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 15 毫秒
1.
Examples of positive correlations between initial 87Sr/86Sr and δ18O have now been shown to be very common in igneous rock series. These data in general require some type of mixing of mantle-derived igneous rocks with high-18O, high-87Sr crustal metamorphic rocks that once resided on or near the Earth's surface, such as sedimentary rocks or hydrothermally altered volcanic rocks. Mixing that involves assimilation of country rocks by magmas, however, is not a simple two-end-member process; heat balance requires appreciable crystallization of cumulates. In such cases, the isotopic compositions may strongly reflect this open-system behavior and indicate the process of assimilation, whereas the major element chemical compositions of the contaminated magmas will be largely controlled by crystal-melt equilibria and crystallization paths fixed by multicomponent cotectics. A variety of oxygen and strontium isotope “mixing” curves were therefore calculated for this process of combined assimilation-fractional crystallization. The positions and characteristics of the resultant curves on δ18O-87Sr/86Sr diagrams markedly diverge from simple two end-member mixing relationships. Based on the above, model calculations can be crudely fitted to two igneous rock suites (Adamello and Roccamonfina in Italy), but the shapes of the calculated curves appear to rule out magmatic assimilation as an explanation for most δ18O-87Sr/86Sr correlations discovered so far, including all of those involving calc-alkaline granitic batholiths and andesitic volcanic rocks. The isotopic relationships in such magma types must be inherited from their source regions, presumably reflecting patterns that existed in the parent rocks (or magmas) prior to or during melting.  相似文献   

2.
87Sr/86Sr ratios of 15 samples of basalt dredged from Loihi Seamount range from 0.70334 to 0.70368. The basalt types range from tholeiite to basanite in composition and can be divided into six groups on the basis of abundances of K2O, Na2O, Rb and Sr and 87Sr/86Sr ratio. The isotopic data require that the various basalt types be derived from source regions differing in Sr isotopic composition. The Loihi basalts may be produced by mixing of isotopically distinct sources, but the tholeiites and alkalic basalts from Loihi do not show a well-developed inverse trend between Rb/Sr and 87Sr/86Sr that is characteristic of the later stages of Hawaiian volcanoes such as Haleakala and Koolau.  相似文献   

3.
143Nd/144Nd,87Sr/86Sr and trace element results are reported for volcanic and plutonic rocks of the Aleutian island arc. The Nd and Sr isotopic compositions plot within the mantle array with εNd values of from 6.5 to 9.1 and87Sr/86Sr ratios of from 0.70289 to 0.70342. Basalts have mildly enriched light REE abundances but essentially unfractionated heavy REE abundances, while andesites exhibit a greater degree of light to heavy REE fractionation. Both the basalts and andesites have significant large ion lithophile element to light rare earth element (LILE/LREE) enrichments. Variations in the isotopic compositions of Nd and Sr are not related to the spatial distribution of volcanoes in the arc, nor are they related to temporal differences. εNd and87Sr/86Sr do not correlate with major element compositions but do, however, correlate with certain LILE/LREE ratios (e.g. BaN/LaN). Plutonic rocks have isotropic and trace element characteristics identical to some of the volcanic rocks. Rocks that make up the tholeiitic, calc-alkaline and alkaline series in the Aleutians do not come from isotopically distinct sources, but do exhibit some differing LILE characteristics.Given these elemental and isotopic constraints it is shown that the Aleutian arc magmas could not have been derived directly from homogeneous MORB-type mantle, or fresh or altered MORB subducted beneath the arc. Mixtures of partially altered MORB with deep-sea sediment can in principle account for the isotopic characteristics and most of the observed LILE/LREE enrichments. However, some samples have exceedingly high LILE/LREE enrichments which cannot be accounted for by sediment contamination alone. For these samples a more complex scenario is considered whereby dehydration and partial melting of the subducted slab, containing less than 8% sediment, produces a LILE-enriched (relative to REE) metasomatic fluid which interacts with the overlying depleted mantle wedge. The isotopic and LILE characteristics of the mantle are extremely sensitive to metasomatism by small percentages of added fluid, whereas major elements are not substantially effected, Major element compositions of Aleutian magmas are dominantly controlled by the partial melting of this mantle and subsequent crystal fractionation; whereas isotopic and LILE characteristics are determined by localized mantle heterogeneities.  相似文献   

4.
Seventeen whole-rock samples, generally taken at 25–50 m intervals from 5 to 560 m sub-basement in Hole 504B, drilled in 6.2 m.y. old crust, were analysed for87Sr/86Sr ratios, Sr and Rb concentrations, and18O/16O ratios. Sr isotope ratios for 8 samples from the upper 260 m of the hole range from 0.70287 to 0.70377, with a mean of 0.70320. In the 330–560 m interval, 5 samples have a restricted range of 0.70255–0.70279, with a mean of 0.70266, the average value for fresh mid-ocean ridge basalts (MORB). In the 260–330 m interval, approximately intermediate Sr isotopic ratios are found.δ18O values (‰) range from 6.4 to 7.8 in the upper 260 m, 6.2–6.4 in the 270–320 m interval, and 5.8–6.2 in the 320–560 m interval. The values in the upper 260 m are typical for basalts which have undergone low-temperature seawater alteration, whereas the values for the 320–560 m interval correspond to MORB which have experienced essentially no oxygen isotopic alteration.The higher87Sr/86Sr and18O/16O ratios in the upper part of the hole can be interpreted as the result of a greater overall water/rock ratio in the upper part of the Hole 504B crust than in the lower part. Interaction of basalt with seawater(87Sr/86Sr=0.7091) increased basalt87Sr/86Sr ratios and produced smectitic alteration products which raised whole-rock δ18O values. Seawater circulation in the lower basalts may have been partly restricted by the greater number of relatively impermeable massive lava flows below about 230 m sub-basement. These flows may have helped to seal off lower basalts from through-flowing seawater.  相似文献   

5.
Oceanic87Sr/86Sr ratios during Jurassic to Pleistocene have been determined by analysing fresh waters from marine limestone and dolomite aquifers. The results are in good agreement with published data from well preserved fossil material. The87Sr/86Sr ratios obtained are 0.7070 for Lower to Middle Jurassic, 0.7075 for Late Cretaceous, 0.7080 for Lower to Middle Eocene and 0.7087 for Pleistocene aquifer waters. The value of87Sr/86Sr for the Eimer and Amend isotopic standard was 0.7082.  相似文献   

6.
Oxygen and strontium isotope ratios have been used to characterize source regions for granitic magmas for a transect across the northern Appalachian orogen in central and eastern Maine. The northwestern plutons (Katahdin and Seboeis) have δ18O values of 10.3–13.3 and initial 87Sr/86Sr ratios of 0.7083 and 0.7066, respectively. The central plutons (Bottle Lake and Center Pond) have lower δ18O values (8.2–9.9) and initial 87Sr/86Sr ratios (0.7043–0.7055). The southeastern plutons (Lucerne and Deblois) have δ18O values (9.0–11.0) but initial 87Sr/86Sr ratios (0.7077 and 0.7041, respectively) which are intermediate between the northwestern and central plutons.Source models derived from these results and other petrological and geochemical data reflect the juxtaposition of discrete source regions by transcurrent faulting, which may be related to oblique plate motions. This model illustrates the importance of microplate accretion in the Palaeozoic history of the northern Appalachian orogen.  相似文献   

7.
The hypothesis that seawater was the source of the hydrothermal fluid which formed the Upper Cretaceous ophiolitic cupriferous pyrite ore desposits of the Troodos Massif (Cyprus) has been tested by analysing the strontium isotopic composition of thirteen mineralized samples from four mines. Initial87Sr/86Sr ratios range from 0.7052 ± 0.0001 to 0.7075 ± 0.0002, the latter value being indistinguishable from that of Upper Cretaceous seawater at 0.7076 ± 0.0006 (2σ). Hence, the mineralized metabasalt samples have been contaminated with87Sr, relative to initial magmatic strontium isotope ratios of the Troodos ophiolitic complex (0.70338 ± 0.00010 to 0.70365 ± 0.00005).Since seawater was the only source of strontium available during formation of the Troodos Complex which was isotopically relatively enriched in87Sr, the data confirm that seawater was the source of the hydrothermal oreforming fluid.  相似文献   

8.
87Sr/86Sr ratios of three hydrothermal waters collected on the East Pacific Rise at 21°N define a mixing line between seawater and a hydrothermal end-member at 0.7030 which is derived by seawater-basalt interaction at ca. 350°C and water/rock ratio of about 1.5. Sr concentrations are not affected in the process while Mg uptake from seawater is almost complete. Up to2/3 of this hydrothermal component is involved in anhydrite precipitation while the Sr isotopic ratio in sulfides (chalcopyrite + sphalerite) cannot be distinguished from that of sulfate. It is estimated that ca. 1 × 1010 moles of strontium are yearly cycled in the hydrothermal systems of mid-oceanic ridges, thereby affecting the87Sr/86Sr budget of seawater. Mass balance between river runoff, limestone precipitation and ridge basalt alteration suggests that the87Sr/86Sr ratios of the river runoff are in the range 0.7097–0.7113, and are largely dominated by limestone alteration.  相似文献   

9.
Oxygen isotope data are reported for 27 igneous rocks of Mesozoic to Quaternary age from the Central Andes. 26–29°S. The plutonic rocks, and most of the volcanics, have δ18O values between 6.2 and 8.3‰.The whole-rock δ18O values show a weak correlation with initial87Sr/86Sr data. This O-Sr array differs from documented trends for calc-alkaline plutonic suites from California, Scotland and northern Italy, but overlaps with data for volcanic and plutonic rocks from Ecuador, northern Chile and southern Perú.The oxygen isotope results indicate that the magmas evolved without significant contamination from supracrustal rocks (e.g., rocks that experienced18O enrichment during surficial weathering). The available O, Sr and Pb isotopic data for these rocks are best explained by magma generation in the upper mantle or lower crust. From the Late Mesozoic on, the87Sr/86Sr values were modified at depth by isotopic exchange between the magma and a continually thickening crust of plutonic rocks of Late Precambrian to early Mesozoic age.  相似文献   

10.
Volcanic rocks of the Sunda and Banda arcs range from tholeiitic through calcalkaline and shoshonitic to leucititic, the widest compositional span of mafic magmatism known from an active arc setting.Mafic rocks in our data set, which includes 315 new analyses of volcanic rocks from twelve Quaternary volcanoes, including Batu Tara in the previously geochemically unknown Flores-Lembata arc sector, are generally similar to those from other island arcs: most contain <1.3 wt. % TiO2 and 16–22 wt. % Al2O3, and have characteristically high K/Nb and La/Nb values. Abundances of P, Ba, Rb, Sr, La, Ce, Nd, Zr and Nb increase sympathetically with increasing K2O contents of mafic rocks but those of Na, Ti, Y and Sc vary little throughout the geochemical continuum from low-K tholeiitic to high-K leucititic rocks.Excluding Sumatra and Wetar, which possess mainly dacitic and rhyolitic volcanics, the Sunda-Banda arc is divisible into four geochemical arc sectors with boundaries that correlate with major changes in regional tectonic setting and geological history. From west to east, the West Java, Bali and Flores arc sectors each comprise volcanoes which become progressively more K-rich eastwards, culminating in the leucitite volcanoes Muriah, Soromundi and Sangenges, and Batu Tara, respectively. In the most easterly Banda sector, the volcanics vary from high- to low-K eastwards around the arc.Correlations between geochemistry and 87Sr/86Sr values show separate trends for each of the four arc sectors, believed to be the result of involvement of at least three geochemically and isotopically distinct components in the source regions of the arc magmatism.A dominant source component with a low K content and a low 87Sr/86Sr value, and common to all sectors, is probably peridotitic mantle. A second component, with low K content but high 87Sr/86Sr value, appears to be crustal material. This component is most apparent in the Banda sector, in keeping with that sector's tectonic setting close to Precambrian Australian continental crust, but it is also present to lesser extents in the West Java and Flores sectors.However, the most marked geochemical and isotopic variations shown by the arc volcanics are primarily due to the involvement of a third component, which is rich in K-group elements but has relatively low 87Sr/86Sr values. This component appears to be mantle-derived and is least overprinted by crustal material in the Bali sector volcanics where the Pb, Be, U-Th and O isotope characteristics of the rocks support the suggestion that their genesis has not involved incorporation of recently subducted, continent-derived sialic material.The high, regionally persistent, Th/U value (about 4.3) of the Sunda subarc mantle, obtained from U-Th isotopic data, suggests a close association could exist between the K-rich component and the southern hemisphere ‘DUPAL’ mantle isotopic anomaly.  相似文献   

11.
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   

12.
To investigate the source, flow paths, and chemistry of rich resources of high‐quality, shallow groundwater in the alluvial fan between the Tedori and Sai rivers in central Japan, we analysed stable isotope ratios of H, O, and Sr and concentrations of major dissolved ions and trace elements in groundwater, river water, and paddy water. The 87Sr/86Sr ratios of the groundwater are related to near‐surface geology: groundwater in sediment from the Tedori River has high 87Sr/86Sr ratios (>0.711), whereas that from the Sai River in the north of the fan has low 87Sr/86Sr ratios (<0.711). δ2H and δ18O values and 87Sr/86Sr ratios indicate that groundwater in the central and southern fans is recharged by the Tedori River, whereas recharge in the north is from the Sai River. Mg2+, Ca2+, Sr2+, HCO3?, and SO42? concentrations and δ2H and δ18O values in the groundwater are high in the central fan and, except for the northern area, tend to increase with distance from the Tedori River. There are linear relationships between 87Sr/86Sr ratio and the reciprocal concentrations of Sr2+, Mg2+, and Ca2+. These geochemical characteristics suggest that as groundwater recharged from the Tedori River flows towards the central fan, it mixes with waters derived from precipitation and paddy water that have become enriched in these components during downward infiltration. These results are consistent with our hydrological analysis and numerical simulation of groundwater flow, thus verifying the validity of the model we used in our simulation of groundwater flow. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

13.
We have investigated 24 whole rocks and mineral separates of five different rock types from the Cantal shield volcano in France, applying high-precision Rb-Sr techniques. The chemical and isotopic systematics suggest the distinction of two series throughout the different rock classes, one practically uncontaminated, the other seriously influenced by wall rock assimilation. The first group comprises basalts and intermediate rocks with87Sr/86Sr= 0.70340–0.70382. The second group in addition includes rhyolites and the corresponding87Sr/86Sr ratios vary between 0.70421 and 0.71270. The data of mineral separates support the hybridization hypothesis and possibly suggest an original87Sr/86Sr ratio of 0.7028 for the magma source region. Moreover they provide internal isochron ages which place a period of extensive volcanic activity at 8.1–8.8 m.y. ago in accord with K-Ar ages of volcanic rocks from the center of the Cantal volcano.  相似文献   

14.
Marble has a great potential to understand a history of various geological events occurring during tectonic processes. In order to decode metamorphic–metasomatic records on C–O isotope compositions of marble at mid-crustal conditions, we conducted a C–O–Sr isotope study on upper amphibolite-facies marbles and a carbonate–silicate rock from the Hida Belt, which was once a part of the crustal basement of the East Asian continental margin. Carbon and oxygen isotope analyses of calcite from marbles (Kamioka area) and a carbonate–silicate rock (Wadagawa area) show a large variation of δ13C [VPDB] and δ18O [VSMOW] values (from −4.4 to +4.2 ‰ and +1.6 to +20.8 ‰, respectively). The low δ13C values of calcites from the carbonate–silicate rock (from −4.4 to −2.9 ‰) can be explained by decarbonation (CO2 releasing) reactions; carbon–oxygen isotope modeling suggests that a decrease of δ13C strongly depends on the amount of silicate reacting with carbonates. The occurrence of metamorphic clinopyroxene in marbles indicates that all samples have been affected by decarbonation reactions. All δ18O values of calcites are remarkably lower than the marine-carbonate values. The large δ18O variation can be explained by the isotope exchange via interactions between marble, external fluids, and/or silicates. Remarkably low δ18O values of marbles that are lower than mantle value (~+5 ‰) suggest the interaction with meteoric water at a later stage. Sr isotope ratios (87Sr/86Sr = 0.707255–0.708220) might be close to their protolith values. One zircon associated with wollastonite in a marble thin-section yields a U–Pb age of 222 ± 3 Ma, which represents the timing of the recrystallization of marble, triggered by H2O-rich fluid infiltration at a relatively high-temperature condition. Our isotope study implies that the upper amphibolite-facies condition, like the Hida Belt, might be appropriate to cause decarbonation reactions which can modify original isotope compositions of marble if carbonates react with silicates.  相似文献   

15.
The annular (6–8 km diameter) Golda Zuelva and Mboutou anorogenic complexes of North Cameroun are composed of a suite of alkaline plutonic rocks ranging from olivine gabbro to amphibole and biotite granite. For the Mboutou complex there are two overlapping centres. In the Golda Zuelva complex the plutonic rocks are associated with a later hawaiite to rhyolite volcanic suite. A Rb/Sr whole rock isochron gives an age of 66±3 Ma for the Golda Zuelva granites, with initial87Sr/86Sr ratio of 0.7020, and demonstrates that plutonism and volcanism were essentially contemporaneous and probably cogenetic. For Golda Zuelva and the north Mboutou centre18O/16O (5.6–6.2),87Sr/86Sr (0.7030–0.7045) and Pb isotopic ratios (207Pb/204Pb: 15.60–15.64) support a mantle origin for the initial magmas. Unlike Sr isotopes, the O isotopic ratios of the granitic end members at Golda Zuelva (～7.5) indicate crustal contamination. Post-magmatic alteration was not significant.For the younger south Mboutou centre the O-, Sr- and Pb-isotopic data indicate more extensive magma-crust interaction and in a different (higher level?) crustal environment with δ18O granite=3.3‰,87Sr/86Sr ratios up to 0.706 and Pb isotopic ratios more markedly displaced from the oceanic volcanic field. The low-18O granites probably record, at least in part, a magmatic process with subsequent minor post-magmatic alteration effects. The major and trace element systematics between the north and south Mboutou centres are directly comparable. The evolution of the magmas were dominated by fractional crystallisation and progressive crustal contamination processes.  相似文献   

16.
The five diogenites, Johnstown, Roda, Ellemeet, Shalka and Tatahouine, give scattered data in the87Rb/86Sr,87Sr/86Sr diagram. This can result from a disturbance which occurred later than 4.45 Ga ago. However, it is shown that if samples of sufficient size were analyzed, there meteorites could plot on the eucrite isochron and are thereby in agreement with a genetic relation between eucrites, howardites and diogenites. The age of eucrite differentiation from diogenites has been computed using data from the two families yielding an age of 4.47±0.1Ga(2σ) (λ=1.42×10?11a?1), the initial87Sr/86Sr ratio being BABI.  相似文献   

17.
New rare earth element (REE) data, Rb and Sr analyses and Sr isotope measurements are presented for pumice clasts collected from some North Chilean ignimbrites of dacite and rhyolite composition. The samples are light-REE enriched with chondrite-normalised Ce (CeN) of 17–98, YbN of 4–14 and CeN/YbN of 2.6–15. While some samples have no Eu anomalies, others do have anomalies with inferred Eu/Eu* values of down to ca. 0.4. Eleven samples have present-day87Sr/86Sr ratios between 0.7053 and 0.7100, and noting that some samples are up to 12 Ma old, initial87Sr/86Sr ratios are below ca. 0.709. These trace element and Sr isotope characteristics resemble those of contemporaneous andesite and dacite lavas, suggesting a common origin for all these rock types. The higher Rb/Sr ratios and larger Eu anomalies in most of the dacitic and rhyolitic ignimbrites are consistent with an origin by plagioclase-dominated fractional crystallization of mantle-derived andesite magma.  相似文献   

18.
We report Sr, Nd and Pb isotope ratios and parent and daughter element concentrations in 34 volcanic rocks from Samoa. The highly undersaturated post-erosional volcanics, which have erupted in Recent to Historic time along a 250-km-long fissure, have isotopic compositions that define fields distinct from those of the tholeiitic to alkalic lavas of the older Samoan shield volcanoes. Most shield lavas have206Pb/204Pb of 18.9–19.4,87Sr/86Sr of 0.7045–0.7055 and87Sr/86Sr (to 0.7075). In general, isotopic compositions of the shield lavas are similar to those of the Marquesas and Society Islands. Post-erosional samples have lower206Pb/204Pb and143Nd/144Nd and higher87Sr/86Sr than most shield lavas. The most striking feature of the post-erosional data is a negative correlation between207Pb/204Pb and206Pb/204Pb. This suggests that post-erosional lavas are derived from mixtures of the shield source and a high-207Pb/204Pb,87Sr/86Sr, low-206Pb/204Pb and143Nd/144Nd post-erosional source which may contain recycled ancient sediment. This enriched mantle domain may also underlie the Ontong-Java and Manihiki Plateaus to the north and west. Although both the Samoan shield and post-erosional lavas show chemical characteristics often associated with mantle plumes, only the shield volcanism can plausibly be related to a plume. The post-erosional eruptions appear to be the result of flexure and rifting due to plate bending at the northern termination of the Tonga Trench.  相似文献   

19.
We have determined K, Rb and Sr concentrations and87Sr/86Sr ratios in fresh surface waters, a rain water sample and five geothermal waters from the Cantal volcanic area in the Massif Central, France. A comparison with appropriate rock types of the region showed no apparent chemical and isotopic fractionation occurring in the fresh water-surface rock system. The thermo-mineral water results suggest that all springs discharge dissolved Sr from the following contributors: Hercynian granito-metamorphic basement, lacustrian sediments underlying the volcano, Miocene-Pliocene volcanic rocks of basaltic to rhyolitic composition.  相似文献   

20.
Analyses of rim-to-interior samples of fresh tholeiitic pillow basalts, deuterically altered holocrystalline basalts, and older, weathered tholeiitic basalts from the deep sea indicate that 87Sr/86Sr ratios of the older basalts are raised by low temperature interaction with strontium dissolved in sea water. 87Sr/86Sr correlates positively with H2O in these basalts; however, there is little detectable modification of the strontium isotope composition in rocks with H2O contents less than 1%. The isotope changes appear to be a function of relatively long-term, low-temperature weathering, rather than high-temperature or deuteric alteration. Strontium abundance and isotopic data for these rocks suggest that strontium content is only slightly modified by interaction with sea water, and it is a relatively insensitive indicator of marine alteration. Average Rb-Sr parameters for samples of apparently unaltered basalt are: Rb= 1.11ppm; Sr= 132ppm; 87Sr/86Sr= 0.70247.  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