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1.
Tumuli are hollow subcircular domes of the most superficial stratum of gypsum, principally found in outcrops of macrocrystalline gypsum. They vary from a few centimetres to several metres in diameter and reach maximum heights of a little more than 1 m. The relationships between the morphostatistical parameters that define these formations are: h = r/3 and e = r/9, where h is the elevation of the raised layer, e its thickness and r the mean radius. Their genesis has caused some controversy over the involvement of phenomena such as hydration of anhydrite, or tectonic processes capable of explaining this folding. This paper shows their genesis linked to the dissolution of macrocrystalline gypsum and reprecipitation of microcrystalline gypsum within the same gypsiferous layer. It has been calculated that to reach the theoretical saturation within the few centimetres' thickness of the cap of the tumulus, water infiltration velocities are required of between 0·002 cm s−1 for an uplifted stratum of 2 cm thickness, and 0·03 cm s−1 for 30 cm thickness. These velocities imply the existence of very slow rates of infiltration and/or capillary movement of water within the gypsiferous layer. The secondary microcrystalline gypsum is precipitated in the intercrystalline and intracrystalline voids of the gypsum crystals, producing an increase in porosity and associated volume that causes the doming of the gypsiferous layer. The development of tumuli is a cyclic process which is favoured by a sequence of short wet and dry intervals which, in turn, facilitate the almost simultaneous processes of dissolution and precipitation. These conditions predominate in arid and semiarid climates where intense evaporation can occur suddenly following sporadic infiltration. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

2.
The precipitation of freshwater carbonates (tufa) along karstic rivers is enhanced by degassing of carbon dioxide (CO2) downstream of karstic springs. However, in most karstic springs CO2 degassing is not enough to force the precipitation of tufa sediments. Little is known about the role of dissolution of gypsum or dolomite in the hydrochemistry of these systems and how this affects the formation of tufa deposits. Here we present a monitoring study conducted over a year in Trabaque River (Spain). The river has typical karst hydrological dynamics with water sinking upstream and re‐emerging downstream of the canyon. Mixing of calcium–magnesium bicarbonate and calcium sulphate waters downstream of the sink enhances the dissolution of carbonates and potentially plays a positive role in the formation of tufa sediments. However, due to the common‐ion effect, dissolution of dolomite and/or gypsum causes precipitation of underground calcite cements as part of the incongruent dissolution of dolomite/dedolomitization process, which limits the precipitation of tufa sediments. Current precipitation of tufa is scant compared to previous Holocene tufa deposits, which likely precipitated from solutions with higher saturation indexes of calcite (SIcc values) than nowadays. Limited incongruent dissolution of dolomite/dedolomitization favours higher SIcc values. This circumstance occurs when waters with relatively high supersaturation of dolomite and low SO42? composition sink in the upper sector of the canyon. In such a scenario, the process of mixing waters enhances the exclusive dissolution of limestones, preventing the precipitation of calcite within the aquifer and favouring the increase of SIcc values downstream of the springs. Such conditions were recorded during periods of high water level of the aquifers and during floods. This research shows that the common‐ion effect caused by the dissolution of gypsum and/or dolomite rocks can limit [or favour] the precipitation of tufa sediments depending on the occurrence [or not] of incongruent dissolution of dolomite/dedolomitization. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

3.
The viscosity of a series of six synthetic dacitic liquids, containing up to 5.04 wt% dissolved water, was measured above the glass transition range by parallel-plate viscometry. The temperature of the 1011 Pa s isokom decreases from 1065 K for the anhydrous liquid, to 864 K and 680 K for water contents of 0.97 and 5.04 wt% H2O. Including additional measurements at high temperatures by concentric-cylinder and falling-sphere viscometry, the viscosity (η) can be expressed as a function of temperature and water content w according to: where η is in Pa s, T is temperature in K, and w is in weight percent. Within the conditions of measurement, this parameterization reproduces the 76 viscosity data with a root-mean square deviation (RMSD) of 0.16 log units in viscosity, or 7.8 K in temperature. The measurements show that water decreases the viscosity of the dacitic liquids more than for andesitic liquids, but less than for rhyolites. At low temperatures and high water contents, andesitic liquids are more viscous than the dacitic liquids, which are in turn more viscous than rhyolitic liquids, reversing the trend seen for high temperatures and low water contents. This suggests that the relative viscosity of different melts depends on temperature and water content as much as on bulk melt composition and structure. At magmatic temperatures, rhyolites are orders of magnitude more viscous than dacites, which are slightly more viscous than andesites. During degassing, all three liquids undergo a rapid viscosity increase at low water contents, and both dacitic and andesitic liquids will degas more efficiently than rhyolitic liquids. During cooling and differentiation, changing melt chemistry, decreasing temperature and increasing crystal content all lead to increases in the viscosity of magma (melt plus crystals). Under closed system conditions, where melt water content can increase during crystallization, viscosity increases may be small. Conversely, viscosity increases are very abrupt during ascent and degassing-induced crystallization.  相似文献   

4.
Generous statistical tests   总被引：1，自引：1，他引：0  
A common statistical problem is deciding which of two possible sources, A and B, of a contaminant is most likely the actual source. The situation considered here, based on an actual problem of polychlorinated biphenyl contamination discussed below, is one in which the data strongly supports the hypothesis that source A is responsible. The problem approach here is twofold: One, accurately estimating this extreme probability. Two, since the statistics involved will be used in a legal setting, estimating the extreme probability in such a way as to be as generous as is possible toward the defendant’s claim that the other site B could be responsible; thereby leaving little room for argument when this assertion is shown to be highly unlikely. The statistical testing for this problem is modeled by random variables {X i } and the corresponding sample mean the problem considered is providing a bound ɛ for which for a given number a 0. Under the hypothesis that the random variables {X i } satisfy E(X i ) ≤ μ, for some 0  < μ < 1, statistical tests are given, described as “generous”, because ɛ is maximized. The intent is to be able to reject the hypothesis that a 0 is a value of the sample mean while eliminating any possible objections to the model distributions chosen for the {X i } by choosing those distributions which maximize the value of ɛ for the test used.  相似文献   

5.
The Tieluping silver deposit, located in the NE-trending faults within the metamorphic basement of the Xiong'er Mountain, is a typical altered fracture type deposit. Its ore-forming process includes three stages with temperatures concentrated at 373°C, 223°C and 165°C respectively. With δD=90‰,\(\delta ^{13} C_{CO_2 } \)=2.0‰ and δ{si18}O=8094‰, the early stage fluid was generated from reworking and metamorphism of the carbonate rich formation; the late one, with δD=?70‰,\(\delta ^{13} C_{CO_2 } \)=-1.2‰, δ18O=1.89‰, was meteoric hydrothermal solution; and the middle. δD=?109‰,\(\delta ^{13} C_{CO_2 } \)=0.1‰, δ18O=1.79‰, might be a hybrid mixed by reworking-metamorphic fluid and meteoric hydrothermal solution. Crystallized rapidly in the condition of fluid-boiling and fluid-mixing, the middle stage minerals have far more fluid inclusions with higher content of ions, higher ratios of H2O/CO2 and KN/MC. Consequently, they have much more ore elements such as gold compared with those of the early and late stages. It was the northward intracontinental subduction along the Machaoying fault during the Mesozoic collision between the South China and North China paleocontinents that intrigued large-scale fluidization and magmatism and led to the appearance of more than 10 large and medium hydrothermal deposits, including the Tieluping silver deposit. The study on ore-forming fluidization of the Tieluping silver deposit proves the CPMF model.  相似文献   

6.
Wave tank experiments with long internal waves of elevation, of different initial length l, moving in a two-fluid system, interacting with a weak slope of 0.045 rad, show an onshore flow of the dense water, at the undisturbed pycnocline-slope intersection, of duration $11.3\sqrt{l/g'}Wave tank experiments with long internal waves of elevation, of different initial length l, moving in a two-fluid system, interacting with a weak slope of 0.045 rad, show an onshore flow of the dense water, at the undisturbed pycnocline-slope intersection, of duration 11.3?{l/g￠}11.3\sqrt{l/g'} (g′ reduced gravity). This period corresponds to that of a strong bottom current event measured in the stratified ocean at the Ormen Lange gas field, at 850 m depth, lasting for 24 hrs, corresponding to 11.2?{l/g￠}11.2\sqrt{l/g'}, using the width l = 300 km of the Norwegian Atlantic Current (NAC) at the site as length scale, suggesting a lateral sloshing motion of the NAC causing the event. The onshore velocity of the dense fluid has a maximal velocity of 0.4?{g￠h2}0.4\sqrt{g'h_2} in laboratory and 0.5 ms-1=0.3?{g￠h2}^{-1}=0.3\sqrt{g'h_2} in the field (h 2 mixed upper layer thickness). Run-up of the dense fluid, beyond the undisturbed pycnocline-slope intersection, has initially a front velocity of 0.35?{g￠h2}0.35\sqrt{g'h_2}, corresponding to the velocity of the head of a density current on a flat bottom. Due to disintegration, an initially depressed pycnocline results in comparatively smaller run-up and velocity. While moving past the turning point, a dispersive wave train is formed in the back part of the depression wave, developing by breaking into a sequence of up to eight boluses moving by the undisturbed pycnocline-slope intersection.  相似文献   

7.

The Tieluping silver deposit, located in the NE-trending faults within the metamorphic basement of the Xiong'er Mountain, is a typical altered fracture type deposit. Its ore-forming process includes three stages with temperatures concentrated at 373°C, 223°C and 165°C respectively. With δD=90‰,\(\delta ^{13} C_{CO_2 } \)=2.0‰ and δ{si18}O=8094‰, the early stage fluid was generated from reworking and metamorphism of the carbonate rich formation; the late one, with δD=−70‰,\(\delta ^{13} C_{CO_2 } \)=-1.2‰, δ18O=1.89‰, was meteoric hydrothermal solution; and the middle. δD=−109‰,\(\delta ^{13} C_{CO_2 } \)=0.1‰, δ18O=1.79‰, might be a hybrid mixed by reworking-metamorphic fluid and meteoric hydrothermal solution. Crystallized rapidly in the condition of fluid-boiling and fluid-mixing, the middle stage minerals have far more fluid inclusions with higher content of ions, higher ratios of H2O/CO2 and KN/MC. Consequently, they have much more ore elements such as gold compared with those of the early and late stages. It was the northward intracontinental subduction along the Machaoying fault during the Mesozoic collision between the South China and North China paleocontinents that intrigued large-scale fluidization and magmatism and led to the appearance of more than 10 large and medium hydrothermal deposits, including the Tieluping silver deposit. The study on ore-forming fluidization of the Tieluping silver deposit proves the CPMF model.

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8.
Strombolian-type volcanic activity is characterized by a series of gas bubbles bursting at the top of a magma column and leading to the ejection of lava clots and gas emission at the surface. The quantitative analysis of physical parameters (e.g., velocity, size, and mass fluxes) controlling the emission dynamics of these volcanic products is very important for the understanding of eruption source mechanisms but remains difficult to obtain in a systematic fashion. Ground-based Doppler radar is found to be a very effective tool for measuring ejecta velocities at a high acquisition rate and close to the emission source. We present here a series of measurements carried out at Mt. Etna’s Southeast crater, using an L-band volcanological Doppler radar, during the 4 July 2001 Strombolian eruptions. Doppler radar data are supplemented by the analysis of video snapshots recorded simultaneously. We provide here a set of physical parameters systematically retrieved from 247 Strombolian explosions spanning 15 min and occurring during the paroxysm of the eruption from 21:30 to 21:45 UT. The time-average values give a maximum particle velocity of Vmaxp = 94.7±24 \textm/s V_{{\max }}^p = {94}.{7}\pm {24} {\text{m/s}} , a bulk lava jet velocity of V\textPW - rad = 37.6±1.9 \textm/s {V_{{{\text{PW - rad}}}}} = {37}.{6}\pm {1}.{9} {\text{m/s}} , and an initial gas velocity at the source vent of V0g = 118.4±36 \textm/s V_0^g = {118}.{4}\pm {36} {\text{m/s}} . The time-averaged particle diameter is found to be about D\textPW - rad = 4.2±2.1 \textcm {D_{{{\text{PW - rad}}}}} = {4}.{2}\pm {2}.{1} {\text{cm}} . The volume and mass gas fluxes are estimated from time-averaged source gas velocities over the sequence duration at Qvg = 3 - 11 ×103\textm3\text/s Q_v^g = {3} - {11} \times {1}{0^{{3}}}{{\text{m}}^{{3}}}{\text{/s}} and Qmg = 0.5 - 2 ×103\textkg/s Q_m^g = 0.{5} - {2} \times {1}{0^{{3}}}{\text{kg/s}} , respectively.  相似文献   

9.
On the basis of Parry’s method (1986), an improved method was established to determine the molar volume (Vm) and compositions (X) of the NaCl-H2O-CO2 (NHC) system inclusion. To use this method, the determination of Vm-X only requires three microthermometric data of a NHC inclusion: partial homog-enization temperature (Th ,CO2), salinity (S) and total homogenization temperature (Th). Theoretically, four associated equations are needed containing four unknown parameters: X CO2, XNaCl, Vm and F (volume fraction of CO2 phase in total inclusion when occurring partial homogenization). When they are released, the Vm-X are determined. The former three equations, only correlated with Th ,CO2, S and F, have simplified expressions:XCO2=f1(Th,CO2,S,F),XNaCl=f2(Th,CO2,S,F),Vm=f3(Th,CO2,S,F). The last one is the thermodynamic relationship of X CO2, XNaCl, Vm and Th:f4(XCO2,XNaCl,Vm,Th)=0.Since the above four associated equations are complicated, it is necessary to adopt iterative technique to release them. The technique can be described by:(i) Freely input a F value (0≤F≤1),with Th ,CO2 and S, into the former three equations. As a result,X CO 2,XNaCl and the molar volume value recorded as Vm1 are derived. (ii) Input the X CO2 and XNaCl gotten in the step above into the last equation, and another molar volume value recorded as Vm2 is determined. (iii) If Vm1 is unequal to Vm2, the calculation will be restarted from “(i)”. The iteration is completed until Vm1 is equal to Vm2, which means that the four associated equations are released. Compared to Parry’s (1986) solution method, the improved method is more convenient to use, as well as more accurate to determine X CO 2. It is available for a NHC inlusion whose partial homogenization temperature is higher than clatherate melting temperature and there are no solid salt crystals in the inclusion at parital homogenization.  相似文献   

10.
Vulnerability maps are designed to show areas of greatest potential for groundwater contamination on the basis of hydrogeological conditions and human impacts. The objective of this research is (1) to assess the groundwater vulnerability using DRASTIC method and (2) to improve the DRASTIC method for evaluation of groundwater contamination risk using AI methods, such as ANN, SFL, MFL, NF and SCMAI approaches. This optimization method is illustrated using a case study. For this purpose, DRASTIC model is developed using seven parameters. For validating the contamination risk assessment, a total of 243 groundwater samples were collected from different aquifer types of the study area to analyze \( {\text{NO}}_{ 3}^{ - } \) concentration. To develop AI and CMAI models, 243 data points are divided in two sets; training and validation based on cross validation approach. The calculated vulnerability indices from the DRASTIC method are corrected by the \( {\text{NO}}_{3}^{ - } \) data used in the training step. The input data of the AI models include seven parameters of DRASTIC method. However, the output is the corrected vulnerability index using \( {\text{NO}}_{3}^{ - } \) concentration data from the study area, which is called groundwater contamination risk. In other words, there is some target value (known output) which is estimated by some formula from DRASTIC vulnerability and \( {\text{NO}}_{3}^{ - } \) concentration values. After model training, the AI models are verified by the second \( {\text{NO}}_{3}^{ - } \) concentration dataset. The results revealed that NF and SFL produced acceptable performance while ANN and MFL had poor prediction. A supervised committee machine artificial intelligent (SCMAI), which combines the results of individual AI models using a supervised artificial neural network, was developed for better prediction of vulnerability. The performance of SCMAI was also compared to those of the simple averaging and weighted averaging committee machine intelligent (CMI) methods. As a result, the SCMAI model produced reliable estimates of groundwater contamination risk.  相似文献   

11.
Hydrate Ridge is located at the second accretion-ary ridge along the Cascadia margin of Oregon in the eastern North Pacific (fig. 1). The Bottom Simulating Reflector (BSR) underlies the entire Hydrate Ridge[1]. The Ocean Drilling Program (ODP) in 1992 at Site 892 and the TECFLUX99 and 2000 showed that the gas hydrate occurs just beneath the thin sediment- covered surface and at the horizon of around 64 meter below seafloor (mbsf) on Hydrate Ridge[25]. The col-lision of the Juan de …  相似文献   

12.
The formation of Namibia's extensive pedogenic gypsum crusts (CaSO4·2H2O) is interpreted in a new light. It is suggested that gypsum primarily precipitates at isolated points of evaporitic concentration, such as inland playas, and that deflation of evaporitic‐rich gypsum dust from these playas contributes to the formation of pedogenic gypsum duricrusts on the coastal gravel plains of the Namib Desert surrounding these playas. This study establishes the nature, extent and distribution of playas in the Central Namib Desert and provides evidence for playa gypsum deflation and gravel plain deposition. Remote sensing shows the distribution of playas, captures ongoing deflation and provides evidence of gypsum deflation. It is proposed that, following primary marine aerosol deposition, both inland playas and coastal sabkhas generate gypsum which through the process of playa deflation and gravel plain redeposition contributes to the extensive pedogenic crusts found in the Namib Desert region. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

13.
Attenuation of P,S, and coda waves in Koyna region,India   总被引：1，自引：0，他引：1  
The attenuation properties of the crust in the Koyna region of the Indian shield have been investigated using 164 seismograms from 37 local earthquakes that occurred in the region. The extended coda normalization method has been used to estimate the quality factors for P waves and S waves , and the single back-scattering model has been used to determine the quality factor for coda waves (Q c). The earthquakes used in the present study have the focal depth in the range of 1–9 km, and the epicentral distance vary from 11 to 55 km. The values of and Q c show a dependence on frequency in the Koyna region. The average frequency dependent relationships (Q = Q 0 f n) estimated for the region are , and . The ratio is found to be greater than one for the frequency range considered here (1.5–18 Hz). This ratio, along with the frequency dependence of quality factors, indicates that scattering is an important factor contributing to the attenuation of body waves in the region. A comparison of Q c and in the present study shows that for frequencies below 4 Hz and for the frequencies greater than 4 Hz. This may be due to the multiple scattering effect of the medium. The outcome of this study is expected to be useful for the estimation of source parameters and near-source simulation of earthquake ground motion, which in turn are required in the seismic hazard assessment of a region.  相似文献   

14.
Estimation of coda wave attenuation in East Central Iran   总被引：1，自引：0，他引：1  
The attenuation of coda waves, Q c , has been estimated in Zarand, Jiroft, and Bam regions of east central Iran using a single back-scattering model of S-coda envelopes. For this purpose, the recordings of 97 earthquakes by three seismic networks and a local strong ground motion network have been used. In this research, the frequency-dependent Q c values are estimated at central frequencies of 1.5, 3, 6, 8, 12, 16, and 24 Hz using different lapse time windows from 20 to 60 s. The frequency-dependent relationships obtained are for Zarand, for Jiroft, and for Bam region. From the strong ground motion data, we obtain the relation . The Q c frequency-dependent relationship for the entire region of east central Iran from all data (both seismograms and accelerograms) is . The average Q c values estimated and their frequency dependent relationships correlate well with a highly heterogeneous and highly tectonically active region. Results also show that the attenuation is higher in Bam region compared to Zarand and Jiroft regions.  相似文献   

15.
The Boteti palaeo‐estuary in northern Botswana is located where the endoreic Boteti river, an overflow from the regional Okavango river system, enters the Makgadikgadi pans. The present work considers diagenetic silica and calcium carbonate dominated transformations. The aims are to help identify precursor conditions for the origin of microcrystalline silcrete–calcrete intergrade deposits while developing insight into pene‐contemporaneous silica and calcite matrix formation. General precursor conditions require the presence of cyclical endoreic freshwater inflow into a saline pan. The pan should be deep enough to sustain a permanent watertable under climatic conditions sufficient to cause carbonate fractionation within the groundwater. Freshwater inflow into a saline pan drives the geochemistry of the system (from freshwater to saline, from neutral to high pH). The geochemistry is controlled by the periodicity of inflow relative to salinity levels of phreatic groundwater in the receptor saline pan. The source of most silica and localized CaCO3 is derived from the dissolution and precipitation of micro‐fossils, while more general CaCO3 enrichment stems from saline pan based carbonate fractionation. Diagenetic change leads to colloidal then more consolidated bSiO2/CaO aggregate formation (amorphous silica) followed by transformations into opaline silica over time. Irregular zones of siliceous sediment forming in otherwise calcareous deposits may relate to the irregular occurrence of biogenic silica in the source sediments, inferring a source for local silica mobilization in intergrade deposits. The distribution of calcareous micro‐fossils may have a similar converse effect. Diagenetic evidence from an intergrade deposit with a low SiO2/CaO ratio suggests that transformation occurred more into the pan, while an intergrade deposit with a high SiO2/CaO ratio more likely formed closer to a land margin and was frequently inundated by freshwater. Pene‐contemporaneous silcrete–calcrete intergrade formation under the above conditions may take place where dissolved silica crystallizes out in the vicinity of calcite crystals due to local decreases in pH. The continuing consolidation of bSiO2/CaO aggregates may be facilitated by the presence of increasing amounts of calcite. It appears that CaCO3 may act as a catalyst leading to pene‐contemporaneous bSiO2/CaO aggregate formation. However the processes involved require further work. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

16.
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17.
Let {Y, Y i , −∞ < i < ∞} be a doubly infinite sequence of identically distributed and asymptotically linear negative quadrant dependence random variables, {a i , −∞ < i < ∞} an absolutely summable sequence of real numbers. We are inspired by Wang et al. (Econometric Theory 18:119–139, 2002) and Salvadori (Stoch Environ Res Risk Assess 17:116–140, 2003). And Salvadori (Stoch Environ Res Risk Assess 17:116–140, 2003) have obtained Linear combinations of order statistics to estimate the quantiles of generalized pareto and extreme values distributions. In this paper, we prove the complete convergence of under some suitable conditions. The results obtained improve and generalize the results of Li et al. (1992) and Zhang (1996). The results obtained extend those for negative associated sequences and ρ*-mixing sequences. CIC Number O211, AMS (2000) Subject Classification 60F15, 60G50 Research supported by National Natural Science Foundation of China  相似文献   

18.
Gypsum and halite crystals, together with saponite and phillipsite, were found in a vein in a basalt sill 625 m below the sea floor at DSDP Site 395A, located 190 km west of the crest of the Mid-Atlantic Ridge. The δ34S value of the gypsum (+19.4‰) indicates a seawater source for the sulfate. The δ18O values of the saponite (+19.9‰) and phillipsite (+18.1‰) indicate either formation from normal seawater at about 55°C or formation from18O-depleted seawater at a lower temperature.The gypsum (which could be secondary after anhydrite) was formed by reaction between Ca2+ released from basalt and SO42? in circulating seawater. The halite could have formed when water was consumed by hydration of basalt under conditions of extremely restricted circulation. A more probable mechanism is that the gypsum was originally precipitated as anhydrite at temperatures above 60°C. As the temperature dropped the anhydrite converted to gypsum. The conversion would consume water, which could cause halite precipitation, and would cause an increase in the volume of solids, which would plug the vein and prevent subsequent dissolution of the halite.  相似文献   

19.
Wind and tidal straining are proposed as key mechanisms influencing the magnitude and timing of the horizontal flux of freshwater across regions of freshwater influence (ROFIs). Evidence for this hypothesis is presented in estimates of the tidally averaged residual current profile, obtained from 5 years of continuous acoustic doppler current profiler measurements in the Liverpool Bay ROFI. The modified horizontal Richardson number (RxwtR_{x}^{wt}), which includes both the tidal and the wind forcing, was assessed as a measure of stratification. RxwtR_{x}^{wt} was found to be a good indicator of the timing of the evolution and destruction of stratification, but was not as successful as an indicator of the magnitude of stratification, both enduring and periodic. The observed mean residual velocities are compared to those predicted by a classical solution, and the eddy viscosity (N z ) is shown to be a control on differences between the observed and predicted circulation. Principal component analysis is used to show that the strongest residual currents occur when the water column periodically alternates between a well-mixed and stratified state, a consequence of straining, rather than simply related to the density gradient. Evidence of wind straining was found in the correspondence between the wind direction and the near surface and near bed residual current direction.  相似文献   

20.
Stable isotopic data are presented for 112 samples of francolite from 18 separate phosphate deposits. Values ofδ13C andδ34S in most offshore deposits suggest formation within oxic or suboxic environments either by carbonate replacement or direct precipitation of francolite from water of normal marine compositions. The exceptions are concretionary francolite from Namibia, which has an isotopic composition in keeping with its formation within organic-rich sediments, and that from offshore Morocco, which has an isotopic signature of the anoxic/suboxic interface. Onshore deposits from Jordan, Mexico, South Africa and, possibly, the Permian Phosphoria Formation in the western U.S.A., are substantially depleted in18O: they appear to be too altered for deductions to be made about their environments of formation. In other onshore deposits which are unaltered, or minimally altered, the isotopic composition suggests that some formed within sulphate-reducing sediments (Sedhura, Morocco) whilst francolite from the Georgina Basin of Australia formed at the oxic/anoxic boundary, where oxidation of biogenic H2S decreases theδ34S of pore water. In general, pelletal samples show non-oxic isotopic signatures, whilst non-pelletal samples show oxic isotopic signatures, but samples from Namibia, Peru (Ica Plateau) and the Californian and Moroccan margins are exceptions to this rule. Morphology may therefore be a misleading indicator of francolite genesis as no definitive relation exists between phosphorite type and isotopic signature.  相似文献   

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