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1.
ABSTRACT This work presents a detailed 87 Sr/86 Sr isotope curve for the interval 7.5–9.7 Ma obtained by a high-resolution analysis (sampling spacing of about 40 kyr) of an astronomically calibrated land-based sedimentary sequence exposed in the central Mediterranean area (Gibliscemi section, southern Sicily). The main aim is to verify a synchronous response of the Mediterranean seawater Sr isotope record to the oceanic forcing on the basis of multiple comparisons of the Gibliscemi record with published coeval 87 Sr/86 Sr curves. A good correlation with the 87 Sr/86 Sr data from the ODP site 926 (equatorial Atlantic ocean), considered to be the Sr chemostratigraphic reference section for the Late Miocene, and from the Pacific DSDP site 590B was registered. Conversely, the comparison of the Gibliscemi Sr isotope data with 87 Sr/86 Sr ratios from the coeval segment of the land-based Sardella section (eastern Mediterranean) shows important differences highlighting a local control on the seawater Sr isotope changes in semi-isolated subbasins within the Late Miocene Mediterranean. 相似文献
2.
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 87 Sr/86 Sr 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 87 Sr/86 Sr among carbonates and represents one of the least radiogenic periods in seawater Sr isotope composition. 相似文献
3.
measurements of 108 sedimentary carbonate rocks have been used to trace variations in the strontium isotopic composition of seawater during the Phanerozoic. The lowest 87Sr/86Sr observed for any suite of carbonates is taken as the best approximation to the value in well-mixed contemporary seawater. Our data support the existence of low in the Cretaceous and Late Jurassic but they do not support further structure beyond a general trend through the Phanerozoic, which may correlate with the continental denudation rate. 相似文献
4.
The isotopic composition of strontium in surface water in continental basins is determined primarily by the geology of the basin and to a lesser extent by climatic conditions. Consequently, the 87Sr/86Sr ratios of brines in such basins can change only as a result of changes in the geology or climate. This principle of isotope geology was studied by analysis of a suite of non-marine carbonate rocks from the Flagstaff Formation (Palaeocene-Eocene) of Utah. The samples were collected from a section in Fairview Canyon of Sanpete County. They include both limestone and dolomite and were selected to have low non-carbonate residues. The concentrations of strontium in calcites averages 383 ± 128 p.p.m., while those of dolomites increase from 354 ± 74 p.p.m. in the lower 43 m of section to a maximum of 2259 p.p.m. higher up. The increase in the strontium content of dolomite is interpreted as evidence for a change from steady-state to progressively more evaporitic conditions. Two dolomites have isotopic compositions of oxygen expressed as δ18O = -2.75‰ (relative to the PDB standard) and are enriched in 18O relative to two calcites whose average δ18O value is -9.9‰. The 87Sr/86Sr ratios of the carbonate minerals range from 0.70890 to 0.71260. These values are clearly greater than the 87Sr/86Sr ratio of marine carbonates of Early Eocene age which is 0.70744. The variation of the 87Sr/86Sr ratio in this section of the Flagstaff Formation is real and reflects the occurrence of geological events which changed the isotopic composition of Sr entering Lake Flagstaff. The non-carbonate fractions of six carbonate rocks and one sandstone fit a straight line on the strontium mixing diagram in co-ordinates of initial 87Sr/86Sr and 1/Sr concentration. These results suggest that the isotopic composition of strontium in Lake Flagstaff may have been modulated by periodic input of volcanogenic detritus of felsic composition. 相似文献
5.
Strontium isotopic composition (87Sr/86Sr) of two petrographically, chemically and isotopically (δ18O and δ13C) distinct phases of burial calcites from the Lincolnshire Limestone are indistinguishable (0.70820± 26). The mean 87Sr/86Sr ratio of these phases is considerably more radiogenic than 87Sr/86Sr ratios of Bajocian marine waters (～0.70725). Neither Bajocian marine waters nor meteoric waters buffered by host marine carbonate in the Limestone could have precipitated the burial spars. Radiogenic strontium may have been contributed from K-feldspar dissolution and/or clay recrystallization, either within clastic portions of the Limestone itself, or from major clastic units adjacent to the Limestone. Alternatively, Palaeozoic marine waters or remobilized Palaeozoic marine carbonate and/or sulphate could have supplied the necessary radiogenic strontium. 相似文献
6.
87Sr/86Sr ratios of fourteen unaltered limestone and six fossil samples show a range of 0.7066-0.7118 for the depositional water of the Middle to Upper Pennsylvanian Madera Formation. The variation of these 87Sr/86Sr ratios may be due to the restricted depositional basin and nature of the source of the Madera Formation.While fossil and limestone samples are equally good for determination of strontium isotopic composition of depositional waters, leaches of calcareous shales contain significant amount of 87Sr leached from the noncarbonate fraction during acid treatment. 相似文献
7.
The isotopic composition of Sr has been measured in 73 formation-water samples from Paleozoic strata in the Illinois basin; 87Sr/86Sr ratios range from 0.7079 to 0.7108. With the exception of four samples, the waters are more radiogenic than corresponding Paleozoic sea-water values. The relatively narrow range of slightly elevated 87Sr/86Sr rations is uniformly distributed in waters throughout the stratigraphic column and in Silurian waters across the basin. Isotopic analyses of core samples from reservoir rocks show an absence of water-rock Sr isotopic equilibration. Basin lithology and analyses of detrital rock units indicate that clay minerals in shales and in quartz sandstone matrices represent the only significant source of radiogenic Sr for the waters. Silurian and Devonian water show a two-component mixing relation which suggests that they comprise a single hydrogeological system that evolved when radiogenic water from New Albany shales entered Silurian-Devonian carbonate rocks and mixed with marine interstitial water. Regional migration of the waters and associated petroleum within the Silurian-Devonian strata, proposed in other studies, is consistent with the Sr isotopic data. Under favorable circumstances subsurface waters are capable of retaining a Sr isotopic recor of their evolution. 相似文献
8.
The Sr isotopic composition of ‘seawater’, as measured on carbonate rocks, shows a composite pattern during geologic history. All known Archaean data are compatible with contemporaneous upper mantle 87Sr/86Sr values. This is followed by a strong increase in the radiogenic component during the 2.5–2.1 b.y. period, a less pronounced increase during the remaining portion of the Proterozoic and a decrease during the Phanerozoic. The trend closely resembles the K2O/Na2O secular variations in composition of igneous and sedimentary rocks (Engelet al, Bull. Geol. Soc. Amer. 85, 843–858, 1974) and probably reflects the fractionation state of the contemporary crust. The data are compatible with recent suggestions of three major tectonic regimes during geologic history: greenstone belts during the Archaean, mobile belts during the Proterozoic and plate tectonics during the Phanerozoic. They also indicate that continental crust during the Archaean contributed only subordinate Sr into the meteoric cycle. 相似文献
9.
Isotopic ratios of strontium in 9 volcanic rocks from the south-eastern part of the Hellenic arc range from 0.7037 to 0.7075. Within individual series of differentiation, there seems to be a correlation between Sr87/Sr86 and K2O/SiO2.All strontium isotope data for the Hellenic arc are reviewed. Comparable (but slightly smaller) ranges of Sr isotope ratios are found in other island arcs with continental basement. To explain the high values of Sr87/Sr86 ratio for the Hellenic arc, a selective addition of Sr87 from the wall rock, and a process of assimilation involving water, perhaps from subducted sediments, are suggested. Since closely-spaced individual volcanic centres of similar ages have very different Sr isotope ratios, and since the range of Sr isotopic composition in individual centres is quite large, the variation is unlikely to be due to primary variation in mantle composition. 相似文献
10.
The concentrations of rubidium and strontium and the isotopic composition of strontium have been determined in minerals separated from ultramaflc nodules occurring in late Tertiary and Quaternary basalts of wide geographic distribution. Clinopyroxene, orthopyroxene and olivine from each of three Iherzolite nodules show a relatively wide range of 87Sr/86Sr disequilibrium and none of the minerals is in isotopic equilibrium with its host basalt. In two cases there is a correlation between 87Sr/86Sr and 87Rb/86Sr ratios of the nodule minerals, indicating apparent isochron relationships which may represent relict mantle events. Clinopyroxene and olivine from each of two wehrlite nodules are not in isotopic equilibrium, although the magnitude of the disequilibrium is smaller than that observed in the Iherzolite nodules. None of these ultramafic nodules can be a crystal cumulate from its host basalt, and it is doubtful that any type of genetic relationship exists. The strontium isotopic disequilibrium between nodule minerals seems to be a primary feature inherited from past mantle histories. 相似文献
11.
Variations in the seawater 87Sr/86Sr curve through time can be caused by fluctuations in the strontium flux or variations in the isotopic ratio from at least six different sources and sinks. Thus, 12 or more parameters control each single measurement although widely accepted assumptions allow this to be reduced to typically six unknowns. Interpreting the causes of time-variation in the seawater 87Sr/86Sr curve is therefore hampered by inherent non-uniqueness. However, this problem is under-constrained rather than unconstrained. As a result, whilst there are an infinite number of possible interpretations, these all come from a few families of very similar solutions. Using this insight, it is possible to find solutions having the smallest possible variations in source flux or source 87Sr/86Sr ratio. Thus, lower-bounds can be placed upon the source variations responsible for the observed fluctuations in the seawater 87Sr/86Sr curve. When applied to the evolution of the Early Jurassic 87Sr/86Sr seawater curve, this approach demonstrates that a short-lived Toarcian event is genuine since it is present in all models, regardless of the values chosen for the unknown source fluxes and unknown source isotope ratios. However, the variations in strontium flux or isotopic ratio necessary to explain the Toarcian event may be significantly smaller than would be predicted assuming modern values for the unknown parameters. 相似文献
12.
Strontium isotopes and other geochemical signatures are used to determine the relationships between CO2-rich thermal (Chaves: 76 °C) and mineral (Vilarelho da Raia, Vidago and Pedras Salgadas: 17 °C) waters discharging along one of the major NNE–SSW trending faults in the northern part of mainland Portugal. The regional geology consists of Hercynian granites (syn-tectonic-310 Ma and post-tectonic-290 Ma) intruding Silurian metasediments (quartzites, phyllites and carbonaceous slates). Thermal and mineral waters have 87Sr/86Sr isotopic ratios between 0.716713 and 0.728035. 87Sr/86Sr vs. 1/Sr define three end-members (Vilarelho da Raia/Chaves, Vidago and Pedras Salgadas thermal and mineral waters) trending from rainfall composition towards that of the CO2-rich thermal and mineral waters, indicating different underground flow paths. Local granitic rocks have 87Sr/86Sr ratios of 0.735697–0.789683. There is no indication that equilibrium was reached between the CO2-rich thermal and mineral waters and the granitic rocks. The mean 87Sr/86Sr ratio of the thermal and mineral waters (0.722419) is similar to the Sr isotopic ratios of the plagioclases of the granitic rocks (0.71261–0.72087). The spatial distribution of Sr isotope and geochemical signatures of waters and the host rocks suggests that the thermal and mineral waters circulate in similar but not the same hydrogeological system. Results from this study could be used to evaluate the applicability of this isotope approach in other hydrogeologic investigations. 相似文献
13.
《Applied Geochemistry》2002,17(3):285-300
Strontium and particularly 87Sr/86Sr ratios in stream water have often been used to calculate weathering rates in catchments. Nevertheless, in the literature, discharge variation effects on the geochemical behavior of Sr are often omitted or considered as negligible. A regular survey of both Sr concentrations and Sr isotope ratios of the Strengbach stream water draining a granite (Vosges mountains, France) has been performed during one year. The results indicate that during low water flow periods, waters contain lower Sr concentrations and less radiogenic Sr isotope ratios (Sr=11.6 ppb and 87Sr/86Sr=0.7246 as an average, respectively) than during high water flow periods (Sr= 13 ppb and 87Sr/86Sr=0.7252 as an average, respectively). This is contrary to expected dilution processes by meteoric waters which have comparatively lower Sr isotopic ratios and lower Sr concentrations. Furthermore, 87Sr/86Sr ratios in stream water behave in 3 different ways depending on moisture and on hydrological conditions prevailing in the catchment. During low water flow periods (discharge < 9 l/s), a positive linear relationship exists between Sr isotope ratio and discharge, indicating the influence of radiogenic waters draining the saturated area during storm events. During high water flow conditions, rising discharges are characterized by significantly less radiogenic waters than the recession stages of discharge. This suggests a large contribution of radiogenic waters draining the deep layers of the hillslopes during the recession stages, particularly those from the more radiogenic north-facing slopes. These results allow one to confirm the negligible instantaneous incidence of rainwater on stream water chemistry during flood events, as well as the existence in the catchment of distinct contributive areas and reservoirs. The influence of these areas or reservoirs on the fluctuations of Sr concentrations and on Sr isotopic variations in stream water depends on both moisture and hydrological conditions. Hence, on a same bedrock type, 87Sr/86Sr ratios in surface waters can be related to flow rate. Consequently, discharge variations must be considered as a pre-requisite when using Sr isotopes for calculating weathering rates in catchments, particularly to define the range of variations of the end-members. 相似文献
14.
The Nd and Sr isotopic ratios on a suite of continental alkali basalts from Marie Byrd Land, West Antarctica, define a change in the source over the range of K/Ar dates between 1 and 28 m.y. ago. The 87Sr/86Sr isotopic ratios (0.7026 to 0.7031) are unusually low for continental alkali basalts, although the corresponding 143Nd/144Nd ratios (0.51283 to 0.51299) are similar to previously reported values. On a 87Sr/86Sr vs. 143Nd/144Nd diagram, they define a trend on the low 87Sr/86Sr side of the “mantle array”, which has a slope steeper than the mantle array. An explanation for the light rare earth elements (LREE) enrichment of the alkali basalts, with high 143Nd/144Nd ratios and low 87Sr/86Sr ratios, is suggested by a model which modifies the source region with a mantle-derived, CO2-enriched metasomatic fluid. 相似文献
15.
Victor A. Melezhik Anthony E. Fallick Dmitry V. Rychanchik Anton B. Kuznetsov 《地学学报》2005,17(2):141-148
This paper addresses global oxygenation and establishment of a marine sulphate reservoir in the Palaeoproterozoic. We report syn-depositional, marine, anhydrite-containing pseudomorphs after Ca-sulphates as widespread throughout the Tulomozero Formation in the SE Fennoscandian Shield, implying that surface waters were oxidized and a large SO marine reservoir was developed as early as 2100 Ma. The Ca-sulphates and associated magnesite and halite precipitated syn-depositionally from oxidized, evolved and modified seawater in coastal playa, sabkha and intertidal flat settings. 87Sr/86Sr and δ13C of associated 13C-rich stromatolitic dolostones were environmentally controlled with the highest ratios occurring in playa and sabkha carbonates. The results imply that the Palaeoproterozoic δ13Ccarb excursion was amplified by 8‰ by local environmental factors and calls into question many observations of putative δ13C global signals reported previously from similar Palaeoproterozoic, evaporitic, dolostones. The local environmental amplification can explain a large regional and intercontinental δ13C discrepancy observed in synchronous carbonates. 相似文献
16.
Gregory F. de Souza Ben C. Reynolds Bernard Bourdon 《Geochimica et cosmochimica acta》2010,74(9):2596-5573
The stable isotope composition of strontium (expressed as δ88/86Sr) may provide important constraints on the global exogenic strontium cycle. Here, we present δ88/86Sr values and 87Sr/86Sr ratios for granitoid rocks, a 150 yr soil chronosequence formed from these rocks, surface waters and plants in a small glaciated watershed in the central Swiss Alps. Incipient chemical weathering in this young system, whether of inorganic or biological origin, has no resolvable effect on the 87Sr/86Sr ratios and δ88/86Sr values of bulk soils, which remain indistinguishable from bedrock in terms of Sr isotopic composition. Although due in part to the chemical heterogeneity of the forefield, the lack of a resolvable difference between soil and bedrock isotopic composition indicates that these soils have thus far witnessed minimal net loss of Sr; a low degree of chemical weathering is also implied by bulk soil chemistry. The isotopic composition of Sr in streamwater is more radiogenic than median soil, reflecting the preferential weathering of biotite in the catchment; streamwater δ88/86Sr values, however, are indistinguishable from bulk soil δ88/86Sr values, implying that no resolvable fractionation of Sr isotopes takes place during release to the weathering flux in the Damma forefield. Analyses of plant tissue reveal that plants (Rhododendron and Vaccinium) preferentially assimilate the lighter isotopes of Sr such that their δ88/86Sr values are significantly lower than those of the soils in which they grow. Additionally, δ88/86Sr values of foliar and floral tissues are lower than those of roots, contrary to observations for Ca, for which Sr is often used as an analogue in weathering studies. We suggest that processes that discriminate against Sr in favour of Ca, due to the different nutritional requirement of plants for these two elements, are responsible for the observed contrast. 相似文献
17.
C. Lerouge E.C. Gaucher P. Negrel C. Guerrot P. Michel S. Buschaert 《Geochimica et cosmochimica acta》2010,74(10):2926-2663
Strontium is a good monitor of geochemical processes in natural clayey formations. In the Callovian-Oxfordian formation of Bure in France, strontium is sorbed on clay minerals and carried by carbonates, detrital minerals and accessory celestite. In order to determine the strontium distribution among these different phases, four-step sequential extractions (1. cobalt hexamine trichloride, 2. acetic acid, 3. EDTA and 4. tri-acid) were performed on samples from different levels of the clayey formation. The leachates were also analyzed for strontium isotopes, in order to determine the strontium origins. This sequential procedure is well suited to determining strontium distribution in claystones, although it is less efficient in clay-rich limestones and in celestite-rich samples. The carbonates (38-47% of the total strontium) show 87Sr/86Sr ratios (0.7070-0.7071) that have recorded the isotopic composition of the Callovian-Oxfordian seawater. Diagenetic carbonates (dolomite, ankerite and siderite) have almost not incorporated any strontium, which has been trapped by celestite during the late diagenesis. The major part of the celestite shows 87Sr/86Sr ratios (0.7069-0.7070) quite close to the primary carbonates. However, a second generation of celestite (0.7074) shows a slight 87Sr-enrichment and is isotopically in equilibrium with the exchangeable strontium (27-48% of the total strontium with a mean 87Sr/86Sr value of 0.70745) and the present-day porewater (0.7074). This very low 87Sr-enrichment could be explained by the partial destabilisation of detrital minerals (feldspars, micas, clays) which exhibit 87Sr/86Sr ratios consistent with their pristine Hercynian origin (0.7229-0.7350). Diffusion of strontium from the subjacent Dogger aquifers (0.7076-0.7082) could also be invoked to explain the slight 87Sr-enrichment. 相似文献
18.
《Applied Geochemistry》2003,18(1):117-125
This paper describes the results of a study that was conducted to determine the relationship between hydrogeochemical composition and 87Sr/86Sr isotope ratios of the Mt. Vulture spring waters. Forty samples of spring waters were collected from local outcrops of Quaternary volcanites. Physico-chemical parameters were measured in the field and analyses completed for major and minor elements and 87Sr/86Sr isotopic ratios. A range of water types was distinguished varying from alkaline-earth bicarbonate waters, reflecting less intense water–rock interaction processes to alkali bicarbonate waters, probably representing interaction with volcanic rocks of Mt. Vulture and marine evaporites. The average 87Sr/86Sr isotope ratios suggest at least 3 different sources. However, some samples have average Sr isotope ratios (0.70704–0.70778) well above those of the volcanites. These ratios imply interaction with other rocks having higher 87Sr/86Sr ratios, probably Triassic evaporites, which is substantiated by their higher content of Na, SO4 and Cl. The Sr isotope ratios for some samples (e.g. Toka and Traficante) are intermediate between the value for the Vulture volcanites and that for the local Mesozoic rocks. The salt content of these samples also lies between the value for waters interacting solely with the volcanites and the value measured in the more saline samples. These waters are thus assumed to result from the mixing of waters circulating in volcanic rocks with waters presumably interacting with the sedimentary bedrock (marine evaporites). 相似文献
19.
现有公布的数据显示,海水87Sr/86Sr值在中三叠世安尼期较为平稳,有别于早三叠世因环境导致的动荡变化,为二叠纪末生物大灭绝之后生物复苏的加速期,而罗平生物群就是该加速期的标志性生物群。本文研究了含罗平生物群层位标准剖面-大凹子剖面上连续沉积的26件碳酸盐岩样品的锶同位素组成与演化,用于了解含罗平生物群层位沉积海水锶同位素的变化趋势。结果显示:(1)碳酸盐岩中Sr平均值大于2000×10-6,主要来自于文石和海洋成岩作用;(2)低Mn和Mn/Sr0.4说明:87Sr/86Sr数据在很大程度上能代表同时期海水中的87Sr/86Sr比值;(3)这些样品的87Sr/86Sr值变化在0.707969～0.708253,平均值为0.708025,锶同位素比值变化曲线呈小幅度的起伏波动,其值明显大于同时期欧洲等地公布的数据,但同华南其他地区公布的数据具有相似性,推测含罗平生物群层位锶同位素组成仅代表区域事件,受印支运动影响。 相似文献
20.
Systematic shifts of oxygen isotopic compositions in the higher grade parts of the high temperature-low pressure Hercynian metamorphic sequence, exposed in the Trois Seigneurs Massif, have previously been explained as a result of an influx of surface-derived water during the prograde part of the metamorphic cycle. It has been suggested that this caused a regional lowering of 87Sr/86Sr in the metamorphic sequence. Mapping of strontium isotopic compositions across a 15 m meta-carbonate horizon in the higher grade pelite-psammite sequence shows that strontium isotopic compositions were homogenised over length scales of metres or less during the Hercynian metamorphism, which brought the carbonate and pelite-psammite to oxygen isotopic equilibrium with a common fluid. Comparison of model pre-Hercynian 87Sr/86Sr profiles across the carbonate (based on a depositional/diagenetic age of 450 Ma and initial 87Sr/86Sr ratio of 0.7086 given by 10 m length scale averaging) with the post-Hercynian 87Sr/86Sr profile (calculated from analysed 87Sr/86Sr and Rb/Sr compositions) implies strontium isotopic diffusion distances of ca. 0.4 m in the carbonate and ca. 7 m in the pelite-psammite. The limited Sr-isotopic diffusion distance of 0.4–0.7 m within the carbonate is compatible with pervasive oxygen-isotopic exchange over distances restricted to 4–15 m if fluid strontium concentrations were between 4 and 50 ppm. The strontium isotopic transport distances are not compatible with pervasive oxygen isotopic alteration over the observed 5 km regional scale. Either the flow was perfectly layer-parallel or, more probably, the regional-scale alteration of oxygen took place by fluid circulation in the brittle regime early in, or prior to, the Hercynian metamorphic event. Flow along cracks with incomplete diffusive exchange between fluid and wall rock would allow greater decoupling of oxygen and strontium isotopic transport than pervasive advective transport with local fluid-solid equilibrium. 相似文献