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1.
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   

2.
The first comprehensive chemical and Sr–Nd–Pb isotopic data set of Plio–Pleistocene tholeiitic and alkaline volcanic rocks cropping out in Sardinia (Italy) is presented here. These rocks are alkali basalts, hawaiites, basanites, tholeiitic basalts and basaltic andesites, and were divided into two groups with distinct isotopic compositions. The vast majority of lavas have relatively high 87Sr/86Sr (0.7043–0.7051), low 143Nd/144Nd (0.5124–0.5126), and are characterised by the least radiogenic Pb isotopic composition so far recorded in Italian (and European) Neogene-to-Recent mafic volcanic rocks (206Pb/204Pb=17.55–18.01) (unradiogenic Pb volcanic rocks, UPV); these rocks crop out in central and northern Sardinia. Lavas of more limited areal extent have chemical and Sr–Nd–Pb isotopic ratios indicative of a markedly different source (87Sr/86Sr=0.7031–0.7040; 143Nd/144Nd=0.5127–0.5129; 206Pb/204Pb=18.8–19.4) (radiogenic Pb volcanic rocks, RPV), and crop out only in the southern part of the island. The isotopic ratios of these latter rocks match the values found in the roughly coeval anorogenic (i.e. not related to recent subduction events in space and time) mafic volcanic rocks of Italy (i.e. Mt. Etna, Hyblean Mts., Pantelleria, Linosa), and Cenozoic European volcanic rocks. The mafic rocks of the two Sardinian rock groups also show distinct trace element contents and ratios (e.g. Ba/Nb>14, Ce/Pb=8–25 and Nb/U=29–38 for the UPV; Ba/Nb<9, Ce/Pb=24–28 and Nb/U=46–54 for the RPV). The sources of the UPV could have been stabilised in the Precambrian after low amounts of lower crustal input (about 3%), or later, during the Hercynian Orogeny, after input of Precambrian lower crust in the source region, whereas the sources of the RPV could be related to processes that occurred in the late Palaeozoic–early Mesozoic, possibly via recycling of proto-Tethys oceanic lithosphere by subduction.  相似文献   

3.
Ocean island basalt (OIB) suites display a wide diversity of major element, trace element, and isotopic compositions. The incompatible trace element and isotopic ratios of OIB reflect considerable heterogeneity in the mantle source regions. In addition to the distinctive Sr, Nd and Pb isotopic signatures of the HIMU, EMI and EMII OIB end-members, differences in incompatible trace element ratios among these end-members are of great help in identifying the nature and origin of their sources. Examination of trace element and isotopic constraints for the three OIB end-members suggests a relatively simple model for their origin. The dominant component in all OIB is ancient recycled basaltic oceanic crust which has been processed through a subduction zone, and which carries the trace element and isotopic signature of a dehydration residue (enrichment in HFSE relative to LILE and LREE, low Rb/Sr, but high U/Pb and Th/Pb ratios leading to the development of radiogenic Pb isotope compositions). HIMU OIB are derived from such a source, with little contamination from other components. Both the EMI and EMII OIB end-members are also dominantly derived from this source, but they contain significant proportions (up to 5–10%) of sedimentary components derived from the continental crust. In the case of EMI OIB, ancient pelagic sediment with high LILE/HFSE, LREE/HFSE, Ba/Th and Ba/La ratios, and low U/Pb ratios, is the contaminant. EMII OIB are also contaminated by a sedimentary component, in the form of ancient terrigenous sediment with high LILE/HFSE and LREE/HFSE ratios, but lacking relative Ba enrichment, and with higher U/Pb and Rb/Sr ratios. A model whereby the source for all OIB is ancient (1–2 Ga old) subducted oceanic crust ± entrained sediment (pelagic and/or terrigenous) is therefore consistent with the trace element and isotopic data. Although subducted oceanic lithosphere will accumulate and be dominantly concentrated within the mesosphere boundary layer, forming the source for hot-spots, such material may also become convectively dispersed within the depleted upper mantle as blobs or streaks, giving rise to small-scale chemical heterogeneities in the upper mantle.  相似文献   

4.
Helium isotope geochemistry of some volcanic rocks from Saint Helena   总被引：6，自引：0，他引：6  
3He/4He ratios have been measured for olivine and clinopyroxene phenocrysts in 7–15 m.y. old basaltic lavas from the island of St. Helena. Magmatic helium was effectively resolved from post-eruptive radiogenic helium by employing various extraction techniques, includingin vacuo crushing, and stepwise heating or fusion of the powders following crushing. The inherited3He/4He ratio at St. Helena is 4.3–5.9 RA. Helium isotope disequilibrium is present within the phenocrysts, with lower3He/4He upon heating and fusion of the powders following crushing, due to radiogenic ingrowth or to -particle implantation from the surrounding(U + Th)-rich lavas.

A single crushing analysis for clinopyroxene in a basalt from Tubuaii gave3He/4He= 7.1 RA.3He/4He ratios at St. Helena and Tubuaii (HIMU hotspots characterized by radiogenic Pb isotope signatures) are similar to3He/4He ratios previously measured at Tristan da Cunha and Gough Island (EM hotspots characterized by low206Pb/204Pb). Overall, the HeSrPb isotope systematics at these islands are consistent with a mantle origin as contiguous, heterogeneous materials, such as recycled crust and/or lithosphere.3He/4He ratios at HIMU hotspots are similar to mantle xenoliths which display nearly the entire range of Pb isotope compositions found at ocean islands, and are only slightly less than values found in mid-ocean ridge basalts (7–9 RA). This suggests that the recycled materials were injected into the mantle within the last 109 yrs.  相似文献   


5.
New U–Pb age-data from zircons separated from a Northland ophiolite gabbro yield a mean 206Pb/238U age of 31.6 ± 0.2 Ma, providing support for a recently determined 28.3 ± 0.2 Ma SHRIMP age of an associated plagiogranite and  29–26 Ma 40Ar/39Ar ages (n = 9) of basalts of the ophiolite. Elsewhere, Miocene arc-related calc-alkaline andesite dikes which intrude the ophiolitic rocks contain zircons which yield mean 206Pb/238U ages of 20.1 ± 0.2 and 19.8 ± 0.2 Ma. The ophiolite gabbro and the andesites both contain rare inherited zircons ranging from 122–104 Ma. The Early Cretaceous zircons in the arc andesites are interpreted as xenocrysts from the Mt. Camel basement terrane through which magmas of the Northland Miocene arc lavas erupted. The inherited zircons in the ophiolite gabbros suggest that a small fraction of this basement was introduced into the suboceanic mantle by subduction and mixed with mantle melts during ophiolite formation.

We postulate that the tholeiitic suite of the ophiolite represents the crustal segment of SSZ lithosphere (SSZL) generated in the southern South Fiji Basin (SFB) at a northeast-dipping subduction zone that was initiated at about 35 Ma. The subduction zone nucleated along a pre-existing transform boundary separating circa 45–20 Ma oceanic lithosphere to the north and west of the Northland Peninsula from nascent back arc basin lithosphere of the SFB. Construction of the SSZL propagated southward along the transform boundary as the SFB continued to unzip to the southeast. After subduction of a large portion of oceanic lithosphere by about 26 Ma and collision of the SSZL with New Zealand, compression between the Australian Plate and the Pacific Plate was taken up along a new southwest-dipping subduction zone behind the SSZL. Renewed volcanism began in the oceanic forearc at 25 Ma producing boninitic-like, SSZ and within-plate alkalic and calc-alkaline rocks. Rocks of these types temporally overlap ophiolite emplacement and subsequent Miocene continental arc construction.  相似文献   


6.
Os isotope systematics in ocean island basalts   总被引：5，自引：0，他引：5  
New ReOs isotopic results for Os-poor basalts from St. Helena, the Comores, Samoa, Pitcairn and Kerguelen dramatically expand the known range of initial 186Os/187Os ratios in OIBs to values as high as 1.7. In contrast to the Os isotopic uniformity of Os-rich basalts from the HIMU islands of Tubuai and Mangaia found by Hauri and Hart [1], our values for St. Helena span most of the known range of Os isotopic variability in oceanic basalts (initial 187Os/186Os ranges from 1.2 to 1.7). Generation of such radiogenic Os in the mantle requires melting of source materials that contain large proportions of recycled oceanic crust. The very low Os concentrations of most of the basalts analyzed here, however, leave them susceptible to modification via interaction with materials containing radiogenic Os in the near-surface environment. Thus the high 186Os/187Os ratios may result from assimilation of radiogenic Os-rich marine sediments, such as Mn oxides, within the volcanic piles traversed by these magmas en route to the surface. Furthermore, the Os isotopic signatures of Os-rich, olivine-laden OIBs may reflect the accumulation of lithospheric olivine, rather than simply their mantle source characteristics. The extent to which these processes alter the view of the mantle obtained via study of ReOs systematics in oceanic basalts is uncertain. These effects must be quantified before ReOs systematics in OIBs can be used with confidence to investigate the nature of mantle heterogeneity and its causes.  相似文献   

7.
Radiogenic isotope variations in lavas from the Cook–Austral volcanic chain have delineated three distinct mantle sources: a HIMU component, a depleted component (DM), and an enriched component (EM). To better constrain the mantle sources for South Pacific hot spot volcanism, we determined lithium isotopic compositions of lavas from Raivavae, Rapa, Mangaia and Tubuai of the volcanic chain. The study includes whole rock and mineral analyses. In general, δ7Li of most olivines resemble bulk rock composition whereas clinopyroxenes are variably lighter. This implies that clinopyroxene is more susceptible to diffusion-induced fractionation, in agreement with previous studies. Olivine δ7Li values span a narrower range than whole rock values, and do not extend to the very heavy compositions previously reported in HIMU bulk lavas. This discrepancy likely reflects alteration of bulk lavas, and suggests that Li-isotope analyses of bulk lavas should be interpreted with caution. Olivines from HIMU lavas have heavy δ7Li values (up to 6.2‰), and extend beyond the range reported for fresh MORB. Correlations between Li-isotopes and radiogenic isotopes suggest that the heavy δ7Li values in HIMU olivines are a source characteristic and not the result of post-magmatic alteration. Although the Li-isotope composition of recycled, dehydrated oceanic crust is currently under debate, our results suggest that HIMU lavas derive from a source containing recycled dehydrated oceanic crust, and that the “heavy” Li-isotope signature of altered oceanic crust is partially preserved during passage through the subduction factory.  相似文献   

8.
Geochemical and isotopic analyses (Sr–Nd–Pb) of late Miocene to Quaternary plateau lavas from the Pali Aike and Morro Chico areas (52°S) were undertaken to constrain the melting processes and mantle sources that contributed to magma generation and the geodynamic evolution of southernmost Patagonia, South America. The Pali Aike and Morro Chico lavas are alkaline (Pali Aike, 45–49 wt.% SiO2; 4.3–5.9 wt.% Na2O+K2O) and subalkaline (Morro Chico, 50.5–50.8 wt.% SiO2; 4.0–4.4 wt.% Na2O+K2O), relatively primitive (Pali Aike, 9.5–13.7 wt.% MgO; Morro Chico, 7.6–8.8 wt.% MgO) mafic volcanic rocks that have typical intraplate ocean island basalt‐like signatures. Incompatible trace element ratios and isotopic ratios of the Pali Aike and Morro Chico lavas differ from those of the majority of Neogene southern Patagonian slab window lavas in showing more enriched characteristics and are similar to high‐μ (HIMU)‐like basalts. The rare earth element (REE) modeling to constrain mantle melting percentages suggests that these lavas were produced by low degrees of partial melting (1.0–2.0% for Pali Aike lavas and about 2.6–2.7% for Morro Chico lavas) of a garnet lherzolite mantle source. The major systematic variations of Sr–Nd–Pb isotopes in southern Patagonian lavas are related to geographic location. The Pali Aike and Morro Chico lavas from the southernmost part of Patagonia have lower 87Sr/86Sr and higher 143Nd/144Nd and 206Pb/204Pb ratios, relative to most of the southern Patagonian lavas erupted north of 49.5°S, pointing to a HIMU‐like signature. An isotopically depleted and HIMU‐like asthenospheric domain may have been the main source of magmas in the southernmost part of Patagonia (e.g. Pali Aike, Morro Chico, and Camusu Aike volcanic field), suggesting the presence of a major discontinuity in the isotopic composition of the asthenosphere in southern Patagonia. On the basis of geochemical and isotope data and the available geological and geotectonic reconstructions, a link between the HIMU asthenospheric mantle domain beneath southernmost Patagonia and the HIMU mega‐province of the southwestern Pacific Ocean is proposed.  相似文献   

9.
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   

10.
himu, em i andem ii are three of the main geochemical mantle components that give rise to oceanic island basalts [1]. They represent the end members that produce the extreme isotopic compositions measured on intraplate volcanics. In French Polynesia, all three mantle components are represented in volcanic rocks. The characteristichimu signature is found in Tubuai, Mangaia and Rurutu,em i is present in the source of Rarotonga and Pitcairn volcanics andem ii dominates the composition of most Society Islands. Intermediate values between the three end members are found on most islands.We suggest that the three components are not independent but are physically related in the mantle. Thehimu component is thought to be recycled oceanic crust that lost part of its Pb through hydrothermal processes prior to and during subduction.em i andem ii are believed to acquire their isotopic and trace element characteristics through entrainment of sediments that were subducted together with the oceanic crust.The trace element pattern and the isotopic composition ofhimu lavas can be quantitatively modelled using a mixture of 25% old recycledmorb crust and 75% mantle peridotite. The extreme Pb composition is modelled assuming that Pb was lost from oceanic crust when hydrothermal alteration at the ridge leached Pb from the basalt to redeposit it as sulphides on top of and throughout the crust, followed by preferential dissolution of sulphides during dehydration in the subduction zone. These processes led to a drastic increase of theU/Pb ratio of the subducted material which evolved over 2 Ga to very radiogenic Pb isotopic compositions. Pb isotopic compositions similar to those ofem i andem ii are modelled assuming that sediments with average crustal Pb isotopic compositions were subducted and recycled into the mantle together with the underlyingmorb oceanic crust. Pelagic sediments (μ 5 andκ 6) account for the Pb isotopic composition ofem i whereas terrigenous sediments (μ 10 andκ 4.5) evolve towards theem ii end member. A few percent of sediment in the recycled crust-sediment mixture will destroy the characteristic Pb isotopic signature of thehimu component. This, together with the low probability of isolating oceanic crust in the mantle for 2 Ga, explains why the extremehimu composition, as seen on Tubuai and St Helena, is sampled so rarely by oceanic volcanism.  相似文献   

11.
New oxygen isotope data are presented for submarine lavas erupted close to the transition between the oceanic Kermadec island arc and the continental Taupo Volcanic Zone, New Zealand. Volcanic glasses display δ18O values ranging from +5.65‰ to +5.83‰, clinopyroxenes range from +5.23‰ to +5.78‰ and olivines range from +4.83‰ to +5.47‰. Coexisting glass and phenocrysts in the lavas are in isotopic equilibrium, with one exception. Oxygen isotope ratios of back-arc lavas erupted through oceanic crust are indistinguishable from mid-ocean ridge basalts or lavas erupted in nearby back-arc settings. Although lavas from the arc front display elevated oxygen isotope ratios, the magnitude of 18O-enrichment is too great to result from recycling of subducted material alone. A single back-arc lava erupted through continental crust is also relatively 18O-rich suggesting that the most likely origin for the high δ18O signature is limited amounts of interaction between continental crust and melts derived from a mantle wedge that has been variably fluxed by recycled oxygen. The results of modelling open system behaviour in this volcanic system highlight the need for strong controls on the composition of local contaminants. Application of ‘average' crustal lithologies, as in other volcanic provinces, may lead to erroneous conclusions regarding the involvement of local basement.  相似文献   

12.
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   

13.
We report new Nd, Hf, Sr, and high-precision Pb isotopic data for 44 lava and tephra samples from Erebus volcano. The samples cover the entire compositional range from basanite to phonolite and trachyte, and represent all three phases of the volcanic evolution from 1.3 Ma to the present. Isotopic analyses of 7 samples from Mt. Morning and the Dry Valley Drilling Project (DVDP) are given for comparison. The Erebus volcano samples have radiogenic 206Pb/204Pb, unradiogenic 87Sr/86Sr, and intermediate 143Nd/144Nd and 176Hf/177Hf, and lie along a mixing trajectory between the two end-member mantle components DMM and HIMU. The Erebus time series data show a marked distinction between the early-phase basanites and phonotephrites, whose Nd, Hf, Sr, and Pb isotope compositions are variable (particularly Pb), and the current ‘phase-three’ evolved phonolitic lavas and bombs, whose Nd, Hf, Sr, and Pb isotope compositions are essentially invariant. Magma mixing is inferred to play a fundamental role in establishing the isotopic and compositional uniformity in the evolved phase-three phonolites. In-situ analyses of Pb isotopes in melt inclusions hosted in an anorthoclase crystal from a 1984 Erebus phonolite bomb and in an olivine from a DVDP basanite are uniform and identical to the host lavas within analytical uncertainties. We suggest that, in both cases, the magma was well mixed at the time melt inclusions were incorporated into the different mineral phases.  相似文献   

14.
Osmium, strontium, neodymium, and lead isotopic data have been obtained for 30 hand picked samples of basaltic glass from the Pacific, Atlantic and Indian mid-oceanic ridges. Large variations in Os isotopic ratios exist in the glasses, from abyssal peridotite-like values to radiogenic compositions similar to oceanic island basalts (187Os/186Os and 187Os/188Os ratios range from 1.06 to 1.36 and from 0.128 to 0.163, respectively). Os isotopic and elemental data suggest the existence of mixing correlations. This relationship might be ascribed to secondary contamination processes; however, such a hypothesis cannot account for the negative correlation observed between Os and Nd isotopes and the existence of complementary covariations between Os and SrPb isotopes. In this case, OsSrNdPb isotopic variations are unrelated to late post-eruption or shallow level contamination. These relationships provide strong evidence that the Os isotopic composition of the samples are derived from the mantle and thus implies a global chemical heterogeneity of the oceanic upper mantle. The results are consistent with the presence of recycled oceanic crust in the mantle sources of mid-ocean ridge basalts, and indicate that the unique composition of the upper mantle below the Indian ocean results from its contamination by a mantle component characterized by radiogenic Os and particularly unradiogenic Nd and Pb isotopic compositions.  相似文献   

15.
Middle Miocene to Quaternary lavas on Kunashir Island in the southern zone of the Kurile Arc were examined for major, trace, and Sr–Nd–Pb isotope compositions. The lavas range from basalt through to rhyolite and the mafic lavas show typical oceanic island arc signatures without significant crustal or sub-continental lithosphere contamination. The lavas exhibit across-arc variation, with increasingly greater fluid-immobile incompatible element contents from the volcanic front to the rear-arc; this pattern, however, does not apply to some other incompatible elements such as B, Sb, and halogens. All Sr–Nd–Pb isotope compositions reflect a depleted source with Indian Ocean mantle domain characteristics. The Nd and Pb isotope ratios are radiogenic in the volcanic front, whereas Sr isotope ratios are less radiogenic. These Nd isotope ratios covary with incompatible element ratios such as Th/Nd and Nb/Zr, indicating involvement of a slab-derived sediment component by addition of melt or supercritical fluid capable of mobilizing these high field-strength elements and rare earth elements from the slab. Fluid mobile elements, such as Ba, are also elevated in all basalt suites, suggesting involvement of slab fluid derived from altered oceanic crust. The Kurile Arc lavas are thus affected both by slab sediment and altered basaltic crust components. This magma plumbing system has been continuously active from the Middle Miocene to the present.  相似文献   

16.
The Lesser Antilles arc is a particularly interesting island arc because it is presently very active, it is located perpendicular to the South American continent and its chemical and isotopic compositions display a strong north–south gradient. While the presence in the south of a thick pile of sedimentary material coming from the old South American continent has long been suspected to explain the geochemical gradient, previous studies failed to demonstrate unambiguously a direct link between the arc lava compositions and the subducted sediment compositions.Here, we present new Nd, Sm, Th, U and Pb concentrations and Nd–Pb isotopic data for over 60 sediments from three sites located in the fore arc region of the Lesser Antilles arc. New data for DSDP Site 543 drill core located east of Dominica Island complement the data published by White et al. [White, W.M., Dupré, B. and Vidal, P., 1985. Isotope and trace element geochemistry of sediments from the Barbados Ridge–Demerara Plain region, Atlantic Ocean. Geochimica et Cosmochimica Acta, 49: 1875–1886.] and confirm their relatively uniform isotopic compositions (i.e., 206Pb/204Pb between 19.13 and 19.53). In contrast, data obtained on DSDP Site 144 located further south, on the edge of the South American Rise and on sediments from Barbados Island are much more variable (206Pb/204Pb ranges from 18.81 to 27.69). The very radiogenic Pb isotopic compositions are found in a 60 m thick black shale unit, which has no age equivalent in the Site 543 drill core. We interpret the peculiar composition of the southern sediments as being due to two factors, (a) the proximity of the South American craton, which contributes coarse grain old detrital material that does not travel far from the continental shelf, and (b) the presence of older sediments including the thick black shale unit formed during Oceanic Anoxic events 2 and 3.The north–south isotopic change known along the Lesser Antilles arc can be explained by the observed geographical changes in the composition of the subducted sediments. About 1% contamination of the mantle wedge by Site 543 sediments explains the composition of the northern islands while up to 10% sediments like those of Site 144 is required in the source of the southern island lavas. The presence of black shales in the subducted pile provides a satisfactory explanation for the very low Δ8/4 values that characterize the Lesser Antilles arc.  相似文献   

17.
High 4He/3He ratios of 100 000 to 160 000 found at HIMU ocean islands (“high μ,” where μ is the U/Pb ratio) are usually attributed to the presence of recycled oceanic crust in the HIMU mantle source. However, significantly higher 4He/3He ratios are expected in recycled crust after residence in the mantle for periods greater than 1 Ga. In order to better understand the helium isotopic signatures in HIMU basalts, we have measured helium and neon isotopic compositions in a suite of geochemically well-characterized basalts from the Cook–Austral Islands. We observe 4He/3He ratios ranging from 56 000 to 141 000, suggesting the involvement of mantle reservoirs both more and less radiogenic than the mantle source for mid-ocean ridge basalts (MORBs). In addition, we find that the neon isotopic compositions of HIMU lavas extend from the MORB range to compositions less nucleogenic than MORBs. The Cook-Austral HIMU He–Ne isotopic compositions, along with Sr, Nd, Pb, and Os isotopic compositions, indicate that in addition to recycled crust, a relatively undegassed mantle end-member (e.g., FOZO) is involved in the genesis of these basalts. The association of relatively undegassed mantle material with recycled crust provides an explanation for the close geographical association between HIMU lavas and EM (enriched mantle)-type lavas from this island chain: EM-type signatures represent a higher mixing proportion of relatively undegassed mantle material. Mixing between recycled material and relatively undegassed mantle material may be a natural result of entrainment processes and convective stirring in deep mantle.  相似文献   

18.
The Pb isotopic compositions and U and Pb concentrations of the lava series (alkali basalt to comendite) and of their plutonic xenoliths (gabbro to alkaline granite) of Ascension Island are reported. The data are used to evaluate the source of the xenoliths which formed two differentiation suites: the acidic and intermediate xenoliths together with most of the lavas on the one hand, and the gabbroic xenoliths and a basaltic tuff on the other hand. The Pb isotopic compositions imply a mantle origin for the source magmas of the xenoliths and confirm the possibility of generating granitic rocks in an oceanic environment by fractional crystallization of a mantle-derived magma whose geochemical and isotopic characteristics are comparable to the source magmas of oceanic island basalts.  相似文献   

19.
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   

20.
Igneous rocks from the Philippine tectonic plate recovered on Deep Sea Drilling Project Legs 31, 58 and 59 have been analyzed for Sr, Nd and Pb isotope ratios. Samples include rocks from the West Philippine Basin, Daito Basin and Benham Rise (40–60 m.y.), the Palau-Kyushu Ridge (29–44 m.y.) and the Parece Vela and Shikoku basins (17–30 m.y.). Samples from the West Philippine, Parece Vela and Shikoku basins are MORB (mid-ocean ridge basalt)-like with 87Sr/86Sr= 0.7026−0.7032, 143Nd/144Nd= 0.51300−0.51315, and 206Pb/204Pb= 17.8−18.1. Samples from the Daito Basin and Benham Rise are OIB (oceanic island basalt)-like with 87Sr/86Sr= 0.7038−0.7040, 143Nd/144Nd= 0.51285−0.51291 and 206Pb/204Pb= 18.8−19.2. All of these rocks have elevated 207Pb/204Pb and 208Pb/204Pb compared to the Northern Hemisphere Regression Line (NHRL) and have δ207Pb values of 0 to +6 and δ208Pb values of +32 to +65. Lavas from the Palau-Kyushu Ridge, a remnant island arc, have 87Sr/86Sr= 7032−0.7035, 143Nd/144Nd= 0.51308−0.51310 and 206Pb/204Pb= 18.4−18.5. Unlike the basin magmas erupted before and after them, these lavas plot along the NHRL and have Pb-isotope ratios similar to modern Pacific plate MORB's. This characteristic is shared by other Palau-Kyushu Arc volcanic rocks that have been sampled from submerged and subaerial portions of the Mariana fore-arc.At least four geochemically distinct magma sources are required for these Philippine plate magmas. The basin magmas tap Source 1, a MORB-mantle source that was contaminated by EMI (enriched mantle component 1 [31]) and Source 2, an OIB-like mantle source with some characteristics of EMII (enriched mantle component 2 [31]). The arc lavas are derived from Source 3, a MORB-source or residue mantle including Sr and Pb from the subducted oceanic crust, and Source 4, MORB-source or residue mantle including a component with characteristics of HIMU (mantle component with high U/Pb [31]). These same sources can account for many of the isotopic characteristics of recent Philippine plate arc and basin lavas. The enriched components in these sources which are associated with the DUPAL anomaly were probably introduced into the asthenosphere from the deep mantle when the Philippine plate was located in the Southern Hemisphere 60 m.y.b.p.  相似文献   

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