We show from elastic neutron diffraction data that anorthite CaAl2Si2O8 (An100) undergoes a primitive to body-centered phase transition at Tc=237±1°C. The transition is reversible, and Tc is well defined. Our measurements demonstrate that the I-lattice at the high temperature phase applies to all structural elements; in other words, the time-averaged lattice is exactly body-centered and not just in the space average of An100, as concluded earlier by other authors. 相似文献
Basalts dredged from the Bouvet triple junction (South Atlantic), from the Mid-Atlantic Ridge near the triple junction, and from a spreading center east of Bouvet Island differ from normal mid-ocean ridge tholeiites by having higher concentrations of K and other large-ion-lithophile elements, higher ratios, and rare earth element distributions which show relative enrichment in the lighter rare earths. The basalts are more fractionated than typical oceanic tholeiites, however, fractional crystallization does not fully account for their chemical characteristics, and it appears that they were derived from special source materials, contaminated perhaps by a mantle plume rising beneath Bouvet Island. 相似文献
The Betts Cove ophiolite includes the components of typical ocean crust: pillow lavas, sheeted dikes, gabbros and ultramafics. However, the trace element geochemistry of basaltic rocks is unusual. Three geochemical units are recognized within the lava and dike members. Within the pillow lavas, the geochemical units correspond to stratigraphic units. Upper lavas have ‘normal’ (i.e., typical for ocean floor basalts) TiO2 contents (0.75 to 2.0 wt%), heavy rare earth elements (HREE) values in the range 6–20× chondrites and chondrite-normalized REE patterns with relative LREE depletion. Intermediate lavas have TiO2 contents between 0.30 and 0.50 wt%, HREE contents from 4–7× chondrites and extreme relative LREE depletion. Lower lavas have anomalously low TiO2 contents (<0.30 wt%) and unusual convex-downwards REE patterns with REE abundances around 2–5 × chondrite. These geochemical differences can be explained if the three groups were derived from different mantle sources. Independent mantle sources for the three units are consistent with their different ratios varying at 480 m.y.B.P. from 0.51222 in a lower lava to 0.51238 in an upper lava. The upper lavas may be partial melts of a source similar in composition to that of modern MORB, the intermediate lavas may be from a very depleted oceanic mantle (second stage melt), and the lower lavas may have formed by melting an extremely depleted mantle that had been invaded by a LREE-enriched fluid. A possible tectonic environment where these different sources could be juxtaposed is a back-arc or inter-arc basin. 相似文献
Post-glacial tholeiitic basalts from the western Reykjanes Peninsula range from picrite basalts (oldest) to olivine tholeiites to tholeiites (youngest). In this sequence there are large systematic variations in rare earth element (REE) abundances (La/Sm normalized to chondrites ranges from 0.33 in the picrite basalts to 1.25 in the fissure tholeiites) and corresponding variations in 143Nd/144Nd (0.51317 in the picrite basalts to 0.51299 in the fissure tholeiites). The large viaration in 143Nd/144Nd, more than one-third the total range observed in most ocean islands and mid-ocean ridge basalts (MORB), is accompanied by only a small variation in 87Sr/86Sr (0.7031–0.7032). These 87Sr/86Sr ratios are within the range of other Icelandic tholeiites, and distinct from those of MORB.We conclude that the mantle beneath the Reykjanes Peninsula is heterogeneous with respect to relative REE abundances and 143Nd/144Nd ratios. On a time-averaged basis all parts of this mantle show evidence of relative depletion in light REE. Though parts of this mantle have REE abundances and Nd isotope ratios similar to the mantle source of “normal” MORB, 87Sr/86Sr is distinctly higher. Unlike previous studies we find no evidence for chondritic relative REE abundances in the mantle beneath the Reykjanes Peninsula; in fact, the data require significant chemical heterogeneity in the hypothesized mantle plume beneath Iceland, as well as lateral mantle heterogeneity from the Reykjanes Ridge to the Reykjanes Peninsula. The compositional range of the Reykjanes Peninsula basalts is consistent with mixing of magmas produced by different degrees of melting in different parts of the heterogeneous mantle source beneath the Reykjanes Peninsula. 相似文献
Tholeiitic basalts dredged from the Mid-Atlantic Ridge (MAR) axis at 43 ° N are enriched in incompatible trace elements compared to the ‘ normal’ incompatible element depleted tholeiites found from 49 ° N to 59 ° N and south of 33 ° N on the MAR. The most primitive 43 ° N glasses have MgO/FeO*= 1.2 and coexist with olivine (Fo90–91) and chrome-rich spinel. The tholeiitic basalts from the MAR 43 ° N are distinct from the strongly incompatible trace element depleted tholeiities found elsewhere in the Atlantic, and have trace element features typical of island tholeiities and MAR axis tholeiites from 45 ° N. Petrographic, major, and compatible trace element trends of the axial valley tholeiites at 43 ° N are consistent with shallow-level fractionation; in particular, evolution from primitive liquids with forsteritic olivine plus chrome spinel as liquidus phases to fractionated liquids with plagioclase plus clinopyroxene as major crystallizing phases. However, each dredge haul has distinctive incompatible trace element abundances. These trace element characteristics require a hetrogeneous mantle or complex processes such as open system fractional crystallization and magma mixing. Alkali basalts (~5% normative nepheline) were dredged from a prominent fracture zone at 43 ° N. Typical of alkali basalts they are strongly enriched (compared to tholeiites) in incompatible elements. Their highly fractionated rare-earth element (REE) abundances require residual garnet during partial melting. The 43 ° N tholeiites and alkali basalts could be derived from a garnet peridotite source with REE contents equal to 2 × chondrites by ~5% and 1% melting, respectively. Alternatively, they could be derived from a moderately light REE enriched source by ~25% and 9.5% melting, respectively. 相似文献
The spatial variability of turbulent flow statistics in the roughness sublayer (RSL) of a uniform even-aged 14 m (= h) tall loblolly pine forest was investigated experimentally. Using seven existing walkup towers at this stand, high frequency velocity, temperature, water vapour and carbon dioxide concentrations were measured at 15.5 m above the ground surface from October 6 to 10 in 1997. These seven towers were separated by at least 100m from each other. The objective of this study was to examine whether single tower turbulence statistics measurements represent the flow properties of RSL turbulence above a uniform even-aged managed loblolly pine forest as a best-case scenario for natural forested ecosystems. From the intensive space-time series measurements, it was demonstrated that standard deviations of longitudinal and vertical velocities (u, w) and temperature (T) are more planar homogeneous than their vertical flux of momentum (u*2) and sensible heat (H) counterparts. Also, the measured H is more horizontally homogeneous when compared to fluxes of other scalar entities such as CO2 and water vapour. While the spatial variability in fluxes was significant (>15 %), this unique data set confirmed that single tower measurements represent the canonical structure of single-point RSL turbulence statistics, especially flux-variance relationships. Implications to extending the moving-equilibrium hypothesis for RSL flows are discussed. The spatial variability in all RSL flow variables was not constant in time and varied strongly with spatially averaged friction velocity u*, especially when u* was small. It is shown that flow properties derived from two-point temporal statistics such as correlation functions are more sensitive to local variability in leaf area density when compared to single point flow statistics. Specifically, that the local relationship between the reciprocal of the vertical velocity integral time scale (Iw) and the arrival frequency of organized structures (/h) predicted from a mixing-layer theory exhibited dependence on the local leaf area index. The broader implications of these findings to the measurement and modelling of RSL flows are also discussed. 相似文献
In order to evaluate factors controlling transpiration of six common eastern deciduous species in North America, a model describing responses of canopy stomatal conductance (GS) to net radiation (RN), vapor pressure deficit (D) and relative extractable soil water (REW) was parameterized from sap flux data. Sap flux was measured in 24 mature trees consisting of the species Carya tomentosa, Quercus alba, Q. rubra, Fraxinus americana, Liriodendron tulipifera, and Liquidambar styraciflua in a bottomland oak-hickory forest in the Duke Forest, NC. Species differences in model coefficients were found during the 1997 growing season. All species showed a reduction in GS with increasing D. RN influenced GS in the overstory shade intolerant L. styraciflua to a larger extent than the other species measured. In addition, despite a severe drought during the study period, only L. tulipifera showed a decline in GS with decreasing REW. The primary effect of the drought for the other species appeared to be early autumn leaf senescence and abscission. As a result, despite the drought in this bottomland forest accustomed to ample water supply, maximum daily transpiration (1.6 mm) and growing season transpiration (264 mm) were similar to a nearby upland forest measured during a year of above average precipitation. These results may aid in assessing differences in water use and the ability of bottomland deciduous species to tolerate alterations in the frequency or amount of precipitation. Results also suggest little variation in water use among forests of similar composition and structure growing in different positions in the landscape and subjected to large interannual variation in water supply. 相似文献
The purpose of this study is to obtain a better understanding of groundwater contamination processes in an arid environment (precipitation of 50 mm/year) due to cultivation. Additional aims were to study the fate of N, K, and other ions along the whole hydrological system including the soil and vadose zone, and to compare groundwater in its natural state with contaminated groundwater (through the drilling of several wells).
A combination of physical, chemical, and isotopic analyses was used to describe the hydrogeological system and the recharge trends of water and salts to the aquifers. The results indicate that intensive irrigation and fertilization substantially affected the quantity and quality of groundwater recharge. Low irrigation efficiency of about 50% contributes approximately 3.5–4 million m3/year to the hydrological system, which corresponds to 0.65 m per year of recharge in the irrigated area, by far the most significant recharge mechanism.
Two main contamination processes were identified, both linked to human activity: (1) salinization due to circulation of dissolved salts in the irrigation water itself, mainly chloride, sulfate, sodium and calcium, and (2) direct input of nitrate and potassium mainly from fertilizers.
The nitrate concentrations in a local shallow groundwater lens range between 100 and 300 mg/l and in the upper sub-aquifer are over 50 mg/l. A major source of nitrate is fertilizer N in the excess irrigation water. The isotopic compositions of δ15N–NO3 (range of 4.9–14.8‰) imply also possible contributions from nearby sewage ponds and/or manure. Other evidence of contamination of the local groundwater lens includes high concentrations of K (20–120 mg/l) and total organic carbon (about 10 mg/l). 相似文献
This paper uses three‐dimensional (3D) seismic data from the continental margin of Israel (Eastern Mediterranean) to describe a series of slump deposits within the Pliocene and Holocene succession. These slumps are linked to the dynamics of subsidence and deformation of the transform margin of the eastern Mediterranean. Repeated slope failure occurred during the post‐Messinian, when a clay‐dominated progradational succession was forming. This resulted in large‐scale slump deposits accumulating in the mid‐lower slope region of the basin at different stratigraphic levels. It is probable that the slumps were triggered by a combination of slope oversteepening, seismic activity and gas migration. The high spatial resolution provided by the 3D seismic data has been used to define a spectrum of internal and external geometries within slump deposits. Importantly, we recognise two main zones for many of the slumps on this margin: a depletion zone and an accumulation zone. The former is characterised by extension and translation, and the latter by complex imbricate thrusts and fold systems. Volume‐based seismic attribute analysis reveals transport directions within the slump deposits, which are predominately downslope, but with subtle variations particularly at the lateral margins. Basal shear surfaces are observed to ramp both up and down stratigraphy. Slump evolution occurs both by retrogressive upslope failure, and by downslope propagation (out‐of‐sequence) failure. Slump anatomy and the combination of factors responsible for slump failure and transport are relatively poorly understood, mainly because of the limited 3D of outcrop control; hence, this subsurface study is an example of how improved understanding of the mechanisms and products can be obtained using this 3D seismic methodology in unstable margin areas. 相似文献