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1.
Lavas erupted behind the volcanic front in southeastern Guatemala have many important distinctions from lavas erupted on the volcanic front. These include: generally higher MgO, Nb, Sr, TiO2, and rare earth element concentrations; higher La/Yb and Nb/Y ratios; and lower Ba/La, La/Nb, Ba/Zr and Zr/Nb ratios. These major and trace element distinctions are caused by reduced fractionation during ascent and storage in the crust, lower degrees of melting in the source, and greatly reduced contributions from the subducted Cocos plate in the source. In addition, because all of these important distinctions are even borne in lavas erupted within 20 km of the front, there is little apparent petrogenetic continuity between front and behind-the-front magmas. What little geochemical continuity exists is in radiogenic isotopes: 143Nd/144Nd falls across the arc, Pb isotopic ratios (except 206Pb/204Pb) rise across the arc, and 87Sr/86Sr rise across the arc after an initial discontinuity within 20 km of the front. These continuous across-arc changes in radiogenic isotopes are caused by increased contamination with older, more isotopically disparate rocks, away from the front. Once the effects of crustal contamination are removed, the remaining isotopic variability behind the front is non-systematic and reflects the inherent isotopic heterogeneity of the source, the mantle wedge. Geochemical disconnection in southeastern Guatemala suggests that behind-the-front magmas are produced by decompression melting near the top of the wedge, not by flux-dominated melting near the base of the wedge.  相似文献   

2.
The Kahoolawe shield volcano produced precaldera and caldera-filling tholeiites and mildly alkalic post-caldera lavas that petrographically and compositionally resemble such lavas from other Hawaiian shield volcanoes. However, Kahoolawe tholeiites display wide ranges in incompatible trace element ratios (e.g., Nb/Th=9–24, Th/Ta=0.6–1.3), 87Sr/86Sr (0.70379–0.70440), 143Nd/144Nd (0.51273–0.51298), and 206Pb/204Pb (17.92–18.37). The isotopic variation exceeds that at any other Hawaiian shield volcano, and spans about half the range for all Hawaiian tholeiites. Quasi-cyclic temporal evolution of Kahoolawe tholeiites is consistent with combined fractional crystallization and periodic recharge by primitive magmas. Ratios of highly incompatible trace elements and Sr, Nd, and Pb isotopic ratios from coherent sub-trends that reflect recurrent interactions between variably evolved magmas and two other mantle components whose compositions are constrained by intersections between these trends. The most MgO-rich Kahoolawe tholeiites are partial melts of a high Nb/Th (23.5) ascending plume, possibly comprising ancient subducted oceanic lithosphere. Slightly evolved tholeiites experienced combined crystal fractionation and assimilation (AFC) of material derived from a distinct reservoir (Nb/Th 9) of asthenospheric derivation. The most evolved tholeiites display compositional shifts toward a third component, having mid ocean ridge basalt-like isotopic ratios but enriched OIB-like trace element ratios, representing part of the lithospheric mantle (or melts thereof). Periodic recurrence of all three magma variants suggests that eruptions may have tapped coeval reservoirs distributed over a large depth range. Kahoolawe provides new evidence concerning the nature of the Hawaiian plume, the distribution of compositional heterogeneities in the suboeanic mantle, and the processes by which Hawaiian tholeiites form and evolve.  相似文献   

3.
We have developed an idealized mathematical model to understand the isotopic variability of the mantle and its relation to the observed variations in isotopic ratios 143Nd/144Nd, 87Sr/86Sr, 176Hf/177Hf, 208Pb/204Pb, 206Pb/204Pb, and 207Pb/204Pb measured on mid-ocean ridge basalt (MORB). We consider a simple box model of mantle processes. A single melt region produces a melt fraction F of melt, and the average time since a given parcel of mantle material last visited this region is given by the time scale τmelt. The melt region fractionates the parent/daughter ratios. Over time this leads to variations in the mantle isotopic ratios as the parent decays to the daughter. Key assumptions are that the half-life of the parent isotope is large compared with τmelt, that the flow is strongly stirring, and that the mantle has reached a statistical steady state. This enables us to neglect the specifics of the underlying flow. Sampling from our model mantle is dealt with by averaging over a large number N of samples to represent the mixing after melting.The model predicts a probability density for isotopic ratios in MORB which, with exception of the Pb isotopes, are consistent with measurements. Fitting the MORB data to this model gives estimates of the model parameters F, τmelt, and N. Small melt fractions with F around 0.5% are essential for a good fit, whereas τmelt and N are less well constrained. τmelt is estimated at around 1.4 to 2.4 Ga, and N is of the order of hundreds. The model predicts a larger variability for the Pb isotopes than that observed. As has been stated by many previous authors, it appears that fundamental differences exist between the dynamics of Pb isotopes and those of Nd, Sr and Hf isotopes.  相似文献   

4.
Re–Os isotope and elemental data have been obtained for 20 historical picrites and basalts (1931–2006) from the Piton de la Fournaise volcano on Réunion Island and two old (>0.78?Ma) cumulates from a drill hole in the eastern part of the volcano. The 187Os/188Os ratios of the historical lava samples, selected to cover the MgO concentration and Pb isotopic ranges of Piton de la Fournaise lavas, range from 0.1311 to 0.1374. This result, together with previous results on 66-Ma-old lavas from the Deccan Traps (Allègre et al. in. Earth Planet Sci Lett, 170:197–204, 1999), supports the idea that the Os isotopic signature of the Réunion plume is relatively uniform and is at the less radiogenic end of the ocean island basalt spectrum. In detail, lavas erupted before 1992 seem to have higher 187Os/188Os than the lavas erupted after the 1992–1998 period of quiescence. Comparison of 187Os/188Os ratios with Pb, Sr and Nd isotopic data on the same set of samples shows no correlation between Os and Sr–Nd isotopes, whereas a broad positive relationship with Pb isotopes is observed, which is interpreted to reflect coupled fractionation of Re/Os and U–Th/Pb in the mantle due to the partitioning of Pb and Os into sulphides. Lavas inferred to be recording the Os isotopic signature of the Réunion plume source have higher 187Os/188Os ratios than the primitive mantle values. While this might be ascribed to melting of a lithologically heterogeneous source comprising recycled oceanic crust and/or continental sediment, the expected coupled Os–Sr–Nd–Pb isotopic variations are not observed. It is thus proposed that the mantle source for Piton de la Fournaise has inherently slightly radiogenic 187Os/188Os values that could reflect a mantle domain almost isolated from recycling processes.  相似文献   

5.
Seven hundred and twenty-five Sr, two hundred and forty-three Nd and one hundred and fifty-one Pb isotopic ratios from seven different Mexican magmatic provinces were compiled in an extensive geochemical database. Data were arranged according to the Mexican geological provinces, indicating for each province total number of analyses, range and mean of values and two times standard deviation (2σ). Data from seven provinces were included in the database: Mexican Volcanic Belt (MVB), Sierra Madre Occidental (SMO), Baja California (BC), Pacific Ocean (PacOc), Altiplano (AP), Sierra Madre del Sur (SMS), and Sierra Madre Oriental (SMOr). Isotopic values from upper mantle and lower crustal xenoliths, basement outcrops and sediments from the Cocos Plate were also compiled. In the MVB the isotopic ratios range as follows:87Sr/86Sr 0.703003-0.70841;143Nd/144Nd 0.512496-0.513098;206Pb/204Pb 18.567-19.580;207Pb/204Pb 15.466-15.647;208Pb/204Pb 38.065-38.632. The SMO shows a large variation in87Sr/86Sr ranging from ∼0.7033 to 0.71387.143Nd/144Nd ratios are relatively less variable with values from 0.51191 to 0.51286. Pb isotope ratios in the SMO are as follows:206Pb/204Pb 18.060-18.860;207Pb/204Pb 15.558-15.636;208Pb/204Pb 37.945-38.625. PacOc rocks show the most depleted Sr and Nd isotopic ratios (0.70232-0.70567 for Sr and 0.512631-0.513261 for Nd). Pb isotopes for PacOc show the following range:206Pb/204Pb 18.049-19.910;207Pb/2047Pb 15.425-15.734;208Pb/204Pb 37.449-39.404. The isotopic ratios of the AP rocks seem to be within the range of those from the PacOc. Most samples with reported Sr and Nd isotopic data are spread within and around the “mantle array”. The SMO seems to have been formed by a mixing process between mantle derived magmas and continental crust. The MVB appears to have a larger mantle component, with AFC as the dominant petrogenetic process for the evolved rocks. There is still a need for Pb isotopic data in all Mexican magmatic provinces and of Nd isotopes in BC, AP, SMS, and SMOr.  相似文献   

6.
We present the first report of geochemical data for submarine basalts collected by a manned submersible from Rurutu, Tubuai, and Raivavae in the Austral Islands in the South Pacific, where subaerial basalts exhibit HIMU isotopic signatures with highly radiogenic Pb isotopic compositions. With the exception of one sample from Tubuai, the 40Ar/39Ar ages of the submarine basalts show no significant age gaps between the submarine and subaerial basalts, and the major element compositions are indistinguishable at each island. However, the variations in Pb, Sr, Nd, and Hf isotopic compositions in the submarine basalts are much larger than those previously reported in subaerial basalts. The submarine basalts with less-radiogenic Pb and radiogenic Nd and Hf isotopic compositions show systematically lower concentrations in highly incompatible elements than the typical HIMU basalts. These geochemical variations are best explained by a two-component mixing process in which the depleted asthenospheric mantle was entrained by the mantle plume from the HIMU reservoir during its upwelling, and the melts from the HIMU reservoir and depleted asthenospheric mantle were then mixed in various proportions. The present and compiled data demonstrate that the HIMU reservoir has a uniquely low 176Hf/177Hf decoupled from 143Nd/144Nd, suggesting that it was derived from an ancient subducted slab. Moreover, the Nd/Hf ratios of the HIMU basalts and curvilinear Nd–Hf isotopic mixing trend require higher Nd/Hf ratios for the melt from the HIMU reservoir than that from the depleted mantle component. Such elevated Nd/Hf ratios could reflect source enrichment by a subducted slab during reservoir formation.  相似文献   

7.
《Lithos》2004,72(1-2):73-96
Petrological, trace element and Sr, Nd, Pb isotopic data are reported for volcanic rocks from the island of Filicudi, Aeolian Arc, Southern Tyrrhenian Sea. The volcano consists of several monogenic and polygenic centres built up through four major phases of explosive and effusive activity started before 1 Ma. Rock composition ranges from calc-alkaline basalts to high-K andesites. There is a negative correlation between silica and MgO, CaO, TiO2, FeOtotal, and a positive trend for K2O, Na2O and P2O5. LILE and HFSE increase with silica, whereas ferromagnesian trace elements have an opposite tendency. Incompatible elements, such as Zr, Ba, Rb, La, display well-defined positive correlations on elemental variation diagrams; weak correlations are shown by the other incompatible elements; Sr and compatible elements define negative, roughly curvilinear trends with incompatible elements. 87Sr/86Sr is poorly but significantly variable (0.704016–0.704740) and shows overall higher values in the mafic than in the sialic rocks. Nd isotope ratios range from 0.512670 to 0.512760 and are negatively correlated with 87Sr/86Sr. Pb isotope ratios cluster around 206Pb/204Pb=19.31–19.67, 207Pb/204Pb=15.64–15.69, 208Pb/204Pb=39.11–39.47.Major, trace element and isotopic variations reveal complex, multistage polybaric evolutionary processes for the Filicudi magmas. It is clear that crystal-liquid fractionation processes determined many of the petrologic and geochemical characteristics of these magmas. However, elemental variations when coupled with isotopic variations (in particular Sr isotopes) demonstrate that mixing processes and interaction of the magmas with older crustal material also played an important role.When compared with other Aeolian arc volcanoes, Filicudi shows petrological and geochemical characteristics similar to those of the nearby islands of Salina and Alicudi. The three islands consist of calc-alkaline rocks, but the degree of magma evolution increases going from the Alicudi to Salina. These variations are likely related to the plumbing system of the three volcanoes. However, trace element and isotopic evidence also suggests significant variations of primary magmas, which reveal a zoned source which suffered different types of metasomatism.  相似文献   

8.
Geochemical Evidence for Slab Melting in the Trans-Mexican Volcanic Belt   总被引：3，自引：0，他引：3  
Geochemical studies of Plio-Quaternary volcanic rocks from theValle de Bravo–Zitácuaro volcanic field (VBZ) incentral Mexico indicate that slab melting plays a key role inthe petrogenesis of the Trans-Mexican Volcanic Belt. Rocks fromthe VBZ are typical arc-related high-Mg andesites, but two differentrock suites with distinct trace element patterns and isotopiccompositions erupted concurrently in the area, with a traceelement character that is also distinct from that of other Mexicanvolcanoes. The geochemical differences between the VBZ suitescannot be explained by simple crystal fractionation and/or crustalassimilation of a common primitive magma, but can be reconciledby the participation of different proportions of melts derivedfrom the subducted basalt and sediments interacting with themantle wedge. Sr/Y and Sr/Pb ratios of the VBZ rocks correlateinversely with Pb and Sr isotopic compositions, indicating thatthe Sr and Pb budgets are strongly controlled by melt additionsfrom the subducted slab. In contrast, an inverse correlationbetween Pb(Th)/Nd and 143Nd/144Nd ratios, which extend to lowerisotopic values than those for Pacific mid-ocean ridge basalts,indicates the participation of an enriched mantle wedge thatis similar to the source of Mexican intraplate basalts. In addition,a systematic decrease in middle and heavy rare earth concentrationsand Nb/Ta ratios with increasing SiO2 contents in the VBZ rocksis best explained if these elements are mobilized to some extentin the subduction flux, and suggests that slab partial fusionoccurred under garnet amphibolite-facies conditions. KEY WORDS: arcs; mantle; Mexico; sediment melting; slab melting  相似文献   

9.
The Pb and Sr isotope ratios of Plio-Pleistocene volcanic rocks from the Aleutian volcanic arc are used as tracers of the lithospheric subduction process at the converging Pacific and Bering plates. Aleutian arc lavas do not have the same Pb isotopic compositions as volcanic rocks of the subducted Pacific ocean crust or the nearby Pribilof Islands, but appear to contain an ‘old continental crustal component’ with high 207Pb/204Pb ratio, as has been found in some other volcanic arcs.87Sr/86Sr ratios in the Aleutian volcanic arc rocks average 0.70322, slightly higher than fresh volcanic rocks from normal ridge segments, but within the range of values from ‘Icelandic’ ridge segments, oceanic islands and the Pribolof Islands. The Pb and Sr isotopic compositions of Aleutian lavas show a positive correlation and the range of values does not change for volcanoes distributed along strike in the arc, even though the crustal type in the hanging wall of the Benioff zone changes from oceanic in the west to continental in the east. Since the basement of the continental arc segment is older than the basement of the oceanic segment, and probably has a different isotopic character, the constancy of isotopic ratios along the arc argues against contamination by wall rocks of the type exposed in the arc.A sufficient explanation for the isotopic data is the mixture of several per cent of continent-derived sediment with melt derived from the underthrust oceanic crust and overlying mantle. This small amount of contaminant is difficult to document by geophysical observations. Such a model implies extensive recycling of Ba, Pb, K and Rb through volcanism at convergent plate margins like the Aleutians.  相似文献   

10.
Summary We present a detailed isotopic study of volcanic rocks emitted from Somma–Vesuvius volcano during three periods of interplinian activity: Protohistoric (3550 y B.P. to 79 A.D.), Ancient Historic (79 to 472 A.D.) and Medieval (472 to 1631 A.D.). Pb isotopic compositions of two acid leached fractions and whole rock residues of 37 whole rock samples (determined by Somma et al., 2001) show that each of the three interplinian periods is distinguished by small, systematic, and unique uranogenic and thorogenic Pb isotopic trends. This key and novel feature is compatible with the notion that the Pb isotopic data reflect small-scale source heterogeneity operating over relatively short periods of time. From this representative group of samples, a selected set of nine whole rocks were analysed for Th isotopes. 232Th/238U ratios in the source can be obtained independently from Pb and from Th isotopes. Those obtained from Pb isotopes represent source ratios, time-integrated over the whole age of the Earth; they range from 3.9 to 4.1. 232Th/238U obtained from Th isotopes are those of the present source. They are lower, and cluster around 3.5; this difference probably indicates recent U enrichment of the present source.The behaviour of Pb, as inferred by its isotopic ratios, is quite distinct from that of Sr and Nd isotopes: Pb isotope variations are not correlated to Sr or Nd isotope variations. The isotopic contrast is compatible with the idea that the isotopes were decoupled during magmatic production, evolution, and ascent through the crust. Thus, the Pb isotopes do not reflect the effects of the same processes as in the case of the Sr and Nd isotopes, or, as we also favor, they do not necessarily reflect the same source contributions into the magmas. Moreover, the Pb isotopic evolution of the interplinian rocks chiefly reflects mixing, driven by processes that are superimposed on, and independent of, other source contributions that determine the isotopic compositions of Sr and Nd. We suggest that reactions between magmas and fluids transported Pb and U, but not Sr. These data show that isotope mixing in the mantle is active at different times and scales.  相似文献   

11.
The Roccamonfina volcano is characterised by two stages of volcanic activity that are separated by volcano-tectonic caldera collapses. Ultrapotassic leucite-bearing rocks are confined to the pre-caldera stage and display geochemical characteristics similar to those of other volcanoes in the Roman Province. After the major sector collapse of the volcano, occurred at ca. 400 ka, shoshonitic rocks erupted from cinder cones and domes both within the caldera and on the external flanks of the pre-caldera Roccamonfina volcano. On the basis of new trace element and Sr–Nd–Pb isotope data, we show that the Roccamonfina shoshonitic rocks are distinct from shoshonites of the Northern Roman Province, but are very similar to those of the Neapolitan volcanoes. The last phases of volcanic activity erupted sub-alkaline magmas as enclaves in trachytic domes, and as lavas within the Monte Santa Croce dome. Ultrapotassic rocks of the pre-caldera composite volcano are plagioclase-bearing leucitites characterised by high levels of incompatible trace elements with an orogenic signature having troughs at Ba, Ta, Nb, and Ti, and peaks at Cs, K, Th, U, and Pb. Initial values of 87Sr/86Sr range from 0.70926 to 0.70999, 143Nd/144Nd ranges from 0.51213 to 0.51217, while the lead isotope rations vary between 18.788–18.851 for 206Pb/204Pb, 15.685–15.701 for 207Pb/204Pb, and 39.048–39.076 for 208Pb/204Pb. Shoshonites show a similar pattern of trace element depletions and enrichments to the earlier ultrapotassic leucite-bearing rocks but have a larger degree of differentiation and lower concentrations of incompatible trace elements. On the other hand, shoshonitic rocks have Sr, Nd, and Pb isotopes consistently different than pre-caldera ultrapotassic leucite-bearing rocks. 87Sr/86Sr ranges from 0.70665 to 0.70745, 143Nd/144Nd ranges from 0.51234 to 0.51238, 206Pb/204Pb ranges from 18.924 to 19.153, 207Pb/204Pb ranges from 15.661 to 15.694, and 208Pb/204Pb ranges from 39.084 to 39.212. High-K calc-alkaline samples have intermediate isotopic values between ultrapotassic plagioclase leucitites and shoshonites, but the lowest levels of incompatible trace element contents. It is argued that ultrapotassic magmas were generated in a modified lithospheric mantle after crustal-derived metasomatism. Interaction between the metasomatic agent and lithospheric upper mantle produced a low-melting point metasomatised veined network. The partial melting of the veins alone produced pre-caldera leucite-bearing ultrapotassic magmas. It was possibly triggered by either post-collisional isotherms relaxation or increasing T°C due increasing heat flow through slab tears. Shoshonitic magmas were generated by further melting, at higher temperature, of the same metasomatic assemblage with addition 10–20% of OIB-like astenospheric mantle material. We suggest that addition of astenospheric upper mantle material from foreland mantle, flowing through slab tearing after collision was achieved. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

12.
The concentrations and isotopic compositions of Sr, Nd, Pb, He and C have been determined for suites of xenoliths from Bullenmerri (Australia), Ichinomegata (Japan), Geronimo (Arizona), and East Africa. The wehrlites and pyroxenites from Bullenmerri have Sr, Nd and Pb isotopic compositions that are generally similar to those found for alkali basalts in the region. The spinel lherzolites, in contrast, have higher 87Sr/86Sr and 206Pb/204Pb and lower 143Nd/144Nd ratios. Whereas the isotopic compositions of He are generally within the range of mid-ocean-ridge basalts (MORB) and do not covary with those of other trace elements, there is an apparent correlation between the 13C/12C and 143Nd/144Nd ratios for each of the two petrologic groups. These relationships, if substantiated for other xenolith suites, greatly limit the possible mechanisms for generating lithophile and volatile isotopic variations in the continental lithosphere. The helium isotopic compositions for all of the xenoliths fall within the range for MORB. This includes those from Ichinomegata, suggesting that the lower 3He/4He ratios found for He sampled at the surface at subduction zones result from mixing mantle He with near-surface crustal He rather than with subducted radiogenic He. Measured C isotopic compositions (relative to Peedee belemnite) for the Ichinomegata xenoliths include values that are both lighter and heavier than those in MORB, and are compatible with contributions from subducted carbon. The Nd and Sr isotopic compositions of the Ichinomegata xenoliths exhibit a correlation over a substantially greater range of values than typically observed for other light-rare-earth-element (LREE)-depleted xenoliths, and include more radiogenic Sr and less radiogenic Nd compositions. The carbon isotopic compositions found for the East African and Geronimo xenoliths extend to values that are lighter than those typically found for MORB.  相似文献   

13.
Major and trace element, and Sr-Nd isotope compositions were determined for Quaternary volcanic rocks from NE Sulawesi (the Sangihe are), Indonesia, in order to examine the origin of across-arc variation in lava and magma source chemistry. The arc is formed in an intraoceanic tectonic setting and is not associated with a backarc basin, thereby minimizing possible contributions from non-arc geochemical reservoirs. The geochemistry of these arc lavas is likely to provide essential information about the chemical characteristics of subduction components. All incompatible elements, except Pb, increase away from the volcancic front. Major element data for Mg-rich lavas together with available experimental data, suggest that primary magmas are produced at higher pressured by smaller degrees of partial melting beneath the backarc-side volcanoes. Rb/K and Ba/Pb are higher, and 87Sr/86Sr and 143Nd/144Nd are lower in backarc-side lavas. These variations may be attributed to generation of hydrous fluids in the downdragged hydrous peridotite layer at the base of the mantle wedge through the following reactions: decompositions of pargasitic amphibole to form phlogopite and breakdown of phlogopite to crystallize K-richterite, beneath the volcanic front and the backarc-side volcanoes, respectively.  相似文献   

14.
The South arm of Sulawesi was an active continental margin from approximately 60 to 10 Ma, when it collided with the microcontinental fragment of Buton. Pre-collisional samples analyzed for this study are characterized by a geochemical signature typical of arc volcanics: high LILE/HFSE ratios; 87Sr/86Sr slightly higher than MORB; 143Nd/144Nd ratios similar to MORB. Syn-collisional samples have more enriched isotopic signatures, and are relatively potassium rich. This is interpreted to reflect a larger contribution from subducted sediments, added to the mantle wedge as a silicic melt. Melting of subducted sediments is interpreted to result from a decrease in subduction rate and an increase of temperature in the slab. Magmatism that postdates the collisional event by 10 Ma is characterized by higher Nb/Y ratios than the pre- or syn-collisional samples, and Sr and Nd isotopic signatures intermediate between these two groups. This is likely to reflect melting of a subduction-modified mantle, with a significant contribution from the sub-continental lithospheric mantle. Comparison with post-collisional magmatism from other areas of the world suggests that trace element signatures are similar, but isotopic characteristics are variable. The latter are likely to reflect both the age of the sub-continental lithospheric mantle and the time lag between cessation of subduction and formation of the post-collisional magmas.  相似文献   

15.
Late Cenozoic intraplate basaltic rocks in northeastern China have been interpreted as being derived from a mantle source composed of DMM and EM1 components. To constrain the origin of the enriched mantle component, we have now determined the geochemical compositions of basaltic rocks from the active Baekdusan volcano on the border of China and North Korea. The samples show LREE-enriched patterns, with positive Eu and negative Ce anomalies. On a trace element distribution diagram, they show typical oceanic island basalt (OIB)-like LILE enrichments without significant Nb or Ta depletions. However, compared with OIB, they show enrichments in Ba, Rb, K, Pb, Sr, and P. The Nb/U ratios are generally within the range of OIB, but the Ce/Pb ratios are lower than those of OIB. Olivine phenocrysts are characterized by low Ca and high Ni contents. The radiogenic isotopic characteristics (87Sr/86Sr = 0.70449 to 0.70554; εNd = −2.0 to +1.8; εHf = −1.7 to +6.1; 206Pb/204Pb = 17.26 to 18.12) suggest derivation from an EM1-like source together with an Indian MORB-like depleted mantle. The Mg isotopic compositions (δ26Mg = −0.39 ± 0.17‰) are generally lower than the average upper mantle, indicating carbonates in the source. The 87Sr/86Sr ratios decrease with decreasing δ26Mg values whereas the 143Nd/144Nd and (Nb/La)N ratios increase. These observations suggest the mantle source of the Baekdusan basalts contained at least two components that resided in the mantle transition zone (MTZ): (1) recycled subducted ancient (∼2.2–1.6 Ga) terrigenous silicate sediments, possessing EM1-like Sr–Nd–Pb–Hf isotopic signatures and relatively high values of δ26Mg; and (2) carbonated eclogites with relatively MORB-like radiogenic isotopic compositions and low values of δ26Mg. These components might have acted as metasomatizing agents in refertilizing the asthenosphere, eventually influencing the composition of the MTZ-derived plume that produced the Baekdusan volcanism.  相似文献   

16.
A total of 17 alkali basalts (alkali olivine basalt, limburgite, olivine nephelinite) and quartz tholeiites, and of 10 peridotite xenoliths (or their clinopyroxenes) were analyzed for Nd and Sr isotopes. 143Nd/144Nd ratios and 87Sr/86Sr ratios of all basalts and of the majority of ultramafic xenoliths plot below the mantle array with a large variation in Nd isotopes and a smaller variation in Sr isotopes. The tholeiites were less radiogenic in Nd than the alkali basalts. Volcanics from the Eifel and Massif Central regions contain Nd and Sr, which is more radiogenic than that of the basalts from the Hessian Depression. Nd and Sr isotopic compositions of all rocks from the latter area, with the exception of one tholeiite and one peridotite plot in the same field of isotope ratios as the Ronda ultramafic tectonite (SW Spain), which ranges in composition from garnet to plagioclase peridotite. The alkali basaltic rocks are products of smaller degrees of partial melting of depleted peridotite, which has undergone a larger metasomatic alteration compared with the source rock of tholeiitic magmas. For the peridotite xenoliths such metasomatic alteration is indicated by the correlation of their K contents and isotopic compositions. We assume that the upper mantle locally can acquire isotopic signatures low in radiogenic Nd and Sr from the introduction of delaminated crust. Such granulites low in radiogenic Nd and Sr are products of early REE fractionation and granite (Rb) separation.  相似文献   

17.
Subduction related basalts display wide ranges in large ion lithophile element ratios (e.g., Rb/Ba and Rb/ Sr) which are unlikely to result from mixing, but suggest a role for small degree partial melting of a relatively Rb-poor mantle wedge source. However, these variations do not correlate with other trace element criteria, such as the depletions of high field strength elements (HFSE) and light rare earth elements (LREE) relative to the LILE, which characterise subduction related magmatism. Integration of radiogenic isotope and trace element data demonstrates that the elemental enrichment cannot be simply related to two component mixtures inferred from isotopic variations. Thus a minimum of three components is required to describe the geochemistry of subduction zone basalts. Two are subduction related: high Sr/Nd material is derived from the dehydration of subducted basaltic ocean crust, and a low Sr/Nd component is thought to be from subducted terrigenous sediment. The third component is in the mantle wedge, it is usually similar to the source of MORB, particularly in its isotopic composition. However, in some cases, notably continental areas, more enriched mantle wedge material with relatively high 87Sr/86Sr, low 143Nd/144Nd and elevated incompatible trace element contents may be involved Mixing of these three components is capable of producing both the entire range of Sr, Nd and Pb isotope signatures observed in destructive margin basalts, and their distinctive trace element compositions. The isotope differences between Atlantic and Pacific island arc basalts are attributed to the isotope compositions of sediments in the two oceans.  相似文献   

18.
New geochemical and isotopic data for post-collisional Early Eocene and Late Miocene adakitic rocks from the eastern part of the Sakarya Zone, Turkey, indicate that slab and lower crustal melting, respectively, played key roles in the petrogenesis of these rocks. The Early Eocene Yoncal?k dacite (54.4 Ma) exhibits high Sr/Y and La/Yb ratios, low Y and HREE concentrations, moderate Mg# (44–65), and relatively high εNd and low ISr values, similar to adakites formed by slab melting associated with subduction. Geochemical composition of the Yoncal?k dacite cannot be explained by simple crystal fractionation and/or crustal contamination of andesitic parent magma, but is consistent with the participation of different proportions of melts derived from subducted basalt and sediments. Sr/Y correlates horizontally with Rb/Y, and Pb/Nd correlates vertically with Nd isotopic composition, indicating that Sr and Pb budgets are strongly controlled by melt addition from the subducting slab, whereas positive correlations between Th/Nd and Pb/Nd, and Rb/Y and Nb/Y point to some contribution of sediment melt. In addition to low concentrations of heavy rare earth elements (~2–3 times chondrite), a systematic decrease in their concentrations and Nb/Ta ratios with increasing SiO2 contents suggests that slab partial melting occurred in the garnet stability field and that these elements were mobilized by fluid flux. These geochemical and isotopic signatures are best explained by slab breakoff and fusion shortly after the initiation of collision. Although the Late Micone Tavda?? rhyolite (8.75 Ma) has some geochemical features identical to adakites, such as high Sr/Y and La/Yb ratios, low Y and HREE concentrations, other requirements, such as sodic andesite and/or dacite with relatively high MgO and Mg# (>50), relatively high Ni and Cr, low K2O/Na2O (<0.4), high Sr (>400 ppm), for slab-derived adakites are not provided. It is sodic in composition and shows no traces of fractionation from dacitic parent magma. Low Nd and high Sr isotope ratios suggest derivation by partial fusion of calc-alkaline, juvenile crust with high Sr/Y and La/Yb ratios.  相似文献   

19.
Cenozoic lamprophyres (minettes, spessartites, kersantite) from the Western Alps, northern Italy, represent small volume, mafic melts with high Mg#s and high Ni and Cr contents. All the lamprophyres show light REE enrichment, high incompatible element contents, and Ta, Ti and Nb troughs on chondrite-normalized diagrams. Age-corrected 87Sr/86Sr isotopic ratios (assuming t = 30 Ma) are highly variable and range from 0.70590 to 0.71884; 143Nd/144Nd ratios range from 0.51203 to 0.51242. Pb isotopic ratios are: 206Pb/204Pb = 18.669–18.895, 207Pb/204Pb = 15.605–15.689 and 208Pb/204Pb = 38.224–39.134. 87Sr/86Sr ratios show a negative correlation with 143Nd/144Nd, and a positive correlation with K, Ba, and Rb as well as with Ti, Th, Ta, Nb and Zr abundances. The primitive nature of the lamprophyres, coupled with their enriched incompatible trace element and isotopic signatures, suggest derivation from a metasomatized upper mantle source. Linear arrays in isotope space and elemental data plots suggest mixing between two distinct end-members in the Italian mantle; an enriched end-member that is isotopically similar to pelagic sediments, and a significantly less enriched end-member that approaches Bulk Earth values. New isotopic data indicate that the mantle source(s) of the lamprophyres from the Western Alps contain a very high proportion of the enriched end-member. The geochemical signature of the enriched end-member is attributed to fluids or melts derived from pelagic sediments subducted during the closure of the Tethyan Ocean in the late Cretaceous to early Tertiary.  相似文献   

20.
Major and trace element, and Sr, Nd and Pb isotopic compositionswere determined for whole-rock samples from the ‘isotopicallyanomalous’ Akagi volcano in the volcanic front of theNE Japan arc. Sr and Nd isotopic compositions of phenocrystswere also analyzed together with their major and trace elementcompositions. Compared with the other volcanoes from the volcanicfront, the whole-rock isotope compositions of Akagi show highlyenriched characteristics; 87Sr/86Sr = 0·7060–0·7088,  相似文献   

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