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601.
Rachel L. KLIMA Carlé M. PIETERS M. Darby DYAR 《Meteoritics & planetary science》2008,43(10):1591-1604
Abstract— The 1.2 μm band in near‐infrared spectra of pyroxenes results from Fe2+in the M1 crystallographic site. The distribution of Fe and Mg between the M1 and M2 sites is in part a function of the cooling rate and thermal history of a pyroxene. Combining near‐infrared and Mössbauer spectra for a series of compositionally controlled synthetic Mg, Fe, Ca pyroxenes, we quantify the strength of the 1.2 μm band as a function of Fe2+in the M1 site. Near‐infrared spectra are deconvolved into component absorptions that can be assigned to the M1 and M2 sites using the modified Gaussian model. The relative strength of the 1.2 μm band is shown to be directly related to the amount of Fe2+in the M1 site measured by Mössbauer spectroscopy. The strength of the 1.2 μm band relative to the combined strengths of the 1.2 and 2 μm bands, or the M1 intensity ratio, is calculated for 51 howardite, eucrite, and diogenite (HED) meteorites. Diogenites and cumulate eucrites exhibit the lowest M1 intensity ratios, consistent with their formation as slowly cooled cumulates. Basaltic eucrites exhibit a large range of M1 intensity ratios, all of which are consistently higher than the diogenites and cumulate eucrites. This example illustrates how the M1 intensity ratio can be a used as a tool for characterizing the cooling history of remotely detected pyroxene‐dominated rocks. 相似文献
602.
603.
A survey of 62 small near-Earth asteroids was conducted to determine the rotation state of these objects and to search for rapid rotation. Since results for 9 of the asteroids were previously published (Pravec, P., Hergenrother, C.W., Whiteley, R.J., Šarounová, L., Kušnirák, P., Wolf, M. [2000]. Icarus 147, 477-486; Pravec, P. et al. [2005] Icarus 173, 108-131; Whiteley, R.J., Tholen, D.J., Hergenrother, C.W. [2002a]. Icarus 157, 139-154; Hergenrother, C.W., Whiteley, R.J., Christensen, E.J. [2009]. Minor Planet Bull. 36, 16-18.), this paper will present results for the remaining 53 objects. Rotation periods significantly less than 2 h are indicative of intrinsic strength in the asteroids, while periods longer than 2 h are typically associated with gravitationally bound aggregates. Asteroids with absolute magnitude (H) values ranging from 20.4 to 27.4 were characterized. The slowest rotator with a definite period is 2004 BW18 with a period of 8.3 h, while 2000 DO8 and 2000 WH10 are the fastest with periods of 1.3 min. A minimum of two-thirds of asteroids with H > 20 are fast rotating and have periods significantly faster than 2.0 h. The percentage of rapid rotators increases with decreasing size and a minimum of 79% of H ? 24 objects are rapid rotators. Slowly-rotating objects, some with periods as long as 10-20 h, make up a small though significant fraction of the small asteroid population. There are three fast rotators with relatively large possible diameters (D): 2001 OE84 with 470 ? D ? 820 m (Pravec, P., Kušnirák, P., Šarounová, L., Harris, A.W., Binzel, R.P., Rivkin, A.S. [2002b]. Large coherent Asteroid 2001 OE84. In: Warmbein, B. (Eds.), Proceedings of Asteroids, Comets, Meteors - ACM 2002. Springer, Berlin, pp. 743-745), 2001 FE90 with 265 ? D ? 594 m (Hicks, M., Lawrence, K., Rhoades, H., Somers, J., McAuley, A., Barajas, T. [2009]. The Astronomer’s Telegrams, # 2116), and 2001 VF2 with a possible D of 145 ? D ? 665 m. Using the diameters derived from nominal absolute magnitudes and albedos, the remainder of the fast rotating population is completely consistent with D ? 200 m. Even when taking into account the largest possible uncertainties in the determination of diameters, the remainder must all have D ? 400 m. With the exceptions of 2001 OE84, this result agrees with previous upper diameter limits for fast rotators in Pravec and Harris (Pravec, P., Harris, A.W. [2000]. Icarus 148, 589-593) and Whiteley et al. (Whiteley, R.J, Tholen, D.J., Hergenrother, C.W. [2002a]. Icarus 157, 139-154. 相似文献
604.
Carl B. Agee 《Earth and Planetary Science Letters》2008,265(3-4):641-654
High pressure experiments using the sink/float method have bracketed the density of hydrous iron-rich ultrabasic silicate melt from 1.35 to 10.0 GPa at temperatures from 1400 to 1860 °C. The silicate melt composition was a 50–50 mixture of natural komatiite and synthetic fayalite. Water was added in the form of brucite Mg(OH)2 and was present in the experimental run products at 2 wt.% and 5 wt.% levels as confirmed by microprobe analyses of total oxygen. Buoyancy marker spheres were olivines and garnets of known composition and density. The density of the silicate melt with 5 wt.% water at 2 GPa and 1500 °C is 0.192 g cm? 3 less than the anhydrous form of this melt at the same P and T. This density difference gives a partial molar volume of water in silicate melt of ~ 7 cm3 mol? 1, which is similar to previous studies at high pressure. The komatiite–fayalite liquids with 0 and 2 wt.% H2O, have extrapolated density crossovers with equilibrium liquidus olivine at 8 and 9 GPa respectively, but there is no crossover for the liquid with 5 wt.% H2O. These results are consistent with the hypothesis that dense hydrous melts could be gravitationally stable atop the 410 km discontinuity in the Earth. The results also support the notion that equilibrium liquidus olivine could float in an FeO-rich hydrous martian magma ocean. Extrapolation of the data suggests that FeO-rich hydrous melt could be negatively buoyant in the Earth's D″-region or atop the core–mantle-boundary (CMB), although experiments at higher pressure are needed to confirm this prediction. 相似文献
605.
Davison Mwale Thian Yew Gan Kevin Devito Carl Mendoza Uldis Silins Richard Petrone 《水文研究》2009,23(21):3040-3056
By applying wavelet‐based empirical orthogonal function (WEOF) analysis to gridded precipitation (P) and empirical orthogonal function (EOF) analysis to gridded air temperature (T), potential evapotranspiration (PET), net precipitation (P‐PET) and runoff (Q), this paper examines the spatial, temporal and frequency patterns of Alberta's climate variability. It was found that only WEOF‐based precipitation patterns, possibly modulated by El Nino Southern Oscillation (ENSO) and Pacific Decadal Oscillation(PDO), delineated Alberta into four major regions which geographically represent northern Alberta Boreal forests, southern Alberta grasslands and Aspen Parklands and the Rocky Mountains and Foothills. The leading mode of wavelet‐based precipitation variability WPC1 showed that between 1900 and 2000, a wet climate dominated northern Alberta with significant 4–8, 11 and 25‐year periodic cycles, while the second mode WPC2 showed that between 1960 and 2000, southern Alberta grasslands were characterized by decreasing precipitation, dominated by 11‐year cycles, and the last two modes WPC3 and WPC4 were characterized by 4–7 and 25‐year cycles and both delineated regions where moisture from the Pacific Ocean penetrated the Rocky Mountains, accounted for much of the sub‐alpine climate. These results show that WEOF is superior to EOF in delineating Alberta precipitation variability to sub‐regions that more closely agree with its eco‐climate regions. Further, it was found that while WPC2 could not explain runoff variations in southern Alberta, WPC1, WPC3 and WPC4 accounted for runoff variability in their respective sub‐regions. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
606.
Muhammad Sadiq Carl C. Tscherning Zulfiqar Ahmad 《Studia Geophysica et Geodaetica》2009,53(3):375-388
This paper deals with the analysis of gravity anomaly and precise levelling in conjunction with GPS-Levelling data for the
computation of a gravimetric geoid and an estimate of the height system bias parameter No for the vertical datum in Pakistan by means of least squares collocation technique. The long term objective is to obtain
a regional geoid (or quasi-geoid) modeling using a combination of local data with a high degree and order Earth gravity model
(EGM) and to determine a bias (if there is one) with respect to a global mean sea surface. An application of collocation with
the optimal covariance parameters has facilitated to achieve gravimetric height anomalies in a global geocentric datum. Residual
terrain modeling (RTM) technique has been used in combination with the EGM96 for the reduction and smoothing of the gravity
data. A value for the bias parameter No has been estimated with reference to the local GPS-Levelling datum that appears to be 0.705 m with 0.07 m mean square error.
The gravimetric height anomalies were compared with height anomalies obtained from GPS-Levelling stations using least square
collocation with and without bias adjustment. The bias adjustment minimizes the difference between the gravimetric height
anomalies with respect to residual GPS-Levelling data and the standard deviation of the differences drops from 35 cm to 2.6
cm. The results of this study suggest that No adjustment may be a good alternative for the fitting of the final gravimetric geoid as is generally done when using FFT methods. 相似文献
607.
Alfonso Benavides I Mark E. Everett Carl Pierce Jr. 《Stochastic Environmental Research and Risk Assessment (SERRA)》2009,23(2):169-179
Self-organizing maps (SOM) are implemented for discrimination of geologic noise, buried metal objects and unexploded ordnance
using the geophysical method of time-domain electromagnetic induction. The learning and misfit measures are based on a Euclidean
metric. The U*-matrix method is shown to be a reliable tool for determining data clusters and cluster boundaries. The performance
of SOM for data-type discrimination was tested using three synthetic, idealized geophysical datasets consisting of exponential,
multi-exponential and stretched-exponential decaying transients. In addition, experimental data were acquired using a modified
Geonics EM63 instrument. Results from the synthetic examples show that SOM clusters the data based on their functional origin,
when represented using U*-matrices. The percentage of correct classification is 100%. Unsupervised learning using the field
dataset obtained with the Geonics EM63 succeeded in producing a multi-clustered map in which the background transients cluster
themselves and are separated from clusters associated with metal clutter objects and UXO. Even though in some cases the SOM
did not produce a single cluster for each type of causative body, it was able to separate clutter data from target data by
producing several small clusters. The results are encouraging in view of the heterogeneity and sparsity of the training dataset. 相似文献
608.
Assessing aquatic macrophyte community change through the integration of palaeolimnological and historical data at Loch Leven,Scotland 总被引:1,自引:0,他引:1
Jorge Salgado Carl Sayer Laurence Carvalho Thomas Davidson Iain Gunn 《Journal of Paleolimnology》2010,43(1):191-204
Submerged macrophytes have a critical role in lake ecosystems affecting nutrient cycling, sediment stability, and community
composition across multiple trophic levels. Consequently temporal changes in the composition of submerged plant populations
can have profound ecological implications and key significance from the perspective of lake conservation. By focusing on macro-remains
of aquatic macrophytes and extensive historical plant records spanning the last approximately 180 years, this study seeks
to evaluate a combined historical-palaeolimnological approach for establishing pre-disturbance macrophytes communities in
Loch Leven, Kinross, Scotland and to provide new information regarding temporal trends in its macrophyte vegetation as well
as potential drivers of change. Some 81% of the species historically recorded for the core site (east side of St. Serf’s Island)
were found as macro-remains. Potamogeton taxa were underrepresented, whereas remains of Elatine hexandra, a small species never recorded historically were found. The core sequence showed good agreement with known floristic changes
including an early (pre- ca. 1850) loss of Isoetes lacustris and Lobelia
dortmanna and a more recent (post-1910) shift to dominance by Potamogeton and Chara taxa associated with eutrophic conditions. A clear pattern in the relationship between macrofossil principal component analysis
(PCA) and loss on ignition suggested a key control of sediment conditions on macrophyte community structure. In particular
the major macrophyte community change of the mid-nineteenth century was concurrent with a substantial increase in organic
matter, likely linked to a historic lake lowering (early 1830s) which would have beached the former gravel-sand shoreline
leading to a much siltier lake littoral. Although recent monitoring data show signs of ecological recovery our study illustrates
that Loch Leven remains a long way from its reference state as a lake with characteristic soft-water macrophytes. To achieve
a full recovery, sediment properties would need to change in addition to nutrient reduction. Consequently restoration strategies
will need to compromise between the desirability of achieving the pre-disturbance state and what is feasible and practicable.
Our study shows the clear potential role of a combined palaeolimnological-historical approach for informing lake management
decisions. 相似文献
609.
Arctic Alaska is a ‘suspect’ terrane that encompasses approximately 20% of Alaska, stretching from the southern Brooks Range all the way to the continental shelves of the Chukchi and Beaufort Seas. Although the origin and subsequent travels of this large crustal fragment are debated among geologists, most researchers agree upon its composite nature and exotic origin. To constrain the early geological history of this terrane, we describe a recent expedition to the Doonerak fenster of the central Brooks Range. This area has long been regarded as a key locality for understanding the structural evolution of the Mesozoic–Cenozoic Brooks Range orogen; however, our target was different: a unique sequence of volcanic and siliciclastic rocks (Apoon assemblage) exposed beneath a profound pre-Mississippian unconformity, which we argue is of key importance to understanding the early Paleozoic tectonic history of northern Alaska and the greater Arctic. 相似文献
610.
Gregory J. Holk Marty Grove Carl E. Jacobson Gordon B. Haxel 《International Geology Review》2017,59(5-6):639-663
ABSTRACTStable isotopes combined with pre-existing 40Ar/39Ar thermochronology at the Gavilan Hills and Orocopia Mountains in southeastern California record two stages of fluid–rock interaction: (1) Stage 1 is related to prograde metamorphism as Orocopia Schist was accreted to the base of the crust during late Cretaceous–early Cenozoic Laramide flat subduction. (2) Stage 2 affected the Orocopia Schist and is related to middle Cenozoic exhumation along detachment faults. There is no local evidence that schist-derived fluids infiltrated structurally overlying continental rocks. Mineral δ18O values from Orocopia Schist in the lower plate of the Chocolate Mountains fault and Gatuna normal fault in the Gavilan Hills are in equilibrium at 490–580°C with metamorphic water (δ18O = 7–11‰). Phengite and biotite δD values from the Orocopia Schist and upper plate suggest metamorphic fluids (δD ~ –40‰). In contrast, final exhumation of the schist along the Orocopia Mountains detachment fault (OMDF) in the Orocopia Mountains was associated with alteration of prograde biotite and amphibole to chlorite (T ~ 350–400°C) and the influx of meteoric-hydrothermal fluids at 24–20 Ma. Phengites from a thin mylonite zone at the top of the Orocopia Schist and alteration chlorites have the lowest fluid δD values, suggesting that these faults were an enhanced zone of meteoric fluid (δD < –70‰) circulation. Variable δD values in Orocopia Schist from structurally lower chlorite and biotite zones indicate a lesser degree of interaction with meteoric-hydrothermal fluids. High fluid δ18O values (6–12‰) indicate low water–rock ratios for the OMDF. A steep thermal gradient developed across the OMDF at the onset of middle Cenozoic slip likely drove a more vigorous hydrothermal system within the Orocopia Mountains relative to the equivalent age Gatuna fault in the Gavilan Hills. 相似文献