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1.
Expanded sedimentary records from the Tethys reveal unique faunal and isotopic changes across the Palaeocene-Eocene (P-E) transition. Unlike in the open oceans, the Tethys exhibits a gradual decrease of 1.5% in δ13C values prior to the rapid δ13C excursion. Associated with the 613C excursion is a decrease in calcite burial, increase in detrital content and appearance of a unique opportunistic planktic foraminifera1 assemblage (e.g. compressed acarininids). The existence of a prelude decrease in δ13C values in the Tethys suggests that the P-E δ13C excursion may have occurred in two steps and over a few hundred thousand years, rather than as one step over a few thousand years as previously suggested. This slower excursion rate is readily explained by changing organic carbon weathering or burial rates and avoids the need of invoking ad hoc scenarios.  相似文献   

2.
The cause of the middle Miocene Badenian salinity crisis in the Central Paratethys is addressed by examining the palaeotemperature evolution of Badenian waters before and after the deposition of evaporites. Selected foraminifer taxa ( Globigerinoides spp., Globigerina bulloides , and Uvigerina ) characterizing, respectively, the near-surface, intermediate, and bottom layers of the water column, were studied in two boreholes of SW Poland. The δ18O and δ13C values for these taxa show distinct differences which can be explained by the temperature difference between surface and bottom waters during deposition. These values also show temporal changes corresponding to the water temperature evolution in the Badenian basin. Different and quickly changing environmental conditions have been inferred from changes in foraminifer assemblages. They explain why biostratigraphic subdivisions based on well-recognized assemblages are the most accurate approach for determining the biostratigraphy of middle Miocene deposits in the Central Paratethys. The results of isotopic studies indicate that evaporites occur in a part of the Badenian section that was characterized by the lowest temperatures in the studied sections.  相似文献   

3.
Stable oxygen and carbon isotope ratios (δ18O and δ13C, respectively) of a planktonic foraminiferal species (Globigerinoides sacculifer) and marble carbonate were measured with and without a variety of pretreatments like ultrasonication, soaking in H2O2, methanol and roasting under vacuum. Additionally, organic matter - carbonate mixtures were also analysed with and without the above pretreatments. No significant difference was found in the isotope ratios of treated and untreated samples. It appears that most of the pretreatments may not be necessary when dealing with planktonic foraminifera, especially G. sacculifer. Analysis of two different size fractions (> 400 μm and 250–400 μm) of this species reveals that the smaller size fraction is enriched in 18O and depleted in 13C relative to the bigger size fraction. It is therefore necessary to choose a proper size fraction for isotopic analyses.  相似文献   

4.
Abstract Positive shifts in global seawater δ13CDIC are related to changes in the ratio of organic relative to inorganic carbon burial in oceanic basins, whereas factors such as climatic cooling and the accumulation of polar ice are known to cause positive shifts in δ18O. Here, an alternative model is proposed for the formation of local positive isotope shifts in shallow-marine settings. The model involves geochemically altered platform-top water masses and the effects of early meteoric diagenesis on carbonate isotopic composition. Both mechanisms are active on modern (sub)tropical carbonate platforms and result in low carbonate δ13C and δ18O relative to typical oceanic values. During high-amplitude transgressive events, the impact of isotopically light meteoric fluids on the carbonate geochemistry is much reduced, and 13C-depleted platform-top water mixes with open oceanic water masses having higher isotope values. Both factors are recorded as a transient increase in carbonate 13C and 18O relative to low background values. These processes must be taken into consideration when interpreting the geochemical record of ancient epeiric seas.  相似文献   

5.
Data from a series of extended analytical sessions using a range of different zircon reference samples have been used to investigate the periodic derivation of abnormal 207Pb/206Pb ages during microbeam analysis%For the Canberra SHRIMP II, this phenomenon probably results from the presence of a signal at mass 204 that is alien to the Pb spectrum; isotopic fractionation is unlikely to be a significant contributor. In contrast, abnormal 207Pb/206Pb ages obtained from SHRIMP I at the same research centre require a different, but as yet unknown explanation. Assessment of the reasons for atypical 207Pb/206Pb measurements and the means of correcting for them should therefore be independently assessed for individual analytical instruments. The use of reference samples with very different ages is an effective way of doing this, because older reference samples are more useful for documenting isotopic fractionation, whereas younger ones are more sensitive indicators of isobaric interference.  相似文献   

6.
Bulk carbonate samples of hemipelagic limestone–marl alternations from the Middle and Upper Triassic of Italy are analysed for their isotopic compositions. Middle Triassic samples are representative of the Livinallongo Formation of the Dolomites, while Upper Triassic hemipelagites were sampled in the Pignola 2 section, within the Calcari con Selce Formation of the Southern Apennines in Southern Italy. Triassic hemipelagites occur either as nodular limestones with chert nodules or as plane‐bedded limestone–marl alternations which are locally silicified. In the Middle Triassic Livinallongo Formation, diagenetic alteration primarily affected the stable isotopic composition of sediment surrounding carbonate nodules, whereas the latter show almost pristine compositions. Diagenesis lowered the carbon and oxygen isotope values of bulk carbonate and introduced a strong correlation between δ13C and δ18O values. In the Middle Triassic successions of the Dolomites, bulk carbonate of nodular limestone facies is most commonly unaltered, whereas carbonate of the plane‐bedded facies is uniformly affected by diagenetic alteration. In contrast to carbonate nodules, plane‐bedded facies often show compaction features. Although both types of pelagic carbonate rocks show very similar petrographic characteristics, scanning electron microscopy studies reveal that nodular limestone consists of micrite (< 5 μm in diameter), whereas samples of the plane‐bedded facies are composed of calcite crystals ca 10 μm in size showing pitted, polished surfaces. These observations suggest that nodular and plane‐bedded facies underwent different diagenetic pathways determined by the prevailing mineralogy of the precursor sediment, i.e. probably high‐Mg calcite in the nodular facies and aragonite in the case of the plane‐bedded facies. Similar to Middle Triassic nodular facies, Upper Triassic nodular limestones of the Lagonegro Basin are also characterized by uncorrelated δ13C and δ18O values and exhibit small, less than 5 μm size, crystals. The alternation of calcitic and aragonitic precursors in the Middle Triassic of the Dolomites is thought to mirror rapid changes in the type of carbonate production of adjacent platforms. Bioturbation and dissolution of metastable carbonate grains played a key role during early lithification of nodular limestone beds, whereby early stabilization recorded the carbon isotopic composition of sea water. The bulk carbonate δ13C values of Middle and Upper Triassic hemipelagites from Italy agree with those of Tethyan low‐Mg calcite shells of articulate brachiopods, confirming that Triassic hemipelagites retained the primary carbon isotopic composition of the bottom sea water. A trend of increasing δ13C from the Late Anisian to the Early Carnian, partly seen in the data set presented here, is also recognized in successions from tropical palaeolatitudes elsewhere. The carbon isotopic composition of Middle and Upper Triassic nodular hemipelagic limestones can thus be used for chemostratigraphic correlation and palaeoenvironmental studies.  相似文献   

7.
For studies of mass-dependent fractionation of calcium isotopes in natural materials, the 48Ca/42Ca ratio is a superior choice to the conventionally measured 44Ca/40Ca ratio for two important reasons. These are (1) mass-dependent fractionation can be determined free from the effects of inherited or ingrown radiogenic 40Ca and (2) this ratio increases the spread of measured isotopic masses by 50%, resulting in statistically better resolution of fractionation, assuming similar precision. A third, though strictly technical, advantage is the inherent ability of a mass spectrometer to measure ratios close to unity (48Ca/42Ca) more precisely than very small or large ratios (44Ca/40Ca). However, because of the very low natural abundance of both 48Ca and 42Ca, their ratio has been very difficult to measure, the only attempt so far being on a high mass resolution MC-ICP-MS with a precision of 0.33%. We report here determination of the 48Ca/42Ca ratio by the more commonly available and user-friendly multi-collector TIMS using a 43Ca-46Ca double-spike, with a significantly better precision of 0.18% (2s). The 48Ca/40Ca or 44Ca/40Ca ratio can also be measured in the same mass spectrometer run to provide complementary information on any radiogenic component.  相似文献   

8.
Zachariah 《地学学报》1998,10(6):312-316
A marble band in the ≈ 2.75 Gyr old Ramagiri schist belt in the Dharwar craton of south India gave a Pb–Pb age of 3.075 ± 0.095 Gyr. The geochemical data, including high Sr and low Ba and Mn indicate seawater origin for the parent rock, and that there was insignificant geochemical exchange between the marble and the surrounding rocks. The calculated initial Nd isotopic composition and μ1 indicate an older continental crustal source for the Nd and Pb. The initial 87Sr/86Sr of the marble is 0.70128, which is higher than the calculated mantle value at ≈ 3 Ga. Although pre-3 Gyr old marine carbonate rocks are thought to be buffered by mantle Sr, the Ramagiri marble contains evolved, crustal Sr. Despite this, the marble has the lowest measured 87Sr/86Sr among carbonates and represents one of the least radiogenic periods in seawater Sr isotope composition.  相似文献   

9.
Abstract Petrological, oxygen isotope and 40Ar/39Ar studies were used to constrain the Tertiary metamorphic evolution of the lower tectonic unit of the Cyclades on Tinos. Polyphase high-pressure metamorphism reached pressures in excess of 15 kbar, based on measurements of the Si content in potassic white mica. Temperatures of 450–500° C at the thermal peak of high-pressure metamorphism were estimated from critical metamorphic assemblages, the validity of which is confirmed by a quartz–magnetite oxygen isotope temperature of 470° C. Some 40Ar/39Ar spectra of white mica give plateau ages of 44–40 Ma that are considered to represent dynamic recrystallization under peak or slightly post-peak high-pressure metamorphic conditions. Early stages in the prograde high-pressure evolution may be documented by older apparent ages in the high-temperature steps of some spectra. Eclogite to epidote blueschist facies mineralogies were partially or totally replaced by retrograde greenschist facies assemblages during exhumation. Oxygen isotope thermometry of four quartz–magnetite pairs from greenschist samples gives temperatures of 440–470° C which cannot be distinguished from those deduced for the high-pressure event. The exhumation and overprint is documented by decreasing ages of 32–28 Ma in some greenschists and late-stage blueschist rocks, and ages of 30–20 Ma in the lower temperature steps of the Ar release patterns of blueschist micas. Almost flat parts of Ar–Ar release spectra of some greenschist micas gave ages of 23–21 Ma which are assumed to represent incomplete resetting caused by a renewed prograde phase of greenschist metamorphism. Oxygen isotope compositions of blueschist and greenschist facies minerals show no evidence for the infiltration of a δ18O-enriched fluid. Rather, the compositions indicate that fluid to rock ratios were very low, the isotopic compositions being primarily controlled by those of the protolith rocks. We assume that the fundamental control catalysing the transformation of blueschists into greenschists and the associated resetting of their isotopic systems was the selective infiltration of metamorphic fluid. A quartz–magnetite sample from a contact metamorphic skarn, taken near the Miocene monzogranite of Tinos, gave an oxygen isotope temperature of 555° C and calculated water composition of 9.1%. The value of δ18O obtained from this water is consistent with a primary magmatic fluid, but is lower than that of fluids associated with the greenschist overprint, which indicates that the latter event cannot be directly related to the monozogranite intrusion.  相似文献   

10.
The 87Sr/86Sr ratios of evaporitic carbonates and sulphates from Miocene sediment cored in the Mediterranean Sea show a depletion of 87Sr when compared to the isotopic composition of the Miocene contemporaneous marine strontium: 0.70803 versus 0.70936. The arrival into the evaporitic environment of strontium brought by continental waters can explain this difference. The variation of the 87Sr/86Sr ratios is, nevertheless, noticeable only when the influence of the continental waters is already well marked. This is proved when one compares the results obtained with strontium, to the results of isotopic analysis made on oxygen, carbon, sulphur and hydrogen taken from the same samples.  相似文献   

11.
The formulas for correcting 14C measurements are explained. The expected ranges of the δ13C values for some types of samples are given. It is pointed out that the corrections are approximate because of the choice of correction formula, and that all samples arc normalized to wood samples instead of to carbon dioxide of the air. Some results from the Uppsala laboratory for plant materials and carbonates are given.  相似文献   

12.
A mammoth tusk find from Toten in Oppland, eastern Norway, has been re-dated by the EDTA method. On examining the tusk for heavy metals, the porous parts were found to have the highest concentration. This fact, together with earlier experience from selecting bone samples for 14C dating, favour the choice of the hard non-porous parts. The influence of possible minor contaminants remaining after pretreatment is discussed. Because of high cosmic-ray intensity the background deviated from the normal stable value. The variations were studied carefully. The final result is that the net activity of the bone is so close to zero that it is questionable whether the age is 'finite'. It is concluded that the tusk is older than 28,000 years.  相似文献   

13.
Ar/Ar thermochronology on 24 hornblendes, 3 biotites, 2 muscovites and 2 K-feldspars, collected along a 400 km-long NW-SE geotraverse through the Grenville Province in western Québec, is employed to provide time constraints on the intermediate and low temperature stages of cooling of part of the Grenville orogen. In the Grenville Front zone, the c. 1000 Ma time of exhumation previously established from thermobarometric and isotopic studies, is supported by the hornblende age data presented here. From 60 km to 160 km SE of the Front, reworked Archaean migmatites of the parautochthonous Réservoir Dozois terrane (RDT; 1004 Ma-old metamorphic monazites) contain hornblendes with 972– 950 Ma cooling ages. Assuming metamorphic geotherms between 25 and 30 °C km?1, calculated cooling and unroofing rates are about 6 °C Ma?1 and 0.33 km Ma?1 in the P–T range 725 °C–800 MPa and 450 °C–400 MPa. Hornblendes from monocyclic rocks of the Mont-Laurier and Morin terranes (MLT and MT; monazite ages c. 1165 Ma) give ages of about 1040 and 1010 Ma, respectively. Calculation of cooling-unroofing rates from peak metamorphic conditions in this area is hampered by thermal perturbations associated with the still poorly dated Grenville collision which took place approximately between 1060 and 1020 Ma. Cooling ages of c. 900 Ma for muscovite and biotite and 860–810 Ma for K-feldspar, show that cooling rates decreased to around 1.5 °C Ma?1 under retrograde greenschist facies conditions in the MLT. On a time vs. distance diagram, the hornblende data define several distinct age ranges, suggesting that each terrane had a characteristic thermal history. Thus, cooling was diachronous and probably non-homogeneous throughout this segment of the Grenville orogen. The time-lag between the cooling history of the parautochthon (972–950 Ma) and the allochthons (1040–1010 Ma) is compatible with an earlier (pre-1040 Ma) peak of metamorphism in the allochthons. The Réservoir Cabonga allochthon was transported toward the NNW from its probable root zone in the MLT during the 1060–1020 Ma Grenvillian collision as a partially cooled slab. The remobilization of the Archaean parautochthon is attributed to this collision. In the Grenville Front zone, slightly older cooling ages and cooling rates initially faster than in the remaining part of the parautochthon are probably as a result of rapid (tectonic?) exhumation shortly after collision. The minor delay (20–30 Ma) in unroofing of the MT compared to the adjacent MLT is most likely related to post-1040 Ma extensional displacement along the Labelle shear zone. In terranes like those described above where metamorphism is diachronous, determination of cooling rates and the history of exhumation may be meaningless without a firm control on the regional structure. However, identification of contrasting cooling histories contributes to unravelling the independent movement of terranes.  相似文献   

14.
Oxygen isotopic compositions of silicates in eclogites and whiteschists from the Kokchetav massif were analyzed by whole‐grain CO2‐laser fluorination methods. Systematic analyses yield extremely low δ18O for eclogites, as low as ?3.9‰ for garnet; these values are comparable with those reported for the Dabie‐Sulu UHP eclogites. Oxygen isotopic compositions are heterogeneous in samples of eclogite, even on an outcrop scale. Schists have rather uniform oxygen isotope values compared to eclogites, and low δ18O is not observed. Isotope thermometry indicates that both eclogites and schists achieved high‐temperature isotopic equilibration at 500–800 °C. This implies that retrograde metamorphic recrystallization barely modified the peak‐metamorphic oxygen isotopic signatures. A possible geological environment to account for the low‐δ18O basaltic protolith is a continental rift, most likely subjected to the conditions of a cold climate. After the basalt interacted with low δ18O meteoric water, it was tectonically inserted into the surrounding sedimentary units prior to, or during subduction and UHP metamorphism.  相似文献   

15.
R. L. Romer 《地学学报》2001,13(4):258-263
The isotopic composition of lead available for incorporation during metamorphic reactions is heterogeneous, depends on the reaction history of the metamorphic rock, and is commonly not accessible for measurement as the precursor minerals have been consumed during the growth of the metamorphic phases. The initial lead composition has a significant effect on the age of low-238U/204Pb metamorphic phases (e.g. garnet, rutile, titanite, staurolite, vesuvianite and ilmenite). Using a distinct value (e.g. leached K-feldspar Pb, model Pb) rather than a geologically reasonably constrained range may result in apparently precise, yet inaccurate ages. Since age data from metamorphic minerals are widely used to unravel the P–T–t–d evolution of orogens, inaccurate ages result in: (1) incorrect timing (duration) of P–T loops and associated with it the heat budget and mass transfer in orogens; (2) arbitrary rates (based on the age difference between core and rim) for mineral growth, P–T evolution and deformation; and (3) apparent sequences of isotopic closure for the U–Pb system of contrasting minerals.  相似文献   

16.
Twenty-three magnesite samples from vein deposits, found in ultramafic rocks related to ophiolites in the north-east Bohemian Massif, have been examined for carbon and oxygen isotope ratios. In these rocks two stages of antigorite serpentinization took place prior to magnesite precipitation. δ18OSMOW in magnesites ranges from 24.5 to 27.9%o (corrected for acid fractionation factor) which leads us to conclude a meteoric origin of the ore-forming solution. This inference is further supported by δ13C values (?18.7 to ?4.4%o) which point to a biogenic origin of carbon in the magnesites.  相似文献   

17.
We present the first trace element and age data combined with new Sr, Nd, and Pb isotope ratios on lavas from San Felix Island in the Southeast Pacific. A 40Ar/39Ar plateau age of 421 ± 18 ka implies young intraplate volcanic activity in this region relative to the ∼22 Ma old volcanism on the neighbouring Easter seamount chain (ESC). The incompatible element compositions of the San Felix magmas are similar to those of EM1-type basalts from Gough, although the isotopic compositions differ. San Felix formed some 20 Ma after the ESC plume affected the plate in this region but no chemical signature of the ESC material is observed in the young volcanic rocks. The composition of the San Felix basalts indicates a mantle source containing old continental lithospheric material from either metasomatized mantle or recycled sediments, which ascends in a weak mantle plume.  相似文献   

18.
We report silicon isotopic determinations for USGS rock reference materials BHVO-1 and BHVO-2 using a Nu Plasma multi-collector (MC)-ICP-MS, upgraded with a new adjustable entrance slit, to obtain medium resolution, as well as a stronger primary pump and newly designed sampler and skimmer cones ("B" cones). These settings, combined with the use of collector slits, allowed a resolution to be reached that was sufficient to overcome the 14N16O and 14N2 interferences overlying the 30Si and the 28Si peaks, respectively, in an earlier set-up. This enabled accurate measurement of both δ30Si and δ29Si. The δ value is expressed in per mil variation relative to the NBS 28 quartz reference material. Based on data acquired from numerous sessions spread over a period of six months, we propose a recommended average δ30Si of −0.33 ± 0.05‰ and −0.29 ± 0.11‰ (2se) for BHVO-1 and BHVO-2, respectively. Our BHVO grand mean silicon isotope composition (δ30Si =−0.31 ± 0.06‰) is significantly more negative than the only published value for BHVO-2, but is in very good agreement with the recently established average value of ocean island basalts (OIB), confirming the conclusion that the OIB reservoir has a distinct isotopic composition from the solar reservoir as sampled by chondrites.  相似文献   

19.
The determination of total organic carbon content and composition in detrital sediments requires careful removal of their carbonate minerals. In detrital sediments containing large amounts of carbonates, including dolomite, this can only be achieved by liquid acid leaching that may solubilise a significant proportion of the organic carbon. For a set of detrital sediments from the Himalayan system and the Amazon River as well as five geological reference materials, we determined the proportion of organic carbon (Corg) solubilised during acid leaching. This proportion is significant for all analysed sediments and generally tends to increase with the organic carbon content. Compared to other types of sediments analysed, clay fractions extracted from river sediments and bed sediments with very low organic carbon content have high and low proportions of acid soluble Corg respectively. In Himalayan and Amazon river sediments, the proportion of Corg solubilised during acid leaching was relatively constant with average values of 14 and 19 % respectively. Thus, it is possible to correct the Corg content for the dissolved organic carbon content measured after decarbonation. Data presented here show that Corg dissolved during liquid acid leaching must be taken into account. After careful calibration, the method presented here should, therefore, be applied to any carbonate-rich detrital sediment.  相似文献   

20.
Abstract The St Malo region in north-west France contains migmatites and anatectic granites derived by partial melting of metasedimentary protoliths during Cadomian orogenesis at c. 540 Ma. Previously reported Rb–Sr model ages for muscovite and biotite range from c. 550 to c. 300 Ma, and suggest variable resetting of mineral isotopic systems. These rocks display microscopic evidence for variably intense Cadomian intracrystalline plastic strain but record no obvious evidence of penetrative Palaeozoic regional deformation. 40Ar/39Ar mineral ages have been determined to evaluate better the extent, timing and significance of Palaeozoic overprinting. Eleven muscovite concentrates and one whole-rock phyllite have been prepared from various units exposed in the St Malo and adjacent Mancellian regions. In the Mancellian region, muscovite from two facies of the Bonnemain Granite Complex record 40Ar/39Ar plateau ages of c. 527 and 521 Ma. An internally discordant 40Ar/39Ar release spectrum characterizes muscovite from protomylonitic granite within the Cadomian Alexain-Deux Evailles-Izé Granite Complex, and probably records the effects of Variscan displacement along the North Armorican Shear Zone. Muscovite concentrates from anatectic granite and from Cadomian mylonites along ductile shear zones within the north-western sector of the St Malo region exhibit internally discordant 40Ar/39Ar release spectra which suggest variable and partial late Palaeozoic rejuvenation. By contrast, muscovite concentrates from samples of variably mylonitic Brioverian metasedimentary rocks exposed within the south-eastern sector of the St Malo region display internally concordant apparent age spectra which define plateaux of 326–320 Ma. A whole-rock phyllite sample from Brioverian metasedimentary rocks exposed along the eastern boundary of the St Malo region displays an internally discordant argon release pattern which is interpreted to reflect the effects of a partial late Palaeozoic thermal overprint. Muscovite from the Plélan granite, part of the Variscan Plélan-Bobital Granite Complex, yields a 40Ar/39Ar plateau age of c. 307 Ma. The 40Ar/39Ar results indicate that Cadomian rocks of the St Malo region have undergone a widespread and variable Palaeozoic (Carboniferous) rejuvenation of intracrystalline argon systems which apparently did not affect the Mancellian region. This rejuvenation was not accompanied by penetrative regional deformation, and was probably of a static thermal–hydrothermal origin. The heat source for rejuvenation was probably either the result of heating during Variscan extension or advection from Variscan granites which are argued to underlie the St Malo region.  相似文献   

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