Beschreibung: The Salamanca Formation of the San Jorge Basin (Patagonia, Argentina) preserves critical records of Southern Hemisphere Paleocene biotas, but its age remains poorly resolved, with estimates ranging from Late Cretaceous to middle Paleocene. We report a multi-disciplinary geochronologic study of the Salamanca Formation and overlying Río Chico Group in the western part of the basin. New constraints include (1) an 40 Ar/ 39 Ar age determination of 67.31 ± 0.55 Ma from a basalt flow underlying the Salamanca Formation, (2) micropaleontological results indicating an early Danian age for the base of the Salamanca Formation, (3) laser ablation HR-MC-ICP-MS (high resolution-multi collector-inductively coupled plasma-mass spectrometry) U-Pb ages and a high-resolution TIMS (thermal ionization mass spectrometry) age of 61.984 ± 0.041(0.074)[0.100] Ma for zircons from volcanic ash beds in the Peñas Coloradas Formation (Río Chico Group), and (4) paleomagnetic results indicating that the Salamanca Formation in this area is entirely of normal polarity, with reversals occurring in the Río Chico Group. Placing these new age constraints in the context of a sequence stratigraphic model for the basin, we correlate the Salamanca Formation in the study area to Chrons C29n and C28n, with the Banco Negro Inferior (BNI), a mature widespread fossiliferous paleosol unit at the top of the Salamanca Formation, corresponding to the top of Chron C28n. The diverse paleobotanical assemblages from this area are here assigned to C28n (64.67–63.49 Ma), ~2–3 million years older than previously thought, adding to growing evidence for rapid Southern Hemisphere floral recovery after the Cretaceous-Paleogene extinction. Important Peligran and " Carodnia " zone vertebrate fossil assemblages from coastal BNI and Peñas Coloradas exposures are likely older than previously thought and correlate to the early Torrejonian and early Tiffanian North American Land Mammal Ages, respectively.

Beschreibung: 40 Ar/ 39 Ar geochronology of basaltic to rhyolitic lavas, domes, and pyroclastic deposits from Ascension Island indicates that the maximum age of subaerially exposed samples is 1094 ka. Thirty-eight 40 Ar/ 39 Ar ages, coupled with new and existing geochemical data, constrain the eruptive histories of the four distinct mafic magma types (high Zr/Nb, low Zr/Nb, intermediate Zr/Nb, and Dark Slope Crater) and document temporal variations in magma sources. Lavas from the eastern felsic complex, previously assumed to be as old as or slightly younger than the 602–1094 ka Middleton Ridge complex, are as young as 52 ka. Basaltic to benmoreitic scoria cones and associated flows of the intermediate Zr/Nb magma type have been inferred to be the most recent eruptive products, yet their eruptive histories extend back to 705 ka. These intermediate Zr/Nb magmas are likely to be parental to the abundant trachytic to rhyolitic lavas and domes, which contradicts previous interpretations that call upon a high Zr/Nb parental basalt. Two distinct fractionation trends are observed in the trace element variations of Ascension trachytes and rhyolites. 40 Ar/ 39 Ar ages of the samples defining the two trends suggest that ilmenite fractionation dominated the Nb budget and thus controlled Zr/Nb ratios in early (〉931 ka) Ascension evolved magmas, whereas zircon–titanite fractionation was predominant in younger felsic magmas. The eruptive sequence and compositions of the subaerial lavas and domes at Ascension Island are unique in comparison with other ocean island volcanoes because of its on-axis location and eruptions of high-SiO 2 trachyte and rhyolite.

Beschreibung: The catastrophic gravitational collapse of the Old Cone of Volcán Parinacota produced a 6 km 3 debris avalanche that traveled ~22 km and covered more than 150 km 2 . The Upper Lauca drainage, a broad high-altitude basin in the Chilean Altiplano, was permanently altered by the collapse. Although the eruptive history of Parinacota before and after the debris avalanche is well known from petrologic and geochronologic studies, previous age limits on the debris avalanche (based on multiple chronometers) span ca. 8–20 ka. New cosmogenic surface-exposure ages from boulders atop the deposit are based on a regionally calibrated production rate of in situ 10 Be and indicate that the avalanche occurred at 8.8 ± 0.5 ka. These data demonstrate that cosmogenic 10 Be surface-exposure dating can be successfully applied to quartz-bearing, volcanic debris avalanche deposits, and that this method offers a distinct advantage over 14 C chronologies that provide only minimum or maximum age limits. The 8.8 ka exposure age for the debris avalanche (1) agrees with 14 C age limits of paleosol material incorporated in the debris avalanche, (2) requires a voluminous initial phase of postcollapse volcanism with an eruptive rate exceeding that of recent cone-building episodes at most continental arc volcanoes, and (3) suggests that volcano collapse did not result in the formation of Lago Chungará, but instead led to a major expansion of a preexisting closed basin.

Beschreibung: A continental inland basin extended in northern Israel and the southwest Golan Heights during the Neogene. Sedimentation of carbonates, marls, and evaporites was accompanied by massive basalt flows and by minor marine ingressions. The latter were recognized in restricted areas of the eastern Lower Galilee and the Kinnerot basin, herein termed together as the SE Galilee basin. This study aims to establish a detailed chronostratigraphic framework for ~5 m.y. of sedimentation between the top of the latest middle Miocene Lower Basalt unit and the base of the earliest Pliocene Cover Basalt unit in the SE Galilee basin, where stratigraphic exposure and lithologic variability are maximized. Data from five outcrop sections, two boreholes, and 27 40 Ar/ 39 Ar plateau ages of volcanic rocks were used. The upper Lower Basalt was eroded from tectonically uplifted blocks. Erosion products mixed with pyroclasts were deposited in structural lows between ca. 12 and 10 Ma, forming the Umm Sabune Formation. A gradual lithological transition to the overlying well-bedded lacustrine/lagoonal Bira Formation occurred between ca. 11 and 10 Ma. The transition to the overlying freshwater Gesher Formation occurred at ca. 7 Ma, around the Tortonian-Messinian boundary; hence, the gypsum beds at this transition predated the Messinian salinity crisis. The Cover Basalt flows started between 5.1 and 4.6 Ma at different locations, contemporaneously with the deposition of the upper part of the Gesher Formation and the Fejjas Tuff unit. A newly discovered unit of conglomerates and paleosols is considered to be the continental equivalent of Messinian salinity crisis evaporite deposition. This chronostratigraphic framework provides a basis for comparison with other similar late Neogene continental basins in the Levant and for the recognition on land of the Tortonian-Messinian boundary and the impact of the Messinian salinity crisis around the Mediterranean.

Beschreibung: Clinopyroxene (Cpx) phenocrysts have the potential to record magmatic water contents and magma ascent rates through their concentration of H 2 O (incorporated as H + and commonly referred to as water). Here we investigate three issues related to the fidelity and utility of the Cpx water record: the partitioning of water between Cpx and melt, the diffusivity of water in natural Cpx phenocrysts, and the possibility for water loss in Cpx erupted in lava flows. Samples studied are from volcanic ash of the 1974 eruption of Volcán de Fuego (Guatemala) and scoria and lava from the 1977 eruption on Seguam Island (Alaska). The partitioning of water was determined by analyzing melt inclusions (MIs) and the adjacent clinopyroxene host by ion microprobe. For seven Cpx-hosted MIs from Seguam, the partition coefficient is well predicted (within 0·1 wt % H 2 O on average) using the temperature-dependent parameterization of O’Leary et al. (2010; Earth and Planetary Science Letters 297 , 111–120). For the determination of diffusivity, H 2 O concentration profiles were measured in oriented Cpx from Fuego by ion microprobe. Water decreases toward the rim, consistent with diffusive re-equilibration during ascent-driven degassing. Using previously estimated durations of ascent (7–12 min), we determined the H + diffusivity (10 –9·7 –10 –10·3 m 2 s –1 at a temperature of 1030 °C) that would satisfy these timescales. These calculated DH+Cpx values are comparable with the medial values determined for natural Cpx (Mg# 〈 92·5) in laboratory diffusion studies. Tephra and lava co-erupted on Seguam bear similar Cpx populations in their major and trace element compositions, but the lava Cpx contain 80% lower H 2 O concentrations on average than the tephra Cpx. Using the DH+Cpx values obtained from the Fuego Cpx, the difference in H 2 O between the lava and tephra Cpx can be attributed to post-eruption H 2 O loss during the estimated ~20 min the sample remained above the H + closure temperature. The results from this study indicate that clinopyroxene from slowly cooled basaltic lavas should not be used to reconstruct initial magmatic water contents. High ascent rates, rapid post-eruptive cooling, large phenocrysts, and cooler magma (e.g. andesites and rhyolites) favor better preservation of water in Cpx. In cases of H + loss, Cpx zonation can be exploited as a chronometer for syn- and post-eruptive volcanic processes.

Beschreibung: This study revises and improves the chronostratigraphic framework for late Turonian through early Campanian time based on work in the Western Interior U.S. and introduces new methods to better quantify uncertainties associated with the development of such time scales. Building on the unique attributes of the Western Interior Basin, which contains abundant volcanic ash beds and rhythmic strata interpreted to record orbital cycles, we integrate new radioisotopic data of improved accuracy with a recently published astrochronologic framework for the Niobrara Formation. New 40 Ar/ 39 Ar laser fusion ages corresponding to eight different ammonite biozones are determined by analysis of legacy samples, as well as newly collected material. These results are complemented by new U-Pb (zircon) chemical abrasion–isotope dilution–thermal ionization mass spectrometry ages from four biozones in the study interval. When combined with published radioisotopic data from the Cenomanian-Turonian boundary, paired 206 Pb/ 238 U and 40 Ar/ 39 Ar ages spanning Cenomanian to Campanian time support an astronomically calibrated Fish Canyon sanidine standard age of 28.201 Ma. Stage boundary ages are estimated via integration of new radioisotopic data with the floating astrochronology for the Niobrara Formation. The ages are determined by anchoring the long eccentricity bandpass from spectral analysis of the Niobrara Formation to radioisotopic ages with the lowest uncertainty proximal to the boundary, and adding or subtracting time by parsing the 405 k.y. cycles. The new stage boundary age determinations are: 89.75 ± 0.38 Ma for the Turonian-Coniacian, 86.49 ± 0.44 Ma for the Coniacian-Santonian, and 84.19 ± 0.38 Ma for the Santonian-Campanian boundary. The 2 uncertainties for these estimates include systematic contributions from the radioisotopic measurements, astrochronologic methods, and geologic uncertainties (related to stratigraphic correlation and the presence of hiatuses). The latter geologic uncertainties have not been directly addressed in prior time scale studies and their determination was made possible by critical biostratigraphic observations. Each methodological approach employed in this study—new radioisotopic analysis, stratigraphic correlation, astrochronology, and ammonite and inoceramid biostratigraphy—was critical for achieving the final result.

Beschreibung: The development of integrated astronomical and radioisotopic time scales from rhythmic strata of the Western Interior Basin (WIB) has played a fundamental role in the refinement of Late Cretaceous chronostratigraphy. In this study, X-ray fluorescence (XRF) core scanning is utilized to develop a new elemental data set for cyclostratigraphic investigation of Cenomanian-Turonian strata in the WIB, using material from the Aristocrat-Angus-12-8 core (north-central Colorado). The XRF data set yields the first continuous 5-mm-resolution analysis of lithogenic, biogenic, and syngenetic-authigenic proxies through the uppermost Lincoln Limestone Member, the Hartland Shale Member, and the Bridge Creek Limestone Member, including oceanic anoxic event 2 (OAE 2). The 40 Ar/ 39 Ar ages from ashes in three biozones, including a new age from the Dunveganoceras pondi biozone (uppermost Lincoln Limestone Member), provide geochronologic constraints for the cyclostratigraphic analysis. Astrochronologic testing of the 5-mm-resolution XRF weight percent CaCO 3 data via average spectral misfit analysis yields strong evidence for astronomical influence on climate and sedimentation. Results from the Bridge Creek Limestone Member are consistent with the previously published astrochronology from the U.S. Geological Survey #1 Portland core (central Colorado), and identification of an astronomical signal in the underlying Hartland Shale Member now permits extension of the WIB astrochronology into the earlier Cenomanian, prior to OAE 2. High rates of sedimentation in the Angus core during the interval of OAE 2 initiation, as compared to the Portland core, allow recognition of a strong precessional control on bedding development. As a consequence, the new results provide a rare high-resolution chronometer for the onset of OAE 2, and the timing of proposed hydrothermal trace metal enrichment as observed in the 5 mm XRF data.

Beschreibung: Prior to the ad 1902 Plinian eruption of 8 km 3 of dacite and subsequent growth of the 〉1 km 3 Santiaguito dacite dome complex, Santa María volcano grew into an 8 km 3 composite cone over ~75 kyr in four phases (at 103–72, 72, 60–46, and 35–25 ka). The 1902 eruption occurred after an ~25 kyr period of repose in growth of the composite cone. To provide context for processes that ultimately led to the 1902 eruption, we present geochemical and isotopic (Sr, Nd, Pb, U-series) data from lavas of the composite cone for which ages are constrained by 40 Ar/ 39 Ar dating. The four cone-building phases comprise basaltic to basaltic-andesite lava (51·4–56·1% SiO 2 ) whose major- and trace-element compositions suggest that crystallization was important in differentiation. Relative to other Central American arc volcanoes, these lavas also have large 238 U excesses and high 207 Pb/ 204 Pb ratios that imply melting of a mantle wedge modified to an unusual extent by fluid from subducted crust and sediment of the Cocos plate. Major- and trace-element and isotopic variations over time imply that mafic recharge and magma mixing were prevalent during early phases of cone-building, whereas assimilation processes were more dominant during the latest stage of cone growth. Indeed, some early erupted basalts have lower 143 Nd/ 144 Nd and higher 87 Sr/ 86 Sr ratios than more SiO 2 -rich basaltic andesites that erupted during the final phase of cone-building. These features point to an assimilant that is not typical continental crust and instead may be more like mid-ocean ridge basalt with respect to major- and trace-element composition and Sr, Nd, Pb, and U–Th isotope ratios. Energy-constrained modeling of a parental basalt that undergoes crystal fractionation, assimilation and periodic recharge with basalt in the lower crust can reproduce lava compositions erupted during phases I–III and the early part of phase IV. Modeling further indicates that assimilation within the lower crust of partially melted garnet-amphibolite metabasalt, without basaltic recharge, may produce the youngest cone-forming lavas in phase IV. These models link the 8 km 3 of cone growth over 75 kyr to the mass flux of magma into the crust. Our findings suggest an along-arc magma flux into the lower crust beneath Santa María of 〉20 km 3 km – 1 Myr – 1 , which is higher than anticipated in recent numerical–thermal approaches to basalt–crust interaction. Consequently, the thermal incubation period needed to produce hybrid basaltic-andesite magma may be only a few tens of thousand years.