Science.gov

Sample records for 40ar 39ar age

  1. Identification of excess 40Ar by the 40Ar 39Ar, age spectrum technique

    USGS Publications Warehouse

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

    1976-01-01

    40Ar 39Ar incremental heating experiments on igneous plagioclase, biotite, and pyroxene that contain known amounts of excess 40Ar indicate that saddle-shaped age spectra are diagnostic of excess 40Ar in igneous minerals as well as in igneous rocks. The minima in the age spectra approach but do not reach the crystallization age. Neither the age spectrum diagram nor the 40Ar 36Ar versus 39Ar 36Ar isochron diagram reliably reveal the crystallization age in such samples. ?? 1976.

  2. 40Ar/39Ar age spectra of some undisturbed terrestrial samples

    USGS Publications Warehouse

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

    1974-01-01

    40Ar/39Ar age spectra and 40Ar/36Ar vs 39Ar/36Ar isochrons were determined by incremental heating for 11 terrestrial rocks and minerals whose geology indicates that they represent essentially undisturbed systems. The samples include muscovite, biotite, hornblende, sanidine, plagioclase, dacite, diabase and basalt and range in age from 40 to 1700 m.y. For each sample, the 40Ar/39Ar ratios, corrected for atmospheric and neutron-generated argon isotopes, are the same for most of the gas fractions released and the age spectra, which show pronounced plateaus, thus are consistent with models previously proposed for undisturbed samples. Plateau ages and isochron ages calculated using plateau age fractions are concordant and appear to be meaningful estimates of the crystallization and cooling ages of these samples. Seemingly anomalous age spectrum points can be attributed entirely to small amounts of previously unrecognized argon loss and to gas fractions that contain too small (less than 2 per cent) a proportion of the 39Ar released to be geologically significant. The use of a quantitative abscissa for age spectrum diagrams is recommended so that the size of each gas fraction is readily apparent. Increments containing less than about 4-5 per cent of the total 39Ar released should be interpreted cautiously. Both the age spectrum and isochron methods of data reduction for incremental heating experiments are worthwhile, as each gives slightly different but complementary information about the sample from the same basic data. Use of a least-squares fit that allows for correlated errors is recommended for 40Ar/36Ar vs 39Ar/36Ar isochrons. The results indicate that the 40Ar/39Ar incremental heating technique can be used to distinguish disturbed from undisturbed rock and mineral systems and will be a valuable geochronological tool in geologically complex terranes. ?? 1994.

  3. Age measurements of potassium-bearing sulfide minerals by the 40Ar/39Ar technique

    USGS Publications Warehouse

    Czamanske, G.K.; Lanphere, M.A.; Erd, Richard C.; Blake, M.C., Jr.

    1978-01-01

    K-Ar ages have been determined for sulfide minerals for the first time. The occurrence of adequate amounts of potassium-bearing sulfides with ideal compositions K3Fe10S14 (???10 wt.% K) and KFe2S3 (???16 wt.% K) in samples from a mafic alkalic diatreme at Coyote Peak, California, prompted an attempt to date these materials. K3Fe10S14, a massive mineral with conchoidal fracture, gives an age of 29.4 ?? 0.5 m.y. (40Ar/39Ar), indistinguishable from the 28.3 ?? 0.4 m.y. (40Ar/39Ar) and 30.2 ?? 1.0 m.y.8 (conventional K-Ar) ages obtained for associated phlogopite (8.7 wt.% K). KFe2S3, a bladed, fibrous sulfide, gives a younger age, 26.5 ?? 0.5 m.y. (40Ar/39Ar), presumably owing to Ar loss. ?? 1978.

  4. 40Ar/39Ar age of Cretaceous-Tertiary boundary tektites from Haiti

    USGS Publications Warehouse

    Izett, G.A.; Dalrymple, G.B.; Snee, L.W.

    1991-01-01

    40Ar/39Ar dating of tektites discovered recently in Cretaceous-Tertiary (K-T) boundary marine sedimentary rocks on Haiti indicates that the K-T boundary and impact event are coeval at 64.5 ?? 0.1 million years ago. Sanidine from a bentonite that lies directly above the K-T boundary in continental, coal-bearing, sedimentary rocks of Montana was also dated and has a 40Ar/39Ar age of 64.6 ?? 0.2 million years ago, which is indistinguishable statistically from the age of the tektites.

  5. 40Ar/39Ar ages of the AD 79 eruption of Vesuvius, Italy

    USGS Publications Warehouse

    Lanphere, M.; Champion, D.; Melluso, L.; Morra, V.; Perrotta, A.; Scarpati, C.; Tedesco, D.; Calvert, A.

    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 (1?? 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. ?? Springer-Verlag 2006.

  6. 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.

  7. Age and origin of carlsbad cavern and related caves from 40Ar/39Ar of alunite

    PubMed

    Polyak; McIntosh; Guven; Provencio

    1998-03-20

    40Ar/39Ar dating of fine-grained alunite that formed during cave genesis provides ages of formation for the Big Room level of Carlsbad Cavern [4.0 to 3.9 million years ago (Ma)], the upper level of Lechuguilla Cave (6.0 to 5.7 Ma), and three other hypogene caves (11.3 to 6.0 Ma) in the Guadalupe Mountains of New Mexico. Alunite ages increase and are strongly correlative with cave elevations, which indicates an 1100-meter decline in the water table, apparently related to tectonic uplift and tilting, from 11.3 Ma to the present. 40Ar/39Ar dating studies of the hypogene caves have the potential to help resolve late Cenozoic climatic, speleologic, and tectonic questions. PMID:9506939

  8. 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

  9. Saddle-shaped 40Ar /39Ar age spectra from young, microstructurally complex potassium feldspars

    NASA Astrophysics Data System (ADS)

    Zeitler, Peter K.; Fitz Gerald, John D.

    1986-06-01

    A suite of young potassium feldspars show markedly saddle-shaped 40Ar /39Ar age spectra as a result of incorporating 10 -10 to 10 -9 mol/g of excess 40Ar. The minima of these age spectra record reasonable cooling ages, based on the known thermal history and geology of the samples. Acid etching of one sample indicates that excess 40Ar is concentrated near grain margins. The release of a substantial portion of this excess Ar at high temperatures in the laboratory requires that this component be situated in a more retentive site than radiogenic 40Ar. Anion vacancies have been proposed to act in this role in plagioclase, and we speculate that this is so in K-feldspar as well. Such a mechanism would explain the observation that relative to radiogenic 40Ar, excess 40Ar is incorporated at low temperatures in nature but is released at high temperatures in the laboratory. Oxygen diffusion provides an appropriate analogy for this phenomenon, being relatively fast under natural, hydrothermal conditions, but extremely slow in anhydrous environments such as an Ar-extraction system. TEM observations made on two of the samples confirm that their effective grain sizes for diffusion are likely to be on the order of ten microns, due to the presence of such microstructures as incoherent exsolution lamellae, dislocations, and stepped twins. TEM observations also reveal the presence in one sample of orthoclase enclaves in a microcline host.

  10. A test of the 40Ar/39Ar age spectrum technique on some terrestrial materials

    USGS Publications Warehouse

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

    1971-01-01

    40Ar/39Ar age spectra were determined for 10 terrestrial rock and mineral samples whose geologic history is known from independent evidence. The spectra for six mineral and whole rock samples, including biotite, feldspar, hornblende, muscovite, and granodiorite, that have experienced post-crystallization heating did not reveal the age of crystallization in any obvious way. Minima in the spectra, however, give reasonable maximum ages for reheating and high-temperature maxima can be interpreted as minimum crystallization ages. High-temperature ages of microcline and albite that have not been reheated are approximately 10% younger than the known crystallization age. Apparently there are no domains in these feldspars that have retained radiogenic 40Ar quantitatively. Spectra from two diabase samples that contain significant quantities of excess argon might mistakenly be interpreted as spectra from reheated samples and do not give the age of emplacement. The 40Ar/39Ar age spectrum technique may be a potentially valuable tool for the study of geologic areas with complex histories, but the interpretation of age spectra from terrestrial samples seems to be more difficult than suggested by some previous studies. ?? 1971.

  11. 40Ar/39Ar age of material returned from asteroid 25143 Itokawa

    NASA Astrophysics Data System (ADS)

    Park, Jisun; Turrin, Brent D.; Herzog, Gregory F.; Lindsay, Fara N.; Delaney, Jeremy S.; Swisher, Carl C.; Uesugi, Masayuki; Karouji, Yuzuru; Yada, Toru; Abe, Masanao; Okada, Tatsuaki; Ishibashi, Yukihiro

    2015-11-01

    The Hayabusa mission to asteroid 25143, Itokawa, brought back 2000 small particles, which most closely resemble material found in LL4-6 chondrites. We report an 40Ar/39Ar age of 1.3 ± 0.3 Ga for a sample of Itokawa consisting of three grains with a total mass of ~2 μg. This age is lower than the >4.0 Ga ages measured for 75% of LL chondrites but close to one for Y-790964 and its pairs. The flat 40Ar/39Ar release spectrum of the sample suggests complete degassing 1.3 Ga ago. Recent solar heating in Itokawa's current orbit does not appear likely to have reset that age. Solar or impact heating 1.3 Ga ago could have done so. If impact heating was responsible, then the 1.3 Ga age sets an upper bound on the time at which the Itokawa rubble pile was assembled and suggests that rubble pile creation was an ongoing process in the inner solar system for at least the first 3 billion years of solar system history.

  12. New high-precision 40Ar/39Ar ages on Oligocene volcanic rocks of northwestern Kenya

    NASA Astrophysics Data System (ADS)

    Brown, Francis H.; Jicha, Brian R.

    2016-02-01

    New, high-precision 40Ar/39Ar ages from volcanic rocks in northwestern Kenya are provided for some areas of exposure in this remote area. We report seven 40Ar/39Ar ages generated from single crystal total fusion experiments on alkali feldspar separated from volcanic rocks in the Mogila, Songot, and Lokwanamur Ranges and the Gatome valley. A rhyolite from the lower part of the sequence in the Mogila Range yielded ages of 32.31 ± 0.06 Ma and 32.33 ± 0.07 Ma, and a rhyolite near the top of that sequence yielded 31.67 ± 0.04 Ma. A single sample from the Songot Range yielded an age of 32.49 ± 0.07 Ma, slightly older than the rocks collected from Mogila. In both ranges the early Oligocene rhyolites are underlain by basalts, as is also the case in the Labur Range. Ages of 25.95 ± 0.03 Ma, 25.91 ± 0.04 Ma, and 27.15 ± 0.03 Ma were measured on alkali feldspar from rhyolites from the Lokwanamur Range, and the nearby Gatome valley. All of these rocks are part of an episode of widespread volcanism in northwestern Kenya in the mid-to late Oligocene that is not currently known from the Ethiopian Rift Valley.

  13. Unmixing 40Ar/39Ar Muscovite Ages Using Powder X-ray Diffraction

    NASA Astrophysics Data System (ADS)

    McAleer, R. J.; Kunk, M. J.; Valley, P. M.; Walsh, G. J.; Bish, D. L.; Wintsch, R. P.

    2014-12-01

    Whole rock powder X-ray diffraction (XRD) experiments from eight samples collected across a retrograde ductile shear zone in the Devonian Littleton Formation near Claremont, NH, exhibit broad and asymmetric to bimodal muscovite 00l reflections. These composite 00l reflections exhibit a systematic change in shape with increasing retrograde strain. Microtextural relationships, electron microprobe quantitative analyses, and element mapping indicate that the change in peak shape reflects progressive dissolution of metastable Na-rich muscovite and the precipitation of stable Na-poor muscovite. 40Ar/39Ar step heating experiments on muscovite concentrates from these samples show a decrease in total gas age from 274 to 258 Ma as the highest strain zone is approached, and steps within individual spectra range in age by ~20 m.y. The correlation between age and 00l peak shape suggests that the argon isotopic system also tracks the dissolution-precipitation process. Furthermore, the variation in age during step heating indicates that these populations exhibit different in-vacuo degassing behavior. Comparison of whole rock and muscovite concentrate XRD patterns from the same samples shows that the mineral separation process can fractionate these muscovite populations. With this knowledge, four muscovite concentrates were prepared from a single hand sample, analyzed by XRD, and dated. Combining modal estimates from XRD experiments with total gas ages, the four splits narrowly define a mixing line that resolves end-member ages of 250 and 300 Ma for the neocrystallized and earlier high grade populations of muscovite, respectively. These ages are consistent with age data from all other samples. The results show that, in some settings, powder XRD provides a powerful and time effective method to both identify the existence of and establish the proportions of multiple compositional populations of muscovite prior to 40Ar/39Ar analysis. This approach will be especially useful in

  14. 40Ar/39Ar age of the Lathrop Wells volcanic center, Yucca Mountain, Nevada

    USGS Publications Warehouse

    Turrin, B.D.; Champion, D.; Fleck, R.J.

    1991-01-01

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

  15. 40Ar/39Ar Age of the Lathrop Wells Volcanic Center, Yucca Mountain, Nevada.

    PubMed

    Turrin, B D; Champion, D; Fleck, R J

    1991-08-01

    Paleomagnetic and (40)Ar/(39)Ar analyses from the Lathrop Wells volcanic center, Nevada, indicate that two eruptive events have occurred there. The ages (136 +/- 8 and 141 +/- 9 thousand years ago) for these two events are analytically indistinguishable. The small angular difference (4.7 degrees ) between the paleomagnetic directions from these two events suggests they differ in age by only about 100 years. These ages are consistent with the chronology of the surficial geological units in the Yucca Mountain area. These results contradict earlier interpretations of the cinder-cone geomorphology and soil-profile data that suggest that at least five temporally discrete eruptive events occurred at Lathrop Wells approximately 20,000 years ago. PMID:17772371

  16. 40Ar/39Ar and cosmic ray exposure ages of plagioclase-rich lithic fragments from Apollo 17 regolith, 78461

    NASA Astrophysics Data System (ADS)

    Das, J. P.; Baldwin, S. L.; Delano, J. W.

    2016-01-01

    Argon isotopic data is used to assess the potential of low-mass samples collected by sample return missions on planetary objects (e.g., Moon, Mars, asteroids), to reveal planetary surface processes. We report the first 40Ar/39Ar ages and 38Ar cosmic ray exposure (CRE) ages, determined for eleven submillimeter-sized (ranging from 0.06 to 1.2 mg) plagioclase-rich lithic fragments from Apollo 17 regolith sample 78461 collected at the base of the Sculptured Hills. Total fusion analysis was used to outgas argon from the lithic fragments. Three different approaches were used to determine 40Ar/39Ar ages and illustrate the sensitivity of age determination to the choice of trapped (40Ar/36Ar)t. 40Ar/39Ar ages range from ~4.0 to 4.4 Ga with one exception (Plag#10). Surface CRE ages, based on 38Ar, range from ~1 to 24 Ma. The relatively young CRE ages suggest recent re-working of the upper few centimeters of the regolith. The CRE ages may result from the effect of downslope movement of materials to the base of the Sculptured Hills from higher elevations. The apparent 40Ar/39Ar age for Plag#10 is >5 Ga and yielded the oldest CRE age (i.e., ~24 Ma). We interpret this data to indicate the presence of parentless 40Ar in Plag#10, originating in the lunar atmosphere and implanted in lunar regolith by solar wind. Based on a chemical mixing model, plagioclase compositions, and 40Ar/39Ar ages, we conclude that lithic fragments originated from Mg-suite of highland rocks, and none were derived from the mare region.

  17. 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

  18. New 40Ar/39Ar Ages From Southwest Bolivia Refine the Timing of APVC Volcanism

    NASA Astrophysics Data System (ADS)

    Salisbury, M.; de Silva, S. L.; Jicha, B.; Singer, B.; Jiménez, N.; Ort, M.

    2008-12-01

    The Altiplano-Puna Volcanic Complex (APVC) of the Central Andes has produced prodigious silicic volcanism (at least 11,000 km3 of magma) over the last 10 Ma including some of the largest known ignimbrites on Earth. Despite excellent exposure, little previous work had been conducted on the timing and distribution of ignimbrite volcanism in the Lípez region of southwestern Bolivia, the heart of the APVC. To address this deficiency we have performed ~612 single crystal laser-fusion 40Ar/39Ar analyses from 39 pumice and bulk matrix samples collected from the main ignimbrite units within the Lípez region. Geochemistry of pumice and mineral samples, and paleomagnetic data are also being used to correlate individual ignimbrite units. Our new 40Ar/39Ar results establish new or refined eruption ages (with 2σ error) from the Vilama caldera at 8.41±0.02 Ma, Pastos Grandes caldera at 5.45±0.02 and 2.94±0.01 Ma, and Guacha caldera at 5.65±0.01, and 3.57±0.02 Ma. New ages were also determined for eruptions from the Panizos ignimbrite shield (6.86±0.03 Ma), Juvina ignimbrite shield (5.23±0.01 Ma), and the Laguna Colorado ignimbrite shield (2.21±0.05 and 1.95±0.03 Ma). The oldest ignimbrite we have found in the area is 10.33±0.64 Ma, a local unit beneath the Vilama ignimbrite. The youngest units have been identified west of the Guacha caldera with eruption ages of 1.70±0.6 Ma and 0.70±0.01 Ma. These results demonstrate that ignimbrite-producing eruptions in the Lípez region span the age of APVC volcanism previously established, with the largest eruptions occurring from long-lived, cyclic supervolcano caldera systems like Guacha and Pastos Grandes. The aggregate data from the APVC support the hypothesis that the APVC developed predominantly during distinct pulses of massive ignimbrite eruptions at ~8, 6, and 4 Ma and attest to episodic behavior of the magmatic system. Ignimbrites of <1 Ma, the cyclical nature of activity, and the continued geothermal presence and

  19. 40Ar/39Ar Ages for the Sentinel-Arlington Volcanic Field, Southwestern Arizona

    NASA Astrophysics Data System (ADS)

    Cave, S. R.; Greeley, R.; Champion, D. E.; Turrin, B. D.

    2007-12-01

    .16-51.48. Geochronology using 40Ar/39Ar method revealed an age of 1.94 +/- 0.85 Ma for Painted Rock Low Shield (New Mexico Geochronology Research Laboratory), 1.64 +/- 0.14 Ma for Theba Low Shield (Rutgers University) and 1.24 +/- 0.040 Ma for Wild Horse Low Shield (Rutgers University). Some ages were precise enough to correspond to the Matuyama reversed polarity epoch, with SAVF initiation possibly within the Olduvai normal polarity event. These dates represent an overall improvement in precision and accuracy over previous dates (values corresponding to 6.20 Ma to 1.28 Ma) collected in the late 1970s and early 1980s using K-Ar technique. The 40Ar/39Ar ages correspond to expected magnetic polarities and stratigraphic sequences.

  20. (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).

  1. 40Ar/39Ar ages from the rhyolite of Alder Creek, California: age of the Cobb Mountain normal-polarity subchron revisited

    USGS Publications Warehouse

    Turrin, B.D.; Donnelly-Nolan, J. M.; Hearn, B.C., Jr.

    1994-01-01

    New 40Ar/39Ar age determinations on sanidine from the rhyolite of Alder Creek, California, indicate a 1.186 ?? 0.006 Ma age for the Cobb Mountain Normal-Polarity Subchron. The hew age is statistically older (?? = 0.05) than the previously reported K-Ar age (1.12 ?? 0.02 Ma) and agrees with the age suggested by the astronomical polarity time scale. Incomplete extraction of radiogenic 40Ar (40Ar*) from the sanidine is the most likely reason for the disparity between the 40Ar/39Ar and K-Ar ages. Because the Cobb Mountain subchron is a worldwide, short-duration event, and because no widely used interlaboratory 40Ar/39Ar standard younger than 27 Ma exists, it is proposed that sanidine from the rhyolite of Alder Creek be considered for use as a new Quaternary 40Ar/39Ar mineral standard. -Authors

  2. A Late Mesoproterozoic 40Ar/39Ar age for a melt breccia from the Keurusselkä impact structure, Finland

    NASA Astrophysics Data System (ADS)

    Schmieder, Martin; Jourdan, Fred; Moilanen, Jarmo; Buchner, Elmar; Öhman, Teemu

    2016-02-01

    Field investigations in the eroded central uplift of the ≤30 km Keurusselkä impact structure, Finland, revealed a thin, dark melt vein that intersects the autochthonous shatter cone-bearing target rocks near the homestead of Kirkkoranta, close to the center of the impact structure. The petrographic analysis of quartz in this melt breccia and the wall rock granite indicate weak shock metamorphic overprint not exceeding ~8-10 GPa. The mode of occurrence and composition of the melt breccia suggest its formation as some kind of pseudotachylitic breccia. 40Ar/39Ar dating of dark and clast-poor whole-rock chips yielded five concordant Late Mesoproterozoic miniplateau ages and one plateau age of 1151 ± 10 Ma [± 11 Ma] (2σ; MSWD = 0.11; P = 0.98), considered here as the statistically most robust age for the rock. The new 40Ar/39Ar age is incompatible with ~1.88 Ga Svecofennian tectonism and magmatism in south-central Finland and probably reflects the Keurusselkä impact, followed by impact-induced hydrothermal chloritization of the crater basement. In keeping with the crosscutting relationships in the outcrop and the possible influence of postimpact alteration, the Late Mesoproterozoic 40Ar/39Ar age of ~1150 Ma should be treated as a minimum age for the impact. The new 40Ar/39Ar results are consistent with paleomagnetic results that suggested a similar age for Keurusselkä, which is shown to be one of the oldest impact structures currently known in Europe and worldwide.

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

    PubMed

    Vasconcelos, P M; Becker, T A; Renne, P R; Brimhall, G H

    1992-10-16

    Supergene cryptomelane [K(1-2)(Mn(3+)Mn(4+))(8)O(16). chiH(2)O] samples from deeply weathered pegmatites in southeastern Brazil subjected to (40)K-(40)Ar and (40)Ar/(39)Ar analysis yielded (40)K-(40)Ar dates ranging from 10.1 +/- 0.5 to 5.6 +/- 0.2 Ma (million years ago). Laser-probe (40)Ar/(39)Ar step-heating of the two most disparate samples yielded plateau dates of 9.94 +/- 0.05 and 5.59 +/- 0.10 Ma, corresponding, within 2 sigma, to the (40)K-(40)Ar dates. The results imply that deep weathering profiles along the eastern Brazilian margin do not reflect present climatic conditions but are the result of a long-term process that was already advanced by the late Miocene. Weathering ages predate pulses of continental sedimentation along the eastern Brazilian margin and suggest that there was a time lag between weathering and erosion processes and sedimentation processes. PMID:17833140

  4. 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.

  5. 40Ar/39Ar ages of lunar impact glasses: Relationships among Ar diffusivity, chemical composition, shape, and size

    NASA Astrophysics Data System (ADS)

    Zellner, N. E. B.; Delano, J. W.

    2015-07-01

    Lunar impact glasses, which are quenched melts produced during cratering events on the Moon, have the potential to provide not only compositional information about both the local and regional geology of the Moon but also information about the impact flux over time. We present in this paper the results of 73 new 40Ar/39Ar analyses of well-characterized, inclusion-free lunar impact glasses and demonstrate that size, shape, chemical composition, fraction of radiogenic 40Ar retained, and cosmic ray exposure (CRE) ages are important for 40Ar/39Ar investigations of these samples. Specifically, analyses of lunar impact glasses from the Apollo 14, 16, and 17 landing sites indicate that retention of radiogenic 40Ar is a strong function of post-formation thermal history in the lunar regolith, size, and chemical composition. This is because the Ar diffusion coefficient (at a constant temperature) is estimated to decrease by ∼3-4 orders of magnitude with an increasing fraction of non-bridging oxygens, X(NBO), over the compositional range of most lunar impact glasses with compositions from feldspathic to basaltic. Based on these relationships, lunar impact glasses with compositions and sizes sufficient to have retained ∼90% of their radiogenic Ar during 750 Ma of cosmic ray exposure at time-integrated temperatures of up to 290 K have been identified and are likely to have yielded reliable 40Ar/39Ar ages of formation. Additionally, ∼50% of the identified impact glass spheres have formation ages of ⩽500 Ma, while ∼75% of the identified lunar impact glass shards and spheres have ages of formation ⩽2000 Ma. Higher thermal stresses in lunar impact glasses quenched from hyperliquidus temperatures are considered the likely cause of poor survival of impact glass spheres, as well as the decreasing frequency of lunar impact glasses in general with increasing age. The observed age-frequency distribution of lunar impact glasses may reflect two processes: (i) diminished

  6. 40Ar/39Ar dating of Pleistocene tuffs: an accurate age for the Matuyama-Brunhes geomagnetic reversal (MBGR)

    NASA Astrophysics Data System (ADS)

    Mark, D. F.; Renne, P. R.; Morgan, L. E.; Deino, A.; Smith, V. C.; Ellis, B. S.; Pearce, N. J.

    2012-12-01

    Recent recalibrations of the 40Ar/39Ar system [1,2] reveal inconsistencies with some previous ages inferred for the MBGR. An Ar/Ar age [3] for the Bishop Tuff (BT) (which post-dates the MBGR by at least 15.3 ± 2.2 ka [3]) recalculated [2] yields an age of 778.0 ± 3.8 ka (1σ, full systematic uncertainty). The age is c. 10 ka older than the BT zircon ID-TIMS U-Pb age [4] and places the MBGR at c. 793 ka, c. 13 and 20 ka older than astronomical ages for the MBGR of 780 ka [5] and 773 ka [6], respectively. To determine an accurate age for the MBGR, we have made a series of 40Ar/39Ar age determinations for Pleistocene tuffs from both Indonesia and North America that have direct relationships to the MBGR. Blind analyses were conducted at SUERC and BGC. We observed excellent inter-laboratory agreement and no systematic offset in data. Ar/Ar ages are reported relative to [2] (1σ, full systematic uncertainty). Drill cores from ODP Site 758 show the precise location of the MBGR. Below the MBGR are two distal tephra horizons that we have identified as products of two temporally distinct Old Toba Tuff (OTT) eruptions (layer d OTT1 and layer D OTT2). Continuous sedimentation between OTT1 (802.8 ± 0.7 ka, n = 100, MSWD 1.2) and OTT2 (796.2 ± 0.8 ka, n = 62, MSWD 1.3) allows for calculation of an accurate sedimentation rate and for extrapolation of an age from OTT2 to the MBGR. Data define an age for the MBGR of 795.2 ± 0.9 ka. Using tephra above the MBGR boundary, the Middle Toba Tuff (layer C) and Young Toba Tuff (layer A), extrapolation down core supports a MBGR age of c. 795 ka. Recent age data for BT sanidine reported relative to FCs at 28.172 Ma (767.4 ± 1.1 Ma) [7] oddly yielded an Ar/Ar age that was indistinguishable from the BT zircon U-Pb age [4], which is consistent with previous 40Ar/39Ar age measurements made relative to FCs at 28.02 Ma [3]. Thus we made a series of 40Ar/39Ar measurements on the exact same sample as used by Rivera et al. [7] and observed

  7. 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

  8. 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.

  9. 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.

  10. 40Ar/ 39Ar ages for the alkaline volcanism and the basement of Gorringe Bank, North Atlantic ocean

    NASA Astrophysics Data System (ADS)

    Féraud, Gilbert; Gastaud, Janine; Auzende, Jean-Marie; Olivet, Jean-Louis; Cornen, Guy

    1982-01-01

    Gorringe Bank is situated on the Europe-Africa plate boundary at the eastern end of the Azores-Gibraltar fracture zone. It has two summits, Gettysburg Bank to the Southwest and Ormonde Bank to the northeast. We applied the 40Ar/ 39Ar stepwise heating method to date six samples of the alkaline volcanic rocks, two gabbros from the Ormonde Bank and a dolerite from the Gettysburg Bank. The results that the alkaline volcanism lasted probably for less than 6 Ma(66-60 Ma). Although the nature of this volcanism precludes any subduction feature during its setting, the alkaline volcanism of Ormonde is probably linked to Upper Cretaceous/Eocene compressive tectonic events. The basement rocks of Gorringe Bank reveal distrubed 40Ar/ 39Ar age spectra. One plagioclase and one biotite from a gabbro give evidence for a thermic event whose age is tentatively estimated at about 75 Ma, and related to a variation in the direction of the relative movement between Europe and Africa. The more probable age given by a plagioclase of another gabbro and by a dolerite (110 Ma) corresponds to tilting northeastward of the Gorringe massif.

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

    SciTech Connect

    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 separate 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.

  12. 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.

  13. 40Ar/39Ar age and chemistry of manganese mineralization in the Moab and Lisbon fault systems, southeastern Utah

    NASA Astrophysics Data System (ADS)

    Chan, Marjorie A.; Parry, William T.; Petersen, Erich U.; Hall, Chris M.

    2001-04-01

    Diagenetic iron and manganese mineralization is associated with the Moab and Lisbon faults and is an important indicator of fluid flow in Jurassic Navajo Sandstone of southeastern Utah. Reducing brines originating from the Pennsylvanian Paradox Formation (with or without hydrocarbons) mobilized disseminated iron and manganese in the Jurassic sandstones and mixed with shallow, oxygenated groundwater to precipitate both iron and manganese mineralization. Mineralization consists of colliform and concretionary hematite, pyrolusite, and cryptomelane-hollandite that contains 1.33 2.12 wt% K. The 40Ar/39Ar dating of vacuum-encapsulated cryptomelane yields age estimates of 25 20 Ma, indicating mineralization coincident with either a Colorado Plateau uplift episode or La Sal Mountains volcanism.

  14. The Manson Impact Structure: 40Ar/39Ar age and its distal impact ejecta in the pierre shale in southeastern South Dakota

    USGS Publications Warehouse

    Izett, G.A.; Cobban, W.A.; Obradovich, J.D.; Kunk, M.J.

    1993-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Izett, G. A.; Cobban, W. A.; Obradovich, J. D.; Kunk, M. J.

    1993-10-01

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

  16. Comparative 40Ar/39Ar and K-Ar dating of illite-type clay minerals: A tentative explanation for age identities and differences

    NASA Astrophysics Data System (ADS)

    Clauer, Norbert; Zwingmann, Horst; Liewig, Nicole; Wendling, Raymond

    2012-10-01

    The 40K/40Ar (K-Ar) and 40Ar/39Ar dating methods are applied here to the same, very small, micrometric illite-type particles that crystallized under low-temperature (< 175 °C) hydrothermal conditions in deeply buried Rotliegend (Permian) gas-bearing sandstones of NW Germany. Four samples with a total of fifteen size fractions from < 2 to 20-40 μm yield K-Ar ages that range from 166.0 ± 3.4 to 214.0 ± 5.9 Ma. The same size fractions dated by the 40Ar/39Ar method give total-gas ages ranging from 173.3 ± 2.0 to 228.8 ± 1.6 Ma. Nearly all 40Ar/39Ar total-gas ages are slightly older, which cannot be explained by the recoil effect only, the impact of which being amplified by the inhomogeneous shape of the clay minerals and their crystallographic characteristics, with varied crystallinity indices, and a particle width about 10 times large than thickness. The 40Ar/39Ar data outline some advantages, such as the plateaus obtained by incremental step heating of the various size fractions, even if not translatable straight as ages of the illite populations; they allow identification of two generations of authigenic illite that formed at about 200 and 175 Ma, and one detrital generation. However, 40Ar/39Ar dating of clay minerals remains challenging as technical factors, such as the non-standardized encapsulation, may have potential unexpected effects. Both dating methods have their limitations: (1) K-Ar dating requires relatively large samples (ca. 10-20 mg) incurring potential sample homogeneity problems, with two aliquots required for K and Ar analysis for an age determination, also inducing a higher analytical uncertainty; (2) an identified drawback of 40Ar/39Ar dating is Ar recoil and therefore potential loss that occurs during neutronic creation of 39Ar from 39K, mostly in the finer mineral particles. If all the recoiled 39Ar is redistributed into adjacent grains/minerals, the final 40Ar/39Ar age of the analyzed size fraction remains theoretically identical, but it

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

    NASA Astrophysics Data System (ADS)

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

    2016-08-01

    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-wise 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. 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.

  18. 40Ar/39Ar ages in deformed potassium feldspar: evidence of microstructural control on Ar isotope systematics

    NASA Astrophysics Data System (ADS)

    Reddy, Steven M.; Potts, Graham J.; Kelley, Simon P.

    2001-05-01

    Detailed field and microstructural studies have been combined with high spatial resolution ultraviolet laser 40Ar/39Ar dating of naturally deformed K-feldspar to investigate the direct relationship between deformation-related microstructure and Ar isotope systematics. The sample studied is a ~1,000 Ma Torridonian arkose from Skye, Scotland, that contains detrital feldspars previously metamorphosed at amphibolite-facies conditions ~1,700 Ma. The sample was subsequently deformed ~430 Ma ago during Caledonian orogenesis. The form and distribution of deformation-induced microstructures within three different feldspar clasts has been mapped using atomic number contrast and orientation contrast imaging, at a range of scales, to identify intragrain variations in composition and lattice orientation. These variations have been related to thin section and regional structural data to provide a well-constrained deformation history for the feldspar clasts. One hundred and forty-three in-situ 40Ar/39Ar analyses measured using ultraviolet laser ablation record a range of apparent ages (317-1030 Ma). The K-feldspar showing the least strain records the greatest range of apparent ages from 420-1,030 Ma, with the oldest apparent ages being found close to the centre of the feldspar away from fractures and the detrital grain boundary. The most deformed K-feldspar yields the youngest apparent ages (317-453 Ma) but there is no spatial relationship between apparent age and the detrital grain boundary. Within this feldspar, the oldest apparent ages are recorded from orientation domain boundaries and fracture surfaces where an excess or trapped 40Ar component resides. Orientation contrast images at a similar scale to the Ar analyses illustrate a significant deformation-related microstructural difference between the feldspars and we conclude that deformation plays a significant role in controlling Ar systematics of feldspars at both the inter- and intragrain scales even at relatively low

  19. Hydrothermal fluids, argon isotopes and mineralization ages of the Fankou Pb-Zn deposit in south China: Insights from sphalerite 40Ar/39Ar progressive crushing

    NASA Astrophysics Data System (ADS)

    Jiang, Ying-De; Qiu, Hua-Ning; Xu, Yi-Gang

    2012-05-01

    Hydrothermal fluid geochemistry and mineralization timing are two important factors in the genesis of a hydrothermal deposit. 40Ar/39Ar analyses of fluid inclusions not only provide time constraints for the mineralization but also help to clarify the K-Ca-Cl-Ar characteristics for the ore-forming fluids. In this study, six sphalerite samples collected from the Fankou lead-zinc sulfide deposit are investigated by 40Ar/39Ar in vacuo crushing. Gases liberated from the early and late crushing steps exhibit distinct Ar isotopic compositions and 40Ar/39Ar apparent ages. Argon released in the early steps has much higher 40Ar and 38ArCl and lower 37ArCa contents than those in the late steps. The significant excess Ar (40ArE) extracted in the early crushing steps shows a strong correlation with 38ArCl with very high 40ArE/38ArCl ratios. In contrast, those of the late steps mainly consist of atmospheric Ar and K-correlated radiogenic Ar with a constant 40ArR/39ArK ratio and the atmospheric initial 40Ar/36Ar ratio. As a result, all samples yield similar declining age spectra: the fore segments with anomalously old apparent ages decline quickly in the early crushing steps and the rear ones are flat with concordant apparent ages in the late crushing steps. The data points of the early steps define linear correlations in plots of 40ArNA/39ArK vs. 38ArCl/39ArK and 38ArCl/40ArNA vs. 39ArK/40ArNA (NA for non-atmospheric) and most yield ages of 240-230 Ma. On the other hand, the data of the late steps always construct well-defined isochrons in the plots of 36ArA/40ArNA vs. 39ArK/40ArNA with consistent ages of ˜300 Ma. We interpret that gases released in the early steps were from the secondary fluid inclusions (SFIs) due to their distribution characteristics along cracks leading to be easily extracted, and those released in the later steps represented the contribution of the primary fluid inclusions (PFIs). The initial 40Ar/36Ar ratios of SFIs, much higher than the modern

  20. An 40Ar/39Ar age for Geomagnetic Instability Recorded at the Albuquerque Volcanoes and Pringle Falls, Oregon

    NASA Astrophysics Data System (ADS)

    Singer, B. S.; Jicha, B. R.; Kirby, B. T.; Zhang, X.; Geissman, J. W.; Herrero-Bervera, E.

    2005-12-01

    The timing and frequency of short-lived geomagnetic events, including excursions and aborted reversal attempts, provide important observational constraints on models of geodynamo behavior as well as calibration points for stratigraphic and paleoclimatic age models derived from marine sediments. The number of potential geomagnetic events during the Quaternary period has proliferated as more detailed paleodirectional and paleointensity data have emerged from sediments worldwide. Yet, determining ages for these events remains a challenge because astronomical dating of sediment cores is subject to assumptions and non-systematic errors that are difficult to quantify and vary from core to core. 40Ar/39Ar geochronology applied to lava flows or ash falls that record geomagnetic excursions can yield radioisotopic ages sufficiently precise to verify individual events, test correlations among seemingly disparate events, and quantify periods between events. Seventeen furnace incremental heating experiments on 100-200 mg groundmass samples from six sites in the transitionally magnetized basalt of the Albuquerque Volcanoes yield an isochron of 211 ± 22 ka* that is within error of previous K-Ar (155 ± 94 ka) and U-Th isochron (156 ± 58 ka) age determinations, but is 3 to 4 times more precise. At Pringle Falls, Oregon, Ash D was deposited during the onset of an excursion recorded by a lacustrine sediment sequence. Sixteen laser incremental heating experiments on 20-40 mg samples of plagioclase crystals from Ash D gave 64 concordant plateau age points that define an 40Ar/39Ar isochron of 211 ± 13 ka which is an order of magnitude more precise than the isochron (198 ± 118 ka) associated with the published plateau age from a single plagioclase age spectrum (221 ± 20 ka). Although the Virtual Geomagnetic Pole (VGP) recorded by the Albuquerque Volcanoes lies near, bot not on, the VGP path of the Pringle Falls excursion, these two sites are 15 arc degrees apart and need not record

  1. Rheasilvia provenance of the Kapoeta howardite inferred from ∼ 1 Ga40Ar/39Ar feldspar ages

    NASA Astrophysics Data System (ADS)

    Lindsay, Fara N.; Delaney, Jeremy S.; Herzog, Gregory F.; Turrin, Brent D.; Park, Jisun; Swisher, Carl C.

    2015-03-01

    We report 40Ar/39Ar ages for several lithological components of the brecciated howardite Kapoeta and compare the ages with results for asteroid 4 Vesta as observed by the Dawn mission. Our Kapoeta sample has an unusual, millimeter wide glass vein that intruded into a complex breccia. The plateau ages of three lithic clasts of basaltic composition that were remote from the glass vein range from 4.2 to 4.5 Ga. Such ages are typical of eucritic material; the oldest reflects early magmatic crystallization (∼ 4.5 Ga), the younger (4.2-4.5 Ga) reflect magmatism associated with protracted cooling. Samples of the glass vein itself, which include relict grains, give apparent ages between 3.1 and 3.9 Ga as do chips from the matrix. We consider both glass and bulk matrix ages as mixing ages; not marking the time of a single event, but dating regolith activity (< 3.1- 4.1 Ga). Eight feldspar grains close to the glass vein give markedly younger plateau ages averaging 1.4 Ga. The Ar release spectra for glass vein and breccia subsamples indicate a disturbance in the last 1.4 Ga. Taken together, these younger ages suggest a recent, major thermal event in the history of howardites that has been under-reported - perhaps the impact that formed the Rheasilvia basin on Vesta.

  2. 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

  3. 40Ar/39Ar impact ages and time-temperature argon diffusion history of the Bunburra Rockhole anomalous basaltic achondrite

    NASA Astrophysics Data System (ADS)

    Jourdan, Fred; Benedix, Gretchen; Eroglu, Ela.; Bland, Phil. A.; Bouvier, Audrey.

    2014-09-01

    The Bunburra Rockhole meteorite is a brecciated anomalous basaltic achondrite containing coarse-, medium- and fine-grained lithologies. Petrographic observations constrain the limited shock pressure to between ca. 10 GPa and 20 GPa. In this study, we carried out nine 40Ar/39Ar step-heating experiments on distinct single-grain fragments extracted from the coarse and fine lithologies. We obtained six plateau ages and three mini-plateau ages. These ages fall into two internally concordant populations with mean ages of 3640 ± 21 Ma (n = 7; P = 0.53) and 3544 ± 26 Ma (n = 2; P = 0.54), respectively. Based on these results, additional 40Ar/39Ar data of fusion crust fragments, argon diffusion modelling, and petrographic observations, we conclude that the principal components of the Bunburra Rockhole basaltic achondrite are from a melt rock formed at ∼3.64 Ga by a medium to large impact event. The data imply that this impact generated high enough energy to completely melt the basaltic target rock and reset the Ar systematics, but only partially reset the Pb-Pb age. We also conclude that a complete 40Ar∗ resetting of pyroxene and plagioclase at this time could not have been achieved at solid-state conditions. Comparison with a terrestrial analog (Lonar crater) shows that the time-temperature conditions required to melt basaltic target rocks upon impact are relatively easy to achieve. Ar data also suggest that a second medium-size impact event occurred on a neighbouring part of the same target rock at ∼3.54 Ga. Concordant low-temperature step ages of the nine aliquots suggest that, at ∼3.42 Ga, a third smaller impact excavated parts of the ∼3.64 Ga and ∼3.54 Ga melt rocks and brought the fragments together. The lack of significant impact activity after 3.5 Ga, as recorded by the Bunburra Rockhole suggests that (1) either the meteorite was ejected in a small secondary parent body where it resided untouched by large impacts, or (2) it was covered by a porous heat

  4. The tectonic evolution of Cenozoic extensional basins, northeast Brazil: Geochronological constraints from continental basalt 40Ar/39Ar ages

    NASA Astrophysics Data System (ADS)

    de Souza, Zorano Sérgio; Vasconcelos, Paulo Marcos; Knesel, Kurt Michael; da Silveira Dias, Luiz Gustavo; Roesner, Eduardo Henrique; Cordeiro de Farias, Paulo Roberto; de Morais Neto, João Marinho

    2013-12-01

    The Boa Vista and Cubati Basins, Paraíba, Brazil, are NW-SE extension-related intracratonic basins that resulted from tectonic stresses after the opening of the South Atlantic. These basins contain lacustrine fossiliferous sediments, bentonite beds, and basalt flows that preserve Cenozoic continental records. 40Ar/39Ar ages for six whole-rocks from two distinct basaltic flows underlying the sediments in the Boa Vista basin are 27.3 ± 0.8 and 25.4 ± 1.3 Ma, while three grains from a basaltic flow overlying the sediments yield 22.0 ± 0.2 Ma. The sediments at the nearby Cubati Basin are overlain by a basalt flow with ages of ˜25.4 Ma. Three whole-rocks from an NE-SW-trending trachytic dyke cross cutting the sediments at the Boa Vista Basin yield 40Ar/39Ar ages of ˜12.45 ± 0.06, 12.59 ± 0.07, and 12.58 ± 0.07 Ma. Three whole-rocks from a nearby volcanic plug (Chupador) yield an age of 23.4 ± 0.1 Ma. The geochronological results combined with stratigraphic correlations between the two basins allow bracketing the age of the main sedimentary and bentonic units within the Boa Vista and Cubati Basins between 25.5 ± 1.3 and 24.9 ± 0.1 Ma. The ages, combined with field observations reveal that the formation of the Boa Vista and Cubati basins is associated with mantle-derived magmas channelled through reactivated Precambrian shear zones. Our geochronological results suggest that a temporal link with the Fernando de Noronha and Saint Helena hot spots can be excluded as possible sources of the Boa Vista and Cubati magmas. Rather, the extensional tectonics in the 30-20 Ma interval, long after Gondwana break-up, may be associated with the re-activation of continental-scale shear zones that channelled small batches of mantle-derived magmas.

  5. 40Ar/39Ar Age Constraints on Caldera Formation of the Emmons Lake Volcanic Center, Alaska Peninsula, Alaska

    NASA Astrophysics Data System (ADS)

    Drake, J.; Layer, P. W.; Mangan, M. T.; Miller, T. P.; Waythomas, C. F.

    2001-12-01

    The Emmons Lake Volcanic Center located on the Alaska Peninsula is a large shield/stratovolcano complex composed of basaltic to andesitic lava flows and dacite to rhyolite pyroclastic flows, domes and ashfall. Two caldera forming eruptions in Pleistocene time each produced more than 50 cubic kilometers of silicic ejecta and created a nested depression measuring 20 km long and 10 km wide. We conducted 40Ar/39Ar whole rock dating of units associated with the first caldera forming event, which because of broad geochemical similarities, has been suggested as a possible source of the Old Crow Tephra, dated throughout interior Alaska and the Yukon at about 140 ka. Samples dated ranged in composition from ~62 to 69 wt % SiO2 and contained 2 - 3 wt % K2O. For each sample, 15 specimens, consisting of small ( ~1 mm) whole rock chips, were fused with an argon ion laser. From these analyses, weighted mean and isochron ages were calculated. For all samples, the initial 40Ar/36Ar ratio was indistinguishable from that of the present-day atmosphere (295.5), indicating that these samples do not contain significant quantities of excess argon. The age of a welded tuff interpreted to be from the opening plinian phase of the eruption is 233 +/- 6 ka, and is identical to the age of a post-collapse rheomorphic tuff (234 +/- 5 ka). A lithic fragment from a syn-collapse lag breccia has an age of 419 +/- 9 ka, which we interpret as representing incorporation of older material. Younger tuffs and domes were dated at 99 +/- 7 ka and 16 +/- 10 ka and imply that the complex was active throughout the late Quaternary. Based on these new age data, and subtle but significant trace element differences in glass and Fe-Ti oxide composition, we conclude that the first major caldera building event occurred at approximately 230 ka, and is probably not responsible for the deposition of the Old Crow tephra.

  6. A Carnian 40Ar/39Ar age for the Paasselkä impact structure (SE Finland)—An update

    NASA Astrophysics Data System (ADS)

    Schwarz, Winfried H.; Schmieder, Martin; Buchner, Elmar; Trieloff, Mario; Moilanen, Jarmo; Öhman, Teemu

    2015-01-01

    A recrystallized band of pale feldspathic impact melt in a gneissic impact breccia from the approximately 10 km Paasselkä impact structure in southeast Finland was dated via 40Ar/39Ar step-heating. The newly obtained plateau age of 228.7 ± 1.8 (2.2) Ma (2σ) (MSWD = 0.32; p = 0.93) is equal to the previously published pseudoplateau age of 228.7 ± 3.0 (3.4) (2σ) for the impact event. According to the current international chronostratigraphic chart and using the most recent published suggestions for the K decay constants, a Carnian (Late Triassic) age for the Paasselkä impact structure of 231.0 ± 1.8 (2.2) Ma (2σ) is calculated and considered the most precise and accurate age for this impact structure. The new plateau age for Paasselkä confirms the previous dating result but is, based on its internal statistics, much more compelling.

  7. 40Ar/ 39Ar mineral ages from basement rocks in the Eastern Kunlun Mountains, NW China, and their tectonic implications

    NASA Astrophysics Data System (ADS)

    Liu, Yongjiang; Genser, Johann; Neubauer, Franz; Jin, Wei; Ge, Xiaohong; Handler, Robert; Takasu, Akira

    2005-04-01

    40Ar/ 39Ar dating and estimates of regional metamorphic P- T conditions were carried out on the basement rocks of the Eastern Kunlun Mountains, Western China. Samples from the Jinshuikou, Xiaomiao, Kuhai, Wanbaogou, and Nachitai groups revealed distinct metamorphic events and four age groups. The age group in the range from 363 to 439 Ma is interpreted to represent cooling after Middle Silurian-Late Devonian granulite(?) and amphibolite facies metamorphism, which is dominated by low-middle pressure/high temperature conditions. This tectono-thermal event is related to the closure of an oceanic basin or marginal sea. An age group of 212-242 Ma represents cooling after Triassic metamorphic overprint, which is probably associated with magmatic intrusions. This thermal event, together with the Permo-Triassic ophiolite zone along the South Kunlun Fault, relates to the closure of a major ocean (between India and Eurasia) and the eventual N-ward accretion of the Qiangtang block in Permo-Triassic times. The significance of the age group of 104-172 Ma may be related to the ductile deformation along the Xidatan fault due to the northward-directed accretion of the Lhasa block. Biotites from Nachitai record a partial isotopic resetting at ca. 32 Ma that is interpreted to represent a late-stage exhumation caused by further crustal shortening.

  8. A 13 ± 3 ka age determination of a tholeiite, Pinacate volcanic field, Mexico, and improved methods for 40Ar/ 39Ar dating of young basaltic rocks

    NASA Astrophysics Data System (ADS)

    Turrin, Brent D.; Gutmann, James T.; Swisher, Carl C., III

    2008-11-01

    Among the youngest lava flows of the Pinacate volcanic field, Sonora, Mexico, is a large outpouring of tholeiite, the Ives flow. This tube-fed pahoehoe flow contrasts sharply with other Pinacate lavas in its great volume, alkali-poor composition and morphologic features, which include novel small structures named "spatter tubes." Despite its K-poor character, young age, and the presence of excess 40Ar, we determined a 40Ar/ 39Ar age on samples of this flow at 13 ± 3 ka. Such an age determination is made possible via careful monitoring of the mass discrimination of the mass spectrometer and by stacking results from multiple incremental-heating experiments into a single, composite isochron. This age is among the youngest ever to be determined with such precision by the 40Ar/ 39Ar method on a K-poor tholeiite.

  9. Carbon isotope stratigraphy, magnetostratigraphy, and 40Ar/39Ar age of the Cretaceous South Atlantic coast, Namibe Basin, Angola

    NASA Astrophysics Data System (ADS)

    Strganac, Christopher; Salminen, Johanna; Jacobs, Louis L.; Polcyn, Michael J.; Ferguson, Kurt M.; Mateus, Octávio; Schulp, Anne S.; Morais, Maria Luísa; Tavares, Tatiana da Silva; Gonçalves, António Olímpio

    2014-11-01

    We present the δ13C and paleomagnetic stratigraphy for marine strata at the coast of southern Angola, anchored by an intercalated basalt with a whole rock 40Ar/39Ar radiometric age of 84.6 ± 1.5 Ma, being consistent with both invertebrate and vertebrate biostratigraphy. This is the first African stable carbon isotope record correlated to significant events in the global carbon record spanning the Late Cenomanian to Early Maastrichtian. A positive ∼3‰ excursion seen in bivalve shells below the basalt indicates the Cenomanian-Turonian Boundary Event at 93.9 Ma, during Oceanic Anoxic Event 2. Additional excursions above the basalt are correlated to patterns globally, including a negative ∼3‰ excursion near the top of the section interpreted as part of the Campanian-Maastrichtian Boundary Events. The age of the basalt ties the studied Bentiaba section to a pulse of Late Cretaceous magmatic activity around the South Atlantic and significant tectonic activity, including rotation, of the African continent.

  10. 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.

  11. 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

  12. New palynology-based astronomical and revised 40Ar/39Ar ages for the Eocene maar lake of Messel (Germany)

    NASA Astrophysics Data System (ADS)

    Lenz, Olaf K.; Wilde, Volker; Mertz, Dieter F.; Riegel, Walter

    2015-04-01

    The annually laminated oil shale from the Eocene maar lake at Messel (Federal State of Hessen, Germany) provides unique paleoenvironmental data for a time interval of ~640 ka during the Paleogene greenhouse phase. As a consequence of orbitally controlled changes in the vegetation in the vicinity of the lake, the lacustrine laminites can now be astronomically tuned. Dating is based on the short eccentricity amplitude modulations of the regional pollen rain and their correlation to the astronomical La2010a/La2010d solutions in combination with a revised 40Ar/39Ar age of a basalt fragment from a lapilli tuff section below the first lacustrine sediments. Depending on different newly suggested ages for the Fish Canyon sanidine used as monitor for neutron irradiation, the age for the eruption at Messel is between 48.27 ± 0.22 and 48.11 ± 0.22 Ma. This allows for the first time the exact correlation of a Paleogene lacustrine sequence to the marine record in Central Europe. The Messel oil shale becomes now slightly older than previously assumed and includes the Ypresian/Lutetian boundary that moves the base of the European Land Mammal Age Geiseltalian (MP 11) into the Lower Eocene. This opens a window for establishing an independent chronostratigraphic framework for Paleogene terrestrial records and their correlation to the marine realm. Furthermore, the study reveals that higher amounts of pollen from "wet" and thermophilous plants indicate less seasonal and more balanced precipitation and slightly higher temperatures during a well-expressed eccentricity minimum.

  13. The tectonic significance of pre-Scandian 40Ar/39Ar phengite cooling ages in the Caledonides of western Norway

    USGS Publications Warehouse

    Andersen, T.B.; Berry, H.N., IV; Lux, D.R.; Andresen, A.

    1998-01-01

    Pre-Silurian continental-margin deposits in western Norway, non-conformably overlying allochthonous continental orthogneisses retain Ordovician 40Ar/39Ar cooling ages for phengites, implying either rapid cooling immediately after a Late Ordovician orogenic event, or less likely, a slow cooling following an Early Ordovician or older orogeny. The Dalsfjord Suite-H??yvik Group basement-cover pair are probably a lateral equivalent to Late Proterozoic sandstones ('sparagmites') covering the Jotun Nappe gneisses of the Middle Allochthon in central-south Norway. The H??yvik Group underwent polyphase deformation, greenschist-facies metamorphism (Tmax <450??C) and exhumation prior to deposition of the unconformably overlying Wenlockian continental-margin deposits of the Herland Group. The H??yvik Group was only weakly metamorphosed during obduction of the Solund-Stavfjord Ophiolite and the Scandian continental collision between Baltica and Laurentia. Phengitic white micas from the H??yvik Group yield cooling ages of 446.1 ?? 3.0, 449.1 ?? 2.2 and 447.5 ?? 4.0 Ma, respectively, identical within experimental error. One sample gives a plateau over 72% of the gas analysed, whereas the other samples were slightly disturbed after initial cooling, as indicated by systematically lower apparent ages at low experimental extraction temperatures. Minor 40Ar loss probably occurred during subsequent Scandian deformation and late to post-orogenic extension. The H??yvik Group rocks were unroofed before the Wenlock time (423-428 Ma) and cooled through the temperature for argon retention in phengite at c. 447 ?? 4 Ma, indicating a maximum cooling rate between 14 and 22??C/Ma-1 through Ashgill and Llandovery times before being subjected to low-grade metamorphism during the Scandian orogeny. Rapid pre-Scandian cooling, combined with peak metamorphic conditions of 450??C or less, may indicate that the Dalsfjord-H??yvik basement-cover pair were affected by an orogenic event during the Late

  14. 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

  15. /sup 40/Ar//sup 39/Ar age of detrital muscovite within Lower Ordovician sandstone in the coastal plain basement of Florida: implications for west African terrane linkages

    SciTech Connect

    Dallmeyer, R.D.

    1987-11-01

    Detrital muscovite was concentrated from a core of Lower Ordovician sandstone recovered from 1282 m in the Sun Oil Company, H.T. Parker No.1 well, Marion County, Florida. The concentrate records a /sup 40/Ar//sup 39/Ar plateau age of 504.1 +/- 2.1 Ma. The Paleozoic sedimentary section penetrated in this well is part of an extensive subsurface Lower Ordovician-Middle Devonian sedimentary succession characterized by Gondwanan paleontological affinities. The succession has been correlated with sequences of similar age in the Bove Basin of west Africa which unconformably overlie metamorphic units of the Bassaride and Rokelide orogens in Senegal and Guinea. Muscovite within these metamorphic rocks records ca. 500-510 Ma postmetamorphic /sup 40/Ar//sup 39/Ar cooling ages and was likely a proximal source for the lower Paleozoic clastic detritus represented in the pre-Mesozoic sedimentary sequences beneath the southeastern US coastal plain.

  16. Thermochronology of economic mineral deposits: dating the stages of mineralization at Panasqueira, Portugal, by high-precision 40Ar/ 39Ar age spectrum techniques on muscovite

    USGS Publications Warehouse

    Snee, L.W.; Sutter, J.F.; Kelly, W.C.

    1988-01-01

    This study is an example of a new and powerful application of 40Ar/39Ar age spectrum dating of muscovite. It is now possible to establish time constraints necessary for solving some of the long-standing problems in economic geology. Beyond this, the unique geologic situation of Panasqueira has allowed us to quantify the thermal characteristics of muscovite. Published fluid inclusion data have been used to estimate a muscovite argon closure temperature of ~325??C during rapid cooling or short reheating and a temperature of ~270??C during slow cooling or extended reheating. Argon-loss patterns displayed by all dated muscovites resulted from reheating after original closure; the mechanism for this argon loss appears to have been argon transport by volume diffusion. Thus, 40Ar/39Ar age spectrum dating of muscovite can be used to evaluate thermal conditions controlling argon diffusion as well as age, duration, and number of episodes of mineralization. -from Authors

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

    PubMed

    Swisher, C C; Grajales-Nishimura, J M; Montanari, A; Margolis, S V; Claeys, P; Alvarez, W; Renne, P; Cedillo-Pardoa, E; Maurrasse, F J; Curtis, G H; Smit, J; McWilliams, M O

    1992-08-14

    (40)Ar/(39)Ar dating of drill core samples of a glassy melt rock recovered from beneath a massive impact breccia contained within the 180-kilometer subsurface Chicxulub crater in Yucatán, Mexico, has yielded well-behaved incremental heating spectra with a mean plateau age of 64.98 +/- 0.05 million years ago (Ma). The glassy melt rock of andesitic composition was obtained from core 9 (1390 to 1393 meters) in the Chicxulub 1 well. The age of the melt rock is virtually indistinguishable from (40)Ar/(39)Ar ages obtained on tektite glass from Beloc, Haiti, and Arroyo el Mimbral, northeastern Mexico, of 65.01 +/- 0.08 Ma (mean plateau age for Beloc) and 65.07 +/- 0.10 Ma (mean total fusion age for both sites). The (40)Ar/(39)Ar ages, in conjunction with geochemical and petrological similarities, strengthen the recent suggestion that the Chicxulub structure is the source for the Haitian and Mexican tektites and is a viable candidate for the Cretaceous-Tertiary boundary impact site. PMID:17789640

  18. sup 40 Ar/ sup 39 Ar polyorogenic mineral age record within the southern Mauritanide orogen (M'Bout-Bakel region) West Africa

    SciTech Connect

    Dallmeyer, R.D. ); Lecorche, J.P. )

    1990-12-01

    The southern Mauritanide orogen exposed between M'Bout and Bakel is characterized by several internally imbricated, polydeformed, and variably metamorphosed infrastructural allochthons. {sup 40}Ar/{sup 39}Ar incremental-release ages recorded by hornblende within undeformed granodiorite of the Guidimakha Complex suggest post-magmatic cooling through appropriate argon closure temperatures at approx 670 Ma. {sup 40}Ar/{sup 39}Ar ages recorded by muscovite within lithologic elements of both the Guidimakha Complex and the Diala-Bouanze Series suggest initial regional metamorphism (associated with Pan-African I orogenesis) was following by cooling through muscovite argon closure temperatures between approx 600 and 620 Ma. Slight rejuvenation of muscovite argon systems occurred locally between approx 325 and 350 Ma. Muscovite and whole-rock slate/phyllite argon systems within metavolcanic and metavolcaniclastic components of the infrastructural calc-alkaline igneous complex (easternmost sectors of the M'Bout Series) record {sup 40}Ar/{sup 39}Ar plateau age of approx 300 to 320 Ma. Muscovite and whole-rock slate/phyllite argon systems within westernmost portions of the study area (western portions of the M'Bout Series) record {sup 40}Ar/{sup 39}Ar plateau ages of approx 267 to 312 Ma. All foreland units within the M'Bout-Bakel area were affected by post-Emsian folding. Effects of this tectonic activity are widespread throughout the parautochthon and western metamorphic sequences. These effects include emplacement of suprastructural ( ) allochthons and local reactivation of older thrust faults within infrastructural units.

  19. 40Ar- 39Ar ages of intrusions in East Greenland: Rift-to-drift transition over the Iceland hotspot

    NASA Astrophysics Data System (ADS)

    Tegner, C.; Brooks, C. K.; Duncan, R. A.; Heister, L. E.; Bernstein, S.

    2008-03-01

    Sixteen 40Ar- 39Ar ages are presented for alkaline intrusions to appraise prolonged post-breakup magmatism of the central East Greenland rifted margin, the chronology of rift-to-drift transition, and the asymmetry of magmatic activity in the Northeast Atlantic Igneous Province. The alkaline intrusions mainly crop out in tectonic and magmatic lineaments orthogonal to the rifted margin and occur up to 100 km inland. The area south of the Kangerlussuaq Fjord includes at least four tectonic lineaments and the intrusions are confined to three time windows at 56-54 Ma, 50-47 Ma and 37-35 Ma. In the Kangerlussuaq Fjord, which coincides with a major tectonic lineament possibly the failed arm of a triple junction, the alkaline plutons span from 56 to 40 Ma. To the north and within the continental flood basalt succession, alkaline intrusions of the north-south trending Wiedemann Fjord-Kronborg Gletscher lineament range from 52 to 36 Ma. We show that post-breakup magmatism of the East Greenland rifted margin can be linked to reconfiguration of spreading ridges in the Northeast Atlantic. Northwards propagation of the proto-Kolbeinsey ridge rifted the Jan Mayen micro-continent away from central East Greenland and resulted in protracted rift-to-drift transition. The intrusions of the Wiedemann Fjord-Kronborg Gletscher lineament are interpreted as a failed continental rift system and the intrusions of the Kangerlussuaq Fjord as off-axis magmatism. The post-breakup intrusions south of Kangerlussuaq Fjord occur landward of the Greenland-Iceland Rise and are explained by mantle melting caused first by the crossing of the central East Greenland rifted margin over the axis of the Iceland mantle plume (50-47 Ma) and later by uplift associated with regional plate-tectonic reorganization (37-35 Ma). The Iceland mantle plume was instrumental in causing protracted rift-to-drift transition and post-breakup tholeiitic and alkaline magmatism on the East Greenland rifted margin, and asymmetry

  20. Dating slate belts using 40Ar/39Ar geochronology and zircon ages from crosscutting plutons: A case study from east-central Maine, USA

    NASA Astrophysics Data System (ADS)

    Ghanem, Hind; Kunk, Michael; Ludman, Allan; Bish, David; Wintsch, Robert

    2016-04-01

    Determining the tectonic significance of slate belts is a persistent problem in many orogenic belts because of the lack of time constraints on the age of deposition and the age(s) of cleavages. We have solved this problem in east-central Maine where the ages of the regional Acadian cleavage (S1) and local ductile fault zone cleavage (S2) were both constrained using 40Ar/39Ar geochronology and the ages of crosscutting plutons. Applying 40Ar/39Ar geochronology to rocks with multiple generations of muscovite was possible because each cleavage-forming muscovite records a crystallization age rather than a cooling age due to the low grade of regional metamorphism. Evidence for metamorphic crystallization in rocks dominated by regional Acadian cleavage (S1) comes from the truncations of detrital and authigenic muscovite and chlorite grains by new muscovite and chlorite grains that define the S1 foliation. In rocks that display two foliations, the evidence comes from the truncations of chlorite and muscovite grains defining all earlier fabrics by new muscovite grains in the younger folia (S2). Step-heating experiments using the 40Ar/39Ar technique on twelve samples all yielded sigmoidal age spectra. The low-temperature steps produced a hump in the age spectra, indicating 39Ar recoil into adjacent interlayered chlorite grains, the latter interlayering confirmed by back-scattered electron imaging. Continuing steps climbed steadily from those with minimum apparent ages as young as ~381 Ma to steps with maximum ages as old as 466 Ma. The samples with the lowest minimum apparent age steps are those in which the S2 cleavage-forming mica population dominates. In contrast, the oldest apparent age steps are from samples that have the highest modal abundance of detrital micas. The Middle Ordovician age of the maximum age steps is interpreted to be the minimum cooling age of the detrital micas. The minimum 40Ar/39Ar age steps of muscovite in the samples that display only S1 cleavage

  1. Evidence from 40Ar/ 39Ar ages of lunar impact glasses for an increase in the impact rate ˜800 Ma ago

    NASA Astrophysics Data System (ADS)

    Zellner, N. E. B.; Delano, J. W.; Swindle, T. D.; Barra, F.; Olsen, E.; Whittet, D. C. B.

    2009-08-01

    Geochemical and 40Ar/ 39Ar data on nine impact glasses from the Apollo 14, 16, and 17 landing sites indicate at least seven distinct impact events with ages ˜800 Ma. Rock fragments analyzed by Barra et al. [Barra F., Swindle T. D., Korotev R. L., Jolliff B. L., Zeigler R. A., and Olsen E. (2006) 40Ar- 39Ar dating of Apollo 12 regolith: implications for the age of Copernicus and the source of nonmare materials, Geochim. Cosmochim. Acta,70, 6016-6031] from the Apollo 12 landing site and some Apollo 12 spherules reported by Levine et al. [Levine J., Becker T. A., Muller R. A., Renne P. R. (2005) 40Ar/ 39Ar dating of Apollo 12 impact spherules, Geophys. Res. Let., 32, L15201, doi: 10.1029/2005GL022874.] show ˜800 Ma ages, close to the accepted age of the Copernicus event, 800 ± 15 Ma [Bogard D. D., Garrison D. H., Shih C. Y., and Nyquist L. E. (1994) 39Ar- 40Ar dating of two lunar granites: The age of Copernicus, Geochim. Cosmochim. Acta, 58, 3093-3100]. These Apollo 12 samples are thought to have been affected by material from the Copernicus event since there is a Copernicus ray going through the Apollo 12 landing site. When all of these data are viewed collectively, including an Apollo 16 glass bomb [Borchardt R., Stöffler D., Spettel B., Palme H. and Wänke H. (1986) Composition, structure, and age of the Apollo 16 subregolith basement as deduced from the chemistry of post-Imbrium melt bombs. In Proceedings, 17th Lunar and Planetary Science Conference, pp. E43-E54], and in the context of diverse compositional range and sample location, there is a suggestion that there may have been a transient increase in the global lunar impact flux at ˜800 Ma. Therefore, the Copernicus impact event could have been one of many. If correct, there should be evidence for this increased impact flux around 800 Ma ago in the age statistics of terrestrial impact samples.

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

    NASA Astrophysics Data System (ADS)

    Righter, K.; Cosca, M. A.; Morgan, L. E.

    2016-09-01

    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.

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

    NASA Astrophysics Data System (ADS)

    Righter, K.; Cosca, M. A.; Morgan, L. E.

    2016-07-01

    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.

  4. 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

    In this study we systematically explore how acid leaching can be used to reduce the negative effects of seawater alteration on the 40Ar/39Ar age dating of submarine basalts. Koppers et al (2000) showed that acid leaching of groundmass samples generated more consistent ages as well as ages more concordant with phenocrystic mineral phases, compared to samples that were left untreated. By studying the effects of progressively increasing the strength and length of acid treatment, we will show how acid leaching of groundmass separates reduces alteration while leaving the initial eruption signature intact. Samples were chosen from the Walvis ridge hotspot trail in the southeast Atlantic. Three samples were selected based on degree and style of alteration. Two samples (basalt and basaltic andesite) appear highly altered in thin section. The basalt contains diffuse iddingsite alteration that is pervasive throughout the groundmass. The basaltic andesite displays focused secondary mineral phases within and around abundant vesicles. The third sample, a trachyte, shows relatively minor degrees of alteration in thin section. These groundmass separates were divided into four splits and treated with a progressively stronger acid and for longer duration. One split from each rock was left untreated to act as a baseline. Of the other three splits from each sample, one was treated with a mild leach (1N HCl and 1N HNO3), one a strong leach (1N HCl, 1N HNO3, 6N HCl, and 3N HNO3), and lastly the strong leach performed twice. The samples were then handpicked to remove any remaining visible alteration. The untreated samples were picked as well, removing the most distinctly altered grains. All splits were analyzed by electron microprobe, x-ray fluorescence (XRF) and the incremental heating 40Ar/39Ar dating method. We will report on the results of an image analysis of microprobe backscatter images and elemental maps taken of individual groundmass grains. This analysis will show the location

  5. 39Ar-40Ar Dating of Thermal Events on Meteorite Parent Bodies

    NASA Astrophysics Data System (ADS)

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

    1999-03-01

    A summary of 39Ar-40Ar ages reveals the impact and thermal history of several meteorite parent bodies, i.e., eucrites, chondrites, mesosiderites, acapulcoites/lodranites, winonaites, enstatites, and IAB and IIE irons.

  6. The Chelyabinsk Meteorite: Variable Shock Effects Recorded by the 40Ar-39Ar System

    NASA Astrophysics Data System (ADS)

    Korochantseva, E. V.; Buikin, A. I.; Hopp, J.; Lorenz, C. A.; Trieloff, M.

    2015-07-01

    Shocked lithologies of the Chelyabinsk LL chondrite have higher apparent 40Ar-39Ar ages than the very young light lithology. We interpret previous impact events made shocked lithologies more retentive and resistant against thermal reset.

  7. 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

  8. An Evaluation of the Complex Age Progression along the Cook-Austral Islands Using High-resolution 40Ar/39Ar Incremental Heating Ages

    NASA Astrophysics Data System (ADS)

    Rose, J.; Koppers, A. A. P.

    2014-12-01

    Until recently, the hotspot hypothesis has been generally accepted to explain the presence of linear volcanic chains. The hypothesis predicts a linear age progression along each chain, as well as consistent angular rotation velocities for all chains on a single plate. While such age progressions have been observed at places such as Hawaii and Louisville, several young (0-30 Ma) volcanic chains that formed on the Pacific plate show age progressions and associated angular velocities that are in disagreement with one another. The Cook-Austral island chain has age distributions that are particularly difficult to resolve based on the hotspot hypothesis, due to its location on the "hotspot highway" (Jackson et al. 2010) and a wide geographic range of recent volcanism. Several of these islands were previously studied by Turner and Jarrard (1982) who interpreted this age progression to suggest the presence of three active hotspots positioned along a "hot line" above a sheet-like upwelling area in the mantle as opposed to a singular "hot spot". However, this study was performed using the K/Ar dating method, and it has been shown that K and/or Ar loss (and addition of K) due to weathering and alteration can have significant effects on the age and uncertainty of samples dated with this technique. Here we present high-resolution 40Ar/39Ar incremental heating ages for several of the same samples previously analyzed in this study, as well as some unpublished samples. Analyses were conducted using the ARGUS-VI multicollector mass spectrometer, employing incremental heating procedures that provide more precise and more accurate ages compared to K/Ar and total fusion 40Ar/39Ar measurements. These new data will be used in conjunction with existing plate motion models and geochemical data to assess whether they support a point source or line source hypothesis. This in turn will allow us to improve our overall knowledge of mantle anomaly geometry and absolute plate motion.

  9. Cadomian vs. Variscan evolution of the Ossa-Morena zone (SW Iberia): field and 40Ar/ 39Ar mineral age constraints

    NASA Astrophysics Data System (ADS)

    Dallmeyer, R. D.; Quesada, C.

    1992-12-01

    Six hornblende and ten muscovite concentrates have been dated from three contrasting tectonic units exposed within the Ossa-Morena zone of the Iberian Massif. These include: (1) the Obejo-Valsequillo domain (north of the Badajoz-Córdoba shear zone); (2) the Sierra Albarrana structural unit (immediately south of the Badajoz-Córdoba shear zone); and (3) the thermal some exposed within the Olivenza-Monesterio antiform (Zafra-Monesterio domain). Hornblende from foliated amphibolite in the Obejo-Valsequillo domain (Siera Negra unit) displays internally discordant 40Ar/ 39Ar age spectra as a result of both intracrystalline contamination with extraneous argon and slight rejuvenation. 36Ar/ 40Ar vs. 39Ar/ 40Ar isotope-correlations are generally well-defined and yield ages of c. 550-560 Ma. These are interpreted to date cooling following late Precambrian-early Paleozoic (Cadomian) tectonothermal activity. This was followed by slight Variscan rejuvenation at c. 375-400 Ma. Muscovite from this unit displays internally discordant age spectra which reflect more extensive Variscan rejuvenation of intracrystalline argon systems which had initially cooled below appropriate blocking temperatures at c. 560 Ma. Hornblende and muscovite from the Sierra Albarrana domain (Sierra Albarrana Group) record 40Ar/ 39Ar plateau and isotope-correlation ages which range between c. 392 Ma and 351 Ma. These results indicate that complete Variscan rejuvenation was followed by relatively rapid post-metamorphic cooling. Hornblende separated from various lithologie elements exposed in the Olivenza-Monesterio Antiform yield variable results. Hornblende within a mafic xenolith in the Monesterio Granodiorite records an isotope-correlation age of c. 553 Ma which is interpreted to date the last cooling through appropriate argon retention temperatures. Hornblende within amphibolite of the Montemolin Series (lower Serie Negra) records internally discordant age spectra which reflect extensive Variscan (c

  10. Evidence of synmagmatic foliation in the Selawik Hills, NW Alaska, based on [sup 40]Ar/[sup 39]Ar age determinations

    SciTech Connect

    Solie, D.N. ); Layer, P.W. . Geophysical Inst.)

    1993-04-01

    Based on [sup 40]Ar/[sup 39]Ar plateau ages from various rock units in the Selawik Hills plutonic complex, northwestern Alaska, the units were emplaced in the order syenite/monzonite, followed by nepheline syenite and then quartz monzonite. There is no evidence of disturbance of the Ar isotopic system in the dated plutonic minerals, and the ages compare fairly well with previously published K/Ar data. A cooling history of about ten m.y. for the Selawk Hills rocks is suggested, based on comparison of [sup 40]Ar/[sup 39]Ar ages with apatite fission-track data (Murphy and Till, 1992). Comparison of hornblende plateau ages between nonfoliated and foliated syenite indicates that foliated rocks crystallized later than nonfoliated rocks, but within the initial cooling history of the complex. Foliated syenite/monzonite has mineralogy similar to nonfoliated, but with generally higher color index. Foliated textures are distributed throughout the complex, but are more prevalent to the north, proximal to a large (about 2 km[sup 2]) xenolithic metamorphic block which is bounded on the north by an east-west-trending fault. The authors suggest that synmagmatic fault movement acted as a mechanism causing plutonic foliation and resulting in possible loss of late fluid from semicrystallized syenitic magma to form kspar-rich dikes. Foundering of the xenolithic block within the magma may also have contributed to development of foliation. Continued fault movement is indicated by cataclastic deformation along the fault trace.

  11. 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

  12. 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, M.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

  13. Resolvable miscalibration of the 40Ar/39Ar geochronometer

    NASA Astrophysics Data System (ADS)

    Mundil, R.; Renne, P. R.; Min, K. K.; Ludwig, K. R.

    2006-12-01

    U/Pb and 40Ar/39Ar isotopic dating techniques are the most widely applied geochronometers, both capable of 0.1% internal precision. A robust intercalibration between the two isotopic systems is fundamental for reconstructing short term processes and events in geologic time. However, whereas the U decay constants are known precisely (to ca 0.1%), the currently used 40K decay constant (5.543×10^{-10}/yr, (1)) is associated with an unstated uncertainty that is about an order of magnitude larger than the former, making high-resolution comparisons of ages from the two isotopic systems impossible. We present an indirect calibration by comparing radio-isotopic ages derived from both isotopic systems of rapidly cooled volcanic rocks in order to minimize effects from protracted cooling history. Eleven data pairs of 206Pb/238U and conventional 40Ar/39Ar ages exhibit a bias between the two isotopic systems ranging from >-1.5% for young rocks to ca -0.5% for rocks as old as 2 Ga (possibly even smaller for rocks >2 Ga), with the 40Ar/39Ar ages being consistently younger. All Mesozoic and Paleozoic samples display a bias of about -1%. Most of this bias is probably the result of miscalibration of the electron capture decay constant of 404→ 40Ar (λ40Kec) by ca -1%, in combination with a miscalibration of smaller magnitude and opposite sense of the β- decay constant (λ40Kβ-) of 40K→ 40Ca. Bias greater than 1% for younger Cenozoic samples probably reflects pre-eruptive zircon saturation (magma residence time) whose effects become proportionately negligible beyond ca. 200 Ma. Whereas the currently used decay constant for 40K (see above) is based on an arguably arbitrary selection from counting experiments associated with large and sometimes incomprehensible uncertainties (mostly from experiments conducted in the 1940s to 1960s) two recent recalibrations of λ40Ktotal using liquid scintillation counting techniques suggest precise and mutually consistent values of 5.553 ± 0

  14. 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

  15. New 40Ar/39Ar Ages for Savai'i Island Reinstate Samoa as a Hotspot Trail with a Linear Age Progression

    NASA Astrophysics Data System (ADS)

    Koppers, A. A.; Russell, J. A.; Staudigel, H.; Hart, S. R.

    2006-12-01

    The volcanic islands and seamounts of the Samoan Archipelago have long been considered problematic in terms of the hotspot hypothesis. Existing K/Ar and 40Ar/39Ar measurements on subaerial samples from the Samoan islands have consistently given ages that are too young by several Myrs, conflicting with the expected linear age progression model. Previous data from the volcanic series and cones on Savai'i Island gave a range of ages between 0.2 and 2.1 Ma. This is in contrast to an age of 5.2 Ma that a Pacific plate motion of 7.1 cm/yr would predict for the onset of the shield building stage on Savai'i (Workman et al. 2004). The oldest shield ages for the islands of Upolu (2.7 My) and Tutuila (1.6 My) young eastward, toward the volcanically active Vailulu'u seamount that marks the current location of the Samoan hotspot (Hart et al. 2000; Staudigel et al. 2006). However, these ages are younger than predicted by the plate-speed model with 4.4 My and 2.7 My, respectively. The omnipresence of only young post-erosional volcanism on Savai'i has lead to extensive discussions on the origin of Samoan volcanism, and is often used as an argument against a possible hotspot and mantle plume origin. We present new 40Ar/39Ar data on volcanic samples from the deep flanks and rifts of Savai'i and a group of Samoan seamounts that were dredged during the ALIA Expedition. Twelve ages from eight different dredge locations confirm the predicted 7.1 cm/yr age progression for the Samoan hotspot. Three different volcanic samples from dredge ALIA-115, on the deepest portion of the SW flank of Savai'i Island, give indistinguishable ages (2σ confidence level) ranging from 4.99 to 5.21 Ma. In addition, a sample from dredge ALIA-128, on the NE flank of Savai'i, gives an age of 4.74 Ma. These results clearly demonstrate that the onset of the shield-building stage on Savai'i occurred much earlier than the oldest volcanics (2.1 Ma) sampled subaerially on the island. Sr-Nd-Pb isotopes and trace

  16. 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.

  17. 40Ar/ 39Ar-ages of phlogopite in mantle xenoliths from South African kimberlites: Evidence for metasomatic mantle impregnation during the Kibaran orogenic cycle

    NASA Astrophysics Data System (ADS)

    Hopp, Jens; Trieloff, M.; Brey, G. P.; Woodland, A. B.; Simon, N. S. C.; Wijbrans, J. R.; Siebel, W.; Reitter, E.

    2008-12-01

    We applied the 40Ar/ 39Ar dating method to an extensive suite of phlogopites from kimberlite-hosted mantle xenoliths (dominantly garnet bearing) from the mines of Bultfontein (South Africa), Letseng-la-Terae and Liqhobong (Lesotho). Argon extraction was performed by conventional high resolution stepwise heating technique, laser incremental heating technique and laser spot analysis. All age spectra obtained by conventional analysis indicate various degrees of 40Ar loss during kimberlite emplacement, but never resulted in a total reset of the argon system. Most intriguingly, the sample-specific maximum apparent ages cluster between 1.0 and 1.22 Ga for the phlogopites with the least disturbed age spectra. A maximum apparent age of 1.02 Ga was observed during laser heating analysis. Individual grains tend to yield older ages in their cores, with successively younger ages at their rims. The range in age obtained via the laser fusion technique and with conventional stepwise heating technique agrees with each other, as well as with literature data. The often inferred presence of excess 40Ar in those phlogopites cannot explain the coherent age pattern in the large suite of samples. Hence, the age constraint of 1.0-1.25 Ga is regarded as geologically meaningful and assigned to metasomatism of the local cratonic mantle during the advent of Kibaran orogenesis (1.00-1.25 Ga). The major consequences of our findings are: (i) The argon system of phlogopite can remain closed for long time scales, even at ambient temperatures of 800-1200 °C within the mantle, most likely because the solid/solid partitioning behaviour of Ar between phlogopite and other major phases in the mantle strongly favours phlogopite, or because conventionally inferred diffusivity of argon in phlogopite is seriously overestimated. Thus, the 40Ar/ 39Ar phlogopite system appears to be a valuable tool for deciphering ancient metasomatic events affecting the lithospheric mantle. (ii) The cratonic lithospheric

  18. 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.

  19. Re-Os and 40Ar/ 39Ar isotope measurements of inclusions in alluvial diamonds from the Ural Mountains: Constraints on diamond genesis and eruption ages

    NASA Astrophysics Data System (ADS)

    Laiginhas, Fernando; Pearson, D. Graham; Phillips, David; Burgess, Ray; Harris, Jeff W.

    2009-11-01

    The Re-Os isotope data for 20 syngenetic sulphide inclusions, recovered from 15 diamonds and the 40Ar/ 39Ar laser probe eruption ages of 7 syngenetic clinopyroxenes recovered from 5 diamonds, all from alluvial placer deposits in the Ural Mountains, have been determined. Six eclogitic sulphide inclusions, two of which coexist in the same diamond, yield an isochron age of 1280 ± 310 Ma (2 σ), with an unusually high initial 187Os/ 188Os ratio of 2.10 ± 0.58 (2 σ). The age is interpreted to date remobilisation of carbon and sulphur, and homogenisation of Os, during rift-related thermal-magmatic events that affected the East European Craton (EEC) at ca. 1.3 Ga. The high initial Os ratio suggests Re-Os evolution over a 100 to 500 Ma period within previously metasomatised lithosphere, most likely the EEC. Five eclogitic clinopyroxenes recovered from four diamonds yielded similar 40Ar/ 39Ar ages averaging 472 ± 28 Ma, which likely approximate the time of source kimberlite/lamproite eruption. This age suggests that the Ural diamonds are not likely to have derived either from the well known diamond-bearing kimberlites of the Siberian craton, nor from presently known Russian and Finnish kimberlite provinces on the EEC. The Urals placer deposits are mainly confined to 397-407 Ma sedimentary rocks along the western side of these mountains, with sediment transportation at that time generally from the north-west. Present evidence suggests the existence of an undiscovered kimberlite/lamproite source, probably on the Volgo-Uralia crustal segment of the EEC, which gave rise to the Urals diamond deposits.

  20. Radial fast-neutron fluence gradients during rotating 40Ar/39Ar sample irradiation recorded with metallic fluence monitors and geological age standards

    NASA Astrophysics Data System (ADS)

    Rutte, Daniel; Pfänder, Jörg A.; Koleška, Michal; Jonckheere, Raymond; Unterricker, Sepp

    2015-01-01

    the neutron-irradiation parameter J is one of the major uncertainties in 40Ar/39Ar dating. The associated uncertainty of the individual J-value for a sample of unknown age depends on the accuracy of the age of the geological standards, the fast-neutron fluence distribution in the reactor, and the distances between standards and samples during irradiation. While it is generally assumed that rotating irradiation evens out radial neutron fluence gradients, we observed axial and radial variations of the J-values in sample irradiations in the rotating channels of two reactors. To quantify them, we included three-dimensionally distributed metallic fast (Ni) and thermal- (Co) neutron fluence monitors in three irradiations and geological age standards in three more. Two irradiations were carried out under Cd shielding in the FRG1 reactor in Geesthacht, Germany, and four without Cd shielding in the LVR-15 reactor in Řež, Czech Republic. The 58Ni(nf,p)58Co activation reaction and γ-spectrometry of the 811 keV peak associated with the subsequent decay of 58Co to 58Fe allow one to calculate the fast-neutron fluence. The fast-neutron fluences at known positions in the irradiation container correlate with the J-values determined by mass-spectrometric 40Ar/39Ar measurements of the geological age standards. Radial neutron fluence gradients are up to 1.8 %/cm in FRG1 and up to 2.2 %/cm in LVR-15; the corresponding axial gradients are up to 5.9 and 2.1 %/cm. We conclude that sample rotation might not always suffice to meet the needs of high-precision dating and gradient monitoring can be crucial.

  1. 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

  2. A late Triassic 40Ar/39Ar age for the El Hammam high-REE fluorite deposit (Morocco): mineralization related to the Central Atlantic Magmatic Province?

    NASA Astrophysics Data System (ADS)

    Cheilletz, Alain; Gasquet, Dominique; Filali, Fouad; Archibald, Douglas A.; Nespolo, Massimo

    2010-04-01

    El Hammam is the only fluorite mine in Morocco (production 100,000 t/year). The fluorite mineralization is in an array of fluorite-calcite veins and is characterized by unusually high REE content in carbonate minerals (1,400 ppm in calcite; up to 2,000 ppm in siderite) and in fluorite (about 600 ppm). Since the 1960s, the genesis of the deposit has been attributed to a mesothermal hydrothermal event connected with late-Variscan granitic intrusions. Precise 40Ar/39Ar dating of hydrothermal K-feldspar yields an age of formation of the El Hammam deposit at 205 ± 1 Ma. Its genesis is therefore associated in time and space with the development of the Triassic-Jurassic basins and the associated anorogenic continental flood basalts of the Moroccan Mesetian Middle Atlas. The source of the hydrothermal mineralization (magmatic and/or metamorphic) is discussed.

  3. 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)

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

    2015-09-01

    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.

  4. 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.

  5. Palaeomagnetism and 40Ar/ 39Ar age from a Cretaceous volcanic sequence, Inner Mongolia, China: Implications for the field variation during the Cretaceous normal superchron

    NASA Astrophysics Data System (ADS)

    Zhu, Rixiang; Pan, Yongxin; He, Huaiyu; Qin, Huafeng; Ren, Shoumai

    2008-08-01

    An integrated palaeomagnetic and 40Ar/ 39Ar dating study was carried out on an Early Cretaceous volcanic lava sequence from the Suhongtu section, Inner Mongolia, to determine the field behavior within the Cretaceous normal superchron (CNS). 40Ar/ 39Ar ages were obtained from 12 lava flows, indicating that the studied lava was formed around 114.1 ± 0.3 Ma for the lower interval and 110.6 ± 0.1 Ma for the upper interval. Rock-magnetic experiments and electron microprobe analyses indicate that the primary Fe-Ti oxides are the main magnetic carriers. All lava flows carry normal palaeomagnetic directions, which can be grouped into 31 units by an F-test, with a Fisher mean characteristic remanent magnetization (ChRM) of D/ I = 12.8/58.6° ( α95 = 2.3°). The corresponding palaeomagnetic pole is located at 80.3°N and 200.3°E ( A95 = 3.2°, K = 64.4), which is indistinguishable at the 95% confidence level from the Eurasia pole derived from the apparent polar wander path for the early Cretaceous. Using a modified Thellier palaeointensity method with stringent acceptance criteria, we obtained two time-series of palaeointensity records from 15 independent palaeomagnetic units (total 136 samples). The virtual dipole moment (VDM) values varied from 2.53 × 10 22 Am 2 to 9.92 × 10 22 Am 2. The mean VDMs for the upper and lower intervals are (5.38 ± 2.06) × 10 22 Am 2 and (4.61 ± 2.67) × 10 22 Am 2, respectively. The observed time-series of palaeointensity, together with the previously available data for the CNS, suggest that magnetic field strength during the CNS fluctuated significantly with time.

  6. LASER MICROPROBE **4**0Ar/**3**9Ar DATING OF MINERAL GRAINS IN SITU.

    USGS Publications Warehouse

    Sutter, J.F.; Hartung, J.B.

    1984-01-01

    A laser-microprobe attached to a mass spectrometer for **4**0Ar/**3**9Ar age determination of single mineral grains in geological materials has been made operational at the US Geological Survey, Reston, VA. This microanalytical technique involves focusing a pulsed laser beam onto a sample contained in an ultra-high vacuum chamber attached to a rare-gas mass spectrometer. Argon in the neutron-irradiated sample is released by heating with the laser pulse and its isotopic composition is measured to yield an **4**0Ar/**3**9Ar age. Laser probe **4**0Ar/**3**9Ar ages of single mineral grains measured in situ can aid greatly in understanding the chronology of many geological situations where datable minerals are present but are not physically separable in quantities needed for conventional age dating.

  7. 40Ar-39Ar Analyses of Antarctic Dust Particles

    NASA Astrophysics Data System (ADS)

    Knott, S. F.; Turner, G.; Maurette, M.

    1993-07-01

    Eleven particles from the 100-400-micrometer-sized fraction of a sample of dust (vial G1-35) retrieved from Antarctica early in 1991 [1] have been analyzed using high-sensitivity noble gas mass spectrometry and, where possible, SEM/EDX techniques. The bulk sample was possibly heavily contaminated with terrestrial material but an attempt was made to preselect angular unmelted grains of extraterrestrial origin. Particles were examined optically and then split, where possible, into three parts to provide samples for 40Ar-39Ar, SEM, and He isotope analysis. Samples for 40Ar-39Ar studies were irradiated in the University of Michigan reactor, where they received a fast neutron fluence of approximately 10^18 cm^-2 (J = 0.0097, beta = 3.16). SEM analyses indicated that three particles (SK64, SK69, and SK72) have chondritic compositions, while a fourth (SK71) was thought to be extraterrestrial on the basis of its morphology. Two particles (SK65 and SK73) appeared to be terrestrial based on their location on an Mg-Fe-Si plot [2]. No SEM analyses are available for five of the samples (SK63, SK66, SK67, SK70, and SK71), and their origins are unknown. Gas was extracted from the samples for the argon analyses using a pulsed Nd laser. Step-heating was performed on each particle by defocusing the laser beam to reduce the heating effect. The laser delivered about 200 mJ per pulse; the initial heating was done with the beam covering approximately 150 micrometers. A broad overview of the data from nine particles analyzed in a seven-day sequence is shown in Fig. 1. Gas release, in units of 10^-12 ccSTP, is plotted as a function of run number with sample analyses interspersed with system blanks. Only two terrestrial particles, with well-defined ages of 200 Ma and 1000 Ma, released large amounts of gas and are omitted from the plot. The remaining particles analyzed so far released very little 40Ar and contrast sharply with the much larger amounts observed by Saxton et al. [3] in a suite

  8. Distinct brief major events in the Karoo large igneous province clarified by new 40Ar/ 39Ar ages on the Lesotho basalts

    NASA Astrophysics Data System (ADS)

    Jourdan, F.; Féraud, G.; Bertrand, H.; Watkeys, M. K.; Renne, P. R.

    2007-10-01

    Recent mineral separate ages obtained on the Karoo large igneous province (southern Africa) suggest that the province was built by several distinct magmatic pulses over a rather long period on the order of 5-6 Ma concerning the main erupted volume [Jourdan, F., Féraud, G., Bertrand, H., Kampunzu, A.B., Tshoso, G., Watkeys, M.K., Le Gall., B., 2005. The Karoo large igneous province: Brevity, origin, and relation with mass extinction questioned by new 40Ar/ 39Ar age data, Geology 33, 745-748]. Although this apparently atypical province is dated in more detail compared to many other large igneous provinces, volumetrically important areas still lack sufficient high-quality data. The timing of the Karoo province is crucial as this event is correlated with the breakup activity of the Gondwana supercontinent. The Lesotho basalts represent a major lava sequence of the province, but have not yet been precisely dated by systematic analysis of mineral separates. We analyzed plagioclase separates from five lava flows encompassing the complete 1.4-km-thick Lesotho sequence from top to bottom using the 40Ar/ 39Ar method. We obtained five plateau and mini-plateau ages statistically indistinguishable and ranging from 182.3 ± 1.6 to 181.0 ± 2.0 Ma (2 σ). We derived an apparent maximum duration for this event of ˜ 0.8 Ma by neglecting correlated errors embedded in the age uncertainties. A critical review of previous ages obtained on the Lesotho sequence [Duncan R.A., Hooper, P.R., Rehacek, J., Marsh, J.S., Duncan, A.R., 1997. The timing and duration of the Karoo igneous event, southern Gondwana. Journal of Geophysical Research 102, 18127-18138] shows that groundmass analyses are unreliable for high-resolution geochronology, due to alteration and 39Ar recoil effects. Discrepancy between our ages and a previous plagioclase age at ˜ 184 Ma obtained by the later workers is tentatively attributed to the heterogeneity of the monitor used and/or cryptic excess 40Ar *. The current age

  9. Implications of a nonlinear 40Ar/39Ar age progression along the Louisville seamount trail for models of fixed and moving hot spots

    NASA Astrophysics Data System (ADS)

    Koppers, Anthony A. P.; Duncan, Robert A.; Steinberger, Bernhard

    2004-06-01

    The Louisville seamount trail has been recognized as one of the key examples of hot spot volcanism, comparable to the classic volcanic Hawaiian-Emperor lineaments. The published total fusion 40Ar/39Ar data of Watts et al. [1988] showed an astonishing linear age progression, firmly establishing Louisville as a fixed hot spot in the South Pacific mantle. We report new 40Ar/39Ar ages based on high-resolution incremental heating 40Ar/39Ar dating for the same group of samples, showing a marked increase in both precision and accuracy. One of the key findings in our reexamination is that the age progression is not linear after all. The new data show a significantly decreased "apparent" plate velocity for the Louisville seamount trail older than 62 Ma but confirm the linear trend between 47 Ma and the present day (although based on only three samples over 2150 km). The most recent volcanic activity in the Louisville seamount trail has now been dated at 1.11 ± 0.04 Ma for the most southeastern seamount located at 50°26'S and 139°09'W. These results indicate that the Louisville age progression should be interpreted on the basis of both plate and hot spot motion. In this paper we examine our new results in conjunction with the numerical mantle flow models of [2004] that also predict marked deviations from simple linear age progressions. With these models we can achieve a good fit to the geometry of both the Hawaiian and Louisville seamount trails and their age progressions as well as the ˜15° paleolatitudinal shift observed by [2003] for the Hawaiian hot spot between 80 and 47 Ma. If the model is restricted to Pacific hot spots only, we can improve the fit to the nonlinear age trend for the Louisville seamount trail by allowing an additional rotation change of the Pacific plate around 62 Ma and by decreasing the initiation age of the Louisville plume from 120 to 90 Ma. This improved model features a significant eastward hot spot motion of ˜5° between 80 and 30 Ma for

  10. 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

  11. Paleointensities and 40Ar/39Ar ages of the Matuyama-Brunhes transition recorded in the lava sequence in the Punaruu valley, Tahiti

    NASA Astrophysics Data System (ADS)

    Mochizuki, N.; Oda, H.; Ishizuka, O.; Yamazaki, T.; Uto, K.; Tsunakawa, H.

    2009-12-01

    We have made a paleointensity study of the Matuyama-Bruhnes (M-B) transition recorded in the lava sequence in the Punaruu valley on Tahiti by using the LTD-DHT Shaw method. Samples were collected from 34 lavas of a section in the northern side of the Punaruu valley. Seventeen lavas of the stratigraphically low part of the section recorded reversed polarity, while the 11 lavas of the middle part showed a transitional behavior comprising multiple directional changes between full polarities. Four lavas in the highest part of the section recorded normal polarity. Three 40Ar/39Ar ages from the transitional zone and full polarity zones underlying and overlying it are indistinguishable and yield a weighted mean age of 769±20 ka (2 sigma). This age estimate is concordant with the reported age of the M-B transition. An 40Ar/39Ar age of 845±18 ka is obtained from the lowest part of the reversed zone. These results indicate the 34 lavas recorded the geomagnetic field ranging from the end of the Matuyama reversed chron to the early Brunhes normal chron. For the reserved period prior to the transitional behavior, the paleointensities show an oscillation-like variation between 6.1 and 41.3 µT corresponding to virtual dipole moments (VDMs) between 1.6 ×1022 and 9.6 ×1022 Am2. For the end of the reversed period and the transitional period, paleointensities were quite weak: 3.3 and 4.8 µT corresponding to VDMs of 0.9 ×1022 and 1.0 ×1022 Am2. For the normal period, paleointensities are 13.5-20.9 µT giving VDMs of 3.5-5.2 ×1022 Am2. A clear linear-like trend is observed in the diagram of VDM versus virtual geomagnetic pole (VGP) latitude. This trend could be resulted from a certain process of the geodynamo at the onset of the M-B transition.

  12. Single-crystal {sup 40}Ar/{sup 39}Ar ages for rocks in the lower part of the frontier formation (Upper Cretaceous), Southwest Wyoming

    SciTech Connect

    M`Gonigle, J.W.; Holmes, C.W.; Dalrymple, G.B.

    1995-04-01

    Five tuff beds in a 150 m (490 ft) thick section within the nonmarine Chalk Creek Member of the Frontier Formation and one bentonite bed within the Allen Hollow Shale Member of the Frontier Formation were sampled for {sup 40}Ar/{sup 39}Ar dating at localities south of Kemmerer, Wyoming. The study area extends from Cumberland Gap northward for 15 km (9.3 mi) past Blason Gap, and includes units 5-43 and unit 91 of the reference section measured by Cobban and Reeside in 1952. The age of the tuff beds ranges from 96.6 {plus_minus} 0.3 to 93.6 {plus_minus} 0.3 Ma and confirms the inferred Cenomanian age of much of the Chalk Creek Member. Previously, the member`s age had been based solely on its stratigraphic position between the Albian-to-lower Cenomanian marine rocks for the Aspen Shale and the lower Turonian marine shales in the middle of the Frontier Formation. The age of biotite crystals from the bentonite in the Allen Hollow Member, 92.1 {plus_minus} 0.2 Ma, confirms the paleontologic Turonian age of the member.

  13. 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

  14. High-precision 40Ar/ 39Ar age constraints on the basal Lanqi Formation and its implications for the origin of angiosperm plants

    NASA Astrophysics Data System (ADS)

    Chang, Su-chin; Zhang, Haichun; Renne, Paul R.; Fang, Yan

    2009-03-01

    Abundant Mesozoic terrestrial fossils were discovered in the Haifanggou Formation and the overlying Lanqi Formation in northeastern China. The recent discovery of Schmeissneria sinensis from the Haifanggou Formation provides evidence that the origin of angiosperms could be much earlier than previously believed. 92 taxa of plant fossils from the Lanqi Formation provide unique opportunities to understand the floral evolution and its diversification in the Mesozoic. Here we present robust high-precision 40Ar/ 39Ar data of 160.7 ± 0.4 Ma and 158.7 ± 0.6 Ma for two tuffs from the lowest part of the Lanqi Formation near the main outcrop of floral fossils in Beipiao City, Liaoning, China. Our age results indicate the whole Lanqi Formation was deposited in the Late Jurassic; consequently, the underlying Haifanggou Formation and Schmeissneria sinensis are at least Middle Jurassic in age. Besides its importance for floral evolution, our high-precision age results for the basal Lanqi Formation indicate the paleoenvironment in the north margin of the North China Craton was dry and hot in the Late Jurassic. Moreover, the new age data for the basal Lanqi Formation suggest that the unearthed fossils from the Haifanggou Formation and Lanqi Formation should be equivalent to the Daohugou Biota in Inner Mongolia, China.

  15. Strike-slip accomodation during the development of the Cantabrian and Central-Iberian oroclines: 40Ar*/39Ar geochronological ages of major shear zones.

    NASA Astrophysics Data System (ADS)

    Gutierrez-Alonso, Gabriel; Pastor-Galán, Daniel; Collins, Alan S.

    2013-04-01

    One of the most striking features found in the West European Variscan Belt is a large strikeslip shear zone/fault system, characterized as "Late-Variscan", that runs parallel to the broad structural trends around the Iberian Armorican Arc. 40Ar*-39Ar ages of micas grown during fabric development in five shear zones of this system (Traguntia-Juzbado; Porto-Tomar; Malpica-Tuy, Punta Langosteira and Ricobayo, both dextral and left lateral, have yielded ages that, within error, cluster at 307 Ma, suggesting that their development took place within the time frame of oroclinal bending constrained by paleomagnetism and structural data, that is to say, coeval with the formation of the Ibero-Armorican Arc. According to our new data and other data from the literature, we interpret the development of the strike-slip shear zone system and the origin of the magmatic pulse at ca 307 Ma as being related to the initiation of the orocline development. These new ages constrain deformation in the outer arc to be penecontemporaneous with thrust-sheet rotations in the inner arc Cantabrian Zone. The 307 Ma strike-slip shear-zones are inferred to have accommodated the vertical axis crustal or lithospheric-block rotations needed to accommodate oroclinal bending. Coeval granitoid ages, clustering at 307 Ma and located in Cantabrian orocline outer arc represent decompressive melting during the mechanical thinning of the mantle lithosphere below the outer arc during bending.

  16. 40Ar/39Ar Interlaboratory Calibration into the Holocene.

    NASA Astrophysics Data System (ADS)

    Heizler, M. T.; Jicha, B.; Koppers, A. A. P.; Miggins, D. P.

    2015-12-01

    Advances in 40Ar/39Ar analytical precision for very young rocks requires collaborative efforts amongst argon geochronology labs to demonstrate age reproducibility commensurate with high precision. NM Tech (NMT), the University of Wisconsin (UW) and Oregon State University (OSU) have each dated Quaternary flux monitor standard AC-2 sanidine (~1.185 Ma), a blind sanidine described as being 50-100 ka (BS) and sanidine from the Qixiangshan (QIX) flow (~10 ka), Changbaishan volcano, China. The samples were irradiated in a single package with FC-2 sanidine (28.201 Ma) as the flux monitor and the irradiated material was distributed amongst the labs. Heizler was present during analysis at both OSU and UW and Jicha attended OSU during analysis. Physical presence was key towards gaining understanding of individual protocols and prompted valuable discussions. Analyses were carried out on single crystals using total fusion and/or step heating approaches. Age agreement was achieved within 2s uncertainty that ranged between (0.03-0.3%, 0.13-0.37% and 1.8-2.6%) for AC-2, BS and QIX, respectively. Each lab found AC-2 to vary somewhat beyond a normal distribution and to yield an age relative to FC-2 of ~1.185 Ma that is ~1.3% (~5-10 sigma) lower than some published estimates. A key cause of the variation between this study and previous results may be variable gas pressure equilibration times between extraction line and mass spectrometer coupled with variable choices to estimate time zero by other laboratories. The majority of our efforts concentrated on the QIX sanidine where prior data obtained by our labs revealed a factor of two spread in age (~11 and 23 ka) based on experiments carried out by total fusion and bulk incremental heating. By conducting single crystal age spectrum analysis we were able to mitigate effects of melt inclusion hosted excess argon and xenocrystic contamination towards obtaining analytical agreement with apparent ages near 10 ka. However, philosophical

  17. SHRIMP and 40Ar/39Ar age constraints for timing of plutonism and mineralization in the Boulder batholith

    USGS Publications Warehouse

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

    2002-01-01

    The 66 Ma age for the quartz monzodiorite of Boulder Baldy and consideration of previous dating studies in the region indicate that small ca. 66 Ma plutonic systems may be common in the Boulder batholith region and especially to the east. The approximately 64 Ma porphyry copper systems at Butte and gold mineralization at Miller Mountain are indicative of regionally important mineralizing systems of this age in the Boulder batholith region. Resolution of the age and probable magmatic source of the Butte pre-Main Stage porphyry copper-molybdenum system and of the silver-rich polymetallic quartz vein systems in the northern part of the Boulder batholith documents that these deposits formed from two discrete periods of hydrothermal mineralization related to two discrete magmatic events.

  18. 40Ar/39Ar age of gold mineralization of the Malomyr deposit (eastern part of the Mongolian-Okhotsk foldbelt)

    NASA Astrophysics Data System (ADS)

    Sorokin, A. A.; Ponomarchuk, A. V.; Buchko, I. V.; Travin, A. V.; Ponomarchuk, V. A.

    2016-01-01

    Reliable age estimation was obtained originally in this study for gold mineralization of the Malomyr deposit (the eastern part of the Mongolian-Okhotsk foldbelt), which is one of the most well-known deposits in the Russian Far East. The data obtained show that the age of hydrothermal process that resulted in the formation of the Malomyr deposit may be estimated as ˜133-132 Ma. Data on magmatism of the same age within the considered region are absent. In the opinion of the authors, mobilization, redistribution of the ore material, and the formation of the Malomyr deposit were mostly controlled by dislocation processes accompanied by hydrothermal activity, which is supported by the results of structural studies.

  19. New [sup 40]Ar/[sup 39]Ar mica ages from eastern New Hampshire and southern Maine: Implications for the exhumation history of the region

    SciTech Connect

    Lux, D.R.; West, D.P. Jr. . Dept. of Geological Science)

    1993-03-01

    It has long been recognized that micas from the high-grade metamorphic terrane of Maine and New Hampshire have anomalously young K-Ar ages. Furthermore, ages show systematic spatial patterns. Samples from western New Hampshire are youngest and become progressively older towards the east. In the Kearsarge-Central Maine Synclinorium (KCMS) of western Maine, ages are oldest along the northern terminus of high grade metamorphism and become progressively younger towards the southwest. In order to understand this peculiar relationship, micas from 20 sites in eastern New Hampshire and southern Maine were dated by the [sup 40]Ar/[sup 39]Ar method. The following relationships are observed: (1) Micas from the KCMS of southern New Hampshire have Permo-Carboniferous ages and coexisting micas are highly discordant, (2) micas from within or very near the Massabesic Gneiss Complex have young ages ([approximately]240--250 Ma) and show little to no discordance, (3) with one exception, micas from south of the Sebago batholith in Maine are also young ([approximately]240--250 Ma) and show little to no discordance. North of the Sebago batholith the transition to older micas is gradual. Mica ages from the Massabesic Gneiss Complex are younger than in surrounding regions and the transition to older ages roughly coincides with the Campbell Hill and Flint Hill faults. Outside the zone of young micas, cooling curves are concave upward for the same temperature interval. The young micas are concordant indicating rapid cooling but they are [approximately]40 Ma younger than the time of Late Paleozoic metamorphism. Therefore the young ages cannot be explained by rapid post-metamorphic cooling. The authors believe the accelerated cooling is the result of regional tectonic exhumation related to the earliest stages of rifting associated with opening of the Atlantic.

  20. 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

  1. High-precision 40Ar/39Ar ages for the Jehol fossil-bearing formations in SE China

    NASA Astrophysics Data System (ADS)

    Chang, S.; Zhang, H.; Hemming, S. R.; Fang, Y.; Mesko, G. T.

    2009-12-01

    The Jehol Biota, defined as the characteristic Eosestheria-Ephemeropsis-Lycoptera assemblage, is known to be widely distributed in East Asia. The fossils of the Jehol Biota are magnificent, exquisitely preserved and extraordinarily diverse. Since the 1990s, abundant and varied fossils, including plants, insects, salamanders, dinosaurs, pterosaurs, choristoderes, birds, mammals and freshwater invertebrates, have been discovered from the Dabeigou, Yixian and Jiufotang Formations in Inner Mongolia, and Liaoning and Hebei Provinces of NE China. Each of these Jehol fossil-bearing formations has preserved a distinct assemblage of invertebrate and vertebrate fossils. Based on major invertebrates groups, the Jehol Biota has been divided into three developing stages and a hypothesis about its distribution and spread has been proposed. There is a clear progression towards greater diversity through the three phases and it corresponds to a progressive paleogeographic expansion through time. In addition to their extensive distribution in Inner Mongolia and NE China, other strata that contain Jehol related fossils have been identified in the central and most provinces of eastern China, the Korean Peninsula, Mongolia and Siberia. However, the detailed correlation between the classic Jehol outcrops and the less-studied localities requires further work, including high-resolution ages. We are analyzing sixteen volcanic samples from Zhejiang and Anhui Provinces to establish a high-precision chronostratigraphy for the less-studied localities across SE China and adjacent regions. Our work will provide important data to test the timing and the duration of three phases of the Jehol radiation. Furthermore, the age results will allow us to understand the temporal relationship among the Jehol localities and test if the later phases of the Jehol radiation had broader geographic distributions, as inferred from existing collections.

  2. 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

  3. 40Ar/39Ar and U-series ages of a Late Pleistocene geomagnetic excursion in Western North America: The Halina Pali event in Western North America?

    NASA Astrophysics Data System (ADS)

    Turrin, B. D.; Champion, D. E.; Mortlock, R. A.; Fairbanks, R. G.; Swisher, C. C.

    2013-12-01

    Here we report 40Ar/39Ar results on two mafic flows, Swift Creek (SC) andesite (17.3 ×1.6ka), located near Mount St. Helens, WA and Tabernacle Hill (TH) basalt (16.9 ×1ka) near Fillmore UT that record paleomagnetic co-latitudes of 28° for SC and 23° for TH, respectively. The remnant directions of these two flows are on the fringe of secular variation and we consider them 'excursionoid'. Because these magnetic directions are somewhat unusual, there are limited matches in the magnetic record preserved in the Mono Lake and Lake Bonneville sediments in this time interval. The best matches to the radiocarbon (calibrated to calendar year age) dated lacustrine magnetic record are at 22 and 18 for the SC flow and 22, 19, or 17 ka for the TH flows. The closest matches for the SC and TH paleomagnetic directions are between 18 and 17 ka, which stratigraphically bracket an excursion recorded in the sediments. Pending additional analyses, our best estimate for the age of the excursion is 17.1 ×1.9ka, which is concordant with the radiocarbon-calibrated age of 17.6 ka for the excursion as determined from the age calibrated paleomagnetic record of the lacustrine sections. In addition to the 40Ar/39Ar ages on the TH flow, we also report an isochron U-Th age of 15 × 4 ka on the outer surface tufa coating, which provides a minimum constraint on the age of the TH lava flows. Discussion: Evidence of a Late Pleistocene geomagnetic excursion event is quite extensive and has been reported from several localities. The earliest report of the excursion, from the Halina Pali scarp, Hawaii, suggested an age of about 17-18 ka [1]. Subsequently, there have been additional reports and age estimates of a similar age excursional event from several other localities; Lake Imurak, Alaska, USA (17-18 ka), Hokkaido, Japan (~23 ka), Lake Baikal, Russia (~20 ka), and from Tule Lake, California, USA (~23 ka), summarized in Oda, [2] and referenced therein, and in China (17-18 ka) [3]. In conclusion

  4. Uplift and cooling history of the NW Himalaya, northern Pakistan - evidence from fission-track and /sup 40/Ar//sup 39/Ar cooling ages

    SciTech Connect

    Zeitler, P.K.

    1983-01-01

    This study reports 145 fission-track and 21 /sup 40/Ar//sup 39/Ar cooling ages from the Himalaya of northern Pakistan. Studies of the Himalaya are important because they provide geologists with an opportunity to test models of orogenesis in an active tectonic setting. As the Himalaya become better known and models become more quantitative, information about thermal histories and rates of uplift and erosion will be needed. The cooling ages suggest, and thermal modelling confirms, that throughout the Tertiary, the cooling history of northern Pakistan was controlled by the effects of accelerating uplift and erosion. On average, from 30 Ma to the present, uplift rates increased from less than 0.1 mm/yr to 0.4 mm/yr. This uplift and erosion, however, has been variable in space as well as time. The association of the Nanga Parbat-Haramosh Massif and Hunza with very young cooling ages and with rapid uplift maintained for a period of several million years is the most striking discovery made by this study. The location of these two areas at the heart of the Pamir-Himalaya Arc suggests that their anomalous behavior is linked in some way to a locally vigorous collision of India and Eurasia, possibly due to a promontory of Indian crust. Several of the cooling ages reported help constrain the emplacement ages of intrusives located in northern Pakistan. In addition, cooling ages from the southern Swat-Hazara region can be interpreted to give the time of final southward thrusting of the Kohistan Arc along the Main Mantle Thrust, at about 30 Ma.

  5. 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.

  6. 40Ar/ 39Ar ages of CAMP in North America: Implications for the Triassic-Jurassic boundary and the 40K decay constant bias

    NASA Astrophysics Data System (ADS)

    Jourdan, F.; Marzoli, A.; Bertrand, H.; Cirilli, S.; Tanner, L. H.; Kontak, D. J.; McHone, G.; Renne, P. R.; Bellieni, G.

    2009-06-01

    The Central Atlantic magmatic province (CAMP) is one of the largest igneous provinces on Earth (> 10 7 km 2), spanning four continents. Recent high-precision 40Ar/ 39Ar dating of mineral separates has provided important constraints on the age, duration, and geodynamic history of CAMP. Yet the North American CAMP is strikingly under-represented in this dating effort. Here we present 13 new statistically robust plateau, mini-plateau and isochron ages obtained on plagioclase and sericite separates from lava flows from the Fundy ( n = 10; Nova Scotia, Canada), Hartford and Deerfield ( n = 3; U.S.A.) basins. Ages mostly range from 198.6 ± 1.1 to 201.0 ± 1.4 Ma (2 σ), with 1 date substantially younger at 190.6 ± 1.0 Ma. Careful statistical regression shows that ages from the upper (199.7.0 ± 1.5 Ma) and bottom (200.1 ± 0.9 Ma) units of the lava pile in the Fundy basin are statistically indistinguishable, confirming a short duration of emplacement (≪ 1.6 Ma; ≤ 1 Ma). Three ages obtained on the Hartford (198.6 ± 2.0 Ma and 199.8 ± 1.1 Ma) and Deerfield (199.3 ± 1.2 Ma) basins were measured on sericite from the upper lava flow units. We interpret these dates as reflecting syn-emplacement hydrothermal activity within these units. Collectively, CAMP ages gathered so far suggest a short duration of the main magmatic activity (2-3 Ma), but also suggest the possibility of a temporal migration of the active magmatic centers from north to south. Such a migration challenges a plume model that would postulate a radial outward migration of the magmatism and is more compatible with other models, such as the supercontinent global warming hypothesis. When compared to the age of the Triassic-Jurassic boundary, the filtered CAMP age database suggests that the onset of the magmatic activity precedes the limit by at least few hundred thousand years, thereby suggesting a causal relationship between CAMP and the end of Triassic mass extinction. An age at 191 Ma possibly suggests

  7. Prolonged volcanic history for the Curaçao Lava Formation inferred from new 40Ar-39Ar ages and trace phase geochemistry

    NASA Astrophysics Data System (ADS)

    Loewen, M. W.; Duncan, R. A.; Krawl, K.; Kent, A. J.; Sinton, C. W.; Lackey, J.

    2011-12-01

    Popular models have suggested that the Caribbean Large Igneous Province (CLIP) formed from the initial plume head of the Galapagos hotspot. A key exposure of CLIP lavas is the Curaçao Lava Formation (CLF), a 5 km section of lavas and sills on the island of Curaçao. The CLF contains a broad compositional range of submarine lavas from picrite pillows at the base of the identified section to plagioclase-clinopyroxene tholeiitic lavas, hyaloclastites, and poikilitic sills at the top. Despite the regional significance of the CLF to the tectonic interpretation of the CLIP, only whole rock geochemistry and a few 40Ar-39Ar ages exist for Curaçao. These ages suggest formation of the lavas around 89 Ma and the poikilitic sills around 75 Ma. However, Mid-Albian (~110 Ma) fossil ammonites from a single locality of intercalated sediments suggest a considerably older formation for the CLF. This older age of formation has been used to justify alternate formation models for the CLIP to a Galapagos plume initiation. Here we present new 40Ar-39Ar ages for the lavas and sills of the CLF that indicate a younger and more extended volcanic history than previously recognized. We have identified lava sequences from several localities on the island that erupted 62-66 Ma as well as lavas and sills that fall within previously recognized age ranges. The youngest ages are obtained on samples directly adjacent to the Mid-Albian ammonite locality. These results suggest that large portions of the CLF (1) significantly postdate initiation of the CLIP, and (2) may not be compatible with an origin related to the initiation of the Galapagos hotspot. It is important to note, however, that the previous 89 Ma ages from Curaçao lavas are closer to ages from CLIP lavas on Haiti that reach 90-94 Ma and are spatially located near the picrites on Curaçao. These diverse ages also require significant reinterpretation of the CLF as a complex association of lavas potentially spanning 30 million years

  8. 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.

  9. Zircon U-Pb And Biotite 40Ar/39Ar Ages Of Kohistan Lower Crustal Tonalite And Their Implications For The History Of Continental Collision

    NASA Astrophysics Data System (ADS)

    Nakajima, T.; Williams, I. S.; Hyodo, H.; Miyazaki, K.; Sano, S.; Kausar, A. B.

    2002-12-01

    The Kohistan block in northern Pakistan exposes a crustal cross section through an ancient oceanic island arc, comprising garnet pyroxenite, garnet granulite, banded amphibolite, norite gabbro, metasediment and metavolcanics. The Dasu Tonalite intrudes the lower crustal Kamila Amphibolite. The tonalite is foliated and folded concordantly with the host amphibolite, indicating syn-tectonic intrusion, and contains abundant magmatic epidotes, indicating high-P crystallization. The Dasu Tonalite is extremely poor in K2O (0.6-0.9 wt.% for SiO2 64-70%) and has a low initial 87Sr/86Sr (0.7037-0.7038, similar to the associated lower crustal amphibolite and granulite), consistent with juvenile granitic magma free of contamination by recycled upper crust. The Dasu Tonalite gave SHRIMP zircon U-Pb ages of 97.6+/-1.0 Ma and 98.0+/-1.1 Ma on two samples, and biotite 40Ar/39Ar ages of 69.7+/-0.7 Ma and 69.7+/-0.9 Ma. The euhedral shape and lack of overgrowth or resorption features in CL images of the zircons suggest a simple magmatic history starting at ca. 98 Ma with no evidence for a later major thermal event. The large discrepancy between the U-Pb and Ar-Ar ages might record the deep crustal residence time of the Dasu Tonalite. The tonalite magma was probably generated and crystallized at ca. 98 Ma, then remained in the lower crust at a temperature of about 700C (which is given by geothermometry of the intercalating Kamila amphibolite), cooling down to ca.300C at 69.7Ma. 69.7Ma is a cooling age during the process of exhumation of the Kohistan arc caused by the Indian collision.

  10. 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.

  11. 40Ar/ 39Ar ages and Sr-Nd-Pb-Os geochemistry of CAMP tholeiites from Western Maranhão basin (NE Brazil)

    NASA Astrophysics Data System (ADS)

    Merle, Renaud; Marzoli, Andrea; Bertrand, Hervé; Reisberg, Laurie; Verati, Chrystèle; Zimmermann, Catherine; Chiaradia, Massimo; Bellieni, Giuliano; Ernesto, Marcia

    2011-03-01

    The Central Atlantic Magmatic Province (CAMP), emplaced at the Triassic-Jurassic (T-J) boundary (~ 200 Ma), is among the largest igneous provinces on Earth. The Maranhão basin in NE Brazil is located around 700 km inland and 2000 km from the site of the earliest Pangea disruption. The CAMP tholeiites occur only in the western part of the basin and have been described as low and high-Ti. Here we document the occurrence of two sub-groups among the high-Ti tholeiites in the Western Maranhão basin. The major and trace elements and the Sr-Nd-Pb isotopic ratios define three chemical groups corresponding to the low-Ti (TiO 2 < 1.3 wt.%), high-Ti (TiO 2 ~ 2.0 wt.%) and evolved high-Ti (TiO 2 > 3 wt.%) western Maranhão basin tholeiites (WMBT). The new 40Ar/ 39Ar plateau ages obtained on plagioclase separates for high-Ti (199.7 ± 2.4 Ma) and evolved high-Ti WMBT (197.2 ± 0.5 Ma and 198.2 ± 0.6 Ma) are indistinguishable and identical to those of previously analyzed low-Ti WMBT (198.5 ± 0.8 Ma) and to the mean 40Ar/ 39Ar age of the CAMP (199 ± 2.4 Ma). We also present the first Re-Os isotopic data for CAMP basalts. The low and high-Ti samples display mantle-like initial ( 187Os/ 188Os) i ranging from 0.1267 to 0.1299, while the evolved high-Ti samples are more radiogenic (( 187Os/ 188Os) i up to 0.184) We propose that the high-Ti WMBT were derived from the sub-lithospheric asthenosphere, and contaminated during ascent by interaction with the subcontinental lithospheric mantle (SCLM). The evolved high-Ti WMBT were derived from the same asthenospheric source but experienced crustal contamination. The chemical characteristics of the low-Ti group can be explained by partial melting of the most fertile portions of the SCLM metasomatized during paleo-subduction. Alternatively, the low-Ti WMBT could be derived from the sub-lithospheric asthenosphere but the resulting melts may have undergone contamination by the SCLM. The occurrences of high-Ti basalts are apparently not

  12. 40Ar/39Ar ages of flood basalt provinces in Russia and China and their possible link to global faunal extinction events: A cautionary tale regarding alteration and loss of 40Ar∗

    NASA Astrophysics Data System (ADS)

    Baksi, Ajoy K.

    2014-04-01

    The hypothesis that the Permo-Triassic boundary (PTB) mass extinctions were caused by flood basalt volcanism in Russia (Siberian Traps) and/or China (the Emeishan Traps) is investigated from the point of view of time of occurrence (40Ar/39Ar ages). Numerous published ages in the literature are rejected as good estimates of the time of crystallization. The filters applied in this respect are (a) statistical reliability of plateau/isochron sections of stepheating data and (b) the alteration state of the material that was dated. Alteration appears to be ubiquitous, unsurprising since most of the material dated was used without acid leaching - a procedure that is effective in yielding fresh(er) samples. Of ˜70 ages in the literature for the main pulse of Siberian Trap volcanism, less than ten prove to be reliable ages. Similar techniques applied to 40Ar/39Ar for the Emeishan Traps, leaves only a single reliable age for the magmatic episode. These ages are compared to both published and new 40Ar/39Ar ages for the PTB as based on analysis of minerals from critical ash beds in China. There is good overlap in the ages (PTB - 250.0 ± 0.1 Ma, Siberian Trap lavas - 250.1 ± 0.4 Ma), lending credence to a genetic link between the formation of the Siberian Traps and the faunal extinction event at the PTB. A similar link for the formation of the Viluy Traps (Russia) and the Late Devonian extinction event is investigated; only a single reliable 40Ar/39Ar age is available for the Viluy Traps, and falls close to the interpolated age for the Frasnian-Fammenian boundary. The use of the unspiked K-Ar technique to yield accurate ages for such (altered) samples is questioned.

  13. 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.

  14. Improvements Needed in the 40Ar/39Ar Study of Geomagnetic Excursion Chronology

    NASA Astrophysics Data System (ADS)

    Champion, D. E.; Turrin, B. D.

    2015-12-01

    Our knowledge of the existence and frequency of brief geomagnetic polarity. excursions only increases with time. Precise and accurate 40Ar/39Ar ages will be required to document this, because 25 or more excursions may have occurred within the Brunhes Epoch (780ky) separated in time by as little as 10ky. Excursions are and will dominantly be discovered in mafic, low K2O rocks. Improvements in the analytical protocol to 40Ar/39Ar date low K2O, "young", and thus low 40Arrad rocks are required. While conventional K/Ar dating "worked", the assumption of perfect atmospheric equilibration is flawed. In particular, using a measured isochron intercept (±2s) to embrace an atmospheric intercept assumption turns a 40Ar/39Ar diffusive extraction into a series of "K/Ar-lite" experiments. The near ubiquitous excess 40Ar exhibited in final steps of "matrix" or "groundmass" fractions from whole-rock experiments (no glass, crystals) suggests equilibration with the atmosphere is not achieved. Removing magnetic sample splits (glass?) thought subject to poor argon retention, and crystals subject to 40Ar inheritance are routinely done without documenting different isochrons. Short 15 to 20 minute irradiation times effectively eliminate recoil and dramatically minimize isotopic corrections, and the assumption of equivalence in Ar isotope recoil behavior. Assuming no pressure dependency and constancy of mass discrimination value ignores knowledge from other gas mass spectroscopy (O, H, He, Ne). Dynamic mass spectroscopy in stable isotopic analysis allows routine per mil and 0.1 per mil ratios to be measured. Maintaining more than daily bracketing air pipette measurements at differing pressures, and controlling the range of pressures from each diffusive step will approximate this dynamic precision. Experiments will be discussed that exhibit aspects of 40Ar/39Ar dating protocols with which precision and accuracy can be improved.

  15. Application of deuteron-deuteron (D-D) fusion neutrons to 40Ar/39Ar geochronology.

    PubMed

    Renne, Paul R; Knight, Kim B; Nomade, Sébastien; Leung, Ka-Ngo; Lou, Tak-Pui

    2005-01-01

    Neutron irradiation of samples for 40Ar/39Ar dating in a 235U fission reactor requires error-producing corrections for the argon isotopes created from Ca, K, and, to a lesser extent, Cl. The fission spectrum includes neutrons with energies above 2-3 MeV, which are not optimal for the 39K(n,p)39Ar reaction. These higher-energy neutrons are responsible for the largest recoil displacements, which may introduce age artifacts in the case of fine-grained samples. Both interference corrections and recoil displacements would be significantly reduced by irradiation with 2.45 MeV neutrons, which are produced by the deuteron-deuteron (D-D) fusion reaction 2H(d,n)3He. A new generation of D-D reactors should yield sufficiently high neutron fluxes (>10(12) n cm(-2)s(-1)) to be useful for 40Ar/39Ar dating. Modeling indicates that irradiation with D-D neutrons would result in scientific benefits of improved accuracy and broader applicability to fine-grained materials. In addition, radiological safety would be improved, while both maintenance and operational costs would be reduced. Thus, development of high-flux D-D fusion reactors is a worthy goal for 40Ar/39Ar geochronology. PMID:15498681

  16. Comparison of conventional K-Ar and 40Ar/39Ar dating of young mafic volcanic rocks

    USGS Publications Warehouse

    Lanphere, M.A.

    2000-01-01

    K-Ar and 40Ar/39Ar ages have been measured on nine mafic volcanic rocks younger than 1 myr from the Snake River Plain (Idaho), Mount Adams (Washington), and Crater Lake (Oregon). The K-Ar ages were calculated from Ar measurements made by isotope dilution and K2O measurements by flame photometry. The 40Ar/39Ar ages are incremental-heating experiments using a low-blank resistance-heated furnace. The results indicate that high-quality ages can be measured on young, mafic volcanic rocks using either the K-Ar or the 40Ar/39Ar technique. The precision of an 40Ar/39Ar plateau age generally is better than the precision of a K-Ar age because the plateau age is calculated by pooling the ages of several gas increments. The precision of a plateau age generally is better than the precision of an isotope correlation (isochron) age for the same sample. For one sample the intercept of the isochron yielded an 40Ar/36Ar value significantly different from the atmospheric value of 295.5. Recalculation of increment ages using the isochron intercept for the composition of nonradiogenic Ar in the sample resulted in much better agreement of ages for this sample. The results of this study also indicate that, given suitable material and modern equipment, precise K-Ar and 40Ar/39Ar ages can be measured on volcanic rocks as young as the latest Pleistocene, and perhaps even the Holocene.

  17. The Karoo triple junction questioned : Evidence from 40Ar/39Ar Jurassic and Proterozoïc ages and geochemistry of the Okavango dyke swarm (Botswana).

    NASA Astrophysics Data System (ADS)

    Jourdan, F.; Féraud, G.; Bertrand, H.; Kampunzu, A. B.; Tshoso, G.; Le Gall, B.; Tiercelin, J. J.

    2003-04-01

    The lower Jurassic Karoo-Ferrar magmatism represents one of the most important Phanerozoic continental flood basalt (CFB) provinces. Karoo CFB is dominated by tholeiitic traps and apparently radiating giant dyke swarms covering altogether ca 3x106 km2. This study focuses on the giant N110° oriented Okavango dyke swarm (ODS) stretching over a distance of 1500 km through Botswana. This dyke swarm represents the main arm of the so-called Karoo triple junction which is generally considered as a key marker linking the Karoo magmatism to a starting mantle plume impact (Campbell and Griffiths, 1990). ODS dolerites yield twelve reliable plagioclase 40Ar/39Ar plateau (and mini-plateau) ages ranging from 178.3 +-1.1 (2 sigma) to 179.3 +-1.2 Ma (Le Gall et al, 2002 and unpublished data). The distribution of the ages along a narrow gaussian curve suggests a short period of magmatic activity centered around 178.9 Ma. In addition, small clusters of plagioclase separated from twenty-five other dykes and measured by total fusion, gave either Karoo or Proterozoïc ages. The Proterozoïc rocks range from 758.2 +-6.6 Ma and 1223.8 +-10.0 Ma (integrated ages) and, although petrographically indistinguishable in some cases, they display clear geochemical differences (e.g. TiO2<2%, Ti/Y<400) compared to the Karoo high-Ti ODS (TiO2>2%, Ti/Y>400). Geochemical data combined with available Ar/Ar dates allowed us to identify the two groups within a total set of seventy-eight dykes investigated: about 15 % of the bulk ODS dykes were emplaced during the Proterozoïc and, thus, the Jurassic Karoo dykes were emplaced along reactivated Proterozoïc structures. The validity of the Karoo triple junction-plume model, should therefore be revisited. Although available data on Proterozoïc dykes along the ODS are not precise enough to assess their exact emplacement age, they indicate that most of the Proterozoïc dykes were emplaced between 900 and 1100 Ma. This age range is the same as dating commonly

  18. A metrological approach to measuring 40Ar* concentrations in K-Ar and 40Ar/39Ar mineral standards

    NASA Astrophysics Data System (ADS)

    Morgan, Leah E.; Postma, Onno; Kuiper, Klaudia F.; Mark, Darren F.; van der Plas, Wim; Davidson, Stuart; Perkin, Michael; Villa, Igor M.; Wijbrans, Jan R.

    2011-10-01

    In geochronology, isotopic ages are determined from the ratio of parent and daughter nuclide concentrations in minerals. For dating of geological material using the K-Ar system, the simultaneous determination of 40Ar and 40K concentrations on the same aliquot is not possible. Therefore, a widely used variant, the 40Ar/39Ar technique, involves the production of 39Ar from 39K by neutron bombardment and the reliance on indirect natural calibrators of the neutron flux, referred to as "mineral standards." Many mineral standards still in use rely on decades-old determinations of 40Ar concentrations; resulting uncertainties, both systematic and analytical, impede the determination of higher accuracy ages using the K-Ar decay system. We discuss the theoretical approach and technical design of a gas delivery system which emits metrologically traceable amounts of 40Ar and will allow for the sensitivity calibration of noble gas mass spectrometers. The design of this system is based on a rigorous assessment of the uncertainty budget and detailed tests of a prototype system. A number of obstacles and proposed resolutions are discussed along with the selection of components and their integration into a pipette system.

  19. 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

  20. 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

  1. Magmatism and Eurekan deformation in the High Arctic Large Igneous Province: 40Ar- 39Ar age of Kap Washington Group volcanics, North Greenland

    NASA Astrophysics Data System (ADS)

    Tegner, C.; Storey, M.; Holm, P. M.; Thorarinsson, S. B.; Zhao, X.; Lo, C.-H.; Knudsen, M. F.

    2011-03-01

    The High Arctic Large Igneous Province is unusual on two counts: first, magmatism was prolonged and has been suggested to include an initial tholeiitic phase (130-80 Ma) and a second alkaline phase (85-60 Ma); second, it was subsequently deformed during the Eurekan Orogeny. New 40Ar-39Ar dating of alkaline volcanics from Kap Kane, part of the Kap Washington Group volcanics at the northern tip of Greenland, provides an emplacement age of 71.2 ± 0.5 Ma obtained from amphibole in lapilli tuffs, and a thermal resetting age of 49-47 Ma obtained in feldspar and whole-rocks from trachyte flows. Patch perthite feldspars and coeval resetting of Rb-Sr isotopes by hydrothermal fluids provide further support for thermal overprinting. This thermal event is interpreted as a result of compressional tectonism of the Kap Cannon Thrust Zone in which older Palaeozoic metasediments were thrusted northwards over the Kap Washington Group volcanics. The formation of the tholeiitic suite (130-80 Ma) is linked to the opening of the Canada Basin and may involve mantle plume action. Formation of the alkaline suite (85-60 Ma) is attributed to continental rifting in the Lincoln Sea area linked to seafloor spreading in the Labrador Sea and the Baffin Bay, and to eastwards displacement of Greenland relative to North America. The alkaline suite, therefore, may be unrelated to the main tholeiitic phase of the High Arctic Large Igneous Province. The subsequent initiation of continental rifting and ensuing seafloor spreading in the Northeast Atlantic resulted in spreading and volcanism (61-25 Ma) on both sides of Greenland, pushing Greenland northwards relative to North America. The tectonic setting in the High Arctic thus changed from extensional to compressional and volcanic activity was terminated. Evaluation of plate kinematic models shows that the relative northwards movement of Greenland culminated in the Eocene, coinciding with thermal resetting. We conclude that compression in North

  2. Independent 40Ar/39Ar and 14C age constraints on the last five glacial terminations from the aggradational successions of the Tiber River, Rome (Italy)

    NASA Astrophysics Data System (ADS)

    Marra, F.; Rohling, E. J.; Florindo, F.; Jicha, B.; Nomade, S.; Pereira, A.; Renne, P. R.

    2016-09-01

    We use 13 new 40Ar/39Ar and 4 new 14C datings of volcanic deposits and organic material found within near-coastal aggradational successions deposited by the Tiber River near Rome, Italy, to integrate a larger dataset previously achieved in order to offer independent age constraints to the sea-level fluctuations associated with Late Quaternary glacial cycles during the last 450 ka. Results are compared with the chronologically independently constrained Red Sea relative sea-level curve, and with the astronomically tuned deep-sea benthic δ18O record. We find good agreements for the timings of change, and in several cases for both the amplitudes and timings of change during glacial terminations T-1, T-2, T-3, and T-5. There is one striking exception, namely for glacial termination T-4 that led into interglacial Marine Isotope Stage (MIS) 9. T-4 in our results is dated a full 18 ka earlier than in the Red Sea and deep-sea benthic δ18O records (which are in good agreement with each other in spite of their independent chronological constraints). The observed discrepancy is beyond the scale of the combined age uncertainties. One possible explanation is that the documented aggradation represents an early phase, triggered by a smaller event in the sea-level record, but the thickness of the aggradational sediment sequence then suggests that the amplitude of this earlier sea-level rise is underestimated in the Red Sea and benthic δ18O records. Also, this would imply that the aggradational succession of the main T-4 deglaciation has not yet been located in the study region, which is hard to reconcile with our extensive fieldwork and borehole coverage, unless unlikely non-deposition or complete erosion. Resolving this discrepancy will improve understanding of the timing of deglaciations relative to the orbitally modulated insolation forcing of climate and will require further focused research, both into the nature and chronology of the Tiber sequences of this period, and into

  3. 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

  4. 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.

  5. 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, M.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

  6. [sup 40]Ar/[sup 39]Ar thermochronology in the northern Bitterroot mylonite zone, Mt

    SciTech Connect

    House, M.A.; Hodges, K.V. . Dept. of Earth, Atmospheric, and Planetary Sciences)

    1993-04-01

    The extensional Bitterroot mylonite zone defines the eastern and southern border of the Bitterroot metamorphic core complex and is generally interpreted to be the major structure which accommodated unroofing of the metamorphic core. The most commonly cited evidence for the age of mylonitization are [sup 40]Ar/[sup 39]Ar ages for hornblend, muscovite, biotite, and potassium feldspar from the southern Bitterroot mylonite zone that indicate rapid cooling of the core rocks between 45.5 and 43.5 Ma. More recently, an [sup 40]Ar/[sup 39]Ar K-feldspar age of 46.4 [+-] 0.8 Ma for an undeformed rhyolite dike that cuts across the mylonitic fabric places a minimum age constraint on the southern part of the shear zone. The authors have obtained new [sup 40]Ar/[sup 39]Ar data for metapelitic rocks and amphibolites from the northeast border of the Bitterroot metamorphic core complex near an area where mylonitized granitoid rocks yielding 48--52 Ma U-Pb zircon crystallization ages constrain the maximum age of mylonitization. Isochran ages of 47.9 [+-] 0.9 and 49 [+-] 1 Ma for hornblende separated from deformed amphibolite pods in the northeast border zone are within analytical uncertainty of the younger mylonitized granitoid crystallization ages and indicate rapid post-crystallization cooling through temperatures of [approximately]780--800 K. They attribute this cooling to denudation related to shear zone development. Muscovite and biotite isochron ages from metapelitic rocks within the shear zone are significantly younger, between 42 and 44 Ms., and generally agree with mica ages obtained by Garmezy and Sutter for the southern part of the shear zone. However, all mica ages from the Bitterroot shear zone are younger than the minimum age of the shear zone deduced from the age of cross-cutting rhyolite dikes.

  7. First 40Ar/39Ar dating of intense Late Palaeogene lateritic weathering in Peninsular India

    NASA Astrophysics Data System (ADS)

    Bonnet, Nicolas J.; Beauvais, Anicet; Arnaud, Nicolas; Chardon, Dominique; Jayananda, Mudlappa

    2014-01-01

    Lateritic surface processes have shaped large platform and cratons of the tropical belt. Constraining the timing of such processes is crucial to decipher their role in cratonic morphogenesis and their response to long-term climatic change and lithospheric deformation. Weathering histories have been documented for South America, Africa and Australia, but precise time constraints of the lateritic weathering processes in South India are still lacking. We present 40Ar/39Ar ages of supergene cryptomelane (K-Mn oxide) formed in the Sandur Mn ore deposits exposed on the highest lateritic paleolandsurface that once covered the Mysore plateau and the adjacent Deccan Traps. Significant 40Ar/39Ar ages are estimated between ∼36 and ∼26 Ma from well-defined plateaus in step heating 39Ar release spectra and from best-fitted inverse isochrones. These ages constitute firm time constraints that document intense late Eocene-Oligocene lateritic weathering over Peninsular India under the influence of warm and wet climate comparable to that prevailing in tropical humid forests. These results imply that Southern India was weathered between ∼36 and 26 Ma and may have been dissected mostly in the Neogene.

  8. 40Ar- 39Ar laser dating of tektites from the Cheb Basin (Czech Republic): Evidence for coevality with moldavites and influence of the dating standard on the age of the Ries impact

    NASA Astrophysics Data System (ADS)

    Di Vincenzo, Gianfranco; Skála, Roman

    2009-01-01

    Moldavites (Central European tektites) are genetically related to the impact event that produced the ˜24-km diameter Ries crater in Germany, representing one of the youngest large impact structures on Earth. Although several geochronological studies have been completed, there is still no agreement among 40Ar- 39Ar ages on both moldavites and glasses from Ries suevites. Even recently published data yielded within-sample mean ages with a nominal spread of more than 0.6 Ma (14.24-14.88 Ma). This age spread, which significantly exceeds current internal errors, must be in part ascribed to geological and/or analytical causes. This study reports the results of a detailed geochronological investigation of moldavites from the Cheb area (Czech Republic), which have never been dated before, and, for comparison, of two samples from type localities, one in southern Bohemia and the other in western Moravia. We used 40Ar- 39Ar laser step-heating and total fusion techniques in conjunction with microscale petrographic and chemical characterization. In addition, with the purpose of ascertaining the influence of the dating standards on the age of the Ries impact and making data from this study and literature consistent with the now widely used Fish Canyon sanidine (FCs) standard, we performed a direct calibration of multi-grain splits of the Fish Canyon biotite (FCT-3) with FCs. The intercalibration factors ( RFCsFCT-3), determined for eight stack positions in one of the three performed irradiations, were indistinguishable within errors and gave an arithmetic mean and a standard deviation of 1.0086 ± 0.0031 (±2 σ), in agreement with previous works suggesting that biotite from the Fish Canyon Tuff is somewhat older (˜0.8%) than the coexisting sanidine. Laser total fusion analysis of milligram to sub-milligram splits of five tektite samples from the Cheb area yielded mostly concordant intrasample 40Ar- 39Ar ages, and within-sample weighted mean ages of 14.66 ± 0.08-14.75 ± 0

  9. 40Ar/39Ar laser fusion and K-Ar ages from Lathrop Wells, Nevada, and Cima, California. The age of the latest volcanic activity in the Yucca Mountain area

    USGS Publications Warehouse

    Turrin, Brent D.; Champion, Duane E.

    1991-01-01

    K-Ar and 40Ar/39Ar ages from the Lathrop Wells volcanic center, Nevada, and from the Cima volcanic field, California, indicate that the recently reported 20-ka age estimate for the Lathrop Wells volcanic center is incorrect. Instead an age of 119??11 to 141??10 ka is indicated for the Lathrop Wells volcanic center. This age corrected is concordant with the ages determined by two independent isotopic geochronometric techniques and with the stratigraphy of surficial deposits in the Yucca Mountain region. In addition, paleomagnetic data and radiometric age data indicate only two volcanic events at the Lathrop Wells volcanic center that are probably closely linked in time, not as many as five as recently reported.

  10. {sup 40}Ar/{sup 39}Ar laser fusion and K-Ar ages from Lathrop Wells, Nevada, and Cima, California: The age of the latest volcanic activity in the Yucca Mountain area

    SciTech Connect

    Turrin, B.D. |; Champion, D.E.

    1991-05-01

    K-Ar and {sup 40}Ar/{sup 39}Ar ages from the Lathrop Wells volcanic center, Nevada, and from the Cima volcanic field, California, indicate that the recently reported 20-ka age estimate for the Lathrop Wells volcanic center is incorrect. Instead, an age of 119 {plus_minus} 11 to 141 {plus_minus} 10 ka is indicated for the Lathrop Wells volcanic center. This age corrected is concordant with the ages determined by two independent isotopic geochronometric techniques and with the stratigraphy of surficial deposits in the Yucca Mountain region. In addition, paleomagnetic data and radiometric age data indicate only two volcanic events at the Lathrop Wells volcanic center that are probably closely linked in time, not as many as five as recently reported. 32 refs., 2 figs., 2 tabs.

  11. 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

  12. 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

  13. The barents sea magmatic province: Geological-geophysical evidence and new 40Ar/39Ar dates

    NASA Astrophysics Data System (ADS)

    Shipilov, E. V.; Karyakin, Yu. V.

    2011-07-01

    Resulting from study of the geological structure of the Franz Josef Land and Svalbard archipelagoes, this work presents new 17 40Ar/39Ar age datings for basalts taken during coastal expeditions in 2006-2010. Radiological age determination for intrusive units (sills) located in the western part of Nordensciold Land (Spitzbergen Island) has been made for the first time. In relation to use of the interpretation results of marine geological-geophysical data, the distribution peculiarities and time ranges for Jurassic-Cretaceous basic magmatism within the studied regions of the Barents Sea continental margin and within the Arctic as a whole are discussed.

  14. Application of the 40Ar/39Ar technique to date the Minoan Tuff, Santorini

    NASA Astrophysics Data System (ADS)

    Wijbrans, J. R.; Kuiper, K.; Morgan, L. E.; Klaver, M.; Vroon, P. Z.

    2012-12-01

    The age of the catastrophic eruption of the volcano of Santorini during the Bronze Age is well established from 14C dating at 3344.9 ± 7.5 a1 (uncertainties quoted as 1-σ). Application of the 40Ar/39Ar technique to products from this eruption is used here to (1) investigate the limits of the technique using conventional single collector mass spectrometry on a MAP215-50 instrument, (2) analyse sources of uncertainty to identify major contributing factors for the uncertainty of young 40Ar/39Ar ages, and (3) provide 40Ar/39Ar ages for a sample that has been previously dated via 14C and dendrochronology to further investigate issues with the accuracy of 40Ar/39Ar dating in the late Quaternary. We have separated the plagioclase fraction from the lower Minoan Tuff that immediately overlies the Cape Riva (rp6) tuff in a bay on the west coast of Thira, NW of the town of Oia. Using the calibration of 40Ar/36Ar of Lee et al.2, the decay constant recommended by Min at al.3, and the FCs age of Kuiper et al.4, we calculate an inverse isochron age of 3.7 ± 1.6 ka and a trapped 40Ar/36Ar intercept of 299.8 ± 1.2, slightly higher than the ratio for atmospheric argon of 298.56 ± 0.31, when all steps with ages > 50 ka are included in the regression. Enrichment in radiogenic 40Ar in the steps used for the isochron is extremely low, given the low concentration of K2O in plagioclase and the extremely young age. The stepwise heating approach proved useful because in all 5 replicate experiments unexpectedly high ages showed up at higher step temperatures, suggesting that in each separate some older contaminant was present. The plateaus of each of the replicate experiments had quite reproducible ages, however, and a pooled age was calculated for 23 out of 48 individual steps. The pooled age for the plateau was 17.6 ± 4.1 ka, which is high due to the slight component of excess 40Ar in the non-radiogenic component, as revealed from regression analysis. refs: 1SW Manning et al. (2006

  15. 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

  16. Potassium isotopic compositions of NIST potassium standards and 40Ar/39Ar mineral standards

    NASA Astrophysics Data System (ADS)

    Morgan, L. E.; Tappa, M.; Ellam, R. M.; Mark, D. F.; Lloyd, N. S.; Higgins, J. A.; Simon, J. I.

    2013-12-01

    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 0.25‰ level (1σ) [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. [1] Hiess

  17. 40Ar-39Ar age determinations of lunar basalt meteorites Asuka 881757, Yamato 793169, Miller Range 05035, La Paz Icefield 02205, Northwest Africa 479, and basaltic breccia Elephant Moraine 96008

    NASA Astrophysics Data System (ADS)

    Fernandes, Vera A.; Burgess, Ray; Morris, Adam

    2009-06-01

    40Ar-39Ar data are presented for the unbrecciated lunar basaltic meteorites Asuka (A-) 881757, Yamato (Y-) 793169, Miller Range (MIL) 05035, LaPaz Icefield (LAP) 02205, Northwest Africa (NWA) 479 (paired with NWA 032), and basaltic fragmental breccia Elephant Moraine (EET) 96008. Stepped heating 40Ar-39Ar analyses of several bulk fragments of related meteorites A-881757, Y-793169 and MIL 05035 give crystallization ages of 3.763 ± 0.046 Ga, 3.811 ± 0.098 Ga and 3.845 ± 0.014 Ga, which are comparable with previous age determinations by Sm-Nd, U-Pb Th-Pb, Pb-Pb, and Rb-Sr methods. These three meteorites differ in the degree of secondary 40Ar loss with Y-793169 showing relatively high Ar loss probably during an impact event ˜200 Ma ago, lower Ar loss in MIL 05035 and no loss in A-881757. Bulk and impact melt glass-bearing samples of LAP 02205 gave similar ages (2.985 ± 0.016 Ga and 2.874 ± 0.056 Ga) and are consistent with ages previously determined using other isotope pairs. The basaltic portion of EET 96008 gives an age of 2.650 ± 0.086 Ga which is considered to be the crystallization age of the basalt in this meteorite. The Ar release for fragmental basaltic breccia EET 96008 shows evidence of an impact event at 631 ± 20 Ma. The crystallization age of 2.721 ± 0.040 Ga determined for NWA 479 is indistinguishable from the weighted mean age obtained from three samples of NWA 032 supporting the proposal that these meteorites are paired. The similarity of 40Ar-39Ar ages with ages determined by other isotopic systems for multiple meteorites suggests that the K-Ar isotopic system is robust for meteorites that have experienced a significant shock event and not a prolonged heating regime.

  18. Laser {sup 40}Ar/{sup 39}Ar microprobe analyses of fine-grained illite

    SciTech Connect

    Onstott, T.C.; Mueller, C.; Vrolijk, P.J.; Pevear, D.R.

    1997-09-01

    Fine-grained (<0.02 {mu}m) to coarse-grained (2.0-0.2 {mu}m) illite separates and finely powdered muscovite standards were analyzed with a microencapsulation technique and an {sup 40}Ar/{sup 39}Ar laser microprobe. The integrated ages of the illite agreed within error with conventional K/Ar analyses, even though the sample sizes, 1-100 micrograms, were at least a 10,000-fold less. Incremental laser heating of an artificial mixture of illite and muscovite of two different ages yielded a stair step profile, where the youngest and oldest incremental ages approximately coincided with their K/Ar ages. The thermally activated argon release rate from illite was distinct from that of the muscovite and may result from differences in grain thickness, lower K concentration, and the presence of cis vs. trans-sited vacancies. Incremental heating, therefore, may prove capable of delineating detrital from authigenic components in illite extracted from shale and sandstone. Microencapsulation and laser {sup 40}Ar/{sup 39}Ar analyses, when combined with sophisticated techniques for separating clays, will permit dating of samples where clay is a minor constituent, such as sandstones and meteorites, and will enhance identification of endmember ages in naturally occurring clay. 45 refs., 9 figs., 2 tabs.

  19. 40Ar/39Ar thermochronology of isotopically zoned micas: Insights from the southwestern USA proterozoic orogen

    NASA Astrophysics Data System (ADS)

    Hodges, K. V.; Bowring, S. A.

    1995-08-01

    We have used three different 40Ar/39Ar laser microprobe methods to explore the distribution of radiogenic 40Ar in 1.0-1.5-mm biotite crystals from the ca. 1680 Ma Horse Mountain monzogranite of central Arizona. Incremental heating of two single crystals with a defocused laser beam produced flat age spectra with near-plateau ages of ˜1190 Ma, showing no indication of intracrystalline 40Ar inhomogeneity. In contrast, total fusion of twenty-five biotite fragments (˜ 100 pm) yielded apparent ages ranging from 1006.7 to 1212.0 Ma. Detailed age mapping in the {001} plane of two crystals, with the laser focused to a minimum spot size, confirms that the age dispersion in the fragment data reflects the existence of 200 m.y. age gradients in single crystals. The two mapped crystals display very different age distribution patterns that suggest radiogenic 40Ar loss through two mechanisms: volume diffusion on a scale comparable to that of the grain radius, and more rapid diffusion along discrete zones of high crystal defect density. Simple inverse modeling of the overall age dispersion in the two mapped crystals and the fragment population is consistent with the development of the observed age gradients by slow cooling at an average rate of ˜0.5 K/m.y. The Horse Mountain results, as well as previously published studies, indicate that conventional, incremental heating of hydrous phases can homogenize intracrystalline gradients in 40Ar, thereby masking important details of the thermal history of analyzed samples. In contrast, detailed isotopic mapping studies offer a wealth of information, and will become more powerful with continued improvement in the spatial resolution of 40Ar/39Ar laser microprobes. Total-fusion studies of crystal fragment populations can be readily automated, making them less labor-intensive than mapping studies. Our preliminary experiment on a limited Horse Mountain fragment population suggests that this procedure has great potential as a reconnaissance

  20. 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, M.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

  1. Tracking the provenance of Greenland-sourced, Holocene aged, individual sand-sized ice-rafted debris using the Pb-isotope compositions of feldspars and 40Ar/39Ar ages of hornblendes

    NASA Astrophysics Data System (ADS)

    White, Lee F.; Bailey, Ian; Foster, Gavin L.; Allen, Georgina; Kelley, Simon P.; Andrews, John T.; Hogan, Kelly; Dowdeswell, Julian A.; Storey, Craig D.

    2016-01-01

    The provenance of sand-sized ice-rafted debris (IRD) sourced from Greenland is currently difficult to determine. Such knowledge, if it could be ascertained with a high degree of certainty, could be applied to the Greenland-proximal marine records to improve both our understanding of modern-day spatial patterns of iceberg rafting and the past history of the Greenland Ice Sheet (GIS). Recent studies have highlighted the utility of the Pb-isotope composition of individual sand-sized feldspars and the 40Ar/39Ar ages of individual sand-sized hornblendes in this regard. However, before any such provenance toolkit can be applied to the palaeo-record, it is necessary first to determine whether this approach can be used to track the sources of known recent Greenland-proximal IRD deposition. To this end we present new records of the Pb-isotope composition and the 40Ar/39Ar ages of individual sand-sized grains of feldspars and hornblendes, respectively, from modern Greenland glacifluvial and fjord sands and Holocene to modern Greenland-proximal marine sediments. These new data demonstrate that sand-sized feldspars and hornblendes glacially eroded by the GIS exhibit distinct intra- and inter-tectonic terrane differences in their Pb-isotope compositions and ages and that these differences are clearly expressed in the geochemistry and geochronology of sand-sized IRD deposited in marine sediments around Greenland. Although overlap exists between some Greenland-proximal IRD 'source fields' defined by these data, our approach has the potential to both better understand spatial patterns of Greenland-derived IRD in the modern day as well as during past episodes of iceberg calving.

  2. New 40Ar/39Ar Ages Support The Dominant Right-Lateral Transform Motion Within The CARIB-SOAM PBZ Since Middle Eocene

    NASA Astrophysics Data System (ADS)

    Altamira, A.; Burke, K.; Copeland, P.; Foster, D.

    2006-12-01

    New 40Ar/3939Ar ages in amphiboles (84±2 to 95±2 Ma) and in white micas (86±3 to 95±2 Ma) from the Villa de Cura Blue Schist Belt (VdC) in Central Venezuela are in agreement with those ages reported by Smith et al., 1999 (96-80 Ma). We found that amphiboles and white micas reached their closure temperature at approximately the same time, suggesting a rapid cooling from 500°C to 300°C. The ages are very similar throughout the VdC, with no evident trend variations in any particular direction. We also report 40Ar/3939Ar amphibole ages from different localities along the Caribbean South America Plate Boundary Zone (CARIB-SOAM PBZ): 1) Batholithic granite in Aruba (88±9 Ma); 2) Ultramafic sliver in the northern coast of the Araya Peninsula, near San Juan de las Galdonas (145±14 Ma); 3) Dragon gneiss, in the extreme eastern tip of the Paria Peninsula (91±5 Ma); 4) Ultramafic rock from the Chacao Complex (122±17 Ma); 5) Three samples from Las Hermanas Formation, North of San Sebastian and South of VdC (134±4, 129±6 and 97±4 Ma). The igneous and high P/T metamorphic ages reported above were sampled in the thrust belts of Venezuela and are older than 70 Ma, when the Great Arc of the Caribbean struck the west coast of SOAM. The Venezuelan island of Los Testigos gave an 40Ar/3939Ar age for amphibole from a diorite of 41±2 Ma; this younger igneous age is located in the northern part of the PBZ; we interpret Los Testigos as a fragment of the southern end of the Lesser Antillean arc dragged into the PBZ as the arc slid by. All the ages are consistent with the predictions made by Burke et al., 2005, in which they proposed dominant right-lateral transform motion with limited flower structural strain-partitioning involving shortening (in thrusts) and extension (in pull-aparts) within the CARIB- SOAM PBZ and do not require a component of oblique convergence for the Caribbean-South America margin.

  3. 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.

  4. 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

  5. On Full Disclosure and Transparent Data Flow from 40Ar/39Ar Geochronology Measurements to Data Reduction to Online Repositories

    NASA Astrophysics Data System (ADS)

    Koppers, A. A. P.

    2015-12-01

    Arguably 40Ar/39Ar geochronology is one of the most versatile techniques available to Earth scientists today for the dating of rocks and minerals and determining the rates of geological processes on Earth and in our solar system. Over the last four decades large quantities of high (and lower) quality 40Ar/39Ar data have been produced using many different generations of mass spectrometry instrumentation. This wealth of data is only as useful as its description and availability of metadata allows. Many online data sets or compilations available in the science literature only carry the resulting product, an age and a related uncertainty in millions of years, for example. These data points are far from desirable as these don't allow recalculation against modern-day age standards, decay constants and other parameters essential in 40Ar/39Ar geochronology. Over time these data will become less useful to the research community and eventually these will be put by the wayside. In this presentation I will emphasize the need for full disclosure of all data and metadata involved in 40Ar/39Ar geochronology. I will give examples of how a complex data flow can be kept transparent from sample preparation to measurement to data reduction and eventually the uploading into online data repositories. Without the full disclosure of our data and a transparent data flow, it is evident that we cannot live up to one of the governing doctrines in the sciences, namely reproducibility of our scientific experiments and findings.

  6. 40Ar-39Ar age constraint on deformation and brittle-ductile transition of the Main Central Thrust and the South Tibetan Detachment zone from Dhauliganga valley, Garhwal Himalaya, India

    NASA Astrophysics Data System (ADS)

    Sen, Koushik; Chaudhury, Reetam; Pfänder, Jörg

    2015-08-01

    40Ar-39Ar data from two sets of mylonitic two-mica granites present in the Main Central Thrust (MCT) and one leucogranite from the South Tibetan Detachment (STD) of Dhauliganga valley, Garhwal Himalaya are presented. The MCT and the STD bound the High Himalayan Crystallines (HHC) and are believed to facilitate its extrusion. Field evidence of ductile deformation in the form of tight isoclinal folding and brittle deformation in the form of back thrusts and transverse fractures are observed. The STD zone shows evidence of pervasive migration of leucogranitic melt through north dipping extensional shear zones. The ∼19.5 Ma old Malari Leucogranite, present adjacent to the STD zone, experienced ductile and brittle deformation related to the tectonics of the STD. Muscovite analysis from the Malari leucogranite gives a cooling age of ∼15.2 Ma suggesting that ductile deformation in the STD zone may have ceased by ∼15 Ma. 40Ar-39Ar chronology of biotite from two mylonitic granites of the MCT yields cooling ages of 10.8 Ma and 9.7 Ma, which we correlate with activity of the MCT at ∼10 Ma that caused rapid exhumation of the HHC. 40Ar-39Ar ages of 6.4 Ma and 6.2 Ma from white mica represent newly crystallized white mica post-dating biotite cooling and indicate late stage deformation. It is inferred that, as the HHC wedge started to exhume and erode rapidly along the MCT zone at ∼10 Ma, the taper angle of the Himalayan wedge decreased to a 'sub-critical' stage. To regain the critical taper angle, the wedge underwent internal deformation in the form of back thrusts and duplex structures. Comparison of our data with earlier results from other sections of the MCT helps us envisage that the ∼6 Ma white mica ages can be correlated with this internal deformation event and also with the transition of deformation regime in the MCT zone from ductile to brittle.

  7. LASER step-heating 40Ar /39Ar age spectra from early Archean (~3.5 Ga) Barberton greenstone belt sediments: A technique for detecting cryptic tectono-thermal events

    NASA Astrophysics Data System (ADS)

    De Ronde, C. E. J.; Hall, C. M.; York, D.; Spooner, E. T. C.

    1991-07-01

    Samples from sediments of the Fig Tree Group located in the central part of the circa 3.2 to 3.5 Ga Barberton greenstone belt (BGB) have been analyzed by the 40Ar /39Ar laser step-heating technique. This technique has enabled previously cryptic thermal overprints to be detected in various sedimentary units which include: reworked chemical sediments (barite), clastic sediments (sandstone/shale), and one stromatolite. In most cases the apparent age plateaux can be identified by corresponding Ca/K and Cl/K ratio plots as belonging to separate mineral phases. Most of the samples exhibit simple Ar diffusion loss during the low temperature part of the experiments while occasionally showing excess Ar, or possible 39Ar recoil, effects. The various sediments analyzed show evidence for distinct overprinting between 2025 and 2090 Ma. One barite sample gave T0 (original blocking age) = 2673 ± 3 Ma (1σ) which is close to a calculated model Ar diffusion-loss age for the same sample of 2688 ± 3 Ma (1σ). This age is interpreted as representing final granitoid activity adjacent to the BGB, and/or craton-scale tectonism associated with the Limpopo Orogeny. Two samples gave T0 ages of ˜2350-2400 Ma which may reflect increased thermal gradients associated with the formation of the thick Transvaal sedimentary basin that may once have covered the BGB. The dominant apparent age plateaux together with modelled diffusive Ar loss ages of 2025-2090 Ma, are thought to represent regional thermal anomalies related to large-scale tectono-thermal activity in the Kaapvaal craton, of which the Bushveld Complex (which covers a surface area of ~ 60,000 km 2) and formation of the Vredefort Structure are obvious manifestations. The strong thermal overprinting recorded by the sediments has effectively removed any ancient atmosphere (i.e., 40Ar /36Ar ratios < 295.5) signatures.

  8. Combined U-Th/He and 40Ar/39Ar geochronology of post-shield lavas from the Mauna Kea and Kohala volcanoes, Hawaii

    SciTech Connect

    Aciego, S.M.; Jourdan, F.; DePaolo, D.J.; Kennedy, B.M.; Renne, P.R.; Sims, K.W.W.

    2009-10-01

    Late Quaternary, post-shield lavas from the Mauna Kea and Kohala volcanoes on the Big Island of Hawaii have been dated using the {sup 40}Ar/{sup 39}Ar and U-Th/He methods. The objective of the study is to compare the recently demonstrated U-Th/He age method, which uses basaltic olivine phenocrysts, with {sup 40}Ar/{sup 39}Ar ages measured on groundmass from the same samples. As a corollary, the age data also increase the precision of the chronology of volcanism on the Big Island. For the U-Th/He ages, U, Th and He concentrations and isotopes were measured to account for U-series disequilibrium and initial He. Single analyses U-Th/He ages for Hamakua lavas from Mauna Kea are 87 {+-} 40 ka to 119 {+-} 23 ka (2{sigma} uncertainties), which are in general equal to or younger than {sup 40}Ar/{sup 39}Ar ages. Basalt from the Polulu sequence on Kohala gives a U-Th/He age of 354 {+-} 54 ka and a {sup 40}Ar/{sup 39}Ar age of 450 {+-} 40 ka. All of the U-Th/He ages, and all but one spurious {sup 40}Ar/{sup 39}Ar ages conform to the previously proposed stratigraphy and published {sup 14}C and K-Ar ages. The ages also compare favorably to U-Th whole rock-olivine ages calculated from {sup 238}U - {sup 230}Th disequilibria. The U-Th/He and {sup 40}Ar/{sup 39}Ar results agree best where there is a relatively large amount of radiogenic {sup 40}Ar (>10%), and where the {sup 40}Ar/{sup 36}Ar intercept calculated from the Ar isochron diagram is close to the atmospheric value. In two cases, it is not clear why U-Th/He and {sup 40}Ar/{sup 39}Ar ages do not agree within uncertainty. U-Th/He and {sup 40}Ar/{sup 39}Ar results diverge the most on a low-K transitional tholeiitic basalt with abundant olivine. For the most alkalic basalts with negligible olivine phenocrysts, U-Th/He ages were unattainable while {sup 40}Ar/{sup 39}Ar results provide good precision even on ages as low as 19 {+-} 4 ka. Hence, the strengths and weaknesses of the U-Th/He and {sup 40}Ar/{sup 39}Ar methods are

  9. 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

    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.

  10. The bombardment history of the Moon as recorded by 40Ar-39Ar chronology

    NASA Astrophysics Data System (ADS)

    Fernandes, V. A.; Fritz, J.; Weiss, B. P.; Garrick-Bethell, I.; Shuster, D. L.

    2013-02-01

    New petrography and 40Ar-39Ar ages have been obtained for 1-3 mm sized rock fragments from Apollo 16 Station 13 soil 63503 (North Ray crater ejecta) and chips from three rocks collected by Apollo 16 and Apollo 17 missions. Selection of these samples was aimed at the old 40Ar-39Ar ages to understand the early history of the lunar magnetic field and impact flux. Fifteen samples were studied including crustal material, polymict feldspathic fragmental breccias, and impact melts. The impact ages obtained range between approximately 3.3 and 4.3 billion years (Ga). Polymict fragmental breccia 63503,1 exhibits the lowest signs of recrystallization observed and a probable old relic age of 4.547 ± 0.027. The plateau age of 4.293 ± 0.044 Ga obtained for impact melt rock 63503,13 represents the oldest known age for such a lithology. Possibly, this age represents the minimum age for the South Pole-Aitken (SPA) Basin. In agreement with literature data, these results show that impact ages >3.9 Ga are found in lunar rocks, especially within soil 63503. Impact exhumation of deep-seated warm crustal material onto the lunar surface is considered to explain the common 4.2 Ga ages obtained for weakly shocked samples from soil 63503 and Apollo 17. This would directly imply that one or more basin-forming events occurred at that time. Some rock fragments showing none to limited petrologic features indicate thermal annealing. These rocks may have lost Ar while resident within the hot-ejecta of a large basin. Concurrent with previous studies, these results lead us to advocate for a complex impact flux in the inner solar system during the initial approximately 1.3 Ga.

  11. 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.

  12. 40Ar-39Ar dating of volcanogenic products from the AND-2A core (ANDRILL Southern McMurdo Sound Project, Antarctica): correlations with the Erebus Volcanic Province and implications for the age model of the core

    NASA Astrophysics Data System (ADS)

    di Vincenzo, Gianfranco; Bracciali, Laura; Del Carlo, Paola; Panter, Kurt; Rocchi, Sergio

    2010-05-01

    The AND-2A drillcore (Antarctic Drilling Program—ANDRILL) was successfully completed in late 2007 on the Antarctic continental margin (Southern McMurdo Sound, Ross Sea) with the aim of tracking ice proximal to shallow marine environmental fluctuations and to document the 20-Ma evolution of the Erebus Volcanic Province. Lava clasts and tephra layers from the AND-2A drillcore were investigated from a petrographic and stratigraphic point of view and analyzed by the 40Ar-39Ar laser technique in order to constrain the age model of the core and to gain information on the style and nature of sediment deposition in the Victoria Land Basin since Early Miocene. Ten out of 17 samples yielded statistically robust 40Ar-39Ar ages, indicating that the AND-2A drillcore recovered ≤230 m of Middle Miocene (˜128-358 m below sea floor, ˜11.5-16.0 Ma) and >780 m of Early Miocene (˜358-1093 m below sea floor, ˜16.0-20.1 Ma). Results also highlight a nearly continuous stratigraphic record from at least 358 m below sea floor down hole, characterized by a mean sedimentation rate of ˜19 cm/ka, possible oscillations of no more than a few hundreds of ka and a break within ˜17.5-18.1 Ma. Comparison with available data from volcanic deposits on land, suggests that volcanic rocks within the AND-2A core were supplied from the south, possibly with source areas closer to the drill site for the upper core levels, and from 358 m below sea floor down hole, with the “proto-Mount Morning” as the main source.

  13. 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., Jr.

    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.

  14. Comparison Between 40Ar/39Ar and U/Pb Geochronometers at ca. 2.1 Ga

    NASA Astrophysics Data System (ADS)

    Renne, P. R.; Renne, P. R.; Min, K.; Schmitz, M. D.; Bowring, S. A.; de Wit, M. J.; Morelli, C.

    2001-12-01

    Recent sudies have revealed 1-2% age bias between conventional calibrations of the 40Ar/39Ar and U/Pb geochronologic methods applied to quickly cooled volcanic rocks whose isotopic systems should be uncomplicated by differential retention of radiogenic daughter isotopes. The U-Pb system serves as an ideal basis for comparison because of its rigorous internal reliability criteria and precisely-determined decay constants via alpha counting. Studies capable of providing useful comparison have been limited to samples younger than 1.1 Ga, which offers useful constraints primarily on 40Ar/40K of 40Ar/39Ar standards and the electron capture decay constant of 40K. The magnitude of observed bias for these samples is within the range of realistically propagated errors in those quantities. The beta decay constant of 40K is comparably poorly constrained, leading to ambiguities about early solar system cooling rates among other issues, and is more difficult to test directly due to a paucity of appropriate (e.g., minimally altered with demonstrably simple thermal history) rocks for comparison. A strikingly fresh hornblende-biotite dacite from the Eglab region of the Requibat massif, West Africa, offers an exceptional opportunity for head-to-head comparison of the two geochronometers at nearly twice the age limit currently available. Zircons from this unit are concordant to nearly concordant.and indicate an age of ca 2076 Ma. 40Ar/39Ar analysis of individual hornblende grains, step-heated with a CO2 laser, reveal some complexities but generally yield plateau ages of 2050-2060 Ma based on IUGS 1977 decay constants and 28.02 Ma for the Fish Canyon sanidine. Thus the bias between 40Ar/39Ar and U/Pb systems in this case is of order 1%, suggesting that relative error in the conventional beta decay constant is somewhat less than that of the electron capture constant for 40K.

  15. Refining lunar impact chronology through high spatial resolution (40)Ar/(39)Ar dating of impact melts.

    PubMed

    Mercer, Cameron M; Young, Kelsey E; Weirich, John R; Hodges, Kip V; Jolliff, Bradley L; Wartho, Jo-Anne; van Soest, Matthijs C

    2015-02-01

    Quantitative constraints on the ages of melt-forming impact events on the Moon are based primarily on isotope geochronology of returned samples. However, interpreting the results of such studies can often be difficult because the provenance region of any sample returned from the lunar surface may have experienced multiple impact events over the course of billions of years of bombardment. We illustrate this problem with new laser microprobe (40)Ar/(39)Ar data for two Apollo 17 impact melt breccias. Whereas one sample yields a straightforward result, indicating a single melt-forming event at ca. 3.83 Ga, data from the other sample document multiple impact melt-forming events between ca. 3.81 Ga and at least as young as ca. 3.27 Ga. Notably, published zircon U/Pb data indicate the existence of even older melt products in the same sample. The revelation of multiple impact events through (40)Ar/(39)Ar geochronology is likely not to have been possible using standard incremental heating methods alone, demonstrating the complementarity of the laser microprobe technique. Evidence for 3.83 Ga to 3.81 Ga melt components in these samples reinforces emerging interpretations that Apollo 17 impact breccia samples include a significant component of ejecta from the Imbrium basin impact. Collectively, our results underscore the need to quantitatively resolve the ages of different melt generations from multiple samples to improve our current understanding of the lunar impact record, and to establish the absolute ages of important impact structures encountered during future exploration missions in the inner Solar System. PMID:26601128

  16. Refining lunar impact chronology through high spatial resolution 40Ar/39Ar dating of impact melts

    PubMed Central

    Mercer, Cameron M.; Young, Kelsey E.; Weirich, John R.; Hodges, Kip V.; Jolliff, Bradley L.; Wartho, Jo-Anne; van Soest, Matthijs C.

    2015-01-01

    Quantitative constraints on the ages of melt-forming impact events on the Moon are based primarily on isotope geochronology of returned samples. However, interpreting the results of such studies can often be difficult because the provenance region of any sample returned from the lunar surface may have experienced multiple impact events over the course of billions of years of bombardment. We illustrate this problem with new laser microprobe 40Ar/39Ar data for two Apollo 17 impact melt breccias. Whereas one sample yields a straightforward result, indicating a single melt-forming event at ca. 3.83 Ga, data from the other sample document multiple impact melt–forming events between ca. 3.81 Ga and at least as young as ca. 3.27 Ga. Notably, published zircon U/Pb data indicate the existence of even older melt products in the same sample. The revelation of multiple impact events through 40Ar/39Ar geochronology is likely not to have been possible using standard incremental heating methods alone, demonstrating the complementarity of the laser microprobe technique. Evidence for 3.83 Ga to 3.81 Ga melt components in these samples reinforces emerging interpretations that Apollo 17 impact breccia samples include a significant component of ejecta from the Imbrium basin impact. Collectively, our results underscore the need to quantitatively resolve the ages of different melt generations from multiple samples to improve our current understanding of the lunar impact record, and to establish the absolute ages of important impact structures encountered during future exploration missions in the inner Solar System. PMID:26601128

  17. Instrumentation development for planetary in situ 40Ar/39Ar geochronology

    NASA Astrophysics Data System (ADS)

    Davidheiser-Kroll, B.; Morgan, L. E.; Munk, M.; Warner, N. H.; Gupta, S.; Slaybaugh, R.; Harkness, P.; Mark, D. F.

    2015-12-01

    The chronology of the Solar System, particularly the timing of formation of extraterrestrial bodies and their features, is a major 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, Farley et al., 2014), the reliability, accuracy, and applicability of the 40Ar/39Ar method makes it by far the most desirable chronometer for dating extraterrestrial bodies. The method however relies on the neutron irradiation of samples, and thus a neutron source. We will 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 analyzing samples are discussed, along with an exploration of limitations such as mass, power, and cost. Two potential solutions for the in situ extraterrestrial deployment of the 40Ar/39Ar method will be 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.

  18. Argon diffusion in Apollo 16 impact glass spherules: Implications for 40Ar/39Ar dating of lunar impact events

    NASA Astrophysics Data System (ADS)

    Gombosi, David J.; Baldwin, Suzanne L.; Watson, E. Bruce; Swindle, Timothy D.; Delano, John W.; Roberge, Wayne G.

    2015-01-01

    The 40Ar/39Ar technique applied to impact glass has been used to date both terrestrial and lunar impact events. The ability to utilize the 40Ar/39Ar technique rests on the assumption that impact glasses are closed to the loss of daughter product, 40Ar∗, after formation. Diffusion experiments were performed on three Apollo 16 lunar impact glasses and yielded activation energies for 39Ar of ∼17 to 20 kcal mol-1 and log10(D0/a2) values of -5.2 to -6.0 s-1. The resulting diffusion coefficients are interpreted as minimum values and the Apollo 16 glass is probably some of the least retentive of lunar glasses, as the degree of non-bridging oxygen is at one end of the range in lunar glasses. At temperatures below the glass transition temperature (i.e., ∼660 °C), the data can be explained by volume diffusion from a single diffusion domain. Modeling shows that Apollo 16 composition glass could lose significant quantities of radiogenic argon (40Ar∗) (∼90-100% over 20-40 Myr assuming a diffusion domain size (a) of 75 μm) due to diurnal temperature variations on the lunar surface, although 40Ar∗ loss is highly sensitive to exposure duration and effective diffusion domain size. Modeling shows that loss from transient thermal events (e.g., heating to ∼200 °C for 102 yr duration) can also cause partial resetting of apparent 40Ar/39Ar ages. In small (a = 75 μm) glasses a maximum of 50-60% of 40Ar∗ is lost over 4 Ga when buried to depths corresponding to temperatures of -15 °C. Results indicate that caution should be exercised in interpreting lunar impact glass 40Ar/39Ar ages, as the assumption of closed system behavior may have been violated, particularly in glasses with low fractions of non-bridging oxygen.

  19. 40Ar/39Ar and U-Th-Pb dating of separated clasts from the Abee E4 chondrite

    USGS Publications Warehouse

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

    1983-01-01

    Determinations of 40Ar/39Ar and U-Th-Pb are reported for three clasts from the Abee (E4) enstatite chondrite, which has been the object of extensive consortium investigations. The clasts give 40Ar/39Ar plateau ages and/or maximum ages of 4.5 Gy, whereas two of the clasts give average ages of 4.4 Gy. Within the range of 4.4-4.5 Gy these data do not resolve any possible age differences among the three clasts. 206Pb measured in these clasts is only ???1.5-2.5% radiogenic, which leads to relatively large uncertainties in the Pb isochron age and in the 207Pb/206Pb model ages. The Pb data indicate that the initial 207Pb/206Pb was no more than 0.08??0.07% higher than this ratio in Can??on Diablo troilite. The U-Th-Pb data are consistent with the interpretation that initial formation of these clasts occurred 4.58 Gy ago and that the clasts have since remained closed systems, but are contaminated with terrestrial Pb. The 40Ar/39Ar ages could be gas retention ages after clast formation or impact degassing ages. The thermal history of Abee deduced from Ar data appears consistent with that deduced from magnetic data, and suggests that various Abee components experienced separate histories until brecciation no later than 4.4 Gy ago, and experienced no appreciable subsequent heating. ?? 1983.

  20. 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

  1. KAr and 40Ar/ 39Ar evidence for a Transamazonian age (2030-1970 Ma) for the granites and emerald-bearing K-metasomatites from Campo Formoso and Carnaíba (Bahia, Brazil)

    NASA Astrophysics Data System (ADS)

    Giuliani, G.; Zimmermann, J.-L.; Montigny, R.

    1994-04-01

    The Campo Formoso and Carnaíba granites belong to a suite of middle Proterozoic magmatic rocks located in the northern part of the São Francisco craton. They intrude the Archaen basement and Lower Proterozoic Jacobina volcanosedimentary series. Emerald-bearing K-metasomatites in the mining districts of Campo Formoso and Carnaíba are developed within serpentinites at the contact with granite-related pegmatitic veins. KAr and 40Ar/ 39Ar measurements were performed on biotites and deuteric muscovites from these two granites, and phlogopites from the K-metasomatites. For the Campo Formoso granite, the biotites yield ages between 1875 ± 45 Ma and 1908 ± 47 Ma (2 σ) and the muscovites yield ages of 1897 ± 34 Ma and 2040 ± 24 Ma (2 σ). For the Carnaíba granite, the biotites and muscovites fit isochrons with ages of 1888 ± 32 and 1979 ± 28 Ma (2 σ), respectively. In contrast, phlogopites from emerald-bearing metasomatites display KAr ages that spread between 1900 and 2000 Ma with an isochron of 1973 ± 20 Ma (2 σ) for Carnaíba. Generally, the youngest biotite and phlogopite ages occur for specimens where these minerals are chloritized. 40Ar/ 39Ar step heating release spectra are complex but give integrated ages in good agreement with the KAr ages. The least disturbed spectrum permits assignment of an age of 2032 ± 10 Ma (2 σ)for the first granitic pulse of the emplacement of the Campo Formoso composite pluton. Since in Carnaíba, deuteric muscovites and chlorite-free phlogopites give similar KAr ages, 1979 ± 28 and 1973 ± 20 Ma (2 σ) respectively, we conclude that emerald mineralization is contemporaneous with the pervasive muscovitization of the granite. The 1979 ± 28 Ma (2 σ) age obtained by KAr on muscovite represents the best estimate of the Carnaíba granite cooling age. A model invoking the pervasive alteration of the upper part of the granitic cupola along the pegmatite veins and serpentinites by the muscovitizing fluids is

  2. 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

  3. Development of a precise and accurate age-depth model based on 40Ar/39Ar dating of volcanic material in the ANDRILL (1B) drill core, Southern McMurdo Sound, Antarctica

    NASA Astrophysics Data System (ADS)

    Ross, J. I.; McIntosh, W. C.; Dunbar, N. W.

    2012-10-01

    High precision 40Ar/39Ar dates on a variety of volcanic materials from the AND-1B drill core provide pinning points for defining the chronostratigraphy for the core. The volcanic materials dated include 1) felsic and basaltic tephra, 2) interior of a ~ 3 m thick intermediate submarine lava flow, and 3) felsic and basaltic volcanic clasts. In the upper 600 m of the core, two felsic tephra, two basaltic tephra and the intermediate laval flow yield precise and depositional ages, with further maximum age constraints from volcanic clasts. Below 600 m in the core, tephric intervals are significantly altered and maximum age constraints only are available from volcanic clasts. The ages for eight stratigraphic intervals are 1) 17.17-17.18 mbsf, basaltic clast (maximum depositional age 0.310 ± 0.039 Ma, all errors quoted at 2σ), 2) 52.80-52.82 mbsf, three basaltic clasts (maximum depositional age 0.726 ± 0.052 Ma), 3) 85.27-85.87 mbsf felsic tephra (1.014 ± 0.008 Ma), 4) ~ 112-145 mbsf sequence of basaltic tephra (1.633 ± 0.057 to 1.683 ± 0.055 Ma), 5) 480.97-481.96 mbsf pumice-rich mudstone (4.800 ± 0.076 Ma), 6) 646.30-649.34 mbsf intermediate lava flow (6.48 ± 0.13 Ma), 7) 822.78 mbsf kaersutite phenocrysts from volcanic clasts (maximum depositional age 8.53 ± 0.53 Ma) and 8) ~ 1280 mbsf, three volcanic clasts (maximum depositional age 13.57 ± 0.13 Ma). Minimum average sediment accumulation rates or 102 and 87 m/Ma for the upper and lower 650 m of core, respectively were calculated using the 40Ar/39Ar analyses. The volcanic material recovered from AND-1B also reveals a general northward progression of volcanism in Southern McMurdo Sound.

  4. 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

  5. 40Ar/ 39Ar dating and preliminary paleointensity determination on a single lava flow from Chifeng, Inner Mongolia

    NASA Astrophysics Data System (ADS)

    Shi, Ruiping; Hill, Mimi J.; Zhu, Rixiang; He, Huaiyu; Shaw, John

    2005-09-01

    A precise 40Ar/ 39Ar age and paleointensity data for the Cretaceous lava flow from Chifeng, southern Inner Mongolia, northeastern China are presented in this study. Detailed rock magnetic investigations including the variation of magnetization with temperature, low temperature susceptibility and hysteresis loops show that pseudo single domain (PSD) grain size high-Ti titanomagnetite is the main magnetic mineral in the studied lava flow. Both the microwave and double heating Thellier techniques were used to determine the paleointensity, yielding mean flow paleointensities of 15.6 ± 3.2 μT and 23.9 ± 8.0 μT, respectively. However, the paleointensity results using the microwave technique are of higher quality (mean q = 12 for microwave compared to q = 2 for Thellier) and yield higher internal consistency for the flow mean (21% standard deviation about the mean for microwave compared to 34% for Thellier). The microwave paleointensity result, 15.6 ± 3.2 μT is therefore deemed the more reliable estimate for the paleointensity of the Niutoushan lava flow. 40Ar/ 39Ar age determination on the lava flow is 106.42 ± 0.48 Ma (2 σ, relative to GA-1550 biotite: 98.79 ± 0.96 Ma). Combining our 40Ar/ 39Ar dating and paleointensity results with other published paleointensity data suggests that the intensity of the Earth's magnetic field during the middle Cretaceous normal superchron (CNS) was weak, but variable throughout the whole CNS.

  6. Petrology, 40Ar/39Ar age, Sr-Nd isotope systematics, and geodynamic significance of an ultrapotassic (lamproitic) dyke with affinities to kamafugite from the easternmost margin of the Bastar Craton, India

    NASA Astrophysics Data System (ADS)

    Rao, N. V. Chalapathi; Atiullah; Burgess, R.; Nanda, Purnendu; Choudhary, A. K.; Sahoo, Samarendra; Lehmann, B.; Chahong, Ngazipmi

    2016-04-01

    We report the mineralogy, bulk-rock geochemistry, 40Ar/39Ar (whole-rock) age and radiogenic (Sr and Nd) isotope composition of an ultrapotassic dyke from Sakri (Nuapada lamproite field) located at the tectonic contact between the easternmost margin of the Bastar craton and Eastern Ghats Mobile Belt, India. The Sakri dyke has a mineralogy which strongly resembles a lamproite sensu stricto (viz.,Ti-rich phlogopite, Na-poor diopside, Fe-rich sanidine, ulvospinel trend and Sr-rich apatite). However, its bulk-rock major element geochemical characteristics (viz., extreme silica-undersaturated nature) resemble sensu lato kamafugite from Toro Ankole, Uganda, East African Rift, and Alto Paranaiba Province, Brazil. The Sakri dyke also displays certain compositional peculiarities (viz., high degree of evolution of mica composition from phlogopite to biotite, elevated titanium and aluminum in clinopyroxene and significantly lower bulk Mg#) when compared to the ultrapotassic rocks from various Indian cratons. 40Ar/39Ar dating gave a plateau age of 1045 ± 9 Ma which is broadly similar to that of other Mesoproterozoic (i) lamproites from the Bastar and Bundelkhand cratons, and (ii) kimberlites from the Eastern Dharwar craton. Initial bulk-rock Sr (0.705865-0.709024) and Nd (0.511063-0.511154) isotopic ratios reveal involvement of an `enriched' source region with long-term incompatible element enrichment and a depleted mantle (TDM) Nd model age of 2.56 Ga straddling the Archaean-Proterozoic chronostratigraphic boundary. The bulk-rock incompatible trace element ratios (Ta/Yb, Th/Yb, Rb/Ba and Ce/Y) of the Sakri ultrapotassic dyke negate any significant influence of crustal contamination. Small-degree melting (1 to 1.5 %) of a mixed garnet-facies and spinel-facies phlogopite lherzolite can account for its observed REE concentrations. Whereas the emplacement of the Sakri ultrapotassic dyke is related to the amalgamation of the supercontinent of Rodinia, its overlapping geochemical

  7. 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

  8. 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.; Champion, Duane E.; Turrin, B.D.; Swisher, C. C., III

    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

  9. 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

  10. 40Ar/39Ar dating of the Siberian Traps, USSR: Evaluation of the ages of the two major extinction events relative to episodes of flood-basalt volcanism in the USSR and the Deccan Traps, India

    SciTech Connect

    Baksi, A.K. ); Farrar, E. )

    1991-05-01

    {sup 40}Ar/{sup 39}Ar incremental-heating studies have been carried out on three whole-rock specimens from the Siberian Traps. A basalt lava flow from the lowermost horizon yields and age of 238.4 {plus minus} 1.4 Ma (1{sigma} error). A second basalt lava flow from the top of the section, {approximately}800 m above the first specimen, yields an age of 229.9 {plus minus} 2.3 Ma, indicating that the duration of volcanism was {approximately}5{minus}10 m.y. A doleritic dike intrusive into the lower parts of the Siberian Traps contains excess argon and yields an isochron age of 234 {plus minus} 7 Ma. Critical reexamination of relevant radiometric data relating two separate episodes of flood-basalt volcanism to global faunal extinctions suggests the volcanic event forming the most voluminous sections of the Deccan Traps, India, was coincident to within {plus minus}1 m.y. with the time of the Cretaceous/Tertiary boundary. However, the onset of volcanism in the Siberian Traps apparently occurred at a time postdating that of the Permian/Triassic boundary.

  11. 40Ar/39Ar constraints on the activity of the Temsamane extensional detachment (eastern Rif, Morocco)

    NASA Astrophysics Data System (ADS)

    Jabaloy Sánchez, A.; Booth-Rea, G.; Azdimousa, A.; Asebriy, L.; Vázquez-Vílchez, M.; Martínez-Martínez, J. M.; Gabites, J.

    2012-04-01

    The subducted North Maghrebian passive margin was exhumed by an upper crustal brittle-ductile extensional detachment and brittle low-angle normal faults in a continental subduction transform setting. The Temsamane detachment in the eastern Rif is defined by a ductile shear zone approximately 100 m thick with a low-angle ramp geometry that cuts down into the Temsamane fold-nappe stack. The shear zone shows southwestward kinematics and separates epizone metapelites of the Temsamane units below from the epizone to diagenetic rocks of the Tanger-Ketama-Aknoul units above. To the east, the detachment becomes brittle, branching into a listric-fan that cuts through 10-6 Ma sediments and volcanoclastics in the Tres Forcas cape. New 40Ar/39Ar radiometric ages on amphiboles and micas from the footwall of the Temsamane detachment indicate that the metamorphic peak was reached in the footwall (Temsamane units) at ca. 21 Ma, producing the amphibolite epidote facies in the Ras Afrou Unit. The cooling of the footwall rocks below the 325 °C occurred between the 16 and 13 Ma, while apatite fission track ages indicate that the cooling below the 120 °C occurred at ca. 11 Ma. The 40Ar/39Ar radiometric ages on amphiboles and micas of the metamorphic klippes over the Temsamene units (Ait-Amrâne massif) indicates that the Jurassic marbles of the Tanger-Ketama Unit reached their metamorphic peak at ca. 80 Ma, in agreement with previously published K/Ar ages in micas. The rocks of the Tanger-Ketama Unit cooled below the 120 °C between 17.0 ± 2.4 Ma and 13.9 ± 1.8 Ma. We interpret the increase of cooling rates of the footwall rocks between 15-13 Ma and 11 Ma as due to the activity of the Temsamane detachment fault. Thus, both the North Maghrebian and the South Iberian subducted passive margins were exhumed in the Betic and Rif branches of the Gibraltar arc by SW-directed brittle-ductile detachments during the Late Miocene in an oblique collisional setting.

  12. Implications of new 40Ar/39Ar data for the Cretaceous-Paleogene boundary

    NASA Astrophysics Data System (ADS)

    Renne, P. R.; Deino, A.; Hilgen, F.; Kuiper, K.; Mark, D. F.; Mitchell, W. S.; Morgan, L. E.; Smit, J.

    2012-12-01

    The cause of the mass extinctions across the Cretaceous-Paleogene boundary (KPB) is still controversial. Previous geochronological data [1-3] appear to preclude the Chicxulub impact as a causal mechanism as they indicate that the KPB predated the impact by 181 ± 71 ka (1σ here and throughout). New 40Ar/39Ar data, determined blind in the BGC and SUERC labs, for the terrestrial KPB (IrZ-coal bentonite) from the Hell Creek area of northeastern Montana yield an age of 66.044 ± 0.011/0.045 Ma (without/with systematic uncertainties, calibrated per [4]). New 40Ar/39Ar data for Haitian tektites, combined with previous work [2,3], yield an age of 66.038 ± 0.025/0.049 Ma thereby establishing synchrony between the terrestrial KPB and the Chicxulub bolide impact to within 33 ka. The absolute boundary age of 66.044 Ma ± 0.045 [4] or 65.837 ± 0.061 Ma [5] allows clear discrimination between Earth's 405 ka orbital eccentricity cycles and both calibrations favor the orbital tuning chronology of [5] for the KPB at Zumaia, Spain. The former calibration is corroborated by U/Pb data [6]. In contrast, a recent orbital chronology proposed for the Zumaia section [7] infers an age of 65.2 Ma for the KPB, suggesting that the tuning missed two 405 ka eccentricity cycles. New data for tuffs above the KPB indicate a dramatic reduction in the post-KPB timescale of faunal recovery during the Puercan1 NALMA substage, and for the restoration of pre-KPB atmospheric δ13C values, to several tens of ka at most. Our new data clearly implicate a significant role for the Chicxulub impact in the KPB extinctions, but this cannot explain the significant pre-KPB climate instability or precursory faunal turnover [8]. The Chicxulub impact likely delivered the final blow to stressed ecosystems rather than being the sole cause of the KPB extinctions. The role of roughly synchronous phenomena such as eruption of the multiphased Deccan Traps [9] remains to be tested via high-precision geochronology. Refs

  13. 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.

  14. ``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

  15. Unique Thermal Histories from Whole-Rock 40Ar/39Ar Step-heating Data

    NASA Astrophysics Data System (ADS)

    Boehnke, P.; Harrison, M.; Heizler, M. T.; Lovera, O. M.; Warren, P. H.

    2014-12-01

    Step-heating 40Ar/39Ar analysis can reveal spatial distributions of 40Ar* at the micron scale imparted by post- crystallization heating events through complex, multi-diffusion domain models. These efforts have largely focused on single-phase, terrestrial samples with only scant attention paid to multi-phase or extra-terrestrial materials. Generalizing these models to incorporate the multiple activation energies (E) expected from bulk rock samples introduces significant interpretational ambiguity. This is because the thermal crossovers explicit in multi-E cases make the age spectrum a function of the lab heating schedule in thermally disturbed samples. A further difficulty is that unique interpretation of the associated Arrhenius plot is no longer possible and a range of E's can be fitted with equal goodness of fit. In order to address these challenges, we developed a new computational approach that simultaneously inverts the Arrhenius spectra and release pattern using a variant of the Adaptive Particle Swarm Optimization (APSO) algorithm for a square-pulse heating event. Our version uses a Levy Flight to break the swarm out of a local minima rather than randomly modifying a single dimension as in the original APSO. Further we explored issues of Pareto efficiency arising from fitting two fitness functions (i.e., the fit to the age spectra and to the Arrhenius plot) and found an adequate resolution to the classic inability to have a single best fit. By utilizing multiple-E samples, we are able to obtain unique thermal history solutions. Application of these methods to high resolution age spectra of the Jilin chondrite and Apollo 16 samples (North Ray Crater) and found fits of sufficiently high fidelity to constrain the absolute temperature of the thermal episode to better than ±10%.

  16. Dating the longevity of ductile shear zones: Insight from 40Ar/39Ar in situ analyses

    NASA Astrophysics Data System (ADS)

    Schneider, Susanne; Hammerschmidt, Konrad; Rosenberg, Claudio L.

    2013-05-01

    We attempt to improve temporal constraints on the longevity and the termination of ductile shear zones by performing texturally-controlled in situ 40Ar/39Ar analyses of pre-kinematic muscovite, biotite and K-feldspars, of syn-kinematic phengite and K-feldspar, and of post-kinematic phengite within the same samples of sinistral shear zones from the western Tauern Window (Eastern Alps). Additionally two samples were dated by the Rb/Sr method (microsampling). Relative sequences of mineral formation based on microstructural, cross-cutting relationships were confirmed by in situ 40Ar/39Ar analyses, showing that syn-kinematic minerals are, in general, younger than pre-kinematic minerals and older or of equal age than the post-kinematic minerals of the same sample. From the rim to the core of the western Tauern Window syn-kinematic phengite and K-feldspar reveal a set of formation ages varying between 33 and 15 Ma for the northernmost and peripheral shear zone (Ahorn Shear Zone), between 24 and 12 Ma for the intermediate shear zone network (Tuxer Shear Zones), and between 20 and 7 Ma for the southernmost and central shear zone (Greiner Shear Zone). The age variation of syn-kinematic phengite and K-feldspar analyses is larger than the analytical error of each age obtained. In addition, isochron calculations of the syn-kinematic minerals reveal atmospheric-like 40Ar/36Ar intercepts. Therefore, the obtained age values of the syn-kinematic minerals are interpreted as formation ages which date increments of a long lasting deformation period. The time range of deformation of each shear zone system is bracketed by the oldest and youngest formation ages of syn-kinematic phengite and K-feldspar. Post-kinematic phengite laths show the youngest formation ages and overlap with the youngest syn-kinematic formation ages. This relationship indicates that post-kinematic growth occurred immediately after syn-kinematic mineral formation at the end of ductile sinistral shear. Hence, the

  17. 40Ar-39Ar ages of bentonite beds in the upper part of the Yazoo Formation (Upper Eocene), west-central Mississippi

    USGS Publications Warehouse

    Obradovich, J.D.; Dockery, D. T., III; Swisher, C. C., III

    1993-01-01

    Bentonite beds recorded from both outcrops and cores in the upper Eocene Yazoo Formation offer opportunities to date the uppermost Eocene of this region and to provide information on the age of the Eocene/Oligocene boundary. This report gives radiometric age dates for three bentonites sampled from the upper Yazoo Formation. Two bentonites are from outcrops near Satartia in western Mississippi and one is from a core hole at Society Ridge in west-central Mississippi. The upper bentonite at Satartia was studied independently at two laboratories using different techniques but with the same results, an age of 34.3 Ma (million years). Results from the Society Ridge bentonite gave the same age. -from Authors

  18. 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.

  19. Laser step-heating sup 40 Ar/ sup 39 Ar age spectra from early Archean (@3. 5 Ga) Barberton greenstone belt sediments: A technique for detecting cryptic tectono-thermal events

    SciTech Connect

    De Ronde, C.E.J.; Hall, C.M.; York, D.; Spooner, E.T.C. )

    1991-07-01

    Samples from sediments of the Fig Tree Group located in the central part of the circa 3.2 to 3.5 Ga Barberton greenstone belt (BGB) have been analyzed by the {sup 40}Ar/{sup 39}Ar laser step-heating technique. This technique has enabled previously cryptic thermal overprints to be detected in various sedimentary units which include: reworked chemical sediments (barite), clastic sediments (sandstone/shale), and one stromatolite. In most cases the apparent age plateau can be identified by corresponding Ca/K and Cl/K ratio plots as belonging to separate mineral phases. Most of the samples exhibit simple Ar diffusion loss during the low temperature part of the experiments while occasionally showing excess Ar, or possible {sup 39}Ar recoil, effects. The various sediments analyzed show evidence for distinct overprinting between 2,025 and 2,090 Ma. One barite sample gave T{sub 0} = 2,673 {plus minus} 3 Ma (1{sigma}) which is close to a calculated model Ar diffusion-loss age for the same sample of 2,688 {plus minus} 3 Ma (1{sigma}). This age is interpreted as representing final granitoid activity adjacent to the BGB, and/or craton-scale tectonism associated with the Limpopo Orogeny. Two samples gave T{sub 0} ages of {approximately}2,350-2,400 Ma which may reflect increased thermal gradients associated with the formation of the thick Transvaal sedimentary basin that may once have covered the BGB. The dominant apparent age plateau together with modeled diffusive Ar loss ages of 2,025-2,090 Ma, are thought to represent regional thermal anomalies related to large-scale tectono-thermal activity in the Kaapvaal craton, of which the Bushveld Complex (which covers a surface area of {approximately}60,000 km{sup 2}) and formation of the Vredefort Structure are obvious manifestations. The strong thermal overprinting recorded by the sediments has effectively removed any ancient atmosphere signatures.

  20. 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

  1. 40Ar/ 39Ar dating of the emplacement of the Muslim Bagh ophiolite, Pakistan

    NASA Astrophysics Data System (ADS)

    Mahmood, Khalid; Boudier, Françoise; Gnos, Edwin; Monié, Patrick; Nicolas, Adolphe

    1995-11-01

    The obduction-related basal part of the Muslim Bagh ophiolite (Baluchistan, Pakistan) and the underlying metamorphic sequence were studied structurally which demonstrated a WSW-ENE-trending thrusting sequence for the initial obduction. 40Ar/ 39Ar measurements on amphiboles and plagioclase from the subophiolitic metamorphic rocks, and on plastically deformed and recrystallized dolerite samples from the base of the sheeted dyke complex give apparent ages between 70.7 ± 5.0 and 65.1 ± 4.1 Ma interpreted as cooling ages dating approximately the formation of the plastic deformation and obduction. The results indicate that the Muslim Bagh ophiolite represents a segment of ocean floor from the small and slow-spreading ocean branch of the Neo-Tethys located between the Indo-Pakistani and the Afro-Arabian plates. The WSW-ENE-oriented obduction of the Muslim Bagh ophiolite onto the Indo-Pakistani continental margin occurred with the convergence of the Neo-Tethys branch during the Late Cretaceous and before the Tertiary collision of the Indo-Pakistani plate with the Eurasian plate.

  2. Geology and 40Ar/39Ar Geochronology of Akutan Volcano, Eastern Aleutian Islands

    NASA Astrophysics Data System (ADS)

    Coombs, M. L.; Jicha, B. R.

    2013-12-01

    40Ar/39Ar dating and new whole-rock geochemical analyses are used to establish an eruptive chronology for Akutan volcano, Akutan Island, in the eastern Aleutian island arc. Akutan Island (166° W, 54.1° N) is the site of long-lived volcanism and the entire island comprises volcanic rocks as old as 3.3 Ma (Richter et al., 1998, USGS Open-File 98-135). Our current focus is on the 225 km2 western half of the island, which is home to the Holocene active cone, Holocene to latest Pleistocene satellite vents, and underlying middle Pleistocene volcanic basement rocks. Eruptive products span the tholeiitic-calc-alkaline boundary, are medium-K, and range from basalt to dacite. Furnace incremental heating experiments on groundmass separates of 38 samples resulted in 29 40Ar/39Ar ages. The remainder did not yield radiogenic 40Ar contents and are likely Holocene in age. The oldest ages (1251×10 and 1385×12 ka) are from a wedge of flat-lying dissected lavas north of the Holocene cone; these likely represent the upper part of the volcanic basement that underlies the entire island. Above a major unconformity lie basaltic andesite to dacite lavas that range from 765× 4 to 522×8 ka. The eroded remnants of the source volcano for these flows appears to crop out as a series of variably hydrothermally altered breccias and domes 5 km east-northeast of the current summit. A 625 m-tall eroded basaltic center, Lava Peak, sits 6 km northwest of the summit; its deeply incised western flank exposes lava flows and a plug. Two flows are dated at 598×16 and 602×15 ka. A high ridge 1.5 km south of the summit is made of oxidized, mostly andesitic lavas 284-249 ka old; these are presumably the remnants of an eruptive center located near the current cone. Flat Top Peak, 3.5 km southwest of the summit, produced almost exclusively basalts and six dated lavas range from 155×8 to 98×18 ka. Lavas from Flat Top (1065 m asl) are deeply eroded suggesting extensive ice cover during marine isotope

  3. Results of 40Ar/39Ar dating of phlogopites from kelyphitic rims around garnet grains (Udachnaya-Vostochnaya kimberlite pipe)

    NASA Astrophysics Data System (ADS)

    Yudin, D. S.; Tomilenko, A. A.; Alifirova, T. A.; Travin, A. V.; Murzintsev, N. G.; Pokhilenko, N. P.

    2016-07-01

    40Ar/39Ar dating of phlogopite from kelyphitic rims around garnet grains from the Udachnaya-Vostochnaya kimberlite pipe in the Sakha (Yakutia) Republic (Russia) revealed that when this mineral has contact with a kimberlite melt its age corresponds (within error limits) to that of the formation of the kimberlite pipe, thus indicating that the method may be used for dating kimberlites and related rocks. In mantle xenoliths, kelyphitic phlogopites rimming garnet grains partially lose radiogenic Ar, which results in a complex age spectrum. Rejuvenation of the K/Ar system in them is determined by the thermal impact of the kimberlite melt on captured rocks.

  4. 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

  5. Volcanic history and 40Ar/39Ar and 14C geochronology of Terceira Island, Azores, Portugal

    USGS Publications Warehouse

    Calvert, A.T.; Moore, R.B.; McGeehin, J.P.; Rodrigues da Silva, A.M.

    2006-01-01

    Seven new 40Ar/39Ar and 23 new radiocarbon ages of eruptive units, in support of new geologic mapping, improve the known chronology of Middle to Late Pleistocene and Holocene volcanic activity on the island of Terceira, Azores and define an east-to-west progression in stratovolcano growth. The argon ages indicate that Cinco Picos Volcano, the oldest on Terceira, completed its main subaerial cone building activity by about 370-380??ka. Collapse of the upper part of the stratovolcanic edifice to form a 7 ?? 9??km caldera occurred some time after 370??ka. Postcaldera eruptions of basalt from cinder cones on and near the caldera floor and trachytic pyroclastic flow and pumice fall deposits from younger volcanoes west of Cinco Picos have refilled much of the caldera. The southern portion of Guilherme Moniz Volcano, in the central part of the island, began erupting prior to 270??ka and produced trachyte domes, flows, and minor pyroclastic deposits until at least 111??ka. The northern part of Guilherme Moniz Caldera is less well exposed than the southern part, but reflects a similar age range. The northwest portion of the caldera was formed sometime after 44??ka. Several well-studied ignimbrites that blanket much of the island likely erupted from Guilherme Moniz Volcano. The Pico Alto Volcanic Center, a tightly spaced cluster of trachyte domes and short flows, is a younger part of Guilherme Moniz Volcano. Stratigraphic studies and our new radiocarbon ages suggest that most of the Pico Alto eruptions occurred during the period from about 9000 to 1000??years BP. Santa Barbara Volcano is the youngest stratovolcano on Terceira, began erupting prior to 29??ka, and has been active historically. ?? 2006.

  6. Chronostratigraphy of Monte Vulture volcano (southern Italy): secondary mineral microtextures and 39Ar-40Ar systematics

    NASA Astrophysics Data System (ADS)

    Villa, Igor M.; Buettner, Annett

    2009-12-01

    The eruptive history of Monte Vulture has been the subject of several geochronological investigations during the past decades, which reliably dated only a small number of eruptions. Understanding the causes of sub-optimum data yield in the past requires an interdisciplinary approach. We re-analyzed samples from previous works and present new data on samples from the main volcano-stratigraphic units of Monte Vulture, so as to provide an improved, consistent chronostratigraphic database. Imaging of minerals by cathodoluminescence and backscattered electrons reveals that heterochemical, high-temperature deuteric reaction textures are ubiquitous. Such observations are common in metamorphic rocks but had not frequently been reported from volcanic rocks. In view of the mineralogical complexity, we base our chronological interpretation on isochemical steps, defined as steps for which the Cl/K and/or the Ca/K ratios are constant. Isochemical steps carry the isotopic signature of chemically homogeneous mineral phases and therefore allow a well-constrained age interpretation. Comparison of old and new 39Ar-40Ar data proves the reproducibility of age spectra and their shapes. This quantifies the analytical reliability of the irradiation and mass-spectrometric analyses. Anomalous age spectra are a reproducible property of some specific samples and correlate with mineralogical anomalies. The present data allow us to fine-tune the age of the volcanostratigraphic units of Monte Vulture during the known interval of main volcanic activity from ca. 740 to 610 ka. After a very long stasis, the volcanic activity in the Monte Vulture area resumed with diatremic eruptions, one of which (Lago Piccolo di Monticchio, the site of a palynological-paleoclimatological drilling) was dated at ca. 140 ka.

  7. Calibration of a Pleistocene Geomagnetic Instability Time Scale (GITS) using 40Ar/39Ar-dated lavas

    NASA Astrophysics Data System (ADS)

    Singer, B. S.; Hoffman, K. A.

    2005-12-01

    Advances in measuring paleomagnetic intensity recorded by marine sediments, and 40Ar/39Ar dating of paleomagnetic directional recordings in lava flows, offer a means of calibrating a global magnetostratigraphy for the last 2 m.y. This involves moving beyond the classic geomagnetic polarity time scale (GPTS) and resolving not only the undisputed polarity reversals, but also the many short-lived geomagnetic "events" or cryptochrons thought to signal brief periods of geodynamo instability. Many short events are distinguished as intensity minima in global sediment records (SINT-800; GLOPIS-75) that are dated by astrochronology. Thus, when the degree of stability of the geodynamo is considered, rather than lengths of polarity intervals, an alternative approach to the GPTS is appropriate. We are developing a Geomagnetic Instability Time Scale (GITS) via 40Ar/39Ar dating of transitionally-magnetized lava flows younger than 2 Ma. As an example, the Laschamp event--expressed as a sharp intensity minimum in the GLOPIS-75 sediment stack--was dated by matching O-isotope variations in North Atlantic sediments to those recorded in annually counted layers of the GISP2 ice core. Matching 14C ages from the sediments to specific varves in the ice core shows the paleointensity minimum to span 1500 yr between 41 and 39 ka. 40Ar/39Ar and unspiked K-Ar dating of two basaltic lava flows that record the event at Laschamps, France yield an age of 40.4±1.1 ka (± 2 sigma, analytical uncertainty). Thus, despite systematic uncertainty in the 40K decay constant, both the accuracy and precision of the K-Ar clock can be remarkably good, i.e., better than 2% for the Pleistocene. Intercomparison of several 40Ar/39Ar-dated geomagnetic events, including the Matuyama-Brunhes polarity reversal (776 ± 2 ka), Big Lost event (579 ± 6 ka) and Pringle Falls/Albuquerque event (211 ± 11 ka) implies either that: 1) the astrochronology-based age models used for the SINT-800 paleointensity stack are

  8. 40Ar/39Ar constraints on the temporal evolution of Graciosa Island, Azores (Portugal)

    NASA Astrophysics Data System (ADS)

    Larrea, Patricia; Wijbrans, Jan R.; Galé, Carlos; Ubide, Teresa; Lago, Marceliano; França, Zilda; Widom, Elisabeth

    2014-02-01

    Lava flows spanning the eruptive record of Graciosa Island (Azores archipelago) and a gabbro xenolith were dated by 40Ar/39Ar in order to constrain the Pleistocene and Holocene volcanic evolution of the island. The results range from 1.05 Ma to 3.9 ka, whereas prior published K-Ar and 14C ages range from 620 to 2 ka. The formation of the Serra das Fontes shield volcano started at minimum 1.05 Ma, and the magmatic system was active for ca. 600 ky, as suggested by the formation of the gabbro xenolith by magmatic differentiation. Evolved magmas making up the Serra das Fontes-Serra Branca composite volcano were generated at ca. 450 ka. After a period of ca. 110 ky of volcanic inactivity and erosion of volcanic edifices, volcanism was reactivated with the formation of the Vitória Unit NW platform. Later, the development of the Vulcão Central Unit started with the formation of monogenetic cones located to the south of the Serra das Fontes-Serra Branca-Vitória Unit. This volcanism became progressively more evolved and was concentrated in a main eruptive center, forming the Vulcão Central stratovolcano with an age older than 50 ka. The caldera related to this stratovolcano is older than 47 ka and was followed by effusion of basaltic magmas into the caldera, resulting in the formation of a lava lake, which ultimately spilled over the caldera rim at ca. 11 ka. The most recent eruptions on Graciosa formed two small pyroclastic cones within the caldera and the Pico do Timão cone within the Vitória Unit at ca 3.9 ka.

  9. 40Ar/ 39Ar ages of mafic dykes from the Mesoproterozoic Chhattisgarh basin, Bastar craton, Central India: Implication for the origin and spatial extent of the Deccan Large Igneous Province

    NASA Astrophysics Data System (ADS)

    Rao, N. V. Chalapathi; Burgess, R.; Lehmann, B.; Mainkar, D.; Pande, S. K.; Hari, K. R.; Bodhankar, N.

    2011-08-01

    We present 40Ar/ 39Ar whole-rock ages of 63.7 ± 2.7 Ma (2σ, 92% Ar release) and 66.6 ± 2.2 Ma (2σ, 96% Ar release) for two samples of sub-surface mafic dykes intrusive into the sedimentary rocks of the Mesoproterozoic Chhattisgarh basin, Bastar craton, Central India. The obtained ages are synchronous with those of the Deccan Traps whose nearest exposures are at a distance of ~ 200 km to the west, and the recently dated diamondiferous orangeites (Group-II kimberlites) of the Mainpur area (located ~ 100 km SE within the Bastar craton). The chemical composition of the Chhattisgarh mafic dykes is indistinguishable from the chemostratigraphic horizons of the upper Deccan lavas of the Wai Subgroup (Ambenali and Poladpur Formations) and confirms them to be a part of the Deccan Large Igneous Province (LIP). The geological setting of the Deccan-age mafic dykes in the Chhattisgarh basin is analogous to that observed in other LIPs of the world such as (i) Pasco Basin of NW U.S.A, (ii) Ellisras sub-basin of southern Africa, (iii) Rift basins of New England in the NE U.S.A and (iv) the West Siberian Basin of Russia where LIP-related basalts and sills have been emplaced in distant domains from the main province. The Deccan-age of the Chhattisgarh dykes and the Mainpur orangeites permits a substantial increase of at least 8.5 × 10 4 km 2 in the spatial extent of the Deccan LIP. The temporal link at ~ 65 Ma between the Deccan Traps and (i) sub-surface mafic dykes within the Chhattisgarh basin and orangeites in the Bastar craton, (ii) Ambadongar carbonatite in western India, (iii) Salma mafic dyke in the Eastern Indian craton, (iv) Rajahmundry Traps off the eastern coast of southern India and (v) tholeiitic dykes and basalts from the Seychelles, suggests a common tectonomagmatic control, via a vast mantle plume-head of the order of 2000-2500 km. Our study has relevance to the (i) origin (plume vs non-plume) of the Deccan LIP, (ii) plumbing system for Deccan dykes and lavas in

  10. 40Ar/39Ar dating of tourmaline as a tool for high-temperature metamorphism thermochronology

    NASA Astrophysics Data System (ADS)

    Jourdan, Fred; Thern, Eric

    2014-05-01

    Tourmaline is an ubiquitous mineral, with properties making it ideal for studying metamorphic processes as well as a useful tool for a wide range of applications (e.g, magmatism, metasomatism, ore deposits [1]), mostly because it is not sensitive to chemical or mechanical alteration and is stable over a wide range of pressure-temperature conditions (up to 6 GPa and 850° C [2]). Typical metamorphic tourmaline types include dravite and shorl which, along with elbaite, belong to the alkali group [1]. The alkali group is notable because tourmalines from this group tend to incorporate trace amounts of K2O and therefore, can be dated using the 40Ar/39Ar technique. In order to understand the maximum temperature below which the K/Ar chronometer stays closed to argon loss by thermally activated diffusion, we carried out temperature controlled furnace diffusion experiments on well-behaved 40Ar/39Ar plateau-forming Archean tourmaline of 2935 ± 9 Ma [3]. Each experiment yielded an Arrhenius profile (Do vs. 1/temperature) that shows that the 39Ar data form two linear arrays with two distinct slopes. The first array only includes a few % of the total gas, has a shallow slope and shows very fast diffusivity at low temperature. We interpret these data as indicating very fast release of argon by cracks and defects. The second array of data points includes most of the gas of each experiment and forms a much steeper slope. These data yielded Ea (activation energy) values ranging from 120 to 157 Kcal/mol and D0 (pre-exponential diffusion factor) values ranging from 1.9x106 to 2.5x109 cm2/s for crystals with an average radius of 100 ± 25 μm. Three additional experiments using a laser (resulting in poor temperature control) suggest similar values although the latter experiments are considered semi-quantitative. The furnace experiments suggest that tourmaline has a weighted mean closure temperature of 804 ± 90 ° C (1σ) for a cooling rate of 10° C/Ma. Monte Carlo simulations using

  11. Combined oxygen, hydrogen, /sup 40/Ar//sup 39/Ar isotopic evaluation of molybdenite mineralization, east Pioneer Mountains, Montana

    SciTech Connect

    Chesley, J.T.; Snee, L.W.; O'Neil, J.R.

    1985-01-01

    The Pear Lake molybdenite (moly) prospect is in the Eastern Pioneer Wilderness Area, 30 km south of the Cannivan Gulch moly deposit. It lies on a NW-SE trend with other prospects that are contemporaneous. Mineralized veins outcrop sporadically over a 4 square mile area, with a major concentration in the center of the study area. Previous studies have suggested potential moly-mineralization at depth. /sup 40/Ar//sup 39/Ar age-spectra on hornblend (hb) from the granodiorite host rock indicate emplacement at 75.0 +/- 1 Ma, with biotite closure at 69.0 +/- .3 Ma, in the vicinity of Pear Lake. Hydrothermal muscovites from the moly-bearing veins have ages of 68.0 +/- .5 Ma. partial resetting of biotite cooling dates in the vicinity of mineralization are observed, but biotites on the outer portion of the mineralizing system are undisturbed. These relations suggest that mineralization occurred after the host pluton had cooled to below 280/sup 0/C. Quartz-magnetite isotopic temperatures are 380 to 420/sup 0/C for moly-veins. These data suggest that magmatic hydrothermal fluids operated in the upper portion of this molysystem. These isotopic data, combined with geologic mapping, suggest that the Pear Lake moly prospect resulted from a weak, short-lived magmatic hydrothermal event at depth, and is unlikely to contain economic levels of molybdenum. The combined /sup 40/Ar//sup 39/Ar-stable isotope approach should prove useful in evaluating other hydrothermal systems.

  12. Termination of major ductile strike-slip shear and differential cooling along the Insubric line (Central Alps): UPb, RbSr and 40Ar /39Ar ages of cross-cutting pegmatites

    NASA Astrophysics Data System (ADS)

    Schärer, Urs; Cosca, Michael; Steck, Albrecht; Hunziker, Johannes

    1996-08-01

    To constrain the age of strike-slip shear, related granitic magmatism, and cooling along the Insubric line, 29 size fractions of monazite and xenotime were dated by the UPb method, and a series of 25 RbSr and 40Ar /39Ar ages were measured on different size fractions of muscovite and biotite. The three pegmatitic intrusions analyzed truncate high-grade metamorphic mylonite gneisses of the Simplon shear zone, a major Alpine structure produced in association with dextral strike-slip movements along the southern edge of the European plate, after collision with its Adriatic indenter. Pegmatites and aplites were produced between 29 and 25 Ma in direct relation to right-lateral shear along the Insubric line, by melting of continental crust having 87Sr /86Sr between 0.7199 and 0.7244 at the time of melting. High-temperature dextral strike-slip shear was active at 29.2 ± 0.2 (2σ) Ma, and it terminated before 26.4 ± 0.1 Ma. During dike injection, temperatures in the country rocks of the Isorno-Orselina and Monte Rosa structural units did not exceed ≈ 500°C, leading to fast initial cooling, followed by slower cooling to ≈ 350°C within several million years. In one case, initial cooling to ≈ 500°C was significantly delayed by about 4 m.y., with final cooling to ≈ 300°C at 20-19 Ma in all units. For the period between 29 and 19 Ma, cooling of the three sample localities was non-uniform in space and time, with significant variations on the kilometre scale. These differences are most likely due to strongly varying heat flow and/or heterogeneous distribution of unroofing rates within the continuously deforming Insubric line. If entirely ascribed to differences in unroofing, corresponding rates would vary between 0.5 and 2.5 mm/y, for a thermal gradient of 30°/km.

  13. 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.

  14. First ^4^0Ar/^3^9Ar geochronology of lateritic manganiferous pisolites: Implications for the Palaeogene history of a West African landscape [rapid communication

    NASA Astrophysics Data System (ADS)

    Colin, F.; Beauvais, A.; Ruffet, G.; Hénocque, O.

    2005-09-01

    A pisolitic superficial formation deriving from the geochemical degradation of a previous massive manganiferous crust has been investigated in West Africa using 40Ar/ 39Ar laser probe analysis on different generations of cryptomelane from Fe-Mn pisolites and their embedding lateritic matrices. Prior to the 40Ar- 39Ar geochronological analysis, a detailed petrographical study has allowed the identification of successive assemblages from Mn-pisolites to (Fe,Mn)-pisolites embedded in Mn-rich and Fe-rich matrices, respectively. The age spectra obtained for pisolites, including cores, cortices and matrices enables the definition of three age clusters around 56-59, 44-47, and 24-27 Ma. The first two age clusters with the petrographical and geochemical results obtained on the different assemblages indicate that the development of the pisolitic crust first induced Mn-leaching and Fe-Al accumulations due to warmer and wetter climatic conditions in the age interval 56-47 Ma that characterizes the Tertiary greenhouse effect period propitious to bauxite formation in West Africa. The absence of 40Ar/ 39Ar ages between 44 and 27 Ma only means that geochemical conditions for cryptomelane crystallisation were not fulfilled, but could still be favourable to Fe- and/or Al-oxy-hydroxides formation, before drier climatic conditions became rather effective at the Oligocene period to sustain mechanical erosion rather than chemical weathering processes. The age cluster 24-27 Ma indicates however a reactivation of the manganiferous lateritic weathering late Oligocene. The 40Ar/ 39Ar dating results are discussed in terms of climatic condition changes during the Palaeogene that were favourable to the genesis of either Al- and Fe- or Mn-oxy-hydroxides in the course of development and evolution of the pisolitic formation. The 40Ar/ 39Ar dating also provides a new highlight to large-scale geomorphological patterns of West Africa.

  15. Mineralogy, 40Ar/ 39Ar dating and apatite fission track dating of rocks along the Castle Mountain fault, Alaska

    NASA Astrophysics Data System (ADS)

    Parry, W. T.; Bunds, M. P.; Bruhn, R. L.; Hall, C. M.; Murphy, J. M.

    2001-07-01

    The Castle Mountain fault is a 200-km-long, right-lateral fault that forms the northern boundary of the Cook Inlet basin and Matanuska Valley, Alaska. Fault gouge and fault rock at six localities contain the clay minerals illite, smectite, chlorite, and interstratified illite/smectite. At one locality, gouge contains deformed illite/smectite with very little wall rock chlorite contamination. Fine (<0.03 μm), medium (0.03-0.2 μm), and coarse (0.2-2.0 μm) illite/smectite from this site were dated using 40Ar/ 39Ar micro-encapsulation and laser microprobe methods. Total gas ages for the three size fractions are 28.21±0.12, 32.42±0.11 and 36.24±0.08 Ma for fine to coarse sizes respectively. Argon retention ages obtained from 40Ar and 39Ar retained in the three size fractions of illite at room temperature during neutron irradiation are 37.36±0.15, 42.11±0.14 and 47.20±0.10 respectively. Apatite fission track ages were measured in arkose at a locality on the fault 60 km west of the gouge locality. Three samples of arkose were dated: one within 10 m of the fault core, one 170 m from the fault, and one 335 m from the fault. The sample nearest to the fault yielded an age of 29.3±2.8 Ma, but it only had four track lengths at 10-13 μm. Two apatite grains from the intermediate sample yielded a pooled age of 34.3±6.1 Ma. The distant sample (25 grains counted, 101 track lengths) yielded an age of 32.0±2.9 Ma. This sample has a broad distribution of track lengths and a broad distribution of individual grain ages ranging from 14.8±5.1 to 67.8±8.8 Ma. Monte Carlo modeling of the apatite age and track length data is consistent with hydrothermal mineralization at 37-39 Ma followed by rapid uplift and cooling after 10 Ma. The 40Ar/ 39Ar total gas ages (K-Ar) are minimum ages, and the argon retention ages are maximum ages. The thermal model derived from the fission track data, and the argon retention age for the finest illite fraction of ˜37 Ma date a hydrothermal

  16. 40Ar/39Ar ages and petrogenesis of the West Iberian Margin onshore magmatism at the Jurassic-Cretaceous transition: Geodynamic implications and assessment of open-system processes involving saline materials

    NASA Astrophysics Data System (ADS)

    Mata, J.; Alves, C. F.; Martins, L.; Miranda, R.; Madeira, J.; Pimentel, N.; Martins, S.; Azevedo, M. R.; Youbi, N.; De Min, A.; Almeida, I. M.; Bensalah, M. K.; Terrinha, P.

    2015-11-01

    The West Iberian Margin (WIM) preserves onshore testimonies of three Mesozoic magmatic cycles. In this paper we present and discuss 40Ar/39Ar ages and geochemical data for the second cycle, which occurred at least from 148 Ma to 140 Ma, during the late stages of an important extensional event associated with the Iberia-Newfoundland rifting. The related lithospheric stretching induced magma genesis by adiabatic decompression. Primitive rocks are mildly alkaline but evolved to SiO2-saturated and oversaturated rocks at "high" pressure. Magmas sampled a source of fairly homogenous composition characterized by Sr and Nd isotopic compositions (εNdi from + 1.6 to + 4.2), more enriched than the typical N-MORB source. Magmas were generated at the top of the garnet zone. Considering the thickness of the lithosphere and the geochemical constraints, an origin by melting of a metasomatized domain of the lithosphere is favored. The composition of these onshore magmas is somewhat distinct from the quasi coeval magmas emplaced offshore, which is interpreted as a result of the less important onshore lithospheric stretching, leading to lower degrees of partial melting. This favored the contribution of lithospheric metasomatized domains to onshore magmas. Rocks intruded two sectors of the Lusitanian Basin separated by the Nazaré Fault and characterized by distinct subsidence rates during the Jurassic. The fact that the rocks to the north of the Nazaré Fault are significantly more evolved indicates the more important development of magma chambers in the north, suggesting distinct thermal profiles for those two sectors. Such magma chambers enabled the "high-pressure" fractionation necessary to drive magma compositions from Ne-normative to SiO2- saturated and -over-saturated. The rocks cropping out south of the Nazaré fault are clearly less evolved, and its variability is mostly due to different partial melting events. Some rocks present evidence of post-magmatic processes involving

  17. Mesozoic thermal history and timing of structural events for the Yukon-Tanana Upland, east-central Alaska: 40Ar/39Ar data from metamorphic and plutonic rocks

    USGS Publications Warehouse

    Dusel-Bacon, C.; Lanphere, M.A.; Sharp, W.D.; Layer, P.W.; Hansen, V.L.

    2002-01-01

    We present new 40Ar/39Ar ages for hornblende, muscovite, and biotite from metamorphic and plutonic rocks from the Yukon-Tanana Upland, Alaska. Integration of our data with published 40Ar/39Ar, kinematic, and metamorphic pressure (P) and temperature (T) data confirms and refines the complex interaction of metamorphism and tectonism proposed for the region. The oldest metamorphic episode(s) postdates Middle Permian magmatism and predates the intrusion of Late Triassic (215-212 Ma) granitoids into the Fortymile River assemblage (Taylor Mountain assemblage of previous papers). In the eastern Eagle quadrangle, rapid and widespread Early Jurassic cooling is indicated by ???188-186 Ma 40Ar/39Ar plateau ages for hornblende from plutons that intrude the Fortymile River assemblage, and for metamorphic minerals from the Fortymile River assemblage and the structurally underlying Nasina assemblage. We interpret these Early Jurassic ages to represent cooling resulting from northwest-directed contraction that emplaced the Fortymile River assemblage onto the Nasina assemblage to the north as well as the Lake George assemblage to the south. This cooling was the final stage of a continuum of subduction-related contraction that produced crustal thickening, intermediate- to high-P metamorphism within both the Fortymile River assemblage and the structurally underlying Lake George assemblage, and Late Triassic and Early Jurassic plutonism in the Fortymile River and Nasina assemblages. Although a few metamorphic samples from the Lake George assemblage yield Jurassic 40Ar/39Ar cooling ages, most yield Early Cretaceous 40Ar/39Ar ages: hornblende ???135-115 Ma, and muscovite and biotite ???110-108 Ma. We interpret the Early Cretaceous metamorphic cooling, in most areas, to have resulted from regional extension and exhumation of the lower plate, previously tectonically thickened during Early Jurassic and older convergence.

  18. 40Ar/39Ar thermochronologic constraints on the tectonothermal evolution of the Northern East Humboldt range metamorphic core complex, Nevada

    USGS Publications Warehouse

    McGrew, A.J.; Snee, L.W.

    1994-01-01

    The northern East Humboldt Range (NEHR) of northeastern Nevada exposes a suite of complexly deformed migmatitic, upper amphibolite-facies rocks in the footwall of the Ruby Mountains-East Humboldt Range (RM-EHR) detachment fault. New 40Ar/39Ar data on hornblende, muscovite, biotite, and potassium feldspar help constrain the kinematic and thermal evolution of this terrain during Tertiary extensional exhumation. Hornblende samples from relatively high structural levels yield discordant age spectra that suggest initial cooling during early Tertiary time (63-49 Ma). When coupled with petrological constraints indicating a strongly decompressional P-T-t path above 550??C, the hornblende data suggest that exhumation of the RM-EHR may have initiated in early Tertiary time, approximately coincident with the initial phases of unroofing in the Wood Hills immediately to the east and with the end of thrusting in the late Mesozoic to early Tertiary Sevier orogenic belt of eastern Nevada and western Utah. This temporal coincidence suggests that gravitational collapse of tectonically thickened crust in the internal zone of the Sevier belt could have driven the initial phases of unroofing. Thermal history during the final stage of exhumation of the NEHR is constrained by discordant hornblende cooling ages of 36-29 Ma from deep structural levels and biotite, muscovite, and potassium feldspar cooling ages of 27-21 Ma from a range of structural levels. Comparison of muscovite, biotite, and potassium feldspar cooling ages with previously published fission-track cooling ages implies very rapid cooling rates at temperatures below the closure temperature for muscovite (270??-350??C), but time gaps of > 7 m.y. between hornblende and mica cooling ages suggest that cooling at higher temperatures was more gradual. In addition, comparison of 40Ar 39Ar mica cooling ages with previously published fission-track apatite cooling ages suggests pronounced thermal gradients between the NEHR and

  19. 40Ar/ 39Ar thermochronologic constraints on the tectonothermal evolution of the Northern East Humboldt range metamorphic core complex, Nevada

    NASA Astrophysics Data System (ADS)

    McGrew, Allen J.; Snee, Lawrence W.

    1994-11-01

    The northern East Humboldt Range (NEHR) of northeastern Nevada exposes a suite of complexly deformed migmatitic, upper amphibolite-facies rocks in the footwall of the Ruby Mountains-East Humboldt Range (RM-EHR) detachment fault. New 40Ar/ 39Ar data on hornblende, muscovite, biotite, and potassium feldspar help constrain the kinematic and thermal evolution of this terrain during Tertiary extensional exhumation. Hornblende samples from relatively high structural levels yield discordant age spectra that suggest initial cooling during early Tertiary time (63-49 Ma). When coupled with petrological constraints indicating a strongly decompressional P-T-t path above 550°C, the hornblende data suggest that exhumation of the RM-EHR may have initiated in early Tertiary time, approximately coincident with the initial phases of unroofing in the Wood Hills immediately to the east and with the end of thrusting in the late Mesozoic to early Tertiary Sevier orogenic belt of eastern Nevada and western Utah. This temporal coincidence suggests that gravitational collapse of tectonically thickened crust in the internal zone of the Sevier belt could have driven the initial phases of unroofing. Thermal history during the final stage of exhumation of the NEHR is constrained by discordant hornblende cooling ages of 36-29 Ma from deep structural levels and biotite, muscovite, and potassium feldspar cooling ages of 27-21 Ma from a range of structural levels. Comparison of muscovite, biotite, and potassium feldspar cooling ages with previously published fission-track cooling ages implies very rapid cooling rates at temperatures below the closure temperature for muscovite (270°-350°C), but time gaps of > 7 m.y. between hornblende and mica cooling ages suggest that cooling at higher temperatures was more gradual. In addition, comparison of {40Ar }/{39Ar } mica cooling ages with previously published fission-track apatite cooling ages suggests pronounced thermal gradients between the NEHR and

  20. 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

  1. Diamond provenance studies from 40Ar/ 39Ar dating of clinopyroxene inclusions: An example from the west coast of Namibia

    NASA Astrophysics Data System (ADS)

    Phillips, D.; Harris, J. W.

    2009-11-01

    The west coast of Namibia is host to substantive detrital diamond deposits located in onshore and offshore beach gravels, desert deflation deposits and lower Orange river terraces. The origin of the Namibian diamonds is controversial, with some studies favouring derivation from distal Cretaceous/Jurassic kimberlites on the Kaapvaal craton, and others arguing that most diamonds originated from proximal Dwyka glacial deposits (~ 300 Ma), which incorporated diamonds from older (≥ 500 Ma), pre-Karoo kimberlites. Previous studies have demonstrated that clinopyroxene inclusions extracted from their host diamonds give 40Ar/ 39Ar ages approaching the time of source kimberlite eruption. This behaviour is attributed to diffusion of argon to lattice defect sites and the diamond/inclusion interface region during mantle residence, with subsequent loss of the latter component on cleaving of the diamond to release the inclusion(s). In this study, we measured 40Ar/ 39Ar ages of extracted clinopyroxene inclusions from Namibian detrital diamonds, in order to determine potential kimberlite sources, craton erosion histories and palaeo-drainage evolution in southern Africa. 40Ar/ 39Ar step-heating data were obtained for eclogitic and peridotitic clinopyroxene inclusions from 50 Namibian diamonds. Low temperature steps produced older apparent ages than high temperature (fusion) steps, consistent with partial retention of pre-eruption argon in defect sites. With one exception, fusion steps yielded younger ages, ranging from 62 ± 30 Ma to 1441 ± 700 Ma. The majority (80%) of inclusions have fusion ages < 300 Ma, indicating that most Namibian detrital diamonds originated from post-Dwyka (< 300 Ma) kimberlites. Six inclusion aliquots (13%) produced ages unique to Cretaceous Group I kimberlites, confirming erosion of diamonds from these sources. The proportion of diamonds sourced from Group II kimberlites is uncertain, although forward modelling suggests roughly equal quantities from

  2. 40Ar/39Ar dating of microgram feldspar grains from the paired feldspathic achondrites GRA 06128 and 06129

    NASA Astrophysics Data System (ADS)

    Lindsay, Fara N.; Herzog, Gregory F.; Park, Jisun; Delaney, Jeremy S.; Turrin, Brent D.; Swisher, Carl C.

    2014-03-01

    40Ar/39Ar ages of single feldspar grains from the paired meteorites Graves Nunatak 06128 (GRA8; 8 grains) and 06129 (GRA9; 26 grains) are presented. Plateau ages (⩾70% of the 39Ar released) ranged from 4000 to 4600 Ma with an average 1-σ uncertainty of ±90 Ma. The most precise ages obtained were 4267 ± 17 Ma for a grain from GRA8 and 4437 ± 19 Ma and 4321 ± 18 Ma for two grains from GRA9. Isotope correlation diagrams yield less precise ages ranging from 3800 to 5200 Ma with an average 1-σ uncertainty of 250 Ma; they indicate a negligible trapped component. Plateau ages, integrated total fusion ages, and isochron ages are internally concordant at the 95% confidence level. The distribution of the plateau ages for GRA9 is bimodal with peaks at 4400 and 4300 Ma. In contrast, the plateau age distribution for GRA8 peaks at about 4260 Ma with broad wings extending toward younger and older ages. To explain the distributions of grain ages we prefer a scenario that includes a major post-formation event about 4400 Ma ago and a later melt intrusion event that heated GRA8 more than some parts of GRA9.

  3. Direct dating of folding events by 40Ar/39Ar analysis of synkinematic muscovite from flexural-slip planes

    NASA Astrophysics Data System (ADS)

    Wang, Yu; Zwingmann, Horst; Zhou, Liyun; Lo, Ching-hua; Viola, Giulio; Hao, Jinhua

    2016-02-01

    Timing of folding is usually dated indirectly, with limited isotopic dating studies reported in the literature. The present study investigated the timing of intracontinental, multi-stage folding in Upper Proterozoic sandstone, limestone, and marble near Beijing, North China, and adjacent regions. Detailed field investigations with microstructural, backscattered electron (BSE) images and electron microprobe analyses indicate that authigenic muscovite and sericite crystallized parallel to stretching lineations/striations or along thin flexural-slip surfaces, both developed during the complex deformation history of the study area, involving repeated compressional, extensional and strike-slip episodes. Muscovite/sericite separates from interlayer-slip surfaces along the limbs and from dilatant sites in the hinges of folded sandstones yield muscovite 40Ar/39Ar plateau ages of ∼158-159 Ma, whereas those from folded marble and limestone samples yield ages of 156 ± 1 Ma. Muscovite from thin flexural-slip planes on fold limbs and hinges yields ages within analytical error of ∼155-165 Ma. Further muscovite samples collected from extensionally folded limestone and strike-slip drag folds yield younger ages of 128-125 Ma with well-defined plateaus. To assess the potential influence of the detrital mica component of the host rock on the age data, two additional muscovite samples were investigated, one from a folded upper Proterozoic-Cambrian sandstone outside the Western Hills of Beijing and one from a folded sandstone sampled 20 cm from folding-related slip planes. Muscovite separates from these samples yield significantly older ages of 575 ± 2 Ma and 587 ± 2 Ma, suggesting that the timing of folding can be directly determined using the 40Ar/39Ar method. This approach enables the identification and dating of distinct deformation events that occur during multi-stage regional folding. 40Ar/39Ar dating can be used to constrain the timing of muscovite and sericite growth at

  4. 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

  5. 40Ar/39Ar dating of the eruptive history of Mount Erebus, Antarctica: volcano evolution

    NASA Astrophysics Data System (ADS)

    Esser, Richard P.; Kyle, Philip R.; McIntosh, William C.

    2004-12-01

    Mt. Erebus, a 3,794-meter-high active polygenetic stratovolcano, is composed of voluminous anorthoclase-phyric tephriphonolite and phonolite lavas overlying unknown volumes of poorly exposed, less differentiated lavas. The older basanite to phonotephrite lavas crop out on Fang Ridge, an eroded remnant of a proto-Erebus volcano and at other isolated locations on the flanks of the Mt. Erebus edifice. Anorthoclase feldspars in the phonolitic lavas are large (~10 cm), abundant (~30 40%) and contain numerous melt inclusions. Although excess argon is known to exist within the melt inclusions, rigorous sample preparation was used to remove the majority of the contaminant. Twenty-five sample sites were dated by the 40Ar/39Ar method (using 20 anorthoclase, 5 plagioclase and 9 groundmass concentrates) to examine the eruptive history of the volcano. Cape Barne, the oldest site, is 1,311±16 ka and represents the first of three stages of eruptive activity on the Mt. Erebus edifice. It shows a transition from sub-aqueous to sub-aerial volcanism that may mark the initiation of proto-Erebus eruptive activity. It is inferred that a further ~300 ky of basanitic/phonotephritic volcanism built a low, broad platform shield volcano. Cessation of the shield-building phase is marked by eruptions at Fang Ridge at ~1,000 ka. The termination of proto-Erebus eruptive activity is marked by the stratigraphically highest flow at Fang Ridge (758±20 ka). Younger lavas (~550 250 ka) on a modern-Erebus edifice are characterized by phonotephrites, tephriphonolites and trachytes. Plagioclase-phyric phonotephrite from coastal and flank flows yield ages between 531±38 and 368±18 ka. The initiation of anorthoclase tephriphonolite occurred in the southwest sector of the volcano at and around Turks Head (243±10 ka). A short pulse of effusive activity marked by crustal contamination occurred ~160 ka as indicated by at least two trachytic flows (157±6 and 166±10 ka). Most

  6. 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 ages40Ar/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.

  7. Constraints on the Jurassic time scale by /sup 40/Ar//sup 39/Ar dating of North Caucasian volcanic rocks

    SciTech Connect

    Hess, J.C.; Lippolt, H.J.; Borsuk, A.M.

    1987-07-01

    /sup 40/Ar//sup 39/Ar age measurements on biotites and high-temperature plagioclases of Jurassic basaltic to rhyolitic subvolcanic rocks from the Northern Great Caucasus (USSR) yielded plateau and total argon ages between 190 and 180 Ma. The dated rocks are intrusive sills, dikes and laccoliths in sediments of the middle to upper Pliensbachian and of the lower Toarcian (Lower Jurassic). Pebbles of the volcanic rocks exist in the basal conglomerates of the Aalenian (base of the Middle Jurassic). Thus, their stratigraphic age is restricted to the Lower Jurassic stages of middle to upper Pliensbachian and Toarcian. Because of the scarcity of tie-points in the Lower Jurassic, the isotopic ages of these volcanic rocks, in spite of their rather large stratigraphic range, may serve as new calibration points for the improvement of the Jurassic time-scale.

  8. High Precision 40Ar/39Ar Geochronology of Servilleta Basalts of the Rio Grande Gorge, New Mexico

    NASA Astrophysics Data System (ADS)

    Cosca, M. A.; Thompson, R. A.; Turner, K. J.

    2014-12-01

    New geologic mapping and high-precision 40Ar/39Ar geochronology within the Taos Plateau in northern New Mexico indicate a period of vigorous volcanic activity between ~5.5 and ~1 Ma. Over 50 visible volcanic centers formed during this time together with an unresolved number of vents, fissures, and volcanic centers buried by intercalated volcanic rock and sedimentary basin fill. Defining the volcanic stratigraphy is essential for models of regional groundwater flow and for understanding the geologic evolution of the Pliocene to Recent Rio Grande rift. A spectacular stratigraphic section of volcanic rock related to Rio Grande rifting is visible from the High Bridge, just a few miles outside of Taos, NM, where a 240 m canyon is incised through the basal, middle, and upper Servilleta basalt flow packages (Dungan et al., 1984). Fresh basalt from a vertical transect of the canyon near the High Bridge were analyzed by 40Ar/39Ar methods on ~3 mm3 rock fragments using an ARGUS VI mass spectrometer and the resulting 40Ar/39Ar ages define a precise emplacement chronology of the entire stratigraphic section. The basal flow package records ages of 4.78 ± 0.03 Ma (relative to FCT sanidine = 28.204 Ma; all errors 2 sigma) at river level, 4.77 ± 0.03 Ma at mid flow, and 4.50 ± 0.04 Ma at the top of the flow. The middle flow package records ages of 4.11 ± 0.03 Ma at the base of the flow, 4.08 ± 0.04 Ma mid flow, and 4.02 ± 0.06 Ma at the top of the flow. The upper basalt package records ages of 3.69 ± 0.06 Ma at the base of the flow and 3.59 ± 0.08 Ma at the top of the flow. These data support rapid effusion of voluminous lava flows on time scales of 100-200 ka. Two reddish paleosols separating the Servilleta packages each developed during a 400 ka period of volcanic quiescence. First order calculations using exposed lava thicknesses in the gorge and areal exposures suggest each flow package represents emplacement of ~200 km3 of basalt. Because no exposed vent of

  9. Resolving the early chronology of Mono Craters volcanism with combined 238U-230Th and 40Ar/39Ar dating

    NASA Astrophysics Data System (ADS)

    Vazquez, J. A.; Calvert, A. T.; Marcaida, M.; Mangan, M.; Lidzbarski, M. I.; Stelten, M. E.

    2013-12-01

    California's largest locus of Pleistocene-Holocene rhyolitic volcanism is the Mono Lake-Long Valley region of eastern California. The Mono Craters chain marks the northern portion of this locus, and is composed of at least 28 individual domes of high-silica rhyolite. The record of Holocene volcanism at Mono Craters is relatively well constrained by tephrostratigraphy and radiocarbon dating. However, the timing and frequency of late Pleistocene dome emplacement is poorly resolved, with most of the chronology based on hydration-rind dating of obsidian. A well-exposed archive of late Pleistocene volcanism from Mono Craters is recorded by tephra beds (ashes numbered 1-19, youngest to oldest) of the informal Wilson Creek formation that accumulated in ancestral Mono Lake. To resolve a precise chronology for late Pleistocene volcanism at Mono Craters and tune the time-series of explosive volcanism preserved by Wilson Creek tephras, we performed ion microprobe 238U-230Th dating of allanite and zircon together with laser-fusion 40Ar/39Ar dating of sanidine from rhyolite domes that yield the oldest hydration rind ages and have relatively subdued morphology. Sanidine from multiple domes, including both hornblende-biotite and fayalite-bearing rhyolite types, yield 40Ar/39Ar ages up to ca. 25 ka. Ion microprobe analyses of unpolished rims on indium-mounted allanite and zircon crystals yield U-Th isochron ages that are indistinguishable from their associated sanidine 40Ar/39Ar ages. However, the interiors of sectioned allanite crystals yield model U-Th ages that may be up to 30 kyr older than their rims. Rims on allanite and zircon from ashes 7-19 in the lower portion of the Wilson Creek stratigraphy yield isochron ages of ca. 27-62 ka [1], which are supported by ages from magnetostratigraphy [2]. Ash 3 contains titanomagnetites that are compositionally distinct from other Wilson Creek tephras, but match those in the hornblende-biotite rhyolite of dome 11. Rims on allanite and

  10. Constraints on the development of Proterozoic basins in central India from 40Ar/39Ar analysis of authigenic glauconitic minerals

    USGS Publications Warehouse

    Conrad, J.E.; Hein, J.R.; Chaudhuri, A.K.; Patranabis-Deb, S.; Mukhopadhyay, J.; Deb, G.K.; Beukes, N.J.

    2011-01-01

    Ages of some key stratigraphic sequences in central Indian Proterozoic basins are based predominantly on lithostratigraphic relationships that have been constrained by only a few radioisotopic dates. To help improve age constraints, single grains of glauconitic minerals taken from sandstone and limestone in two Proterozoic sequences in the Pranhita-Godavari Valley and the Chattisgarh basin were analyzed by the 40Ar/39Ar incremental heating method. Analysis of the age spectra distinguishes between ages that are interpreted to reflect the time of glauconite formation, and anomalous ages that result from inherited argon or postcrystallization heating. The analyses indicate an age of 1686 ± 6 Ma for the Pandikunta Limestone and 1566 ± 6 Ma for the Ramgundam Sandstone, two units in the western belt of Proterozoic sequences in Pranhita-Godavari Valley. Glauconite from the Chanda Limestone, in the upper part of this sequence, contains inherited 40Ar but is interpreted to reflect an age of ca. 1200 Ma. Glauconite from the Somanpalli Group in the eastern belt of the Pranhita-Godavari Valley gives an age of 1620 ± 6 Ma. In the Chattisgarh basin, glauconite from two units gives disturbed ages that suggest a period of regional heating in the Chattisgarh basin at ca. 960–1000 Ma. These new ages indicate that these sequences are 200–400 m.y. older than previously recognized, which has important implications for geochemical studies of Mesoproterozoic ocean redox conditions in addition to providing important constraints on regional tectonics and lithostratigraphy.

  11. New 40Ar/39Ar Chronostratigraphy for Olduvai Gorge, Tanzania, and Orbital Forcing of Lacustrine Sedimentation

    NASA Astrophysics Data System (ADS)

    Deino, A.

    2011-12-01

    The Pleistocene sedimentary and volcanic deposits of Olduvai Gorge, north-central Tanzania, have been a focus of geological, paleontological, paleoanthropological and archaeological investigation since Louis Leakey initiated fossil and artifact excavations there in 1931. Although more than half a century of effort has been applied to radiometric dating of the volcaniclastic strata within this sequence, vexing questions persist regarding details of the chronostratigraphy. Renewed dating efforts have enabled revision of the chronostratigraphy of Bed I, providing a more accurate evaluation of the absolute ages of volcanic events, and the rates of geological processes. The new 40Ar/39Ar geochronology, based primarily on phenocrystic K-feldspar from interbedded marker tuffs, resolves some of the anomalies apparent in earlier Olduvai basin Bed I chronostratigraphies. A new age of 1.88 ± 0.05 for Tuff IA near the base of Bed I, while relatively imprecise, agrees with a projected age 1.91 Ma based on magnetostratigraphy and sedimentation rates toward the base of Bed I. The key marker tuff IB in the upper part of Bed I has a well-determined age of 1.848 ± 0.003 Ma. Comparative dating of early and late erupted phases of this unit yields identical ages, resolving the conundrum of an apparent hiatus of ~80 ka between the crystal-rich fallout base and the body of the Tuff IB ignimbrite in the eastern exposures proposed by Walter et al. (1991, 1992). New ages for tuffs above Tuff IB to the top of Bed I (i.e., Tuff IE 'vitric,' Tuff IE, Ng'eju tuff, and Tuff IF), fall in stratigraphic order and are consistent with the paleomagnetic age constraint of 1.778 Ma imposed by the top of the Olduvai Subchron in overlying Bed II. The new high-precision chronostratigraphy provides age control for a series of wet/dry paleoclimate indicators previously documented in upper Bed I (Hay, 1976; Hay and Kyser, 2001; Sikes and Ashley, 2007). The most prominent event is a major lake expansion

  12. The Taili-Yiwulüshan metamorphic core complex corridor: Diachronous exhumation and relationships to the adjacent basins based on new 40Ar/39Ar and (U-Th-Sm)/He mineral ages

    NASA Astrophysics Data System (ADS)

    Liang, Chenyue; Neubauer, Franz; Liu, Yongjiang; Genser, Johann; Dunkl, István; Heberer, Bianca; Jin, Wei; Zeng, Zuoxun; Li, Weimin; Wen, Quanbo; Li, Jing

    2015-04-01

    The Xingcheng-Taili ductile shear zone (western Liaoning Province in China) formed during latest Jurassic to Early Cretaceous crustal extension of the eastern North China craton, and exhumed low to medium metamorphic grade Archean, Upper Triassic and Upper Jurassic granitic rocks. The Mesozoic Yiwulüshan metamorphic core complex (Yiwulüshan MCC) is dominated by a NNE-SSW elongated dome with a left-lateral shear zone, which is located in the northeastern part of Xingcheng-Taili ductile shear zone, and combine as Taili-Yiwulüshan metamorphic core complex corridor. To the east, it is bounded by the NNE-trending Cretaceous to Eocene Liaohe basin (the northern extension of the Bohai Bay basin), and to the west by the Cretaceous-aged Fuxin-Yixian basin, which could potentially interpreted as supra-detachment basins. Here, we present results from a multi-method thermochronological study and coupled with structural investigations and sections of adjacent supra-detachment basins, which constrain the timing of regional deformation as well as the cooling history and exhumation processes of the low- to middle-grade metamorphic complex in the Taili-Yiwulüshan MCC corridor, in order to understand the mode of lithospheric scale reactivation, extension and thinning of the North China craton. The new40Ar/39Ar muscovite, biotite, K-feldspar and (U-Th)/He apatite ages from granitic rocks help constrain the thermal evolution during its exhumation. The thermochronologic studies have shown at least three stages of exhumation and cooling from late Jurassic to Eocene in Xingcheng-Taili shear zone should be distinguished, e.g., ~ 150-130 Ma, 130-115 Ma and 115-52 Ma, respectively. Diachronous onset and subsequent parallel cooling and exhumation characterize the early thermal history. The Yiwulüshan MCC has a similar exhumation history from 135 to 97 Ma with a similar cooling history. The development of Taili-Yiwulüshan MCC corridor is associated with synkinematic emplacement

  13. 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

    The 40Ar/39Ar laser step-heating method potentially can be used to provide absolute ages for a number of formerly undatable, low-temperature ore deposits. This study demonstrates the use of this method by determining absolute ages for Zn-Pb-Ag sediment-hosted massive sulfide deposits and vein-breccia occurrences found throughout a 300-km-long, east-west-trending belt in the northern Brooks Range, Alaska. Massive sulfide deposits are hosted by Mississippian to Pennsylvanian(?) black carbonaceous shale, siliceous mudstone, and lesser chert and carbonate turbidites of the Kuna Formation (e.g., Red Dog, Anarraaq, Lik (Su), and Drenchwater). The vein-breccia occurrences (e.g., Husky, Story Creek, West Kivliktort Mountain, Vidlee, and Kady) are hosted by a deformed but only weakly metamorphosed package of Upper Devonian to Lower Mississippian mixed continental and marine clastic rocks (the Endicott Group) that stratigraphically underlie the Kuna Formation. The vein-breccias are mineralogically similar to, but not spatially associated with, known massive sulfide deposits. The region's largest shale-hosted massive sulfide deposit is Red Dog; it has reserves of 148 Mt grading 16.6 percent zinc, 4.5 percent lead, and 77 g of silver per tonne. Hydrothermally produced white mica in a whole-rock sample from a sulfide-bearing igneous sill within the Red Dog deposit yielded a plateau age of 314.5 Ma. The plateau age of this whole-rock sample records the time at which temperatures cooled below the argon closure temperature of the white mica and is interpreted to represent the minimum age limit for massive sulfide-related hydrothermal activity in the Red Dog deposit. Sulfide-bearing quartz veins at Drenchwater crosscut a hypabyssal intrusion with a maximum biotite age of 337.0 Ma. Despite relatively low sulfide deposition temperatures in the vein-breccia occurrences (162°-251°C), detrital white mica in sandstone immediately adjacent to large vein-breccia zones was partially to

  14. sup 40 Ar- sup 39 Ar dating of the Beja gabbro: Timing of the accretion of southern Portugal

    SciTech Connect

    Ruffet, G. )

    1990-11-01

    The {sup 40}Ar-{sup 39}Ar dating of the amphibole from the Beja gabbro (Southern Portugal) yields a plateau age at 336.4 {plus minus} 0.8 Ma (2{sigma} level). The corresponding calculated isotopic closure temperature is around 800C. The comparison of this temperature with the magnetic blocking temperature ({approximately}570C) allows an estimation of a probable thermoremanent acquisition age for the characteristic magnetization component of the Beja gabbro between 335Ma and 315Ma, assuming cooling rates between 10C/Ma and 100C/Ma. These results, combined wtih paleomagnetic results from the Beja gabbro and Late Paleozoic rocks from Southern Portugal (Perroud et al., 1985), suggest that the southermost part of Spain and Portugal was separated from Northern Iberia in Early Carboniferous times and was accreted to Europe during the Late Carboniferous.

  15. 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.

  16. Geology and preliminary [sup 40]Ar/[sup 39]Ar geochronology of the Sliderock Mountain volcano, south-central Montana

    SciTech Connect

    Du Bray, E.A.; Harlan, S.S. )

    1993-04-01

    The Sliderock Mountain Volcano is a deeply eroded, Upper Cretaceous basaltic andesite stratovolcano complex located along the northeastern margin of the Laramide Beartooth uplift of south-central Montana. Historically, these rocks have been included in the Livingston Group and correlated with Upper Cretaceous, dominantly epiclastic sedimentary rocks of the Livingston Group in the Crazy Mountains Basin. Recent geologic mapping has identified several map units including: basaltic andesite of Derby Ridge (lava flows, and minor interbedded pyroclastic flows including welded tuff, block and ash flows, and lahars); volcaniclastic sedimentary rocks; lahar deposits; dioritic plutons and sills (including the diorite of Sliderock Mountain); basaltic andesite lavas; and basaltic trachyandesite dikes. Stratigraphic relations indicate that initial volcanic activity was dominated by eruption of the basaltic andesite of Derby Ridge. Cross-cutting relations indicate that dioritic plutons and sills are younger than the basaltic andesite of Derby Ridge and the lahars but age relations with the second set of basaltic andesite lavas are indeterminate. The volcanic and dike rocks of the stratovolcano are cpx-plag rocks, characterized by limited compositional variation, whereas intrusive rocks are hbl-plag rocks whose compositions are principally that of diorite, but range to granite. [sup 40]Ar/[sup 39]Ar isotopic analysis of hornblende from the Lodgepole intrusion, a hypabyssal intrusion that may constitute part of the volcano's solidified magma chamber, gives an age of 76.2 [+-] 0.3 Ma (1[sigma]), significantly younger than a previously reported K-Ar biotite age of 82 Ma. [sup 40]Ar/[sup 39]Ar analyses of sericite from weakly mineralized Au-bearing quartz veins hosted by the diorite of Sliderock Mountain give slightly younger isochron ages of 73--74 Ma indicating that gold mineralization is probably associated with the late stages of cooling of the Sliderock Mountain magma system.

  17. 40Ar/39Ar and U-Pb Ages and Isotopic Data for Oligocene Ignimbrites, Calderas, and Granitic Plutons, Southern Stillwater Range and Clan Alpine Mountains: Insights into the Volcanic-Plutonic Connection and Crustal Evolution in Western Nevada

    NASA Astrophysics Data System (ADS)

    John, D. A.; Watts, K. E.; Henry, C.; Colgan, J. P.; Cousens, B.

    2014-12-01

    Calderas in the southern Stillwater Range (SSR) and Clan Alpine Mountains (CAM) were formed during the mid-Tertiary ignimbrite flareup and subsequently tilted (40->90°) by large-magnitude extension. New geologic mapping, geochemistry, and 40Ar/39Ar and SHRIMP U-Pb zircon dating document 2 periods of magmatism resulting in 4 nested calderas and related granitoid plutons in sections up to 10 km thick. The first period included pre-caldera rhyolite lava domes (30(?) Ma), ~5 km of pre- and post-collapse intermediate lavas and rhyolite tuff that filled the Job Canyon caldera (~29.4 to 28.8 Ma), and the >4-5 km thick, geochemically similar IXL pluton (28.9±0.4 Ma) that intruded the Job Canyon caldera. The second period included pre-caldera rhyolite lava domes and dikes (~25.5 Ma) and 3 ignimbrite units in 3 calderas: tuff of the Louderback Mountains (low-silica rhyolite; ≥600 m thick; ~25.2 Ma); tuff of Poco Canyon (high-silica rhyolite; up to 4.3 km thick; 25.27±0.05 Ma); and ≥2000 km3 tuff of Elevenmile Canyon (trachydacite to rhyolite; up to 4.5 km thick; 25.12±0.01 Ma). The composite Freeman Creek pluton (granite, 24.8±0.4 Ma; granodiorite, 25.0±0.2 Ma) and Chalk Mountain rhyolite porphyry (25.2±0.2 Ma) and granite (24.8±0.3 Ma) plutons intruded the Poco Canyon and Elevenmile Canyon calderas. Early (30 Ma) rhyolites have the least radiogenic compositions (Sri~0.7040), whereas other units are relatively homogeneous (Sri~0.7050, ENd~0.0). Oxygen isotope compositions for SSR and CAM calderas are highly variable (d18Oquartz=5.6-8.2‰, d18Osanidine=5.5-7.0‰, d18Ozircon= 4.1-6.3‰), corresponding to a magmatic range of 5.7-7.9‰. U-Pb dating of zircons indicates homogeneous age populations and few/no xenocrysts and antecrysts. These data show that (1) thick plutons (>2-5 km) underlie compositionally and temporally related caldera-filling ignimbrites, (2) caldera-forming cycles are isotopically variable, requiring divergent magmatic sources in relatively

  18. The Berkeley Instrumental Neutron Generator (BINGE) for 40Ar/39Ar geochronology

    NASA Astrophysics Data System (ADS)

    Renne, P. R.; Becker, T. A.; Bernstein, L.; Firestone, R. B.; Kirsch, L.; Leung, K. N.; Rogers, A.; Van Bibber, K.; Waltz, C.

    2014-12-01

    The Berkeley Instrumental Neutron Generator (BINGE) facility is the product of a consortium involving the Berkeley Geochronology Center (BGC), the U.C. Berkeley Nuclear Engineering Dept. (UCB/NE), and Lawrence Berkeley (LBNL) and Lawrence Livermore (LLNL) National Labs. BINGE was initially designed (and funded by NSF) for 40Ar/39Ar geochronology. BINGE uses a plasma-based deuteron ion source and a self-loading Ti-surfaced target to induce deuteron-deuterium (DD) fusion via the reaction 2H(d,n)3He, producing 2.45 MeV neutrons. The limited neutron energy spectrum is aimed at reducing recoil effects, interfering nuclear reactions, and unwanted radioactive byproducts, all of which are undesirable consequences of conventional irradiation with 235U fission spectrum neutrons. Minimization of interfering reactions such as 40Ca(n,na)36Ar greatly reduces penalties for over-irradiation, enabling improved signal/background measurement of e.g. 39Ar. BINGE will also be used for a variety of nuclear physics and engineering experiments that require a high flux of monoenergetic neutrons. Neutron energies lower than 2.45 MeV can be obtained via irradiation ports within and external to polyethylene shielding. Initial commissioning produced a neutron flux of 108 n/sec/cm2 at 1 mA source current and 100 kV anode voltage, as expected. When scaled up to the 1 A source current as planned, this indicates that BINGE will achieve the design objective neutron flux of 1011 n/sec/cm2. Further progress towards this goal will be reported. Supported by NSF (grant #EAR-0960138), BGC, UCB/NE, University of California Office of the President, and DOE through LLNL under contract #DE-AC52-07NA27344 and LBNL under contract #DE-AC02-05CH11231.

  19. Paleomagnetic and 40Ar/39Ar geochronologic data from late Proterozoic mafic dikes and sills, Montana and Wyoming

    USGS Publications Warehouse

    Harlan, Stephen S.; Geissman, John William; Snee, Lawrence W.

    1997-01-01

    Paleomagnetic and 40Ar/39Ar results from mafic dikes and sills in northwestern Wyoming and western Montana yield similar virtual geomagnetic poles and isotopic dates. In combination with paleomagnetic and geochronologic data from elsewhere in the western Cordillera, these data provide evidence for a regional mafic magnetic event at 780 to 770 Ma that affected a large area of western North America.

  20. Geochemical and 40Ar/39Ar constraints on the evolution of volcanism in the Woodlark Rift, Papua New Guinea

    NASA Astrophysics Data System (ADS)

    Catalano, Joseph P.

    The tectonic mechanisms producing Pliocene to active volcanism in eastern Papua New Guinea (PNG) have been debated for decades. In order to assess mechanisms that produce volcanism in the Woodlark Rift, we evaluate the evolution of volcanism in eastern PNG using 40Ar/39Ar thermochronology and whole rock geochemistry. Active volcanism in southeastern Papua New Guinea occurs on the Papuan Peninsula (Mt. Lamington, Mt. Victory and Waiwa), in the Woodlark Rift (Dobu Island, SE Goodenough Island, and Western Fergusson Island), and in the Woodlark Basin. In the Woodlark Basin, seafloor spreading is active and decompression melting of the upper mantle is producing basaltic magmatism. However, the cause of Pliocene and younger volcanism in the Woodlark Rift is controversial. Two hypotheses for the tectonic setting have been proposed to explain Pliocene and younger volcanism in the Woodlark Rift: (1) southward subduction of Solomon Sea lithosphere beneath eastern PNG at the Trobriand Tough and (2) decompression melting of mantle, previously modified by subduction, as the lithosphere undergoes extension associated with the opening of the Woodlark Basin. A comparison of 40Ar/39Ar ages with high field strength element (HFSE) concentrations in primary magmas indicates that HFSE concentrations correlate with age in the Woodlark rift. These data support the hypothesis that Pliocene to active volcanism in the Woodlark Rise and D'Entrecasteaux Islands results from decompression melting of a relict mantle wedge. The subduction zone geochemical signatures (negative HFSE anomalies) in Woodlark Rift lavas younger than 4 m.y. are a relict from older subduction beneath eastern Papua, likely in the middle Miocene. As the lithosphere is extended ahead of the tip of the westward propagating seafloor spreading center in the Woodlark Basin, the composition of volcanism is inherited from prior arc magmatism (via flux melting) and through time evolves toward magmatism associated with a rifting

  1. The Detrital White Mica 40Ar/39Ar Record of the Katawaz Remnant Ocean Basin, Pakistan, and Tectonic Implications for the Himalayan Source Region

    NASA Astrophysics Data System (ADS)

    Zhuang, G.; Najman, Y.; Wijbrans, J. R.; Millar, I.; Carter, A.

    2014-12-01

    The Paleogene-Neogene sedimentary rocks in the Katawaz remnant ocean basin, Pakistan were thought to be a product of a fan-deltaic system, analogous to the modern Indus River and delta. A preliminary detrital zircon U-Pb study (Carter et al 2010) supported materials derived from the nascent western Himalaya and associated magmatic arc but that study was based on too few samples to fully characterize the whole series. Moreover, the chronology in the Katawaz basin was previously not well constrained, which impedes accurate comparison to other Himalaya foreland records. Here, we present a densely sampled study of detrital white mica 40Ar/39Ar. This study aims to: (1) constrain sedimentary ages of major lithostratigraphic units, (2) understand the exhumation history of the source region, and (3) reconstruct the paleodrainage system in NW Himalayan foreland. New 40Ar/39Ar data, together with a complementary study of detrital zircon U-Pb, constrain the sand-rich, fluvial-dominated Shaigalu Member to span from <34-36 Ma (basal sample) to <22 Ma (uppermost sample). The basal Shaigalu Member demonstrates similarity of ages of detrital zircon U-Pb and detrital white mica 40Ar/39Ar; both are characterized by a dominant peak of ca. 37 Ma. The dominant 37 Ma peak of detrital white mica 40Ar/39Ar ages has also been identified in the late Eocene Balakot Formation (Najman et al. 2001), the oldest terrestrial unit in the Himalayan peripheral foreland basin, and which is Himalayan-derived. We interpret the similarity in youngest age peak (37 Ma) between U-Pb and 40Ar/39Ar as a signal of rapid exhumation related to a rising western Himalaya. Our new 40Ar/39Ar data also reveal that sediment sources changed through time, as demonstrated by the disappearance of the 37 Ma population up-section and re-occurrence at the top. This could be related to either migration of the drainage system and/or changes in sediment sources. Finally, our study indicates that the latest Eocene rapid

  2. WA1ms: A ∼2.61 Ga muscovite standard for 40Ar/39Ar dating

    NASA Astrophysics Data System (ADS)

    Jourdan, Fred; Frew, Adam; Joly, Aurore; Mayers, Celia; Evans, Noreen J.

    2014-09-01

    The 40Ar/39Ar dating technique requires the use of neutron fluence monitors (standards) to allow calculation of the age of a sample. Precise calibration of these standards is crucial to obtaining accurate ages and decreasing the uncertainties associated with 40Ar/39Ar dates. Few fully intercalibrated 40Ar/39Ar standards with a sufficient total fusion grain-to-grain reproducibility are currently in use in the argon community. For Precambrian samples, only Hb3gr hornblende (∼1.08 Ga) yields sufficient grain-to-grain reproducibility and has an appropriate age for acceptable argon isotopic ratio measurements. Here, we present chemical and intercalibration results for a new ∼2.61 Ga standard. WA1ms is a muscovite extracted from an Archaean shear zone in the Lake Johnston greenstone belt, Western Australia. In situ trace element analysis by ELA-ICPMS revealed consistent K contents, subtle zonation and intra-grain and grain-to-grain heterogeneities in Rb, Sr, Ti, and Fe but a lack of mineral inclusions.WA1ms has been investigated over 3 irradiations ranging from 25 to 40 h, in two reactors, with several disc positions and three grains sizes and has been calibrated against FCs and GA1550, and Hb3gr. Overall, we carried out 48 total fusion and 4 step-heating experiments of WA1ms crystals. Flat age spectra and average F-value (40Ar∗/39ArK) relative standard deviations ranging from of 0.43% to 0.60% (P = 0.15-0.83) for 47/48 analyses demonstrate the reproducibility of WA1ms and its suitability as a reliable 40Ar/39Ar standard. We calculated R[WA1ms/FCs] = 205.59 ± 0.25, R[WA1ms/GA1550] = 57.25 ± 0.06 and R[WA1ms/Hb3gr] = 3.9713 ± 0.014 (all with P > 0.14) allowing direct comparison between WA1ms and any standards in used in the community, provided that they have been calibrated against any of the three standards used in the calibration and regardless of the age adopted for each of these standards. The recently revised decay constant values and standard ages proposed

  3. High resolution sup 40 Ar/ sup 39 Ar chronostratigraphy of the Late Cretaceous El Gallo Formation, Baja California del Norte, Mexico

    SciTech Connect

    Fulford, M.M.; Busby-Spera, C. ); Renne, P.R.

    1991-03-01

    Laser probe {sup 40}Ar/{sup 39}Ar analyses of individual sanidine grains from four tuffs in the alluvial Late Cretaceous (Campanian) El Gallo Formation yield statistically distinct mean dates ranging from 74.87 {plus minus} 0.05 Ma to 73.59 {plus minus} 0.09 Ma. The exceptional precision of these dates permits calculation of statistically significant sediment accumulation rates that are much higher than passive sediment loading would cause, implying rapid tectonically induced subsidence. The dates bracket tightly the age of important dinosaur and mammalian faunas previously reported from the El Gallo Formation. The dates support an age less than 73 Ma for the Campanian/Maastrichtian stage boundary, younger than indicated by several currently used time scales. Further application of the single grain {sup 40}Ar/{sup 39}Ar technique may be expected to greatly benefit stratigraphic studies of Mesozoic sedimentary basins and contribute to calibration of biostratigraphic and magnetostratigraphic time scales.

  4. Tectonic burial and exhumation cycles tracked by muscovite and K-feldspar 40Ar/39Ar thermochronology in a strike-slip fault zone, central Turkey

    NASA Astrophysics Data System (ADS)

    Idleman, Lauren; Cosca, Michael A.; Heizler, Matthew T.; Thomson, Stuart N.; Teyssier, Christian; Whitney, Donna L.

    2014-02-01

    Muscovite and K-feldspar 40Ar/39Ar ages from the eastern margin of the Niğde massif in central Anatolia track the timing of initial exhumation, reburial, and final exhumation and cooling of metamorphic rocks deformed within a strike-slip fault zone. Although the ages of initial and final cooling were known from previous studies, our new results document the timing of the reheating/reburial event. Muscovite from four of eight gneiss samples have Late Cretaceous 40Ar/39Ar ages that date initial cooling at ~ 75 Ma. The remaining samples have perturbed spectra that climb to Late Cretaceous ages with increasing extraction temperatures during analysis. These perturbed samples are located beneath a faulted unconformity overlain by Paleogene sedimentary deposits that were derived in part from the metamorphic rocks, then buried, metamorphosed, and deformed under greenschist facies conditions. Samples close to the faulted unconformity are more perturbed than structurally deeper samples. The age of the thermal perturbation is determined at 30 ± 5 Ma using multi-diffusion domain modeling of K-feldspar 40Ar/39Ar data from two gneiss samples, one located close to the unconformity and one at a structurally deeper level. Muscovite 40Ar/39Ar results and modeled K-feldspar temperature-time histories show that the eastern margin of the Niğde massif experienced a reheating event that peaked at ~ 30 Ma. The thermal pulse has been attributed to reburial associated with transpression in the Ecemiş segment of the Central Anatolian Fault Zone along the eastern margin of the Niğde massif. Activity of this fault zone may represent a far-field expression of the onset of collision of Arabia with Eurasia in SE Anatolia.

  5. Cerro Toledo Rhyolite, Jemez Volcanic Field, New Mexico: {sup 40}Ar/{sup 39}Ar geochronology of eruptions between two caldera-forming events

    SciTech Connect

    Spell, T.L. |; McDougall, I.; Doulgeris, A.P.

    1996-12-01

    The Cerro Toledo Rhyolite comprises a group of domes and tephra which were erupted during the interval between two caldera-forming ignimbrites, the Tshirege Member and Otowi Member of the Bandelier Tuff, in the Jemez Volcanic Field, New Mexico. To provide a chronologic framework for geochemical and isotopic studies on these rhyolites, which record the evolution of the Bandelier magma system during this interval, a {sup 40}Ar/{sup 39}Ar geochronology study was undertaken. Pumice from major pyroclastic fall deposits within the rhyolite tephra and samples from the rhyolite domes were dated as well as the stratigraphically bracketing Bandelier Tuff. The {sup 40}Ar/{sup 39}Ar ages for the two members of the Bandelier Tuff Yield and interval of 380{+-}20 k.y. between these caldera forming eruptions. During this interval nine major pyroclastic pumice units were deposited in the sections studies, for which six yield isochron ages, one a weighted mean age, one a maximum age, and one no reliable age due to lack of sanidine. {sup 40}Ar/{sup 39}Ar dates on pumice fall units within the Cerro Toledo Rhyolite tephra indicate that eruptive activity occurred at > 1.59, 1.54, 1.48, 1.37 and 1.22 Ma. {sup 40}Ar/{sup 39}Ar dating of Cerro Toledo Rhyolite domes indicates these were erupted within the caldera at 1.54, 1.45, 1.38-1.34, and 1.27 Ma. The dates obtained indicate that eruptive activity occurred throughout the 380 k.y. interval between the two members of the Bandelier Tuff, but suggest that eruptions producing both tephra and domes occurred during discrete intervals at ca. 1.54, 1.48 and 1.38-1.34 Ma. 43 refs., 5 figs., 3 tabs.

  6. Linking Late Pleistocene alpine glacial erosion and continental margin sedimentation: Insights from 40Ar/39Ar dating of silt-sized sediment, Canterbury Basin, New Zealand

    NASA Astrophysics Data System (ADS)

    Villaseñor, Tania; Jaeger, John M.; Foster, David A.

    2016-01-01

    Quaternary climatic and eustatic cycles in mid-latitude regions have led to more extensive alpine glaciations and continental shelf progradation, respectively. However, the glacial influence on sediment fluxes to the ocean creating continental margin strata is poorly documented. This contribution analyzes the provenance of fine sediment accumulating on the continental shelf during the Late Pleistocene to evaluate the influence of glacial cycles on sediment erosion and routing to the continental shelf. Taking advantage of the contrasting bedrock ages exposed across the Southern Alps, New Zealand, we perform 40Ar/39Ar incremental heating on the bulk silt-size sediment from three drill sites of IODP Expedition 317, Canterbury Basin, New Zealand. The results suggest that a large proportion of sediment accumulating on the continental shelf results from erosion within the Main Divide fault zone of the Southern Alps. Sediment 40Ar/39Ar age fluctuations over this time period suggest that bedrock with various 40Ar/39Ar cooling ages has been differentially eroded in the upper Waitaki River catchment and mixed in the Waitaki-Canterbury sediment-routing system. Across-shelf variations in sediment 40Ar/39Ar age reflect changing modes of sediment dispersal on the continental shelf. Fluvial material, likely derived from the main drainage divide zone, preferentially accumulates in the middle continental shelf, whereas material representing erosion of older bedrock (Torlesse Terrane), located lower in the drainage basin, is dispersed uniformly across the shelf. The age signature of the muddy sediment accumulating on the continental shelf reflects Late Pleistocene landscape evolution of the Southern Alps and its influence on sediment dispersal to the continental shelf.

  7. Direct dating of weathering phenomena by [sup 40]Ar/[sup 39]Ar and K-Ar analysis of supergene K-Mn oxides

    SciTech Connect

    Vasconcelos, P.M.; Brimhall, G.H. ); Renne, P.R.; Becker, T.A. )

    1994-03-01

    Potassium-bearing manganese oxides, cryptomelane, K[sub 1-2](Mn[sup 3+]Mn[sup 4+])[sub 8] O[sub 16] [center dot] xH[sub 2]O, and hollandite, (K,Ba)[sub 1-2](Mn[sup 3+],Mn[sup 4+])[sub 8] O[sub 16] [center dot] xH[sub 2]O, are often authigenically precipitated in weathering profiles. Dating of these phases allows timing of the progression of oxidation fronts during weathering and pedogenic processes. Potential problems in manganese oxide dating, such as Ar and/or K losses, excess argon, [sup 39]Ar loss by recoil during neutron irradiation, etc. are addressed. The K-Ar and [sup 40]Ar/[sup 39]Ar analytical results indicate that Ar and/or K losses, excess [sup 40]Ar, and [sup 39]Ar recoil seem not to pose problems in manganese oxide dating. This investigation suggests that the fine scale, laser-probe [sup 40]Ar/[sup 39]Ar technique is most appropriate for dating of weathering phenomena because this technique permits identification of contaminating phases and the presence of multiple generations of weathering minerals in the inherently complex mineral assemblage characteristic of weathering profiles. K-Ar and [sup 40]Ar/[sup 39]Ar dating of supergene K-bearing manganese oxides formed during lateritization of Archean and Proterozoic bedrocks in the Carajas Region, Amazonia, Brazil, indicates that weathering started before 72 [+-] 6 Ma. Petrographic, electron microscope, and electron microprobe investigation reveal multiple generations of manganese oxide precipitation. Age clusters at 65-69, 51-56, 40-43, 33-35, 20, 24, 12-17 Ma, and zero-age (0.2 [+-] 0.2 Ma) suggest episodic precipitation of K-Mn oxides resulting form changing weathering conditions in the Amazon throughout the Cenozoic. K-Ar and [sup 40]Ar/[sup 39]Ar dating of supergene cryptomelane from weathering profiles in eastern Minas Gerais, southeastern Brazil, suggests continuous weathering from 10 to 5.6 Ma ago, possibly reflecting local climatic conditions due to the proximity with the Atlantic Ocean.

  8. Dating transitionally magnetized lavas of the late Matuyama Chron: Toward a new 40Ar/39Ar timescale of reversals and events

    NASA Astrophysics Data System (ADS)

    Singer, Bradley S.; Hoffman, Kenneth A.; Chauvin, Annick; Coe, Robert S.; Pringle, Malcolm S.

    1999-01-01

    The K-Ar based geomagnetic polarity timescale was constructed using data from lavas and tuffs that bracketed, but rarely dated, the transitions between polarity intervals. Subsequent 40Ar/39Ar dating indicated that the ages of some polarity transitions had been underestimated by about 6%. Although the accepted ages of the polarity chron boundaries have increased, their precise temporal definition remained uncertain. We have taken a different approach and used incremental-heating techniques to obtain 18 new 40Ar/39Ar ages from basaltic lavas within flow sequences at Punaruu Valley, Tahiti, and Haleakala volcano, Hawaii. These lavas record transitional paleomagnetic directions corresponding to four mid-Pleistocene polarity reversals or events. Three lavas from Punaruu Valley previously thought to record the Cobb Mountain Normal Polarity Subchron (CMNS) gave a mean age of 1.105+/-0.005Ma, indicating that they were erupted about 76 kyr after the CMNS; this period of transitional field behavior is designated the Punaruu event. In addition, seven new 40Ar/39Ar ages from the Punaruu Valley indicate that the Jaramillo Normal Polarity Subchron (JNS) lasted about 67 kyr, starting at 1.053+/-0.006Ma and ending 0.986+/-0.005Ma. This agrees with astronomical estimates but conflicts with JNS ages proposed by Spell and McDougall [1992] and Izett and Obradovich [1994] on the basis of 40Ar/39Ar dating of rhyolite domes in the Valles Caldera. Indistinguishable 40Ar/39Ar ages of seven lavas, including one from Punaruu Valley and six from Haleakala that record broadly similar intermediate paleodirections, suggest that the Kamikatsura event occurred at 0.886+/-0.003Ma. Moreover, these data indicate that the Kamikatsura event occurred 20-40 kyr after another geomagnetic event, most probably taking place at 0.92 Ma. We designate this earlier field behavior the Santa Rosa event, adopting its name from that of a transitionally magnetized rhyolite dome which happened to figure prominently

  9. 40Ar/39Ar dating of basaltic dykes swarm in Western Cameroon: Evidence of Late Paleozoic and Mesozoic magmatism in the corridor of the Cameroon Line

    NASA Astrophysics Data System (ADS)

    Tchouankoue, Jean Pierre; Simeni Wambo, Nicole Armelle; Kagou Dongmo, Armand; Li, Xian-Hua

    2014-05-01

    40Ar/39Ar ages of three basalt dykes that intrude the Precambrian basement in the southern continental part of the Cretaceous Cameroon Line are presented. Specimen were sampled at Dschang, Maham and Kendem (Cameroon). The ages obtained are 421.3 ± 3.5 Ma (Dschang), 404.22 ± 3.51 Ma (Maham), and 192.10 ± 7.45 Ma (Kendem). The Dschang and Maham samples yield a relatively undisturbed spectrum while the Kendem sample shows an excess of argon but with plateau ages in the frame of the Mesozoic. Plateau ages at Dschang, Maham and Kendem represent more than 80% of the total 39Ar released and are interpreted as emplacement ages. 40Ar/39Ar dating results confirm Devonian and Jurassic K/Ar ages obtained from similar dykes of the same region. Geochemically, the basalt dykes are subalkaline in composition with 45-50 wt.% SiO2. Incompatible trace elements and rare earth elements are lower than that of the Cameroon Line basalts. Overall geochemical characteristics of the basalt dykes much more closely resemble those of tholeiites of the Benue Through in Nigeria that are interpreted as related to the opening of the Atlantic Ocean. The combination of 40Ar/39Ar ages, major, trace and rare earth elements geochemistry data demonstrate a magmatic phase that is significantly older and different of that of the Cretaceous Cameroon Line and younger than the dominantly granitic Neoproterozoic to early Paleozoic magmatism in the region. These findings offer new clues for a better understanding of the tectonic history of the region, particularly the origin of the Cameroon Line and Africa-South America pre-drift reconstitutions.

  10. 40Ar/39Ar dates from alkaline intrusions of the northern Crazy Mountains, south-central Montana

    NASA Astrophysics Data System (ADS)

    Harlan, S. S.

    2005-05-01

    The Crazy Mountains basin of south-central Montana is a complex foreland basin that formed during the interaction of thin-skinned, decollement-style folds of the Montana thrust belt and the basement-involved folds and thrust faults of the Rocky Mountain foreland province. Near the depositional center of the basin, synorogenic strata of the Paleocene Fort Union Formation have been intruded and locally thermally metamorphosed by strongly alkaline to subalkaline Tertiary intrusive rocks. The subalkaline rocks are found mostly in the southern Crazy Mountains and form stocks (Big Timber stock, Loco Mountain stock), radiating dikes and sills. With the exception of the Ibex Mountain sill (?), the alkaline rocks are restricted to the northern Crazy Mountains. New 40Ar/39Ar dates are reported from the strongly alkaline rocks, including the Comb Creek stock and dike swarm, the Ibex Mountain sill(?), and sills from the Robinson anticline intrusive complex. The alkaline rocks of the Robinson anticline intrusive complex are exposed in the easternmost folds of the Cordilleran fold and thrust belt, but despite their arcuate and apparently folded map geometry they have been shown to post-date folding. Hornblende from a trachyte sill in the Robinson anticline intrusive complex yielded a relatively simple age spectrum with a weighted mean of 50.61 ± 0.14 Ma (2σ), which probably records the age of sill emplacement. Nepheline syenite and mafic nepheline syenites of the Comb Creek stock and a dike from its radial dike swarm, two sills from the Robinson antlicline intrusive complex, and the Ibex Mountains sill(?) gave biotite plateau dates ranging from 50.03 to 50.22 Ma, with 2σ errors of ± 0.11 to 0.19 Ma. Because these dates are from fairly small, hypabyssal intrusions, they must have cooled quickly and thus these dates closely approximate the emplacement age of the intrusions. These data indicate that the strongly alkaline intrusions were emplaced during a fairly restricted

  11. Laser /39/Ar-/40/Ar dating of two clasts from consortium breccia 73215

    NASA Technical Reports Server (NTRS)

    Eichhorn, G.; Schaeffer, O. A.; James, O. B.; Mueller, H. W.

    1978-01-01

    A laser Ar-39-Ar-40 study of the components of an ANT-suite anorthositic gabbro and a black aphanite from a consortium breccia is reported. A wide range of K-Ar ages is found for the plagioclase in the anorthositic gabbro; at the centers of the largest grains is material showing the greatest age (older than 4.11 billion years) while the youngest material (3.81-3.88 billion years) is found near the grain margins. Partial outgassing of the clasts upon incorporation into the breccia could account for the age patterns. The black aphanite clast appears to be cogenetic with the aphanite that forms the breccia matrix. The time of crystallization of a lunar granite has also been measured by the laser technique.

  12. The Thermo Scientific HELIX-SFT noble gas mass spectrometer: (preliminary) performance for 40Ar/39Ar geochronology

    NASA Astrophysics Data System (ADS)

    Barfod, D. N.; Mark, D. F.; Morgan, L. E.; Tomkinson, T.; Stuart, F.; Imlach, J.; Hamilton, D.

    2011-12-01

    The Thermo Scientific HELIX-platform Split Flight Tube (HELIX-SFT) noble gas mass spectrometer is specifically designed for simultaneous collection of helium isotopes. The high mass spur houses a switchable 1011 - 1012 Ω resistor Faraday cup and the low mass spur a digital pulse-counting secondary electron multiplier (SEM). We have acquired the HELIX-SFT with the specific intention to measure argon isotopes for 40Ar/39Ar geochronology. This contribution will discuss preliminary performance (resolution, reproducibility, precision etc.) with respect to measuring argon isotope ratios for 40Ar/39Ar dating of geological materials. We anticipate the greatest impact for 40Ar/39Ar dating will be increased accuracy and precision, especially as we approach the techniques younger limit. Working with Thermo Scientific we have subtly modified the source, alpha and collector slits of the HELIX-SFT mass spectrometer to improve its resolution for resolving isobaric interferences at masses 36 to 40. The enhanced performance will allow for accurate and precise measurement of argon isotopes. Preliminary investigations show that we can obtain a valley resolution of >700 and >1300 (compared to standard HELIX-SFT specifications of >400 and >700) for the high and low mass spurs, respectively. The improvement allows for full resolution of hydrocarbons (C3+) at masses 37 - 40 and almost full resolution at mass 36. The HELIX-SFT will collect data in dual collection mode with 40Ar+ ion beams measured using the switchable 1011 - 1012 Ω resistor Faraday cup and 39Ar through 36Ar measured using the SEM. The HELIX-SFT requires Faraday-SEM inter-calibration but negates the necessity to inter-calibrate multiple electron multipliers. We will further present preliminary data from the dating of mineral standards: Alder Creek sanidine, Fish Canyon sanidine and Mount Dromedary biotite (GA1550).

  13. Joint determination of 40K decay constants and 40Ar∗/ 40K for the Fish Canyon sanidine standard, and improved accuracy for 40Ar/ 39Ar geochronology

    NASA Astrophysics Data System (ADS)

    Renne, Paul R.; Mundil, Roland; Balco, Greg; Min, Kyoungwon; Ludwig, Kenneth R.

    2010-09-01

    40Ar/ 39Ar and K-Ar geochronology have long suffered from large systematic errors arising from imprecise K and Ar isotopic data for standards and imprecisely determined decay constants for the branched decay of 40K by electron capture and β - emission. This study presents a statistical optimization approach allowing constraints from 40K activity data, K-Ar isotopic data, and pairs of 238U- 206Pb and 40Ar/ 39Ar data for rigorously selected rocks to be used as inputs for estimating the partial decay constants ( λ ɛ and λ β) of 40K and the 40Ar∗/ 40K ratio ( κFCs) of the widely used Fish Canyon sanidine (FCs) standard. This yields values of κFCs = (1.6418 ± 0.0045) × 10 -3, λ ɛ = (0.5755 ± 0.0016) × 10 -10 a -1 and λ β = (4.9737 ± 0.0093) × 10 -10 a -1. These results improve uncertainties in the decay constants by a factor of >4 relative to values derived from activity data alone. Uncertainties in these variables determined by our approach are moderately to highly correlated (cov( κFCs, λ ɛ) = 7.1889 × 10 -19, cov( κFCs, λ β) = -7.1390 × 10 -19, cov( λ ɛ, λ β) = -3.4497 × 10 -26) and one must take account of the covariances in error propagation by either linear or Monte Carlo methods. 40Ar/ 39Ar age errors estimated from these results are significantly reduced relative to previous calibrations. Also, age errors are smaller for a comparable level of isotopic measurement precision than those produced by the 238U/ 206Pb system, because the 40Ar/ 39Ar system is now jointly calibrated by both the 40K and 238U decay constants, and because λ ɛ( 40K) < λ( 238U). Based on this new calibration, the age of the widely used Fish Canyon sanidine standard is 28.305 ± 0.036 Ma. The increased accuracy of 40Ar/ 39Ar ages is now adequate to provide meaningful validation of high-precision U/Pb or astronomical tuning ages in cases where closed system behavior of K and Ar can be established.

  14. Late-stage volcano geomorphic evolution of the Pleistocene San Francisco Mountain, Arizona (USA), based on high-resolution DEM analysis and 40Ar/39Ar chronology

    NASA Astrophysics Data System (ADS)

    Karátson, Dávid; Telbisz, Tamás; Singer, Brad S.

    2010-09-01

    The cone-building volcanic activity and subsequent erosion of San Francisco Mountain, AZ, USA, were studied by using high-resolution digital elevation model (DEM) analysis and new 40Ar/39Ar dating. By defining remnants or planèzes of the volcano flanks in DEM-derived images, the original edifice can be reconstructed. We propose a two-cone model with adjacent summit vents which were active in different times. The reconstructed cones were 4,460 and 4,350 m high a.s.l., corresponding to ˜2,160 and 2,050 m relative height, respectively. New 40Ar/39Ar data allow us to decipher the chronological details of the cone-building activity. We dated the Older and Younger Andesites of the volcano that, according to previous mapping, built the stage 2 and stage 3 stratocones, respectively. The new 40Ar/39Ar plateau ages yielded 589-556 ka for the Older and 514-505 ka for the Younger Andesites, supporting their distinct nature with a possible dormant period between. The obtained ages imply an intense final (≤100 ka long) cone-building activity, terminating ˜100 ka earlier than indicated by previous K-Ar ages. Moreover, 40Ar/39Ar dating constrains the formation of the Inner Basin, an elliptical depression in the center of the volcano initially created by flank collapse. A 530 ka age (with a ±58.4 ka 2σ error) for a post-depression dacite suggests that the collapse event is geochronologically indistinguishable from the termination of the andesitic cone-building activity. According to our DEM analysis, the original cone of San Francisco Mountain had a volume of about 80 km3. Of this volume, ˜7.5 km3 was removed by the flank collapse and subsequent glacial erosion, creating the present-day enlarged Inner Basin, and ˜2 km3 was removed from the outer valleys by erosion. Based on volumetric analysis and previous and new radiometric ages, the average long-term eruption rate of San Francisco Mountain was ˜0.2 km3/ka, which is a medium rate for long-lived stratovolcanoes. However

  15. Coherence of the Dabie Shan UHPM terrane investigated by Lu-Hf and 40Ar/39Ar dating of eclogites

    NASA Astrophysics Data System (ADS)

    Brouwer, F. M.; Groen, M.; Nebel, O.; Wijbrans, J. R.; Qiu, H.

    2009-12-01

    The Central China Orogenic Belt is the largest known ultrahigh-pressure metamorphic (UHPM) belt. Currently exposed UHP metamorphic rocks reflect subduction of massive swathes of continental crust to depths exceeding 100 km. Subsequent uplift exposed the voluminous sequence more or less intact. Deciphering responsible exhumation processes requires well constrained P-T-time paths. Most workers accept Triassic (~240 and 220-200 Ma) peak UHP metamorphism on the basis of zircon U-Pb ages in Central and Eastern Dabie Shan, while Western Dabie Shan, Qinling, North Qaidam and Altyn Tagh exhibit Ordovician (420-500 Ma) UHPM. However, contrasting reports of Carboniferous and Ordovian UHPM in Eastern Dabie Shan (Jian et al. 2001; Qiu & Wijbrans, 2006, 2008), and Ordovician, Carboniferous and Triassic (U)HPM in Western Dabie Shan (Wu et al. 2009) question this simple East-West gradient. Here, we investigate PTt-paths for localities throughout Dabie Shan to determine how far west the Triassic UHP event is documented, and how far east the Carboniferous and Ordovician events can be traced. Based on this distribution we aim to establish whether the Dabie Shan terrane is an amalgam of blocks that underwent UHPM at different times rather than a single coherent terrane. Eclogite samples are investigated for thermobarometry, Lu-Hf Grt-Cpx geochronology, and 40Ar/39Ar thermochronology. For fresh eclogites Thermocalc was used to establish equilibration conditions of the UHPM assemblage, Grt and Cpx of which were subsequently used for Lu-Hf isotope analysis. In addition, retrogressed eclogites, two fresh eclogites and two orthogneisses were analysed for 40Ar/39Ar isotope distributions in Phg, Bt, Amp and Kfs. Four fresh eclogites, all collected at reported UHP-localities confirm established PT-estimates for peak-metamorphism above the Coe-in reaction at 450-680 °C, with higher T for eastern Dabie. In one sample this is confirmed by the presence of a Coe inclusion in Cpx. Lu-Hf Grt

  16. Evolution of the Southwest Indian continental divergent margin: Constraints from 40Ar-39Ar dating of lateritic paleolandsurfaces

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

    The western continental passive margin of Peninsular India is marked by the Western Ghats escarpment, which separates a coastal lowland from an East-dipping highland plateau and is carved both into the 63-Ma old Deccan traps and their Archean basement. Previous studies suggested establishment of the escarpment by differential erosion across an elevated rift shoulder, and thermochronologic models predicted escarpment formation from higher denudation in the coastal lowland than on the plateau until ~ 50 Ma. We provided complementary time constraints on the evolution of the passive margin by 40Ar-39Ar dating of supergene K-Mn oxides (cryptomelane) sampled in lateritic formations exposed on paleosurfaces, which are preserved as relicts on both sides of the escarpment. Three main lateritic paleosurfaces were identified in the highland at altitude ranges of 1200-1000 m (S1), 1000-900 m (S2) and 850-600 m (S3), and a lower paleosurface in the lowland at 150-50 m (S4). All the 40Ar-39Ar ages obtained on either side of the escarpment document major weathering periods for each paleosurface: 53 to 45 Ma (S1-S4) synchronously with the bauxitic weathering, 40 to 32 Ma (S2), 30 to 23 Ma (S3), and 24 to 19 Ma (S4). These ages indicate that most of the incision and dissection of plateau landsurfaces S1, S2, and S3 must therefore have taken place after 45, 32 and 23 Ma respectively, while the coastal lowland surface S4 was incised after 19 Ma. Preservation of laterites as old as 47 Ma in the coastal lowland implies that the escarpment already existed in the Mid-Eocene while intense bauxitic weathering was taking place on both sides of the escarpment. The ages obtained in the lowland are also indicative of limited erosion (~ 4 m Ma-1) at the foot of the escarpment since 45 Ma, and particularly low incipient incision of the lowland (~ 5 m Ma-1) since 19 Ma. Ages obtained on the highland plateau indicate further Neogene denudation inland but at less than 15 m Ma-1 since 45 Ma, and

  17. 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

  18. 40Ar/39Ar dating of the Honghuaqiao Formation in SE China

    NASA Astrophysics Data System (ADS)

    Chang, S.; Zhang, H.; Hemming, S. R.; Mesko, G. T.; Fang, Y.

    2010-12-01

    The Jehol Biota, defined as the characteristic Eosestheria-Ephemeropsis-Lycoptera assemblage (Grabau, 1923, Bulletin of the Geological Survey of China), is widely distributed in eastern and central Asia (Li et al., 1982, Acta Geologica Sinica; Chen, 1988, Acta Palaeontologica Sinica). Abundant and varied fossils of the terrestrial Jehol Biota, including plants, insects, dinosaurs, birds, mammals, and freshwater invertebrates, have been discovered from the Dabeigou, the Yixian and the Jiufotang Formations (or their correlative strata) in northeast China from the Liaoning and Hebei Provinces and Inner Mongolia (Chen and Jin, 1999, Acta Palaeontologica Sinica). In addition, strata that may be correlative with the classic Jehol fossil-bearing formations have been identified extensively in central and eastern China, the Korean Peninsula, Mongolia, and Siberia. In the past three decades mollusk, conchostracan, ostracod, insect, fish, and plant fossils from localities in southeastern China, interpreted as related to the Jehol biota of the northeast, have been found (Mateer and Chen, 1986, Cretaceous Research; Li, 2003, Chinese Science Bulletin; Chen, Li and Batten, 2007, Geological Journal). However, a detailed correlation between the classic Jehol outcrops and the more recently found localities to the South and West has yet to emerge. Volcanic rocks from the Honghuaqiao fossil-bearing Formation in Tuzhou City of eastern Anhui Province, southeastern China provide an excellent opportunity to rectify this situation. Preliminary results of a pilot study suggest that the Honghuaqiao Formation is equivalent to the Longwanshan Formation of Anhui Province, southeastern China and the Yixian Formation, northeastern China (Chang et al., 2009, AGU abstract). Initial 40Ar/39Ar results indicate that conchostracans from the upper Honghuaqiao Formation are approximately 130 Ma. Our ongoing work aims to establish a high-resolution chronostratigraphy for Tuzhou City in Anhui Province

  19. Dating low-temperature deformation by 40Ar/39Ar on white mica, insights from the Argentera-Mercantour Massif (SW Alps)

    NASA Astrophysics Data System (ADS)

    Sanchez, Guillaume; Rolland, Yann; Schneider, Julie; Corsini, Michel; Oliot, Emilien; Goncalves, Philippe; Verati, Chrystèle; Lardeaux, Jean-Marc; Marquer, Didier

    2011-07-01

    In order to date low-temperature deformation, intensely strained muscovite porphyroclasts and neocrystallized shear band phengite from greenschist-facies shear zones have been dated by 40Ar/39Ar method in the Argentera-Mercantour massif. Shear zones are featured by gradual mylonitization of a Variscan granite, gneiss and Permian pelite protolith (300-315 Ma) during the Alpine orogenic event. Mineralogical and textural observations indicate that phengites and chlorites developed from biotite and plagioclase in fluid system during deformation following dissolution-transport-precipitation reactions of the type biotite + plagioclase + aqueous fluid = chlorite + albite + phengite + quartz + titanite + K-bearing fluid in the granite-gneiss mylonite. Contrariwise, phengite developed at the expense of clays following substitution reaction in pelite mylonite. Based on conventional thermobarometry on phengite and chlorite and Pressure-Temperature-aqueous fluid (P-T-MH2O) pseudosections calculated with shear zone bulk compositions, the conditions during shear deformation were estimated at 375 ± 30 °C and 4.8-7 ± 1 kbar in an H2O-satured system. In this low temperature environment, 40Ar/39Ar analysis of the Variscan muscovite for various grades of ductile strain intensity shows a limited 40Ar/39Ar isotopic resetting, all ages scattering between 296 and 315 Ma. Under conditions of intense ductile deformation and large-scale fluid circulation, muscovite grains formed during the Variscan retain their much older ages. 40Ar/39Ar dating of very fine grained synkinematic phengite grains, neoformed during the Alpine history, give consistent plateau ages (34-20 Ma) for each shear zone. In detail, 40Ar excess can be detected in the pelite mylonitic sample where phengites crystallized by substitution process while the other mylonitic samples where phengites grow from fluid-induced reactions do not evidence any 40Ar excess. These results demonstrate that the 40Ar/39Ar dating of

  20. Preservation of Sub-Microscopic Scale Structural Relics in Biotite: Implications for 40AR/39AR Geochronology

    NASA Astrophysics Data System (ADS)

    Beltrando, M.; Di Vincenzo, G.; Ferraris, C.

    2012-12-01

    Relic crystals of magmatic biotite that undewent eclogite facies metamorphism were investigated by laser step-heating and laser in-situ 40Ar/39Ar techniques, in conjunction with SEM, EMP and TEM analyses, to elucidate the influence of recrystallization on K-Ar systematics. Samples were collected from meta-granitoids of the Gran Paradiso Massif (Western Alps), emplaced at upper crustal depths (P~0.25-0.35 GPa) in the Permian and metamorphosed at P>2.0 GPa and T~500-600°C in the Middle-Late Eocene. Two different samples, which underwent different degrees of Alpine re-equilibration, were examined. Sample JT1007 still preserves the original cm-sized magmatic quartz, k-feldspar and brown biotite (Bt 1), rarely rimmed by small green biotite crystals (Bt 2) related to Alpine metamorphism. Sample JT1008, instead, is characterized by a larger degree of Alpine re-equilibration, with static re-crystallization of the original magmatic quartz into aggregates of fine-grained polygonal crystals and common green biotite rims around brown Bt 1. Compositional profiling across Bt 1 in sample JT1007 reveals a progressive decrease in Ti content towards the crystal rims, which overlap with the composition of Bt 2. Compositional profiling across Bt 1 in sample JT1008, instead, reveals a homogeneous Ti content across the grain. This compositional pattern is mirrored by TEM investigation at the sub-micron scale, revealing a systematic zoning in Bt 1 from sample JT1007, with high-temperature magmatic politypes, surrounded by 1M politypes, still preserved in crystal cores. Crystal rims, instead, are characterized by 1M politypes only. Biotites from JT1008, instead, are homogeneous, solely consisting of 1M politypes. Infrared laser step-heating experiments on mineral separates from the two samples gave contrasting results: JT1008 yielded a flat spectrum, for more than 90% of the released 39ArK, with an error-weighted mean age of 36.5±0.3 Ma; JT1007 yielded a slightly saddle-shaped profile

  1. Structural and Temporal Requirements for Geomagnetic Field Reversal Deduced From 40Ar/39Ar Dated Lava Flows

    NASA Astrophysics Data System (ADS)

    Singer, B. S.; Hoffman, K. A.; Coe, R. S.; Brown, L. L.; Jicha, B. R.; Pringle, M. S.; Chauvin, A.

    2004-12-01

    40Ar/39Ar dating of lavas on Tahiti, long thought to record the primary part of the Matuyama-Bruhnes (M-B) reversa1, gives an age of 795+/- 7 ka, indistinguishable from that of transitional lavas in Chile and La Palma, but older than the accepted age for the reversal. Only the transitional lavas on Maui and one from La Palma (dated at 776 +/- 2 ka), agree with the astronomical age for the M-B reversal. Virtual geomagnetic poles (VGPs) associated with the Tahitian and Chilean lavas cluster near Australia, as do VGPs recorded on Tahiti during the Big Lost and Punaruu events, two apparently unsuccessful reversals. These findings, suggestive of a recurring, mantle-held flux pattern at the outer core surface during reversal attempts, are also theoretically equivalent to the situation that would arise today if the axial dipole were to continue to weaken and vanish. Hence, we propose that the 795 ka lavas record the onset of a dynamo process--one which only on occasion would result in polarity change. This initial instability, associated with the first of two decreases in field intensity, began 18 kyrs prior to the actual polarity switch. These data may provide the first observational support to the claim that complete reversals require a significant interval of time for magnetic flux to escape from the solid inner core and sufficiently weaken its stabilizing effect.

  2. The sup 40 Ar/ sup 39 Ar thermochronology of the eastern Mojave Desert, California, and adjacent western Arizona with implications for the evolution of metamorphic core complexes

    SciTech Connect

    Foster, D.A.; Harrison, T.M. ); Miller, C.F. ); Howard, K.A. )

    1990-11-10

    The application of {sup 40}Ar/{sup 39}Ar thermochronology provides information about the timing and nature of thrusting, plutonism, metamorphism, denudation, and detachment faulting. The {sup 40}Ar/{sup 39}Ar ages of 175 to 125 Ma from the Clipper, Piute, Turtle, Mohave, Bill Williams, and Hualapai Mountains are interpreted to be the result of a middle Mesozoic thermal event(s) caused by crustal thickening. The {sup 40}Ar/{sup 39}Ar data from the Clipper and Piute Mountains suggest that this thermal event was followed by a period of cooling at rates of 1-5C/m.y. Orogenesis culminated during the Late Cretaceous when rocks exposed in the Old Woman-Piute, Chemehuevi, and Sacramento Mountains attained temperatures >500C which reset the K-Ar systems of minerals from Proterozoic rocks. High-grade metamorphism in the Old Woman Mountains area was caused by the intrusion of the Old Woman-Piute batholith at 73 {plus minus} 1 Ma. Cooling rates following batholith emplacement in the Old Woman Mountains were {approximately}100C/m.y. between 73 and 70 Ma and 5-10C/m.y. from 70 to {approximately}30 Ma. By 30 Ma, rocks exposed in the Old Woman-Piute, Marble, Ship, Clipper, and Turtle Mountains were below {approximately}100C. The {sup 49}Ar/{sup 39}Ar ages from the Sacramento Mountains suggest that mylonization caused by the onset of regional extension occurred at 23 {plus minus} 1 Ma. When extension started in the Chemehuevi Mountains, rocks exposed in the southwestern and northeastern portions of footwall to the Chemehuevi detachment fault were at {approximately}180C and {approximately}350C, respectively. Unroofing of the footwalls to detachment faults in the Sacramento and Chemehuevi Mountains resulted in average cooling rates of 10-50C/m.y. between 22 and 15 Ma.

  3. Single-crystal sup 40 Ar/ sup 39 Ar dating of the Eocene-Oligocene transition in North America

    SciTech Connect

    Swisher, C.C. III ); Prothero, D.R. )

    1990-08-17

    Explanations for the causes of climatic changes and associated faunal and floral extinctions at the close of the Eocene Epoch have long been controversial because of, in part, uncertainties in correlation and dating of global events. New single-crystal laser fusion (SCLF) {sup 40}Ar/{sup 39}Ar dates on tephra from key magnetostratigraphic and fossil-bearing sections necessitate significant revision in North American late Paleogene chronology. The Chadronian-Orellan North American Land Mammal Age boundary, as a result, is shifted from 32.4 to 34.0 Ma (million years ago), the Orellan-Whitneyan boundary is shifted from 30.8 to 32.0 Ma, and the Whitneyan-Arikareean boundary is now approximately 29.0 Ma. The new dates shift the correlation of Chron C12R from the Chadronian to within the Orellan-Whitneyan interval, the Chadronian becomes late Eocene in age, and the North American Oligocene is restricted to the Orellan, Whitneyan, and early Arikareean. The Eocene-Oligocene boundary, and its associated climate change and extinction events, as a result, correlates with the Chadronian-Orellan boundary, not the Duchesnean-Chadronian boundary. 30 refs., 1 fig., 1 tab.

  4. 40Ar/39Ar geochronology and geochemical reconnaissance of the Eocene Lowland Creek volcanic field, west-central Montana

    USGS Publications Warehouse

    Dudas, F.O.; Ispolatov, V.O.; Harlan, S.S.; Snee, L.W.

    2010-01-01

    We report geochronological and geochemical data for the calc-alkalic Lowland Creek volcanic field (LCVF) in westcentral Montana. 40Ar/ 39Ar age determinations show that the LCVF was active from 52.9 to 48.6 Ma, with tuff-forming eruptions at 52.9 ?? 0.14 and 51.8 ?? 0.14 Ma. These dates span the age range of vigorous Eocene igneous activity in the Kamloops-Absaroka-Challis belt. The LCVF evolved upward from basal rhyolites (SiO 2>71 wt%) to dacites and andesites (SiO 2 > 62 wt%). Compositional change parallels a transition from early explosive volcanism to late effusive activity. Four geochemical components can be detected in the rocks. A component with 206Pb/204Pb < 16.5 and epsilon;Nd near-15 is predominant in anhydrous, two-pyroxene dacites; hydrous rhyolites, rhyodacites, and dacites with epsilon;Nd below-10 are dominated by a second component; hydrous rocks with 206Pb/ 204Pb > 18.3 and epsilon;Nd>-9 contain a third component; and an andesite with low Nd content and epsilon;Nd near-9 probably contains a fourth component. The first three components probably derive from the lower and middle crust, whereas the fourth is probably from the lithospheric mantle. ?? 2010 by The University of Chicago.

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

    PubMed

    Swisher, C C; Prothero, D R

    1990-08-17

    Explanations for the causes of climatic changes and associated faunal and floral extinctions at the close of the Eocene Epoch have long been controversial because of, in part, uncertainties in correlation and dating of global events. New single-crystal laser fusion (SCLF) (40)Ar/(39)Ar dates on tephra from key magnetostratigraphic and fossilbearing sections necessitate significant revision in North American late Paleogene chronology. The Chadronian-Orellan North American Land Mammal "Age" boundary, as a result, is shifted from 32.4 to 34.0 Ma (million years ago), the Orellan-Whitneyan boundary is shifted from 30.8 to 32.0 Ma, and the Whitneyan-Arikareean boundary is now approximately 29.0 Ma. The new dates shift the correlation of Chron C12R from the Chadronian to within the Orellan-Whitneyan interval, the Chadronian becomes late Eocene in age, and the North American Oligocene is restricted to the Orellan, Whitneyan, and early Arikareean. The Eocene-Oligocene boundary, and its associated climate change and extinction events, as a result, correlates with the Chadronian-Orellan boundary, not the Duchesnean-Chadronian boundary. PMID:17756788

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

    NASA Astrophysics Data System (ADS)

    Swisher, Carl C., III; Prothero, D. R.

    1990-08-01

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

  7. 40Ar/39Ar and K/Ar dating of low grade metamorphism: examples on metabasites from Central Chile

    NASA Astrophysics Data System (ADS)

    Aguirre, L.; Feraud, G.; Fuentes, F.; Delbar, M.; Morata, D.

    2003-04-01

    Dating low to very low-grade burial metamorphic assemblages is often difficult because of (1) few mineral phases compositionally suitable to apply the 40Ar/39Ar and K-Ar methods, and (2) small amount in which these phases are commonly found. K-feldspar adularia, sericitic mica, and celadonite are the best known K-bearing secondary minerals. We present some successful attempts to analyse two distinct secondary phases from a same volcanic formation that allow to test the validity of the measured ages. These ages have been also compared with the crystallisation age of the volcanic rocks in which the secondary phases were lately developed. Adularia and sericite were selected from basic lava flows from a 3 to 13 km thick Cretaceous sequence from the Coastal Range of central Chile, at two different locations: the Bustamante Hill (west from Santiago), and the Cordón de Chacana, c. 80 km further north. Adularia came from a low-variance assemblage with pumpellyite, chlorite and low-albite contained in amygdules whereas sericite was present in milky-white strongly sericitized plagioclase crystals. While small clusters of rare fresh plagioclase grains from lava flows from Bustamante and Chacana displayed concordant plateau ages 119.4 ± 2.4 (2 sigma) and 118.7 ± 0.6 Ma, respectively, the adularia from the same formations gave sensibly younger ages around 94 Ma (high temperature steps), and 96.8 ± 0.2 Ma (plateau age) in Bustamante and Chacana, respectively. Sericite ages were measured in situ into single crystals of strongly transformed plagioclases. The relative proportion of sericite and plagioclase corresponding to each degasing step was monitored by measuring the Ca/K ratio (deduced from 37ArCa/39Ar_K). While intermediate ages were measured on some sericite of both sites (corresponding to a variable but permanent contribution of plagioclase on each step), a plateau age of 97.0 ± 1.6 Ma (concordant with adularia) could be obtained on a strongly sericitized plagioclase

  8. Duration of Louisville hotspot volcanism at IODP 330 sites Canopus, Burton, and Rigil via 40Ar/39Ar geochronology

    NASA Astrophysics Data System (ADS)

    Cohen, B. E.; Vasconcelos, P. M.; Koppers, A. A.; Thiede, D. S.

    2013-12-01

    The Louisville seamount trail is a chain of intraplate volcanoes constructed over the past ~80 million years, as the Pacific plate moved 4300 km over a long-lived mantle melting anomaly. During IODP Expedition 330, over 800 m of alkaline mafic volcanic lavas, as well as interbedded and overlying sediments, were recovered from five of these seamounts in the older (~80-50 Ma) part of the chain. In this study we have undertaken geochronology of the volcanic units to provide time constraints for the magmatic evolution of the volcanoes. Sixteen of these drilled lava flows have been successfully dated using MAP-215-50 spectrometers, with six samples analyzed at Oregon State University (Koppers et al. 2012) and 12 flows analyzed at The University of Queensland. To check for consistency, two lava flows were dated at both laboratories; both samples yielded results within 2σ error. To minimize the effects of seawater alteration, only samples with well-crystallized groundmass were picked, and material cleaned via HNO3 and HCl acid pretreatment. Plateaus comprise 61 to 87% of the 39Ar released, and 40Ar/36Ar vs. 39Ar/36Ar correlation diagrams reveal all samples contained trapped argon within error of modern-day atmosphere. All ages determined are consistent with stratigraphic constraints, and we interpret the results to be reliable estimates of eruption ages. Units from Burton Guyot (site U1376A) yield ages from 70.8 × 0.5 to 64.1 × 0.5 Ma (2σ, using the atmospheric argon ratio and decay constants from Steiger & Jäger (1978) and a Fish Canyon sanidine age of 28.02 (Renne et al. 1998)). This long (~7 Ma) duration is consistent with petrologic evidence for substantial post-shield volcanic activity at this site. Meanwhile, at Canopus (site U1372A) and Rigil (sites U1373A and U1374A), lavas from the base, middle, and top of the respective volcanic piles yielded ages within analytical error. At the two deepest sites (U1372A and 1374A) 187 and 505 m of volcanic rocks were

  9. (40)Ar/(39)Ar dating of Bed I, Olduvai Gorge, Tanzania, and the chronology of early Pleistocene climate change.

    PubMed

    Deino, Alan L

    2012-08-01

    (40)Ar/(39)Ar dating of tuffs and lavas of the late Pleistocene volcanic and sedimentary sequence of Olduvai Gorge, north-central Tanzania, provides the basis for a revision of Bed I chronostratigraphy. Bed I extends from immediately above the Naabi Ignimbrite at 2.038 ± 0.005 Ma to Tuff IF at 1.803 ± 0.002 Ma. Tuff IB, a prominent widespread marker tuff in the basin and a key to understanding hominin evolutionary chronologies and paleoclimate histories, has an age of 1.848 ± 0.003 Ma. The largest lake expansion event in the closed Olduvai lake basin during Bed I times encompassed the episode of eruption and emplacement of this tuff. This lake event is nearly coincident with the maximum precessional insolation peak of the entire Bed I/Lower Bed II interval, calculated from an astronomical model of the boreal summer orbital insolation time-series. The succeeding precessional peak also apparently coincides with the next youngest expansion of paleo-Lake Olduvai. The extreme wet/dry climate shifts seen in the upper part of Bed I occur during an Earth-orbital eccentricity maximum, similar to episodic lake expansions documented elsewhere in the East African Rift during the Neogene. PMID:22809744

  10. Systematic analysis of K-feldspar 40Ar/ 39Ar step heating results II: relevance of laboratory argon diffusion properties to nature

    NASA Astrophysics Data System (ADS)

    Lovera, Oscar M.; Grove, Marty; Harrison, T. Mark

    2002-04-01

    We examine a database containing the results of 40Ar/ 39Ar step-heating experiments performed on 194 basement K-feldspars to recover thermal history information. Qualitative examination of 40Ar/ 39Ar systematics reveals that about half of the K-feldspars examined are sufficiently well behaved to be suitable for thermal history analysis. Correlation algorithms are developed to quantitatively assess the degree to which age and 39Ar release spectra are compatible with the same volume diffusion process. Upon applying these methods, we find that 65% of all samples yield correlation coefficients in excess of 0.8, whereas roughly 40% give values above 0.9. We further compare the observed correlation behavior with that predicted from the multidiffusion domain model and find good agreement for samples with correlation coefficients above 0.9. In contrast, hydrous phases unstable under in vacuo heating and K-feldspars with highly disturbed age spectra yield poorly correlated age and diffusion properties. The high degree of correlation exhibited by the majority of K-feldspars we have analyzed validates extrapolation of experimentally determined diffusion properties to conditions attending natural Ar loss within the crust. Despite this, a significant number of basement K-feldspars analyzed by the step-heating method yield 40Ar/ 39Ar systematics that are clearly problematic for thermal history analysis. We numerically explore the effects of low-temperature alteration of K-feldspar on thermochronological analysis and identify a range of conditions under which information is progressively lost. Finally, we demonstrate the insensitivity of thermal history calculations to detailed knowledge of the diffusion mechanism by introducing the heterogeneous diffusion model. We find that the multidiffusion domain approach can successfully recover imposed thermal histories from heterogeneous diffusion-type crystals and conclude that most details of the interpretive model employed are of

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

    SciTech Connect

    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 of 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.

  12. Insights into the late Cenozoic configuration of the Laurentide Ice Sheet from 40Ar/39Ar dating of glacially transported minerals in midcontinent tills

    NASA Astrophysics Data System (ADS)

    Roy, Martin; Clark, Peter U.; Duncan, Robert A.; Hemming, Sidney R.

    2007-09-01

    Glacial sedimentary sequences in the north central United States record multiple advances of the Laurentide Ice Sheet (LIS) since ˜2 Ma. Although the tills found in these sequences were deposited by southward flowing glacial lobes, little information is available on the geometry of flow lines in the interior of the LIS during any one glaciation, and the provenance of glacial deposits older than the last ice advance is largely unknown. Systematic changes in the composition of midcontinent tills and other paleogeographic considerations, however, raise the possibility of significant shifts in the trajectory of flow lines feeding the lobes of the southwestern LIS margin. Here we constrain till provenance using 40Ar/39Ar ages of individual hornblende and feldspar grains retrieved from tills representing several glaciations since ˜2 Ma. Hornblende grains show 40Ar/39Ar ages that indicate erosion of Paleoproterozoic (˜1.7-2.0 Ga) and late Archean (>2.5 Ga) rock sources, whereas feldspar grains show a broad range of Paleoproterozoic ages (˜1.4-2.4 Ga). Dating of hornblende and feldspar minerals in single pebbles suggests that this latter distribution of ages is related to the greater sensitivity of feldspars to thermal resetting during minor tectonic events. Accordingly, the range of 40Ar/39Ar ages for the predominant population of Paleoproterozoic hornblende and feldspar grains in our samples is consistent with a source from terrains forming the Churchill province of the Canadian Shield, while the small population of Archean-age grains likely reflects a source from the southwestern tip of the Archean Superior province that crops out near the study area. These results indicate that midcontinent tills were deposited by ice derived from the northwestern (Keewatin) sector of the LIS. The nearly identical distribution of hornblende and feldspar ages in the till samples identifies the Keewatin ice dome and the related ice flow to the midcontinent as long-standing features

  13. Polychronous (Early Cretaceous to Palaeogene) emplacement of the Mundwara alkaline complex, Rajasthan, India: 40Ar/39Ar geochronology, petrochemistry and geodynamics

    NASA Astrophysics Data System (ADS)

    Pande, Kanchan; Cucciniello, Ciro; Sheth, Hetu; Vijayan, Anjali; Sharma, Kamal Kant; Purohit, Ritesh; Jagadeesan, K. C.; Shinde, Sapna

    2016-07-01

    The Mundwara alkaline plutonic complex (Rajasthan, north-western India) is considered a part of the Late Cretaceous-Palaeogene Deccan Traps flood basalt province, based on geochronological data (mainly 40Ar/39Ar, on whole rocks, biotite and hornblende). We have studied the petrology and mineral chemistry of some Mundwara mafic rocks containing mica and amphibole. Geothermobarometry indicates emplacement of the complex at middle to upper crustal levels. We have obtained new 40Ar/39Ar ages of 80-84 Ma on biotite separates from mafic rocks and 102-110 Ma on whole-rock nepheline syenites. There is no evidence for excess 40Ar. The combined results show that some of the constituent intrusions of the Mundwara complex are of Deccan age, but others are older and unrelated to the Deccan Traps. The Mundwara alkaline complex is thus polychronous and similar to many alkaline complexes around the world that show recurrent magmatism, sometimes over hundreds of millions of years. The primary biotite and amphibole in Mundwara mafic rocks indicate hydrous parental magmas, derived from hydrated mantle peridotite at relatively low temperatures, thus ruling out a mantle plume. This hydration and metasomatism of the Rajasthan lithospheric mantle may have occurred during Jurassic subduction under Gondwanaland, or Precambrian subduction events. Low-degree decompression melting of this old, enriched lithospheric mantle, due to periodic diffuse lithospheric extension, gradually built the Mundwara complex from the Early Cretaceous to Palaeogene time.

  14. 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.

  15. Molybdenite Re-Os and muscovite 40Ar/39Ar dating of the Xihuashan tungsten deposit, central Nanling district, South China

    NASA Astrophysics Data System (ADS)

    Hu, Rui-Zhong; Wei, Wen-Feng; Bi, Xian-Wu; Peng, Jian-Tang; Qi, You-Qiang; Wu, Li-Yan; Chen, You-Wei

    2012-10-01

    The Xihuashan tungsten deposit in the central Nanling region, South China, is an important vein-type ore deposit hosted in Cambrian strata and Mesozoic granitic intrusions. Wolframite and molybdenite are the principal ore minerals. The gangue minerals are mainly quartz and muscovite. Wolframite and molybdenite are products of the first stage hydrothermal activity, whereas muscovite formed dominantly at the second stage. Molybdenite Re-Os and muscovite 40Ar/39Ar dating have been carried out to investigate the age of mineralization. Re-Os isotopic dating for molybdenite associated with wolframite yield a precise, well-constrained isochron age of 157.8 ± 0.9 Ma (MSWD = 1.5). Ar-Ar isotopic analyses of muscovite yield a plateau age of 152.8 ± 1.6 Ma, in agreement with an inverse isochron age of 152.8 ± 1.6 Ma, which is ~ 5 mys younger than the Re-Os age. The molybdenite Re-Os age is interpreted as the age of tungsten mineralization. This age coincides well with the zircon U-Pb age of the host granitic intrusion reported previously. The ~ 5 mys difference between molybdenite Re-Os and muscovite 40Ar/39Ar ages probably represents the duration of hydrothermal activity. The results show that the Xihuashan tungsten deposit is one of many important tungsten-tin deposits formed during 150 to 160 Ma associated with large-scale lithospheric extension in South China.

  16. 40Ar/39Ar dating of the eruptive history of Mount Erebus, Antarctica: Summit flows, tephra, and caldera collapse

    USGS Publications Warehouse

    Harpel, C.J.; Kyle, P.R.; Esser, R.P.; McIntosh, W.C.; Caldwell, D.A.

    2004-01-01

    Eruptive activity has occurred in the summit region of Mount Erebus over the last 95 ky, and has included numerous lava flows and small explosive eruptions, at least one plinian eruption, and at least one and probably two caldera-forming events. Furnace and laser step-heating 40Ar/39Ar ages have been determined for 16 summit lava flows and three englacial tephra layers erupted from Mount Erebus. The summit region is composed of at least one or possibly two superimposed calderas that have been filled by post-caldera lava flows ranging in age from 17 ?? 8 to 1 ?? 5 ka. Dated pre-caldera summit flows display two age populations at 95 ?? 9 to 76 ?? 4 ka and 27 ?? 3 to 21 ??4 ka of samples with tephriphonolite and phonolite compositions, respectively. A caldera-collapse event occurred between 25 and 11 ka. An older caldera-collapse event is likely to have occurred between 80 and 24 ka. Two englacial tephra layers from the flanks of Mount Erebus have been dated at 71 ?? 5 and 15 ?? 4 ka. These layers stratigraphically bracket 14 undated tephra layers, and predate 19 undated tephra layers, indicating that small-scale explosive activity has occurred throughout the late Pleistocene and Holocene eruptive history of Mount Erebus. A distal, englacial plinian-fall tephra sample has an age of 39 ?? 6 ka and may have been associated with the older of the two caldera-collapse events. A shift in magma composition from tephriphonolite to phonolite occurred at around 36 ka. ?? Springer-Verlag 2004.

  17. 40Ar/39Ar dating of the eruptive history of Mount Erebus, Antarctica: summit flows, tephra, and caldera collapse

    NASA Astrophysics Data System (ADS)

    Harpel, Christopher J.; Kyle, Philip R.; Esser, Richard P.; McIntosh, William C.; Caldwell, David A.

    2004-12-01

    Eruptive activity has occurred in the summit region of Mount Erebus over the last 95 ky, and has included numerous lava flows and small explosive eruptions, at least one plinian eruption, and at least one and probably two caldera-forming events. Furnace and laser step-heating 40Ar/39Ar ages have been determined for 16 summit lava flows and three englacial tephra layers erupted from Mount Erebus. The summit region is composed of at least one or possibly two superimposed calderas that have been filled by post-caldera lava flows ranging in age from 17 ± 8 to 1 ± 5 ka. Dated pre-caldera summit flows display two age populations at 95 ± 9 to 76 ± 4 ka and 27 ± 3 to 21 ± 4 ka of samples with tephriphonolite and phonolite compositions, respectively. A caldera-collapse event occurred between 25 and 11 ka. An older caldera-collapse event is likely to have occurred between 80 and 24 ka. Two englacial tephra layers from the flanks of Mount Erebus have been dated at 71 ± 5 and 15 ± 4 ka. These layers stratigraphically bracket 14 undated tephra layers, and predate 19 undated tephra layers, indicating that small-scale explosive activity has occurred throughout the late Pleistocene and Holocene eruptive history of Mount Erebus. A distal, englacial plinian-fall tephra sample has an age of 39 ± 6 ka and may have been associated with the older of the two caldera-collapse events. A shift in magma composition from tephriphonolite to phonolite occurred at around 36 ka.

  18. Radiometric ages of the Fire Clay tonstein [Pennsylvanian (Upper Carboniferous), Westphalian, Duckmantian]: A comparison of U-Pb zircon single-crystal ages and 40Ar/39Ar sanidine single-crystal plateau ages

    USGS Publications Warehouse

    Lyons, P.C.; Krogh, T.E.; Kwok, Y.Y.; Davis, D.W.; Outerbridge, W.F.; Evans, H.T., Jr.

    2006-01-01

    The Fire Clay tonstein [Pennsylvanian (Upper Carboniferous), Westphalian Series, Duckmantian Stage]-a kaolinized, volcanic-ash deposit occurring in Kentucky, West Virginia, Tennessee, and Virginia-is the most widespread bed in the Middle Pennsylvanian of the central Appalachian basin, USA. A concordant single-crystal U-Pb zircon datum for this tonstein gives a 206Pb/238U age of 314.6 ?? 0.9 Ma (2??). This age is in approximate agreement with a mean sanidine plateau age of 311.5 ?? 1.3 Ma (1??, n = 11) for the Fire Clay tonstein. The difference between the two ages may be due to bias between the 40K and 238U decay constants and other factors. The age of the Fire Clay tonstein has important implications for Duckmantian Stage (Westphalian Series) sedimentation rates, correlations with the Westphalian Series of Europe, Middle Pennsylvanian volcanic events, and the late Paleozoic time scale. ?? 2006 Elsevier B.V. All rights reserved.

  19. Detailed 40Ar/39Ar chronology of the Tancítaro Volcanic Field, Michoacán, Mexico

    NASA Astrophysics Data System (ADS)

    Ownby, S.; Delgado-Granados, H.; Lange, R.; Hall, C. M.

    2005-12-01

    The Tancítaro volcanic field (TVF) is characterized by over 300 cinder cones and fissure fed lava flows, in addition to the ~10 shield volcanoes and 1 large andesite stratovolcano, ~60 km3 Volcán Tancítaro. The TVF is part of the larger Michoacán Guanajuato volcanic field (MGVF) in the Trans Mexican Volcanic Belt, related to subduction of the Cocos plate. We report new 40Ar/39Ar age constraints on the most recent activity from Volcán Tancítaro. Previously, there was only one K-Ar date 530±60 ka for this volcano; Ban et al. (1992). It has been the site of at least two debris avalanche deposits (Capra et al., 2002; Garduno-Monroy, 1999; Ownby et al., 2004), and continues to threaten the cities of Uruapan and Apatzingán in the state of Michoacán, which have a combined population of >300,000. The most recent activity produced a thin blanket of ash (~1-5 m thick along the flanks of the volcano); this ash is tightly bracketed by dates on two nearby shield volcanoes, one underneath (268±14 ka) and the other on top (267±12 ka) of this ash layer. It appears to have triggered a large debris avalanche deposit off the steep slopes of V. Tancítaro (the distinctive ash is mixed in with this avalanche deposit), which itself is bracketed by two cinder cones, one underneath (425±45 ka) and the other on top (179±77 ka). The timing of this ash eruption is close to that for four different andesite lavas from near the summit of V. Tancítaro, which yielded ages of 251±25 ka, 241±25 ka, 228±16 ka, 223±23 ka, respectively. Other dates from the main edifice of V. Tancítaro reveal two earlier episode of activity at ~450 and ~700 ka. We also report an additional set of ~50 40Ar/39Ar ages on various cinder cones, shields, and fissure-fed flows that are peripheral to V. Tancítaro. The samples range in age from ~980 ka to the present, with no obvious breaks in time. They range continuously from 51-62 wt% SiO2, with no breaks in composition. It appears that neither dacite

  20. High Precision 40Ar/39Ar Geochronology of Servilleta basalt flows in the Rio Grande Gorge, New Mexico

    NASA Astrophysics Data System (ADS)

    Cosca, M. A.; Lee, J. P.; Thompson, R. A.; Turner, K. J.

    2013-12-01

    New geologic mapping and high-precision 40Ar/39Ar geochronology within the Taos Plateau in northern New Mexico indicate a period of vigorous volcanic activity between ~5.5 and ~1 Ma. Over 50 volcanic centers formed during this time together with an unresolved number of vents, fissures, and volcanic centers buried by intercalated volcanic rock and sedimentary basin fill. Defining the volcanic stratigraphy is essential for models of regional groundwater flow and for understanding the geologic evolution of the Pliocene to Recent Rio Grande rift. A spectacular stratigraphic section of volcanic rock related to Rio Grande rifting is visible from the High Bridge on NM State Hwy 64 , approximately 24 km northwest of Taos, NM, where a 240 m canyon is incised through the basal, middle, and upper Servilleta basalt flow packages (Dungan et al., 1984). Fresh basalt from a vertical transect of the canyon near the High Bridge was collected and sized into ~3 mm3 fragments and irradiated with fast neutrons at the USGS TRIGA reactor. The irradiated rock fragments were incrementally heated with a CO2 laser and Ar isotopes were measured using both MAP 215-50 and ARGUS-VI mass spectrometers. Only the argon isotope measurements using the ARGUS-VI had sufficient precision to make accurate corrections for argon isotopic sources and the resulting 40Ar/39Ar ages define a precise emplacement chronology of the entire stratigraphic section. The basal flow package records ages of 4.76 × 0.03 Ma (FCT sanidine = 28.201 Ma; all errors 2 sigma) at river level, 4.75 × 0.04 Ma at mid flow package, and 4.47 × 0.04 at the top of the flow package. The middle flow package records ages of 4.10 × 0.03 Ma at the base of the flow package, 4.14 × 0.10 Ma mid flow package, and 3.98 × 0.08 at the top of the flow package. The upper basalt package records ages of 3.69 × 0.07 Ma at the base of the flow package and 3.59 × 0.10 Ma at the top. The upper and middle flow packages were erupted rapidly, whereas

  1. Paleozoic accretionary orogenesis in the eastern Beishan orogen: constraints from zircon U-Pb and 40Ar/39Ar geochronology

    NASA Astrophysics Data System (ADS)

    Ao, Songjian; Xiao, Wenjiao; Windley, Brian; Mao, Qigui

    2016-04-01

    The continental growth mechanism of the Altaids in Central Asia is still in controversy between models of continuous subduction-accretion versus punctuated accretion by closure of multiple oceanic basins. The Beishan orogenic belt, located in the southern Altaids, is a natural laboratory to address this controversy. Key questions that are heavily debated are: the closure time and subduction polarity of former oceans, the emplacement time of ophiolites, and the styles of accretion and collision. This paper reports new structural data, zircon ages and Ar-Ar dates from the eastern Beishan Orogen that provide information on the accretion process and tectonic affiliation of various terranes. Our geochronological and structural results show that the younging direction of accretion was northwards and the subduction zone dipped southwards under the northern margin of the Shuangyingshan micro-continent. This long-lived and continuous accretion process formed the Hanshan accretionary prism. Our field investigations show that the emplacement of the Xiaohuangshan ophiolite was controlled by oceanic crust subduction beneath the forearc accretionary prism of the Shuangyingshan-Mazongshan composite arc to the south. Moreover, we address the age and terrane affiliation of lithologies in the eastern Beishan orogen through detrital zircon geochronology of meta-sedimentary rocks. We provide new information on the ages, subduction polarities, and affiliation of constituent structural units, as well as a new model of tectonic evolution of the eastern Beishan orogen. The accretionary processes and crustal growth of Central Asia were the result of multiple sequences of accretion and collision of manifold terranes. Reference: Ao, S.J., Xiao, W., Windley, B.F., Mao, Q., Han, C., Zhang, J.e., Yang, L., Geng, J., Paleozoic accretionary orogenesis in the eastern Beishan orogen: Constraints from zircon U-Pb and 40Ar/39Ar geochronology. Gondwana Research, doi: http://dx.doi.org/10.1016/j

  2. Resetting of Neoarchaean hornblendes from the Murmansk Terrane (Kola Peninsula, Russia) revealed by a combined 40Ar/39Ar and Rb-Sr analysis

    NASA Astrophysics Data System (ADS)

    de Jong, K.; Timmerman, M. J.; Cliff, R. A.; Wijbrans, J. R.; Daly, J. S.; Balagansky, V. V.

    2003-04-01

    We present isotope data from amphiboles from the 2.6--2.8 Ga Murmansk Terrane in the northern foreland of the 1.9 Ga Lapland-Kola Orogen of northern Fennoscandia. Most amphiboles are zoned with tschermakitic cores that progressively change to actinolite in 5--10 μm wide areas in rims, defect zones or adjacent to biotite. Biotite chiefly occurs along cleavages, fractures and grain boundaries of amphibole, showing that its hydration and fluid ingress are confined to lattice imperfections. Furnace step heating of hornblende separate MT-11 gave spectra with increasing apparent ages and Ca/K ratios (a proxy for 37ArCa/39Ar_K). Hornblende is intergrown with biotite that also occurs in the matrix. Yet, laser step heating of single hornblende grain MT-11 yielded flat spectra with Neoarchaean apparent ages and constant Ca/K ratios. This suggests that the hornblende grain that was drilled from a thin section and which was not affected by biotite growth retained its Neoarchaean age. In contrast, the hornblende separate with intergrown biotite from this sample has a partially reset Neoarchaean isotope system. Hornblende MT-27 has Neoarchaean apparent ages and lacks low Ca/K ratios in both single grain and mineral separate spectra; it is not affected by biotite growth and this mineral is also absent from the matrix. Age spectra of other hornblende separates have increasing apparent ages to 2.56--2.65 Ga during final 39Ar release; their Ca/K ratio spectra similarly increase. Low Ca/K ratios for gas release below 950^oC imply degassing of included biotite. Apparent ages of the first heating increments may be as young as 1.8 Ga, comparable to the K-Ar and 40Ar/39Ar ages of the youngest biotites. Hornblende-plagioclase pairs from aliquots used for 40Ar/39Ar furnace step heating yielded sharply discordant Rb-Sr ages. MT-11 yielded an 1881 ± 23 Ma Rb-Sr age that compares well with the K-Ar and 40Ar/39Ar ages of biotite in the sample. In contrast, hornblende of sample MT-27 has a

  3. A volcanological and geochemical investigation of Boa Vista, Cape Verde Islands; 40Ar/ 39Ar geochronology and field constraints

    NASA Astrophysics Data System (ADS)

    Dyhr, Charlotte T.; Holm, Paul M.

    2010-01-01

    Boa Vista, the easternmost island in the Cape Verde archipelago, consists of volcanic products, minor intrusions and a thin partial sedimentary cover. The first 15 age results from 40Ar/ 39Ar incremental heating analysis of groundmass separates from volcanic and plutonic rocks from Boa Vista are presented. The combination of age results and field observations demonstrates that the volcanic activity that formed the island occurred in three main stages: (1) > 16 Ma, (2) 15-12.5 Ma and (3) 9.5-4.5 Ma. The first stage, restricted to the north eastern part of the island, is dominated by ankaramitic lavas. The second stage, consisting of evolved lavas of phonolitic-trachytic compositions and nepheline syenites, makes up large central parts of the island. The large volume of evolved rocks and the extended eruption period of several Ma make stage 2 in Boa Vista unique to Cape Verde. Mainly basanites and nephelinites were erupted during the third stage, initially dominated by eruption of subaerial mafic lavas around 9 Ma. Pillow lavas are erupted around 7 Ma whereupon dominantly subaerial mafic lavas were erupted. Stage 3 saw volcanism in many centres distributed mainly along the present coastline and with activity partly overlapping in time. The volcanic evolution of Boa Vista constrains the initiation of volcanic activity in the Cape Verde archipelago to the eastern islands. Major and trace element geochemistry of 160 volcanic and plutonic rocks representing the entire exposed chronological sequence on Boa Vista is presented, revealing an extremely well developed Daly Gap. Only a little was modified from the mafic magmas that rose in small batches from the mantle. Compositional variation distinguishes each volcanic complex and was to a large extent present in the mantle melts. The highly evolved stage 2 phonolites and trachytes are related through the fractional crystallization of three compositionally distinct magmas. Two of these may have been derived by crystal

  4. {sup 40}Ar/{sup 39}Ar thermochronology and thermobarometry of metamorphism, plutonism, and tectonic denudation in the Old Woman Mountains area, California

    SciTech Connect

    Foster, D.A.; Miller, C.F.; Harrison, T.M.; Hoisch, T.D.

    1992-02-01

    Discrimination of individual tectonometamorphic events in polymetamorphosed terranes requires a comprehensive understanding of the relative timing and conditions of metamorphism and plutonism. We have applied a combination of {sup 40}Ar/{sup 39} Ar thermochronology, petrology, and thermobarometry to reconstruct the complex Early Proterozoic through early Cenozoic tectonic and metamorphic evolution of continental crust in the Old Woman Mountains area, southeastern California. Strong Mesozoic thermal events obscure the earlier history in much of the Old Woman Mountains area. In those areas where Early Proterozoic rocks underwent only lower-greenschist-facies metamorphism during the Mesozoic, thermobarometry of pelitic schists indicates that Proterozoic metamorphism occurred at 9 to 11 kbar and {approximately}700 {degrees}C. {sup 40}Ar/{sup 39}Ar ages of hornblende from samples of interbedded Proterozoic amphibolite indicate that this high-grade metamorphism took place before 1600 Ma. The relatively high-pressure conditions of Early Proterozoic metamorphism in the Old Woman Mountains area contrast with the low-pressure granulite-facies metamorphism that occurred elsewhere in the Mojave Desert at this time. {sup 40}Ar/{sup 39}Ar analyses of hornblende from Proterozoic rocks within Mesozoic shear zones and hornblende barometry from Jurassic intrusive rocks suggest that tectonism and burial of Paleozoic strata to >10 km began between 170 and 150 Ma. This tectonism resulted in regional greenschist-facies metamorphism. Late-stage mineral assemblages in Proterozoic and Paleozoic pelitic rocks in the Old Woman Mountains area indicate an increase in metamorphic grade from greenschist to upper amphibolite facies toward Later Cretaceous Plutons of the 73 Ma Old Woman-Piute batholith. Barometric calculations from garnet-bearing metamorphic rocks suggest that this Cretaceous metamorphism took place at 3.5 to 5.0 kbar in the Old Woman Mountains. 68 refs., 11 figs., 3 tabs.

  5. 40Ar/39Ar dated climatic and hydrological variability between MIS20 and MIS18 at Sulmona Basin (central Italy)

    NASA Astrophysics Data System (ADS)

    Zanchetta, Giovanni; Giaccio, Biagio; Eleonora, Reagattieri; Nomade, Sebastien; Renne, Paul R.; Sprain, Courtney J.; Drysdale, Russell N.; Tzedakis, Polychronis C.; Messina, Paolo; Scardia, Giancarlo; Sposato, Andrea

    2015-04-01

    Understanding spatial-temporal variability, magnitude and different expressions of Quaternary millennial-scale palaeoclimatic changes is one of the frontier challenges of modern palaeoclimatology. Addressing this issue requires the acquisition of regionally representative, and ideally independently-dated, records of climatic variability. Multiproxy record (stable isotopes, XRF, MS, %CaCO3) from lacustrine succession of Sulmona basin (central Italy), highlights climatic and hydrological variability at orbital to millennial scales between MIS20 and MIS18. The record highlights the presence of interesting millennial scale variability within MIS19, considered to be the best orbital analogue of the current interglacial. The presence of several tephra layers precisely dated by 40Ar/39Ar technique, allow placement of the record within a robust time frame. Assembling a high-resolution paleoclimatic record for MIS19 anchored to a high-precision 40Ar/39Ar chronology, it is possible to show that the MIS 19c interglacial started shortly before the boreal summer insolation and obliquity maximum/precession minimum at 790-788 ka, and ended 11.6 ± 2.3 kyr later, when orbital parameters assumed a configuration similar to the present one.

  6. 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

  7. 40Ar/39Ar dating of unusual minerals (tourmaline, K-richterite, yimengite, wadeite and priderite) and applicability to the geological record.

    NASA Astrophysics Data System (ADS)

    Jourdan, F.; Thern, E.; Wilde, S. A.; Frewer, L.

    2012-04-01

    One of the advantages of the 40Ar/39Ar technique is that it relies on the decay of K, one of the most abundant elements in the Earth's crust. As such, many minerals can be dated with this technique, with the corresponding age data reflecting either the emplacement age or a cooling age. Here we present robust well-defined 40Ar/39Ar plateau ages obtained on a suite of minerals with variable potassium concentration which, despite their potential to unravel numerous geological problems, have seldom been (if ever; e.g. priderite) analyzed using the 40Ar/39Ar technique. Tourmaline: Multi- and single-grain aliquots of tourmaline were extracted from three sets of cross-cutting and stratiform quartz-tourmaline veins throughout the siliciclastic metasedimentary rocks of the Archean Illaara granite-greenstone belt, Western Australia [1]. Tourmaline samples yielded 9 plateau and 1 inverse isochron ages defining two distinct age populations of 2939 ± 11 Ma (P=0.64) and 2642 ± 16 Ma (P=0.50). Tourmaline deposits are a common occurrence in the geological record [e.g. 2], and the mineral may be useful in assigning minimum depositional ages (e.g. cross-cutting veins) and in dating hydrothermal fluid circulations, as well as its potential for use in detrital mineral studies. Yimengite: This mineral is a K-oxide and has been recovered from the Turkey Well kimberlite pipe, Yilgarn Craton, Australia. We obtained two concordant well-defined 40Ar/39Ar plateaus with a mean age of 2128 ± 5 Ma (P=1.0) interpreted as the emplacement age of the Kimberlite. No excess Ar is present suggesting that this mineral can be used to date Kimberlite emplacement as a viable alternative to phlogopite, which commonly retains part of its mantle history [e.g. 3, 4] thus leading to uncertainties in assigning a precise age. K-Richterite, wadeite and priderite: K-richterite is a K-bearing sodic amphibole, wadeite is K-Zr silicate and Priderite is a K-oxide. These minerals have been recovered from Wolgidee

  8. 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. PMID:11795971

  9. 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.

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

    NASA Astrophysics Data System (ADS)

    McIntosh, William C.; Sutter, John F.; Chapin, Charles E.; Kedzie, Laura L.

    1990-11-01

    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 1σ precision of±0.25% 0.4% (±0.07 0.14 Ma). Replicate plateau age determinations for eight different samples show within-sample 1σ precisions averaging ±0.25%. Plateau ages from multiple ( n=3 8) samples of individual ignimbrites show 1σ 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.

  11. KAr and {40Ar }/{39}Ar study of metamorphic rocks associated with the Oman ophiolite: Tectonic implications

    NASA Astrophysics Data System (ADS)

    Montigny, R.; Le Mer, O.; Thuizat, R.; Whitechurch, H.

    1988-09-01

    K-Ar analyses on extracted minerals are reported for a variety of metamorphic rocks associated with the Sumail ophiolite. Amphibolites lying at the sole of the ophiolite yield ages of 95-100 Ma, which are viewed as reflecting times of crystallization. High-pressure metamorphics of the Saih Hatat reveal complex results: white micas range in age from 80 to 131 Ma whereas blue amphiboles indicate ages that are systematically lower than those of coexisting white micas. Investigation of a few white micas by the {40Ar }/{39Ar } step heating method yields rather intricate age spectra, featuring low apparent ages in the first and the last stages of gas release and high apparent ages in between. Two explanations can be equally envisaged for these convex-upward age spectra. The first is the mixing of two generations of micas, corresponding to two main metamorphisms. The first one ( M1) is a low- to medium-temperature, high-pressure event which conceivably occurred between 130 and 114 m.y. ago. The second ( M2) overprints M1 and has produced rocks typical of the greenschist facies. It took place 80 m.y. ago and also affected the sole of the ophiolites. The second is the presence of excess argon in mica mixtures with complex degassing properties. Thus, the two metamorphic phases identified by microscopic inspection are not significantly different in age. They occurred in the 70-80 Ma interval. Moreover, K-Ar dates on amphibole from gabbroic dikes intersecting the peridotites suggest that they are genetically linked to the mafic part of the ophiolites. Assuming that metamorphism is a tracer of tectonic events, we view the infraophiolitic amphibolites as the result of an intraoceanic thrusting which took place near a spreading center. Nevertheless, the uncertainty as to the age of the blueschist metamorphism precludes the possibility of indicating a timetable, based on metamorphic ages, for the motion shift of Africa relative to Eurasia during the Late Cretaceous. A tentative

  12. (40)Ar/(39)Ar dating, paleomagnetism, and tephrochemistry of Pliocene strata of the hominid-bearing Woranso-Mille area, west-central Afar Rift, Ethiopia.

    PubMed

    Deino, Alan L; Scott, Gary R; Saylor, Beverly; Alene, Mulugeta; Angelini, Joshua D; Haile-Selassie, Yohannes

    2010-02-01

    (40)Ar/(39)Ar dating of tuffs and mafic lavas, tephra geochemistry, and paleomagnetic reversal stratigraphy have been used to establish the chronostratigraphy of the Pliocene hominid-bearing fossiliferous succession at Woranso-Mille, a paleontological study area in the western part of the central Afar region of Ethiopia. The succession in the northwestern part of the study area ranges in (40)Ar/(39)Ar age from 3.82-3.570 Ma, encompassed by paleomagnetic subchron C2Ar (4.187-3.596 Ma). One of the major tuff units, locally named the Kilaytoli tuff, is correlative on the basis of age and geochemistry to the Lokochot Tuff of the Turkana Basin. A hominid partial skeleton (KSD-VP-1) was found in strata whose precise stratigraphic position and age is still under investigation, but is believed to correspond to the later part of this interval. Woranso-Mille fills a significant gap in the fossil record of northeastern Africa at the time of the lower to middle Pliocene transition, when many extant species lineages of African fauna were established. PMID:20034653

  13. 40Ar/39Ar dating of tuff vents in the Campi Flegrei caldera (southern Italy): Toward a new chronostratigraphic reconstruction of the Holocene volcanic activity

    USGS Publications Warehouse

    Fedele, L.; Insinga, D.D.; Calvert, A.T.; Morra, V.; Perrotta, A.; Scarpati, C.

    2011-01-01

    The Campi Flegrei hosts numerous monogenetic vents inferred to be younger than the 15 ka Neapolitan Yellow Tuff. Sanidine crystals from the three young Campi Flegrei vents of Fondi di Baia, Bacoli and Nisida were dated using 40Ar/39Ar geochronology. These vents, together with several other young edifices, occur roughly along the inner border of the Campi Flegrei caldera, suggesting that the volcanic conduits are controlled by caldera-bounding faults. Plateau ages of ∼9.6 ka (Fondi di Baia), ∼8.6 ka (Bacoli) and ∼3.9 ka (Nisida) indicate eruptive activity during intervals previously interpreted as quiescent. A critical revision, involving calendar age correction of literature 14C data and available 40Ar/39Ar age data, is presented. A new reference chronostratigraphic framework for Holocene Phlegrean activity, which significantly differs from the previously adopted ones, is proposed. This has important implications for understanding the Campi Flegrei eruptive history and, ultimately, for the evaluation of related volcanic risk and hazard, for which the inferred history of its recent activity is generally taken into account.

  14. Paleoclimate change in the Nakuru basin, Kenya, at 119 - 109 ka derived from δ18Odiatom and diatom assemblages and 40Ar/39Ar geochronology

    NASA Astrophysics Data System (ADS)

    Bergner, Andreas; Deino, Alan; Leng, Melanie; Gasse, Francoise

    2016-04-01

    A 4.5m-thick diatomite bed deposited during the cold interval of the penultimate interglacial at ~119 - 109 ka documents a period in which a deep freshwater lake filled the Nakuru basin in the Central Kenya Rift (CKR), East Africa. Palaeohydrological conditions of the basin are reconstructed for the paleolake highstand using δ18Odiatom and characterization of diatom assemblages. The age of the diatomite deposit is established by precise 40Ar/39Ar-dating of intercalated pumice tuffs. The paleolake experienced multiple hydrological fluctuations on sub-orbital (~1,500 to 2,000 years) time scales. The magnitude of the δ18Odiatom change (+/- 3‰) and significant changes in the plankton-littoral ratio of the diatom assemblage (+/- 25%) suggest that the paleolake record can be interpreted in the context of long-term climatic change in East Africa. Using 40Ar/39Ar age control and nominal diatomite-sedimentation rates we establish a simplified age model of paleohydrological vs. climatic change, from which we conclude that more humid conditions prevailed in equatorial East Africa during the late Pleistocene over a relatively long time interval of several thousands years. Then, extreme insolation at eccentricity maximum and weakened zonal air-pressure gradients in the tropics favored intensified ITCZ-like convection over East Africa and deep-freshwater lake conditions.

  15. Argon behaviour in an inverted Barrovian sequence, Sikkim Himalaya: The consequences of temperature and timescale on 40Ar/39Ar mica geochronology

    NASA Astrophysics Data System (ADS)

    Mottram, Catherine M.; Warren, Clare J.; Halton, Alison M.; Kelley, Simon P.; Harris, Nigel B. W.

    2015-12-01

    40Ar/39Ar dating of metamorphic rocks sometimes yields complicated datasets which are difficult to interpret in terms of timescales of the metamorphic cycle. Single-grain fusion and step-heating data were obtained for rocks sampled through a major thrust-sense shear zone (the Main Central Thrust) and the associated inverted metamorphic zone in the Sikkim region of the eastern Himalaya. This transect provides a natural laboratory to explore factors influencing apparent 40Ar/39Ar ages in similar lithologies at a variety of metamorphic pressure and temperature (P-T) conditions. The 40Ar/39Ar dataset records progressively younger apparent age populations and a decrease in within-sample dispersion with increasing temperature through the sequence. The white mica populations span ~ 2-9 Ma within each sample in the structurally lower levels (garnet grade) but only ~ 0-3 Ma at structurally higher levels (kyanite-sillimanite grade). Mean white mica single-grain fusion population ages vary from 16.2 ± 3.9 Ma (2σ) to 13.2 ± 1.3 Ma (2σ) from lowest to highest levels. White mica step-heating data from the same samples yields plateau ages from 14.27 ± 0.13 Ma to 12.96 ± 0.05 Ma. Biotite yield older apparent age populations with mean single-grain fusion dates varying from 74.7 ± 11.8 Ma (2σ) at the lowest structural levels to 18.6 ± 4.7 Ma (2σ) at the highest structural levels; the step-heating plateaux are commonly disturbed. Temperatures > 600 °C at pressures of 0.4-0.8 GPa sustained over > 5 Ma, appear to be required for white mica and biotite ages to be consistent with diffusive, open-system cooling. At lower temperatures, and/or over shorter metamorphic timescales, more 40Ar is retained than results from simple diffusion models suggest. Diffusion modelling of Ar in white mica from the highest structural levels suggests that the high-temperature rocks cooled at a rate of ~ 50-80 °C Ma- 1, consistent with rapid thrusting, extrusion and exhumation along the Main

  16. 40Ar-39Ar dating and tectonic implications of volcanic rocks recovered at IODP Hole U1342A and D on Bowers Ridge, Bering Sea

    NASA Astrophysics Data System (ADS)

    Sato, Keiko; Kawabata, Hiroshi; Scholl, David W.; Hyodo, Hironobu; Takahashi, Kozo; Suzuki, Katsuhiko; Kumagai, Hidenori

    2016-03-01

    During the Integrated Ocean Drilling Program (IODP), a total of 41.54 m of basement rock, consolidated volcaniclastic sediment, was recovered beneath a thin sediment cover. The drilled site is at the eastern end of the crestal area of Bowers Ridge, a north and westward sweeping offshoot of the Aleutian Arc into the Bering Sea. The volcanic sequence recovered from Holes U1342A and U1342D was divided into six major lithologic units. We used the single grain 40Ar-39Ar dating method performed by step-wise heated laser fusion technique to date andesites of Unit 1. Thereby two ages of Oligocene volcanism (34-32 Ma, 28-26 Ma) were distinguished each other according to our 40Ar-39Ar data. These ages refute a hypothesized Cretaceous origin in the North Pacific as an exotic arc massif or sector of the Hawaiian-Emperor chain and indicate that the Bowers Ridge is a Bering-Sea formed arc or remnant arc that ceased forming in the latest Oligocene to the earliest Miocene time.

  17. Inherited argon in a Pleistocene andesite lava: 40Ar/39Ar incremental-heating and laser-fusion analyses of plagioclase

    NASA Astrophysics Data System (ADS)

    Singer, B. S.; Wijbrans, J. R.; Nelson, S. T.; Pringle, M. S.; Feeley, T. C.; Dungan, M. A.

    1998-05-01

    By using 40Ar/39Ar incremental-heating and laser-fusion techniques, xenocrystic plagioclase was discovered in a late Pleistocene andesitic lava that erupted through the Andean Cordillera. Inherited argon in the xenocrysts is as much as ˜450 times older than the host lava, the age of which is independently known, and is an obstacle to dating the lava by using a whole-rock sample. The xenocrysts are impossible to identify from petrography or chemical parameters such as their K/Ca ratios. Holocrystalline groundmass, carefully separated from plagioclase and other phenocrysts, gives an accurate 40Ar/39Ar age for the lava. The xenocrysts could not have been degassed for more than several days in the magma and probably were assimilated from Paleozoic rocks buried under kilometers of Mesozoic and Tertiary arc rocks composing the Cordillera in central Chile. The required magma ascent velocity, on the order of kilometers/day, is extraordinarily high compared to the 10-4 km/day minimum implied by the 226Ra excess in continental arc lavas. These data permit magma migration and storage in the mantle and lower crust for as much as thousands of years, followed abruptly by rapid ascent to the surface.

  18. Defining the Tristan-Gough Hotspot: High-Resolution 40Ar/39Ar Dating of Volcanism at Tristan da Cunha

    NASA Astrophysics Data System (ADS)

    Schnur, S.; Koppers, A. A. P.

    2015-12-01

    Explaining the spatial distribution of intra-plate volcanism is an important geologic problem. The Walvis Ridge is a uniquely-shaped hotspot trail in the South Atlantic that is not fully explained by the prevailing mantle plume paradigm. About halfway through its 130 Myr history, Walvis shows a morphological shift from a continuous ridge to a diffuse region of guyots arranged in two volcanic tracks. Recent volcanism at both Tristan da Cunha and Gough Island suggests these tracks are produced by two hotspots sourced from a single plume. However, the islands are located more than 400 km apart, which does not conform to our understanding of plumes as narrow, semi-stationary upwellings. It remains unclear which of the two islands better represents the current plume position. New ages from previously unstudied seamounts show that Tristan is younger than surrounding volcanism, whereas Gough appears to fit the local age progression (Schnur et al. 2014). Modern radiometric ages suggest the main island of Tristan may have been active for up to 1.3 ± 0.2 Myr (O'Connor and le Roex 1992). However, the seemingly older Inaccessible, Nightingale and Middle islands have yet to be reliably dated and could be up to 18 ± 4 Ma based on K-Ar ages (Miller in Baker et al. 1964). In order to confidently delineate the duration of volcanism at Tristan, we present the results of 29 new 40Ar/39Ar step-heating experiments on biotite, hornblende, plagioclase and groundmass separates from rocks collected on Inaccessible, Nightingale and Middle islands. Our results show that volcanism on all three islands is young, in most cases < 600 ka. Previous ages from Nightingale and Middle islands are therefore too old and should be ignored in interpretations of plume dynamics. These results also show that magma was being supplied simultaneously to both Tristan and Gough over recent geologic time. Two possible explanations for this are that (1) there is a broad plume underlying the area, with focus

  19. Deciphering post-Deccan weathering and erosion history of South Indian Archean rocks from cryptomelane 40Ar-39Ar dating

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

    Since the extrusion of Deccan traps ~ 63 Ma ago, weathering and erosion processes have shaped the landscapes of this Peninsula India. This resulted in pervasive bauxitic weathering on traps and deep lateritic weathering of their basement on either side of the Western Ghats Escarpment, which separates a coastal lowland from an East-dipping highland plateau. Mn-rich lateritic profiles formed by supergene weathering of Late Archean manganiferous protores in the different greenstone belts are exposed on relict paleosurfaces, which are preserved at different elevations on the highland plateau and in the coastal lowland, allowing for direct comparison of paleosurfaces and geomorphological processes across one of the most prominent relief in the Indian peninsula. Detailed petrological and geochemical investigations of samples collected in the different Mn-rich lateritic profiles allowed for precise characterization of cryptomelane [Kx Mn8-xIV MnxIII O16, nH2O], a Mn-oxide suitable for 40Ar-39Ar dating. The ages obtained document major weathering periods at ~ 53-50 Ma, ~ 40-32 Ma, and ~ 30-23 Ma in the highland profiles, and ~ 47-45 Ma, ~ 24-19 Ma and a younger age at ~ 9 Ma in the coastal lowland profiles. The age clusters are in good agreement with major regional and global Cenozoic paleoclimatic events, e.g., the Eocene climatic optimum and the early beginnings of Asian monsoons at ~ 40 Ma. The old ages obtained both in the coastal lowland and high plateau indicate synchronous lateritic (mostly bauxitic) weathering on both sides of the escarpment. The ages also indicate that most of the incision and dissection of plateau landsurfaces must have taken place during successive periods after 45, 32 and 23 Ma, while the coastal lowland surface was only weakly incised after 19 Ma. Our results thus document post-Eocene divergent erosion and weathering histories across the escarpment since it was formed at least 47 Ma ago, suggesting installation of a dual climatic regime on

  20. Rb-Sr whole-rock and mineral ages, K-Ar, 40Ar/39Ar, and U-Pb mineral ages, and strontium, lead, neodymium, and oxygen isotopic compositions for granitic rocks from the Salinian Composite Terrane, California:

    USGS Publications Warehouse

    Kistler, R.W.; Champion, D.E.

    2001-01-01

    This report summarizes new and published age and isotopic data for whole-rocks and minerals from granitic rocks in the Salinian composite terrane, California. Rubidium-strontium whole-rock ages of plutons are in two groups, Early Cretaceous (122 to 100 Ma) and Late Cretaceous (95 to 82 Ma). Early Cretaceous plutons occur in all granitic rock exposures from Bodega Head in the north to those from the Santa Lucia and Gabilan Ranges in the central part of the terrane. Late Cretaceous plutons have been identified in the Point Reyes Peninsula, the Santa Lucia and the Gabilan Ranges, and in the La Panza Range in the southern part of the terrane. Ranges of initial values of isotopic compositions are 87Sr/86Sr, 0.7046-0.7147, δ18O, +8.5 to +12.5 per mil, 206Pb/204Pb, 18.901-19.860, 207Pb/204Pb, 15.618-15.814, 208Pb/204Pb, 38.569- 39.493, and εNd, +0.9 to -8.6. The initial 87Sr/86Sr=0.706 isopleth is identified in the northern Gabilan Range and in the Ben Lomond area of the Santa Cruz Mountains, in Montara Mountain, in Bodega Head, and to the west of the Farallon Islands on the Cordell Bank. This isotopic boundary is offset about 95 miles (160km) by right-lateral displacements along the San Gregorio-Hosgri and San Andreas fault systems.

  1. High Resolution 40Ar/39Ar Geochronology of the Tuvalu Seamount Chain: Implications for Hotspot Longevity and Pacific Plate Motion.

    NASA Astrophysics Data System (ADS)

    Konrad, K.; Finlayson, V. A.; Koppers, A. A. P.; Konter, J.; Jackson, M. G.

    2014-12-01

    The Tuvalu seamount chain is a Mid-Pacific (4-11oS, 175-179oE) linear volcanic chain that was previously poorly sampled. Absolute plate motion (APM) models predict a long-lived relationship with hotspot activity in French Polynesia. The lack of detailed age data therefore results in a key chronologic gap in the geologic history of this hotspot and current APM models. Depending on the set of assumptions employed, previous APM models have disagreed on which known hotspot chain, if any, the Tuvalu volcanoes are associated with. Based on APM modeling and geochemical affinities (HIMU, 206Pb/204Pb > 20), Konter et al. (2008) argue that Rurutu Island (French Polynesia) represents the modern location of the hotspot that contributed volcanism to the Tuvalu seamounts. This model traces the hotspot chain from Rurutu through the region of modern day Samoa, the Tuvalu seamounts, the Gilbert ridge, and into the North & South Wake islands. This hypothesis suggests that a single HIMU mantle reservoir can exist and remain relatively geochemically consistent over 100 Myrs. On the contrary, the Wessel and Kroenke (2008) APM model suggest the Tuvalu seamounts and N & S Wake are unrelated. This model requires the N & S Wake chains to rotate significantly at the young end of the Gilbert Ridge resulting in a current hotspot location around 13-15oS and 156-155oW, away from any known active volcanism. During the summer of 2013, 25 Tuvaluan seamounts and 9 seamounts near the current Samoan chain were dredged onboard the R.V. Roger Revelle (expedition RR1310). Here we present 43 new 40Ar-39Ar ages covering 19 Tuvaluan seamounts and four seamounts within the Samoan hotspot track. These ages provide insights into the contributing hotspot for Tuvaluan volcanism and provide a new reference frame for constraining Pacific APM models. The corresponding chemical analyses for a subset of these seamounts will be presented by Finlayson et al. (this volume). Konter, J. G. et al. One hundred million

  2. [sup 40]Ar/[sup 39]Ar geochronology of Cenozoic magmatism and faulting, Yerington and northern Wassuk Range Nevada

    SciTech Connect

    Dilles, J.H. . Geosciences Dept.); Gans, P.B. . Dept. of Geological Sciences)

    1993-04-01

    [sup 40]Ar/[sup 39]Ar radiometric ages ([+-] sigma) refine the ages of magmatism and both normal and strike-slip faulting in the Yerington district and to the east in the northern Wassuk Range portion of the Walker Lane. The oldest rhyolite ignimbrites (28.58 [+-] 0.04 Ma (san)) underlie the Mickey Pass Tuff. Two rhyolite ignimbrites yielded 24.66 [+-] 0.02 (san) and 24.60 [+-] 0.02 (san) Ma and lie in angular unconformity upon the [approximately]26 Ma Singatse Tuff within a NW-striking fault zone in that N. Wassuk Ra., recording the earliest tectonism in the Walker Lane here. In this area, an ignimbrite from the upper part of the Hu-Pwi Rhyodacite yielded 23.09 [+-] 0.04 Ma (bi), and is cut by faults intruded by pyroxene andesites dated at 22.16 [+-] 0.27 Ma (wr). The andesite of Lincoln Flat is closely associated with tectonism throughout the Yerington-N. Wassuk area, and yielded four ages: 14.95 [+-] 0.24 (hbl), 14.08 [+-] 0.23 (hbl), 12.85 [+-] 0.33 (hbl), and 13.83 [+-] 0.17 (hbl) Ma. The 14.95 Ma-andesite is late-tectonic, whereas the 14.08 and 12.85 Ma andesites post-date early, northwest-striking vertical (strike-slip ) and normal faults in the N. Wassuk portion of the Walker Lane. In summary, tectonism initiated in the region within the Walker Lane portion of the northern Wassuk Ra. at [approximately]25 Ma, was active between 23 and 22 Ma, and in the interval prior to 14 Ma. Rapid crustal extension migrated westward to the Yerington district, where normal faulting and 30--40 W -- tilting of the upper crust initiated at 14 Ma and proceeded rapidly for the next 1--2 m.y. This period was succeeded by normal-oblique slip and strike-slip faults bounding Wassuk Group sedimentary basins in the Wassuk Ra. at [approximately]9 Ma and synchronously tilting them 10--35 W. Since [approximately]6 Ma, modern range-front normal faults and NW-striking strike-slip faults have yielded reduced rates of crustal extension and tilting.

  3. Ar-39/Ar-40 and Space Exposure Ages of the Unique Portales Valley H-Chondrite

    NASA Technical Reports Server (NTRS)

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

    2001-01-01

    The space exposure age of the unique Portales Valley H-chondrite is approx. 40-45 Myr. The 39 Ar-40 Ar ages of two samples are 4.477 +/- 0.016 and 4.46 +/- 0.02 Ga and show no evidence of more recent disturbance, in contrast to previous radiometric determinations Additional information is contained in the original extended abstract..

  4. 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

  5. Long-lived structural control of Mt. Shasta's plumbing system illuminated by 40Ar/39Ar geochronology

    NASA Astrophysics Data System (ADS)

    Calvert, A. T.; Christiansen, R. L.

    2013-12-01

    Mt. Shasta is the largest stratovolcano in the Cascade Arc, surpassed in volume only by the large rear-arc Medicine Lake and Newberry composite volcanoes. Including the material in the ~350 ka debris avalanche, it has produced more than 500 km3 of andesite and dacite from several superimposed central vents over its 700-850 kyr history. Earlier, between at least 970 to 1170 ka, the Rainbow Mountain volcano of similar composition and size occupied this latitude of the arc ~20 km further east. This shift of magmatic focus from within the arc axis (as defined by 6 Ma and younger calc-alkaline centers) to the arc front is poorly understood, but the current center's location appears structurally controlled. Most identifiable volcanic vents on Mt. Shasta lie within 1 km of a N-S line through the active summit cone. 40Ar/39Ar ages of map units occupying the vent alignment range from the Holocene (5×1 ka) current summit dome to at least the Middle Pleistocene (464×9 ka McKenzie Butte). The vast majority of eruptions have issued from central vents (Sargents Ridge, 300-135 ka; Misery Hill, 100-15 ka; and Hotlum, <10 ka), each 500 to 1000m north of its predecessor. A central vent for the pre-avalanche edifice is impossible to locate precisely, but was possibly on the same N-S trend and certainly no more than 4 km to the west, likely south of the Sargents Ridge central vent. ~15 of ~25 mapped flank vents lie on the alignment and the other ten lie west of the line. No identified volcanic vents lie east of the line until >12 km from Mt. Shasta (Ash Creek Butte, 227 ka; Basalt of McCloud River, 38 ka; The Whaleback, 102 ka), and monogenetic and polygenetic centers further east and northeast. From these observations we infer that: (1) magmas are localized along a ~20 km, long-lived, N-S trending structure running through the summit; (2) the upper crustal structure appears impermeable to magmas and resistant to dikes on its eastern side; (3) the western half of the area beneath

  6. Mesozoic rift magmatism in the North Sea region: 40Ar/39Ar geochronology of Scanian basalts and geochemical constraints

    NASA Astrophysics Data System (ADS)

    Bergelin, Ingemar; Obst, Karsten; Söderlund, Ulf; Larsson, Kent; Johansson, Leif

    2011-06-01

    More than 100 volcanic necks composed of basanites and melanephelinites occur in Scania, southern Sweden, at the junction of two major tectonic lineaments, the Phanerozoic Sorgenfrei-Tornquist Zone (STZ) and the Proterozoic Protogine Zone. New 40Ar/39Ar isotope analyses of whole rock fragments of nine selected basalt necks suggest that the Mesozoic alkaline volcanism in the Scanian province commenced earlier than previously reported and comprised three separate volcanic episodes that span a total period of ca. 80 Myr: a first Jurassic (191-178 Ma), a second at the Jurassic/Cretaceous boundary (ca. 145 Ma), and a final middle Cretaceous episode (ca. 110 Ma). The new results allow for precise time correlations between eruption events in the Scanian and those in the North Sea volcanic provinces. The older, early Jurassic event in Scania is largely synchronous with that in the Egersund Basin and the Forties field whereas the event at ca. 145 Ma is correlated with activity in the Central Graben. These volcanic episodes also correlate in age with Kimmerian tectonic activity. Volcanic activity in the middle Cretaceous period has also been dated in the triple junction in the North Sea and offshore in the Netherland Sector. The correlation of basalt volcanism in Scania with the Egersund nephelinites strongly suggest that volcanism was triggered by repeated tectonic activity along the STZ. Geochemical data of alkaline mafic rocks in the Scanian and the North Sea volcanic provinces imply that different provinces have largely unique geochemical signatures in favour of a heterogeneous mantle in the North Sea volcanic region. However, basalts of different generations in one and the same province cannot be readily separated on the basis of geochemistry, suggesting that the same lithospheric mantle was the source of repeated volcanism over time in each province. The data suggest a low degree of melting of a volatile-bearing mantle lherzolite enriched in incompatible elements with

  7. Multi-method low-temperature geochronology from the bergell pluton, italian alps: Utilizing K-feldspar 40Ar/39Ar multidiffusion domain modeling, conventional 40Ar/39Ar biotite dating, U-Th-He apatite and fission-track dating.

    NASA Astrophysics Data System (ADS)

    Page, L.; Dunai, T.; Wijbrans, J.; Andriessen, P.

    2003-04-01

    The Bergell batholith of SE Switzerland and NE Italy is a deep-seated igneous body which intruded previously emplaced and metamorphosed Alpine nappes at c. 32 Ma (U-Pb Zircon, von Blanckenburg 1990) The intrusion is located at the junction between the Eastern, Southern and Central Alps and is located along the late Alpine Insubric Fault Zone which has been linked with deep crustal lithospheric processes in the Alps. .K-Feldspar 40Ar/39Ar MDD geochronology from a suite of samples from different elevations in the Bergell Pluton has been obtained as part of an on-going multi-method geochronology study which also includes conventional 40Ar/39Ar step-heating of biotites, U-Th-He dating and Fission Track dating to 1) fully constrain the tectonothermal history of the region and 2) test the different isotopic methods against each other. The results to date from high-resolution K-Feldspar samples from a vertical profile of c. 1300 m demonstrate that the first-order characteristics of the release spectra are consistent with different splits and show a trend with elevation with highest elevations leveling out at a 30 Ma maximum and lower elevations reaching 25 Ma. K-Feldspar multidiffusion domain modeling confirms the height vs. cooling age relationship and yields discernable changes in the cooling rate from c. 10 C/MY to c. 60 C/MY which may represent discrete tectonic episodes. The 40Ar/39Ar biotite ages range from 23-26 Ma and are generally consistent with the K-Feldspar results. U-Th-He apatite ages from the region range from 7-15 Ma. New fission-track ages from these samples will be presented.

  8. 40Ar/39Ar geochronology of the Neogene-Quaternary Harrat Al-Madinah intercontinental volcanic field, Saudi Arabia: Implications for duration and migration of volcanic activity

    NASA Astrophysics Data System (ADS)

    Moufti, M. R.; Moghazi, A. M.; Ali, K. A.

    2013-01-01

    New 40Ar/39Ar ages, based on incremental heating techniques for groundmass separates of 25 samples, are presented for the Harrat Al-Madinah volcanic field, part of Harrat Rahat in the north western part of the Arabian plate. This area is an active volcanic field characterized by the occurrence of two historical eruptions approximately in 641 and 1256 AD. Field investigations of the main volcanic landforms indicate dominantly monogenetic strombolian eruptions, in addition to local more explosive eruptions. The lavas consist mainly of olivine basalt and hawaiite flows with minor evolved rocks of mugearite, benmoreite, and trachyte that occur mainly as domes, tuff cones and occasionally as lava flows. Previous K/Ar dating shows that the Harrat Al-Madinah lava flows and associated domes comprise seven units spanning an age range of ca. 1.7 Ma-Recent. The new 40Ar/39Ar age determinations confirm, to a great extent, the previously obtained K/Ar ages in the sense that no major systematic biases were found in the general stratigraphy of the different flow units. However, the 40Ar/39Ar plateau ages show that volcanism in this area began in the Neogene (˜10 Ma) and continued to Recent, with the most voluminous eruptions occurring in the Quaternary. Neogene volcanism occurred in at least three pulses around 10, 5 and 2 Ma, whereas Quaternary volcanism produced at least seven units reflecting lava flow emplacement in the time period of 1.90 Ma-Recent. Thus, the whole duration of volcanic activity in the Harrat Al-Madinah (10 Ma-Recent) appears much longer than that previously identified. The longevity of volcanism in the same part of the moving Arabian plate and absence of evidence for uni-directional migration of volcanic activity indicate that there is no fixed plume beneath this region. The NNW-trending distribution of the volcanic vents is parallel to the Red Sea, and suggests their origin is related to periodic extensional episodes along the reactivated Red Sea fault

  9. Evidence for pre-Taconic metamorphism in the Potomac terrane, Maryland and Virginia: Hornblende and Muscovite [sup 40]Ar/[sup 39]Ar results

    SciTech Connect

    Becker, J.L.; Wintsch, R.P. . Dept. of Geological Sciences); Kunk, M.J.; Drake, A.A. Jr. )

    1993-03-01

    New [sup 40]Ar/[sup 39]Ar age spectra of hornblende and white mica from the Great Falls area of the Potomac terrane of Maryland and Virginia indicate pre-Taconic metamorphism. Age spectra of hornblende samples are interpreted to represent cooling from peak metamorphic conditions through their closure temperatures for argon diffusion ([approximately]500C) at about 490 Ma. These older Ordovician postmetamorphic cooling ages strongly contrast with younger post-Ordovician metamorphic cooling ages now being reported in the Blue Ridge and Goochland terranes to the west and east respectively. A late phyllitic sheen observed on rocks in the field and petrographic observations of undulose plagioclase and amphibole, and older muscovite, and kinked primary muscovite in the Bear Island Granodiorite reflect a younger retrogressive metamorphism involving the growth of secondary muscovite (Fisher's S4 ). [sup 40]Ar/[sup 39]Ar Age spectra of white micas from the Bear Island Granodiorite are complex and probably indicate both primary and secondary white mica, the latter apparently growing below the closure temperature for retention of argon in muscovite ([approximately]350C). The age spectra permit an estimate of a minimum age of 420 Ma for cooling through closure of the older generation of white mica. The above ages of hornblende and muscovite closure imply a minimum cooling rate of [approximately]2C/m.y., and exhumation rate of about 1 mm/yr. The projected time of peak metamorphism at upper amphibolite facies for the Great Falls area clearly predates the Ordovician Taconic orogeny and suggests that these rocks escaped this event and largely escaped younger Paleozoic metamorphic events, which are well documented in adjacent terranes.

  10. Reconstructing western Grand Canyon's lava dams and their failure mechanisms: new insights from geochemical correlation and 40Ar/39Ar dating

    NASA Astrophysics Data System (ADS)

    Crow, R.; Karlstrom, K. E.; McIntosh, W. C.; Peters, L.; Dunbar, N. W.

    2010-12-01

    New geochemical analyzes and 40Ar/39Ar dating of lava dam remnants allows for the more accurate reconstruction of the timing, extent, and structure of western Grand Canyon’s lava dams. Whole-rock major, trace, and rare-earth element (REE) analyzes on over 60 basaltic lava dam remnants, cascades, plugs, and basaltic alluvium, show compositional variation from basanites to alkali basalts to tholeiites. Whitmore Canyon flows, for example, are some of the only tholeiitic flows and have a distinguishable trace and REE composition, which allows for correlation of dam remnants. Over 30 new high-precision 40Ar/39Ar dates also aid in remnant correlation and establish a better-constrained sequence of intra-canyon lava dams. Reliable 40Ar/39Ar dates on western Grand Canyon’s intra-canyon basalts range from ca. 100 ka to 840 ka (new date). The best understood lava dam formed from tholeiitic flows that erupted on the north rim, flowed down Whitmore side canyon and blocked a 6-km-long reach of the Grand Canyon. The youngest of these flows is unique because we know its age (200ka), its composition (tholeiitic), and the exact area where it entered Grand Canyon. The highest flow in the resulting dam, Whitmore Cascade, is capped with very coarse basaltic alluvium that previous workers have attributed to an upstream catastrophic dam failure event at about 200 ka. However, strong similarities between the geochemistry and age of the alluvium with the underlying Whitmore Cascade flow suggest that the alluvial deposit is related to failure of the 200 ka Whitmore Cascade dam itself. Similarly the 100 ka Upper Gray Ledge flow is commonly overlain by a balsaltic alluvium that is indistinguishable in terms of age and geochemistry from the underlying Upper Gray Ledge flow. These observations lead to a new model for Grand Canyon lava dams by which lava dams undergo multi-staged failure where the upstream parts of dams fail quickly (sometimes catastrophically) but downstream parts are

  11. Contrasting tectonothermal domains and faulting in the Potomac terrane, Virginia-Maryland - Discrimination by 40Ar/39Ar and fission-track thermochronology

    USGS Publications Warehouse

    Kunk, M.J.; Wintsch, R.P.; Naeser, C.W.; Naeser, N.D.; Southworth, C.S.; Drake, A.A., Jr.; Becker, J.L.

    2005-01-01

    New 40Ar/39Ar data reveal ages and thermal discontinuities that identify mapped and unmapped fault boundaries in the Potomac terrane in northern Virginia, thus confirming previous interpretations that it is a composite terrane. The rocks of the Potomac terrane were examined along the Potomac River, where it has been previously subdivided into three units: the Mather Gorge, Sykesville, and Laurel Formations. In the Mather Gorge Formation, at least two metamorphic thermal domains were identified, the Blockhouse Point and Bear Island domains, separated by a fault active in the late Devonian. Early Ordovician (ca. 475 Ma) cooling ages of amphibole in the Bear Island domain reflect cooling from Taconic metamorphism, whereas the Blockhouse Point domain was first metamorphosed in the Devonian. The 40Ar/39Ar data from muscovites in a third (eastern) domain within the Mather Gorge Formation, the Stubblefield Falls domain, record thrusting of the Sykesville Formation over the Mather Gorge Formation on the Plummers Island fault in the Devonian. The existence of two distinctly different thermal domains separated by a tectonic boundary within the Mather Gorge argues against its status as a formation. Hornblende cooling ages in the Sykesville Formation are Early Devonian (ca. 400 Ma), reflecting cooling from Taconic and Acadian metamorphism. The ages of retrograde and overprinting muscovite in phyllonites from domain-bounding faults are late Devonian (Acadian) and late Pennsylvanian (Alleghanian), marking the time of assembly of these domains and subsequent movement on the Plummers Island fault. Our data indicate that net vertical motion between the Bear Island domain of the Mather Gorge complex and the Sykesville Formation across the Plummers Island fault is east-side-up. Zircon fission-track cooling ages demonstrate thermal equillbrium across the Potomac terrane in the early Permian, and apatite fission-track cooling ages record tilting of the Potomac terrane in the Cretaceous

  12. A re-examination of petrogenesis and 40Ar/39Ar systematics in the Chain of Ponds K-feldspar: "diffusion domain" archetype versus polyphase hygrochronology

    NASA Astrophysics Data System (ADS)

    Chafe, Alex N.; Villa, Igor M.; Hanchar, John M.; Wirth, Richard

    2014-05-01

    K-feldspar (Kfs) from the Chain of Ponds Pluton (CPP) is the archetypal reference material, upon which thermochronological modeling of Ar diffusion in discrete "domains" was founded. We re-examine the CPP Kfs using cathodoluminescence and back-scattered electron imaging, transmission electron microscopy, and electron probe microanalysis. 40Ar/39Ar stepwise heating experiments on different sieve fractions, and on handpicked and unpicked aliquots, are compared. Our results reproduce the staircase-shaped age spectrum and the Arrhenius trajectory of the literature sample, confirming that samples collected from the same locality have an identical Ar isotope record. Even the most pristine-looking Kfs from the CPP contains successive generations of secondary, metasomatic/retrograde mineral replacements that post-date magmatic crystallization. These chemically and chronologically distinct phases are responsible for its staircase-shaped age spectra, which are modified by handpicking. While genuine within-grain diffusion gradients are not ruled out by these data, this study demonstrates that the most important control on staircase-shaped age spectra is the simultaneous presence of heterochemical, diachronous post-magmatic mineral growth. At least five distinct mineral species were identified in the Kfs separate, three of which can be traced to external fluids interacting with the CPP in a chemically open system. Sieve fractions have size-shifted Arrhenius trajectories, negating the existence of the smallest "diffusion domains." Heterochemical phases also play an important role in producing nonlinear trajectories. In vacuo degassing rates recovered from Arrhenius plots are neither related to true Fick's Law diffusion nor to the staircase shape of the age spectra. The CPP Kfs used to define the "diffusion domain" model demonstrates the predominance of metasomatic alteration by hydrothermal fluids and recrystallization in establishing the natural Ar distribution among different

  13. Integrating 40Ar-39Ar, 87Rb-87Sr and 147Sm-143Nd geochronology of authigenic illite to evaluate tectonic reactivation in an intraplate setting, central Australia

    NASA Astrophysics Data System (ADS)

    Middleton, Alexander W.; Uysal, I. Tonguç; Bryan, Scott E.; Hall, Chris M.; Golding, Suzanne D.

    2014-06-01

    The Warburton-Cooper basins, central Australia, include a multitude of reactivated fracture-fault networks related to a complex, and poorly understood, tectonic evolution. We investigated authigenic illites from a granitic intrusion and sedimentary rocks associated with prominent structural features (Gidgealpa-Merrimelia-Innamincka Ridge and the Nappamerri Trough). These were analysed by 40Ar-39Ar, 87Rb-87Sr and 147Sm-143Nd geochronology to explore the thermal and tectonic histories of central Australian basins. The combined age data provide evidence for three major periods of fault reactivation throughout the Phanerozoic. While Carboniferous (323.3 ± 9.4 Ma) and Late Triassic ages (201.7 ± 9.3 Ma) derive from basin-wide hydrothermal circulation, Cretaceous ages (∼128 to ∼86 Ma) reflect episodic fluid flow events restricted to the synclinal Nappamerri Trough. Such events result from regional extensional tectonism derived from the transferral of far-field stresses to mechanically and thermally weakened regions of the Australian continent. Specifically, Cretaceous ages reflect continent-wide transmission of tensional stress from a >2500 km long rifting event on the eastern (and southern) Australian margin associated with break-up of Gondwana and opening of the Tasman Sea. By integrating 40Ar-39Ar, 87Rb-87Sr and 147Sm-143Nd dating, this study highlights the use of authigenic illite in temporally constraining the tectonic evolution of intracontinental basins that would otherwise remain unknown. Furthermore, combining Sr- and Ar-isotopic systems enables more accurate dating of authigenesis whilst significantly reducing geochemical pitfalls commonly associated with these radioisotopic dating methods.

  14. Temporal variations in the cooling and denudation history of the Hunza plutonic complex, Karakoram Batholith, revealed by 40Ar/39Ar thermochronology

    NASA Astrophysics Data System (ADS)

    Krol, Michael A.; Zeitler, Peter K.; Poupeau, GéRard; Pecher, Arnaud

    1996-04-01

    The 40Ar/39Ar thermochronology of the Late Cretaceous Hunza plutonic complex reveals an episodic cooling and denudation history for this regional-scale pluton. The 40Ar/39Ar analyses of biotites from a vertical relief section of >3200 m reveal a pulse of rapid cooling at ˜20 Ma. In the interval of 110-27 Ma, age-elevation distributions suggest denudation rates of the order of 0.02 ± 0.003 mm/yr. At ˜20 Ma, denudation rates increased significantly to 2.7 ± 0.7 mm/yr, then returned to much slower rates until 12 Ma. A second pulse of rapid cooling beginning at 12 Ma is revealed by inverse numerical modeling of multidiffusion domain alkali feldspars from a vertical section of 1700 m. Decreasing in elevation, these samples record the onset of rapid cooling at 12, 9, and 7 Ma, respectively. All of the alkali feldspars record a period of nearly isothermal conditions prior to the onset of rapid cooling when rates increased to 30°C/m.y. Assuming a geothermal gradient of 30°C/km, these cooling rates translate into denudation rates of 1.0 mm/yr. Apatite fission track analysis indicates denudation rates of 0.7 ± 0.1 mm/yr over the interval of 6.6 Ma to 2.4 Ma in agreement with the alkali feldspar data. These data suggest denudation of 2.9 ± 0.4 km since the Plio-Pleistocene. Together, the alkali feldspar and apatite data indicate that a minimum of 10 km of overburden has been removed since the mid-late Miocene. An electronic supplement of this material may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN to AGU's FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system).(Paper 95TC02424, Temporal variations in the cooling and denudation history of the Hunza plutonic complex, Karakoram Batholith, revealed by 40Ar/39Ar thermochronology, M.A. Krol, P.K. Zeitler, G

  15. Application of U/Th and 40Ar/39Ar Dating to Orgnac 3, a Late Acheulean and Early Middle Palaeolithic Site in Ardèche, France

    PubMed Central

    Michel, Véronique; Shen, Guanjun; Shen, Chuan-Chou; Wu, Chung-Che; Vérati, Chrystèle; Gallet, Sylvain; Moncel, Marie-Hélène; Combier, Jean; Khatib, Samir; Manetti, Michel

    2013-01-01

    Refined radio-isotopic dating techniques have been applied to Orgnac 3, a Late Acheulean and Early Middle Palaeolithic site in France. Evidence of Levallois core technology appeared in level 4b in the middle of the sequence, became predominant in the upper horizons, and was best represented in uppermost level 1, making the site one of the oldest examples of Levallois technology. In our dating study, fourteen speleothem samples from levels 7, 6 and 5b, were U/Th-dated. Four pure calcite samples from the speleothem PL1 (levels 5b, 6) yield ages between 265 ± 4 (PL1-3) and 312 ± 15 (PL1-6) thousand years ago (ka). Three samples from the top of a second stalagmite, PL2, yield dates ranging from 288 ± 10 ka (PL2-1) to 298 ± 17 ka (PL2-3). Three samples from the base of PL2 (level 7) yield much younger U/Th dates between 267 and 283 ka. These dates show that the speleothems PL1 and PL2 are contemporaneous and formed during marine isotope stage (MIS) 9 and MIS 8. Volcanic minerals in level 2, the upper sequence, were dated by the 40Ar/39Ar method, giving a weighted mean of 302.9 ± 2.5 ka (2σ) and an inverse isochron age of 302.9 ± 5.9 ka (2σ). Both 40Ar/39Ar dating of volcanic sanidines and U/Th dating of relatively pure and dense cave calcites are known to be well established. The first parallel application of the two geochronometers to Orgnac 3 yields generally consistent results, which point to the reliability of the two methods. The difference between their age results is discussed. PMID:24349273

  16. Application of U/Th and 40Ar/39Ar dating to Orgnac 3, a Late Acheulean and Early Middle Palaeolithic site in Ardèche, France.

    PubMed

    Michel, Véronique; Shen, Guanjun; Shen, Chuan-Chou; Wu, Chung-Che; Vérati, Chrystèle; Gallet, Sylvain; Moncel, Marie-Hélène; Combier, Jean; Khatib, Samir; Manetti, Michel

    2013-01-01

    Refined radio-isotopic dating techniques have been applied to Orgnac 3, a Late Acheulean and Early Middle Palaeolithic site in France. Evidence of Levallois core technology appeared in level 4b in the middle of the sequence, became predominant in the upper horizons, and was best represented in uppermost level 1, making the site one of the oldest examples of Levallois technology. In our dating study, fourteen speleothem samples from levels 7, 6 and 5b, were U/Th-dated. Four pure calcite samples from the speleothem PL1 (levels 5b, 6) yield ages between 265 ± 4 (PL1-3) and 312 ± 15 (PL1-6) thousand years ago (ka). Three samples from the top of a second stalagmite, PL2, yield dates ranging from 288 ± 10 ka (PL2-1) to 298 ± 17 ka (PL2-3). Three samples from the base of PL2 (level 7) yield much younger U/Th dates between 267 and 283 ka. These dates show that the speleothems PL1 and PL2 are contemporaneous and formed during marine isotope stage (MIS) 9 and MIS 8. Volcanic minerals in level 2, the upper sequence, were dated by the (40)Ar/(39)Ar method, giving a weighted mean of 302.9 ± 2.5 ka (2σ) and an inverse isochron age of 302.9 ± 5.9 ka (2σ). Both (40)Ar/(39)Ar dating of volcanic sanidines and U/Th dating of relatively pure and dense cave calcites are known to be well established. The first parallel application of the two geochronometers to Orgnac 3 yields generally consistent results, which point to the reliability of the two methods. The difference between their age results is discussed. PMID:24349273

  17. NW termination of the West Cycladic Detachment System on the Lavrion Peninsula, Greece: results from mica 40Ar/39Ar geochronology

    NASA Astrophysics Data System (ADS)

    Schneider, David; Dubosq, Renelle; Spalding, Jennifer; Grasemann, Bernhard; Soukis, Kostis

    2015-04-01

    The West Cycladic Detachment System (WCDS) has been mapped from the western Cycladic islands to the Lavrion peninsula, where several top-to-SSW low-angle normal faults at different structural levels are observed. The dominant detachment juxtaposes lower plate schistose to mylonitic rocks of the Kamariza Unit against the upper plate Lavrion Unit. The Kamariza Unit exhibits a NNE-SSW stretching lineation whereas the main foliation in the Lavrion Unit is a compressional crenulation cleavage with ENE-WSW to NE-SW stretching and intersection lineations. Kinematic indicators reveal top-to-SSW sense of shear, and along the detachment both units are overprinted by cataclastic deformation and high-temperature metallic ore mineralization. White mica-bearing schists and marbles were collected for microstructural and 40Ar/39Ar geochronology. Quartz crystals in all samples show subgrain rotation and bulging of grains, indicated by lobate grain boundaries. These same rocks contain interconnected elongated mica crystals, which are kinked or internally deformed by C'-type shear zones. White mica is rarely prismatic, and chemical mapping highlights the chemical zonation of the muscovite. Calcite, when present, exhibits curved and tapered twins. Glaucophane and chlorite pseudomorphs of glacophane are preserved in the Lavrion Unit. New single-crystal 40Ar/39Ar geochronology on muscovite from the upper plate Lavrion Unit of the peninsula yields ages between 35-28 Ma, and together with published zircon (U-Th)/He dates of 16-12 Ma and preservation of glaucophane suggests these rocks did not witness the dominant Miocene greenschist facies deformation that characterizes the WCDS. One muscovite sample along the west coast at Thimari yielded an 40Ar/39Ar age of c. 175 Ma (duplicate analyses) and maybe part of the Sub-Pelagonian Berzekos Unit. Comparatively, the Kamariza Unit yields Early to Middle Miocene 40Ar/39Ar ages, and coupled with Late Miocene zircon (U-Th)/He ages and top

  18. 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

    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.

  19. High precision 40Ar/39Ar dating of Oligocene rhyolites from the Mogollon-Datil Volcanic Field using a continuous laser system

    NASA Astrophysics Data System (ADS)

    Dalrymple, G. Brent; Duffield, Wendell A.

    1988-05-01

    Replicate 40Ar/39Ar analyses of sanidine from seven Oligocene (28.2 Ma) rhyolite flows using a continuous laser dating system routinely yield analytical precisions of less than 0.45% (1s) on samples smaller than 0.5 mg. This is as good or better than has been obtained by any other method even though the sample mass is several orders of magnitude smaller. The system has the potential of resolving differences in apparent age of as little as 0.2% for high K/Ca samples as young as a few Ma. The analyses suggest that the seven volcanic units analyzed were erupted in an interval of about 0.1 m.y. or less.

  20. High Spatial Resolution 40Ar/39Ar Geochronology of Impact Melt Breccias from Apollo 17 Boulders at Stations 2, 6, and 7

    NASA Astrophysics Data System (ADS)

    Mercer, C. M.; Hodges, K. V.; Jolliff, B. L.; Van Soest, M. C.; Wartho, J. A.; Weirich, J. R.

    2015-12-01

    Several boulders located at the bases of the North and South Massifs were among the primary field targets of the Apollo 17 mission to the Taurus-Littrow Valley on the Moon [1]. Some boulders are polylithologic, including Boulder 1 at Station 2 and the boulders at Stations 6 and 7. These boulders were the subjects of consortium studies [2, 3] that included 40Ar/39Ar geochronology to determine the ages of distinct lithologies within each boulder [e.g., 4-6]. We report new 40Ar/39Ar data for the impact melt breccias 72255, 76315, 77075, and 77135 obtained using the UV laser ablation microprobe (UVLAMP) methods of [7]. For 72255, we obtained a preliminary isochron date ca. 3814 Ma from 22 melt analyses, which is younger than published plateau dates (e.g., 3951-3835 Ma [4, 8]). Fifteen melt analyses of 76315 yield a preliminary isochron date ca. 3850 Ma, younger than the 3900 ± 16 Ma date reported by [8]. Melt analyses of 77075 yield preliminary dates between ca. 3797-3584 Ma, possibly reflecting partial loss of 40Ar. In this case, the oldest date may provide a minimum age for the formation of melt in 77075. Finally, the UVLAMP dates for the 77135 melt range from 3810-3361 Ma and corresponding Ca/K ratios range from ca. 100-6. Electron microprobe analyses of small (ca. 10s of microns wide) pockets of K-rich materials show that both K-rich glass and K-feldspar are present. The UVLAMP dates for 77135 likely reflect spatially variable 40Ar loss, consistent with published step heating results [e.g., 6]. References: [1] Schmitt (1973) Science, 182, 681-690. [2] Ryder (1993). Catalog of Apollo 17 Rocks: Volume 1 - Stations 2 and 3 (South Massif). LPI. [3] Ryder (1993). Catalog of Apollo 17 Rocks: Volume 4 - North Massif. LPI. [4] Leich et al. (1975) The Moon, 14, 407-444. [5] Cadogan & Turner (1976). LPSC, 7, 2267-2285. [6] Stettler et al. (1978). LPSC, 9, 1113-1115. [7] Mercer et al. (2015) Sci. Adv., 1, e1400050. [8] Dalrymple & Ryder (1996). JGR, 101, 26069-26084.

  1. Single Phase 40Ar/39Ar Dating of Rajahmundry Trap Basalts Contemporaneous With Late Stage Deccan Trap Volcanism

    NASA Astrophysics Data System (ADS)

    Knight, K. B.; Knight, K. B.; Renne, P. R.; Renne, P. R.; Halkett, A.; White, N.

    2001-12-01

    The Rajahmundry Traps of eastern peninsular India, often considered to be outliers of the Deccan Traps, occupy ~35 km2 centered on the Krishna-Godavari Basin and extending offshore in the sub-surface. Onshore exposures average 60m in thickness, including a laterally continuous sedimentary interlayer of laterite, limestone and shale ( ~2m thick, total) separating `upper' flows from `lower' flows. 40Ar/39Ar CO2 laser incremental heating analysis of twelve plagioclase separates from Rajahmundry Trap basalts reveal an age of ~64.6 Ma for the entire sequence based on the FCs standard at 28.02 Ma. Flows chosen for dating include 8 sites spanning both the `upper' and `lower' flow sequences. Paleontological studies of sediments adjacent to the basalt at depth in Krishna-Godavari Basin, e.g. Jaiprakash et al. (1993), suggest that the period of time covered by the two flows and intertrappean sediments is up to ca. 6 myr. Dates obtained for this study, however, show that ages for both upper and lower flows are indistinguishable within 2σ error from one another, and span ~2 Ma at most, pointing to a substantial hiatus in the sedimentary record at the top of the upper basalt flows. Extremely high Ca/K ratios (up to ~400) in several samples limits precision due to error propagation attending the large correction necessary for reactor produced 36Ar from Ca. However, plateau ages as precise as 64.8 +/- 0.4 and 65.5 +/- 0.8 from above and below (respectively, 2σ errors) the sedimentary interlayer have been obtained. Samples with both high and low Ca/K ratios confirm rapid eruption of the entire Rajahmundry Trap sequence. A petrogenetic link between these basalts and the Deccan Trap basalts (the remains of which lie over 300 km from the nearest exposure of Rajahmundry Trap) has been suggested but has yet to be substantiated. These new data clearly place the eruption of the Rajahmundry Traps temporally close to the K-T boundary, coincident with late stage Deccan volcanism and

  2. 40Ar/39Ar Dating of the Brunhes-Matuyama Geomagnetic Field Reversal.

    PubMed

    Baksi, A K; Hsu, V; McWilliams, M O; Farrar, E

    1992-04-17

    Magnetostratigraphic studies are widely used in conjunction with the geomagnetic polarity time scale (GPTS) to date events in the range 0 to 5 million years ago. A critical tie point on the GPTS is the potassium-argon age of the most recent (Brunhes-Matuyama) geomagnetic field reversal. Astronomical values for the forcing frequencies observed in the oxygen isotope record in Ocean Drilling Project site 677 suggest that the age of this last reversal is 780 ka (thousand years ago), whereas the potassium-argon-based estimate is 730 ka. Results from 4039; Ar incremental heating studies on a series of lavas from Maui that straddle the Brunhes-Matuyama reversal give an age of 783 + 11 ka, in agreement with the astronomically derived value. The astronomically based technique appears to be a viable tool for dating young sedimentary sequences. PMID:17743111

  3. Timing and duration of supergene mineralization at the Xinrong manganese deposit, western Guangdong Province, South China: cryptomelane 40Ar/39Ar dating

    NASA Astrophysics Data System (ADS)

    Li, Jian-Wei; Vasconcelos, Paulo; Zhang, Wei; Deng, Xiao-Dong; Duzgoren-Aydin, Nurdan; Yan, Dai-Rong; Zhang, Jian-Qiang; Hu, Ming-An

    2007-04-01

    Supergene Mn-oxide deposits are widely distributed in Guangxi, Guangdong, Yunnan, and Hunan Provinces, South China, accounting for 18% of the total Mn reserves in the country. Direct dating of supergene Mn enrichment, however, is lacking. In this paper, we present high-resolution 40Ar/39Ar ages of Mn oxides from the Xinrong Mn deposit, western Guangdong, to place numerical constraints on the timing and duration of supergene Mn enrichment. A total of ten cryptomelane samples, spanning a vertical extent of 67 m, were dated using the 40Ar/39Ar laser incremental heating technique, with seven samples yielding well-defined plateau or pseudo-plateau ages ranging from 23.48 ± 0.91 to 2.06 ± 0.05 Ma (2 σ). One sample yields a staircase spectrum that does not reach a plateau; the spectrum, however, indicates the presence of two or more generations of Mn oxides in the sample, whose ages are best estimated at 22.34 ± 0.31 and 10.2 ± 0.86 Ma, respectively. The remaining two samples gave meaningless or uninterpretable results due to significant 39Ar recoil and contamination by old phases. The 40Ar/39Ar data thus reveal a protracted history of weathering and supergene Mn enrichment that started at least in the end of the Oligocene or beginning of Miocene and extending into the latest Pliocene. Staircase-apparent age spectra, resulting from banded or botryoidal samples, yield an average growth rate of Mn oxides at 0.6-0.7 × 10-3 mm kyr-1. The values indicate that a 1-mm grain of Mn oxides may host minerals precipitated during a time span of ca. 1.5 m.y., and accumulation of Mn oxides to form economic deposits under weathering environments may take millions of years. The distribution of weathering ages shows that the oldest Mn oxides occur on the top of the profile, whereas the youngest minerals are found at the bottom, suggesting downward propagation of weathering fronts. However, two samples located at the intermediate depths of the profile yield ages comparable with those

  4. 40Ar/39Ar dating of Quaternary feldspar: examples from the Taupo Volcanic Zone, New Zealand

    USGS Publications Warehouse

    Pringle, M.S.; McWilliams, M.; Houghton, B.F.; Lanphere, M.A.; Wilson, C.J.N.

    1992-01-01

    Using a continuous laser and resistance furnace, we have measured ages on Quaternary plagioclase with an absolute precision of about ??30 ka and on Quaternary sanidine with a relative precision of better than 1%. Such precision was achieved by using low-temperature heating steps to remove much of the nonradiogenic argon contamination. Plagioclase is one of the most common mineral phases in volcanic rocks; thus, these procedures will be widely applicable to many problems for which precise radiometric age control has not been available. We studied plagioclase and plagioclase-sanidine concentrates from the oldest and the three largest silicic ash-flow deposits of the Taupo Volcanic Zone, New Zealand, one of the world's largest and most active volcanic systems. The results are in close agreement with new magnetostratigraphic data, suggesting that existing fission-track age determinations significantly underestimate the age of older units, and shift the inception of Taupo Vaolcanic Zone volcanism back to at least 1600 ka. -from Authors

  5. High-Precision 40Ar/39Ar Geochronology and Geology of St. George Island, Pribilof Islands, Alaska: Implications for Eruption Rates in the Bering Sea Basalt Province

    NASA Astrophysics Data System (ADS)

    Feeley, T. C.; Cosca, M. A.; Hamblock, J. M.; Underwood, S. J.

    2007-12-01

    New high-precision 40Ar/39Ar ages and geologic mapping establish an eruptive chronology for St. George Island, Pribilof Islands, Alaska. St. George is part of the Bering Sea basalt province (BSBP), a group of 15 late Cenozoic (mostly < 6 Ma) alkalic to tholeiitic basaltic volcanic fields widely distributed on islands in the Bering Sea, along the west coast of Alaska, and along the coast of northeast Russia. Twelve samples of washed, but otherwise untreated, whole-rock basalts from St. George were cut with a micro-wire saw into chips ~3 mm3 in size and irradiated for 40Ar/39Ar analysis. The chips were incrementally heated with a CO2 laser equipped with an integrator lens, and analyzed using a NU Instruments Noblesse mass spectrometer equipped with a Faraday cup and two ion counting electron multipliers. Detector intercalibration was done using automated air pipettes. A minimum of 20 heating steps were measured per sample, with the data often defining age plateaux. Isochron plots of the data yield ages ranging from 1.57 ± 0.04 to 2.89 ± 0.11 Ma, with trapped 40Ar/36Ar ratios ranging from 312 to 330. The stratigraphic positions of the dated rocks are known directly from field relations and there are no discrepancies between the 40Ar/39Ar ages and this sequence. Geochemical data combined with the age data indicate no progressive petrologic trends during evolution of the magmatic system, except for intermittent eruption of distinctive plagioclase-phyric basalts with low to moderate MgO contents (7 - 5 wt%) beginning at ~2.0 Ma. The new age data combined with volume estimates indicate an average subaerial eruption rate of ~107 m3km-2yr-1, which is adjusted for 3% sedimentary and ultramafic basement rocks beneath the volcanic pile, an average vesicularity of 5%, and an assumed surficial erosion value of 20%. This rate is identical to the estimate (110 m3km-2yr-1) by Mukasa et al. (JGR 112, 2007) for St. George Island. Both estimates, however, do not account for

  6. 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.

  7. 40Ar-39Ar step-heating of impact glasses from the Nördlinger Ries impact crater—Implications on excess argon in impact melts and tektites

    NASA Astrophysics Data System (ADS)

    Schwarz, Winfried H.; Lippolt, Hans J.

    2014-06-01

    Seven impact melts from various places in the Nördlinger Ries were dated by 40Ar-39Ar step-heating. The aim of these measurements was to increase the age data base for Ries impact glasses directly from the Ries crater, because there is only one Ar-Ar step-heating spectrum available in the literature. Almost all samples display saddle-shaped age spectra, indicating the presence of excess argon in most Ries glass samples, most probably inherited argon from incompletely degassed melt and possibly also excess argon incorporated during cooling from adjacent phases. In contrast, moldavites usually contain no inherited argon, probably due to their different formation process implying solidification during ballistic transport. The plateau age of the only flat spectrum is 14.60 ± 0.16 (0.20) Ma (2σ), while the total age of this sample is 14.86 ± 0.20 (0.22) Ma (isochron age: 14.72 ± 0.18 [0.22] Ma [2σ]), proofing the chronological relationship of the Ries impact and moldavites. The total ages of the other samples range between 15.77 ± 0.52 and 20.4 ± 1.0 Ma (2σ), implying approximately 2-40% excess 40Ar (compared to the nominal age of the Ries crater) in respective samples. Thus, the age of 14.60 ± 0.16 (0.20) (2σ) (14.75 ± 0.16 [0.20 Ma] [2σ], calculated using the most recent suggestions for the K decay constants) can be considered as reliable and is within uncertainties indistinguishable from the most recent compilation for the age of the moldavite tektites.

  8. Geodynamic interpretation of the 40Ar/39Ar dating of ophiolitic and arc-related mafics and metamafics of the northern part of the Anadyr-Koryak region

    USGS Publications Warehouse

    Palandzhyan, S.A.; Layer, P.W.; Patton, W.W., Jr.; Khanchuk, A.I.

    2011-01-01

    Isotope datings of amphibole-bearing mafics and metamafics in the northern part of the Anadyr-Koryak region allow clarification of the time of magmatic and metamorphic processes, which are synchronous with certain stages of the geodynamic development of the northwest segment of the Pacific mobile belt in the Phanerozoic. To define the 40Ar/39Ar age of amphiboles, eight samples of amphibole gabbroids and metamafics were selected during field work from five massifs representing ophiolites and mafic plutons of the island arc. Rocks from terranes of three foldbelts: 1) Pekulnei (Chukotka region), 2) Ust-Belaya (West Koryak region), and 3) the Tamvatnei and El'gevayam subterranes of the Mainits terrane (Koryak-Kamchatka region), were studied. The isotope investigations enabled us to divide the studied amphiboles into two groups varying in rock petrographic features. The first was represented by gabbroids of the Svetlorechensk massif of the Pekulnei Range and by ophiolites of the Tamvatnei Mts.; their magmatic amphiboles show the distribution of argon isotopes in the form of clearly distinguished plateau with an age ranging within 120-129 Ma. The second group includes metamorphic amphiboles of metagabbroids and apogabbro amphibolites of the Ust-Belaya Mts., Pekulnei and Kenkeren ranges (El'gevayam subterranes). Their age spectra show loss of argon and do not provide well defined plateaus the datings obtained for them are interpreted as minimum ages. Dates of amphiboles from the metagabbro of the upper tectonic plate of the Ust-Belaya allochthon points to metamorphism in the suprasubduction environment in the fragment of Late Neoproterozoic oceanic lithosphere in Middle-Late Devonian time, long before the Uda-Murgal island arc system was formed. The amphibolite metamorphism in the dunite-clinopyroxenite-metagabbro Pekulnei sequence was dated to occur at the Permian-Triassic boundary. The age of amphiboles from gabbrodiorites of the Kenkeren Range was dated to be Early

  9. 40Ar/39Ar dating of the Valsequillo volcanic deposits, Central Mexico: Resolution of an ongoing archaeological controversy and implications for the first human colonization of the 'New World'

    NASA Astrophysics Data System (ADS)

    Mark, Darren

    2010-05-01

    It is currently accepted that the Clovis culture was the first to migrate into the New World at 13.1 ka [1]. However, archeological evidence in the form of stone tools, linguistics, craniometrics and genetics suggest that the first Americans were ethnically diverse and a few sites dated to 15-16 ka BP challenge the 'Clovis First' model. Perhaps the biggest challenge to the 'Clovis First' model was the reported presence of human footprints within a basaltic ash (Xalnene Ash) dated to 38.04 ± 8.57 ka using optically stimulated luminescence (OSL) [2]. However, Renne et al. [3] challenged the validity of the footprints by dating lapilli from the Xalnene ash using 40Ar/39Ar and reported an age of 1.30 ± 0.03 Ma (2σ). They also reported a reversed palaeomagnetic polarity for the ash, consistent with deposition during chron C1r.2r. Such antiquity casts considerable doubt on the interpretation of the impressions as human footprints. Gonzalez et al. [4] questioned the validity of the 40Ar/39Ar age and highlighted the heterogeneous nature of the lapilli as a potential problem for 40Ar/39Ar geochronology. The lapilli contain abundant phenocrysts and xenocrysts. Olivine phenocrysts can be contaminated with excess Ar (40ArE) [5] and hence the dating of 40ArE-bearing lapilli and xenocrystic material may potentially produce anomalously old 40Ar/39Ar ages. Gonzalez et al. [4] also dismissed the significance of the reversed palaeomagnetic polarity as the proposed age of the ash (38.04 ± 8.57 ka) overlapped with the Laschamp Geomagnetic Excursion at 40.4 ± 1.1 ka. Subsequently there has been support for both sides of the debate. The OSL age presented was questioned [6] and reconfirmed by [7]. The OU 40Ar/39Ar laboratory showed the presence of 40ArE in the samples although they were unable to date the ash [2]. Palaeomagnetic data has both supported emplacement of the Xalnene Ash during the LGE [8,9] and at 1.3 Ma [10]. The age of the 'alleged' footprint-bearing Xalnene ash and

  10. Pyroclastic chronology of the Sancy stratovolcano (Mont-Dore, French Massif Central): New high-precision 40Ar/39Ar constraints

    NASA Astrophysics Data System (ADS)

    Nomade, Sébastien; Scaillet, Stéphane; Pastre, Jean-François; Nehlig, Pierre

    2012-05-01

    The Sancy (16 km2) is the youngest of the two stratovolcanoes that constitute the Mont-Dore Massif (Massif Central, France). The restricted number of high precision radio-isotopic ages currently limits our knowledge of the pyroclastic chronology of this edifice which is the source of many tephra layers detected in middle Pleistocene sequences in southeast Europe. To improve our knowledge of the building phases of this stratovolcano, we collected thirteen pyroclastic units covering the entire proximal record. We present 40Ar/39Ar single grain laser dating performed in the facility hosted at the LSCE (Gif-sur-Yvette, France). The 40Ar/39Ar ages range from 1101 ± 11 ka to 392 ± 7 ka (1σ external). Four pyroclastic cycles lasting on average 100 ka were identified (C. I to C. IV). C. I corresponds to the earlier explosive phase between 1101 ka and 1000 ka and starts about 100 ka earlier than previously thought. The second pyroclastic cycle (C. II) is the main pyroclastic episode spanning from 818 to 685 ka. This cycle is constituted of a minimum of 8 major pyroclastic eruptions and includes a major event that corresponds to a large plinian eruption at 719 ± 10 ka (1σ external) and recorded as a 1.4 m thick layer 60 km south-east of the Sancy volcano. The link between this large eruption and formation of a caldera stays however, hypothetical. The third pyroclastic cycle (C. III) found in the northeastern part of the Sancy (Mont-Dore valley) spanned from 642 to 537 ka. Finally, the youngest pyroclastic cycle (C. IV) starts at 392 ka and probably ends around 280 ka. The age versus geographic location of each pyroclastic cycle indicates three preferential directions of channeling of the pyroclastic events and/or collapse of the volcanic edifice: northwest to west (C. I), southeast (C. II) and finally north to northeast (C. III and IV). The new high precision 40Ar/39Ar age for the Queureuilh bas pyroclastic unit (642 ± 9 ka) is identical within error with the U/Pb age

  11. 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

  12. 40Ar/39Ar dating and paleoenvironmental reconstruction of the Lower Pleistocene sequence of Kvemo-Orozmani (Republic of Georgia): New chronological constraints for Dmanisi

    NASA Astrophysics Data System (ADS)

    Nomade, S.; Messager, E.; Voinchet, P.; Mgeladze, A.; Guillou, H.; Ferring, R.; Lordkipanidze, D.

    2010-12-01

    Discovery of Early Pleistocene hominid remains about 15 years ago in Dmanisi (southwestern part of the actual Republic of Georgia) provides evidence on an early expansion of hominid out of Africa as early as the Olduvai subchron period (Gabunia et al., 2001). Two other Early Pleistocene sequences only few kilometers from Dmanisi: Zemo and Kvemo Orozmani are of prime interest to improve the dating of this exceptional site. They both display similar sediments than Dmanisi, but contrary to it, they both are overly by a lava flow allowing to precisely bracketing these sequences using radio-isotopic methods. In this contribution, we present the first high precision 40Ar/39Ar dating and paleoecological reconstruction (phytoliths record) of the Kvemo-Orozmani sequence. The 40Ar/39Ar ages we obtained on the lava flow bracketing the Kvemo Orozmani sequence are: 1.83 ± 0.02Ma and 1.77 ± 0.02Ma (95% confidence, relative to the ACR2 standard at 1.194 Ma). These numerical ages place the sequence exactly at the top of the Olduvai subchron. Furthermore, the lowermost lava flow (c.a. 1.83Ma) is only marginally younger than the lava flow found below the Dmanisi site and dated at 1.85 ± 0.01Ma (Gabunia et al.,(2000)), whereas, the uppermost one displays the same age than the one covering the Zemo Orozmani sequence (Gabunia et al., 2000) located only 2km East. Phytoliths analyses (silica opal produced by plants) show that lower part of the sequence is associated with herbaceous vegetations composed of both temperate and sub-tropical taxa whereas the upper part of the sequence shows an absence of subtropical phytoliths taxa suggesting dryer condition. The shift in the phytoliths assemblage we found in Kvemo-Orozmani is similar to the one described in Dmanisi at the top of the A stratum and corresponds paleomagnetically to the top of the Olduvai subchron (Messager et al., 2010). Both numerical ages and phytoliths assemblages we obtained suggest that the Kvemo Orozmani sequence

  13. Thermal History of the Felsite Unit, Geysers Geothermal Field, From Thermal Modeling of 40Ar/39Ar Incremental Heating Data

    SciTech Connect

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

    1999-08-19

    An Ar-40/Ar-39 and U-Pb study was performed of the Geysers plutonic complex of the Geysers Geothermal Field in California. Sixty-nine ion microprobe spot analyses of zircons from four granite samples from the plutonic complex that underlies the Geysers geothermal field yielded Pb-207/Pb-206 vs. U-238/Pb-206 concordia ages ranging from 1.13 {+-} 0.04 Ma to 1.25 {+-} 0.04 Ma. The U-Pb ages coincide closely with Ar-40/Ar-39 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 {approximately}0.9-1.0 Ma. Interpretation of the feldspar Ar-40/Ar-39 age data using multi-diffusion domain theory indicates that post-emplacement rapid cooling was succeeded either by slower cooling from 350-300 C between 1.0 and 0.4 Ma or transitory reheating to 300-350 C at about 0.4-0.6 Ma. Heat flow calculations constrained with K-feldspar thermal histories and the pre sent elevated regional heal 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 1.2-0.6 Ma.

  14. Early Pleistocene climate cycles in continental deposits of the Lesser Caucasus of Armenia inferred from palynology, magnetostratigraphy, and 40Ar/39Ar dating

    NASA Astrophysics Data System (ADS)

    Joannin, Sebastien; Cornée, Jean-Jacques; Münch, Philippe; Fornari, Michel; Krijgsman, Wout; Nahapetyan, Samuel; Gabrielyan, Ivan; Ollivier, Vincent; Roiron, Paul; Chataignier, Christine

    2010-05-01

    The Lesser Caucasus in Armenia is an active volcanic and tectonic zone which resulted from the collision of the Arabian and the Eurasian plates since Neogene times. The During Quaternary, Lesser Caucasus was uplifted (0.3 mm/yr; Mitchell and Westaway, 1999) and experienced extensional tectonics times. Large lakes developed in graben structures. The diatomitic sequences of the Shamb paleo-lake (South Armenia) offer a rare opportunity to give new insights of Western Asia paleo-climate. Based on macroflora analysis, Bruch and Gabrielyan (2002) proposed a cooling and drying general climate trend through Pleistocene times in relation with a general uplift of the chain. Several questions have to be answer for this poorly investigated region. Did the climate record humid glacials and arid interglacials as suggested northward in Kazakhstan? What are the vegetation and climate responses to orbital parameters and to the monsoon? Moreover the lesser Caucasus is known as the entrance way used by the first hominids in Eurasia during Pleistocene time. How was the environment at this time? We present an integrated palynological, 40Ar/39Ar isotopic and magnetostratigraphic study for the most complete section (Joannin et al., in press). 40Ar/39Ar dating of two volcaniclastic layers provided ages of 1.24 ± 0.03 and 1.16 ± 0.02 Ma. Magnetostratigraphic data show that the entire Shamb section is of reversed polarity which correlates with part of the Matuyama period (1.785-1.070 Ma). Pollen assemblages and macroremains diversity revealed an alternation of glacial and interglacial phases that are compared with climate changes inferred from the global isotopic curve. The Shamb section ranges from approximately 1.300 to 1.080 Ma in age (marine isotopic stages 40 to 31). The vegetation of the Lesser Caucasus developed in a mosaic pattern in a Pleistocene continental, mostly arid climate, similar to the present-day climate. The vegetation changes record a dominant climate response to the

  15. New 40Ar/ 39Ar dating of the Grande Ronde lavas, Columbia River Basalts, USA: Implications for duration of flood basalt eruption episodes

    NASA Astrophysics Data System (ADS)

    Barry, T. L.; Self, S.; Kelley, S. P.; Reidel, S.; Hooper, P.; Widdowson, M.

    2010-08-01

    Grande Ronde Basalt (GRB) lavas represent the most voluminous eruptive pulse of the Columbia River-Snake River-Yellowstone hotspot volcanism. With an estimated eruptive volume of 150,000 km 3, GRB lavas form at least 66% of the total volume of the Columbia River Basalt Group. New 40Ar/ 39Ar dates for GRB lavas reveal they were emplaced within a maximum period of 0.42 ± 0.18 My. A well-documented stratigraphy indicates at least 110 GRB flow fields (or individual eruptions), and on this basis suggests an average inter-eruption hiatus of less than 4000 years. Isotopic age-dating cannot resolve time gaps between GRB eruptions, and it is difficult to otherwise form a picture of the durations of eruptions because of non-uniform weathering in the top of flow fields and a general paucity of sediments between GR lavas. Where sediment has formed on top of GRB lavas, it varies in thickness from zero to 20-30 cm of silty to fine-sandy material, with occasional diatomaceous sediment. Individual GRB eruptions varied considerably in volume but many were greater than 1000 km 3 in size. Most probably eruptive events were not equally spaced in time; some eruptions may have followed short periods of volcanic repose (perhaps 10 2 to 10 3 of years), whilst others could have been considerably longer (many 1000 s to > 10 4 years). Recent improvements in age-dating for other continental flood basalt (CFB) lava sequences have yielded estimates of total eruptive durations of less than 1 My for high-volume pulses of lava production. The GRB appears to be a similar example, where the main pulse occupied a brief period. Even allowing for moderate to long-duration pahoehoe flow field production, the amount of time the system spends in active lava-producing mode is small — less than c. 2.6% (based on eruption durations of approximately 10,000 years, compared to the duration of the entire eruptive pulse of c. 420,000 years). A review of available 40Ar/ 39Ar data for the major voluminous phases

  16. 40Ar/ 39Ar dating of the Jurassic volcanic province of Patagonia: migrating magmatism related to Gondwana break-up and subduction

    NASA Astrophysics Data System (ADS)

    Féraud, G.; Alric, V.; Fornari, M.; Bertrand, H.; Haller, M.

    1999-10-01

    The Mesozoic large igneous province (LIP) of Patagonia (southern South America), which is one of the largest silicic provinces on Earth has been investigated by the 40Ar/ 39Ar method. Twenty-seven ages considered as valid, including twenty plateau ages, show that the volcanic activity, ranging from 187 to 144 Ma, occurred between and contemporaneously with the initial break-up of Gondwana (starting with the Karoo-Antarctic-Tasmanian (KAT) flood basalt province) in the east, and a subduction in the west. The data display a regular decreasing of ages from the ENE (187 Ma) to the WSW (144 Ma) along about 650 km, apparently related to the tectonic structure in half-grabens oriented NNW-SSE. The good fitting of this trend with the opening of the Rocas Verdes-Sarmiento marginal basin favors a space time evolution of this continental volcanism culminating towards the SSW in a continental disruption behind the magmatic arc. The observed age progression of volcanism may be the result of the variations of the physical characteristics of the subduction. The spreading and thermal effect of the KAT plume may have an additional effect and also could account for the unusually large volume of magma.

  17. 40Ar/39Ar and (U-Th)/He - 4He/3He geochronology of landscape evolution and channel iron deposit genesis at Lynn Peak, Western Australia

    NASA Astrophysics Data System (ADS)

    Vasconcelos, Paulo M.; Heim, Jonathan A.; Farley, Kenneth A.; Monteiro, Hevelyn; Waltenberg, Kathryn

    2013-09-01

    (U-Th)/He geochronology of authigenic goethite cements from the Lynn Peak channel iron deposit (CID), Hamersley Province, Western Australia, reveals a history of mineral precipitation ranging from ca. 33 to 14 Ma. Massive goethites from nearby weathering profiles at Roy Hill North, a possible source of detrital material during the aggradation of the Lynn Peak channels, yield (U-Th)/He results as old as ca. 64 Ma. The combination of (U-Th)/He geochronology with incremental outgassing 4He/3He studies on proton-irradiated samples reveals that Lynn Peak goethites host radiogenic 4He in low retentivity (LRD) and high retentivity (HRD) domains and that the HRDs account for most of the sample mass and have lost very little of their original 4He over geologic time. Such high retentivity is especially notable given the goethites were collected from the surface, where they were subject to significant heating by solar irradiation. Minor contamination by detrital fragments of potentially 4He-rich primary phases (e.g., rutile, ilmenite, zircon) occurs in some samples. Fortunately, the 4He/3He method permits characterization of this extraneous 4He component, which is small (<10 wt.% of the total 4He in the goethite) and can be corrected out in estimating the goethite formation age. These results indicate that the Lynn Peak channel was already aggraded and undergoing goethite cementation by ca. 33 Ma. The history of aggradation and channel cementation independently measured through 40Ar/39Ar geochronology is consistent with that obtained from the (U-Th)/He and 4He/3He record. Laser incremental-heating 40Ar/39Ar geochronology of detrital and authigenic Mn oxides, primarily cryptomelane (KMn8O16·xH2O), from the same locality in the Lynn Peak channel reveals that detrital oxides are older than ca. 44 Ma (and as old as ca. 65 Ma) and authigenic oxides are younger than ca. 35 Ma and as young as ca. 16 Ma. Authigenic cryptomelane precipitation and channel cementation occurred

  18. 40Ar/39Ar whole-rock data constraints on Acadian diagenesis and Alleghanian cleavage in the Martinsburg Formation, eastern Pennsylvania

    USGS Publications Warehouse

    Wintsch, R.P.; Kunk, M.J.; Epstein, J.B.

    1996-01-01

    A comparison of 40Ar/39Ar age spectra of whole-rock mudstone and slate samples from the Ordovician Martinsburg Formation at Lehigh Gap, Pennsylvania, and stratigraphic and thermal constraints support an Alleghanian age for regional slaty cleavage and a late Acadian age for diagenesis in these rocks. Age spectra from mudstones have a sigmoidal shape, with slopes that climb steeply from apparent Mesozoic ages to intermediate saddle regions with Devonian apparent ages, and then climb steeply again to Late Proterozoic apparent ages. The steps with these oldest apparent ages are interpreted to be dominated by Late Proterozoic detrital muscovite. The saddle region of the mudstone samples gives very Late Silurian to earliest Devonian ages, which are maximum ages of diagenetic micas and which eliminate a Taconic age for the cleavage. The ages of the saddle regions of the slate samples containing cleavage-forming muscovite is age of this mica and requires an Alleghanian age for the cleavage. These age constraints are supported by ages of individual mica components calculated with knowledge of the total gas ages and mass fractions of the micas and by predictions from thermal modeling. We conclude that the Taconic orogeny in the Martinsburg Formation in eastern Pennsylvania was a very mild event. Not only is the cleavage in these rocks not Taconic in age, but even the mild (???100C) diagenetic growth of illite was Silurian or younger. Thus the Taconic event in these rocks is limited to loading of less than about 3 km.

  19. 40Ar/39Ar Geochronology of the Pleistocene to Historic Puyehue-Cordon Caulle Volcanic Complex, Andean Southern Volcanic Zone, Chile

    NASA Astrophysics Data System (ADS)

    Harper, M. A.; Singer, B. S.; Moreno Roa, H. A.

    2003-12-01

    Linking magma reservoir processes beneath arc volcanoes to their surficial expression as eruptive events is hampered, in part, by an incomplete knowledge of long-term eruptive flux. To address this problem, we have begun to quantify the eruptive history of the 105 km3 Puyehue-Cordon Caulle Volcanic Complex at 40.5° S in the Andean Southern Volcanic Zone (SVZ) using a combination of 40Ar/39Ar dating and geologic mapping. Puyehue-Cordon Caulle has erupted low-medium K basaltic to rhyolitic magma that spans the largest range of major element compositions in the southern SVZ. Low radiogenic argon contents necessitate the incremental heating of multiple aliquots (200-800 mg) of groundmass from each lava using a low-blank resistance furnace. New 40Ar/39Ar plateau ages from 26 lavas suggest that volcanism began ca. 250 ka. Numerous Mid- to Late Pleistocene vents were active between 250 and 33 ka and produced at least 85 km3 of basaltic to andesitic lava, tephra, and ignimbrites. More recently, the large Puyehue stratovolcano, built between 45 and 2.7 ka, erupted 18-21 km3 of basaltic to rhyolitic lava and tephra. Puyehue began to grow after 45 ka with the eruption of 8-10 km3 of dacitic and rhyodacitic lava followed by several km3 of basalt and basaltic andesite that erupted between 15.1 +/- 2.6 ka and 10.0 +/- 1.1 ka. The final stage of cone growth, comprising 1-2 km3 of rhyolitic and rhyodacitic lava and tephra, began by 10 ka and culminated at 2.66 +/- 0.19 ka (14C date) with the plinian eruption of several km3 of airfall deposits zoned from dacite to basaltic andesite. The average eruptive rate for Puyehue volcano is 0.4-0.5 km3/k.y., twice that of Tatara-San Pedro volcano located ˜ 500 km to the north. Glaciers eroded extensive parts of the complex between 200 and 15 ka, making our growth rate a minimum estimate. The Cordon Caulle fissural zone, which extends for 15 km to the NW of the modern Puyehue edifice, has erupted 2 km3 of rhyodacitic to rhyolitic lavas

  20. Tephrochronology of the Mont-Dore volcanic Massif (Massif Central, France): new 40Ar/39Ar constraints on the Late Pliocene and Early Pleistocene activity

    NASA Astrophysics Data System (ADS)

    Nomade, Sébastien; Pastre, Jean-François; Nehlig, Pierre; Guillou, Hervé; Scao, Vincent; Scaillet, Stéphane

    2014-03-01

    The Mont-Dore Massif (500 km2), the youngest stratovolcano of the French Massif Central, consists of two volcanic edifices: the Guéry and the Sancy. To improve our knowledge of the oldest explosive stages of the Mont-Dore Massif, we studied 40Ar/39Ar-dated (through single-grain laser and step-heating experiments) 11 pyroclastic units from the Guéry stratovolcano. We demonstrate that the explosive history of the Guéry can be divided into four cycles of explosive eruption activity between 3.09 and 1.46 Ma (G.I to G.IV). We have also ascertained that deposits associated with the 3.1-3.0-Ma rhyolitic activity, which includes the 5-km3 "Grande Nappe" ignimbrite, are not recorded in the central part of the Mont-Dore Massif. All the pyroclastites found in the left bank of the Dordogne River belong to a later explosive phase (2.86-2.58 Ma, G.II) and were channelled down into valleys or topographic lows where they are currently nested. This later activity also gave rise to most of the volcanic products in the Perrier Plateau (30 km east of the Mont-Dore Massif); three quarters of the volcano-sedimentary sequence (up to 100 m thick) was emplaced within less than 20 ky, associated with several flank collapses in the northeastern part of the Guéry. The age of the "Fournet flora" (2.69 ± 0.01 Ma) found within an ash bed belonging to G.II suggests that temperate forests already existed in the French Massif Central before the Pliocene/Pleistocene boundary. The Guéry's third explosive eruption activity cycle (G.III) lasted between 2.36 and 1.91 Ma. It encompassed the Guéry Lake and Morangie pumice and ash deposits, as well as seven other important events recorded as centimetric ash beds some 60 to 100 km southeast of the Massif in the Velay region. We propose a general tephrochronology for the Mont-Dore stratovolcano covering the last 3.1 My. This chronology is based on 44 40Ar/39Ar-dated events belonging to eight explosive eruption cycles each lasting between 100 and 200

  1. Repeated thermal resetting of phengites in the Mulhacen Complex (Betic Zone, southeastern Spain) shown by 40Ar/ 39Ar step heating and single grain laser probe dating

    NASA Astrophysics Data System (ADS)

    de Jong, Koen; Wijbrans, Jan R.; Féraud, Gilbert

    1992-05-01

    This study reports the results of the first 40Ar/ 39Ar combined induction furnace and laser probe dating of phengites from the Mulhacen HP/LT metamorphic complex in the Betic Cordilleras, southern Spain. Laser step heating and spot fusion analyses on different halves of a split single grain were made with a continuous laser probe. Spot fusion analysis resulted in ages of about 30-31 Ma in the core and ages as low as 25-26 Ma in the rim. Laser step heating on the other half of the grain gave a spectrum with apparent ages increasing from about 25 Ma to 29.5 Ma. The age spectrum and the decreasing ages towards the rim of the grain may imply that resetting essentially occurred by volume diffusion of radiogenic 40Ar due to late stage reheating resulting from extensional tectonics. Ages around 30 Ma in the core of the grain are interpreted as minimum estimates of the cooling age of the main tectono-metamorphic phase D 2. Induction furnace step heating on phengite separates from mica schists and one gneiss resulted in two types of age spectra. Type I spectra show monotonously rising apparent ages from 14.5 ± 1.9 Ma to 20.7 ± 0.2 Ma, and in a second sample from 16.9 ± 0.7 to 29.7 ± 0.2 Ma. Type II spectra are characterized by plateaus of 14.4 ± 0.1 Ma (the gneiss sample), 17.3 ± 0.1 Ma and 17.6 ± 0.1 Ma. Type II spectra show low temperature apparent ages significantly below the plateau age, implying resetting subsequent to initial cooling. Modelling of the age spectra demonstrated that the plateau ages are possibly the result of strong resetting (75-85% of argon loss) of an older isotope system. Total fusion of a number of phengite single grains from marbles taken close to type II mica schists yielded ages of 15.4 ± 1.2 Ma and 17.0 ± 0.7 Ma. The observed repeated resetting is coeval with major volcanic activity in basins adjacent to the metamorphic ranges, pointing to a resetting by advective fluid transport related to volcanism.

  2. A palaeomagnetic and 40Ar/39Ar study of late precambrian sills in the SW part of the Amazonian craton: Amazonia in the Rodinia reconstruction

    NASA Astrophysics Data System (ADS)

    Elming, S.-Å.; D'Agrella-Filho, M. S.; Page, L. M.; Tohver, E.; Trindade, R. I. F.; Pacca, I. I. G.; Geraldes, M. C.; Teixeira, W.

    2009-07-01

    A new key palaeomagnetic pole (Plat. = 64.3°S, Plon. = 271.0°E, N = 14, A95 = 9.2° Q = 5) is calculated from a primary magnetization isolated in early Neoproterozoic Aguapei basic sills and dykes hosted by 1.3-1.0 Ga sedimentary rocks from the southwestern part of the Amazon craton. The characteristic remanence carried by stable, pseudo-single domain titanomagnetite shows two antipodal polarities that pass a reversals test. Magnetic anisotropy for most sites shows fabric orientations that are typical of sills, with horizontal magnetic foliations concordant to the flat-lying bedding of the host sedimentary rocks. 40Ar/39Ar analyses for one of the sills reveal a well-defined plateau age at 981 +/- 2 Myr. A tectonic reconstruction for Amazonia relative Laurentia based on this new pole `is consistent with' a position of the present northwestern part of Amazonia attached with eastern Laurentia close to Greenland at ca. 981 Ma. On basis of palaeomagnetic and geological data, we propose a scenario where Amazonia moved northeastwards along the present southeast coast of Laurentia from ca. 1200 to 980 Ma. By 980 Ma, Amazonia is placed alongside Laurentia and Baltica, in a position similar to other reconstructions of Rodinia but with a significantly different orientation.

  3. [sup 40]Ar/[sup 39]Ar evidence for delayed post-Acadian cooling in the southernmost Connecticut Valley Synclinorium

    SciTech Connect

    Moecher, D.P. . Dept. of Geological Science); Cosca, M.A. )

    1992-01-01

    Available Ar-40/Ar-39 data for the Connecticut Valley Synclinorium (CVS) of the New England segment of the Appalachian Orogen indicate rapid post-Acadian cooling. However, new data indicate this pattern does not extend the entire length of the CVS. Ar-40/Ar-39 ages obtained from hornblende and muscovite in The Straits Schist indicate delayed cooling and a more complex post-Acadian thermal history. Data for the Seymour area are consistent with the studies above for the vicinity of the Waterbury Dome. The data farther south indicate one or more of the following: (1) slow (2--3C/Ma) post-Acadian cooling and uplift through the Permian; (2) post-Acadian cooling through Hbl closure in the Mississippian with a subsequent Alleghanian metamorphism that did not exceed 500 C; or (3) post-Acadian cooling with subsequent metamorphism that approached 500 C or involved ductile recrystallization, partly resetting hornblende and totally resetting muscovite south of Derby. Petrologic evidence supporting (2) or (3) consists of widespread but not pervasive greenschist facies retrogression of Hbl + Pl + Sph assemblages in amphibolites to Act + Ep, and Grt + Ky + St assemblages in metapelites to Chl + Bt + Qz. The present data cannot resolve between (2) or (3). However, both are consistent with results of a study in the Bridgeport Synform that yield (1) a U-Pb monazite age of 296 [+-] 2 Ma from the Ansonia Leucogranite, implying the occurrence of an Alleghanian thermal event that promoted monazite growth; and, (2) a U-Pb cooling age of 360 Ma from sphene in the Pumpkin Ground Granodiorite, indicating that Alleghanian events did not exceed ca. 550 C.

  4. [sup 40]Ar/[sup 39]Ar isotopic dates from the Cripple Creek gold-Telluride district, Colorado: Constraints on the timing of magmatism and mineralization

    SciTech Connect

    Kelley, K.D.; Snee, L.W. ); Thompson, T.B. . Dept. of Earth Resources)

    1993-04-01

    The Cripple Creek district is within a Tertiary diatreme-intrusive complex, a steep-walled basin in Proterozoic pelitic and igneous rocks that is filled with terrigenous sedimentary rocks, volcanic and hydrothermal breccias, and tuffs. The orebodies occur as veins in Proterozoic and Tertiary rocks or as deposits localized within hydrothermal breccia bodies or disseminated in diatreme breccias. [sup 40]Ar/[sup 39]Ar dates from igneous rocks demonstrate the approximately contemporaneous emplacement of the most differentiated phonolitic rocks. Three sanidine samples from phonolite yield apparent ages ranging from 30.9 [+-] 0.1 to 31.8 [+-] 0.1 Ma (1 sigma). Biotite and sanidine age spectra from relatively less differentiated tephriphonolite are discordant; the emplacement age is estimated to be between 31.4 [+-] 0.1 and 32.5 [+-] 0.1 Ma. A maximum age of 31.5 [+-] 0.1 Ma was obtained on a whole-rock sample of trachyandesite. The mafic phonolitic rocks are relatively younger. A sample of the Isabella dike, a phonotephrite dike cutting phonolite, yields a whole-rock age of 28.7 [+-] 0.04 Ma. The data suggest that mineralization both predates and postdates emplacement of the mafic phonolitic rocks. Hydrothermal biotite in a vein cutting phonolite yields an age of 29.9 [+-] 0.1 Ma. The age spectrum of adularia from a vein cutting volcaniclastic rocks is difficult to interpret due to the presence of excess argon, but an age is estimated to be between 29.5 and 30.4 Ma. In the vicinity of the phonotephrite dike, field evidence suggests that vein mineralization postdates emplacement of the dike; potassium feldspar from potassium altered phonolite in the vicinity of mineralized rock yields ages of 28.2 [+-] 0.1 and 28.8 [+-] 0.1 Ma.

  5. New SHRIMP U-Pb and 40Ar/39Ar constraints on the crustal stabilization of southern South America, from the margin of the Rio de Plata (Sierra de Ventana) craton to northern Patagonia

    NASA Astrophysics Data System (ADS)

    Tohver, E.; Cawood, P. A.; Rossello, E.; Lopez de Luchi, M. G.; Rapalini, A.; Jourdan, F.

    2008-12-01

    Two models exist to explain the late Paleozoic tectonic history for southern South America: an accretionary model of crustal growth through magmatism and a collisional model involving pre-existing continental elements, namely, the Rio de Plata craton and the possibly allochthonous terrane(s) of Patagonia, the Northern Patagonia Massif and the Deseado Massif. We report new U-Pb and 40Ar/39Ar results from rocks within a posited collision zone between the SW edge of the Rio de Plata craton and the northern margin of the Northern Patagonia Massif. Igneous basement samples from the Sierra de Ventana region, Buenos Aires province, were dated by ion microprobe (SHRIMP) analysis of zircon. A previously unrecognized occurrence of Paleoproterozoic basement indicates that the Rio de Plata craton extends ca.250 km farther west than considered. The majority of the basement rocks are shallow mid-Cambrian granitoids and rhyolites, including the rocks of the Cerro Colorado granite, which is intrusive into the sediments of the Curamalal Gp, signifying that these mature quartzites and conglomerates are older than early Cambrian in age, possibly correlated with the low-grade sedimentary rocks of the Tandilia Range that includes the La Tinta Fm. The 40Ar/39Ar ages from biotite, muscovite, and sericite from three different sheared basement localities demonstrates deformation in the latest Permian (265-260 Ma), ca. 20 Ma after the foreland deposition of the synorogenic Tunas Fm. in the upper Pilahuinco Gp, constrained by 282.4 ± 2.8 Ma zircon ages in volcanic ashbeds. Farther south, along the northern margin of the Northern Patagonian Massif, late Ordovician 40Ar/39Ar cooling ages of granites intrusive into the Cambro-Ordovician Nahuel Niyeu Fm. are consistent with the presence of Ordovician magmatism along the W edge of the Rio de Plata craton. These ages alternate with late Permian 40Ar/39Ar cooling ages from undeformed granites and pegmatites, as well as early Jurassic cross

  6. Ar-39-Ar-40 ages of four ureilites

    NASA Technical Reports Server (NTRS)

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

    1994-01-01

    Ureilites Novo Urei, Havero, and Kenna show strong evidence of one or more Ar-40 degassing events in the time period of 3.3-4.1 Ga ago. These ages may be compared to current interpretations of ureilite chronology. These include the suggestion of metasomatic activity on the parent body 3.7 Ga ago that reset some Sm-Nd ages and the suggestion that ureilites have experienced terrestrial contamination of several trace elements (including Pb and LREE), which makes suspect ages younger than approximately 4.5 Ga. Because the K-Ar chronometer can be sensitive to metamorphic events, we made Ar-39-Ar-40 determinations on bulk samples (0.12-0.14 g each) of four ureilites. The Ar-39-Ar-40 age spectra and K/Ca ratios as a function of cumulative Ar release from stepwise temperature extractions for the four ureilites analyzed are shown. Because Ar-39-Ar-40 ages shown by low and high temperature extractions may be suspect, we examined the intermediate temperature extractions. Although interpretation of these spectra is obviously uncertain, we believe that the most recent times of Ar degassing can be roughly inferred. These times are approximately 3.3 Ga for Havero, 3.3-3.7 Ga for Novo Urei, and approximately 4.1 Ga for Kenna, for which Ar degassing may not have been complete. The indication of Ar-39-Ar-40 degassing ages of 3.3-4.1 Ga for three ureilites that also contain an enhanced LREE component and (excepting Havero) produce a 3.74 Ga Sm-Nd age, suggests that both chronometers may have responded to the same parent body event. On the other hand, it is also possible that the Ar data reflect one or more separate events that did not strongly affect the Sm-Nd system, a situation that commonly occurs in eucrites. Thus the existence of reset Ar ages does not require similarly reset Sm-Nd ages.

  7. The Early Andean Magmatic Province (EAMP): 40Ar/ 39Ar dating on Mesozoic volcanic and plutonic rocks from the Coastal Cordillera, northern Chile

    NASA Astrophysics Data System (ADS)

    Oliveros, Verónica; Féraud, Gilbert; Aguirre, Luis; Fornari, Michel; Morata, Diego

    2006-10-01

    The Early Andean Magmatic Province (EAMP), consists of about 150 000 km 3 of volcanic and plutonic units in the Coastal Cordillera of northern Chile and southern Peru and represents a major magmatic Mesozoic event in the world, for which the precise age of the thick volcanic series was unknown. Thirty 40Ar/ 39Ar analyses were carried out on primary mineral phases of volcanic and plutonic rocks from northern Chile (18°30'-24°S). Reliable plateau and "mini plateau" ages were obtained on plagioclase, amphibole and biotite from volcanic and plutonic rocks, despite widespread strong alteration degree. In the Arica, Tocopilla and Antofagasta (700 km apart) regions, the ages obtained on lava flows constrain the volcanic activity between 164 and 150 Ma and no N-S migration of volcanism is observed. The uppermost lava flows of the volcanic sequence at the type locality of the La Negra Formation extruded at ca. 153-150 Ma, suggesting the end of the volcanic activity of the arc at that time. The oldest volcanic activity occurred probably at ca. 175-170 Ma in the Iquique area, although no plateau age could be obtained. The plutonic bodies of the same regions were dated between ca. 160 and 142 Ma, indicating that they were partly contemporaneous with the volcanic activity. At least one volcanic pulse around 160 Ma is evidenced over the entire investigated reach of the EAMP, according to the ages found in Arica, Tocopilla, Michilla and Mantos Blancos regions. The episodic emplacement of huge amounts of subduction related volcanism is observed throughout the whole Andean history and particularly during the Jurassic (southern Peru, northern Chile and southern Argentina). These events probably correspond to periodic extensional geodynamic episodes, as a consequence of particular subduction conditions, such as change of obliquity of the convergence, change in the subduction angle, slab roll back effect or lower convergence rate, that remain to be precisely defined.

  8. 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.

  9. Geochronological constraints (40Ar/39Ar and U/Pb) on the thermal history of the Tolumne Intrusive Suite (Sierra Nevada, California)

    NASA Astrophysics Data System (ADS)

    Mundil, R.; Nomade, S.; Paterson, S. R.; Renne, P. R.

    2004-12-01

    The Tuolumne Intrusive Suite (TIS) in the Eastern Sierra Nevada is considered a type example of a batholith and represents a spectacularly exposed, protracted record of internal differentiation and plutonic assembly in a large, open-system, continental arc magma chamber. One of the recent advances in our understanding of magmatic systems is the recognition that a substantial number are constructed episodically over timescales of up to millions of years for larger plutons. The main objective of this study is to investigate the episodic growth and evolution of magmatic systems by integrating thermal, geochronologic, geochemical, and crystal size distribution (CSD) studies with ongoing field studies of the TIS. Here we present high-resolution U/Pb and 40Ar/39Ar geochronology from the TIS (which was assembled between 93 and 85 Ma, Coleman et al., 2004) and adjacent older units in order to unravel the time scales of its assemblage and thermal history. 25 Samples were collected along a SW-NE corridor (ca 30 km) across the TIS, including older plutons to the SW (El Capitan) and the NE (Soldier Lake (SDL) and Green Lake plutons (GRL)). So far, conventional U/Pb single-zircon analyses yield weighted mean 206Pb/238U ages of 165.0 ± 0.3 Ma for the GRL and a preliminary age of ca. 95 Ma for the SDL, which are interpreted as emplacement ages (all uncertainties are given at the 2σ level). 40Ar/39Ar analyses were performed on two different biotite and hornblende grain size fractions (800-900μ m and 150-180μ m) from each sample. As expected, isotherms in the eastern pendant of the Sierra Nevada move towards the TIS as a result of its cooling between 85 to 80 Ma. The gradient of temperature at the time of the emplacement of the Cathedral Peak (CP) Pluton (U/Pb zircon age of ca 88 Ma, Coleman, 2004) was about 150° C to 200° C per 5 km. The western margin of the GRL (at 5 km distance from the TIS) is thermally affected by the TIS as indicated by biotite ages that are reset (ca

  10. Laser probe 40Ar/39Ar dating of pseudotachylyte from the Sudbury Structure: evidence for post-impact thermal overprinting in the North Range

    NASA Astrophysics Data System (ADS)

    Thompson, Lucy M.; Spray, John G.; Kelley, Simon P.

    1998-11-01

    Ten pseudotachylyte samples from the North Range of the 1850 Ma Sudbury impact structure have been analyzed by the 40Ar/39Ar laser spot fusion method. Field and petrological evidence indicate that the pseudotachylytes were formed at 1850 Ma by comminution and frictional melting due to impact-induced faulting. The cryptocrystalline to microcrystalline grain size (<30 _?m) of the pseudotachylyte matrices, and the predominance of orthoclase as the main K-bearing phase, have rendered the rocks particularly susceptible to argon loss. The age determinations range from ?1850 Ma to ?1000 Ma, with some samples yielding multiple ages that cannot be correlated with known geological events in the area. However, if the finite-difference algorithm of Wheeler (1996) is used to calculate combined argon loss and the accumulation of radiogenic argon for the K-bearing phases, it is possible to reproduce the range of observed ages. The model infers that the long-term volume diffusion of Ar has occurred and that, as a result, the Ar system cannot be treated with a conventional closure temperature approach. The algorithm requires burial of the impact structure to 5-6 km depth and 160-180 deg C at 1850 Ma, followed by exhumation at ?1000 Ma. These ages may be equated with two events: Penokean thin-skinned overthrusting in the North Range, immediately following impact, and exhumation ?850 Ma later, coincident with the Grenville orogeny to the southeast. The results suggest that, contrary to previously accepted paradigms, the North Range has been affected by a protracted period of post-impact, low-grade thermal metamorphism. If this event also involved tectonic shortening within the North Range, as has been documented for the South Range, then the original size of the Sudbury impact structure has been underestimated.

  11. Morphological modifications of the Kerguelen Islands (South Indian Ocean) in response to Neogene climate change: evidence from 40Ar/39Ar and (U-Th)/He thermochronology

    NASA Astrophysics Data System (ADS)

    Ahadi, Floriane; Delpech, Guillaume; Gautheron, Cécile; Nomade, Sébastien; Pinna-jamme, Rosella; Ponthus, Léandre; Guillaume, Damien

    2016-04-01

    The processes driving erosion in geodynamic contexts in which regional tectonics is of minor importance, such as in oceanic islands, can be seen as a combination of positive/negative retroactions between climate change, isostasy or dynamic topography. The Kerguelen Islands (48-50° S, 68.5-70.5° E) are of particular interest to understand the impact of Cenozoïc climatic variations on the long-term geomorphological evolution of emerged reliefs at mid-latitudes. The Kerguelen Islands (6700 km2) are the emerged part of the vast Kerguelen oceanic plateau and reach a maximum height of 1852m asl. The archipelago is mostly made up of Oligocene basaltic traps (≈25 Ma) up to 1000m asl that are cross-cut by a dense network of large and deep valleys. The impact of glacial erosion during the last Quaternary glaciations on the landscape morphology is attested by the occurrence of U-shaped valleys, abundant moraines, erratic blocs and glacial lakes, as well as remnants of glaciers. Numerous plutonic complexes of various age (25-4.5 Ma) locally intrude theses traps and cover about 15% of the main island's surface; the largest being located in the Rallier du Baty peninsula (800 km2). This plutonic complex is mainly constituted of syenites with minor occurrence of gabbros and monzonites. The southern part of this complex has a laccolith structure with satellites plutons and formed between 13.7 and 8.0 Ma. The cooling history of syenites from the Rallier du Baty plutonic complex was investigated in order to identify one or several denudation periods and to understand the potential role of climate change on the geomorphological evolution of the islands since the Oligocene. We conducted the first thermochronological study on the Kerguelen Islands using the biotite 40Ar/39Ar thermochronometer and the apatite (U-Th)/He thermochronometer (AHe). The 40Ar/39Ar ages range from 9.44 ± 0.13 Ma to 13.84 ± 0.07 Ma for the various parts of the southern complex. These ages are identical to

  12. sup 40 Ar- sup 39 Ar and K-Ar dating of K-rich rocks from the Roccamonfina volcano, Roman Comagmatic Region, Italy

    SciTech Connect

    Di Brozolo, F.R.; Di Girolamo, P.; Turi, B.; Oddone, M. )

    1988-06-01

    Roccamonfina is the northernmost Volcano of the Campanian area of the K-rich Roman comagmatic Region of Italy. It erupted a huge amount of pyroclastics and lavas belonging to both the Leucite-Basanite and Leucitite Series (LBLS) and the Shoshonite Series (SS), spread over an area of about 300 km{sup 2}. The above series correspond to the High-K Series (HKS) and Low-K Series (LKS) of Appleton (1971), respectively. {sup 40}Ar-{sup 39}Ar and K-Ar dating of samples from both series gave ages ranging from 0.656 to 0.096 Ma for the SS and from 1.03( ) to 0.053 Ma for the LBLS. These results indicate that the products of the two series were outpoured together at least between 0.7 and 0.1 Ma age, i.e. during both the so-called pre-caldera phase and the post-caldera phase of activity. The latest products of the volcanism at Roccamonfina were erupted just before the deposition of the Grey Campanian Ignimbrite, which erupted from vents located about 50 km to the south in the Phlegrean Fields near Naples and has an age of about 33,000 years. Taking into account all the available all the available radiometric data the authors conclude that Roccamonfina was active between 1.5 and 0.05 Ma ago, in excellent agreement with the stratigraphic evidence. In this same time span is concentrated the activity of all the centers of the Roman Region north of Naples.

  13. 40Ar/39Ar geochronology of the Fernando de Noronha Archipelago and implications for the origin of alkaline volcanism in the NE Brazil

    NASA Astrophysics Data System (ADS)

    Perlingeiro, Gabriela; Vasconcelos, Paulo M.; Knesel, Kurt M.; Thiede, David S.; Cordani, Umberto G.

    2013-01-01

    The Fernando de Noronha archipelago, centered ~ 250 km off the northeastern coast of Brazil, is comprised of a diverse suite of alkaline volcanic rocks commonly associated with a mantle-plume origin. Although previous K-Ar determinations divide the three main volcanic formations of the archipelago (Remédios, Quixaba and São José formations) into two age groups, a few ages conflicting with the stratigraphic framework were suspected to suffer from excess argon. To evaluate the presence or absence of excess Ar and to improve the geochronological database for the archipelago, we have dated, by the laser incremental-heating 40Ar/39Ar method, the exact same hand specimens previously dated by K-Ar. The 22 samples studied here yield plateau ages for at least one of the two grains analyzed and none of the specimens contain significant excess Ar. Our results derive a chronostratigraphic sequence for the archipelago that is consistent with the earlier K-Ar determinations. The main discrepancy is related to some basanitic rocks of São José formation, interpreted as the youngest eruptive products which are in fact coeval with the oldest subareal volcanic activity at Fernando de Noronha. Our revised eruptive chronology defines a hiatus of nearly 3 Ma separating an older period of volcanism between 12.5 ± 0.1 and 9.0 ± 0.1 Ma comprising the Remédios and São José formations and a younger episode forming the Quixaba formation between 6.2 ± 0.1 and 1.3 ± 0.1 Ma. Moreover, these results confirm that much of the activity at Fernando de Noronha was contemporaneous with alkaline volcanism well onshore in northeastern Brazil, supporting the suggestion that this hotspot may be a product of small-scale, plate-driven convection in the upper mantle.

  14. UV-laser ablation 40Ar/39Ar dating of pseudotachylite provides time constraints on exhumation of coesite-bearing Dora Maira whiteschists

    NASA Astrophysics Data System (ADS)

    Cosca, M.; Caby, R.

    2003-04-01

    At or near the roof of the coesite-bearing ultra high pressure (UHP) unit in the Dora Maira Massif, Italy, a ca. 50 m thick, gently west dipping (ca 15^o) band of gneissic rock containing cataclasite, protomylonite, and pseudotachylite is exposed over more than 1 km along an EW section. All kinematic critera observed in the gneiss, including late stage ductile deformation and later brittle structures and fault zones, are consistent with top-to-the west extensional shear. The youngest observable textural features are pseudotachylite veins up to 1 cm thick rooting in composite (ultramylonite/cataclasite/pseudotachylite) bands roughly parallel to the mylonitic foliation. Some pseudotachylite intrudes fractures within the gneiss at angles roughly perpendicular to the foliation. A polished thick section of a 1 cm band of pseudotachylite was prepared and analyzed by in situ UV-laser ablation 40Ar/39Ar dating. Different parts of the pseudotachylite yield analytically indistinguishable (2s) ages with a weighted mean of 20.1 ± 0.5 Ma. These data are consistent with recent zircon fission track ages from the gneiss (29.9 ± 1.4 Ma, Gebauer et al., 1997), and provide unequivocal evidence that the UHP rocks were in a near-surface position at this time. The pseudotachylite thus appears to represent a late manifestation of the rapid exhumation, which has been estimated at 2.0 to 2.4 cm/a for these rocks (Gebauer et al. 1997). Pseudotachylites form by localized deformation at high slip rates (> 0.1 m/s). The ˜1 cm thick pseudotachylite from the border of the UHP rocks represents significant localized deformation and melting, probably indicating that, even at relatively shallow crustal depths, exhumation of the UHP rocks may have occurred by discontinuous displacements of large magnitude.

  15. The Central Atlantic Magmatic Province at the Triassic-Jurassic boundary: paleomagnetic and 40Ar/ 39Ar evidence from Morocco for brief, episodic volcanism

    NASA Astrophysics Data System (ADS)

    Knight, K. B.; Nomade, S.; Renne, P. R.; Marzoli, A.; Bertrand, H.; Youbi, N.

    2004-11-01

    The Central Atlantic Magmatic Province (CAMP), one of the largest known flood basalt provinces formed in the Phanerozoic, is associated with the pre-rift stage of the Atlantic Ocean at the Triassic-Jurassic boundary ca. 200 Ma. Paleomagnetic sampling targeted packages of CAMP lava flows in Morocco's High Atlas divided into four basic units (the lower, intermediate, upper, and recurrent units) from sections identified on the basis of field observations and geochemistry. Oriented cores were demagnetized using both alternating field (AF) and thermal techniques. Paleomagnetic results reveal wholly normal polarity interrupted by at least one brief reversed chron located in the intermediate unit, and reveal distinct pulses of volcanic activity identified by discrete changes in declination and inclination. These variations in magnetic direction are interpreted as a record of secular variation, and they may provide an additional correlative tool for identification of spatially separated CAMP lava flows within Morocco. 40Ar/39Ar analyses of Moroccan CAMP lavas yield plateau ages indistinguishable within 2σ error limits, sharing a weighted mean age of 199.9±0.5 Ma (2σ), reinforcing the short-lived nature of these eruptions despite the presence of sedimentary horizons between them. Correlation of our sections with the E23n, E23r, E24 sequence reported in the Newark basin terrestrial section and St. Audrie's Bay marine section is suggested. Brief volcanism in sudden pulses is a potential mechanism for volcanic-induced climatic changes and biotic disruption at the Triassic-Jurassic boundary. Combination of our directional group (DG) poles yields an African paleomagnetic pole at 200 Ma of λ(°N)=73.0°, ϕ(°E)=241.3° (Dp=5.0°, Dm=18.5°).

  16. New 40Ar/39Ar isotopic dates from Miocene volcanic rocks in the Lake Mead area and southern Las Vegas Range, Nevada

    USGS Publications Warehouse

    Harlan, S.S.; Duebendorfer, E.M.; Deibert, J.E.

    1998-01-01

    New 40Ar/39Ar dates on volcanic rocks interlayered with synextensional Miocene sedimentary rocks in the western Lake Mead area and southern end of the Las Vegas Range provide tight constraints on magmatism, basin formation, and extensional deformation in the Basin and Range province of southern Nevada. Vertical axis rotations associated with movement along the Las Vegas Valley shear zone occurred after 15.67??0.10 Ma (2??), based on a 40Ar/39Ar date from a tuff in the Gass Peak formation in the southern Las Vegas Range. Basaltic magmatism in the western Lake Mead area began as early as 13.28??0.09 Ma, based on a date from a basalt flow in the Lovell Wash Member of the Horse Spring Formation. Isotopic dating of a basalt from the volcanic rocks of Callville Mesa indicates that these rocks are as old as 11.41??0.14 Ma, suggesting that volcanic activity began shortly after formation of the Boulder basin, the extensional basin in which the informally named red sandstone unit was deposited. The red sandstone unit is at least as old as 11.70??0.08 Ma and contains megabreccia deposits younger than 12.93??0.10 Ma. This results shows that formation of the Boulder basin was associated with development of topographic relief that was probably generated by movement along the Saddle Island low-angle normal fault. Stratal tilting associated with extension occurred both prior to and after 11.5 Ma.

  17. Cryptochron C2r.2r-1 recorded 2.51 Ma in the Koolau Volcano at Halawa, Oahu, Hawaii, USA: Paleomagnetic and 40Ar/ 39Ar evidence

    NASA Astrophysics Data System (ADS)

    Herrero-Bervera, Emilio; Browne, Edward J.; Valet, Jean Pierre; Singer, Brad S.; Jicha, Brian R.

    2007-02-01

    New paleomagnetic measurements, coupled with Argon-Argon ( 40Ar/ 39Ar) radioisotopic dating, are revolutionizing our understanding of the geodynamo by providing detailed terrestrial lava records of the short-term behavior of the paleomagnetic field. As part of an investigation of the Koolau Volcano, Oahu, and the short-term behavior of the geomagnetic field, we have sampled a long volcanic section located on the volcano's buttressed flank within Halawa Valley. Prior paleomagnetic and Potassium-Argon (K-Ar) investigations of the Koolau Volcanic Series revealed excursional directions The alkaline composition of lava flows, easy access, and close geographical proximity to K-Ar dated lava flows made this newly studied 120-m thick sequence of flows an excellent candidate for detailed paleomagnetic analysis. At least eight samples, collected from each of 28 successive flow sites, were stepwise demagnetized by both alternating field (5 mT to 100 mT) and thermal (from 28 °C to 575-650 °C) methods. Mean directions were obtained by principal component analysis. All samples yielded a strong and stable ChRM trending towards the origin of vector demagnetization diagrams based on seven or more demagnetization steps, with thermal and AF results differing insignificantly. Low-field susceptibility vs. temperature ( k- T) analysis conducted on individual lava flows indicated approximately half with reversible curves. Curie point determinations from these analyses revealed a temperature close to or equal to 580 °C, indicative of almost pure magnetite ranging from single domain (SD) to pseudosingle domain (PSD) grain sizes for most of the flows. The mean directions of magnetization of the entire section sampled indicate a reversed polarity, with ˜ 10 m of the section characterized by excursional directions (5 lava flows). The corresponding VGPs are located off the southeastern part of Africa, close to Madagascar. 40Ar/ 39Ar incremental heating experiments on groundmass from nine

  18. 40Ar/39Ar geochronology of subaerial lava flows of Barren Island volcano and the deep crust beneath the Andaman Island Arc, Burma Microplate

    NASA Astrophysics Data System (ADS)

    Ray, Jyotiranjan S.; Pande, Kanchan; Bhutani, Rajneesh

    2015-06-01

    Little was known about the nature and origin of the deep crust beneath the Andaman Island Arc in spite of the fact that it formed part of the highly active Indonesian volcanic arc system, one of the important continental crust forming regions in Southeast Asia. This arc, formed as a result of subduction of the Indian Plate beneath the Burma Microplate (a sliver of the Eurasian Plate), contains only one active subaerial magmatic center, Barren Island volcano, whose evolutional timeline had remained uncertain. In this work, we present results of the first successful attempt to date crustal xenoliths and their host lava flows from the island, by incremental heating 40Ar/39Ar method, in an attempt to understand the evolutionary histories of the volcano and its basement. Based on concordant plateau and isochron ages, we establish that the oldest subaerial lava flows of the volcano are 1.58 ± 0.04 (2σ) Ma, and some of the plagioclase xenocrysts have been derived from crustal rocks of 106 ± 3 (2σ) Ma. Mineralogy (anorthite + Cr-rich diopside + minor olivine) and isotopic compositions (87Sr/86Sr < 0.7040; ɛNd > 7.0) of xenoliths not only indicate their derivation from a lower (oceanic) crustal olivine gabbro but also suggest a genetic relationship between the arc crust and the ophiolitic basement of the Andaman accretionary prism. We speculate that the basements of the forearc and volcanic arc of the Andaman subduction zone belong to a single continuous unit that was once attached to the western margin of the Eurasian Plate.

  19. The geology and 40Ar/ 39Ar geochronology of magmatic activity and related mineralization in the Nevados del Famatina mining district, La Rioja province, Argentina

    NASA Astrophysics Data System (ADS)

    Losada-Calderón, A. J.; McBride, S. L.; Bloom, M. S.

    1994-01-01

    The Nevados del Famatina mining district (NFMD) is located in La Rioja province, Argentina. This district contains porphyry-style mineralization (Nevados del Famatina) and high sulfidation veins (La Mejicana). The stratigraphic column in the NFMD begins with Cambrian siltstones which were metamorphosed during the Late Ordovician - Early Silurian and intruded by Late Ordovician-Silurian granitic rocks. These units were covered by Upper Paleozoic and Tertiary continental sedimentary rocks which are intercalated with and overlain by dacitic-rhyodacitic porphyritic rocks (Mogote Formation) emplaced during the Pliocene. All these units are covered by Pleistocene sediments and Quaternary alluvial and colluvial deposits. Magmatic activity and related mineralization in the NFMD have been dated by the 40Ar/ 39Ar technique. Step heating studies of orthoclase and biotite phenocrysts from the Mogote Formation in the NFMD suggest that the igneous rocks were emplaced around 5.0±0.3 Ma ago. However, plateau ages of biotite from the outer carapace of the subjacent granodioritic magma chamber and of muscovite from quartz-sericite alteration at both Nevados del Famatina and La Mejicana are around 3.8±0.2 Ma. Emplacement of the shallow stocks is separated from cooling of the outer carapace of the subjacent granodioritic magma chamber to temperatures below 350° C by a time span of approximately 1 Ma. During this interval, a convective hydrothermal system was established proximal to the granodioritic magma chamber, which resulted in porphyry molybdenumcoppergold mineralization adjacent to the igneous rocks and more distal high sulfidation veins located in fault zones.

  20. Volcano-stratigraphic and structural evolution of Brava Island (Cape Verde) based on 40Ar/ 39Ar, U-Th and field constraints

    NASA Astrophysics Data System (ADS)

    Madeira, José; Mata, João; Mourão, Cyntia; Brum da Silveira, António; Martins, Sofia; Ramalho, Ricardo; Hoffmann, Dirk L.

    2010-10-01

    Three volcano-stratigraphic units were identified at Brava Island in the Cape Verde Archipelago on the basis of field relationships, geologic mapping and 40Ar/ 39Ar and U-Th ages. The Lower Unit comprises a 2-to-3 Ma-old submarine volcanic sequence that represents the seamount stage. It is composed of nephelinitic/ankaramitic hyaloclastites and pillow lavas, which are cut by abundant co-genetic dikes. Plutonic rocks of an alkaline-carbonatite complex, which intruded the submarine sequence 1.8 to 1.3 Ma ago, constitute the Middle Unit. A major erosional surface developed between 1.3 and ~ 0.25 Ma. The post-erosional volcanism recorded in the Upper Unit started 0.25 Ma ago and is dominated by phonolitic magmatism. This phase is characterised by explosive phreato-magmatic and magmatic activity that produced block and ash flow, surge, and pyroclastic fall deposits and numerous phreato-magmatic craters. Effusive events are represented by lava domes and coulées. One peculiarity of Brava is the occurrence of carbonatites in both the plutonic complex and the post-erosional phase as extrusive volcanics. The intrusive carbonatites are younger than those occurring on Fogo, Santiago and Maio islands. Young (Upper Pleistocene to Holocene) extrusive carbonatites occurring in the late stages of volcanism are unknown in other Cape Verde islands. The occurrence of pillow lavas and hyaloclastites above the present sea level (up to 400 m) and raised Upper Pleistocene beaches indicates continuous uplift of Brava since the seamount stage. By dating raised marine markers, uplift rates were estimated at between 0.2 and 0.4 mm/a. The evolution of Brava was controlled by faults with directions similar to those described for Fogo, suggesting a common stress field. A detailed geological map (1/25,000) of Brava is presented.

  1. Cosmic markers, 40Ar/ 39Ar dating and paleomagnetism of the KT sections in the Anjar Area of the Deccan large igneous province

    NASA Astrophysics Data System (ADS)

    Courtillot, V.; Gallet, Y.; Rocchia, R.; Féraud, G.; Robin, E.; Hofmann, C.; Bhandari, N.; Ghevariya, Z. G.

    2000-10-01

    Bhandari et al. [Bhandari et al., Geophys. Res. Lett. 22 (1995) 433-436; Bhandari et al., Geol. Soc. Am. Spec. Paper 307 (1996) 417-424] reported the discovery of iridium-bearing sediments sandwiched between basalt flows in the Anjar area (Kutch province, India). They concluded that the signature of the K/T impact had been recorded and that onset of volcanism in the Deccan traps preceded the K/T boundary, excluding the possibility of a causal connection. This paper reports complementary analyses of Anjar outcrops by a joint Indo-French team, where we focused on cosmic markers (iridium and spinels) in the intertrappean sediments and 40Ar/ 39Ar dating and paleomagnetism of the lava flows. Anomalous Ir concentrations (up to 0.4 ng/g) are confirmed, with up to three thin and patchy enriched layers which cannot be traced throughout the exposed sections. Despite careful search, no Ni-rich spinels were found. Eight basalt samples provided 40Ar/ 39Ar results, four on plagioclase bulk samples, four on whole rocks. Spectra for whole rocks all indicate some amount of disturbance, and ages based on plagioclase bulk samples seem to be consistently more reliable [Hofmann et al., Earth Planet. Sci. Lett. 180 (2000) 13-28]. The three flows underlying the Ir-bearing sediments are dated at ˜66.5 Ma, and two overlying flows at ˜65 Ma. Magnetic analyses (both thermal and by alternating fields) uncovered clear reversed primary components in the upper flows, and more disturbed normal components in the lower flows, with evidence for an additional reversed component. There are reports [Bajpai, Geol. Soc. India Mem. 37 (1996) 313-319; Bajpai, J. Geol. Soc. London 157 (2000) 257-260] that the intertrappean sediments contain uppermost Maastrichtian dinosaur and ostracod remains above the uppermost Ir-bearing level, and may not be mechanically disturbed. We propose the following scenario to interpret these multiple field and analytical observations. Deccan trap volcanism started within

  2. Rates of burial and exhumation of lawsonite blueschist/eclogite in subduction zones from in situ UV laser ablation 40Ar/39Ar phengite geochronology

    NASA Astrophysics Data System (ADS)

    Fornash, K.; Cosca, M. A.; Whitney, D. L.; Teyssier, C. P.

    2014-12-01

    Lawsonite eclogites and blueschists are accessible records of processes that occur at depth in subducting slabs and can therefore provide information about the chemical and physical evolution of subduction zones. In composite blueschist-eclogite terranes, blueschists may have formed (1) by prograde metamorphism (pre-eclogite), (2) at the same P-T conditions as eclogite-facies metamorphism as a result of differences in bulk composition, H2O content, or oxidation state, or (3) from retrogression of eclogite, e.g. during exhumation. Field and petrologic observations of lawsonite eclogite and blueschist in the Sivrihisar Massif, Turkey, suggest that some blueschist formed from eclogite during exhumation in the subduction channel, whereas results from thermobarometry suggest that some blueschist formed at the same P-T conditions as eclogite. To test the age, petrologic, and tectonic relationship of coexisting eclogite and blueschist, we applied in situ UV laser ablation 40Ar/39Ar phengite geochronology to eclogite- and blueschist-facies rocks representing different structural positions and displaying different phengite textures and coexisting mineral assemblages. Phengite from fresh lawsonite eclogite yield an age of 93 ± 2 Ma and have the narrowest spread in ages (<12 Ma) of any rock type analyzed. Retrogressed (epidote) eclogite yields a mean weighted age of 82 ± 2 Ma. In contrast to the tightly constrained ages obtained in eclogite pods, blueschists and blueschist-facies quartzite exhibit discrete age populations ranging from 82 Ma to 110 Ma. Deformed phengite clusters from lawsonite garnet blueschist record age populations at 82 Ma and 92 Ma. Phengite from lawsonite-garnet veins and glaucophane-rich margins of eclogite pods also record 92 Ma. Omphacite-bearing lawsonite blueschist and a blueschist-facies quartzite from the same structural position contain age populations at 97 Ma and 110 Ma. These results document a sequence of events from prograde blueschist

  3. Cenozoic tectonics in the Buruanga Peninsula, Panay Island, Central Philippines, as constrained by U-Pb, 40Ar/39Ar and fission track thermochronometers

    NASA Astrophysics Data System (ADS)

    Walia, M.; Yang, T. F.; Knittel, U.; Liu, T.-K.; Lo, C.-H.; Chung, S.-L.; Teng, L. S.; Dimalanta, C. B.; Yumul, G. P.; Yuan, W. M.

    2013-01-01

    Buruanga Peninsula forms the westernmost part of Panay Island, Central Philippines and is a part of the Palawan Continental Terrane (PCT), which was formerly attached to south-eastern China. It acted as the leading edge of the continental fragment and collided with the Philippine Mobile Belt (PMB) followed by convergence beneath the latter. Dating of the collision is crucial for understanding the evolution of the archipelago. Samples collected from Buruanga Peninsula were dated using U-Pb, 40Ar/39Ar and fission track dating (FTD) techniques to constrain the timing of the tectonic events related to the collision of the PMB with the PCT. These techniques have enabled us to obtain ages over a range of closure temperatures from about 700 °C to about 110 °C. Paleoproterozoic and Permian zircon U-Pb ages from Saboncogon Formation emphasize derivation of the western part of Buruanga Peninsula from SE China; zircon and apatite fission track ages of 51 Ma and 16 Ma, respectively, constrain the exhumation of this formation. The age data suggest tectonic events at ~ 14 Ma, ~ 11-12 Ma and about 7-8 Ma following intrusive activity at about 18 Ma. Uplift and exhumation at ~ 14 Ma are thought to be the result of subduction of low-density crustal rocks, at 11 Ma to be the result of isostatic uplift as a consequence of crustal thickening and at ~ 8 Ma to be due to the isostatic re-equilibration of the sediments overlying the former suture. Hence, collision is constrained to have started at about 14-15 Ma and to have ended before 8 Ma. Multi-element patterns of the 18 Ma Patria-Diorite from Buruanga Peninsula show enrichment in LILE (Rb, Sr, and K) and LREE and depletion in HFSE elements (Ti, Nb, and Ta) similar to those from Luzon volcanics and the volcanic rocks of Negros Island. These arc-signatures indicate a subduction related environment for the emplacement of this intrusive body and show that the diorite belongs to the PMB. The age constraints of the present study neither

  4. Hydrogen and 40Ar/39Ar isotope evidence for multiple and protracted paleofluid flow events within the long-lived North Anatolian Keirogen (Turkey)

    NASA Astrophysics Data System (ADS)

    Boles, Austin; van der Pluijm, Ben; Mulch, Andreas; Mutlu, Halim; Uysal, I. Tonguç; Warr, Laurence N.

    2015-06-01

    We present a new approach to identifying the source and age of paleofluids associated with low-temperature deformation in the brittle crust, using hydrogen isotopic compositions (δD) and 40Ar/39Ar geochronology of authigenic illite in clay gouge-bearing fault zones. The procedure involves grain-size separation, polytype modeling, and isotopic analysis, creating a mixing line that is used to extrapolate to δD and age of pure authigenic and detrital material. We use this method on samples collected along the surface trace of today's North Anatolian Fault (NAF). δD values of the authigenic illite population, obtained by extrapolation, are -89 ± 3‰, -90 ± 2‰, and -97 ± 2‰ (VSMOW) for samples KSL, RES4-1, and G1G2, respectively. These correspond to δD fluid values of -62‰ to -85‰ for the temperature range of 125°C ± 25°, indistinguishable from present-day precipitation values. δD values of the detrital illite population are -45 ± 13‰, -60 ± 6‰, and -64 ± 6‰ for samples KSL, G1G2, and RES4-1, respectively. Corresponding δD fluid values at 300°C are -26‰ to -45‰ and match values from adjacent metamorphic terranes. Corresponding clay gouge ages are 41.4 ± 3.4 Ma (authigenic) and 95.8 ± 7.7 Ma (detrital) for sample G2 and 24.6 ± 1.6 Ma (authigenic) and 96.5 ± 3.8 Ma (detrital) for sample RES4-1, demonstrating a long history of meteoric fluid infiltration in the area. We conclude that today's NAF incorporated preexisting, weak clay-rich rocks that represent earlier mineralizing fluid events. The samples preserve at least three fluid flow pulses since the Eocene and indicate that meteoric fluid has been circulating in the upper crust in the North Anatolian Keirogen since that time.

  5. 40Ar* loss in experimentally deformed muscovite and biotite with implications for 40Ar/39Ar geochronology of naturally deformed rocks

    USGS Publications Warehouse

    Cosca, Michael; Stunitz, Holger; Bourgiex, Anne-Lise; Lee, John 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, ~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 degrees 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.

  6. [sup 40]Ar/[sup 39]Ar analysis of supergene jarosite and alunite: Implications to the paleoweathering history of the western USA and West Africa

    SciTech Connect

    Vasconcelos, P.M.; Brimhall, G.H. ); Becker, T.A.; Renne, P.R. )

    1994-01-01

    Supergene alunite (KAl[sub 3](SO[sub 4])[sub 2](OH)[sub 6]) and jarosite (KFe[sub 3](SO[sub 4])[sub 2](OH)[sub 6]) are often precipitated during the oxidation of sulfide-bearing rocks by meteoric solutions. Dating of these phases by the [sup 40]Ar/[sup 39]Ar method allows timing of the progression of the oxidation front during chemical weathering. Fine-scale laser-heating [sup 40]Ar/[sup 39]Ar dating of hypogene alunite and supergene jarosites allows to precisely and accurately time hydrothermal alteration and subsequent supergene oxidation in Goldfield, Nevada. The results indicate that pervasive weathering occurred in the western USA during the Late Miocene ([approximately] 10 Ma ago). Similar application of this technique to the study of weathering and laterite formation in West Africa indicates that the last pervasive oxidation event recorded in the weathering profile in this area also occurred in the Miocene ([approximately] 13 Ma ago). The occurrence of a pervasive Mid to Late Miocene oxidation event recorded in these weathering profiles in the western USA and Africa, and also previously measured in Brazil and Chile, indicates that climatic conditions at that time were conducive to worldwide development of deep weathering sequences. Subsequent weathering processes have not been as pervasive as the late Miocene event, indicating a general climatic transition to cooler, drier climates in most of the areas studied. The results indicate that deep weathering profiles reflect past climatic conditions and may not be directly linked to the present climates.

  7. Cooling and inferred uplift/erosion history of the Grenville Orogen, Ontario: Constraints from sup 40 Ar/ sup 39 Ar thermochronology

    SciTech Connect

    Cosca, M.A.

    1989-01-01

    Thermochronological ({sup 40}Ar/{sup 39}Ar) data are presented from 76 mineral separates of hornblende, muscovite, biotite, phlogopite, and K-feldspar. Samples were selected from regionally metamorphosed gneiss, amphibolite, metasediment, marble, metagabbro and pegmatite across the two major metamorphic belts of the Grenville Province, the Central Metasedimentary Belt (CMB) and the Central Gneiss Belt (CGB). When combined with published temperature estimates for closure to argon diffusion in the phases analyzed, cooling rates from {approximately}500 C to {approximately}120 C of 1-4 C/MA are calculated across the entire Grenville Province of Ontario. Regional uplift/erosion rates for the Grenville Orogen of Ontario have been estimated from the {sup 40}Ar/{sup 39}Ar data, a retrograde P-T path for rocks of the CGB, and an upper time constraint provided by flat, overlying Cambro-Ordovician sediments. Twenty-two of the hornblendes used for thermochronology have been quantitatively analyzed for major elements by microprobe, Fe{sup 2+}/Fe{sup 3+} by wet chemistry, and for H{sub 2}O by manometric measurement. Water activities calculated from hornblende equilibria are typically low (<0.01) because of the exponential dilutions in hornblende (tremolite) activity required by present activity-composition models. An oxyamphibole component of 25% further reduces any amphibole component and the H{sub 2}O activity by as much as 50% below that calculated with simplifying assumption. These findings indicate that different amphibole normalization schemes have a marked effect on the activity calculated for a specific amphibole or H{sub 2}O, and should be carefully evaluated.

  8. Evidence for an Alleghanian (Early Carboniferous to Late Permian) tectonothermal event in the New Jersey Coastal Plain basement from 40Ar/39Ar biotite data, geochemistry and gravity modeling

    USGS Publications Warehouse

    Maguire, T.J.; Volkert, R.A.; Swisher, C. C., III; Sheridan, R.E.

    2009-01-01

    40Ar/39Ar dating of biotite from felsic orthogneiss recovered from the -3890-foot level of the Island Beach State Park (IBSP) well beneath the outer New Jersey Coastal Plain was accomplished using CO2 laser incremental-heating techniques. Over 75% of the Ar released from the incremental-heating experiment form a well-behaved plateau with a calculated age of 243.98 ?? 0.10 Ma. The new 244 Ma biotite age reported here is a cooling age younger than the metamorphic event that crystallized or reheated the biotite. We consider reheating of older biotite to be unlikely because the concordant 40Ar/39Ar spectrum upon repeated incremental laser heating showed a well-developed plateau. Thus, biotites from the IBSP gneiss are interpreted as having crystallized during a single thermal event, followed by cooling to below 300 ??C. The IBSP well falls on a structural and geophysical anomaly trend that is along strike with rocks of the Bronson Hill anticlinorium to the north of the IBSP gneiss. Locally graphitic metasedimentary schists and gneisses recovered from New Jersey wells inboard of the IBSP well gneiss correlate to similar lithologies of the Connecticut Valley synclinorium west of the Hartford basin. Our reinterpretation of the IBSP gneiss as metamorphosed dacite or dacitic tuff is consistent with a correlation to some rocks of the Bronson Hill magmatic arc east of the Hartford basin. If correct, this would imply a Late Ordovician age for the protolith of the IBSP gneiss. Reported 40Ar/39Ar biotite ages of 235-253 Ma from southwestern Rhode Island, and of 238-247 Ma from southeastern Connecticut, are interpreted as cooling ages following a tectonothermal event associated with the Alleghanian orogeny (Early Carboniferous to Late Permian). Cooling ages of Alleghanian age (Early Carboniferous to Late Permian) are not recognized west of the Bronson Hill volcanic arc in either central Connecticut or in Massachusetts. Therefore, the 244 Ma cooling age presented here, and the

  9. Triassic High-P Metamorphism of the central Qiangtang terrane, Tibet; constraints using mineral equilibria modelling and 40Ar/39Ar geochronology

    NASA Astrophysics Data System (ADS)

    Rajkumar, A.; Hui, L.; Clarke, G. L.; Aitchison, J. C.; Forster, M. A.

    2014-12-01

    The SE-trending Qiangtang metamorphic belt (QMB) stretches more than 500 km through the Qiangtang terrane in central Tibet and comprises tectonically disrupted blueschist and eclogite in lower-grade garnet-phengite-bearing schist and quartzite. These rocks record the closure of a paleo-Tethyan Triassic ocean that formerly separated Cathaysian and Gondwana components of Asia, now forming the northern and southern Qiangtang blocks. Eclogite is extensively recrystallized to high-P amphibolite and greenschist facies assemblages, formed during water ingression that accompanied terrane uplift. P-T pseudosections constructed in Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O (NCKMASHTO) in the context of petrography and mineral chemistry provides the ability to recover a dynamic PT history for the eclogite facies assemblages. Prograde (S1) assemblages for the Gemu Co eclogite are predicted to have formed at P≈21.5 kbars and T≈505°C and involved garnet, glaucophane, omphacite, rutile, lawsonite and chlorite, based on garnet composition and inferred pseudomorphs after lawsonite. Peak (S2) assemblages of garnet, barroisite, omphacite, rutile, epidote and quartz reflect P≈15 kbars and T≈570°C. Based on textural relations, post-peak stages can be divided into epidote-amphibolite and greenschist facies. The geothermal gradient for the prograde S1 assemblage and the peak S2 assemblage is 7.1 and 11.5°C/km respectively.40Ar/39Ar geochronology of phengitic mica using step heating in recrystallized eclogite components and surrounding garnet-mica schist components both yield maximum ages ranging 230-220 Ma. The congruency in ages of the deeply subducted high-pressure eclogites to the surrounding garnet phengite schists indicate they were the most probable source of fluids to extensively recrystallize most of the high-pressure eclogite components in the high-pressure belt. The P-T history of the high-P rocks of the QMB records the deep subduction of paleo-Tethyan oceanic crust to

  10. Volcán Tancítaro, Michoacán, Mexico, 40Ar/ 39Ar constraints on its history of sector collapse

    NASA Astrophysics Data System (ADS)

    Ownby, Steven; Delgado Granados, Hugo; Lange, Rebecca A.; Hall, Chris M.

    2007-03-01

    Volcán Tancítaro is a 97 ± 3 km 3 stratovolcano located in the Michoacán Guanajuato volcanic field (MGVF), part of the Trans Mexican Volcanic Belt. Prior to this study, there was only one K-Ar date (530 ± 60 ka; [Ban, M., Hasenaka, T., Delgado-Granados, H., Takaoka, N., 1992. K-Ar ages of lavas from shield volcanoes in the Michoacán-Guanajuato volcanic field, Mexico. Geofisica Internacional 31 (4), 467-473.] and one sector-collapse event reported for this volcano in the literature [Garduño-Monroy V.H., Corona-Chavéz, P., Israde-Alcantara, I., Mennella, L., Arreygue, E., Bigioggero, B., Chiesa, S., 1999. Carta Geológica de Michoacán, scale 1:250,000. Universidad Michoacana de San Nicolás de Hidalgo.; Capra, L., Macías, J.L., Scott, K.M., Abrams, M., Garduño-Monroy, V.H., 2002. Debris avalanches and debris flows transformed from collapses in the Trans-Mexican Volcanic Belt, Mexico — Behavior, and implications for hazard assessment. Journal of Volcanology and Geothermal Research 113, 81-110.]. Twenty-six new 40Ar/ 39Ar ages indicate that Volcán Tancítaro became active ≥ 793 ± 22 ka and that the most recent effusive activity occurred at 237 ± 34 ka. Two catastrophic sector-collapse events are identified and dated; the first one occurred on the west side between 694 and 571 ka, whereas the second one occurred on the east side between 261 and 238 ka. The older collapse produced a 2.3-3.4 km 3 debris-avalanche and laharic deposit spread over ˜ 567 km 2, whereas the more recent collapse left a 3-km wide, horseshoe-shaped scar on the eastern flank and produced a 3.6-7.0 km 3 debris-avalanche and laharic deposit that covers ˜ 654 km 2. Reconstruction of the main edifice of Volcán Tancítaro using ArcGIS software and digital elevation models indicates that the volume removed during the eastern sector collapse was ˜ 4.7 km 3.

  11. Status Report on the 40Ar/39Ar and U/Pb Dating of Tuffs in the Dewey Lake Formation of West Texas Towards Constraining the Permo-Triassic Magnetostratigraphic Time Scale

    NASA Astrophysics Data System (ADS)

    Chang, S.; Renne, P. R.; Mundil, R.

    2007-12-01

    A detailed magnetic polarity time scale for the Permo-Triassic Boundary interval, critical for correlating events in marine and terrestrial paleoenvironments, is not yet well-established. Recently, late Permian magnetostratigraphic studies have been reported for non-marine sections in Europe and South Africa (Szurlies et al., 2003; Nawrocki, 2004; Ward et al., 2005). However, these sections are devoid of index fossil suitable for correlation with marine successions and also lack age constraints from radioisotopic dating methods. In other words, it is dubious to correlate these magnetostratigraphic data with the GSSP Permo-Triassic boundary and mass extinction. The Dewey Lake red beds formation of West Texas, believed to be the youngest Permian formation in North America, has yielded high-quality paleomagnetic data (Molina-Garza et al., 1989; Steiner, 2001) and contains several silicic tuffs potentially enabling high-resolution calibration of the magnetic polarity time scale in this critical age range. The tuffs have yet to be placed into a regional stratigraphic or magnetostratigraphic framework, and it is unclear exactly how many distinct eruptive units are represented by the 7 distinct samples collected to date from widely separated (>160 km) localities. 40Ar/39Ar (sanidine and biotite) and U/Pb (zircon) studies reveal that all 7 sampled tuffs were probably erupted within several hundred ka of the Permo-Triassic boundary as dated at the Meishan GSSP section (Renne et al., 1995; Mundil et al., 2004) but results thus far are inadequate to convincingly resolve age differences between the various samples. U/Pb dating of some samples is severely challenged by Pb-loss from the zircons despite application of the Mattinson (2005) annealing/chemical abrasion technique. 40Ar/39Ar data have been obtained from as many as four different irradiations in order to reduce neutron fluence related error. We observe the familiar ~1% bias between U/Pb and 40Ar/39Ar ages. Biotite

  12. Magnetostratigraphy and 39Ar/40Ar studies of the Lana'i Long Volcanic Sequence (ca. 1.606+/-0.063 Ma), Hawaii, USA

    NASA Astrophysics Data System (ADS)

    Herrero-Bervera, E.; Jicha, B.; Valet, J.

    2013-12-01

    Previous published work on Lanai indicated that the volcano was formed mainly during the Matuyama Chron (Herrero-Bervera et al., 2000). In order to constrain further the timing of the active phases of the Lanai volcano, we conducted a paleomagnetic and rock magnetic study involving a ~500-m vertical thick sequence of lava flows that were erupted between 0.76+/-0.66 Ma and 1.6+/-0.09 Ma according to previous K/Ar and 40Ar/39Ar dating (Leonhardt et al., 2009). Low-field susceptibility versus temperature (k-T) and SIRM experiments performed on a dozen flows indicate that magnetite dominates the remanent magnetization (575°C). In a few cases, a low-temperature mineral phase (300-400°C) could reflect the presence of titanomagnetite with low Ti content, but the presence of maghemite or pyrrhotite cannot be completely excluded. Additional investigations are in progress on this matter. All specimens were step-wise demagnetized by alternating fields from 5 to 100 mT. Companion specimens from the same samples were demagnetized at 15 temperature steps. The demagnetization diagrams obtained with each technique showed a stable direction of remanence. In all cases, the characteristic (ChRM) component was clearly defined from at least seven successive directions isolated during step-wise demagnetization. The succession of the mean directions calculated for each lava flow reveals the existence of at least one polarity interval. Based on radiometric dates, they were assigned to the Gilsa, "excursion" (1.606+/-0.063 Ma). Thus, the present results, along with the radiometric ages of the lavas, indicate that the tholeiitic flows that formed the Lanai volcano were erupted over a short time period, and only during the Matuyama Chron (0.780-2.58 Ma). No eruptions have occurred during the Brunhes Chron (0.78 Ma) as previously indicated from K-Ar data on lavas in the Maunalei Gulch. The excursional VGPs from the onset of the Gilsa excursion recorded on Lanai are situated near the

  13. 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

  14. 40Ar/39Ar geochronology of hypabyssal igneous rocks in the Maranon Basin of Peru - A record of thermal history, structure, and alteration

    USGS Publications Warehouse

    Prueher, L.M.; Erlich, R.; Snee, L.W.

    2005-01-01

    Hypabyssal andesites and dacites from the Balsapuerto Dome in the Mara?on Basin of Peru record the thermal, tectonic, and alteration history of the area. The Mara?on Basin is one of 19 sub-Andean foreland basins. The hypabyssal rocks in the Balsapuerto Dome are one of four known occurrences of subvolcanic rocks along the deformation front in Peru. This dome is a potential petroleum structural trap. Petroleum seeps near the dome indicate that a source for the petroleum is present, but the extent and amount of petroleum development is unknown. The Balsapuerto hypabyssal rocks are plagioclase-, hornblende-, pyroxene-phyric andesites to dacites. Some parts of the dome are pervasively altered to a hydrothermal assemblage of quartz-sericite-pyrite. 40Ar/39Ar geochronology shows that thermal activity related to emplacement of these subvolcanic rocks took place between 12-10 Ma, subsequent to the major periods of Andean folding and faulting, previously assumed to have occurred about 9 Ma. Eleven argon mineral age-spectrum analyses were completed. Argon apparent ages on amphibole range from 12.7 to 11.6 Ma, and the age spectra are simple, which indicates that the ages are very close to emplacement ages. Potassium feldspar yields an argon age spectrum ranging in age from 12.5 to 11.4 Ma, reflecting the period during which the potassium feldspar closed to argon diffusion between the temperature range of 350?C to about 150?C; thus the potassium feldspar age spectrum reflects a cooling profile throughout this temperature range. This age range is consistent with ages of emplacement for the entire igneous complex indicating that an increased thermal state existed in the area for at least 1.0 m.y. Combined with the coexisting hornblende age, this rock cooled from ~580?C to ~150?C in ~1.2 m.y. resulting in an average cooling rate of 358?C /m.y. White mica, or sericite, formed as a later alteration phase associated with quartz- sericite- pyrite and propylitic alteration in some

  15. Comment on “40Ar / 39Ar dating of the Rajahmundry Traps, eastern India and their relationship to the Deccan Traps” by Knight et al. [Earth Planet Sci. Lett. 208 (2003) 85 99

    NASA Astrophysics Data System (ADS)

    Baksi, Ajoy K.

    2005-11-01

    Knight et al. presented age and chemical data on two (sets of) lava flows from the Rajahmundry area, on either bank of the Godavari River. The age and petrogenesis of these flows and their possible link to sections of the main Deccan Province are of importance to the understanding of many aspects of flood basalt volcanism. I comment on (a) the use of geochemical fingerprints for lava identification/correlation at Rajahmundry, superceding (apparent) field relations, (b) their 40Ar / 39Ar data and its refinement based on statistical tests and the alteration state of the samples (c) correlation of age data and the magnetic polarity of the lavas to the geomagnetic polarity time scale and (d) the possibility that both lavas at Rajahmundry were formed by intracanyon flows derived from ˜1000 km away.

  16. 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.

  17. 40Ar/ 39Ar dating constraints on the high-angle normal faulting along the southern segment of the Tan-Lu fault system: An implication for the onset of eastern China rift-systems

    NASA Astrophysics Data System (ADS)

    Wang, Yu; Zhou, Su

    2009-01-01

    High-angle normal faulting in eastern China was an important tectonic process responsible for the rifting of the eastern Asian continental margin. Along the southern segment of the Tan-Lu fault system, part of the eastern China rift-system, 55-70° east-dipping normal faults are the oldest structures within this rift-system. Chlorite, pseudotachylite, and fault breccia are found in fault zones, which are characterized by microstructures and syn-deformation chlorite minerals aligned parallel to a down-dip stretching lineation. 40Ar/ 39Ar dating of syn-deformation chlorite and K-feldspar from the fault gouge zone yields cooling ages of ˜75-70 Ma, interpreted as the timing of slip along the normal faults. This age is older than that of opening of the Japanese sea and back-arc extension in the west Pacific, but similar to the onset of the Indo-Asian (soft?) collision.

  18. Evidence of a close link between petrology and isotope records: constraints from SEM, EMP, TEM and in situ 40Ar- 39Ar laser analyses on multiple generations of white micas (Lanterman Range, Antarctica)

    NASA Astrophysics Data System (ADS)

    Di Vincenzo, Gianfranco; Ghiribelli, Barbara; Giorgetti, Giovanna; Palmeri, Rosaria

    2001-10-01

    K-Ar ages from white mica are commonly interpreted to record cooling below a certain temperature with the implicit assumption that all the requirements of the volume diffusion theory are fulfilled. Nevertheless, studies on metamorphic white micas have highlighted discrepancies with previously inferred closure temperatures and have evidenced a close link between petrology and argon isotope age records. This study uses the in situ 40Ar- 39Ar laserprobe method in conjunction with scanning electron microscopy, electron microprobe and transmission electron microscopy (TEM) techniques to examine the relations between argon isotope records and microtextural, microchemical and microstructural variations in white mica. Gneisses and micaschists belonging to three different tectono-metamorphic complexes of the Lanterman Range (Antarctica) contain multiple generations of potassic white micas and are well-suited to investigate the relation between petrology and argon dating. Texturally resolvable white mica generations show microchemical and microstructural (TEM scale) variations that suggest development under different P- T deformation regimes, ranging from an eclogite facies stage down to low greenschist facies conditions. In situ 40Ar- 39Ar laserprobe analyses on white mica samples from the three complexes reveal a complex intragrain and intergrain spatial distribution of argon ages which is closely linked to microtextural, microchemical and microstructural variations: texturally, compositionally and microstructurally older generations yield older ages whereas the younger ones yield younger ages. Results show that in the absence of re-crystallisation, white mica preserves argon isotope records pertaining to the high-pressure stage which survived amphibolite retrogression at temperatures of 550-650°C. The texture, petrology and isotope record of white micas in the studied samples preserve a nearly continuous record of P, T and deformation history within the same orogenic

  19. Eruptive History of Volcán Tepetiltic, Mexico: Evidence for Remelting of Silicic Ashflows Revealed by 40Ar/39Ar Geochronology

    NASA Astrophysics Data System (ADS)

    Frey, H. M.; Lange, R. A.; Hall, C. M.; Nelson, S. A.; Granados, H. D.

    2004-12-01

    Volcán Tepetiltic (VT) is located in the northwestern part of the Trans-Mexican Volcanic Belt and features an elliptical caldera (5 x 2.5 km). Previous detailed 40Ar/39Ar geochronology studies at V. Tequila and V. Ceboruco, showed that cone-building events occur within narrow time intervals (< 25 kyrs) and eruptive phases may be separated by hiatuses of more than 100 kyrs. At those volcanoes, our studies were restricted to surface flows. However, at VT, a rhyolitic Plinian eruption created a caldera which exposed ˜600 m of stratified andesitic and dacitic lava flows and thus, allowed the opportunity to study the cone-building events and compositional evolution of this arc stratocone. A suite of samples from a stratigraphic section of the southern caldera wall were dated to determine the eruptive history during construction of the stratovolcano. Samples from andesite flows at the base of the caldera wall (516 ± 11 and 529 ± 18 ka), mid-way up the wall (533 ± 32 ka) and the second highest flow of the wall (528 ± 15 ka) yielded indistinguishable ages. Similar ages were obtained from the northern rim of the caldera wall (502 ± 26 ka), an andesite flank flow north of the stratocone (552 ± 18 ka) and a vertical dike cutting flows in the southern caldera wall (505 ± 10 ka). Therefore the bulk of the edifice was built within a 20 kyr interval (given 1 sigma errors) at ˜524 ka. The stratigraphically highest sample in the caldera wall yielded an age of 231 ± 36 ka. Thus, there may have been a hiatus of >200 kyr between cone building episodes along the southern flank of the volcano. Following construction of the main stratocone (dominated by andesitic effusive activity), there was a clustering of rhyolite eruptions, including the caldera forming event which produced rhyolitic ashflow and airfall deposits. The age of the eruption is constrained by stratigraphic and cross-cutting relations. The eruption must be younger than 231 ± 36 kyr, based on the uppermost

  20. On the Discovery of Cryptochron C2r.2r-l (ca. 2.42-2.44 Ma) Recorded on Koolau Volcano at Halawa: Evidence from Paleomagnetic and 40Ar/39Ar studies

    NASA Astrophysics Data System (ADS)

    Browne, E. J.; Herrero-Bervera, E.; Singer, B.

    2004-12-01

    New paleomagnetic measurements, coupled with precise 40Ar/39Ar radioisotopic dating, are revolutionizing our understanding of the geodynamo by providing detailed terrestrial lava records of the short-term behavior of the paleomagnetic field. As part of an investigation of the Koolau Volcano, Oahu, and the short-term behavior of the geomagnetic field, we have sampled a long volcanic section located on the volcanoś buttressed flank within Halawa Valley. Prior paleomagnetic and K-Ar investigations of the Koolau (Volcano) Series revealed excursional directions (Site F of Doell and Dalrymple, 1973). The alkaline composition of lava flows, easy access, and close geographical proximity to K-Ar dated lava flows made this newly studied 120 m thick sequence of flows an excellent candidate for detailed paleomagnetic analysis. At least eight samples, collected from each of 28 successive flow-sites, were stepwise demagnetized by both alternating field (5mT to 100mT) and thermal (from 28oC to 575-650o C) methods. Mean directions were obtained by principal component analysis. All samples yielded a strong and stable ChRM trending towards the origin based on no less than seven to nine steps, with thermal and AF results agreeing to a very high degree. Low field susceptibility vs. temperature (k-T) analyses were conducted for individual lava flows, with approximately half showing reversible curves. Curie point determinations revealed a temperature close to or equal to 580o C, indicative of almost pure magnetite for most of the flows. The mean directions of magnetization of the entire section sampled indicate that about 10 m of the section are characterized by excursional directions (5 lava flows). The corresponding VGPs are located off the southeast part of Africa, close to Madagascar. Initial 40Ar/39Ar incremental heating experiments on groundmass from four flow-sites located at different stratigraphic levels yielded isochron ages ranging from 2.68+/-0.25 to 2.40+/-0.46 Ma

  1. 40Ar/ 39Ar dating of the pre-evaporitic Messinian marine sequences of the Melilla basin (Morocco): a proposal for some biosedimentary events as isochrons around the Alboran Sea

    NASA Astrophysics Data System (ADS)

    Roger, S.; Münch, Ph.; Cornée, J. J.; Saint Martin, J. P.; Féraud, G.; Pestrea, S.; Conesa, G.; Ben Moussa, A.

    2000-06-01

    In the Mediterranean region, the Melilla basin (NE Morocco) represents a key area that recorded biosedimentary events and environmental changes relative to the pre-evaporitic Messinian times. 40Ar/ 39Ar dating of volcanic tuffs interbedded all along the Melilla Messinian shelf carbonates and coeval basin deposits has been performed in order to date accurately three main pre-evaporitic biosedimentary events: the prograding bioclastic deposition and the oligospecific prograding Porites coral reef buildings with coeval Halimeda blooms, both within the platform, and the diatomite deposition basinward. The new age data allow a precise chronological framework to be established for the pre-evaporitic sequence of Melilla basin demonstrating that diatomitic deposits are coeval with both prograding bioclastic and reefal units. The prograding bioclastic carbonate unit related to boreal influences in coeval basinal diatomites, began at least at 6.73±0.02 Ma and ended at 6.46±0.03 Ma. The oligospecific prograding Porites coral-reefs and Halimeda beds and coeval warm-water diatomites began at least at 6.46±0.03 Ma. Previous 40Ar/ 39Ar ages indicate that they ended prior to 6.0±0.1 Ma. Both biosedimentological similarities and chronological accordance within several platforms and adjacent basins all around the Alboran Sea show that these main bioevents, dated in Melilla, are synchronous over the Alboran realm. This accurate time scale for these pre-evaporitic biosedimentary events (6.9-6.0 Ma) is in accordance with the most recent work on the latter period corresponding to the Messinian Salinity Crisis.

  2. Paleomagnetic, Anisotropy of Magnetic Susceptibility, and 40AR/39AR Data from the Cienega Volcano, Cerros del Rio Volcanic Field, New Mexico

    NASA Astrophysics Data System (ADS)

    Foucher, M. S.; Petronis, M. S.; Lindline, J.; Van Wyk de Vries, B.

    2012-12-01

    exposure level), the magma flowed laterally away from the ascent location. Magma transport was, therefore, not solely directed into the central conduit but actually involved numerous conduits away from the vent area. The in situ results for eight paleomagnetic sites provide a group mean of D=354.2°, I=19.7°, α95= 4.8°, 5/8. The virtual geomagnetic pole dispersion of the group mean yields a value of 1.19, significantly lower than the predicted VGP dispersion estimate of 15° for the paleolatitude of the site (35.7°N). The results indicate that the different dikes were emplaced within a short period of time (<100 years) relative to a secular variation of the geomagnetic field. Four new whole rock 40Ar/39Ar age determinations from both vent and dike facies are indistinguishable at 2.75 Ma. These ages place the eruptive events in the latest Pliocene and indicate rapid magma injections and cone construction relative to secular variation.

  3. The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge

    NASA Astrophysics Data System (ADS)

    Cluzel, Dominique; Jourdan, Fred; Meffre, SéBastien; Maurizot, Pierre; Lesimple, StéPhane

    2012-06-01

    Amphibolite lenses that locally crop out below the serpentinite sole at the base of the ophiolite of New Caledonia (termed Peridotite Nappe) recrystallized in the high-temperature amphibolite facies and thus sharply contrast with blueschists and eclogites of the Eocene metamorphic complex. Amphibolites mostly display the geochemical features of MORB with a slight Nb depletion and thus are similar to the youngest (Late Paleocene-Eocene) BABB components of the allochthonous Poya Terrane. Thermochronological data from hornblende (40Ar/39Ar), zircon, and sphene (U-Pb) suggest that these mafic rocks recrystallized at ˜56 Ma. Using various geothermobarometers provides a rough estimate of peak recrystallization conditions of ˜0.5 GPa at ˜800-950°C. The thermal gradient inferred from the metamorphic assemblage (˜60°C km-1), geometrical relationships, and geochemical similarity suggest that these mafic rocks belong to the oceanic crust of the lower plate of the subduction/obduction system and recrystallized when they subducted below young and hot oceanic lithosphere. They were detached from the down-going plate and finally thrust onto unmetamorphosed Poya Terrane basalts. This and the occurrence of slab melts at ˜53 Ma suggest that subduction inception occurred at or near to the spreading ridge of the South Loyalty Basin at ˜56 Ma.

  4. Alpha / Mendeleev Ridge and Chukchi Borderland 40Ar/39Ar Geochronology and Geochemistry: Character of the First Submarine Intraplate Lavas Recovered from the Arctic Ocean

    NASA Astrophysics Data System (ADS)

    Mukasa, Samuel B.; Mayer, Larry A.; Aviado, Kimberly; Bryce, Julie; Andronikov, Alex; Brumley, Kelley; Blichert-Toft, Janne; Petrov, Oleg; Shokalsky, Sergey

    2015-04-01

    At least three episodes of magmatic activity have been recognized on the basis of 40Ar/39Ar age determinations in the submarine basaltic samples dredged, drilled or grabbed with a manipulation arm from Alpha / Mendeleev Ridge and Chukchi Borderland of the Arctic Ocean by US Coast Guard Icebreaker Healy, in August-September 2008, and Russian research vessel Captain Dranitsin in August-October 2012: ca. 112 Ma, ca. 100 Ma and ca. 85-73 Ma. Major-oxide and trace-element concentrations, and Pb, Sr, Nd, and Hf isotopic ratios of the recovered lavas provide important constraints on the composition and sources for the original melts. Lavas erupted at ca. 112 Ma (Group 1) have alkali basalt major-oxide compositions. Their low degree of rare-earth-element (REE) fractionation (CeN/YbN = 1.7-2.5), combined with high overall HREE (22-24 times chondrite) and Mg# ~54, suggest derivation from a garnet-free source followed by only minimal crystal fractionation for this group. Pb-Sr-Nd-Hf isotopic systematics of the lavas (206Pb/204Pb = 18.73-18.79; 207Pb/204Pb = 15.54-15.56; 208Pb/204Pb = 38.28-38.35; 143Nd/144Nd = 0.512594-0.512610; 87Sr/86Sr = 0.709458-0.709601; 176Hf/177Hf = 0.283224), together with ratios of highly incompatible trace elements (Th/Ce = 0.09-0.11; Ce/Nb = 2.58-3.09; Th/Nb = 0.24-0.33), point toward a lithospheric source for the magmas. Eruptions at ca. 100 Ma and 85-73 Ma produced two types of lavas: low-Ti tholeiitic basalts - LT, and high-Ti alkali basalts - HT, both assigned to Group 2. This distribution of low- and high-Ti lavas is common in continental flood basalt (CFB) provinces elsewhere, and has been attributed to plume activity in some studies. The trace-element abundance patterns for these Group 2 Arctic lavas are also very similar to those of CFBs elsewhere. Their low degrees of REE fractionation (CeN/YbN = 2.0-3.3) accompanied by progressively decreasing Mg#s (from 53 to 33) suggest a garnet-free source, with the derivative magmas experiencing

  5. 40Ar-39Ar dating of the Manson impact structure: A cretaceous-tertiary boundary crater candidate

    USGS Publications Warehouse

    Kunk, M.J.; Izett, G.A.; Haugerud, R.A.; Sutter, J.F.

    1989-01-01

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

  6. The edge of time: Dating young volcanic ash layers with the 40Ar- 39Ar laser probe

    USGS Publications Warehouse

    Chen, Y.; Smith, P.E.; Evensen, N.M.; York, D.; Lajoie, K.R.

    1996-01-01

    Argon-40-argon-39 single-crystal dating of young (5000 to 30,000 years ago) volcanic ash layers erupted from the Mono Craters, California, shows that the method can yield meaningful ages in Holocene tephra. Because of ubiquitous xenocrystic contamination, the data do not form isochrons but plot in wedge-shaped regions on an argon isotopic diagram. The upper boundary of the region is an isochron matching the 14C-derived age of the eruption. Such contamination-related patterns may be common in dating young materials by the single-crystal method. Argon dating by this method can help refine the time scale of physical and biological evolution over the past 100,000 years.

  7. sup 40 Ar- sup 39 Ar dating of the manson impact structure: A cretaceous-tertiary boundary crater candidate

    SciTech Connect

    Kunk, M.J.; Sutter, J.F. ); Izett, G.A. ); Haugerud, R.A. )

    1989-06-30

    The mineralogy of shocked mineral and lithic grains in the Cretaceous-Tertiary (K-T) boundary claystone worldwide is most consistent with a bolide impact on a continent. Both the concentrations and sizes of these shocked grains are greatest in the western interior of North America. These data suggest that the Manson impact structure in north-central Iowa is a viable candidate for the K-T boundary impact event. Argon-40-argon-39 age spectrum dating of shocked microcline from the crystalline central uplift of the Manson impact structure indicates that there was severe argon-40 loss at 65.7 {plus minus} 1.0 million years ago, an age that is indistinguishable from that of the K-T boundary, within the limits of analytical precision. 36 refs., 2 figs., 1 tab.

  8. A Human Deciduous Tooth and New 40Ar/39Ar Dating Results from the Middle Pleistocene Archaeological Site of Isernia La Pineta, Southern Italy.

    PubMed

    Peretto, Carlo; Arnaud, Julie; Moggi-Cecchi, Jacopo; Manzi, Giorgio; Nomade, Sébastien; Pereira, Alison; Falguères, Christophe; Bahain, Jean-Jacques; Grimaud-Hervé, Dominique; Berto, Claudio; Sala, Benedetto; Lembo, Giuseppe; Muttillo, Brunella; Gallotti, Rosalia; Thun Hohenstein, Ursula; Vaccaro, Carmela; Coltorti, Mauro; Arzarello, Marta

    2015-01-01

    Isernia La Pineta (south-central Italy, Molise) is one of the most important archaeological localities of the Middle Pleistocene in Western Europe. It is an extensive open-air site with abundant lithic industry and faunal remains distributed across four stratified archaeosurfaces that have been found in two sectors of the excavation (3c, 3a, 3s10 in sect. I; 3a in sect. II). The prehistoric attendance was close to a wet environment, with a series of small waterfalls and lakes associated to calcareous tufa deposits. An isolated human deciduous incisor (labelled IS42) was discovered in 2014 within the archaeological level 3 coll (overlying layer 3a) that, according to new 40Ar/39Ar measurements, is dated to about 583-561 ka, i.e. to the end of marine isotope stage (MIS) 15. Thus, the tooth is currently the oldest human fossil specimen in Italy; it is an important addition to the scanty European fossil record of the Middle Pleistocene, being associated with a lithic assemblage of local raw materials (flint and limestone) characterized by the absence of handaxes and reduction strategies primarily aimed at the production of small/medium-sized flakes. The faunal assemblage is dominated by ungulates often bearing cut marks. Combining chronology with the archaeological evidence, Isernia La Pineta exhibits a delay in the appearance of handaxes with respect to other European Palaeolithic sites of the Middle Pleistocene. Interestingly, this observation matches the persistence of archaic morphological features shown by the human calvarium from the Middle Pleistocene site of Ceprano, not far from Isernia (south-central Italy, Latium). In this perspective, our analysis is aimed to evaluate morphological features occurring in IS42. PMID:26457581

  9. A Human Deciduous Tooth and New 40Ar/39Ar Dating Results from the Middle Pleistocene Archaeological Site of Isernia La Pineta, Southern Italy

    PubMed Central

    Peretto, Carlo; Arnaud, Julie; Moggi-Cecchi, Jacopo; Manzi, Giorgio; Nomade, Sébastien; Pereira, Alison; Falguères, Christophe; Bahain, Jean-Jacques; Grimaud-Hervé, Dominique; Berto, Claudio; Sala, Benedetto; Lembo, Giuseppe; Muttillo, Brunella; Gallotti, Rosalia; Thun Hohenstein, Ursula; Vaccaro, Carmela; Coltorti, Mauro; Arzarello, Marta

    2015-01-01

    Isernia La Pineta (south-central Italy, Molise) is one of the most important archaeological localities of the Middle Pleistocene in Western Europe. It is an extensive open-air site with abundant lithic industry and faunal remains distributed across four stratified archaeosurfaces that have been found in two sectors of the excavation (3c, 3a, 3s10 in sect. I; 3a in sect. II). The prehistoric attendance was close to a wet environment, with a series of small waterfalls and lakes associated to calcareous tufa deposits. An isolated human deciduous incisor (labelled IS42) was discovered in 2014 within the archaeological level 3 coll (overlying layer 3a) that, according to new 40Ar/39Ar measurements, is dated to about 583–561 ka, i.e. to the end of marine isotope stage (MIS) 15. Thus, the tooth is currently the oldest human fossil specimen in Italy; it is an important addition to the scanty European fossil record of the Middle Pleistocene, being associated with a lithic assemblage of local raw materials (flint and limestone) characterized by the absence of handaxes and reduction strategies primarily aimed at the production of small/medium-sized flakes. The faunal assemblage is dominated by ungulates often bearing cut marks. Combining chronology with the archaeological evidence, Isernia La Pineta exhibits a delay in the appearance of handaxes with respect to other European Palaeolithic sites of the Middle Pleistocene. Interestingly, this observation matches the persistence of archaic morphological features shown by the human calvarium from the Middle Pleistocene site of Ceprano, not far from Isernia (south-central Italy, Latium). In this perspective, our analysis is aimed to evaluate morphological features occurring in IS42. PMID:26457581

  10. Early Permian stage of formation of gold-ore deposits of northeastern Transbaikalia: Isotope-geochronological (Rb-Sr and 39Ar-40Ar) data for the Uryakh ore field

    NASA Astrophysics Data System (ADS)

    Chugaev, A. V.; Nosova, A. A.; Abramov, S. S.; Chernyshev, I. V.; Bortnikov, N. S.; Larionova, Yu. O.; Goltsman, Yu. V.; Moralev, G. V.; Volfson, A. A.

    2015-08-01

    This work presents the first results of geochronological study of metasomatic rocks accompanying gold-bearing quartz veins of the Uryakh ore field (UOF). Based on the Rb-Sr and 39Ar-40Ar geochronological data, it is shown that hydrothermal metasomatic processes in the ore field occurred about 280 Ma ago (Early Permian) and they are correlated with the terminal phases of formation of the Angara-Vitim batholith.

  11. Protracted tectono-metamorphic history of the SE Superior Province : contribution of 40Ar/39Ar thermochronology in the Abitibi-Opatica contact zone, Québec, Canada

    NASA Astrophysics Data System (ADS)

    Daoudene, Yannick; Tremblay, Alain; Ruffet, Gilles; Leclerc, François; Goutier, Jean

    2015-04-01

    Archean orogens mainly consist of greenstone belts juxtaposing deeper crustal domains of TTG-type plutonic rocks. The greenstone belts show regional folds, penetrative steeply-dipping fabrics, and localised shear zones, whereas the plutonic belts predominantly display dome structures. Concurrently, rocks in Archean orogens undergone MT/HT-LP/MP metamorphic conditions that vary, from upper to lower crustal domains, between greenschist- and granulite-facies, respectively. These structural and metamorphic variations are well-documented, but modes of deformation related to such orogens is still debated. Some studies suggest that the Archean tectonic processes were comparable to present-day plate tectonics and the Archean greenstone belts were interpreted as tectonic collages commonly documented in Phanerozoic subduction/collision zones. Alternative models propose that the Archean tectonics were different from those predicted by the plate tectonics paradigm, mainly due to the existence of a hotter mantle and a mechanically weak crust. In such models, the burying and exhumation of crustal rocks are attributed to the vertical transfer of material, resulting in the development of pop-down and domes structures. As a contribution of the study of mechanisms that might have operated during the Archean, we present a structural and metamorphic study of the contact zone between the Abitibi subprovince (ASP), which contains greenstone belts, and the Opatica subprovince (OSP), which is dominated by plutonic rocks, of the Superior Province. The 40Ar/39Ar dating of amphiboles and micas is used to constrain the age and duration of regional metamorphism and associated deformations. On the basis of seismic profiling, showing a north-dipping lithospheric-scale reflector, the ASP-OSP contact has been interpreted as the surficial trace of an Archean subduction zone. However, our structural analysis suggest that the ASP overlies the OSP and that the ASP-OSP contact does not show evidences

  12. Eocene tectono-thermal rejuvenation of an upper Paleozoic-lower Mesozoic terrane in the Cordillera de Carabaya, Puno, southeastern Peru, revealed by KAr and 40Ar/ 39Ar dating

    NASA Astrophysics Data System (ADS)

    Kontak, D. J.; Farrar, E.; Clark, A. H.; Archibald, D. A.

    KAr dates for muscovites and biotites in granitoid rocks and hydrothermal ore deposits of the northeastern parts of the plutons making up the Triassic Carabaya batholith, underlying the axial Cordillera Oriental of northern Puno Department, southeastern Peru, are markedly variable and mutually discordant. Steep transverse gradients are defined in the apparent ages of both micas, which decrease systematically from SW to NE, delimiting a ca. 25-km-wide, longitudinal zone of anomalously young Mesozoic to Paleocene dates. Age minima of 37±1 Ma are attained in three of the four studied transects. 40Ar/ 39Ar step-heating analyses of selected micas confirm the occurrence of a thermal disturbance, and modeling of the spectra suggests that argon loss in muscovites attains at least ca. 75% in the northeastern part of the zone. A single K-feldspar spectrum yielded a minimum at 31 Ma, and apatite fission-track age cluster at ca. 31 and 18.5 Ma. The affected granitoid rocks generally display little megascopic evidence of tectonism, but microscopic deformational fabrics increase in intensity with apparent decreasing KAr age, paralleling a marked increase in alkali feldspar ordering. Secondary fluid inclusions trapped within the microfabrics reveal that the plutonic rocks were penetrated by a homogeneous H 2OCO 2CH 4NaCl fluid at ca. 300-400°C and 0.7-2 kbar. This fluid is implicated in the degassing of the rocks. These diverse data are interpreted as evidence for a major, but moderate-temperature (400°C) and brief, tectono-thermal event at ca. 37±1 Ma (biotite closure temperature)— i.e., at the Eocene-Oligocene boundary. The K-feldspar 40Ar/ 39Ar data and the Oligocene fission-track dates may record the later stages in the event, whereas the Miocene fission-track dates are tentatively ascribed to a distinct Neogene episode. Essentially identical geochronological and petrological relationships have been documented in the Cordillera Real of northwestern

  13. 40Ar/39Ar incremental heating experiments on celadonite from the Skessa Tuff, eastern Iceland: Thermochronology of low-temperature alteration of a flood basalt pile during burial metamorphism

    NASA Astrophysics Data System (ADS)

    Riishuus, M. S.; Miggins, D. P.; Koppers, A. A. P.; Duncan, R. A.

    2014-12-01

    Celadonite is a low-temperature (<~50 °C) alteration mineral that fills voids and fractures within buried and metasomatized volcanic rocks. The common occurrence, mineral chemistry and structural properties of celadonite (a K2O-rich and Ar retentive phyllosilicate) make it attractive as a monitor of not only spatial and temporal variability of low-temperature hydrothermal fluid circulation and alteration, but also potentially of burial rates of lava piles. The Neoge