associated with NE Kibaran belt (Karagwe-Ankolean) in NW Tanzanian are in the range of about 945-700 Ma, much less than the estimated age of the granites. Age gradients in the muscovite spectra are indicative of partial gas loss as a result of thermal overprinting. Evidence for at least two tectonothermal events, at ca. 950 Ma and ca. 700 Ma, is noted. The older of these correlates with the formation of tin-bearing pegmatites and hydrothermal veins in the Kibaran belt; the younger with vein emplacements in the Burundian and/or a deformational episode. Correlation of 40Ar/ 39Ar age data with K-Ar and Rb-Sr data from other parts of the Kibaran belt in Burundi, Rwanda and Zaire indicates that the NE Kibaran belt, east of the Western Rift, experienced a tectonothermal history similar to that of the western part of the during the late-Proterozoic.

The effect of thermal resetting and recrystallisation on white mica 40Ar/39Ar ages during retrograde metamorphism on Syros, Greece

NASA Astrophysics Data System (ADS)

Uunk, Bertram; Wijbrans, Jan; Brouwer, Fraukje

2015-04-01

White mica 40Ar/39Ar dating is a proven powerful tool for constraining timing of metamorphism, deformation and exhumation. However, in high-pressure metamorphic rocks, dating often results in wide age ranges which are not in agreement with constraints from other isotopic systems, indicating that geological and chemical processes complicate straightforward 40Ar/39Ar dating. In this research project, white mica ages from rocks of the Cycladic Blueschist Unit on Syros, Greece with contrasting rheology and strain mechanisms are compared, in order to better understand the role of deformation, recrystallization and fluid flow on 40Ar/39Ar ages of white mica during retrograde metamorphism. Resulting ages vary along different sections on the island, inconsistent with other isotopic constraints on eclogite-blueschist metamorphism (55-50 Ma) and greenschist overprinting (41-30 Ma). Two end-member models are possible: 1) Results represent continuous crystallization of white mica while moving from blueschist to greenschist conditions in the metamorphic P-T loop, or 2) white mica equilibrated in eclogite-blueschist conditions and their diffusion systematics were progressively perturbed during greenschist overprinting. The single grain fusion analyses yielded contrasting age distributions, which indicate contrasts in degree of re-equilibration during retrograde metamorphism. Step wise heating of larger grain populations resulted in flat plateau shapes, providing no evidence for partial resetting. Electron microprobe measurements of Si per formula unit, as a proxy for pressure during crystallisation, do not explain age variation within sections or on the island scale. The previously unreported north-south age trend and age ranges per sample, as shown only in the 40Ar/39Ar system of the metapelitic and marble lithologies, contains key information that will allow us to test between different scenarios for age formation. Excess argon infiltration at this stage seems to have been of

Illitization and paleothermal regimes in the Middle Ordovician St. Peter Sandstone, Central Michigan Basin: K-Ar, Oxygen Isotope, and fluid inclusion data

DOE Office of Scientific and Technical Information (OSTI.GOV)

Girard, J.P.; Barnes, D.A.

1995-01-01

Hydrocarbon reservoirs occur in the Middle Ordovician St. Peter Sandstone in the central Michigan basin at depths of 1.5-3.5 km and are diagenetically altered. Latest diagenetic cements include saddle dolomite, pervasive microcrystalline illite and chlorite, and quartz. A K-Ar and {sup 18}O/{sup 16}O study of the fine-grained authigenic illite in 25 samples from 16 wells covering a large area within the basin yields K-Ar ages ranging from 367 to 322 Ma and {delta}{sup 18}O values between 12.7 and 16.9% SMOW. The {delta}{sup 18}O values of diagenetic quartz overgrowths range from 15.2 to 18.9%. Fluid inclusion temperatures in the quartz cementmore » range from 70 to 170{degrees}C, reflecting multiple generations of diagenetic quartz and/or precipitation over most of the diagenetic history. Reequilibrated fluid inclusions in the saddle dolomite cement yield temperatures ranging from 90 to 150{degrees}C. A regionally significant episode of illitization occurred during the Late Devonian-Mississipian. Temperatures of illite formation are indirectly estimated to be in the range of 125-170{degrees}C and most paleodepths of illitization are between 2.8 and 3.2 km. These results imply that (1) illite formed from {sup 18}O-rich fluids, and (2) elevated geothermal gradients, i.e., greater than 34% C/km, existed in the Michigan basin in the late Paleozoic. The K-Ar ages and the {delta}{sup 18}O values are not correlated to present depths of the samples or paleodepths of illitization. Illites with young ages and low {delta}{sup 18}O values tend to be geographically distributed along the north-south branch of the buried Precambrian rift. The {delta}{sup 18}O values of the diagenetic quartz follow a similar trend. The spread of illite K-Ar ages and {delta}{sup 19}O values, and their geographic distribution, are best explained as reflecting abnormally high thermal regimes in the part of the basin located above the presumably highly fractured basement along the rift.« less

Tectono-metamorphic evolution of high-P/T and low-P/T metamorphic rocks in the Tia Complex, southern New England Fold Belt, eastern Australia: Insights from K-Ar chronology

NASA Astrophysics Data System (ADS)

Fukui, Shiro; Tsujimori, Tatsuki; Watanabe, Teruo; Itaya, Tetsumaru

2012-10-01

The Tia Complex in the southern New England Fold Belt is a poly-metamorphosed Late Paleozoic accretionary complex. It consists mainly of high-P/low-T type pumpellyite-actinolite facies (rare blueschist facies) schists, phyllite and serpentinite (T = 300 °C and P = 5 kbar), and low-P/high-T type amphibolite facies schist and gneiss (T = 600 °C and P < 5 kbar) associated with granodioritic plutons (Tia granodiorite). White mica and biotite K-Ar ages distinguish Carboniferous subduction zone metamorphism and Permian granitic intrusions, respectively. The systematic K-Ar age mapping along a N-S traverse of the Tia Complex exhibits a gradual change. The white mica ages become younger from the lowest-grade zone (339 Ma) to the highest-grade zone (259 Ma). In contrast, Si content of muscovite changes drastically only in the highest-grade zone. The regional changes of white mica K-Ar ages and chemical compositions of micas indicate argon depletion from precursor high-P/low-T type phengitic white mica during the thermal overprinting and recrystallization by granitoids intrusions. Our new K-Ar ages and available geological data postulate a model of the eastward rollback of a subduction zone in Early Permian. The eastward shift of a subduction zone system and subsequent magmatic activities of high-Mg andesite and adakite might explain formation of S-type granitoids (Hillgrove suite) and coeval low-P/high-T type metamorphism in the Tia Complex.

Targeted suppression of AR-V7 using PIP5K1α inhibitor overcomes enzalutamide resistance in prostate cancer cells.

PubMed

Sarwar, Martuza; Semenas, Julius; Miftakhova, Regina; Simoulis, Athanasios; Robinson, Brian; Gjörloff Wingren, Anette; Mongan, Nigel P; Heery, David M; Johnsson, Heather; Abrahamsson, Per-Anders; Dizeyi, Nishtman; Luo, Jun; Persson, Jenny L

2016-09-27

One mechanism of resistance of prostate cancer (PCa) to enzalutamide (MDV3100) treatment is the increased expression of AR variants lacking the ligand binding-domain, the best characterized of which is AR-V7. We have previously reported that Phosphatidylinositol-4-phosphate 5-kinase alpha (PIP5Kα), is a lipid kinase that links to CDK1 and AR pathways. The discovery of PIP5Kα inhibitor highlight the potential of PIP5K1α as a drug target in PCa. In this study, we show that AR-V7 expression positively correlates with PIP5K1α in tumor specimens from PCa patients. Overexpression of AR-V7 increases PIP5K1α, promotes rapid growth of PCa in xenograft mice, whereas inhibition of PIP5K1α by its inhibitor ISA-2011B suppresses the growth and invasiveness of xenograft tumors overexpressing AR-V7. PIP5K1α is a key co-factor for both AR-V7 and AR, which are present as protein-protein complexes predominantly in the nucleus of PCa cells. In addition, PIP5K1α and CDK1 influence AR-V7 expression also through AKT-associated mechanism dependent on PTEN-status. ISA-2011B disrupts protein stabilization of AR-V7 which is dependent on PIP5K1α, leading to suppression of invasive growth of AR-V7-high tumors in xenograft mice. Our study suggests that combination of enzalutamide and PIP5K1α may have a significant impact on refining therapeutic strategies to circumvent resistance to antiandrogen therapies.

Targeted suppression of AR-V7 using PIP5K1α inhibitor overcomes enzalutamide resistance in prostate cancer cells

PubMed Central

Sarwar, Martuza; Semenas, Julius; Miftakhova, Regina; Simoulis, Athanasios; Robinson, Brian; Wingren, Anette Gjörloff; Mongan, Nigel P.; Heery, David M.; Johnsson, Heather; Abrahamsson, Per-Anders; Dizeyi, Nishtman; Luo, Jun; Persson, Jenny L.

2016-01-01

One mechanism of resistance of prostate cancer (PCa) to enzalutamide (MDV3100) treatment is the increased expression of AR variants lacking the ligand binding-domain, the best characterized of which is AR-V7. We have previously reported that Phosphatidylinositol-4-phosphate 5-kinase alpha (PIP5Kα), is a lipid kinase that links to CDK1 and AR pathways. The discovery of PIP5Kα inhibitor highlight the potential of PIP5K1α as a drug target in PCa. In this study, we show that AR-V7 expression positively correlates with PIP5K1α in tumor specimens from PCa patients. Overexpression of AR-V7 increases PIP5K1α, promotes rapid growth of PCa in xenograft mice, whereas inhibition of PIP5K1α by its inhibitor ISA-2011B suppresses the growth and invasiveness of xenograft tumors overexpressing AR-V7. PIP5K1α is a key co-factor for both AR-V7 and AR, which are present as protein-protein complexes predominantly in the nucleus of PCa cells. In addition, PIP5K1α and CDK1 influence AR-V7 expression also through AKT-associated mechanism dependent on PTEN-status. ISA-2011B disrupts protein stabilization of AR-V7 which is dependent on PIP5K1α, leading to suppression of invasive growth of AR-V7-high tumors in xenograft mice. Our study suggests that combination of enzalutamide and PIP5K1α may have a significant impact on refining therapeutic strategies to circumvent resistance to antiandrogen therapies. PMID:27588408

Electron microprobe evaluation of terrestrial basalts for whole-rock K-Ar dating

USGS Publications Warehouse

Mankinen, E.A.; Brent, Dalrymple G.

1972-01-01

Four basalt samples for whole-rock K-Ar dating were analyzed with an electron microprobe to locate potassium concentrations. Highest concentrations of potassium were found in those mineral phases which were the last to crystallize. The two reliable samples had potassium concentrated in fine-grained interstitial feldspar and along grain boundaries of earlier formed plagioclase crystals. The two unreliable samples had potassium concentrated in the glassy matrix, demonstrating the ineffectiveness of basaltic glass as a retainer of radiogenic argon. In selecting basalt samples for whole-rock K-Ar dating, particular emphasis should be placed on determining the nature and condition of the fine-grained interstitial phases. ?? 1972.

40Ar/39Ar geochronology, paleomagnetism, and evolution of the Boring volcanic field, Oregon and Washington, USA

USGS Publications Warehouse

Fleck, Robert J.; Hagstrum, Jonathan T.; Calvert, Andrew T.; Evarts, Russell C.; Conrey, Richard M.

2014-01-01

The 40Ar/39Ar investigations of a large suite of fine-grained basaltic rocks of the Boring volcanic field (BVF), Oregon and Washington (USA), yielded two primary results. (1) Using age control from paleomagnetic polarity, stratigraphy, and available plateau ages, 40Ar/39Ar recoil model ages are defined that provide reliable age results in the absence of an age plateau, even in cases of significant Ar redistribution. (2) Grouping of eruptive ages either by period of activity or by composition defines a broadly northward progression of BVF volcanism during latest Pliocene and Pleistocene time that reflects rates consistent with regional plate movements. Based on the frequency distribution of measured ages, periods of greatest volcanic activity within the BVF occurred 2.7–2.2 Ma, 1.7–0.5 Ma, and 350–50 ka. Grouped by eruptive episode, geographic distributions of samples define a series of northeast-southwest–trending strips whose centers migrate from south-southeast to north-northwest at an average rate of 9.3 ± 1.6 mm/yr. Volcanic activity in the western part of the BVF migrated more rapidly than that to the east, causing trends of eruptive episodes to progress in an irregular, clockwise sense. The K2O and CaO values of dated samples exhibit well-defined temporal trends, decreasing and increasing, respectively, with age of eruption. Divided into two groups by K2O, the centers of these two distributions define a northward migration rate similar to that determined from eruptive age groups. This age and compositional migration rate of Boring volcanism is similar to the clockwise rotation rate of the Oregon Coast Range with respect to North America, and might reflect localized extension on the trailing edge of that rotating crustal block.

Mineralogy and Ar-39 - Ar-40 of an old pristine basalt: Thermal history of the HED parent body

NASA Technical Reports Server (NTRS)

Takeda, Hiroshi; Mori, Hiroshi; Bogard, Donald D.

1994-01-01

Previous investigations of mineral chemistry and Rb-Sr and Sm-Nd ages indicated that clast,84 from eucrite Yamato 75011 had preserved the pristine nature of its initial crystallization during an early stage of the HED parent body. Microscale mineralogy and Ar-39-Ar-40 ages of this clast, however, revealed local disturbance of microtextures and partially reset ages. This evidence suggests that, in addition to initial crystallization and rapid cooling, the Y75011,84 clast experienced shock deformation, reheating of short duration at higher temperature, and brecciation. These characteristics suggest two or more impact events. Fe-rich olivine filling fractures in pyroxene may have been introduced during the accompanying shock fracturing. The inferred Ar-39-Ar-40 degassing ages for Y75011 matrix and clast, 84 are 3.94 +/- 0.04 Ga and 3.98 +/- 0.03 Ga, respectively. The suggested degassing age for a clast from Y790020, believed to be paired with Y75011, is approximately 4.03 Ga, but could be younger. We consider it likely that all three samples experienced a common degassing event 3.95 +/- 0.05 Ga ago, but we cannot rule out two or more events spaced over a approximately 0.1 Ga interval. Higher temperature extractions of the two clast samples show significantly older apparent ages up to approximately 4.5 Ga and suggest that the time/temperature regime of this event was not sufficient to degas Ar totally. Most likely, the K-Ar ages were reset by thermal metamorphism associated with one or more impact events associated with shock fracturing, formation of Fe-rich olivine veins, and/or meteorite brecciation. The pyroxene annealing that commonly occurs in many eucrites is likely to be a much earlier process than the impact-produced textural changes and reset K-Ar ages observed in these meteorites. The existence of mineralogical and chronological evidence for metamorphism in an otherwise pristine eucrite suggests that the HED parent body experienced an extensive degree of

40Ar/39Ar chronology and paleomagnetism of Quaternary basaltic lavas from the Perşani Mountains (East Carpathians)

NASA Astrophysics Data System (ADS)

Panaiotu, C. G.; Jicha, B. R.; Singer, B. S.; Ţugui, A.; Seghedi, I.; Panaiotu, A. G.; Necula, C.

2013-08-01

Quaternary volcanism in the Perşani Mountains forms an Na-alkali basaltic province inside the bend area of the Carpathians in the southeastern part of Europe. Previous K-Ar ages and paleomagnetic data reveal several transitional virtual geomagnetic poles, which were tentatively associated with the Cobb Mountain subchron and a Brunhes chron excursion. We report a new paleomagnetic and rock-magnetic study coupled with 40Ar/39Ar geochronology to better constrain the age of geomagnetic reversals or excursions that might be recorded and the timing of volcanism. Of the paleomagnetic directions obtained from sampled lava flows 4 are reversed polarity, 19 are normal polarity and 16 have transitional polarity. 40Ar/39Ar plateau ages determined from incremental heating experiments on groundmass indicate that two of the reversely magnetized lavas erupted at 1142 ± 41 and 800 ± 25 ka, four of the normally magnetized lavas erupted at 1060 ± 10, 1062 ± 24, 684 ± 21, and 683 ± 28 ka, and two transitionally magnetized lavas formed at 1221 ± 11 and 799 ± 21 ka. Both the new 40Ar/39Ar ages and the paleomagnetic data suggest at least five episodes of volcanic activity with the most active periods during the Jaramillo and Brunhes chrons. This results shows that the last phases of alkalic and calc-alkaline magmatism in the South-East Carpathians were contemporaneous. The age of the older transitionally magnetized lava flow is within error of recent unspiked K-Ar and astrochronologic ages for the reversal that defines the onset of the Cobb Mountain normal polarity subchron. The age of the younger transitional lava is similar to that of an excursion that preceded the Matuyama-Brunhes polarity reversal and which has come to be known as the Matuyama-Brunhes precursor. Omitting the excursion data, the dispersion of the virtual geomagnetic poles (around 19°) is larger than the expected value around 45°N from the global compilation, but closer to the value obtained only from the

40Ar/39Ar ages for the fossil-bearing Gyeongsang Supergroup in South Korea

NASA Astrophysics Data System (ADS)

Chang, S. C.; Hemming, S. R.

2016-12-01

Since the 1970s, abundant vertebrate fossils have been documented from the Cretaceous Gyeongsang Supergroup in the Gyeongsang Basin and some small nearby basins of the Korean Peninsula, including dinosaurs, pterosaurs, crocodilians, turtles and fish. In addition to body fossils, well-preserved dinosaur, bird and pterosaur tracks have been found from these formations. Well-preserved and extensive vertebrate ichnofaunas from the Gyeongsang Supergroup represent the largest known concentration of Cretaceous vertebrate track sites reported from the Asian continent. Determining the age of the Gyeongsang Supergroup is critical to understanding several fundamental questions related to evolution and paleo-biogeography. However, limited radioisotopic studies for the Gyeongsang Supergroup have been previously reported. Additionally, the large uncertainties of previous data and the incomplete stratigraphic description of the samples limit their value for high-resolution chronostratigraphy. In this study, we aim to establish high-precision 40Ar/39Ar ages for two well-known tuffs from the middle and the upper part of the Gyeongsang Supergroup, and one rhyolite from the uppermost Gyeongsang Supergroup. Our preliminary 40Ar/39Ar data for the Kusandong Tuff indicates that the middle part of the Gyeongsang Supergroup is 78-82 Ma. This is consistent with the hypothesized extension of the Jehol biota into Korea and the preliminary results suggest that refinement of the time scale for these strata is a practical goal. The Gyeongsang Supergroup sample has great potential for substantially increasing our knowledge of Mesozoic terrestrial ecosystems.

Empirical test of an illite/muscovite 40Ar/39Ar thermochronometer

NASA Astrophysics Data System (ADS)

Verdel, C.; van der Pluijm, B. A.; Niemi, N. A.; Hall, C. M.

2010-12-01

Minerals which both preserve age information and indicate metamorphic conditions are particularly useful in thermochronology. Variations in sub-greenschist facies metamorphism have traditionally been quantified in terms of the illite to muscovite transition, a transformation which involves the growth of crystallites of increasing thickness at higher metamorphic temperatures. Thickness variations may influence Ar retention within these K-rich minerals, both in nature and during neutron irradiation. Along a transect in the southwestern US from the Grand Canyon to Death Valley, metamorphic conditions of a stratigraphic interval (the Middle Cambrian Bright Angel Shale and laterally equivalent Carrara Fm.) range from zeolite facies in the east to greenschist facies in the west, as determined by estimating illite crystallite thickness with X-ray diffraction. 40Ar/39Ar step-heating experiments were conducted on illite/muscovite-rich, micron to submicron grain sizes of these shales that were encapsulated in quartz tubes prior to irradiation. The proportion of 39Ar expelled during irradiation decreases in these samples as both crystallite thickness and grain size increases. Spectra from the least metamorphosed samples (diagenetic zone) are staircase-shaped and reach maximum ages that appear to reflect the age of detrital muscovite. Spectra from the highest grade samples (epizone) display partial plateaus and yield much younger maximum ages. Based on these findings we conclude that Ar can escape from illite via two processes: loss from low retention sites on crystallite edges and c-axis perpendicular volume diffusion. Based on our empirical data, the closure temperature of illite appears to lie at or near the anchizone-epizone bounday, or roughly 200-300 °C. Illite/muscovite thickness and 40Ar/39Ar data may therefore be useful for studies of detrital muscovite geochronology in very low grade shales and as a thermochronometer for higher grade pelites.

Study of inelastic processes in Li+-Ar, K+-Ar, and Na+-He collisions in the energy range 0.5-10 keV

NASA Astrophysics Data System (ADS)

Lomsadze, Ramaz A.; Gochitashvili, Malkhaz R.; Kezerashvili, Roman Ya; Schulz, Michael

2017-11-01

Absolute cross sections are measured for charge-exchange, ionization, and excitation processes within the same experimental setup for the Li{}+-Ar, K{}+-Ar, and Na{}+-He collisions in the ion energy range of 0.5-10 keV. The results of the measurements and schematic correlation diagrams are used to analyze and determine the mechanisms for these processes. The experimental results show that the charge-exchange processes occur with high probabilities and electrons are predominantly captured in ground states. The contributions of various partial inelastic channels to the total ionization cross section are estimated, and a primary mechanism for the process is identified. In addition, the energy-loss spectrum is applied in order to estimate the relative contribution of different inelastic channels, and to determine the mechanisms for the ionization and for some excitation processes of Ar resonance lines for the {{{K}}}+-Ar collision system. The excitation cross sections for the helium and for the sodium doublet lines for the Na{}+-He collision system both reveal some unexpected features. A mechanism to explain this observation is suggested.

40Ar/39Ar geochronology of submarine Mauna Loa volcano, Hawaii

NASA Astrophysics Data System (ADS)

Jicha, Brian R.; Rhodes, J. Michael; Singer, Brad S.; Garcia, Michael O.

2012-09-01

New geochronologic constraints refine the growth history of Mauna Loa volcano and enhance interpretations of the petrologic, geochemical, and isotopic evolution of Hawaiian magmatism. We report results of 40Ar/39Ar incremental heating experiments on low-K, tholeiitic lavas from the 1.6 km high Kahuku landslide scarp cutting Mauna Loa's submarine southwest rift zone, and from lavas in a deeper section of the rift. Obtaining precise40Ar/39Ar ages from young, tholeiitic lavas containing only 0.2-0.3 wt.% K2O is challenging due to their extremely low radiogenic 40Ar contents. Analyses of groundmass from 45 lavas yield 14 new age determinations (31% success rate) with plateau and isochron ages that agree with stratigraphic constraints. Lavas collected from a 1250 m thick section in the landslide scarp headwall were all erupted around 470 ± 10 ka, implying an extraordinary period of accumulation of ˜25 mm/yr, possibly correlating with the peak of the shield-building stage. This rate is three times higher than the estimated vertical lava accumulation rate for shield-building at Mauna Kea (8.6 ± 3.1 mm/yr) based on results from the Hawaii Scientific Drilling Project. Between ˜470 and 273 ka, the lava accumulation rate along the southwest rift zone decreased dramatically to ˜1 mm/yr. We propose that the marked reduction in lava accumulation rate does not mark the onset of post-shield volcanism as previously suggested, but rather indicates the upward migration of the magma system as Mauna Loa evolved from a submarine stage of growth to one that is predominantly subaerial, thereby cutting off supply to the distal rift zone. Prior to ˜250 ka, lavas with Loihi-like isotopic signatures were erupted along with lavas having typical Mauna Loa values, implying greater heterogeneity in the plume source earlier in Mauna Loa's growth. In addition to refining accumulation rates and the isotopic evolution of the lavas erupted along the southwest rift zone, our new40Ar/39Ar results

Chemical, petrographic, and K-Ar age data to accompany reconnaissance geologic strip map from Kingman to south of Bill Williams Mountain, Arizona

DOE Office of Scientific and Technical Information (OSTI.GOV)

Arney, B.; Goff, F.; Eddy, A.C.

1985-04-01

As part of a reconnaissance mapping project, 40 chemical analyses and 13 potassium-argon age dates were obtained for Tertiary volcanic and Precambrian granitic rocks between Kingman and Bill Williams Mountain, Arizona. The dated volcanic rocks range in age from 5.5 +- 0.2 Myr for basalt in the East Juniper Mountains to about 25 Myr for a biotite-pyroxene andesite. The date for Picacho Butte, a rhyodacite in the Mt. Floyd volcanic field, was 9.8 +- 0.07 Myr, making it the oldest rhyodacite dome in that volcanic field. Dated rocks in the Fort Rock area range from 20.7 to 24.3 Myr. Nomore » ages were obtained on the Precambrian rocks. Compositionally, the volcanic rocks analyzed range from alkali basalt to rhyolite, but many rocks on the western side of the map area are unusually potassic. The granites chosen for analysis include syenogranite from the Hualapai Mountains, a muscovite granite from the Picacho Butte area, and two other granites. The chemical and K-Ar age data and petrographic descriptions included in this report accompany the reconnaissance geologic strip map published as LA-9202-MAP by Goff, Eddy, and Arney. 9 refs., 4 figs., 2 tabs.« less

The tectono-thermal evolution of the Waterbury dome, western Connecticut, based on U-Pb and 40Ar/39Ar ages

USGS Publications Warehouse

Dietsch, Craig; Kunk, Michael J.; Aleinikoff, John; Sutter, John F.

2010-01-01

Level 3 nappes were emplaced over the Waterbury dome along an Acadian décollement synchronous with the formation of a D3 thrust duplex in the dome. The décollement truncates the Ky + Kfs-in (migmatite) isograd in the dome core and a St-in isograd in level 3 nappes, indicating that peak metamorphic conditions in the dome core and nappe cover rocks formed in different places at different times. Metamorphic overgrowths on zircon from the felsic orthogneiss in the Waterbury dome have an age of 387 ± 5 Ma. Rocks of all levels and the décollement are folded by D4 folds that have a strongly developed, regional crenulation cleavage and D5 folds. The Waterbury dome was formed by thrust duplexing followed by fold interference during the Acadian orogeny. The 40Ar/39Ar ages of amphibole, muscovite, biotite, and K-feldspar from above and below the décollement are ca. 378 Ma, 355 Ma, 360 Ma (above) and 340 (below), and 288 Ma, respectively. Any kilometer-scale vertical movements between dome and nappe rocks were over by ca. 378 Ma. Core and cover rocks of the Waterbury dome record synchronous, post-Acadian cooling.

Preservation of ancient impact ages on the R chondrite parent body: 40Ar/39Ar age of hornblende-bearing R chondrite LAP 04840

USGS Publications Warehouse

Righter, Kevin; Cosca, Michael A.; Morgan, Leah

2016-01-01

The hornblende- and biotite-bearing R chondrite LAP 04840 is a rare kind of meteorite possibly containing outer solar system water stored during metamorphism or postshock annealing deep within an asteroid. Because little is known regarding its age and origin, we determined 40Ar/39Ar ages on hornblende-rich separates of the meteorite, and obtained plateau ages of 4340(±40) to 4380(±30) Ma. These well-defined plateau ages, coupled with evidence for postshock annealing, indicate this meteorite records an ancient shock event and subsequent annealing. The age of 4340–4380 Ma (or 4.34–4.38 Ga) for this and other previously dated R chondrites is much older than most impact events recorded by ordinary chondrites and points to an ancient event or events that predated the late heavy bombardment that is recorded in so many meteorites and lunar samples.

Metasomatism in the Chain of Ponds K-feldspars: Reassessing Discrete Domain 39Ar-40Ar Modelling

NASA Astrophysics Data System (ADS)

Villa, I. M.; Chafe, A. N.; Hanchar, J. M.; Wirth, R.

2012-12-01

The post-crystallization petrology of K-feldspar (Kfs) is mostly controlled by fluids. Accordingly, [1] documented that successive mineral generations in Kfs grains of the Aar metagranite can be concordantly identified by both cathodoluminescence (CL) and back-scattered electron (BSE) imaging, elemental, and multi-isotope techniques. Imaging microstructures is a particularly powerful tool appreciated by many U-Pb geochronologists, and its use in 39Ar-40Ar dating is beginning to show beneficial progress [2]. However, a dissenting reviewer of [1] argued that the Aar sample was not typical of "orthodox" Kfs and the results could not be generalized to all Kfs [3]. On a different front, [4] demonstrated that the mathematical modeling that assumed Fickian diffusion in discrete domains, defined once and for all by [5], lacked internal consistency on several counts. As Chain of Ponds Pluton (CPP) Kfs sample MH-10 played a foundational role in the development of the mathematical model by [5], we decided to obtain direct evidence whether the numerous internal inconsistencies of the model are due to the previously undescribed petrological history of MH-10. We collected sample JH-02-01 on the same CPP outcrop as MH-10 [6]. All age spectra of different sieve fractions of the Kfs separate, both handpicked and unpicked, show a staircase shape. The Arrhenius diagram of apparent diffusivity agrees with the original MH-10 [5] and shows the same apparent r/r0 behavior. However, Arrhenius trajectories for all size fractions are parallel to each other and self-similar, as predicted by [4], instead of being joined at low temperature ("small domains") and diverging only at high T ("largest domain"), as would be implicit in the model by [5]. The CL and BSE images demonstrate several successive Kfs generations of diverse luminescence and chemical and isotopic properties. Microchemical analyses document patchy Ba enrichment, a tell-tale fingerprint of deuteric fluid interaction and

Interpretation of 40Ar/39Ar Age-spectra in Low-grade Polymetamorphic Rocks: The Importance of Petrologic constraints

NASA Astrophysics Data System (ADS)

Kunk, M. J.

2007-12-01

40Ar/39Ar age-spectra of white micas from low-grade polymetamorphic terranes can be difficult to interpret, because these low-grade rocks frequently have multiple generations of cleavage-forming white mica that grew at temperatures below their closure for diffusion of argon (~350°C). Under such conditions, each generation of cleavage-forming white mica will retain its original growth age. In addition, rocks from metamorphic terranes can also contain populations of detrital and diagenetic white micas which also have distinct ages associated with them. A final complication, frequently found in these low-grade polymetamorphic samples is the presence of inseparable, intergrown chlorite and the problems with associated 39Ar recoil during irradiation of the samples, prior to sample analysis. The shapes of 40Ar/39Ar age spectra of white micas separated from such samples are frequently complex and are quite variable in shape. Some age spectra show a steady increase in age with increase in the temperature of release, while others have sigmoidal or saddle-shaped patterns. The age of most of the steps in such age spectra are mixtures of the various age populations of white mica in the sample, and as such are geologically meaningless. Nonetheless, useful constraints on the ages of at least some events recorded in these complex low-grade polymetamorphic rocks can be discerned with the help of petrographic observations, and by placing the results in a spatial context to test for reproducibility and/or the occurrence of meaningful patterns. If no chlorite is present and the sample contains no detrital white mica, an estimate of the maximum age of the youngest component and the minimum age of the oldest cleavage forming white mica populations can frequently be discerned. If the sample contains detrital white mica, a minimum age for its cooling or crystallization can be estimated. The presence of a small amount of intimately intergrown chlorite masks the age of the youngest white

3-D Reconstructions of Subsurface Pleistocene Basalt Flows from Paleomagnetic Inclination Data and 40Ar/39Ar Ages in the Southern Part of the Idaho National Laboratory (INL), Idaho (USA)

NASA Astrophysics Data System (ADS)

Hodges, M. K.; Champion, D. E.; Turrin, B. D.; Swisher, C. C.

2012-12-01

The U. S. Geological Survey, in cooperation with the U.S. Department of Energy, is mapping the distribution of basalt flows and sedimentary interbeds at the Idaho National Laboratory in three dimensions to provide data for refining numerical models of groundwater flow and contaminant transport in the eastern Snake River Plain aquifer. Paleomagnetic inclination and polarity data from basalt samples from 47 coreholes are being used to create a three-dimensional (3-D) model of the subsurface of the southern part of the INL. Surface and sub-surface basalt flows can be identified in individual cores and traced in three dimensions on the surface and in the subsurface for distances of more than 20 km using a combination of paleomagnetic, stratigraphic, and 40Ar/39Ar data. Eastern Snake River Plain olivine tholeiite basalts have K2O contents of 0.2 to 1.0 weight per cent. In spite of the low-K content, high-precision 40Ar/39Ar ages were obtained by applying a protocol that employs short irradiation times (minimizing interferences from Ca derived 36Ar), frequent measurement of various size atmospheric Ar pipettes to monitor and correct for temporal variation, and signal size dependent nonlinearity in spectrometer mass bias, resulting in age dates with resolution generally between 2 to 10% of the age. 3-D models of subsurface basalt flows are being used to: (1) Estimate eruption volumes; (2) locate the approximate vent areas and extent of sub-surface flows; and (3) Help locate high and low transmissivity zones. Results indicate that large basalt eruptions (>3 km3) occurred at and near the Central Facilities Area between 637 ka and 360 ka; at and near the Radioactive Waste Management Complex before 540 ka; and north of the Naval Reactors Facility at about 580 ka. Since about 360 ka, large basalt flows have erupted along the Arco-Big Southern Butte Volcanic Rift Zone and the Axial Volcanic Zone, and flowed northerly towards the Central Facilities Area. Basalt eruptions shifted

3-D reconstructions of subsurface Pleistocene basalt flows from paleomagnetic inclination data and 40Ar/39Ar ages in the southern part of the Idaho National Laboratory (INL), Idaho (USA)

USGS Publications Warehouse

Hodges, Mary K. V.; Champion, Duane E.; Turrin, B.D.; Swisher, C. C.

2012-01-01

The U. S. Geological Survey, in cooperation with the U.S. Department of Energy, is mapping the distribution of basalt flows and sedimentary interbeds at the Idaho National Laboratory in three dimensions to provide data for refining numerical models of groundwater flow and contaminant transport in the eastern Snake River Plain aquifer. Paleomagnetic inclination and polarity data from basalt samples from 47 coreholes are being used to create a three-dimensional (3-D) model of the subsurface of the southern part of the INL. Surface and sub-surface basalt flows can be identified in individual cores and traced in three dimensions on the surface and in the subsurface for distances of more than 20 km using a combination of paleomagnetic, stratigraphic, and 40Ar/39Ar data. Eastern Snake River Plain olivine tholeiite basalts have K2O contents of 0.2 to 1.0 weight per cent. In spite of the low-K content, high-precision 40Ar/39Ar ages were obtained by applying a protocol that employs short irradiation times (minimizing interferences from Ca derived 36Ar), frequent measurement of various size atmospheric Ar pipettes to monitor and correct for temporal variation, and signal size dependent nonlinearity in spectrometer mass bias, resulting in age dates with resolution generally between 2 to 10% of the age. 3-D models of subsurface basalt flows are being used to: (1) Estimate eruption volumes; (2) locate the approximate vent areas and extent of sub-surface flows; and (3) Help locate high and low transmissivity zones. Results indicate that large basalt eruptions (>3 km3) occurred at and near the Central Facilities Area between 637 ka and 360 ka; at and near the Radioactive Waste Management Complex before 540 ka; and north of the Naval Reactors Facility at about 580 ka. Since about 360 ka, large basalt flows have erupted along the Arco-Big Southern Butte Volcanic Rift Zone and the Axial Volcanic Zone, and flowed northerly towards the Central Facilities Area. Basalt eruptions shifted

Rotationally inelastic collisions of He and Ar with NaK: Theory and Experiment

NASA Astrophysics Data System (ADS)

Price, T. J.; Towne, A. C.; Richter, K.; Jones, J.; Hickman, A. P.; Huennekens, J.; Faust, C.; Malenda, R. F.; Ross, A. J.; Crozet, P.; Talbi, D.; Forrey, R. C.

2016-05-01

Rotationally inelastic thermal collisions of NaK A1Σ+ molecules with He and Ar have been studied at Lehigh and Lyon. In both laboratories, a pump laser excites a particular ro-vibrational level A1Σ+ (v , J). Strong transitions from the pumped (v , J) level and weaker transitions from collisionally-populated levels (v ,J' = J + ΔJ) occur. Ratios of line intensities yield information about population and orientation transfer. At Lyon, we also identify v changing collisions. A strong propensity for ΔJ = even transitions is observed for He and Ar. Theoretical calculations are underway; we've calculated He-NaK and Ar-NaK potential surfaces using GAMESS and performed coupled channel scattering calculations for JM -->J'M' transitions. Semiclassical formulas for the cross sections have been obtained and agree well with our quantum mechanical calculations. Using the vector model, where J precesses with polar angle θ about the z-axis, we derived the distribution of final polar angles θ' and final M' states. We identify a special case where the θ' distribution is a Lorentzian centered at θ. Work supported by NSF, XSEDE and CNRS (PICS).

sup 40 Ar/ sup 39 Ar ages of six Apollo 15 impact melt rocks by laser step heating

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dalrymple, G.B.; Ryder, G.

1991-06-01

The authors have obtained 15 high resolution (21-51 step) {sup 40}Ar/{sup 39}Ar age spectra on six Apollo 15 impact melt rocks of different compositions using a continuous laser system on submilligram subsamples and on single crystal plagioclase clasts. Four of the six samples gave reproducible age spectra with well-defined intermediate temperature plateaus over 48% or more of the {sup 39}AR released; the plateaus are interpreted as crystallization ages. Samples 15304,7,69, 15294,6,21, and 15314,26,156 gave virtually identical plateau ages whose weighted mean is 3,870 {plus minus} 6 Ma. These three melt rocks differ in composition and likely formed in three separatemore » impact events. Sample 15356,9 gave replicate plateau ages that average 3,836 {plus minus} 12 Ma and date a fourth and younger impact event. The age spectra for samples 15308,9 and 15414,3,36 increase with increasing increment temperature and may have been formed in or affected by impacts at about 2,700 Ma and 3,870 Ma, respectively. So far there continues to be no convincing evidence in the lunar record for impact melts older than about 3.9 Ga.« less

40Ar/39Ar Data for White Mica, Biotite, and K-Feldspar Samples from Low-Grade Metamorphic Rocks in the Westminster Terrane and Adjacent Rocks, Maryland

USGS Publications Warehouse

Kunk, Michael J.; McAleer, Ryan J.

2008-01-01

This report contains reduced 40Ar/39Ar data of white mica and K-feldspar mineral separates and matrix of a whole rock phyllite, all from low-grade metamorphic rocks of the Westminster terrane and adjacent strata in central Maryland. This report presents these data in a preliminary form, but in more detail than can be accommodated in todays professional journals. Also included in this report is information on the location of the samples and a brief description of the samples. The data contained herein are not interpreted in a geological context, and care should be taken by readers unfamiliar with argon isotopic data in the use of these results; many of the individual apparent ages are not geologically meaningful. This report is primarily a detailed source document for subsequent publications that will integrate these data into a geological context.

On the origin of (4)He and (40)Ar in natural gold

NASA Technical Reports Server (NTRS)

Eugster, O.; Hofmann, B.; Niedermann, S.; Thalmann, CH.

1993-01-01

In a first report on our investigations of noble gases in native gold we demonstrated that placer gold contains an excess of radiogenic (4)He and (40)Ar relative to the concentrations expected from in situ decay of U, Th, and K, respectively, during the geologic age of about 30 Ma of the samples. We also showed that the U/Th-(4)He age of 36 Ma of vein-type gold from the Southern Alps agrees with its K-Ar formation age derived from associated muscovite and biotite. We now studied the question of the origin of the (4)He and (40)Ar excesses of placer gold. We conclude that gold contains two components of noble gases, a low-temperature component from fluid inclusions or phases which release noble gases at less than 800 C and a high-temperature component released when gold melts (1064 C). In some samples extremely high U and K concentrations or an unreasonably high formation age would be required to explain the observed (4)He abundances. Thus, trapped (4)He and (40)Ar must be present in gold.

A high-precision 40Ar/39Ar age for the Nördlinger Ries impact crater, Germany, and implications for the accurate dating of terrestrial impact events

NASA Astrophysics Data System (ADS)

Schmieder, Martin; Kennedy, Trudi; Jourdan, Fred; Buchner, Elmar; Reimold, Wolf Uwe

2018-01-01

40Ar/39Ar dating of specimens of moldavite, the formation of which is linked to the Ries impact in southern Germany, with a latest-generation ARGUS VI multi-collector mass spectrometer yielded three fully concordant plateau ages with a weighted mean age of 14.808 ± 0.021 Ma (± 0.038 Ma including all external uncertainties; 2σ; MSWD = 0.40, P = 0.67). This new best-estimate age for the Nördlinger Ries is in general agreement with previous 40Ar/39Ar results for moldavites, but constitutes a significantly improved precision with respect to the formation age of the distal Ries-produced tektites. Separates of impact glass from proximal Ries ejecta (suevite glass from three different surface outcrops) and partially melted feldspar particles from impact melt rock of the SUBO 18 Enkingen drill core failed to produce meaningful ages. These glasses show evidence for excess 40Ar introduction, which may have been incurred during interaction with hydrothermal fluids. Only partially reset 40Ar/39Ar ages could be determined for the feldspathic melt separates from the Enkingen core. The new 40Ar/39Ar results for the Ries impact structure constrain the duration of crater cooling, during the prevailing hydrothermal activity, to locally at least ∼60 kyr. With respect to the dating of terrestrial impact events, this paper briefly discusses a number of potential issues and effects that may be the cause for seemingly precise, but on a kyr-scale inaccurate, impact ages.

The Azores plume influence on the SASC-Great Meteor and MAR: the importance for the Portuguese Extension of the Continental Shelf Project (PECSP)

NASA Astrophysics Data System (ADS)

Ribeiro, Luisa P.; Madureira, Pedro; Hildenbrand, Anthony; Martins, Sofia; Mata, João

2017-04-01

The Southern Azores Seamount Chain (SASC) is a group of large seamounts located south of the Azores Plateau and east of the Mid-Atlantic Ridge (MAR) and part of the natural prolongation of the Azores land mass. The SASC, including the Great Meteor Seamount (aprox. 1000km south of São Miguel), is rooted on a flat, gently SE dipping Terrace, surrounded by steep scarps with almost 2000 m high. Only a few studies from the 70-80's discuss the geologic and/or geodynamic evolution of this region based on scarce bathymetry and geophysical data. Wendt et al. (1976) presented geochemical data and K-Ar ages on three basalt from the Great Meteor Seamount (<16Ma old), later analyzed for Sr-Nd-Pb isotopes by Geldmacher et al. (2006). Given the rarity of geochemical data, the origin of the seamounts and the regional evolution of this large area of the Atlantic, remains largely unknown. During the preparatory work of the PECSP, the EMEPC promoted three oceanographic campaigns to the SASC (2007, 2008 and 2009) with multidisciplinary teams. Within these cruises, more than 120 samples were dredged or collected with the Luso ROV (rated to 6000m depth) although less than 50 were suitable for major and trace elements analysis, for Sr-Nd-Pb-Hf isotopes and for K-Ar radiometric dating. Early studies relating the SASC with the New England Seamounts can be refuted by geophysical data and kinematic models presented by Gente et al. (2003) and, also by our new isotopic data, which shows that isotope ratios are clearly distinct from New England (Ribeiro et al., in prep). However, the analyzed SASC basalts display isotope ratios that overlap the Azores isotopic signature. Two new K-Ar ages (unspiked Cassignol-Gillot technique on fresh separated groundmass and/or plagioclase microlites) on the seamounts show coeval volcanism at Plato Seamount SE flank (33.4±0.5 Ma) an at Small Hyeres Seamount (31.7±0.5Ma). The SASC basalts erupted on the Terrace through an oceanic crust with 26Ma and 43Ma

40Ar/39Ar age-spectrum data for hornblende, biotite, white mica, and K-feldspar samples from metamorphic rocks in the Great Smoky Mountains of North Carolina and Tennessee

USGS Publications Warehouse

Kunk, Michael J.; McAleer, Ryan J.

2011-01-01

This report contains reduced 40Ar/39Ar data of hornblende, biotite, white mica and (or) sericite, and potassium-feldspar mineral separates and phyllite groundmass samples from metamorphic rocks of the Great Smoky Mountains in North Carolina and Tennessee. Included in this report are information on the location of the samples and a brief description of the samples. The data contained herein are not interpreted in a geological context, and care should be taken by users unfamiliar with argon isotopic data in the use of these results. No geological meaning is implied for any of the apparent ages presented below, and many of the individual apparent ages are not geologically meaningful. This report is primarily a detailed source document for subsequent publications that will integrate these data into a geological context. All the samples in this report were collected in and around the Great Smoky Mountain National Park in western North Carolina and eastern Tennessee.

40Ar/39Ar ages for deep (˜3.3 km) samples from the Hawaii Scientific Drilling Project, Mauna Kea volcano, Hawaii

NASA Astrophysics Data System (ADS)

Jourdan, Fred; Sharp, Warren D.; Renne, Paul R.

2012-05-01

The Hawaii Scientific Drilling Project recovered core from a 3.5 km deep hole from the flank of Mauna Kea volcano, providing a long, essentially continuous record of the volcano's physical and petrologic development that has been used to infer the chemical and physical characteristics of the Hawaiian mantle plume. Determining a precise accumulation rate via 40Ar/39Ar dating of the shield-stage tholeiites, which constitute 95-98% of the volcano's volume is challenging. We applied40Ar/39Ar dating using laser- and furnace-heating in two laboratories (Berkeley and Curtin) to samples of two lava flows from deep in the core (˜3.3 km). All determinations yield concordant isochron ages, ranging from 612 ± 159 to 871 ± 302 ka (2σ; with P ≥ 0.90). The combined data yield an age of 681 ± 120 ka (P = 0.77) for pillow lavas near the bottom of the core. This new age, when regressed with 40Ar/39Ar isochron ages previously obtained for tholeiites higher in the core, defines a constant accumulation rate of 8.4 ± 2.6 m/ka that can be used to interpolate the ages of the tholeiites in the HSDP core with a mean uncertainty of about ±83 ka. For example at ˜3300 mbsl, the age of 664 ± 83 ka estimated from the regression diverges at the 95% confidence level from the age of 550 ka obtained from the numerical model of DePaolo and Stolper (1996). The new data have implications for the timescale of the growth of Hawaiian volcanoes, the paleomagnetic record in the core, and the dynamics of the Hawaiian mantle plume.

AR-12 suppresses dengue virus replication by down-regulation of PI3K/AKT and GRP78.

PubMed

Chen, Hsin-Hsin; Chen, Chien-Chin; Lin, Yee-Shin; Chang, Po-Chun; Lu, Zi-Yi; Lin, Chiou-Feng; Chen, Chia-Ling; Chang, Chih-Peng

2017-06-01

Dengue virus (DENV) infection has become a public health issue of worldwide concern and is a serious health problem in Taiwan, yet there are no approved effective antiviral drugs to treat DENV. The replication of DENV requires both viral and cellular factors. Targeting host factors may provide a potential antiviral strategy. It has been known that up-regulation of PI3K/AKT signaling and GRP78 by DENV infection supports its replication. AR-12, a celecoxib derivative with no inhibiting activity on cyclooxygenase, shows potent inhibitory activities on both PI3K/AKT signaling and GRP78 expression levels, and recently has been found to block the replication of several hemorrhagic fever viruses. However the efficacy of AR-12 in treating DENV infection is still unclear. Here, we provide evidence to show that AR-12 is able to suppress DENV replication before or after virus infection in cell culture and mice. The antiviral activities of AR-12 are positive against infection of the four different DENV serotypes. AR-12 significantly down-regulates the PI3K/AKT activity and GRP78 expression in DENV infected cells whereas AKT and GRP78 rescue are able to attenuate anti-DENV effect of AR-12. Using a DENV-infected suckling mice model, we further demonstrate that treatment of AR-12 before or after DENV infection reduces virus replication and mice mortality. In conclusion, we uncover the potential efficacy of AR-12 as a novel drug for treating dengue. Copyright © 2017 Elsevier B.V. All rights reserved.

Improvement of Vergence Movements by Vision Therapy Decreases K-ARS Scores of Symptomatic ADHD Children.

PubMed

Lee, Sun Haeng; Moon, Byeong-Yeon; Cho, Hyun Gug

2014-02-01

[Purpose] To determine whether the improvement of vergence movements by vision therapy can decrease the K-ARS scores of symptomatic ADHD children. [Methods] Eighty-one out of 1,123 children surveyed using the K-ARS, a parents'-reported questionnaire, led to 16 of these 81 children being showed scores of ≥19, and measurement of binocular function diagnosed as having convergence insufficiency. The 16 children were divided equally into a control group and a vision therapy group. [Results] After vision therapy for 12 weeks, near point convergence (4.38±0.69 cm) significantly neared compared to the near point convergence before vision therapy (11.50±2.28 cm), and both the break point (32.38±2.53 Δ) and recovery point (19.75±2.11 Δ) of near positive fusional vergence significantly improved compared to their values before vision therapy (15.88±2.64 Δ, 6.38±6.70 Δ, respectively). Near exophoria after vision therapy (7.81±2.00 Δ BI) significantly decreased compared to its value before vision therapy (12.00±1.16 Δ BI). The K-ARS scores referring to symptomatic ADHD significantly decreased after vision therapy (17.13±2.84) compared to before vision therapy (23.25±1.49). [Conclusions] Convergence insufficiency symptoms are closely related to symptoms screened for ADHD, and vision therapy to improve vergence movements is an effective method of decreasing the K-ARS scores.

An Early Jurassic 40Ar/39Ar Age for the Puchezh-Katunki Impact Structure (Russia) — No Causal Link to an Extinction Event

NASA Astrophysics Data System (ADS)

Holm-Alwmark, S.; Alwmark, C.; Lindström, S.; Ferrière, L.; Scherstén, A.; Masaitis, V. L.; Mashchak, M. S.; Naumov, M. V.

2016-08-01

We propose a revised age of 192.0 ± 0.8 Ma for the formation of the Puchezh-Katunki impact structure, Russia, based on 40Ar/39Ar step-heating analyses of five impact melt rock samples. This age does not correlate with any known extinction event.

Ar-40/Ar-39 age of the Shergotty achondrite and implications for its post-shock thermal history

NASA Technical Reports Server (NTRS)

Bogard, D. D.; Nyquist, L. E.; Husain, L.

1979-01-01

Ar-40/Ar-39 measurements are used to determine the age of the Shergotty achondrite and the chronology of the shock event responsible for the complete conversion of its plagioclase to maskelynite is discussed. Apparent ages are found to vary between 240 and 640 million years for the whole rock sample, with a plateau age of 254 million years for a maskelynite separate. The Rb-Sr age of 165 million years determined by Nyquist at al (1978) suggests that the maskelynite as well as the whole rock was incompletely degassed. Argon diffusion characteristics indicate a post-shock cooling time greater than 1000 years and a burial depth greater than 300 m for a thermal model of a cooling ejecta blanket of variable thickness. It is concluded that the shock event which degassed the argon and reset the Rb-Sr systematics occurred between 165 and 250 million years ago when the parent body experienced a collision in the asteroid belt.

Age of the youngest volcanism at Eagle Lake, northeastern California—40Ar/39Ar and paleomagnetic results

USGS Publications Warehouse

Clynne, Michael A.; Calvert, Andrew T.; Champion, Duane E.; Muffler, L.J.P.; Sawlan, Michael G.; Downs, Drew T.

2017-03-22

The age of the youngest volcanism at Eagle Lake, California, was investigated using stratigraphic, paleomagnetic, and 40Ar/39Ar techniques. The three youngest volcanic lava flows at Eagle Lake yielded ages of 130.0±5.1, 127.5±3.2 and 123.6±18.7 ka, and are statistically indistinguishable. Paleomagnetic results demonstrate that two of the lava flows are very closely spaced in time, whereas the third is different by centuries to at most a few millennia. These results indicate that the basalt lava flows at Eagle Lake are not Holocene in age, and were erupted during an episode of volcanism at about 130–125 ka that is unlikely to have spanned more than a few thousand years. Thus, the short-term potential for subsequent volcanism at Eagle Lake is considered low. 

40Ar/39Ar Dating of Zn-Pb-Ag Mineralization in the Northern Brooks Range, Alaska

USGS Publications Warehouse

Werdon, Melanie B.; Layer, Paul W.; Newberry, Rainer J.

2004-01-01

completely recrystallized. The 40Ar/39Ar age spectra and inverse isochron plots of the multicomponent whole-rock sandstone samples are more complex than those of single minerals. However, different minerals have different Ca/K and Cl/K ratios and closure temperatures, and these properties were used to identify portions of spectra dominated by argon release from specific minerals. 40Ar/39Ar laser step-heating analyses of Late Devonian sandstone whole rocks produced spectra that record a two-stage resetting history: a Carboniferous hydrothermal event first and later Mesozoic to Tertiary events, which are in agreement with geologic constraints. The 40Ar/39Ar ages and the similar mineralogy, lead isotope composition, and relative stratigraphic positions support the interpretation that the shale-hosted massive sulfide deposits and most vein-breccia occurrences are temporally and genetically related, and that they are different expressions of Carboniferous basinal dewatering.

High-precision 40Ar/39Ar sanidine geochronology of ignimbrites in the Mogollon-Datil volcanic field, southwestern New Mexico

USGS Publications Warehouse

McIntosh, W.C.; Sutter, J.F.; Chapin, C.E.; Kedzie, L.L.

1990-01-01

40Ar/39Ar age spectra have been obtained from 85 sanidine separates from 36 ignimbrites and one rhyolitic lava in the latest Eocene-Oligocene Mogollon-Datil volcanic field of southwestern New Mexico. Of the 97 measured age spectra, 94 yield weighted-mean plateau ages each giving single-spectrum 1?? precision of??0.25%-0.4% (??0.07-0.14 Ma). Replicate plateau age determinations for eight different samples show within-sample 1?? precisions averaging ??0.25%. Plateau ages from multiple (n=3-8) samples of individual ignimbrites show 1?? within-unit precision of ??0.1%-0.4% (??0.04-0.13 Ma). This within-unit precision represents a several-fold improvement over published K-Ar data for the same ignimbrites, and is similar to the range of precisions reported from single-crystal laser fusion studies. A further indication of the high precision of unit-mean 40Ar/30Ar ages is their close agreement with independently established stratigraphic order. Two samples failed to meet plateau criteria, apparently due to geologic contamination by older feldspars. Effects of minor contamination are shown by six other samples, which yielded slightly anomalous plateau ages. 40Ar/39Ar plateau ages permit resolution of units differing in age by 0.5% (0.15 Ma) or less. This high resolution, combined with paleomagnetic studies, has helped to correlate ignimbrites among isolated ranges and has allowed development of an integrated timestratigraphic framework for the volcanic field. Mogollon-Datil ignimbrites range in age from 36.2 to 24.3 Ma. Ignimbrite activity was strongly episodic, being confined to four brief (<2.6 m.y.) eruptive episodes separated by 1-3 m.y. gaps. Ignimbrite activity generally tended to migrate from the southeast toward the north and west. ?? 1990 Springer-Verlag.

Mitigation of ^{42}Ar/^{42}K background for the GERDA Phase II experiment

NASA Astrophysics Data System (ADS)

Lubashevskiy, A.; Agostini, M.; Budjáš, D.; Gangapshev, A.; Gusev, K.; Heisel, M.; Klimenko, A.; Lazzaro, A.; Lehnert, B.; Pelczar, K.; Schönert, S.; Smolnikov, A.; Walter, M.; Zuzel, G.

2018-01-01

Background coming from the ^{42}Ar decay chain is considered to be one of the most relevant for the Gerda experiment, which searches for the neutrinoless double beta decay of ^{76}Ge. The sensitivity strongly relies on the absence of background around the Q-value of the decay. Background coming from ^{42}K, a progeny of ^{42}Ar, can contribute to that background via electrons from the continuous spectrum with an endpoint at 3.5 MeV. Research and development on the suppression methods targeting this source of background were performed at the low-background test facility LArGe . It was demonstrated that by reducing ^{42}K ion collection on the surfaces of the broad energy germanium detectors in combination with pulse shape discrimination techniques and an argon scintillation veto, it is possible to suppress ^{42}K background by three orders of magnitude. This is sufficient for Phase II of the Gerda experiment.

Experimental characterization of elastomeric O-rings as reusable seals for mass spectrometric measurements: Application to in situ K-Ar dating on Mars

NASA Astrophysics Data System (ADS)

Cho, Yuichiro; Kameda, Shingo; Okuno, Mamoru; Horiuchi, Misa; Shibasaki, Kazuo; Wagatsuma, Ryo; Aida, Yusuke; Miura, Yayoi N.; Yoshioka, Kazuo; Okazaki, Ryuji; Sugita, Seiji

2017-10-01

Mass spectrometry has been widely used in lander missions to characterize the volatiles in rocks and soils on planetary surfaces. A good vacuum seal is very important for introducing such solid samples to a vacuum chamber and ejecting them. However, multiple measurements require many metal gaskets, leading to extra weight and complexity for the instruments. In this study, we investigate the capability of three kinds of elastomeric O-rings (Viton, Nexus-SLT, and Nexus-FV) as vacuum seals for mass spectrometric measurements, particularly for in situ K-Ar dating on Mars. First, thermal cycle tests revealed that low-temperature-resistant O-rings can maintain pressure <10-5 Pa at -60 °C under 1 bar ambient pressure, whereas Viton O-rings leaked at -25 °C. Then, the amount of 40Ar due to outgassing from the O-rings and permeation under the ambient pressure of 650 Pa or 3 Pa was measured and compared with the amounts of 40Ar that a flight-equivalent laser would liberate from potential target Martian rocks. The measured amounts were <1% of that a target rock with 5000 ppm K2O and an age of 4.2 Ga would yield. These results suggest that a Viton O-ring can maintain the Ar blank low under the Mars atmospheric pressure when temperatures are higher than -25 °C. A double O-ring seal using the low-temperature-resistant elastomers would be an alternative approach at lower temperatures. The elastomeric O-rings would be useful for constructing a small and light-weighted mass spectrometric instrument for in situ K-Ar dating on Mars.

High-resolution 40Ar 39Ar chronology of Oligocene volcanic rocks, San Juan Mountains, Colorado

USGS Publications Warehouse

Lanphere, M.A.

1988-01-01

The central San Juan caldera complex consists of seven calderas from which eight major ash-flow tuffs were erupted during a period of intense volcanic activity that lasted for approximately 2 m.y. about 26-28 Ma. The analytical precision of conventional K-Ar dating in this time interval is not sufficient to unambiguously resolve this complex history. However, 40Ar 39Ar incremental-heating experiments provide data for a high-resolution chronology that is consistent with stratigraphie relations. Weighted-mean age-spectrum plateau ages of biotite and sanidine are the most precise with standard deviations ranging from 0.08 to 0.21 m.y. The pooled estimate of standard deviation for the plateau ages of 12 minerals is about 0.5 percent or about 125,000 to 135,000 years. Age measurements on coexisting minerals from one tuff and on two samples of each of two other tuffs indicate that a precision in the age of a tuff of better than 100,000 years can be achieved at 27 Ma. New data indicate that the San Luis caldera is the youngest caldera in the central complex, not the Creede caldera as previously thought. ?? 1988.

Successive reactivation of older structures under variable heat flow conditions evidenced by K-Ar fault gouge dating in Sierra de Ambato, northern Argentine broken foreland

NASA Astrophysics Data System (ADS)

Nóbile, Julieta C.; Collo, Gilda; Dávila, Federico M.; Martina, Federico; Wemmer, Klaus

2015-12-01

The Argentine broken foreland has been the subject of continuous research to determine the uplift and exhumation history of the region. High-elevation mountains are the result of N-S reverse faults that disrupted a W-E Miocene Andean foreland basin. In the Sierra de Ambato (northern Argentine broken foreland) the reverse faults offset Neogene sedimentary rocks (Aconquija Fm., ˜9 Ma) and affect the basement comprising Paleozoic metamorphic rocks that have been dated at ˜477-470 Ma. In order to establish a chronology of these faults affecting the previous continuous basin we date the formation age of clay minerals associated with fault gouge using the K-Ar dating technique. Clay mineral formation is a fundamental process in the evolution of faults under the brittle regime (<<300 °C). K-Ar ages (9 fractions from 3 samples collected along a transect in the Sierra de Ambato) vary from Late Devonian to Late Triassic (˜360-220 Ma). This age distribution can be explained by a long lasting brittle deformation history with a minimum age of ˜360 Ma and a last clay minerals forming event at ˜220 Ma. Moreover, given the progression of apparent ages decreasing from coarse to fine size fractions (˜360-311 Ma for 2-1 μm grain size fraction, ˜326-286 Ma for 1-0.2 μm and ˜291-219 Ma of <0.2 μm), we modeled discrete deformation events at ˜417 Ma (ending of the Famatinian cycle), ˜317-326 Ma (end of Gondwanic orogeny), and ˜194-279 Ma (Early Permian - Jurassic deformation). According to our data, the Neogene reactivation would not have affected the K-Ar system neither generated a significant clay minerals crystallization in the fault gouge, although an exhumation of more than 2 Km is recorded in this period from stratigraphic data.

40Ar 39Ar age constraints on neogene sedimentary beds, Upper Ramparts, half-way Pillar and Canyon village sites, Porcupine river, east-central Alaska

USGS Publications Warehouse

Kunk, Michael J.; Rieck, H.; Fouch, T.D.; Carter, L.D.

1994-01-01

40Ar 39Ar ages of volcanic rocks are used to provide numerical constraints on the age of middle and upper Miocene sedimentary strata collected along the Porcupine River. Intercalated sedimentary rocks north of latitude 67??10???N in the Porcupine terrane of east-central Alaska contain a rich record of plant fossils. The fossils are valuable indicators of this interior region's paleoclimate during the time of their deposition. Integration of the 40Ar 39Ar results with paleomagnetic and sedimentological data allows for refinements in estimating the timing of deposition and duration of selected sedimentary intervals. 40Ar 39Ar plateau age spectra, from whole rock basalt samples, collected along the Upper Ramparts and near Half-way Pillar on the Porcupine River, range from 15.7 ?? 0.1 Ma at site 90-6 to 14.4 ?? 0.1 Ma at site 90-2. With exception of the youngest basalt flow at site 90-2, all of the samples are of reversed magnetic polarity, and all 40Ar 39Ar age spectrum results are consistent with the deposition of the entire stratigraphic section during a single interval of reversed magnetic polarity. The youngest flow at site 90-2 was emplaced during an interval of normal polarity. With age, paleomagnetic and sedimentological data, the ages of the Middle Miocene sedimentary rocks between the basalt flows at sites 90-1 and 90-2 can be assigned to an interval within the limits of analytical precision of 15.2 ?? 0.1 Ma; thus, the sediments were deposited during the peak of the Middle Miocene thermal maximum. Sediments in the upper parts of sites 90-1 and 90-2 were probably deposited during cooling from the Middle Miocene thermal maximum. 40Ar 39Ar results of plagioclase and biotite from a single tephra, collected at sites 90-7 and 90-8 along the Canyon Village section of the Porcupine River, indicate an age of 6.57 ?? 0.02 Ma for its time of eruption and deposition. These results, together with sedimentological and paleomagnetic data, suggest that all of the Upper

In situ dating on Mars: A new approach to the K–Ar method utilizing cosmogenic argon

DOE PAGES

Cassata, William S.

2013-08-22

In this paper, cosmogenic argon isotopes are produced in feldspars via nuclear reactions between cosmic rays and Ca and K atoms within the lattice. These cosmogenic isotopes can be used as proxies for K and Ca, much like nuclear reactor-derived 39Ar and 37Ar are used as proxies for K and Ca, respectively, in 40Ar/ 39Ar geochronology. If Ca and K are uniformly distributed, then the ratio of radiogenic 40Ar ( 40Ar*) to cosmogenic 38Ar or 36Ar ( 38Ar cos or 36Ar cos) is proportional to the difference between the radioisotopic and exposure ages, as well as the K/Ca ratio ofmore » the degassing phase. Thus cosmogenic, radiogenic, and trapped Ar isotopes, all of which can be measured remotely and are stable over geologic time, are sufficient to generate an isochron-like diagram from which the isotopic composition of the trapped component may be inferred. Finally, such data also provide a means to assess the extent to which the system has remained closed with respect to 40Ar*, thereby mitigating otherwise unquantifiable uncertainties that complicate the conventional K–Ar dating method.« less

Call for Development of New Mineral Standards for 40Ar/39Ar Dating

NASA Astrophysics Data System (ADS)

Deino, A. L.; Turrin, B. D.; Renne, P. R.; Hemming, S. R.

2015-12-01

Age determination via the 40Ar/39Ar dating method relies on the intercomparison of measured 40Ar*/39ArK ratios of geological unknowns with those of co-irradiated mineral standards. Good analytical procedure dictates that these ratios (and the evolution of the Ar ion beams underpinning them) be as similar as practical for the greatest accuracy. Unfortunately, throughout several intervals of the geological time scale this 'best practice' cannot be achieved with existing widely used standards. Only two internationally utilized sanidine standards are available for the middle to late Cenozoic: the Alder Creek Rhyolite sanidine (ACs), at ~1.2 Ma (Turrin et al., 1994; Nomade et al., 2005), and the Fish Canyon Tuff sanidine (FCs) at ~28.2 Ma (e.g., Kuiper et al., 2008; Renne et al, 2011). The situation is even worse throughout much of the rest of the Phanerozoic, as the next oldest standard in common use is the Hb3gr hornblende standard with an age of ~1.1 Ga (Turner, 1971; Jourdan et al., 2006). We propose, as a community effort, the development a set of standards covering the entire target range of high-precision 40Ar/39Ar dating, i.e. the Phanerozoic. Their ages would be stepped in a regular fashion with no more than approximately a factor of 3 between standards, such that in the worse case the 40Ar*/39Ar ratios of standards and unknown need differ by no more than a factor of two. While somewhat arbitrary, an approximately 3 X age progression allows the entire time scale to be covered by a manageable number of standards. Anchoring the progression in the widely used ACs, FCs, and Hb3gr (in bold, below) yields the following set of suggested standard ages: 0.4, 1.2, 3.3, 9.4, 28.2, 95, 320, and 1100 Ma. A suitable standard should be highly reproducible in age at the grain-to-grain and sub-grain levels, and highly radiogenic. The mineral should be abundant and easily separated from the host rock. These criteria may be most easily achieved by focusing on sanidine phenocrysts

Phanerozoic polyphase orogenies recorded in the northeastern Okcheon Belt, Korea from SHRIMP U-Pb detrital zircon and K-Ar illite geochronologies

NASA Astrophysics Data System (ADS)

Jang, Yirang; Kwon, Sanghoon; Song, Yungoo; Kim, Sung Won; Kwon, Yi Kyun; Yi, Keewook

2018-05-01

We present the SHRIMP U-Pb detrital zircon and K-Ar illite 1Md/1M and 2M1 ages, suggesting new insight into the Phanerozoic polyphase orogenies preserved in the northeastern Okcheon Belt, Korea since the initial basin formation during Neoproterozoic rifting through several successive contractional orogens. The U-Pb detrital zircon ages from the Early Paleozoic strata of the Taebaeksan Zone suggest a Cambrian maximum deposition age, and are supported by trilobite and conodont biostratigraphy. Although the age spectra from two sedimentary groups, the Yeongwol and Taebaek Groups, show similar continuous distributions from the Late Paleoproterozoic to Early Paleozoic ages, a Grenville-age hiatus (1.3-0.9 Ga) in the continuous stratigraphic sequence from the Taebaek Group suggests the existence of different peripheral clastic sources along rifted continental margin(s). In addition, we present the K-Ar illite 1Md/1M ages of the fault gouges, which confirm fault formation/reactivation during the Late Cretaceous to Early Paleogene (ca. 82-62 Ma) and the Early Miocene (ca. 20-18 Ma). The 2M1 illite ages, at least those younger than the host rock ages, provide episodes of deformation, metamorphism and hydrothermal effects related to the tectonic events during the Devonian (ca.410 Ma) and Permo-Triassic (ca. 285-240 Ma). These results indicate that the northeastern Okcheon Belt experienced polyphase orogenic events, namely the Okcheon (Middle Paleozoic), Songrim (Late Paleozoic to Early Mesozoic), Daebo (Middle Mesozoic) and Bulguksa (Late Mesozoic to Early Cenozoic) Orogenies, reflecting the Phanerozoic tectonic evolution of the Korean Peninsula along the East Asian continental margin.

Experimental Determination of the Cosmogenic Ar Production Rate From Ca

NASA Astrophysics Data System (ADS)

Niedermann, S.; Schäfer, J. M.; Wieler, R.; Naumann, R.

2005-12-01

Cosmogenic 38Ar is produced in terrestrial surface rocks by spallation of target nuclides, in particular K and Ca. Though the presence of cosmogenic Ar in Ca-rich minerals has been demonstrated earlier [1], is has proven difficult to establish its production rate. To circumvent problems connected to 36Ar production by 35Cl neutron capture and different production rates from K and Ca, we have analyzed the noble gases in seven pyroxene separates (px) from the Antarctic Dry Valleys which are essentially free of Cl and K. The px were obtained from dolerite rocks, for which 3He and 21Ne exposure ages from 1.5 to 6.5 Ma have been reported [2]. The noble gases were extracted in two or three heating steps at GFZ Potsdam, yielding 38Ar/36Ar ratios up to 0.2283 ± 0.0008 (air: 0.1880). Ca (3.7-11.2 wt. %) is expected to be the only relevant target element for Ar production in the five pure px (<0.05% K); the production rate from Fe is at least two orders of magnitude lower than that from Ca [e.g. 3]. Assuming an 38Ar/36Ar production ratio of 1.5 ± 0.2, we obtain cosmogenic 38Ar concentrations between 130 and 530x106 atoms/g. The 38Ar production rate was calculated based on 21Ne exposure ages [2], corrected for elevated nuclide production in Antarctica due to prevailing low air pressure and for the revised 21Ne production rate from Si. We obtain values between 188 ± 17 and 243 +110/-24 atoms (g Ca)-1 a-1 at sea level and high (northern) latitudes for four out of the five pure px, while one yields a very high value of 348 ± 70 atoms (g Ca)-1 a-1. Values above 250 atoms (g Ca)-1 a-1 are also obtained from two less pure px containing 0.3 and 0.9% K and from one feldspar/quartz accumulate, indicating that the production rate from K may be higher than that from Ca. The weighted mean (excluding the outlier) of ~200 atoms (g Ca)-1 a-1 is in excellent agreement with Lal's [3] theoretical estimate. [1] Renne et al., EPSL 188 (2001) 435. [2] Schäfer et al., EPSL 167 (1999) 215. [3

The age of the Keystone thrust: laser-fusion 40Ar/39Ar dating of foreland basin deposits, southern Spring Mountains, Nevada

USGS Publications Warehouse

Fleck, R.J.; Carr, M.D.

1990-01-01

Nonmarine sedimentary and volcaniclastic foreland-basin deposits in the Spring Mountains are cut by the Contact and Keystone thrusts. These synorogenic deposits, informally designated the Lavinia Wash sequence by Carr (1980), previously were assigned a Late Jurassic to Early Cretaceous(?) age. New 40Ar.39Ar laser-fusion and incremental-heating studies of a tuff bed in the Lavinia Wash sequence support a best estimate age of 99.0 ?? 0.4 Ma, indicating that the Lavinia Wash sequence is actually late Early Cretaceous in age and establishing a maximum age for final emplacement of the Contact and Keystone thrust plates consistent with the remainder of the Mesozoic foreland thrust belt. -from Authors

40Ar/ 39Ar age and oxygen isotope temperature of the Centinela Formation, southwestern Argentina: An Eocene age for crustacean-rich "Patagonian" beds

NASA Astrophysics Data System (ADS)

Casadío, Silvio; Feldmann, Rodney M.; Foland, Kenneth A.

2000-05-01

The presence of a prominent volcanic ash enclosed within sediments of the Centinela Formation, southwestern Argentina, permits establishment of an 40Ar/ 39Ar age of approximately 46 Ma for these rocks. Oxygen isotopic analysis of shell material from the oyster Crassostrea? hatcheriOrtmann, 1897, suggests seasonal fluctuation of temperature from about 15°C to about 21°C. In concordance with this, a diverse crustacean fauna, including nine families within the order Decapoda and one within Isopoda, bears strong affinities with temperate and subtropical faunas of the Atlantic Ocean basin and documents the southernmost extension of low latitude oceanographic influence on this region during the Paleogene. Establishment of an Eocene age for the rocks in the Centinela Formation provides the first definitive documentation of rocks of that age in the region of Calafate, permits correlation of these rocks with Eocene strata southward to the vicinity of Río Turbio, and suggests that these rocks, assigned to the Patagonian sequence, may be substantially older than those in eastern Argentina.

In Situ UV-Laser Ablation 40Ar/39Ar Muscovite Thermochronology Reveals Excess Argon and 405-399 Ma Age for the Nordfjord-Sogn Detachment Zone, Hornelen region, Norway

NASA Astrophysics Data System (ADS)

Johnston, S. M.; Hacker, B. R.; Eide, E. A.; Hendriks, B.

2006-12-01

The Nordfjord-Sogn Detachment Zone (NSDZ) is a 2-6 km thick, normal-sense shear zone that juxtaposes the eclogite-bearing Western Gneiss Complex in its footwall with Devonian sediments in its hangingwall, and is widely cited as the primary structure responsible for the exhumation of the Norwegian ultrahigh-pressure (UHP) rocks. Critical to our understanding of this crustal-scale detachment, detailed thermochronology places essential constraints on the rates of exhumation and documents spatial variability in the timing of exhumation- related cooling. Regional thermochronology has shown that 40Ar/39Ar muscovite ages increase gradually from 405-390 Ma across the Western Gneiss Complex and into the lower part of the NSDZ, but at higher structural levels within the NSDZ, these ~400 Ma ages are mixed with an older population at 420-418 Ma. Although neither excess argon nor incomplete resetting of argon in these older samples has been ruled out, these results have been interpreted to suggest that extension and exhumation within the NSDZ may have initiated as early as 420 Ma. Here, we investigate these older 40Ar/39Ar muscovite ages by comparing resistance furnace (RF) ages from bulk muscovite separates with in situ UV-laser ablation ages from a suite of 6 samples that transect the NSDZ. RF spectra from 5 of the 6 analyzed samples yield plateau ages that range from 405-399 Ma; in situ ages of these same 5 samples reveal no age zonation within large grains and no differences in ages among texturally different grains. In contrast, the structurally highest sample yields a RF plateau age of 425±6 Ma, and in situ ages of 474-417 Ma in grain cores and c. 400 Ma in grain rims and small grains. On an inverse isochron diagram, these in situ ages define two distinct populations, with grain cores distributed along a line between an excess argon component of 40Ar/36Ar = 660±60 and an age of 404±7 Ma. Analyses from rims and small grains plot in triangle defined by these two apices

Paleotemperatures at the lunar surfaces from open system behavior of cosmogenic 38Ar and radiogenic 40Ar

DOE PAGES

Shuster, David L.; Cassata, William S.

2015-02-10

The simultaneous diffusion of both cosmogenic 38Ar and radiogenic 40Ar from solid phases is controlled by the thermal conditions of rocks while residing near planetary surfaces. Combined observations of 38Ar/ 37Ar and 40Ar/ 39Ar ratios during stepwise degassing analyses of neutron-irradiated Apollo samples can distinguish between diffusive loss of Ar due to solar heating of the rocks and that associated with elevated temperatures during or following impact events; the data provide quantitative constraints on the durations and temperatures of each process. From sequentially degassed 38Ar/ 37Ar ratios can be calculated a spectrum of apparent 38Ar exposure ages versus the cumulativemore » release fraction of 37Ar, which is particularly sensitive to conditions at the lunar surface typically over ~106–108 year timescales. Due to variable proportions of K- and Ca-bearing glass, plagioclase and pyroxene, with variability in the grain sizes of these phases, each sample will have distinct sensitivity to, and therefore different resolving power on, past near-surface thermal conditions. Furthermore, we present the underlying assumptions, and the analytical and numerical methods used to quantify the Ar diffusion kinetics in multi-phase whole-rock analyses that provide these constraints.« less

Early Thermal History of Eucrites by Ar-39-Ar-40

NASA Technical Reports Server (NTRS)

Bogard, D. D.; Garrison, D. H.

2001-01-01

Ar-39-Ar-40 ages for Piplia Kalan (3.58 +/- 0.02 Ga) and two other eucrites indicate later impact resetting. Older Ar-39-Ar-40 ages exist for the Moama cumulate eucrite (4.42 +/- 0.01 Ga) and the PCA82502 (4.506 +/- 0.009 Ga) and PCA91007 non-brecciated eucrites. Additional information is contained in the original extended abstract.

New Ar/Ar single grain mineral ages from Korean orogenic belts with implications for the Triassic cooling and exhumation history

NASA Astrophysics Data System (ADS)

de Jong, Koenraad; Ruffet, Gilles; Han, Seokyoung

2013-04-01

The Korean peninsula is located in the eastern margin of the Eurasian continent where major late Palaeozoic to early Mesozoic continental collision zones, like the Central Asian Orogenic Belt and the Qinling-Dabie-Sulu Belt, merge with circum-Pacific subduction-accretion systems. Deciphering the tectonic evolution of Korea is thus crucial for the understanding of the amalgamation of East Asia. Classically, research in Korea has focused on the search for (ultra)high-pressure metamorphic rocks and their isotopic dating, most recently applying SHRIMP on Th- and U-bearing accessory minerals, in order to substantiate links with the Qinling-Dabie-Sulu Belt across the Yellow Sea in China. Instead of trying to date peak pressure conditions we focused on 40Ar/39Ar laser-probe step-heating dating of single grains of the fabric-forming minerals muscovite, biotite and amphibole, formed during retrograde recrystallisation and exhumation. This is a big advantage as their growth can be straightforwardly correlated to major phases of the tectono-metamorphic evolution of rocks. This approach helps to meet the major geochronological challenge of obtaining age estimates for the timing of specific tectono-metamorphic events in the Korean orogenic belts. The Korean peninsula comprises a number of Palaeoproterozoic high-grade gneiss terranes; only one of which has been affected by Permo-Triassic metamorphism: the Gyeonggi Massif. We concentrated on the uppermost Gyeonggi Massif and the overlying Imjingang Belt, to the North, and the ill-defined Hongseong zone to the West, both constituted by younger metamorphic rocks. Both belts contain rare lenses of mafic rocks with relics of high-pressure metamorphism. Hornblende from a corona-textured amphibolite from the lowermost part of the Imjingang Belt yielded a U-shaped age spectrum, the base of which is formed by four concordant steps with a weighted mean age of 242.8 ± 2.4 Ma (15% 39Ar release). Muscovites from strongly retrogressed and

Instrumentation development for In Situ 40Ar/39Ar planetary geochronology

USGS Publications Warehouse

Morgan, Leah; Munk, Madicken; Davidheiser-Kroll, Brett; Warner, Nicholas H.; Gupta, Sanjeev; Slaybaugh, Rachel; Harkness, Patrick; Mark, Darren

2017-01-01

The chronology of the Solar System, particularly the timing of formation of extra-terrestrial bodies and their features, is an outstanding problem in planetary science. Although various chronological methods for in situ geochronology have been proposed (e.g., Rb-Sr, K-Ar), and even applied (K-Ar), the reliability, accuracy, and applicability of the 40Ar/39Ar method makes it by far the most desirable chronometer for dating extra-terrestrial bodies. The method however relies on the neutron irradiation of samples, and thus a neutron source. Herein, we discuss the challenges and feasibility of deploying a passive neutron source to planetary surfaces for the in situ application of the 40Ar/39Ar chronometer. Requirements in generating and shielding neutrons, as well as analysing samples are described, along with an exploration of limitations such as mass, power and cost. Two potential solutions for the in situ extra-terrestrial deployment of the 40Ar/39Ar method are presented. Although this represents a challenging task, developing the technology to apply the 40Ar/39Ar method on planetary surfaces would represent a major advance towards constraining the timescale of solar system formation and evolution.

40Ar/39Ar geochronology of terrestrial pyroxene

NASA Astrophysics Data System (ADS)

Ware, Bryant; Jourdan, Fred

2018-06-01

Geochronological techniques such as U/Pb in zircon and baddeleyite and 40Ar/39Ar on a vast range of minerals, including sanidine, plagioclase, and biotite, provide means to date an array of different geologic processes. Many of these minerals, however, are not always present in a given rock, or can be altered by secondary processes (e.g. plagioclase in mafic rocks) limiting our ability to derive an isotopic age. Pyroxene is a primary rock forming mineral for both mafic and ultramafic rocks and is resistant to alteration process but attempts to date this phase with 40Ar/39Ar has been met with little success so far. In this study, we analyzed pyroxene crystals from two different Large Igneous Provinces using a multi-collector noble gas mass spectrometer (ARGUS VI) since those machines have been shown to significantly improve analytical precision compared to the previous single-collector instruments. We obtain geologically meaningful and relatively precise 40Ar/39Ar plateau ages ranging from 184.6 ± 3.9 to 182.4 ± 0.8 Ma (2σ uncertainties of ±1.8-0.4%) and 506.3 ± 3.4 Ma for Tasmanian and Kalkarindji dolerites, respectively. Those data are indistinguishable from new and/or published U-Pb and 40Ar/39Ar plagioclase ages showing that 40Ar/39Ar dating of pyroxene is a suitable geochronological tool. Scrutinizing the analytical results of the pyroxene analyses as well as comparing them to the analytical result from plagioclase of the same samples indicate pure pyroxene was dated. Numerical models of argon diffusion in plagioclase and pyroxene support these observations. However, we found that the viability of 40Ar/39Ar dating approach of pyroxene can be affected by irradiation-induced recoil redistribution between thin pyroxene exsolution lamellae and the main pyroxene crystal, hence requiring careful petrographic observations before analysis. Finally, diffusion modeling show that 40Ar/39Ar of pyroxene can be used as a powerful tool to date the formation age of mafic

Advancements in cosmogenic 38Ar exposure dating of terrestrial rocks

NASA Astrophysics Data System (ADS)

Oostingh, K. F.; Jourdan, F.; Danišík, M.; Evans, N. J.

2017-11-01

Cosmogenic exposure dating of Ca-rich minerals using 38Ar on terrestrial rocks could be a valuable new dating tool to determine timescales of geological surface processes on Earth. Here, we show that advancement in analytical precision, using the new generation multi-collector ARGUSVI mass spectrometer on irradiated pyroxene and apatite samples, allows determination of cosmogenic 38Ar abundances above background values, as well as discrimination of 38Ar/36Ar ratios (1σ absolute precision of ±0.3%) from the non-cosmogenic background value. Four statistically significant cosmochron (38Ar/36Ar vs37Ar/36Ar) diagrams could be constructed for southeast Australian pyroxene samples from the Mt Elephant scoria cone for which a combined apparent exposure age of 313 ± 179 ka (2σ) was obtained when using a 38Ar production rate (Ca) of 250 atoms /g Ca/ yr. This exposure age overlaps within error with the known 40Ar/39Ar eruption age of 184 ± 15 ka (2σ). Although apatite shows much larger 38Ar abundances than pyroxene, our modelling and analyses of unirradiated apatite suggest that apatite suffers from both natural and reactor-derived chlorogenic as well as natural nucleogenic contributions of 38Ar. Hence, we suggest that cosmogenic 38Ar exposure dating on irradiated Ca-rich (and eventually K-rich), but Cl-free, terrestrial minerals is a potential valuable and accessible tool to determine geological surface processes on timescales of a few Ma. Calculations show that with the new generation multi-collector mass spectrometers an analytical uncertainty better than 5% (2σ) can be achieved on samples with expected exposure ages of >4 Ma.

Timing of Hydrocarbon Fluid Emplacement in Sandstone Reservoirs in Neogene in Huizhou Sag, Southern China Sea, by Authigenic Illite 40Ar- 39Ar Laser Stepwise Heating

NASA Astrophysics Data System (ADS)

Hesheng, Shi; Junzhang, Zhu; Huaning, Qiu; yu, Shu; Jianyao, Wu; Zulie, Long

Timing of oil or gas emplacements is a new subject in isotopic geochronology and petroleum geology. Hamilton et al. expounded the principle of the illite K-Ar age: Illite is often the last or one of the latest mineral cements to form prior to hydrocarbon accumulation. Since the displacement of formation water by hydrocarbons will cause silicate diagenesis to cease, K-Ar ages for illite will constrain the timing of this event, and also constrain the maximum age of formation of the trap structure. In this study, the possibility of authigenic illites 40Ar- 39Ar dating has been investigated. The illite samples were separated from the Tertiary sandstones in three rich oil reservoir belts within the Huizhou sag by cleaning, fracturing by cycled cooling-heating, soxhlet-extraction with solvents of benzene and methanol and separating with centrifugal machine. If oil is present in the separated samples, ionized organic fragments with m/e ratios of 36 to 40 covering the argon isotopes will be yielded by the ion source of a mass spectrometer, resulting in wrong argon isotopic analyses and wrong 40Ar- 39Ar ages. The preliminary experiments of illite by heating did show the presence of ionized organic fragments with m/e ratios of 36 to 44. In order to clean up the organic gases completely and obtain reliable analysis results, a special purification apparatus has been established by Qiu et al. and proved valid by the sequent illite analyses. All the illite samples by 40Ar- 39Ar IR-laser stepwise heating yield stair-up age spectra in lower laser steps and plateaux in higher laser steps. The youngest apparent ages corresponding to the beginning steps are reasonable to be interpreted for the hydrocarbon accumulation ages. The weighted mean ages of the illites from the Zhuhai and Zhujiang Formations are (12.1 ± 1.1) Ma and (9.9 ± 1.2) Ma, respectively. Therefore, the critical emplacement of petroleum accumulation in Zhujiang Formation in Huizhou sag took place in ca 10 Ma. Late

40Ar/(39)Ar dating of Chemeron Formation strata encompassing the site of hominid KNM-BC 1, Tugen Hills, Kenya.

PubMed

Deino, Alan L; Hill, Andrew

2002-01-01

A fossil hominid temporal bone (KNM-BC 1) from surface exposures at Baringo Paleontological Research Project site BPRP#2 in the Chemeron Formation outcropping in a tributary drainage of the Kapthurin River west of Lake Baringo, Kenya has been attributed to Homo sp. indet. K-feldspar phenocrysts from lapilli tuffs bracketing the inferred fossiliferous horizon yield single-crystal(40)Ar/(39)Ar ages of 2.456+/-0.006 and 2.393+/-0.013 Ma. These age determinations are supported by stratigraphically consistent ages on higher tuff horizons and from nearby sections. In addition, new(40)Ar/(39)Ar ages on tuffaceous units near the base and top of the formation along the Kapthurin River yield 3.19+/-0.03 and 1.60+/-0.05 Ma respectively. The base of the formation along the Kapthurin River is thus approximately 0.5 Ma younger than the uppermost Chemeron Formation strata exposed at Tabarin, 23 km to the north-northwest. The upper half of the formation along the Kapthurin River was deposited at an average rate of approximately 11 cm/ka, compared to 21-23 cm/ka at Tabarin. Copyright 2002 Academic Press.

Ar/Ar age data of muscovite from the Keivy Terrane (central Kola Peninsula, arctic European Russia) imply a prolonged fluid-assisted recrystallisation

NASA Astrophysics Data System (ADS)

de Jong, K.; Ruffet, G.; Marker, M.

2012-04-01

Single grain muscovite 40Ar/39Ar age data from metasediments of the Keivy Terrane point to a prolonged recrystallisation, and imply that the younger age set in metamorphic terranes with a long history cannot always be simply interpreted as due to late and slow cooling. The Keivy terrane is an element of the Palaeoproterozoic Lapland-Kola collisional belt developed along the northern margin of the Fennoscandian (Baltic) Shield. It comprises a lower series of late Archaean meta-volcanic rocks, intruded by earliest Palaeoproterozoic alkali granites that are covered by strongly deformed quartz-rich kyanite-staurolite-garnet-micaschists of the Keivy unit that have yielded magmatic zircons as young as ~2.35 Ga, which were derived from the substratum's alkaline granite. 40Ar/39Ar step-heating dating with a defocussed laser beam of muscovite grains from seven metasediments of the Keivy unit yielded saddle-shaped age spectra in most experiments. In five out of seven cases the base of the saddle corresponded to a plateau age in the range of 1667 to 1593 Ma (60-90% of the gas release; 1 sigma errors: 1.0-1.2 Ma). We do not simply interpret these 40Ar/39Ar ages in the classical way as due to cooling, because the saddle shape of the spectra enables a more complete and detailed interpretation. Saddle-shaped age spectra may result from the presence of different argon reservoirs in partially recrystallised and chemically distinct micas that degas over a different energy interval: a primary, not recrystallised or inherited domain (low and high temperature steps) and a newly formed or recrystallised one (saddle minimum in the intermediate steps). The younger subdomains formed by growth or recrystallisation could characterise the last isotopic record during an extended (re)crystallisation history. It is striking that 1612 and 1615 Ma saddle minimum ages in two samples correspond to a plateau age of 1612 Ma in another sample. Also elevated high and/or low temperature apparent ages of

The 40Ar/39Ar age record and geodynamic significance of Indo-Madagascar and Deccan flood basalt volcanism in the Sarnu-Dandali alkaline complex, Rajasthan, northwestern India

NASA Astrophysics Data System (ADS)

Vijayan, Anjali; Pande, Kanchan; Sheth, Hetu; Kant Sharma, Kamal

2017-04-01

The Sarnu-Dandali alkaline complex in Rajasthan, northwestern India, is considered to represent early, pre-tholeiite magmatism in the Deccan Traps continental flood basalt (CFB) province, based on a single 40Ar/39Ar age of 68.57 Ma. Rhyolites found in the complex are considered to be 750 Ma Malani basement. Our new 40Ar/39Ar ages of 88.9-86.8 Ma (for syenites, nephelinite, phonolite and rhyolite) and 66.3 ± 0.4 Ma (2σ, melanephelinite) provide clear evidence that whereas the Sarnu-Dandali complex has Deccan-age components, it is dominantly an older (by ˜20 million years) alkaline complex, with rhyolites included. Sarnu-Dandali is thus an alkaline igneous center active at least twice in the Late Cretaceous, and also much before as suggested by a basalt flow underlying the Early Cretaceous Sarnu Sandstone. The 89-86 Ma 40Ar/39Ar ages fully overlap with those for the Indo-Madagascar CFB province formed during continental break-up between India (plus Seychelles) and Madagascar. Recent 40Ar/39Ar work has shown polychronous emplacement (over ≥ 45 million years) of the Mundwara alkaline complex in Rajasthan, 100 km from Sarnu-Dandali, and 84-80 Ma ages obtained from Mundwara also arguably represent late stages of the Indo-Madagascar CFB volcanism. Remnants of the Indo-Madagascar CFB province are known from several localities in southern India but hitherto unknown from northwestern India 2000 km away. Additional equivalents buried under the vast Deccan Traps are highly likely. We relate the Sarnu-Dandali and Mundwara complexes to decompression melting of ancient, subduction-fluxed, enriched mantle lithosphere due to periodic lithospheric extension during much of the Cretaceous, and hundreds of kilometers inland from the India-Madagascar and India-Seychelles rifted margins.

Studies of rotationally inelastic collisions of NaK and NaCs with Ar and He perturbers

NASA Astrophysics Data System (ADS)

Jones, J.; Faust, C.; Richter, K.; Wolfe, C. M.; Ashman, S.; Malenda, R. F.; Weiser, P.; Carlus, S.; Fragale, A.; Hickman, A. P.; Huennekens, J.

2013-05-01

We report studies of rotationally inelastic collisions of Ar and He atoms with the molecules NaK and NaCs prepared in various ro-vibrational levels of the A1Σ+ electronic state. We use laser induced fluorescence (LIF) and polarization labeling (PL) spectroscopy in a pump-probe, two step excitation process. The pump excites the molecule to a ro-vibrational level (v , J) in the A state. The probe laser is scanned over transitions to the 31 Π state in NaK or the 53 Π state in NaCs. In addition to strong direct lines, we observe weak satellite lines that arise from collision-induced transitions of the A state level (v , J) to (v , J + ΔJ) . The ratio of intensities of the satellite line to the direct line in LIF and PL yields information about population and orientation transfer. Preliminary results show a strong propensity for collisions with ΔJ =even for NaK; the propensity is larger for He than for Ar. Collisions of NaCs with He show a similar propensity, but collisions of NaCs with Ar do not. Theoretical calculations are also underway. For He-NaK, we have completed potential surface calculations using GAMESS and coupled channel scattering calculations of rotational energy transfer and transfer of orientation. Work supported by NSF and XSEDE.

Single and double capture in F9+ + Ar collisions: Comparison of total capture with capture occurring from the Ar K shell

NASA Astrophysics Data System (ADS)

La Mantia, David; Kumara, Nuwan; Kayani, Asghar; Simon, Anna; Tanis, John

2016-05-01

Total cross sections for single and double capture, as well as the corresponding cross sections for capture resulting in the emission of an Ar K x ray, were measured. This work was performed at Western Michigan University with the use of the tandem Van de Graaff accelerator. A 45 MeV beam of fully-stripped fluorine ions was collided with argon gas molecules in a differentially pumped cell. Surface barrier detectors were used to observe the charge changed projectiles and a Si(Li) x-ray detector, placed at 90o to the incident beam, were used to measure coincidences with Ar K x rays. The total capture cross sections are compared to previously measured cross sections in the existing literature. The coincidence cross sections, considerably smaller than the total cross sections, are found to be nearly equal for single and double capture in contrast to the total cross sections, which vary by about an order of magnitude. Possible reasons for this behavior are discussed. Supported in part by the NSF.

Ars Moriendi: Coping with death in the Late Middle Ages.

PubMed

Espi Forcén, Fernando; Espi Forcén, Carlos

2016-10-01

The Ars moriendi was a book written in the early 15th century with the goal of assisting friars in their work of helping the dying. The aim of our study was to review the current literature on the Ars Moriendi concerning the field of medicine, to analyze the psychological mechanisms for coping with death anxiety within Ars Moriendi, and to explore parallels between the strategies used in the medieval book and in contemporary literature about death and dying. A review of literature using Pubmed, EMBASE, JSTOR, Project MUSE, and the New York Public Library was undertaken first. The primary source was then interpreted from a medical/psychological point of view. Seven articles were selected by literature review. These works comment on the importance of the Ars Moriendi in its historical context and explore the possibility of retrieving the principles of the text in contemporary society. The original text of Ars Moriendi, the primary source, presents death as a relief from the sufferings of earthly life and a gateway to eternal glory. According to the author, a good death implied the triumph over five demonic temptations in agonizing people: a lack of faith, despair, impatience, pride and greed. Analyzed from a modern psychiatric perspective, the Ars Moriendi offers descriptions of behavioral manifestations compatible with delirium, mood and anxiety disorders that characterize people with terminal illnesses. Moreover, we also explored parallels between the strategies used to cope with death anxiety in the Late Middle Ages and in contemporary society.

K/T age for the popigai impact event

NASA Technical Reports Server (NTRS)

Deino, A. L.; Garvin, J. B.; Montanari, S.

1991-01-01

The multi-ringed POPIGAI structure, with an outer ring diameter of over 100 km, is the largest impact feature currently recognized on Earth with an Phanerozoic age. The target rocks in this relatively unglaciated region consist of upper Proterozoic through Mesozoic platform sediments and igneous rocks overlying Precambrian crystalline basement. The reported absolute age of the Popigai impact event ranges from 30.5 to 39 Ma. With the intent of refining this age estimate, a melt-breccia (suevite) sample from the inner regions of the Popigai structure was prepared for total fusion and step-wise heating Ar-40/Ar-39 analysis. Although the total fusion and step-heating experiments suggest some degree of age heterogeneity, the recurring theme is an age of around 64 to 66 Ma.

A reconnaissance Rb-Sr, Sm-Nd, U-Pb, and K-Ar study of some host rocks and ore minerals in the West Shasta Cu- Zn district, California ( USA).

USGS Publications Warehouse

Kistler, R.W.; McKee, E.H.; Futa, K.; Peterman, Z.E.; Zartman, R.E.

1985-01-01

The Copley Greenstone, Balaklala Rhyolite, and Mule Mountain stock in the West Shasta Cu-Zn district, California, have Rb-Sr, Sm-Nd, U-Pb, and K-Ar systematics that indicate they are a cogenetic suite of ensimatic island-arc rocks about 400 Ma. Pervasive alteration and mineralization of these rocks, for the most part, was syngenetic and the major component of the mineralizing fluid was Devonian seawater. K-Ar ages of quarz-sericite concentrates from ore horizons and Rb-Sr systematics of a few rock and ore specimens record a later thermal and mineralizing event in the district of about 260 Ma. Contamination of some rocks with pelagic sediments is indicated by the Sm-Nd data. -Authors

Ar-Ar Ages of Detrital Hornblendes from Glacial Sediments of the North Sea Trough Mouth Fan

NASA Astrophysics Data System (ADS)

Hemming, S. R.; Haflidason, H.; Sejrup, H. P.

2007-12-01

Determining the relative timing of major iceberg calving from different ice sheet margins around the North Atlantic remains an important goal that will lead to a better understanding of causes and consequences of rapid climate variability during the last glacial period. Characterization of the composition of potential contributors is a necessary step towards this goal. The North Sea trough mouth fan is one of the largest glaciogenic debris flow complexes in the North Atlantic/Arctic region, with an approximate area of 142,000 square km (King et al., 1998, Marine Geology v. 152, pp. 217-246; Nygard et al., 2007, Geology, pp. 395-398). The large ice stream trough crosses the shelf along the southern margin of Norway. The crystalline rocks along the southern margin of Norway are Grenville (approximately 1 Ga old orogen). We undertook a study of the Ar-Ar age populations of individual detrital hornblende grains from a sediment sample of the glacigenic debris lobe created during the last phases of the last glacial maximum from the North Sea trough mouth fan. The goal is to test the hypothesis that the ice stream that fed this fan is the source of abundant Grenville age grains found on Bjorn drift site ODP984, at times when North American Grenville sources are not found in the North Atlantic ice rafted detritus belt (Hemming et al., 2005, AGU Spring meeting, PP23A-04). Hornblende grains from North Sea TMF core NH071-B01\\SC1 (1) (63.24N, 3.36E, 1049m) have a dominant age population of Grenville (921 Ma, 19 of 48 grains) with subordinate populations of 1108 Ma (n=3) and 1779 Ma (n=4). Accordingly they lend support to the hypothesis that this ice stream could be the source of IRD on the Bjorn drift. These results could additionally shed light on the pathways of fine grain sediment transport to the Bjorn drift which would contribute a better understanding of sediment processes in the region. For example, the provenance implied for the IRD by the Ar-Ar hornblende ages is

On the 40Ar/39Ar Dating of Low-Potassium Ocean Crust Basalt from IODP Expedition 349, South China Sea

NASA Astrophysics Data System (ADS)

Koppers, A. A. P.

2014-12-01

Accurate age dates for the basement rocks in the South China Sea (SCS) basins were lacking before the execution of International Ocean Discovery Program (IODP) Expedition 349 in early 2014. This left a large margin of error in estimated opening ages for the SCS and rendered various hypotheses regarding its opening mechanism and history untested, hampering our understanding of East Asian tectonic and paleoenvironmental evolution. Therefore, high-precision 40Ar/39Ar age dating lies at the heart of Expedition 349, which in particular aimed to determine the timing of the start and cessation of seafloor spreading in the SCS. In addition, the recovery of a complete seamount apron section at Site U1431 allows 40Ar/39Ar dating of abundantly present plagioclase and biotite crystals to help establish a detailed chronology of the sedimentary and volcaniclastic sequences cored. Here we present the first 40Ar/39Ar incremental heating ages on the low-potassium (~0.1-0.2 wt% K2O) and the least altered (loss on ignition < 1.5%) mid-ocean ridge basalt (MORB) from the SCS. Plagioclase and groundmass samples were prepared using conventional mineral separation techniques, acid-leaching and hand-picking. Analyses were carried out using a new ARGUS-VI multi-collector noble gas mass spectrometer. Ages are expected to have precisions ranging between 0.1-0.3 Ma (2σ), which will allow us to precisely and accurately date the final emplacement of basalts at Sites U1431, U1433 and U1434 in the SCS basin, just prior to the cessation of spreading as all sites were slightly offset from the relict spreading center.

Recurrent Early Cretaceous, Indo-Madagascar (89-86 Ma) and Deccan (66 Ma) alkaline magmatism in the Sarnu-Dandali complex, Rajasthan: 40Ar/39Ar age evidence and geodynamic significance

NASA Astrophysics Data System (ADS)

Sheth, Hetu; Pande, Kanchan; Vijayan, Anjali; Sharma, Kamal Kant; Cucciniello, Ciro

2017-07-01

The Sarnu-Dandali alkaline complex in Rajasthan, northwestern India, is considered to represent early, pre-flood basalt magmatism in the Deccan Traps province, based on a single 40Ar/39Ar age of 68.57 Ma. Rhyolites found in the complex are considered to be 750 Ma Malani basement. Our new 40Ar/39Ar ages of 88.9-86.8 Ma (for syenites, nephelinite, phonolite and rhyolite) and 66.3 ± 0.4 Ma (2σ, melanephelinite) provide clear evidence that whereas the complex has Deccan-age (66 Ma) components, it is dominantly an older (by 20 million years) alkaline complex, with rhyolites included. Basalt is also known to underlie the Early Cretaceous Sarnu Sandstone. Sarnu-Dandali is thus a periodically rejuvenated alkaline igneous centre, active twice in the Late Cretaceous and also earlier. Many such centres with recurrent continental alkaline magmatism (sometimes over hundreds of millions of years) are known worldwide. The 88.9-86.8 Ma 40Ar/39Ar ages for Sarnu-Dandali rocks fully overlap with those for the Indo-Madagascar flood basalt province formed during continental breakup between India (plus Seychelles) and Madagascar. Recent 40Ar/39Ar work on the Mundwara alkaline complex in Rajasthan, 120 km southeast of Sarnu-Dandali, has also shown polychronous emplacement (over ≥ 45 million years), and 84-80 Ma ages obtained from Mundwara also arguably represent post-breakup stages of the Indo-Madagascar flood basalt volcanism. Remnants of the Indo-Madagascar province are known from several localities in southern India but hitherto unknown from northwestern India 2000 km away. Additional equivalents buried under the vast Deccan Traps are highly likely.

40Ar-39Ar Age Constraints on Volcanism and Tectonism in the Terror Rift of the Ross Sea, Antarctica

USGS Publications Warehouse

2007-01-01

Volcanic sills and dikes inferred from seismic reflection profiles and geophysical studies of the Ross Sea are thought to be related to the rift basins in the region, and their emplacement to be coeval with extension. However, lack of precise geochronology in the Terror Rift of the Ross Sea region has left these inferred relationships poorly constrained and has hindered neotectonic studies, because of the large temporal gaps between seismic reflectors of known ages. New 40Ar/39Ar geochronology presented here for submarine volcanic rocks provides better age constraints for neotectonic interpretations within the Terror Rift. Several samples from seamounts yielded young ages between 156 ± 21 and 122 ± 26 Ka. These ages support interpretations that extension within the Terror Rift was active at least through the Pleistocene. Three evenly spaced samples from the lowermost 100 m of Franklin Island range in age from 3.28 ± 0.04 to 3.73 ± 0.05 Ma. These age determinations demonstrate that construction of a small volcanic edifice such as Franklin Island took at least several hundred thousand years, and therefore that much larger ones in the Erebus Volcanic Province are likely to have taken considerably longer than previously inferred. This warrants caution in applying a limited number of age determinations to define the absolute ages of events in the Ross Sea region

40Ar/(39)Ar geochronology and paleomagnetic stratigraphy of the Lukeino and lower Chemeron Formations at Tabarin and Kapcheberek, Tugen Hills, Kenya.

PubMed

Deino, Alan L; Tauxe, Lisa; Monaghan, Marc; Hill, Andrew

2002-01-01

(40)Ar/(39)Ar single-crystal laser-fusion dating, K-Ar dating, and paleomagnetic reversal stratigraphy have been used to determine the chronostratigraphy of the Kabarnet Trachyte, Lukeino Formation, Kaparaina Basalt Formation, and Chemeron Formation at the sites of Kapcheberek (BPRP#77) and Tabarin (BPRP#77) in the Tugen Hills, Kenya. The succession ranges in age from 6.56-3.8 Ma. The upper Lukeino Formation at Kapcherberek, including the fauna from the site BPRP#76, was deposited during chron C3r and can be constrained to the interval 5.88-5.72 Ma. The Chemeron Formation at Tabarin includes at the base an ignimbrite and associated basal air-fall tuff with a combined age of 5.31+/-0.03 Ma. Sedimentary and volcaniclastic rocks of the Chemeron Formation which unconformably overlie the ignimbrite record chrons C3n.2n through C2Ar. The combined(40)Ar/(39)Ar and paleomagnetic data constrain the age of this sequence to 4.63-3.837 Ma. The age of the Tabarin mandible fragment (KNM-TH 13150) and associated fauna at site BPRP#77 in the Chemeron Formation is 4.48-4.41 Ma, marginally older than similar early hominids from Aramis, Ethiopia. Basin subsidence appears to be defining an overall accumulation rate of about 17 cm/ka over the 2.7 Ma represented at Tabarin and Kapcheberek, despite episodes of rapid accumulation and hiatuses. Copyright 2002 Academic Press.

40Ar* loss in experimentally deformed muscovite and biotite with implications for 40Ar/39Ar geochronology of naturally deformed rocks

USGS Publications Warehouse

Cosca, M.; Stunitz, H.; Bourgeix, A.-L.; Lee, J.P.

2011-01-01

The effects of deformation on radiogenic argon (40Ar*) retentivity in mica are described from high pressure experiments performed on rock samples of peraluminous granite containing euhedral muscovite and biotite. Cylindrical cores, ???15mm in length and 6.25mm in diameter, were drilled from granite collected from the South Armorican Massif in northwestern France, loaded into gold capsules, and weld-sealed in the presence of excess water. The samples were deformed at a pressure of 10kb and a temperature of 600??C over a period 29 of hours within a solid medium assembly in a Griggs-type triaxial hydraulic deformation apparatus. Overall shortening in the experiments was approximately 10%. Transmitted light and secondary and backscattered electron imaging of the deformed granite samples reveals evidence of induced defects and for significant physical grain size reduction by kinking, cracking, and grain segmentation of the micas.Infrared (IR) laser (CO2) heating of individual 1.5-2.5mm diameter grains of muscovite and biotite separated from the undeformed granite yield well-defined 40Ar/39Ar plateau ages of 311??2Ma (2??). Identical experiments on single grains separated from the experimentally deformed granite yield results indicating 40Ar* loss of 0-35% in muscovite and 2-3% 40Ar* loss in biotite. Intragrain in situ ultraviolet (UV) laser ablation 40Ar/39Ar ages (??4-10%, 1??) of deformed muscovites range from 309??13 to 264??7Ma, consistent with 0-16% 40Ar* loss relative to the undeformed muscovite. The in situ UV laser ablation 40Ar/39Ar ages of deformed biotite vary from 301 to 217Ma, consistent with up to 32% 40Ar* loss. No spatial correlation is observed between in situ 40Ar/39Ar age and position within individual grains. Using available argon diffusion data for muscovite the observed 40Ar* loss in the experimentally treated muscovite can be utilized to predict average 40Ar* diffusion dimensions. Maximum 40Ar/39Ar ages obtained by UV laser ablation overlap those

Crater Retention Ages from (4) Vesta Matching Independent Ar-Ar Ages of HED Meteorites

NASA Astrophysics Data System (ADS)

Schmedemann, Nico; Kneissl, Thomas; Ivanov, Boris A.; Michael, Gregory G.; Neukum, Gerhard; Nathues, Andreas; Sierks, Holger; Wagner, Roland; Krohn, Katrin; Le Corre, Lucille; Reddy, Vishnu; Ruesch, Ottaviano; Hiesinger, Harald; Jaumann, Ralf; Raymond, Carol A.; Russell, Christopher T.

2013-04-01

.054/-0.087) Ga and 3.63 (+0.058/-0.096) Ga. We also find seismic (miniscule ejecta blanket from Rheasilvia) resurfacing events in the time frame of ~3.56 to ~3.59 Ga at several areas in the northern hemisphere, indicative for a major seismic activity probably connected to the Rheasilvia formation. An antipodal activity is also suggested by hydrocode modeling [9]. By summation of age probability curves of measurements we link to the Rheasilvia formation, we find 3.58 (+0.07/-0.12) Ga. Using a similar attempt we find 3.75 (+0.05/-0.21) Ga for the Veneneia formation. Both crater retention ages correspond within the error bars with prominent peaks of independent Ar-Ar ages of Vesta related HED meteorites [10]. Acknowledgement: This work has been supported by the German Space Agency (DLR) on behalf of the Federal Ministry of Economics and Technology, grants 50OW1101(NS,TK), 50QM1001 (GM) and 50OW1102 (OR,HH). References: [1] Russell et al. (2007): Advances in Space Research 40(2): pp193-201, 2007. [2] Neukum and Ivanov: In: Gehrels T (ed) "Hazards due to comets and asteroids". University of Arizona Press, Tucson, 359-416, 1994. [3] O'Brien and Greenberg (2005): Icarus 178(1): 179-212. [4] Nesvorny et al. (2009): Icarus 200(2): 698-701. [5] Ivanov (2001): Chronology and Evolution of Mars 96, 87-104, 2001. [6] Schmedemann et al. (2012): 43.LPSC, The Woodlands, #1659. [7] Morbidelli et al. (2003): Icarus 162, 328-336. [8] Marchi et al. (2012): Science 336, 690. [9] Bowling et al. (2012): 75th Annual Meeting of the Meteoritical Society, 2012, Cairns, Australia. Meteoritics and Planetary Science Supplement, id.5256. [10] Bogard, D. D. (2011): Chemie der Erde - Geochemistry, vol. 71, issue 3: 207-226.

Paraná flood basalt volcanism primarily limited to 1 Myr beginning at 135 Ma: New 40Ar/39Ar ages for rocks from Rio Grande do Sul, and critical evaluation of published radiometric data

NASA Astrophysics Data System (ADS)

Baksi, Ajoy K.

2018-04-01

40Ar/39Ar step heating analyses were carried out on seven rocks (five basalts, an andesite and a rhyolite) from the southern Paraná Province ( 28°S-30°S); they yield plateau/isochron ages of 135-134 Ma, in good agreement with published step heating data on rocks from the same area. Critical review of laser spot isochron ages for rocks from the Province, ranging from 140 to 130 Ma, are shown to be unreliable estimates of crystallization ages, as the rocks were substantially altered; step heating results on three of these rocks thought to yield good plateau ages, are shown to be incorrect, as a result of a technicality in dating procedures followed. U-Pb ages on zircon and baddeleyite separated from a variety of rock types ( 30°S-23°S) fall in the range 135 to 134 Ma. All reliable 40Ar/39Ar and U-Pb ages indicate volcanism was sharply focused, initiated at 135 Ma, and 1 Myr in duration; no variation of age with either latitude or longitude is noted, Scrutiny of published 40Ar/39Ar ages on the Florianopolis dykes shows they cannot be used as reliable crystallization ages. U-Pb work shows that this dyke swarm was formed coevally with the main part of the Parana province. Most of the published 40Ar/39Ar ages on the Ponta Grossa dyke swarm are unreliable; a few ages appear reliable and suggest the magmatic event in this area, may have postdated the main Paraná pulse by 1-2 Myr. A single 40Ar/39Ar age from a high-Nb basalt in the southernmost part ( 34°S) of the Paraná at 135 Ma, highlights the need for further radiometric work on other areas of this flood basalt province. The Paraná Province postdates the time of the Jurassic-Cretaceous bound­ary by 10 Myr.

[sup 40]Ar/[sup 39]Ar mineral ages from southwestern Penobscot Bay, Maine: Evidence for Silurian metamorphism

DOE Office of Scientific and Technical Information (OSTI.GOV)

West, D.P. Jr.; Guidotti, C.V.; Lux, D.R.

1992-01-01

The nature and timing of metamorphic events in the Coastal Lithotectonic Block of Maine remain poorly understood. Immediately west and southwest of Penobscot Bay the rocks are polymetamorphic showing evidence for at least two episodes of amphibolite facies metamorphism and later, perhaps regionally extensive, retrograde events. Hornblende mineral separates from two amphibolites din the Port Clyde area have identical Ar-40/Ar-39 plateau ages of 414.0 [+-] 3.3 and 414.0 [+-] 3.9 Ma. These ages are interpreted to reflect the time of cooling following the last significant thermal event in this area. Biotite from an amphibolite in the Port Clyde area givesmore » a total gas age of 346.5 [+-] 3.2 Ma. Hornblende from an amphibolite 7 km to the west near Friendship gives a nearly concordant release spectrum with a plateau age of 369.0 [+-] 3.7 Ma. Coexisting biotite from this amphibolite gives a total gas age of 289.2 [+-] 2.7 Ma. Muscovite from the Waldoboro pluton has a nearly concordant release spectrum with a plateau age of 306.3 [+-] 2.2 Ma. Biotite from this sample gives a total gas age of 288.9 [+-] 2.2 Ma. The 414.0 Ma hornblende cooling ages from the Port Clyde area reflect cooling following a significant high grade Silurian thermal event. This Silurian metamorphism is the same age as tectonothermal events in the Nashoba Terrane in eastern Massachusetts, the Kingston Complex in southern New Brunswick, the Aspy Terrane in Cape Breton island, Nova Scotia, and the Hermitage Flexure in southern Newfoundland.d Thus a distinctive Silurian tectonothermal province located along the western edge of the Avalon Zone appears to extend discontinuously from Massachusetts to Newfoundland.« less

40Ar ∗ loss in experimentally deformed muscovite and biotite with implications for 40Ar/ 39Ar geochronology of naturally deformed rocks

NASA Astrophysics Data System (ADS)

Cosca, Michael; Stunitz, Holger; Bourgeix, Anne-Lise; Lee, John P.

2011-12-01

The effects of deformation on radiogenic argon ( 40Ar ∗) retentivity in mica are described from high pressure experiments performed on rock samples of peraluminous granite containing euhedral muscovite and biotite. Cylindrical cores, ˜15 mm in length and 6.25 mm in diameter, were drilled from granite collected from the South Armorican Massif in northwestern France, loaded into gold capsules, and weld-sealed in the presence of excess water. The samples were deformed at a pressure of 10 kb and a temperature of 600 °C over a period 29 of hours within a solid medium assembly in a Griggs-type triaxial hydraulic deformation apparatus. Overall shortening in the experiments was approximately 10%. Transmitted light and secondary and backscattered electron imaging of the deformed granite samples reveals evidence of induced defects and for significant physical grain size reduction by kinking, cracking, and grain segmentation of the micas. Infrared (IR) laser (CO 2) heating of individual 1.5-2.5 mm diameter grains of muscovite and biotite separated from the undeformed granite yield well-defined 40Ar/ 39Ar plateau ages of 311 ± 2 Ma (2σ). Identical experiments on single grains separated from the experimentally deformed granite yield results indicating 40Ar ∗ loss of 0-35% in muscovite and 2-3% 40Ar ∗ loss in biotite. Intragrain in situ ultraviolet (UV) laser ablation 40Ar/ 39Ar ages (±4-10%, 1σ) of deformed muscovites range from 309 ± 13 to 264 ± 7 Ma, consistent with 0-16% 40Ar ∗ loss relative to the undeformed muscovite. The in situ UV laser ablation 40Ar/ 39Ar ages of deformed biotite vary from 301 to 217 Ma, consistent with up to 32% 40Ar ∗ loss. No spatial correlation is observed between in situ40Ar/ 39Ar age and position within individual grains. Using available argon diffusion data for muscovite the observed 40Ar ∗ loss in the experimentally treated muscovite can be utilized to predict average 40Ar ∗ diffusion dimensions. Maximum 40Ar/ 39Ar ages

Reliable high-power injection locked 6kHz 60W laser for ArF immersion lithography

NASA Astrophysics Data System (ADS)

Watanabe, Hidenori; Komae, Shigeo; Tanaka, Satoshi; Nohdomi, Ryoichi; Yamazaki, Taku; Nakarai, Hiroaki; Fujimoto, Junichi; Matsunaga, Takashi; Saito, Takashi; Kakizaki, Kouji; Mizoguchi, Hakaru

2007-03-01

Reliable high power 193nm ArF light source is desired for the successive growth of ArF-immersion technology for 45nm node generation. In 2006, Gigaphoton released GT60A, high power injection locked 6kHz/60W/0.5pm (E95) laser system, to meet the demands of semiconductor markets. In this paper, we report key technologies for reliable mass production GT laser systems and GT60A high durability performance test results up to 20 billion pulses.

The tectonothermal evolution of the Venezuelan Caribbean Mountain System: 40Ar/39Ar age insights from a Rodinian-related rock, the Cordillera de la Costa and Margarita Island

NASA Astrophysics Data System (ADS)

Fournier, Herbert W.; Lee, James K. W.; Urbani, Franco; Grande, Sebastián

2017-12-01

is 5 Ma younger than the last magmatic activity in the Aves Arc (c. 59 Ma) along its southern edge and related to the opening of the Grenada Basin. In the Cordillera de la Costa, magnesiohornblende, phengite, magnesian siderophyllite, ferroan phlogopite and K-feldspar from rock units of the Carayaca and Ávila terranes yield a wide range of ages (275-20 Ma). The oldest ages (275 Ma and 120 Ma) obtained from the Caruao Metatonalite suggest at least two thermal events at these times and are in agreement with the amalgamation of Pangaea and fluid infiltration, respectively. The remaining ages (35-20 Ma) are consistent with previous results and reflect short-lived fluid infiltrations related to tectonothermal events. A phengite age of 35 Ma from an omphacitite lens in the Antímano Marble of the Carayaca Terrane indicates a younger HP/LT event than previously stated. The preservation of this phengite age and constant Si-content profile indicate a short-lived HP/LT event followed by a very rapid exhumation. At 35-20 Ma rocks from the Ávila Terrane remained at shallower crustal levels than the ones of the Carayaca Terrane, but were thermally affected by tectonism and the infiltration of relatively cold and 40Ar-free fluids causing widespread chloritization of trioctahedral micas.

The single grain fusion dating approach: Determining the factors that control white mica 40Ar/39Ar age formation during HP metamorphism of the Cycladic Blueschist Unit, Greece

NASA Astrophysics Data System (ADS)

Uunk, Bertram; Wijbrans, Jan; Brouwer, Fraukje

2017-04-01

White mica 40Ar/39Ar dating is a proven powerful tool for constraining the timing and rate of metamorphism, deformation and exhumation. However, for high-pressure metamorphic rocks dating often results in wide age ranges, which are not in agreement with constraints from other isotopic systems, indicating that geological and chemical processes complicate straightforward 40Ar/39Ar dating. Despite hosting one of the largest geochronological datasets in the world, the Cycladic Blueschist Unit in Greece is presently one of the focal areas in the discussion on the interpretation of metamorphic 40Ar/39Ar ages. Previous phengite multi grain step heating experiments commonly yielded undulating age spectra ranging between 20 - 60 Ma. While some studies attempt to assign geological significance to these ages, others argue the ages are geologically meaningless and the result of the interplay between partial diffusive resetting and continued crystallization. By taking an alternative approach of multiple single grain fusion experiments, this study investigates age heterogeneity between samples of contrasting metamorphic facies, rheology and strain from the Cycladic islands of Syros and Sifnos. Comparing the size and shape of single grain fusion age distributions at the grain, rock, outcrop and island scale allows determination of the scale at which different age-forming processes operate. Resulting ages show a previously unreported consistent variation between different outcrops, moving from the eclogite-blueschist facies (55-45 Ma) to greenschist overprinting (40-30 Ma). This indicates that outcrop scale homogeneous resetting is the dominant processes for age formation in the CBU. Single grain age variation at the sample and outcrop scale is only limited to 10 Ma, indicating a smaller but observable role for local age perturbing processes of incomplete resetting, continued (re)crystallization or infiltration of excess argon. Some of the partially overprinted samples show

``Smoking From The Same Pipe": Developement of an 40Ar/39Ar Datting Intercalibration PIpette System (Invited)

NASA Astrophysics Data System (ADS)

Turrin, B. D.; Swisher, C. C.; Deino, A.; Hemming, S. R.; Hodges, K.; Renne, P. R.

2010-12-01

The precision and accuracy of Ar isotope ratio measurements is one of the main limiting factors in the uncertainties of an 40Ar/39Ar age. Currently, it is relatively common to measure Ar isotopic ratios to a precision of 1-2‰ or better on an intralaboratory basis. This level of analytical precision equates to a comparable level of precision (1-3‰) in the calculated age, depending on the extent of atmospheric Ar contamination, importance of nucleogenic interference corrections, and other factors. However, it has become clear that improving the precision of mass spectrometry is not the only bottleneck towards improving the accuracy and precision of 40Ar/39Ar dating in general. Rather, the most urgent issue is interlaboratory reproducibility. This became obvious in a recent EARTHTIME initiative undertaken to intercalibrate two commonly used 40Ar/39Ar standards [the Fish Canyon sanidine (FCs) and the Alder Creek sanidine (ACs)]. This effort revealed variations amongst laboratories (at the 1-2% level), an order of magnitude greater than the internal analytical precisions. To address these issues, we have proposed (to NSF) to construct two identical pipette systems loaded to identical starting pressures and with identical isotopic compositions. One pipette system will travel between participating 40Ar/39Ar labs and the second system will not travel and serve as the “Master” system to test for any fractionation or undocumented depletion of the traveling pipette system. In order to ensure delivery of uniform amounts of homogenous gas, the pipette system will be computer-controlled with preprogrammed routines and lockouts to prevent compromising the reservoirs. The pipette systems will deliver three gas samples with different isotopic ratios at two different pressures/concentrations. One pipette bulb will be of atmospheric isotopic composition, and the other two pipette bulbs will have 40Ar*/39ArK ratios corresponding to co-irradiated ACs and FCs fixed by their

Feldspar 40Ar/39Ar dating of ICDP PALEOVAN cores

NASA Astrophysics Data System (ADS)

Engelhardt, Jonathan Franz; Sudo, Masafumi; Stockhecke, Mona; Oberhänsli, Roland

2017-11-01

Volcaniclastic fall deposits in ICDP drilling cores from Lake Van, Turkey, contain sodium-rich sanidine and calcium-rich anorthoclase, which both comprise a variety of textural zoning and inclusions. An age model records the lake's history and is based on climate-stratigraphic correlations, tephrostratigraphy, paleomagnetics, and earlier 40Ar/39Ar analyses (Stockhecke et al., 2014b). Results from total fusion and stepwise heating 40Ar/39Ar analyses presented in this study allow for the comparison of radiometric constraints from texturally diversified feldspar and the multi-proxy lacustrine age model and vice versa. This study has investigated several grain-size fractions of feldspar from 13 volcaniclastic units. The feldspars show textural features that are visible in cathodoluminescence (CL) or back-scattered electron (BSE) images and can be subdivided into three dominant zoning-types: (1) compositional zoning, (2) round pseudo-oscillatory zoning and (3) resorbed and patchy zoning (Ginibre et al., 2004). Round pseudo-oscillatory zoning records a sensitive alternation of Fe and Ca that also reflects resorption processes. This is only visible in CL images. Compositional zoning reflects anticorrelated anorthite and orthoclase contents and is visible in BSE. Eleven inverse isochron ages from total fusion and three from stepwise heating analyses fit the age model. Four experiments resulted in older inverse isochron ages that do not concur with the model within 2σ uncertainties and that deviate from 1 ka to 17 ka minimum. C- and R-type zoning are interpreted as representing growth in magma chamber cupolas, as wall mushes, or in narrow conduits. Persistent compositions of PO-type crystals and abundant surfaces recording dissolution features correspond to formation within a magma chamber. C-type zoning and R-type zoning have revealed an irregular incorporation of melt and fluid inclusions. These two types of zoning in feldspar are interpreted as preferentially

Rotationally inelastic collisions of He and Ar with NaK: Theory and Experiment

NASA Astrophysics Data System (ADS)

Richter, K.; Price, T. J.; Jones, J.; Faust, C.; Hickman, A. P.; Huennekens, J.; Malenda, R. F.; Ross, A. J.; Harker, H.; Crozet, P.; Forrey, R. C.

2015-05-01

Rotationally inelastic collisions of NaK A1Σ+ molecules with He and Ar are studied. At Lehigh, we use pump-probe polarization labeling (PL) and laser-induced fluorescence (LIF) spectroscopy. At Lyon, Fourier transform (FT)-resolved LIF spectra are recorded. In both cases, the pump laser excites a particular ro-vibrational level A1Σ+ (v , J). We observe strong direct lines corresponding to transitions from the (v , J) level pumped, and weak satellite lines corresponding to transitions from collisionally-populated levels (v ,J' = J + ΔJ). The ratios of satellite to direct line intensities in LIF and PL yield population and orientation transfer information. A strong propensity for ΔJ = even transitions is observed for both He and Ar perturbers. In the FT fluorescence experiment we also observe v-changing collisions. Ab initio potential surface and scattering calculations are underway for collisions in the A1Σ+ and X1Σ+ states. For He-NaK we have calculated potential surfaces using GAMESS and carried out coupled channel scattering calculations of transfer of population, orientation, and alignment. Calculations of v-changing collision cross sections are also in progress. Work supported by NSF, XSEDE and CNRS (PICS).

Upper Triassic mafic dykes of Lake Nyos, Cameroon (West Africa) I: K-Ar age evidence within the context of Cameroon Line magmatism, and the tectonic significance

NASA Astrophysics Data System (ADS)

Aka, Festus Tongwa; Hasegawa, Takeshi; Nche, Linus Anye; Asaah, Asobo Nkengmatia Elvis; Mimba, Mumbfu Ernestine; Teitchou, Isidore; Ngwa, Caroline; Miyabuchi, Yasuo; Kobayashi, Tetsuo; Kankeu, Boniface; Yokoyama, Tetsuya; Tanyileke, Gregory; Ohba, Takeshi; Hell, Joseph Victor; Kusakabe, Minoru

2018-05-01

The hydrodynamic fragmentation that formed Lake Nyos in northwest Cameroon did not only make it the most unpopular lake in the world from a gas disaster perspective, it also opened a rare and formidable window through which much of the geology of Cameroon can be studied in a single locality. The Cambrian quartz monzonite cliff excavated by the maar-forming explosion and exposed in its northeastern shore is intruded by mafic dykes, two of which we dated. Even though close to one another, the dykes are different in composition. The alkaline dyke yields a slightly older (Carnian) K-Ar fedspar age of 231.1 ± 4.8 Ma, while the sub alkaline dyke yields an age of 224.8 ± 4.7 Ma (Norian). Based on radioisotopic age data available over the last 48 years (347 data) for the Cameroon Line magmatism comprising eruptives and volcano-plutonic complexes, the Nyos dykes are way older than the Cameroon Line, and even pre-date the Lower Cretaceous initiation of west Gondwana fragmentation in Equatorial Atlantic domain. They would therefore not have been directly linked to the formation of the Cameroon Line. Alternatively, they might be associated with the development of intra-continental rift systems in West Central Africa that pre-dated west Gondwana breakup to form the Atlantic Ocean.

Late Cretaceous remagnetization of Proterozoic mafic dikes, southern Highland Mountains, southwestern Montana: A paleomagnetic and 40Ar/39Ar study

USGS Publications Warehouse

Harlan, S.S.; Geissman, J.W.; Snee, L.W.; Reynolds, R.L.

1996-01-01

Paleomagnetic results from Early Proterozoic metabasite sills and Middle Proterozoic diabase dikes from the southern Highland Mountains of southwestern Montana give well-defined, dual-polarity magnetizations that are statistically identical to those from a small Late Cretaceous pluton that cuts the dikes. The concordance of paleomagnetic directions from rocks of three widely separated ages indicates that the Proterozoic rocks were remagnetized, probably during Late Cretaceous time. Paleomagnetic, rock magnetic, and petrographic observations from the metabasite and diabase samples indicate that remanence is carried primarily by low-Ti magnetite. Combining virtual geomagnetic poles from metabasite sills, diabase dikes, and the Late Cretaceous pluton, we obtain a paleomagnetic pole at 85.5??N, 310.7??E (K = 19.9, A95 = 9.1??, N = 14 sites) that is similar to a reference pole from the 74 Ma Adel Mountain Volcanics of western Montana. Biotite and hornblende 40Ar/39Ar isotopic dates from host basement geneiss and a hornblende from a remagnetized metabasite sill yield ages of ca. 1800 Ma; these dates probably record cooling of the southern Highland Mountains following high-grade metamorphism at 1.9-1.8 Ga. The gneiss and metabasite age spectra show virtually no evidence of disturbance, indicating that the basement rocks were never heated to temperatures sufficient to cause even partial resetting of their argon systems. Thus, the overprint magnetization of the Highland Mountains rocks is not a thermoremanent magnetization acquired during conductive cooling of nearby Late Cretaceous plutons. Remagnetization of the metabasite sills and diabase dikes was probably caused by localized thermochemical and thermoviscous effects during circulation of Late Cretaceous hydrothermal fluids related to epithermal mineralization. The absence of significant disturbance to the 40Ar/39Ar age spectrum from the remagnetized metabasite hornblende indicates that some secondary magnetizations may

40Ar/39Ar dating and zircon chronochemistry for the Izu-Bonin rear arc, IODP site U1437

NASA Astrophysics Data System (ADS)

Schmitt, A. K.; Konrad, K.; Andrews, G. D.; Horie, K.; Brown, S. R.; Koppers, A. A. P.; Busby, C.; Tamura, Y.

2016-12-01

The scientific objective of IODP Expedition 350 drilling at Site U1437 (31°47.390'N, 139°01.580'E) was to reveal the "missing half of the subduction factory": the rear arc of a long-lived intraoceanic subduction zone. Site U1437 lies in a 50 km long and 20 km wide volcano-bounded basin, 90 km west of the Izu arc front, and is the only IODP site drilled in the rear arc. The Izu rear arc is dominated by Miocene basaltic to dacitic seamount chains, which strike at a high angle to the arc front. Radiometric dating targeted a single igneous unit (1390 mbsf), and fine to coarse volcaniclastic units for which we present zircon and 40Ar/39Ar (hornblende, plagioclase, and groundmass) age determinations. All zircons analyzed as grain separates were screened for contamination from drill-mud (Andrews et al., 2016) by analyzing trace elements and, where material was available, O and Hf isotope compositions. Igneous Unit 1 is a rhyolite sheet and yielded concordant in-situ and crystal separate U-Pb zircon ages (13.7±0.3 Ma; MSWD = 1.3; n = 40 spots), whereas the 40Ar/39Ar hornblende plateau age (12.9±0.3; MSWD = 1.1; n = 9 steps) is slightly younger, possibly reflecting pre-eruptive zircon crystallization, or alteration of hornblende. U-Pb zircon and 40Ar/39Ar plateau ages from samples above igneous Unit 1 are concordant with biostratigraphic and paleomagnetic ages (available to 1300 mbsf), but plagioclase and groundmass samples below 1300 m become younger with depth, hinting at post-depositional alteration. A single zircon from 1600 mbsf yielded a U-Pb age of 15.4±1.8 Ma; its trace element composition resembles other igneous zircons from U1437, and is tentatively interpreted as a Middle Miocene age for the lowermost lithostratigraphic unit VII. Oxygen and Hf isotopic values of igneous zircon indicate mantle origins, with some influence of assimilation of hydrothermally altered oceanic crust evident in sub-mantle oxygen isotopic compositions. Lessons from site U1437 are

SHRIMP U-Pb and 40Ar/39Ar age constraints for relating plutonism and mineralization in the Boulder batholith region, Montana

USGS Publications Warehouse

Lund, K.; Aleinikoff, J.N.; Kunk, Michael J.; Unruh, D.M.; Zeihen, G.D.; Hodges, W.C.; du Bray, E.A.; O'Neill, J. M.

2002-01-01

The composite Boulder batholith, Montana, hosts a variety of mineral deposit types, including important silver-rich polymetallic quartz vein districts in the northern part of the batholith and the giant Butte porphyry copper-molybdenum pre-Main Stage system and crosscutting copper-rich Main Stage vein system in the southern part of the batholith. Previous dating studies have identified ambiguous relationships among igneous and mineralizing events. Mineralizing hydrothermal fluids for these types of deposits and magma for quartz porphyry dikes at Butte have all been considered to be late-stage differentiates of the Boulder batholith. However, previous dating studies indicated that the Boulder batholith plutons cooled from about 78 to 72 Ma, whereas copper-rich Main Stage veins at Butte were dated at about 61 Ma. Recent efforts to date the porphyry copper-molybdenum pre-Main Stage deposits at Butte resulted in conflicting estimates of both 64 and 76 Ma for the mineralizing events. Silver-rich polymetallic quartz vein deposits elsewhere in the batholith have not been dated previously. To resolve this controversy, we used the U.S. Geological Survey, Stanford, SHRIMP RG ion mic??roprobe to date single-age domains within zircons from plutonic rock samples and 40Ar/39Ar geochronology to date white mica, biotite, and K-feldspar from mineral deposits. U-Pb zircon ages are Rader Creek Granodiorite, 80.4 ?? 1.2 Ma; Unionville Granodiorite, 78.2 ?? 0.8 Ma; Pulpit Rock granite, 76.5 ?? 0.8 Ma; Butte Granite, 74.5 ?? 0.9 Ma; altered Steward-type quartz porphyry dike (I-15 roadcut), 66.5 ?? 1.0 Ma; altered Steward-type quartz porphyry dike (Continental pit), 65.7 ?? 0.9 Ma; and quartz monzodiorite of Boulder Baldy (Big Belt Mountains), 66.2 ?? 0.9 Ma. Zircons from Rader Creek Granodiorite and quartz porphyry dike samples contain Archean inheritance. The 40Ar/39Ar ages are muscovite, silver-rich polymetallic quartz vein (Basin district), 74.4 ?? 0.3 Ma; muscovite, silver

Paleomagnetism and 40Ar/39Ar ages from volcanics extruded during the Matuyama and Brunhes Chrons near McMurdo Sound, Antarctica

USGS Publications Warehouse

Tauxe, L.; Gans, Philip B.; Mankinen, Edward A.

2004-01-01

Maps of virtual geomagnetic poles derived from international geomagnetic reference field models show large lobes with significant departures from the spin axis. These lobes persist in field models for the last few millenia. The anomalous lobes are associated with observation sites at extreme southerly latitudes. To determine whether these features persist for millions of years, paleomagnetic vector data from the continent of Antarctica are essential. We present here new paleomagnetic vector data and 40Ar/39Ar ages from lava flows spanning the Brunhes and Matuyama Chrons from the vicinity of McMurdo Sound, Antarctica. Oriented paleomagnetic samples were collected from 50 lava flows by E. Mankinen and A. Cox in the 1965–1966 austral summer season. Preliminary data based largely on the natural remanent magnetization (NRM) directions were published by Mankinen and Cox [1988]. We have performed detailed paleomagnetic investigations of 37 sites with multiple fully oriented core samples to investigate the reliability of results from this unique sample collection. Of these, only one site fails to meet our acceptance criteria for directional data. Seven sites are reversely magnetized. The mean normal and reverse directions are antipodal. The combined mean direction has  = 12,  = −86, α = 4, κ = 37 and is indistinguishable from that expected from a GAD field. We obtained reproducible absolute paleointensity estimates from 15 lava flows with a mean dipole moment of 49 ZAm2 and a standard deviation of 28 ZAm2. 40Ar/39Ar age determinations were successfully carried out on samples from 18 of the flows. Our new isotopic ages and paleomagnetic polarities are consistent with the currently accepted geomagnetic reversal timescales.

Assessing the 40K decay constant by intercalibration with U-Pb, Rb-Sr and K-Ca chronometers

NASA Astrophysics Data System (ADS)

Naumenko-Dèzes, M. O.; Nagler, T. F.; Mezger, K.; Villa, I. M.

2016-12-01

Ar-Ar is one of the most used dating systems and its accuracy plays an important role in constraining the age of planets, durations of processes and their sequence of occurrence. This system has been reported to give ages that are ca. 1% younger than U-Pb ages. The discrepancies between the two mostly used and precise geochronometers, U-Pb and K-Ar, have been a subject of critical reviewing (e.g. Renne et al. [1] and ref. therein) and were attributed to a systematic offset of the 40K decay constant. Multiple attempts to recalibrate it did not achieve consistency.We attempted to intercalibrate three dating systems: U-Pb (the main reference), Rb-Sr (the consistency check) and K-Ca (the unknown). We examined 11 natural samples, but only a phlogopite from the Phalaborwa carbonatite complex met all requirements of a geological "point-like" event [2]. The Rb-Sr age of this sample is 2058.9±5.2 Ma and agrees with the age determined by Nebel et al [3] and with published U-Pb ages. The K-Ca age for the same sample calculated with constants of Steiger&Jäger [4] is 2040±13 Ma, ca.1% younger age than U-Pb reference age of the complex. Since we improved the Ca measurements [5] the K-Ca age has a low uncertainty of 0.6%. This age constrains the decay constant of the Ca branch of the 40K decay. However, the total decay constant is given by one equation with two unknowns, the branching ratio B and the decay constant of the Ar branch.Within the range of published branching ratios (B = 0.892 to 0.896 [6]) the best total 40K decay constant lies mid-way between the values proposed by Renne et al [7] and Min et al [8] (fig. 1).Figure 1. K-Ca age of Phalaborwa phlogopite changes along sloping lines as a function of assumed branching ratio B, calculated with the total 40K decay constant of [3]. [5], [6].[1] Renne et al. (2010) Geochim. Cosmochim Acta 74, 5349-5367; [2] Begemann et al. (2001) Geochim. Cosmochim Acta 65, 111-121; [3] Nebel et al (2010) GCA 74, 5349; [4] Steiger

[sup 40]Ar/[sup 39]Ar ages of Challis volcanic rocks and the initiation of Tertiary sedimentary basins in southwestern Montana

DOE Office of Scientific and Technical Information (OSTI.GOV)

M'Gonigle, J.W.; Dalrymple, G.B.

1993-10-01

[sup 40]Ar/[sup 39]Ar ages on single sanidine crystals from rhyolitic tuffs and ash flow tuffs within the uppermost and lowermost parts of the volcanic sequence of the Horse Prairie and Medicine Lodge topographic basins, southwestern Montana, show that these volcanic rocks were emplaced between about 48.8[+-]0.2 Ma and 45.9[+-]0.2 Ma, and are correlative with the Eocene Challis Volcanic Group of central Idaho. Sanidine ages on tuffs at the base of the Tertiary lacustrine, paludal, and fluvial sedimentary sequence, which unconformably overlies the volcanic sequence, suggest that sedimentation within an ancestral sedimentary basin that predated the development of the modern Horsemore » Prairie and Medicine Lodge basins began in the middle Eocene. 22 refs., 3 figs., 2 tabs.« less

Age of the Hawaiian-Emperor bend

USGS Publications Warehouse

Dalrymple, G.B.; Clague, D.A.

1976-01-01

40Ar/39Ar age data on alkalic and tholeiitic basalts from Diakakuji and Kinmei Seamounts in the vicinity of the Hawaiian-Emperor bend indicate that these volcanoes are about 41 and 39 m.y. old, respectively. Combined with previously published age data on Yuryaku and Ko??ko Seamounts, the new data indicate that the best age for the bend is 42.0 ?? 1.4 m.y. Petrochemical data indicate that the volcanic rocks recovered from bend seamounts are indistinguishable from Hawaiian volcanic rocks, strengthening the hypothesis that the Hawaiian-Emperor bend is part of the Hawaiian volcanic chain. 40Ar/39Ar total fusion ages on altered whole-rock basalt samples are consistent with feldspar ages and with 40Ar/39Ar incremental heating data and appear to reflect the crystallization ages of the samples even though conventional K-Ar ages are significantly younger. The cause of this effect is not known but it may be due to low-temperature loss of 39Ar from nonretentive montmorillonite clays that have also lost 40Ar. ?? 1976.

Diffusion in the Muscovite 40K Decay System (Invited)

NASA Astrophysics Data System (ADS)

Harrison, T. M.

2010-12-01

The considerable potential of muscovite for thermochronological applications is beginning to be fully exploited following the belated publication of Ar kinetic data. Muscovite’s high potassium content, low solubility for excess 40Ar*, and ubiquitous presence in regionally metamorphosed terranes make it an important phase for 40Ar/39Ar thermochronometry, particularly in light of recognition that both age spectra and vacuum-step-heating-derived 39Ar Arrhenius plots reflect Ar release via the same volume diffusion mechanism. Thus instead of assuming a nominal closure temperature to estimate a single T-t datum, continuous and accurate thermal histories can be inferred in a similar fashion to that well-documented for K-feldspar using the multi-diffusion domain (MDD) model. The Arrhenius parameters for Ar diffusion in muscovite (E=64 kcal/mol, Do=4 cm2/s) correspond to an effective intragrain closure temperature range of ~500 to 300oC for ca. 100 μm grains cooling at ~10oC/Ma at 5 kbar. However, even greater exploitation of the 40K decay system remains possible as only one of every ten 40K atoms decay to 40Ar. The other 90% decay to 40Ca giving the 40K-40Ca branch, in principle, greater sensitivity for dating high K/Ca minerals such as muscovite. The advent of the ‘double-plus’ SIMS 40K++-40Ca++ dating method, which permits analysis of Ca isotopes at an MRP of ~4k rather than the ~25k required for full separation of 40K+ from 40Ca+, opens up the prospect of directly revealing 40K-40Ca closure profiles in muscovite (as opposed to their indirect inference from inversion of 40Ar/39Ar data through the MDD model) at a gain of enhanced precision and accuracy in thermal history reconstruction. We have used SIMS to observe K-Ca age variations in natural muscovites pressed into In. Translating this data into thermal history information, however, requires knowledge of the Arrhenius parameters for Ca tracer diffusion in muscovite. We are undertaking hydrothermal piston

New 40Ar/39Ar age progression for the Louisville hot spot trail and implications for inter-hot spot motion

NASA Astrophysics Data System (ADS)

Koppers, Anthony A. P.; Gowen, Molly D.; Colwell, Lauren E.; Gee, Jeffrey S.; Lonsdale, Peter F.; Mahoney, John J.; Duncan, Robert A.

2011-12-01

In this study we present 42 new 40Ar/39Ar incremental heating age determinations that contribute to an updated age progression for the Louisville seamount trail. Louisville is the South Pacific counterpart to the Hawaiian-Emperor seamount trail, both trails representing intraplate volcanism over the same time interval (˜80 Ma to present) and being examples of primary hot spot lineaments. Our data provide evidence for an age-progressive trend from 71 to 21 Ma. Assuming fixed hot spots, this makes possible a direct comparison to the Hawaiian-Emperor age progression and the most recent absolute plate motion (APM) model (WK08G) of Wessel and Kroenke (2008). We observe that for the Louisville seamount trail the measured ages are systematically older relative to both the WK08G model predictions and Hawaiian seamount ages, with offsets ranging up to 6 Myr. Taking into account the uncertainty about the duration of eruption and magmatic succession at individual Louisville volcanoes, these age offsets should be considered minimum estimates, as our sampling probably tended to recover the youngest lava flows. These large deviations point to either a contribution of inter-hot spot motion between the Louisville and Hawaiian hot spots or to a more easterly location of the Louisville hot spot than the one inferred in the WK08G model. Both scenarios are investigated in this paper, whereby the more eastern hot spot location (52.0°S, 134.5°W versus 52.4°S, 137.2°W) reduces the average age offset, but still results in a relatively large maximum offset of 3.7 Myr. When comparing the new ages to the APM models (S04P, S04G) by Steinberger et al. (2004) that attempt to compensate for the motion of hot spots in the Pacific (Hawaii) or globally (Hawaii, Louisville, Reunion and Walvis), the measured and predicted ages are more in agreement, showing only a maximum offset of 2.3 Myr with respect to the S04G model. At face value these more advanced APM models, which consider both plate and

Inter-monitor standard calibration and tests for Ar-Ar biases

NASA Astrophysics Data System (ADS)

Hemming, S. R.; Turrin, B. D.; Swisher, C. C.; Cox, S. E.; Mesko, G. T.; Chang, S.

2010-12-01

A major issue facing the geochronology community is that there are biases between chronometers that have become significant as we interrogate the rock record with ever increasing levels of precision. Despite much progress there are still major issues with building a timescale with multiple chronometers and for testing synchroneity of anomalous events in Earth history. Improvements in methods for determining U-Pb zircon dates has led to their application at precisions of 0.2% or better in rocks even younger than a million years (e.g., Crowley et al., 2007, Geology), and significantly better than 0.1% in some cases (e.g., Bowring et al., 2006, Paleontological Society Papers, Volume 12). Additionally, the inter-calibration experiments for U-Pb using the EARTHTIME tracer have yielded excellent agreement among labs (0.05%) and these values are traceable back to SI units through the EARTHTIME tracer calibration experiment (e.g., Condon et al., in press, Geochimica et Cosmochimica Acta). These advances have greatly extended the need for cross calibrations of the two chronometers and ultimately seamless integration into the Geologic Time Scale. The direct comparison of ages using different chronometers and laboratories is the central aspect in the quest for a highly resolved and accurate time scale of Earth History. A significant obstacle to high precision inter-comparison of U-Pb and Ar-Ar age results is the current inability of Ar-Ar labs to achieve agreement on monitor standard ages at the 0.1% level. At the heart of Ar-Ar geochronology is the assumption of a known absolute age of a standard, to which all applicable unknowns are referenced. While individual labs are able to achieve highly precise apparent ages on monitor standards, the lack of a “gold standard” for Ar-Ar dating means that we do not know who is, or indeed if anybody is correct. In order to improve our understanding of factors that may lead to biases in our own laboratories at Lamont-Doherty Earth

Ar-Ar Thermochronlogy of Apollo 12 Impact-Melt Breccia 12033,638-1

NASA Technical Reports Server (NTRS)

Crow, C. A.; Cassata, W. S.; Jolliff, B. L.; Ziegler, R. A.; Borg, L. E.; Shearer, C. K.

2017-01-01

We have undertaken an Ar-Ar thermochronology investigation as part of a coordinated multichronometer analysis of a single Apollo 12 impact- melt breccia to demonstrate the wide range of information that can be obtained for a single complex rock. This has implications for the age of formation, component makeup, and subsequent impact/shock and exposure history of the sample. This study also serves as a capabilities demonstration for the proposed MoonRise Mission [1]. The goal of this investigation is to elucidate the history of this sample through coordinated 40Ar*/39Ar, Sm-Nd, Rb-Sr and zircon 207Pb-206Pb ages along with geochemical and petrographic context on a relatively small (approximately 450 mg) sample. Here, we report preliminary results of the Ar-Ar thermochronology.

Rotationally inelastic collisions of He and Ar with NaK: Theory and Experiment

NASA Astrophysics Data System (ADS)

Richter, K.; Price, T.; Jones, J.; Faust, C.; Hickman, A. P.; Huennekens, J.; Malenda, R. F.; Ross, A. J.; Crozet, P.

2014-05-01

Rotationally inelastic collisions of NaK (A1Σ+) molecules with He and Ar have been studied. At Lehigh, we use a pump-probe scheme (the probe is scanned over transitions to the 31 Π state) with either polarization labeling (PL) or laser-induced fluorescence (LIF) spectroscopy. At Lyon, one-laser excitation is used with Fourier Transform (FT) fluorescence spectroscopy. In both cases, the pump laser excites a particular ro-vibrational level A1Σ+ (v , J). We observe strong direct lines corresponding to transitions from the (v , J) level pumped, and weak satellite lines corresponding to transitions from collisionally-populated levels (v ,J' = J + ΔJ). The ratios of satellite to direct line intensities in LIF and PL yield information about population and orientation transfer. A strong propensity for ΔJ = even transitions is observed for both He and Ar perturbers. In the FT fluorescence experiment we also observe v changing collisions. Theoretical calculations are also underway for collisions in both the A1Σ+ and X1Σ+ states. For He-NaK we have calculated potential surfaces using GAMESS and carried out coupled channel scattering calculations of transfer of population, orientation, and alignment. Work supported by NSF, XSEDE and CNRS (PICS).

Eurekan deformation on Prins Karls Forland, Svalbard - new insights from Ar40/Ar39 muscovite dating

NASA Astrophysics Data System (ADS)

Faehnrich, Karol; Schneider, David; Manecki, Maciej; Czerny, Jerzy; Myhre, Per Inge; Majka, Jarosław; Kośmińska, Karolina; Barnes, Christopher; Maraszewska, Maria

2017-04-01

Eurekan deformation has been proven to be a complex sequence of tectonic episodes, dominated by compression in the Circum Arctic region. It was associated with early Cenozoic collision of Eurasia, North America and Greenland plates producing fold-thrust belt style of deformation. Timing of this enigmatic event has not yet been extensively resolved by radiometric dating (Piepjohn et al. 2016, Journal of the Geological Society, 173(6), 1007-1024). Reinhardt et al. (2013, Z. Dt. Ges. Geowiss., 164 (1), 131-147) dated syn-tectonic volcanic ashes at c. 60 Ma and 54 Ma on Ellesmere Island, Canada. Tagner et al. (2011, Earth and Planetary Science Letters, 303(3), 203-214) interpreted c. 49-47 Ma 40Ar/39Ar ages on trachyte flows in northern Greenland as peak compression during the Eurekan event. On Svalbard, Tessensohn et al. (2001, Geologisches Jahrbuch, B 91, 83-104) reported K/Ar whole rock ages ranging from c. 67 to 49 Ma for the slates from Svartfjella-Eidembukta-Daudmannsodden Lineament. Bentonite layers in the Central Tertiary Basin are as young as c. 56 Ma (Charles et al. 2011, Geochem. Geophys. Geosyst., 12, 1-19), predating latest deformation. Moreover, Barnes et al (2017, in prep.) applied (U-Th)/He thermochronology along the western margin of Svalbard and resolved Early to Middle Eocene heating, likely documenting burial related to thrusting. Here we present new results from 40Ar/39Ar muscovite dating of ductile to brittle shear zone on Prins Karls Forland, Svalbard, indicating Eurekan age of thrusting. Prins Karls Forland is dominated by Neoproterozoic siliciclastic metasediments (comprising Caledonian basement) regionally metamorphosed to greenschist facies conditions. A ˜1 km wide ductile to brittle shear zone (the Bouréefjellet shear zone) separates the amphibolite facies Pinkie Unit from the lower grade upper structural unit, the Grampianfjella Formation (Faehnrich et al. 2016, EGU 2016). The age of the amphibolite facies metamorphism (c. 370-355 Ma

The contemporary degassing rate of 40Ar from the solid Earth.

PubMed

Bender, Michael L; Barnett, Bruce; Dreyfus, Gabrielle; Jouzel, Jean; Porcelli, Don

2008-06-17

Knowledge of the outgassing history of radiogenic (40)Ar, derived over geologic time from the radioactive decay of (40)K, contributes to our understanding of the geodynamic history of the planet and the origin of volatiles on Earth's surface. The (40)Ar inventory of the atmosphere equals total (40)Ar outgassing during Earth history. Here, we report the current rate of (40)Ar outgassing, accessed by measuring the Ar isotope composition of trapped gases in samples of the Vostok and Dome C deep ice cores dating back to almost 800 ka. The modern outgassing rate (1.1 +/- 0.1 x 10(8) mol/yr) is in the range of values expected by summing outgassing from the continental crust and the upper mantle, as estimated from simple calculations and models. The measured outgassing rate is also of interest because it allows dating of air trapped in ancient ice core samples of unknown age, although uncertainties are large (+/-180 kyr for a single sample or +/-11% of the calculated age, whichever is greater).

The contemporary degassing rate of 40Ar from the solid Earth

PubMed Central

Bender, Michael L.; Barnett, Bruce; Dreyfus, Gabrielle; Jouzel, Jean; Porcelli, Don

2008-01-01

Knowledge of the outgassing history of radiogenic 40Ar, derived over geologic time from the radioactive decay of 40K, contributes to our understanding of the geodynamic history of the planet and the origin of volatiles on Earth's surface. The 40Ar inventory of the atmosphere equals total 40Ar outgassing during Earth history. Here, we report the current rate of 40Ar outgassing, accessed by measuring the Ar isotope composition of trapped gases in samples of the Vostok and Dome C deep ice cores dating back to almost 800 ka. The modern outgassing rate (1.1 ± 0.1 × 108 mol/yr) is in the range of values expected by summing outgassing from the continental crust and the upper mantle, as estimated from simple calculations and models. The measured outgassing rate is also of interest because it allows dating of air trapped in ancient ice core samples of unknown age, although uncertainties are large (±180 kyr for a single sample or ±11% of the calculated age, whichever is greater). PMID:18550816

AR-40 AR-39 Age of an Impact-Melt Lithology in DHOFAR 961

NASA Technical Reports Server (NTRS)

Frasl, B.; Cohen, B. A.; Li, Z.-H.; Jolliff, B.; Korotev, R.; Zeigler, R.

2016-01-01

, although they note that further age dating of all the stones is required to test this hypothesis. We received a split of Dhofar 961 from R. Zeigler consisting of a large clast of IM Lithology B, with some light-colored, friable matrix clinging to the external margins of the impact-melt clast. This lithology was not present in the samples investigated by Joy et al. and thus does not have corresponding U-Pb ages on it. We created multiple subsplits of both the IM and matrix lithologies, each weighing several tens of micrograms. We conducted Ar-40 Ar-39 dating of this candidate SPA material by high-resolution step heating and comparing it with the regolith that surrounds it.

The sup 40 Ar/ sup 39 Ar geochronology of the Pelona schist and related rocks, southern California

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jacobson, C.E.

1990-01-10

Seventeen {sup 40}Ar/{sup 39}Ar ages for hornblende, celadonitic muscovite, and biotite from the Pelona, Orocopia, Rand, and Portal Ridge (POR) schists range from 39 to 85 Ma. Two muscovites and one hornblende from the Rand Schist have ages of 72 to 74 Ma, indistinguishable from the K-Ar age of 74 Ma for hornblende from a posttectonic granodiorite that intrudes the schist, but younger than the 70 Ma U-Pb age of the intrusion. Four muscovite and two hornblende ages for schist and mylonite from the East Fork area of the San Gabriel Mountains range from 55 to 61 Ma. Concordance ofmore » schist and upper plate ages confirms structural and metamorphic evidence that the Vincent thrust in the San Gabriel Mountains has not undergone significant postmetamorphic disruption. Ages from the Orocopia Mountains are 75 Ma for hornblende from nonmylonitic upper plate, 52 Ma for muscovite from structurally high Orocopia Schist that is mylonitic, and 41 Ma for muscovite from nonmylonitic Orocopia Schist. These are consistent with field evidence that the Orocopia thrust is a postmetamorphic normal fault. Muscovite and hornblende from the Gavilan Hills have ages of 48 to 50 Ma, younger than ages from the San Gabriel Mountains but similar to schist ages from the Orocopia Mountains. The geochronologic and structural complexities of the Vincent, Chocolate Mountains, Orocopia, and Rand thrusts imply that previously cited northeastward vergence may not relate to prograde metamorphism (subduction) of the POR schists. The data indicate substantial uplift of the POR schists prior to middle Tertiary detachment faulting, which confirms other geochronologic evidence of uplift in southern California and southern Arizona during the Late Cretaceous-early Tertiary.« less

Sm-Nd, K-Ar and petrologic study of some kimberlites from eastern United States and their implication for mantle evolution

USGS Publications Warehouse

Basu, A.R.; Rubury, E.; Mehnert, H.; Tatsumoto, M.

1984-01-01

We provide new data on Sm-Nd systematics, K-Ar dating and the major element chemistry of kimberlites from the eastern United States (mostly from central New York State) and their constituent mineral phases of olivine, clinopyroxene, garnet, phlogopite and perovskite. In addition, we report Nd-isotopes in a few kimberlites from South Africa, Lesotho and from the eastern part of China. The major element compositions of the New York dike rocks and of their constituent minerals including a xenolith of eclogite are comparable with those from the Kimberley area in South Africa. The K-Ar age of emplacement of the New York dikes is further established to be 143 Ma. We have analyzed the Nd-isotopic composition of the following kimberlites and related rocks: Nine kimberlite pipes from South Africa and Lesotho, two from southern India; one from the U.S.S.R., fifteen kimberlite pipes and related dike rocks from eastern and central U.S. and two pipes from the Shandong Province of eastern China. The age of emplacement of these kimberlites ranges from 1300 million years to 90 million years. The initial Nd-isotopic compositions of these kimberlitic rocks expressed as e{open}NdIwith respect to a chondritic bulk-earth growth-curve show a range between 0 and +4, with the majority of the kimberlites being in the range 0 to +2. This range is not matched by any other suite of mantle-derived igneous rocks. This result strengthens our earlier conclusion that kimberlitic liquids are derived from a relatively primeval and unique mantle reservoir with a nearly chondritic Sm/Nd ratio. ?? 1984 Springer-Verlag.

Xe-129 - Xe-128 and Ar-40 - Ar-39 chronology of two Antarctic enstatite meteorites

NASA Technical Reports Server (NTRS)

Honda, M.; Bernatowicz, T. J.; Podosek, F. A.

1983-01-01

Xe-129 - Xe-128 and Ar-40 - Ar-39 analyses has been performed on two Antarctic enstatite meteorites, the chondrite Y-691 and the aubrite (enstatite achondrite) ALH-78113. Both meteorites have complex Ar-40 - Ar-39 release patterns to which no unambiguous age assignment is possible. Both give apparently satisfactory Xe-129 - Xe-128 correlations corresponding to unusual ages. The I-Xe age of the chondrite Y-691 is 16 Ma after Bjurbole, not unusual for chondrites in general but 10 Ma later than previously known ages for enstatite chondrites. The I-Xe age of the aubrite ALH-78113 is 210 Ma after Bjurbole, the latest age (rather than a limit) so far observed by the I-Xe technique, but this age assignment must be considered tentative because of the possibility that it is significantly influenced by terrestrial I contamination.

Mineralogy, Petrology, Chemistry, and Ar-39 - Ar-40 and Ages of the Caddo County IAB Iron: Evidence for Early Partial Melt Segregation of a Gabbro Area Rich in Plagioclase-Diopside

NASA Technical Reports Server (NTRS)

Takeda, Hiroshi; Bogard, Donald D.; Mittlefehldt, David W.; Garrison, Daniel H.

2000-01-01

We found coarse-grained gabbroic material rich in plagioclase and diopside in the Caddo County IAB iron meteorite. The polished thin sections studied were made from areas rich in Al and Ca detected by a micro-focus X-ray fluorescence (XRF) mapping technique. The gabbro is not a clast within a breccia, but rather this area is located mainly at silicate-metal boundaries only a few cm away from an area with fine-grained, ultramafic silicate similar to winonaites. Medium-grained orthopyroxene and olivine are found in transitional areas showing no disturbance of their crystalline textures. A vein-like region, starting at the area rich in fine-grained mafic silicate, extends towards the gabbroic area with a gradual increase in abundance of plagioclase and diopside. This texture and our accumulated knowledge of the formation mechanism of IAB/winonaltes meteorites, suggest that the gabbroic materials were formed by inhomogeneous segregation of partial melts of chondritic source materials. Compositional data on two mineralogically distinct samples of the gabbro-rich portion of the inclusion were obtained by INAA. Compared to an average of LAB silicate inclusions or winonaites, the Caddo County gabbro is enriched in the incompatible lithophile elements Na, Ca, Sc, REE and Hf, which is consistent with a melt origin for the gabbro. The cosmogenic space exposure age of Caddo County (511 Ma) is significantly younger than exposure ages of some other IAB meteorites, An 39Ar-40Ar age determination of the gabbroic material indicates a series of upward steps in age from 4.516 Ga to 4.523 Ga, with a few high temperature ages up to 4.54 Ga. The older age could approximate the primary recrystallization age of silicates. The stepped Ar age spectrum may indicate differences in Ar closure temperatures during slow cooling of -2-20'C/Myr in the parent body. Alternatively, the younger Ar-Ar ages may date a shock event which occurred while Caddo County was hot and which also created textures

U-Pb and K-Ar geochronology in Paleozoic and Mesozoic intrusive rocks of the Coastal Cordillera, Valparaiso, Chile

USGS Publications Warehouse

Gana, Paulina; Tosdal, Richard M.

1996-01-01

The U-Pb and K-Ar geochronology applied to intrusive rocks from the Coastal Batholith of Central Chile, demonstrates the existence of a basement block of the Mirasol Unit, with a crystallization age of 299??10 Ma, exposed in the northern block of the Melipilla Fault. The age of 214??1 Ma obtained in the 'Dioritas Gne??isicas de Cartagena Unit', indicates that a Late Triassic magmatism took place in this region; it coincides with the end of an extensive crustal melting period, proposed for northern Chile. The ages of the Jurassic plutonic units (Laguna Verde, Sauce, Pen??uelas and Limache) are restricted to the 156-161 Ma interval, showing in certain cases, inherited zircons from an unknown source. The difference between ages obtained using both chronological methods is a few million years, indicating that a short time passed between the crystallization and the cooling of the plutonic bodies, as well as a fast magmatic differentiation process. The Laguna Verde and Sauce Units, experienced a fast uplift, probably as a result of an extensional tectonic process in the magmatic arc, or induced by the magmatic pressure through fracture zones during Middle Jurassic.

The role of mitochondrial fusion and StAR phosphorylation in the regulation of StAR activity and steroidogenesis.

PubMed

Castillo, Ana F; Orlando, Ulises; Helfenberger, Katia E; Poderoso, Cecilia; Podesta, Ernesto J

2015-06-15

The steroidogenic acute regulatory (StAR) protein regulates the rate-limiting step in steroidogenesis, i.e. the delivery of cholesterol from the outer (OMM) to the inner (IMM) mitochondrial membrane. StAR is a 37-kDa protein with an N-terminal mitochondrial targeting sequence that is cleaved off during mitochondrial import to yield 30-kDa intramitochondrial StAR. StAR acts exclusively on the OMM and its activity is proportional to how long it remains on the OMM. However, the precise fashion and the molecular mechanism in which StAR remains on the OMM have not been elucidated yet. In this work we will discuss the role of mitochondrial fusion and StAR phosphorylation by the extracellular signal-regulated kinases 1/2 (ERK1/2) as part of the mechanism that regulates StAR retention on the OMM and activity. Copyright © 2014 Elsevier Ireland Ltd. All rights reserved.

Implications of new ^{40}Ar/^{39}Ar age of Mallapur Intrusives on the chronology and evolution of the Kaladgi Basin, Dharwar Craton, India

NASA Astrophysics Data System (ADS)

Pillai, Shilpa Patil; Pande, Kanchan; Kale, Vivek S.

2018-04-01

The Kaladgi Basin on the northern edge of the Dharwar craton has characters diverse from the other epicratonic Purana basins of Peninsular India. Sedimentological studies in the basin have established the presence of three cycles of flooding separated by an event of intra-basinal deformation accompanied by low grade incipient metamorphism. The overall structural configuration of the basin indicates its development by supracrustal extension accompanied by shearing in a trans-tensional regime during the Mesoproterozoic. This was followed by sagging that yielded Neoproterozoic sedimentation in a successor nested basin. ^{40}Ar/^{39}Ar dating of an intrusive mafic dyke along the axial plane of a fold has yielded a plateau age of 1154{± }4 Ma. This helps constraint the age of the various events during the evolution of this basin.

Evidence for shock heating and constraints on Martian surface temperatures revealed by 40Ar/ 39Ar thermochronometry of Martian meteorites

NASA Astrophysics Data System (ADS)

Cassata, William S.; Shuster, David L.; Renne, Paul R.; Weiss, Benjamin P.

2010-12-01

The thermal histories of Martian meteorite are important for the interpretation of petrologic, geochemical, geochronological, and paleomagnetic constraints that they provide on the evolution of Mars. In this paper, we quantify 40Ar/ 39Ar ages and Ar diffusion kinetics of Martian meteorites Allan Hills (ALH) 84001, Nakhla, and Miller Range (MIL) 03346. We constrain the thermal history of each meteorite and discuss the resulting implications for their petrology, paleomagnetism, and geochronology. Maskelynite in ALH 84001 yields a 40Ar/ 39Ar isochron age of 4163 ± 35 Ma, which is indistinguishable from recent Pb-Pb ( Bouvier et al., 2009a) and Lu-Hf ages ( Lapen et al., 2010). The high precision of this result arises from clear resolution of a reproducible trapped 40Ar/ 36Ar component in maskelynite in ALH 84001 ( 40Ar/ 36Ar = 632 ± 90). The maskelynite 40Ar/ 39Ar age predates the Late Heavy Bombardment and likely represents the time at which the original natural remanent magnetization (NRM) component observed in ALH 84001 was acquired. Nakhla and MIL 03346 yield 40Ar/ 39Ar isochron ages of 1332 ± 24 and 1339 ± 8 Ma, respectively, which we interpret to date crystallization. Multi-phase, multi-domain diffusion models constrained by the observed Ar diffusion kinetics and 40Ar/ 39Ar age spectra suggest that localized regions within both ALH 84001 and Nakhla were intensely heated for brief durations during shock events at 1158 ± 110 and 913 ± 9 Ma, respectively. These ages may date the marginal melting of pyroxene in each rock, mobilization of carbonates and maskelynite in ALH 84001, and NRM overprints observed in ALH 84001. The inferred peak temperatures of the shock heating events (>1400 °C) are sufficient to mobilize Ar, Sr, and Pb in constituent minerals, which may explain some of the dispersion observed in 40Ar/ 39Ar, Rb-Sr, and U-Th-Pb data toward ages younger than ˜4.1 Ga. The data also place conservative upper bounds on the long-duration residence

Rb-Sr and K-Ar age of globular phyllosilicates and biostratigraphy of the Riphean deposits of the Olenek Uplift (North Siberia)

NASA Astrophysics Data System (ADS)

Zaitseva, T. S.; Gorokhov, I. M.; Semikhatov, M. A.; Ivanovskaya, T. A.; Kuznetsov, A. B.; Dorzhieva, O. V.

2017-11-01

This work presents results of the complex mineralogical, geochemical, and isotope-geochronological investigation of globular dioctahedral 2: 1 phyllosilicates (GPS) of the illite-glauconite series from the Riphean sequences of the Olenek Uplift. It is established that GPS (glauconite, Al-glauconite, Fe-illite) in deposits of the Arymass, Debengda, and Khaipakh formations are represented by mixed-layer varieties of two types: (1) with relatively low (<10%) and (2) higher (10-20%) contents of expandable layers. Among the mixed-layer varieties are those with disordered alternation of micaceous and smectite layers (R = 0), as well as with tendency to ordering (R ≥ 1). The parameter b of an elementary cell of minerals varies from 9.18 to 9.72 Å. The Rb-Sr age dating of GPS was first carried out in combination with the calculation of theoretical pattern of the cation distribution in the mineral structure and comparison of the calculation results obtained with the Mössbauer and IR spectroscopy data. This approach is based on the assumption that development and evolution of isotope systems in GPS are synchronous with the evolution of the crystalline structure of the mineral at various stages of the geological and geochemical history of the development of sedimentary units. Analysis of the obtained data allows us to state that the structural features of the Riphean GPS from the Olenek section reflect the early diagenetic stages of the formation of the minerals studied. The 87Sr/86Sr initial ratios in the studied sediments are consistent with the range of variations in this ratio in the Middle Riphean Ocean (0.7049-0.7061). The Rb-Sr and K-Ar ages of the GPS of the Arymass (1305 ± 8 and 1302 Ma, respectively), Debengda (1265 ± 12 and 1284 ± 22 Ma), and Khaipakh (1172 ± 18 and 1112 ± 24 Ma) formations in the Olenek Uplift section are close to the accumulation time of corresponding deposits and, correspondingly, have significance for stratigraphic correlations.

Geomagnetic field excursion recorded 17 ka at Tianchi Volcano, China: New 40Ar/39Ar age and significance

NASA Astrophysics Data System (ADS)

Singer, Brad S.; Jicha, Brian R.; He, Huaiyu; Zhu, Rixiang

2014-04-01

New 40Ar/39Ar dating of a comenditic lava atop Tianchi Volcano, China, indicates eruption at 17.1 ± 0.9 ka. The flow interior records a pair of transitional virtual geomagnetic poles and a low paleointensity of ~25 μT. Thus, it records a geomagnetic field excursion that is younger than the 41 ka Laschamp or 32 ka Auckland excursions. Implications are: (1) following a repose of several tens of kyr, Tianchi Volcano became highly active immediately following termination of the last glaciation maximum. The flare-up of silicic eruptions may reflect rapid deglaciation of the edifice. (2) A 17 ka age for the Tianchi excursion provides the first direct radioisotopic evidence that excursional behavior, which is imprecisely dated and less well documented magnetically at several other sites, is a global feature of geodynamo behavior. (3) During the Brunhes chron, 13 well-dated excursions cluster into two periods, including seven between 17 and 212 ka, and six between about 530 and 730 ka.

An Impact Event on the Lunar Surface at ~3.3 Ga Inferred From 40Ar/39Ar Thermochronometry of Multiple Samples in Apollo 16 Soil 63503

NASA Astrophysics Data System (ADS)

Shuster, D. L.; Barrick-Bethell, I.; Fernandes, V. A.; Weiss, B. P.

2008-12-01

High-Resolution 40Ar/39Ar thermochronometry conducted on 8 separate whole-rock samples including shocked anorthosites, anorthositic impact melts, and fragmented breccias from Apollo 16 soil 63503 reveals an apparently short-duration thermal disturbance at ~3.3 Ga. Using feedback-controlled laser heating, we quantified the Ar diffusion kinetics and spatial distribution of radiogenic 40Ar (primarily contained in plagioclase) for each rock. Five of the samples show clear evidence for a partial resetting event in the K-Ar system at ~3.3 ± 0.1 Ga, despite two of them having 40Ar/39Ar "plateau" ages of ~3.9 Ga, and 3 having ages of ~4.2 Ga. Assuming that the observed Ar diffusion kinetics applies over all time, we constrained a time-temperature (t-T) pair required to diffusively perturb each of the 40Ar distributions at 3.3 Ga and result in the observed 40Ar/39Ar age spectra today. For each sample, this calculation quantified the non-dimensional parameter Dt/a2, with an estimated uncertainty. With each sample's Ar diffusion kinetics, the value of Dt/a2 corresponds to a non- unique family of t-T pairs (shown as continuous curves in t versus T); each pair would result in equivalent diffusive disturbance of the 40Ar spatial distribution at 3.3 Ga. Assuming there exists a best- fit solution for all 8 samples (i.e., that all samples experienced a common thermal event at 3.3 Ga), we constrain the unique t-T solution by minimizing the differences between each. We find that an instantaneous increase in temperature from ambient lunar near-surface conditions to ~530 °C for ~2.5 hours at 3.3 Ga would best explain the entire dataset, although longer durations (~days) spent cooling down to lower temperatures are permissible within the data uncertainty. The dataset also excludes the possibility of significant thermal disturbances after 3.3 Ga. We interpret the best-fit solution to represent the mean thermal state of the rocks during an impact event at ~3.3 Ga. This event may have

The Effects of B, K10, and AR Chromosomes on the Resistance of Maize to Viral Infection

PubMed Central

McGirr, Scott C.; Endrizzi, J. E.

1978-01-01

Studies were conducted to determine if accessory (B) chromosomes, the abnormal tenth (K10) chromosome or the aberrant ratio (AR) phenomenon of maize (Zea mays L.) affect the resistance of the plants to viral infection. Genetically similar stocks of maize with and without these elements were compared to determine what effect they would have on the plants response to Brome Mosaic Virus (BMV), Maize Dwarf Mosaic Virus (MDMV), Wheat Streak Mosaic Virus (WSMV) and Barley Stripe Mosaic Virus (BSMV).—The test results with BSMV were not found to be conclusive. With BMV and MDMV, neither the B orK10 chromosomes were found to alter infections; however, these chromosomes were found to affect the resistance of the plants to WSMV infection. The B chromosomes were found to delay the onset of leaf necrosis by 15%, while the K10 chromosome was found to increase the susceptibility to necrosis by 100%. The AR phenomenon was not found to alter the resistance of maize to BMV infection. However, it was found to increase the susceptibility of maize to MDMV infection by 36% and to decrease the susceptibility of maize to WSMV infection by 92%. PMID:17248865

The Effects of B, K10, and AR Chromosomes on the Resistance of Maize to Viral Infection.

PubMed

McGirr, S C; Endrizzi, J E

1978-10-01

Studies were conducted to determine if accessory (B) chromosomes, the abnormal tenth (K10) chromosome or the aberrant ratio (AR) phenomenon of maize (Zea mays L.) affect the resistance of the plants to viral infection. Genetically similar stocks of maize with and without these elements were compared to determine what effect they would have on the plants response to Brome Mosaic Virus (BMV), Maize Dwarf Mosaic Virus (MDMV), Wheat Streak Mosaic Virus (WSMV) and Barley Stripe Mosaic Virus (BSMV).-The test results with BSMV were not found to be conclusive. With BMV and MDMV, neither the B orK10 chromosomes were found to alter infections; however, these chromosomes were found to affect the resistance of the plants to WSMV infection. The B chromosomes were found to delay the onset of leaf necrosis by 15%, while the K10 chromosome was found to increase the susceptibility to necrosis by 100%. The AR phenomenon was not found to alter the resistance of maize to BMV infection. However, it was found to increase the susceptibility of maize to MDMV infection by 36% and to decrease the susceptibility of maize to WSMV infection by 92%.

Decreased H3K9ac level of StAR mediated testicular dysplasia induced by prenatal dexamethasone exposure in male offspring rats.

PubMed

Liu, Min; Chen, Biao; Pei, Linguo; Zhang, Qi; Zou, Yunfei; Xiao, Hao; Zhou, Jin; Chen, Liaobin; Wang, Hui

2018-06-11

Prenatal dexamethasone exposure (PDE) could induce testicular developmental toxicity in adults. The present study aims to confirm its intrauterine origination, and to explore its potential intrauterine programming mechanism. The pregnant rats were respectively injected subcutaneously with 0.2 and 0.8 mg/kg⋅d dexamethasone during gestational days (GD) 9 to 20. The testes and serum of offspring rats were collected on GD20 and postnatal week (PW) 12. In vivo, PDE significantly induced the abnormal testicular morphology in offspring from GD20 to PW12. Moreover, the serum and intratesticular testosterone levels and the expression of testicular steroidogenic acute regulatory protein (StAR) were reduced by PDE. The expression levels of glucocorticoid receptor (GR) and histone deacetylase 7 (HDAC7) were increased in fetal testes. Furthermore, the histone 3 lysine 9 acetylation (H3K9ac) level in the StAR promoter was decreased by PDE from GD20 to PW12. In vitro, mouse Leydig tumour cell line (MLTC-1) cells were treated with dexamethasone (20, 100 and 500 nM), and the testosterone production and StAR expression were reduced. Moreover, dexamethasone increased the expression of HDAC7 by activating GR, which decreased the H3K9ac level in the StAR promoter. Taken together, PDE caused testicular dysplasia before and after birth in male offspring rats, and its mechanism was related to the low-expressional programming of StAR mediated by decreasing H3K9ac level. Copyright © 2018. Published by Elsevier B.V.

High Spatial Resolution 40Ar/39Ar Geochronology of Lunar Impact Melt Rocks

NASA Astrophysics Data System (ADS)

Mercer, Cameron Mark

Impact cratering has played a key role in the evolution of the solid surfaces of Solar System bodies. While much of Earth’s impact record has been erased, its Moon preserves an extensive history of bombardment. Quantifying the timing of lunar impact events is crucial to understanding how impacts have shaped the evolution of early Earth, and provides the basis for estimating the ages of other cratered surfaces in the Solar System. Many lunar impact melt rocks are complex mixtures of glassy and crystalline “melt” materials and inherited clasts of pre-impact minerals and rocks. If analyzed in bulk, these samples can yield complicated incremental release 40Ar/39Ar spectra, making it challenging to uniquely interpret impact ages. Here, I have used a combination of high-spatial resolution 40Ar/39Ar geochronology and thermal-kinetic modeling to gain new insights into the impact histories recorded by such lunar samples. To compare my data to those of previous studies, I developed a software tool to account for differences in the decay, isotopic, and monitor age parameters used for different published 40Ar/39Ar datasets. Using an ultraviolet laser ablation microprobe (UVLAMP) system I selectively dated melt and clast components of impact melt rocks collected during the Apollo 16 and 17 missions. UVLAMP 40Ar/39Ar data for samples 77135, 60315, 61015, and 63355 show evidence of open-system behavior, and provide new insights into how to interpret some complexities of published incremental heating 40Ar/39Ar spectra. Samples 77115, 63525, 63549, and 65015 have relatively simple thermal histories, and UVLAMP 40Ar/39Ar data for the melt components of these rocks indicate the timing of impact events—spanning hundreds of millions of years—that influenced the Apollo 16 and 17 sites. My modeling and UVLAMP 40Ar/39Ar data for sample 73217 indicate that some impact melt rocks can quantitatively retain evidence for multiple melt-producing impact events, and imply that such

The 40Ar/39Ar dating of core recovered by the Hawaii Scientific Drilling Project (phase 2), Hilo, Hawaii

NASA Astrophysics Data System (ADS)

Sharp, Warren D.; Renne, Paul R.

2005-04-01

The Hawaii Scientific Drilling Project, phase 2 (HSDP-2), recovered core from a ˜3.1-km-thick section through the eastern flanks of Mauna Loa and Mauna Kea volcanoes. We report results of 40Ar/39Ar incremental heating by broad-beam infrared laser of 16 basaltic groundmass samples and 1 plagioclase separate, mostly from K-poor tholeiites. The tholeiites generally have mean radiogenic 40Ar enrichments of 1-3%, and some contain excess 40Ar; however, isochron ages of glass-poor samples preserve stratigraphic order in all cases. A 246-m-thick sequence of Mauna Loa tholeiitic lavas yields an isochron age of 122 ± 86 kyr (all errors 2σ) at its base. Beneath the Mauna Loa overlap sequence lie Mauna Kea's postshield and shield sequences. A postshield alkalic lava yields an age of 236 ± 16 kyr, in agreement with an age of 240 ± 14 kyr for a geochemically correlative flow in the nearby HSDP-1 core hole, where more complete dating of the postshield sequence shows it to have accumulated at 0.9 ± 0.4 m/kyr, from about 330 to <200 ka. Mauna Kea's shield consists of subaerial tholeiitic flows to a depth of 1079 m below sea level, then shallow submarine flows, hyaloclastites, pillow lavas, and minor intrusions to core bottom at 3098 m. Most subaerial tholeiitic flows fail to form isochrons; however, a sample at 984 m yields an age of 370 ± 180 kyr, consistent with ages from similar levels in HSDP-1. Submarine tholeiites including shallow marine vitrophyres, clasts from hyaloclastites, and pillow lavas were analyzed; however, only pillow lava cores from 2243, 2614, and 2789 m yield reliable ages of 482 ± 67, 560 ± 150, and 683 ± 82 kyr, respectively. A linear fit to ages for shield samples defines a mean accumulation rate of 8.6 ± 3.1 m/kyr and extrapolates to ˜635 kyr at core bottom. Alternatively, a model relating Mauna Kea's growth to transport across the Hawaiian hot spot that predicts downward accelerating accumulation rates that reach ˜20 m/kyr at core bottom (De

Etching Enhancement Followed by Nitridation on Low-k SiOCH Film in Ar/C5F10O Plasma

NASA Astrophysics Data System (ADS)

Miyawaki, Yudai; Shibata, Emi; Kondo, Yusuke; Takeda, Keigo; Kondo, Hiroki; Ishikawa, Kenji; Okamoto, Hidekazu; Sekine, Makoto; Hori, Masaru

2013-02-01

The etching rates of low-dielectric-constant (low-k), porous SiOCH (p-SiOCH) films were increased by nitrogen-added Ar/C5F10O plasma etching in dual-frequency (60 MHz/2 MHz)-excited parallel plate capacitively coupled plasma. Previously, perfluoropropyl vinyl ether [C5F10O] provided a very high density of CF3+ ions [Nagai et al.: Jpn. J. Appl. Phys. 45 (2006) 7100]. Surface nitridation on the p-SiOCH surface exposed to Ar/N2 plasma led to the etching of larger amounts of p-SiOCH in Ar/C5F10O plasma, which depended on the formation of bonds such as =C(sp2)=N(sp2)- and -C(sp)≡N(sp).

Reappraisal of Los Humeros Volcanic Complex by New U/Th Zircon and 40Ar/39Ar Dating: Implications for Greater Geothermal Potential

NASA Astrophysics Data System (ADS)

Carrasco-Núñez, G.; Bernal, J. P.; Dávila, P.; Jicha, B.; Giordano, G.; Hernández, J.

2018-01-01

Longevity and size of magmatic systems are fundamental factors for assessing the potential of a geothermal field. At Los Humeros volcanic complex (LHVC), the first caldera-forming event was reported at 460 ± 40 ka. New zircon U/Th and plagioclase 40Ar/39Ar dates of pre-, syn- and postcaldera volcanics allow a reappraisal of the evolution of the geothermally active LHVC. The age of the voluminous Xaltipan ignimbrite (115 km3 dense rock equivalent [DRE]) associated with the formation of the Los Humeros caldera is now constrained by two geochronometers (zircon U/Th and plagioclase 40Ar/39Ar dating) to 164 ± 4.2 ka, which postdates a long episode of precaldera volcanism (rhyolitic domes), the oldest age of which is 693.0 ± 1.9 ka (40Ar/39Ar). The inferred short residence time (around 5 ka) for the paroxysmal Xaltipan ignimbrite is indicative of rapid assembly of a large magma body and rejuvenation of the system due to recurrent recharge magmas, as it has been occurred in some other large magmatic systems. Younger ages than previously believed have been obtained also for the other voluminous explosive phases of the Faby fall tuff at ˜70 ka and the second caldera-forming Zaragoza ignimbrite with 15 km3 DRE, which erupted immediately after. Thus, the time interval that separates the two caldera-forming episodes at Los Humeros is only 94 kyr, which is a much shorter interval than suggested by previous K-Ar dates (410 kyr). This temporal proximity allows us to propose a caldera stage encompassing the Xaltipan and the Zaragoza ignimbrites, followed by emplacement at 44.8 ± 1.7 ka of rhyolitic magmas interpreted to represent a postcaldera, resurgent stage. Rhyolitic eruptions have also occurred during the Holocene (<7.3 ± 0.1 ka) along with olivine-rich basalts that suggest recharge of the system. The estimated large volume magmatic reservoir for Los Humeros (>˜1,200 km3) and these new ages indicating much younger caldera-forming volcanism than previously believed are

Concordant ages for the Lava Creek Tuff from high-spatial-resolution U-Pb dating of zircon rim faces and single-crystal sanidine 40Ar/39Ar dating

NASA Astrophysics Data System (ADS)

Matthews, N. E.; Vazquez, J. A.; Calvert, A. T.

2013-12-01

The last great explosive supereruption from the Yellowstone Plateau formed present-day Yellowstone caldera and ejected the >1000 km3 of rhyolite that composes the Lava Creek Tuff (LCT). The LCT eruption blanketed much of the western United States in ash, and consequently is a key chronostratigraphic marker bed for delimiting Quaternary uplift rates, the age of middle Pleistocene glacial and pluvial deposits, and tephra correlation in North America. Previous 40Ar/39Ar dating of the two mineralogically distinct LCT members (A & B) yield ages ranging from ca. 600 ka (Gansecki et al., 1998) to ca. 640 ka (Lanphere et al., 2002). To resolve the timing of eruption and crystallization timescale for the LCT magma, we dated both LCT members using a dual-method approach as follows: (1) ion microprobe (SHRIMP-RG) U-Pb dating and trace-element characterization of the final few micrometers of zircon crystallization by analysis of unpolished rims on indium-mounted crystals, and dating of the onset of zircon crystallization by traditional analysis of sectioned crystal interiors, and (2) laser-fusion 40Ar/39Ar dating of single sanidine crystals from bulk LCT ignimbrite and pumice. The unpolished rims of zircon from LCT members A & B yield indistinguishable ages, with a mean age of 621.8 × 2.5 ka (1σ) after correction for initial 230Th disequilibrium as constrained by ion-probe analyses of LCT melt inclusions. Single sanidine crystals from LCT-B yield a mean age of 624.9 × 2.6 ka (FCT=28.17 Ma) that is indistinguishable from the zircon rim ages for both members. These results indicate that LCT members A & B erupted over a geologically brief interval, which is supported by the direct and gradational contact of their equivalent fallout in distal lacustrine deposits and a lack of field evidence for a significant time-break between the LCT A & B in proximal deposits (Christiansen, 2001), but contrasts with older Yellowstone ignimbrite (e.g., Huckleberry Ridge) that may have erupted

Noble Gases in Alpine Gold: U/Th-He Dating and Excesses of Radiogenic He and AR

NASA Astrophysics Data System (ADS)

Eugster, O.; Hofmann, B.; Krahenbuhl, U.; Neuenschwander, J.

1992-07-01

Gold precipitates in hydrothermal fluids along with other heavy elements, such as Ag and Pt. In order to explore the possibility of dating the formation of gold we determined the concentrations of U, Th, and their decay product ^4He, as well as the K and ^40Ar concentrations in vein-type gold and in placer gold samples. The gold-quartz veins at Brusson in the south-western alps were formed approximately 32 Ma ago during an episode of tectonic uplift (Diamond, 1990). Alpine material was deposited as sediment layers in the region of central Switzerland and placer gold is thus relatively abundant in the rivers of the Napf area. We washed placer gold from the river Grosse Fontanne in 1990 and 1991. Placer gold that had been collected from the river Kruempelgraben in 1933 and a sample of vein-type free gold grown on quartz rock from the Brusson area (Val d'Ayas) have been obtained from the Museum of Natural History in Bern. Table 1 gives the results. Most of the ^4He is released above 1050 degrees C, that is when gold melts, indicating that gold is extremely well retentive for He. From the ^4He concentration of (269 +- 20) x 10^-8 cm^3 STP/g, (0.4 +- 0.1) ppm U, and (0.9 +- 0.3) ppm Th for vein-type gold we calculate a U/Th-He age of (36 +- 8)Ma. This age agrees within errors with the proposed age of 32 Ma. The data given in Table 1 show that all placer gold samples contain excesses of radiogenic ^4He and ^40Ar relative to the concentrations expected from the U/Th and K decay, respectively, if we assume a formation age of 32 Ma. The quartz sample is depleted in ^4He but strongly enriched in radiogenic ^40Ar. The excess of ^40Ar(sub)rad is easier to explain than that of ^4He. Vein-type gold and placer gold contain quartz inclusions (Schmid, 1973). The high ^40Ar(sub)rad content of quartz (Table 1) indicates that the ^40Ar(sub)rad excess of gold originates from quartz inclusions. Excess ^4He in gold must be of radiogenic origin. Taking ^20Ne and ^36Ar as a measure for the

U-Pb zircon and 40Ar/39Ar geochronology of sericite from hydrothermal alteration zones: new constraints for the timing of Ediacaran gold mineralization in the Sukhaybarat area, western Afif terrane, Saudi Arabia

NASA Astrophysics Data System (ADS)

Harbi, Hesham M.; Ali, Kamal A.; McNaughton, Neal J.; Andresen, Arild

2018-04-01

The Sukhaybarat East and Red Hill deposits, in the northeastern part of the Arabian Shield, are mesothermal vein-type gold deposits hosted by late Cryogenian-Ediacaran intrusive rocks of the Idah suites (diorite, tonalite, granodiorite) and, at Sukhaybarat East, also by Ediacaran metasedimentary rocks. Gold mineralization comprises quartz-arsenopyrite veins (Sukhaybarat East), quartz-carbonate-pyrite veins (Red Hill), and subordinate gold-base metal sulfide veins. In the Red Hill deposit, alteration is complicated due to multiple overprinting hydrothermal events and is characteristically affected by pervasive, pink quartz-K-feldspar-hematite alteration which is overprinted by potassic alteration characterized by a quartz-biotite-carbonate-muscovite/sericite-rutile-apatite assemblage. This assemblage is associated with molybdenite veins which appear to form late in the paragenetic sequence and may represent either evolution of the ore fluid composition, or a later, unrelated mineralized fluids. Hydrothermal alteration at the Sukhaybarat East deposit is dominated by quartz-carbonate-sericite-arsenopyrite assemblages. Zircon from ore-hosting tonalite at Sukhaybarat East yields a U-Pb age of 629 ± 6 Ma, and biotite from the same rock gives an 40Ar/39Ar age of 622 ± 23 Ma. The 40Ar/39Ar age is within the uncertainty range for the U-Pb age of the host intrusion and is interpreted as a minimally disturbed cooling age for the tonalite. In the Red Hill area, granodiorite was emplaced at 615 ± 5 Ma, whereas muscovite/sericite separated from a mineralized sample of a quartz-carbonate-pyrite vein, that was overprinted by molybdenite-bearing veinlets, yields an 40Ar/39Ar age of 597 ± 8 Ma. We interpreted this age to represent the maximum age of the molybdenite mineralization and the probable minimum age of gold mineralization in the Red Hill deposit.

Detrital K-feldspar thermochronology of the Nanaimo Group: Characterization of Basement and Extraregional Basin Contributions

NASA Astrophysics Data System (ADS)

Isava, V.; Grove, M.; Mahoney, J. B.; Kimbrough, D. L.

2016-12-01

The Late Cretaceous-Early Paleogene Nanaimo Group covers the contact between Triassic basement Wrangellia terrane and the Jurassic-Cretaceous Coast Plutonic Complex (CPC) in southern British Columbia. Prior detrital zircon U-Pb and Hf studies indicate a change in sediment source for the Nanaimo basin, from the primitive CPC in Santonian-Early Campanian time to an isotopically evolved continental extraregional source during the late Campanian/Maastrictian. Two notably different areas have been proposed as potential source regions: (1) the Idaho/Boulder batholith and Belt Supergroup, and (2) the Mojave/Salinia segment of structurally disrupted late Cretaceous southern California margin. Single crystal 40Ar/39Ar laser fusion of ca. 100-200 grains apiece from seven detrital K-feldspar samples from Santonian-Maastrichtian strata of the northern Nanaimo Group constrain the history of the sediments' source regions. The two oldest samples, from the K-feldspar poor Comox and Extension Fms., display a monotonic increasing distribution of cooling ages 80-125 Ma that reflects shallow erosion of the CPC. In contrast, Late Campanian strata of the Cedar District and De Courcy Fms. exhibit a more pronounced cluster of cooling ages 80-95 Ma as well as a greater proportion of Jurassic ages that represent progressively deeper erosion of the CPC. Evidence for an extraregional sediment source appears abruptly in the Geoffrey Fm. by 72 Ma, matching the time of local-to-extraregional shift indicated in detrital zircon U-Pb studies. Over 90% of the detrital K-feldspars from these arkosic sandstones yield cooling ages of 70-80 Ma, with sparse older ages associated with the CPC. Samples from the successively younger Spray and Gabriola Fms. also yield >90% K-feldspar ages younger than 80 Ma and exhibit age maxima of 68 Ma and 65 Ma, respectively. These results are distinct from detrital zircon U-Pb and K-feldspar 40Ar/39Ar ages of the southern Sierra Nevada, Mojave/Salina, and northern

Ar-40/Ar-39 and U-Th-Pb dating of separated clasts from the Abee E4 chondrite

NASA Technical Reports Server (NTRS)

Bogard, D. D.; Unruh, D. M.; Tatsumoto, M.

1983-01-01

Ar-40/Ar-39 and U-Th-Pb are investigated for three clasts from the Abee (E4) enstatite chondrite, yielding Ar-40/Ar-39 plateau ages (and/or maximum ages) of 4.5 Gy, while two of the clasts give average ages of 4.4 Gy. The 4.4-4.5 Gy range does not resolve possible age differences among the clasts. The U-Th-Pb data are consistent with the interpretation that initial clast formation occurred 4.58 Gy ago, and that the clasts have since remained closed systems which have been contaminated with terrestrial Pb. The thermal history of Abee deduced from Ar data seems consistent with that deduced from magnetic data, suggesting that various Abee components experienced separate histories until brecciation no later than 4.4 Gy ago, experiencing no significant subsequent heating.

Transition from subduction- to exhumation-related fabrics in glaucophane-bearing eclogites, Oman: evidence from relative fabric chronology and 40Ar/ 39Ar ages

NASA Astrophysics Data System (ADS)

Gray, David R.; Miller, John McL.; Foster, David A.; Gregory, Robert T.

2004-09-01

Controversy over the age of peak metamorphism and therefore the tectonic evolution of the Arabian margin relates to the polydeformed and polymetamorphosed nature of glaucophane-bearing eclogites from the Saih Hatat window beneath the allochthonous Samail ophiolite in NE Oman. These eclogites contain relicts of earlier fabrics, structures and metamorphic assemblages and provide a record of change from subduction to exhumation. The eclogites are part of a mafic layer that was disrupted into boudins up to 0.5 km in length within a lower plate shear zone (As Sifah shear zone). The megaboudins not only preserve the relicts of the highest grade of metamorphism but also an early ENE-trending lineation and sheathlike isoclines enveloped by the flat-lying schistosity. The boudin-bearing layer is isoclinally folded with calc-schist, mafic schist and quartz-mica schist, where the regional folds have axes parallel to the NE-trending stretching lineation (a-type folds). Textural evidence suggests multiple growth events for garnet and clinopyroxene, requiring polymetamorphism of the mafic layers that formed the eclogite megaboudins. The surrounding calc-schist and quartz-mica schist are both intensely deformed with transposition foliation containing an NE-trending lineation in phengite and asymmetric shear indicators such as C'-type shear bands and asymmetric pressure shadows around garnets, that give top-to-the-NE sense of shear. Although consistent ENE-trending lineations in all the boudins suggest that they have largely acted as passive, nonrotating rigid bodies, the presence of NE-vergent asymmetric mesofolds, extensive dynamic recrystallisation, multiple generations of phengites and a range of 40Ar- 39Ar apparent ages within the megaboudins suggest, however, that they have not acted entirely passively during the later deformation. Phengites isolated from the high- P/low- T fabrics show groupings in 40Ar- 39Ar apparent ages interpreted as distinct metamorphic

40Ar/39Ar systematics and argon diffusion in amber: implications for ancient earth atmospheres

USGS Publications Warehouse

Landis, G.P.; Snee, L.W.

1991-01-01

Argon isotope data indicate retained argon in bulk amber (matrix gas) is radiogenic [40Ar/39Ar ???32o] than the much more abundant surface absorbed argon [40Ar/39Ar ???295.5]. Neutron-induced 39Ar is retained in amber during heating experiments to 150?? -250??C, with no evidence of recoiled 39Ar found after irradiation. A maximum permissible volume diffusion coefficient of argon in amber (at ambient temperature) D???1.5 x 10-17 cm2S-1 is calculated from 39Ar retention. 40Ar/39Ar age calculations indicate Dominican Republic amber is ??? 45 Ma and North Dakota amber is ??? 89 Ma, both at least reasonable ages for the amber based upon stratigraphic and paleontological constraints and upon the small amount of radiogenic 40Ar. To date, over 300 gas analyses of ambers and resins of Cretaceous to Recent age that are geographically distributed among fifteen noted world locations identify mixtures of gases in different sites within amber (Berner and Landis, 1988). The presence of multiple mixing trends between compositionally distinct end-members gases within the same sample and evidence for retained radiogenic argon within the amber argue persuasivley against rapid exchange by diffusion of amber-contained gases with moder air. Only gas in primary bubbles entrapped between successive flows of tree resin has been interpreted as original "ancient air", which is an O2-rich end-member gas with air-like N2/Ar ratios. Gas analyses of these primary bubbles indicate atmospheric O2 levels in the Late Cretaceous of ??? 35%, and that atmospheric O2 dropped by early Tertiary time to near a present atmospheric level of 21% O2. A very low argon diffusion coefficient in amber persuasively argues for a gas in primary bubbles trapped in amber being ancient air (possibly modified only by O2 reaction with amber). ?? 1991.

A new high-precision 40Ar/39Ar age for the Rochechouart impact structure: At least 5 Ma older than the Triassic-Jurassic boundary

NASA Astrophysics Data System (ADS)

Cohen, Benjamin E.; Mark, Darren F.; Lee, Martin R.; Simpson, Sarah L.

2017-08-01

The Rochechourt impact structure in south-central France, with maximum diameter of 40-50 km, has previously been dated to within 1% uncertainty of the Triassic-Jurassic boundary, at which time 30% of global genera became extinct. To evaluate the temporal relationship between the impact and the Triassic-Jurassic boundary at high precision, we have re-examined the structure's age using multicollector ARGUS-V 40Ar/39Ar mass spectrometry. Results from four aliquots of impact melt are highly reproducible, and yield an age of 206.92 ± 0.20/0.32 Ma (2σ, full analytical/external uncertainties). Thus, the Rochechouart impact structure predates the Triassic-Jurassic boundary by 5.6 ± 0.4 Ma and so is not temporally linked to the mass extinction. Rochechouart has formerly been proposed to be part of a multiple impact event, but when compared with new ages from the other purported "paired" structures, the results provide no evidence for synchronous impacts in the Late Triassic. The widespread Central Atlantic Magmatic Province flood basalts remain the most likely cause of the Triassic-Jurassic mass extinction.

Planar defects as Ar traps in trioctahedral micas: A mechanism for increased Ar retentivity in phlogopite

NASA Astrophysics Data System (ADS)

Camacho, A.; Lee, J. K. W.; Fitz Gerald, J. D.; Zhao, J.; Abdu, Y. A.; Jenkins, D. M.; Hawthorne, F. C.; Kyser, T. K.; Creaser, R. A.; Armstrong, R.; Heaman, L. W.

2012-08-01

The effects of planar defects and composition on Ar mobility in trioctahedral micas have been investigated in samples from a small marble outcrop (∼500 m2) in the Frontenac Terrane, Grenville Province, Ontario. These micas crystallized during amphibolite-facies metamorphism at ∼1170 Ma and experienced a thermal pulse ∼100 Ma later at shallow crustal levels associated with the emplacement of plutons. 87Rb/86Sr ages of the phlogopites range from ∼950 to ∼1050 Ma, consistent with resetting during the later thermal event. The same phlogopites however, give 40Ar/39Ar ages between ∼950 and 1160 Ma, spanning the age range of the two thermal events. This result is intriguing because these micas have undergone the same thermal history and were not deformed after peak metamorphic conditions. In order to understand this phenomenon, the chemical, crystallographical, and microstructural nature of four mica samples has been characterized in detail using a wide range of analytical techniques. The scanning electron microscope (SEM), electron microprobe (EMP), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) data show that the micas are chemically homogeneous (with the exception of Ba) and similar in composition. The Fourier transform infrared spectroscopy and Mossbauer results show that the M sites for three of the micas are dominated by divalent cations and the Fe3+/(Fe2++Fe3+) ratio for all four phlogopites ranges from 0.10 to 0.25. The stable-isotopic data for calcite indicate that this outcrop was not affected by hydrothermal fluids after peak metamorphism. No correlation between chemical composition and 87Rb/86Sr and 40Ar/39Ar age or between crystal size and 40Ar/39Ar age is observed. The only major difference among all of the micas was revealed through transmitted electron microscope (TEM), which shows that the older 1M micas contain significantly more layer stacking defects, associated with crystallization, than the younger micas. We

Differential unroofing within the central metasedimentary Belt of the Grenville Orogen: constraints from 40Ar/39Ar thermochronology

USGS Publications Warehouse

Cosca, M.A.; Essene, E.J.; Kunk, Michael J.; Sutter, J.F.

1992-01-01

An 40Ar/39Ar thermochronological investigation of upper greenschist to granulite facies gneiss, amphibolite and marble was conducted in the Central Metasedimentary Belt (CMB), Ontario, to constrain its cooling history. Incremental 40Ar/39Ar release spectra indicate that substantial differential unroofing occurred in the CMB between ??? 1000 and ??? 600 Ma. A consistent pattern of significantly older hornblende and phlogopite 40Ar/3Ar cooling ages on the southeast sides of major northeast striking shear zones is interpreted to reflect late displacement due to extensional deformation. Variations in hornblende 40Ar/39Ar age plateaus exceeding 200 Ma occur over distances less than 50 km with major age discontinuities occurring across the Robertson Lake shear zone and the Sharbot Lake mylonite zone which separate the Sharbot Lake terrane from the Elzevir and Frontenac terranes. Extensional displacements of up to 14 km are inferred between the Frontenac and Elzevir terranes of the CMB. No evidence for significant post argon-closure vertical displacement is indicated in the vicinity of the Perth Road mylonite within the Frontenac terrane. Variations of nearly 100 Ma in phlogopite 40Ar/39Ar plateau ages occur in undeformed marble on either side of the Bancroft Shear Zone. Phlogopites from sheared and mylonitized marble within the shear zone yield 40Ar/39Ar diffusional loss profiles, but have older geologically meaningless ages thought to reflect incorporation of excess argon. By ??? 900 Ma, southeast directed extension was occurring throughout the CMB, possibly initiated along previous zones of compressional shearing. An easterly migration of active zones of extension is inferred, possibly related to an earlier, overall easterly migration of active zones of regional thrusting and easterly migration of an ancient subduction zone. The duration of extensional shearing is not well constrained, but must have ceased before ??? 600 Ma as required by the deposition of overlying

Paleointensity and 40Ar/39Ar geochronology of basalts at Gamarri, Ethiopia: Correlation with the Réunion subchron and Huckleberry Ridge excursion?

NASA Astrophysics Data System (ADS)

Kindley, C.; Macho, A.; Tsegaye, M. A.; Feinberg, J. M.; Singer, B. S.; Jicha, B. R.; Brown, M. C.; Birke, T. K.

2012-12-01

Characterization of the geomagnetic field during subchrons, reversals, and excursions is vital to understanding geodynamo processes and interactions across the core-mantle boundary. Moreover, an accurate timescale for geomagnetic field instabilities is critical to global high resolution stratigraphy. The Réunion subchron and Huckleberry Ridge excursion are ideal candidates for study due to globally distributed recordings in both sedimentary and igneous rocks. We present new full-vector paleomagnetic data for 30 basaltic flows from the Gamarri volcanic section in the Afar region of Ethiopia and 11 40Ar/39Ar ages. Paleointensities were calculated using the LTD-DHT Shaw technique and results generally agree with those of Carlut et al. (1999). Two geomagnetic instabilities are recorded, an older excursion and a younger period of normal polarity within the reversed Matuyama chron. Our results show a longer duration of low (<20 μT) paleointensity in the oldest flows and more variable low paleointensity values in the younger flows, and are generally lower than Thellier-style values of Carlut et al. (1999). Relative to 28.201 Ma Fish Canyon sanidine, plateau 40Ar/39Ar ages of the youngest (GB21) and oldest (GA02) flows are 2.029 ± 0.041 (2σ) and 2.410 ± 0.130 Ma, respectively. This eruptive duration is longer than that reported by Kidane et al. (1999), where the unspiked K-Ar method yields ages for GB23 (2 flows overlying GB21) and GA02 of 2.02 ± 0.08 (2σ) and 2.14 ± 0.12 Ma, respectively. 40Ar/39Ar ages of 4 lavas within the normal polarity zone in the upper section are between 2.063 ± 0.044 and 2.118 ± 0.057 Ma, but are indistinguishable at 2σ. These flows may sample the Huckleberry Ridge excursion (2.086 ± 0.016 Ma, Singer et al. 2004), the Réunion subchron (2.153-2.115 Ma, Channell et al. 2003), or both. Given several 40Ar/39Ar ages >2.2 Ma, the older excursion in the Gamarri section is not consistent with the Réunion subchron, and can be linked to any of

40Ar/39Ar geochronology and petrogenesis of the Table Mountain Shoshonite, Golden, Colorado, U.S.A.

USGS Publications Warehouse

Millikin, Alexie E. G.; Morgan, Leah; Noblett, Jeffery

2018-01-01

The Upper Cretaceous and Lower Paleogene Table Mountain Shoshonite lava flows and their proposed source, the Ralston Buttes intrusions, provide insight into the volcanic history of the Colorado Front Range. This study affirms the long-held hypothesis linking the extrusive Table Mountain lava flows and their intrusive equivalents at Ralston Buttes through major- and trace- element geochemistry. Systematic 40Ar/39Ar geochronology from all flows and intrusive units refines the eruptive history, improves precision on previously reported ages, and provides tighter constraints on the position of the K-Pg boundary in this location. Four flows are recognized on North and South Table mountains outside of Golden, Colorado. Flow 1 (66.5 ± 0.3 Ma, all ages reported with 2σ uncertainty) is the oldest, most compositionally distinct flow and is separated from younger flows by approximately 35 m of sedimentary deposits of the Denver Formation. Stratigraphically adjacent flows 2 (65.8 ± 0.2 Ma), 3 (65.5 ± 0.3 Ma), and 4 (65.9 ± 0.3 Ma) are compositionally indistinguishable. Lavas (referred to here as unit 5) that form three cone-shaped structures (shown by this study to be volcanic vents of a new unit 5) on top of North Table Mountain are compositionally similar to other units, but yield an age almost 20 m.y. younger (46.94 ± 0.15 Ma). Geochemistry and geochronology suggest that the rim phase of the Ralston plug (65.4 ± 0.2 Ma) is a reasonable source for flows 2, 3, and 4. All units are shoshonites—potassic basalts containing plagioclase, augite, olivine, and magnetite phenocrysts—and plot in the continental-arc field in tectonic discrimination diagrams. A continental-arc setting coupled with Late Cretaceous to early Paleogene ages suggest the high-K magmatism is associated with Laramide tectonism.

Excess Ar-40 in the Zagami Shergottite: Does It Reveal Crystallization History?

NASA Technical Reports Server (NTRS)

Bogard, Donald D.; Park, Jisun

2007-01-01

The Zagami basaltic shergottite has fine- and coarse-grained (FG & CG) areas, which may reflect partial crystallization in a deep, slowly cooled magma chamber to form Mg-rich pyroxene cores, followed by entrainment of these crystals into a magma that rose and crystallized near the surface. Late-stage melt pockets formed mesostasis and feldspar (maskelynite) having a range of compositions, but low water abundance. Higher I(sub Sr) in the FG portion may result from the second stage having incorporated old crustal rocks that failed to reach isotopic equilibrium. Zagami, like other shergottites, contains excess Ar-40(sub xs) beyond that expected from internal decay of K-40 during its Sm-Nd age of 177 Myr. We suggest that at least a portion of this Ar-40(sub xs) in Zagami and some other shergottites was inherited from the magma, much as is the case of MORBs on Earth. We made Ar-39-Ar-40 age determinations on feldspar and pyroxene separates from both the FG and CG portions of Zagami. If Zagami experienced an evolving fractional crystallization history, including possible crustal contamination of the magma, that might be indicated in differing amounts of Ar-40(sub xs) between mineral phases and between FG and CG portions.

High Precision 40K/39K Ratio Determination

NASA Astrophysics Data System (ADS)

Naumenko, M. O.; Mezger, K.; Nagler, T. F.; Villa, I. M.

2012-12-01

Potassium is one of the eight most abundant chemical elements in the Earth's crust and a major element in many rock-forming minerals. The isotope 40K is radioactive and undergoes β- decay to 40Ca (ca. 89.3%) and electron capture to 40Ar (ca. 10.7%). Both decays can potentially be used as dating systems. The most commonly used branch is the decay of 40K to 40Ar because it can yield highly precise ages. Both decay schemes rely on the knowledge of the 40K branching ratio and the natural 40K abundance. A 40K abundance of 0.011672±41 % was measured on terrestrial material [1]. The relative uncertainty of 0.35 % has not been improved since. Recent improvements in the precision of mass spectrometric measurements have led to the situation that the uncertainties on the K decay constant and the abundance of 40K are a major source of uncertainty on the measured ages. A more precise definition of the 40K decay constant was attempted by different research groups within the last decade [2-9] but the goal of obtaining 0.1 % relative uncertainty on K-Ar ages for geological materials, as requested by the EARTHtime initiative, has not been achieved yet. In order to improve on this situation we studied the abundances of the K isotopes in terrestrial standards. A ThermoFischer Triton+ thermal ionisation mass spectrometer was used for K isotope ratio measurements of the NIST SRM 918b K standard loaded on Ta filaments with 0.1M phosphoric acid. Three techniques were applied: (A) dynamic measurement with in-run normalisation to the IUPAC value 41K/39K=0.072168; (B) a simple total evaporation procedure; (C) the "NBL-modified" total evaporation [10]. The 40K ion beam was measured in a Faraday cup with a 1E12 Ω resistor; 39K and 41K were collected in Faraday cups with 1E11 Ω resistors. Amplifier gains were intercalibrated by supplying fixed voltages off-line. Different measurement techniques were combined with different loading procedures. We also tested ionisation yields for the

Higenamine protects ischemia/reperfusion induced cardiac injury and myocyte apoptosis through activation of β2-AR/PI3K/AKT signaling pathway

PubMed Central

Wu, Mei-ping; Zhang, Yi-shuai; Zhou, Qian-mei; Xiong, Jian; Dong, Yao-rong; Yan, Chen

2016-01-01

Cardiomyocyte apoptosis contributes to ischemic cardiac injury and the development of heart failure. Higenamine is a key component of the Chinese herb aconite root that has been prescribed for treating symptoms of heart failure for thousands of years in the oriental Asian countries. It has been shown that higenamine has anti-apoptotic effects in a few cell types including cardiomyocytes. However, the pharmacological target and molecular mechanism of higenamine in the heart are still not fully illustrated. Herein, we report that higenamine protected myocyte apoptosis and ischemia/reperfusion (I/R) injury through selective activation of beta2-adrenergic receptor (β2-AR). In particular, we show that higenamine significantly reduced I/R-induced myocardial infarction in mice. In both primary neonatal rat and adult mouse ventricular myocytes, we show higenamine inhibited cell apoptosis and also reduced biochemical markers of apoptosis such as cleaved caspase 3 and 9. More importantly, we show that the anti-apoptotic effects of higenamine in cardiomyocytes were completely abolished by β2-AR but not β1-AR antagonism. Furthermore, we confirmed that higenamine attenuated I/R-induced myocardial injury and reduced cleaved caspases in a β2-AR dependent manner in intact mouse hearts. Higenamine stimulated AKT phosphorylation and required PI3K activation for the anti-apoptotic effect in cardiomyocytes. These findings together suggest that anti-apoptotic and cardiac protective effects of higenamine are mediated by the β2-AR/PI3K/AKT cascade. PMID:26746354

Demonstration of a Groundwater Age Determination Using 39Ar in Support of a Multi-Tracer Groundwater Analysis of Wells in Fresno, CA

NASA Astrophysics Data System (ADS)

Wurstner White, S.; Brandenberger, J. M.; Kulongoski, J. T.; Aalseth, C.; Williams, R. M.; Mace, E. K.; Humble, P.; Seifert, A.; Cloutier, J. M.

2015-12-01

Argon-39 has a half-life of 269 years, making it an ideal tracer for groundwater dating in the age range of 50-1000 years. In September 2014, two production wells within the San Joaquin Valley Aquifer System, located in Fresno, CA were sampled and analyzed for a suite of inorganic and organic contaminants and isotopic constituents. The radiotracers 3H (< 50 years) and 14C (> 1000 years) are routinely measured as part of the U. S. Geological Survey (USGS) National Water Quality Assessment (NAWQA) Enhanced Trends Network project. Adding 39Ar to the suite of tracers provides age data in the intermediate range to refine the groundwater age distribution of mixed waters and establishes groundwater residence times and flow rates. Characterizing the groundwater recharge and flow rate is of particular interest at these wells for determining the sources and movement of contaminants in groundwater, particularly nitrate, DBCP, and perchlorate. The sampled wells were pumped and purged. The sample collection for the 39Ar measurements required extracting the dissolved gases from 3000-5000 L of groundwater using a membrane degasification system with a maximum flow rate of 50 gpm (11.4 m^3/hr). The membranes are plastic hollow fibers that are hydrophobic. The gas was collected in duplicate large aluminum coated plastic sample bags. The gas was purified and then counted via direct beta counting using ultra-low background proportional counters loaded with a mixture of geologic Ar and methane to enhance the sensitivity for Ar measurements. The activity of 39Ar is 1.01 Bq/kg Ar, corresponding to an abundance of 0.808 ppq. The estimated absolute ages of the samples from the two groundwater wells were 23.3 and 27.0 percent of modern Ar. The comparison of the groundwater residence times determined using the suite of radiotracers (3H, 39Ar, and 14C) highlighted the value of knowing the intermediate age of groundwater when determining contaminant fate and transport pathways.

Southernmost Andes and South Georgia Island, North Scotia Ridge: Zircon U-Pb and muscovite {40Ar }/{39Ar } age constraints on tectonic evolution of Southwestern Gondwanaland

NASA Astrophysics Data System (ADS)

Mukasa, Samuel B.; Dalziel, Ian W. D.

1996-11-01

Zircon U-Pb and muscovite {40Ar }/{39Ar } isotopic ages have been determined on rocks from the southernmost Andes and South Georgia Island, North Scotia Ridge, to provide absolute time constraints on the kinematic evolution of southwestern Gondwanaland, until now known mainly from stratigraphic relations. The U-Pb systematics of four zircon fractions from one sample show that proto-marginal basin magmatism in the northern Scotia arc, creating the peraluminous Darwin granite suite and submarine rhyolite sequences of the Tobifera Formation, had begun by the Middle Jurassic (164.1 ± 1.7 Ma). Seven zircon fractions from two other Darwin granites are discordant with non-linear patterns, suggesting a complex history of inheritances and Pb loss. Reference lines drawn through these points on concordia diagrams give upper intercept ages of ca. 1500 Ma, interpreted as a minimum age for the inherited zircon component. This component is believed to have been derived from sedimentary rocks in the Gondwanaland margin accretionary wedge that forms the basement of the region, or else directly from the cratonic "back stop" of that wedge. Ophiolitic remnants of the Rocas Verdes marginal basin preserved in the Larsen Harbour complex on South Georgia yield the first clear evidence that Gondwanaland fragmentation had resulted in the formation of oceanic crust in the Weddell Sea region by the Late Jurassic (150 ± 1 Ma). The geographic pattern in the observed age range of 8 to 13 million years in these ophiolitic materials, while not definitive, is in keeping with propagation of the marginal basin floor northwestward from South Georgia Island to the Sarmiento Complex in southern Chile. Rocks of the Beagle granite suite, emplaced post-tectonically within the uplifted marginal basin floor, have complex zircon U-Pb systematics with gross discordances dominated by inheritances in some samples and Pb loss in others. Of eleven samples processed, only two had sufficient amounts of zircon for

Age distribution of Serra Geral (Paraná) flood basalts, southern Brazil

USGS Publications Warehouse

Fodor, R.V.; McKee, E.H.; Roisenberg, A.

1989-01-01

We evaluated 193 K-Ar ages (10 newly determined) of basaltic and differentiated rocks of the Serra Geral (Paraná) flood-basalt province for indications of magmatism occurring systematically with progressive rifting and complete separation ( ≈130-105 Ma) of South America from Africa. The K-Ar ages represent basalt emplacement between 35° and 19°S covering about 1,200,000 km2. We note that volcanism appears ubiquitous across the province between about 140 and 115 Ma, and that there are no significant age differences within that relate directly to progressive south-to-north tectonism. On the other hand, the oldest samples, about 140–160 Ma, are among those nearest the Brazil coastline (rift margin), perhaps suggesting migration of activity away from the rift with time. Studies of other flood-basalt provinces now indicate short (<3 m.y.) eruption periods, thereby pointing to the need for re-examination of Serra Geral ages by 40Ar-39Ar incremental heating techniques.

Ar-39 - Ar-40 Dating of Two Angrites and Two Brachnites

NASA Technical Reports Server (NTRS)

Garrison, Daniel; Bogard, Donald

2003-01-01

Angrites are a rare group (approx.7 known) of igneous meteorites with basalt-like composition, which probably derive from a relatively small parent body that differs from those of other igneous meteorites. Angrites show evidence for extinct Mn-53, Sm-146, and Pu-244, and precise U-Pb, and Pb-Pb ages of 4.558 Gyr for two angrites define the time of early parent body differentiation. The S-147 - Nd-143 ages of two angrites range between 4.53+/-0.04 and 4.56+/-0.04 Gyr, but no Ar-39 - Ar-40 or Rb-Sr ages have been reported. Most angrites show no evidence for either shock brecciation or metamorphism.

Revised age for Midway volcano, Hawaiian volcanic chain

USGS Publications Warehouse

Dalrymple, G.B.; Clague, D.A.; Lanphere, M.A.

1977-01-01

New conventional K-Ar, 40Ar/39Ar, and petrochemical data on alkalic basalt pebbles from the basalt conglomerate overlying tholeiitic flows in the Midway drill hole show that Midway evolved past the tholeiitic shield-building stage and erupted lavas of the alkalic suite 27.0 ?? 0.6 m.y. ago. The data also show that previously published conventional K-Ar ages on altered samples of tholeiite are too young by about 9 m.y. These results remove a significant anomaly in the age-distance relationships of the Hawaiian chain and obviate the need for large changes in either the rate of rotation of the Pacific plate about the Hawaiian pole or the motion of the plate relative to the Hawaiian hot spot since the time of formation of the Hawaiian-Emperor bend. All of the age data along the Hawaiian chain are now reasonably consistent with an average rate of volcanic propagation of 8.0 cm/yr and with 0.83??/m.y. of angular rotation about the Hawaiian pole. ?? 1977.

Toward a high-resolution 40Ar/39Ar geochronology of the Tatun Volcano Group, Taiwan

NASA Astrophysics Data System (ADS)

Mesko, G. T.; Song, S.; Chang, S.; Hemming, S. R.; Turrin, B. D.

2010-12-01

The Tatun Volcano Group [TVG] consists of five volcanic subgroups of which ~30 edifices have been identified, all in close proximity to the densely populated Taipei Basin to its south (Song et al., 2000, Journal of the Geological Society of China, in Chinese). Evidence of eruptions is in the form of mostly lava flows, with pyroclastic flows, and ash deposition (Tsai et al., 2010, TAO), consistent with vulcanian and plinian eruptions that are only minimally preserved because of the region’s high weathering rate (Belousov et al., 2010, Journal of Volcanology and Geothermal Research). The TVG is made up of calc-alkaline andesite, with few interspersed basaltic lava flows that bear geochemical signatures consistent with subduction volcanism, yet due to tectonic location Teng (1996, Geology) describes it as Ryukyu back-arc basin volcanism, and still others attribute volcanism here to post-collisional collapse of the Taiwan orogen (Wang et al., 1999, Tectonophysics and 2004, Journal of Petrology). Various TVG samples were previously K-Ar dated by Juang and Chen (1989, Bulletin of Central Geological Survey, in Chinese), Tsao (1994, Bulletin of Central Geological Survey, in Chinese), and 40Ar/39Ar whole rock analyses by Lee (1996, masters thesis, National Taiwan University) to suggest volcanism from 2.8-2.5Ma and then from 1.5-.22Ma after which volcanic events ceased. In contrast, radiocarbon dates obtained from charcoal in related sediment by Chen et al. (2010, TAO) and Belousov et al. (2010, Journal of Volcanology Geothermal Research) suggest volcanic activity was present at 20ka and 6ka respectively. The andesite samples are microcrystalline; therefore hand picked aliquots of groundmass from the hand magnetic fraction were subjected to several iterations of sonic rinse in glycine-based soap, then 4N HNO3, then quartz-distilled water in a preparation modified from Nicolaysen et al. (2000, EPSL). Samples were co-irradiated at the USGS facility in Denver using Alder

A reconnaissance 40Ar/39Ar geochronologic study of ore-bearing and related rocks, Siberian Russia

USGS Publications Warehouse

Dalrymple, G.B.; Czamanske, G.K.; Fedorenko, V.A.; Simonov, O.N.; Lanphere, M.A.; Likhachev, A.P.

1995-01-01

40Ar/39Ar age spectra of biotite from a mineralized vein in the ore-bearing, Noril'sk I intrusion and from picritic-like gabbrodolerite from the weakly mineralized, Lower Talnakh intrusion show that these bodies were emplaced at 249 ?? 2 Ma, which is not significantly different from the age of the Permian-Triassic boundary. The ore-bearing intrusions postdate the lower third of the flood-basalt sequence in the Noril'sk area and, on the basis of geochemistry, can best be correlated with lavas slightly younger than those which they cut. Thus, flood basalt was erupted at the time of the Permian-Triassic mass extinction event, although its role in this event is, as yet, ill defined. Additional new 40Ar/39Ar age data for a group of intrusive and extrusive rocks on the western margin of the Siberian craton are discussed. -from Authors

New determinations of 40Ar/39Ar isotopic ages and flow volumes for Cenozoic volcanism in the Terror Rift, Ross Sea, Antarctica

NASA Astrophysics Data System (ADS)

Rilling, S.; Mukasa, S.; Wilson, T.; Lawver, L.; Hall, C.

2009-12-01

This study provides new determinations of 40Ar/39Ar isotopic ages and flow volumes for submarine and subaerial Neogene volcanism developed within the Terror Rift, Ross Sea, Antarctica, the youngest segment of the West Antarctic Rift System. The study is based on the first dredged samples from seven seamounts north of Ross Island, as well as new data from Franklin and Beaufort Islands. The sampled foidite and basanitic lavas range in age from Quaternary (90 ± 66 ka) on a small seamount ˜10 km north of Franklin Island to 6.80 ± 0.05 Ma on Beaufort Island. These ages are consistent with ages of volcanism in both the Melbourne and Erebus Volcanic Provinces and significantly expand the documented area of Neogene magmatism in Victoria Land. There is no geographic progression of volcanism through time, but volcanism was voluminous in the Pliocene and particularly widespread during the Pleistocene. Two of the dredges sampled edifices comprised of less than 0.2 km3 of volcanic materials. The largest seamount in the study area has 58.8 km3 of volcanic material and represents growth over a period of several thousand years. Estimated minimum eruption rates range from 2 × 10-4 km3 y-1 to 2 × 10-3 km3 y-1, consistent with rates proposed for other rift systems and nearby Mt. Erebus. Recent estimates of extension magnitude for the Terror Rift correspond to minimal decompression of only 0.10 to 0.22 GPa and therefore limited melt output of a typical peridotite source.

Geological and 40Ar/39Ar age constraints on late-stage Deccan rhyolitic volcanism, inter-volcanic sedimentation, and the Panvel flexure from the Dongri area, Mumbai

NASA Astrophysics Data System (ADS)

Sheth, Hetu C.; Pande, Kanchan

2014-04-01

Post-K-Pg Boundary Deccan magmatism is well known from the Mumbai area in the Panvel flexure zone. Represented by the Salsette Subgroup, it shows characters atypical of much of the Deccan Traps, including rhyolite lavas and tuffs, mafic tuffs and breccias, spilitic pillow basalts, and "intertrappean" sedimentary or volcanosedimentary deposits, with mafic intrusions as well as trachyte intrusions containing basaltic enclaves. The intertrappean deposits have been interpreted as formed in shallow marine or lagoonal environments in small fault-bounded basins due to syn-volcanic subsidence. We report a previously unknown sedimentary deposit underlying the Dongri rhyolite flow from the upper part of the Salsette Subgroup, with a westerly tectonic dip due to the Panvel flexure. We have obtained concordant 40Ar/39Ar ages of 62.6 ± 0.6 Ma (2σ) and 62.9 ± 0.2 Ma (2σ) for samples taken from two separate outcrops of this rhyolite. The results are significant in showing that (i) Danian inter-volcanic sedimentary deposits formed throughout Mumbai, (ii) the rock units are consistent with the stratigraphy postulated earlier for Mumbai, (iii) shale fragments known in some Dongri tuffs were likely derived from the sedimentary deposit under the Dongri rhyolite, (iv) the total duration of extrusive and intrusive Deccan magmatism was at least 8-9 million years, and (v) Panvel flexure formed, or continued to form, after 63 Ma, possibly even 62 Ma, and could not have formed by 65-64 Ma as concluded in a recent study.

Cosmogenic Ne-21 Production Rates in H-Chondrites Based on Cl-36 - Ar-36 Ages

NASA Technical Reports Server (NTRS)

Leya, I.; Graf, Th.; Nishiizumi, K.; Guenther, D.; Wieler, R.

2000-01-01

We measured Ne-21 production rates in 14 H-chondrites in good agreement with model calculations. The production rates are based on Ne-21 concentrations measured on bulk samples or the non-magnetic fraction and Cl-36 - Ar-36 ages determined from the metal phase.

Anomalous isotopic compositions of Sr, Ar and O in the Mesozoic diabase dikes of Liberia, West Africa

NASA Astrophysics Data System (ADS)

Mauche, Renée; Faure, Gunter; Jones, Lois M.; Hoefs, Jochen

1989-01-01

The Mesozoic diabase dikes of Liberia are tholeiites whose 87Sr/86Sr and 87Rb/86Sr ratios scatter widely on the Rb-Sr isochron diagram. The problem is attributed to differences in the initial 87Sr/86Sr ratios of these rocks which range from 0.70311 to 0.70792, assuming a uniform age of 186 Ma for the dikes and using λ(87Rb)=1.42 × 10-11y-1. The range of values is similar to that observed in the Mesozoic basalt flows and dikes of other Gondwana continents. New whole-rock K-Ar dates confirm previous conclusions that the diabase dikes in the Liberian and Pan-African age provinces of Liberia absorbed extraneous 40Ar after intrusion. Only the dikes in the Paynesville Sandstone have K-Ar dates that range from 117 Ma to 201 Ma and may not contain extraneous 40Ar. However, dikes from all three age provinces of Liberia have elevated initial 87Sr/86Sr ratios. These results indicate that contamination with radiogenic 87Sr occurred primarily before intrusion of the magma whereas the addition of extraneous 40Ar occurred after emplacement and reflects the age and mineral composition of the country rock. The δ 18O values of the Liberian diabase range from +5.6/% to +9.10/% and correlate positively with initial 87Sr/86Sr ratios. The data can be modeled by fractional crystallization and simultaneous assimilation of crustal rocks by the magma. However, samples containing amphibole and biotite replacing pyroxene deviate from the Sr-O isotope trajectories of the model and appear to have been depleted in 18O and enriched in 87Sr by interactions with groundwater at high temperature.

Ar-39 - Ar-40 Evidence for an Approximately 4.26 Ga Impact Heating Event on the LL Parent Body

NASA Technical Reports Server (NTRS)

Dixon, E. T.; Bogard, D. D.; Rubin, A. E.

2003-01-01

Miller Range 99301 is a type 6, unbrecciated LL chondrite. MIL 99301 is of interest because some compositional and petrographic features suggest it experienced rather high shock grades, whereas other features suggest it is relatively unshocked. Inconsistent shock indicators could be explained if MIL 99301 was shocked but then partly annealed by heat produced by impacts on the parent body. The hypothesis that MIL 99301 experienced high temperature metamorphism (type 6) followed by a later shock event that heated, but did not melt, the constituent feldspar can be evaluated using (39)Ar-(40)Ar chronology. This is because (39)Ar-(40)Ar ages of shocked ordinary chondrites are generally <4.2 Ga, whereas (39)Ar-(40)Ar ages of unshocked meteorites are generally older, and between 4.52 - 4.38 Ga.

Regional patterns of Mesozoic-Cenozoic magmatism in western Alaska revealed by new U-Pb and 40Ar/39Ar ages: Chapter D in Studies by the U.S. Geological Survey in Alaska, vol. 15

USGS Publications Warehouse

Bradley, Dwight C.; Miller, Marti L.; Friedman, Richard M.; Layer, Paul W.; Bleick, Heather A.; Jones, James V.; Box, Steven E.; Karl, Susan M.; Shew, Nora B.; White, Timothy S.; Till, Alison B.; Dumoulin, Julie A.; Bundtzen, Thomas K.; O'Sullivan, Paul B.; Ullrich, Thomas D.

2017-03-02

porphyry copper deposit along the Gulf of Alaska continental margin. More than half of the new ages fall between 75 and 65 Ma, confirming the existence, based on conventional potassium-argon (K-Ar) ages, of a 70-Ma igneous flare-up across southwestern Alaska. Our new ages hint that during this pulse, the locus of magmatism shifted toward the Gulf of Alaska, that is, toward a more outboard position. This shift is consistent with the hypothesis that magmatism was the product of rollback of a subducted slab, which at that time would have been the Resurrection Plate. Intrusive rocks in the Taylor Mountains and Sleetmute quadrangles in the age range of 63 to 59 Ma were emplaced shortly before the onset of ridge subduction as dated by near-trench plutons in the adjacent part of the Chugach accretionary complex. Southwestern Alaska at this time would have been positioned above a very young subducted slab belonging to the Resurrection Plate; magmas, in this scenario, were generated near the edge of the slab window related to ridge subduction. A 56.3±0.2 Ma granite in Taylor Mountains quadrangle and a 54.7±0.7 Ma ashfall tuff in McGrath quadrangle were likely emplaced above the Resurrection-Kula slab window, which by this time is inferred to have entered the region. Another ashfall tuff in McGrath quadrangle, at 42.8±0.5 Ma, likely belongs to the Meshik Arc, the product of renewed subduction after inferred passage of the slab window. A 49.0±0.3-Ma rhyolite in Taylor Mountains quadrangle is about the age of the transition from slab window to renewed subduction. Two plutons in the western Alaska Range, at 31.8±0.4 and 30.9±0.6 Ma, belong to a suite of gabbro to peralkaline granite of unknown origin. Finally, a 4.6±0.1-Ma basalt from a flow in Taylor Mountains quadrangle belongs to the Neogene basaltic province of western Alaska. These rocks were erupted in a distal retroarc setting; the cause of magmatism is unknown. 

Rotationally inelastic collisions of He and Ar with NaK: Experiment and theory

NASA Astrophysics Data System (ADS)

Malenda, R. F.; Jones, J.; Faust, C.; Richter, K.; Wolfe, C. M.; Hickman, A. P.; Huennekens, J.; Talbi, D.; Gatti, F.

2012-06-01

We are investigating collisions of the ground (X^1&+circ;) and first excited (A^1&+circ;) electronic states of NaK using both experimental and theoretical methods. Potential surfaces for HeNaK (fixed NaK bond length) are used for coupled channel calculations of cross sections for rotational energy transfer and also for collisional transfer of orientation and alignment. Additional calculations use the MCTDH wavepacket method. The measurements of the A state collisions involve a pump--probe excitation scheme using polarization labeling and laser-induced fluorescence spectroscopy. The pump excites a particular ro-vibrational level (v,J) of the A state from the X state, and the probe laser is scanned over various transitions to the 3^1π state. In addition to strong direct transitions, weak satellite lines are observed that arise from collisionally-induced transitions from the (v,J) level to (v,J'=J+δJ). This method provides information about the cross sections for transfer of population and orientation for A state levels, and it can be adapted to transitions starting in the X state. For the A state we observe a strong δJ=even propensity for both He and Ar perturbers. Preliminary results for the X state do not show this propensity.

Ar-Ar Dating of Martian Chassignites, NWA2737 and Chassigny, and Nakhlite MIL03346

NASA Technical Reports Server (NTRS)

Bogard, D. D.; Garrison, D. H.

2006-01-01

Until recently only three nakhlites and one chassignite had been identified among martian meteorites. These four exhibit very similar radiometric ages and cosmic ray exposure (CRE) ages, indicating that they may have derived from a common location on Mars and were ejected into space by a single impact. This situation is quite different from that of martian shergottites, which exhibit a range of radiometric ages and CRE ages (1). Recently, several new nakhlites and a new martian dunite (NWA2737) have been recognized. Here we report our results of Ar-39-Ar-40 dating for the MIL03346 nakhlite and the NWA2737 "chassignite", along with new results on Chassigny.

Reconsideration of evolutionary model of the Hawaiian-type volcano: 40Ar/39Ar ages for lavas from deep interior of Oahu Island and alkali basalts from the North Arch volcanic field

NASA Astrophysics Data System (ADS)

Uto, K.; Ishizuka, O.; Garcia, M. O.; Clague, D. A.; Naka, J.

2002-12-01

Growth history of Hawaiian-type volcanoes is typified into four stages: pre-shield, shield-forming, post-shield and rejuvinated. Duration of volcanism from pre-shield to post-shield stage is considered to be at most two million years, and is followed by the rejuvinated-stage after the dormance of one to two million years. There are, however, considerable amount of volcanic products hidden beneath the surface, and the above model may not be real due to the limited observation. US-Japan joint research on Hawaiian volcanism using ROV {\\KAIKO} and submersible {\\SHINKAI6500} of JAMSTEC has revealed many unknown volcanic processes of Hawaii. We challenge the well-established growth model of Hawaiian volcanoes from 40Ar/39Ar dating on rocks collected from the deep root of the submarine cliff of Oahu Island and from the widespread lava field off the coast of Oahu. Northern slope of Oahu Island is a deeply dissected steep wall from the ridge 1,000 m above the sea level to 3,000 m beneath the sea level. We expected to discover the deeper part of volcanic products forming Oahu Island. We obtained 6 40Ar/39Ar ages for tholeiitic lavas collected from 3,000 m to 2,600 m below the sea level. Ages are 5.7 and 6 Ma for two samples from the depth of 2,800 - 3,000 m, 4 Ma for a sample from 2,630 m, 3 Ma for a rock dredged between 2,500 and 2,800m, and 2.2 Ma for a sample from 2,602 m. Ages between 2.2 and 4 Ma are compatible with existing ages on subaerial shield-forming lavas on Koolau and Waianae volcano on Oahu, but ages of 5.7 and 6 Ma are about two million years older. Duplicate analyses gave concordant results and isochron ages have atmospheric 40Ar/36Ar initials. We, therefore, consider that these ages represent eruptive ages of samples. Current results suggest that tholeiitic volcanism forming Oahu Island continued almost 4 million years, which is far longer than ever considered. Considering the 8.7 cm/y of plate velocity, volcanism continued while Oahu Island moved 350 km

Accessory mineral U-Th-Pb ages and 40Ar/39Ar eruption chronology, and their bearing on rhyolitic magma evolution in the Pleistocene Coso volcanic field, California

USGS Publications Warehouse

Simon, J.I.; Vazquez, J.A.; Renne, P.R.; Schmitt, A.K.; Bacon, C.R.; Reid, M.R.

2009-01-01

We determined Ar/Ar eruption ages of eight extrusions from the Pleistocene Coso volcanic field, a long-lived series of small volume rhyolitic domes in eastern California. Combined with ion-microprobe dating of crystal ages of zircon and allanite from these lavas and from granophyre geothermal well cuttings, we were able to track the range of magma-production rates over the past 650 ka at Coso. In ??? 230 ka rhyolites we find no evidence of protracted magma residence or recycled zircon (or allanite) from Pleistocene predecessors. A significant subset of zircon in the ???85 ka rhyolites yielded ages between ???100 and 200 Ma, requiring that generation of at least some rhyolites involves material from Mesozoic basement. Similar zircon xenocrysts are found in an ???200 ka granophyre. The new age constraints imply that magma evolution at Coso can occur rapidly as demonstrated by significant changes in rhyolite composition over short time intervals (???10's to 100's ka). In conjunction with radioisotopic age constraints from other young silicic volcanic fields, dating of Coso rhyolites highlights the fact that at least some (and often the more voluminous) rhyolites are produced relatively rapidly, but that many small-volume rhyolites likely represent separation from long-lived mushy magma bodies. ?? The Author(s) 2009.

K/Ar Dating of Fine Grained Sediments Near Prydz Bay, Antarctica: East Antarctic Ice Sheet Behavior During the Middle-Miocene Climate Transition

NASA Astrophysics Data System (ADS)

Duchesne, A. E.; Pierce, E. L.; Williams, T.; Hemming, S. R.; Johnson, D. L.; May, T.; Gombiner, J.; Torfstein, A.

2012-12-01

¶ The Middle Miocene Climate Transition (MMCT) (~14 Ma) represents a time of major East Antarctic Ice-Sheet (EAIS) expansion, with research suggesting major global sea level fall on the order of ~60 meters (John et al., 2011, EPSL). Ocean Drilling Program (ODP) core data from Site 1165B near Prydz Bay shows an influx of cobbles deposited ~13.8-13.5 Ma, representing a sudden burst of ice-rafted detritus (IRD) during the MMCT. Based on 40Ar/39Ar dating of hornblendes and/or biotite grains, 5 of 6 dated pebbles from a companion study show Wilkes Land origins, indicating transport from over 1500 kilometers away. However, samples throughout this time interval have an anomalously low abundance of sand, thus we seek to understand the sedimentary processes that led to the deposition of these isolated dropstones in a fine matrix through provenance studies of the core's terrigenous fine fraction. Geochemical provenance studies of the terrigenous fraction of marine sediments can aid in identifying past dynamic EAIS behavior; the few outcrops available on the continent provide specific rock characterizations and age constraints from which cored marine sediments can then be matched to using established radiogenic isotope techniques. Here we apply the K/Ar dating method as a provenance tool for identifying the source area(s) of fine-grained terrigenous sediments (<63 μm) deposited during the MMCT. ¶ After source area characterization, we find that the fine-grained sediments from the mid-Miocene show a mixture of both local Prydz Bay sourcing (~400 Ma signature) and Wilkes Land provenance (~900 Ma signature). While locally-derived Prydz Bay sediments are likely to have been delivered via meltwater from ice and deposited as hemipelagic sediments (with some possible bottom current modification, as this is a drift site), sediments sourced from Wilkes Land required transport via large icebergs. Future work will involve further provenance determination on both the fine

AR-39Ar-40 dating of basalts and rock breccias from Apollo 17 and the malvern achondrite

NASA Technical Reports Server (NTRS)

Kirsten, T.; Horn, P.

1977-01-01

The principles and the potential of the Ar-39/Ar-40 dating technique are illustrated by means of results obtained for 12 Apollo 17 rocks. Emphasis is given to methodical problems and the geological interpretation of lunar rock ages. Often it is ambigious to associate a given lunar breccia with a certain formation, or a formation with a basin. In addition, large-scale events on the Moon have not necessarily reset radiometric clocks completely. One rock fragment has a well-defined plateau age of 4.28 b.y., but the ages of two Apollo 17 breccias define an upper limit for the formation age of the Serenitatis basin at 4.05 b.y. Ages derived from five mare basalts indicate cessation of mare volcanism at Taurus-Littrow approximately 3.78 b.y. ago. Ca/Ar-37 exposure ages show that Camelot Crater was formed by an impact approximately 95 m.y. ago. After a short summary of the lunar timetable as it stands at the end of the Apollo program, we report about Ar-39/Ar-40 and rare gas studies on the Malvern meteorite. This achondrite resembles lunar highland breccias in texture as well as in rare-gas patterns. It was strongly annealed at some time between 3.4 and 3.8 b.y. ago. The results indicate that very similar processes have occurred on the Moon and on achondritic parent bodies at comparable times, leading to impact breccias with strikingly similar features, including the retention of rare-gas isotopes from various sources.

K-shell Photoionization of Na-like to Cl-like Ions of Mg, Si, S, Ar, and Ca

NASA Technical Reports Server (NTRS)

Witthoeft, M. C.; Garcia, J.; Kallman, T. R.; Bautista, M. A.; Mendoza, C.; Palmeri, P.; Quinet, P.

2010-01-01

We present R-matrix calculations of photoabsorption and photoionization cross sections across the K edge of Mg, Si, S, Ar, and Ca ions with more than 10 electrons. The calculations include the effects of radiative and Auger damping by means of an optical potential. The wave functions are constructed from single-electron. orbital bases obtained using a Thomas-Fermi-Dirac statistical model potential. Configuration interaction is considered among all states up to n = 3. The damping processes affect the resonances converging to the K-thresholds causing them to display symmetric profiles of constant width that smear the otherwise sharp edge at the photoionization threshold. These data are important for the modeling of features found in photoionized plasmas.

40Ar/39Ar thermochronology of mesoproterozoic metamorphism in the Colorado Front Range

USGS Publications Warehouse

Shaw, C.A.; Snee, L.W.; Selverstone, J.; Reed, J.C.

1999-01-01

A low-pressure metamorphic episode in the Colorado Front Range has been identified by the presence of staurolite, andalusite, cordierite, and garnet porphyroblasts overprinting earlier assemblages. The overprinting assemblages and reaction textures are most consistent with porphyroblast growth on a prograde metamorphic path with peak temperatures exceeding ~525??C. Twenty-eight 40Ar/39Ar dates on hornblende, muscovite, biotite, and microcline were used to infer the age and thermal conditions of metamorphism. Muscovite and biotite 40Ar/39Ar ages fall mainly in the interval 1400-1340 Ma, consistent with cooling through the closure temperature interval of micas (~400??-300??C) after about 1400 Ma. In contrast, hornblende apparent ages (T(c)~500??-550??C) between 1600 and 1390 Ma reflect variable retention of radiogenic argon. Forward modeling of argon diffusion shows that the distribution of hornblende and mica ages is consistent with the partial resetting of argon systematics ca. 1400 Ma by a thermal pulse reaching maximum temperatures around 550??C and decaying within <20 m.yr. These temperatures match the conditions inferred from the overprinting assemblage; thus, muscovite and biotite ages are interpreted to date the cooling phase of this metamorphic event. This late metamorphism is broadly coeval with the intrusion of ca. 1400-Ma granitic plutons in the study area and throughout the southwestern United States. However, thermal effects are observed far from pluton margins, suggesting pervasive, regional crustal heating rather than restricted contact metamorphism. Our results suggest that ca. 1400-Ma metamorphism and plutonism are manifestations of a regional thermal episode that both partially melted the lower crust and pervasively metamorphosed middle crustal rocks.

Geochemical composition, petrography and 40Ar/39Ar age of the Heldburg phonolite: implications on magma mixing and mingling

NASA Astrophysics Data System (ADS)

Abratis, Michael; Viereck, Lothar; Pfänder, Jörg A.; Hentschel, Roland

2015-11-01

Differentiated magmatic rocks such as trachyte and phonolite are volumetrically subordinate to mafic volcanic rocks within the Cenozoic Central European Volcanic Province (exceptions are the East Eifel and the Rhön volcanic fields). Within the volcanic field of the "Heldburg dike swarm" (Heldburger Gangschar), the phonolite of the Burgberg near Heldburg represents the only known occurrence of differentiated magmatic rocks. However, the Heldburg phonolite is famous foremost for containing mantle xenoliths (spinel lherzolite). Former studies proposing a cogenetic relationship between the phonolite and the peridotites concluded that the phonolite magma must have evolved under upper mantle conditions. Herewith, we present petrographic and geochemical evidence for magma mixing and mingling in the Heldburg phonolite melt due to the intrusion of mantle-derived basanitic magma, which is exposed today as dikes at the foot of the Heldburg Burgberg. During this process, the mantle xenoliths were introduced into the phonolite melt as they all contain rims of basanitic magma. Extensive mingling features (e.g., schlieren layers, load casts, flame structures, mafic enclaves) are developed, indicating that the basanite and the zoned phonolitic body were melts at the time of mixing. These petrographic and geochemical indications of two coeval melts of different composition are substantiated by 40Ar/39Ar dating, revealing identical ages of ca. 15 Ma.

Oldest human footprints dated by Ar/Ar

NASA Astrophysics Data System (ADS)

Scaillet, Stéphane; Vita-Scaillet, Grazia; Guillou, Hervé

2008-11-01

Fossilized human trackways are extremely rare in the geologic record. These bear indirect but invaluable testimony of human/hominid locomotion in open air settings and can provide critical information on biomechanical changes relating to bipedalism evolution throughout the primitive human lineage. Among these, the "Devil's footsteps" represent one of the best preserved human footprints suite recovered so far in a Pleistocene volcanic ash of the Roccamonfina volcano (southern Italy). Until recently, the age of these footprints remained speculative and indirectly correlated with a loosely dated caldera-forming eruption that produced the Brown Leucitic Tuff. Despite extensive hydrothermal alteration of the pyroclastic deposit and variable contamination with excess 40Ar, detailed and selective 40Ar/ 39Ar laser probe analysis of single leucite crystals recovered from the ash deposit shows that the pyroclastic layer and the footprints are 345 ± 6 kyr old (1 σ), confirming for the first time that these are the oldest human trackways ever dated, and that they were presumably left by the modern human predecessor, Homo heidelbergensis, close to Climatic Termination IV.

A tale of 10 plutons - Revisited: Age of granitic rocks in the White Mountains, California and Nevada

USGS Publications Warehouse

McKee, E.H.; Conrad, J.E.

1996-01-01

40Ar/39Ar incremental heating analysis and conventional K-Ar age determinations on plutonic rocks of the White Mountains define two stages of magmatic emplacement: Late Cretaceous, between ca. 90 Ma and 75 Ma, and Middle-Late Jurassic, between ca. 180 and 140 Ma. The Jurassic stage can be divided into two substages, 180-165 Ma and 150-140 Ma. Thermal effects of the younger plutons on the older granitoids partially to completely reset ages, making it difficult to determine the age of emplacement and cooling of several of the plutons even by 40Ar/39Ar incremental heating analyses. New data together with published ages and regional geochronological synthesis of the Sierra Nevada batholith indicate that regions within the batholith have coherent periods or episodes of magmatic activity. In the White Mountains and Sierra Nevada directly to the west there was little or no activity in Early Jurassic and Early Cretaceous time; magmatism took place during relatively short intervals of 15 m.y. or less in the Middle and Late Jurassic and Late Cretaceous periods. The new K-Ar and 40Ar/39Ar analyses of granitoids from the White Mountains help, but do not completely clarify the complex history of emplacement, cooling, and reheating of the batholith.

Ultrastructural and biochemical evidence for the presence of mature steroidogenic acute regulatory protein (StAR) in the cytoplasm of human luteal cells.

PubMed

Sierralta, Walter D; Kohen, Paulina; Castro, Olga; Muñoz, Alex; Strauss, Jerome F; Devoto, Luigi

2005-10-20

The distribution of the steroidogenic acute regulatory protein (StAR) inside thecal and granulosa-lutein cells of human corpus luteum (CL) was assessed by immunoelectron microscopy. We found greater levels of StAR immunolabeling in steroidogenic cells from early- and mid-than in late luteal phase CL and lower levels in cells from women treated with a GnRH antagonist in the mid-luteal phase. Immunoelectron microscopy revealed significant levels of StAR antigen in the mitochondria and in the cytoplasm of luteal cells. The 30 kDa mature StAR protein was present in both mitochondria and cytosol (post-mitochondrial) fractions from homogenates of CL at different ages, whereas cytochrome c and mitochondrial HSP70 were detected only in the mitochondrial fraction. Therefore, we hypothesized that either appreciable processing of StAR 37 kDa pre-protein occurs outside the mitochondria, or mature StAR protein is selectively released into the cytoplasm after mitochondrial processing. The presence of mature StAR in the cytoplasm is consonant with the notion that StAR acts on the outer mitochondrial membrane to effect sterol import, and that StAR may interact with other cytoplasmic proteins involved in cholesterol metabolism, including hormone sensitive lipase.

Episodic Holocene eruption of the Salton Buttes rhyolites, California, from paleomagnetic, U-Th, and Ar/Ar dating

USGS Publications Warehouse

Wright, Heather M.; Vazquez, Jorge A.; Champion, Duane E.; Calvert, Andrew T.; Mangan, Margaret T.; Stelten, Mark E.; Cooper, Kari M.; Herzig, Charles; Schriener Jr., Alexander

2015-01-01

In the Salton Trough, CA, five rhyolite domes form the Salton Buttes: Mullet Island, Obsidian Butte, Rock Hill, North and South Red Hill, from oldest to youngest. Results presented here include 40Ar/39Ar anorthoclase ages, 238U-230Th zircon crystallization ages, and comparison of remanent paleomagnetic directions with the secular variation curve, which indicate that all domes are Holocene. 238U-230Th zircon crystallization ages are more precise than but within uncertainty of 40Ar/39Ar anorthoclase ages, suggesting that zircon crystallization proceeded until shortly before eruption in all cases except one. Remanent paleomagnetic directions require three eruption periods: (1) Mullet Island, (2) Obsidian Butte, and (3) Rock Hill, North Red Hill, and South Red Hill. Borehole cuttings logs document up to two shallow tephra layers. North and South Red Hills likely erupted within 100 years of each other, with a combined 238U-230Th zircon isochron age of: 2.83 ± 0.60 ka (2 sigma); paleomagnetic evidence suggests this age predates eruption by hundreds of years (1800 cal BP). Rock Hill erupted closely in time to these eruptions. The Obsidian Butte 238U-230Th isochron age (2.86 ± 0.96 ka) is nearly identical to the combined Red Hill age, but its Virtual Geomagnetic Pole position suggests a slightly older age. The age of aphyric Mullet Island dome is the least well constrained: zircon crystals are resorbed and the paleomagnetic direction is most distinct; possible Mullet Island ages include ca. 2300, 5900, 6900, and 7700 cal BP. Our results constrain the duration of Salton Buttes volcanism to between ca. 5900 and 500 years.

40Ar/(39)Ar dating of the Kapthurin Formation, Baringo, Kenya.

PubMed

Deino, Alan L; McBrearty, Sally

2002-01-01

The(40)Ar/(39)Ar radiometric dating technique has been applied to tuffs and lavas of the Kapthurin Formation in the Tugen Hills, Kenya Rift Valley. Two variants of the(40)Ar/(39)Ar technique, single-crystal total fusion (SCTF) and laser incremental heating (LIH) have been employed to date five marker horizons within the formation: near the base, the Kasurein Basalt at 0.61+/-0.04 Ma; the Pumice Tuff at 0.543+/-0.004 Ma; the Upper Kasurein Basalt at 0.552+/-0.015 Ma; the Grey Tuff at 0.509+/-0.009 Ma; and within the upper part of the formation, the Bedded Tuff at 0.284+/-0.012 Ma. The new, precise radiometric age determination for the Pumice Tuff also provides an age for the widespread Lake Baringo Trachyte, since the Pumice Tuff is the early pyroclastic phase of this voluminous trachyte eruption. These results establish the age of fossil hominids KNM-BK 63-67 and KNM-BK 8518 at approximately 0.510-0.512 Ma, a significant finding given that few Middle Pleistocene hominids are radiometrically dated. The Kapthurin hominids are thus the near contemporaries of those from Bodo, Ethiopia and Tanzania. A flake and core industry from lacustrine sediments in the lower part of the formation is constrained by new dates of 0.55-0.52 Ma, a period during which the Acheulian industry, characterized by handaxes, is known throughout East Africa. Points, typical of the Middle Stone Age (MSA), are found in Kapthurin Formation sediments now shown to date to between 0.509+/-0.009 Ma and 0.284+/-0.012 Ma. This date exceeds previous estimates for the age of the MSA elsewhere in East Africa by 49 ka, and establishes the age of Acheulian to MSA transition for the region. Evidence of the use of the Levallois technique for the manufacture of both small flakes and biface preforms, the systematic production of blades, and the use and processing of red ochre also occurs in this interval. The presence of blades and red ochre at this depth is important as blades signify a high degree of technical

Cathepsin K knockout alleviates aging-induced cardiac dysfunction

PubMed Central

Hua, Yinan; Robinson, Timothy J; Cao, Yongtao; Shi, Guo-Ping; Ren, Jun; Nair, Sreejayan

2015-01-01

Aging is a major risk factor for cardiovascular disease. It has previously been shown that protein levels of cathepsin K, a lysosomal cysteine protease, are elevated in the failing heart and that genetic ablation of cathepsin K protects against pressure overload-induced cardiac hypertrophy and contractile dysfunction. Here we test the hypothesis that cathepsin K knockout alleviates age-dependent decline in cardiac function. Cardiac geometry, contractile function, intracellular Ca2+ properties, and cardiomyocyte apoptosis were evaluated using echocardiography, fura-2 technique, immunohistochemistry, Western blot and TUNEL staining, respectively. Aged (24-month-old) mice exhibited significant cardiac remodeling (enlarged chamber size, wall thickness, myocyte cross-sectional area, and fibrosis), decreased cardiac contractility, prolonged relengthening along with compromised intracellular Ca2+ release compared to young (6-month-old) mice, which were attenuated in the cathepsin K knockout mice. Cellular markers of senescence, including cardiac lipofuscin, p21 and p16, were lower in the aged-cathepsin K knockout mice compared to their wild-type counterpart. Mechanistically, cathepsin K knockout mice attenuated an age-induced increase in cardiomyocyte apoptosis and nuclear translocation of mitochondrial apoptosis-inducing factor (AIF). In cultured H9c2 cells, doxorubicin stimulated premature senescence and apoptosis. Silencing of cathepsin K blocked the doxorubicin-induced translocation of AIF from the mitochondria to the nuclei. Collectively, these results suggest that cathepsin K knockout attenuates age-related decline in cardiac function via suppressing caspase-dependent and caspase-independent apoptosis. PMID:25692548

Age of the Lava Creek supereruption and magma chamber assembly at Yellowstone based on 40Ar/39Ar and U-Pb dating of sanidine and zircon crystals

USGS Publications Warehouse

Matthews, Naomi E.; Vazquez, Jorge A.; Calvert, Andrew T.

2015-01-01

The last supereruption from the Yellowstone Plateau formed Yellowstone caldera and ejected the >1000 km3 of rhyolite that composes the Lava Creek Tuff. Tephra from the Lava Creek eruption is a key Quaternary chronostratigraphic marker, in particular for dating the deposition of mid Pleistocene glacial and pluvial deposits in western North America. To resolve the timing of eruption and crystallization history for the Lava Creek magma, we performed (1) 40Ar/39Ar dating of single sanidine crystals to delimit eruption age and (2) ion microprobe U-Pb and trace-element analyses of the crystal faces and interiors of single zircons to date the interval of zircon crystallization and characterize magmatic evolution. Sanidines from the two informal members composing Lava Creek Tuff yield a preferred 40Ar/39Ar isochron date of 631.3 ± 4.3 ka. Crystal faces on zircons from both members yield a weighted mean 206Pb/238U date of 626.5 ± 5.8 ka, and have trace element concentrations that vary with the eruptive stratigraphy. Zircon interiors yield a mean 206Pb/238U date of 659.8 ± 5.5 ka, and reveal reverse and/or oscillatory zoning of trace element concentrations, with many crystals containing high U concentration cores that likely grew from highly evolved melt. The occurrence of distal Lava Creek tephra in stratigraphic sequences marking the Marine Isotope Stage 16–15 transition supports the apparent eruption age of ∼631 ka. The combined results reveal that Lava Creek zircons record episodic heating, renewed crystallization, and an overall up-temperature evolution for Yellowstone's subvolcanic reservoir in the 103−104 year interval before eruption.

Facts about Vitamin K

MedlinePlus

... the amount of vitamin K they contain (USDA- ARS, 2015). Table 2. Sources of vitamin K. Food ... U.S. Department of Agriculture, Agricultural Research Service USDA-ARS. (2015). National Nutrient Database for Standard Reference, Release ...

40Ar/39Ar geochronology and paleomagnetism of Independence volcano, Absaroka volcanic supergroup, Beartooth mountains, Montana

USGS Publications Warehouse

Harlan, S.S.; Snee, L.W.; Geissman, J.W.

1996-01-01

Independence volcano is a major volcanic complex in the lower part of the Absaroka Volcanic Supergroup (AVS) of Montana and Wyoming. Recently reported Rb-Sr mineral dates from the complex give apparent ages of 91 and 84 Ma, whereas field relationships and the physical and compositional similarity of the rocks with other dated parts of the AVS indicate an Early to Middle Eocene age for eruption and deposition. To resolve the conflict between age assignments based on stratigraphic correlations and Rb-Sr dates, we report new paleomagnetic data and 40Ar/39Ar dates for Independence volcano. Paleomagnetic data for the stock and an and andesite plug that cuts the stock are well grouped, of reverse polarity, and yield a virtual geomagnetic pole that is essentially identical to Late Cretaceous and Tertiary reference poles. The reverse polarity indicates that the magnetization of these rocks is probably younger than the Cretaceous normal superchron, or less than about 83.5 Ma. Hornblende from a volcanic breccia near the base of the volcanic pile gives a 40Ar/39Ar age of 51.57 Ma, whereas biotites from a dacite sill and a granodiorite stock that forms the core of the volcano give dates that range from 49.96 to 48.50 Ma. These dates record the age of eruption and intrusion of these rocks and clearly show that the age of Independence volcano is Early to Middle Eocene, consistent with stratigraphic relations. We suggest that the Rb-Sr mineral dates from the Independence stock and related intrusions are unreliable.

40Ar/39Ar dating of Glacial Termination VI: constraints on the duration of Marine Isotopic Stage 13.

PubMed

Marra, Fabrizio; Florindo, Fabio; Jicha, Brian R

2017-08-21

We present four new 40 Ar/ 39 Ar ages of tephra layers from an aggradational succession (Valle Giulia Formation) near the mouth of the Tiber Valley in Rome that was deposited in response to sea-level rise during Marine Isotopic Stage (MIS) 13. These new ages, integrated with seven previously determined ages, provide the only extant independent, radioisotopic age constraint on glacial termination VI and on the duration of MIS 13 sea-level rise. The new geochronologic constraints suggest a long duration for the period of sea-level rise (533 ± 2 through 498 ± 2 ka) encompassing two consecutive positive peaks of the δ 18 O curve (substages 13.3 and 13.1). Consistently, the litho-stratigraphic features of the sedimentary record account for two aggradational phases separated by an intervening erosional phase. Moreover, the ages obtained for this study give us the opportunity to compare the timing of the sea-level fluctuations inferred from the stratigraphic record and that provided by the astrochronologic calibration of the Oxygen isotopic curves, and to assess the calibrations of 40 Ar/ 39 Ar standards. Results of this comparison indicate that the best match is for an age of 1.186 Ma for the Alder Creek Rhyolite sanidine and 28.201 Ma for the Fish Canyon Tuff sanidine.

Mineralogical, chemical and K-Ar isotopic changes in Kreyenhagen Shale whole rocks and <2 µm clay fractions during natural burial and hydrous-pyrolysis experimental maturation

USGS Publications Warehouse

Clauer, Norbert; Lewan, Michael D.; Dolan, Michael P.; Chaudhuri, Sambhudas; Curtis, John B.

2014-01-01

Large amounts of smectite layers in the illite–smectite mixed layers of the pyrolyzed outcrop <2 μm fraction remain during thermal experiments, especially above 310 °C. With no illitization detected above 310 °C, smectite appears to have inhibited rather than promoted generation of expelled oil from decomposition of bitumen. This hindrance is interpreted to result from bitumen impregnating the smectite interlayer sites and rock matrix. Bitumen remains in the <2 μm fraction despite leaching with H2O2. Its presence in the smectite interlayers is apparent by the inability of the clay fraction to fully expand or collapse once bitumen generation from the thermal decomposition of the kerogen is completed, and by almost invariable K–Ar ages confirming for the lack of any K supply and/or radiogenic 40Ar removal. This suggests that once bitumen impregnates the porosity of a progressively maturing source rock, the pore system is no longer wetted by water and smectite to illite conversion ceases. Experimental attempts to evaluate the smectite conversion to illite should preferentially use low-TOC rocks to avoid inhibition of the reaction by bitumen impregnation.

Histone demethylase JMJD1A promotes alternative splicing of AR variant 7 (AR-V7) in prostate cancer cells.

PubMed

Fan, Lingling; Zhang, Fengbo; Xu, Songhui; Cui, Xiaolu; Hussain, Arif; Fazli, Ladan; Gleave, Martin; Dong, Xuesen; Qi, Jianfei

2018-05-15

Formation of the androgen receptor splicing variant 7 (AR-V7) is one of the major mechanisms by which resistance of prostate cancer to androgen deprivation therapy occurs. The histone demethylase JMJD1A (Jumonji domain containing 1A) functions as a key coactivator for AR by epigenetic regulation of H3K9 methylation marks. Here, we describe a role for JMJD1A in AR-V7 expression. While JMJD1A knockdown had no effect on full-length AR (AR-FL), it reduced AR-V7 levels in prostate cancer cells. Reexpression of AR-V7 in the JMJD1A-knockdown cells elevated expression of select AR targets and partially rescued prostate cancer cell growth in vitro and in vivo. The AR-V7 protein level correlated positively with JMJD1A in a subset of human prostate cancer specimens. Mechanistically, we found that JMJD1A promoted alternative splicing of AR-V7 through heterogeneous nuclear ribonucleoprotein F (HNRNPF), a splicing factor known to regulate exon inclusion. Knockdown of JMJD1A or HNRNPF inhibited splicing of AR-V7, but not AR-FL, in a minigene reporter assay. JMJD1A was found to interact with and promote the recruitment of HNRNPF to a cryptic exon 3b on AR pre-mRNA for the generation of AR-V7. Taken together, the role of JMJD1A in AR-FL coactivation and AR-V7 alternative splicing highlights JMJD1A as a potentially promising target for prostate cancer therapy.

Geochemical correlation and 40Ar/39Ar dating of the Kern River ash bed and related tephra layers: Implications for the stratigraphy of petroleum-bearing formations in the San Joaquin Valley, California

USGS Publications Warehouse

Baron, D.; Negrini, R.M.; Golob, E.M.; Miller, D.; Sarna-Wojcicki, A.; Fleck, R.J.; Hacker, B.; Erendi, A.

2008-01-01

The Kern River ash (KRA) bed is a prominent tephra layer separating the K and G sands in the upper part of the Kern River Formation, a major petroleum-bearing formation in the southern San Joaquin Valley (SSJV) of California. The minimum age of the Kern River Formation was based on the tentative major-element correlation with the Bishop Tuff, a 0.759??0.002 Ma volcanic tephra layer erupted from the Long Valley Caldera. We report a 6.12??0.05 Ma 40Ar/39Ar date for the KRA, updated major-element correlations, trace-element correlations of the KRA and geochemically similar tephra, and a 6.0??0.2 Ma 40Ar/39Ar age for a tephra layer from the Volcano Hills/Silver Peak eruptive center in Nevada. Both major and trace-element correlations show that despite the similarity to the Bishop Tuff, the KRA correlates most closely with tephra from the Volcano Hills/Silver Peak eruptive center. This geochemical correlation is supported by the radiometric dates which are consistent with a correlation of the KRA to the Volcano Hills/Silver Peak center but not to the Bishop Tuff. The 6.12??0.05 Ma age for the KRA and the 6.0??0.2 Ma age for the tephra layer from the Volcano Hills/Silver Peak eruptive center suggest that the upper age of the Kern River Formation is over 5 Ma older than previously thought. Re-interpreted stratigraphy of the SSJV based on the new, significantly older age for the Kern River Formation opens up new opportunities for petroleum exploration in the SSJV and places better constraints on the tectonostratigraphic development of the SSJV. ?? 2007 Elsevier Ltd and INQUA.

Low-k SiOCH Film Etching Process and Its Diagnostics Employing Ar/C5F10O/N2 Plasma

NASA Astrophysics Data System (ADS)

Nagai, Mikio; Hayashi, Takayuki; Hori, Masaru; Okamoto, Hidekazu

2006-09-01

We proposed an environmental harmonic etching gas of C5F10O (CF3CF2CF2OCFCF2), and demonstrated the etching of low-k SiOCH films employing a dual-frequency capacitively coupled etching system. Dissociative ionization cross sections for the electron impact ionizations of C5F10O and c-C4F8 gases have been measured by quadrupole mass spectroscopy (QMS). The dissociative ionization cross section of CF3+ from C5F10O gas was much higher than those of other ionic species, and 10 times higher than that of CF3+ from C4F8 gas. CF3+ is effective for increasing the etching rate of SiO2. As a result, the etching rate of SiOCH films using Ar/C5F10O/N2 plasma was about 1000 nm/min, which is much higher than that using Ar/C4F8/N2 plasma. The behaviours of fluorocarbon radicals in Ar/C5F10O/N2 plasma, which were measured by infrared diode laser absorption spectroscopy, were similar to those in Ar/C4F8/N2 plasma. The densities of CF and CF3 radicals were markedly decreased with increasing N2 flow rate. Etching rate was controlled by N2 flow rate. A vertical profile of SiOCH with a high etching rate and less microloading was realized using Ar/C5F10O/N2 plasma chemistry.

Lifetime of an ocean island volcano feeder zone: constraints from U-Pb dating on coexisting zircon and baddeleyite, and 40/39Ar age determinations, Fuerteventura, Canary Islands

USGS Publications Warehouse

Allibon, James; Ovtcharova, Maria; Bussy, Francois; Cosca, Michael; Schaltegger, Urs; Bussien, Denise; Lewin, Eric

2011-01-01

High-precision isotope dilution - thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon and baddeleyite ages from the PX1 vertically layered mafic intrusion Fuerteventura, Canary Islands, indicate initiation of magma crystallization at 22.10 ± 0.07 Ma. The magmatic activity lasted a minimum of 0.52 Ma. 40Ar/39Ar amphibole dating yielded ages from 21.9 ± 0.6 to 21.8 ± 0.3, identical within errors to the U-Pb ages, despite the expected 1% theoretical bias between 40Ar/39Ar and U-Pb dates. This overlap could result from (i) rapid cooling of the intrusion (i.e., less than the 0.3 to 0.6 Ma 40Ar/39Ar age uncertainties) from closure temperatures (Tc) of zircon (699-988 °C) to amphibole (500-600 °C); (ii) lead loss affecting the youngest zircons; or (iii) excess argon shifting the plateau ages towards older values. The combination of the 40Ar/39Ar and U/Pb datasets implies that the maximum amount of time PX1 intrusion took to cool below amphibole Tc is 0.8 Ma, suggesting PX1 lifetime of 520,000 to 800,000 Ma. Age disparities among coexisting baddeleyite and zircon (22.10 ± 0.07/0.08/0.15 Ma and 21.58 ± 0.15/0.16/0.31 Ma) in a gabbro sample from the pluton margin suggest complex genetic relationships between phases. Baddeleyite is found preserved in plagioclase cores and crystallized early from low silica activity magma. Zircon crystallized later in a higher silica activity environment and is found in secondary scapolite and is found close to calcite veins, in secondary scapolite that recrystallised from plagioclase. close to calcite veins. Oxygen isotope δ18O values of altered plagioclase are high (+7.7), indicating interaction with fluids derived from host-rock carbonatites. The coexistence of baddeleyite and zircon is ascribed to interaction of the PX1 gabbro with CO2-rich carbonatite-derived fluids released during contact metamorphism.

New 40Ar / 39Ar age and geochemical data from seamounts in the Canary and Madeira volcanic provinces: Support for the mantle plume hypothesis

NASA Astrophysics Data System (ADS)

Geldmacher, J.; Hoernle, K.; Bogaard, P. v. d.; Duggen, S.; Werner, R.

2005-08-01

The role of mantle plumes in the formation of intraplate volcanic islands and seamount chains is being increasingly questioned. Particular examples are the abundant and somewhat irregularly distributed island and seamount volcanoes off the coast of northwest Africa. New 40Ar / 39Ar ages and Sr-Nd-Pb isotope geochemistry of volcanic rocks from seamounts northeast of the Madeira Islands (Seine and Unicorn) and northeast of the Canary Islands (Dacia and Anika), however, provide support for the plume hypothesis. The oldest ages of shield stage volcanism from Canary and Madeira volcanic provinces confirm progressions of increasing age to the northeast. Average volcanic age progression of ∼1.2 cm/a is consistent with rotation of the African plate at an angular velocity of ∼0.20° ± 0.05 /Ma around a common Euler pole at approximately 56° N, 45° W computed for the period of 0-35 Ma. A Euler pole at 35° N, 45° W is calculated for the time interval of 35-64 Ma. The isotope geochemistry further confirms that the Madeira and Canary provinces are derived from different sources, consistent with distinct plumes having formed each volcanic group. Conventional hotspot models, however, cannot easily explain the up to 40 m.y. long volcanic history at single volcanic centers, long gaps in volcanic activity, and the irregular distribution of islands and seamounts in the Canary province. A possible explanation could involve interaction of the Canary mantle plume with small-scale upper mantle processes such as edge-driven convection. Juxtaposition of plume and non-plume volcanism could also account for observed inconsistencies of the classical hotspot concept in other volcanic areas.

40K-40Ca and 87Rb-86Sr Dating by SIMS: The Double-Plus Advantage

NASA Astrophysics Data System (ADS)

Harrison, T. M.; McKeegan, K. D.; Schmitt, A. K.

2009-12-01

The decay of 40K to 40Ar forms the basis of the potassium-argon dating method, although only one out of every 10 parent atoms decays to daughter 40Ar. The other 90% decay to 40Ca giving, in principle, the 40K-40Ca decay system great potential for dating samples with high K/Ca. This method, however, has not been utilized as an ion-microprobe-based geochronometer, largely because these isotopes require a very high mass resolving power (MRP) of ~25k for full separation. We found that limiting secondary ion transmission in our ims1270 ion microprobe to ~20% permits sufficient separation of 40K from 40Ca (MRP≈ 20k) to permit isotope ratio analysis, albeit with 40Ca+ on the shoulder of the more intense 40K+ peak. A pegmatitic muscovite from Jack Hills (K-Ca age = 2.54 Ga; Fletcher et al., Chem. Geol. 138, 289) yields ~104 cps of both 40K+ and 40Ca+ with a 15 μm primary spot size and O- beam current of 10 nA. The 40Ca+ signal is >90% radiogenic and reflects a “common” Ca content of ≤ 100 ppm. However, application of the relative sensitivity factor (RSF) calculated from the Jack Hills muscovite to unknowns yields relatively high age dispersion, perhaps related to the incompletely separated mass interferences. Theorizing that the noble gas electronic structure of K+ would likely resist further electron loss, we investigated an alternative approach involving analysis of Ca++/K++. The double-plus method provides an important advantage in that K++ species are suppressed by a factor of ~103 relative to K+, thereby effectively removing 40K++ from the spectrum at m/e≈ 20 and leaving 40Ca++ free from any significant interferences at an MRP≈ 4k. Measurement of the much more abundant 39K++ then permits 40Ca++/40K++ to be calculated from the known 39K/40K ratio. We applied this approach to Precambrian muscovite samples obtaining ages similar to, but generally younger than, their associated 40Ar/39Ar ages. This could reflect a minor matrix effect or a lower intrinsic

Prevalence of acute repetitive seizures (ARS) in the United Kingdom.

PubMed

Martinez, Carlos; Sullivan, Tim; Hauser, W Allen

2009-12-01

"Acute repetitive seizures" (ARS) is a term to describe a condition manifest by multiple seizures occurring over a relatively brief period of time -generally 24 hours- in patients with epilepsy. There is limited information regarding the epidemiology of ARS in the general population. We performed a historical cohort study using data from the United Kingdom General Practice Research Database (GPRD) to identify all incident and prevalent cases of active epilepsy in 2005. From among this group, we identified individuals at risk for ARS. This included those with "catastrophic epilepsy syndromes of childhood" (CE), and those with a history of seizure clustering in the context of other epilepsy syndromes. We identified 21,010 people with active epilepsy in the GPRD in 2005; prevalence 7.2/1000; age adjusted to the European Standard Population, 6.7; incidence 50/100,000 per year, age-adjusted 48/100,000. We identified 665 people at risk for ARS. The prevalence of CE in the general population was 1.2/10,000 and that of cluster seizures was 1.1/10,000. We estimated the crude prevalence of ARS in the general population to be 2.3/10,000; age adjusted 2.5 (CI, 2.3-2.7. The prevalence of ARS was highest in those 0-4 years of age (5.9/10,000) and fell with advancing age to 0.5/10,000 in those age 70 and older). This is the first population-based study to provide information on the prevalence of ARS. ARS affects about 3% of the population with epilepsy and 0.02% of the general population. More studies are needed to further evaluate this serious epilepsy phenomenon.

Cooling, exhumation, and deformation in the Hindu Kush, NW Pakistan: New constraints from preliminary 40Ar/39Ar and fission track analyses

NASA Astrophysics Data System (ADS)

Faisal, Shah; Larson, Kyle P.; Camacho, Alfredo; Coutand, Isabelle

2018-06-01

Asian crust in the Hindu Kush region in northern Pakistan records a protracted history of rifting, subduction and collision not commonly preserved within the Himalaya. Because of this, it is key to understanding the development of the southern Eurasian margin both prior to and after collision with India. New mica 40Ar/39Ar and apatite fission track geochronologic data from this region provide constraints on the kinematics of the Hindu Kush. 40Ar/39Ar muscovite and biotite ages from the late Cambrian Kafiristan pluton are 379.7 ± 1.7 Ma and 47.2 ± 0.3 Ma, respectively. The muscovite age may record cooling or partial resetting, while the biotite age is interpreted to record a thermal disruption associated with the early stages of continental collision in the Himalayan system. A 111.0 ± 0.6 Ma muscovite age from the northern part of the Tirich Mir pluton (∼123 Ma old; U-Pb) is interpreted to indicate a recrystallization event ∼12 Myrs after its intrusion. In addition, a younger muscovite age of 47.5 ± 0.2 Ma was derived from the opposite side of the same pluton in the immediate hanging wall of the Tirich Mir fault. This Eocene age is interpreted to represent the time of recrystallization during fault (re)activation in the early stages of India-Asia continent-continent collision. 40Ar/39Ar biotite analysis from the Buni-Zom pluton yields an age of 61.6 ± 1.1 Ma and is interpreted to reflect cooling at mid-upper crustal levels subsequent to the pluton's emplacement in the middle Cretaceous. Finally, 17.1-21.3 Ma 40Ar/39Ar ages from the Garam Chasma pluton and surrounding metapelites indicate cooling immediately following crystallization of the leucogranite body in the earliest Miocene/latest Oligocene. The younger cooling history is resolved by fission track dating of apatite (AFT). In the vicinity of the bounding Tirich Mir fault, the Tirich Mir pluton yields an AFT age of 1.4 ± 0.3 Ma, which is consistent with active exhumation associated with the surface

75 FR 49005 - In the Matter of Geotec, Inc., InnoPet Brands Corp., Marbledge Group, Inc. (n/k/a AR Growth...

Federal Register 2010, 2011, 2012, 2013, 2014

2010-08-12

... SECURITIES AND EXCHANGE COMMISSION [File No. 500-1] In the Matter of Geotec, Inc., InnoPet Brands Corp., Marbledge Group, Inc. (n/k/a AR Growth Finance Corp.), Phlo Corp., Pliant Systems, Inc., Southeast Banking Corp., TNX Television Holdings, Inc., and WestPoint Stevens, Inc.; Order of Suspension of Trading August 10, 2010. It appears to the...

Organic reactions mediated by electrochemically generated ArS+.

PubMed

Matsumoto, Kouichi; Suga, Seiji; Yoshida, Jun-ichi

2011-04-21

Low-temperature electrochemical oxidation of ArSSAr was carried out to generate a pool of "ArS(+)". Spectroscopic studies ((1)H NMR and CSI-MS) of the resulting solution revealed the accumulation of ArS(ArSSAr)(+). The resulting "ArS(+)" pool reacted with alkenes and alkynes to give diarylthio-substituted products. The "ArS(+)" pool rapidly reacted with thioacetals to give the corresponding alkoxycarbenium ion pools, which reacted with various carbon nucleophiles (indirect cation pool method). The reaction of the alkoxycarbenium ion pools with stilbene derivatives in the presence of ArSSAr gave thiochroman derivatives. In addition to such stoichiometric reactions, a catalytic amount of "ArS(+)" serves as an initiator and a chain carrier of some cationic chain reactions involving intramolecular carbon-carbon bond formation. In situ generation of "ArS(+)" by electrochemical oxidation of ArSSAr with a catalytic amount of electricity in the presence of a substrate is also effective for such cationic chain reactions.

Microglial K+ Channel Expression in Young Adult and Aged Mice

PubMed Central

Schilling, Tom; Eder, Claudia

2015-01-01

The K+ channel expression pattern of microglia strongly depends on the cells' microenvironment and has been recognized as a sensitive marker of the cells' functional state. While numerous studies have been performed on microglia in vitro, our knowledge about microglial K+ channels and their regulation in vivo is limited. Here, we have investigated K+ currents of microglia in striatum, neocortex and entorhinal cortex of young adult and aged mice. Although almost all microglial cells exhibited inward rectifier K+ currents upon membrane hyperpolarization, their mean current density was significantly enhanced in aged mice compared with that determined in young adult mice. Some microglial cells additionally exhibited outward rectifier K+ currents in response to depolarizing voltage pulses. In aged mice, microglial outward rectifier K+ current density was significantly larger than in young adult mice due to the increased number of aged microglial cells expressing these channels. Aged dystrophic microglia exhibited outward rectifier K+ currents more frequently than aged ramified microglia. The majority of microglial cells expressed functional BK-type, but not IK- or SK-type, Ca2+-activated K+ channels, while no differences were found in their expression levels between microglia of young adult and aged mice. Neither microglial K+ channel pattern nor K+ channel expression levels differed markedly between the three brain regions investigated. It is concluded that age-related changes in microglial phenotype are accompanied by changes in the expression of microglial voltage-activated, but not Ca2+-activated, K+ channels. PMID:25472417

Age of the Lava Creek supereruption and magma chamber assembly at Yellowstone based on 40Ar/39Ar and U-Pb dating of sanidine and zircon crystals

NASA Astrophysics Data System (ADS)

Vazquez, J. A.; Matthews, N. E.; Calvert, A. T.

2015-12-01

The last supereruption from the Yellowstone Plateau formed Yellowstone caldera and ejected the >1000 km3 of rhyolite that composes the Lava Creek Tuff (LCT). Tephra from the eruption blanketed much of the western United States, and is a key Quaternary chronostratigraphic marker, in particular for dating deposition of mid-Pleistocene glacial and pluvial deposits in western North America. We performed 40Ar/39Ar dating of single sanidines to delimit eruption age, and ion microprobe U-Pb and trace-element analyses of crystal faces on single zircons to characterize magmatic evolution and date near-eruption crystallization, as well as analyses of crystal interiors to date the interval of zircon crystallization. Sanidines from the two LCT members A and B yield an 40Ar/39Ar isochron date of 631 ± 4 ka (2σ). Crystal faces on zircons from both members yield a weighted mean 206Pb/238U date of 627 ± 6 ka (2σ) and have trace element concentrations that vary with eruptive stratigraphy. Zircon interiors yield a weighted mean 206Pb/238U date of 660 ± 6 ka, and reveal reverse and/or oscillatory zoning of trace element concentrations, with many crystals containing high-U concentrations and dark cathodoluminescence (CL) cores. These crystals with high-U cores are possibly sourced from 'defrosting' of melt-impregnated margins of the growing subvolcanic reservoir. LCT sanidines mirror the variation of zircon composition within the eruptive stratigraphy, with crystals from upper LCT-A and basal LCT-B having bright-CL rims with high Ba concentrations, suggesting late crystallization after addition of less evolved silicic magma. The occurrence of distal LCT in stratigraphic sequences marking the Marine Isotope Stage 16-15 transition supports the apparent eruption age of ca. 631 ka. These results reveal that Lava Creek zircons record episodic heating, renewed crystallization, and an overall up-temperature evolution for Yellowstone's subvolcanic reservoir in the 103-104 year interval