Initial Isotopic Compositions Research Articles

A model for the relationships between precipitation D/H ratios and precipitation intensity

A simple quantitative model has been developed which predicts that a parametric correlation between the isotopic composition of precipitation and precipitation intensity can exist as a consequence of the condensation process when certain conditions are met. These conditions include the following: (1) approximate constancy, from one sampling interval to the next, of the initial isotopic composition of the water vapor in precipitation‐producing air masses; (2) approximate constancy of the initial condensation temperature; (3) approximate constancy of the rate of vertical ascent of the precipitating air mass; and (4) approximate constancy of the ratio of condensed water to water vapor. Such conditions are probably rare in nature for the short collection intervals represented by existing samples. However, with the probable exception of condition 3, tropical oceanic islands may satisfy the preceding conditions on the month‐to‐month time scale for which precipitation samples have generally been collected for isotopic analysis. Precipitation samples from Wake and Johnston islands that plot on the meteoric water line were used to compare monthly δD values with corresponding precipitation intensity. In general, there appears to be a convergence of the data at high values of δD (low intensity) and a divergence at low values of δD (high intensity). Such behavior is predicted by the isotopeintensity model largely in terms of variation in the vertical rate of ascent. For Wake Island this divergence of the data appears to be bimodal in character. However, for Johnston Island, there is considerable scatter in the data. This scatter is roughly constrained by the envelope defined by two vertical ascent rates that approximately characterize the presumably bimodal Wake Island trends. Thus much of the scatter in the Johnston Island data may be explained by month‐to‐variability in vertical ascent rates. The data for these two islands suggest that a simple precipitation δD versus intensity correlation is unlikely for monthly sample intervals. However, the issue remains unresolved for longer time scales.

A direct determination of the oxygen isotope fractionation between quartz and magnetite at 600 and 800°C and 5 kbar

The partitioning of oxygen isotopes between quartz and magnetite was measured at 600 and 800 C and 5 kbar, using an internally heated pressure vessel equipped with a Shaw-membrane. Through adjustment of the hydrogen fugacity the starting material, fayalite, was decomposed to form quartz and magnetite. The isotopic composition difference between the separated quartz and magnetite fraction was measured directly. As the mineral separates were not completely pure, a correction was applied to the measured fractionations. The independence of the measured isotopic composition differences of run length, the initial isotopic composition of the starting materials as well as the isotopic composition difference between them suggests that the fractionations, 1000 In α Q- M = 7.8 ± 0.4% at 600°C and 1000 In α Q- M = 6.1 ± 0.2% at 800°C, represent equilibrium values.

REE geochemistry and isotopic data of Archean silicic volcanics and granitoids from the Pilbara Block, Western Australia: implications for the early crustal evolution

Eighteen silicic volcanic rocks of the Warrawoona Group and ten associated plutonic rocks from the Pilbara Block, Western Australia, have been chosen for geochemical and isotopic studies. Silicic volcanics of the UNSB (Upper member of North Star Basalt) are dated at 3.56—3.57 ▪, by both the Rb-Sr and the Sm-Nd methods. The respective 1 (initial isotopic composition) values are 0.7005 ± 5 (Sr) and 0.50810 ± 39 (Nd). This age is consistent with the stratigraphic interpretation that the TalgaTalga Subgroup, in which the North Star Basalt occupies the lowermost position, is overlain by the Duffer Formation, whose age was earlier established at 3.45 ▪ by the zircon U-Pb method. The new Rb-Sr data on six silicic lava samples from the Duffer Formation yield an isochron of 3.23 ± 0.28 (2v). Though imprecise, this age agrees with the zircon age within error limits. Rb-Sr ages of 2.3–2.4.▪ obtained for the ‘Panorama’ rocks and the Wyman Formation do not correspond to their initial eruption ages. Chemical arguments suggest that these ages represent the time of metasomatism associated with the widespread thermal event in this region about 2.3–2.4 ▪ ago.Geochemically, most of these analyzed rocks (volcanic and plutonic) are of tonalite-trondhjemitegranodiorite (TTG) composition, a typical feature found in many other Archean terrains. They generally show fractionated REE patterns, except the Panorama Formation rocks. Furthermore, the Wyman Formation rhyolites and the post-tectonic adamellites show significant negative Eu anomalies, suggesting a similar mode of magma generation and a probable genetic link. Theoretical considerations suggest that most of these TTG rocks could have been generated by partial melting of amphibolitic or basaltic sources, followed by fractional crystallization.Although the Archean granitic gneisses often possess mantle-like Isr values, the trace element data indicate that they could not have been derived by direct melting of upper mantle materials. The immediate tectonic implication is that in any Archean terrain, the formation of Na-rich continental crust of TTG suite must be preceded by the presence of basaltic crust. The occurrence of this basaltic crust is a matter of controversy. Such crust might have been totally destroyed by repeated melting processes, or its remnants are now represented by some of the mafic-ultramafic enclaves within the tonalite-trondhjemite batholiths.

A reply to the discussion of the [ I, S] diagram by V.M. Oversby

Despite Oversby's [1] belief, the [ I, S] diagram [2] is as much sensitive as the [α, β] diagram since both of them are based on the same isotopic evolution equations. Also, we do insist again on the greater simplicity of the [ I, S] diagram in process in Pb-Pb data for whole rock series to study global isotopic evolution, and most particularly in the case where initial isotopic composition for the series considered cannot be measured.

Archaean mantle heterogeneity: evidence from chemical and isotopic abundances in Archaean igneous rocks

The problem of compositional heterogeneity in the Archaean upper mantle is evaluated from the existing data of isotopic compositions (Sr, Pb and Nd), trace element abundances (rare earth elements (r.e.e.)) and some critical major element chemistry in Archaean igneous rocks. To assess a large-scale mantle heterogeneity, we have emphasized the chemical characteristics of undoubtedly mantle-derived komatiites. The initial isotopic compositions of Sr and Pb ( I Sr and I Pb ) in Archaean igneous rocks show a significant variation, suggesting that a long-term heterogeneity in Rb/Sr and U /Pb ratios might have existed in their mantle sources. In contrast, the available I Nd values show little variation from the expected evolution in a chondritic uniform reservoir. These contrasting results, i.e. the variable I Sr and I Pb values as against the uniform I Nd values, are in part attributed to the fact that mobile-refractory pairs in the Rb/Sr and the U /Pb systems but only the refractory-refractory pair in the Sm/Nd system have been involved in geochemical differentiation. The r.e.e. distributions in komatiitic rocks are not uniform. Many show strong depletion in light r.e.e. The degree of light r.e.e. depletion in these komatiitic rocks is comparable with that found in present mid-ocean ridge basalts. Regarding the major element chemistry, we have demonstrated that the CaO / Al 2 O 3 ratios are significantly different in the komatiitic suites of two adjacent cratons (Kaapvaal and Rhodesia) in southern Africa. Both trace and major elements data suggest that the Archaean upper mantle was heterogeneous, but whether the heterogeneity truly represents a long-term phenomenon remains to be resolved by more isotopic analyses.

The carbon isotopic composition of diamonds: relationship to diamond shape, color, occurrence and vapor composition

Three hundred and thirty new 13C analyses of diamonds are presented, indicating, in conjunction with earlier published work, a range of about 30%. in the carbon isotopic composition of diamonds. The frequency distribution of diamond δ 13 C analyses shows a very pronounced mode at −5 to −6%.vs PDB, a large negative skewness, and a sharp boundary at about −1%.. Analyses of diamonds from the Premier and Dan Carl mines, South Africa, demonstrate that: (1) differences in 13C content that can be related to diamond color and shape are smaller than 1%.; (2) the mean 13C content of kimberlite carbonates is 1–2%. lower than that of associated diamonds; (3) significant differences in 13C content exist between the mean isotopic compositions of diamonds from these two pipes; (4) the variability in δ 13 C differs from one mine to the other. Computations were carried out evaluating the effect on the 13C content of diamonds of: (i) various precipitation processes; (ii) the abundance of the species H 2, H 2O, CH 4, CO, CO 2 and O 2 in the vapor; (iii) the initial isotopic composition variability of the source carbon; (iv) variations of the carbon isotope effects resulting from changes in pressure and temperature and (v) reservoir effects (Rayleigh fractionation). Fifty-eight genetic models were investigated for compatibility with the 13C distribution in diamonds and associated carbonate. The modeling does not permit an unambiguous answer to the question whether or not a vapor participated in diamond formation, although the presence of methane during diamond formation is compatible with the carbon isotopic composition data, possible oxygen fugacities in the mantle and with the composition of gases liberated from diamonds. In all probability carbon isotope effects in the diamond formation process were small, and the very large range in δ 13 C observed was inherited from the source carbon.

On the fitting of parallel isochrons and the method of maximum likelihood

Radiometric age data involving isochron techniques sometimes indicate that a suite of rocks may be coeval but have different initial isotopic compositions. The common age may be estimated by adapting previous regression procedures to the fitting of parallel lines. An alternative to the previous approaches is to use the methodology of maximum likelihood estimation (MLE), the results for which differ only in the standard errors. All the approaches are approximations in this respect but MLE is preferred for various reasons: it is currently in more prevalent use in statistical literature; the formulae for the standard errors are comparatively simple; in the case of the common-age problem it leads to a convenient approximate test of whether the lines are parallel. The methods are illustrated and compared on three well-known sets of lunar data and the results from a previous method of D. York and from the MLE approach are quite comparable. Whichever method is used, it is important that the numbers of samples representing the populations are large in order that the error estimates are meaningful.

Neodymium isotopes in flood basalts from the Siberian Platform and inferences about their mantle sources

The initial isotopic compositions of Nd and Sr in basalts from the Central Siberian Plateau and other major continental flood basalts are reported. The continental flood basalts appear to be the product of partial melting of mantle sources that consist of relatively primitive undifferentiated material and are clearly distinct from midocean ridge basalts, which sample mantle reservoirs that have been modified by extraction of continental crust earlier in earth history. These observations provide fundamental constraints on models of mantle structure and dynamics. Isotopic effects of crustal contamination are clearly recognizable in some continental flood basalts, but these effects can be distinguished from isotopic patterns inherited from the mantle magma sources.

Oxygen isotope fractionation in decarbonation metamorphism: the Mottled Zone event

Calculated univariant equilibria and oxygen isotope compositions of silicates and carbonates support the proposal that the “Mottled Zone Event” is a low-pressure (1–25 atm), high-temperature (200° < T < 1300°C) metamorphism of calcareous siliceous sediments in which the thermal energy is provided by combustion of organic matter. δ 18O of silicates decreases systematically with increasing metamorphic grade from averages of 18.1‰ in protolith shales, to 16.6‰ in grossular-diopside-zeolite rocks, 15.6‰ in wollastonite and anorthite-diopside-gehlenite-grossular fels, 14.1‰ in spurrite-brownmillerite marbles and 11.7‰ in the highest-grade larnite-gehlenite-brownmillerite assemblages. Decarbonation is the principal mechanism influencing the oxygen isotope compositions. The progressive decrease of δ 18O in silicates can be modelled as a Rayleigh distillation of CO 2 approximately 16‰ enriched in 18O relative to whole rock assemblages i.e., of initial isotopic composition 8.5‰ heavier than the parent carbonates. The mineral assemblage of one sample with an unusual granoblastic texture is in apparent isotopic equilibrium at a temperature of 540°C.

Isotope separation of oxygen-17, oxygen-18, carbon-13, and deuterium by ion laser induced formaldehyde photopredissociation

Near ultraviolet photopredissociation of formaldehyde has been verified as a highly selective method for enrichment of the rare stable isotopes of oxygen, hydrogen, and carbon contained in this molecule. High resolution absorption spectra of rare-isotope-substituted fomaldehyde show coincidences to within the Doppler profile with strong ion laser transitions in Xeiii at 345 nm and Neii at 332 and 338 nm. By tuning each ion laser transition with an intracavity etalon, optimum enrichment of each isotope was selected by monitoring formaldehyde fluorescence. Oxygen-18 was enriched up to ninefold using natural neon lasing at 332.374 nm, while substituting Ne-22 permitted up to ninefold O-17 enrichment and 33-fold C-13 enrichment at 332.371 nm. Use of D2CO instead of H2CO permitted 44-fold O-18 and 27-fold O-17 enrichment. Hydrogen photoproducts showed 24-fold HD enrichment at 337.822 nm, 60-fold HD enrichment at 345.426 nm, and 180-fold D2 enrichment at 332.377 nm. Single vibronic level photolysis of HDCO shows greater than 90% molecular photoproducts at wavelengths longer than 330 nm and a sudden rise to greater than 50% radical photoproducts below 320 nm. Photopredissociation of D2CO at 332 nm yields less than 1% radical photoproducts. Isotope scrambling in HDCO is about 40% at 325 nm, but drops to less than 6% at 338 nm. Isotope scrambling is 24% for both C-13 and O-18 at 332 nm. Enrichment of deuterium, carbon-13, and oxygen-18 was carried out using formaldehyde with initial isotope composition ranging from natural abundance to 50% to yield photoproduct with up to 88% isotopic purity.

Isotopic and geochemical studies on crustal effects in the genesis of the woodlawn Pb-Zn-Cu deposit

Lead (and U-Pb) isotope data for sulphides and whole rocks, U-3Pb data for zircons, and Rb-Sr data for whole rocks have been determined in an attempt to elucidate the processes by which the volcanic Pb-Zn-Cu deposit at Woodlawn, southeastern N.S.W., was formed, and to relate this information to current theories of crustal effects in the genesis of volcanic Pb-Zn deposits. The lead isotope compositions of pyrite, galena, sphalerite, and chalcopyrite from the ore horizon are the same, and identical to the initial lead isotope compositions of pyrite in the host volcanics. Linear relations are obtained for plots of 207Pb/ 204Pb vs 206Pb/204Pb, 208Pb/204Pb vs 204Pb/204Pb, and 238U/204Pb vs 206Pb/204Pb for least altered whole rock volcanics, indicating no loss or gain of U to these rocks since their formation. The similar initial 206Pb/204Pb ratios in the sulphides and host volcanics suggest a common source for the lead. However, acid leach experiments, carried out to remove the ore lead influence, suggest the lead to be a complex mixture of ore and rock lead. The differences observed for high and low lead samples in the acid leaching experiments suggest that these tests may provide an additional tool in prospecting for base-metal sulphides of this type. U-Pb data for zircons from the whole rocks give a spectrum of apparent ages ranging from 428 to 477 m.y. reflecting varying amounts of older zircons or resetting of the U-Pb systems in the volcanics. The older zircons are present either as discrete rounded crystals or cores surrounded by new euhedral growth. The populations and U-Pb data suggest the rocks were pre-existing volcanics with a possible detrital component, whose ages have not been fully reset during remelting. In contrast to the zircon data, U-Pb and Rb-Sr whole rock data define lines proportional to ages of 413±6 m.y. and 409±4 m.y. (1.39×10−11/yr) respectively and the strontium has a relatively high initial ratio of ∼0.710. The complex zircon population, high initial Sr ratio, Th/U ratios and rare earth data suggest the Woodlawn volcanics were formed by multistage remelting of material of similar chemical composition. Consequently, conformable deposits of this type should not be discussed in terms of the Growth Curve and single stage models of lead development. Mineralization is thought to have occurred at ∼420 m.y., either from solutions associated with the volcanism and/or concentrated from the volcanics and sediments by circulating sea water, in a shallow convective cell and soon after, or during, the formation of the pile. Alternatively, the multistage processes operating during formation of the Woodlawn volcanics acted as an effective homogeniser of lead isotopes.

129I in iodyrite and marshite

The concentrations of 129I incorporated in iodyrite (AgI) and marshite (CuI) are determined by measurements of radiogenic 129Xe. In addition to radiogenic 129Xe, some of these iodine-rich minerals contain excesses of 128Xe, 126Xe and 124Xe which are assigned to ( n, γβ −), ( n, 2 nβ −) and spallation reactions on 127I, respectively. The concentrations of radiogenic 129Xe indicate that the initial isotopic composition of iodine in iodyrite was (1.48±0.33)10 −14 ≤ 129 I 127 I ≤ (9.8±2.2)10 −14 and in marshite it was (0.32±0.24)10 −14 ≤ 129 I 127 I .

The geochemistry of central Australian granulites in relation to the chemical and isotopic effects of granulite facies metamorphism

Metamorphism to intermediate-pressure granulite grade had a minimal effect on the geochemistry of layered gneisses in central Australia. The overall composition of the terrain is granodioritic and major element compositions have equivalents in igneous and sedimentary supracrustal rocks. K, Rb, Sr and probably Th concentrations, and K/Rb ratios are normal; the initial isotopic composition of Sr shows the usual range of crustal rocks. However, U is strongly depleted and was lost by a pervasive process, probably dehydration, rather than by anatexis. Comparison with other areas in which major chemical depletions and unusually low initial Sr isotopic ratios are postulated leads to alternative interpretations of these areas which do not involve large scale chemical migration. An intermediate composition for the lower crust may result from a high density of basic intrusions rather than chemical processes.

Initial strontium isotopic composition of volcanic rocks from Jan Mayen and Spitsbergen

A comparison of the initial 87 Sr/ 86 Sr ratios between recent lavas from Jan Mayen Island and Spitsbergen reveals significant differences. These islands are close to each other, but the first belongs to the mid-ocean ridge system, the second to the continental margin. Jan Mayen basalts have normal mantle values of 87 Sr/ 86 Sr close to 0.7037. On the contrary 87 Sr/ 86 Sr ratios of samples from Spitsbergen vary between 0.7044–0.7104. Close geological, petrographic and chemical relations between the Spitsbergen samples exclude derivation from different source regions and prove that they are cogenetic. Contamination by bulk assimilation or by wall rock effects are also excluded because of the absence of a correlation between 87 Sr/ 86 Sr on the one hand, major element chemistry and K/Rb and Rb/Sr ratios on the other. Therefore it is proposed that the samples have been contaminated with radiogenic strontium alone, mobilized from the continental basement of Spitsbergen by heating during the ascent of the magma. The results suggest that, in certain cases, 87 Sr/ 86 Sr ratios cannot be used to determine cogenetic volcanic series.

Migration of radiogenic strontium during metamorphism

Study of the concentration and isotopic composition of strontium and rubidium in hornblende diorite dikes and sills of Precambrian age from the Panamint Mountains of California showed that these rock systems were enriched in radiogenic Sr during a late Mesozoic metamorphism. Enrichments in radiogenic Sr were observed for total-rock samples which yielded apparent ages of up to 34,000 m.y., although there is no obvious petrographic evidence for such metasomatic changes. These results indicate that some caution is necessary in interpreting the Sr isotopic composition of rocks in terms of their original source or in assuming that ‘total rocks’ form closed systems. In general, the initial isotopic Sr composition should be determined, not assumed.