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Sample records for 40ar 39ar ages

  1. 39Ar-40Ar "ages" and origin of excess 40Ar in Martian shergottites

    NASA Astrophysics Data System (ADS)

    Bogard, Donald; Park, Jisun; Garrison, Daniel

    2009-06-01

    We report new 39Ar-40Ar measurements on 15 plagioclase, pyroxene, and/or whole rock samples of 8 Martian shergottites. All age spectra suggest ages older than the meteorite formation ages, as defined by Sm-Nd and Rb-Sr isochrons. Employing isochron plots, only Los Angeles plagioclase and possibly Northwest Africa (NWA) 3171 plagioclase give ages in agreement with their formation ages. Isochrons for all shergottite samples reveal the presence of trapped Martian 40Ar (40Arxs), which exists in variable amounts in different lattice locations. Some 40Arxs is uniformly distributed throughout the lattice, resulting in a positive isochron intercept, and other 40Arxs occurs in association with K-bearing minerals and increases the isochron slope. These samples demonstrate situations where linear Ar isochrons give false ages that are too old. After subtracting 40Ar*that would accumulate by 40K decay since meteorite formation and small amounts of terrestrial 40Ar, all young age samples give similar 40Arxs concentrations of ˜1-2 × 10-6cm3/g, but a variation in K content by a factor of ˜80. Previously reported NASA Johnson Space Center data for Zagami, Shergotty, Yamato (Y-) 000097, Y-793605, and Queen Alexandra Range (QUE) 94201 shergottites show similar concentrations of 40Arxs to the new meteorite data reported here. Similar 40Arxs in different minerals and meteorites cannot be explained as arising from Martian atmosphere carried in strongly shocked phases such as melt veins. We invoke the explanation given by Bogard and Park (2008) for Zagami, that this 40Arxs in shergottites was acquired from the magma. Similarity in 40Arxs among shergottites may reveal common magma sources and/or similar magma generation and emplacement processes.

  2. Interpretation of discordant 40Ar/39Ar age-spectra of mesozoic tholeiites from antarctica

    USGS Publications Warehouse

    Fleck, R.J.; Sutter, J.F.; Elliot, D.H.

    1977-01-01

    Conventional K-Ar ages of tholeiitic basalts of the Ferrar Group in the central Transantarctic Mountains indicate significant loss of radiogenic 40Ar from this unit over much of its outcrop area. Argon loss varies inversely with amount of devitrified matrix in the basalts, which have not been thermally or tectonically disturbed since extrusion. 40Ar/19Ar age-spectra of these tholeiites are generally discordant and indicate significant inhomogeneity in the distribution of radiogenic 40Ar with respect to 39Ar, but are distinctly different from release patterns of thermally disturbed samples. Amounts of argon redistribution vary directly with amounts of devitrification and are reflected in progressive modification of the age spectra. A model of redistribution of radiogenic 40Ar by devitrification of originally glassy matrix is suggested that is consistent with disturbance of the conventional K-Ar systematics as well as the 40Ar/39Ar age-spectra. Samples with substantial redistribution but minor loss of radiogenic argon yield age spectra whose apparent ages decrease from low-temperature to high-temperature steps, similar to those reported for some lunar basalts, breccias, and soils. Modification of all the age spectra is attributed to redistribution of radiogenic 40Ar during progressive devitrification, although 39Ar-recoil effects suggested by Turner and Cadogan (1974) may be a factor in some cases. Where devitrification involves most potassium sites within the basalt, 40Ar/39Ar age-plateaux may be formed that have no geologic significance. ?? 1977.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 40Ar/39Ar Age of Cretaceous-Tertiary Boundary Tektites from Haiti.

    PubMed

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

    1991-06-14

    (40)Ar/(39)Ar 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 (40)Ar/(39)Ar age of 64.6 +/- 0.2 million years ago, which is indistinguishable statistically from the age of the tektites.

  8. 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.

  9. Metrologically-Calibrated 40Ar Concentrations and Ages of Mineral Standards Used in 40Ar/39Ar Geochronology

    NASA Astrophysics Data System (ADS)

    Morgan, L. E.; Davidheiser-Kroll, B.; Kuiper, K.; Wijbrans, J. R.; Mark, D. F.

    2015-12-01

    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 present results for the 40Ar concentrations and ages of mineral standards determined based on a modern gas delivery system (Morgan et al. 2011), which delivers metrologically-traceable amounts of 40Ar and thus allows for the sensitivity calibration of noble gas mass spectrometers.

  10. 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.

  11. 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

  12. 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.

  13. 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.

  14. 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.

  15. 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

  16. 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

  17. 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.

  18. 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.

  19. An alternative hypothesis for high-T 40Ar/39Ar age spectrum discordance in polyphase extraterrestrial materials

    NASA Astrophysics Data System (ADS)

    Cassata, W. S.; Shuster, D. L.; Renne, P. R.; Weiss, B. P.

    2009-12-01

    A common feature observed in 40Ar/39Ar age spectra of extraterrestrial (ET) rocks is a conspicuous decrease in the ages of high temperature extractions relative to lower temperature steps and a correlated increase in Ca/K, often succeeded by a monotonic increase in ages. This feature is routinely attributed to recoil-implanted 39Ar from a potassium (K)-rich donor phase into a K-poor receptor phase (e.g., 1,2). While 39Ar recoil redistribution is undoubtedly manifested in many terrestrial and ET 40Ar/39Ar whole-rock age spectra, it cannot easily explain the magnitude of high release temperature 40Ar*/39ArK anomalies observed in Martian meteorites ALH 84001 and Nakhla, as well as other course-grained meteorites and lunar rocks. Depending on the aliquot and sample, 50 - 100% of the pyroxene release spectra in ALH 84001 and Nakhla appear strongly perturbed to lower ages. As the mean recoil distance of 39Ar ~0.1 µm, the recoil hypothesis demands that a high-K phase be ubiquitously distributed amongst sub-micron to micron sized pyroxene crystals to account for the observed pyroxene age spectra. However, in both Nakhla and ALH 84001, pyroxene is often completely isolated from high-K phases and individual grains commonly exceed 100 µm in diameter. 40Ar/39Ar analyses of pyroxene-bearing terrestrial basalts, wherein fine-grained pyroxene and plagioclase are intimately adjoined, show that recoil-implanted 39Ar into pyroxene produces much less precipitous anomalies in 40Ar*/39ArK, as predicted by the recoil lengthscale. An alternative hypothesis is that whole-rock age spectra of ET samples with anomalously low ages at high temperatures may reflect diffusive 40Ar distributions within considerably degassed pyroxene grains. Owing to apparent differences in activation energies between glass and/or plagioclase and pyroxene, 40Ar may diffuse more rapidly from pyroxene under certain high-temperature conditions (i.e., above the temperature at which the extrapolated Ar Arrhenius

  20. 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.

  1. Duration of sedimentation of Creede Formation from 40Ar/39Ar ages

    USGS Publications Warehouse

    Lanphere, Marvin A.

    2000-01-01

    The Oligocene Creede Formation was deposited in the moat of the Creede caldera, which formed as a result of eruption of ythe Snowshoe Mountains Tuff. The Creede Formation in the two moat drill holes contains ash layers that are considered fallout tuffs derived from Fisher Dacite volcanoes that were erupting during accumulation of the Creede Formation. The duration of sedimentation of the Creede Formation could hnot be determinted directly by measuring the ages of the ash layers because 40Ar/39Ar ages of biotite from the asj layers do not stack in the correct stratigraphic order, indicating that the ash layers have been contaminated by biotite from older units. The duration of sedimentation is constrained by the ages of volcanic unites that stratigraphically bracket the Creede Formation. Pooling all ages for the underlyinh Snowshoe Mountain Tuff yields an age of 26.92 ± 0.07 Ma for the unit. The age of the stratigraphically highest lavas of Fisher Dacite, which overlie the Creede Formation, is 26.26 ± 0.04 Ma. The two limits give a maximum duration for sedimentation of the Creede Formation of 0.66 m.y. Using the ages of older Fisher Dacite lavas, on which some beds of the Creede Formation were deposited, a more realistic maximum duration of 0.34 m.y. for sedimentation of the Creede Formation can be determined.

  2. 40Ar/39Ar Age Dating of Two Chondrules from the Bjurböle (L/LL4) Chondrite

    NASA Astrophysics Data System (ADS)

    Park, J.; Lindsay, F. N.; Turrin, B. D.; Herzog, G. F.; Delaney, J. S.; Swisher, C. C.

    2016-08-01

    We present detailed 40Ar/39Ar results for two Bjurböle chondrules. Younger Ar/Ar step ages for one Bjurböle chondrule suggest that the analysis of single chondrules can yield more accurate meteorite ages than are obtainable from bulk samples.

  3. 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

  4. 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.

  5. (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).

  6. 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

  7. The Next Generation Cretaceous Time Scale: How to integrate 40Ar/39Ar, U-Pb and Astrochronologic ages? (Invited)

    NASA Astrophysics Data System (ADS)

    Singer, B. S.; Condon, D. J.; Siewert, S. E.; Sageman, B. B.; Sawyer, D. A.; Obradovich, J. D.; Meyers, S. R.; Jicha, B.

    2010-12-01

    TThe Cretaceous Time Scale of Obradovich (1993), based on 40Ar/39Ar dating of ash beds within a framework of ammonite biostratigraphy, achieved a resolution of ~0.5%. To build from this toward the EARTHTIME goal of 0.1% resolution, we are undertaking new 40Ar/39Ar dating of these and previously undated ash beds, coupled with U-Pb dating of zircons and newly developed astrochronologic age models and methods for key intervals. Initial focus is on Cenomanian to Campanian stages which: span 37 ammonite zones, include a GSSP associated with OAE2, and yield, through spectral analysis of cores, floating astrochronologies for ~7 Ma. 40Ar/39Ar ages of sanidine from newly collected samples are identical to those obtained from legacy samples of Obradovich (1993). Although individual 40Ar/39Ar analyses, each derived from laser fusion of 1- 5 sanidine crystals, yield 2 sigma analytical precisions of ˜2 Ma, large numbers (33-103) of such analyses reveal few outliers (analyses <97% 40Ar*), give Guassian distributions from as many as three separate sites in a single ash bed, and have wtd. mean analytical uncertainties of 100 -170 ka, i.e., <0.2%. Using the ET535 tracer and CA-TIMS methods multiple zircons (single crystals and fragments) have also been measured in samples from 8 of the ash beds for which we have new 40Ar/39Ar data. The high precision of each single crystal 206Pb/238U age of about 0.1% is a strength and each sample yields zircon 206Pb/238U ages that typically span 0.5 to 1.0 Ma, reflecting inheritance or recycling of some zircon during protracted evolution of the volcanic sources. The selection of which group of the youngest zircons from several of these ash beds best estimates the eruptive age requires some choice as to whether chemical abrasion has eliminated all zircon affected by Pb loss. Choosing the ‘youngest zircons’ in 6 of these ash beds yields wtd. mean 206Pb/238U ages that are indistinguishable from the wtd. mean 40Ar/39Ar ages, provided the latter

  8. 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.

  9. 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.

  10. 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.

  11. 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

  12. High-precision 40Ar/39Ar Age of the Janisjärvi Impact Structure (Russia)

    NASA Astrophysics Data System (ADS)

    Jourdan, F.; Renne, P. R.; Reimold, U. W.

    2007-12-01

    The ~14 km Jänisjärvi impact structure occurs within the Svecofennian Proterozoic terrains, in the southeastern part of the Baltic shield, Karelia, Russia. Previous K/Ar and 40Ar/39Ar studies were interpreted to give ages of 700 ± 5 Ma and 698 ± 22 Ma respectively, both results being difficult to interpret. Recent paleomagnetic results challenged those ages and propose instead ages of either 500 Ma or 850-900 Ma. In order to better constrain the age of the Jänisjärvi impact structure, we present new 40Ar/39Ar data for melt rocks from the crater. We obtained five concordant isochron ages (based on a total decay constant of 5.543 x 10-10/y and an age of 28.03 Ma for the FCs standard) that yield a combined isochron age of 682 ± 4 Ma (2 sigma) with a MSWD of 1.2, P = 0.14 and 40Ar/36Ar intercept of 475 ± 3. We suggest that this date indicates the age of the impact and therefore can be used in conjunction with existing paleomagnetic results to refine the position of the Baltica paleocontinent at this time. Argon isotopic results imply that melt homogenization has been achieved at the hundred-micron scale certainly because of the low-silica content of the molten target rock that allows fast 40Ar* diffusion in the melt. However, the large range of F(40Ar*inherited) (3 to 8 percents) observed for seven grains show that complete isotopic homogenization was not reached at the centimeter and perhaps millimeter scale. This result is in good agreement with previous Rb and Sr isotopic data.

  13. 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

  14. 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.

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

    PubMed

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

    2012-11-13

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

  16. A model for meteoritic and lunar 40Ar/39Ar age spectra: Addressing the conundrum of multi-activation energies

    NASA Astrophysics Data System (ADS)

    Boehnke, P.; Harrison, T. Mark; Heizler, M. T.; Warren, P. H.

    2016-11-01

    Results of whole-rock 40Ar/39Ar step-heating analyses of extra-terrestrial materials have been used to constrain the timing of impacts in the inner solar system, solidification of the lunar magma ocean, and development of planetary magnetic fields. Despite the importance of understanding these events, the samples we have in hand are non-ideal due to mixed provenance, isotopic disturbances from potentially multiple heating episodes, and laboratory artifacts such as nuclear recoil. Although models to quantitatively assess multi-domain, diffusive 40Ar* loss have long been applied to terrestrial samples, their use on extra-terrestrial materials has been limited. Here we introduce a multi-activation energy, multi-diffusion domain model and apply it to 40Ar/39Ar temperature-cycling, step-heating data for meteoritic and lunar samples. We show that age spectra of extra-terrestrial materials, the Jilin chondrite (K-4) and Apollo 16 lunar breccia (67514 , 43), yielding seemingly non-ideal behavior commonly interpreted as either laboratory artifacts or localized shock heating of pyroxene, are meaningful and can be understood in context of the presence of multi-diffusion domains containing multiple activation energies. Internally consistent results from both the meteoritic and lunar samples reveal high-temperature/short duration thermal episodes we interpret as due to moderate shock heating.

  17. Part II. Evaluation of 40Ar- 39Ar quartz ages: Implications for fluid inclusion retentivity and determination of initial 40Ar/ 36Ar values in Proterozoic samples

    NASA Astrophysics Data System (ADS)

    Kendrick, M. A.; Miller, J. McL.; Phillips, D.

    2006-05-01

    The argon isotope systematics of vein-quartz samples with two different K-reservoirs have been evaluated in detail. Potassium is hosted by ultra-high-salinity fluid inclusions in quartz samples from the Eloise and Osborne iron-oxide-copper-gold (IOCG) deposits of the Mt Isa Inlier, Australia. In contrast, K is hosted by accidentally trapped mica within lower-salinity fluid inclusions of a sample selected from the Railway Fault, 13 km south of the Mt Isa copper mine, Australia. Imprecise apparent ages have been obtained for all of the samples studied and conclusively demonstrate that quartz fluid inclusions are retentive to Ar and have not leaked over billions of years. IOCG samples that host K in fluid inclusions only, have K/Cl values of <1 and the ages obtained represent the maximum ages for mineralization. In contrast, the Railway Fault samples that include accidentally trapped mica have K/Cl values of ≫1. Excess 40Ar E plus Cl hosted by fluid inclusions, and radiogenic 40Ar R plus K, are strongly correlated in these samples and define a plane in 3D 40Ar- 36Ar-K-Cl space. In this case, the plane yields an 'excess 40Ar E' corrected age of ˜1030 Ma that is 100's of Ma younger than nearby Cu-mineralization at Mt Isa. The age is interpreted to reflect 40Ar-loss from the accidentally trapped mica into the surrounding fluid inclusions, and is not related to the samples' age of formation. The initial 40Ar/ 36Ar value of fluid inclusions is widely used to provide information on fluid origin. For the IOCG samples that host K in fluid inclusions only, the initial 40Ar/ 36Ar values are close to the measured values at every temperature of stepped heating experiments. For samples that include accidentally trapped mica, the correction for post-entrapment radiogenic 40Ar R production is significant. Furthermore, because 39Ar K present in accidentally trapped mica crystals is released at different temperatures to radiogenic 40Ar R lost to the surrounding fluid inclusions

  18. 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.

  19. 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.

  20. 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.

  1. sup 40 Ar/ sup 39 Ar age calibration of the litho- and paleomagnetic stratigraphies of the Ngorora Formation, Kenya

    SciTech Connect

    Deino, A.; Drake, R. ); Tauxe, L. ); Monaghan, M. )

    1990-07-01

    Precise eruptive ages have been determined by the laser-fusion, single-crystal {sup 40}Ar/{sup 39}Ar method for juvenile volcanic feldspars from reworked and contaminated volcaniclastic rocks of the middle Miocene Ngorora Formation, Kenya Rift Valley. These ages range from 13.06 Ma at the base to 10.51 Ma toward the top of the type section near Kabarsero. Correlation of the local paleomagnetic stratigraphies with the geomagnetic reversal time scale yields magnetochronologic age estimates that are younger than the isotopic ages by an average of 0.18 Ma. Much of the discrepancy can be eliminated if an inferred change in sea-floor spreading rate occurred at 13 Ma or earlier, rather than at 10.42 Ma as previously suggested. Sedimentation rates at Kabarsero calculated from the {sup 40}Ar/{sup 39}Ar results decrease from initial values of {approximately}25 cm/1,000 yr to {approximately}5 cm/1,000 yr toward the top of the section. The initial rapid sedimentation rates characterize the first 0.1 to 0.3 m.y. following emplacement of the underlying, voluminous, basin-filling Tiim Phonolites, indicating that the Baringo Basin at this time may not have existed as a rift valley created by extensional tectonics, but instead may have been a subsidence feature formed in response to removal of large volumes of magma from the lithosphere. A premolar tentatively identified as Proconsul sp. indet. found in the Ngorora Formation near the village of Bartabwa has been dated at {approximately}12.42 Ma, representing perhaps the last known occurrence of this genus in the fossil record.

  2. 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.

  3. The 40Ar/39Ar ages and tectonic setting of the Middle Eocene northeast Nevada volcanic field

    USGS Publications Warehouse

    Brooks, W.E.; Thorman, C.H.; Snee, L.W.

    1995-01-01

    Widespread middle to late Eocene calc-alkalic volcanism, which formed the Northeast Nevada Volcanic Field, marks the earliest Tertiary volcanism in the northern Basin and Range. The central part of this major field in northest Nevada and adjacent Utah is herein defined by 23 40Ar/39Ar ages that arange from 42.6 to 39.0 Ma, rock chemistry from 12 localities, stratigraphic position of the volcanic rocks above a regional middle Eocene unconformity, volcanic setting, and lithology. The type area is at Nanny Creek, in the northern Pequop Mountains, Nevada. In the western and southeastern parts of the field these middle Eocene volcanic rocks rest with depositional angular discordance on lower Eocene lacustrine strata of the Elko and White Sage Formations, respectively. This angular discordance documents a middle Eocene deformational event previously unrecognized in the region. -from Authors

  4. 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.

  5. 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.

  6. 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

  7. 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

  8. 40Ar/39Ar Dating of Volcanic Glass

    NASA Astrophysics Data System (ADS)

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

    2007-12-01

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

  9. Ultra-high precision 40Ar/39Ar ages for Fish Canyon Tuff and Alder Creek Rhyolite sanidine: New dating standards required?

    NASA Astrophysics Data System (ADS)

    Phillips, D.; Matchan, E. L.

    2013-11-01

    The 40Ar/39Ar dating technique is a high precision (<0.1%) method with wide application to geological samples. However, the method is predicated on the availability of natural mineral standards of known age. Widely used 40Ar/39Ar standards include sanidine from the (ca. 28 Ma) Fish Canyon Tuff (FCT) and the (ca. 1.2 Ma) Alder Creek Rhyolite (ACR). Despite common usage, the ages of FCT and ACR sanidine have proven contentious, with reported values varying by >2%; well outside the ±0.1% aspiration of EARTHTIME (http://www.earth-time.org).

  10. 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

  11. 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

  12. Precise K-Ar, 40Ar/39Ar, Rb-Sr and U/Pb mineral ages from the 27.5 Ma fish canyon tuff reference standard

    USGS Publications Warehouse

    Lanphere, M.A.; Baadsgaard, H.

    2001-01-01

    The accuracy of ages measured using the 40Ar/39Ar technique is affected by uncertainties in the age of radiation fluence-monitor minerals. At present, there is lack of agreement about the ages of certain minerals used as fluence monitors. The accuracy of the age of a standard may be improved if the age can be measured using different decay schemes. This has been done by measuring ages on minerals from the Oligocene Fish Canyon Tuff (FCT) using the K-Ar, 40Ar/39Ar. Rb-Sr and U/Pb methods. K-Ar and 40Ar/39Ar total fusion ages of sanidine, biotite and hornblende yielded a mean age of 27.57 ?? 0.36 Ma. The weighted mean 40Ar/39Ar plateau age of sanidine and biotite is 27.57 ?? 0.18 Ma. A biotite-feldspar Rb-Sr isochron yielded an age of 27.44 ?? 0.16 Ma. The U-Pb data for zircon are complex because of the presence of Precambrian zircons and inheritance of radiogenic Pb. Zircons with 207Pb/235U < 0.4 yielded a discordia line with a lower concordia intercept of 27.52 ?? 0.09 Ma. Evaluation of the combined data suggests that the best age for FCT is 27.51 Ma. Published by Elsevier Science B.V.

  13. 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

  14. 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.

  15. 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

  16. Revised 40Ar/39Ar age of Aleutian Island arc formation implies high rate of magma production

    NASA Astrophysics Data System (ADS)

    Jicha, B. R.; Scholl, D. W.; Singer, B. S.; Yogodzinski, G. M.; Kay, S. M.

    2005-12-01

    40Ar/39Ar incremental heating data from subaerial and submarine volcanic and plutonic rocks in the Aleutian Island arc provide insight into the timing of arc formation in the late Eocene. Groundmass and plagioclase separates from the Finger Bay volcanics, the oldest exposed rocks in the arc, gave a weighted mean isochron age of 37.4 ± 0.6 Ma, that is 12-17 m.y. younger than a widely cited age of 55-50 Ma. Twenty-six 40Ar/39Ar ages agree with existing K-Ar ages and constrain the duration of arc magmatism to the last 40 m.y. The initiation of magmatism at this time is in agreement with the late Eocene to early Oligocene ages for the fossiliferous sequence of sedimentary deposits on northern Adak Island that overlie 37-38 Ma units, yet it is at odds with all of the existing models of Aleutian arc formation because no major tectonic events in the north Pacific occurred at that time. We have also identified three main pulses of arc-wide plutonism and volcanism at 38-29, 16-11, and 6-0 Ma. The geochronology--in concert with new-generation transverse and arc-parallel seismic constraints on the composition and structure of the Aleutian Island arc and volumetric estimates of crust generated and eroded over the last 40 m.y.--leads to astonishingly high time-averaged magma production rates of 110-205 km3/km/m.y. for the entire arc. This exceeds magma production rates based on older geophysical and petrologic paradigms for the Aleutian arc by almost an order of magnitude. Because the majority of crustal growth likely occurred during the first few m.y. of the arc's history, magma productivity may have been as high as that of mid-ocean ridge spreading centers or continental batholiths (e.g., near 1000 km3/km/m.y.). Rapid Eocene arc growth has recently been proposed for both the Izu Bonin-Mariana and Tonga island arcs in the western Pacific Ocean. Determining whether extraordinarily high rates of island arc magma production in the Eocene reflects increased plate velocities and

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

    NASA Astrophysics Data System (ADS)

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

    2016-08-01

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

  18. 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.

  19. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

  1. The 40Ar/39Ar and U/Pb dating of young rhyolites in the Kos-Nisyros volcanic complex, Eastern Aegean Arc, Greece: Age discordance due to excess 40Ar in biotite

    NASA Astrophysics Data System (ADS)

    Bachmann, O.; Schoene, B.; Schnyder, C.; Spikings, R.

    2010-08-01

    High-precision dating of Quaternary silicic magmas in the active Kos-Nisyros volcanic center (Aegean Arc, Greece) by both 40Ar/39Ar on biotite and U/Pb on zircon reveals a complex geochronological story. U/Pb ID-TIMS multi and single-grain zircon analyses from 3 different units (Agios Mammas and Zini domes, Kefalos Serie pyroclasts) range in age from 0.3 to 0.5 to 10-20 Ma. The youngest dates provide the maximum eruption age, while the oldest zircons indicate inheritance from local continental crust (Miocene and older). Step-heating 40Ar/39Ar experiments on 1-3 crystals of fresh biotite yielded highly disturbed Ar-release patterns with plateau ages typically older than most U/Pb ages. These old plateau ages are probably not a consequence of inheritance from xenocrystic biotites because Ar diffuses extremely fast at magmatic temperatures and ratios are reset within a few days. On the basis of (1) elevated and/or imprecise 40Ar/36Ar ratios, (2) shapes of the Ar release spectra, and (3) a high mantle 3He flux in the Kos-Nisyros area, we suggest that biotite crystals retained some mantle 40Ar that led to the observed, anomalously old ages. In contrast, sanidine crystals from the only sanidine-bearing unit in the Kos-Nisyros volcanic center (the caldera-forming Kos Plateau Tuff) do not appear to store any excess 40Ar relative to atmospheric composition. The eastern edge of the Aegean Arc is tectonically complex, undergoing rapid extension and located close to a major structural boundary. In such regions, which are characterized by high fluxes of mantle volatiles, 40Ar/39Ar geochronology on biotite can lead to erroneous results due to the presence of excess 40Ar and should be checked either against 40Ar/39Ar sanidine or U/Pb zircon ages.

  2. 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

  3. 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.

  4. 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.; 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

  5. 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

  6. /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.

  7. 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.

  8. 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

  9. 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.

  10. An Astronomically Dated Standard in 40Ar/39Ar Geochronology?

    NASA Astrophysics Data System (ADS)

    Kuiper, K.; Hilgen, F.; Krijgsman, W.; Wijbrans, J.

    2003-12-01

    The standard geological time scale of Berggren et al. (1995) and Cande and Kent (1995) is calibrated with different absolute dating techniques, i.e. the Plio - Pleistocene relies on astronomical tuning, and older parts of the time scale are based on radio-isotopic (40Ar/39Ar and U/Pb) calibration methods. In the new edition of the standard geological timescale (Lourens et al., to be published in 2004) the entire Neogene will rely on astronomical dating. Therefore, it is of crucial importance that all dating methods produce equivalent absolute ages when the same geological event is dated. The Mediterranean Neogene provides an excellent opportunity to compare different dating methods by isotopic dating (40Ar/39Ar, U/Pb) of volcanic ash layers intercalated in astronomically dated sediments. Here we will show that in spite of potential errors in all methods, we succeeded to intercalibrate the 40Ar/39Ar and astronomical methods, arriving at astronomically calibrated age of 28.24 +/- 0.01 Ma for the in 40Ar/39Ar geochronology commonly used standard FCT sanidine. The advantage of an astronomically calibrated FCT above a K/Ar calibrated standard is a smaller error in the absolute age due to the lack of uncertainties related to 40K and radiogenic 40Ar contents in the primary standard and a decreasing influence of errors in the decay constant (branching ratio is not required). In addition to an astronomically calibrated FCT age we propose to introduce an astronomically dated standard. A direct astronomically dated standard can be regarded as a "primary" standard and does not require intercalibration with other standards, thus reducing analytical (and geological) uncertainties. Ash layers intercalated in sedimentary sequences in the Melilla Basin, Morocco appear to be the most suitable for this purpose. A reliable astronomical time control is available and intercalated ash layers contain sanidine phenocrysts up to 2 mm. Four ash layers are not or barely affected by

  11. 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

  12. New 40Ar/ 39Ar dating results from the Shanwang Basin, eastern China: Constraints on the age of the Shanwang Formation and associated biota

    NASA Astrophysics Data System (ADS)

    He, Huaiyu; Deng, Chenglong; Pan, Yongxin; Deng, Tao; Luo, Zhaohua; Sun, Jimin; Zhu, Rixiang

    2011-07-01

    The fluvio-lacustrine sequence of the Shanwang Basin, eastern China, preserves a rich and important terrestrial fossil fauna and flora; the exceptional preservation of these fossils reveals the dynamics of ancient mammalian ecosystems and plant biology. However, the timing of this sedimentary sequence has been the subject of debate for decades. Here we contribute to this debate by presenting the detailed results of 40Ar/ 39Ar analysis of the basalts above, below, and within the Shanwang Formation. These dates place stringent constraints on the age of Shanwang Formation and associated biota. 40Ar/ 39Ar ages obtained from basalts of the Niushan and Yaoshan Formations, which underlie and overlie the Shanwang Formation, are 21.0 ± 2.5 Ma (2σ, full external error) and 17.3 ± 1.5 Ma (2σ, full external error), respectively. The 40Ar/ 39Ar age of the basalt in the Shanwang Formation is 17-18 Ma. Given the age constraints of the basalts of the Yaoshan and Shanwang Formations, the age of the Shanwang biota is estimated to be ca. 17 Ma, late Burdigalian of the Early Miocene, indicating that the deposition of this fauna coincided with the onset of the mid-Miocene Climatic Optimum. The results provide new age constraints on the Shanwang mammal fauna, and independently support interpretations that this fauna can be assigned to chronozone MN4, and correlated with middle Orleanian of the European Land Mammal Age, and to late Hemingfordian of the North American Land Mammal Age. Biological diversity of the Shanwang Formation could reflect the global-scale mid-Miocene Climatic Optimum.

  13. 40Ar/ 39Ar and RbSr analyses from ductile shear zones from the Atacama Fault Zone, northern Chile: the age of deformation

    NASA Astrophysics Data System (ADS)

    Scheuber, Ekkehard; Hammerschmidt, Konrad; Friedrichsen, Hans

    1995-11-01

    The influence of deformation on the K-Ar and the Rb-Sr isotope system is investigated. It is assumed that, due to the diffusion processes involved, deformation has a similar effect on isotopic equilibrium as has temperature. In order to examine the influence of deformation on the K-Ar and the Rb-Sr isotope systems two shear zones from the Atacama Fault Zone (AFZ), situated in the north Chilean Coastal Cordillera, have been investigated. The AFZ, which was active as a sinistral strike-slip fault during the Mesozoic, has two sets of shear zones, one formed under amphibolite (SZ1), one under greenschist facies conditions (SZ2), Rb-Sr and 40Ar/ 39Ar age determinations were conducted on samples from cross sections of each set. In SZ1 the hornblendes and bioties from a weakly deformed sample reveal cooling ages of 153-152 and 150 ± 1 Ma, respectively. Biotite from the center of the shear zone of SZ1 gave an isochron of 143.9 ± 0.3 Ma (MSWD = 0.04) which is interpreted as the age of deformation which produced resetting of the mineral system. In SZ2 hornblendes yielded 40Ar/ 39Ar plateau (cooling) ages of ˜ 138 Ma. Biotites from undeformed samples gave Rb-Sr and 40Ar/ 39Ar total degassing ages of 130 ± 1 Ma, whereas biotite from the mylonitic rocks yielded 126-125 Ma which dates the time of deformation. Sr isotope homogenization occurred in the mylonitic rocks, and is most likely a result of deformation. The formation of SZ1 can be correlated to the Araucanian (= Nevadan) phase. The deformation in SZ2 is related to the onset of uplift and cooling of the Coastal Cordilleran magmatic arc.

  14. 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.

  15. 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

  16. 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.

  17. 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.

  18. 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

  19. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  1. 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.

  2. 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

  3. 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

  4. A 47 ka 40Ar/39Ar age for the Rotoiti Eruption, New Zealand, Measured by Multi-collection Noble gas Mass Spectrometry

    NASA Astrophysics Data System (ADS)

    Storey, M.

    2006-12-01

    The recent availability of commercial multi-collector noble gas mass spectrometers provides new opportunities for improved precision in 40Ar/39Ar dating, particularly for young Quaternary aged samples, where precise measurement of the 40Ar/36Ar ratio is critical. A Nu Instruments Noblesse multi-collector noble gas mass spectrometer was used to investigate the age of the Rotoiti eruption, the last major caldera-forming event at the Haroharo caldera, Okataina volcanic centre, New Zealand. Ash derived from the Rotoiti eruption is an important regional stratigraphic marker, but has proved difficult to date by a variety of methods, with estimates ranging from 45-65ka, at or beyond the useful range of 14C. The Rotoiti eruption is notable for the occurrence of cognate K-feldspar-biotite-glass-bearing granitoid lithics. K- feldspars were separated from a previously studied sample of a Rotoiti granitoid (103/2-1 and along with neutron fluence monitor Alder Creek sanidine (ACs = 1.194 +/- 0.007 Ma) were irradiated for 10 minutes in the Cd-lined facility at the OSU TRIGA reactor. Unknowns and monitor minerals were measured in multi-collection mode using the same detector configuration. Mass fractionation and detector discrimination for 40Ar/36Ar was monitored by repeated measurement of 1.2 x 10-13 mole air aliquots. Single crystal laser fusion ages, for K- feldspar that contain more than 10 percent *40Ar, range from 45-100 ka. On an isotope correlation diagram, the data for the younger population defines an isochron of 47 +/- 2 ka (eruption age), with an initial 40Ar/36Ar = 299.32 +/- 0.91 (MSWD of 1.1). K-feldspars with higher apparent ages, which are excluded from the isochron calculation, are interpreted to be partly reset crystals from earlier crystallization event/s within the Haroharo Caldera.

  5. 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

  6. 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

  7. 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.

  8. 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

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

    SciTech Connect

    West, D.P. Jr.; Guidotti, C.V.; Lux, D.R. . Dept. of Geological Sciences)

    1992-01-01

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

  10. 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.

  11. 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.

  12. 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

  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

    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.

  14. 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.

  15. 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.

  16. 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.

  17. Noble gas composition and 40Ar/39Ar age in eclogites from the main hole of the Chinese Continental Scientific Drilling project

    NASA Astrophysics Data System (ADS)

    Hopp, Jens; Schwarz, Winfried H.; Trieloff, Mario; Meyer, Hans-Peter; Hanel, Michael; Altherr, Rainer

    2016-10-01

    We present the first comprehensive noble gas study on eclogites. The four eclogite samples were recovered during the Chinese Continental Scientific Drilling and are from two distinct profile depth sections differing in their degree of interaction with meteoric water, based on their δ 18O-values (surface related and of mantle-type). Hence, noble gas analyses offer the potential to further discriminate between shallow (meteoric) and deep (mantle) fluid sources. Noble gas compositions reveal typical crustal fluid compositions, characterized by a variable mixture of atmospheric gases with significant contributions of nucleogenic neon, radiogenic 4He*, radiogenic 40Ar*, fissiogenic 131-136Xe, and presumably bariogenic 131Xe, but no significant addition of mantle gases. This signature can be also considered to represent one endmember component of eclogitic diamonds. Concentrations of non-radiogenic noble gases are rather low, with depletion of light relative to the heavier noble gases. Eclogites from lower depth which experienced a higher degree of interaction with meteoric water also showed higher contributions of atmospheric gas compared with eclogites recovered from greater depth. This is interpreted to result from interaction with high-salinity fluids during ultrahigh pressure (UH P). It demonstrates that the atmospheric noble gas abundance is a proxy for interaction with surface related fluids. 40Ar/39Ar (inverse) isochron ages of two phengite separates (241.2 ± 0.4 Ma and 275.0 ± 1.8 Ma, 1 σ-errors) predate the main phase of UH P metamorphism (ca. 220 Ma). Biotite yields an integrated age of about 1100 Ma. These age values are interpreted to reflect the likely addition of excess 40Ar without any chronological meaning.

  18. Diffusion of sup 40 Ar and sup 39 Ar in irradiated orthoclase

    SciTech Connect

    Foland, K.A.; Xu, Yuping )

    1990-11-01

    The important concerns of whether neutron irradiation affects Ar diffusion behavior in minerals and whether the diffusivities of natural radiogenic {sup 40}Ar and induced {sup 39}Ar are identical are considered. Both issues are addressed with isothermal and incremental-heating experiments on natural, homogeneous orthoclase from Benson Mines which was subjected to irradiation similar to typical {sup 40}Ar/{sup 39}Ar measurements. Previous study of this feldspar shows that laboratory {sup 40}Ar loss occurs by volume diffusion with physical grain sizes as the effective transport dimensions following a single Arrhenius relation. Isothermal-heating experiments on irradiated feldspar show the same loss and apparent {sup 40}Ar diffusion coefficients, within uncertainty, as unirradiated sample. For these heating times and temperatures, the experiments indicatet that the defects accompanying irradiation have only very minor, if any, effects on Ar behavior with regard to both diffusion kinetics and effective transport domains and that {sup 39}Ar and {sup 40}Ar diffusivities do not differ radically. Step-heating experiments yield essential flat spectra but with minor yet significant apparent discordance which is the same for different grain sizes. The spectra show high ages for small, initial low-temperature fractions and a slight increase in age with progressive Ar release. While these variations are potentially explicable by sample heterogeneities, the preferred explanation is that they result from {sup 39}Ar recoil loss and a slightly higher diffusivity of {sup 39}Ar relative to {sup 40}Ar. The results have important implications for diffusion processes in feldspar and the application of the {sup 40}Ar/{sup 39}Ar method.

  19. 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

  20. 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

  1. 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.

  2. 40Ar/39Ar ages and Sr-Nd-Pb isotopic compositions of alkaline and tholeiitic rocks from the northern Deccan Traps

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  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)

    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

  4. Detrital muscovite 40Ar/39Ar ages from Carboniferous sandstones of the British Isles: Provenance and implications for the uplift history of orogenic belts

    NASA Astrophysics Data System (ADS)

    Stuart, Finlay M.; Bluck, Brian J.; Pringle, Malcolm S.

    2001-04-01

    Major progradations of clastic sediments are recorded in the sedimentary record of the Famennian and the Visean-Namurian of the United Kingdom and surrounding waters. We have determined 40Ar/39Ar ages of 162 detrital muscovites from 11 coarse sandstones which were deposited between 370 and 465 Ma, spanning both progradations. Detrital mica ages are dominated by a peak at 415 Ma, with minor peaks at 440 Ma and 390 Ma. The 415 Ma muscovites are derived from the unroofing of the Scandian nappes during the compressional phase of the Caledonian orogeny in Scandinavia. The 440 Ma muscovites record pre-Scandian orogenic activity, which is rarely preserved in the orogenic record. Thermochronological evidence suggests that episodic postorogenic uplift, and exhumation events kept the Scandian orogen a major topographic feature and likely a sediment source for over 100 million years after nappe emplacement and implicates tectonic rather than climatic control on the clastic sediment progradations. The near total absence of detrital muscovites with ages <415 Ma suggests that the Scandian nappes had not been entirely eroded despite repeated uplift during the postorogenic extension. The river(s) which supplied the sediments probably ran parallel to the strike of the major Scandian thrusts, along the length of the Caledonian orogen, in a manner analogous to the major river systems of contemporary orogenic highlands.

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

    USGS Publications Warehouse

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

    2010-01-01

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

  6. 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

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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

  12. The Deccan Trap - Cretaceous-Paleogene boundary connection; new 40Ar/39Ar ages and critical assessment of existing argon data pertinent to this hypothesis

    NASA Astrophysics Data System (ADS)

    Baksi, Ajoy K.

    2014-04-01

    The Cretaceous-Paleogene boundary (KPgB) was dated by the 40Ar/39Ar method herein from the western interior of North America at 65.48 ± 0.12 Ma (1σ), in good agreement with other recent published estimates. For the Deccan Traps, India, new argon ages as well as others available in the literature, are assessed for reliability based on (a) statistical reliability of plateau/isochron sections and (b) freshness of material dated utilizing the alteration index method. From tholeiitic lavas from the Composite Western Ghats Section (CWGS), only six ages are found to be reliable estimates of the time of crystallization. These ages along with the magnetic polarity of the lavas agree with the geomagnetic polarity time scale (GPTS) at ˜67-64 Ma. Alkaline rocks from the Anjar area of Kutch, provide three reliable ages that suggest a hiatus in lava extrusion around KPgB. For the Rajahmundry basalts, the upper flow's age defines its formation during chron 29n; a single age from the lower reversed polarity flow appears somewhat dichotomous when plotted against the GPTS. The reliable lava ages indicate the most voluminous (reversed polarity) sections of the CWGS were extruded at a time statistically indistinguishable from that of the KPgB. The Deccan Trap - KPgB faunal extinction hypothesis remains plausible, but must compete with the latest report, favoring a very close temporal connection (˜0.03 m.y.) between the Chixculub (Impact) Crater and the KPgB.

  13. 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.

  14. 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

  15. 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.

  16. 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

  17. 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.

  18. 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

  19. Palaeomagnetism and 40Ar/39Ar age of a Pliocene lava flow sequence in the Lesser Caucasus: record of a clockwise rotation and analysis of palaeosecular variation

    NASA Astrophysics Data System (ADS)

    Caccavari, Ana; Calvo-Rathert, Manuel; Goguitchaichvili, Avto; Huaiyu, He; Vashakidze, Goga; Vegas, Néstor

    2014-06-01

    A palaeomagnetic and rock-magnetic investigation has been carried out on a Pliocene lava flow sequence in the Djavakheti Highland in the central Lesser Caucasus in the Republic of Georgia. In addition, a 40Ar/39Ar dating and electronic microscopic studies were performed on samples of this sequence, named the Saro section, which consists of 39 successive lava flows of doleritic basalts. A characteristic magnetization could be isolated in all studied 39 flows, yielding reverse-polarity directions in all cases, a mean direction D = 202.2°, I = -60.6° (N = 39, α95 = 2.0°, k = 138) being obtained. Thermomagnetic experiments (strong-field versus temperature curves) suggested low-Ti titanomagnetites and low Curie-temperature titanomagnetites with a rather high titanium content (x ≈ 0.5-0.7) as the main carriers of remanence. Their domain structure is characterized by a mixture of single- and multidomain grains. 40Ar/39Ar dating yielded an age of 1.73 ± 0.03 Ma, interpreted as the eruption age of the uppermost lava flow of the sequence. Analysis of palaeomagnetic results and radiometric data from the present and a previous study allows two different explanations about the time of emplacement of the section: (i) The lower 36 flows of the sequence might have been emitted between the normal-polarity Reunion and Olduvai chrons, and the upper three flows after the Olduvai chron, with a long hiatus in volcanic activity of more than 150 kyr or (ii) The whole sequence has been emitted between 1.778 and 1.73 ± 0.03 Ma, after the Olduvai chron. Comparison of the palaeomagnetic results obtained in this study with the expected direction shows that while inclination values agree well, declination shows an eastward deviation of 19.2° ± 5.8°. This discrepancy can be explained with a clockwise vertical-axis rotation of the sequence, which might have been produced by extensional structures with strike-slip component, which can be found in the study area. Virtual geomagnetic pole

  20. 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.

  1. Whole-Rock 40Ar/39Ar Step-heating Analyses, Problems and Potential

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

    Whole-rock 40Ar/39Ar step-heating analyses of extra-terrestrial materials are used to constrain the impact history of the inner solar system, the formation age of the Moon, and timing of paleomagnetic fields. Despite the importance of knowing the timing of these important events, the samples we have in hand are usually disturbed through mixing, (multiple?) impact events, and perhaps recoil loss. Extra-terrestrial 40Ar/39Ar data are typically interpreted through the assignment of essentially arbitrary plateau ages rather than through a robust physical model. Although the use of models capable of quantitatively assessing diffusive 40Ar* loss in extra-terrestrial samples has been around for nearly 50 years, this early advance has been widely ignored. Here we present implications of applying a robust, multi-activation energy, multi-diffusion domain model to step-heated 40Ar/39Ar data, with temperature cycling. Our findings show that for even a single heating event, "plateau" ages are unlikely to record meaningful ages. Further, if the sample has experienced multiple heating events or contains inherited clasts, recovering a unique solution may be impossible. Indeed the most readily interpretable portion of the age spectrum is the early heating steps which represents a maximum age estimate of the last re-heating event. Our results challenge the chronologic validity of 40Ar/39Ar "plateau" ages and by extension the hypotheses that are based on this data (e.g., the Late Heavy Bombardment). Future work will require new analytical procedures, interpretative frameworks, and (potentially) the combination of multiple chronometers to derive a robust impact history for the early solar system.

  2. 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.

  3. 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.

  4. 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.

  5. 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

  6. Preliminary 40Ar-39Ar thermochronological study of Dien Bien Phu Fault, northern Vietnam

    NASA Astrophysics Data System (ADS)

    Lin, T.; Lo, C.; Wang, P.; Lee, T.; Chung, S.; Lan, C.

    2003-12-01

    The Song Ma belt, located along the suture zone between the South China and Indochina blocks, was offset by the Dien Bien Phu Fault (DBF) in northern Vietnam. Many previous studies suggested that the Dien Bien Phu Fault is one of the Tertiary shear zones, resulted from the Cenozoic extrusion tectonics due to the Eurasian-Indian collision. However, the timing of the movement of DBF is remained unclear due to lacking of age data. In the present study, schists and mylonitic granites from DBF, and undeformed granites from the granitic belt along the south China block, were analysed by 40Ar-39Ar method. K-feldspar, hornblende, and biotite samples form granite plutons along south edge of South China block displayed different age ranges between opposite sides of DBF. Samples from plutonic body which is located in west side exhibit plateau ages ranging form 207Ma to 229Ma. Whereas, hornblende separates from granites on the west side exhibit much older plateau dates at around 277Ma. More detail studies were required to get better understand about the discrepancy of dating results between granitic masses located in different sides of DBF. On the other hand, Laser 40Ar-39Ar single-grain analyses conducted on deformed biotites and muscovites separated from the schists in DBF, show 40Ar-39Ar ages ranging form 185Ma to 205Ma, which are much younger than the plateau dates of those undeformed granitic samples. In summary, K-feldspar, hornblende and biotite samples from the undeformed granites, exhibit plateau dates in the range of 207- 277Ma, reflecting the ages of plutonism during the Indosinian orogeny. Whereas, muscovites and biotites extracted from schists and deformed granites in DBF, display young 40Ar-39Ar ages in the range of 205-185 Ma. This may suggest that although the DBF might have been reactivated during the Cenozoic extrusion tectonics, but it was in fact active in the Mesozoic.

  7. 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

  8. 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

  9. 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

  10. 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.

  11. 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

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

    USGS Publications Warehouse

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

    2000-01-01

    Precise dating of sanidine from proximal ash flow Bishop Tuff and air fall Bishop pumice and ash, California, can be used to derive an absolute age of the Matuyama Reversed-Brunhes Normal (M-B) paleomagnetic transition, identified stratigraphically close beneath the Bishop Tuff and ash at many sites in the western United States. An average age of 758.9 ?? 1.8 ka, standard error of the mean (SEM), was obtained for individual sanidine crystals or groups of several crystals, determined from ???70 individual analyses of sanidine separates from 11 sample groups obtained at five localities. The basal air fall pumice (757.7 ?? 1.8 ka) and overlying ash flow tuff (762.2 ?? 4.7 ka) from near the source yield essentially the same dates within errors of analysis, suggesting that the two units were emplaced close in time. A date on distal Bishop air fall ash bed at Friant, California, ???100 km to the west of the source area, is younger, 750.1 ?? 4.3 ka, but not significantly different within analytical error (??1 standard deviation). Previous dates of the Bishop Tuff, obtained by others using conventional K-Ar and the fission track method on zircons, ranged from ???650 ka to ???1.0 Ma. The most recent, generally accepted date by the K-Ar method on sanidine was 738 ?? 3 ka. We infer, as others before, that many K-Ar dates on sanidine feldspar are too young owing to incomplete degassing of radiogenic Ar during fusion in the K-Ar technique and that many older K-Ar dates are too old owing to detrital or xenocrystic contamination in the larger samples that are necessary for the technique. The new dates are similar to recent 40Ar/39Ar ages of the Bishop Tuff determined on individual samples by others but are derived from a larger proximal sample population and from multiple analysis of each sample. The results provide a definitive and precise age calibration of this widespread chronostratigraphic marker in the western United States and northeastern Pacific Ocean. We calculated the

  13. [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.

  14. 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.

  15. 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.

  16. {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.

    1991-12-31

    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{+-}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.

  17. 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.

  18. {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.

  19. 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

  20. 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

  1. 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

  2. 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

  3. 40Ar-39Ar Results of Lunar Meteorites Dhofar 025, 280, 309, 730, 733, 1436, 1442, SAU 449, NWA 6888

    NASA Astrophysics Data System (ADS)

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

    2016-08-01

    40Ar-39Ar ages vary between 3.0 and 4.2 Ga common for lunar rocks, but only Dho 025, Dho 309 and Dho 1436 are consistent with the LHB. Dho 280, 733, 1442, and NWA 6888 were affected by young impact events ≤1 Ga ago, partially also reset of CRE ages.

  4. 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.

  5. 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

  6. 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.

  7. 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

  8. 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.

  9. Investigation of the Gardnos Impact Structure: 40Ar/39Ar Results

    NASA Astrophysics Data System (ADS)

    Grier, J. A.; Swindle, T. D.; Kring, D. A.; Melosh, H. J.

    1995-09-01

    Introduction: The Gardnos structure (Norway) [1], has recently been identified as an impact crater [2,3], despite being deformed by a series of tectonic processes [2]. The age of the structure is uncertain, although attempts to date the impact event have been made on the basis of stratigraphic relationships [2,3]. Unfortunately, these estimates differ by more than 100 million years. The first study suggested that the impact corresponded roughly with the Cambro-Ordovician boundary, ~500 Ma [2], while a second stratigraphic interpretation suggested a Precambrian age of 650 Ma [3]. To try and resolve this discrepancy, we attempted to determine the age of the structure with radiometric techniques, which typically have an error of < 10 Ma. Method: We examined three samples from the crater, in an attempt to determine the age using the 40Ar/39Ar dating method. The samples (plagioclase feldspar, alkali feldspar, and shards of gray, semi-transparent, glassy-looking material) were obtained from the suevite breccia unit of [2]. This unit should be the most promising for 40Ar/39Ar dating, since it is melt bearing, and no other unit is likely to be less affected by subsequent tectonism and sedimentation. Results: Our experiments yielded complicated age spectra interpreted to reflect a thermal event at 385+/-5 Ma and another possible event at 312+/-5 Ma (Fig. 1). All three samples yielded similar results. If the stratigraphic interpretations are correct, it thus seems clear that the impact event itself was not dated. The 385 Ma age corresponds to the end of the Caledonian Orogeny, which significantly altered this section of Norway [4]. It is suspected that late Caledonian folding occurred during this time [4], and we therefore interpret the 385 Ma age as the thermal metamorphic overprint of the Caledonian Orogeny. Another event clearly occurred afterwards, which is responsible for the 312 Ma signature. The cause of this thermal overprint has not yet been identified, since

  10. New constraints on the release of noble gases during in vacuo crushing and application to scapolite Br-Cl-I and 40Ar/ 39Ar age determinations

    NASA Astrophysics Data System (ADS)

    Kendrick, M. A.; Phillips, D.

    2009-10-01

    The release of irradiation-produced noble gas isotopes ( 38Ar Cl, 80Kr Br, 128Xe I and 39Ar K) during in vacuo crushing scapolite has been investigated and is compared to quartz. Three thousand crushing strokes released ˜98% of fluid inclusion-hosted noble gas from quartz. In comparison, 3000 crushing strokes released only ˜4% of the lattice-hosted 38Ar Cl from a scapolite gem. In vacuo crushing released lattice Ar preferentially relative to lattice Kr or Xe and prolonged crushing released ˜88% of the lattice-hosted noble gas in 96,000 crushing strokes. We suggest fast diffusion pathways generated by crushing are an important noble gas release mechanism and we demonstrate two applications of prolonged in vacuo crushing on irradiated scapolite. Firstly, scapolite molar Br/Cl and I/Cl values are shown to vary over a similar range as crustal fluids. The Cl-rich scapolite gem from Hunza, Pakistan has Br/Cl of 0.5-0.6 × 10 -3 and I/Cl values of 0.3-2 × 10 -6, that are similar to fluids that have dissolved evaporites. In contrast, three out of four skarn-related scapolites from the Canadian Grenville Province have molar Br/Cl values of 1.5-2.4 × 10 -3, and I/Cl values of 11-24 × 10 -6, that are broadly consistent with skarn formation by magmatic fluids. The fourth Grenvillian scapolite, with only 0.02 wt% Cl, has an exceptionally elevated molar Br/Cl value of up to ˜54 × 10 -3 and I/Cl of 284 × 10 -6. It is unclear if these values reflect the composition of fluids formed during metamorphism or preferential incorporation of Br and I in Cl-poor meionitic scapolite. Secondly, the Grenvillian scapolites give plateau ages of between 830 Ma and 400 Ma. The oldest ages post-date regional skarn formation by ˜200 Myr, but are similar to feldspar cooling ages in the Province. The age variation in these samples is attributed to a combination of factors including variable thermal history and the presence of mineral sub-grains in some of the samples. These sub

  11. 40Ar/39Ar geochronology of submarine Mauna Loa volcano, Hawaii

    NASA Astrophysics Data System (ADS)

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

    2012-09-01

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

  12. 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.

  13. 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.

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

    NASA Astrophysics Data System (ADS)

    Jacobson, Carl E.

    1990-01-01

    Seventeen 40Ar/39Ar ages for hornblende, celadonitic muscovite, and biotite from the Pelona, Orocopia, Rand, and Portal Ridge (POR) schists range from 39 to 85 Ma. The 85 Ma is from Portal Ridge, consistent with the proposed correlation of that body with the schist of Sierra de Salinas. 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 79 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, similar to K-Ar ages from nearby upper plate rocks. A third hornblende has an age of 73 Ma but is not well constrained. 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 40Ar/39Ar 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.

  15. 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.

  16. 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

  17. Age of Lucy and the First Family: Single-crystal 40Ar/39Ar dating of the Denen Dora and lower Kada Hadar members of the Hadar Formation, Ethiopia

    NASA Astrophysics Data System (ADS)

    Walter, Robert C.

    1994-01-01

    Single-crystal laser-fusion 40Ar/39Ar analyses on previously undatable tephra deposits provide the first reliable dates for the important hominid fossils popularly called "Lucy" and the "First Family,"here dated to 3.18 and 3.20 Ma, respectively. These results help to establish an age framework for hominid and biostratigraphic evolution for the fossil-rich Hadar Formation. The abundance of well-preserved vertebrate fossils at Hadar can be attributed, in part, to an unusually high sediment-accumulation rate of approximately 80 cm/ka, which might be due to increased tectonic subsidence of the Hadar basin between 3.4 and 3.2 Ma. The new dates also provide supportive evidence that the boundary ages for the Kaena and Mammoth subchrons, of the geomagnetic polarity time scale, need to be increased by as much as 4% to 5%.

  18. 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.

  19. Direct dating of tin-tungsten mineralization of the Piaotang tungsten deposit, South China, by 40Ar/39Ar progressive crushing

    NASA Astrophysics Data System (ADS)

    Bai, Xiu-Juan; Wang, Min; Jiang, Ying-De; Qiu, Hua-Ning

    2013-08-01

    Dating ore minerals is the most direct method to determine the mineralization age of a deposit, but only a few ore minerals can be directly dated by traditional isotopic geochronometers. The aim of this study is to investigate the possibility of direct dating of the ore minerals cassiterite and wolframite from the Piaotang tungsten deposit using the 40Ar/39Ar stepwise crushing technique, comparing the age to coexisting K-rich muscovite dated by 40Ar/39Ar laser stepwise heating. The cassiterite, wolframite and muscovite samples were separated from four pieces of ore hand-specimens. The 40Ar/39Ar isochron ages of cassiterite and wolframite are concordant with ages of their coexisting muscovite, indicating that cassiterite and wolframite are suitable ore minerals for directly dating the ore-forming event by 40Ar/39Ar stepwise crushing.

  20. 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.

  1. 40Ar/39Ar dates from the West Siberian Basin: Siberian flood basalt province doubled.

    PubMed

    Reichow, Marc K; Saunders, Andrew D; White, Rosalind V; Pringle, Malcolm S; Al'Mukhamedov, Alexander I; Medvedev, Alexander I; Kirda, Nikolay P

    2002-06-01

    Widespread basaltic volcanism occurred in the region of the West Siberian Basin in central Russia during Permo-Triassic times. New 40Ar/39Ar age determinations on plagioclase grains from deep boreholes in the basin reveal that the basalts were erupted 249.4 +/- 0.5 million years ago. This is synchronous with the bulk of the Siberian Traps, erupted further east on the Siberian Platform. The age and geochemical data confirm that the West Siberian Basin basalts are part of the Siberian Traps and at least double the confirmed area of the volcanic province as a whole. The larger area of volcanism strengthens the link between the volcanism and the end-Permian mass extinction. PMID:12052954

  2. 40Ar/39Ar thermochronology of mesoproterozoic metamorphism in the Colorado Front Range

    USGS Publications Warehouse

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

    1999-01-01

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

  3. Correlation diagrams in 40 Ar/39Ar dating: is there a correct choice?

    USGS Publications Warehouse

    Dalrymple, G.B.; Lanphere, M.A.; Pringle, M.S.

    1988-01-01

    Contrary to published assertions, the 2 types of correlation diagrams used in the interpretation of 40Ar/39Ar incremental-heating data yield the same information provided the correct mathematics are used for estimating correlation coefficients and for the least squares fit. The choice is simply between 2 illustrative, graphical displays, neither of which is fundamentally superior to the other. -Authors

  4. 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.

  5. 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

  6. 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

  7. 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.

  8. Single-crystal sup 40 Ar/ sup 39 Ar dating of the Olorgesailie Formation, southern Kenya rift

    SciTech Connect

    Deino, A. ); Potts, R. )

    1990-06-10

    Single-crystal laser fusion {sup 40}Ar/{sup 39}Ar analyses and several conventional bulk fusion {sup 40}K- {sup 40}Ar dates have been used to determine the age of volcaniclastic strata within the Olorgesailie Formation and of associated volcanic and sedimentary units of the southern Kenya rift. In the principal exposures along the southern edge of the Legemunge Plain, the formation spans the interval from approximately 500 to 1,000 ka. Deposition continued to the east along the Ol Keju Nyiro river where a tuff near the top of the formation has been dated at 215 ka. In these exposures, the formation is unconformably overlain by sediments dated at 49 ka. A possible source for the Olorgesailie tephra, the Ol Doinyo Nyokie volcanic complex, contains as ash flow dated at {approximately} 1 Ma, extending the known age range of this complex to encompass that of virtually the entire Olorgesailie Formation in the Legemunge Plain. These geologic examples illustrate the importance of the single-crystal {sup 40}Ar/{sup 39}Ar dating technique whereby contaminant, altered, or otherwise aberrant grains can be identified and eliminated from the determination of eruptive ages for reworked or altered pyroclastic deposits. The authors have presented a computer-modeling procedure based on an inverse-isochron analysis that promotes a more objective approach to trimming {sup 40}Ar/{sup 39}Ar isotope data sets of this type.

  9. 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.

  10. Geochronology of the Porgera gold deposit, Papua New Guinea: Resolving the effects of excess argon on K-Ar and sup 40 Ar/ sup 39 Ar age estimates for magmatism and mineralization

    SciTech Connect

    Richards, J.P.; McDougall, I. )

    1990-05-01

    Mesothermal/epithermal gold mineralization at Porgera in the highlands of Papua New Guinea (PNG), occurs in structurally controlled veins and disseminations, which overprint and cross-cut a suite of shallow-level, comagmatic, mafic alkaline stocks and dykes and their sedimentary host rocks. Conventional K-Ar apparent ages of twelve hornblende separates from eight different intrusions scatter between 7 and 14 Ma, but four biotite separates are concordant at 6.02 {plus minus} 0.29 Ma (2{sigma}). {sup 40}Ar/{sup 39}Ar step-heating experiments on six of the hornblende separates reveal saddle-shaped age spectra, which indicate the presence of excess {sup 40}Ar. One of these samples yields a well-defined plateau with an apparent age of 5.96 {plus minus} 0.25 Ma (2{sigma}). Conventional K-Ar analyses of six separates of hydrothermal illite and roscoelite associated with gold mineralization yield apparent ages of between 5.1 and 6.1 Ma and indicate that ore deposition occurred within 1 Ma of magmatism at Porgera. Evidence for the evolution of a magmatic volatile phase, and the presence of excess {sup 40}Ar both in the intrusives and in hydrothermal fluids associated with the orebody, suggest that magmatic fluids may have had some involvement in metallogenesis, but the exact nature of this involvement is not yet clear. Late Miocene magmatism and mineralization at Porgera are thought to have occurred shortly prior to or during the initiation of continent/arc collision and to pre-date associated Pliocene uplift and foreland deformation in the highlands.

  11. 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.

  12. 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.

  13. 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

  14. 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

  15. 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

  16. 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.

  17. 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.

    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

  18. 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

  19. 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

  20. 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

  1. 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

  2. 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.

  3. 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.

  4. 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%.

  5. ``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

  6. 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.; Swisher, C. C.

    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

  7. 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.

  8. 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.

  9. 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

  10. New chronometers for the metamorphism of ophiolitic rocks: 40Ar/39Ar neptunite and 232Th/208Pb joaquinite

    NASA Astrophysics Data System (ADS)

    Borneman, N.; Hodges, K. V.; Van Soest, M. C.

    2015-12-01

    Both primary magmatism and metamorphism of ophiolite sequences are difficult to date, due in large part to the fact that the majority of lithologies present tend to contain very low concentrations of radioactive elements. As a result, researchers are often forced to process large amounts of material to search for accessory phases like zircons in gabbro, or to employ geochronometers that often yield multiply interpretable results (e.g., 40Ar/39Ar glaucophane or phengite), or to rely on indirect evidence for inferring ages. Here, we introduce two new options for chronometery of metamorphosed ophiolites: 40Ar/39Ar neptunite and 232Th/208Pb joaquinite. The best known locality for these rare minerals is the New Idria serpentinite diapir, found within the southern Diablo Range of the Coast Range Province of California. Here, both the joaquinite and neptunite chronometers record indistinguishable dates that we interpret to be the crystallization age of the phases during diapir ascent, based on the demonstrated low temperature history of the diapir as whole and the agreement of dates from chromonometers that almost certainly have different closure temperatures. This age is generally inferred to be coincident with the timing of the passage of the Mendocino Triple Junction and associated initiation of the San Andreas fault in this area. We propose that the mean40Ar/39Ar neptunite plateau date of 12.375 ± 0.082 Ma and corroborating 232Th/208Pb joaquinite date (12.08 ± 0.59 Ma) may represent a high-precision constraint on the timing of this event. We also report a second application of these chronometers to samples from the Yarlung suture, which formed at the time of initial India-Eurasia collision in southern Tibet. Here, both chronometers record indistinguishable dates of ca. 52 Ma, which we also interpret as the crystallization age. This age is consistent with most previously published estimates for the timing of the India-Eurasia collision.

  11. 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.

  12. The new 39Ar/40Ar dating facility of the LSCE, background and performances.

    NASA Astrophysics Data System (ADS)

    Scaillet, S.; Nomade, S.; Guillou, H.; Scaillet-Vita, G.

    2007-12-01

    Precise and accurate timescales are increasingly needed in most disciplines of the Earth sciences. To contribute to this challenging task, a new 39Ar/40Ar laboratory, specifically devoted to the dating of very recent (down to 1 ka) volcanic products, has been developed at the LSCE (CEA-IPSL-UVSQ, France). In this contribution, we present the first data obtained from this new facility. The laboratory comprises a VG 5400 mass spectrometer coupled with a high sensitivity and high dynamic range ion counting system. Gas extraction is achieved with a 25 watts C02 laser or a double vacuum furnace depending of the analyzed samples. The full metal vacuum and purification line feature a GP50 and a compact Ti flash getters which permit extremely low blank for all Argon isotopes (e.g. ~ 3.0 10e-19 Moles for 36Ar). Both analytical protocols and hardware were specifically developed and optimized to date extremely young samples. Analytical performances including protocols, flux monitoring as well mass spectrometer discrimination correction method will be presented in the light of data obtained over the last 10 months. All samples were irradiated, under cadmium, in the Β-1 position (~1.0 10e+13 fast n cm-2 s-1) of the 70MWh-1 OSIRIS reactor (Pierre Süe laboratory, CEA-Saclay, France). Irradiation package is composed of home-design Aluminium disks constituting a 4 cm stack (10 to 30 unknowns/irradiation). Analyzed neutrons flux standards indicate less than 1% variation along the 4cm stack and validate the use of this reactor for high-precision 39Ar/40Ar dating. The precision and accuracy of the facility has been checked from cross-comparison of international single grain standards including FCs (28.02Ma), ACR-2 (1.194Ma) and TCR (28.32Ma) using the most recent recommended values for these monitors. A total of 80 grains in two irradiations (10 and 120 minutes) will be presented in details. Results from single-grain analyses agree within errors with those proposed by Renne et al

  13. Higher Precision: Opening A New 40Ar/39Ar Can Of Worms

    NASA Astrophysics Data System (ADS)

    Heizler, M. T.

    2012-12-01

    Advances in technology often lead to advances in science and this is true for the noble gas community that is currently being inundated with new multi-collector mass spectrometers. For the 40Ar/39Ar community, the ARGUS VI mass spectrometers are yielding age precision on individual age spectrum steps or single grain fusion ages that is commonly an order of magnitude improved over the MAP-215-50 instruments. Just as age spectrum analyses did in the 1960's, and single crystal laser fusion did in the 1980's, new data now yields another level of insight about isotopic behavior and an improved geological understanding, but also reveals difficult to understand complexity. Ongoing with geochronology studies, NM Tech is revisiting many of the commonly used 40Ar/39Ar flux monitor standards used throughout the argon community. Single grain analysis of AC-2, FC-2 sanidine, TCR-2 sanidine, GA1550 biotite, Fireclay sanidine and PP20 hornblende all yield scattered distributions except for AC-2. Typical single crystal 1 sigma precision for FC-2 and TCR sanidine is 0.02 and 0.06%, respectively with both samples heterogeneous at the 25 ka level. GA1550 biotite is measured to 0.02% and as governed by an MSWD value of 3.5 has a relatively tight age population. The ca. 313 Ma Fireclay sanidine grains yields ~0.1 Ma age precision, but scatters by more than 1 Ma. PP20 hornblende single grains (~1175 Ma) vary by ~25 Ma however most scatter by ±5 Ma. AC-2 is the youngest standard (~1.18 Ma) and we obtain single grain precision of 0.15% and commonly a normal distribution about a total age error of <1 ka 1 sigma. Incremental heating of single grain sanidine standards yields a variety of age spectrum shapes that are commonly not flat. For instance FC-2 shows disturbance for individual steps at about 0.3 Ma that is well resolved with high precision analysis. In general, nearly all sanidines step heated on the ARGUS VI show non-ideal behavior indicating that combinations of slight argon loss

  14. 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

  15. Some Footnotes to the Optimization-based Calibration of the 40Ar/39Ar System

    NASA Astrophysics Data System (ADS)

    Renne, P. R.; Deino, A.

    2011-12-01

    Calibration of the 40Ar/39Ar system presented by Renne et al. (2010) provides a robust synthesis of data from disintegration counting, isotopic composition of a standard, and normalization to the precisely-determined 238U decay constant. Recognition of the incompatibility of liquid scintillation counting (LSC) data for the total 40K decay constant, and subsequent exclusion of these data, provides a minor improvement to accuracy that is most important for pre-Phanerozoic samples (Renne et al., 2011). Discrepancies remain, however, with some results from the literature recalculated using this calibration. To investigate these discrepancies, we have analyzed sanidine from (1) the Bishop Tuff (Sarna et al., 2002) and (2) the IrZ coal (Cretaceous-Paleogene boundary; KPB) (Swisher et al., 1993). Preliminary results give 775.8 ± 1.7 ka (1σ here and throughout, including systematic errors) for the Bishop Tuff and 66.041 ± 0.046 Ma for the IrZ coal, contrasting with ages of 778.1 ± 3.7 ka and 66.214 ± 0.060 (respectively) recalculated from the original work by Renne et al. (2011). Our Bishop Tuff results, though younger than recalculated previous results, still conflict with the U-Pb zircon age (767.1 ± 0.5 ka) reported by Crowley et al. (2007), and with the age of the Brunhes-Matuyama geomagnetic polarity transition inferred by Channell et al. (2010). Our age for the IrZ coal (KPB) agrees with the preferred astronomically tuned age of 65.957 ± 0.040 Ma inferred for the KPB by Kuiper et al. (2008), although the recent work of Laskar et al. (2011) may undermine this result. Some misconceptions regarding the approach and results of Renne et al. (2010) need to be rectified. First, the age of the Fish Canyon sanidine (FCs) standard arising from this approach is not the primary output of the approach but rather a consequence of output parameters having correlated errors- it should not be used in conjunction with other decay constants such as those of Steiger and J

  16. 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.

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

    USGS Publications Warehouse

    Calvert, Andrew T.; Moore, Richard B.; McGeehin, John P.; Rodrigues da Silva, Antonio

    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.

  18. Sedimentology and taphonomy of the upper Karoo-equivalent Mpandi Formation in the Tuli Basin of Zimbabwe, with a new 40Ar/ 39Ar age for the Tuli basalts

    NASA Astrophysics Data System (ADS)

    Rogers, Raymond R.; Rogers, Kristina Curry; Munyikwa, Darlington; Terry, Rebecca C.; Singer, Bradley S.

    2004-10-01

    Karoo-equivalent rocks in the Tuli Basin of Zimbabwe are described, with a focus on the dinosaur-bearing Mpandi Formation, which correlates with the Elliot Formation (Late Triassic-Early Jurassic) in the main Karoo Basin. Isolated exposures of the Mpandi Formation along the banks of the Limpopo River consist of red silty claystones and siltstones that preserve root traces, small carbonate nodules, and hematite-coated prosauropod bones. These fine-grained facies accumulated on an ancient semi-arid floodplain. Widespread exposures of quartz-rich sandstone and siltstone representing the upper Mpandi Formation crop out on Sentinel Ranch. These strata preserve carbonate concretions and silicified root casts, and exhibit cross-bedding indicative of deposition via traction currents, presumably in stream channels. Prosauropod fossils are also preserved in the Sentinel Ranch exposures, with one particularly noteworthy site characterized by a nearly complete and articulated Massospondylus individual. An unconformity caps the Mpandi Formation in the study area, and this stratigraphically significant surface rests on a laterally-continuous zone of pervasive silicification interpreted as a silcrete. Morphologic, petrographic, and geochemical data indicate that the Mpandi silcrete formed by intensive leaching near the ground surface during prolonged hiatus. Chert clasts eroded from the silcrete are intercalated at the base of the overlying Samkoto Formation (equivalent to the Clarens Formation in the main Karoo Basin), which in turn is overlain by the Tuli basalts. These basalts, which are part of the Karoo Igneous Province, yield a new 40Ar/ 39Ar plateau age of 186.3 ± 1.2 Ma.

  19. 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

  20. Thermal history of the Pan-African/Brasiliano Borborema Province of northeast Brazil deduced from 40Ar/ 39Ar analysis

    NASA Astrophysics Data System (ADS)

    Corsini, M.; de Figueiredo, L. Lambert; Caby, R.; Féraud, G.; Ruffet, G.; Vauchez, A.

    1998-02-01

    Detailed step heating 40Ar/ 39Ar analyses were performed on different minerals from magmatic and metamorphic rocks of the Borborema Province. This region, located in northeast Brazil, belongs to the Pan-African/Brasiliano belt which is about 1000 km wide and 600 km long. Twenty-six single grains of amphibole, muscovite and biotite were extracted from eighteen samples selected in an area of about 17,500 km 2 (250 km by 70 km) in the Patos region. This region has been affected by a continental-scale shear zone system. Well-defined 40Ar/ 39Ar plateau ages and an U sbnd Pb analysis on zircon [Leterrier, J., Jardim de Sà, E.F, Bertrand, J.M. and Pin, C., 1994. Ages U sbnd Pb sur zircon de granitoïdes brasilianos de la ceinture du Serido (Province Borborema, NE brésil). C.R. Acad. Sci., Paris, 318: 1505-1511] allow the definition of a homogeneous and unusual slow cooling history (3-4°C/Ma) and suggest a rather slow uplift rate between 580 and 500 Ma, followed by a fast cooling of the whole studied area around 500 Ma. Rapid cooling is suggested by concordant plateau ages on muscovite and biotite. The existence of such an event at the end of the belt history is in agreement with data obtained on other major belts such as the Alpine-Himalayan system or the Hercynian belt.

  1. 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

  2. 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

  3. Observation of micron to centimetre scale argon in alkali feldspars: implications for 40Ar/39Ar thermochronology

    NASA Astrophysics Data System (ADS)

    Kelley, S. P.; Flude, S.

    2012-04-01

    New data from gem quality K-feldspar from Itrongay, Madagascar indicates that such crystals behave as a single diffusion domain for Ar diffusion. A study of such a grain allows us to measure natural Ar diffusion from micron to centimetre scales in gem quality feldspars. UV-laserprobe 40Ar/39Ar ages from single gem quality grains indicate that the natural crystal surface acted as the main diffusion domain boundary. UV-Laser ablation 40Ar/39Ar dating of 1 cm whole crystal of gem quality K-feldspar was undertaken at a range of length scales. Micron scale UV laser depth profiling was used to determine Ar diffusion adjacent to the natural crystal surface (presumed to have formed as the pegmatite crystalised). UV laser spot dating was used to measure the age variations on a length scales of 10s of microns to mm and even cm. The high potassium content and age of the Itrongay sample make it possible to measure natural argon diffusion at high precision and high spatial resolution, to address some of the issues surrounding Ar diffusion, at a scale that can be imaged in the laboratory. The analysis reveals the presence of age gradients in the Itrongay feldspar spanning more than 50Ma - between 420 and 470 Ma. As previous work on Itrongay feldspar has tended to be carried out on mm-sized fragments without knowledge of the original crystal boundaries, the variation in radiometric ages in the published literature is likely due to these internal age variations. These gradients are interpreted as diffusion profiles caused by the diffusion loss of 40Ar from the crystal and we have modeled the likely differences between slow cooling/storage at elevated temperature, episodic loss, or different diffusion mechanisms. The age gradients appear to be in agreement with previous thermochronology in the area.

  4. 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.

  5. K-Ar and 40Ar/ 39Ar ages of dikes emplaced in the onshore basement of the Santos Basin, Resende area, SE Brazil: implications for the south Atlantic opening and Tertiary reactivation

    NASA Astrophysics Data System (ADS)

    Guedes, Eliane; Heilbron, Monica; Vasconcelos, Paulo M.; de Morisson Valeriano, Cláudio; César Horta de Almeida, Júlio; Teixeira, Wilson; Thomaz Filho, Antonio

    2005-03-01

    New K-Ar and 40Ar/ 39Ar data of tholeiitic and alkaline dike swarms from the onshore basement of the Santos Basin (SE Brazil) reveal Mesozoic and Tertiary magmatic pulses. The tholeiitic rocks (basalt, dolerite, and microgabbro) display high TiO 2 contents (average 3.65 wt%) and comprise two magmatic groups. The NW-oriented samples of Group A have (La/Yb) N ratios between 15 and 32.3 and range in age from 192.9±2.2 to 160.9±1.9 Ma. The NNW-NNE Group B samples, with (La/Yb) N ratios between 7 and 16, range from 148.3±3 to 133.9±0.5 Ma. The alkaline rocks (syenite, trachyte, phonolite, alkaline basalts, and lamprophyre) display intermediate-K contents and comprise dikes, plugs, and stocks. Ages of approximately 82 Ma were obtained for the lamprophyre dikes, 70 Ma for the syenite plutons, and 64-59 Ma for felsic dikes. Because Jurassic-Early Cretaceous basic dikes have not been reported in SE Brazil, we might speculate that, during the emplacement of Group A dikes, extensional stresses were active in the region before the opening of the south Atlantic Ocean and coeval with the Karoo magmatism described in South Africa. Group B dikes yield ages compatible with those obtained for Serra Geral and Ponta Grossa magmatism in the Paraná Basin and are directly related to the breakup of western Gondwana. Alkaline magmatism is associated with several tectonic episodes that postdate the opening of the Atlantic Ocean and related to the upwelling of the Trindade plume and the generation of Tertiary basins southeast of Brazil. In the studied region, alkaline magmatism can be subdivided in two episodes: the first one represented by lamprophyre dykes of approximately 82 Ma and the second comprised of felsic alkaline stocks of approximately 70 Ma and associated dikes ranging from 64 to 59 Ma.

  6. New 40Ar/39Ar age determinations and paleomagnetic results bearing on the tectonic and magmatic history of the northern Madison Range and Madison Valley region, southwestern Montana, U.S.A

    USGS Publications Warehouse

    Kellogg, K.S.; Harlan, S.S.

    2007-01-01

    Detailed 40Ar/39Ar dating and paleomagnetic analysis of dacite porphyry sills and dikes that intrude Cretaceous sedimentary rocks in the northern Madison Range in southwestern Montana show that Laramide shortening was essentially complete by ???69 Ma. A negative paleomagnetic fold test indicates that Laramide folding occurred before cooling of the dacite sills and dikes at ???69 Ma. Laramide deformation began synchronous with deposition of the Livingston Formation rocks at ???79 Ma. These results are consistent with previous observations in the region that show the onset of Laramide deformation in the northern Rocky Mountains becoming progressively younger toward the east. 40Ar/39Ar dating of additional igneous rocks in the northern Madison Valley and around Norris, Montana better define post-Laramide tectonomagmatic events in the region, including Eocene-Oligocene volcanism and Basin and Range crustal extension. Dates from three rhyolitic intrusions near Red Mountain are between 48.71 ?? 0.18 Ma and 49.42 ?? 0.18 Ma, similar to the dates from basal silicic flows of the Virginia City volcanic field (part of the southwest Montana volcanic province), suggesting that the Red Mountain intrusions may have been the sources for some of the early extrusive rocks. Magmatism in the Virginia City volcanic field became generally more mafic with time, and a ???30-Ma basalt flow near Norris is considered a late, outlying member of the volcanic field. A tuff along the east side of the Madison Valley half graben yielded a early middle Miocene date (16.2 ?? 0.19 Ma), suggesting that accelerated crustal extension and associated rapid basin sedimentation probably began in the early Miocene, slightly earlier than previous estimates.

  7. 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.

  8. 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.

  9. 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

  10. 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

  11. 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.

  12. 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

  13. 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

  14. 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

  15. 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.

  16. 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

  17. 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

  18. 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

  19. 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.

  20. 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.

  1. 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

  2. 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.

  3. 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.

  4. 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

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. 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

  10. 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

  11. 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

  12. The cooling history of the Acapulco meteorite as recorded by the 244Pu and 40Ar- 39Ar chronometers

    NASA Astrophysics Data System (ADS)

    Pellas, Paul; Fiéni, Christine; Trieloff, Marig; Jessberger, Elmar K.

    1997-08-01

    244Pu fission track densities recorded in phosphates (merrillite and apatite) and in orthopyroxenes adjacent to phosphates, along with 40Ar- 39Ar dating, were used to retrace the cooling evolution in the low temperature regime of the Acapulco meteorite over a time interval of ˜110 Ma. High resolution 40Ar- 39Ar dating yielded a plateau age of 4514 ± 16 Ma and a precise 40Ar-closure temperature of 560 ± 20 K in plagioclase. This Ar closure temperature is essentially indistinguishable from the 50% retention temperature of fission tracks registered in orthopyroxene (550 ± 25 K). This allows us to anchor the floating relative 244Pu timescale to the absolute timescale defined by the 40Ar- 39Ar chronometer. Plutonium contents of bulk phosphates inferred from fission xenon are >2X higher than those inferred from tracks in the same phosphates, reflecting the low track retention temperatures of phosphates. However, the Pu contents from tracks in orthopyroxenes adjacent to phosphates agree with those from fission xenon, suggesting similar retention temperatures and times. Our results, together with an estimated peak metamorphic temperature (˜1300 K), and the U/Pb sbnd Pb datum of Acapulco phosphates (Göpel et al., 1992), outline an early thermochronological history of Acapulco, that is more detailed than obtained by earlier attempts and spans a time of ˜160 Ma. Fast cooling in the high temperature regime (1300-720 K) of 100 ± 40 K/Ma was followed by a drastic change of the cooling rate between 720 and 560 K (3.7 K/Ma), down to a very slow cooling (1.7 ± 0.5 K/Ma from 550 to 360 K). The fast cooling at high temperatures shared also by other acapulcoite-lodranites (A-L) suggests that the parent body of Acapulco was distinctly smaller than the H chondrite asteroid. In contrast, the very slow cooling in the low temperature regime should imply that acapulco-like material became effectively insulated during a later stage of its history (>6 Ma after the asteroid formation

  13. 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.

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. 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).

  19. 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.

  20. 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

  1. 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.

  2. 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

  3. 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

  4. 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.

  5. 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.

  6. 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.

  7. 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

  8. 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.

  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.

  10. (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

  11. Mineralogy and 40Ar/39Ar geochronology of supergene Mn-oxides in the Dongxiangqiao deposit, Hunan Province, South China

    NASA Astrophysics Data System (ADS)

    Deng, Xiao-Dong; Li, Jian-Wei

    2016-09-01

    The Dongxiangqiao Mn deposit in Hunan Province is one of numerous supergene Mn-oxide deposits in central South China. This deposit is derived from chemical weathering of Permian Mn-rich shales and limestones. Mn-oxide samples from the Dongxiangqiao deposit consist mainly of pyrolusite, lithiophorite, and cryptomelane, with minor amounts of hollandite and nsutite. Cobalt and Ni are enriched in lithiophorite and cryptomelane-hollandite. Our studies suggest that Co occurs mainly in the structural and adsorption sites of lithiophorite. Cobalt in lithiophorite accounts for ~80 % of the bulk Mn-oxide ores, thus it can be comprehensively utilized when lithiophorite has been separated by a suitable physical beneficiation process. Four cryptomelane-dominated grains from the saprolite zone yield 40Ar/39Ar plateau ages ranging from 10.3 ± 0.3 to 4.19 ± 0.08 Ma (2σ). This indicates that intense enrichment of supergene Mn-oxides has prevailed at least in the late Miocene and persisted into the Pliocene at Dongxiangqiao. When combined sedimentalogical and thermochronoloical data, our dating results suggest that central South China has a relatively rapid surface uplift rate.

  12. Effects of fluid-rock interaction on 40Ar/39Ar geochronology in high-pressure rocks (Sesia-Lanzo Zone, Western Alps)

    NASA Astrophysics Data System (ADS)

    Halama, Ralf; Konrad-Schmolke, Matthias; Sudo, Masafumi; Marschall, Horst R.; Wiedenbeck, Michael

    2014-02-01

    In situ UV laser spot 40Ar/39Ar analyses of distinct phengite types in eclogite-facies rocks from the Sesia-Lanzo Zone (Western Alps, Italy) were combined with SIMS boron isotope analyses as well as boron (B) and lithium (Li) concentration data to link geochronological information with constraints on fluid-rock interaction. In weakly deformed samples, apparent 40Ar/39Ar ages of phengite cores span a range of ˜20 Ma, but inverse isochrons define two distinct main high-pressure (HP) phengite core crystallization periods of 88-82 and 77-74 Ma, respectively. The younger cores have on average lower B contents (˜36 μg/g) than the older ones (˜43-48 μg/g), suggesting that loss of B and resetting of the Ar isotopic system were related. Phengite cores have variable δ11B values (-18‰ to -10‰), indicating the lack of km scale B homogenization during HP crystallization. Overprinted phengite rims in the weakly deformed samples generally yield younger apparent 40Ar/39Ar ages than the respective cores. They also show variable effects of heterogeneous excess 40Ar incorporation and Ar loss. One acceptable inverse isochron age of 77.1 ± 1.1 Ma for rims surrounding older cores (82.6 ± 0.6 Ma) overlaps with the second period of core crystallization. Compared to the phengite cores, all rims have lower B and Li abundances but similar δ11B values (-15‰ to -9‰), reflecting internal redistribution of B and Li and internal fluid buffering of the B isotopic composition during rim growth. The combined observation of younger 40Ar/39Ar ages and boron loss, yielding comparable values of both parameters only in cores and rims of different samples, is best explained by a selective metasomatic overprint. In low permeability samples, this overprint caused recrystallization of phengite rims, whereas higher permeability in other samples led to complete recrystallization of phengite grains. Strongly deformed samples from a several km long, blueschist-facies shear zone contain mylonitic

  13. 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.

  14. 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

  15. Evidence for Extended (5-10 Ma) Emplacement of Ferrar Dolerite from 40Ar-39Ar Geochronology

    NASA Astrophysics Data System (ADS)

    Knight, K. B.; Renne, P. R.

    2005-12-01

    Antarctica's Ferrar large igneous province (LIP) is dominated by dolerites linked in time, space and geochemical character with the nearby Kirkpatrick basalts, and more broadly, with magmas preserved in Dronning Maud Land (Antarctica), the Karoo LIP (southern Africa) and the Chon Aike LIP (Patagonia). Together, these magmatic bodies represent an estimated emplacement volume of ~3 x 106 km3, correlated with the break-up and post-break-up activity of the Gondwana supercontinent in the middle Jurassic. Plagioclase separates from 9 Ferrar dolerites exposed across the Dry Valleys of the Transantarctic Mountains were analyzed using the 40Ar-39Ar method. Detailed apparent age spectra reveal a range of well-defined plateau ages. Four samples yield indistinguishable ages with a weighted mean age 180.0 ± 0.5 Ma. The remaining 5 samples yield statistically distinct and significantly younger plateau ages, decreasing to as young as ca. 173 Ma. These ages may represent emplacement and/or cooling ages (below ~300°C, the approximate closure temperature for argon in plagioclase). Spectacular exposure and preservation of the Ferrar intrusive complex has led to increased interest in development of models of magma mush formation and emplacement. These data provide quantitative constraints for models of magma emplacement for the Ferrar, suggesting either the presence of long-lived magma chambers or sufficient thermal insulation or input to allow heterogeneous slow cooling of the Ferrar dolerites. The suggestion of protracted (5-10 Ma) intrusive activity supports previous geochronology data reported from Dronning Maud Land and the Ferrar, as well as the broad temporal picture arising around the time of Gondwana break-up.

  16. 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.

  17. 40Ar- 39Ar geochronology of the charnockites and granulites of the Kan Nack complex, Kon Tum Massif, Vietnam

    NASA Astrophysics Data System (ADS)

    Maluski, Henri; Lepvrier, Claude; Leyreloup, André; Tich, Vu Van; Thi, Phan Truong

    2005-07-01

    The Truong Son Belt forms the eastern rim of the Indochina Block in Southeast Asia. The age of the metamorphism, mainly along NW-SE mylonitic shear zones that affects this belt, has been formerly determined at about 240-250 Ma. This age corresponds to the Indosinian tectonometamorphic episode. The Kon Tum Massif, situated to the south of this belt, comprises high-temperature rocks, the Kan Nack Complex, including charnockites and granulites. The main charnockitic outcrops, restricted to the Song Ba Valley, establish the intrusive nature of these magmatic rocks within granulite facies material. Basic charnockitic rocks are mainly quartz enderbites to norites and hornblende-pyroxene granulite facies rocks. The 40Ar- 39Ar age of intrusion-cooling of charnockitic magmas is determined from primary magmatic biotites at about 245 Ma. In the east of the Kan Nack Complex some granulite facies rocks exhibit relicts of primary granulite facies parageneses, whereas others show evidence of overprinting by a retrogressive low-grade metamorphism. Ar-Ar dating confirm this evolution, giving ages of 400 Ma for primary relict granulite facies phases and 260-270 Ma from the most retrogressed samples establishing the youngest limit for the granulite facies metamorphism. Granulites intruded by charnockites in the Song Ba Valley yield ages of about 250 Ma, equivalent to the ages of the charnockites, and have evidently been completely reset by these high temperature intrusions. Therefore, the Kan Nack Complex of the Kon Tum Massif is not an independent unit with respect to the Indosinian orogen, but represents the deep-crustal part of this belt.

  18. Tectono-thermal evolution of the Western Pamir Mountains, using 40Ar/39Ar thermochronology on modern river sands

    NASA Astrophysics Data System (ADS)

    Lukens, C. E.; Carrapa, B.; Schoenbohm, L. M.; Singer, B. S.; Jicha, B.

    2009-12-01

    The Pamir Mountains of Central Asia are among the highest mountains on Earth, resulting from Indo-Asia collision in the early Cenozoic. Other processes such as intra-continental subduction are currently at work beneath the western Pamir, and have likely contributed to the late-stage tectonic evolution of the orogen. Timing and mechanisms related to both collision and intra-continental subduction are poorly constrained. One way to investigate the timing of tectono-thermal events is by applying thermochronology to sediment in rivers currently draining the mountain belt. The age of material removed from the source reflects provenance from different structural terranes and cooling related to tectonic and erosional processes. New multi-crystal laser fusion 40Ar/39Ar thermochronologic data from detrital white mica in samples from five rivers that drain the Western Pamir in Tajikistan (Vanch, Yazgulem, Bartang, Gunt, and Murghab Rivers) give ages significantly younger than previously published thermo- and geo-chronologic data from the magmatic belt. Apparent ages range from 14 to 348 Ma with distinctive populations of young crystals at 15, 17, 26, and 39 Ma. These new data suggest 1) the presence of undocumented mid-late Cenozoic plutons within the western Pamirs, and/or 2) deep mid-late Cenozoic exhumation. The fact that previously reported crystallization ages of plutonic rocks within the Pamir are all older than 41 Ma suggests that white mica in these rivers represent exhumation signals rather than crystallization ages; this will be further tested by U-Pb dating of zircons from the same sediment samples. Moreover, rivers draining separate terranes within the source yield discrete age populations for the youngest crystals, suggesting reactivation of terrane-bounding Mesozoic sutures during the early-mid Miocene. Though open questions remain on the mechanism responsible for Miocene exhumation, processes related to intra-continental subduction are favored.

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

    NASA Astrophysics Data System (ADS)

    Koppers, A. A. P.

    2014-12-01

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

  20. 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.

  1. 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.

    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.

  2. 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

  3. {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.

  4. Paleomagnetism and 40Ar / 39Ar Geochronology of Yemeni Oligocene volcanics: Implications for timing and duration of Afro-Arabian traps and geometry of the Oligocene paleomagnetic field

    NASA Astrophysics Data System (ADS)

    Riisager, Peter; Knight, Kim B.; Baker, Joel A.; Ukstins Peate, Ingrid; Al-Kadasi, Mohamed; Al-Subbary, Abdulkarim; Renne, Paul R.

    2005-09-01

    A combined paleomagnetic and 40Ar / 39Ar study was carried out along eight stratigraphically overlapping sections in the Oligocene Afro-Arabian flood volcanic province in Yemen (73 sites). The composite section covers the entire volcanic stratigraphy in the sampling region and represents five polarity zones that are correlated to the geomagnetic polarity time scale based on 40Ar / 39Ar ages from this and previous studies. The resulting magnetostratigraphy is similar to that of the conjugate margin in Ethiopia. The earliest basaltic volcanism took place in a reverse polarity chron that appears to correspond to C11r, while the massive rhyolitic ignimbrite eruptions correlated to ash layers in Oligocene Indian Ocean sediment 2700 km away from the Afro-Arabian traps, appear to have taken place during magnetochron C11n. The youngest ignimbrite was emplaced during magnetochron C9n. Both 40Ar / 39Ar and paleomagnetic data suggest rapid < 1 Ma eruption of the basal basalt units and punctuated eruption of the upper silicic units over a duration potentially as long as 3 Ma with interspersed eruptive hiatuses. Eruption of the basal basalts may have preceded the Oi2 cooling event. The paleomagnetic pole λ = 74.2°N, φ = 249.1°E (A95 = 3.6°; N = 48) is supported by a positive reversal test. Paleosecular variation, estimated as the angular standard deviation of the VGP distribution 14.2° + 2.3° / - 1.7°, is close to expected, suggesting that the paleomagnetic pole represents a time-averaged field. The pole is in excellent accord with the paleomagnetic poles obtained from the Ethiopian part of the Afro-Arabian province, after closure of the Red Sea. By analyzing Afro-Arabian paleomagnetic data in conjunction with contemporaneous paleomagnetic poles available from different latitudes we argue that the Oligocene paleomagnetic field was dominated by the axial dipole with insignificant non-dipole field contributions.

  5. 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

  6. 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.

  7. 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

  8. 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.

  9. 40Ar/39Ar and U/Pb chronology of the Green Lake Pluton (Eastern Sierra Nevada, California)

    NASA Astrophysics Data System (ADS)

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

    2003-12-01

    The aim of this study is to understand the cooling (and emplacement) history of the Cretaceous Tuolumne Intrusive Suite (TIS, eastern Sierra Nevada, California) by studying its thermal effects on adjacent plutonic bodies such as the Green Lake pluton (GLP), which is a small granodioritic intrusion 5 km to the east of the TIS (ca. 5 km in diameter). Nine samples were collected from the center of the GLP towards the TIS. U/Pb zircon single-crystal analyses display an upper intercept age of 168 +/- 5 Ma (uncertainties are given at the 2σ level). 40Ar/39Ar analyses were performed on two different biotite and hornblende grain size fractions (800-900 μ m and 150-180 μ m) from each sample. Step heating experiments on large biotite and hornblende as well as total fusion analyses on 20 (biotite) to 25 (hornblende) single grains yield the following ages (relative to FCs at 28.02 Ma): (1) Biotite plateau ages (100 to 95% 39Ar) display a trend between 118.2 +/- 0.8 Ma in the center of the GLP to 83.9 +/- 1.5 Ma closest to the TIS. (2) Total fusion analyses of the 150 to 180 μ m biotite fractions yield younger ages ranging from 89 to 82 Ma. We interpret these ages as the result of partial and/or total resetting of the K/Ar system at the time of TIS emplacement at around 90 Ma (Coleman et al, 2002). (3) Age spectra from large hornblende crystals are highly discordant due to "younger" (i.e. degassed) biotite inclusions. Hornblende (150 to 180 μ m) total fusion experiments display a wide range of ages (92 to 172 Ma) with most of the ages ranging from 164 to 170 Ma (Ca/K of 15, weighted mean age of 166.2 +/- 1.6 Ma). This age is in agreement with the above reported U/Pb age and interpreted to be the "true cooling age" of hornblende devoid of biotite inclusions and unaffected by the TIS intrusion. The results demonstrate the possibility for erroneous conclusions if techniques are used which fail to reveal these spatial complexities and partial resetting of the K/Ar system. The

  10. (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.

  11. (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

  12. 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.

  13. 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.

  14. New 40Ar/39Ar dating of the Clearwater Lake impact structures (Québec, Canada) - Not the binary asteroid impact it seems?

    NASA Astrophysics Data System (ADS)

    Schmieder, Martin; Schwarz, Winfried H.; Trieloff, Mario; Tohver, Eric; Buchner, Elmar; Hopp, Jens; Osinski, Gordon R.

    2015-01-01

    The two Clearwater Lake impact structures (Québec, Canada) are generally interpreted as a crater doublet formed by the impact of a binary asteroid. Here, arguments are presented that raise important questions about the proposed double impact scenario. New 40Ar/39Ar dating of two virtually fresh impact melt rock samples from the ⩾36 km West Clearwater Lake impact structure yielded two statistically robust Early Permian plateau ages with a weighted mean of 286.2 ± 2.2 (2.6) Ma (2σ; MSWD = 0.33; P = 0.57). In contrast, 40Ar/39Ar results for two chloritized melt rocks from the ∼26 km East Clearwater Lake impact structure produced disturbed age spectra suggestive of a distinct extraneous argon component. Although individually weakly robust, age spectra corrected for the trapped argon component and inverse isochron plots for the East Clearwater melt rocks consistently yielded apparent ages around ∼460-470 Ma. No Permian signal was found in either of these melt aliquots. Our new 40Ar/39Ar results reproduce earlier 40Ar/39Ar plateau ages (∼283 Ma and ∼465 Ma, respectively) for the two impact structures by Bottomley et al. (1990) and are in conflict with a previous, statistically non-robust Rb-Sr age of 287 [293] ± 26 Ma for East Clearwater. The combined cluster of apparent ages of ∼460-470 Ma, derived from four different samples across the impact melt sheet, is very unlikely to represent a 'false age effect' due to the incorporation of extraneous argon into the melt; instead, it strongly favors a Middle Ordovician age for the East Clearwater impact and impact-induced hydrothermal chloritization. Moreover, the Clearwater impact structures are characterized by different natural remanent magnetizations testifying to separate geologic histories, an effect unexpected in the case of a Permian double impact. Whereas the West Clearwater impact affected Ordovician carbonates incorporated into the impact breccia, drill core reports from the 1960s concluded that

  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 cooling history of the Albany Mobile Belt, Albany-Fraser Orogen, Western Australia

    NASA Astrophysics Data System (ADS)

    Scibiorski, Elisabeth; Tohver, Eric; Jourdan, Fred

    2013-04-01

    The Albany-Fraser Orogen of southwestern Australia is a Grenville-age orogenic belt that marks the suturing of the Yilgarn Craton of Western Australia to the Mawson Craton of South Australia and Antarctica. The Albany Mobile Belt is situated in the west of the orogen and consists of three geological domains: the Nornalup Zone, the Biranup Zone and the Northern Foreland. The crustal genesis and nature of boundaries between these domains is unknown. 40Ar/39Ar thermochronology of biotite and muscovite grains from a 250 km transect across all three domains in the Albany Mobile Belt is used to study the exhumation and cooling history of the amphibolite to granulite facies orogenic root. Previously published geochronological data dates peak amphibolite or granulite facies metamorphism in the Nornalup Zone, Biranup Zone and Northern Foreland at ca. 1170 Ma, ca. 1180 Ma and ca. 1210 - 1180 Ma respectively. All samples reported in this study yielded well defined plateau ages consistent with Stage II of the Albany-Fraser Orogeny (1215 - 1140 Ma). Four biotites from the Nornalup Zone give cooling ages ranging from 1144 ± 5 Ma to 1168 ± 5 Ma, one biotite from the Biranup Zone gives a cooling age of 1159 ± 5 Ma, and four muscovites from the Northern Foreland give statistically indistinguishable cooling ages ranging from 1157 ± 6 Ma to 1164 ± 5 Ma, with a weighted mean age of 1159 ± 6 Ma (P = 0.10). The new cooling ages imply that the three domains had been brought to a similar structural level (12 - 17 km depth) by ca. 1158 Ma, and have shared a common geological history since that time. This suggests that Stage II tectonic activity may have ended at ca. 1158 Ma in the Albany Mobile Belt, 20 Myr earlier than previously assumed. A cooling rate of 25°C/Myr for the Biranup Zone was calculated based on the 20 Myr interval between peak granulite-facies metamorphism and the cooling of the domain through the estimated biotite closure temperature (ca. 300°C) by ca. 1159 Ma

  17. 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.

  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. 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.

  20. [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.

  1. The Cambrian Kalkarindji Large Igneous Province: Extent and characteristics based on new 40Ar/ 39Ar and geochemical data

    NASA Astrophysics Data System (ADS)

    Evins, Lena Z.; Jourdan, Fred; Phillips, David

    2009-06-01

    The Early Cambrian Kalkarindji Continental Flood Basalt Province in northern Australia is an important Large Igneous Province (LIP) both in size and timing. Being the earliest Phanerozoic LIP it may have had a severe effect on the Early Cambrian biota. Here, we investigate the extent of this province by testing the hypothesis that the extensive Table Hill Volcanics in south-central Australia are a southern extension of the Kalkarindji province. The Table Hill Volcanics have long been considered coeval with the Antrim Plateau Volcanics and related volcanics in the Kalkarindji province; however precise age data is lacking. Here, we present new 40Ar/ 39Ar geochronological, and major and trace element data to investigate this possibility. One Table Hill sample yielded a plateau age of 504.6 ± 2.5 Ma (2 σ), within error of recently published age data from the Kalkarindji LIP. Samples from both the Table Hill Volcanics and the northern part of the province are low-Ti tholeiites with MgO-content ranging from 3 to 9 wt.% and all samples are highly enriched in incompatible elements compared to primitive mantle. This, coupled with a negative Nb anomaly, suggests crustal contamination at an early stage in magma evolution or a significant contribution from the sub-continental lithospheric mantle. Subsequent fractionation produced further elevation of the REE in the later stages of the eruptions. Subtle differences in some incompatible element ratios between the Table Hill Volcanics and the northern Kalkarindji indicate some variation likely related to the assimilated crustal component or minor mantle source heterogeneity. The new geochronology data and analogous geochemical signatures confirm that the Table Hill Volcanics are a southern counterpart of the Kalkarindji province. These new data permit the areal extent of the province to be estimated at > 2.1 × 10 6 km 2. This makes the Kalkarindji province one of the largest LIPs of the Phanerozoic. Volatile outgassing from a

  2. 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

  3. Millennial-scale phase relationships between ice-core and Mediterranean marine records: insights from high-precision 40Ar/39Ar dating of the Green Tuff of Pantelleria, Sicily Strait

    NASA Astrophysics Data System (ADS)

    Scaillet, S.; Vita-Scaillet, G.; Rotolo, S. G.

    2013-10-01

    With the advent of annually-resolved polar ice records extending back to 70 ka, marine and continental paleoclimate studies have now matured into a discipline where high-quality age control is essential for putting on an equal pace layer-counted timescale models and Late Quaternary sedimentary records. High-resolution U-Th dating of speleothem records and 40Ar/39Ar dating of globally recorded geomagnetic excursions have recently improved the time calibration of Quaternary archives, reflecting the cross-disciplinary effort made to synchronize the geologic record at the millennial scale. Yet, tie-points with such an absolute age control remain scarce for paleoclimatic time-series extending beyond the radiocarbon timescale, most notably in the marine record. Far-travelled tephra layers recorded both onland and offshore provide an alternative in such instance to synchronize continental and marine archives via high-resolution 40Ar/39Ar dating of the parent volcanic eruption. High-resolution 40Ar/39Ar data are reported herein for one such volcanic marker, the Green Tuff of Pantelleria and its Y-6 tephra equivalent recorded throughout the Central and Eastern Mediterranean. Published radiochronometric and δ18O orbitally-tied ages for this marker horizon scatter widely from about 41 ka up to 56 ka. Our new 40Ar/39Ar age at 45.7 ± 1.0 ka (2σ) reveals that previous estimates are biased by more than their reported errors would suggest, including recent orbital tuning of marine records hosting the tephra bed that are reevaluated in the context of this study. This improved estimate enables potential phase lags and leads to be studied between deep-sea and terrestrial archives with unrivaled (near-millennial) 40Ar/39Ar precision in the marine record.

  4. Timing and processes for exhumation of HP/LT rocks of the southern Brooks Range (AK): Insight from combined geochemistry and 40Ar/39Ar thermochronology of white mica

    NASA Astrophysics Data System (ADS)

    O'Brien, T.; Miller, E. L.; Grove, M. J.; Hayden, L. A.

    2015-12-01

    The obduction of an island arc onto the Arctic Alaska continental margin in the Jura-Cretaceous led to southward subduction of continental crust and high-pressure/low-temperature (HP/LT) epidote-blueschist facies metamorphism in the southern Brooks Range (BR). A regionally developed greenschist facies normal-sense shear zone system along the southern margin of the BR suggests that extensional faulting had an influential role in the exhumation of the metamorphic core. To better constrain the exhumation history of the metamorphic core of the BR, samples were collected from a N-S transect through the metamorphic core of the orogen. Electron microprobe (EMP) analyses of white micas reveal that they are composed of phengite (Si > 3.0 pfu) or a combination of phengite + paragonite. Si-content of phengites reveal a southward increase in Si from 3.1 to 3.4 pfu (corresponding to an increase in pressure). Si-contents in higher-P phengites are scattered, reflecting subsequent muscovite growth. The Si trend is matched by a southward increase in the 40Ar/39Ar total gas ages of white micas. Phengite from the north are characterized by younger ages (~115 Ma) and flatter 40Ar/39Ar spectra. Farther south, phengites and paragonites yield older 40Ar/39Ar ages. These samples yield convex, staircase 40Ar/39Ar spectra that initiate ~115-120 Ma and flatten-out ~130-138 Ma. Modeling using MacArgon proposes that an initial cooling of HP/LT metamorphism occurred ~130-138 Ma, recorded in the high-Si phengites and paragonites. Following initial cooling, modeling suggests HP/LT rocks stalled in the greenschist facies field before subsequent exhumation, resulting in the staircase 40Ar/39Ar spectra. Flatter 40Ar/39Ar spectra recorded by the northern samples and modeling of 40Ar/39Ar results from the southern samples suggest that these rocks from metamorphic core of the BR were exhumed to temperatures < 300°C by ~115 Ma.

  5. 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

  6. 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

  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. Timescales and mechanisms of plume-lithosphere interactions: 40Ar/ 39Ar geochronology and geochemistry of alkaline igneous rocks from the Paraná-Etendeka large igneous province

    NASA Astrophysics Data System (ADS)

    Gibson, S. A.; Thompson, R. N.; Day, J. A.

    2006-11-01

    We have determined high-precision 40Ar/ 39Ar ages for alkaline igneous rocks from the western margin of the Early-Cretaceous Paraná-Etendeka large igneous province (Paraguay). These show that small-fraction melt generation occurred beneath the region in two phases; at 145 Ma and 127.5 Ma, i.e. before and at the end of the 139-127.5 Ma Paraná-Etendeka flood-basalt eruptions. Previously published 40Ar/ 39Ar ages for alkaline igneous rocks on the proto-Atlantic coastal margins range from 134 to 128 Ma and indicate that small-fraction melt generation in the east of the province was either synchronous or slightly later than the main pulse of tholeiitic volcanism (between 134 and 132 Ma). Our new 40Ar/ 39Ar phlogopite ages confirm that: (i) the earliest melts associated with the initial impact of the Tristan plume were generated in the west of the Paraná-Etendeka large igneous province and (ii) igneous activity was long lived and immediately predates continental break-up. The Early-Cretaceous Paraguayan alkaline magmas are silica-undersaturated, enriched in incompatible-trace elements, have very-low initial ɛNd values and probably represent melts of phlogopite-bearing, carbonate-metasomatised peridotite in the subcontinental lithospheric mantle. Our simple one-dimensional, conductive-heating models suggest that the early-phase (145 Ma) alkaline magmas were emplaced on the margins of the Rio de La Plata craton at the time of sublithospheric impact of the proto-Tristan plume. The late phase (127.5 Ma) of Paraguayan alkaline magmatism is concentrated in an intra-cratonic rift zone and melt generation appears to have been triggered by lithospheric extension, perhaps facilitated by conductive heating and thermal weakening associated with the upwelling Tristan plume. The location and timing of both alkaline and tholeiitic melt generation in the Paraná-Etendeka province appear to have been significantly influenced by the non-uniform composition and thickness of the South

  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. 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.; 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

  11. Tectono-thermal evolution of the Palaeoproterozoic Granites-Tanami Orogen, North Australian Craton: Implications from hornblende and biotite 40Ar/39Ar geochronology

    NASA Astrophysics Data System (ADS)

    Li, Ben; Bagas, Leon; Jourdan, Fred

    2014-10-01

    The Palaeoproterozoic Granites-Tanami Orogen (GTO) hosts a number of gold deposits located in the southern margin of the North Australian Craton. The major stratigraphic succession is the Palaeoproterozoic Tanami Group which is subdivided into the Dead Bullock Formation and conformably overlying Killi Killi Formation. New geochemical data for the ca. 1864 Coora and Groundrush dolerite sills in the Dead Bullock Formation suggests that they have the same characteristics with the enriched back-arc basin basaltic rocks from the former Stubbins Formation, such as tholeiitic affinity, high TiO2 contents (0.94 to 1.24 wt.%) and low Mg# (41-45), slightly enriched LILE (Rb, Th, U, and K), weakly depleted HFSE (Nb, Ta), and relatively flat REE patterns. Their magma was generated by high degree decompressional melting (5-15%) of the asthenosphere source with an input of 3-4% subduction-related material. The petrological and geochemical similarities of igneous rocks provide new evidence for the assignment of the ca. 1864 Ma former Stubbins Formation and the Mount Charles Formations in the Dead Bullock Formation of the Tanami Group. These conclusions confirmed that the extensive Palaeoproterozoic Tanami Group was deposited in a back-arc basin environment. Hornblende and biotite 40Ar/39Ar geochronological study identified three major tectono-thermal events in the GTO since the deposition of the Dead Bullock Formation. The ca. 1840 Ma 40Ar/39Ar cooling age of metamorphic hornblende from the Coora and Groundrush dolerite sills in the Dead Bullock Formation provided precise age constraint for the first Palaeoproterozoic tectono-thermal event during the evolution of the Granites-Tanami back-arc basin. This age is highly consistent with the ca. 1850-1840 Ma subduction and peak metamorphism events in the North Australian Craton (NAC) associated with the Halls Creek Orogeny in the Halls Creek Orogen, and the Tennant Orogeny in the Tennant Creek Inlier. The 40Ar/39Ar age of 1753 ± 8 Ma

  12. 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.

  13. 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.

  14. 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

  15. 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

  16. 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.

  17. 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.

  18. 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

  19. 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

  20. 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.

  1. Cooling pattern and mineralization history of the Saint Sylvestre and western Marche leucogranite pluton, French Massif Central: I. 40Ar/39Ar isotopic constraints

    NASA Astrophysics Data System (ADS)

    Scaillet, S.; Cheilletz, A.; Cuney, M.; Farrar, E.; Archibald, D. A.

    1996-12-01

    The Saint Sylvestre-western Marche Leucogranite Complex (LC) is a composite pluton emplaced 324 m.y. ago in the Variscan continental crust of the northwestern Limousin, French Massif Central. Petrographic, structural, and new isotopic data are used to constrain the cooling pattern of the pluton in the temperature range for Ar retention in muscovite (Part I) and the genetic relationships with the subsequent uranium mineralization postdating the time of intrusion by 40-50 m.y.(Part II). A suite of thirty-seven muscovite and nine biotite concentrates selected over the entire plutonic complex were analyzed by the 40Ar/39Ar technique. Muscovite is characterized by well-defined plateau ages ranging between 314 and 301 Ma, while biotite gives less well-behaved age spectra with apparent ages varying between 320 and 300 Ma. The muscovite age variations are systematic and clearly associated with regional trends at the scale of the pluton. Field, petrographic, gravimetric, and structural relationships indicate that the age pattern is controlled by the intrusion shape and the regional, post-cooling fault network cutting through the laccolith. When restored to their approximate pre-faulting geometry, the age variations are shown to correlate positively with sample elevation, with a top-to-base difference as large as 10-14 m.y. over a mean intrusion thickness of ∼3 km. This pattern is interpreted to reflect cooling driven by erosion and exhumation of the thickened Variscan crust during the Westphalian and is confirmed by two-dimensional thermal modelling (Part II). The data show that 40Ar/39Ar mica ages can postdate the time of crystallization of the host pluton by up to 23 Ma depending on the level sampled at the surface; they warn against interpreting K/Ar mica ages from similar, low-relief igneous settings as closely postdating intrusive events without a precise knowledge of the thermal structure, cooling mechanism, and post-cooling faulting of the pluton. In turn, provided

  2. 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.; 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

  3. 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.

  4. 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

  5. 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

  6. 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

  7. Paleomagnetism and 40ar/39ar Geochronology of the Plio-Pleistocene Boring Volcanic Field: Implications for the Geomagnetic Polarity Time Scale

    NASA Astrophysics Data System (ADS)

    Hagstrum, J. T.; Fleck, R. J.; Evarts, R. C.; Calvert, A. T.; Conrey, R. M.

    2014-12-01

    The Boring volcanic field (BVF) in western Oregon and Washington has been the subject of a recently completed investigation that included detailed geologic mapping, petrographic and geochemical analyses, and 40Ar/39Ar geochronologic and paleomagnetic studies. At least 80 monogenetic volcanic centers compose the BVF, each of which erupted small volumes of magma ranging from basalt to mafic andesite over short intervals of time. More than 140 40Ar/39Ar determinations for lava flows and intrusions in the BVF range in age from ~3100 ka to ~60 ka. Oriented samples for paleomagnetic analysis were collected at an equivalent number of localities (>160) coincident with, or within the same unit proximal to, the geochronologic sampling sites. Based on the frequency distribution of ages, the most significant episodes of Boring volcanism occurred between 2700 and 2200 ka, 1700 and 500 ka, and 350 and 60 ka. A systematic determination of the BVF's eruptive history was undertaken mainly to assess its anomalous neotectonic setting west of the Cascade arc axis, as well as the magnitude of its concomitant volcanic hazards within the greater Portland and Vancouver metropolitan areas. Our paleomagnetic and geochronologic data, however, also have significant implications for the timing of geomagnetic field reversals and excursions during the Late Pliocene and Pleistocene epochs. The BVF data are more numerous higher in the section, and they capture transitional fields at two polarity boundaries allowing precise age determinations to be made for these reversals: the Brunhes-Matuyama transition is thus dated at 773±5 ka, and the upper Jaramillo-Matuyama transition at 973±6 ka. The lower Jaramillo-Matuyama transition occurred prior to 1068±8 ka, and the normal Cobb subchron must have occurred between reversed-polarity Matuyama flows dated at 1159±14 ka and 1207±6 ka. The lower Olduvai-Matuyama transition occurred prior to 1927±4 ka, and the Matuyama-Gauss transition prior to 2616

  8. 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

  9. Episodic rapid uplift in the Himalaya revealed by sup 40 Ar/ sup 39 Ar analysis of detrital K-feldspar and muscovite, Bengal fan

    SciTech Connect

    Harrison, T.M.; Copeland, P. )

    1990-04-01

    Detrital K-feldspar and muscovite samples from Ocean Drilling Program Leg 116 cores have been dated by the {sup 40}Ar/{sup 39}Ar technique and have depositional ages from 0 to 18 Ma. From 4 to 13 individual K-feldspars and 1 to 12 individual muscovites have been dated from 7 stratigraphic levels. In every level at least one K-feldspar and one muscovite yielded a minimum age identical, within uncertainty, to the age of deposition. These results indicate that a significant portion of the material in the Bengal fan is first-cycle detritus derived from the Himalaya. Therefore, the substantial amount of sediment deposited in the distal fan in early to middle Miocene time can be ascribed to a significant pulse of uplift and erosion in the collision zone at this time. Moreover, these data indicate that throughout the Neogene, some part of the Himalayan orogen was undergoing rapid erosion (1 to 10 mm/yr); this erosion must have been less than or equal to uplift relative to sea level. The lack of granulite facies rocks in the eastern Himalaya and Tibetan plateau suggests to us that very rapid uplift must have been distributed in brief pulses over different parts of the mountain belt. These data are incompatible with tectonic models in which the Himalaya and Tibetan plateau are uplifted either uniformly over the past 40 m.y. or mostly within the past 2 to 5 m.y.

  10. 40Ar/39Ar dating of a Langhian biotite-rich clay layer in the pelagic sequence of the Cònero Riviera, Ancona, Italy

    NASA Astrophysics Data System (ADS)

    Mader, Dieter; Montanari, Alessandro; Gattacceca, Jérôme; Koeberl, Christian; Handler, Robert; Coccioni, Rodolfo

    2001-12-01

    A nearly complete and undisturbed Miocene carbonate sequence is present in the easternmost part of the Umbria-Marche basin, Italy, which is ideal for detailed and integrated stratigraphic investigations of the Miocene Epoch. In this study, we were trying to obtain evidence for the presence or absence of distal ejecta from the 15 Ma Ries impact structure in southern Germany, located about 600 km to the north-northwest of the Umbria-Marche basin. The first step is to find coeval strata in the Umbria-Marche sequence. At the La Vedova section, Cònero Riviera, we dated a volcaniclastic biotite-rich clay layer, the Aldo Level, which is situated within planktonic foraminiferal Zone N8, at 14.9±0.2 Ma, using the 40Ar/39Ar method. Together with detailed geologic and stratigraphic information about the Aldo Level, the resulting age can be used confidentially to calibrate the Langhian stage. Besides providing new constraints on Miocene geochronology, this age can now be used for impact stratigraphic studies. To directly correlate the biotite ages of the La Vedova section with rocks from the Ries impact event, Ries impact glass was also analyzed and found to be coeval. Although unrelated to this impact event, the biotite-rich clay layer should help in the search for evidence of distal ejecta related to the Ries crater.

  11. 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

  12. 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.

  13. Growth of mica porphyroblasts under low-grade metamorphism - A Taiwanese case using in-situ40Ar/39Ar laser microprobe dating

    NASA Astrophysics Data System (ADS)

    Chen, Chih-Tung; Chan, Yu-Chang; Lo, Ching-Hua; Lu, Chia-Yu

    2016-11-01

    Mica porphyroblasts, a common metamorphic microstructure, are analyzed in the slate belt of northern Taiwan where large fish-like growths are found within a meta-pyroclastics. With constraints on the time-temperature history from deposition through peak metamorphic state to exhumation, in-situ40Ar/39Ar laser microprobe dating was carried out on muscovite and corrensite fibers of mm-scale mica porphyroblasts grown on a pressure-solution seam. Because the peak metamorphic temperature and the porphyroblast formation condition (∼250 °C) remained far below the closure temperature of the K-Ar radioisotope system in muscovite, and the absence of muscovite in the mafic protolith, the dating results likely document the growth of the mica porphyroblast fabrics. The syn-kinematic nature of the analyzed porphyroblasts is confirmed by the ∼6 to ∼2.5 Ma growth ages, suggesting that the host rock was continuously deformed during the earlier two-thirds of the Taiwan Orogeny. The pattern of fiber growth, in contrast to outward-decreasing ages normally observed in peripheral recrystallization, appears random and resembles void fills in boudin openings. We postulate that syntaxial crack-seal following tensile micro-boudinage, along with slips on sub-grain boundaries, as a viable mechanism for the development of mica porphyroblasts and fish especially in lower-grade metamorphic rocks.

  14. 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.

  15. [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.

  16. [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.

  17. Preservation of sub-microscopic structural relicts in micas from the Gran Paradiso Massif (Western Alps): Implications for 40Ar-39Ar geochronology

    NASA Astrophysics Data System (ADS)

    Beltrando, Marco; Di Vincenzo, Gianfranco; Ferraris, Cristiano

    2013-10-01

    Permian igneous biotite and white mica that were re-heated at 500-550 °C in the Middle-Late Eocene were investigated by laser step-heating and in situ 40Ar-39Ar techniques, in conjunction with scanning electron microscopy (SEM), electron microprobe (EMP) and transmission electron microscopy (TEM) to assess the influence of short-lived metamorphism on K-Ar systematics. Large intra- and inter-grain age variations, ranging from the Triassic to the Late Eocene, are primarily related to the extent of metamorphic re-equilibration. Brown biotite porphyroclasts from weakly re-equilibrated samples are characterized by core-to-rim zoning in major element composition, mineral structure and 40Ar/39Ar ratios. Titanium concentration and Fe/(Mg + Fe) ratios decrease from crystal cores towards the rims, which are compositionally indistinguishable from green biotite aggregates in metamorphic coronas. Compositional changes are coupled with structural modifications towards the crystal edges, where the disordered igneous 1M-2M1 stacks are replaced topotactically by highly ordered metamorphic 1M polytypes. Apparent ages up to ∼66 Ma were determined in crystal cores by in situ laser-probe analyses, while ages as young as ∼45 Ma are typical of crystal rims. Step-heating experiments of these zoned biotites yielded a discordant saddle-shaped age spectrum, with a concordant central segment yielding an error-weighted mean age of 44.9 ± 0.3 Ma. Biotite porphyroclasts from a more re-equilibrated specimen gave markedly different results, with uniform major elements compositional profiles, homogeneous argon distribution throughout the crystals and a flat age spectrum at 36.5 ± 0.3 Ma. TEM investigations of these optically homogeneous porphyroclasts revealed that the original igneous structure had been replaced by highly ordered 1M polytypes during Alpine metamorphism. White mica yielded comparable results, with spot ages up to ∼218 Ma in domains characterized by high Na/(Na + K) ratios

  18. 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.

  19. "New 40Ar/39Ar dating of the Grande Ronde lavas, Columbia River Basalts, USA: Implications for duration of flood basalt eruption episodes" by Barry et al. (2010) - Discussion

    NASA Astrophysics Data System (ADS)

    Baksi, Ajoy K.

    2012-08-01

    Barry et al. (2010) presented 40Ar/39Ar ages to argue for a very limited duration (~ 0.4 m.y.) for the eruption of the Grande Ronde lavas. Critical examination of their data sets shows (1) a number of their plateau ages are statistically invalid and (2) many of their groundmass matrix samples were altered, and did not yield valid estimates of the time of crystallization. In general, their ages err on the young side, reflecting partial 40Ar * loss, due to alteration, and there is no compelling evidence therein, to argue for very limited duration for the Grande Ronde Basalt. The Imnaha, Grande Ronde and Wanapum Basalt erupted within ~ 0.7 m.y., starting at ~ 16.4 Ma. By contrast, the "ages" of Barry et al. (2010) span the interval ~ 16.6 to ~ 14.7 Ma and, in some instances, are out of stratigraphic sequence.

  20. 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

  1. 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.

  2. 40Ar-39Ar laser dating of ductile shear zones from central Corsica (France): Evidence of Alpine (middle to late Eocene) syn-burial shearing in Variscan granitoids

    NASA Astrophysics Data System (ADS)

    Di Vincenzo, Gianfranco; Grande, Antonietta; Prosser, Giacomo; Cavazza, William; DeCelles, Peter G.

    2016-10-01

    The island of Corsica (France) plays a central role in any reconstruction of Western Mediterranean geodynamics and paleogeography but several key aspects of its geological evolution are still uncertain. The most debated topics include the interpretation of the Corsican orogen as the result of an east- or west-directed subduction, and the actual involvement of the Variscan basement of Corsica in the Alpine orogenic cycle. This study integrates 40Ar-39Ar laserprobe, mesostructural, microtextural, and microchemical analyses and places relevant constraints on the style, P-T conditions, and timing of Alpine-age, pervasive ductile shear zones which affected the Variscan basement complex of central Corsica, a few kilometers to the west of the present-day front of the Alpine nappes. Shear zones strike ~ NNE-SSW, dip at a high angle, and are characterized by a dominant sinistral strike-slip component. Two of the three investigated shear zones contain two texturally and chemically resolvable generations of white mica, recording a prograde (burial) evolution: (1) deformed celadonite-poor relicts are finely overgrown by (2) a celadonite-rich white mica aligned along the main foliation. White mica from a third sample of another shear zone, characterized by a significantly lower porphyroclast/matrix ratio, exhibits a nearly uniform high-celadonite content, compositionally matching the texturally younger phengite from the nearby shear zones. Mineral-textural analysis, electron microprobe data, and pseudosection modeling constrain P-T conditions attained during shearing at ~ 300 °C and minimum pressures of ~ 0.6 GPa. In-situ 40Ar-39Ar analyses of coexisting low- and high-celadonite white micas from both shear zones yielded a relatively wide range of ages, ~ 45-36 Ma. Laser step-heating experiments gave sigmoidal-shaped age profiles, with step ages in line with in-situ spot dates. By contrast, the apparently chemically homogenous high-celadonite white mica yielded concordant in

  3. 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

  4. Metallogenic features of Miocene porphyry Cu and porphyry-related mineral deposits in Ecuador revealed by Re-Os, 40Ar/39Ar, and U-Pb geochronology

    NASA Astrophysics Data System (ADS)

    Schütte, Philip; Chiaradia, Massimo; Barra, Fernando; Villagómez, Diego; Beate, Bernardo

    2012-04-01

    Mineralization and alteration events at ten Miocene porphyry Cu and porphyry-related epithermal mineral deposits in southern, central, and northern Ecuador were dated by means of molybdenite Re-Os, biotite and alunite 40Ar/39Ar, and titanite U-Pb geochronology. Most of these hydrothermal events show a spatio-temporal correlation with porphyry intrusion emplacement as constrained by zircon U-Pb ages. The total age range for these events spans the 23.5-6.1 Ma period, without displaying systematic along- or across-arc age distribution trends. While epithermal deposits tend to be spatially associated with volcanic rocks of a similar age, porphyry Cu deposits in Ecuador are frequently spatially associated with deeper-seated basement units and batholith-scale precursor intrusive systems assembled over ≥5 m.y. time periods. In most cases, formation of the porphyry Cu deposits is related to the youngest magmatic (-hydrothermal) event in a given area, postdating batholith construction at a regional scale. The majority of Miocene deposits occurs in southern Ecuador where areally extensive, post-mineralization (late Miocene to recent) volcanic sequences with the potential to conceal mineralization at depth are lacking. Only few Miocene deposits occur in northern-central Ecuador, where they mainly crop out in the Western Cordillera, west of the productive present-day volcanic arc. The surface distribution of post-mineralization arc volcanism reflects along-arc variations in subducting slab geometry. Porphyry Cu and epithermal deposits in Ecuador define a Miocene metallogenic belt broadly continuous with its coeval counterpart in northern-central Peru. Although both belt segments were formed in an overall similar tectonomagmatic and metallogenic setting, their respective metal endowments differ significantly.

  5. 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.

  6. 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

  7. An 40Ar/39Ar geochronology on a mid-Eocene igneous event on the Barton and Weaver peninsulas: Implications for the dynamic setting of the Antarctic Peninsula

    NASA Astrophysics Data System (ADS)

    Wang, Fei; Zheng, Xiang-Shen; Lee, Jong I. K.; Choe, Won Hie; Evans, Noreen; Zhu, Ri-Xiang

    2009-12-01

    The genesis of basaltic to andesitic lavas, mafic dikes, and granitoid plutons composing the subaerial cover on the Barton and Weaver peninsulas, Antarctica, is related to arc formation and subduction processes. Precise dating of these polar rocks using conventional 40Ar/39Ar techniques is compromised by the high degree of alteration (with loss on ignition as high as 8%). In order to minimize the alteration effects we have followed a sample preparation process that includes repeated acid leaching, acetone washing, and hand picking, followed by an overnight bake at 250°C. After this procedure, groundmass samples can yield accurate age plateaus consisting of 70%-100% of the total 39Ark released using high-resolution heating schedules. The different rock types studied on the Barton and Weaver peninsulas yielded almost coeval ages, suggesting a giant igneous event in the Weaver and Barton peninsulas at 44.5 Ma. A compilation of newly published ages indicate that this event took place throughout the whole South Shetland Islands, suggesting a dynamic incident occurred at this stage during the arc evolution history. We related this igneous event to a mantle delamination mechanism during Eocene times. The delamination process began at ˜52 Ma, and the resultant upwelling of asthenosphere baffled the subduction of Phoenix plate, causing an abrupt decrease in convergence rate. Then multiple magmatic sources were triggered, resulting in a culminating igneous activity during 50-40 Ma with a peak at ˜45 Ma along the archipelago. The delamination also caused the extension regime indicated by the dike swarm, plugs and sills all over the archipelago, and the uplift of Smith metamorphic complex and Livingston Island. Delamination process may have finished at some time during 40-30 Ma, leaving a weak igneous activity at that stage and thereafter. The convergence rate then recovered gradually, as indicated by the magnetic anomaly identifications. This model is supported by seismic

  8. 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.

  9. 40Ar- 39Ar and K-Ar dating of K-rich rocks from the Roccamonfina Volcano, Roman comagmatic Region, Italy

    NASA Astrophysics Data System (ADS)

    Radicati di Brozolo, Filippo; Di Girolamo, Pio; Turi, Bruno; Oddone, Massimo

    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 2. The above series correspond to the High-K. Series (HKS) and Low-K Series (LKS) of APPLETON (1972), respectively. 40Ar- 39Ar 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 ago, 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 radiometric data, we conclude that Roccamonfina was active between 1.5 and 0.05 Ma ago, in excellent agreement with the stratigraphie evidence. In this same time span is concentrated the activity of all the centers of the Roman Region north of Naples.

  10. 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.

  11. 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.

  12. High-precision 40Ar/39Ar geochronology and the advent of North America’s Late Cretaceous terrestrial fauna

    PubMed Central

    Cifelli, Richard L.; Kirkland, James I.; Weil, Anne; Deino, Alan L.; Kowallis, Bart J.

    1997-01-01

    A densely sampled, diverse new fauna from the uppermost Cedar Mountain Formation, Utah, indicates that the basic pattern of faunal composition for the Late Cretaceous of North America was already established by the Albian-Cenomanian boundary. Multiple, concordant 40Ar/39Ar determinations from a volcanic ash associated with the fauna have an average age of 98.39 ± 0.07 million years. The fauna of the Cedar Mountain Formation records the first global appearance of hadrosaurid dinosaurs, advanced lizard (e.g., Helodermatidae), and mammal (e.g., Marsupialia) groups, and the first North American appearance of other taxa such as tyrannosaurids, pachycephalosaurs, and snakes. Although the origin of many groups is unclear, combined biostratigraphic and phylogenetic evidence suggests an Old World, specifically Asian, origin for some of the taxa, an hypothesis that is consistent with existing evidence from tectonics and marine invertebrates. Large-bodied herbivores are mainly represented by low-level browsers, ornithopod dinosaurs, whose radiations have been hypothesized to be related to the initial diversification of angiosperm plants. Diversity at the largest body sizes (>106 g) is low, in contrast to both preceding and succeeding faunas; sauropods, which underwent demise in the Northern hemisphere coincident with the radiation of angiosperms, apparently went temporarily unreplaced by other megaherbivores. Morphologic and taxonomic diversity among small, omnivorous mammals, multituberculates, is also low. A later apparent increase in diversity occurred during the Campanian, coincident with the appearance of major fruit types among angiosperms, suggesting the possibility of adaptive response to new resources. PMID:9326579

  13. Subduction controls on Miocene back-arc lavas from Sierra de Huantraico and La Matancilla and new 40Ar/39Ar dating from the Mendoza Region, Argentina

    NASA Astrophysics Data System (ADS)

    Dyhr, Charlotte T.; Holm, Paul M.; Llambías, Eduardo J.; Scherstén, Anders

    2013-10-01

    Back-arc volcanism in the western Argentinian provinces of Mendoza and Neuquén has been widespread from the Miocene to historic times. We present a detailed investigation of profiles through two of the major Miocene volcanic areas of the region, the neighboring Huantraico and La Matancilla plateaus, including new 40Ar/39Ar age results of major and trace elements as well as Nd, Sr and Pb isotopic data. Four million years of eruptions from 24.4 ± 0.3 Ma (2σ) of alkali olivine basalts with OIB-type incompatible trace element enrichments at La Matancilla (~ 36.50°S) provide evidence for the presence of back-arc mantle devoid of subduction-related components. In contrast, the lower Huantraico lavas (~ 37.30°S) require an atypical back-arc mantle, almost devoid of arc-like components (e.g. low La/Ta = 15-18 and Ba/La = 12-18), but with a more depleted isotopic signature (e.g. 87Sr/86Sr, 0.7033-0.7037) than observed elsewhere in the Andean back-arc. The Lower to Upper Series development in the Huantraico sequence represents a gradual change from basaltic to trachyandesitic back-arc lavas with a weak but temporally increasing arc geochemical signature (e.g. La/Ta = 15-21; Ba/La = 12-45), which is accompanied by Sr, Nd and Pb isotopic compositions approaching present day values of the Andes arc. The compositional change is accompanied by a gradually decreasing role for garnet in the mantle source, a decreasing degree of melting, but also simultaneously increasing influence from subducted fluids, probably as the slab geometry changes through time. The volcanism at Huantraico ceased when a flat slab was established around 15 Ma.

  14. 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.

  15. 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

  16. Matching conjugate volcanic rifted margins: 40Ar/ 39Ar chrono-stratigraphy of pre- and syn-rift bimodal flood volcanism in Ethiopia and Yemen

    NASA Astrophysics Data System (ADS)

    Ukstins, Ingrid A.; Renne, Paul R.; Wolfenden, Ellen; Baker, Joel; Ayalew, Dereje; Menzies, Martin

    2002-05-01

    40Ar/ 39Ar dating of mineral separates and whole-rock samples of rhyolitic ignimbrites and basaltic lavas from the pre- and syn-rift flood volcanic units of northern Ethiopia provides a temporal link between the Ethiopian and Yemen conjugate rifted volcanic margins. Sixteen new 40Ar/ 39Ar dates confirm that basaltic flood volcanism in Ethiopia was contemporaneous with flood volcanism on the conjugate margin in Yemen. The new data also establish that flood volcanism initiated prior to 30.9 Ma in Ethiopia and may predate initiation of similar magmatic activity in Yemen by ˜0.2-2.0 Myr. Rhyolitic volcanism in Ethiopia commenced at 30.2 Ma, contemporaneous with the first rhyolitic ignimbrite unit in Yemen at ˜30 Ma. Accurate and precise 40Ar/ 39Ar dates on initial rhyolitic ignimbrite eruptions suggest that silicic flood volcanism in Afro-Arabia post-dates the Oligocene Oi2 global cooling event, ruling out a causative link between these explosive silicic eruptions (with individual volumes ≥200 km 3) and climatic cooling which produced the first major expansion of the Antarctic ice sheets. Ethiopian volcanism shows a progressive and systematic younging from north to south along the escarpment and parallel to the rifted margin, from pre-rift flood volcanics in the north to syn-rift northern Main Ethiopian Rift volcanism in the south. A dramatic decrease in volcanic activity in Ethiopia between 25 and 20 Ma correlates with a prominent break-up unconformity in Yemen (26-19 Ma), both of which mark the transition from pre- to syn-rift volcanism (˜25-26 Ma) triggered by the separation of Africa and Arabia. The architecture of the Ethiopian margin is characterized by accumulation and preservation of syn-rift volcanism, while the Yemen margin was shaped by denudational unloading and magmatic starvation as the Arabian plate rifted away from the Afar plume. A second magmatic hiatus and angular unconformity in the northern Main Ethiopian Rift is evident at 10.6-3.2 Ma, and is

  17. 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

  18. 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.

  19. 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.

  20. [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.

  1. 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.

  2. 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.; 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

  3. 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

    NASA Astrophysics Data System (ADS)

    Maguire, Timothy J.; Volkert, Richard A.; Swisher, Carl, III; Sheridan, Robert E.

    2009-07-01

    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 CO 2 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

  4. 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

  5. 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

  6. 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

  7. Contrasting Oligocene and Miocene thermal histories from the hanging wall and footwall of the South Tibetan detachment in the central Himalaya from 40Ar/39Ar thermochronology, Marsyandi Valley, central Nepal

    NASA Astrophysics Data System (ADS)

    Coleman, Margaret E.; Hodges, Kip V.

    1998-10-01

    In the Marsyandi valley of central Nepal, a major strand of the South Tibetan detachment system, the 18-22 Ma Chame detachment, places epidote-amphibolite to amphibolite facies calc-silicate rocks and marbles of the Tibetan sedimentary sequence on amphibolite facies pelitic gneisses and calc-silicate rocks of the Greater Himalayan sequence. Although the resulting metamorphic discontinuity is minor and sometimes cryptic, 40Ar/39Ar thermochronologic results from the area reveal that the hanging wall and footwall of the detachment had distinctive thermal histories. Hanging wall phlogopites and biotites yield cooling ages of 27.1 - 29.9 Ma, compared with footwall biotite ages of 14.1 - 16.6 Ma. U-Pb monazite thermochronology demonstrates that the Greater Himalayan sequence experienced peak amphibolite facies conditions at approximately 22 Ma, but the 40Ar/39Ar results require hanging wall metamorphism to be an Oligocene (or older) phenomenon. These events are interpreted as representing the "Neohimalayan" and "Eohimalayan" metamorphic phases proposed by previous workers in the central Himalaya. Some of the dated hanging wall phlogopites grew synchronously with development of SW vergent macroscopic folds in the Tibetan sedimentary sequence, implying that Eohimalayan metamorphism was associated with an important phase of crustal shortening in this sector of the Himalaya. Despite the intensity of Neohimalayan metamorphism below the Chame detachment, evidence for Eohimalayan metamorphism and igneous activity is preserved in the footwall rocks of the Marsyandi drainage. Inherited approximately 35 Ma monazites of either metamorphic or igneous origin have been found in the upper Greater Himalayan sequence in this area, and one hornblende separate from the uppermost footwall yields a 40Ar/39Ar age of 30.0 ± 3.0 Ma. This hornblende date and a similar result from the same structural level in the nearby Kali Gandaki valley, if robust, suggest that the duration of the

  8. 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.

  9. 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.

  10. 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

  11. Magmatic-hydrothermal evolution of the Cretaceous Duolong gold-rich porphyry copper deposit in the Bangongco metallogenic belt, Tibet: Evidence from U-Pb and 40Ar/ 39Ar geochronology

    NASA Astrophysics Data System (ADS)

    Li, Jinxiang; Qin, Kezhang; Li, Guangming; Xiao, Bo; Zhao, Junxing; Chen, Lei

    2011-06-01

    The Duolong gold-rich porphyry copper deposit was recently discovered and represents a giant prospect (inferred resources of 4-5 Mt fine-Cu with a grade of 0.72% Cu; 30-50 t fine-gold with a grade of 0.23 g/t Au) in the Bangongco metallogenic belt, Tibet. Zircon SHRIMP and LA-ICP-MS U-Pb geochronology shows that the multiple porphyritic intrusions were emplaced during two episodes, the first at about 121 Ma (Bolong mineralized granodiorite porphyry (BMGP) and barren granodiorite porphyry (BGP)) and the second about 116 Ma (Duobuza mineralized granodiorite porphyry (DMGP)). Moreover, the basaltic andesites also have two episodes at about 118 Ma and 106 Ma, respectively. One andesite yields an U-Pb zircon age of 111.9 ± 1.9 Ma, indicating it formed after the multiple granodiorite porphyries. By contrast, the 40Ar/ 39Ar age of 115.2 ± 1.1 Ma (hydrothermal K-feldspar vein hosted in DMGP) reveals the close temporal relationship of ore-bearing potassic alteration to the emplacement of the DMGP. The sericite from quartz-sericite vein (hosted in DMGP) yields a 40Ar/ 39Ar age of 115.2 ± 1.2 Ma. Therefore, the ore-forming magmatic-hydrothermal evolution probably persisted for 6 m.y. Additionally, the zircon U-Pb ages (106-121 Ma) of the volcanic rocks and the porphyries suggest that the Neo-Tethys Ocean was still subducting northward during the Early Cretaceous.

  12. New Insights Into Volcanic Hazards in Western Mexico: Multiple Cone-Building Episodes at Arc Stratovolcanoes Revealed by 40Ar/39Ar Geochronology

    NASA Astrophysics Data System (ADS)

    Frey, H. M.; Lewis-Kenedi, K.; Lange, R. A.; Hall, C. M.; Delgado-Granados, H.

    2003-12-01

    The detailed eruptive histories of two andesitic stratocones, Volcáns Ceboruco and Tequila, in the western Mexican arc have been documented using 40Ar/39Ar geochronology. The volumes of these volcanoes were obtained with mapping, airphotos, and digital elevation models. The age and volume data constrain the rate and duration of major cone-building events, which bears on the longevity of the underlying upper-crustal magma chambers that fed the eruptions. The results indicate that at each stratovolcano there were two discrete cone-building events, separated by a hiatus. At V. Tequila, six samples from the edifice yielded dates (196 +/- 8, 196 +/- 19, 178 +/- 8, 191 +/- 13, 216 +/- 11, and 198 +/- 11 ka; errors are 1 sigma) with a mean eruption age of 196 +/- 12 ka. Thus the bulk of the main edifice ( ˜31 km3) erupted within 24 kyrs (at the 2 sigma level), leading to a cone-building rate of > 1.3 km3/kyr. After a hiatus of ˜110 kyrs, ˜14 km3 of andesite erupted along the NW and SE flanks of V. Tequila at 90 +/- 19 ka. The last activity at V. Tequila produced a ˜2 km3 parasitic cone at ˜60 ka. Since an eruption has not occurred in the last 60 kyrs, V. Tequila is often considered an extinct volcano. This may be the view held by the > 75,000 inhabitants of the town of Tequila located on the northern flanks. A similar history of two discrete cone-building events is found at V. Ceboruco, ˜75 km to the NW. Seven samples taken from various parts of the edifice, including the inner caldera wall, indicate an initial cone-building event at ˜45 ka in which ˜37 km3 of andesite erupted. After a hiatus of nearly 44 kyrs, a second eruptive period began ˜1000 years ago. The first eruption to occur after the hiatus was Plinian and released 3-4 km3 of dacite. In the last 1 kyr, 9.5 km3 of andesite and dacite erupted effusively, culminating in the historic 1870 flow. The sobering conclusion, in terms of volcanic hazards assessment, is that the only Plinian eruption to occur

  13. 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.

  14. 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.

  15. 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

  16. 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

  17. 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.

  18. 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

  19. U-Pb, Re-Os, and 40Ar/39Ar geochronology of the Nambija Au-skarn and Pangui porphyry Cu deposits, Ecuador: implications for the Jurassic metallogenic belt of the Northern Andes

    NASA Astrophysics Data System (ADS)

    Chiaradia, Massimo; Vallance, Jean; Fontboté, Lluis; Stein, Holly; Schaltegger, Urs; Coder, Joshua; Richards, Jeremy; Villeneuve, Mike; Gendall, Ian

    2009-05-01

    New U-Pb, Re-Os, and 40Ar/39Ar dates are presented for magmatic and hydrothermal mineral phases in skarn- and porphyry-related ores from the Nambija and Pangui districts of the Subandean zone, southeastern Ecuador. Nambija has been one of the main gold-producing centers of Ecuador since the 1980s due to exceptionally high-grade ores (average 15 g/t, but frequently up to 300 g/t Au). Pangui is a recently discovered porphyry Cu-Mo district. The geology of the Subandean zone in southeastern Ecuador is dominated by the I-type, subduction-related, Jurassic Zamora batholith, which intrudes Triassic volcanosedimentary rocks. The Zamora batholith is in turn cut by porphyritic stocks, which are commonly associated with skarn formation and/or porphyry-style mineralization. High precision U-Pb and Re-Os ages for porphyritic stocks (U-Pb, zircon), associated prograde skarn (U-Pb, hydrothermal titanite), and retrograde stage skarn (Re-Os, molybdenite from veins postdating gold deposition) of the Nambija district are all indistinguishable from each other within error (145 Ma) and indicate a Late Jurassic age for the gold mineralization. Previously, gold mineralization at Nambija was considered to be Early Tertiary based on K-Ar ages obtained on various hydrothermal minerals. The new Jurassic age for the Nambija district is slightly younger than the 40Ar/39Ar and Re-Os ages for magmatic-hydrothermal minerals from the Pangui district, which range between 157 and 152 Ma. Mineralization at Nambija and Pangui is associated with porphyritic stocks that represent the last known episodes of a long-lived Jurassic arc magmatism (˜190 to 145 Ma). A Jurassic age for mineralization at Nambija and Pangui suggests that the Northern Andean Jurassic metallogenic belt, which starts in Colombia at 3° N, extends down to 5° S in Ecuador. It also adds a new mineralization style (Au-skarn) to the metal endowment of this belt.

  20. 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.

  1. 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

  2. Evolution of West Rota Volcano in the Southern Mariana Arc: Evidence from Swathmapping, Seafloor Robotics, and 40Ar/39Ar Geochronology

    NASA Astrophysics Data System (ADS)

    Stern, R. J.; Tamura, Y.; Embley, R. W.; Ishizuka, O.; Merle, S.; Basu, N. K.; Kawabata, H.; Bloomer, S. H.

    2006-12-01

    West Rota volcano (WRV) is a large (25 km base), extinct submarine volcano in the southern Mariana arc. Its shallowest point lies 300m bsl; before caldera collapse WRV probably was a small island. Several bathymetric and sonar backscatter mapping campaigns reveal a large caldera, 6 x 10 km in diameter, with a maximum of 1km relief. WRV lies near the northern termination of a major NNE-trending normal fault. This and a second, parallel fault just west of the volcano separate uplifted, thick crust beneath the frontal arc to the east from subsiding, thin back-arc basin crust to the west. The youthful morphology of basin-margin faults indicate that the southern Mariana arc is tectonically active. Compared to other Mariana arc volcanoes, WRV is remarkable for 4 reasons: 1) It consists of a lower, predominantly andesite section overlain by a bimodal rhyodacite-basalt layered sequence; 2) Andesitic rocks are locally intensely altered and mineralized; 3) It has a large caldera; and 4) WRV is built on a major fault. Large calderas are commonly associated with volcanoes that erupt voluminous felsic lava (WRV rhyodacite pumice contains 72% SiO2). Such volcanoes are common in the Izu and Kermadec arcs but are otherwise unknown from the Marianas and other primitive, intra- oceanic arcs. WRV's caldera diameter of 6x10 km is large compared with Izu and Kermadec felsic calderas. Robotic seafloor examination has concentrated on understanding the volcanic history exposed in the caldera walls. One dive was carried out with ROPOS during TT167 in April 2004 (R785), followed by 4 dives with Hyperdolphin 3K during NT0517 in Oct. 2005 (HD482-484, 489). 40Ar/39Ar dating indicates that andesitic volcanism formed the lower volcanic section ca. 330,000-550,000 years ago, whereas eruption of the upper rhyodacites and basalts occurred 37,000-51,000 years ago. Four sequences of rhyodacite pyroclastics each are 20-75m thick, are unwelded, and show reverse grading, indicating submarine eruption of

  3. Tectonic and economic implications of trace element, 40Ar/ 39Ar and Sm-Nd data from mafic dykes associated with orogenic gold mineralisation in central Victoria, Australia

    NASA Astrophysics Data System (ADS)

    Bierlein, F. P.; Hughes, M.; Dunphy, J.; McKnight, S.; Reynolds, P.; Waldron, H.

    2001-08-01

    Mafic to intermediate dykes are spatially and temporally closely associated with major post-tectonic granitic complexes in the western Lachlan Orogen of SE Australia. These dykes, which range petrographically from basaltic to andesitic, are concentrated within several, north- to northwest-trending zones and were emplaced during two broad intervals of extensive magmatic activity during the Silurian-Devonian period. Geochemical and Sm-Nd isotopic characteristics of these mafic intrusives are consistent with their formation in a complex subduction-related tectonic setting. Interaction between mantle-wedge material, sinking oceanic crust and input from the overlying continental crust resulted in the petrological and geochemical variations displayed by these and more felsic dykes throughout the study region. Field evidence and 40Ar/ 39Ar data show that in the eastern part of the Stawell Zone and in the northwest portion of the Bendigo Zone, mafic dyke were intruded between 410 and 400 Ma (Late Silurian/Early Devonian). Further emplacement in the Bendigo Zone and the eastern part of the Melbourne Zone took place at between 375 and 365 Ma (Middle to Late Devonian). Episodic mantle-derived magmatism was possibly related to step-wise rollback, slab detachment or changes in the angle and rate of westward subduction in response to periodically occurring accretionary pulses. A close spatial and temporal relationship also exists between the dykes and orogenic gold mineralisation in the central Victorian gold province. Mafic to intermediate dykes both crosscut, and are host to, mineralisation in a number of goldfields. Although there is little evidence for a direct genetic association, the two processes are linked by the common utilisation of translithospheric structures, which facilitated the rapid ascent into shallow crustal levels of both mantle-derived magma and crustal-scale ore-forming fluid systems. Previous studies have suggested that transfer of heat into the crust via

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

    NASA Astrophysics Data System (ADS)

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

    2016-03-01

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

  5. Relief history and denudation evolution of the northern Tibet margin: Constraints from 40Ar/39Ar and (U-Th)/He dating and implications for far-field effect of rising plateau

    NASA Astrophysics Data System (ADS)

    Wang, Fei; Feng, Huile; Shi, Wenbei; Zhang, Weibin; Wu, Lin; Yang, Liekun; Wang, Yinzhi; Zhang, Zhigang; Zhu, Rixiang

    2016-04-01

    How does the rising Tibetan Plateau affect its peripheral region? The current understanding of the mechanism of orogenic plateau development is incomplete and thus no consensus yet exists in this regard. However, our new 40Ar/39Ar and (U-Th)/He dataset presented in this study may shed some light on this issue. 40Ar/39Ar dating, on two vertical transects from the massif between Nuomuhong and Golmud, indicates that the Eastern Kunlun Range was built-up and exhumated during the later Triassic initially, and a minimum overburden of ~ 11.7-14.0 km has been eroded since ~ 220 Ma. (U-Th)/He age-elevation relationships (AERs) indicate a rapid exhumation event at ~ 40 Ma following a long period of slow exhumation phase from late Mesozoic to early Eocene time. In this study, two scenarios - one assuming a single stage and the other assuming multiple stages of evolution history - are modeled. Modeling of a multiple stage scenario is reasonable and is able to reflect the "actual" situation, which reveals the entire denudation and relief history of the northern Tibet from late Mesozoic to the present time. After prolonged denudation before 50 Ma, a low topography (~ 0.17 times the relief of the present) developed by 50 Ma with an erosion rate of 0.013-0.013+ 0.025 mm/yr. The highest relief (~ 1.82 times the relief of the present) of the Cenozoic time came into being at 40 Ma with an erosion rate of 0.052 ± 0.025 mm/yr, which was possibly a result of the collision between India and Eurasia. Subsequently, the relief steadily decreased to the present level due to continued denudation. This suggests that deformation propagation from the continued convergence boundary between India and Eurasia was insignificant after the construction of the highest relief. This observation is broadly consistent with published accounts on the stratigraphic, cooling, and faulting histories of the northern Tibet margin.

  6. Timing of detachment faulting in the Bullfrog Hills and Bare Mountain area, southwest Nevada: Inferences from40Ar/39Ar, K-Ar, U-Pb, and fission track thermochronology

    USGS Publications Warehouse

    Hoisch, T.D.; Heizler, M.T.; Zartman, R.E.

    1997-01-01

    Crustal extension in the Bullfrog Hills and Bare Mountain area of southwest Nevada is associated with movement along a regional detachment fault. Normal faulting in the upper plate and rapid cooling (denudation) of the lower plate were coeval with Miocene silicic volcanism and with west-northwest transport along the detachment fault. A west-northwest progression of tilting along upper plate normal faults is indicated by ages of the volcanic rocks in relation to angular unconformities. Near the breakaway, tilting in the upper plate occurred between 12.7 and 11.6 Ma, continued less strongly past 10.7 Ma, and was over by 8.2 Ma. Ten to 20 km west of the breakaway, tilting occurred between 10.7 and 10.33 Ma, continued less strongly after 10.33 Ma, and was over by 8.1 Ma. The cooling histories of the lower plate metamorphic rocks were determined by thermochronologic dating methods: K-Ar and 40Ar/39Ar on muscovite, biotite, and hornblende, 40Ar/39Ar on K-feldspar, U-Pb on apatite, zircon, and sphene, and fission track on apatite, zircon, and sphene. Lower plate rocks 10 km west of the breakaway cooled slowly from Early Cretaceous lower-amphibolite facies conditions through 350??50?? to 300??50??C between 57 and 38 Ma, then cooled rapidly from 205??50?? to 120??50??C between 12.6??1.6 and 11.1??1.9 Ma. Lower plate rocks 20 km west of the breakaway cooled slowly from Early Cretaceous upper-amphibolite facies conditions through 500??50??C at 78-67 Ma, passed through 350??50?? to 300??50??C between 16.3??0.4 and 10.5??0.3 Ma, then cooled rapidly from 285??50?? to 120??50??C between 10.2 and 8.6 Ma. Upper plate tilting and rapid cooling (denudation) of the lower plate occurred simultaneously in the respective areas. The early slow-cooling part of the lower plate thermal histories was probably related to erosion at the Earth's surface, which stripped off about 9 km of material in 50 to 100 m.y. The results indicate an initial fault dip ???30?? and a 12 mm yr-1 west

  7. 39Ar/40Ar Chronology and Volumes of Eruptive Products Over the Last 1 Myr in the Tequila Volcanic Field, Jalisco, Mexico

    NASA Astrophysics Data System (ADS)

    Lewis-Kenedi, C. B.; Lange, R. A.; Hall, C. M.; Delgado-Granados, H.

    2002-12-01

    The Tequila volcanic field, located within the western Trans-Mexican arc, covers an area of 1036 km2 and includes a central, andesitic stratocone, Volc\\­_{a}n Tequila, as well as cinder cones, domes, and fissure-fed flows. Sixty-nine high precision 39Ar-40Ar dates reveal that major activity in the Tequila volcanic field began at approximately 1 Ma. From 1 Ma to 200 ka, rhyolite (> 73 wt. % SiO2) and alkali basalt (­š 51 wt. % SiO2) were the only compositions erupted in significant volumes (29 +/- 5.7 km3 and 12 +/- 1.2 km3, respectively). At approximately 200 ka, the andesite comprising Volc\\­_{a}n Tequila erupted within 30-40 kyr, producing a volume of 30 +/- 2.0 km3. Additional andesitic flows (11 +/- 1.4 km3) erupted to the northwest and southeast of the stratocone between 140 and 20 ka. The total volume of dacite that erupted at the Tequila volcanic field is small (1.3 +/- 0.03 km3) and occurred largely (88%) within the last 70 kyrs. Unlike the andesites and dacites, the basalts and rhyolites did not erupt within narrow time intervals, but extruded over the entire last 1 Myr, producing a total volume of 12.6 +/- 1.2 km3 and 32 +/- 6.1 km3, respectively. This detailed eruptive history, combined with the observed phenocryst assemblages (0-10 vol. %) in the small-volume andesite, dacite, and alkali basalt flows, suggest that they were erupted directly from the lower (or middle) crust, without prior storage in an upper crustal chamber. In contrast, the voluminous burst of andesitic volcanism that produced the phenocryst-rich (35-45 vol. %) lavas of Volc\\­_{a}n Tequila was likely fed from a short-lived (­š 40 kyrs) upper crustal chamber. This scenario is supported by the complex, disequilibrium textures seen in the phenocryst assemblage of the Volc\\­_{a}n Tequila lavas, indicative of magma mingling within an upper crustal chamber (Wallace and Carmichael, 1994). The total volume of erupted material at the Tequila volcanic field is 89 +/- 12 km3, of which

  8. Combined 40Ar/39Ar and (U-Th)/He geochronological constraints on long-term landscape evolution of the Second Paraná Plateau and its ruiniform surface features, Paraná, Brazil

    NASA Astrophysics Data System (ADS)

    Riffel, Silvana B.; Vasconcelos, Paulo M.; Carmo, Isabela O.; Farley, Kenneth A.

    2015-03-01

    Regional correlation of dated weathered land surfaces provides the necessary constraints to test long-term continental landscape evolution models, but major challenges remain in properly dating these surfaces. The geomorphological province of Second Paraná Plateau, Paraná State, Brazil, is a high elevation (ca. 800 m) land surface characterized by widely distributed deep saprolites and scattered lateritic profiles (e.g., Vila Velha and Serra das Almas). Prolonged exposure to weathering and erosion has promoted the pseudo-karstic and ruiniform features that are characteristic of this landscape. In this study, 40Ar/39Ar laser incremental heating geochronology on 22 grains of supergene Mn oxyhydroxides from lateritic profiles at Vila Velha yielded results ranging from 17.2 ± 0.7 to 9.1 ± 0.7 Ma. (U-Th)/He geochronology on 28 goethite grains from the same profile yielded results ranging from 36.4 ± 3.6 to 1.0 ± 0.1 Ma, with an age cluster lying within the 17.2 ± 0.7 to 7.9 ± 0.8 Ma interval. (U-Th)/He geochronology on 17 goethite grains from the Serra das Almas lateritic profile, located 20 km from Vila Velha, yield results ranging from 35.1 ± 3.5 to 14.1 ± 1.4 Ma. The combined results for the two sites reveal a common weathering history that started ca. 35 Ma, suggesting that the Second Paraná Plateau results from regional fluvial incision and denudation before ~ 35 Ma, followed by a decline in denudation rates and proportionally more intense weathering. Consistent with the laterite profile central ages, weathering was particularly intense during the Miocene (17-8 Ma). Denudation intensified after the Pliocene.

  9. New geologic mapping combined with geochemical, paleomagnetic, and high-precision 40Ar/39Ar analyses reveal multiple overlapping calderas formed 16.4-15.7 Ma at High Rock caldera complex, northwestern Nevada

    NASA Astrophysics Data System (ADS)

    Coble, M. A.; Mahood, G. A.

    2012-12-01

    We present new evidence from 1:100,000- and 1:24,000-scale geologic mapping for the presence of at least four overlapping calderas, 24 to 40 km in diameter, that formed in an interval of only 0.7 m.y. during the mid-Miocene at High Rock caldera complex in northwest Nevada and southern Oregon. In total, an estimated minimum volume of ~725 km3 of rhyolitic magma erupted from the complex between 16.5 and 15.5 Ma, covering an area of ~9,000 km2. Rapid eruption of numerous units at volumetric rates as high as 3,000-4,000 km3/m.y., strong welding of lithic-poor ignimbrites, extensive vapor-phase alteration of lavas and ignimbrites alike, a limited range of phenocryst content and assemblage, silicification along faults, and a lack of well-exposed stratigraphic sections has hindered previous reconnaissance-scale mapping and identification of caldera centers. Calderas are located based on truncation of precaldera rhyolitic lavas by caldera topographic walls, by arcuate patterns of rhyolite lavas that erupted along buried caldera ring faults, and by the presence of pumiceous caldera lake sediments. We attribute formation of the Virgin Valley, Badger Mountain, Hanging Rock, and Cottonwood Creek Calderas to collapse on eruption, respectively, of the ca. 16.37 Ma Idaho Canyon Tuff, the 16.34 Ma Summit Lake Tuff, the 16.0 Ma Soldier Meadows Tuff, and the 15.7 Ma Tuff of Yellow Rock Canyon. Additional smaller-volume pyroclastic units erupted during emplacement of geochemically similar rhyolitic lavas. More than 60 new 40Ar/39Ar ages were obtained on ignimbrites, fall deposits, and rhyolitic, trachytic and basaltic lavas. Many of the eruptive units in the HRCC differ in age by less than 100 k.y., which, at ca. 16 Ma, requires precision at the 1-2‰ (2σ standard error) level to distinguish units using 40Ar/39Ar geochronology. The high-precision of the analyses of sanidine in the rhyolites, coupled with geochemical and paleomagnetic measurements, allowed us to correlate far

  10. Paleomagnetic and 40Ar/39Ar results from the Grant intrusive breccia and coparison to the Permian Downeys Bluff Sill; evidence for Permian igneous activity at Hicks Dome, southern Illinois Basin

    USGS Publications Warehouse

    Reynolds, Richard L.; Goldhaber, Martin B.; Snee, Lawrence W.

    1997-01-01

    Igneous processes at Hicks dome, a structural upwarp at lat 37.5 degrees N., long 88.4 degrees W. in the southern part of the Illinois Basin, may have thermally affected regional basinal fluid flow and may have provided fluorine for the formation of the Illinois-Kentucky Fluorspar district. The timing of both igneous activity and mineralization is poorly known. For this reason, we have dated an intrusive breccia at Hicks dome, the Grant intrusion, using 40Ar/39Ar geochronometric and paleomagnetic methods. Concordant plateau dates, giving Permian ages, were obtained from amphibole (272.1+or-0.7 [1 sigma] Ma) and phlogopite (272.7+or-0.7 [1 sigma] Ma). After alternating-field (AF) demagnetization, specimens that contain titanomagnetite-bearing igneous rock fragments give a mean remanent direction of declination (D)=168.4 degrees; inclination (I)=-8 degrees; alpha 95=8.6 degrees; number of specimens (N)=10; this direction yields a virtual geomagnetic pole (VGP) at lat 54.8 degrees N., long 119.0 degrees E., delta p=4.4 degrees, delta m=8.7 degrees, near the late Paleozoic part of the North American apparent pole wander path. A nearly identical magnetization was found for the nearby Downeys Bluff sill (previously dated at about 275+or-24 Ma by the Rb-Sr method), in southern Illinois. Both AF and thermal demagnetization isolated shallow, southeasterly remanent directions carried by magnetite in the sill and from pyrrhotite in the baked contact of the Upper Mississippian Downeys Bluff Limestone: D=158.6 degrees; I=-11.8 degrees; alpha 95=3.8 degrees; N=15, yielding a VGP at lat 53.0 degrees N., long 128.7 degrees E., delta p=2.0 degrees, delta m=3.9 degrees. The paleomagnetic results, isotopic dates, and petrographic evidence thus favor the acquisition of thermal remanent magnetization by the Grant breccia and the Downeys Bluff sill during the Permian. The isotopic dates record rapid cooling from temperatures greater than 550 degrees C to less than 300 degrees C (the

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

    NASA Technical Reports Server (NTRS)

    Park, Jisun; Garrison, Daniel; Bogard, Donald

    2007-01-01

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

  12. Reconstruction of fault zone evolution from 40Ar/39Ar white mica, zircon and apatite fission track, and apatite U/Th-He thermochronology: 65 million years of fault activity along the Lavanttal Fault Zone (Eastern Alps)?

    NASA Astrophysics Data System (ADS)

    Kurz, Walter; Woelfler, Andreas; Rabitsch, Robert; Genser, Johann

    2010-05-01

    By applying distinct thermochronological methods with closure temperatures ranging from ~450° to ~40°C we reveal the thermochronological evolution of the Lavanttal Fault Zone (LFZ) and the adjacent Koralm Complex (Eastern Alps).The LFZ is generally described to be related to Miocene orogen-parallel escape tectonics in the Eastern Alps. 40Ar/39Ar dating on white mica, zircon and apatite fission track, and apatite U/Th-He thermochronology were carried out on host rocks and fault- related rocks (cataclasites and fault gouges) directly adjacent to the undeformed host rock. The main part of 40Ar/39Ar muscovite ages provided in this study is in accordance to the ages described from the adjacent Koralm Complex. The related plateau ages are therefore interpreted to represent the cooling of the host rocks during Late Cretaceous times (80-90 Ma). Muscovites derived from cataclastic shear zones show Argon release spectra characterized by reduced incremental ages for the first heating steps. This probably indicates Argon loss along the grain boundaries during shearing and lattice distortion. Samples from fault-related catclasites are characterized by a plateau age of ca. 78 Ma and highly reduced incremental ages, respectively, far below the protolith cooling ages described above. This indicates lattice distortion and related Argon loss during cataclastic shearing, and incomplete subsequent resetting. As the incremental ages are in parts highly erratic statements about the timing of shearing remain speculative. Zircon fission track ages range between 77.6±5.5 and 64.8±4.6 Ma both within fault- and host rocks. Although all four fault/host rock sample-pair ages do overlap within the 1σ error, there is a clear trend of descending ages to the fault rocks. Apatite fission track protolith ages range between 51.1±2.3 in the central Koralm massif, and 37.7±4.3 Ma along its western margin, ages from fault- related rocks vary between 46.6 ± 4.7 and 43.3 ± 4.2 in the central part

  13. Early carboniferous wrenching, exhumation of high-grade metamorphic rocks and basin instability in SW Iberia: Constraints derived from structural geology and U-Pb and 40Ar-39Ar geochronology

    NASA Astrophysics Data System (ADS)

    Pereira, M. Francisco; Chichorro, Martim; Silva, J. Brandão; Ordóñez-Casado, Berta; Lee, James K. W.; Williams, Ian S.

    2012-08-01

    New U-Pb and 40Ar-39Ar geochronology and structural data from high- to medium grade metamorphic shear zones of the Ossa-Morena Zone, and structural data from Early Carboniferous basins (Ossa-Morena Zone and South-Portuguese Zone), place additional constraints on the Variscan tectonics in SW Iberia. A zircon U-Pb age of 465 ± 14 Ma (Middle Ordovician) measured on migmatite from the Coimbra-Cordoba shear zone is interpreted as the age of protolith crystallization. This age determination revises the information contained in the geological map of Portugal, in which these rocks were considered to be Proterozoic in age. This paper describes the evolution of Variscan wrench tectonics related to the development of shear zones, exhumation of deep crustal rocks and emplacement of magma in the Ossa-Morena Zone basement. In the Coimbra-Cordoba shear zone (transpressional), migmatites were rapidly exhumed from a depth of 42.5 km to 16.6 km over a period of ca. 10 Ma in the Viséan (ca. 340-330 Ma), indicating oblique slip exhumation rates of 8.5 to 10.6 mm/yr (Campo Maior migmatites) and 3.2 mm/yr (Ouguela gneisses) respectively. In the Évora Massif, the gneisses of the Boa Fé shear zone (transtensional) were exhumed from 18.5 to 7.4 km depth in the period ca. 344-334 Ma (Viséan), with exhumation oblique slip rates of 2.8 to 4.2 mm/yr. At the same time, the Early Carboniferous basins of SW Iberia were filled by turbidites and olistoliths, composed mostly of Devonian rocks. The presence of olistoliths indicates significant tectonic instability during sedimentation with large-scale mass movement, probably in the form of gravity slides. Deformation and metamorphism dated at 356 ± 12 Ma, 321 ± 13 Ma and 322 ± 29 Ma respectively suggests that Variscan wrench movements were active in SW Iberia during the Early Carboniferous for a period of at least 35 Ma.

  14. Assessing the volcanic hazard for Rome: 40Ar/39Ar and In-SAR constraints on the most recent eruptive activity and present-day uplift at Colli Albani Volcanic District

    NASA Astrophysics Data System (ADS)

    Marra, F.; Gaeta, M.; Giaccio, B.; Jicha, B. R.; Palladino, D. M.; Polcari, M.; Sottili, G.; Taddeucci, J.; Florindo, F.; Stramondo, S.

    2016-07-01

    We present new 40Ar/39Ar data which allow us to refine the recurrence time for the most recent eruptive activity occurred at Colli Albani Volcanic District (CAVD) and constrain its geographic area. Time elapsed since the last eruption (36 kyr) overruns the recurrence time (31 kyr) in the last 100 kyr. New interferometric synthetic aperture radar data, covering the years 1993-2010, reveal ongoing inflation with maximum uplift rates (>2 mm/yr) in the area hosting the most recent (<200 ka) vents, suggesting that the observed uplift might be caused by magma injection within the youngest plumbing system. Finally, we frame the present deformation within the structural pattern of the area of Rome, characterized by 50 m of regional uplift since 200 ka and by geologic evidence for a recent (<2000 years) switch of the local stress-field, highlighting that the precursors of a new phase of volcanic activity are likely occurring at the CAVD.

  15. The Oldest Known Caldera Associated with the Yellowstone Hotspot: New Geologic Mapping, Geochemistry, and 40Ar/39Ar Geochronology for the Northern McDermitt Volcanic Field, Northern Nevada and Southeastern Oregon

    NASA Astrophysics Data System (ADS)

    Benson, T. R.; Mahood, G. A.

    2015-12-01

    McDermitt Volcanic Field (MVF) of Nevada and Oregon is one of three major caldera centers associated with Mid-Miocene Steens/Columbia River flood basalts. Pioneering geologic mapping of MVF by Rytuba and McKee (1984) and subsequent work established four main ignimbrites within the field. Our new 40Ar/39Ar ages (FCT=28.02 Ma) are 16.41±0.02 (±2σ) Ma for Tuff of Oregon Canyon, 16.35±0.04 Ma for Tuff of Trout Creek Mountains, 16.30±0.04 Ma for Tuff of Long Ridge, and 15.56±0.08 Ma for Tuff of Whitehorse Creek. We have mapped two previously unrecognized overlapping calderas that we interpret as sources for Tuff of Oregon Canyon and Tuff of Trout Creek. These ~20-km diameter calderas lie north of the well-known McDermitt Caldera; a smaller 7-km caldera that formed on eruption of the Tuff of Whitehorse Creek is nested within them. Argon ages and geochemistry of alkali rhyolite lava domes in the northern MVF define two populations: ~16.6-16.3 Ma associated with the newly recognized calderas, and ~15.5-15.3 Ma outlining the margins of the younger Whitehorse Caldera. Consistent with both ignimbrites erupting from the same evolving magma system, the high-silica alkali rhyolite Tuff of Oregon Canyon lies on compositional trends defined by the Tuff of Trout Creek, which is zoned from a moderately crystal-rich high-silica alkali rhyolite to a strongly porphyritic low-silica alkali rhyolite. They both are distinguished from the Tuff of Long Ridge from McDermitt Caldera by their higher Zr/Rb, and relatively high FeO* concentrations distinguish all MVF ignimbrites from ignimbrites from the nearby High Rock Caldera Complex, where the oldest caldera formed on eruption of the Idaho Canyon Tuff at 16.38±0.02 Ma (Coble and Mahood, in review). The Tuff of Trout Creek rests conformably on the Tuff of Oregon Canyon west and southwest of the calderas, where they overlie a thick stack of Steens Basalt lavas. To the east and southeast the two ignimbrites are separated by as much as

  16. Ar-40-Ar-39 and Rb-Sr age determinations on Quaternary volcanic rocks

    NASA Technical Reports Server (NTRS)

    Radicati Di Brozolo, F.; Huneke, J. C.; Papanastassiou, D. A.; Wasserburg, G. J.

    1981-01-01

    Ages of leucite and biotite separates from samples of the potassic volcanics of the Roman Comagmatic region are derived by the stepwise degassing variant of the Ar-39-Ar-40 dating method and compared with those derived from Rb-Sr dating in order to evaluate the abilities of the methods to date Quaternary geological events. Six of the leucite separates are found to contain Ar with very high bulk 40/36 ratios and to have well correlated Ar-40 and Ar-39 contents, yielding ages of approximately 338,000 years. Two leucites observed to contain Ar with lower bulk 40/36 ratios and Ar-40/Ar-36 ratios significantly lower than atmospheric are found to have ages in substantial agreement with those of the other leucites despite the uncertainty in the composition of the trapped component. Ages obtained for the biotites are not as precise as those of the leucites, due to difficulties in obtaining a good separation of in situ radiogenic Ar-40 from trapped Ar-40. Ages determined from Rb-Sr measurements for selected tuff samples are found to be in good agreement with the Ar-40-Ar-39 ages of the leucites. Results demonstrate the possibility of attaining precisions of better than 5% in the dating of rocks 350,000 years old by both the Ar-40-Ar-39 and the Rb-Sr methods.

  17. New constraints on the subsurface geology of the Mexico City Basin: The San Lorenzo Tezonco deep well, on the basis of 40Ar/39Ar geochronology and whole-rock chemistry

    NASA Astrophysics Data System (ADS)

    Arce, J. L.; Layer, P. W.; Morales-Casique, E.; Benowitz, J. A.; Rangel, E.; Escolero, O.

    2013-10-01

    The San Lorenzo Tezonco deep well, drilled in 2012 by "Sistema de Aguas de la Ciudad de México" (Water Supply System of Mexico City) in the Mexico Basin to a depth of 2008 m, offers an excellent opportunity to explore the subsurface stratigraphy and general geology of the region. Unfortunately, only chipped samples from this well were recovered, which were examined and analyzed for whole-rock chemistry, 40Ar/39Ar, and U-Pb zircon geochronology, in order to reconstruct the lithology of the well. Contrary to previous deep wells of the Mexico Basin, no basement sedimentary rocks were found in this one. While the upper 70 m are composed of lacustrine sediments associated with Lake Texcoco, volcanic rocks make up the majority of the well and range in age from more than 18 Ma to 0.25 Ma. Andesitic lavas are the most abundant products of the stratigraphic column, followed by acidic products represented by dacitic and rhyolitic lavas and ignimbrite deposits. Less abundant are basaltic andesite lavas appearing in the upper and lower parts of the column. The thickest sequence of the well is represented not only by Miocene volcanic rocks ranging from 5 to 17 Ma, suggesting a period of intense volcanic activity in this area producing mainly andesitic lavas, but also by thick rhyolitic ignimbrite deposits dated at 5 Ma. These deposits suggest the presence of a caldera structure, probably buried by subsequent volcanic products and lacustrine sediments. Trace element concentrations suggest that volcanism is likely produced in a subduction environment with typical negative anomalies of Nb, Ta, Ti, and P and positive anomalies in Pb and Cs. We correlated the well units with units outcropping in mountain ranges in the surrounding area, with the recognition of the following units or formations: Eocene Andesite, San Nicolás Basaltic Andesite, Tepoztlán Formation, Miocene Volcanism, Sierra de las Cruces, and Chichinautzin Volcanic Field products. By correlating the two closest deep

  18. 40Ar/39Ar evidence for Middle Proterozoic (1300-1500 Ma) slow cooling of the southern Black Hills, South Dakota, midcontinent, North America: Implications for Early Proterozoic P-T evolution and posttectonic magmatism

    NASA Astrophysics Data System (ADS)

    Holm, Daniel K.; Dahl, Peter S.; Lux, Daniel R.

    1997-08-01

    40Ar/39Ar total gas and plateau dates from moscovite and biotite in the southern Black Hills, South Dakota, provide evidence for a period of Middle Proterozoic slow cooling. Early Proterozoic (1600-1650 Ma) mica dates were obtained from metasedimentary rocks located in a synformal structure between the Harney Peak and Bear Mountain domes and also south of Bear Mountain. Metamorphic rocks from the dome areas and undeformed samples of the ˜1710 Ma Harney Peak Granite (HPG) yield Middle Proterozoic mica dates (˜1270-1500 Ma). Two samples collected between the synform and Bear Mountain dome yield intermediate total gas mica dates of ˜1550 Ma. We suggest two end-member interpretations to explain the map pattern of cooling ages: (1) subhorizontal slow cooling of an area which exhibits variation in mica Ar retention intervals or (2) mild folding of a Middle Proterozoic (˜1500 Ma) ˜300°C isotherm. According to the second interpretation, the preservation of older dates between the domes may reflect reactivation of a preexisting synformal structure (and downwarping of relatively cold rocks) during a period of approximately east-west contraction and slow uplift during the Middle Proterozoic. The mica data, together with hornblende data from the Black Hills published elsewhere, indicate that the ambient country-rock temperature at the 3-4 kbar depth of emplacement of the HPG was between 350°C and 500°C, suggesting that the average upper crustal geothermal gradient was 25°-40°C/km prior to intrusion. The thermochronologic data suggest HPG emplacement was followed by a ˜200 m.y. period of stability and tectonic quiescence with little uplift. We propose that crust thickened during the Early Proterozoic was uplifted and erosionally(?) thinned prior to ˜1710 Ma and that the HPG magma was emplaced into isostatically stable crust of relatively normal thickness. We speculate that uplift and crustal thinning prior to HPG intrusion was the result of differential thinning of

  19. Grain-size effects on the closure temperature of white mica in a crustal-scale extensional shear zone - Implications of in-situ 40Ar/39Ar laser-ablation of white mica for dating shearing and cooling (Tauern Window, Eastern Alps)

    NASA Astrophysics Data System (ADS)

    Scharf, Andreas; Handy, Mark R.; Schmid, Stefan M.; Favaro, Silvia; Sudo, Masafumi; Schuster, Ralf; Hammerschmidt, Konrad

    2016-04-01

    In-situ 40Ar/39Ar laser ablation dating of white-mica grains was performed on samples from the footwall of a crustal-scale extensional fault (Katschberg Normal Fault; KNF) that accommodated eastward orogen-parallel displacement of Alpine orogenic crust in the eastern part of the Tauern Window. This dating yields predominantly cooling ages ranging from 31 to 13 Myr, with most ages clustering between 21 and 17 Myr. Folded white micas that predate the main Katschberg foliation yield, within error, the same ages as white-mica grains that overgrow this foliation. However, the absolute ages of both generations are older at the base (20 Myr) where their grain size is larger (300-500 μm), than at the top and adjacent to the hangingwall (17 Myr) of this shear zone where grain size is smaller (< 100-300 μm). This fining-upward trend of white-mica grain size within the KNF is associated with a reduction of the closure temperature from the base (~ 445 °C) to the top (< 400 °C) and explains the counter-intuitive trend of downward-increasing age of cooling in the footwall. The new data show that rapid cooling within the KNF of the eastern Tauern Window started sometime before 21 Myr according to the 40Ar/39Ar white-mica cooling ages and between 25-21 Myr according to the new Rb/Sr white-mica ages, i