Bruce E. Taylor

Sulphur isotope evidence for an oxic Archaean atmosphere

The presence of mass-independently fractionated sulphur isotopes (MIF-S) in many sedimentary rocks older than ∼2.4 billion years (Gyr), and the absence of MIF-S in younger rocks, has been considered the best evidence for a dramatic change from an anoxic to oxic atmosphere around 2.4 Gyr ago. This is because the only mechanism known to produce MIF-S has been ultraviolet photolysis of volcanic sulphur dioxide gas in an oxygen-poor atmosphere. Here we report the absence of MIF-S throughout ∼100-m sections of 2.76-Gyr-old lake sediments and 2.92-Gyr-old marine shales in the Pilbara Craton, Western Australia. We propose three possible interpretations of the MIF-S geologic record: (1) the level of atmospheric oxygen fluctuated greatly during the Archaean era; (2) the atmosphere has remained oxic since ∼3.8 Gyr ago, and MIF-S in sedimentary rocks represents times and regions of violent volcanic eruptions that ejected large volumes of sulphur dioxide into the stratosphere; or (3) MIF-S in rocks was mostly created by non-photochemical reactions during sediment diagenesis, and thus is not linked to atmospheric chemistry.

Determination of molar absorptivities for infrared absorption bands of H2O in andesitic glasses

Abstract We have determined infrared molar absorptivities for water absorption bands in Fe-bearing and Fe-free andesitic glasses. Water dissolves in andesitic glasses as both hydroxyl groups and molecular water as observed in other silicate glasses. Concentrations of molecular water and hydroxyl species are a strong function of total water content. IR molar absorptivities for Fe-bearing andesite are ε3570 = 62.32 ± 0.42 L/mol·cm, ε4500 = 0.79 ± 0.07 L/mol·cm, ε5200 = 1.07 ± 0.07 L/mol·cm, and ε1630 = 42.34 ± 2.77 L/mol·cm. Molar absorptivities for Fe-free andesite are 69.21 ± 0.52 L/mol·cm for e3570, 0.89 ± 0.07 L/mol·cm for e4500, 1.46 ± 0.07 L/mol·cm for e5200, and 52.05 ± 2.85 L/mol·cm for ε1630. Molar absorptivities show significant compositional dependencies that can be predicted based on tetrahedral cation (Si+4, Al+3)/total cation fraction

Ostwald ripening of clays and metamorphic minerals.

Analyses of particle size distributions indicate that clay minerals and other diagenetic and metamorphic minerals commonly undergo recrystallization by Ostwald ripening. The shapes of their particle size distributions can yield the rate law for this process. One consequence of Ostwald ripening is that a record of the recrystallization process is preserved in the various particle sizes. Therefore, one can determine the detailed geologic history of clays and other recrystallized minerals by separating, from a single sample, the various particle sizes for independent chemical, structural, and isotopic analyses.

Carbon isotope constraints on degassing of carbon dioxide from Kilauea Volcano

Abstract We examine models for batch-equilibrium and fractional-equilibrium degassing of CO2 from magma at Kilauea Volcano. The models are based on 1. (1) the concept of two-stage degassing of CO2 from magma supplied to the summit chamber, 2. (2) C isotope data for CO2 in eruptive and noneruptive (quiescent) gases from Kilauea and 3. (3) data for the isotopic fractionation of C between CO2 and C dissolved in tholeiitic basalt melt. The results of our study indicate that 1. (1) both eruptive and noneruptive degassing of CO2 most closely approach a batch equilibrium process, 2. (2) the δ13C of parental magma supplied to the summit chamber is in the range −4.1 to−3.4‰ and 3. (3) the δ13C of melt after summit chamber degassing is in the range −7 to −8‰, depending upon the depth of equilibration. We also present δ13C data for CO2 in eruptive gases from the current East Rift Zone eruption. These are the first C isotope data for CO2 in high-temperature (>900°C) eruptive gases from Kilauea; they have a mean δ13C value of −7.82 ± 0.24‰ and are similar to those predicted for the melt after summit chamber degassing. The minor role played by fractional degassing of ascending magma at Kilauea means that exsolved CO2 tends to remain entrained in and coherent with its host melt during ascent from both mantle source regions and crustal magma reservoirs. This has important implications for magma dynamics at Kilauea.

Sulfur isotope systematics of basaltic lavas from Indonesia: implications for the sulfur cycle in subduction zones

Abstract We report sulfur isotope compositions of basaltic and basaltic andesite lavas from selected volcanoes in the Indonesian arc system covering the spectrum from low-K tholeiitic to high-K calc–alkaline compositions. The results of 25 samples from seven volcanoes, which are associated with different subduction regimes, show a range in δ34S values of +2.0–+7.8‰ (VCDT) with an average of +4.7±1.4‰ (1σ). Averages and within-suite variations of two larger sets of samples from Batur and Soputan volcanoes (+4.2±1.3‰ with n=9 and +5.7±1.4‰ with n=7, respectively) are comparable to those of the entire sample set. Sulfur concentrations are low (mostly between 2 and 74 ppm, average=19 ppm) and do not show correlations with sulfur isotope composition and whole-rock chemistry, or systematic changes with time in any of the lava suites. From model calculations we infer that basaltic magmas will undergo sulfur isotope fractionation during degassing, most commonly towards lower δ34S values, but that the extent is limited at P–T conditions and oxidation states of interest. Hence, δ34S signatures of basaltic lavas will generally be within a few permil from primary magmatic values, even in cases of extensive sulfur loss. Consequently, magmas in the Indonesian arc system originate from mantle sources that are enriched in 34S relative to MORB and OIB sources and are likely to have δ34S values of about +5–+7‰. The enrichment in 34S is considered to reflect addition of slab-derived material, presumably from sediments rather than altered oceanic crust, with fluids being the most likely transport medium. Absence of correlation between δ34S values of Indonesian basalts and chemical proxies for source components or processes at the slab–wedge interface suggests that sulfur isotopes are relatively insensitive to variations in subduction setting and dynamics. This is supported by the modest range in δ34S of the Indonesian volcanoes studied despite significant variations in the nature and amount of subducted material, and by the similarity with average 34S enrichments in other oceanic arc systems.

Variations in the sulfur isotope composition of troilite from the Cañon Diablo iron meteorite

Abstract Canon Diablo troilite (CDT), although accepted as the reference for the sulfur isotope scale, is not a Standard Reference Material manufactured for international distribution, and its sulfur isotope composition is not well characterized. We report high-precision sulfur isotope analyses of troilite from three different samples of Canon Diablo meteorite, and interlaboratory comparison of one CDT sample and of a reference SF6 gas. The sulfur isotope composition of CDT displays a range in δ 34 S values of 0.4%., significantly larger than our analytical uncertainty (0.05%.). CDT should not be used for calibration of the sulfur isotope scale. These results support the definition of a new sulfur isotope scale relative to a hypothetical Vienna-CDT, as recently proposed by the International Atomic Energy Agency.

High precision and spatial resolution sulfur isotope analysis using MILES laser microprobe

The MILES (Micro In situ Laser Extraction System) laser microprobe permits high spatial resolution (< 10−3mm3, or < 0.2 μmol S) in situ sampling of geological material for sulfur isotope analysis. Sulfides are combusted in F2 by absorption of CO2 laser radiation and converted to sulfur hexafluoride (SF6). The product SF6 is purified by cryogenic distillation. In combination with a high-sensitivity dualinlet isotope ratio mass spectrometer, sulfur isotope analyses of powders of pyrite, galena, and sphalerite yield δ34Scdt values with a high precision, ranging from 0.03 to 0.09%. The sulfur isotope ratios measured are accurate and exhibit no matrix-dependent sulfur isotope effects over the range of 62%. A minimum F2 pressure of 20 kPa (for MILES) is required to mediate against small isotopic fractionations between multiple sulfur species apparently caused by laser isotope separation and/or reaction with oxygen during analysis. The precision and accuracy of δ34Scdt values from in situ analyses are good (

Sulfur isotope and trace element data from ore sulfides in the Noranda district (Abitibi, Canada): implications for volcanogenic massive sulfide deposit genesis

0.2‰), but isotopically homogeneous working standards or intercomparison materials are not available thus far. Sulfur isotope ratios derived by conventional-SO2 and laser-SF6 are well correlated (r2 = 0.99999), but a slope different from unity (m = 1.035) arises, probably due to inadequate corrections to SO2 data for oxygen isobaric interferences. Sulfur isotope isopleths in a large, cubic metamorphic pyrite porphyroblast, determined from 79 in situ analyses, are discordant to crystallographic zonation. Concordance between crystallographic and isotopic zonation needs be tested using high precision and spatial resolution analyses such as those described here. Sampling crystallographic zones in minerals can result in erroneous conclusions if isotopic and crystallographic zoning are not coincident.

Chapter 20 – Fluorination Methods in Stable Isotope Analysis

We examine models for volcanogenic massive sulfide (VMS) mineralization in the ~2.7-Ga Noranda camp, Abitibi subprovince, Superior Province, Canada, using a combination of multiple sulfur isotope and trace element data from ore sulfide minerals. The Noranda camp is a well-preserved, VMS deposit-rich area that is thought to represent a collapsed volcanic caldera. Due to its economic value, the camp has been studied extensively, providing a robust geological framework within which to assess the new data presented in this study. We explore previously proposed controls on mineralization within the Noranda camp and, in particular, the exceptional Au-rich Horne and Quemont deposits. We present multiple sulfur isotope and trace element compositional data for sulfide separates representing 25 different VMS deposits and “showings” within the Noranda camp. Multiple sulfur isotope data for this study have δ34SV-CDT values of between −1.9 and +2.5 ‰, and Δ33SV-CDT values of between −0.59 and −0.03 ‰. We interpret the negative Δ33S values to be due to a contribution of sulfur that originated as seawater sulfate to form the ore sulfides of the Noranda camp VMS deposits. The contribution of seawater sulfate increased with the collapse and subsequent evolution of the Noranda caldera, an inference supported by select trace and major element analyses. In particular, higher concentrations of Se occur in samples with Δ33S values closer to 0 ‰, as well as lower Fe/Zn ratios in sphalerite, suggesting lower pressures and temperatures of formation. We also report a relationship between average Au grade and Δ33S values within Au-rich VMS deposits of the Noranda camp, whereby higher gold grades are associated with near-zero Δ33S values. From this, we infer a dominance of igneous sulfur in the gold-rich deposits, either leached from the volcanic pile and/or directly degassed from an associated intrusion.

Productivity of volcanic-hosted massive sulfide districts: New constraints from the δ18O of quartz phenocrysts in cogenetic felsic rocks

Publisher Summary This chapter reviews the most common fluorination methods applied in stable isotope geochemistry, incorporating both conventional and the more-recently applied laser-induced heating techniques. It describes the construction and utilization of the basic apparatus used for fluorination employing fluorine (F2), or interhalogen fluorides with or without the hydrogen halide. It summarizes the basic components and methods that are reliable and functional. Fluorine is the most oxidizing element known and possesses the highest electronegativity of all elements. Fluorination is a reliable process for oxygen isotope analysis of silicates, oxides, and phosphates and for sulfur isotope analysis of sulfides. Laser-assisted micro-analysis by fluorination facilitates high spatial resolution sampling, permitting isotopic analysis of minerals within their textural context. The marked reduction in sample size over conventional techniques demands close attention to sources of blanks, adsorption, and isotopic fractionation in an effort to produce samples of high purity. Gas chromatography based purification, and helium carrier gas introduction of very small samples to the mass spectrometer have pushed technical innovations and have taken the field of laser-assisted micro-analysis beyond simply a miniaturization of conventional methodology.