Evaluating crustal contamination in continental basalts: the isotopic composition of the Picture Gorge Basalt of the Columbia River Basalt Group   总被引：4，自引：0，他引：4  

Alan D. Brandon  Peter R. Hooper  Gordon G. Goles  Richard St J. Lambert 《Contributions to Mineralogy and Petrology》1993,114(4):452-464

Crustal contamination of basalts located in the western United States has been generally under-emphasized, and much of their isotopic variation has been ascribed to multiple and heterogeneous mantle sources. Basalts of the Miocene Columbia River Basalt Group in the Pacific Northwest have passed through crust ranging from Precambrian to Tertiary in age. These flows are voluminous, homogenous, and underwent rapid effusion, all of which are disadvantages for crustal contamination while en route to the surface. The Picture Gorge Basalt of the Columbia River Basalt Group erupted through Paleozoic and Mesozoic oceanic accreted terranes in central Oregon, and earlier studies on these basalts provided no isotopic evidence for crustal contamination. New Sr, Nd, Pb, and O isotopic data presented here indicate that the isotopic variation of the Picture Gorge Basalt is very small, ⁸⁷Sr/⁸⁶Sr=0.70307–0.70371, _Nd=+7.7-+4.8, ¹⁸O=+5.6±6.1, and ²⁰⁶Pb/²⁰⁴Pb=18.80–18.91. Evaluation of the Picture Gorge compositional variation supports a model where two isotopic components contributed to Picture Gorge Basalt genesis. The first component (C1) is reflected by low ⁸⁷Sr/⁸⁶Sr, high _Nd, and nonradiogenic Pb isotopic compositions. Basalts with C1 isotopic compositions have large MgO, Ni, and Cr contents and mantle-like ¹⁸O=+5.6. C1 basalts have enrichments in Ba coupled with depletions in Nb and Ta. These characteristics are best explained by derivation from a depleted mantle source which has undergone a recent enrichment by fluids coming from a subducted slab. This C1 mantle component is prevalent throughout the Pacific Northwest. The second isotopic component has higher ⁸⁷Sr/ ⁸⁶Sr and ¹⁸O, lower _Nd, and more radiogenic Pb isotopic compositions than C1. There is a correlation in the Picture Gorge data of Sr, Nd, and Pb isotopes with differentiation indicators such as decreasing Mg#, and increasing K₂O/TiO₂, Ba, Ba/Zr, Rb/Sr, La/Sm, and La/Yb. Phase equilibrium and mineralogical constraints indicate that these compositional characteristics were inherited in the Picture Gorge magmas at crustal pressures, and thus the second isotopic component is most likely crustal in origin. Mixing and open-system calculations can produce the isotopic composition of the most evolved Picture Gorge flows from the most primitive compositions by 8 to 21% contamination of isotopic compositions similar to accreted terrane crust found in the Pacific Northwest. Therefore, in spite of the disadvantages for crustal contamination and their narrow range in isotopic compositions, the process controlling isotopic variation within the Picture Gorge Basalt is primarily crustal contamination. We suggest that comprehensive analyses for basaltic suites and careful consideration of these data must be made to test for crustal contamination, before variation resulting from mantle heterogeneity can be assessed.Deceased  相似文献   

Use of fully softened shear strength in slope stability analysis     

Bernardo A. Castellanos  Thomas L. Brandon  Daniel R. VandenBerge 《Landslides》2016,13(4):697-709

Fully softened shear strength is an important empirical engineering concept for slope stability analyses of cuts in stiff clays and compacted embankments constructed of high-plasticity clays. This concept has been used to explain many first-time failures for which the back calculated shear strength is below the peak strength measured in the laboratory. A comprehensive review of case histories and laboratory studies related to fully softened shear strength was used to assess the application of this concept. The case history data were also used to provide guidelines on the soil types for which fully softened shear strength is appropriate, how and when this shear strength should be used in slope stability analysis, and how the fully softened failure envelope should be characterized. This paper contains specific guidelines on when and how the fully softened shear strength concept should be used in slope stability analyses.  相似文献   

Unusual carbon and oxygen isotopic ratios of ostracodal calcite from last interglacial (Sangamon episode) lacustrine sediment in Raymond Basin, Illinois, USA     

B. Brandon Curry  Thomas F. Anderson  Kyger C. Lohmann 《Journal of Paleolimnology》1997,17(4):421-435

The stable isotopic records of ostracode valves deposited during the last interglaciation in Raymond Basin, Illinois, have ¹³C and ¹⁸O values as high as +16.5 and +9.2 respectively, the highest values yet reported from continental ostracodal calcite. Located in south-central Illinois, Raymond, Pittsburg, Bald Knob, and Hopwood Farm basins collectively have yielded important long pollen and ostracode records that date from about 130000 years ago to the present. Although fossils from the present-day interglaciation are not well preserved, these records constitute the only described, conformable, fossiliferous successions of this age from the interior of glaciated North America.The high ¹³C values from Raymond Basin are attributed to the residual effects of methane loss either by ebullition or by emission through the stems of senescent emergent aquatic vegetation. A mass balance model suggests that an increase in ¹³C of dissolved inorganic carbon on the order of +15 is possible within a few hours given modest rates of methanogenesis of about 0.02 mol m^-2 d^-1. The ¹³C records from other studies of ostracode valves have values approaching, but not exceeding about +14 suggesting a limiting value to ¹³C enrichment due to simultaneous inputs and outputs of dissolved inorganic carbon.Values of ¹⁸O in ostracodal calcite are quite variable (–4 to +9) in sediment from the late Sangamon subepisode. A model of isotopic enrichment in a desiccating water body implies that a reduction in reservoir volume of 20% could produce this range of isotopic values. High humidity and evaporation probably account for most of the ¹⁸O variability.  相似文献   

Climate change in southern Illinois, USA, based on the age and δC of organic matter in cave sediments     

Samuel V. Panno  B. Brandon Curry  Hong Wang  Keith C. Hackley  Chao-Li Liu  Craig Lundstrom  Juanzuo Zhou 《Quaternary Research》2004,61(3):301-313

Matrix-supported diamicton and uniform to laminated, silty, fine-grained sediment deposited from about 42,500 to 27,600 cal yr B.P. under slackwater conditions nearly filled two caves in southwestern Illinois. At some point, most of the sediment was flushed from the caves and from about 22,700 to 4000 cal yr B.P., floods deposited a drape of sandy and silty sediment on remnant slackwater successions, cobbly alluvium, and bedrock (especially from 7700 to 4000 cal yr B.P.). Clay mineral analyses of the slackwater cave sediment reveal a provenance of chiefly Petersburg Silt, a smectite- and illite-rich proglacial lacustrine unit present in the overlying Illinois Episode glacial succession. Today, remnants of the ancient subterranean slackwater deposits nearly fill several secondary passages and, in at least two locations, cover a cobble-mantled strath terrace 1.3 to 1.5 m above active stream channels. Slumping and sinkhole formation appear to have been important mechanisms for deposition of the ancient subterranean deposits. Slumping of these surficial deposits and associated vegetation can occur along the flanks of sinkholes (in addition to sinkhole formation) and enter caves; however, the finer organics, some of them comminuted during transport into the caves, become part of the cave alluvium. This finer organic fraction is the modern analog of the humified organic matter disseminated in slackwater sediment dated in this investigation by radiocarbon methods. Twenty-four ¹⁴C ages on humified organic matter provide chronologic control. The δ¹³C values of the organic matter reflect the proportion of C₄-type to C₃-type vegetation growing in and around swallets and sinkholes at the time of redeposition. Drought-tolerant C₄-type vegetation was more prevalent relative to C₃-type vegetation from 42,500 to 31,200 cal yr B.P. compared to conditions from 28,800 cal yr B.P. to the present. The δ¹³C values are consistent with the results from other investigations of speleothems and organic matter from loessial paleosols.  相似文献   

Characterizing supraglacial meltwater channel hydraulics on the Greenland Ice Sheet from in situ observations          下载免费PDF全文

Colin J. Gleason  Laurence C. Smith  Vena W. Chu  Carl J. Legleiter  Lincoln H. Pitcher  Brandon T. Overstreet  Asa K. Rennermalm  Richard R. Forster  Kang Yang 《地球表面变化过程与地形》2016,41(14):2111-2122

Supraglacial rivers on the Greenland Ice Sheet (GrIS) transport large volumes of surface meltwater toward the ocean, yet have received relatively little direct research. This study presents field observations of channel width, depth, velocity, and water surface slope for nine supraglacial channels on the south‐western GrIS collected between July 23 and August 20, 2012. Field sites are located up to 74 km inland and span 494–1485 m elevation, and contain measured discharges larger than any previous in situ study: from 0.006 to 23.12 m³/s in channels 0.20 to 20.62 m wide. All channels were deeply incised with near vertical banks, and hydraulic geometry results indicate that supraglacial channels primarily accommodate greater discharges by increasing velocity. Smaller streams had steeper water surface slopes (0.74–8.83%) than typical in terrestrial settings, yielding correspondingly high velocities (0.40–2.60 m/s) and Froude numbers (0.45–3.11) with supercritical flow observed in 54% of measurements. Derived Manning's n values were larger and more variable than anticipated from channels of uniform substrate, ranging from 0.009 to 0.154 with a mean value of 0.035 ± 0.027 despite the absence of sediment, debris, or other roughness elements. Ubiquitous micro‐depressions in shallow sections of the channel bed may explain some of these roughness values. However, we find that other, unobserved sources of flow resistance likely contributed to these elevated Manning's n values: future work should explicitly consider additional sources of flow resistance beyond bed roughness in supraglacial channels. We conclude that hydraulic modeling for these channels must allow for both subcritical and supercritical flow, and most importantly must refrain from assuming that all ice‐substrate channels exhibit similar hydraulic behavior, especially for Froude numbers and Manning's n. Finally, this study highlights that further theoretical and empirical work on supraglacial channel hydraulics is necessary before broad scale understanding of ice sheet hydrology can be achieved. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

Evaluating the capabilities of the CASI hyperspectral imaging system and Aquarius bathymetric LiDAR for measuring channel morphology in two distinct river environments          下载免费PDF全文

Carl J. Legleiter  Brandon T. Overstreet  Craig L. Glennie  Zhigang Pan  Juan Carlos Fernandez‐Diaz  Abhinav Singhania 《地球表面变化过程与地形》2016,41(3):344-363

Remote sensing is a powerful tool for examining river morphology. This study used detailed field surveys to assess the capability of the CASI hyperspectral imaging system and Aquarius bathymetric LiDAR to measure bed elevations in rivers with disparate optical characteristics. Field measurements of water column optical properties in the clear Snake River, the more complex Blue and Colorado, and highly turbid Muddy Creek were used to calculate depth retrieval precision and dynamic range. Differences in depth of a few centimeters were detectable via passive optical techniques in the clearest stream, but precision was greatly reduced under turbid conditions. The bathymetric LiDAR evaluated in this study could not detect shallow depths or differences in depth smaller than 11 cm owing to the difficulty of distinguishing water surface and bottom returns in laser waveforms. In clear water and with high radiometric resolution, hyperspectral systems such as CASI could detect depths approaching 10 m, but semi‐empirical analysis of the Aquarius LiDAR indicated that maximum detectable depths were of the order of 2–3 m in the clear‐flowing Snake River, and closer to 1 m in the more turbid streams. Turbidity also constrained spectrally based depth retrieval, and depth estimates from the Blue/Colorado were far less reliable than on the Snake. Both sensors yielded positively biased (0.03 m for CASI, 0.08 m for Aquarius) bed elevations on the Snake, with precisions of 0.16–0.17 m. For the Blue/Colorado, mean errors were of the order of 0.2 m, biased shallow for optical data and biased deep for LiDAR, although no Aquarius laser returns were recorded from the deepest parts of these channels; precisions were reduced to 0.29–0.32 m. Both approaches have advantages and limitations, and prospective users must understand the capabilities and constraints associated with various types of remote sensing to ensure efficient use of these evolving technologies. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Small‐scale variability in surface moisture on a fine‐grained beach: implications for modeling aeolian transport     

Brandon L. Edwards  Steven L. Namikas 《地球表面变化过程与地形》2009,34(10):1333-1338

Small‐scale variations in surface moisture content were measured on a fine‐grained beach using a Delta‐T Theta probe. The resulting data set was used to examine the implications of small‐scale variability for estimating aeolian transport potential. Surface moisture measurements were collected on a 40 cm × 40 cm grid at 10 cm intervals, providing a total of 25 measurements for each grid data set. A total of 44 grid data sets were obtained from a representative set of beach sub‐environments. Measured moisture contents ranged from about 0% (dry) to 25% (saturated), by weight. The moisture content range within a grid data set was found to vary from less than 1% to almost 15%. The magnitude of within‐grid variability varied consistently with the mean moisture content of the grid sets, following an approximately normal distribution. Both very wet and very dry grid data sets exhibited little internal variability in moisture content, while intermediate moisture contents were associated with higher levels of variability. Thus, at intermediate moisture contents it was apparent that some portions of the beach surface could be dry enough to allow aeolian transport (i.e. moisture content is below the critical threshold), while adjacent portions are too wet for transport to occur. To examine the implications of this finding, cumulative distribution functions were calculated to model the relative proportions of beach surface area expected to be above or below specified threshold moisture levels (4%, 7%, and 14%). It was found that the implicit inclusion of small‐scale variability in surface moisture levels typically resulted in changes of less than 1% in the beach area available for transport, suggesting that this parameter can be ignored at larger spatial scales. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

Exhuming the Alps through time: clues from detrital zircon fission-track thermochronology     

M. Bernet  M. Brandon  J. Garver†  M. L. Balestieri‡  B. Ventura§  M. Zattin¶ 《Basin Research》2009,21(6):781-798

The European Alps are a mountain belt that is characterized by a series of discrete orogenic events, which have long been recognized. Despite the inherent episodic nature of orogenic evolution, the Alps have been continuously exhumed, mainly by erosion, but also by normal faulting. Since continental collision started in the late Eocene/Early Oligocene evidence for ongoing erosional exhumation has been preserved in synorogenic sediments that accumulated in basins adjacent to the pro- and retro-side of this double-vergent mountain belt. This long-term erosion record can be used to determine exhumation rates. Lag-times calculated from fission-track (FT) ages of detrital zircon from synorogenic sediments are fairly constant for the European Alps since the Oligocene–Late Miocene. Although the fast exhuming areas were unroofed at rates of 0.4–0.7 km Myr⁻¹, the overall average exhumation rate is between 0.2 and 0.3 km Myr⁻¹ on a regional scale. The detrital and bedrock zircon FT data of the Alps do not detect the increase in erosion rates since the Pliocene over the past ∼5 Myr, as shown elsewhere. This increase cannot be detected yet with the detrital zircon FT method because not enough rock has been removed to widely expose zircons with Pliocene or younger cooling ages in the Alps. Long term (30 Myr) exhumation rates appear to have been approximately constant when averaged over a sliding time window of about 8 Myr, or depth window of 5 to 10 km (ZFT closure depths); shorter-term fluctuations are not identified using this method.  相似文献   

Radiogenic helium in xenoliths from Simcoe, Washington, USA: implications for metasomatic processes in the mantle wedge above subduction zones     

Allen Dodson  Alan D. Brandon   《Chemical Geology》1999,160(4):18-385

We report ³He/⁴He ratios from 10 peridotite xenoliths considered to represent samples of the uppermost mantle wedge above the downgoing Juan de Fuca Plate. Helium isotopic ratios in all but two of the xenoliths are similar to many arc magmas, roughly 7 Ra (1 Ra=atmospheric value). Based on decoupling of He from Sr, Nd and Os in these samples, similar He ratios in olivines from rims of larger xenoliths, and modeling of helium exchange between xenoliths and magmas, we interpret this ratio as that of helium in the host basalt. ³He/⁴He ratios as low as 4.2 Ra are found in olivines from the cores of the two largest xenoliths. These results cannot be reasonably explained by interaction with crustal material or post-eruptive ingrowth of ⁴He, but have been produced by interaction between mantle peridotite and a ⁴He-rich melt or fluid. Either ⁴He already present in the subducting oceanic crust has been retained to significant depths below Simcoe and then directly released behind the arc to interact with the mantle wedge, or, more likely, ⁴He has been produced by decay of U and Th in metasomatized mantle directly above the slab; a He-rich fluid or melt from this source has then ascended and modified the region of mantle represented by the xenoliths. This latter model is supported by estimates of residence time for the Simcoe metasomatic agent from U–Th–Pb isotopic systematics of pyroxenes from the Simcoe peridotites, estimated U and Th concentrations in the source of the fluid or melt, and commonly assumed patterns of helium behavior. This model is also consistent with higher ³He/⁴He ratios typically measured in arc samples; the portion of sub-arc mantle with such low He isotope ratios may be quite small, but the Simcoe xenoliths record a much larger volumetric contribution of the He-rich metasomatic agent than do arc lavas.  相似文献   

Geomorphic response to gravel augmentation and high‐flow dam release in the Trinity River,California          下载免费PDF全文

David Gaeuman  Robert Stewart  Brandon Schmandt  Cort Pryor 《地球表面变化过程与地形》2017,42(15):2523-2540

The geomorphic effect of introducing a gravel augmentation totaling 520 m³ into a gravel‐bed stream during a dam‐controlled flood in May of 2015 was monitored with bedload transport measurements, an array of seismometers, and repeated topographic surveys. Half of the augmented gravel was injected into the flow with front‐end loaders on the rising limb of the flood and the other half was injected on the first day of the peak. Virtually all of the gravel transported past the injection point was deposited within about 7 to 10 channel widths of the injection point. Most of the injected gravel deposited along the left bank of the river whereas the right half of the channel bed was dominated by scour. The downstream third of the depositional area consisted of a small dune field that developed prior to the second gravel injection and subsequently migrated about one channel width downstream. A second depositional front was observed upstream from the gravel injection point, where a delta‐like wedge of bed material developed in the first hours of the flow release and changed little over the remainder of the release. These two depositional areas represent small‐scale bed‐material storage reservoirs with the potential to accumulate and periodically release packets of bed material. Interactions with such storage reservoirs are hypothesized to cause large bed‐material pulses to disperse by fragmenting into multiple smaller pulses. As a refinement to the conceptual model that views sediment pulse evolution in terms of dispersion and translation, the concept of pulse fragmentation has practical implications for gravel management. It implies that gravel augmentations can produce morphologic changes at locations that are separated from the augmentation point by arbitrarily long reaches, and it highlights the dependence of pulse propagation rates on the nature and distribution of the bed‐material storage reservoirs in the channel system. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献