Showing papers in "Chemical Geology: Isotope Geoscience Section in 1985"

Comparison of zeta calibration baselines for fission-track dating of apatite, zircon and sphene

Abstract: A.J. Hurford and P.F. Green gave a description of the “zeta” method of calibrating fission-track dating against other radiometric dating techniques. In the study reported here, the work is extended to a large number of zircon samples, and applied for the first time to apatite and sphene. Values of ζ for the three minerals are determined with respect to three different uranium glass dosimeters, and reduced to a common scale using precisely determined conversion factors for the relative fission-track response of each glass dosimeter. All determinations show a high level of consistency. On a scale appropriate to SRM612, overall weighted mean zeta (OWMZ) values of 381.8 ± 10.3 for zircon, 353.5 ± 7.8 for apatite and 320.0 ±12.4 for sphene are determined, showing unexpected differences between the three minerals. Some factors which might introduce such differences are considered but none seems to produce effects of the observed magnitude, and they remain unexplained. An equivalent range of values are in common use by different workers. Further work is necessary to resolve these differences. The results are discussed in terms of the suitability of fission-track dating for placing numerical constraints on the geologic time scale. The study illustrates the need for all fission-track age determinations to be derived from a calibration scale based on the use of age standards. Some recommendations are made for newcomers to the technique. Consistent fission-track counting must be achieved in controlled experiments before reliable ages may be determined on unknown samples.

Construction of the seawater 87Sr86Sr curve for the cenozoic and cretaceous: Supporting data

Abstract: We present the data used to construct the Cenozoic and Cretaceous portion of the Phanerozoic curve of seawater 87 Sr 86 Sr that had been given in summary form by W.H. Burke coworkers. All Cenozoic samples (128) and 22 Cretaceous samples are foram-nannofossil oozes and limestones from DSDP cores distributed among 13 sites in the Atlantic, Pacific and Indian Oceans, and the Caribbean Sea. Non-DSDP Cretaceous samples (126) include limestone, anhydrite and phosphate samples from North America, Europe and Asia. Determination of the 87 Sr 86 Sr value of seawater at particular times in the past is based on comparison of ratios derived from coeval marine samples from widely separated geographic areas. These samples are characterized by a wide variety of diagenetic and burial histories. The large size and cosmopolitan nature of the data set decreases the likelihood that, among coeval data, systematic error has been introduced by a similar pattern of diagenetic alteration of the ratios. There is good clustering of data points throughout the Cenozoic and Cretaceous curve. The consistency of data is illustrated by Cenozoic and Cretaceous data plots that include a separate symbol for each DSDP site and non-DSDP sample location. More than 98% of the data points are enclosed by upper and lower lines that define a narrow band. For any given time, the correct seawater ratio probably lies within this band. A line drawn within the band represents our estimate of the actual seawater ratio as a function of time. The general configuration of the Cenozoic and Cretaceous curve appears to be strongly influenced by the history of plate interactions and sea-floor spreading. Specific rises and falls in the 87 Sr 86 Sr of seawater, however, may be caused by a variety of factors such as variation in lithologic composition of the crust exposed to weathering, configuration and topographic relief of continents, volcanic activity, rate of sea-floor spreading, extent of continental inundation by epeiric seas, and variations in both climate and paleooceanographic conditions. Many or all of these factors are probably related to global tectonic processes, yet their combined effect on the temporal variation of seawater 87 Sr 86 Sr can complicate a direct platetectonic interpretation for portions of the seawater curve.

Exchange of oxygen isotopes in carbon dioxide-phosporic acid systems

Abstract: The rate of exchange of isotopes of oxygen between solutions of concentrated phosphoric acid and CO2 was measured as a function of temperature, acid strength (pressure of water in equilibrium with the solution), pressure of CO2, and surface area of the reaction vessel. At 75° C, significant exchange was found to occur even for the “anhydrous” phosphoric acids, those in which the nominal percentage of H3PO4 in solution is equal to or exceeds 100%. Exchange is much slower at 25° C, but isotopic shifts as large as 0.1%, can be observed in 95% H3PO4 at equilibration times approaching 1000 hr. Rates of exchange were found to be dependent upon the vapor pressure of water in equilibrium with the acid solutions. Exchange was found to occur primarily on the surface of the reaction vessel above the solution, with no dependence on total CO, pressure. These observations indicate that phosphoric acids with nominal concentrations of H3PO4 approaching 105% are preferable for the minimization of exchange between CO2 samples and acid solutions during phosphorolyses of carbonate materials. Moreover, with such acids, significant time—temperature trade-offs are possible, allowing rapid preparation of CO2 at elevated temperatures.

Deuterium and oxygen-18 in European groundwaters — Links to atmospheric circulation in the past

Abstract: A thorough discussion of deuterium and 18O isotope ratios found in modern infiltration waters and in old groundwaters recharged during the final stages of the last glaciation in Western and Central Europe is presented. Old European groundwaters are depleted in deuterium by ∼ 12%. when compared to modern infiltration waters. The continental gradient in deuterium content found in these waters is very similar to that presently observed in infiltration waters and in precipitation. This fact strongly suggests a constant circulation regime of the atmosphere over Europe for the past 35,000 yr. Relative constancy of atmospheric circulation during that time makes a quantitative interpretation of the depletion in heavy-isotope content of glacial waters meaningful in terms of changes in surface air temperature over Europe at the transition from the last ice age to the Holocene climate. The shift in air temperature deduced from the above-mentioned depletion in deuterium content reaches ∼ 7°C. This value is in good agreement with previous estimates based on simulation of the ice age atmosphere and on measurements of noble-gas contents in European groundwaters. The relation between δ D- and δ18O-values found in old European groundwaters is essentially identical with the well-known relationship δ D = 8δ 18O + 10, which is proved to be valid for modern precipitation and infiltration waters. It means further that deuterium excess in European meteoric waters, defined as d = δ D − 8δ 18 O did not undergo substantial changes for at least 35,000 yr. This fact provides a strong argument in favour of the hypothesis that the increase in relative humidity by ∼ 1090 over the glacial ocean, which is a widely accepted opinion nowadays, was probably not a world-wide phenomenon.

Low-temperature alteration of volcanic glass: Hydration, Na, K, 18O and Ar mobility

Abstract: The low-temperature alteration of siliceous volcanic glass by meteoric water involves considerable hydrogen exchange for Na+ or K+ ions. This can occur with little alteration of the other major- or trace-element composition of the glass. As much as 40% of the alkali sites can be occupied by H+ ion. Most water in these glasses is present as molecular water rather than as hydroxyl groups. This hydration is accompanied by oxygen isotope exchange resulting in an oxygen isotopic enrichment to over 20‰ (relative to SMOW) for this suite of samples. The high degree of alkali mobility during this alteration process affects the KAr age dating method. In these glasses, Ar appears to be less mobile than alkali ions, resulting in discrepant KAr ages for glass with significant hydration and oxygen isotopic exchange.

Stable isotope studies of cave seepage water

Abstract: D/H and 18O16O ratios for vadose seepage and precipitation were determined at three cave sites in the eastern U.S.A. Samples were collected over a period of ∼ 1 yr. between the summers of 1978 and 1979. In addition, isotopic determinations were made on a number of individual vadose seepage samples from a variety of caves predominantly located throughout North America. The results of the former portion of the study suggest that the isotopic composition of the recharge water is constant and approximately equal to that of the weighted mean annual precipitation in the locality. In the latter part of the study, the notion of the constancy of isotopic composition of vadose seepage was applied to a number of cave sites and the results agree with other precipitation data. Since it has been shown that caves record mean annual surface temperatures, the relationship between mean temperature and δ18O of precipitation can also be determined. There is good agreement between the relationship based on our data and that based on meteorological data and isotopic analyses by I.A.E.A. The study demonstrates that vadose seepage provides an effective means of assessing mean annual isotope ratios of precipitation in most continental regions.

The relationship between deuterium and oxygen-18 delta values in leaf water

Abstract: The relationship between 180 and deuterium delta values in plant leaves has been studied under laboratory conditions. Our results, in agreement with some earlier studies on fruit juices, show that lines of slope much lower than expected from experiments with evaporating water bodies, are obtained. For needles of Pinus radiata grown under semi-arid conditions, slopes as low as 1.3 were obtained. These low slopes can be reconciled by using a model similar to one developed for lakes subject to evaporation under differing conditions.

Geochemistry, geochronology and petrology of Monte Mucrone: An example of EO-alpine eclogitization of Permian granitoids in the Sesia-Lanzo Zone, Western Alps, Italy

Abstract: A Permian granitoid intrusion, exposed on Monte Mucrone, Sesia-Lanzo Zone, Italy, was metamorphosed under high-pressure conditions. To elucidate the metamorphic history of this granitoid complex, investigations have been made into the chemistry of the protolith, possible mass transfer during metamorphism, phase chemistry and implications for P-T conditions, and the age of metamorphic events. The metagranitoids contain Na-pyroxene, quartz, white mica, garnet and zoisite, and vary from rocks showing a well-preserved granitic texture to strongly deformed schists and gneisses. The rocks are of granitic to granodioritic composition and originated as an I-type granite pluton. With the exception of addition of H2O, the rock series behaved isochemicaliy throughout high-pressure metamorphism and later tectonic and retrograde transformations. P,T estimates based on Na-pyroxenes, garnet-clinopyroxene and garnet-biotite pairs give minimum pressures of ∼ 14 kbar at 600°C for the peak of the eclogite metamorphism. Age determinations by RbSr and KAr methods give 129-Ma for the rejuvenation of the total rock and 114-Ma isochrons for garnet and clinopyroxene, the first eclogite phase minerals. White-mica cooling ages scatter from 90 to 60 Ma.

40Ar39Ar and K/Ar dating of the bishop and fish canyon tuffs: Calibration ages for fission-track dating standards

Abstract: Ambiguous values of the 238U spontaneous fission decay constant and complexities of absolute neutron dosimetry underline the dependency of fission-track dating upon age standards. New K/Ar and 40Ar39Ar step-heating data are presented for two putative standards: the Bishop Tuff, California and the Fish Canyon Tuff, Colorado. Measured K/Ar ages of 0.741 ± 0.014 Ma for Bishop Tuff sanidine and 27.4 ± 0.4 Ma for the Fish Canyon Tuff (mean of concordant results on biotite, sanidine, hornblende and plagioclase) show good agreement with previous results. 40Ar39Ar plateaus for Bishop Tuff sanidine (plateau age 0.734 ±0.024 Ma) and Fish Canyon Tuff biotite (plateau age 27.8 ± 0.2 Ma) suggest minimal thermal disturbance since deposition of the tuffs, supporting the underlying assumption of equality of their fission-track and argon ages.

The oxygen isotope composition of landsnail shells as a climatic indicator: Applications to hydrogeology and paleoclimatology

Abstract: The oxygen isotopic compositions of the carbonate shells of landsnails either Prosobranchia or Putmonata are directly related to the annual mean oxygen isotopic composition of meteoric precipitation by the following relation: δOp(SMOW) = 117δ18OSaragonite(PDB) −591 Moreover, the present study leads for the first time to the establishment of a map of the 18O equal-contents contours of meteoric precipitation in France and on some other areas of western Europe and northern Africa The calculated values are compared with the local isotopic data of precipitation In addition, this study provides a tool for hydrogeology to make quick determinations of the annual isotopic mean composition of meteoric precipitation within a given area This method also allows the quantification of continental paleoclimatic parameters, eg temperature and δ 18O of precipitations

The age and origin of some Norwegian eclogites: A UPb zircon and REE study

Abstract: UPb zircon data from five different eclogites and one amphibolite, all occurring as boudinaged lenses within the 1.7-Ga old Basal Gneisses of western Norway, are reported along with eleven REE patterns on eclogites, amphibolite and country rocks. Based on the size, morphology and zoning patterns of zircon as well as the degree of UPb discordance, protoliths of both basaltic or gabbroic origin can be distinguished. Whereas zircons from eclogites formed from tholeftic and talc-alkaline basaltic protoliths allow dating of the Caledonian eclogite-facies event at ∼ 400 Ma, no precise information on their primary magmatic age is possible. On the other hand, eclogites formed from gabbroic precursors allowed the dating of both the primary magmatic age at ∼ 1.5 Ga, and in one case at more than 1.76 Ga, as well as a Caledonian eclogitization. One amphibolite sample, probably of prograde metamorphic origin, yielded a Sveconorwegian primary age (∼ 950 Ma) and a Caledonian age of metamorphism. REE analyses of the dated amphibolite and eclogites along with REE data on further eclogites within the Basal Gneisses, but also within the younger 1.5-Ma old mangeritic gneisses, are not in agreement with a pre-dominantly continental origin of the respective protoliths. The latter must be postulated if a simple in situ hypothesis for the origin of eclogites is correct. Instead, the observed REE patterns, ranging from LREE-depleted via flat to LREE-enriched patterns with both unfractionated and fractionated HREE, best fit an back arc—island are setting. Major- and trace-element data as well as initial ϵNd-values are in agreement with this hypothesis. Consequently, a tectonic interdigitation of 1.5-Ga old rocks, formed at various depths in a suboceanic mantle, into the Svecofennian grey gneiss basement is suggested. Based on the UPb systematics of some of the analysed zircon fractions together with published age results from the country rocks, a Sveconorwegian tectonometamorphic collisional event is most probable. In the course of such an event not only part of the non-metamorphic precursors of the eclogites but also some ultramafic mantle material, mangerites and other 1.5-Ga old island-arc-derived rocks could have been sub- and/or ob-ducted at various P-T conditions, resulting in a tectonic interdigitation of rocks of different metamorphic and magmatic history and origin. The non-thrusted part of the island arc, however, is thought to be welded to the continental margin of the Baltic Shield. Sveconorwegian gabbroic rocks (∼950 Ma) are transformed into amphibolites in the course of the Caledonian continent-continent collision. However, with the present amount of information on these rocks, it is only possible to say that they probably did not originate in a subcontinental mantle. After closure of the Iapetus Ocean in the Caledonian period, crustal thickening occurred due to subduction of the island-arc system and parts of the attached Baltic Shield underneath the North American-Greenland plate. This led to increasingly higher pressure metamorphism in the subducted rocks. Due to the relative eastward movement of the overriding plate, lithospheric plate thrusting, starting from the lowest eclogitefacies part of the subducted island-arc system, brought eclogites, and probably also ultramafic mantle material as well as other island-arc and continental rocks, from various depths into the crustal level where we observe them today. Similarly, also eclogites derived from older protoliths (> 1.76 Ga) which were already part of the Svecofennian continental crust were involved in such a scenario.

UPb, Nd Isotope and REE geochemistry in eclogites from the Cabo Ortegal Complex, Galicia, Spain: An example of REE immobility conserving MORB-like patterns during high-grade metamorphism

Abstract: REE abundances and Nd isotopic compositions were determined on representative samples of eclogite from the Cabo Ortegal Complex of northern Spain. Zircons were also separated from a whole-rock eclogite and analysed by the UPb radiometric method. Results indicate that eclogite facies metamorphism occurred between 480 and 420 Ma ago, but no precise constraint can be placed on the protolith age. The REE patterns observed suggest that there has been no significant alteration of the protolith whole-rock systems and that high-grade metamorphism has had little effect on the more mobile LREE. The eclogite protoliths were probably derived from ancient mantle sources with geochemical characteristics very similar to present-day MORB sources. This implies that LREE-depleted (N-type) tholeiites have been erupted at constructive plate margins since at least the early Palaeozoic and possibly long before. The Cabo Ortegal eclogites are allochthonous. They have been thrust up on to the continent and thus they can be compared to other eclogites which also show MORB-like characteristics (e.g., 90% of the eclogites of Vendee area of western France).

Noble-gas state in the earth's interior — Some constraints on the present state

Abstract: The noble-gas state in the present Earth's interior is discussed on the basis of selected data on recent samples. For this purpose, careful sample selection, experimental procedures and the usage of suitable isotope systems are discussed in detail. Based on the 3He/4He-40Ar/36Ar diagram, we can infer at least four typical sources for noble-gas isotopes: MORB type (M-type), plume type (P-type), atmosphere type (A-type) and crust type (C-type). Among them, only M- and P-type sources are directly related to the mantle materials. The M-type source is assigned to the depleted mantle, whereas the P-type source to the fertile mantle which is supposed to be located beneath the former. The P-type source materials are characterized by higher 3He4He and lower 40Ar36Ar ratios compared to the MORB's values. All observed data for terrestrial samples seem to be explained by the mixing of these components including those derived from the arc areas and from the hot-spot areas. The apparent lesser heterogeneity of noble-gas isotopes in the Earth's deep interior probably reflect the easier mobility of noble gases at high temperatures compared with those of solid elements. Based on higher 3He4He and lower 40Ar36Ar ratios in the P-type source materials compared with MORB's values, we infer that noble gases (and probably volatiles) are still retained in the Earth's deep interior with significant amounts, which might play an important role to form mantle plumes and some other processes. The 40Ar36Ar ratio for the total Earth is estimated to be ∼350 or less at present. Compared with the values for the other planets, it is conjectured that the K36Ar ratio for each planet might have been already determined during the accretion process.

Oxygen isotope ratios in modern African gastropod shells: a data base for paleoclimatology

Abstract: The oxygen isotope ratios of ∼ 80 modern gastropod shells, collected throughout Africa, show a fair correlation with rainfall patterns as signalled by permanent vegetative cover, but some regions show substantial anomalies. Gastropods from the East African Rift Valley lakes showed oxygen isotope ratios 3–6%o more positive, and those from the northeastern regions of South Africa were both more positive and more variable than expected. Temperature effects on precipitation and evaporative isotope enrichment of rainfall and water bodies are the principal causes of these discrepancies. The possibilities for the use of isotope ratios of fossil shells for paleoclimatology are discussed.

Dissolution of barite for the analysis of strontium isotopes and other chemical and isotopic variations using aqueous sodium carbonate

Abstract: A simple procedure for preparing barite samples for chemical and isotopic analysis is described. Sulfate ion, in barite, in the presence of high concentrations of aqueous sodium carbonate, is replaced by carbonate. This replacement forms insoluble carbonates with the cations commonly in barite: Ba, Sr, Ca and Pb. Sulfate is released into the solution by the carbonate replacement and is separated by filtration. The aqueous sulfate can then be reprecipitated for analysis of the sulfur and oxygen isotopes. The cations in the carbonate phase can be dissolved by acidifying the solid residue. Sr can be separated from the solution for Sr isotope analysis by ion-exchange chromatography. The sodium carbonate used contains amounts of Sr which will affect almost all barite 87 Sr 86 Sr ratios by less than 0.00001 at 1.95θ of the mean. The procedure is preferred over other techniques used for preparing barite samples for the determination of 87 Sr 86 Sr ratios because it is simple, rapid and enables simultaneous determination of many compositional parameters on the same material.

Origin of eclogites from South Brittany, France: A SmNd isotopic and REE study

Abstract: Rare-earth element (REE) distribution patterns and Sr and Nd isotopic compositions have been obtained on 10 whole-rock samples (5 eclogites, 2 garnet-pyroxenites, 3 metagabbros) from the Armorican massif in the Hercynian fold-belt of western France. The REE distribution patterns suggest that: (1) the protoliths of the eclogites originated in an oceanic environment; and (2) severe REE mobility seems not to have occurred either during high-grade metamorphism, or during post-metamorphic alteration. The Nd isotopic data support the above conclusions and further indicate that distinct mantle sources existed for the eclogite protoliths 1.3 Ma ago, and that enrichment in LREE probably occurred in the source rocks before the melting event leading to the formation of the protolith magmas.

Isotopic composition of the earth's helium and the problem of the motive forces of tectogenesis

Abstract: The isotopic composition of helium ( 3 He 4 He = R ) contained in underground fluids was studied in different geotectonic provinces. Provincial mean values ( R ) were estimated by a routine statistic procedure. The R - values were found to depend on the age of tectono-magnetic activity (t): as t increases, R decreases. R min of ∼2 · 10−8 is observed in the ancient (pre-Baikalian) structures of continents. Much higher R-values (≈1.2 · 10−5) are characteristic for the oceanic rift zones. R max of ∼2 · 10−5 is established in Iceland which is considered as a “hot spot”. The source of these types of He isotopic composition appear to be continental crust, depleted mantle reservoir and undepleted mantle. Regional characteristics of the R distribution are similar to those of a terrestrial heat flow (q). Correlation of the values of R and q observed at each station of He sampling was revealed at the 95% probability level. Quasi-synchronous changes of q and R with geologic time reflect two (geophysical and geochemical) aspects of one and the same general geologic process — tectogenesis. The triple relationship of q , R and t demonstrates that this process is related to the influx of thermal energy and silicate material by the heat-mass flow from the mantle into the lithosphere. The silicate composition of this flow is confirmed by the correlation between R and 87 Sr 86 Sr . In the course of geologic time these evidences of the mantle heat-mass flow are gradually reduced because of cooling of the newly formed blocks of the lithosphere, the dissipation of the mantle He into space, and the generation of the radiogenic isotopes.

REE, RbSr and SmNd studies of Norwegian eclogites

Abstract: REE analyses and isotopic studies have been carried out on Norwegian eclogites and garnet peridotites to determine the age of metamorphism and the origin and history of the protoliths. Whole-rock REE patterns of eclogites in gneiss vary greatly, even within single outcrops. Many show positive Eu anomalies, indicating plagioclase fractionation and thus gabbroic protoliths. The irregular REE patterns and the common depletion of La and Ce relative to the middle REE, may reflect metasomatic effects during high-P metamorphism. The distribution of REE between clinopyroxene (cpx) and garnet (gnt) in the garnet peridotites is consistent with equilibration at lower T than the corresponding phases of gnt-lherzolite nodules in kimberlites. If the Norwegian gnt-peridotites are fragments of subcontinental mantle, they have undergone post-emplacement equilibration. Most eclogites that have high TMET (> 700°C) also show equilibrium distribution of REE between cpx and gnt, with large spreads in Sm/Nd. Gnt-cpx pairs from five such eclogites give SmNd ages from 407–447 Ma. In lower-T eclogites (∼ 600°C) the REE have not equilibrated between gnt and cpx; no spread in Sm-Nd is observed, and no dates have been obtained. ISr varies from 0.7025 to 0.7245 in 21 samples of eclogites in gneisses. Most orthopyroxene (opx)-free eclogites fall in the range 0.7025–0.7060, while 7 or 8 opx-bearing ones have ISr > 0.715. While most opx-free eclogites may be derived from gabbroic protoliths, the opx-eclogites must either be derived from old Rb-rich protoliths, or have interacted extensively with the surrounding gneisses. The Sr and Nd isotopic data suggest that the rocks studied here all had complex crustal histories prior to the eclogite-forming metamorphism. Mica-cpx or mica-whole rock (WR) RbSr “ages” on Norwegian eclogites range from 1200 to < 400 Ma. Many samples show marked isotopic disequilibrium and the detailed significance of these “ages” is debatable. However, a five-point mineral isochron on one kyanite eclogite gives an isochron age of 398±1 Ma, with ISr = 0.7034. The eclogite-forming event in western Norway was apparently part of theCaledonian continent-continent collision, in which the Western Norwegian Gneiss terrane was overridden by the Greenland plate. The Wenlockian SmNd mineral ages may refer to the beginning of uplift after this collision, while the RbSr mineral ages probably reflect a later stage of the uplift-cooling cycle.

RbSr geochronology and geochemistry of torlesse metasediments from the Central North Island, New Zealand

Abstract: Petrographic, chemical and isotopic data are presented for a suite of metasedimentary rocks from the Torlesse terrane, North Island, New Zealand. Detrital mineral modes and chemical trends indicate that the rocks are the end-products of hydraulic sorting of dominantly granitic detritus by turbidity currents. Subsequent diagenetic and metamorphic alteration has only slightly modified the original bulk-rock chemistry although important mineralogical changes have occurred. A RbSr whole-rock isochron for the argillitic end-members of the suite gives an age of 141 ± 3 Ma and is interpreted to represent the timing of low-grade metamorphism of the rocks. The study presents evidence to show that near-complete equilibration of Sr isotopes is possible in coarse-grained greywackes as well as in fine-grained argillites during prehnite-pumpellyite to lower-greenschist facies metamorphism. The process relies mainly on the albitisation of plagioclase and recrystallisation of the matrix accompanied by interaction of the released Sr with circulating pore fluids within relatively small domains.

Evidence for old Precambrian relics shown by UPd zircon dating of eclogites and associated rocks in the Hercynian belt of South Brittany, France

Abstract: UPb dating of zircon from an eclogite of the Champtoceaux Complex, south Brittany, has yielded an upper intercept age of 2208 ± 39 Ma. This age is interpreted as representing contamination processes or the emplacement of an eclogite protolith having geochemical affinities with intermediate rocks of talc-alkaline magmatic series. The lower intercept on the concordia diagram is interpreted as the age of the high-pressure-high-temperature metamorphic event at 413 ± 16 Ma. Results obtained on a serpentinite from the Baie d'Audierne Complex indicate an ultrabasic protolith crystallization age older than 840 Ma. Taken in conjunction with published upper intercept ages of 1300 Ma for the eclogite protoliths from the Vendee and the Baie d'Audierne, the present data would suggest the existence of relics of a Precambrian basement in south Brittany. The geological history of this basement can be summarized as follows: 1. (1) a magma of intermediate composition emplaced during the early Proterozoic, around 2.2 Ga ago, in an active margin-type tectonic environment. This date is in agreement with the possible existence of continental crust as old as Archaean in south Brittany, as shown by the presence of inherited zircons in Palaeozoic formations with upper intercept ages around 2.5 Ga; 2. (2) eclogite protoliths were preserved in a continental crustal environment as far as 440-380 Ma; 3. (3) around 400 Ma eclogite-facies metamorphism affected the ancient basement rocks of south Brittany, probably as a result of continental collision during the early stage of the Hercynian orogeny. The regional zonation of the Siluro-Devonian high-pressure event (436-413-384 Ma) throughout the south Brittany area could be due to a succession of small collisions or related to the convergence of plates with irregular outlines.

Terrestrial organic carbon in marine sediment: A preliminary balance for a mangrove environment derived from 13C

Abstract: The δ 13 C-values of organic material in the marine sediments off Hinchinbrook Island, Australia, indicate a widespread (> 230 km 2 ) influence of terrestrial organic carbon, C T . Extensive mangrove forests on the island are known to export T day −1 of leaf litter and 7.5 Mg C T day −1 of micro-particulate organic carbon (POC) (1 Mg = 10 6 g). 210 Ph sedimentation rates, organic-carbon contents and the stable-isotope ratios of organic matter in the sediment were used to calculate an accumulation rate of 13–29 Mg C T day −1 for these marine sediments. This preliminary balance indicates that terrestrial organic carbon in this marine environment can be accounted for by terrestrial export although significant losses do occur. Shoreline redeposition of floating leaves, etc., and organic matter degradation to CO 2 and/or CH 4 followed by bubble ebullition are likely to be major loss terms. Considered with the work of Hedges and Parker (1976) , this work suggests that leaf litter may be the most important single source of terrestrial organic carbon in the marine environment, although it is realized that mangroves represent an extreme example of terrestrial productivity.

Carbon and oxygen isotopes in eclogites, amphibolites, veins and marbles from the Western Gneiss region, Norway

Abstract: The 13C12C and 18O16O ratios have been measured in carbonates and silicates from pods of eclogite, garnet-amphibolite and marbles and also some veins which outcrop in the Western Gneiss Region, Norway, in an attempt to clarify the origin of these rocks. The eclogite carbonates (magnesite or dolomite) display carbon and oxygen isotope values (−9‰ < δ 13C < −2.4‰ PDB; +6.8‰ < δ 18O < +10.4‰ SMOW) which lie within the field of mantle-effluxed carbonate as represented by primary igneous carbonatites. Two garnet-amphibolites have heavier oxygen in the same δ 13C range. Two marbles have heavier carbon and differing δ 18O. Three types of carbonate-bearing vein show different isotopic relationships to the carbonate of their host rocks. Oxygen isotope fractionations amongst minerals indicate preservation of a high-temperature equilibration (650–850°C) comparable to the temperature previously deduced from FeMg partitioning amongst silicates (700–850°C). The eclogite carbonate carbon and oxygen isotope compositions may have been produced either by metamorphism of crustal carbonates or by carbonation of carbonate-free rocks by mantle-effluxed CO2 before or during the eclogitisation processes. The whole-rock δ 18O-values (4–8‰ SMOW) are within the range of crustal basalts and gabbros thus supporting a crustal origin for these eclogites. Several other lines of petrological and geochemical evidence also support the model of crustal protoliths of these rocks. These data may contribute to the geodynamic modelling of the carbon cycle in indicating that subducted carbon reinjected into the mantle could have essentially the same isotope composition as that of carbon effluxed from the mantle and that a crustal protolith origin for some erupted xenoliths gains in credibility.

The solubility of rare gases in silicate melts and implications for KAr dating

Abstract: The solubilities of Ne, Ar and Kr in tholeiitic basalt, alkali-olivine basalt and basaltic andesite have been measured as functions of the pressure of the gas (for Ar only) and the temperature of the melt. The melts were equilibrated with the gases for 8 hr., quenched and the amount of gas incorporated in each specimen was then determined. Within the range of these measurements (0 < p < 50 Torr, 1000°C < T < 1400°C), the solubilities depend on the chemical compositions of the melts, increase with both temperature and pressure; and decrease with increasing atomic weight of the gas.

The closed-system approximation for evolution of argon and helium in the mantle, crust and atmosphere

Abstract: The atmosphere formed by the outgassing of the depleted mantle, leaving a remnant of non-degassed mantle which forms the source of some ocean-island basalts such as Hawaii and Iceland. The 40Ar in the atmosphere degassed from 50% to 90% of the mantle possibly synchronous with sea-floor spreading, ocean-ridge hydrothermal activity and continent formation. The bulk of the degassing ended 1.2–1.8 Ga ago. The similarity of the 40Ar36Ar ratio between the atmosphere and non-degassed mantle suggests both have approximated closed systems. On the other hand, more than 99% of He outgassnd from the mantle has been lost to space from the upper atmosphere. Portions of the oceanic crust and mantle contaminated by atmospheric noble gases are distinguished from non-degassed mantle by this He depletion.

Practical considerations on carbon isotope studies on soil carbon dioxide

Abstract: Field and laboratory investigations show that the absorption of carbon dioxide in sodium hydroxide solutions is only reproducible with total absorption. Otherwise significant kinetic isotope effects do occur. Long-term experiments suggest that isotopic equilibria between “aqueous” carbonate in the alkaline solution and gaseous carbon dioxide are approached.

RbSr geochronology of coarse-grained greywackes and argillites from the Coffs Harbour Block, Eastern Australia

Abstract: The Coffs Harbour Block, eastern Australia, includes a sequence of turbidity-current-deposited grey-wackes and argillites dominated by quartz-poor to quartz-intermediate, volcanic-lithic and feldspathic types. The rocks have a common provenance consisting of dacite with minor andesite and rhyolite, derived from a continental margin volcanic arc. RbSr whole-rock dating of coarse-grained greywackes and argillites suggests that low-grade regional metamorphism (prehnite-pumpellyite to lowest greenschist facies) has completely equilibrated sediments of all grain sizes, giving an age of 318 ± 8 Ma. A subsequent metamorphism which produced thermal biotite on a regional scale over the southern part of the area, re-equilibrated only the fine-grained sediments 238 ± 5 Ma ago; the coarse-grained sediments were unaffected isotopically and still lie on the older isochron. The wide variations in sandstone petrographic modes imply that the older isochron represents neither the age of volcanism in the magmatic arc nor the age of sedimentation at the deposition site, but rather dates the time of earlier metamorphism.

The stable-isotopic composition of water and sulfate from the Raudhatain and Umm Al Aish freshwater fields, Kuwait

Abstract: The desert plain of eastern Arabia slopes gently northeast over Kuwait where it is interrupted only by low hills, depressions, escarpments and shallow wadis. These wadis carry occasional runoff and drainage is mainly internal. Rainfall is variable and averages 100 mm yr. −1 most of it falling during intermittent winter thunderstorms. Average winter lows are 9°C and summer highs 45°C Evaporation potential far exceeds precipitation. High evaporation and low rainfall lead to the formation of a getch layer: a duricrust of gypsum and/or calcite within the desert sands. Underlying Kuwait Group sediments (450 m of sand, gravel and limestone) and Dammam Formation (200 m of dolostone) are the main aquifers in Kuwait. They are underlain by the Rus Formation (125 m of anhydrite) which is an aquiclude. Lenses of fresh groundwater (salinity 200–1400 mg 1 −1 ) occur in the depressions of Raudhatain and Umm Al Aish. Brackish and ultimately salty waters occur in the Kuwait Group and Dammam aquifers. Stable-isotopic analyses of rainwater show a deuterium excess of 10–20%. and reflect influences from both the Indian Ocean and the Mediterranean. Raudhatain and Umm Al Aish samples also lie about meteoric water lines (on a δ 18 0 vs. δ D plot) with averages: δ 18 O = −3 ± 1%., δ D = −10%. and deuterium excess 17%o. They represent rainwater recharge during flooding; no effects of surface evaporation can be detected. In contrast, the brackish paleowaters of the Kuwait Group and Dammam Formation have much lower δ 18 0- (−4.5%.) and δ D- (−35%.) values and are modified meteoric water from a cool wet period in eastern Arabia 11,000–60,000 yr. B.P. No isotopic evidence can be seen for the mixing of the fresh and brackish waters. The Umm Al Aish waters (salinity 200–500 mg 1 −1 are more dilute than Raudhatain (up to 1400 mg 1 −1 where greater draw-off may have caused some intrusion of brackish water. The isotopic compositions of sulfate in the Raudhatain and Umm Al Aish waters (δ 34 S = +15.3–16%. and δ 18 0 = +13.6–15%.) group away from Arabian Gulf seawater and the Rus anhydrite. The source of the sulfate appears to be rainwater-sulfate plus sulfate leached from the getch.

Areal distribution of 3He/4He ratios in the Tohoku district, Northeastern Japan

Abstract: Natural gas samples, including various types of gases from hot springs, mineral springs and water wells, were collected at nine sampling sites in the Tohoku district, northeastern Japan. The 3 He 4 He and 20 Ne 4 He ratios of these samples were measured using a mass spectrometer. The ranges of observed 3 He 4 He and 20 Ne 4 He ratios of gas samples are from 3.56·10 −7 to 9.01·10 −6 and from 2.9·10 −3 to 5.2·10 −1 , respectively. The temporal variations in 3 He 4 He ratios observed for two sampling sites show little fluctuation over a period of 28 months. Samples collected in the forearc region have radiogenic He ratios, being about one-third of the atmospheric value, whereas in the volcanic arc region, the ratios are high with values 2.5–6.4 times the atmosphere. The 3 He 4 He ratios in the Tohoku district show good correlation to the subduction-related tectonic structure of the island-arc system. These observed variations in 3 He 4 He ratios may reflect the state of material flow associated with magma generation.

Age of secondary uranium mineralizations in the basement rocks of northeastern Bavaria, F.R.G.

Abstract: More or less intensive supergene redeposition is displayed in the upper parts of all of the U occurrences in the basement rocks of northeastern Bavaria. The secondary U minerals formed during this redeposition — uranophane, torbernite, autunite, uranocircite and saleeite — were dated using the U/Pb and ionium-uranium methods. All of the samples yielded discordant U Pb ages. The upper intersection of the discordia with the concordia in the graph of 207 Pb 206 Pb vs. 238U/206Pb can be regarded only in some cases as the age of the primary pitchblende from which the U mineral was formed, whereas in all cases the lower intersection gives the age of the secondary U mineralization. Ages between 0.110 ± 0.006 Ma (by the ionium—uranium method) and 8.6 Ma were obtained; these can be divided into three groups: Group I comprises all of the minerals whose 230 Th 234 U activity ratio was lower than 1. Assuming that there was no 230Th present at the time of mineral formation, ages between 0.110 and 0.164 Ma were calculated for three torbernite occurrences. Group II consists of those U minerals which are in a state of radioactive equilibrium (i.e. older than 0.4 Ma), but which yielded U Pb ages between 0 and 3 Ma. Owing to the limited precision of the lower intersections, more accurate dates cannot be given. Group III contains the oldest of the secondary U minerals and is characterized by well-defined ages between 5.6 and 8.4 Ma.

Interpretation of 234U/238U activity ratios in groundwaters

Abstract: The quasi-steady variation in 234 U 238 U activity ratio has been determined along an idealised homogeneous one-dimensional aquifer in which the transfer of radionuclides from the aquifer material to solution occurs by both α-recoil and ion exchange. The results highlight the importance of considering these two effects in predicting the variation of 234 U/U activity ratios in a flowing system. In particular, it was shown that such ratios in old waters should achieve the moderate values indicated by measurements.