Redistribution Of Isotopes Research Articles

Chemical and isotopic redistribution of hydrocarbons during migration: Laboratory simulation experiments

Recent investigation of shale-sandstone sequences isolated from cores indi cate that considerable compositional fractionation of hydrocarbons may occur during migration. In the present study, the effects of laboratory simulated migration on the chromatographic distribution and stable carbon isotopic composition of hydrocarbon constituents of crude oilsand organic-rich shales are examined. Experimental results indicate that a redistribution of the C 15+ aliphatic hydrocarbon fractions in favor of the lower molecular weight components occures with increased distance of migration. With increased migration distance δ 13C values for the aromatic hydrocarbon fractions remained relatively unchanged. The aliphatic and NSO fractions, however, become slightly depleted in 13C with increased distance of migration. This phenomenon, in some cases, might become accentuated with increasing migration distance and should be considered when attempting oil to oil or oil to source rock correlations.

Temperature dependence of [formula omitted] for protium, deuterium and tritium in niobium

The thermal diffusion of protium, deuterium and tritium in niobium was investigated experimentally at average temperatures of about 70, 150 and 250 °C. The redistribution of hydrogen isotopes was examined as a function of the duration of the thermal gradient. The values of the heat Q ∗ of transport and the self-diffusion coefficient D were determined by analysing these data. It was shown that the values of Q ∗ decrease linearly with temperature and that the temperature coefficients for protium, deuterium and tritium are almost equal. These results are discussed on the basis of the biased diffusion model, and it is shown that the bias effect decreases with temperature. The diffusion coefficients determined for protium, deuterium and tritium are compared with those based on the Gorsky effect and good agreement is observed.

Nature of the catalytic cycle in iridium-complex catalysed hydrogenation of unsaturated alcohols

The endo face specific reduction of endo -6-methylenebicyclo[2,2,2]octan-2-ol and endo -6-methy[bicyclo [2,2,2]-oct-5-en-2-ol with D 2 and iridium catalysts is accompanied by deep-seated isotopic redistribution through an intramolecular mechanism, although only two deuterium atoms are incorporated on average. Individual isotopomers of the product may be identified in the 13C N.m.r. spectrum at 125 MHz. and their ratio is generally consistent with a mechanism in which product is formed by breakdown of an alkyliridium trihydride. Iridium (and rhodium) catalysts part-isomerise the exocyclic olefin to its endocyclic isomer via an Ir-allyl intermediate without incorporation of deuterium. The reduction of 3-methylcyclohex-2-enol is likewise accompanied by considerable scrambling, with isotopic enrichment occurring at C2, C3, C4 and C5 of the product, trans -3-methylcyclohexanol. Deuteration occurs exclusively on the hydroxyl-bearing face of the molecule.

Accessory minerals and the geochemical evolution of crustal magmatic systems: a summary and prospectus of experimental approaches

It has recently been recognized that accessory minerals in continental rocks may contain a substantial fraction of the whole-rock complement of trace elements and geochemically important isotopes. This observation is in itself interesting, but its importance is magnified many fold when one considers the possible consequences to the geochemical rearrangement that accompanies crustal melting. If, for example, an accessory phase is insoluble in early-formed partial melts, it may effectively retain in the residue many incompatible elements (e.g., REE) and daughter isotopes (e.g., radiogenic Pb) that would otherwise be partitioned into the melt. In addition, should an unmelted accessory mineral (e.g., zircon) be entrained in a departing melt fraction, it may carry with it radiogenic components (Pb∗, Nd 143, Hf 176) characteristic of the source region. The effectiveness of phases such as zircon, apatite, sphene, allanite-epidote, and monazite in the roles noted above can only be gauged from knowledge of the three “fundamental accessory-phase parameters,” i.e. (1) the solubilities of the accessory minerals in crustal melts; (2) the equilibrium mineral/liquid partition coefficients for the trace element and isotopes of interest; and (3) the diffusivities that govern the rates at which equilibrium will be approached. In general, these parameters can only be measured in laboratory experiments, and relatively few data have been accumulated to date. It does seem clear, however, that apatite generally is residual (insoluble) during crustal melting and zircon can be, depending on melting conditions and abundance of zircon in the source. Petrographic and geochemical studies indicate that monazite, allanite, and epidote may also be highly insoluble in some cases, but the specific circumstances that promote insolubility remain unknown. The few experimentally-determined partition coefficients now available for apatite and zircon agree qualitatively with natural rock data on mineral separates. However, preliminary diffusion measurements and inferences from natural systems suggest that trace-element and isotopic equilibrium between undissolved accessories and crustal melts is in fact rarely approached. Ironically, it is this very disequilibrium in the form of preserved diffusion profiles, that may afford the most information on time-temperature histories of melting events. Continued experimental interest in the fundamental accessory-phase parameters will eventually provide the tools that will enable rigorous assessment not only of the timing and duration of crustal melting episodes, but also of the extent of melting and re-distribution of trace elements and radiogenic isotopes.

Nd isotopic systematics and chemistry of Central Australian sapphirine granulites: an example of rare earth element mobility

Lower Proterozoic sapphirine-bearing and associated granulites from Central Australia exhibit the greatest range of present-day 143Nd/ 144Nd ratios (∈ Nd(O)= −26.5 to +112.3) yet reported for rocks believed to be cogenetic. The Nd isotopic data and REE abundances of these rocks demonstrate extreme fractionation of the rare earths during the formation of stratiform Cu Pb Zn sulfide deposits with which they are closely associated. Field relationships, petrography and chemistry of the sapphirine granulites suggest that their protoliths comprised chlorite-rich rocks which were generated by hydrothermal alteration of a range of rock types prior to metamorphism; calculations employing REE abundances of the sapphirine granulites and associated rocks, combined with bulk solid-fluid distribution coefficient data yield high fluid/rock ratios, consistent with a pre-metamorphic hydrothermal origin for the unusual REE patterns. The Sm Nd data for these rocks define an age of 1760±75Ma, which is significantly younger than the crust formation age of the terrain ( 2070±125Ma) but indistinguishable from the Rb Sr whole rock age for granulite facies metamorphism (1790±35Ma). These data are interpreted in terms of major hydrothermal fractionation of the rare earths shortly (perhaps tens of millions of years) before granulite facies metamorphism, followed by redistribution of Nd isotopes or small fractionations of the Sm/Nd ratio during the granulite facies event, and possibly also during intense retrogression which reset Rb Sr whole rock and U Pb zircon and monazite systematics at about 1700 Ma.

Redistribution of uranium and thorium series isotopes during isovolumetric weathering of granite

Previous studies of the distribution of U and Th in parent versus weathered granites have shown both depletion and enrichment of these elements during weathering. In this study, the distribution of U and Th decay series isotopes was determined in a weathering profile of a granitic saprolite, which showed textural preservation indicating isovolumetric weathering. Two types of dissolution methods were used: a whole-rock dissolution and a sodium-citrate dithionite leach to preferentially attack noncrystalline phases of weathering products. Using volume-based activities, 45–70 percent of the total 232Th was gradually removed during weathering. Although the whole-rock 228Th 232Th activity ratios were in equilibrium, there were large excesses of 228Th in the leachable fraction of both parent rock ( 228Th 232Th = 2.06 ) and partially weathered saprolite ( 228Th 232Th = 3–6.5 ), due to alpha recoil and release of daughter 228Th to the weathering rind of the mineral grain. For the most weathered sample, 81 percent of the thorium was in the teachable fraction and 228Th 232Th = 1 , indicating that even the more resistant minerals were attacked. The total U activities showed as much variation in the six parent rock samples as in the weathered profile, and 234U 238U were in equilibrium in both the whole-rock and leachable fractions. 230Th was deficient relative to 234U and 226Ra in both fractions, suggesting recent addition of U and Ra to the entire profile. The large variation in U was not from absorption onto the intermediate weathering products, because only 11–23 percent of the U was in the leachable fraction.

Rb−Sr age of Gaik granite, Ladakh batholith, northwest Himalaya

The Gaik Granite is a part of the Ladakh batholith outcropping between Gaik and Kiari in NW Himalaya. This is a pink porphyritic granite rich in biotite and poor in hornblende. Rb-Sr analyses have been made on six whole-rock samples of the Gaik Granite. Though the samples are poorly enriched in radiogenic Sr, they define a reliable isochron corresponding to an age of 235±13 (2σ) m.y. and initial87Sr/86Sr ratio of 0·7081±0·0004 (2σ). Biotite, plagioclase and potash feldspar fractions separated from two of the samples have yielded a much younger mineral isochron at 30±1·5 m.y. indicating a nearly complete redistribution of Sr isotopes between mineral phases at a time much later than the primary emplacement of the granite. The present results show that at least some components of the Ladakh batholith are of Permo-Triassic age. These rocks were isotopically re-equilibrated on a mineral scale during Upper Oligocene in response to the Himalayan orogeny.

Determination of the Fragmentation Pattern of HOD

Abstract During our studies of the methanation reaction over Ni/SiO2 catalyst we have employed mass spectroscopy to follow the redistribution of isotopes among the reactant and product species. When deuterium was employed in the studies,1 a wide variety of isotopic species resulted. Among the species produced are H2O, D2O, and HOD. If accurate sensitivity and fragmentation data are available on the species in question, matrix algebra may be used to transform mass fragment data into concentrations. For example see the text by Roboz.2

Kinetic equations of isotope redistribution in an elementary reaction

Kinetic equations for the redistribution of isotopic molecules in an elementary reaction have been derived on the basis of atom distribution matrices.

The geochronology of Iqna Granite (wadi Kid pluton), Southern Sinai

The wadi Kid pluton of Iqna Granite, Southern Sinai, which was intruded during the last Precambrian magmatic phase, yields a Rb-Sr total rock isocrhon age of 580±23 m.y., and an initial 87Sr/ 86Sr ratio of 0.7028±0.0028. The magma of the Iqna Granite was derived from a low Rb/Sr source shortly before its crystallization. Partial resetting of biotite ages is detected by both Rb-Sr and K-Ar methods. Mineral isochrons yield higher initial values (0.7045–0.7065) as a result of Sr isotopic redistribution within a closed total rock system.

Isotopic Structures in Solar System Materials

For quite some time one of the major boundary conditions in considera­ tions of the origin and evolution of the solar system has been the notion that at some stage in its history the solar system had a unique uniform composition and that every sample now accessible has evolved from composition . This is a simple idea but a very powerful one. In review I use the term primordial to denote composition, specifically at the time of of meteorites (where formation means closure to isotopic redistribution, the usual geochronological definition). The principal evidence for primordial uniformity concerns isotopic ratios : an extensive body of data is consistent with the proposition that all isotopic variation in samples of the solar system can be understood in terms of modification of a once uniform composition: This is not a trivial observation . The stuff of the solar system has been made in a variety of times, locations, and astrophysical environments, and we can expect the interstellar medium from which the solar system formed to have been isotopically heterogeneous on all scales. The isotopes have nevertheless been well mixed and, as put by Reynolds (1967) in an earlier review, this mixing is the most important single thing to be said about isotQpes, in that any theory for the origin of the solar system is required to account for isotopic homogenization. It is usually considered that the of the solar system involved a nebular stage in which temperatures were high enough to vaporize any preexisting solids and erase any pre­ existing heterogeneity (Suess 1 965). Within framework we can distinguish four categories of isotopic variation: 1 . Fractionation 2. Decay of primordial radionuclides

Strontium isotope evidence for mantle events in the continental lithosphere

Following current interpretation of isotope data from oceanic basalts, the case for inheritance of Rb/Sr and 87 Sr/ 86 Sr ratios in continental igneous rocks from long-lived heterogeneities in the mantle is reviewed, both from the point of view of necessary conditions and the evidence of published 'mantle isochrons'. Such inheritance seems unlikely for magmas undergoing plagioclase fractionation and can hardly be claimed for rocks where crustal contamination (especially selective contamination with trace elements or 87 Sr alone) is a feasible alternative. Nevertheless, some unfractionated transitional or undersaturated basic volcanics do show remarkable co-variance of Rb/Sr and initial 87 Sr/ 86 Sr ratios, indicating ages far in excess of extrusion. These and other arguments suggest that heterogeneous mantle, both enriched and depleted in lithophile trace elements, is incorporated into the continental lithosphere during major crust-mantle differentiation events. Re-distribution of isotopes and incompatible elements in fluid phases or small partial melts may be an important mechanism, although others have suggested addition of enriched material from below the asthenosphere.

40Ar- 39Ar age determinations on the Owyhee basalt of the Columbia Plateau

40Ar/ 39Ar step-heating analyses have been performed on 11 samples of basalt from sites near Owyhee Reservoir of southeastern Oregon, U.S.A. These rocks were extruded during the great flood basalt episode of the Pacific Northwest. The whole-rock points are highly correlated on a plot of 40Ar/ 36Ar versus 39Ar/ 36Ar, corresponding to a common age of the samples of 14.3 ± 0.3 m.y. Inspite of this, individual “plateau” plots of the age versus fraction of 39Ar released do not give good plateaux. These age spectra exhibit to varying degrees a common structure in which lower age values are found at higher temperatures. This pattern may result from a closed-system redistribution of the argon isotopes. The usefulness of grinding the basalts in removing a loosely held atmospheric argon component is confirmed.

Anion exchange of uranium from aqueous sulphuric acid solutions: Diffusion kinetics

AbstractThe ion exchange kinetics of uranium sorption from acidic sulphate solutions onto weak and strong base resins has been studied. In order to facilitate experimentation and analysis, isotopic redistribution experiments of uranyl sulphate ions (with U‐235 as a tracer) under conditions of particle diffusion control have been performed. Also, self‐diffusion experiments for sulphate ions (with S‐35 as a tracer) and counter diffusion measurements involving uranyl sulphate‐sulphate exchange have been performed. The ion exchange of uranium is a slow reaction and appears to be particle diffusion controlled at all solution conditions of practical interest. For a weak base resin (Relite MG‐1), the self‐diffusion coefficient of uranyl sulphate ions changes with particle size, increasing from 1.49 to 4.88 × 10−9 cm2/s as the particle diameter is reduced from 0.146 to 0.054 cm. With a strong base resin (Dowex‐1×8), the self‐diffusion coefficient of uranyl sulphate ions remains reasonably constant with varying particle diameter at a value of 1.76 × 10−9 cm2/s. Sulphate ion self‐diffusion coefficients were also constant with particle size (2.50 × 10−7 cm2/s) and the counter exchange diffusivity lies between the values for the individual species; a typical value is 1.53 × 10−8 cm2/s. It is argued that resin matrix structure has a significant effect on uranium kinetics and that a knowledge of the relevant diffusion coefficients will provide a rational basis for the design of ion exchange processes and plant equipment.

The effects of diagenesis on the redistribution of strontium isotopes in shales

The rubidium-strontium method has often been used in attempts to date shales, but has yielded mixed results. In order to explore the effects of diagenesis on this system, samples of a Miocene shale section collected from an oil well drilled in coastal Louisiana were separated into fractions and analyzed for Sr 87/Sr 86, Rb and Sr. The diagenetic changes involve the destruction of detrital mica and feldspar and the formation of an illite-rich clay from smectite. Strontium from interstitial waters, calcium carbonate and adsorbed on the clay minerals is released and then sequestered by the newly forming phases, together with the strontium mobilized from the silicate minerals. Attending these changes there is a trend towards the homogenization of the shale with respect to its strontium isotopes, but diagenesis and homogenization are not complete in the deepest part (5523 m) of the section studied. Therefore, the interpretation of rubidium-strontium shale dates remains uncertain, but the dates in many cases most probably represent the time equal to or younger than the time of the major construction of new phases and the attendant homogenization of the strontium isotopes.

Geochronology of a polymetamorphic and anatectic gneiss region: The moldanubicum of the area Lam-Deggendorf, eastern Bavaria, Germany

Rb-Sr isotopic analyses of whole-rocks and biotite and U-Th-Pb analyses of zircon and monazite reveal regional metamorphic events for the Ordovician (Caledonian metamorphism) and the Carboniferous (Variscan=Hercynian orogeny), both accompanied by anatexis. The extent of the Caledonian and Variscan anatexis, however, cannot be evaluated, so far, because the field petrographic criteria are not sufficient to distinguish clearly between early and late Paleozoic anatexites. Evidence for a Precambrian metamorphism has not been found. Rb-Sr whole-rock isochrons obtained on leucosomes and melanosomes of partially molten paragneisses are interpreted as a minimum age of the second, early Variscan anatexis. The alternative explanation of the isochrons as a result of local Sr isotopic redistribution without a melt involved is considered less likely. Concordant and nearly concordant zircon ages (318–335 m.y.) of a coarse-grained granite and of “diatexites” are regarded as evidence for an intensive early Variscan granitization and palingenesis. Concordant zircon ages of “diorite” dykes, crosscutting the anatexites, establish a lower time limit of 309–312 m.y. for the Variscan anatexis. Rb-Sr ages of biotite (310-290 m.y.) indicate the end of the Variscan metamorphism.

OROGENÉSES SUPERPOSÉES DANS LE PRECAMBRIEN DU BRÉSIL SUD -ORIENTAL (États de Rio de Janeiro et de Minas Gerais)

The ages of three precambrian orogenic cycles have been defined hy means of U/Ph, Rb/Sr and K/Ar determinations carried out on igneous and metamorphic rocks of southeast Brazil. This work confirms the preliminary results obtained by the same authors on the granitoid rocks of Serra dos Orgaos (which belong to the Brazilian orogenic cycle, about 620 m.y.), and on the granulitic gneisses of the Paraiba do Sul Formation (Trans-Amazonian orogenic cycle, about 2070 m.y.). For the Mantiqueira gneisses, considered, by field relationships, to be the oldest rocks of the region, new results establish an age of about 2800 m.y. (λ Rb 87 = 1,47 X 10 - 11 years -1 ). The Juiz de Fora Formation has the same petrologic characteristics as the Paraiba do Sul Formation, and it is supposed to be discordant over the Mantiqueira gneisses. However, the geochronological results seem to demonstrate that it is, at least in part, as old as the latter formation. In addition, this work demonstrates a practically complete redistribution of Sr isotopes among the minerals of all rocks of the region, including the oldest ones, during the late precambrian Brazilian orogeny.

Oxygen Isotopic Fractionation within Ultrabasic Clasts of Lunar Breccia 15445

Ultrabasic clasts in a lunar breccia have internal oxygen isotopic fractionations reflecting an igneous origin with no evidence of isotopic redistribution in subsequent metamorphism or recrystallization.

Homogeneous catalysis of olefin isomerisation. Part III. The isomerisation of pent-1-ene catalysed by solutions of tetrakis(triethyl phosphite)-nickel(o) and of hydridochlorocarbonyltris(triphenylphosphine)osmium(II)

The isomerisation of pent-1-ene to cis- and trans-pent-2-ene is catalysed at 35 °C by solutions of Ni{P(OEt)3}4 in benzene containing trifluoroacetic acid, and at 80 °C by solutions of OsHCl(CO)(PPh3)3 in pure benzene. The initial rate of the nickel-catalysed reaction is not diminished by the addition of triethyl phosphite, and NiH{P(OEt)3}4+ is suggested as the catalyst. The initial rate of the osmium-catalysed reaction is retarded by the addition of triphenylphosphine, and catalyst generation is suggested to occur by dissociation of triphenylphosphine from the original complex.The initial cis:trans-ratios in the pent-2-ene were 2·3(Ni) and 6·0(Os).Isomerisation of [1,2-2H2]pent-1-ene has shown that, for each reaction, isotopic redistribution in the reactant is rapid, and involves only the intermolecular exchange of hydrogen and deuterium bonded to the olefinic carbon atoms. Initially, pent-2-ene contained more deuterium than the starting material; no deuterium entered the ethyl group.The suggested mechanism involves pentyl and 1-methylbutyl intermediates. Dissociation or displacement of pent-2-ene from the catalyst occurs before the formation of 1-ethylpropyl complexes. Steric hindrance about the carbon-metal bond in 1-methylbutyl complexes is suggested in order to reconcile the conclusion (i) that the formation of 1-methylbutyl complexes from pent-1-ene is a favoured process, and (ii) that the rate of conversion of 1-methylbutyl complexes into pent-2-ene is relatively slow.

Homogeneous catalysis of olefin isomerisation. Part I. Reactions of trans-[2H2]ethylene catalysed by complexes of cobalt, ruthenium, rhodium, and palladium

Reactions of trans-[2H2]ethylene with solutions of CoHN2(PPh3)3, RuHCl(PPh3)3, RuCl2(PPh3)3, and of RhH(CO)-(PPh3)3 in benzene at 50° have been investigated. Redistribution of hydrogen isotopes in the ethylene occurred, leading eventually to a random distribution, the reaction proceeding via ethyl intermediates. Solutions of the hydrido-complexes were of comparable activity, and were more active than those of RuCl2(PPh3)3. Hydrogen exchange was observed between deuterium atoms of the olefin and protium atoms in the ortho-positions of the triphenylphosphine ligands of RuHCl(PPh3)3.A solution of PdCl2(PhCN)2 in benzene at 50° catalysed the conversion of trans-into cis-[2H2]ethylene without simultaneous isotope redistribution. Routes by which this can occur are discussed.