Effects of cation-exchange treatment and acid leaching on the Rb-Sr system of illite from Fithian,Illinois

Size fractions <5 μm of Fithian illite were treated with 1 N HCl, H⁺- from cation-exchange resin, and 5% NH₄-EDTA-solution, and analyzed for their Rb-Sr isotopic composition. In comparison to the effects of HCl, the ion-exchange resin leached the same amount of Sr, but significantly less Rb and radiogenic ⁸⁷Sr from poorly-crystallized 1 M illite.An age of 284 ± 8 Ma (I.R. 0.7109 ± 6 is more or less in agreement with the Missourian (Stephanian) age of deposition. The agreement is fortuitous because of the presence of small amounts of detrital 2 M illite and kaolinite, thus the true age of diagenesis might be somewhat less.${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio measurements of Tithonian Solnhofen limestone demonstrate that, even though the silicate component constitutes less than 5% of the total, the majority of the radiogenic ⁸⁷Sr is leached from “broken-bond” surfaces of the nearly pure kaolinite-residues during the HCl treatment. Ion-exchange and EDTA treatment can be useful in this situation to obtain a precise separation of environmental Sr (in carbonate) from radiogenic ⁸⁷Sr (in silicates), and an adequate spread of $\text{Rb}\text{Sr}$ values from argillaceous fine fractions in the construction of isochrons.     

87Sr/86Sr composition of seawater during the Phanerozoic

${}^{87}\text{Sr}{}^{86}\text{Sr}$ 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 ${}^{87}\text{Sr}{}^{86}\text{Sr}$ 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.     

A strontium isotopic study of Smackover brines and associated solids,southern Arkansas

The isotopic composition of Sr has been measured in brine samples from the Upper Jurassic Smackover Formation in southern Arkansas; ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios range from 0.7071 to 0.7101. With one exception, the 32 Smackover brines contain Sr which is significantly more radiogenic than the Sr in Late Jurassic sea water, indicating sizable Sr contributions from detrital sources. Isotopic analyses of core samples from rock units associated with the brines and regional stratigraphic relationships suggest that the radiogenic Sr was released from detrital minerals in Bossier shale to interstitial fluids expelled from the underlying Louann Salt in the North Louisiana salt basin. These fluids migrated through the Bossier Formation updip to the South Arkansas shelf, where they entered the upper Smackover carbonate grainstone. The radiogenic fluids mixed with Sr-rich interstitial marine waters that had the isotopic composition of Late Jurassic sea water; mixing in variable proportions resulted in the random distribution pattern of variable ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios that is observed in Smackover brines within the 5000 km² study area. Isotopic analyses of nonskeletal carbonate grains and coexisting coarse calcspar cement from the upper Smackover grainstone imply that the grains were diagenetically stabilized in the presence of interstitial marine waters, whereas the calcspar cement is a relatively late diagenetic phase precipitated after the arrival of radiogenic fluids.     

The RbSr isotope system as an index of origin and diagenetic evolution of southern Pacific red clays

Morphological, mineralogical, chemical and RbSr isotopic studies have been made on Fesmectites (nontronites) from southern Pacific red clays cored near the Marquisas Islands. These minerals have at the top of the core, an ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of 0.70917 ± 0.00007, which indicates an authigenic origin in isotopic equilibrium with seawater. Weak leaching experiments with 1N HCl show that the nontronites also contain a volcanic component with a lower ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio which, combined with the morphology of the particles, suggests a transportation by bottom currents of clay formed elsewhere.During burial, the nontronites experience diagenetic modifications resulting in an increase in Fe, K and Rb contents, a concomitant decrease of Mg, Ca, Ti, Na and Sr, and a preferential migration of radiogenic ⁸⁷Sr from the clays into the surrounding pore waters.The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of the Sr adsorbed on the outermost surfaces of the nontronites does not change with depth in the core, and is, therefore, independent of diagenetic influence, which is rather characterized by the ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of the interstitial waters. The isotopic composition of both the adsorbed Sr and that of the pore fluids may yield useful information on the crystallization environment and subsequent history of deep sea red clays.     

Strontium isotopic studies of the volcanic rocks of the Saunda arc,Indonesia, and their petrogenetic implications

Pleistocene and Recent lavas from the Sunda arc range from those showing affinities with the island arc tholeiitic series, through a spectrum of calc-alkaline to high-K alkaline rocks. The tholeiitic rocks have relatively low ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios averaging 0–7043; the calc-alkaline rocks show a wide range (from 0.7038 to 0.7059, averaging 0.7048); the high-K alkaline rocks average 0.7045. A rhyolitic ignimbrite from Sumatra has an ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of 0.7139.The relationship between ${}^{87}\text{Sr}{}^{86}\text{Sr}$ and major and trace element geochemistry is variable and complex. Lavas from the same volcano sometimes show significant differences in ${}^{87}\text{Sr}{}^{86}\text{Sr}$ despite close geochemical relationships. Rocks of the calc-alkaline suite show a regular decrease in ${}^{87}\text{Sr}{}^{86}\text{Sr}$ from West Java to Bali and there is some evidence for increasing ${}^{87}\text{Sr}{}^{86}\text{Sr}$ with increasing depth to the Benioff zone. Calc-alkaline and tholeiitic rocks from the Sunda arc have significantly higher ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios than those from other island arcs, except from those arcs where continental crustal involvement has been inferred (e.g. New Zealand).A model of ⁸⁷Sr enrichment due to isotopic equilibration of oceanic crust with sea water and disequilibrium melting in the slab and/or mantle is favoured to explain the Sr isotopic composition of the tholeiitic and normal calc-alkaline lavas. Calc-alkaline lavas with high ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios are best explained by either sialic contamination, or the presence of alkali basalt as a component of the downgoing slab. The Sr isotopic data for the high-K alkaline lavas suggest a mantle origin. The high ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio in the Lake Toba rhyolite implies a crustal origin.     

Strontium isotope ratios in volcanic rocks from the northwestern part of the Hellenic arc

${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios have been determined in fifteen volcanic rocks from the northwestern part of the Hellenic arc. They range from 0.7041 to 0.7134. There is no apparent correlation of strontium isotope values with any major chemical component or with Rb/Sr ratios. The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios appear to increase in a general way with increasing depth to the Benioff zone. The strontium isotope ratios are higher than from most island arcs; this is believed to be due to contamination.     

Strontium isotopic contamination and oxidation during ocean floor hydrothermal metamorphism of the ophiolitic rocks of the Troodos Massif,Cyprus

Twenty-six new high precision ${}^{87}\text{Sr}{}^{86}\text{Sr}$ratio determinations and existing analyses are used to discuss the strontium isotopic composition of the Upper Cretaceous ophiolitic rocks of the Troodos Massif, Cyprus. Relative to initial magmatic ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios (0.70338 ± 0.00010 to 0.70365 ± 0.00005), the hydrothermally metamorphosed pillow lavas and dyke complex have been contaminated by isotopically heavier strontium.This observation confirms the hypothesis that hydrothermal metamorphism was a consequence of sea water-rock interaction, since sea water was the only readily accessible reservoir of isotopically heavier strontium. The fact that metagabbros and altered trondhjemites were also Sr isotopically contaminated shows that sea water penetrated approximately 2 km into the oceanic crust represented by the ophiolitic sequence.The amount of Sr isotopic contamination requires that the bulk sea water: rock ratio was at least ~15:1 and shows that water-rock interaction occurred in a flow system. The degree of oxidation decreases with increasing depth. This shows that the vertical component of fluid flow was downward. The absolute bulk water/rock ratio (for water at S.T.P.), as estimated from the oxidation profile, may have been as large as ~3 × 10³:1 —a large figure which independently confirms that rocks showing strong δ¹⁸O shifts have interacted with large volumes of water.The sites of discharge of the hot fluid, which must have come out of the system, are identified as the cupriferous pyrite ore deposits. This process of mass transfer corresponds to hydrothermal convection in a permeable medium with an open upper boundary surface.     

Strontium isotope geology of the South Mountain batholith,Nova Scotia

The South Mountain batholith of southwestern Nova Scotia is a large, peraluminous, granodiorite-granite complex which intrudes mainly greenschist facies metasediments of the Cambro-Ordovician Meguma Group. Using Rb-Sr isochrons constructed from whole rocks and mineral separates, the present study shows a variation in age and initial ratios of the intrusive phases of the batholith as follows: biotite granodiorite (371.8 ± 2.2 Ma, (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{\mathrm{i}}$ ranges from 0.7076 ± 0.0003 to 0.7090 ± 0.0003, with the average = 0.7081); adamellite (364.3 ± 1.3 Ma, $\text{(}{}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{\mathrm{i}}\text{= 0.70942 ± 35}$); porphyry (361.2 ± 1.4 Ma, $\text{(}{}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{\mathrm{i}}\text{= 0.71021 ± 119}$); using λ⁸⁷Rb = 1.42 × 10^?11yr^?1.A suite of Meguma country rock samples showed a variation of ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.7113?0.7177}$ at the time of intrusion of the batholith. A number of xenoliths of this material occurring in the marginal granodiorite had partially equilibrated isotopically with the granodiorite at a higher ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio than elsewhere in the granodiorites. This evidence demonstrates that isotopic (and probably some accompanying bulk chemical) contamination by the Meguma rocks has been an important factor in determining the ultimate chemical composition and mineralogy of the South Mountain batholith.The $\text{(}{}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{372}\text{= 0.7081}$ of the early granodiorites indicates that the parent magma of the South Mountain batholith was derived from a source unlike the Meguma Group. The precise nature of the source region cannot be determined by Rb-Sr work unless the degree of contamination with Megumalike material is known.     

Strontium-isotope composition in the Tethys Sea,Euboea, Greece

We have determined ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of the Tethys Sea by analyzing limestones of Permian to Late Cretaceous age from the overthrusted sequence of central and northern Euboea. The results are in satisfactory agreement with ratios obtained by other investigators on unreplaced fossil shells of similar age collected elsewhere. The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of Mesozoic marbles from the autochtone metamorphic rocks of southern Euboea are more variable, perhaps because of increases in this ratio during regional metamorphism. Nevertheless, the lowest values of suites of isochronous samples may approach the correct ratio. Four samples of the Argyron Marble taken at different localities in southern Euboea and on the Attic peninsula have a concordant ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of 0.70688±0.00006 (1 $\text{}\text{?}$gs), relative to 0.7080 for the Eimer and Amend isotope standard. This value indicates a Jurassic age for this formation even though the fragmentary fossil evidence favors a Late Cretaceous (Turonian) age.     

Strontium isotopic composition of several oilfield brines from Kansas and Colorado

The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of several oilfield brines associated with Paleozoic dolomites, cherty dolomites and sandstones from Kansas and Colorado range from 0.7113 to 0.7341. Two brines in the Mississippian dolomites from Colorado were found to contain the most radiogenic strontium. The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of brines in a small oilfield in eastern Kansas are constant, and the ratios may suggest that in this field there may be only one oil pool. The isotopic composition of strontium in subsurface waters could be useful in determining hydrologic continuity among reservoirs and for obtaining additional information on the origin and migration of these fluids.     

Comment on “the statistical analysis and interpretation of imperfectly-fitted Rb-Sr isochrons from polymetamorphic terrains” by M. Cameron et al.

Cameronet al. (1981) proposed a “Free Line Model” for calculating formation ages for rock systems that have undergone local scale homogenization by ⁸⁷Sr migration. This model is valid only if the variation in the ${}^{87}\text{Rb}{}^{86}\text{Sr}$ ratio is independent of the variation in Sr-content within the rock system. If Sr increases linearly with increasing ${}^{87}\text{Rb}{}^{86}\text{Sr}$ the calculated age will be too high. If Sr decreases linearly with increasing ${}^{87}\text{Rb}{}^{86}\text{Sr}$, the calculated age will be systematically too low. If the rate of change in Sr with increasing ${}^{87}\text{Rb}{}^{86}\text{Sr}$ varies systematically through a rock system the “isochron” will be curved and the calculated ages will be younger or older than the real age, depending on the position of samples in relation to the curvature of the “isochron.” This problem with the “Free Line Model” is inherited in both the “Bulk Earth Model 1” and the “Bulk Earth Model 2.”     

Sr and Nd isotopic evidence for petrogenesis of mid-tertiary felsic volcanism in the mineral district of Zacatecas,Zac. (Sierra Madre Occidental), Mexico

Twelve samples of mid-Tertiary felsic volcanic rocks from Zacatecas and San Luis Potosí (both belonging to the Sierra Madre Occidental) and one sample of Lower Tertiary porphyritic andesite from Zacatecas are analyzed for ${}^{87}\text{Sr}{}^{86}\text{Sr}$, K, Rb, and Sr. Eight selected samples are also analyzed for ${}^{143}\text{Nd}{}^{144}\text{Nd}$. A linear regression of the present-day ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{and}{}^{87}\text{Rb}{}^{86}\text{Sr}$ of the felsic volcanic rocks in Zacatecas gives an approximate date of 30 ± 8 Ma. The initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios are high and widely distributed (from 0.705 to 0.712 or higher) whereas the initial ${}^{143}\text{Nd}{}^{144}\text{Nd}$ ratios are somewhat low and show a narrow range (0.5125–0.5127). The available isotopic and trace-element data are best explained in terms of a binary mixing model in which the magmas derived from a slightly depleted-mantle fractionate and mix with varying proportions of the overlying middle/upper continental crust and undergo further shallow-level fractional crystallization before eruption. This model is also compatible with the trace-element and Sr isotopic data published from other areas of the Sierra Madre Occidental for which a purely mantle origin has been proposed.     

87Sr86Sr in Late Proterozoic carbonates: evidence for a “mantle” event at ∼900 Ma ago

Strontium isotopic measurements were made on Late Proterozoic carbonates from West African Craton. Comparison of samples with acceptable trace element patterns with coeval data from southern Africa and with the published Australian results suggests that the ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of the Late Proterozoic sea water evolved in the following manner about 0.7075 at 1000 ± 50 Ma, 0.7056 to 0.7074 at 900 ± 50 Ma, 0.7068 to 0.7091(0.7106) at 800 ± 50 Ma, 0.7074 to 0.7077 at 700 ± 50 Ma, and 0.7076 to 0.7089(0.7096) at 600 ± 50 Ma ago. The variations are comparable in magnitude and frequency to those described previously for the Phanerozoic. Strontium isotopic values in the radiogenic part of this range suggest that the continental river flux of Sr into Late Proterozoic oceans was of comparable isotopic composition to its present day counterpart (～0.711). Consequently, the non-radiogenic ${}^{87}\text{Sr}{}^{86}\text{Sr}$ value at ～900 ± 50 Ma ago signifies a large flux of “mantle” strontium into the ocean at this time. Because the present time resolution is only about 75 ± 25Ma, additional sampling as well as better stratigraphie resolution and more definite selection criteria are required for construction of a more detailed Late Proterozoic sea water curve.     

The variation of the marine 87Sr86Sr ratio during Phanerozonic time: interpretation using a flux model

The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio in sea water has varied over geologic time due to the addition of strontium to the sea from rocks with a variety of ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios. The measurements by Petermanet al. (Geochim. Cosmochim. Acta34, 105–120, 1970) of the value of the marine ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio have been confirmed by several other workers and by some new measurements on JOIDES samples. They form the basis of a model calculation of the relative proportions of ‘basaltic’ (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.704}$) and ‘granitic’ (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.718}$) strontium being supplied to the sea. For the last 200 million years, the proportions of these two sources appear to reflect the history of global tectonics; ‘basaltic’ during rifting and increasingly ‘granitic’ during the present episodes of uplift and continental collision     

Strontium isotope geochemistry of Icelandic geothermal systems and implications for sea water chemistry

Chemical and Sr isotopic analyses have been made of waters from 16 geothermal sites in Iceland with particular reference to the systems at Reykjanes and Svartsengi for which compositions of geothermal sea water and fresh and hydrothermally-altered rocks have been compared. The alkalies display mixing relationships indicating a hydrothermal input of Rb and K to local meteoric and sea waters as do results for Sr and Ca involving high-temperature fluids. ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of the geothermal waters of meteoric origin parallel those of associated rocks but are higher. Ratios for geothermal sea waters are 0.7042 (Reykjanes) and 0.7040 (Svartsengi), lower than for normal sea water (0.7092) because of leaching of Sr from rocks followed by partial removal into alteration minerals, of which epidote and chlorite may be most important. Consequently, associated hydrothermally-altered rocks have been subject to significant Sr isotopic contamination by sea water Sr raising ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios from 0.7032 for fresh rock to 0.7038–0.7042 (Reykjanes) and to 0.7039–0.7041 (Svartsengi). Altered basalt is only ~50% equilibrated isotopically with geothermal sea water, at a water/rock ratio of ~2, but is internally equilibrated whereas palagonitized rocks (water/rock ratio of 3 to 4) are close to Sr isotopic equilibrium with associated sea water but show significant internal Sr disequilibrium. Hydrothermal input is unlikely to be important in the oceanic mass balance of Sr but is likely to be highly significant in controlling the strontium isotopic composition of sea water.     

Systematic variations of sialic and volcanic detritus in piston cores from the Red Sea

The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios and Sr concentrations of the non-carbonate fractions of sediment from two cores taken in the median valley of the Red Sea indicate that the detrital fractions are mixtures of two components originating from old sialic and young volcanic rocks of the surrounding land areas. The mixing equations were derived from the data and were used to estimate the Sr concentrations of the two components. The volcanogenic detritus has a Sr content between 770 and 800 ppm while the component derived from old sialic rocks contains from 50 to 70 ppm. The volcanic component consists primarily of unweathered particles of alkali basalt and volcanic glass while the sialic component is represented by clay minerals and X-ray amorphous material. Systematic variations of the concentration of volcanogenic detritus were used to define layers of sediment which correlate over a distance of about 200 km separating the two cores. The concentrations of quartz, feldspar, amphibole, illite, kaolinite and chlorite were found to vary systematically with the content of volcanogenic detritus as calculated from the Sr data. The carbonate concentrations of the sediment range from 33 to 87% and appear to increase in a southerly direction. The rate of carbonate deposition increased at times of more efficient input of sediment derived from old sialic rocks. This relationship is explained by the hypothesis that both were influenced indirectly by brief warming trends during the Würm glaciation. The occurrence of sediment layers enriched in sediment derived from old sialic rocks correlates approximately with interstadials of the Würm glaciation in northern Europe. Increased input of volcanogenic detritus occurred around 37,000 and 26,000yr ago and during the past 5000 yr. A plot of ${}^{87}\text{Sr}{}^{86}\text{Sr}$ and ${}^{87}\text{Rb}{}^{86}\text{Sr}$ ratios defines a line on the Rb-Sr isochron diagram yielding a fictitious date of about 230 million yr.     

Composition isotopique du strontium d'eaux interstitielles extraites de sédiments récents: un argument en faveur de l'homogénéisation isotopique des minéraux argileux

The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of interstitial waters squeezed from recent sediments of the Pacific Ocean increases from 0.70920 ± 0.00017 (2σ) to 0.70960 ± 0.0023 and 0.70984 ± 0.00040 at 1.50 and 1.60 m depth—reference value for sea water: 0.70910 ± 0.00035. This variation underlines the likely existence of exchanges between the clays and the interstitial environment. The isotopic homogenization of strontium between the clays and their environment, result of these changes, becomes thus a credible phenomenon. This is a new argument for the dating of clays by the $\text{Rb}\text{Sr}$ method.     

Isotopic composition of strontium in Indian kimberlites

Strontium isotopic studies on twenty three whole rock kimberlites from two petrographic provinces in India show variation of initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios from 0.7027 to 0.7102. The variation is unrelated to the degree of alteration. Between the micaceous and basaltic varieties there is some overlap in the Sr isotopic ratios. Leaching experiments on whole rock samples showed more highly radiogenic Sr in leaches compared to the bulk samples.In two diatremes, the initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios show a positive correlation with $\text{Rb}\text{Sr}$ which is believed to reflect a source event earlier than the formation of the kimberlites. The observed Sr isotopic data can be caused by (i) melting of a heterogeneous source or (ii) disequilibrium partial melting in the source region. In the former case, variable isotopic composition would be a necessary consequence of melting in small subsystems.