On yoderite: Using calculated phase equilibria to investigate its rarity in the geological record of whiteschists          下载免费PDF全文

Brandon L. Alessio  David E. Kelsey 《Journal of Metamorphic Geology》2018,36(3):297-314

A new activity–composition model is presented for green (Mn³⁺‐absent) yoderite for use in the latest internally consistent thermodynamic data set used by THERMOCALC, for calculations primarily in MgO–Al₂O₃–SiO₂–H₂O–O system, where O is a proxy for Fe₂O₃. P–T grids calculated with our model in the MASH and MASHO system feature invariant points and univariant reaction bundles that are consistent with existing experimental results. Using this new model, we have explored the stability of yoderite in whiteschists, a rare type of high‐pressure rock that conforms closely to the MASHO system. Using a series of calculated models in which composition varies, it is shown that yoderite stability is a function of bulk‐rock SiO₂, MgO and Al₂O₃, where the most important component for stabilizing yoderite is a function of pressure and temperature. The rarity of yoderite in naturally occurring whiteschists is largely related to these compositional factors, with most whiteschists having rock compositions that are too SiO₂‐rich and Al₂O₃‐poor to allow yoderite formation. However, in addition to compositional factors, the calculated P–T stability field of yoderite occurs over thermal gradients that are generally too high to occur in modern‐style subduction zones. As nearly all known whiteschist occurrences are Phanerozoic in age, the near‐complete absence of yoderite in late Neoproterozoic–Phanerozoic whiteschists may be at least partially due to modern subduction systems failing to produce the hotter thermal gradients needed to stabilize yoderite. The provision of this new a–x model for green yoderite allows for more rigorous P–T–X investigations of all whiteschists.  相似文献   

Triaxial shear testing of polished slickensided surfaces   总被引：1，自引：1，他引：0  

Christopher L. Meehan  Binod Tiwari  Thomas L. Brandon  J. Michael Duncan 《Landslides》2011,8(4):449-458

A series of consolidated-drained triaxial tests were performed on precut and polished clay specimens to measure their drained residual strength. Two soils were tested during this research program: Rancho Solano Clay and San Francisco Bay Mud. Specimens were tested using a specially modified triaxial compression device which was developed to minimize the effects of end-platen restraint on the measured strengths. Special attention was paid to the influence of changing specimen area and membrane effects during the test. Triaxial test results were compared with baseline measurements of drained residual shear strength that were made for each of the clay soils using Bromhead ring shear tests and polished-specimen direct shear tests. Residual strength values measured in the triaxial device were higher than the drained residual strengths measured in the Bromhead ring shear apparatus and the direct shear device, indicating that this test approach is more challenging than the use of direct shear tests conducted on polished slickensided surfaces. Comparison of single stage and multistage triaxial test data indicates that multistage triaxial testing may work well for specimens that fail along a well-defined plane, provided that careful attention is given to the effects of end platen restraint, membrane restraint, and changes in specimen area during shear.  相似文献   

Quantifying the influence of channel sinuosity on the depositional mechanics of channelized turbidity currents: A laboratory study     

Kyle M. Straub  David MohrigJames Buttles  Brandon McElroyCarlos Pirmez 《Marine and Petroleum Geology》2011,28(3):744-760

Here we present results from a suite of laboratory experiments that highlight the influence of channel sinuosity on the depositional mechanics of channelized turbidity currents. We released turbidity currents into three channels in an experimental basin filled with water and monitored current properties and the evolution of topography via sedimentation. The three channels were similar in cross-sectional geometry but varied in sinuosity. Results from these experiments are used to constrain the run-up of channelized turbidity currents on the outer banks of moderate to high curvature channel bends. We find that a current is unlikely to remain contained within a channel when the kinetic energy of a flow exceeds the potential energy associated with an elevation gain equal to the channel relief; setting an effective upper limit for current velocity. Next we show that flow through bends induces a vertical mixing that redistributes suspended sediment back into the interiors of depositional turbidity currents. This mixing counteracts the natural tendency for suspended sediment concentration and grain size to stratify vertically, thereby reducing the rate at which sediment is lost from a current via deposition. Finally, the laboratory experiments suggest that turbidity currents might commonly separate from channel sidewalls along the inner banks of bends. In some cases, sedimentation rates and patterns within the resulting separation zones are sufficient to construct bar forms that are attached to the channel sidewalls and represent an important mechanism of submarine channel filling. These bar forms have inclined strata that might be mistaken for the deposits of point bars and internal levees, even though the formation mechanism and its implications to channel history are different.  相似文献   

High-precision osmium isotopes in enstatite and Rumuruti chondrites     

David van Acken  Alan D. Brandon 《Geochimica et cosmochimica acta》2011,75(14):4020-4036

Isotopic heterogeneity within the solar nebula has been a long-standing issue. Studies on primitive chondrites and chondrite components for Ba, Sm, Nd, Mo, Ru, Hf, Ti, and Os yielded conflicting results, with some studies suggesting large-scale heterogeneity. Low-grade enstatite and Rumuruti chondrites represent the most extreme ends of the chondrite meteorites in terms of oxidation state, and might thus also present extremes if there is significant isotopic heterogeneity across the region of chondrite formation. Osmium is an ideal tracer because of its multiple isotopes generated by a combination of p-, r-, and s-process and, as a refractory element; it records the earliest stages of condensation.Some grade 3-4 enstatite and Rumuruti chondrites show similar deficits of s-process components as revealed by high-precision Os isotope studies in some low-grade carbonaceous and ordinary chondrites. Enstatite chondrites of grades 5-6 have Os isotopic composition identical within error to terrestrial and solar composition. This supports the view of digestion-resistant presolar grains, most likely SiC, as the major carrier of these anomalies. Destruction of presolar grains during parent body processing, which all high-grade enstatite chondrites, but also some low-grade chondrites seemingly underwent, makes the isotopically anomalous Os accessible for analysis. The magnitude of the anomalies is consistent with the presence of a few ppm of presolar SiC with a highly unusual isotopic composition, produced in a different stellar environment like asymptotic giant branch stars (AGB) and injected into the solar nebula. The presence of similar Os isotopic anomalies throughout all major chondrite groups implies that carriers of Os isotopic anomalies were homogeneously distributed in the solar nebula, at least across the formation region of chondrites.  相似文献   

A GIS-based method for defining snow zones: application to the western United States     

Cara Moore  Brandon Stone  Eric Richer 《国际地球制图》2015,30(1):62-81

This study maps the geographic extent of intermittent and seasonal snow cover in the western United States using thresholds of 2000–2010 average snow persistence derived from moderate resolution imaging spectroradiometer snow cover area data from 1 January to 3 July. Results show seasonal snow covers 13% of the region, and intermittent snow covers 25%. The lower elevation boundaries of intermittent and seasonal snow zones increase from north-west to south-east. Intermittent snow is primarily found where average winter land surface temperatures are above freezing, whereas seasonal snow is primarily where winter temperatures are below freezing. However, temperatures at the boundary between intermittent and seasonal snow exhibit high regional variability, with average winter seasonal snow zone temperatures above freezing in west coast mountain ranges. Snow cover extent at peak accumulation is most variable at the upper elevations of the intermittent snow zone, highlighting the sensitivity of this snow zone boundary to climate conditions.  相似文献   

A revised evolutionary history of armadillos (Dasypus) in North America based on ancient mitochondrial DNA          下载免费PDF全文

Beth Shapiro  Russell W. Graham  Brandon Letts 《Boreas: An International Journal of Quaternary Research》2015,44(1):14-23

The large, beautiful armadillo, Dasypus bellus, first appeared in North America about 2.5 million years ago, and was extinct across its southeastern US range by 11 thousand years ago (ka). Within the last 150 years, the much smaller nine‐banded armadillo, D. novemcinctus, has expanded rapidly out of Mexico and colonized much of the former range of the beautiful armadillo. The high degree of morphological similarity between these two species has led to speculation that they might be a single, highly adaptable species with phenotypical responses and geographical range fluctuations resulting from environmental changes. If this is correct, then the biology and tolerance limits for D. novemcinctus could be directly applied to the Pleistocene species, D. bellus. To investigate this, we isolated ancient mitochondrial DNA from late Pleistocene‐age specimens of Dasypus from Missouri and Florida. We identified two genetically distinct mitochondrial lineages, which most likely correspond to D. bellus (Missouri) and D. novemcinctus (Florida). Surprisingly, both lineages were isolated from large specimens that were identified previously as D. bellus. Our results suggest that D. novemcinctus, which is currently classified as an invasive species, was already present in central Florida around 10 ka, significantly earlier than previously believed.  相似文献   

Spatial interpolation of snow water equivalency using surface observations and remotely sensed images of snow‐covered area     

Brian J. Harshburger  Karen S. Humes  Von P. Walden  Troy R. Blandford  Brandon C. Moore  Raymond J. Dezzani 《水文研究》2010,24(10):1285-1295

As demand for water continues to escalate in the western Unites States, so does the need for accurate monitoring of the snowpack in mountainous areas. In this study, we describe a simple methodology for generating gridded‐estimates of snow water equivalency (SWE) using both surface observations of SWE and remotely sensed estimates of snow‐covered area (SCA). Multiple regression was used to quantify the relationship between physiographic variables (elevation, slope, aspect, clear‐sky solar radiation, etc.) and SWE as measured at a number of sites in a mountainous basin in south‐central Idaho (Big Wood River Basin). The elevation of the snowline, obtained from the SCA estimates, was used to constrain the predicted SWE values. The results from the analysis are encouraging and compare well to those found in previous studies, which often utilized more sophisticated spatial interpolation techniques. Cross‐validation results indicate that the spatial interpolation method produces accurate SWE estimates [mean R² = 0·82, mean mean absolute error (MAE) = 4·34 cm, mean root mean squared error (RMSE) = 5·29 cm]. The basin examined in this study is typical of many mid‐elevation mountainous basins throughout the western United States, in terms of the distribution of topographic variables, as well as the number and characteristics of sites at which the necessary ground data are available. Thus, there is high potential for this methodology to be successfully applied to other mountainous basins. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Mineralogy and petrogenesis of lunar magnesian granulitic meteorite Northwest Africa 5744          下载免费PDF全文

Jeremy J. Kent  Alan D. Brandon  Katherine H. Joy  Anne H. Peslier  Thomas J. Lapen  Anthony J. Irving  Daniel M. Coleff 《Meteoritics & planetary science》2017,52(9):1916-1940

Lunar meteorite Northwest Africa (NWA) 5744 is a granulitic breccia with an anorthositic troctolite composition that may represent a distinct crustal lithology not previously described. This meteorite is the namesake and first‐discovered stone of its pairing group. Bulk rock major element abundances show the greatest affinity to Mg‐suite rocks, yet trace element abundances are more consistent with those of ferroan anorthosites. The relatively low abundances of incompatible trace elements (including K, P, Th, U, and rare earth elements) in NWA 5744 could indicate derivation from a highlands crustal lithology or mixture of lithologies that are distinct from the Procellarum KREEP terrane on the lunar nearside. Impact‐related thermal and shock metamorphism of NWA 5744 was intense enough to recrystallize mafic minerals in the matrix, but not intense enough to chemically equilibrate the constituent minerals. Thus, we infer that NWA 5744 was likely metamorphosed near the lunar surface, either as a lithic component within an impact melt sheet or from impact‐induced shock.  相似文献   

Use of a vertical δ18O profile to constrain hydraulic properties and recharge rates across a glacio-lacustrine unit, Nantucket Island, Massachusetts, USA     

Mark Person  Andee Marksamer  Brandon Dugan  Peter E. Sauer  Kyle Brown  David Bish  Kathy J. Licht  Mark Willett 《Hydrogeology Journal》2012,20(2):325-336

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An assessment of geologic sequestration potential in the panhandle of Florida USA     

Christopher J. Brown  Brandon K. Poiencot  Nick Hudyma  Barry Albright  Richard A. Esposito 《Environmental Earth Sciences》2014,71(2):793-806

One alternative to reduce global greenhouse gas emissions is to store the emissions in underground geologic sequestration repositories. The efficacy of this approach has been favorably evaluated by numerous authors over the last 15 years. This paper discusses an assessment of the overall feasibility of storing emissions in three different repositories in the Florida panhandle located in the Southeastern United States. The feasibility assessment evaluates both saline aquifers and oil reservoirs located in the panhandle region. The overall feasibility is driven by the available geologic sequestration capacity, the transportation cost to deliver emissions to a respective repository, and other engineering and regulatory issues. The geologic sequestration capacity is generally controlled by the so-called storage efficiency, a variable dependent on the site-specific geology, reservoir conditions, and the injected fluid characteristics. For this paper, storage efficiency for saline repositories was assessed in more detail using numerical modeling. Based on the work completed, the 3 repositories studied have at least 4.55 gigatonnes of capacity to sequester CO₂.  相似文献