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1.
Solubility of Ti4+ in ureyite (cosmochlor, NaCrSi2O6) was experimentally studied at 1 atmosphere and ≈1000°C, using sodium disilicate as flux. Microprobe analyses indicate that at low titanium concentrations the substitution of titanium in ureyite is almost exclusively in the M1 site, coupled with a vacancy in the M2 site. At higher TiO2 contents, a small additional amount enters the tetrahedral site. If the solubility of titanium is similar in jadeite and acmite, the □TiSi2O6 substitution could contribute significantly to the vacancy content of natural titanium bearing omphacites.  相似文献   

2.
To determine the removal of regenerated nitrogen by estuarine sediments, we compared sediment N2 fluxes to the stoichiometry of nutrient and O2 fluxes in cores collected in the Childs River, Cape Cod, Massachusetts. The difference between the annual PO4 3− (0.2 mol P m−2 yr−1) and NH4 + (1.6 mol N m−2 yr−1) flux and the Redfield N∶P ratio of 16 suggested an annual deficit of 1.5 mol N m−2 yr−1. Denitrification predicted from O2∶NH4 + flux ratios and measured as N2 flux suggested a nitrogen sink of roughly the same magnitude (1.4 mol N m−2 yr−1). Denitrification accounted for low N∶P ratios of benthic flux and removed 32–37% of nitrogen inputs entering the relatively highly nutrient loaded Childs River, despite a relatively brief residence time for freshwater in this system. Uptake of bottom water nitrate could only supply a fraction of the observed N2 flux. Removal of regenerated nitrogen by denitrification in this system appears to vary seasonally. Denitrification efficiency was inversely correlated with oxygen and ammonium flux and was lowest in summer. We investigated the effect of organic matter on denitrification by simulating phytoplankton deposition to cores incubated in the lab and by deploying chambers on bare and macroaglae covered sediments in the field. Organic matter addition to sediments increased N2 flux and did not alter denitrification efficiency. Increased N2 flux co-varied with O2 and NH4 + fluxes. N2 flux (261±60 μmol m−2 h−1) was lower in chambers deployed on macroalgal beds than deployed on bare sediments (458±70 μmol m−2 h−1), and O2 uptake rate was higher in chambers deployed on macroalgal beds (14.6±2.2 mmol m−2 h−1) than on bare sediments (9.6±1.5 mmol m−2 h−1). Macroalgal cover, which can retain nitrogen in the system, is a link between nutrient loading and denitrification. Decreased denitrification due to increasing macroalgal cover could create a positive feedback because decreasing denitrification would increase nitrogen availability and could increase macroalgae cover.  相似文献   

3.
Bojorquez Lagoon (BL), located on the Mexican Caribbean, has received sewage and dredging impacts as a result of tourism development. The lagoon supports a high diversity of primary producers compared to sheltered adjacent lagoons dominated byThalassia testudinum communities. The Diurnal Curve Method (Odum and Hoskin 1958) was used to measure community metabolism and assess eutrophication in BL by comparing it to the nonimpacted lagoons and to other systems studied with this method. Dissolved oxygen community input to the water column in BL ranged between 8.3 g O2 m?2 d?1 and 41.5 g O2 m?2 d?1 during 1985 and 1986, and averaged 17.1, whereas dissolved oxygen community consumption ranged from 6.4 g O2 m?2 d?1 during 1985 and 1986, and averaged 17.1, whereas dissolved oxygen community consumption ranged from 6.4 g O2 m?2 d?1 to 37.6 g O2 m?2 d?1 and averaged 15.2. These values are higher than those found for the adjacent lagoons and similar coastal lagoons, and are similar to results from other lagoons with sewage or seafood waste discharge. Net flux of oxygen from the community to the water column averaged 1.9 g O2 m?2 d?1 and ranged from ?9.8 g O2 m?2 d?1 to 8.1 g O2 m?2 d?1. These values are low compared to the adjacent lagoons, and close to zero, as in dystrophic environments. Primary productivity, as estimated by oxygen input, increased in BL during the period of study, indicating that eutrophication is proceeding, but the lagoon has not reached yet a level of “critical eutrophication” as defined by Mee (1988).  相似文献   

4.
Waters from high‐altitude alpine lakes are mainly recharged by meteoric water. Because of seasonal variations in precipitation and temperature and relatively short hydraulic residence times, most high‐altitude lakes have lake water isotopic compositions (δ18Olake) that fluctuate due to seasonality in water balance processes. Input from snowmelt, in particular, has a significant role in determining lake water δ18O. Here we compare two high‐resolution δ18Odiatom records from lake sediments in the Swedish Scandes with instrumental data from the last century obtained from nearby meteorological stations. The time period AD 1900–1990 is characterised by an increase in winter precipitation and high winter/summer precipitation ratios and this is recorded in δ18Odiatom as decreasing trends. Lowest δ18Odiatom values and highest amount of winter precipitation are found around AD 1990 when the winter North Atlantic Oscillation index was above +2. We conclude that for the last 150 a the main factor affecting the δ18Odiatom signal in these sub‐Arctic high‐altitude lakes with short residence times has been changes in amount of winter precipitation and that δ18Odiatom derived from high‐altitude lakes in the Swedish Scandes can be used as a winter precipitation proxy. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

5.
The detailed hydro-chemical study of meltwater draining from Khangri glacier Arunachal Pradesh has been carried out to evaluate the major ion chemistry and weathering processes in the drainage basin. The investigative results shows that the meltwater is almost neutral to slightly acidic in nature with Mg–HCO3-dominated hydro-chemical facies. In glacial meltwater, Ca+?2 is the most dominated cation followed by Mg+2, Na+, and K+, while HCO3? is the most dominant anion followed by SO42?, NO3?, and Cl?. The dominant cations such as Ca+2 and Mg+2 show a good relation with the minerals abundance of the rocks. Calcite (CaCO3) and biotite [K(Mg,Fe)3AlSi3O10(F,OH)2] are the most abundant minerals in the deformed carbonate-rich metasedimentary rocks near to the snout with some K feldspar (KAlSi3O8) and quartz (SiO2). This suggests Ca+2 have definitely entered into the water due to the dissolution of calcite and Ca feldspar (CaAl2Si2O8), while one of the source of Mg+2 is biotite. Na feldspar (NaAlSi3O8) has contributed towards the availability of sodium ion, while potassium ion is derived from the chemical weathering of K feldspar and biotite. The chemical weathering is the foremost mechanism controlling the hydro-chemistry of the Khangri glacier because of the least anthropogenic interferences. The mineralogy of surrounding rocks is studied to understand better, the rock–water interaction processes, and their contribution towards ionic concentration of meltwater. The meltwater discharge and individual ion flux of the catchment area have also been calculated, to determine the ionic denudation rate for the ablation season. The high elemental ratio of (Ca?+?Mg)/(Na?+?K) (7.91?±?0.39 mg/l) and low elemental ratio of (Na?+?K)/total cations (0.11?±?0.004) indicate that the chemical composition of meltwater is mainly controlled by carbonate weathering and moderately by silicate weathering. The scatter plot result between (Ca?+?Mg) and total cations confirms that carbonate weathering is a major source of dissolved ions in Khangri glacier meltwater. In addition, the statistical analysis was also used to determine the correlation between physical parameters of glacier meltwater which controlled the solute dynamics.  相似文献   

6.
腾冲新生代火山区温泉CO2气体排放通量研究   总被引：6，自引：6，他引：0  
近期研究表明,不仅火山喷发期会向当时的大气圈输送大量的温室气体,火山间歇期同样会释放大量的温室气体。在火山活动间歇期,火山区主要以喷气孔、温(热)泉以及土壤微渗漏等形式向大气圈释放温室气体。腾冲是我国重要的新生代火山区,同时也是重要的水热活动区,那里出露大量的温泉,然而目前未见腾冲火山区温泉气体排放通量的研究报道。本文利用数字皂膜通量仪测量了腾冲新生代火山区温泉中CO2的排放通量。研究结果表明,腾冲新生代火山区温泉向当今大气圈输送的CO2通量达3.58×103 t·a-1,相当于意大利锡耶纳Bassoleto地热区温泉中CO2的排放规模。腾冲火山区温泉的CO2释放通量主要受深部岩浆囊、断裂分布、地下水循环、围岩成分等多方面因素的影响。本文根据温泉中CO2的排放特征,将腾冲温泉分为南北两区,南区温泉CO2通量远高于北区的温泉,热海地热区的通量为腾冲CO2通量的最大值。在北温泉区,CO2通量主要受控于断裂的分布;而在南温泉区,除受到断裂控制外,热海地热区底部的岩浆囊及其与围岩的相互作用成为CO2气体的重要物质来源,同时高温的岩浆囊为温泉及CO2的形成提供了重要热源。  相似文献   

7.
Mineralization of organic matter and the subsequent dissolution of calcite were simulated for surface sediments of the upper continental slope off Gabon by using microsensors to measure O2, pH, pCO2 and Ca2+ (in situ), pore-water concentration profiles of NO3−, NH4+, Fe2+, and Mn2+ and SO42− (ex situ), as well as sulfate reduction rates derived from incubation experiments. The transport and reaction model CoTReM was used to simulate the degradation of organic matter by O2, NO3−, Fe(OH)3 and SO42−, reoxidation reactions involving Fe2+ and Mn2+, and precipitation of FeS. Model application revealed an overall rate of organic matter mineralization amounting to 50 μmol C cm−2 yr−1, of which 77% were due to O2, 17% to NO3− and 3% to Fe(OH)3 and 3% to SO42−. The best fit for the pH profile was achieved by adapting three different dissolution rate constants of calcite ranging between 0.01 and 0.5% d−1 and accounting for different calcite phases in the sediment. A reaction order of 4.5 was assumed in the kinetic rate law. A CaCO3 flux to the sediment was estimated to occur at a rate of 42 g m−2 yr−1 in the area of equatorial upwelling. The model predicts a redissolution flux of calcite amounting to 36 g m−2 yr−1, thus indicating that ∼90% of the calcite flux to the sediment is redissolved.  相似文献   

8.
Seafloor methane emission from the Quepos Slide on the submarine segment of the Costa Rica fore-arc margin was estimated by extrapolating flux measurements from individual seeps to the total area covered by bacterial mats. This approach is based on the combination of detailed mapping to determine the abundance of seeps and the application of a numerical model to estimate the amount of benthic methane fluxes. Model results suggest that the majority of the studied seeps transport rather limited amount of methane (on average: ~177 μmol cm?2 a?1) into the water column due to moderate upward advection, allowing for intense anaerobic oxidation of methane (AOM; on average: 53 % of the methane flux is consumed). Depth-integrated AOM rates (56–1,538 μmol CH4 cm?2 a?1) are comparable with values reported from other active seep sites. The overall amount of dissolved methane released into the water column from the entire area covered by bacterial mats on the Quepos Slide is estimated to be about 0.28 × 106 mol a?1. This conservative estimate which relies on rather accurate determinations of seafloor methane fluxes emphasizes the potential importance of submarine slides as sites of natural methane seepage; however, at present the global extent of methane seepage from submarine slides is largely unknown.  相似文献   

9.
 Eh, pH, salinity, total alkalinity, dissolved O2, NO2 –, PO4 –3, SiO2 and NH4 + of waters from a mangrove forest, an estuary and a creek connecting the mangrove forest and the estuary have been measured. Further, the chemistry of interstitial waters of surficial and core sediments from the mangrove forest have been analyzed for the above parameters, except dissolved oxygen. To understand the flux of nutrients from the mangrove forest to the adjoining estuary, creek waters were monitored during tidal phases. PO4 –3, SiO2 and NH4 + were found to be at elevated levels in mangrove waters whereas NO2 – shows no variation compared to the estuary. Dissolved O2 is low in mangrove waters. PO4 –3, NH4 + and SiO2 are several times higher in interstitial waters than in overlying waters. Several fold enrichment of PO4 –3, NH4 + and, to some extent, SiO2 were measured in creek waters during ebbing relative to flooding, indicating that mangroves act as a perennial source for the above nutrients. Received: 26 May 1998 · Accepted: 21 July 1998  相似文献   

10.
Based on in situ microprofiles, chamber incubations and eddy covariance measurements, we investigated the benthic carbon mineralization and nutrient regeneration in a ~65-m-deep sedimentation basin of Loch Etive, UK. The sediment hosted a considerable amount of infauna that was dominated by the brittle star A. filiformis. The numerous burrows were intensively irrigated enhancing the benthic in situ O2 uptake by ~50 %, and inducing highly variable redox conditions and O2 distribution in the surface sediment as also documented by complementary laboratory-based planar optode measurements. The average benthic O2 exchange as derived by chamber incubations and the eddy covariance approach were similar (14.9 ± 2.5 and 13.1 ± 9.0 mmol m?2 day?1) providing confidence in the two measuring approaches. Moreover, the non-invasive eddy approach revealed a flow-dependent benthic O2 flux that was partly ascribed to enhanced ventilation of infauna burrows during periods of elevated flow rates. The ratio in exchange rates of ΣCO2 and O2 was close to unity, confirming that the O2 uptake was a good proxy for the benthic carbon mineralization in this setting. The infauna activity resulted in highly dynamic redox conditions that presumably facilitated an efficient degradation of both terrestrial and marine-derived organic material. The complex O2 dynamics of the burrow environment also concurrently stimulated nitrification and coupled denitrification rates making the sediment an efficient sink for bioavailable nitrogen. Furthermore, bioturbation mediated a high efflux of dissolved phosphorus and silicate. The study documents a high spatial and temporal variation in benthic solute exchange with important implications for benthic turnover of organic carbon and nutrients. However, more long-term in situ investigations with like approaches are required to fully understand how environmental events and spatio-temporal variations interrelate to the overall biogeochemical functioning of coastal sediments.  相似文献   

11.
Although the flux of molecular O2 between the atmosphere and the subsurface is intrinsically linked to the net soil production of greenhouse gasses, few studies have focused on the controls affecting the isotopic composition of O2 in the subsurface. Here, we developed and tested a stable oxygen isotope tracer technique and gas transport modeling approach to evaluate O2 cycling and fluxes from the subsurface that used an environmentally controlled soil mescosm. We measured the O2 and δ18O2 profiles in a model unsaturated soil zone and quantified the O2 consumption rates and the O2 isotope fractionation factors resulting from the combined processes of subsurface microbial (including bacteria, fungi, and protozoa) consumption of O2 and diffusive influx of O2 from the atmosphere. We found that at high respiration rates in the mesocosm, there appeared to be very little isotope fractionation of O2 by soil microorganisms. Although the mesocosm respiration rates are not typical of natural soils in northern temperate climes, they may be more representative of soils in warm and moist tropical environments. Our findings caution against the indiscriminate application of laboratory-determined oxygen isotope fractionation factors to field settings. The oxygen isotope tracer and modeling approach demonstrated here may be applied to gain a better understanding of biogenic gas production and O2 cycling in subsurface systems and soils.  相似文献   

12.
Neutron- and gamma irradiation-induced paramagnetic centers in natural colorless topaz from four different Brazilian localities were studied by electron paramagnetic resonance (EPR) and optical absorption as a function of irradiation dose and thermal treatment. Gamma irradiation doses were applied up to 1,000 kGy with a dose rate of 15 kGy h−1. For the neutron irradiation experiments, a neutron flux of 4 × 1012 cm−2 s−1 was used with an integrated flux of up to 1 × 1018 cm−2. From the experiments, it is concluded that brownish colors are induced by gamma-rays and may be associated with a single broad isotropic EPR line with g = 2.015(2). Both the EPR line and the related optical absorption band at 460 nm (2.7 eV) are lost during thermal treatments between 150 and 200°C. Fast neutrons create the paramagnetic peroxyl radicals O2 − and the paramagnetic O− hole centers. The O− centers have the same thermal stability as the optical absorption band at 620 nm (2.0 eV). It is confirmed that the absorption due to the O− center is responsible for the blue color in topaz. Both color centers and their absorption bands are discussed in the context of O− bound small polarons.  相似文献   

13.
The combined water (H2O+) in 30 geochemical standards having known and unknown water contents was determined by an elemental analyzer on 1 day during each of 6 weeks over a period of several months. The analysis of variance of the data measured in the form of a Youden square shows an extremely significant variation due to some unknown cause during the weeks in which the data were obtained. The higher water contents of the first two sets were obtained in the fall season when this area (Reston, Va.) has high and variable humidity, whereas the last four sets were measured during winter months. Humidity, however, was not included as a variable in the design and was not measured. The first two of three average blanks for H2O+, measured after determining H2O+ contents of about 11 percent for two samples, are noticeably higher than the third average blank. Similarly, the first of two average blanks for H2O+, measured after determining the H2O+ content (4.70 percent) of PCC-1, is higher than the second. Data for the last 4 weeks were used to calculate the line of regression and the extremely significant correlation coefficient. The line is used to predict suggested revisions of the “best” H2O+ contents of the standards. Coefficients of variation calculated from the data for the last 4 weeks indicate that there is no severe sampling problem due to the small sample size (20 mg) taken for the determinations. The coefficients follow the general trend of large coefficients for samples containing the least amount of the constituent (BHVO-1: x?= 11.76% H2O+; C.V. = 7.3%) and of small coefficients for samples containing the greatest amount (BX-N: x?= 11.76% H2O+; C.V. = 0.4%).  相似文献   

14.
《Gondwana Research》2013,23(3-4):799-809
Externally derived, pure CO2 that mixes with a carbon-(under)saturated C-O-H fluid in lower crustal granulites may result in graphite precipitation if the host-rock oxygen fugacity (fO2rock) is below the upper fO2 limit of graphite. The maximum relative amount of graphite that can precipitate varies between a few mol% up to more than 25 mol%, depending on pressure, temperature, and host-rock redox state. The maximum relative amount of graphite that can precipitate from an infiltrating CO2 fluid into a dry granulite (CO fluid system) varies between zero and a few mol%. Thermodynamic evaluation of the graphite precipitation process shows that CO2 infiltration into lower crustal rocks does not always result in a carbon (super)saturated fluid. In that case, graphite precipitation is only possible if carbon saturation can be reached as a result of the reaction CO2 → CO + ½ O2. Graphite that has been precipitated during granulite facies metamorphic conditions can subsequently be absorbed by a COH fluid during retrograde metamorphism. It is also possible, however, that significant amounts of graphite precipitate from a COH fluid during retrograde metamorphism. This study shows that interpreting the presence or absence of graphite in granulites with respect to CO2 infiltration requires detailed information on the P–T–fO2rock conditions, the relative amount of CO2 that infiltrates into the rock, and whether H2O is present or not.  相似文献   

15.
In situ measured microprofiles of Ca2+, pCO2, pH and O2 were performed to quantify the CaCO3 dissolution and organic matter mineralization in marine sediments in the eastern South Atlantic. A numerical model simulating the organic matter decay with oxygen was used to estimate the calcite dissolution rate. From the oxygen microprofiles measured at four stations along a 1300-m isobath of the eastern African margin and one in front of the river Niger at a water depth of 2200 m the diffusive oxygen uptake (DOU) and oxygen penetration depth (OPD) was calculated. DOU rates were in the range of 0.3 to 3 mmol m−2 d−1 and showed a decrease with increasing water depth, corresponding to an increase in OPD. The calculated amount of degradated organic matter is in the range of 1 to 8.5 gC m−2 a−1. The metabolic CO2, released from mineralization of the organic matter drives calcite dissolution in these sediments overlain by calcite-supersaturated water. Fluxes across the sediment water interface calculated from the in situ Ca2+ microprofiles were 0.6 mmol m−2 d−1 for two stations at a water depth of 1300 m. The ratio of calcite dissolution flux and organic C degradation is 0.53 and 0.97, respectively. The microprofiles indicate that CO2 produced within the upper oxic sediment layer dissolves up to 85% of the calcite rain to the seafloor. Modeling our O2, pH and Ca2+ profiles from one station predicted a calcite dissolution rate constant for this calcite-poor site of 1000 mol kgw−1 a−1 (mol per kg water and year), which equals 95% d−1. This rate constant is at the upper end of reported in situ values.  相似文献   

16.
Analyses of major element and volatile components of amphiboles from Vulcan's Throne, a Recent volcano on the north rim of the Grand Canyon, Arizona, USA, have been performed by using the electron microprobe and high temperature mass spectrometry. The amphiboles occur as megacrysts, as oikocrysts in peridotite and pyroxenite xenoliths, in amphibole-rich selvages on lherzolite xenoliths, and as grains in hornblendite xenoliths. Total volatiles range from 1.27 to 1.75 wt.%. In all samples, H2O is the principal volatile species. Lesser amounts of structurally bound fluorine, chlorine, and oxygen were also released. The amphiboles studied are hydroxyl-deficient. The O(3) site is probably partially occupied by O2?, which was detected as O2 during degassing of the amphibole. Ti shows a strong positive correlation with the amount of hydroxyl deficiency in the amphiboles except for one oxidized sample. Thus, Ti probably is significant in charge balancing the substitution of O2? for OH? and the substitution probably occurred during crystallization rather than by dehydrogenation. Small amounts of both oxidized and reduced carbon and sulfur-bearing volatile species (e.g., CO2, CO, CH4, SO2, H2S) were detected in all samples. The observation of reduced carbon species supports the hypothesis that the oxygen fugacity of at least portions of the upper mantle is probably less than the quartz-fayalite-magnetite buffer.  相似文献   

17.
Lakes worldwide are commonly oversaturated with CO2, however the source of this CO2 oversaturation is not well understood. To examine the magnitude of the C flux to the atmosphere and determine if an excess of respiration (R) over gross primary production (GPP) is sufficient to account for this C flux, metabolic parameters and stable isotopes of dissolved O2 and C were measured in 23 Québec lakes. All of the lakes sampled were oversaturated with CO2 over the sampling period, on average 221 ± 25%. However, little evidence was found to conclude that this CO2 oversaturation was the result of an excess of pelagic R over GPP. In lakes Croche and à l’Ours, where CO2 flux, R and GPP were measured weekly, the annual difference between pelagic GPP and R, or net primary production (NPP), was not sufficient to account for the size of the CO2 flux to the atmosphere. In Lac Croche average annual NPP was 14.4 mg C m−2 d−1 while the average annual flux of CO2 to the atmosphere was 34 mg C m−2 d−1. In Lac à l’Ours average annual NPP was −9.1 mg C m−2 d−1 while the average annual flux of CO2 to the atmosphere was 55 mg C m−2 d−1. In all of the lakes sampled, O2 saturation averaged 104.0 ± 1.7% during the ice-free season and the isotopic composition of dissolved O2 (δ18ODO) was 22.9 ± 0.3‰, lower than atmospheric values and indicative of net autotrophy. Carbon evasion was not a function of R, nor did the isotopic signature of dissolved CO2 in the lakes present evidence of excess R over GPP. External inputs of C must therefore subsidize the lake to explain the continued CO2 oversaturation. The isotopic composition of dissolved inorganic C (δ13CDIC) indicates that the CO2 oversaturation cannot be attributed to in situ aerobic respiration. δ13CDIC reveals a source of excess C enriched in 13C, which may be accounted for by anaerobic sediment respiration or groundwater inputs followed by kinetic isotope fractionation during degassing under open system conditions.  相似文献   

18.
Current photochemical models suggest that oxygen levels in the prebiological atmosphere were extremely low, most probably remaining in the range 10−8–10−14 PAL (present atmospheric level). It is, therefore, reasonable to assume that only life processes were able to overwhelm these minor O2-pressures, with free oxygen resulting from the reduction of carbon dioxide to the carbohydrate level during photoautotrophic carbon fixation using water as an electron donor (
Full-size image (1K)
). It is by now well established that reduced (organic) carbon is a common constituent of sedimentary rocks from the very start of the geological record 3.8 Ga ago. Both direct assays and inferences derived from a carbon isotope mass balance suggest that the Corg-content of Archaean sediments was not basically different from that of geologically younger rocks. This poses the problem of the existence 3.5 Ga ago of an oxidation equivalent of such a formidable ancient Corg-reservoir which, depending on the model adopted for the growth of the sedimentary mass through time, might have amounted to between 20 and 100% of the present one. Low atmospheric oxygen pressures in the Early Precambrian that are inferred from retarded oxidation reactions, notably in the ancient continental weathering cycle, are likely, therefore, to indicate extremely rapid processes of oxygen consumption in other parts of the system (e.g., hydrosphere) rather than the general absence of photosynthetic oxidation equivalents during this time.  相似文献   

19.
Generally, P–T pseudosections for reduced compositional systems, such as K2O–FeO–MgO–Al2O3–SiO2–H2O, Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O and MnO–K2O–FeO–MgO–Al2O3–SiO2–H2O, are well suited for inferring detailed P–T paths, comparing mineral assemblages observed in natural rocks with those calculated. Examples are provided by P–T paths inferred for four metapelitic samples from a 1 m2 wide outcrop of the Herbert Mountains in the Shackleton Range, Antarctica. The method works well if the bulk composition used is reconstituted from average mineral modes and mineral compositions (AMC) or when X‐ray fluorescence (XRF) data are corrected for Al2O3 and FeO. A plagioclase correction is suitable for Al2O3. Correction for FeO is dependent on additional microscopic observations, e.g. the kind and amount of opaque minerals. In some cases, all iron can be treated as FeOtot, whereas in others a magnetite or hematite correction yields much better results. Comparison between calculated and observed mineral modes and mineral compositions shows that the AMC bulk composition is best suited to the interpretation of rock textures using P–T pseudosections, whereas corrected XRF data yield good results only when the investigated sample has few opaque minerals. The results indicate that metapelitic rocks from the Herbert Mountains of the Northern Shackleton Range underwent a prograde P–T evolution from about 600 °C/5.5 kbar to 660 °C/7 kbar, followed by nearly adiabatic cooling to about 600 °C at 4.5 kbar.  相似文献   

20.
Low-temperature heat capacity measurements for MgCr2O4 have only been performed down to 52 K, and the commonly quoted third-law entropy at 298 K (106 J K−1 mol−1) was obtained by empirical extrapolation of these measurements to 0 K without considering the magnetic or electronic ordering contributions to the entropy. Subsequent magnetic measurements at low temperature reveal that the Néel temperature, at which magnetic ordering of the Cr3+ ions in MgCr2O4 occurs, is at ∼15 K. Hence a substantial contribution to the entropy of MgCr2O4 has been missed. We have determined the position of the near-univariant reaction MgCr2O4+SiO2=MgSiO3+Cr2O3. The reaction, which has a small positive slope in P-T space, has been bracketed at 100 K intervals between 1273 and 1773 K by reversal experiments. The reaction is extremely sluggish, and lengthy run times with a flux (H2O, BaO-B2O3 or K2O-B2O3) are needed to produce tight reversal brackets. The results, combined with assessed thermodynamic data for Cr2O3, MgSiO3 and SiO2, give the entropy and enthalpy of formation of MgCr2O4 spinel. As expected, our experimental results are not in good agreement with the presently available thermodynamic data. We obtain Δ f H ∘ 298=−1759.2±1.5 kJ mol−1 and S ∘ 298=122.1±1.0 J K−1 mol−1 for MgCr2O4. This entropy is some 16 J K−1 mol−1 more than the calorimetrically determined value, and implies a value for the magnetic entropy of MgCr2O4 consistent with an effective spin quantum number (S') for Cr3+ of 1/2 rather than the theoretical 3/2, indicating, as in other spinels, spin quenching. Received: 9 May 1997 / Accepted: 28 July 1997  相似文献   

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