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1.
The Hidra Massif (Rogaland complex, S.W. Norway) is a massif-type anorthositic-charnockitic body. It consists of undeformed anorthosites and leuconorites, grading into fine-grained jotunites at the contact with the granulite facies gneisses of the metamorphic envelope. A stockwork of charnockitic dykes cross-cuts the massif. The Pb isotopic compositions of the anorthosites and leuconorites are comparable or slightly less radiogenic than those of the jotunites ( from 18.079 to 19.307,( from 15.568 to 15.657 and from 37.617 to 38.493). These values are compatible with an upper mantle origin for the parental magma of jotunitic composition and for the plagioclasic cumulates, but show the incorporation of lower crustal material (U-depleted and thus less radiogenic). The charnockitic dykes have significantly less radiogenic Pb isotopic compositions ( from 17.472 to 19.171, from 15.489 to 15.620and from 36.991 to 40.922) which can be explained by a larger proportion of lower crustal contamination material. The contaminant could be the granulite facies gneisses of the metamorphic envelope. This interpretation is compatible with the K-Rb relationships of these rocks and with the O and Sr isotopic geochemistry.The proportion of contaminating lead in the charnockitic dykes can be estimated at 55 ± 15% considering the border facies jotunite as the uncontaminated parental magma and the least radiogenic gneiss of the metamorphic envelope as the contaminant. 相似文献
2.
Asish R. Basu 《Geochimica et cosmochimica acta》1978,42(6):659-668
New analyses of K, Rb, Sr and Ba contents and the ratios of eight amphiboles, one phlogopite, two diopsides and one host alkalic basalt for an amphibole are reported: The samples are mostly inclusions in alkalic basalts and occur in association with peridotite inclusions. Two of the samples are from alpine-type peridotite bodies — one from the Etang de Lhers massif in the French Pyrenees and the other from the Finero massif in the Ivrea zone in northern Italy. The kaersutites come from the following localities: Hoover Dam, Arizona; Deadman Lake, California; Massif Central, France; Queensland; Spring Mountain, New South Wales.The data indicate that kaersutitic amphiboles are genetically unrelated to their host basalts. The isotopic and trace element data of these amphiboles further strengthens the suggestion of BASU and MURTHY (1977) that kaersutites play a significant role in ocean ridge basalt genesis. In addition, pargasitic amphibole with higher ratios, if present, may be important in the source regions of alkalic basalts.The bulk amphibole lherzolite from Lherz has the ratio and ratio appropriate for source material of ridge tholeiites. If the diopside and the amphibole in this rock had isotopically equilibrated under upper mantle conditions, the data show the time of last equilibration to be approximately 735 m.y., in contrast to the young emplacement age of the ultramafic massif.The coexisting phlogopite and diopside in the spinel lherzolite inclusion from Kilbourne Hole, New Mexico, show, surprisingly, isotopic equilibration under upper mantle conditions despite their drastically different ratios. The data show that the phlogopite must have formed very recently in the upper mantle. This phlogopite also has a high ratio (1133), contrary to the commonly held view that mantle phlogopites have low ratios. The coexisting diopside shows high K content (778 ppm) and a lower ratio than the phlogopite. This phlogopite lherzolite has trace elemental and isotopic characteristics that may be adequate for the origin of alkalic basalts upon partial melting. 相似文献
3.
D.J Whitford 《Geochimica et cosmochimica acta》1975,39(9):1287-1302
Pleistocene and Recent lavas from the Sunda arc range from those showing affinities with the island arc tholeiitic series, through a spectrum of calc-alkaline to high-K alkaline rocks. The tholeiitic rocks have relatively low ratios averaging 0–7043; the calc-alkaline rocks show a wide range (from 0.7038 to 0.7059, averaging 0.7048); the high-K alkaline rocks average 0.7045. A rhyolitic ignimbrite from Sumatra has an ratio of 0.7139.The relationship between and major and trace element geochemistry is variable and complex. Lavas from the same volcano sometimes show significant differences in despite close geochemical relationships. Rocks of the calc-alkaline suite show a regular decrease in from West Java to Bali and there is some evidence for increasing with increasing depth to the Benioff zone. Calc-alkaline and tholeiitic rocks from the Sunda arc have significantly higher ratios than those from other island arcs, except from those arcs where continental crustal involvement has been inferred (e.g. New Zealand).A model of 87Sr enrichment due to isotopic equilibration of oceanic crust with sea water and disequilibrium melting in the slab and/or mantle is favoured to explain the Sr isotopic composition of the tholeiitic and normal calc-alkaline lavas. Calc-alkaline lavas with high ratios are best explained by either sialic contamination, or the presence of alkali basalt as a component of the downgoing slab. The Sr isotopic data for the high-K alkaline lavas suggest a mantle origin. The high ratio in the Lake Toba rhyolite implies a crustal origin. 相似文献
4.
We have analyzed samples from the Adirondack Marcy massif for Rb-Sr and Sm-Nd isotopes in an attempt to determine directly the primary crystallization age of a Proterozoic massif-type anorthosite rock suite. The oldest age obtained (1288 ± 36Ma) is from a 4 point Sm-Nd isochron defined by igneous-textured whole-rock and mineral separate data from a local layered sequence gradational from oxiderich pyroxenite to leuconorite. This age is older than Silver's (1969) 1113 Ma zircon age of associated charnockites, but is within the window of permissible anorthosite ages based on previous geochronology and field relationships. As such, 1288 Ma may represent the time of crystallization of the massif. For the most part, however, both Sm-Nd and Rb-Sr isotopic systems did not survive granulite facies metamorphism. Internal isochrons based on whole rocks and minerals yield ages between 995 and 919 Ma. These isotopic data suggest that the granulite fades metamorphism experienced by the massif was a prograde event that occurred a minimum of 100 Ma and as much as 350 Ma after crystallization of the massif. The relatively large range in Rb abundance, and in calculated initial (0.7039–0.7050) and ratios among anorthosite suite rocks, particularly those at or near the contacts of the Marcy massif is explicable by variable contamination with “crustal” materials and/or fluids, derived from surrounding acidic metaplutonic rocks, paragneisses, and marbles. Despite uncertainies caused by crustal contamination and metamorphic resetting of primary ages, Marcy samples have epsilon Nd values between +0.44 and +5.08, implying a source for the massif with long-term depletion in light rare earth elements. A probable source material would be depleted mantle. 相似文献
5.
measurements of 108 sedimentary carbonate rocks have been used to trace variations in the strontium isotopic composition of seawater during the Phanerozoic. The lowest 87Sr/86Sr observed for any suite of carbonates is taken as the best approximation to the value in well-mixed contemporary seawater. Our data support the existence of low in the Cretaceous and Late Jurassic but they do not support further structure beyond a general trend through the Phanerozoic, which may correlate with the continental denudation rate. 相似文献
6.
Garrett W. Brass 《Geochimica et cosmochimica acta》1976,40(7):721-730
The ratio in sea water has varied over geologic time due to the addition of strontium to the sea from rocks with a variety of ratios. The measurements by Petermanet al. (Geochim. Cosmochim. Acta34, 105–120, 1970) of the value of the marine ratio have been confirmed by several other workers and by some new measurements on JOIDES samples. They form the basis of a model calculation of the relative proportions of ‘basaltic’ () and ‘granitic’ () strontium being supplied to the sea. For the last 200 million years, the proportions of these two sources appear to reflect the history of global tectonics; ‘basaltic’ during rifting and increasingly ‘granitic’ during the present episodes of uplift and continental collision 相似文献
7.
Basalts from the Columbia River flood basalt province of the northwestern U.S.A. show a large diversity in chemical and Nd and Sr isotopic compositions. ranges from 0.51303 to 0.51208 and is strongly correlated with variations in . This correlation suggests mixing between two end member compositions, one characterized by and , and the other with and . The more radiogenic component could be mantle enriched in incompatible elements during the Precambrian, or Precambrian materials of the continental crust. A quartz-rich xenolith found in the Columbia lavas has Rb-Sr and Sm Nd model ages of ≈ 1.4Æ, implying the existence of old, isotopically evolved crustal basement which could serve as contaminant. Nevertheless, crustal contamination alone cannot explain the chemical variation of the samples studied, and other fractionation processes must have occurred simultaneously. A model involving combined assimilation and crystal fractionation reproduces the chemical and isotopic characteristics of the volumetrically dominant Grande Ronde unit for an assumed crystallizing component of plagioclase, low calcium pyroxene and minor olivine. The data are not consistent with the suggestion that a ‘primordial’ mantle is the source for this continental flood basalt province. Rather they suggest that the main volume of these lavas was originally derived from a mantle similar in isotopic composition to island arc and ocean island basalts of the north Pacific. The primary magma was modified chemically and isotopically by crystal fractionation and assimilation of sialic crustal materials during its transport through, or storage in the continental crust. 相似文献
8.
9.
We have determined ratios of the Tethys Sea by analyzing limestones of Permian to Late Cretaceous age from the overthrusted sequence of central and northern Euboea. The results are in satisfactory agreement with ratios obtained by other investigators on unreplaced fossil shells of similar age collected elsewhere. The ratios of Mesozoic marbles from the autochtone metamorphic rocks of southern Euboea are more variable, perhaps because of increases in this ratio during regional metamorphism. Nevertheless, the lowest values of suites of isochronous samples may approach the correct ratio. Four samples of the Argyron Marble taken at different localities in southern Euboea and on the Attic peninsula have a concordant ratio of 0.70688±0.00006 (1 gs), relative to 0.7080 for the Eimer and Amend isotope standard. This value indicates a Jurassic age for this formation even though the fragmentary fossil evidence favors a Late Cretaceous (Turonian) age. 相似文献
10.
Ján Veizer William Compston Norbert Clauer Manfred Schidlowski 《Geochimica et cosmochimica acta》1983,47(2):295-302
Strontium isotopic measurements were made on Late Proterozoic carbonates from West African Craton. Comparison of samples with acceptable trace element patterns with coeval data from southern Africa and with the published Australian results suggests that the ratio of the Late Proterozoic sea water evolved in the following manner about 0.7075 at 1000 ± 50 Ma, 0.7056 to 0.7074 at 900 ± 50 Ma, 0.7068 to 0.7091(0.7106) at 800 ± 50 Ma, 0.7074 to 0.7077 at 700 ± 50 Ma, and 0.7076 to 0.7089(0.7096) at 600 ± 50 Ma ago. The variations are comparable in magnitude and frequency to those described previously for the Phanerozoic. Strontium isotopic values in the radiogenic part of this range suggest that the continental river flux of Sr into Late Proterozoic oceans was of comparable isotopic composition to its present day counterpart (～0.711). Consequently, the non-radiogenic value at ～900 ± 50 Ma ago signifies a large flux of “mantle” strontium into the ocean at this time. Because the present time resolution is only about 75 ± 25Ma, additional sampling as well as better stratigraphie resolution and more definite selection criteria are required for construction of a more detailed Late Proterozoic sea water curve. 相似文献
11.
Surendra P. Verma 《Chemical Geology》1984,46(1):37-53
Twelve samples of mid-Tertiary felsic volcanic rocks from Zacatecas and San Luis Potosí (both belonging to the Sierra Madre Occidental) and one sample of Lower Tertiary porphyritic andesite from Zacatecas are analyzed for , K, Rb, and Sr. Eight selected samples are also analyzed for . A linear regression of the present-day of the felsic volcanic rocks in Zacatecas gives an approximate date of 30 ± 8 Ma. The initial ratios are high and widely distributed (from 0.705 to 0.712 or higher) whereas the initial ratios are somewhat low and show a narrow range (0.5125–0.5127). The available isotopic and trace-element data are best explained in terms of a binary mixing model in which the magmas derived from a slightly depleted-mantle fractionate and mix with varying proportions of the overlying middle/upper continental crust and undergo further shallow-level fractional crystallization before eruption. This model is also compatible with the trace-element and Sr isotopic data published from other areas of the Sierra Madre Occidental for which a purely mantle origin has been proposed. 相似文献
12.
ratios measured in Quaternary lavas from Java and the Banda arc of Indonesia range from 0.51242 to 0.51280 and exhibit an inverse correlation with . Isotopically, the Indonesian samples resemble Andean rather than island arc lavas. The samples from Java plot either within, or adjacent to the mantle array, towards higher ratios. Samples from the Banda arc and the anomalous calc-alkaline volcano Papandajan are characterized by relatively low and high ratios. These characteristics are consistent with the interpretation that subducted terrigenous material was involved in the genesis of these lavas. Furthermore the Banda arc samples appear to lie on a mixing line between isotopic compositions characteristic of the mantle and upper continental crust. A high-K trachyte from the alkaline volcano Muriah, Java, has isotopic characteristics of the mantle (, ), which implies that the extreme enrichment in large-ion-lithophile elements in its source must have occurred only shortly before its formation. The inferred ratio of the unmodified mantle beneath Java and the Banda arc is lower than that observed in mid-ocean ridge basalt, which may have important implications for a better understanding of the geochemical structure of the mantle. 相似文献
13.
The South Mountain batholith of southwestern Nova Scotia is a large, peraluminous, granodiorite-granite complex which intrudes mainly greenschist facies metasediments of the Cambro-Ordovician Meguma Group. Using Rb-Sr isochrons constructed from whole rocks and mineral separates, the present study shows a variation in age and initial ratios of the intrusive phases of the batholith as follows: biotite granodiorite (371.8 ± 2.2 Ma, ( ranges from 0.7076 ± 0.0003 to 0.7090 ± 0.0003, with the average = 0.7081); adamellite (364.3 ± 1.3 Ma, ); porphyry (361.2 ± 1.4 Ma, ); using λ87Rb = 1.42 × 10?11yr?1.A suite of Meguma country rock samples showed a variation of at the time of intrusion of the batholith. A number of xenoliths of this material occurring in the marginal granodiorite had partially equilibrated isotopically with the granodiorite at a higher ratio than elsewhere in the granodiorites. This evidence demonstrates that isotopic (and probably some accompanying bulk chemical) contamination by the Meguma rocks has been an important factor in determining the ultimate chemical composition and mineralogy of the South Mountain batholith.The of the early granodiorites indicates that the parent magma of the South Mountain batholith was derived from a source unlike the Meguma Group. The precise nature of the source region cannot be determined by Rb-Sr work unless the degree of contamination with Megumalike material is known. 相似文献
14.
Norbert Clauer Michel Hoffert Anne-Marie Karpoff 《Geochimica et cosmochimica acta》1982,46(12):2659-2664
Morphological, mineralogical, chemical and RbSr isotopic studies have been made on Fesmectites (nontronites) from southern Pacific red clays cored near the Marquisas Islands. These minerals have at the top of the core, an ratio of 0.70917 ± 0.00007, which indicates an authigenic origin in isotopic equilibrium with seawater. Weak leaching experiments with 1N HCl show that the nontronites also contain a volcanic component with a lower ratio which, combined with the morphology of the particles, suggests a transportation by bottom currents of clay formed elsewhere.During burial, the nontronites experience diagenetic modifications resulting in an increase in Fe, K and Rb contents, a concomitant decrease of Mg, Ca, Ti, Na and Sr, and a preferential migration of radiogenic 87Sr from the clays into the surrounding pore waters.The ratio of the Sr adsorbed on the outermost surfaces of the nontronites does not change with depth in the core, and is, therefore, independent of diagenetic influence, which is rather characterized by the ratios of the interstitial waters. The isotopic composition of both the adsorbed Sr and that of the pore fluids may yield useful information on the crystallization environment and subsequent history of deep sea red clays. 相似文献
15.
William K. Hart 《Geochimica et cosmochimica acta》1985,49(1):131-144
Combined elemental and Sr, Nd, Pb and O isotopic data for late Cenozoic olivine tholeiite lavas from the northwestern Great Basin indicate derivation from at least two chemically and isotopically distinct mantle source regions with no significant modification by interaction with continental crust. The lack of crustal involvement is a direct reflection of the extensional tectonic environment which favors rapid ascent of magmas, minimal residence time in crustal magma chambers and scattered fissure eruptions.The observed chemical and isotopic variations in the tholeiite suite are attributed to mixing between depleted oceanic type mantle ( and ) and old, chemically heterogeneous, isotopically enriched subcontinental mantle ( and ). Model incompatible element concentrations suggest strong similarities between the depleted mantle and the mantles beneath normal oceanic ridge segments and back-arc basins and between the enriched mantle and the mantle beneath enriched oceanic ridge segments such as the Azores. Superimposed upon the characteristics derived from the two component mixing model may be the effects of a third mantle source which is identifiable only by its apparent radiogenic ratios. If present, this third source may reflect a component derived from the downgoing slab of an ancient subduction zone. 相似文献
16.
The performance characteristics of PANURGE, a modified CAMECA IMS3F ion microprobe, have been studied at a mass resolving power of 5000 for the purpose of determining isotopic ratios at a precision level approaching that of counting statistics using beam switching. The techniques used for this type of measurement are described. Using this approach, the isotopic composition of Mg and Si and the atomic ratio of in minerals from the Allende inclusion WA and the Allende FUN inclusion Cl have been measured with the ion microprobe at high mass resolving power. Enrichments in 26Mg of up to 260%. have been found. Mg and measurements on cogenetic spinel inclusions and host plagioclase crystals yield Mg-Al isochrons in excellent agreement with precise mineral isochrons determined by thermal emission mass spectrometry. The measurements confirm the presence of substantial excess 26Mg in WA () and its near absence in Cl (). In WA plagioclase, data for which define a linear array with and with initial composition 30%. greater than in high Mg phases. This suggests a metamorphic reequilibration of Mg in Allende plagioclase at least 0.6 my after WA formation. There were no variations in detected in WA plagioclase associated with concentration of 26Mg1 into isolated clusters. We have confirmed by ion probe measurements that the Mg composition in Allende Cl is highly fractionated and is uniform among pyroxene, melilite, plagioclase, spinel crystals and spinel included in melilite and plagioclase crystals. Likewise, the Si composition is mass fractionated and is the same in pyroxene, melilite and plagioclase. 相似文献
17.
G.G. Pe 《Chemical Geology》1975,15(1):53-60
ratios have been determined in fifteen volcanic rocks from the northwestern part of the Hellenic arc. They range from 0.7041 to 0.7134. There is no apparent correlation of strontium isotope values with any major chemical component or with Rb/Sr ratios. The ratios appear to increase in a general way with increasing depth to the Benioff zone. The strontium isotope ratios are higher than from most island arcs; this is believed to be due to contamination. 相似文献
18.
Large amounts of diorite—tonalite magma were intruded into the island-arc successions of the southern Arabian shield between ca. 900 and 700 Ma ago. Major oxide, trace element, rare earth (REE) and isotopic data are presented for two plutons exemplifying older and younger members of this plutonic phase. The Thurrat pluton, which was emplaced into virtually unmetamorphosed volcanics of sequence B, has yielded a 10-point Rb-Sr isochron indicating emplacement 744 ± 22 Ma ago and an initial ratio of 0.70281. It consists of gabbros, diorites, quartz diorites, tonalites and low-Al2O3 trondhjemites which are compositionally primitive, with depleted lithophile element contents and flat REE patterns with a negative Eu anomaly in the trondhjemites. The magma was most probably mantle-derived, and analogies with other trondhjemitic plutons suggest that it was probably emplaced in an island-arc setting. The Bidah pluton, which was emplaced into a compositionally very immature succession of metamorphosed volcanics and volcaniclastics of sequence C, has yielded a nine-point near-isochron (MSWD = 2.86) indicating an age of 901 ± 37Ma and an initial ratio of 0.70246. This date is accepted as the age of emplacement. The rocks are compositionally primitive gabbros, diorites, quartz diorites, tonalites and granodiorites with depleted lithophile element contents, and flat REE patterns with a negative Eu anomaly in the more siliceous components. The country-rock volcanics were formed in an immature island-arc environment, and the composition of the Bidah pluton is suggestive of a mainly mantle-derived magma emplaced into that arc. The data for these two plutons therefore provide further evidence that most of the rock material added to the Arabian shield between 900 and 700 Ma ago was derived from the mantle. 相似文献
19.
D.K Paul 《Geochimica et cosmochimica acta》1979,43(3):389-394
Strontium isotopic studies on twenty three whole rock kimberlites from two petrographic provinces in India show variation of initial ratios from 0.7027 to 0.7102. The variation is unrelated to the degree of alteration. Between the micaceous and basaltic varieties there is some overlap in the Sr isotopic ratios. Leaching experiments on whole rock samples showed more highly radiogenic Sr in leaches compared to the bulk samples.In two diatremes, the initial ratios show a positive correlation with which is believed to reflect a source event earlier than the formation of the kimberlites. The observed Sr isotopic data can be caused by (i) melting of a heterogeneous source or (ii) disequilibrium partial melting in the source region. In the former case, variable isotopic composition would be a necessary consequence of melting in small subsystems. 相似文献
20.
Seventy-two basalts from 58 dredge stations located along the Mid-Atlantic Ridge from 29°N to 59°N have been analyzed for and for K, Rb, Cc, Sr and Ba. The Sr-isotope profile along the ridge has three distinct maxima, one coinciding with the Azores platform (0.70345), one at 45°N (0.70340) and the third at 35°N, in the vicinity of the Oceanographer Fracture Zone. Basalts from ridge segments between 29°N and 33°N, and 49°N and 59°N have ratios typical of ‘normal’ mid-ocean ridge basalts (0.70230–0.70280). Profiles of K, Rb, Cs, Sr, Bz, Rb/Sr and Ba/Sr are similar to the profile, but Rb/K, Cs/K and Ba/K show broad maxima between 35°N and 45°N.These variations result from chemical and isotopic heterogeneity in the mantle, and are interpreted as caused by a mantle plume beneath the Azores which mixes with the LIL-element-depleted asthenosphere. Additional plumes may exist beneath 45°N and 35°N.Compared to the LIL-element-depleted asthenosphere, the Azores mantle plume is 10 to 30 times enriched in LIL elements with very small (? 0.1) bulk crystal/melt partition coefficients (Rb, Cs, Ba, La). Mildly incompatible elements (0.1 < D < 1) (Sr, Sm, Yb) are only 0.8–3 times enriched. These, observations suggest that LIL element differences between these two mantle reservoirs resulted from processes involving solid-liquid equilibria and not vapor-solid or vapor-liquid equilibria. Isotope systematics indicate that neither mantle reservoir remained a closed system since the formation of the Earth, but it is not possible to determine the time at which heterogeneity first developed. 相似文献