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1.
Instrumental neutron activation and X-ray fluorescence analyses of minettes from New Brunswick, Canada, indicate that these rocks are strongly enriched in REE, especially the LREE, and other incompatible elements as well as Cr and Co. The geochemistry of minettes precludes their formation by anatexis or assimilation of crustal rocks, contamination of mantle-derived basalts by non-crystalline residua of granite crystallization, or any process involving fractional crystallization of feldspar. Their peculiar geochemical characteristics must be a direct function of their origin in the mantle.Ultrapotassic rocks, kimberlites, and, to a lesser extent, carbonatites are strikingly similar to minettes in their rare earth and other trace element contents, suggesting genetic links among these rock types. It is difficult to explain the temporal and spatial constancy of this similarity by post-anatectic late enrichment of diversely produced magmas by volatile transport. We tentatively propose that the process best able to account for their unique geochemistry is limited partial melting of the subcontinental mantle following and dependent on the metasomatic introduction of K, Ti, Fe, REE, halogens, P, and other elements as well as H2O and/or CO2. If the enriched mantle is H2O-rich, minette magma is produced; if it is CO2-rich or has an intermediate ratio, carbonatitic-kimberlitic and/or ultrapotassic magmas result. 相似文献
2.
William E. Glassley 《Geochimica et cosmochimica acta》1983,47(3):597-616
Compositional differences between granulite facies rocks and equivalent amphibolite facies rocks and the observation of CO2-rich fluid inclusions in granulites, have led to the suggestion that CO2 must play a role in modifying the composition of deep continental crust. How CO2 effects this change has remained unclear. Using the thermodynamic properties of aqueous ions in a fluid of evolving ratio, it is possible to model the incongruent dissolution of feldspars under conditions appropriate for granulite facies metamorphism. The results demonstrate that dissolution will be strongly enhanced at high ratios, with ion solubilities being Na+ >K+ ? Ca++. This enhancement is compatible with the reported compositional contrasts between granulite and amphibolite facies rock, but requires large fluid volumes.To test the dissolution model, a detailed field and petrologic study was conducted in a well exposed granulite facies terrane in West Greenland. Strong correlation between fluid composition and bulk rock chemistry can be documented; CO2-rich regions contain rocks which consistently have low ratios, while H2O-rich regions consistently have high ratios. Magnetite rims on sulfide grains are ubiquitous in high regions and are absent in high regions, and they provide evidence that CO2 was introduced into the region. These correlations and observations are predictable from the properties of the dissolution process. These considerations, along with observations regarding graphite petrogenesis, provide strong arguments that the total fluid volume interacting with the rock during metamorphism was very large, in some cases equaling or exceeding total rock volume. Such large fluid volumes can lead to significant compositional modification of the crust, and will mask the original protolith chemistry. Such processes should lead to Ca- and Al-enriched, Na-, K-, S- and Si-depleted residues in the deep crust. 相似文献
3.
Jrgen Gutzon Larsen 《Lithos》1981,14(4):241-262
Megacrysts and polymineralic fragments of extraordinary diversity from a Tertiary monchiquitic dyke of Ubekendt Ejland comprise three groups: (1) Cr-diopside-fassaitic diopside + olivine, Fo90.5?81.5 + CrAl spinels. (II) Fassaitic salite-ferrisalite + KTi-pargasite-ferropargasite + apatite + AlTi-magnetite, (III) Scapolite + hyalophane + potassium feldspar + nepheline + analcime. By comparison with mineralogy and phase relations in the host rock and experimental data from alkaline rocks the megacrysts are related to a sequence of crystallization from primitive monchiquitic to potassic phonolitic magmas rich in H2O and CO2 at 5–11 kb. Group I megacrysts formed at temperatures of 1300-1150°C and group II between ? bar at the latter temperature. High Pco2 may have stabilized the scapolite in the more evolved liquid and K-feldspar and nepheline began to crystallize at ca. 800°C possibly together with the ferrisalite. 相似文献
4.
Calibration of five gas geothermometers is presented, three of which used CO2, H2S and H2 concentrations in fumarole steam, respectively. The remaining two use and ratios. The calibration is based on the relation between gas content of drillhole discharges and measured aquifer temperatures. After establishing the gas content in the aquifer, gas concentrations were calculated in steam formed by adiabatic boiling of this water to atmospheric pressure to obtain the gas geothermometry functions. It is shown that the concentrations of CO2, H2S and H2 in geothermal reservoir waters are fixed through equilibria with mineral buffers. At temperatures above 230°C epidote + prehnite + calcite + quartz are considered to buffer CO2. Two buffers are involved for H2S and H2 and two functions are, therefore, presented for the geothermometers involving these gases. For waters containing less than about 500 ppm chloride and in the range 230–300°C pyrite + pyrrholite + epidote + prehnite seem to be involved, but pyrite + epidote + prehnite + magnetite or chlorite for waters above 300°C and waters in the range 230–300°C, if containing over about 500 ppm.The gas geothermometers are useful for predicting subsurface temperatures in high-temperature geothermal systems. They are applicable to systems in basaltic to acidic rocks and in sediments with similar composition, but should be used with reservation for systems located in rocks which differ much in composition from the basaltic to acidic ones. The geothermometry results may be used to obtain information on steam condensation in upflow zones, or phase separation at elevated pressures.Measured aquifer temperatures in drillholes and gas geothermometry temperatures, based on data from nearby fumaroles, compare well in the five fields in Iceland considered specifically for the present study as well as in several fields in other countries for which data were inspected. The results of the gas geothermometers also compare well with the results of solute geothermometers and mixing models in three undrilled Icelandic fields. 相似文献
5.
The chemical composition of gas mixtures emerging in thermal areas can be used to evaluate the deep thermal temperatures. Chemical analyses of the gas compositions for 34 thermal systems were considered and an empirical relationship developed between the relative concentrations of H2S, H2, CH4 and CO2 and the reservoir temperature. The evaluated temperatures can be expressed by: where (concentrations in % by volume) and β = 7 logPco2 相似文献
6.
J.R Holloway 《Geochimica et cosmochimica acta》1973,37(3):651-666
The stability of the amphibole pargasite [NaCa2Mg4Al(Al2Si6))O22(OH)2] in the melting range has been determined at total pressures (P) of 1.2 to 8 kbar. The activity of H2O was controlled independently of P by using mixtures of H2O + CO2 in the fluid phase. The mole fraction of H2O in the fluid () ranged from 1.0 to 0.2.At P < 4 kbar the stability temperature (T) of pargasite decreases with decreasing at constant P. Above P ? 4 kbar stability T increases as is decreased below one, passes through a T maximum and then decreases with a further decrease in . This behavior is due to a decrease in the H2O content of the silicate liquid as decreases. The magnitude of the T maximum increases from about 10°C (relative to the stability T for ) at P = 5 kbar to about 30°C at P = 8 kbar, and the position of the maximum shifts from at P = 5 kbar to at P = 8 kbar.The H2O content of liquid coexisting with pargasite has been estimated as a function of at 5 and 8 kbar P, and can be used to estimate the H2O content of magmas. Because pargasite is stable at low values of at high P and T, hornblende can be an important phase in igneous processes even at relatively low H2O fugacities. 相似文献
7.
Charles D Byers David M Christie David W Muenow John M Sinton 《Geochimica et cosmochimica acta》1984,48(11):2239-2245
Volatiles and major elements in abyssal glasses ranging in composition from basalt, ferrobasalt, andesite to rhyodacite from the Galapagos Spreading Center (GSC) near 95°W were analyzed using electron microprobe and high temperature mass spectrometry. Total volatile content ranged from 0.32 wt.% to 2.74 wt.%. Volatile abundances of MORB glasses from the 95.5°W propagating rift are similar to those from the adjacent normal rift (avg. 0.34 wt.%) and lower than those of N-type MORB from the Mid-Atlantic Ridge (avg. 0.49 wt.%). Although both propagating and non-propagating rift glasses contain trace amounts of methane (<0.01 wt.%) and carbon monoxide (0.04 wt.%), significantly higher 100 ratios are observed for the primitive propagating rift glasses. Water contents of the most primitive GSC glasses are ~0.09 wt.% suggesting a water content for the mantle source of ~0.02 wt.% which indicates that source masses with very low water content can be involved in the generation of MORB.In fractionated ferrobasalt, andesite and rhyodacite glasses from the 95.5°W propagating rift, increasing abundances of H2O, Cl and F indicate highly incompatible behavior, whereas CO2 and reduced carbon species appear to decrease in abundance with increasing differentiation. Ferric-ferrous ratios increase from basalt to andesite and reduce to near zero in the rhyodacite. These values are not distinguishable from those previously reported for similar fractionated glasses from the Galapagos 85°W propagating rift, despite the apparent suppression of oxide precipitation in the 85°W suite. 相似文献
8.
CaCO3Ca(OH)2CaS serves as a model system for sulfide solubility in carbonatite magmas. Experiments at 1 kbar delineate fields for primary crystallization of CaCO3, Ca(OH)2 and CaS. The three fields meet at a ternary eutectic at 652°C with liquid composition (wt%): CaCO3 = 46.1%, Ca(OH)2 = 51.9%, CaS = 2.0%. Two crystallization sequences are possible for liquids that precipitate calcite, depending upon whether the liquid is on the low-CaS side, or the high-CaS side of the line connecting CaCO3 to the eutectic liquid. Low-CaS liquids precipitate no sulfide until the eutectic temperature is reached leading to sulfide enrichment. The higher-CaS liquids precipitate some sulfide above the eutectic temperature, but the sulfide content of the melt is not greatly depleted as the eutectic temperature is approached. Theoretical considerations indicate that sulfide solubility in carbonate melts will be directly proportional to and inversely proportional to ; it also is likely to be directly proportional to melt basicity, defined here by . A strong similarity exists in the processes which control sulfide solubility in carbonate and in silicate melts. By analogy with silicates, ferrous iron, which was absent in our experiments, may also exert an important influence on sulfide solubility in natural carbonatite magmas. 相似文献
9.
Fluid inclusion analyses leave little doubt that solutions containing large concentrations of H2O, CO2, and electrolytes are involved in a wide range of geologic processes. Although the miscibility gap in the system H2O-CO2 occurs only at low temperatures, experimental data reported by Takenouchi and Kennedy (1965) and Gehrig (1980) indicate that the addition of 6 weight percent NaCl relative to H2O + Nacl extends the region of immiscibility in the system H2O-CO2-NaC] to ≥700°C at 500 bars and mole fractions of CO2 (XCO2) ? 0.1. In contrast, addition of 20 weight percent NaCl relative to H2O + NaCl at 700°C and 500 bars expands the miscibility gap to XCO2 ? 0.2. At 2000 bars, addition of 20 and 35 weight percent NaCl relative to H2O + NaCl causes the miscibility gap to extend to ~500° and ~700°C, respectively, at . The existence of the immiscible region in this high-pressure/temperature environment has a profound effect on temperatures of equilibration for metamorphic mineral assemblages (Bowers and Helgeson, 1983). To determine the extent to which nonideality in the ternary system affects these equilibria, the modified Redlich-Kwong (MRK.) equation of state was fit to pressure-volume-temperature data taken from Gehrig (1980) along pseudobinaries for which is constant. Fugacity coefficients of the components were then generated from the fugacity coefficient analog of the MRK equation of state and these coefficients were used together with solubility data to determine the compositions of the coexisting immiscible phases. The tie lines connecting the coexisting phases shift in orientation from nearly parallel to the H2O-CO2 binary at low temperatures to almost perpendicular to this binary at high temperatures. 相似文献
10.
Major-element, trace-element and isotopic compositions of approximately 1200 basalts (< 53 wt. % SiO2) from intra-oceanic island arcs have been compiled to assess the nature and possible sources of primitive island-arc basalts (IAB). The chemical characteristics of IAB are examined with reference to those of mid-ocean ridge basalts (MORB) and intraplate oceanic basalts (IPB). Major-element compositions of primitive [] IAB and MORB are similar, but differ significantly from IPB. In general, IAB do not have higher Al2O3, lower TiO2 or a lack of Fe enrichment compared to primitive MORB but many do have greater K2O contents. Differences in major- and minor-element contents between more evolved IAB and MORB result from the dominance of plagioclase + olivine crystal fractionation in MORB magmas vs. clinopyroxene + olivine controlled fractionation in IAB suites. This difference in crystallization history may be related to the higher PH2O or greater depth of crystallization of IAB magmas compared to those inferred for MORB.IAB are characteristically enriched in large-ion-lithophile (LIL) elements and depleted in high-field-strength ions (e.g., Zr, Nb and Hf) relative to normal MORB (N-type) and IPB. The enrichment of some LIL elements (e.g., Sr, Rb, Ba and Pb) relative to the rare-earth elements in IAB is difficult to explain by simple partial melting alone and suggests a multistage petrogenesis involving an LIL-enriched component. Low abundances of high-field-strength ions in evolved IAB are explicable in terms of fractional crystallization, but the cause for consistently low abundances in primitive IAB remains problematic.Island-arc lavas contain greater concentrations of volatiles and have higher and Cl/F ratios than either MORB or IPB, suggesting involvement of a slab-derived volatile component. However, this is not consistent with data which indicate that only near-trench volcanics have been significantly affected by dehydration of the oceanic crust.Sr-, Nd-, Pb- and O-isotopic data, in conjunction with the trace-element data, clearly indicate that IAB are derived from heterogeneous, LIL-depleted mantle sources most similar to those which give rise to enriched MORB (E-type). The marked shift towards higher in IAB compared to oceanic lavas with similar values cannot be explained simply by the addition of radiogenic Sr from the slab. Variable degrees of contamination from a crustally-derived sedimentary component is consistent with the isotopic and trace-element data from a number of arcs. However, the lack of correlation between LIL/REE ratios and more radiogenic isotopic ratios suggests that this enrichment/contamination process is complex. A multi-stage petrogenetic model involving subducted oceanic crust (± sediments), dehydration/volatile transfer, and partial melting of metasomatized mantle beneath island arcs is considered the most reasonable, although least constrained, method to generate a variety of primitive IAB. 相似文献
11.
The fractionation factors () for the reduction of CO2 to CH4 by pure cultures of methane-producing bacteria are, for Methanosarcina barkeri at 40°C, 1.045 ± 0.002; for Methanobacterium strain M.o.H. at 40°C, 1.061 ± 0.002; and, for Methanobacterium thermoautotrophicum at 65°C, 1.025 ± 0.002. These observations suggest that the acetic acid used by acetate dissimilating bacteria, if they play an important role in natural methane production, must have an intramolecular isotopic fractionation () approximating the observed fractionation. 相似文献
12.
The solvus in the system CO2-H2O-2.6 wt% NaCl-equivalent was determined by measuring temperature of homogenization in fluid inclusions which contained variable but the same amount of salt dissolved in the aqueous phase at room temperature. The critical point of the solvus is at 340 ± 5°C, at pressures between 1 and 2 kbar; this is about 65°C higher than for the pure CO2-H2O system. The solvus is assymetrical, with a steeper H2O-rich limb and with the critical point at mole fraction of water between 0.65 and 0.8. 相似文献
13.
N Guilhaumou P Dhamelincourt J-C Touray J Touret 《Geochimica et cosmochimica acta》1981,45(5):657-673
Optical and analytical studies were performed on 400 N2 + CO2 gas bearing inclusions in dolomites and quartz from Triassic outcrops in northern Tunisia. Other fluids present include brines (NaCl and KCl bearing inclusions) and rare liquid hydrocarbons. At the time of trapping, such fluids were heterogeneous gas + brine mixtures. In hydrocarbon free inclusions the mole ratio was determined using two different non-destructive and punctual techniques: Raman microprobe analysis, and optical estimation of the volume ratios of the different phases selected at low temperatures. In the observed range of compositions, the two methods agree reasonably well.The N2 + CO2 inclusions are divided into three classes of composition: (a) : Liquid nitrogen is always visible at very low temperature and homogenisation occurs in the range ?151°C to ? 147°C (nitrogen critical temperature) dry ice (solid CO2) sublimates between ?75°C and ?60°C; (b) : liquid nitrogen is visible at very low temperature but dry ice melts on heating; liquid and gas CO2 homogenise to liquid phase between ?51°C to ?22°C; (c) : liquid nitrogen is not visible even at very low temperature (?195°C) and liquid and gas CO2 homogenise to liquid phase between ?22°C and ?15°C. The observed phases changes are used to propose a preliminary phase diagram for the system CO2-N2 at low temperatures.Assuming additivity of partial pressures, isochores for the CO2-N2 inclusions have been computed. The intersection of these isochores with those for brine inclusions in the same samples may give the P and T of trapping of the fluids. 相似文献
14.
R.W. Boyle 《Journal of Geochemical Exploration》1974,3(4):345-369
The behaviour of the ratios K2O/Na2O, SiO2/CO2, and in the alteration envelopes of a variety of epigenetic deposits is documented. It is concluded that the ratio K2O/Na2O is the most suitable for estimating proximity to ore during exploration drilling programs. The other two ratios are useful in indicating proximity to ore only under certain geological conditions. Before the use of ratios is contemplated in detailed exploration programs within a given mineral belt orientation studies should be carried out to determine the trend of the ratios with proximity to mineralization. 相似文献
15.
Thermodynamic calculations for selected silicate-oxide-fluorite assemblages indicate that several commonly occurring fluorite-bearing assemblages are restricted to relatively narrow fields at constant P?T. The presence of fayalite-ferrohedenbergite-fluorite-quartz ± magnetite and ferrosalite-fluorite-quartz-magnetite assemblages in orthogneisses from Au Sable Forks, Wanakena and Lake Pleasant, New York, buffered fluorine and oxygen fugacities during the granulite facies metamorphism in the Adirondack Highlands. These buffering assemblages restrict to 10?29 ± 1 bar and to 10?16 ± 1 bar at the estimated metamorphic temperature of 1000K and pressure of 7 kbar. The assemblage biotite-magnetite-ilmenite-K-feldspar, found in the same Au Sable Forks outcrop as the fayalite-fluorite-ferrohedenbergite-quartz-magnetitie assemblage, restricts H2O fugacities to less than 103·3 bar. These fugacities limit H2 and HF fugacities to less than 101 bar for the Au Sable outcrop. The data indicate that relative to H2O, O2, H2, F2 and HF are not major species in the fluid equilibrated with Adirondack orthogneisses. The calculated F2 fugacilies are similar to the upper limits possible for plagioclase-bearing rocks and probably represent the upper limit for metamorphism in the Adirondacks and in other granulite facies terranes. 相似文献
16.
Rapid sea floor spreading has taken place over the last 8 Myr behind the South Sandwich island arc, producing a regular set of magnetic lineations. Suites of fresh basalts have been dredged from four widely separated localities along the spreading axis. Dredges 20 and 23 yielded sub-alkaline olivine tholeiites, dredge 22 recovered vesicular tholeiites with minor normative olivine, while dredge 24 contained a fractionated suite of highly vesicular quartz-normative basalts with higher . The concentrations of the incompatible elements Ti, P, Zr, Hf, Nb, Ta, Y and the REE increase systematically from dredge 24 through dredges 22 and 20 to dredge 23 and there is a comparable increase in . Quantitative modelling suggests that all the basalts can be derived from an essentially similar mantle source (with respect to these elements) through varying degrees of partial melting, but involving some residual clinopyroxene. Basalts from dredge 24 have unusually low concentrations of Ti, P, Zr, Nb, Y, REE and Ni, similar to the values in arc tholeiites, and the more primitive dredge 24 liquids seem to have been generated through high degrees of partial melting (~ 30%) leaving a dunitic residue. Transitional arc tholeiite characteristics are also apparent in the relatively high K, Rb, Ba contents and ratios of dredge 24 and 22 basalts, though Nd isotope ratios are uniform. It is considered that fluids derived from the dehydrating subducted slab may have locally penetrated the source regions of the back-arc basalts, carrying K, Rb, Ba and seawater-enriched 87Sr, and producing conditions of magma generation similar to that of arc tholeiites. However, it is unlikely that the sources for these and other marginal basin basalts differ fundamentally from the range of mantle sources feeding normal mid-ocean ridges. 相似文献
17.
Walter Riesen Heinz Gamsjäger Paul W. Schindler 《Geochimica et cosmochimica acta》1977,41(9):1193-1200
The carbonato and hydrogencarbonato complexes of Mg2+ were investigated at 25 and 50° in solutions of the constant ClO4? molality (3 M) consisting preponderantly of NaClO4. The experimental data could be explained assuming the following equilibria: Mg2+ + CO2B + H2O ag MgHCO+3 + H+, , Mg2+ + 2 CO2g + 2 H2Oag Mg(HCO3)02 ± 2 H+, , ?15.37 ± 0.39 (50°), Mg2+ + CO2g + H2Oag MgCO03 + 2 H+, ,?15.23 ± 0.02 (50°), with the assumption γMgCO30 = γMg(HCO3)02, ΔG0(I = 0) for the reaction MgCO03 + CO2g + H2O = Mg(HCO3)02 was estimated to be ?3.91 ± 0.86 and 0.6 ± 2.4 kJ/mol at 25 and 50°C, respectively. The abundance of carbonate linked Mg(II) species in fresh water systems is discussed. 相似文献
18.
19.
Masao Nomura Tadao Kanzaki Takejiro Ozawa Makoto Okamoto Hidetake Kakihana 《Geochimica et cosmochimica acta》1982,46(11):2403-2406
Boron samples from 40 fumarolic condensates from volcanoes in the Ryukyu arc (Satsuma Iwo-jima and Shiratori Iwo-yama) and the North-east Japan arc (Usu-shinzan, Showa-shinzan, Esan and Issaikyo-yama) all have ratios close to 4.07. Higher values, from 4.09 to 4.13, were only observed in condensates from volcanoes in the southernmost end of the North-east Japan arc (Nasu-dake), the northern part of the Izu-Bonin arc (Hakone), and the North Mariana arc (Ogasawara Iwo-jima). These higher values suggest geological interaction of the magmas with sea-water enriched in 11B. 相似文献