Incremental Heating Research Articles

Experimental and geologic evaluation of monazite (U–Th)/He thermochronometry: Catnip Sill, Catalina Core Complex, Tucson, AZ

Monazite is a petrologically important and analytically promising target for (U–Th)/He thermochronology. Previous studies have reported highly variable He diffusion results from monazite from a single sample and demonstrated that composition can significantly affect He diffusion parameters. In this study, we performed incremental heating of single monazite grains to experimentally determine the 4He diffusion properties of reference monazite ‘554’ that occurs within a peraluminous two-mica granite from the Catnip Sill within the Catalina Core complex, Arizona. Assuming that the grain size defines the diffusion geometry, the six experiments yielded Ea values of 212 to 238±5 kJmol−1 (1σ) and Do values of 15.7 to 103 cm2s−1 with one value of 784 cm2s−1. Monazite (U–Th)/He data from five grains yielded closure temperatures of 291 to 262 °C (± c. 15 °C) and ages of 23.8–20.3(±∼1.2;2σ) Ma; the weighted mean age is 21.8±0.73 (MSWD=1.83, n=5) and the weighted mean closure temperature is 282±6°C (MSWD=0.96, n=5;1σ). We tested the accuracy of these results by comparing our monazite thermochronology data with monazite Th/Pb depth profiling results, the 40Ar/39Ar thermal history for the Catnip Sill constrained using coexisting muscovite, biotite, and K-feldspar, and published regional zircon and apatite fission track results. The monazite Th/Pb data indicate emplacement of the Catnip Sill at ∼45 Ma. The 40Ar/39Ar muscovite and biotite data indicate cooling from 460 to 350 °C from 27 to 26 Ma. K-feldspar MDD modeling suggests cooling from 360 to 240 °C from 26 to 24 Ma. Zircon fission track data indicate cooling through 250 °C between 29 and 20 Ma. Additional cooling through 110 °C is recorded by apatite fission track ages of 19–16 Ma. Because the monazite thermochronology results are reproducible and consistent with the thermal history constrained by the other chronometers, our results 1) confirm the accuracy of the 4He diffusion kinetics from monazite ‘554,’ and 2) demonstrate that monazite can be an effective (U–Th)/He thermochronometer, provided that He diffusion properties are either measured or reasonably assumed for each sample.

Geochronology of the Baye Mn oxide deposit, southern Yunnan Plateau: Implications for the late Miocene to Pleistocene paleoclimatic conditions and topographic evolution

40Ar/39Ar laser incremental heating analyses of supergene K-Mn oxides from weathering profiles at the Baye Mn deposit, southern Yunnan Plateau, SW China, were carried out to place constraints on the timing of weathering and derive insights into local paleoclimatic and landscape evolution. Weathering profiles in the Baye Mn mine are dominated by 20–30m thick saprolites, which are locally covered by unconsolidated alluvial deposits. We analyzed 70 grains from 35 hand samples collected from four sites located at distinct elevations. In most cases, different grains from the same hand sample and different samples from the same site have reproducible 40Ar/39Ar plateau ages. The plateau ages of individual grains are also consistent with the respective isochron, integrated, and ideogram ages. This age consistency attests to the reliability of the geochronological results as numerical constraints on the formation and evolution of these weathering profiles.Sixty-four grains from 32 hand samples collected from weathering profiles at four sites (A to D) yield well-defined plateau or pseudoplateau ages ranging from 2.98±0.07 to <0.16Ma (2σ). The age results show that Mn oxides become progressively younger towards lower elevations: samples collected from the highest elevation site (∼1855m a.s.l.) have the oldest ages at 2.98±0.07 to 2.83±0.13Ma; samples from intermediate elevations (1821 to 1815m a.s.l.) yield ages ranging from 2.91±0.10 to 2.08±0.11Ma; and those from the lowest sites (1768 to 1753m a.s.l.) are younger than 1.25±0.08Ma. The age versus elevation relationship indicates fast downward propagation of weathering front since the late Pliocene, and permits estimating the weathering rates at 3.3±3.8, 6.6±1.2, and 11.1±1.9mMyr−1 during the time periods of 2.98–2.83, 2.91–2.08, and 1.25 to <0.16Ma, respectively. The average river incision rates estimated from the surface exposed ages range from 235±10 to 416±19mMyr−1 during the past 3 million years. The incision rate calculated from minimum exposed ages at sites B–D is 33±2mMyr−1 for the time period of 2.91–1.25Ma, which is consistent with the slow incision of streams on the ridges over the Yunnan Plateau since the early Pleistocene.Three Mn oxide pebbles contained in the alluvial deposits overlying the saprolite-dominated weathering profiles at the two highest sites provided additional information on the timing of weathering. Four grains from two pebbles yield plateau ages of 6.32±0.19 to 5.27±0.10Ma, whereas the other two grains from the third pebble indicate the minimum formation ages of 8.2±0.4 and 9.3±0.3Ma. These ages confirm the existence of older weathering profiles, now dismantled, in the region. Manganese oxide 40Ar/39Ar ages of the Baye deposit, when combined with results from other localities, indicate that lateritic weathering and supergene Mn enrichment and, by inference, warm and humid climates conducive to intense weathering have prevailed over the Plateau since the middle to late Miocene. The climatic conditions inferred from the weathering geochronology are consistent with multiple independent marine and terrestrial sedimentary and paleontological records, confirming that supergene Mn oxides can be used as a useful proxy for past climate. Age clusters of Mn oxides at 2.9–2.4, 1.2–0.8, and 0.6–0.4Ma are broadly coincident with and thus likely reflect intensification events of the Indian Summer Monsoon that brings moisture and abundant precipitation to the Yunnan Plateau. These clusters also coincide with the periods of significant surface uplift in the Yunnan Plateau, demonstrating a causal link between topographic evolution, plateau uplift, and intensification of the monsoonal climate.

Eocene to Miocene igneous activity in NE Greenland: northward younging of magmatism along the East Greenland margin

Radiometric dating by the 40 Ar– 39 Ar incremental heating method was carried out on lavas, sills, dykes, and a central intrusion from NE Greenland. Eighteen samples gave acceptable crystallization ages. Lavas of both Lower and Upper Plateau Lava Series gave ages in the range 55.5–53.5 Ma and cannot be constrained to better than 56–53 Ma. Sills and dykes from Traill Ø to Shannon, with compositions fairly similar to those of the lavas, gave ages of 55.1–51.3 Ma, contemporaneous with and slightly younger than the lavas. Alkaline lavas on inland nunataks have ages of 53–50 Ma, and the Kap Broer Ruys intrusive centre has an age of 48.7 ± 0.5 Ma. An alkaline sill on Hvalrosø is much younger at 20.3 ± 0.1 Ma. There are no pre-breakup lavas onshore NE Greenland. We surmise that the hot mantle of the Iceland plume arrived and melted extensively beneath the northern basins only at the time of breakup around 55 Ma. Post-breakup intrusive events in NE Greenland coincided with plate-tectonic events such as reorganization, uplift and opening in the north. The Hvalrosø sill represents a local small melting event that may be related to coeval opening of the Lena Trough. Supplementary materials: Details of the source data, results, and the compositions and locations of the dated samples, are available at www.geolsoc.org.uk/SUP18738 .

Fuerteventura – Assessment of a calibration site for cosmogenic 3He exposure dating with the 40Ar/39Ar incremental heating method

In situ Terrestrial Cosmogenic Nuclides (hereafter TCNs) are increasingly important for absolutely dating terrestrial events and processes. This study aimed at improving our knowledge of the production rate of Terrestrial Cosmogenic 3He formed in situ in rock surfaces at low latitude and sea level as well as re-evaluation of the Canary Islands as a calibration site for TCNs. For this purpose, we sampled basaltic lava flows from some of the youngest and yet undated volcanic sites and used the 40Ar/39Ar incremental heating method on groundmass samples and in situ cosmogenic 3He on olivine and clinopyroxene phenocrysts. 40Ar/39Ar analysis was done on a Hiden HAL Series 1000 triple filter quadrupole mass spectrometer with extraction furnace. Incremental heating data shows ages in the Late Pleistocene from 52.7 ± 21.6 ka to 398.6 ± 27.6 ka.We measured cosmogenic 3He concentrations in olivine and clinopyroxene phenocrysts from flow top samples on a MAP 215-50 sector mass spectrometer with a crushing device and a diode laser extraction system. Exposure age calculations yielded ages in the range 38.9 ± 4.0 ka to 62.3 ± 6.7 ka for the youngest lava flow and the data series is in broad agreement with the argon data up to 250 ka and reveals a more continuous time line of volcanism during the late Pleistocene on the island. However, the dataset was not sufficient for calculation of production rates for in situ Terrestrial Cosmogenic 3He as many samples showed signs of erosion. Calculated erosion rates range from none to as high as 7.3 mm/kyr assuming a rock density of 2.9 g/cm2. This finding puts a constraint on the use of Fuerteventura as a calibration site for exposure histories older than 50–100 ka. A comparison with cosmogenic 36Cl data supports these findings and indicates substantial weathering.

Prolonged plume volcanism in the Caribbean Large Igneous Province: New insights from Curaçao and Haiti

We present 36 new 40Ar‐39Ar incremental heating age determinations from the Caribbean Large Igneous Province (CLIP) providing evidence for extended periods of volcanic activity and suggest a new tectonomagmatic model for the province's timing and construction. These new 40Ar‐39Ar ages for the Curaçao Lava Formation (CLF) and Haiti's Dumisseau Formation show evidence for active CLIP volcanism from 94 to 63 Ma. No clear changes in geochemical character are evident over this period. The CLF has trace element signatures (e.g., Zr/Nb = 10–20) and flat rare earth element (REE) trends consistent with plume volcanism. The Dumisseau Formation also has plume‐like geochemistry and steeper REE trends similar to ocean island basalts. Volcanism in the Dumisseau Formation appears to have largely ceased by 83 Ma while at Curaçao it continued until 63 Ma. A rapidly surfacing and melting plume head alone does not fit this age distribution. Instead, we propose that the residual Galapagos plume head, following initial ocean plateau construction, was advected eastward by asthenospheric flow induced by subducting oceanic lithosphere. Slab rollback at the Lesser Antilles and Central America subduction zones created an extensional regime within the Caribbean plate. Mixing of plume with upwelling asthenospheric mantle provided a source for intermittent melting and eruption through the original plateau over a ∼30 Ma period.

Bimodal volcanism of the High Lava Plains and Northwestern Basin and Range of Oregon: Distribution and tectonic implications of age‐progressive rhyolites

Multiple episodes of Oligocene and younger silicic volcanism are represented in the high lava plateau of central and southeastern Oregon. From 12 Ma to Recent, volcanism is strongly bimodal with nearly equal volumes of basalt and rhyolite. It is characterized by moderate to high silica (SiO2 &gt;72 wt. %) rhyolitic tuffs and domes that are younger to the west, and widespread, tholeiitic basalts that show no temporal pattern. We report 18 new 40Ar/39Ar incremental heating ages on rhyolites, and establish that the timing of the age‐progressive rhyolites is decoupled from basaltic pulses. This work expands on that of previous workers by clearly linking the High Lava Plains (HLP) and northwestern‐most Basin and Range (NWBR) rhyolite volcanism into a single age‐progressive trend. The spatial‐temporal relationship of the rhyolite outcrops and regional tectonics indicate that subsidence due to increasingly dense crust creates large, primarily sediment‐filled basins within the more volcanically active HLP. The west‐northwest age progression in rhyolitic volcanism is counter to the trend expected for a quasi‐stationary mantle upwelling relative to North American plate motion. We attribute the rhyolitic age progression to mantle upwelling in response to slab rollback and steepening, and this is consistent with mantle anisotropy under the region and analog slab rollback models. This removes the necessity of deep mantle plume involvement. Laboratory experimental studies indicate that the geometry of the downgoing slab can focus upwelling or asthenospheric counterflow into a constricted band, resulting in greater volcanic volumes in the HLP as compared to the NWBR.

Stratigraphy and age of the Eocene Igtertivâ Formation basalts, alkaline pebbles and sediments of the Kap Dalton Group in the graben at Kap Dalton, East Greenland

A NE–SW-trending graben at Kap Dalton on the Blosseville Kyst contains an at least 600 m thick succession of Eocene basalt lavas and sediments. The succession has been investigated by new field work, geochemical analysis and radiometric dating by the 40Ar-39Ar incremental heating method. The results show that the volcanic succession comprises about 220 m of the uppermost plateau basalt formation, the Skrænterne Formation. This is separated from the overlying lava flows of the Igtertivâ Formation by 7 m of sediments that represent a period of around six million years. The two formations can be distinguished by different trace element ratios. The Igtertivâ Formation comprises an at least 300 m thick main succession of flows dated to 49.09 ± 0.48 Ma, overlain by sediments of the Bopladsdalen Formation. A basal conglomerate in the sediments contains pebbles of alkaline igneous rocks of which three were dated at 49.17 ± 0.35 Ma, 47.60 ± 0.25 Ma, and 46.98 ± 0.24 Ma. The sediments are thus younger than 47 Ma. Above 30 m of sediments occur two Igtertivâ Formation lava flows dated to 43.77 ± 1.08 Ma. The overlying sediments of the Bopladsdalen and Krabbedalen Formations are therefore not older than about 44 Ma and palynological evidence shows that they are also not much younger than this. Use of the Geological Time Scale 2012 has resulted in good agreement between radiometric and palynological ages. The Igtertivâ Formation lava flows were fed from a regional coast-parallel dyke swarm indicating a new rifting episode at 49–44 Ma. This coincides with a major mid-Eocene plate reorganisation event in the North Atlantic and the start of northward-propagation of the Reykjanes Ridge through the continent. The Igtertivâ rift may have been directly instrumental for the initiation of this process.

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

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±25ka, four of the normally magnetized lavas erupted at 1060±10, 1062±24, 684±21, and 683±28ka, and two transitionally magnetized lavas formed at 1221±11 and 799±21ka. 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 Time Averaged geomagnetic Field Initiative studies.

40Ar/39Ar dating of cryptomelane from the Baye manganese deposit, SW Yunnan, China: Implications for growth rate of supergene Mn-oxide veins

Supergene Mn-oxide deposits are commonly related to thick weathering profiles and form as a result of intense and prolonged weathering of Mn-bearing precursor rocks, processes facilitated by a combination of multiple factors including warm and wet climate, relatively quiescent tectonic environment, and favorable geomorphic setting. Precise age constraints of supergene Mn-oxides may provide valuable information on the climatic, tectonic, and geomorphic conditions prevailing in the geological past. 40Ar/39Ar laser incremental heating analysis of cryptomelane from the Baye manganese deposit, western Yunnan-Guizhou Plateau, SW China, was conducted to provide numerical constraints on the ages of supergene Mn enrichment and growth rates of Mn-oxide veins. In this study, two Mn-oxide veins of 3–6 cm thickness, termed as Vein A and B, respectively, were collected for 40Ar/39Ar dating. Vein A is subhorizontal and present in fractures parallel to schistosity of the Proterozoic spessartine schists, whereas Vein B is filled in nearly vertical fractures penetrating the schistosity. A combination of optical microscopy, XRD, SEM, and EMP analyses demonstrate that mineral grains extracted from both veins consist of pure, well-crystallized cryptomlane, which has acicular crystal forms and contains 0.96%–4.70% K2O. Fourteen cryptomelane subsamples extracted from different growth bands of Vein A yield well-defined plateau ages ranging from 1.35 ± 0.05 to 1.16 ± 0.04 Ma (2σ), whereas seven subsamples obtained from Vein B have plateau ages ranging from 1.23 ± 0.05 to 1.01 ± 0.04 Ma (2σ). The age results permit estimation for the growth rates of both veins at ca. 115–153 and 34–67 mm Ma−1, respectively. Distinct growth rates between Veins A and B are likely reflective of different fluid pressures during their formation governed by the orientation of the fractures hosting the veins. The growth rates calculated for both veins are 1–2 orders of magnitude higher than the values (<1–10 mm Ma−1) estimated for Mn-oxide nodules in supergene Mn deposits elsewhere. The high growth rates obtained for the Baye samples are consistent with direct precipitation of tetravalent Mn-oxides from weathering solution in open cavities or fractures, characterized by incremental growth of Mn-oxide bands from the margin to the center of the cavities or fractures. In contrast, Mn-oxide nodules with low growth rates from other deposits formed through self-oxidation and self-catalization, and grow outwards from the nuclei. In addition, prevalence of warm and humid climate and prolonged tectonic uplift in the Yunnan Plateau may have also partly contributed to the rapid growth of Mn-oxides in the Baye deposit.

In situ X-ray crystallographic study of the structural evolution of colloidal crystals upon heating.

The structural evolution of colloidal crystals made of polystyrene hard spheres has been studied in situ upon incremental heating of a crystal in a temperature range below and above the glass transition temperature of polystyrene. Thin films of colloidal crystals having different particle sizes were studied in transmission geometry using a high-resolution small-angle X-ray scattering setup at the P10 Coherence Beamline of the PETRA III synchrotron facility. The transformation of colloidal crystals to a melted state has been observed in a narrow temperature interval of less than 10 K.

Timing and composition of volcanic activity at Harrat Lunayyir, western Saudi Arabia

Harrat Lunayyir is an alkali basaltic, continental volcanic field in NW Saudi Arabia. Lava flows and cinder cones are basanite to alkali olivine basalt to trachy-basalt in composition. The field contains about 50 volcanic cones fed by fissures through Precambrian crystalline rocks along a N–S axis, lying about 200km east of the Red Sea spreading center. One of cones erupted as recently as the 10th century AD. Analysis of a recent earthquake swarm (2007–2009) indicates a ~10-km, NW-trending cluster of events at both shallow and deep crustal locations, concentrated in regions of higher velocity material. Six volcano-stratigraphic units are identified, based on super-position and morphology (degree of erosion). New 40Ar–39Ar incremental heating age determinations indicate that the entire volcanic history occurred within the last 600ka, with eruption rate decreasing with time. Major and minor element compositional variations are due almost entirely to crustal level fractionation (of mainly olivine, plagioclase, and clinopyroxene), or small differences in mantle partial melting. Primitive liquid composition, estimated by adding olivine to parental magma compositions, is consistent with ~10% melting of an upper mantle peridotitic source in the depth range of spinel to garnet stability (80–60km). There is no evidence for crustal assimilation. Trace element variations (in Dy/Yb, Ce/Yb) are consistent with shallowing of the asthenospheric melting region with time. Regional variations in trace element compositions among other harrats indicate a strong influence of the lithosphere–asthenosphere boundary in controlling mantle melting.

An age for the Korath Range, Ethiopia and the viability of 40Ar/39Ar dating of kaersutite in Late Pleistocene volcanics

The Korath Range is the northernmost of five Quaternary volcanic fields along the axis of the Turkana rift of northern Kenya and southern Ethiopia. It is located within the Omo River valley of Ethiopia, a culturally diverse region that contains abundant Plio-Pleistocene human remains, which have been essential to refining our understanding of hominid evolution. Yet, very little geochronologic information is available for the Korath Range. We have undertaken 40Ar/39Ar incremental heating experiments, using two techniques, on kaersutite phenocrysts from a pyroclastic flow and obtained a weighted mean age of 91 ± 15 ka (2σ). The results, although imprecise, suggest that 40Ar/39Ar geochronology of these Ti-rich amphiboles can yield useful age information.

Systematic variations of argon diffusion in feldspars and implications for thermochronometry

Coupled information about the time-dependent production and temperature-dependent diffusion of radiogenic argon in feldspars can be used to constrain the thermal evolution attending a host of Earth and planetary processes. To better assess the accuracy of thermal models, an understanding of the mechanisms and pathways by which argon diffuses in feldspars is desirable. Here we present step-heating Ar diffusion experiments conducted on feldspars with diverse compositions, structural states, and microstructural characteristics. The experiments reveal systematic variations in diffusive behavior that appear closely related to these variables, with apparent closure temperatures for 0.1–1mm grains of ∼200–400°C (assuming a 10°C/Ma cooling rate). Given such variability, there is no broadly applicable set of diffusion parameters that can be utilized in feldspar thermal modeling; sample-specific data are required. Diffusion experiments conducted on oriented cleavage flakes do not reveal directionally-dependent diffusive anisotropy to within the resolution limits of our approach (approximately a factor of 2). Additional experiments aimed at constraining the physical significance of the diffusion domain are presented and indicate that unaltered feldspar crystals with or without coherent exsolution lamellae diffuse at the grain-scale, whereas feldspars containing hydrothermal alteration and/or incoherent sub-grain intergrowths do not. Arrhenius plots for argon diffusion in plagioclase and alkali feldspars appear to reflect a confluence of intrinsic diffusion kinetics and structural transitions that occur during incremental heating experiments. These structural transitions, along with sub-grain domain size variations, cause deviations from linearity (i.e., upward and downward curvature) on Arrhenius plots. An atomistic model for Arrhenius behavior is proposed that incorporates the variable lattice deformations of different feldspars in response to heating and compression. The resulting implications for accurately extrapolating laboratory-derived diffusion parameters to natural settings and over geologic time are discussed. We find that considerable inaccuracies may exist in published thermal histories obtained using multiple diffusion domain (MDD) models fit to Arrhenius plots for exsolved alkali feldspar, where the inferred Ar partial retention zones may be spuriously hot.

Nitric oxide‐mediated cutaneous microvascular function is impaired in polycystic ovary sydrome but can be improved by exercise training

Polycystic ovary syndrome (PCOS) is associated with cardiovascular disease. The contribution of the nitric oxide (NO) dilator system to cutaneous endothelial dysfunction is currently unknown in PCOS. Our aim was to examine whether women with PCOS demonstrate impaired cutaneous microvascular NO function and whether exercise training can ameliorate any impairment. Eleven women with PCOS (age, 29 ± 7 years; body mass index, 34 ± 6 kg m(-2)) were compared with six healthy obese control women (age, 29 ± 7 years; body mass index, 34 ± 5 kg m(-2)). Six women with PCOS (30 ± 7 years; 31 ± 6 kg m(-2)) then completed 16 weeks of exercise training. Laser Doppler flowmetry, combined with intradermal microdialysis of l-N(G)-monomethyl-l-arginine, a nitric oxide antagonist, in response to incremental local heating of the forearm was assessed in women with PCOS and control women, and again in women with PCOS following exercise training. Cardiorespiratory fitness, homeostasis model assessment for insulin resistance, hormone and lipid profiles were also assessed. Differences between women with PCOS and control women and changes with exercise were analysed using Student's unpaired t tests. Differences in the contribution of NO to cutaneous blood flow [expressed as a percentage of maximal cutaneous vasodilatation (CVCmax)] were analysed using general linear models. At 42°C heating, cutaneous NO-mediated vasodilatation was attenuated by 17.5%CVCmax (95% confidence interval, 33.3, 1.7; P = 0.03) in women with PCOS vs. control women. Exercise training improved cardiorespiratory fitness by 5.0 ml kg(-1) min(-1) (95% confidence interval, 0.9, 9.2; P = 0.03) and NO-mediated cutaneous vasodilatation at 42°C heating by 19.6% CVCmax (95% confidence interval, 4.3, 34.9; P = 0.02). Cutaneous microvascular NO function is impaired in women with PCOS compared with obese matched control women but can be improved with exercise training.

Refining the Quaternary Geomagnetic Instability Time Scale (GITS): Lava flow recordings of the Blake and Post-Blake excursions

The Blake excursion was among the first recognized with directional and intensity behavior known mainly from marine sediment and Chinese loess. Age estimates for the directional shifts in sediments are poorly constrained to about 118−100 ka, i.e., at the marine isotope stage (MIS) 5e/5d boundary. Moreover, sediments at Lac du Bouchet maar, France and along the Portuguese margin reveal what may be a "post-Blake" excursion at about 105−95 ka. The excursional directions are associated with a prominent paleointensity minimum between about 125 and 95 ka in global stacked records. Lava flow recordings of the Blake excursion(s) have, however, been questionable because precise ages required for correlation with these sediment records are lacking. To establish new, independent records of the Blake excursion, and link these into a larger Quaternary GITS, we have undertaken 40Ar/39Ar incremental heating and unspiked K-Ar experiments on groundmass from the transitionally magnetized Inzolfato flow on Lipari Island. We also obtained 40Ar/39Ar incremental heating results for a lava flow on Amsterdam Island originally thought to record the Mono Lake excursion and from the transitionally magnetized El Calderon basalt flow, New Mexico that was K-Ar dated by Champion et al. (1988) at 128 ± 66 ka.Unspiked K-Ar ages of four samples from the Inzolfato flow are 102.5 ± 4.7, 101.3 ± 3.3, 97.1 ± 2.6, and 96.8 ± 3.1 ka and thus indistinguishable from one another. 40Ar/39Ar results are more complex, with three samples yielding discordant age spectra. Based on incremental heating data obtained in both the UW-Madison and Gif-sur-Yvette 40Ar/39Ar laboratories, a fourth sample yields six concordant age plateaus and a weighted mean age of 105.2 ± 1.4 ka that we take as the best estimate of time since the flow erupted. Five 40Ar/39Ar incremental heating experiments on the Amsterdam Island lava yield a plateau age of 120 ± 12 ka, whereas ages from two sites in the Calderon flow are 112 ± 23 and 101 ± 14 ka, together giving a weighted mean of 104 ± 12 ka. The age of 120 ± 12 ka from Amsterdam Island, though imprecise, correlates with the Blake excursion. In contrast, the 104–105 ka age obtained from both Lipari and New Mexico indicates that these lavas record a younger period of dynamo instability, most probably associated with the post-Blake excursion. These radioisotopic ages are consistent with the astronomical ages of two paleointensity minima in the PISO-1500 global stack. Our findings indicate that the Blake and post-Blake excursions are both global features of past geodynamo behavior and support the hypothesis that Brunhes chron excursions are temporally clustered into two groups of at least a half-dozen each spanning over 220 to 30 ka and 720 to 520 ka.

Long-term low latitude cosmogenic 3He production rate determined from a 126 ka basalt from Fogo, Cape Verdes

An ankaramite lava from Fogo, Cape Verdes, has been used to determine the long-term cosmogenic 3He (3Hecos) production rate at low latitude (14.9°N). 40Ar/39Ar incremental heating yield an eruption age of 125.8±7ka. Cosmogenic 3He abundances yield local production rates of 75.6±7.0atoms/g/year in olivine (1σ, n=10) and 70.1±2.9atoms/g/yr in pyroxene (1σ, n=11). The lower 3Hecos production rate in pyroxene is consistent with compositional control on production. When scaled to sea level high latitude using available algorithms, the olivine 3Hecos production rate ranges from 86–109atoms/g/yr. These values overlap the long-term production rate determined at the Canary Islands (99atoms/g/yr). They are, however, lower than previously reported long-term production rates from Hawaii and southern South America (129–152atoms/g/yr), and the range of <20ka calibration sites (110–130atoms/g/yr). Erosion-burial and high atmospheric pressure appear to be of negligible importance. The likeliest explanation is that the geomagnetic field strength was higher in the past. The agreement between the Fogo geomagnetic field strength-corrected production rate scaled using various published scaling algorithms suggests that differences in incorporation of the non-dipole component of the Earth magnetic field in the scaling algorithms are negligible for long-term production rate determinations at low elevation and low latitude. Longitudinal variation in long-term cosmogenic nuclide production rates between the central Atlantic Ocean and the Americas (Hawaii and Patagonia) may exist. This has implications for exposure dating of old surfaces. It will require that locally determined production rates should be used where available, and that scaling schemes incorporating longitudinal variations should be used if nearby calibration sites are not available. Furthermore, if long-term longitudinal production rate variations exist, it implies that variation in the geomagnetic field may not average out after around 20ka, but may require significantly longer periods (up to 100ka).

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

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

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

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 constrain the eruption of the Ninole Basalts from 227 to 108 ka and provide maximum estimates on the timing of the Ka Lae and South Kona landslides.

Deccan volcanism in Rajasthan: 40Ar–39Ar geochronology and geochemistry of the Tavidar volcanic suite

The Tavidar volcanic suite in western Rajasthan, India, comprises a group of lava flows (and subordinate pyroclastic deposits) of highly diverse compositions ranging from basalt through trachyte to rhyolite. We have dated five samples of the Tavidar volcanic rocks by the 40Ar–39Ar incremental heating technique. One trachyte and two rhyolite samples yield very good plateau, isochron and inverse isochron ages of 67–65Ma, typical of the Deccan Traps large igneous province. A subalkalic basalt and a basaltic trachyandesite yield saddle-shaped argon release spectra and show evidence for excess argon. Importantly, all five samples have very similar Nd–Sr isotopic ratios, and constitute a broadly cogenetic magmatic suite with the rhyolites possibly derived by closed-system fractional crystallization of trachytic magmas. The Tavidar rocks’ isotopic data overlap, or are very close to, those of the Mahabaleshwar and Panhala Formation basalts in the Western Ghats type section 700–800km to the southeast. We therefore infer that the Tavidar rocks, having initial Nd values of +3.2 to +0.7, have incorporated only small amounts of lower continental crust. The Tavidar volcanic suite attests to the great areal extent of the Deccan Traps, and reaffirms the great compositional diversity evident in the northwestern Deccan Traps.

In-Depth Study on the Effect of Active-Area Scale-Down of Solution-Processed $\hbox{TiO}_{x}$

The effect of active-area scale-down and improved memory performance of solution-processed TiO_x were investigated using devices with active areas ranging from 50 × 50 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> to 200 × 200 nm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . As the active area decreases, higher operation voltages were required owing to the reduction of unintended extrinsic defects resulting from solution processing. Moreover, faster switching speeds were observed with decreasing active area, which is induced by incremental Joule heating. These scale-down effects provided enhanced reliability characteristics such as highly uniform operation voltages and resistance states and improved pulse endurance by minimizing extrinsic defect-related nonuniformity and introducing additional heating-assisted filamentary switching.