首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 725 毫秒
1.
The currently active off-rift central volcano Öræfajökull in south-east Iceland sits unconformably on much older (10–12 Ma) and eroded crust. The composition of recent volcanics ranges from basalt to rhyolite, but the series is more sodic alkaline than the common rift zone tholeiitic suites. In this study we present Sr, Nd, Pb and O isotopic data for a suite of Öræfajökull samples. The complete suite shows typical mantle values for oxygen isotopes. The 87Sr/86Sr ratios (average of 15 SAMPLES=0.703702) of the modern Öræfajökull rocks (basalts as well as rhyolites) are much higher than observed so far for any other Icelandic rocks. The 143Nd/144Nd ratios (average=0.512947; n=15) are lower than for rift rocks, but similar to rocks of the off-rift Snæfellsnes volcanic zone. Furthermore, the Öræfajökull rocks are enriched in the 207Pb/204Pb and 208Pb/204Pb isotope ratios compared to Icelandic rift basalts. The enriched nature of the suite indicates that Öræfajökull samples a source component that has characteristics common with EM2 type mantle. Furthermore, it is concluded that the silicic rocks of Öræfajökull formed by fractional crystallization from mafic melts rather than by partial melting of older crust.  相似文献   

2.
Peridotite xenoliths from the Eifel can be divided into incompatible element-depleted and -enriched members. The depleted group is restricted to dry lherzolites whereas the enriched group encompasses dry harzburgites, dry websterite and amphibole and/or phlogopite-bearing peridotites. Isotopically the depleted group is very diverse with143Nd/144Nd ranging from 0.51302 to 0.51355 and87Sr/86Sr from 0.7041 to 0.7019, thus occupying a field larger than expected for oceanic-type subcontinental mantle. These xenoliths are derived from a mantle which appears to have diverged from a bulk-earth Nd and Sr isotopic evolution path 2 Ga ago as a consequence of partial melting. The combination of high143Nd/144Nd with high87Sr/86Sr in some members of the depleted-xenoliths suite is likely to be the result of incipient reaction with incompatible element-enriched fluids in the mantle. In the enriched group such reactions have proceeded further and erased any pre-enrichment isotope memory resulting in a smaller isotopic diversity (143Nd/144Nd 0.51256–0.51273,87Sr/86Sr 0.7044–0.7032). An evaluation of SmHf and YbHf relationships suggests that the amphibole-bearing lherzolites and harzburgites acquired their high enrichment of light rare earth elements by fluid infiltration into previously depleted peridotite rather than by silicate melt-induced metasomatism. Upper mantle composed of such metasomatized peridotites does not represent a potential source for the basanites and nephelinites from the Eifel. The isotopic and chemical diversity of the subcontinental lithospheric part of the mantle may result from it having remained isolated from the convecting mantle for times > 1 Ga.  相似文献   

3.
Pumice samples from Fukutoku-oka-no-ba in the Izu–Bonin – Mariana (IBM) arc were analysed for 40 trace elements and Sr, Nd, and Pb isotopic compositions. These samples are shoshonites (59.4–61.8 wt% SiO2), characterized by high contents of K2O (3.74–4.64 wt%), Ba (1274–1540 p.p.m.), Rb (91–105 p.p.m.), and light rare earth elements. The characteristics of alkali-element enrichment are similar to those of other parts of the Alkalic Volcano Province (AVP) in the northern Mariana and southernmost Volcano arcs. Sr (87Sr/86Sr = 0.7036–0.7038) and Pb isotopic compositions (206Pb/204Pb = 19.08–19.11, 207Pb/204Pb = 15.62–15.63, 208Pb/204Pb = 38.85–38.91) of Fukutoku-oka-no-ba pumice are relatively radiogenic, whereas Nd is unradiogenic (143Nd/144Nd = 0.51283–0.51286). Fukutoku-oka-no-ba is isotopically distinct from Iwo Jima and is similar to the Hiyoshi Volcanic Complex, suggesting that Fukutoku-oka-no-ba might have a magma source similar to that of the Hiyoshi volcanic complex. Plots of Pb and Nd isotopes for AVP lavas trend toward the fields of ocean island basalt (OIB) source and pelagic sediments, which are possible sources of AVP enrichments.  相似文献   

4.
Alkali basalts and nephelinites from the southern end of the East African Rift (EAR) in northern Tanzania have incompatible trace element compositions that are similar to those of ocean island basalts (OIB). They define a considerable range of Sr, Nd and Pb isotopic compositions (87Sr/86Sr= 0.7035−0.7058,εNd = −5to+3, and206Pb/204Pb= 17.5−21.3), each of which partially overlaps the range found in OIB. However, they occupy a unique position in combined Nd, Sr and Pb isotopic compositional space. Nearly all of the lavas have radiogenic Pb, similar to HIMU with high time-integrated238U/204Pb coupled with unradiogenic Nd (+2 to −5) and radiogenic Sr (>0.704), similar to EMI. This combination has not been observed in OIB and provides evidence that these magmas predominantly acquired their Sr, Nd and Pb in the subcontinental lithospheric mantle rather than in the convecting asthenosphere. These data contrast with compositions for lavas from farther north in the EAR. The Pb isotopic compositions of basalts along the EAR are increasingly radiogenic from north to south, indicating a fundamental change to sources with higher time-integratedU/Pb, closer to the older cratons in the south. An ancient underplated OIB melt component, isolated for about 2 Ga as enriched lithospheric mantle and then remelted, could generate both the trace element and isotopic data measured in the Tanzanian samples. Whereas the radiogenic Pb in Tanzanian lavas requires a source with high time-integratedU/Pb, most continental basalts that are thought to have interacted with the continental lithospheric mantle have unradiogenic Pb, requiring a source with a history of lowU/Pb. Such lowU/Pb is readily accomplished with the addition of subduction-derived components, since the lower averageU/Pb of arc basalts (0.15) relative to OIB (0.36) probably reflects addition of Pb from subducted oceanic crust. If the subcontinental lithosphere is normally characterized by low time-integratedU/Pb it would appear that subduction magmatism is more important than OIB additions in supplying the Pb inventory of the lithospheric mantle. However,U/Pb ratios of xenoliths derived from the continental lithospheric mantle suggest that both processes may be important. This apparent discrepancy could be because xenoliths are not volumetrically representative of the subcontinental lithospheric mantle, or, more likely, that continental lithospheric mantle components in basalts are normally only identified as such when the isotopic ratios are dissimilar from MORB or OIB. Lithospheric enrichment from subaccreted OIB components appears to be more significant than generally recognized.  相似文献   

5.
143Nd/144Nd ratios, and Sm and Nd abundances, are reported for particulates from major and minor rivers of the Earth, continental sediments, and aeolian dusts collected over the Atlantic, Pacific, and Indian Oceans. Overall, Sm/Nd ratios and Nd isotopic compositions in contemporary continental erosion products vary within the small ranges of 147Sm/144Nd= 0.115 ± 0.01 and143Nd/144Nd= 0.51204 ± 0.0002 (εNd = −11.4 ± 4). The average period of residence in the continental crust is estimated to be1.70 ± 0.35Ga.

These results combined with data from the literature have implications for the age, history, and composition of the sedimentary mass and the continental crust: (1) The average “crustal residence age” of the whole sedimentary mass is about 1.9 Ga. (2) The range of Nd isotope compositions in the continent derived particulate input to the oceans is the same as Atlantic sediments and seawater, but lower than those of the Pacific, demonstrating the importance of Pacific volcanism to Pacific Nd chemistry. (3) The average ratio of Sm/Nd is about 0.19 in the upper continental crust, and has remained so since the early Archean. This precludes the likelihood of major mafic to felsic or felsic to mafic trends in the overall composition of the upper continental crust through Earth history. (4) Sediments appear to be formed primarily by erosion of continental crust having similar Sm/Nd ratios, rather than by mixing of mafic and felsic compositions. (5) The average ratio of 143Nd/144Nd≈ 0.5117 (εNd ≈ −17) in the upper continental crust, assuming its mean age is about 2 Ga. (6) The uniformity of the SmNd isotopic systematics in river and aeolian particulates primarily reflects efficient recycling of old sediment by sedimentary processes on a short time scale compared to the amount of time the material has resided in the crust.  相似文献   


6.
Basalts from young seamounts situated within 6.8 m.y. of the East Pacific Rise, between 9° and 14°N latitude, display significant variations in 143Nd/144Nd (0.51295–0.51321), 87Sr/86Sr (0.7025–0.7031), and(La/Sm)N (0.415–3.270). Nd and Sr isotope ratios are anti-correlated and form a trend roughly parallel to the “mantle array” on a143Nd/144Nd vs.87Sr/86Sr variation diagram. Nd and Sr isotope ratios display negative and positive correlations, respectively, with(La/Sm)N. The geochemical variations observed at the seamounts are nearly as great or greater than those observed over several hundred kilometers of the Reykjanes Ridge, or at the islands of Iceland or Hawaii.

Samples from one particular seamount, Seamount 6, display nearly the entire observed range of chemical variations, offering an ideal opportunity to constrain the nature of heterogeneities in the source mantle. Systematics indicative of magma mixing are recognized when major elements, trace elements, trace element ratios, and isotope ratios are compared with each other in all possible permutations. The source materials required to produce the end-member magmas are: (1) a typical MORB-source-depleted peridotite; and (2) a relatively enriched material which may represent ancient mantle segregations of basaltic melt, incompletely mixed remnants of subducted ocean crust, or metasomatized peridotite such as that found at St. Paul's Rocks or Zabargad Island. Due to the proximity of the seamounts to the East Pacific Rise (EPR), the source materials are thought to comprise an intimate mixture in the mantle immediately underlying the seamounts and the adjacent EPR. Lavas erupted at the ridge axis display a small range of isotopic and incompatible trace element compositions because the large degrees of melting and presence of magma chambers tend to average the chemical characteristics of large volumes of mantle.

If the postulated mantle materials, with large magnitude, small-scale heterogeneities, are ubiquitous in the upper mantle, chemical variations in basalts ranging from MOR tholeiites to island alkali basalts may reflect sampling differences rather than changes in bulk mantle chemistry.  相似文献   


7.
Geochemical variations in mid-ocean ridge basalts have been attributed to differing proportions of compositionally distinct mantle components in their sources, some of which may be recycled crust. Oxygen isotopes are strongly fractionated by near-surface interactions of rocks with the hydrosphere, and thus provide a tracer of near-surface materials that have been recycled into the mantle. We present here oxygen isotope analyses of basaltic glasses from the mid-Atlantic ridge south of and across the Azores platform. Variations in δ18O in these samples are subtle (range of 0.47‰) and may partly reflect shallow fractional crystallization; we present a method to correct for these effects. Relatively high fractionation-corrected δ18O in these samples is associated with geochemical indices of enrichment, including high La/Sm, Ce/Pb, and 87Sr/86Sr and low 143Nd/144Nd. Our results suggest two first-order conclusions about these enriched materials: (1) they are derived (directly or indirectly) from recycled upper oceanic crustal rocks and/or sediments; and (2) these materials are present in the north Atlantic MORB sources in abundances of less than 10% (average 2–5%). Modeling of variations of δ18O with other geochemical variables further indicates that the enriched component is not derived from incorporation of sediment or bulk altered oceanic crust, from metasomatism of the mantle by hydrous or carbonate-rich fluids, or from partial melting of subducted sediment. Instead, the data appear to require a model in which the enriched component is depleted mantle that has been metasomatized by small-degree partial melts of subducted, dehydrated, altered oceanic crust. The age of this partial melting is broadly constrained to 250 Ma. Reconstructed plate motions suggest that the enriched component in the north Atlantic mantle may have originated by subduction along the western margin of Pangea.  相似文献   

8.
The igneous rocks of the Pongola Supergroup (PS) and Usushwana Intrusive Suite (UIS) represent a case of late Archaean continental magmatism in the southeastern part of the Kaapvaal craton of South Africa and Swaziland.

U-Pb dating on zircons from felsic volcanic rocks of the PS yields a concordia intercept age of 2940 ± 22Ma that is consistent with a Sm-Nd whole rock age of 2934 ± 114Ma determined on the PS basalt-rhyolite suite. The initial εNd of−2.6 ± 0.9 is the lowest value so far reported for Archaean mantle-derived rocks. Rb-Sr whole rock dating of the PS yields a younger isochron age of 2883 ± 69Ma, which is not significantly different form the accepted U-Pb zircon age.

An internal (cpx-opx-plag-whole rock) isochron for a pyroxenite from the younger UIS yields an age of 2871 ± 30 Ma and initial 143Nd/144Nd that lies off the CHUR growth curve by εNd −2.9 ± 0.2. However, Sm-Nd whole-rock data for the UIS yield an excessively high age of 3.1 Ga that conflicts with firm geological evidence showing the UIS to be intrusive into the PS.

The negative deviations of initialεNd from the chondritic Nd evolution curve suggest significant contamination of the PS and UIS melts by older continental crust. A mixing process with continental crust after magma segregation is supported by a high initial 87Sr/86Sr ratio of0.703024 ± 24 for a clinopyroxene sample from a UIS pyroxenite, compared with an expected value of 0.701 for the 2.9 Ga mantle. We therefore interpret the linear array of data points for the UIS gabbros as a mixing line between 2.87 Ga old magma and older continental crust.

Parallel LREE-enriched REE patterns, negative Nb-Ti anomalies, a distinctive and uniform ratio of Ti/Zr 46 and a narrow span of initial Nd indicate a common source for both the PS and UIS suites which is different from primitive mantle.  相似文献   


9.
Primitive basaltic single eruptions in the Big Pine Volcanic Field (BPVF) of Owens Valley, California show systematic temporal–compositional variation that cannot be described by simple models of fractional crystallization, partial melting of a single source, or crustal contamination. We targeted five monogenetic eruption sequences in the BPVF for detailed chemical and isotopic measurements and 40Ar/39Ar dating, focusing primarily on the Papoose Canyon sequence. The vent of the primitive (Mg# = 69) Papoose Canyon sequence (760.8 ± 22.8 ka) produced magmas with systematically decreasing (up to a factor of two) incompatible element concentrations, at roughly constant MgO (9.8 ± 0.3 (1σ) wt.%) and Na2O. SiO2 and compatible elements (Cr and Ni) show systematic increases, while 87Sr/86Sr systematically decreases (0.7063–0.7055) and εNd increases (− 3.4 to − 1.1). 187Os/188Os is highly radiogenic (0.20–0.31), but variations among four samples do not correlate with other chemical or isotopic indices, are not systematic with respect to eruption order, and thus the Os system appears to be decoupled from the dominant trends. The single eruption trends likely result from coupled melting and mixing of two isotopically distinct sources, either through melt-rock interaction or melting of a lithologically heterogeneous source. The other four sequences, Jalopy Cone (469.4 ± 9.2 ka), Quarry Cone (90.5 ±17.6 ka), Volcanic Bomb Cone (61.6 ± 23.4 ka), and Goodale Bee Cone (31.8 ± 12.1 ka) show similar systematic temporal decreases in incompatible elements. Monogenetic volcanic fields are often used to decipher tectonic changes on the order of 105–106 yr through long-term changes in lava chemistry. However, the systematic variation found in Papoose Canyon (100–102 yr) nearly spans that of the entire volcanic field, and straddles cutoffs for models of changing tectonic regime over much longer time-scales. Moreover, ten new 40Ar/39Ar ages combined with chemistry from all BPVF single eruption sequences show the long-term trend of BPVF evolution comprises the overlapping, temporal–compositional trends of the monogenetic vents. This suggests that the single eruption sequences contain the bulk of the systematic chemical variation, whereas their aggregate compositions define the long-term trend of volcanic field evolution.  相似文献   

10.
Abstract Middle Miocene basalts and basaltic andesites of the Matsue Formation outcrop within a 5 km radius of Matsue city in eastern Shimane Prefecture. Despite their limited outcrop and age (11.0 ± 1.5 Ma), they show a wide range in 87Sr-86Sr(0.70370–0.70593), 143Nd-144Nd(0.512904–0.512471) and large ion lithophile element (LILE) contents, but a relatively narrow range for some high field strength elements (HFSE) such as Nb and Ti. These basalts and andesites can be divided into three groups based on petrography, major element, trace element and isotope chemistry. Although one group has undergone some fractional crystallization, isotope chemistry precludes linkage of the groups by a closed-system process. Crustal contamination can explain isotope chemistry, but is not consistent with trace element variations. The most satisfactory model is eruption of two compositionally distinct magmas, with limited magma mixing and fractional crystallization. Published experimental work shows that one end-member resulted from shallow melting of upwelling mantle at ∼25 km. The simultaneous eruption of the other end member magma in the same area points towards a heterogeneous mantle. The isotopic composition of Matsue Formation basalts and andesites covers the entire range of Late Miocene mafic volcanic rocks of southwest Japan. Such gross heterogeneity developed on a local scale has implications for models that deal with regional chemical variations of mafic volcanic rocks in southwest Honshu.  相似文献   

11.
Seasonal variations in the origin of lead in snow at Dye 3, Greenland   总被引：1，自引：0，他引：1  
The isotopic composition and concentration of lead has been measured in fresh and slightly aged snow collected at Dye 3 in southern Greenland during one full year. The lead concentration displayed large variations ranging from 14–3016 pg/g in April (spring) to 3–6 pg/g in September (summer) while the isotopic ratios changed in regular manner during the year. The 206Pb/207Pb ratios were 1.15 from spring to mid-summer snow, and increased in late summer to early autumn, reaching 1.20 in winter. These isotopic data indicate that the lead in the autumn to winter snow originated in North America, while that in spring to mid-summer snow is from Eurasia.  相似文献   

12.
Measurements of 87Sr/86Sr ratios of interstitial waters from leg 25, site 245 and leg 38, site 336 of the Deep Sea Drilling Project show that the enrichment of Sr2+ with depth is caused both by the alteration of volcanic material and by the introduction of strontium derived from calcium carbonate. 87Sr/86Sr ratios range from 0.70913 to 0.70794 at site 245 and from 0.70916 to 0.70694 at site 336. The low ratios compared with contemporaneous seawater reflect the release of Sr from a volcanic source having, according to material-balance calculations, a 87Sr/86Sr ratio of about 0.7034 at site 336. At this site the source appears to be volcanic ash and not basaltic basement which acts as a sink for Sr2+ during in situ low-temperature weathering. The volcanic contribution to the strontium enrichment in the basal interstitial waters varies from <10% at site 245 to >50% at site 336. The remaining Sr2+ is derived from Sr-rich biogenic carbonate during diagenetic recrystallization to form Sr-poor calcite.  相似文献   

13.
Melt inclusions from four individual lava samples representing the HIMU (Mangaia Island), EMI (Pitcairn Island) and EMII (Tahaa Island) end member components, have heterogeneous Pb isotopic composition larger than that defined by the erupted lavas in each island. The broad linear trend in 207Pb/206Pb–208Pb/206Pb space produced by the melt inclusions from Mangaia, Tahaa and fPitcairn samples reproduces the entire trend defined by the Austral chain, the Society islands and the Pitcairn island and seamount groups. The inclusions preserve a record of melt composition of far greater isotopic diversity than that sampled in whole rock basalts. These results can be explained by mixing of a common depleted component with the HIMU, EMI and EMII lavas, respectively. We favor a model that considers the oceanic lithosphere to be that common component. We suggest that the Pb isotopic compositions of the melt inclusions reflect wall rock reaction of HIMU, EMI and EMII melts during their percolation through the oceanic lithosphere. Under these conditions, the localized rapid crystallization of olivine from primitive basalt near the reaction zone would allow the entrapment of melt inclusions with different isotopic composition.  相似文献   

14.
A review of O, C, Sr and S isotope trends for the entire Phanerozoic shows that the present-day values of isotope signals are similar to those at the Proterozoic termination. The sharp rise in 87Sr/86Sr since 65 Ma has been attributed to an uplift and subsequent metamorphism and erosion associated with the Himalayas and Tibet. This orogenic evolution has been postulated to have influenced the global organic and inorganic carbon cycles and climate as well. A similar large-scale orogeny, the Pan-African event, also dominated the Neoproterozoic (Vendian) times, and the similarity of modern and Neoproterozoic isotope values for seawater may therefore have had a comparable tectonic cause. In this contribution, we present the results of a numerical model of the coupled C–alkalinity–S–Sr cycles suggesting that the early Paleozoic (from early Cambrian to late Devonian) evolution of Sr, O, C and S seawater isotope signals could have been the consequence of progressive oxidation of a large reduced carbon reservoir exhumed during the Pan-African orogeny. The δ18O measured in brachiopod shells is used as a forcing of the model, postulating that any change in the oxygen isotopic composition of seawater is the result of a disequilibrium in the organic carbon subcycle through the coupling of the oxygen isotopic and carbon cycles. The calculated δ13C, 87Sr/86Sr and δ34S are in good agreement with the data, as is the reasonable calculated history for atmospheric pCO2 and its relation to global climate.  相似文献   

15.
This paper presents new O and Sr isotope data for lavas from the northern part of the Roman perpotassic province. The samples comprise the tephritic leucititic to leucite phonolitic lavas and the saturated lavas from the Vulsinian District, the olivine leucite melilitite of San Venanzo, and the kalsilite diopside melilitite of Cupaello. Previous oxygen isotope work on the lavas of the Vulsinian District suggested crustal contamination of “normal” mantle-derived magmas. The new data cover the ranges previously found. O and Sr isotope ratios of evolved lavas of the undersaturated suite indicate assimilation in variable amounts of up to ca. 10% of continental crustal material. The saturated lavas probably assimilated large amounts (up to ca. 50%) of crust. Lavas chemically identified as corresponding to little modified mantle-derived liquids are high in both87Sr/86Sr andδ18O: 0.7103−0.7107, +7.8 to +9.4 (Vulsini), 0.7104, +12.3 (San Venanzo) and 0.7112, +14.4 (Cupaello). These high values are interpreted to have been inherited from a metasomatized parental mantle. Hydrous fluids enriched in large-ion lithophile elements and high inδ18O and87Sr/86Sr are thought to have mixed with mantle of “normal”δ18O and87Sr/86Sr. The fluids probably origi dehydration of continent-derived sediments, which were subducted beneath a mantle wedge in the continent-continent collision of the Corsica-Sardinia block and the Adriatic (Italian) plate. This hypothesis is supported by Pb and Nd isotopic evidence and is probably valid for the entire Roman Province.  相似文献   

16.
New Sr and C isotopic data, both obtained on the same samples of marine carbonates, provide a relatively detailed record of isotopic variation in seawater through the latest Proterozoic and allow, for the first time, direct correlation of these isotopic changes in the Vendian ( 540–610 Ma). The strong isotope variations determined in this study record significant environmental and tectonic changes. Together with a fairly poorly constrained Nd isotopic record, the Sr and C isotopic records can be used to constrain rates of erosion, hydrothermal alteration and organic C burial. Further, comparison of these records with those of the Cenozoic permit investigation of the general relationship between global tectonics and continental glaciation. In particular, results of this study show a very large change in the 87Sr/86Sr of marine carbonates from low pre-Vendian ( > 610 Ma) values ( 0.7066) to high Middle Cambrian values ( 0.7090). This change is greater in magnitude than the significant increase in seawater 87Sr/86Sr through the Cenozoic. Both changes are attributed to high erosion rates associated with continent-continent collisions (Pan-African and Himalayan-Tibetan). In the latest Proterozoic these high erosion rates, probably coupled with high organic productivity and anoxic bottom-water conditions, contributed to a significant increase in the burial rate of organic C. Ice ages mark both the Neoproterozoic and Cenozoic, but different stratigraphic relationships between the Sr isotopic increase and continental glaciation indicate that uplift-driven models proposed to explain Cenozoic climatic change cannot account for the latest Proterozoic ice ages.  相似文献   

17.
We present the first sulfur and oxygen isotopic data for tephra from the catastrophic 1883 eruption of Krakatau. Sulfur isotopic ratios in unaltered Krakatau tephra erupted August 26–27, 1883 are markedly enriched in 34S relative to mantle sulfur. High δ34S values of +6.3 to +16.4‰ can best be explained by open-system or multi-stage degassing of SO2 from the oxidized rhyodacitic and gray dacitic magmas with 34S enrichment of SO2−4 remaining in the melt. Lower whole-rock δ34S values of +2.6‰ and +4.0‰ in two oxidized gray dacitic samples indicate more primitive subarc mantle sulfur in the 1883 magma chamber. Initial δ34S of the rhyodacitic magma was probably in the +1.5‰ to +4.0‰ range and similar to δ34S values measured in arc volcanic rocks from the Mariana Arc.  相似文献   

18.
Large-scale compositional domains at DSDP/ODP drill sites 417A, 417D and 418A were analyzed for O, Sr and Nd isotope ratios, and REE, U, K, Rb and Sr abundances, to constrain the bulk chemical composition of the oceanic crust that is recycled at subduction zones. The combination of the three sites gives the composition of the upper oceanic crust in this region over a distance of about 8 km. The δ18OSMOW and87Sr/86Srmeas of compositional domains 10–100 m in size correlate well, with a range of 7.7–19.2 and 0.70364–0.70744, and mean of 9.96 and 0.70475, respectively. The Rb inventory of the upper crust increases by about an order of magnitude, while Sr contents remain constant. U abundances increase moderately under oxidizing alteration conditions and nearly triple in the commonly reducing alteration environments of the upper oceanic crust. REEs are influenced by alteration only to a small extent, and recycled oceanic crust is similar to MORB with respect to143Nd/144Nd. Even though the average composition of the upper oceanic crust is well defined, the large scale composition varies widely. Highly altered compositional domains may not have a large impact on the average composition of the oceanic crust, but they may preferentially contribute to fluids or partial melts derived from the crust by prograde metamorphic reactions.  相似文献   

19.
Late Miocene (7–9 Ma) basaltic rocks from the Monbetsu‐Kamishihoro graben in northeast Hokkaido have chemical affinities to certain back‐arc basin basalts (referred to herein as Hokkaido BABB). Pb‐, Nd‐ and Sr‐isotopic compositions of the Hokkaido BABB and arc‐type volcanic rocks (11–13 Ma and 4–4.5 Ma) from the nearby region indicate mixing between the depleted mantle and an EM II‐like enriched component (e.g. subducted pelagic sediment) in the magma generation. At a given 87Sr/86Sr, Hokkaido BABB have slightly lower 143Nd/144Nd and slightly less radiogenic 206Pb/204Pb compared with associated arc‐type lavas, but both these suites are difficult to distinguish solely on the basis of isotopic compositions. These isotopic data indicate that while generation of the Hokkaido BABB involves smaller amounts of the EM II‐like enriched component than do associated arc lavas, Hokkaido BABB are isotopically distinct from basalts produced at normal back‐arc basin spreading centers. Instead, northeast Hokkaido BABB are more similar to basalts erupted during the initial rifting stage of back‐arc basins. The Monbetsu‐Kamishihoro graben may have developed in association with extension that formed the Kurile Basin, suggesting that opening of the basin continued until late Miocene (7–9 Ma).  相似文献   

20.
Chemical and isotopic ratio (He, C, H and O) analysis of hydrothermal manifestations on Pantelleria island, the southernmost active volcano in Italy, provides us with the first data upon mantle degassing through the Sicily Channel rift zone, south of the African–European collision plate boundary. We find that Pantelleria fluids contain a CO2–He-rich gas component of mantle magmatic derivation which, at shallow depth, variably interacts with a main thermal (100°C) aquifer of mixed marine–meteoric water. The measured 3He/4He ratios and δ13C of both the free gases (4.5–7.3 Ra and −5.8 to −4.2‰, respectively) and dissolved helium and carbon in waters (1.0–6.3 Ra and −7.1 to −0.9‰), together with their covariation with the He/CO2 ratio, constrain a 3He/4He ratio of 7.3±0.1 Ra and a δ13C of ca. −4‰ for the magmatic end-member. These latter are best preserved in fluids emanating inside the active caldera of Pantelleria, in agreement with a higher heat flow across this structure and other indications of an underlying crustal magma reservoir. Outside the caldera, the magmatic component is more affected by air dilution and, at a few sites, by mixing with either organic carbon and/or radiogenic 4He leached from the U–Th-rich trachytic host rocks of the aquifer. Pantelleria magmatic end-member is richer in 3He and has a lower (closer to MORB) δ13C than all fluids yet analyzed in volcanic regions of Italy and southern Europe, including Mt. Etna in Sicily (6.9±0.2 Ra, δ13C=−3±1‰). This observation is consistent with a south to north increasing imprint of subducted crustal material in the products of Italian volcanoes, whose He and C (but also O and Sr) isotopic ratios gradually evolve towards crustal values northward of the African–Eurasian plate collision boundary. Our results for Pantelleria extend this regional isotopic pattern further south and suggest the presence of a slightly most pristine or ‘less contaminated’, 3He-richer mantle source beneath the Sicily Channel rift zone. The lower than MORB 3He/4He ratio but higher than MORB CO2/3He ratio of Pantelleria volatile end-member are compatible with petro-geochemical evidence that this mantle source includes an upwelling HIMU–EM1-type asthenospheric plume component whose origin, according to recent seismic data, may be in the lower mantle.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号