87Sr86Sr Ratios Research Articles

Isotopic and trace element composition of the upper mantle beneath a young continental rift: Results from Kilbourne Hole, New Mexico

Clinopyroxenes (cpx) separated from discrete spinel lherzolite xenoliths from Kilbourne Hole, New Mexico, are compositionally and isotopically heterogeneous. On a Nd-Sr isotope correlation diagram, the cpx plot largely within the mantle array, from near Bulk Earth to depleted MORB values. None of the bulk xenoliths are equivalent to primitive mantle; all have undergone one or more depletion events and some have been enriched in incompatible elements. The lherzolites as well as the cpx show significant variations in incompatible element ratios including Sm/ Nd and Sr/Nd; moreover, 147Sm 144Nd and 143Nd 144Nd ratios of the cpx are positively correlated and suggestive of an 0.4 Ga fractionation event. Cpx from spinel pyroxenite dikes and lherzolite wallrocks of composite xenoliths are relatively homogeneous (isotopically) and similar to OIB and some MORB. The wallrocks are isotopically equilibrated with the pyroxenites or nearly so, and have negative Nd model ages; we infer that the pyroxenite-forming event caused enrichment in incompatible elements in the contiguous wallrock. The pyroxenite parent magma was probably a primitive basanite characterized by low Hf/Sm and Ti/Sm ratios relative to primitive mantle as a consequence of residual garnet. Our data confirm the presence of a MORB-related component in the mantle beneath Kilbourne Hole. This component is fertile with respect to basaltic constituents, is relatively LREE-depleted, and is isotopically similar to MORB; it is probably derived from asthenosphere. The isotopic heterogeneity of the discrete lherzolites requires a second, enriched component characterized by relatively low 143Nd 144Nd and high 87Sr 86Sr ratios and unsupported 143Nd 144Nd ratios. Old but disparate Sr and Nd model ages require that MORB-related spinel lherzolites have had a complicated history and differentiated from primitive mantle more than 1 b.y. ago. Two plausible models, one involving more than one depletion event and the second involving mixing of mantle components depleted at distinct times, can explain the common observation that Sr model ages are older than Nd model ages.

Hydrogen and oxygen isotope variations in the high himalaya peraluminous Manaslu leucogranite: Evidence for heterogeneous sedimentary source

The Manaslu granite belongs to the High Himalaya leucogranitic belt which was produced by melting of the crust during postcollisional thrusting. δD and δ 18 O values have been determined for whole rock and coexisting minerals from the ~8 km thick Manaslu massif and its 50 km long dyke sheet, its country rocks and the Formation 1 (F1) paragneisses which are the source of the granite. For the granite, δD musc range from −70 to −85%. and δ 18 O W.R. from 10.9 to 12.8%. H and O-isotope fractionations among minerals are consistent with high temperature equilibrium and, for oxygen, closed system evolution. A few samples, coming mainly from the dyke swarm, have very low δD values (down to −188%.) and biotite-muscovite H-isotope fractionations indicative of disequilibrium; the D H ratios of associated magmatic tourmaline are essentially unmodified. From the distribution of δD values in the granite and its country rocks, circulation of very low deuterium meteoric hydrothermal waters was extremely localized. Because these waters are depleted in deuterium by up to 50%. relative to modern meteoric waters, the Manaslu area was either at an altitude substantially higher than that of today (2500–6000 m for the analyzed samples) or a mountain chain once existed to the south. The F1 gneisses have δ 18 O quartz between 12 and 14.3%. which confirms that the granite was generated from F1, but δD values are ≈20%. higher than in the granite. Such a difference can be a result of degassing of the magma and/or introduction of fluid in the melting zone. Infiltration of low δD fluid (≈−90%.) into the hot but dry F1 probably triggered partial melting; these fluids could have come from the dehydration of the Midlands sediments which are separated from the overlying F1 by the Main Central Thrust. The correlations among δ 18 O, ( 87Sr 86Sr) 20 Ma and ϵNd values in both F1 and the granite indicate that the variations of these isotopic ratios in the Manaslu are inherited from those in F1 at the time of melting. In turn, these ratios in F1 are related to the proportion of quartz and phyllosilicates for the isotopic ratios of Nd and O, and to the quantity of radiogenic Sr generated within the sediment, which is a function of age and Rb content (amount of phyllosilicate). Some other Himalayan leucogranites require either other crustal source rocks or the δ 18 O and 87Sr 86Sr ratios of F1 vary along the Himalaya.

RbSr and SmNd systematics of cherts and other siliceous deposits

We have analyzed a suite of marine and lacustrine cherts to determine the concentration of Rb, Sr, Sm and Nd and the isotopic composition of Sr and Nd. The samples ranged from Pliocene to Silurian in age and included nodular cherts in carbonates as well as bedded cherts. Massive samples were leached in acid to remove carbonate and both the leach and residue analyzed. The residues contained 0.1 to 17 ppm Rb and 0.5 to 20 ppm Sr. All samples have high Rb Sr ratios (~0.1 to 0.8) and thus show enrichment of Rb relative to Sr of 6 to 60 times that of seawater. The Rb and Sr in pegmatitic quartz are much lower by factors of 10 to 10 3, although these also may have high Rb Sr . The site of Rb and Sr in the cherts was not established but appears to be correlated with Al content. The high Rb Sr ratios permit the use of cherts for dating in favorable circumstances. The Nd contents range from 9 to 1000 ppb with Sm Nd ~ 0.16 to 0.19, typical of continental sources. The initial 87Sr 86Sr ratio for a Pacific deep sea chert is equal to that of associated carbonates and hence reflects the seawater environment. In shelf and marginal basin environments, the initial 87Sr 86Sr ratios of chert are distinctly more radiogenic than seawater. This is most obvious where a substantial clay component is present, but is observed even in samples with Al 2O 3 contents less than 0.1%. These results show the Rb-Sr system in chert to be rather isolated from the associated carbonate with very little Sr exchange. This separate behavior appears to persist through both early and late recrystallization stages. These data suggest that, in many cases, cherts are formed by “dissolved” SiO 2 nucleating on clay-like material which preserves the isotopic characteristics of the detrital source. The detrital component required is less than 1% of the total chert. In some cases, it is possible that the cherts were formed from silica-saturated waters from a nonmarine source, possibly in coastal environments or during uplift. These waters contained radiogenic Sr in solution that was not exchanged with the carbonate matrix. This latter mechanism is also indicated by oxygen isotopic studies ( Knauth and Epstein, 1976; Kolodny and Epstein, 1976). The Nd isotopic signature of the deep sea samples reflects the seawater composition in agreement with the initial 87Sr 86Sr data on the same samples. Samples with excesses of 87Sr relative to seawater show low ϵ Nd, both sets of data thus indicating inheritance of an old detrital component. The results demonstrate that the Rb-Sr and Sm-Nd systems may be used in elucidating the origin and evolution of cherts and that the Rb-Sr system may be used for dating, if consideration is given to the initial Sr composition and to diagenetic changes. This opens up numerous possibilities for sedimentological-geochemical studies.

Local and regional heterogeneity in MORB from the Mid-Atlantic Ridge between 54.5°S and 51°S: Evidence for geochemical enrichment

Basaltic lavas from the southern Mid-Atlantic Ridge between the Bouvet triple junction at 54.5°S 1°W and 51°S, show a range in texture from aphyric to highly olivine or plagioclase porphyritic. Olivine ± Cr-spinel ± plagioclase are the dominant phenocryst phases, and clinopyroxene occurs as a phenocryst or microphenocryst phase in many lavas. The range in differentiation can be attributed to crystal fractionation accumulation ; basalts from fracture zones show a greater range than basalts from ridge segments. The fracture zone basalts also show a minor transform fault effect in that their compositions are consistent with lower degrees of partial melting than the ridge axis basalts. This difference is, however, not manifested in incompatible trace element ratios. The incompatible trace element ratios range from normal depleted (N-type) MORB through transitional varieties to moderately enriched (E-type) MORB, yet have higher Sr and lower Nd isotopic ratios than is generally accepted for depleted MORB. Characteristic trace element and isotopic ratios of these lavas are: Zr Nb = 8–38 , Y Nb = 2.1–15.7 , Zr Y = 2.0–4.0 , 143Nd 144Nd = 0.51303–0.51282 , 87Sr 86Sr = 0.70290–0.70364 . The geochemical signature of enriched MORB from this region is distinct from that associated with the Bouvet mantle plume and enriched MORB from the Southwest Indian and the American-Antarctic Ridges. This is best illustrated in terms of the covariation of Zr Nb ratio and 143Nd 144Nd and 87Sr 86Sr ratios. The source of enrichment in these lavas may be the recently proposed Shona hotspot. The enriched signature results from source region mixing achieved by veining of normal depleted mantle by low volume partial melts associated with the rising Shona plume. Subsequent partial melting, during upwelling below the ridge axis, has led to the geochemical enrichment observed in MORB from this region.

Archean depleted mantle: Evidence from Nd and Sr initial isotopic ratios of carbonatites

Initial 87Sr 86Sr and 143Nd 144Nd ratios of some carbonatites and related alkalic rocks intruded into the Canadian Shield between 2700 Ma and 110 Ma ago define development lines on a plot of initial ratio versus age. The isotopic data show that these complexes have been derived from a mantle source with a Rb Sr ratio of 0.020 ± 0.002 and a Sm Nd ratio of 0.358 ± 0.008, which are lower and higher respectively than the values for the silicate portion of the Earth. An estimate of the mean age of formation of this LIL-depleted reservoir, based on the intersection of the development lines with those of bulk Earth, is about 2900 Ma, a figure similar to the age of most Superior Province rocks. A model is proposed that involves differentiation, ca. 2900 Ma ago, of mantle similar in chemical composition to bulk Earth, into granitoid rocks (the Superior Province), and a depleted, residual upper mantle. The source for the Nd and Sr contained in the carbonatites is thought to lie within this depleted sub-continental lithosphere. Measured Sm Nd ratios of about 0.15 for the carbonatites are much lower than the calculated Sm Nd ratio of their source, and support enrichment of the light REE either slightly before or at the same time as carbonatite magma generation.

Geochronology and isotopic variation of the early Archaean Amitsoq gneisses of the Isukasia area, southern West Greenland

Relatively unaltered samples of the Amitsoq gneiss complex in the Isukasia area were identified and analysed despite the effects of metamorphism and local Proterozoic hydrothermal alteration and contamination. The parents of the three lithologies that make up the Amitsoq gneisses of the area were emplaced over about 320 Ma. Taking a weighted average of dates from U-Pb on zircons, Rb-Sr on WR, Sm-Nd on WR and Pb-Pb on WR, the most probable ages of intrusion are: grey gneisses at 3690 ± 50 Ma, white gneisses at 3590 ± 40 Ma and pegmatitic gneisses at 3370 ± 60 Ma. The Rb-Sr isochron initial 87Sr 86Sr ratios for the grey, white and pegmatitic gneisses are 0.7006 ± 1, 0.7016 ± 1 and 0.8248 ± 3, respectively. The corresponding average ϵ Nd ( T) values are +1.0, +0.7 and −0.2. Since the REE chemistry of the rocks that were the direct parents of the grey gneisses is not known, the +1.0 average ϵ Nd ( T) of the grey gneisses need not imply generation from a depleted mantle. The Sr initial ratio and ϵ Nd ( T) values for the white and pegmatitic gneisses are compatible with consecutive derivation at two discrete intervals by partial melting of crust mainly comprised of rocks similar to the grey gneisses. The progressive evolution of ϵ Nd with T for the Amitsoq gneisses in the early Archaean shows that values of ϵ Nd ( T) near zero in granitoids do not necessarily indicate derivation from the “undepleted” mantle (CHUR).

The Archean-Proterozoic transition: Evidence from the geochemistry of metasedimentary rocks of Guyana and Montana

Chemical and isotopic compositions of ancient sedimentary rocks have in some cases provided evidence of first-order changes yin the composition and extent of continental crust, from the Archean to the Proterozoic. We reversed the typical sampling pattern, comparing metapelites and carbonates from Proterozoic greenstone belts with those from an Archean continental margin sequence, and found the reverse of the typical age-composition relations. Metapelites and volcaniclastic metagreywackes from the Early Proterozoic greenstone belts of northern Guyana resemble those of many Archean greenstone belts in REE, trace, and major element compositions (mean ratios: La Sm 5.0, La Th 5.1, Eu Eu ∗ 0.80, Th U 3.4; median K 2O Na 2O 0.8). Schists from an Archean continental marginal sequence in Montana have compositions like many post-Archean metapelites derived from continental sources (mean ratios: La Sm 6.8, La Th 2.7, Eu Eu ∗ 0.67, Th U 5.2; median K 2O Na 2O 1.7). Other Archean continental basin sediments also resemble their post-Archean counterparts, implying that upper continental crusts of these eras may not have differed significantly in these respects. However, temporal change may have occurred in upper crustal contents of Ni and Cr many Archean clastic metasedimentary rocks of both continental and greenstone belt provenance have contents of these compatible elements higher than in their post-Archean equivalents. Dolomitic limestone from a greenstone belt of northern Guyana has a very low 87Sr 86Sr ratio of 0.7007. Such low values may reflect the local influence of mantle-derived volcanic rocks during carbonate deposition, diagenesis, or metamorphism and they may not be reliable indicators of the evolution of Sr in seawater. Archean dolomitic marbles from the continental marginal sequence of Montana have 87Sr 86Sr ratios of 0.7065 and higher, significantly more radiogenic than many accepted values for carbonates of Archean greenstone belts.

Effects of diagenesis on strontium, carbon, nitrogen and oxygen concentration and isotopic composition of bone

Paleodietary analysis based on variations in the trace element and stable isotopic composition of inorganic and organic phases in fossil bone depends on the assumption that measured values reflect in vivo values. To test for postmortem alteration, we measured 87Sr 86Sr , 13C 12C , 18O 16O and 15N 14N ratios and Sr concentrations in modern and prehistoric (610 to 5470 yr old) bones of animals with marine or terrestrial diets from Greenland. Bones from modern terrestrial feeders have substantially lower Sr concentrations and more radiogenic 87Sr 86Sr ratios than those from modern marine feeders. This contrast was not preserved in the prehistoric samples, which showed almost complete overlap for both Sr concentration and isotopic composition in bones from the two types of animals. Leaching experiments, X-ray diffraction analysis and infrared spectroscopy indicate that alteration of the Sr concentration and isotopic composition in prehistoric bone probably results from nearly complete exchange with groundwater. Oxygen isotope ratios in fossil apatite carbonate also failed to preserve the original discrimination between modern terrestrial and marine feeders. The C isotope ratio of apatite carbonate did not discriminate between animals with marine or terrestrial diets in the modern samples. Even so, the ranges of apatite δ 13C values in prehistoric bone are more scattered than in modern samples for both groups, suggesting alteration had occurred. δ 13C and δ 15N values of collagen in modern bone are distinctly different for the two feeding types, and this distinction is preserved in most of the prehistoric samples. Our results suggest that postmortem alteration of dietary tracers in the inorganic phases of bone may be a problem at all archaeological sites and must be evaluated in each case. While collagen analyzed in this study was resistant to alteration, evaluation of the possibility of diagenetic alteration of its isotopic composition in bones from other contexts is also warranted.

Diagenetic history of Eocene Wilcox sandstones, South-Central Texas

Sandstones and shales of the lower Eocene Wilcox Group extend from outcrop to depths greater than 5 km where temperatures exceed 200°C. Eight authigenic minerals have been identified. Quartz, kaolinite, calcite, ankerite and albitized plagioclase are volumetrically significant, whereas chlorite, illite, albite overgrowths and dolomite are present only in trace amounts. Quartz, kaolinite, and ankerite cements are present over the entire depth range sampled (1528 to 4446 m). Calcite cement occurs only in sandstones shallower than 3300 m. Albitized plagioclase is the predominant feldspar in samples deeper than 2600 m whereas detrital potassium feldspar has been completely dissolved out of sandstones deeper than about 3200 m. No systematic variations in the chemical composition of calcite with depth, no zoning within calcite or ankerite, and no compositional differences between pore-filling and grain-replacing calcite or ankerite were found. Calcite cements have 87Sr 86Sr ratios generally less than 0.708, whereas ankerite cements are significantly enriched in 87Sr. The oxygen isotopic composition of carbonate cement ranges between −9 and −14%. (PDB) for calcite and between −8 and −12%. for ankerite with no systematic variation with depth. Calculated δ 18O values for authigenic quartz and kaolinite range from +22 to +31%. (SMOW) and from +14 to +17%., respectively. Volumetrically important amounts of authigenic quartz and kaolinite precipitated during early burial before calcite cementation, secondary porosity generation, or smectite to illite transformation in encasing shales. The isotopic composition of strontium in calcite cement shows most of it to be of Eocene or older marine carbonate origin. Ankerite cementation followed the generation of secondary porosity and occurred after the products of organic maturation and shale diagenesis, plagioclase albitization or potassium feldspar dissolution significantly affected the sandstone-pore water system.

Plutonic and metasedimentary rocks from the Coastal Range of northern Chile: Rb[sbnd]Sr and U[sbnd]Pb isotopic systematics

More than 50% of the surface in the Coastal Range of northern Chile between the towns of Taltal and Chan˜aral consists of granitoid rocks. Age determinations by means of UPb isotopic analyses on sized zircons and by means of RbSr isotopic measurements, in conjuction with other geochemical studies, reveal a complex pattern for the development of the plutonites. Though closely associated, two groups of intrusive rocks can be distinguished on the basis of isotopic and geochemical data:In a west-east transect southeast of Chan˜aral, several calc-alkaline plutonites with basic to acid members intruded mainly into Paleozoic metasedimentary rocks. The UPb data of the zircons from the plutonites yielded concordant ages ranging from Lower Jurassic to Lower Cretaceous. UPb ages are identical with RbSr biotite ages within the error limits and almost coincide with the KAr mineral ages. Both the ages and the initial87Sr86Sr ratios decrease continuously from west to east. The low initial Sr isotopic ratios (0.703–0.705) and other geochemical parameters indicate I-type characteristics.In the west-east segment northeast of Chan˜aral, mainly alkaline plutonic rocks intruded in an interval from Upper Permian to Upper Triassic. The UPb data of zircons are slightly discordant and indicate a small portion of inherited crustal material> 1 Ga old. RbSr biotite ages were reset by thermal pulses due to the emplacement of younger granitoid rocks. The initial 87Sr86Sr ratios of < 0.710 point to an origin of these rocks by anatexis of crustal rocks (S-type).The oldest rocks in the area of Chan˜aral consist of thick sequences of metasediments. The material for these rocks was derived from a source> 2.5 Ga old as indicated by the UPb data of the zircons. The lower intercept age of the zircons points to metamorphism of the source area at about 600 Ma. Due to uplift in the Early Paleozoic the source area supplied the material for the sediments which were deposited in the Devonian. Contact metamorphism in the Triassic transformed the sediments into metasedimentary rocks as demonstrated by RbSr whole-rock data. This event was caused by the emplacement of the S-type granitoid rocks which probably originated, however, from a deeper level of the crust.

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 143Nd 144Nd ratio (I Nd) of 0.51002±9 (2 σ), corresponding to ϵ 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 87Sr 86Sr 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.

Strontium and its isotopes in Canadian rivers: Fluxes and global implications

Systematic sampling of the 39 largest Canadian rivers shows that the weighted average 87Sr86Sr ratio in the dissolved load is 0.7111, similar to previous measurements on such large rivers as the Amazon and Mississippi. Consequently, we believe that the above estimate is likely representative of the global average. This imposes a limit of 6.5 × 1011g yr−1 on the amount of Sr exchanged with basalts in hydrothermal cells on mid-oceanic ridges. Evaluation of geological information suggests that Sr from silicate sources is of considerable importance for all but the largest Canadian rivers. The latter have chemical and isotopic composition consistent with ~4:1 carbonate vs. silicate derivation of Sr, but such interpretation is not unique. In terms of their water discharge, the 39 Canadian rivers studied account for 4.2% of the world total and their weighted average concentrations for other dissolved solutes are: TDS 176 ppm, Ca 18 ppm, Cl 6.8 ppm and Sr 84 ppb.

40Ar 36Ar in MORB glasses: constraints on atmosphere and mantle evolution

We performed argon isotopic composition measurements of MORB glassy samples from the Pacific, Atlantic and Indian Oceans. There is a very large scatter in the 40Ar 36Ar ratio, from 980 up to 24,400 for bulk rock analyses, which is mainly due to atmospheric contamination. Using the stepwise heating technique, very high ratios are obtained, from 15,000 up to 25,250 which is the highest 40Ar 36Ar ratio ever measured in MORB. We establish a negative correlation between the highest 40Ar 36Ar results from stepwise heating and 87Sr 86Sr ratios, which is perfectly c consistent with a two-layered mantle structure. From both 40Ar and 129Xe [1] MORB systematics a model is proposed for the kinetics of degassing: a very early and extensive burst, with a time constant of ≈ 4My, is followed by a slower process of present day type, with a time constant of ≈ 0.5Gy. The mean age of the atmosphere is so determined to be around 4.4 Gy.

The age and emplacement of obducted oceanic crust in the Urals from Sm [sbnd]Nd and Rb [sbnd]Sr systematics

The Urals contain a 2000 km belt of mafic-ultramafic bodies. The Sm Nd and Rb Sr systematics of two of these bodies, the Kempersai Massif in the South Ural Mountains and the Voykar-Syninsky Ophiolite Complex in the Polar Ural Mountains have been examined. These data confirm the hypothesis that these bodies represent fragments of pre-collision oceanic crust and establish constraints on the nature and timing of events in the Uralian Orogeny. Two Kempersai gabbros define Sm Nd internal isochrons of 397 ± 20My and 396 ± 33My with ε Nd(T) = +8.7 ∓ 0.6 and +8.4 ∓ 1.3, respectively. Whole rock samples of pillow basalt, diabase, gabbros, troctolite, and a metasediment give Sm Nd values which lie on this isochron indicating that these rocks are genetically related and have an igneous crystallization age of 397 My. Whole rock samples of Voykar-Syninsky diabase, gabbros, and clinopyroxenite give Sm Nd values which lie on or within ∼ 1 ε-unit of this isochron indicating an age and ε Nd(T) virtually identical to those of Kempersai. ε Nd(T) for the Kempersai and Voykar-Syninsky mafic samples range from +7.3 to +9.0 with an average value of +8.4. This indicates that the Urals ophiolites are derived from an ancient depleted mantle source and are most plausibly pieces of the oceanic crust and lithosphere. The fact that a metasediment has the same ε Nd(397 My) as the other samples indicates derivation from an oceanic source with negligible continental input. ε Nd(T) for the massifs is ∼ 1.5 ε-units lower than the average for modern MORBs. This may be due to the differential evolution of the MORB source over the past 397 My and in conjunction with data for other ophiolites and Mesozoic MORB suggests that over the past 750 My the source for MORB has evolved at a rate less than or equal to its rate of evolution averaged over the age of the earth. Initial 87Sr 86Sr ratios are highly variable ranging from ε Sr(T) = −25.2 for a gabbro to +70.3 for a highly serpentinized harzburgite. This reflects the effects of seawater alteration which is particularly strong on ultrabasic rocks. We conclude that the long belt of mafic-ultramafic rocks in the Urals, which includes the Kempersai and Voykar-Syninsky Massifs, represents segments of Siluro-Devonian oceanic crust. Our igneous age for Kempersai in conjunction with other age constraints suggest that these segments of oceanic crust formed at least 80 My before the collision that produced the Urals.

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; 87Sr 86Sr 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 87Sr 86Sr ratios that is observed in Smackover brines within the 5000 km 2 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.

Petrography, chemistry and isotopic ages of Peninsular Gneiss, Dharwar acid volcanic rocks and the Chitradurga granite with special reference to the late Archean evolution of the Karnataka craton, southern India

New petrographic, major and trace element, and isotopic age data are presented for four suites of rocks from the low- to medium-grade part of the South Indian Archaean craton in central Karnataka. Two suites are from the Peninsular Gneiss basement to the late Archaean Dharwar Supergroup, the third is a suite of Dharwar potassic rhyolites and the fourth is from the Chitradurga Granite which has intrusive contacts with the Dharwar Supergroup. The Chikmagalur Granite (granodiorite), which with its host gneisses forms part of the basement (Peninsular Gneiss) to the Dharwar Supergroup in the Bababudan basin, yields RbSr and Pb/Pb whole-rock isochron dates of 3080 ± 110 Ma and 3175 ± 45 Ma, respectively, an initial 87Sr86Sr ratio of 0.7013 ± 0.0009 and a μ1 value (238U204Pb) of 7.98. Host tonalitic gneisses to the Chikmagalur Granite give an RbSr whole-rock isochron date of 3060 ± 160 Ma with an initial 87Sr86Sr ratio of 0.7015 ± 0.0004 and a Pb/Pb whole-rock isochron date of 3190 ± 100 Ma with a model μ1 value of 8.00. A suite of granites and granodioritic-tonalitic gneisses (Chitradurga granitic rocks and gneisses) forming part of the Peninsular Gneiss basement to the Dharwar Supergroup west of the Chitradurga belt yields RbSr and Pb/Pb whole-rock isochron dates of 2970 ± 100 Ma and 3028 ± 28 Ma, respectively, with an initial 87Sr86Sr ratio of 0.7035 ± 0.0013 and a model μ1 value of 7.62. The suite of Dharwar acid volcanic rocks from north of Honnali defines a poorly fitted 207Pb206Pb linear array (MSWD 15.8) which gives an apparent age of 2565 ± 28 Ma and a model μ1 value of 7.46. The poor fit of this ‘isochron’ shows that a closed system was not maintained, a conclusion supported by high K abundances which are attributed to a syn- or post-magmatic ion exchange process. The Chitradurga Granite which intrudes the older rocks of the Dharwar Supergroup in the Chitradurga belt yields a well-fitted Pb/Pb whole-rock isochron date of 2605 ± 18 Ma with a model μ1 value of 7.68. The UPb relations in these suites from Karnataka are in marked contrast with the commonly severe U depletion and, consequently, unradiogenic Pb isotopic compositions observed in deeply eroded, high-grade Archaean gneiss terranes such as those bordering the North Atlantic. This contrast indicates that the cratonic rocks in central Karnataka represent a relatively high level in the original Archaean continental crust.

Sr and Nd isotope geochronology, geologic history, and origin of the Adirondack Anorthosite

We have analyzed samples from the Adirondack Marcy massif for Rb-Sr and Sm-Nd isotopes in an attempt to determine directly the primary crystallization age of a Proterozoic massif-type anorthosite rock suite. The oldest age obtained (1288 ± 36Ma) is from a 4 point Sm-Nd isochron defined by igneous-textured whole-rock and mineral separate data from a local layered sequence gradational from oxiderich pyroxenite to leuconorite. This age is older than Silver's (1969) 1113 Ma zircon age of associated charnockites, but is within the window of permissible anorthosite ages based on previous geochronology and field relationships. As such, 1288 Ma may represent the time of crystallization of the massif. For the most part, however, both Sm-Nd and Rb-Sr isotopic systems did not survive granulite facies metamorphism. Internal isochrons based on whole rocks and minerals yield ages between 995 and 919 Ma. These isotopic data suggest that the granulite fades metamorphism experienced by the massif was a prograde event that occurred a minimum of 100 Ma and as much as 350 Ma after crystallization of the massif. The relatively large range in Rb abundance, and in calculated initial 87Sr 86Sr (0.7039–0.7050) and 143Nd 144Nd ratios among anorthosite suite rocks, particularly those at or near the contacts of the Marcy massif is explicable by variable contamination with “crustal” materials and/or fluids, derived from surrounding acidic metaplutonic rocks, paragneisses, and marbles. Despite uncertainies caused by crustal contamination and metamorphic resetting of primary ages, Marcy samples have epsilon Nd values between +0.44 and +5.08, implying a source for the massif with long-term depletion in light rare earth elements. A probable source material would be depleted mantle.

Strontium isotope evidence on the history of oilfield brines, Mediterranean Coastal Plain, Israel

The isotopic composition of Sr in oil field brines from the Mediterranean Coastal Plain was determined in 18 drillholes (13 of which are oil producing wells). The brines are characterized by salinities ranging from 35 to 93 g/l (TDS), Sr from 28 to 350 mg/l, Sr Ca molar ratios from 0.011 to 0.053 and 87Sr 86Sr ratios from 0.7075 to 0.7090. E & A = 0.7081 ± 0.0004 (2σ). The brines are classified into two groups: 1. (a) Mavqi'im group—brines with relatively high 87Sr 86Sr ratios (0.7087 and 0.7090), sampled from elastics, dolomites and anhydrites of Upper Miocene age. 2. (b) Heletz group—brines with relatively low 87Sr 86Sr ratios (0.7075 to 0.7081), sampled from sandstones and dolomites of Lower Cretaceous age. Equations were derived to show the relations between 87Sr 86Sr ratio of the brines and the processes through which they evolved (dolomitization and dissolution-reprecipitation). It is suggested that both groups of brines originated from Mediterranean evaporated seawater during the Messinian desiccation. The strontium isotope composition of the seawater is reflected in that of both groups of brines, the Mavqi'im group containing the original 87Sr 86Sr ratio (~0.7088). The Heletz group evolved later on, through exchange reactions of those primary brines with a carbonate sequence of Cretaceous age and consequently new 87Sr 86Sr ratios (~0.7078) could have been developed. The effect of the dissolution reprecipitation reactions on the carbonate rocks is estimated to be very slight, only around 1% (by weight).

Formula omitted] and [formula omitted] ratios, interstitial water chemistry and diagenesis in deep-sea carbonate sediments of the Ontong Java Plateau

Interstitial waters and sediments from DSDP sites 288 and 289 contain information on the chemistry and diagenesis of carbonate in deep-sea sediments and on the role of volcanic matter alteration processes. Sr Ca ratios are species dependent in unaltered foraminifera from site 289 and atom ratios (1.2–1.6 × 10 −3) exceed those predicted by distribution coefficent data (~0.4 × 10 −3). During diagenesis Sr Ca ratios of carbonates decrease and reach the theoretical distribution at a depth which is identical to the depth of Sr isotopic equilibration, where 87Sr 86Sr ratios of interstitial waters and carbonates converge. Mg Ca ratios in the carbonates do not increase with depth as found in some other DSDP sites, possibly because of diagenetic re-equilibration with interstitial waters showing decreasing Mg 2+/Ca 2+ ratios with depth due to Ca input and Mg removal by alteration of volcanic matter. Interstitial 18O 16O ratios increase with depth at site 289 to δ 18 O = 0.67%. (SMOW), reflecting carbonate recrystallization at elevated temperatures ( $ ̆ = 20°C), the first recorded evidence of this effect in interstitial waters. Interstitial Sr 2+ concentrations reach high levels, up to 1 mM, chiefly because of carbonate recrystallization. However, 87Sr 86Sr ratios decrease from 0.7092 to less than 0.7078, lower than for contemporaneous sea water, showing that there is a volcanic input of strontium at depth. This volcanic component is recorded in the Sr isotopic composition of recrystallized calcites. Isotopic compositions of the unrecrystallized calcites suggests that the rate of increase of the 87Sr 86Sr ratio of sea water with time has been faster since 3 my ago than in the preceding 13 my.

Geochemical investigations of the origin of the Manaslu leucogranite (Himalaya, Nepal)

The Himalayan orogeny is characterized by an intraplate subduction zone. The thrusting of the Tibetan Slab over the foreland is probably responsible for the origin of the Manaslu-type leucogranites. This geochemical study tries to examine the crustal origin of the granite, to verify whether the paragneisses of the Tibetan Slab are the most probable parent material and to determine the subsequent petrogenetic processes. The Rb-Sr systematics suggest that 1. (1) the scatter of data points in the 87Sr 86Sr vs. 87Rb 86Sr diagram may be due to the heterogeneity of the initial 87Sr 86Sr ratios. 2. (2) the initial isotopic ratios are very high (0.730 → 0.760) and are in agreement with those of the gneisses of the Tibetan Slab at the time of the melting. Pb isotopic ratios are homogeneous. They are also consistent with a crustal prehistory. The REE and Th contents are unexpectedly low in comparison with the much higher contents of the parent material. Monazite left in the residue of melting or more probably extracted in an early stage of the magmatic history may explain the observed REE and Th abundances. Alternatively, field observations show that the emplacement of the granite has been accompanied by a very strong hydrothermal activity. In consequence, the low REE and Th contents of the granite could also be due to the loss by solution during prolonged fluid circulation throughout the granitic melt and the Sr and Pb isotope distribution to the effect of late to post magmatic fluids the isotopic character of which was determined by the underlying Tibetan Slab.