Neodymium isotopic composition of Quaternary island arc lavas from Indonesia

${}^{143}\text{Nd}{}^{144}\text{Nd}$ ratios measured in Quaternary lavas from Java and the Banda arc of Indonesia range from 0.51242 to 0.51280 and exhibit an inverse correlation with ${}^{87}\text{Sr}{}^{86}\text{Sr}$. Isotopically, the Indonesian samples resemble Andean rather than island arc lavas. The samples from Java plot either within, or adjacent to the mantle array, towards higher ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios. Samples from the Banda arc and the anomalous calc-alkaline volcano Papandajan are characterized by relatively low ${}^{143}\text{Nd}{}^{144}\text{Nd}$ and high ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios. These characteristics are consistent with the interpretation that subducted terrigenous material was involved in the genesis of these lavas. Furthermore the Banda arc samples appear to lie on a mixing line between isotopic compositions characteristic of the mantle and upper continental crust. A high-K trachyte from the alkaline volcano Muriah, Java, has isotopic characteristics of the mantle (${}^{143}\text{Nd}{}^{144}\text{Nd}\text{= 0.51270}$, ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.70424}$), which implies that the extreme enrichment in large-ion-lithophile elements in its source must have occurred only shortly before its formation. The inferred ${}^{143}\text{Nd}{}^{144}\text{Nd}$ ratio of the unmodified mantle beneath Java and the Banda arc is lower than that observed in mid-ocean ridge basalt, which may have important implications for a better understanding of the geochemical structure of the mantle.     

Columbia River volcanism: the question of mantle heterogeneity or crustal contamination

Basalts from the Columbia River flood basalt province of the northwestern U.S.A. show a large diversity in chemical and Nd and Sr isotopic compositions. ${}^{143}\text{Nd}{}^{144}\text{Nd}$ ranges from 0.51303 to 0.51208 and is strongly correlated with variations in ${}^{87}\text{Sr}{}^{86}\text{Sr}$. This correlation suggests mixing between two end member compositions, one characterized by ${}^{143}\text{Nd}{}^{144}\text{Nd}\text{> 0.51303}$ and ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{< 0.7035}$, and the other with ${}^{143}\text{Nd}{}^{144}\text{Nd}\text{< 0.5120}$ and ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{> 0.715}$. The more radiogenic component could be mantle enriched in incompatible elements during the Precambrian, or Precambrian materials of the continental crust. A quartz-rich xenolith found in the Columbia lavas has Rb-Sr and Sm Nd model ages of ≈ 1.4Æ, implying the existence of old, isotopically evolved crustal basement which could serve as contaminant. Nevertheless, crustal contamination alone cannot explain the chemical variation of the samples studied, and other fractionation processes must have occurred simultaneously. A model involving combined assimilation and crystal fractionation reproduces the chemical and isotopic characteristics of the volumetrically dominant Grande Ronde unit for an assumed crystallizing component of plagioclase, low calcium pyroxene and minor olivine. The data are not consistent with the suggestion that a ‘primordial’ mantle is the source for this continental flood basalt province. Rather they suggest that the main volume of these lavas was originally derived from a mantle similar in isotopic composition to island arc and ocean island basalts of the north Pacific. The primary magma was modified chemically and isotopically by crystal fractionation and assimilation of sialic crustal materials during its transport through, or storage in the continental crust.     

Chemical and isotopic evidence for mixing between depleted and enriched mantle,northwestern U.S.A.

Combined elemental and Sr, Nd, Pb and O isotopic data for late Cenozoic olivine tholeiite lavas from the northwestern Great Basin indicate derivation from at least two chemically and isotopically distinct mantle source regions with no significant modification by interaction with continental crust. The lack of crustal involvement is a direct reflection of the extensional tectonic environment which favors rapid ascent of magmas, minimal residence time in crustal magma chambers and scattered fissure eruptions.The observed chemical and isotopic variations in the tholeiite suite are attributed to mixing between depleted oceanic type mantle (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{~ 0.7030}$ and ${}^{143}\text{Nd}{}^{144}\text{Nd}\text{~ 0.51305}$) and old, chemically heterogeneous, isotopically enriched subcontinental mantle (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{~ 0.7078}$ and ${}^{143}\text{Nd}{}^{144}\text{Nd}\text{~ 0.51233}$). Model incompatible element concentrations suggest strong similarities between the depleted mantle and the mantles beneath normal oceanic ridge segments and back-arc basins and between the enriched mantle and the mantle beneath enriched oceanic ridge segments such as the Azores. Superimposed upon the characteristics derived from the two component mixing model may be the effects of a third mantle source which is identifiable only by its apparent radiogenic ${}^{206}\text{Pb}{}^{204}\text{Pb}$ ratios. If present, this third source may reflect a component derived from the downgoing slab of an ancient subduction zone.     

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.     

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.     

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.     

Importance of the Lu-Hf isotopic system in studies of planetary chronology and chemical evolution

The ¹⁷⁶Lu-¹⁷⁶Hf isotope method and its applications in earth sciences are discussed. Greater fractionation of Lu/Hf than Sm/Nd in planetary magmatic processes makes ${}^{176}\text{Hf}{}^{177}\text{Hf}$ a powerful geochemical tracer. In general, proportional variations of ${}^{176}\text{Hf}{}^{177}\text{Hf}$ exceed those of ${}^{143}\text{Nd}{}^{\mathrm{l44}}\text{Nd}$ by factors of 1.5–3 in terrestrial and lunar materials. Lu-Hf studies therefore have a major contribution to make in understanding of terrestrial and other planetary evolution through time, and this is the principal importance of Lu-Hf. New data on basalts from oceanic islands show unequivocally that whereas considerable divergences occur in ${}^{176}\text{Hf}{}^{177}\text{Hf}\text{-}{}^{87}\text{Sr}{}^{86}\text{Sr}$ and ${}^{143}\text{Nd}{}^{\mathrm{l44}}\text{Nd}\text{-}{}^{87}\text{Sr}{}^{86}\text{Sr}$ diagrams, ${}^{176}\text{Hf}{}^{177}\text{Hf}$ and ${}^{143}\text{Nd}{}^{144}\text{Nd}$ display a single, linear isotopic variation in the suboceanic mantle. These discordant ${}^{87}\text{Sr}{}^{86}\text{Sr}$ relationships may allow, with the acquisition of further Hf-Nd-Sr isotopic data, a distinction between processes such as mantle metasomatism, influence of seawater-altered material in the magma source, or recycling of sediments into the mantle. In order to evaluate the Hf-Nd isotopic correlation in terms of mantle fractionation history, there is a need for measurements of Hf distribution coefficients between silicate minerals and liquids, and specifically for a knowledge of Hf behavior in relation to rareearth elements. For studying ancient terrestrial Hf isotopic variations, the best quality Hf isotope data are obtained from granitoid rocks or zircons. New data show that very U-Pb discordant zircons may have upwardly-biased ${}^{176}\text{Hf}{}^{177}\text{Hf}$, but that at least concordant to slightly discordant zircons appear to be reliable carriers of initial ${}^{176}\text{Hf}{}^{177}\text{Hf}$. Until the controls on addition of radiogenic Hf to zircon are understood, combined zircon-whole rock studies are recommended. Lu-Hf has been demonstrated as a viable tool for dating of ancient terrestrial and extraterrestrial samples, but because it offers little advantage over existing methods, is unlikely to find wide application in pure chronological studies.     

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.     

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.     

Rb—Sr and Sm—Nd geochronology of lower Proterozoic granite—greenstone terrains in French Guiana,South America

Nine samples of metavolcanic rock from the lower parts of greenstone belts in central French Guiana (the Paramaca series) and 14 granitic samples from the intrusive gneisses (the Degrad Roche and Arawa gneisses) were selected for Sm—Nd and Rb—Sr analysis.The Sm—Nd results from the metavolcanic series (including two tholeiites, five peridotitic komatiites and two andesites) yield an isochron age of 2.11±0.09 (2 σ) Ga with an initial ${}^{143}\text{Nd}{}^{144}\text{Nd}$ ratio (I_Nd) of 0.51002±9 (2 σ), corresponding to ?_Nd(T) = + 2.1 ± 1.8. This isochron is interpreted as representing the age of initial volcanism of the Paramaca series. Acid intrusives were dated by the Rb—Sr method. A whole rock Rb—Sr isochron, including data points from both the Degrad Roche and Arawa gneisses, yields an age of 2.00±0.07 (2 σ) Ga with initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio (I_Sr value) of 0.7019±4 (2 σ). This result is considered to be the time of emplacement of the orthogneiss protoliths.The positive ε_Nd value (+ 2.1 ± 1.8) obtained from the metavolcanic rocks of French Guiana suggests that their mantle sources have evolved in reservoirs slightly depleted in Light Rare Earth Elements (LREE). This result confirms the possible existence of ancient LREE-depleted reservoirs within the lower Proterozoic mantle. Moreover, the high ε_Nd(T) value for these rocks excludes any significant crustal contamination during magma genesis.The French Guianese orthogneisses yield a low I_Sr value (0.7019±4 (2 σ)) which, together with geochemical considerations, suggests that their granitic protoliths could have originated by partial melting of short-lived crustal precursors of basaltic to granodioritic composition.The present geochronological and isotopic study suggests that the Guiana Shield may represent a major continental accretion event during the lower Proterozoic.     

Isotopic and incompatible element constraints on the genesis of island arc volcanics from Cold Bay and Amak Island,Aleutians, and implications for mantle structure

Cold Bay and Amak Island, two Quaternary volcanic centers in the eastern Aleutians, are orthogonal relative to the trench and separated by ~50 km. Sr, Nd and Pb isotopic compositions of the calc-alkaline andesite magmas show no sign of contamination from continental crust (average ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.70323}$, ${}^{143}\text{Nd}{}^{144}\text{Nd}\text{= 0.51301}$, ${}^{206}\text{Pb}{}^{204}\text{Pb}\text{= 18.82}$, ${}^{207}\text{Pb}{}^{204}\text{Pb}\text{= 15.571}$). These samples plot within the mantle arrays for Sr-Nd and for Pb and are similar to arcs such as the Marianas and New Britain (Sr-Nd) and Marianas and Tonga (Pb). Incompatible element ratios for the Aleutian andesites (K/Rb ~ 332, K/Cs ~ 10,600, K/Sr ~ 22.4, K/Ba ~ 18.3, Ba/La ~ 60) are within the range reported for arc basalts, despite the difference in degree of fractionation.Average K content, K/Rb, K/Ba and K/Sr are approximately the same for basalts from arcs and from oceanic islands (OIB); K/Cs is a factor of 4 lower and Ba/La almost 3 times higher in arcs. Abundance ratio correlations indicate that arcs are enriched in Cs and depleted in La relative to OIB, with other incompatible element abundances very similar. Histograms of Sr and Nd isotopic compositions for MORB, OIB, and intraoceanic arcs show remarkably similar peaks and distribution patterns for intraoceanic arcs and OIB.A “plum pudding” model for the upper mantle best accommodates a) geochemical coherence of OIB and IAV, b) the existence of mantle plumes at some oceanic islands, and c) the presence of a MORB-type source at back arc spreading centers. In this model, OIB plums are imbedded in a MORB matrix; small degrees of melting generate OIB-type magmas while larger degrees of melting dilute the OIB magma with MORB matrix melts.OIB plums are merely less robust lower mantle plumes (i.e., blobs) which are distributed throughout the upper mantle by convection. The existence of at least two types of OIB, as indicated by Sr, Nd, and Pb isotopes, suggests that nuggets of recycled oceanic lithosphère may coexist with lower-mantle plums and that both may be tapped in arcs and intraplate environments.     

Rb-Sr and U-Th-Pb systematics of alkaline rocks: the alkaline rocks from Italy

The isotopic composition of Pb and Sr and the abundances of Rb, Sr, U, Th, and Pb were determined for whole rock samples from all major volcanic centres of the Cenozoic alkaline volcanism of Central and South Italy, together with some samples from the contemporaneous anatectic Tuscan volcanism. The Sr and Pb isotopic compositions of the alkaline rocks show a negative correlation combined with a regional trend: the ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios decrease from 0.711 in the north-west to 0.704 in the south-east, while the ${}^{206}\text{Pb}{}^{204}\text{Pb}$ ratios increase from 18.7 to 20.0. Variations in both isotopic compositions are generally small throughout erupted rock sequences for any volcanic centre.The Pb and Sr isotopic abundance variations are interpreted on the basis of two alternative models, which correspond to two groups of geological processes: variations can result (i) from a time dependent development in subsystems with different $\text{Rb}\text{Sr}$ or $\text{U(Th)}\text{Pb}$ ratios or, (ii) from mixing of Sr or Pb with different isotopic compositions. Combining both Pb and Sr isotope abundance measurements it is shown that the source of each volcanic centre is formed by various degrees of mixing between two components. One component and the most southern Tuscan anatectic rocks most likely have a common source, whereas the other component of the mixing process is suggested to be a liquid fraction derived from a small degree of partial fusion of a hydrous mantle. Thus at least a two-stage evolution of the Italian alkaline rocks is indicated: first a mixing process leading to the formation of the parental material followed by differentiation processes leading to the formation of the erupted rock sequences.The geodynamic model which explains the data best is that of a lateral inhomogeneous mantle. The lateral inhomogeneities in the mantle would be the result of mixing between originally mantle and crustal derived material. The mixing process itself would not have any primary connection with the Quarternary volcanic activity.     

A study of strontium in core 119K,Discovery deep,Red Sea

The isotopic composition of strontium of pore water and of authigenic minerals leached from the sediment of core 119K with hot aqua regia is similar to that of the brine in the Discovery deep and differs from that of normal seawater. The average ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of strontium removed by acid leaching is 0.7077 ± 0.0007 (1σ) compared to a value of 0.70904 for the Red Sea. The detrital silicate fraction exhibits an approximate inverse correlation between ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios and strontium concentrations which provides tentative support for a model in which the detrital silicate fraction of deep-sea sediment is considered to be a mixture of terrigenous dust of sialic composition enriched in radiogenic ⁸⁷Sr and of volcanogenic material of basaltic composition and low ⁸⁷Sr abundance. The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of the shells of foraminifers and pteropods, expressed as δ ⁸⁷Sr‰ relative to 0.70904 for seawater, decrease from $\text{?0.23 ± 0.17‰}$ at 90 cm to $\text{?0.82 ± 0.17‰}$ at 273 cm and remain constant at this value to a depth of 450 cm. The lowering of the δ ⁸⁷Sr values is attributed both to the presence of aragonite overgrowths on pteropod shells and to possible isotope exchange with strontium in the connate fluid.     

The New England Batholith: constraints on its derivation from Nd and Sr isotopic studies of granitoids and country rocks

Nd and Sr isotopic compositions are reported for the granitic suites which comprise the late Palaeozoic to earliest Mesozoic New England Batholith of eastern Australia. Some of the granitic suites are typically I-type in their mineralogy, chemistry and isotopic compositions, implying a derivation from igneous (infracrustal) source rocks, whereas other suites have characteristics consistent with a derivation from a protolith which was predominantly sedimentary and relatively felsic (S-types). The I-type granitoids of the Nundle Suite have ?_Nd values (+3.3 to +6.1) that are amongst the most primitive yet documented for a relatively felsic (SiO₂ ~ 65%) plutonic suite and these values imply a derivation from either a depleted upper mantle source or, more probably, a complex source region involving both volcanic-arc rocks and detrital material. Their compositions are distinctly more primitive than those of the New England Super-Suite which constitutes the Permian ‘core’ of the batholith. This extensive Super-Suite (comprising granitoids traditionally designated as I-type) has an overall range in initial Nd and Sr isotopic compositions of ?1.7 to +4.6 and 0.70458 to 0.70624 respectively, although the majority of plutons have initial Nd isotopic compositions which fall into a very narrow range (+1.0 ±1.5 ?units). This limited range is remarkable considering the extreme lithological diversity and range in chemical composition of the analysed samples (SiO₂ 47%–74%) and implies a source region of considerable volume having reasonably uniform isotopic compositions but variation in chemistry. A similarly uniform source isotopically is also indicated for the S-type granitoids of the Carboniferous Hillgrove Suite and Carboniferous-Permian Bundarra Suite with initial ?_Nd values of +0.8 to +2.3 and initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ compositions of 0.70474 to 0.70577 showing only limited ranges.Five pelites, three ‘felsic’ and four ‘mafic’ greywackes, representing typical country rocks from different stratigraphic levels have initial ?_Nd values (?1.7 to +6.6) and initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ compositions (0.70378 to 0.70585) which essentially mirror the compositional variation in the granitoids. A chemical, mineralogical and isotopic bimodality in these sediments indicates two very distinctive sources, one felsic (rhyodacitic) the other relatively mafic (andesitic), which were separated spatially and temporally in many but not all areas of early- to mid-Palaeozoic New England. A model is presented in which the S-type granitoids are derived from a predominantly felsic source, i.e. pelites and ‘felsic’ greywackes, whereas some of the granitoids belonging to the New England Super-Suite may have been derived from source rocks consisting of both felsic and mafic sedimentary components.The distinction between many S-type and I-type granitoids in New England is unclear for two principal reasons; (a) because the granitoids and their respective source rocks are relatively young geologically so that their isotopic systems have not evolved to any considerable extent, and (b) because of the intrinsic igneous chemical compositions of any sedimentary component that may be involved in their genesis.     

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.     

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.