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1.
Iron ore and host rocks have been sampled (90 oriented samples from 19 sites) from the Las Truchas mine, western Mexico. A broad range of magnetic parameters have been studied to characterize the samples: saturation magnetization, Curie temperature, density, susceptibility, remanence intensity, Koenigsberger ratio, and hysteresis parameters. Magnetic properties are controlled by variations in titanomagnetite content, deuteric oxidation, and hydrothermal alteration. Las Truchas deposit formed by contact metasomatism in a Mesozoic volcano-sedimentary sequence intruded by a batholith, and titanomagnetites underwent intermediate degrees of deuteric oxidation. Post-mineralization hydrothermal alteration, evidenced by pyrite, epidote, sericite, and kaolin, seems to be the major event that affected the minerals and magnetic properties. Magnetite grain sizes in iron ores range from 5 to >200 μm, which suggest dominance of multidomain (MD) states. Curie temperatures are 580±5°C, characteristic of magnetite. Hysteresis parameters indicate that most samples have MD magnetite, some samples pseudo-single domain (PSD), and just a few single domain (SD) particles. AF demagnetization and IRM acquisition indicate that NRM and laboratory remanences are carried by MD magnetite in iron ores and PSD–SD magnetite in host rocks. The Koenigsberger ratio falls in a narrow range between 0.1 and 10, indicating the significance of MD and PSD magnetites. 相似文献
2.
Forty-five samples have been collected at nine sites on the 42.5 Ma Quxu pluton (90°50′E, 29°20′N) in the Gangdese batholith. Westerly declination ( D = −48°and−83°) is observed in primary magnetizations from two sites about 25 km from the Indus-Zangbo suture zone after thermal demagnetization. This direction is consistent with the westerly paleomagnetic directions of the crustal blocks in other areas along the Indus-Zangbo suture zone. The Quxu pluton of the Gangdese Belt was rotated in a “domino style” deformation process as a part of a long (840 km) and narrow (less than 100 km) deformed zone between the India-Eurasia continents associated with the collision of India since 42.5 Ma. The pluton, between 11 km and 14 km from the suture acquired the secondary magnetization ( D = −28°and−39°) during a cataclastic metamorphic process at sometime during the ‘domino style’ deformation. The primary magnetization was completely destroyed in the pluton within 11 km of the suture during slow cooling at the uplift stage and was replaced by thermoviscous remanent magnetization parallel to the present axial dipole field. 相似文献
3.
Thermal effects related to burial and hydrothermal alteration leads to chemical remanent magnetization (CRM). We present an experimental study of CRM production by heating claystones at 95 °C. A vertical magnetic field of 2 mT was applied to the claystones during heating and the evolution of the remanence during heating in air is monitored intermittently for up to four months. Solid fragments (9 to 26 g) of claystones are included in a Teflon holder that is placed in the oven under a controlled atmosphere. Newly formed grains acquire a CRM and a thermoviscous magnetization (TVRM), both being parallel to the applied magnetic field. CRM is related to the amount of newly formed grains that pass the critical volume during the reaction. To measure the acquired remanence, the claystones are first cooled in a zero magnetic field and then measured using a 2G SQUID magnetometer.In the frame of the research programme on the feasibility of radioactive waste disposal in a deep geological formation, we investigate the magnetic transformation of Mont Terri Lower Dogger claystones (Switzerland) due to thermal imprinting at 95 °C. We simulate the dehydration that occurs in the walls of galleries after excavation when interstitial water evaporates and rehydration when the galleries are refilled allowing water to move towards dehydrated zones. During dehydration, the remanence gains one order of magnitude at the beginning of the experiment and then it follows a linear rate of 0.23 ± 0.07 mA m − 1/day between 3 and 14 days. The magnetic susceptibility increases by a few percent. The increase of the remanence and of the magnetic susceptibility stops after 15 days. Mass monitoring indicates that interstitial water evaporates when remanence and magnetic susceptibility stabilizes. During rehydration, the remanence increases again whilst magnetic susceptibility drops by a few percent. After 20 days, the remanence during rehydration follows a rate of 0.42 ± 0.15 mA m − 1/day. By contrast, when rehydration takes place later, after 66 days, the rate is much lower (0.09 ± 0.04 mA m − 1/day). Low temperature investigation of magnetic properties indicates an initial magnetic assemblage of magnetite and pyrrhotite. Newly formed magnetite and hematite carry the remanence. We propose that magnetite is formed at the expense of pyrite. Hematite results from the progressive oxidation of newly formed magnetite. Our results suggest the possibility that any claystones that pass the oil window can be remagnetized due to the unique action of temperature. 相似文献
4.
The morphology and composition of spinel in rapidly quenched Pu’u ’O’o vent and lava tube samples are described. These samples contain glass, olivine phenocrysts (3–5 vol.%) and microphenocrysts of spinel (0.05 vol.%). The spinel surrounded by glass occurs as idiomorphic octahedra 5–50 μm in diameter and as chains of octahedra that are oriented with respect to each other. Spinel enclosed by olivine phenocrysts is sometimes rounded and does not generally form chains. The temperature before quenching was calculated from the MgO content of the glass and ranges from 1150°C to 1180°C. The oxygen fugacity before quenching was calculated by two independent methods and the log fO2 ranged from −9.2 to −9.9 (delta QFM=−1). The spinel in the Pu’u ’O’o samples has a narrow range in composition with Cr/(Cr+Al)=0.61 to 0.73 and Fe 2+/(Fe 2++Mg)=0.46 to 0.56. The lower the calculated temperature for the samples, the higher the average Fe 2+/(Fe 2++Mg), Fe 3+ and Ti in the spinel. Most zoned spinel crystals decrease in Cr/(Cr+Al) from core to rim and, in the chains, the Cr/(Cr+Al) is greater in the core of larger crystals than in the core of smaller crystals. The occurrence of chains and hopper crystals and the presence of Cr/(Cr+Al) zoning from core to rim of the spinel suggest diffusion-controlled growth of the crystals. Some of the spinel crystals may have grown rapidly under the turbulent conditions of the summit reservoir and in the flowing lava, and the crystals may have remained in suspension for a considerable period. The rapid growth may have caused very local (μm) gradients of Cr in the melt ahead of the spinel crystal faces. The crystals seem to have retained the Cr/(Cr+Al) ratio that developed during the original growth of the crystal, but the Fe 2+/(Fe 2++Mg) ratio may have equilibrated fairly rapidly with the changing melt composition due to olivine crystallization. Six of the samples were collected on the same day at various locations along a 10-km lava tube and the calculated pre-collection temperatures of the samples show a 5°C drop with distance from the vent. The average Fe 2+/(Fe 2++Mg) of the spinel in these samples shows a weak positive correlation with decreasing MgO in the glass of these samples. The range in Cr 2O 3 (0.041–0.045 wt.%) of the glass for these six samples is too small to distinguish a consistent change along the lava tube. The spinel in the Pu’u ’O’o samples shows a zoning trend in a Cr–Al–Fe 3+ diagram almost directly away from the Cr apex. This compares with a zoning trend in rapidly quenched MORB samples away from Cr coupled with decreasing Fe 3+. The trend away from Cr displayed by spinel in rapidly quenched samples is in marked contrast to the trend of increasing Fe 3+ shown by spinel in slowly cooled lava. 相似文献
5.
Garnets in an amphibolite-facies metasediment from Sulitjelma, North Norway yield precise and concordant SmNd, UPb and RbSr ages that relate directly to the pressure ( P) and temperature ( T) conditions of mineral growth. Differential mineral reaction between graphitic and non-graphitic layers within this sample preserves a record of the P-T and time ( t) history experienced during Barrovian regional metamorphism. Garnets in graphitic layers grew during prograde metamorphism at 462 ± 16°C and 5.2 ± 0.5 kbar under conditions of low aH2O, and yield indistinguishable 147Sm 143Nd and 238U 206Pb ages of 434.1 ± 1.2 Ma and 433.9 ± 1.0 Ma, respectively. In contrast, garnet growth in adjacent graphite-free layers did not occur until P-T conditions of 540 ± 18°C and 8.0 ± 1.0 kbar were attained, with continued growth in response to minor heating and decompression with final matrix equilibration at 544 ± 16°C and 7.0 ± 1.0 kbar. The inclusion-free garnet rims in this assemblage record indistinguishable 147Sm 143Nd and 238U 206Pb ages of 424.6 ± 1.2 Ma and 423.4± 1.7 Ma, respectively. These results provide precise estimates for average heating and burial rates during prograde metamorphism of 8.6 −4.4+7.5°C Ma −1 and 0.8 −0.5+0.9 km Ma −1, respectively. Rb and Sr exchange between coexisting silicates in the graphite-free assemblage continued for some 37 Ma after the “peak” of metamorphism, and require an average cooling rate of about 4.0°C Ma −1 during uplift. These results illustrate a clear relationship between reaction history and the timing of mineral growth and provide definitive constraints on the rates of thermal and tectonic processes accompanying regional metamorphism. 相似文献
6.
The possibility that the parent body of the SNC meteorites is Mars implies that the magnetic properties of these meteorites may provide evidence concerning ancient Martian magnetic fields. EETA 79001 possesses a weak, very stable primary magnetization, the properties of which are consistent with its acquisition in an ambient magnetic field either during the meteorite's formation or during the severe shock event later in its history. The samples of ALHA 77005 studied possessed no measurable primary magnetization: the observed remanence appears to be a viscous magnetization acquired in local laboratory fields. The magnetic carriers in the meteorites are fine-grained magnetite and a lower Curie point mineral, probably titanomagnetite or pyrrhotite, present to the extent of less than 0.1% by weight. Estimates of the strength of the magnetizing field for EETA 79001 are in the range 1–10 μT. 相似文献
7.
Electron microprobe and reflected light microscopic examinations confirm the presence of composite grains of ferrian ilmenite with Xilm = 0.53 and titanomagnetite with Xusp = 0.13 in a dacite with self-reversed TRM. A parallel TRM component associated with titanomagnetite and a reversed component associated with self-reversing ferrian ilmenite are the principal NRM components. A subordinate, parallel component is associated with ferrian ilmenite which is not magnetically coupled to an “χ-phase”. The natural self-reversing properties are mainly a consequence of the dacite's high quenching temperature, calculated at 862–864°C using the Fe—Ti oxide geothermometer, and are most consistent with the self-reversal mechanism proposed by Lawson et al. [9].The conduction of thermal demagnetization and TRM induction tests in air had a much greater effect on the Fe—Ti oxides than did natural cooling, and resulted in significant oxidation with the consequent modification of some magnetic properties and the formation of another reversed TRM component. The subdivision of titanomagnetite grains by oxidation along fractures decreased its effective grain size and caused an apparent increase in its magnetic intensity, in addition to a slight increase in its resistance to alternating field demagnetization. The χ-phase associated with the reversed NRM component, with 0.42 > Xilm 0.31, became Fe-enriched during the earlier stages of heat treatment. It is suggested that after heating at 600°C for two hours or more, this χ-phase exsolves as titanohematite with Xilm < 0.33. The ferrian ilmenite host is consequently enriched in Ti, and another χ-phase much closer in composition to the host generates a reversed TRM component with Tb < 200°C. 相似文献
8.
Summary The magnetic properties of basaltic rocks are dominated by the contained primary Fe–Ti oxides. At solidus temperature (1000°C) the composition of these primary oxides is restricted to titanomagnetite (Fe 3-xTi xO 4) and hemoilmenites (Fe 2-yTi yO 3). The examination of 269 chemical analyses of the primary Fe–Ti oxides in basalts (in sensu lato) gives an average of x=0.61 ( T
c=168°C) for the titanomagnetites and y=0.89 ( T
c=–121°C) for the hemoilmenites. If distinction is made between tholeiites, alkali basalts and andesites, a clear difference for the x-values is observed: the average for tholeiites x=0.64 ( T
c=144°C), for alkali basalts x=0.52 ( T
c=253°C), for andesites x=0.38 ( T
c=341°C).Environment of crystallization and cooling rate are major interrelated factors influencing subsequent changes in the mineralogy of the primary Fe–Ti oxides and resulting magnetic properties. This has been tested by studying the variation of magnetization and some of its parameters in three different basalt rock units: a dike, 180 cm, and two lava flows, 3 m and 33 m thick, respectively. Grain size and oxidation state of the titanomagnetites control the variation of magnetization in these basalt units. 相似文献
9.
Curie temperatures, hysteresis, alternating field properties and anhysteretic and ordinary susceptibilities have been used to characterize the titanomagnetites in a large collection of continental granites, diorites, syenites, anorthosites, gabbros, diabases and basalts. Low-Curie-point titanomagnetites or titanomaghemites were found only in basalts. In all shallow and deep-seated intrusive rocks, the predominant magnetic phase was nearly-titanium-free titanomagnetite with a Curie point of 520–580°C. Most felsic plutonic rocks owed their magnetic properties to coarse, discrete titanomagnetites with truly multidomain properties. Many mafic plutonic rocks (anorthosites, gabbros, norites) displayed bimodal magnetic properties, strong-field properties being due to the discrete titanomagnetites and weak-field properties being due to fine magnetite inclusions in deuterically altered silicates. The Lowrie-Fuller test and the anhysteretic induction curve were the most diagnostic tests of this bimodal behaviour. Grain-size variation within a single diabase dike or sill had a strong expression in all magnetic properties, except HR/ Hc and the Lowrie-Fuller test. On the other hand, the Lowrie-Fuller test was a sensitive indicator of changes in “effective” grain size in basalts due to the subdivision of grains by ilmenite lamellae. 相似文献
10.
Magnetic hysteresis of coarse-grained titanomagnetites at room temperature is characterised by low coercive force, low relative remanence, and a high ratio of coercivity of remanence to coercive force. These properties are generally interpreted in terms of multidomain structure. At low temperatures, however, ulvöspinel-rich compositions exhibit hysteresis properties similar to those of single-domain assemblages, and on this basis Radhakrishnamurty and Deutsch have proposed an alternative interpretation of the domain structure of titanomagnetites having x 0.3 in terms of a mixture of single-domain and superparamagnetic particles. Low apparent Curie temperatures are attributed to the effects of thermal agitation above the blocking temperature. We have examined theoretically the effects of thermal agitation on the low- and high-field thermomagnetic curves and find that observed Curie temperatures in general represent an intrinsic property of the magnetic mineral present, rather than reflecting thermal agitation. The high coercive force and relative remanence at low temperatures for titanomagnetites having x > 0.5 can be explained on the basis of the interaction of domain walls with crystal defects when the large increases in magnetocrystalline anisotropy and magnetostriction with decreasing temperature are taken into account. We discuss the evidence for the existence of domain walls in coarse-grained ulvöspinel-rich titanomagnetites and conclude that multidomain structure is well established. It is also shown that fine titanomagnetite grains may have more than one blocking temperature. In any temperature interval for which superparamagnetic grains are present they will disproportionately influence susceptibility and low-field hysteresis. 相似文献
11.
The precise knowledge of the initial 26Al/ 27Al ratio [( 26Al/ 27Al) 0] is crucial if we are to use the very first solid objects formed in our Solar System, calcium–aluminum-rich inclusions (CAIs) as the “time zero” age-anchor and guide future work with other short-lived radio-chronometers in the early Solar System, as well as determining the inventory of heat budgets from radioactivities for early planetary differentiation. New high-precision multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) measurements of 27Al/ 24Mg ratios and Mg-isotopic compositions of nine whole-rock CAIs (six mineralogically characterized fragments and three micro-drilled inclusions) from the CV carbonaceous chondrite, Allende yield a well-defined 26Al– 26Mg fossil isochron with an ( 26Al/ 27Al) 0 of (5.23 ± 0.13) × 10 − 5. Internal mineral isochrons obtained for three of these CAIs ( A44A, AJEF, and A43) are consistent with the whole-rock CAI isochron. The mineral isochron of AJEF with ( 26Al/ 27Al) 0 = (4.96 ± 0.25) × 10 − 5, anchored to our precisely determined absolute 207Pb– 206Pb age of 4567.60 ± 0.36 Ma for the same mineral separates, reinstate the “canonical” ( 26Al/ 27Al) 0 of 5 × 10 − 5 for the early Solar System. The uncertainty in ( 26Al/ 27Al) 0 corresponds to a maximum time span of ± 20 Ka (thousand years), suggesting that the Allende CAI formation events were culminated within this time span. Although all Allende CAIs studied experienced multistage formation history, including melting and evaporation in the solar nebula and post-crystallization alteration likely on the asteroidal parent body, the 26Al– 26Mg and U–Pb-isotopic systematics of the mineral separates and bulk CAIs behaved largely as closed-system since their formation. Our data do not support the “supra-canonical” 26Al/ 27Al ratio of individual minerals or their mixtures in CV CAIs, suggesting that the supra-canonical 26Al/ 27Al ratio in the CV CAIs may have resulted from post-crystallization inter-mineral redistribution of Mg isotopes within an individual inclusion. This redistribution must be volumetrically minor in order to satisfy the mass balance of the precisely defined bulk CAI and bulk mineral data obtained by MC-ICP-MS.The radiogenic 208Pb/ 206Pb ratio obtained as a by-product from the Pb–Pb age dating is used to estimate time-integrated 232Th/ 238U ratio ( κ value) of CAIs. Limited κ variations among the minerals within a single CAI, contrasted by much larger variations among the bulk CAIs, suggest Th/U fractionation occurred prior to crystallization of igneous CAIs. If interpreted as primordial heterogeneity, the κ value can be used to calculate the mean age of the interstellar dust from which the CAIs condensed. 相似文献
12.
We investigated the response of phytoplankton and zooplankton to experimental alteration of nitrate and phosphate levels in outdoor enclosures. Experiments were conducted in summer and winter and in the absence and presence of a layer of soil. The tubs (12 in all) except the two plain water controls were manured initially with a mixture of fresh cowdung (50g 1 −1), mustard oil cake (25 g l −1) and poultry wastes (25 g l −1; mostly excreta), prior to enrichment. Water samples were collected from the experimental tubs twice a week to measure selected physico-chemical and biological variables. Water temperature in the summer experiments ranged from 20–30 °C and during the winter experiments from 11–15 °C. The pH values ranged from 8.0 to 9.5 and the dissolved oxygen levels from 8.2–10.0 mg l −1. The levels of soluble reactive phosphorus and nitrate nitrogen ranged from undetectable levels to 1800 μg l −1 and 6000 μg l −1, respectively. The increase in chlorophyll-a following enrichment was rapid (3–7 days) during summer, but slower in winter (7–14 days). The predominant phytoplankton species observed in the tubs belong to the genera Sphaerocystis, Chlorella, Scenedesmus, Cosmarium, Ulothrix, Zygnema, Gonium and Pandorina. The rotifer species observed were Brachionus calyciflorus, Rotaria neptunia, Lecane bulla, L. luna, L. unguitata, Euchlanis dilatata, Asplanchna intermedia, Pseudoharringia spp., Eosphora spp., Lepadella ovalis, Epiphanes brachionus, Hexarthra mira and Cephalodella gibba. The cladocerans observed were Macrothrix spp. and Alona spp. 相似文献
13.
Isotopic compositions were determined for hydrothermal quartz, calcite, and siderite from core samples of the Newberry 2 drill hole, Oregon. The δ15O values for these minerals decrease with increasing temperatures. The values indicate that these hydrothermal minerals precipitated in isotopic equilibrium with water currently present in the reservoirs. The δ18O values of quartz and calcite from the andesite and basalt flows (700–932 m) have isotopic values which require that the equilibrated water δ18O values increase slightly ( − 11.3 to −9.2‰) with increasing measured temperatures (150–265°C). The lithic tuffs and brecciated lava flows (300–700 m) contain widespread siderite. Calculated oxygen isotopic compositions of waters in equilibrium with siderite generally increase with increasing temperatures (76–100°C). The δ18O values of siderite probably result from precipitation in water produced by mixing various amounts of the deep hydrothermal water ( − 10.5 ‰) with meteoric water ( − 15.5 ‰) recharged within the caldera. The δ13C values of calcite and siderite decrease with increasing temperatures and show that these minerals precipitated in isotopic equilibrium with CO 2 of about −8 ‰.The δ18O values of weakly altered (<5% alteration of plagioclase) whole-rock samples decrease with increasing temperatures above 100°C, indicating that exchange between water and rock is kinetically controlled. The water/rock mass ratios decrease with decreasing temperatures. The δ18O values of rocks from the bottom of Newberry 2 show about 40% isotopic exchange with the reservoir water.The calculated δ18O and δD values of bottom hole water determined from the fluid produced during the 20 hour flow test are −10.2 and −109‰, respectively. The δD value of the hydrothermal water indicates recharge from outside the caldera. 相似文献
14.
Pressure–volume–temperature relations have been measured to 32 GPa and 2073 K for natural magnesite (Mg 0.975Fe 0.015Mn 0.006Ca 0.004CO 3) using synchrotron X-ray diffraction with a multianvil apparatus at the SPring-8 facility. A least-squares fit of the room-temperature compression data to a third-order Birch–Murnaghan equation of state (EOS) yielded K0 = 97.1 ± 0.5 GPa and K′ = 5.44 ± 0.07, with fixed V0 = 279.55 ± 0.02 Å 3. Further analysis of the high-temperature compression data yielded the temperature derivative of the bulk modulus (∂ KT/∂ T) P = −0.013 ± 0.001 GPa/K and zero-pressure thermal expansion α = a0 + a1T with a0 = 4.03 (7) × 10 −5 K −1 and a1 = 0.49 (10) × 10 −8 K −2. The Anderson–Grüneisen parameter is estimated to be δT = 3.3. The analysis of axial compressibility and thermal expansivity indicates that the c-axis is over three times more compressible ( KTc = 47 ± 1 GPa) than the a-axis ( KTc = 157 ± 1 GPa), whereas the thermal expansion of the c-axis ( a0 = 6.8 (2) × 10 −5 K −1 and a1 = 2.2 (4) × 10 −8 K −2) is greater than that of the a-axis ( a0 = 2.7 (4) × 10 −5 K −1 and a1 = −0.2 (2) × 10 −8 K −2). The present thermal EOS enables us to accurately calculate the density of magnesite to the deep mantle conditions. Decarbonation of a subducting oceanic crust containing 2 wt.% magnesite would result in a 0.6% density reduction at 30 GPa and 1273 K. Using the new EOS parameters we performed thermodynamic calculations for magnesite decarbonation reactions at pressures to 20 GPa. We also estimated stability of magnesite-bearing assemblages in the lower mantle. 相似文献
15.
Sulfur isotope effects during the SO 2 disproportionation reaction to form elemental sulfur (3SO 2+3H 2O→2HSO 4−+S+2H +) at 200–330°C and saturated water vapor pressures were experimentally determined. Initially, a large kinetic isotopic fractionation takes place between HSO 4− and S, followed by a slow approach to equilibrium. The equilibrium fractionation factors, estimated from the longest run results, are expressed by 1000 ln α HSO4−–S=6.21×10 6/ T2+3.62. The rates at which the initial kinetic fractionation factors approach the equilibrium ones were evaluated at the experimental conditions. δ34S values of HSO 4− and elemental sulfur were examined for active crater lakes including Noboribetsu and Niseko, (Hokkaido, Japan), Khloridnoe, Bannoe and Maly Semiachik (Kamchatka), Poás (Costa Rica), Ruapehu (New Zealand) and Kawah Ijen and Keli Mutu (Indonesia). Δ HSO4−–S values are 28‰ for Keli Mutu, 26‰ for Kawah Ijen, 24‰ for Ruapehu, 23‰ for Poás, 22‰ for Maly Semiachik, 21‰ for Yugama, 13‰ for Bannoe, 9‰ for Niseko, 4‰ for Khloridonoe, and 0‰ for Noboribetsu, in the decreasing order. The SO 2 disproportionation reaction in the magmatic hydrothermal system below crater lakes where magmatic gases condense is responsible for high Δ HSO4−–S values, whereas contribution of HSO 4− produced through bacterial oxidation of reduced sulfur becomes progressively dominant for lakes with lower Δ HSO4−–S values. Currently, Noboribetsu crater lake contains no HSO 4− of magmatic origin. A 40-year period observation of δ 34S HSO4− and δ 34S S values at Yugama indicated that the isotopic variations reflect changes in the supply rate of SO 2 to the magmatic hydrothermal system. This implies a possibility of volcano monitoring by continuous observation of δ 34S HSO4− values. The δ18O values of HSO 4− and lake water from the studied lakes covary, indicating oxygen isotopic equilibration between them. The covariance gives strong evidence that lake water circulates through the sublimnic zone at temperatures of 140±30°C. 相似文献
16.
The cooling of a magmatic intrusion is simulated by a simple model of a non-homogeneous earth, with thermal properties depending on temperature, in which heat transfer is assumed to take place by conduction only. The mathematical problem consists in solving a non-linear partial differential equation with continuity conditions on temperature and heat flux imposed at the contacts between different rocks. This has been done numerically by a finite difference method. The model is then adopted as “reality” against which a number of commonly used approximations are tested. It is found that the effect of latent heat liberation can be reasonably taken into account by attributing an effective initial temperature to the magma (errors within 20°C for t > 10 5 years, when the temperature of the magma is still as high as 600°C); the effective specific heat approximation does not work as well. The dependence of thermal conductivity and specific heat on temperature may be eliminated by maintaining the errors within 30°C for t < 5 × 10 5 years. The assumption that magma and country rocks have the same thermal properties allows an estimate of the temperature field in the host rocks with errors of 50°C at most. The assumption that all rocks have the same constant conductivity yields results that are far from “reality” (errors of 100–200°C even at shallow depth). 相似文献
17.
Gas concentrations and isotopic compositions of water have been measured in hydrothermal waters from 13°N on the East Pacific Rise. In the most Mg-depleted samples ( 5 × 10 −3 moles/kg) the gas concentrations are: 3–4.5 × 10 −5 cm 3 STP/kg helium, 0.62–1.24 cm 3 STP/kg CH 4, 10.80–16.71 × 10 −3 moles/kg CO 2. The samples contain large quantities (95–126 cm 3/kg) of H 2 and some carbon monoxide (0.26–0.36 cm 3/kg) which result from reaction with the titanium sampling bottles. δ 13C in methane and CO 2 (−16.6 to −19.5 and −4.1 to −5.5 respectively) indicate temperatures between 475 and 550°C, whereas δ 13C CO is compatible with formation by reduction of CO 2 on Ti at 350°C close to the sampling temperature. 3He/ 4He are very homogeneous at (7.5 ± 0.1)R A( 3He/ 4He = 1.0 × 10 −5) and very similar to already published data as well as CH 4/ 3He ratios between 1.4 and 2.1 × 10 6. 18O and D in water show enrichments from 0.39 to 0.69‰ and from 0.62 to 1.49‰ respectively. These values correspond to W/R ratios of 0.4–7. The distinct 18O enrichments indicate that the isotopic composition of the oceans is not completely buffered by the hydrothermal circulations. The 3He-enthalpy relationship is discussed in terms of both hydrothermal heat flux and 3He mantle flux. 相似文献
18.
The bulk magnetic properties, including Curie temperature, susceptibility and hysteresis at various temperatures, and the Mössbauer spectra of coarse synthetic members of the titanomagnetite series are compared as a function of titanium content, using some previously reported and some new results. It has been noted previously that the titanium-rich members fail to show the expected “true multidomain” behaviour observed in pure magnetite. One possible explanation is that the behaviour observed may be due to a tendency to inhibit domain wall formation in some titanomagnetites. In this paper we discuss some other possible mechanisms to account for the magnetic properties of such titanomagnetites observable even when domain-wall-related structures do form in them. These mechanisms suggest that magnetocrystalline anisotropy and its temperature dependence in titanomagnetites may be related to local magnetic inhomogeneities, and do not depend only on crystal structure as in the case of classical ferromagnetic materials. 相似文献
19.
Distribution, abundance and life history characteristics of Mysis relicta were studied in the Feldberg Lake District (Lake Breiter Luzin, Lake Schmaler Luzin, Lake Zansen) located in northeastern Germany. Between July 2001 and November 2002 mysids were collected by vertical net hauls. In order to determine the impact of the current trophic conditions on the distribution of mysids in these lakes, oxygen concentration, total phosphorus, chlorophyll a and water transparency were also measured. All investigated lakes are mesotrophic at present. Lake Breiter Luzin exhibited great seasonal and spatial variations in mysid abundance. Density of adults and juveniles had a mean of 44.9 ± 57.1 and 68.7 ± 99.6 m −2, respectively. Highest abundance of adults was 110.4 ± 76.5 m −2 in summer, lowest abundances of 2.0 ± 4.0 m −2 occurred in spring. For juveniles, highest density of 218.4 ± 174.6 m −2 was detected in summer and lowest of 0.8 ± 1.8 m −2 in winter. No mysids were caught in any of the daytime hauls, but they were widely distributed throughout the water column at night. Size frequency distribution of mysids suggested that reproduction occurred year-round, the most consistent influx of juveniles occurred in early summer and a smaller second cohort in autumn. Highest mysid abundance was 189.2 ± 318.6 adults and 127.0 ± 66.3 juveniles m −2 in Lake Schmaler Luzin, and 59.6 ± 5.6 adults and 79.4 ± 11.2 juveniles m −2 in Lake Zansen. There were great spatial differences in abundance in both lakes. 相似文献
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