Strontium isotope geochemistry of Icelandic geothermal systems and implications for sea water chemistry     

Henry Elderfield  Mervyn J Greaves 《Geochimica et cosmochimica acta》1981,45(11):2201-2212

Chemical and Sr isotopic analyses have been made of waters from 16 geothermal sites in Iceland with particular reference to the systems at Reykjanes and Svartsengi for which compositions of geothermal sea water and fresh and hydrothermally-altered rocks have been compared. The alkalies display mixing relationships indicating a hydrothermal input of Rb and K to local meteoric and sea waters as do results for Sr and Ca involving high-temperature fluids. ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of the geothermal waters of meteoric origin parallel those of associated rocks but are higher. Ratios for geothermal sea waters are 0.7042 (Reykjanes) and 0.7040 (Svartsengi), lower than for normal sea water (0.7092) because of leaching of Sr from rocks followed by partial removal into alteration minerals, of which epidote and chlorite may be most important. Consequently, associated hydrothermally-altered rocks have been subject to significant Sr isotopic contamination by sea water Sr raising ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios from 0.7032 for fresh rock to 0.7038–0.7042 (Reykjanes) and to 0.7039–0.7041 (Svartsengi). Altered basalt is only ~50% equilibrated isotopically with geothermal sea water, at a water/rock ratio of ~2, but is internally equilibrated whereas palagonitized rocks (water/rock ratio of 3 to 4) are close to Sr isotopic equilibrium with associated sea water but show significant internal Sr disequilibrium. Hydrothermal input is unlikely to be important in the oceanic mass balance of Sr but is likely to be highly significant in controlling the strontium isotopic composition of sea water.  相似文献   

A study of strontium in core 119K,Discovery deep,Red Sea     

M.O.U. Ikpeama  P.D. Boger  G. Faure 《Chemical Geology》1974,13(1):11-22

The isotopic composition of strontium of pore water and of authigenic minerals leached from the sediment of core 119K with hot aqua regia is similar to that of the brine in the Discovery deep and differs from that of normal seawater. The average ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of strontium removed by acid leaching is 0.7077 ± 0.0007 (1σ) compared to a value of 0.70904 for the Red Sea. The detrital silicate fraction exhibits an approximate inverse correlation between ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios and strontium concentrations which provides tentative support for a model in which the detrital silicate fraction of deep-sea sediment is considered to be a mixture of terrigenous dust of sialic composition enriched in radiogenic ⁸⁷Sr and of volcanogenic material of basaltic composition and low ⁸⁷Sr abundance. The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of the shells of foraminifers and pteropods, expressed as δ ⁸⁷Sr‰ relative to 0.70904 for seawater, decrease from $\text{?0.23 ± 0.17‰}$ at 90 cm to $\text{?0.82 ± 0.17‰}$ at 273 cm and remain constant at this value to a depth of 450 cm. The lowering of the δ ⁸⁷Sr values is attributed both to the presence of aragonite overgrowths on pteropod shells and to possible isotope exchange with strontium in the connate fluid.  相似文献   

⁸⁷Sr/⁸⁶Sr composition of seawater during the Phanerozoic     

Ján Veizer  William Compston 《Geochimica et cosmochimica acta》1974,38(9):1461-1484

${}^{87}\text{Sr}{}^{86}\text{Sr}$ 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 ${}^{87}\text{Sr}{}^{86}\text{Sr}$ 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   

A strontium isotopic study of Smackover brines and associated solids,southern Arkansas     

Alan M. Stueber  Paul Pushkar  Ernest A. Hetherington 《Geochimica et cosmochimica acta》1984,48(8):1637-1649

The isotopic composition of Sr has been measured in brine samples from the Upper Jurassic Smackover Formation in southern Arkansas; ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios range from 0.7071 to 0.7101. With one exception, the 32 Smackover brines contain Sr which is significantly more radiogenic than the Sr in Late Jurassic sea water, indicating sizable Sr contributions from detrital sources. Isotopic analyses of core samples from rock units associated with the brines and regional stratigraphic relationships suggest that the radiogenic Sr was released from detrital minerals in Bossier shale to interstitial fluids expelled from the underlying Louann Salt in the North Louisiana salt basin. These fluids migrated through the Bossier Formation updip to the South Arkansas shelf, where they entered the upper Smackover carbonate grainstone. The radiogenic fluids mixed with Sr-rich interstitial marine waters that had the isotopic composition of Late Jurassic sea water; mixing in variable proportions resulted in the random distribution pattern of variable ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios that is observed in Smackover brines within the 5000 km² study area. Isotopic analyses of nonskeletal carbonate grains and coexisting coarse calcspar cement from the upper Smackover grainstone imply that the grains were diagenetically stabilized in the presence of interstitial marine waters, whereas the calcspar cement is a relatively late diagenetic phase precipitated after the arrival of radiogenic fluids.  相似文献   

The RbSr isotope system as an index of origin and diagenetic evolution of southern Pacific red clays     

Norbert Clauer  Michel Hoffert  Anne-Marie Karpoff 《Geochimica et cosmochimica acta》1982,46(12):2659-2664

Morphological, mineralogical, chemical and RbSr 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 ${}^{87}\text{Sr}{}^{86}\text{Sr}$ 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 ${}^{87}\text{Sr}{}^{86}\text{Sr}$ 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 ${}^{87}\text{Sr}{}^{86}\text{Sr}$ 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 ${}^{87}\text{Sr}{}^{86}\text{Sr}$ 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   

Strontium and its isotopes in Canadian rivers: Fluxes and global implications     

Moire A. Wadleigh  Ján Veizer  Christopher Brooks 《Geochimica et cosmochimica acta》1985,49(8):1727-1736

Systematic sampling of the 39 largest Canadian rivers shows that the weighted average ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio in the dissolved load is 0.7111, similar to previous measurements on such large rivers as the Amazon and Mississippi. Consequently, we believe that the above estimate is likely representative of the global average. This imposes a limit of 6.5 × 10¹¹g yr^?1 on the amount of Sr exchanged with basalts in hydrothermal cells on mid-oceanic ridges. Evaluation of geological information suggests that Sr from silicate sources is of considerable importance for all but the largest Canadian rivers. The latter have chemical and isotopic composition consistent with ~4:1 carbonate vs. silicate derivation of Sr, but such interpretation is not unique. In terms of their water discharge, the 39 Canadian rivers studied account for 4.2% of the world total and their weighted average concentrations for other dissolved solutes are: TDS 176 ppm, Ca 18 ppm, Cl 6.8 ppm and Sr 84 ppb.  相似文献   

87Sr86Sr in Late Proterozoic carbonates: evidence for a “mantle” event at ∼900 Ma ago     

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 ${}^{87}\text{Sr}{}^{86}\text{Sr}$ 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 ${}^{87}\text{Sr}{}^{86}\text{Sr}$ 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   

The statistical analysis and interpretation of imperfectly-fitted Rb-Sr isochrons from polymetamorphic terrains     

M. Cameron  K.D. Collerson  W. Compston  R. Morton 《Geochimica et cosmochimica acta》1981,45(7):1087-1097

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Strontium isotopic studies of the volcanic rocks of the Saunda arc,Indonesia, and their petrogenetic implications     

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 ${}^{87}\text{Sr}{}^{86}\text{Sr}$ 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 ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of 0.7139.The relationship between ${}^{87}\text{Sr}{}^{86}\text{Sr}$ and major and trace element geochemistry is variable and complex. Lavas from the same volcano sometimes show significant differences in ${}^{87}\text{Sr}{}^{86}\text{Sr}$ despite close geochemical relationships. Rocks of the calc-alkaline suite show a regular decrease in ${}^{87}\text{Sr}{}^{86}\text{Sr}$ from West Java to Bali and there is some evidence for increasing ${}^{87}\text{Sr}{}^{86}\text{Sr}$ with increasing depth to the Benioff zone. Calc-alkaline and tholeiitic rocks from the Sunda arc have significantly higher ${}^{87}\text{Sr}{}^{86}\text{Sr}$ 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 ${}^{87}\text{Sr}{}^{86}\text{Sr}$ 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 ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio in the Lake Toba rhyolite implies a crustal origin.  相似文献   

The variation of the marine ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio during Phanerozonic time: interpretation using a flux model     

Garrett W. Brass 《Geochimica et cosmochimica acta》1976,40(7):721-730

The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio in sea water has varied over geologic time due to the addition of strontium to the sea from rocks with a variety of ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios. The measurements by Petermanet al. (Geochim. Cosmochim. Acta34, 105–120, 1970) of the value of the marine ${}^{87}\text{Sr}{}^{86}\text{Sr}$ 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’ (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.704}$) and ‘granitic’ (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.718}$) 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   

Strontium isotope ratios in volcanic rocks from the northwestern part of the Hellenic arc     

G.G. Pe 《Chemical Geology》1975,15(1):53-60

${}^{87}\text{Sr}{}^{86}\text{Sr}$ 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 ${}^{87}\text{Sr}{}^{86}\text{Sr}$ 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   

Isotopic composition of strontium in Indian kimberlites     

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 ${}^{87}\text{Sr}{}^{86}\text{Sr}$ 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 ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios show a positive correlation with $\text{Rb}\text{Sr}$ 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   

Composition isotopique du strontium d'eaux interstitielles extraites de sédiments récents: un argument en faveur de l'homogénéisation isotopique des minéraux argileux     

Norbert Clauer  Michel Hoffert  Daniel Grimaud  Georges Millot 《Geochimica et cosmochimica acta》1975,39(11):1579-1582

The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of interstitial waters squeezed from recent sediments of the Pacific Ocean increases from 0.70920 ± 0.00017 (2σ) to 0.70960 ± 0.0023 and 0.70984 ± 0.00040 at 1.50 and 1.60 m depth—reference value for sea water: 0.70910 ± 0.00035. This variation underlines the likely existence of exchanges between the clays and the interstitial environment. The isotopic homogenization of strontium between the clays and their environment, result of these changes, becomes thus a credible phenomenon. This is a new argument for the dating of clays by the $\text{Rb}\text{Sr}$ method.  相似文献   

Columbia River volcanism: the question of mantle heterogeneity or crustal contamination     

R.W Carlson  G.W Lugmair  J.D Macdougall 《Geochimica et cosmochimica acta》1981,45(12):2483-2499

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. ${}^{143}\text{Nd}{}^{144}\text{Nd}$ ranges from 0.51303 to 0.51208 and is strongly correlated with variations in ${}^{87}\text{Sr}{}^{86}\text{Sr}$. This correlation suggests mixing between two end member compositions, one characterized by ${}^{143}\text{Nd}{}^{144}\text{Nd}\text{> 0.51303}$ and ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{< 0.7035}$, and the other with ${}^{143}\text{Nd}{}^{144}\text{Nd}\text{< 0.5120}$ and ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{> 0.715}$. 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   

Détermination de ${}^{87}\text{Sr}{}^{86}$Sr dans les andésites cénozoïques et les laves associées de Sardaigne Nord occidentale (Italie)     

C Dupuy  R.H McNutt  C Coulon 《Geochimica et cosmochimica acta》1974,38(8):1287-1296

Initial isotopic ratios of strontium have been determined in some calc-alkali rocks of Sardinia. The values of these ratios are from 0.7044 and 0.7047 in two basalts, and from 0.7063 to 0.7081 in five andesites. A dacite and a rhyolite have intermediate values of 0.7056 and 0.7058.It is suggested that increase of radiogenic strontium in andesite is most likely related to a contamination process involving water expelled from an underthrusting plate. This water also contributes to the increase in the potassium and related trace element contents in andesites.In spite of the${}^{87}\text{Sr}{}^{86}$Sr difference between basalt and andesite, a cogenetic relationship between these two kinds of rocks cannot be precluded  相似文献   

Strontium isotope geology of the South Mountain batholith,Nova Scotia     

D.B. Clarke  A.N. Halliday 《Geochimica et cosmochimica acta》1980,44(8):1045-1058

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, (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{\mathrm{i}}$ ranges from 0.7076 ± 0.0003 to 0.7090 ± 0.0003, with the average = 0.7081); adamellite (364.3 ± 1.3 Ma, $\text{(}{}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{\mathrm{i}}\text{= 0.70942 ± 35}$); porphyry (361.2 ± 1.4 Ma, $\text{(}{}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{\mathrm{i}}\text{= 0.71021 ± 119}$); using λ⁸⁷Rb = 1.42 × 10^?11yr^?1.A suite of Meguma country rock samples showed a variation of ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.7113?0.7177}$ 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 ${}^{87}\text{Sr}{}^{86}\text{Sr}$ 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 $\text{(}{}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{372}\text{= 0.7081}$ 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   

Strontium isotopic composition of several oilfield brines from Kansas and Colorado     

S. Chaudhuri 《Geochimica et cosmochimica acta》1978,42(3):329-331

The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of several oilfield brines associated with Paleozoic dolomites, cherty dolomites and sandstones from Kansas and Colorado range from 0.7113 to 0.7341. Two brines in the Mississippian dolomites from Colorado were found to contain the most radiogenic strontium. The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of brines in a small oilfield in eastern Kansas are constant, and the ratios may suggest that in this field there may be only one oil pool. The isotopic composition of strontium in subsurface waters could be useful in determining hydrologic continuity among reservoirs and for obtaining additional information on the origin and migration of these fluids.  相似文献   

Geochemical investigations of the origin of the Manaslu leucogranite (Himalaya,Nepal)   总被引：1，自引：0，他引：1  

Ph. Vidal  A. Cocherie  P. Le Fort 《Geochimica et cosmochimica acta》1982,46(11):2279-2292

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Sr and Pb isotopes,U and Th chemistry of the alkaline Monteregian and White Mountain igneous provinces,eastern North America     

G.Nelson Eby 《Geochimica et cosmochimica acta》1985,49(5):1143-1153

The Monteregian Hills and younger White Mountain alkaline intrusions were emplaced into the Cambro-Ordovician sediments of the St. Lawrence Lowlands and the folded and thrusted Lower Paleozoic sequence of the Appalachian orogen. Age relations indicate that there is a fine-scale structure to the igneous activity, with slightly undersaturated to critically saturated rocks emplaced between 141 and 128 Ma and strongly undersaturated rocks emplaced between 121 and 117 Ma.Sr and Pb isotopic data for the mantle-derived alkali picrite, alkali olivine basalt and basanite magmas, indicate derivation from a depleted mantle similar to that which produces present-day oceanic island basalts. For the most isotopically primitive samples, decay-corrected ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.7030–0.7037}$, ${}^{206}\text{Pb}{}^{204}\text{Pb}\text{= 19.05–19.72}$, ${}^{207}\text{Pb}{}^{204}\text{Pb}\text{= 15.56–15.65}$, and ${}^{208}\text{Pb}{}^{204}\text{Pb}\text{= 38.64–39.26}$. On Pb-Sr isotope correlation diagrams the data define trends similar to those for MOR basalts, implying mantle heterogeneity which requires the presence of a component enriched in radiogenic Pb relative to Sr. The interaction of these isotopically primitive magmas with the crust can be defined in terms of a three component system: depleted mantle-Grenville age crust-Lower Paleozoic age crust. The granitic magmas were apparently derived from the Lower Paleozoic crust of the Appalachian orogen.For the mantle-derived magmas, Th/U ratios vary from 2.5 (estimated ratio for MORB source) to 5.1, with the mean value near that of the bulk earth. The variations in Th/U suggest mantle heterogeneity on a local scale, and the high Th/U of some samples suggests that the mantle was enriched in incompatible elements shortly before melting. The magmas derived by partial melting of the crust have Th/U of 3.3 to 8.7, and the higher ratios are associated with rocks crystallized from magmas that originated by melting of Lower Paleozoic sediments.The Sr and Pb isotopic data support the conclusion of Bellet al. (1982) that the subcontinental mantle under eastern Canada underwent a Precambrian depletion event. This depleted mantle apparently extends under the White Mountain province and is isotopically similar to the mantle which gives rise to oceanic island basalts. In contrast, Pb isotopic ratios for the New England Seamount chain (TARAS and HART, 1983), which apparently represents the oceanic extension of this magmatic activity, are significantly more radiogenic. It is possible that a mantle plume provided the heat energy, and perhaps metasomatic fluids, to trigger melting in the subcontinental mantle, whereas in the case of the oceanic extension the plume directly contributed to the observed magmatism, as reflected in the more radiogenic Pb ratios.  相似文献   

Chemical and isotopic evidence for mixing between depleted and enriched mantle,northwestern U.S.A.     

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 (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{~ 0.7030}$ and ${}^{143}\text{Nd}{}^{144}\text{Nd}\text{~ 0.51305}$) and old, chemically heterogeneous, isotopically enriched subcontinental mantle (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{~ 0.7078}$ and ${}^{143}\text{Nd}{}^{144}\text{Nd}\text{~ 0.51233}$). 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 ${}^{206}\text{Pb}{}^{204}\text{Pb}$ ratios. If present, this third source may reflect a component derived from the downgoing slab of an ancient subduction zone.  相似文献