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1.
Archean metasedimentary rocks occur as components of the Isua supracrustals, Akilia association and Malene supracrustals of southern West Greenland. Primary structures in these rocks have been destroyed by metamorphism and deformation. Their chemistry and mineralogy is consistent with a sedimentary origin, but other possible parents (e.g. acid volcanics, altered pyroclastic rocks) cannot be excluded for some of them. There is little difference in the composition of metasedimentary rocks from the early Archean Isua supracrustals and probable correlative Akilia association. Both have a wide range in rare earth element (REE) patterns with ranging from 0.61?5.8. The REE pattern of one Akilia sample, with low , compares favourably with that of associated tholeiites and it is likely that such samples were derived almost exclusively from basaltic sources. Other samples with very steep REE patterns are similar to felsic volcanic boulders found in a conglomeratic unit in the Isua supracrustals. Samples with intermediate REE patterns are best explained by mixing of basaltic and felsic end members. Metasedimentary rocks from the Malene supracrustals can be divided into low silica (≤55% SiO2) and high silica (>77% SiO2) varieties. These rocks also show much variation in (0.46?14.0) and their origin is explained by derivation from a mixture of mafic volcanics and felsic igneous rocks. The wide range in trace element characteristics of these metasedimentary rocks argues for inefficient mixing of the various source lithologies during sedimentation. Accordingly, these data do not rigorously test models of early Archean crustal composition and evolution. The systematic variability in trace element geochemistry provides evidence for the bimodal nature of the early Archean crust. 相似文献
2.
Analyses of Sr and REE in apatites from a variety of mantle-derived parageneses are used in conjunction with trace element data from the literature to investigate relationships between alkali basalts and apatite-rich materials in upper-mantle source regions. Despite difficulties in interpretation, positive P-anomalies in the hygromagmatophile element abundance patterns of some continental primary alkali basalts suggest either P-enrichment of their source or assimilation of P-rich material, or both. Amphibole- and apatite-rich xenoliths occur in several alkali-basalt provinces, and by virtue of the P and LREE enrichment represent a probable source of the P anomalies and part of the other trace element enrichments of these magmas. Incorporation of such apatite-rich materials by later primary magmas would be enhanced by the high P2O5 concentrations required to achieve apatite saturation in basaltic liquids.In the early stages of mantle diapirism an undersaturated magma, produced by slight partial melting of garnet peridotite, might fractionate as it rises to the range of amphibole stability. Hygromagmatophile element patterns of clinopyroxenite xenoliths indicate that clinopyroxene fractionation could produce P-enriched liquids which might subsequently crystallize amphibole- and apatite-rich materials now represented by xenoliths. During generation of later primary magma, apatite-rich materials might preferentially contaminate the liquids, to yield positive P-anomalies. This model requires that magmas undergo prolonged fractionation at considerable depth (~ 100 km), a process which is apparently most probable in subcontinental environments.An apatite- and zircon-bearing mica-clinopyroxenite xenolith from Matsoku provides a link between the S. African MARID suite and amphibole and apatite-rich xenoliths from various alkali basalt provinces. Unusual REE patterns () of apatites in this xenolith suggest a link between the MARID suite xenoliths and postulated pre-Karroo mantle metasomatism. 相似文献
3.
The Archean Yellowknife Supergroup (Slave Structural Province. Canada) is composed of a thick sequence of supracrustal rocks, which differs from most Archean greenstone belts in that it contains a large proportion ( ~ 80%) of sedimentary rocks. Felsic volcanics of the Banting Formation are characterized by HREE depletion without Eu-anomalies, indicating an origin by small degrees of partial melting of a mafic source, with minor garnet in the residua. Granitic rocks include synkinematic granites [HREE-depleted; low ()I], post-kinematic granites [negative Eu-anomalies, high ()I] and granitic gneisses with REE patterns similar to the post-kinematic granites. Sedimentary rocks (turbidites) of the Burwash and Walsh Formations have similar chemical compositions and were derived from 20% mafic-intermediate volcanics, 55% felsic volcanics and 25% granitic rocks. Jackson Lake Formation lithic wackes can be divided into two groups with Group A derived from 50% mafic-intermediate volcanics and 50% felsic volcanics and Group B, characterized by HREE depletion, derived almost exclusively from felsic volcanics.REE patterns of Yellowknife sedimentary rocks are similar to other Archean sedimentary REE patterns, although they have higher . These patterns differ significantly from typical post-Archean sedimentary REE patterns, supporting the idea that Archean exposed crust had a different composition than the present day exposed crust. 相似文献
4.
Stephen E Haggerty 《Geochimica et cosmochimica acta》1973,37(4):857-867
Electron microprobe analyses of the spinel mineral group, ilmenite and rutile have been carried out on part of the Luna 20 soil sample. The spinel group shows an almost continuous trend from MgAl8O4 to FeCr2O4 and a discontinuous trend from FeCr2O4 to Fe2TiO4. Well defined non-linear relationships exist within the spinel group for Fe-Mg substitution, for divalent () versus trivalent (), and for divalent versus . For Cr-Al substitution the relationship is linear and is negative for Mg-rich spinel and positive for Fe-Ti rich spinel. In general a combination of aluminous-rich chromite and ulvöspinel in the Luna 16 samples, combined with the chromian-pleonaste in Apollo 14 define comparable major compositional trends to those observed in Luna 20. Ilmenite is present in trace amounts. It is exsolved from pleonaste and pyroxene, is present in subsolidusreduced ulvöspinel and has undergone reequilibration to produce oriented intergrowths of chromite + rutile. Primary ilmenite is among the most magnesian-rieh (6 wt.% MgO) yet found in the lunar samples. The high MgO, inferred high Cr2O3 concentrations and the iron content of rutile (2.5 wt.% FeO) suggest crystallization at high temperatures and pressures for some components of the Luna 20 soil. 相似文献
5.
A new technique for the determination of intrinsic oxygen fugacities () of single and polyphase geological samples with solid ZrO2, oxygen-specific electrolytes is described. Essentially the procedure involves isolating the emf signal from the sample from that unavoidably imposed by the residual atmosphere inside the sample-bearing sensor. By varying the of the residual atmosphere, it is possible to determine a ‘plateau’ value of constant recorded from the sensor which represents a reversed intrinsic measurement for the sample alone, and where the extent of the plateau reflects the innate buffering capability of the sample. A measure of the precision and accuracy of the data obtained is the fact that identical values are obtained whether on a heating or cooling cycle of the sample + compatible atmosphere system.These techniques have been applied to measurements of the intrinsic of spinels from peridotites and megacryst assemblages from Australia, West Germany and the U.S.A. Oxidation states range from ～- 0.25 log10 units more oxidized to 1 log10 unit more reduced than the iron-wüstite (IW) buffer. The overall reduced nature of the spinels and the range of 's obtained are striking features of the data. One implication of the results is that the majority of mantle-derived magmas are initially highly reduced, and the relatively oxidized values observed at surface (～- 4–5 log10 orders more oxidized than IW) reflect late-stage alteration, perhaps by H2 loss (Sato, 1978). 相似文献
6.
Isotopic, major and trace element studies of loess deposits from America, China, Europe and New Zealand show general uniformity of composition. Silica, Zr and Hf are enriched relative to estimates of bulk composition of the upper continental crust. The REE data are indistinguishable from those of average shales, confirming the concept that these REE patterns () represent the upper crustal average. Sm-Nd model ages are variable but <1700 m.y. They reflect derivation from younger elevated erogenic areas subject to Pleistocene glaciation. Although Sm-Nd model ages vary by a factor of two, the REE patterns remain constant. This indicates that processes responsible for formation of the upper crust have produced no secular change in composition since the mid-Proterozoic. 相似文献
7.
Robert L. Cullers Subramanian Ramakrishnan Pieter Berendsen Tom Griffin 《Geochimica et cosmochimica acta》1985,49(6):1383-1402
Lamproite sills and their associated sedimentary and contact metamorphic rocks from Woodson County, Kansas have been analyzed for major elements, selected trace elements, and strontium isotopic composition. These lamproites, like lamproites elsewhere, are alkalic (molecular ), are ultrapotassic , are enriched in incompatible elements (LREE or light rare-earth elements, Ba, Th, Hf, Ta, Sr, Rb), and have moderate to high initial strontium isotopic compositions (0.7042 and 0.7102). The silica-saturated magma (olivine-hypersthene normative) of the Silver City lamproite could have formed by about 2 percent melting of a phlogopite-garnet lherzolite under high ratios in which the Iherzolite was enriched before melting in the incompatible elements by metasomatism. The Rose Dome lamproite probably formed in a similar fashion although the extreme alteration due to addition of carbonate presumably from the underlying limestone makes its origin less certain. Significant fractional crystallization of phases that occur as phenocrysts (diopside, olivine, K-richterite, and phlogopite) in the Silver City magma and that concentrate Co, Cr, and Sc are precluded as the magma moved from the source toward the surface due to the high abundances of Co, Cr, and Sc in the magma similar to that predicted by direct melting of the metasomatized Iherzolite.Ba and, to a lesser extent, K and Rb and have been transported from the intrusions at shallow depth into the surrounding contact metamorphic zone. The Silver City lamproite has vertical fractionation of some elements due either to volatile transport or to variations in the abundance of phenocrysts relative to groundmass most probably due to flow differentiation although multiple injection or fractional crystallization cannot be conclusively rejected. 相似文献
8.
Archean clastic sedimentary rocks are well exposed in the Pilbara Block of Western Australia. Shales from turbidites in the Gorge Creek Group (ca. 3.4 Ae) and shales from the Whim Creek Group (ca. 2.7 Ae) have been examined. The Gorge Creek Group samples, characterized by muscovite-quartzchlorite mineralogy, are enriched in incompatible elements (K, Th, U, LREE) by factors of about two, when compared to younger Archean shales from the Yilgarn Block. Alkali and alkaline earth elements are depleted in a systematic fashion, according to size, when compared with an estimate of Archean upper crust abundances. This depletion is less notable in the Whim Creek Group. Such a pattern indicates the source of these rocks underwent a rather severe episode of weathering. The Gorge Creek Group also has fairly high B content (85 ± 29 ppm) which may indicate normal marine conditions during deposition.Rare earth element (REE) patterns for the Pilbara samples are characterized by light REE enrichment () and no or very slight Eu depletion (). A source comprised of about 80% felsic igneous rocks without large negative Eu-anomalies (felsic volcanics, tonalites, trondhjemites) and 20% mafic-ultramafic volcanics is indicated by the trace element data. Very high abundances of Cr and Ni cannot be explained by any reasonable provenance model and a secondary enrichment process is called for. 相似文献
9.
Diopside-melt and forsterite-melt rare earth (REE) and Ni partition coefficients have been determined as a function of bulk compositions of the melt. Available Raman spectroscopic data have been used to determine the structures of the melts coexisting with diopside and forsterite. The compositional dependence of the partition coefficients is then related to the structural changes of the melt.The melts in all experiments have a ratio of nonbridging oxygens to tetrahedral cations () between 1 and 0. The quenched melts consist of structural units that have, on the average, 2 (chain), 1 (sheet) and 0 (three-dimensional network) nonbridging oxygens per tetrahedral cation. The proportions of these structural units in the melts, as well as the overall , change as a function of the bulk composition of the melt.It has been found that Ce, Sm, Tm and Ni crystal-liquid partition coefficients () decrease linearly with increasing . The values of the individual REE crystal-liquid trace element partition coefficients have different functional relations to , so that the degree of light REE enrichment of the melts would depend on their .The solution mechanisms of minor oxides such as CO2, H2O, TiO2, P2O5 and Fe2O3 in silicate melts are known. These data have been recast as changes of of the melts with regard to the type of oxide and its concentration in the melt. From such data the dependence of crystal-liquid partition coefficients on concentration and type of minor oxide in melt solution has been calculated. 相似文献
10.
Rare-earth elements in the rock-forming minerals of melilitic rocks in alkaline-ultrabasic complexes
I.T Rass 《Geochimica et cosmochimica acta》1982,46(9):1477-1488
The rare-earth distributions in melilites and other coexisting minerals of melilite-containing rocks from five alkaline-ultrabasic complexes were analysed by partition paper chromatography and neutron activation. Relatively late differentiates are characterized by elevated relative alkalinity and reduced absolute basicity. The characteristics of melilitic sövites markedly differ from established trends. The minerals studied are richer in LREE than chondrites and REE concentrations in these minerals increase in the late differentiates of the melilitic series. A proportionality between logarithms of La and Sm concentrations in pyroxene and melilite imply that melilite-containing rocks (except melilitic sövite) belong to a similar comagmatic series. The degree of LREE-enrichment in rock-forming minerals directly depends on the alkalinity of the rock. The distribution of REE between coexisting melilite and pyroxene may be used as an additional genetic criterion: p.e. magmatic ratio is 0.91, while in metasomatic rocks this ratio is between 1.80 and 1.97. 相似文献
11.
Direct measurements on water samples from several CO2-charged warm springs are significantly higher than values calculated from field pH and alkalinity (and other constituents). In addition, calcite saturation indices calculated from field pH and solution composition indicated supersaturation in samples which, on the basis of hydrogeologic concepts, should be near saturation or undersaturated. We attribute these discrepancies to uncertainties in field pH, resulting from CO2 outgassing during pH measurement. Because samples for direct measurement can be taken with minimal disturbance to the water chemistry, we have used the measured to back calculate an estimate of the field pH and the carbon isotopic composition of the water before outgassing. By reconstructing water chemistry in this way, we find generally consistent grouping of , pH, and degree of calcite saturation in samples taken from the same source at different times, an observation which we expect based on our understanding of the hydrogeology and geochemistry of the ground-water systems. This suggests that for very careful geochemical work, particularly on ground-waters much above ambient temperature, measurements may provide more information on the system and a better estimate of its state of saturation with respect to carbonate minerals than can field measurements of pH. 相似文献
12.
Charles D Byers David W Muenow Michael O Garcia 《Geochimica et cosmochimica acta》1983,47(9):1551-1558
Glasses from submarine lavas recovered by the ALVIN submersible from the Galapagos Spreading Center (GSC) near 86°W have been analyzed by electron microprobe for major elements and by high-temperature mass spectrometry for volatiles. The samples studied range in composition from basalt to andesite and are more evolved than typical MORBs. Previous studies indicate that they are related to normal MORB by extensive crystal fractionation in small, isolated magma chambers. The H2O, Cl and F contents of these lavas are substantially higher than any previously reported for MORBs. H2O, Cl and F abundances increase linearly with P2O5 content, which is used as an indicator of the extent of crystal fractionation. The ratios measured in the andesite glasses progressively decrease with increasing P2O5 content and are probably related to fractionation of Fe-Tioxides. Reduced carbon gas species, principally CH4 and CO, were discovered in these glasses. The presence of reduced carbon species in GSC glasses may be indicative of a more reduced oxidation state of the upper mantle than is commonly assumed. 相似文献
13.
Constant M.G. van den Berg 《Geochimica et cosmochimica acta》1984,48(12):2613-2617
The stability constants, , for borate complexes with the ions of Cu, Pb, Cd and Zn are determined in this work by DPASV in 0.7 M KNO3 at metal concentrations of 10?7 M. The acidity constants of the Cu2+ ion are determined by DPASV in the same conditions. The following values for log () have been obtained: CuB: 3.48, CuB2: 6.13, PbB: 2.20, PbB2: 4.41, ZnB: 0.9, ZnB2: 3.32, CdB: 1.42, and CdB2: 2.7, while the values for the acidity constants of Cu are and . At the low concentration of boron in 35%. S sea-water complexes with borate represent only about 0.2% Cu, 0.03% Pb, 0.02% Zn and 0.003% Cd. 相似文献
14.
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
15.
The solid metal/silicate melt partition coefficient for P, D(P), has been determined experimentally at 1190°C and 1300°C. The dependence of the partition coefficient on oxygen fugacity has been investigated, and is consistent with a valence state of 5 for P in the silicate melt. The experimental partition coefficients are given by: The experimentally determined partition coefficients may be used to interpret the low ratios of the Earth, Moon and eucrites relative to C1 chondrites. The low ratios in the eucrites may be explained by partitioning of P into 5% to 25% of a sulfur-bearing metallic liquid assuming equilibration and separation of the liquid metal from the silicates at low degrees of partial melting of the silicates. The low ratios in the eucrites compared to C1 chondrites require the separation of an additional 2% to 10% solid metal.The lowering of both and ratios in the Moon may be explained by partitioning of P and W into metal during formation of a small core by separation of liquid metal from silicate at low degrees of partial melting of the silicates. The ratios in the Earth and Moon are virtually indistinguishable, while ratios differ by a factor of two. The concentrations of FeO also appear to be different. These observations are difficult to reconcile with the hypothesis of a terrestrial origin of the Moon following formation of the Earth's core, but are consistent with independent formation of the Earth and Moon. 相似文献
16.
Nd and Sr isotopic compositions as well as trace element concentrations have been determined on a suite of alkali basalts from the Massif Central, in France. Samples show a typical enrichment in incompatible elements. In particular, the REE patterns exhibit a strong fractionation characterized by a ( ratio of about 20. The YbN content is about 10 times chondrite. The ratios exhibit a range from 0.512775 to 0.512989, values quite comparable to those from oceanic island basalts. The ratios vary between 0.70338 and 0.70458 and are anti-correlated with the Nd isotopic ratio.The isotopic and the trace element (in particular REE) data have been used in order to quantitatively model the genesis of the alkali basalts. Among the several types of models tested here, the most likely one appears to be the model of mantle metasomatism. A semi-quantitative approach shows that the source of alkali basalts from the Massif Central was metasomatized prior to melting. In such a model, the basalts could be produced by rather high degrees of partial melting (such as 10 or 15%) of the metasomatically enriched mantle. 相似文献
17.
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
18.
Water samples collected from a slope station and two deep stations in the western basin of the Black Sea were analyzed for stenols and stanols by glass capillary gas chromatography. These results were used in conjuction with hydrographic, particulate organic carbon, and chlorophyll a data to better understand sterol sources and their transport and transformation mechanisms in anoxic basins.The total free sterol concentrations found in the surface waters were 450–500 ng/l dropping rapidly to values well below 100 ng/l at depths below the interface. In the upper 200 m of the water column a strong association of sterols with particulate matter is suggested. Structural elucidation by a gas chromatograph-mass spectrometer-computer system revealed the presence of at least sixteen different stenols and stanols in the surface waters of the Black Sea. Cholesterol, 24-methylenecholesterol and 24-methylcholesta-5,22-dien-3β-ol were the major sterols in the surface waters. Cholesterol and 24-ethylcholesterol both exhibited a subsurface maximum at the interface. In the anoxic deep waters (200–2000 m) only cholesterol and 24-ethylcholesterol were found. Two stenols were found that have not been reported in seawater: a C26 stenol with a saturated C7H15 side chain (presumably 24-norcholesterol) and 24-ketocholesterol. At least six 5α-stanols could be identified in the surface samples, each of them comprising about 10–20% of the concentration of the corresponding Δ5-stenol. From these comparatively high surface values the stanol concentrations drop rapidly to values near zero at the interface. Except for very low concentrations of 5α-cholestanol (< 4ng/l) no other stanols could be detected in the anoxic zone.From this data it appears that no detectable stenol → stanol conversion is occurring at the interface or in the deep anoxic waters of the Black Sea. 相似文献
19.
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
20.
Solubility product determinations suggest that the hydrous phosphates of the rare earths, REPO4 · , are important in controlling the sea water REE concentrations. Two of these solids, rhabdophane, (P6222) and “hydrous xenotime”, (141/amd), have been synthesized at 100°C via the acid hydrolysis of the respective REE pyrophosphate. The solubility products at infinite dilution were determined to be , (La at 25°C); 26.0, (Pr at 100°C); 25.7, (Nd at 100°C): and 25.5, (Er at 100°C). On the basis of calculations involving the reaction of RE3+ with apatite to form the hydrous phosphate, the lanthanum concentration in sea water is predicted to be about 140 pmol/L. Laboratory experiments support the hypothesis that apatite is a substrate for reactions with dissolved REE. 相似文献