Douglas Rumble

New insights into Archean sulfur cycle from mass-independent sulfur isotope records from the Hamersley Basin, Australia

We have measured multiple sulfur isotope ratios ( 34 S/ 33 S/ 32 S) for sulfide sulfur in shale and carbonate lithofacies from the Hamersley Basin, Western Australia. The v 33 S values (v 33 SWN 33 S30.515UN 34 S) shift from 31.9 to +6.9x over a 22-m core section of the lower Mount McRae Shale (V2.5 Ga). Likewise, sulfide sulfur analyses of the Jeerinah Formation (V2.7 Ga) yield v 33 S values of 30.1 to +8.1x over a 50-m section of core. Despite wide variations in v 33 S and N 34 S, these two shale units yield a similar positive correlation between v 33 S and N 34 S. In contrast, pyrite sulfur analyses of the Carawine Dolomite (V2.6 Ga) yield a broad range in N 34 S (+3.2 to +16.2x) but a relatively small variation and negative values in v 33 S( 32.5 to 31.1x). The stratigraphic distribution of N 33 S, N 34 S, and v 33 S in Western Australia allows us to speculate on the sulfur isotopic composition of Archean sulfur reservoirs and to trace pathways in the Archean sulfur cycle. Our data are explained by a combination of massindependent fractionation (MIF) in the atmosphere and biological mass-dependent fractionation in the ocean. In the Archean, volcanic, sulfur-bearing gas species were photolysed by solar ultraviolet (UV) radiation in an oxygen-free atmosphere, resulting in MIF of sulfur isotopes. Aerosols of S8 (with v 33 Ss 0) and sulfuric acid (with v 33 S6 0) formed from the products of UV photolysis and carried mass-independently fractionated sulfur into the hydrosphere. The signatures of atmospheric photolysis were preserved by precipitation of pyrite in sediments. Pyrite precipitation was mediated by microbial enzymatic catalysis that superimposed mass-dependent fractionation on mass-independent atmospheric effects. Multiple sulfur isotope analyses provide new insights into the early evolution of the atmosphere and the evolution and distribution of early sulfur-metabolizing organisms. A 2003 Elsevier Science B.V. All rights reserved.

The impact and recovery of asteroid 2008 TC 3

In the absence of a firm link between individual meteorites and their asteroidal parent bodies, asteroids are typically characterized only by their light reflection properties, and grouped accordingly into classes. On 6 October 2008, a small asteroid was discovered with a flat reflectance spectrum in the 554–995 nm wavelength range, and designated 2008 TC3 (refs 4–6). It subsequently hit the Earth. Because it exploded at 37 km altitude, no macroscopic fragments were expected to survive. Here we report that a dedicated search along the approach trajectory recovered 47 meteorites, fragments of a single body named Almahata Sitta, with a total mass of 3.95 kg. Analysis of one of these meteorites shows it to be an achondrite, a polymict ureilite, anomalous in its class: ultra-fine-grained and porous, with large carbonaceous grains. The combined asteroid and meteorite reflectance spectra identify the asteroid as F class, now firmly linked to dark carbon-rich anomalous ureilites, a material so fragile it was not previously represented in meteorite collections.

Unusually low δ18O ultra-high-pressure metamorphic rocks from the Sulu Terrain, eastern China

Eclogite and quartzite interlayers, containing ultra-high-pressure (UHP) metamorphic mineral assemblages, occur at Qinglongshan in the Sulu terrain, eastern China. These rocks exhibit some of the most negative oxygen isotope compositions that have ever been reported for metamorphic rocks. The δ18 value of minerals in eclogite ranges from −10.4 to −9.0%o and in quartzite ranges from −10.2 to −7.3‰ Near isotopic equilibrium between minerals and rocks suggests that these rocks must have acquired such low oxygen isotope compositions before the UHP metamorphism, through near surface meteoric water-rock interactions. The rocks were then subducted to a great depth during continent-continent collision, forming UHP minerals with concomitant local oxygen isotope redistribution. It is significant that the rocks have retained such negative oxygen isotope compositions despite subduction into the upper mantle.

Low δ18O zircons, U-Pb dating, and the age of the Qinglongshan oxygen and hydrogen isotope anomaly near Donghai in Jiangsu Province, China

Zircons from metamorphosed granites exposed near Qinglongshan have δ18OVSMOW values of −7 to 0‰ in both grain rims and cores. The concordant 238U/206Pb ages of zircon cores are 684 to 754 Ma with rims at 221 Ma. Discordant 238U/206Pb ages range from 242 to 632 Ma. Results demonstrate a Neoproterozoic age for the origin of the Qinglongshan oxygen and hydrogen isotope anomaly. The low δ18O values were imprinted on the rocks by a hydrothermal system charged with meteoric water from a cold climate. Groundwater circulation was driven by heat from cooling granitic magma. The geologic age of the hydrothermal system correlates with that of the Nantuo tillite in the Sinian strata of the South China block, suggesting that Qinglongshan’s cold climate may be a manifestation of Neoproterozoic “snowball Earth.”

The Qinglongshan oxygen and hydrogen isotope anomaly near Donghai in Jiangsu Province, China

The Qinglongshan isotope anomaly has unusually low values of both δ18O and δD. Grnets from coesite-bearing eclogite are as low as −11‰ (VSMOW) and rutiles are −15‰. Phengites have δD of −120‰ (VSMOW). The low values were acquired in an ancient geothermal system prior to subduction during Triassic continental collision. New data shows that depleted isotope values in different rock types extend over distances of 20 × 40 km demonstrating that the geothermal system retained structural coherence throughout subduction and exhumation. The finding of structural coherence suggests that not only eclogites, with their characteristic ultra-high pressure mineral assemblages, but also gneisses, meta-granites, and meta-sediments were all subjected to metamorphism in the coesite-eclogite facies. Oxygen isotope fractionation between quartz and garnet gives estimates of peak metamorphic temperatures at 754–893°C. The spatial extent of oxygen isotopic equilibration between different rock types was limited to tens of centimeters. The persistence of hydrous minerals such as epidote and phengite approaching high temperature oxygen isotope exchange equilibrium with ultrahigh pressure minerals in coesite-bearing rocks shows that mineral-bound water can be subducted into the upper mantle.

Oxygen isotope evidence for rapid mixing of the HED meteorite parent body

Abstract The 16 O, 17 O and 18 O abundances of howardites, eucrites, and diogenites have been used to assign them to a single ‘HED’ parent body, thought to be asteroid 4 Vesta. We report the first evidence of oxygen isotopic heterogeneity among HED meteorites indicating incompletely mixed sources. New high-precision oxygen isotope measurements of 34 HED meteorites reveal that most have the same Δ 17 O′, consistent with a very rapid early history of large-scale mixing on Vesta. However, howardites are on average very slightly enriched in 16 O, whereas Ibitira, Caldera, Pasamonte, and ALHA78132 are 16 O-depleted compared to other investigated eucrites. The Δ 17 O′ of Ibitira is completely different from all other HEDs measured. Some of the results for eucrites and diogenites can be explained by partial melting and rapid mixing of the interior of Vesta. Others require a separate parent body or indicate that parts of the outer layer of Vesta retained some primary isotopic heterogeneity. The oxygen isotopic composition of howardites provides an upper limit for the amount of admixed carbonaceous chondritic material into the HED parent body regolith.

Atmospheric Sulfur in Archean Komatiite-Hosted Nickel Deposits

Early Ore Formation Ore deposits contain most of the worlds metal resources, from commonly used metals such as iron, to precious and expensive metals such as platinum. Understanding how these ancient deposits form may lead to more efficient metal extraction and give clues about early Earth. Bekker et al. (p. 1086) studied sulfur and iron isotopes in 2.7-billion-year old Fe-Ni sulfide deposits from Canada and Australia and found that most of the metal-scavenging sulfur was originally atmospheric in origin. Photochemical reactions in the ancient oxygen-free atmosphere produced sulfide that eventually circulated to the sea floor and mixed with newly erupted komatite magmas. Thus, global surface processes in the oceans, atmosphere, and on continents are geochemically linked to ore-forming processes within Earth. The source of sulfur in economic iron-nickel sulfide deposits is primarily derived from the atmosphere. Some of Earth’s largest iron-nickel (Fe-Ni) sulfide ore deposits formed during the Archean and early Proterozoic. Establishing the origin of the metals and sulfur in these deposits is critical for understanding their genesis. Here, we present multiple sulfur isotope data implying that the sulfur in Archean komatiite-hosted Fe-Ni sulfide deposits was previously processed through the atmosphere and then accumulated on the ocean floor. High-temperature, mantle-derived komatiite magmas were then able to incorporate the sulfur from seafloor hydrothermal sulfide accumulations and sulfidic shales to form Neoarchean komatiite-hosted Fe-Ni sulfide deposits at a time when the oceans were sulfur-poor.

STABLE ISOTOPE CHARACTERISTICS OF ECLOGITES FROM THE ULTRA-HIGH-PRESSURE METAMORPHIC TERRAIN, EAST-CENTRAL CHINA

Abstract A stable isotope study was made for the ultra-high-pressure (UHP) metamorphic rocks in the Dabie and Sulu terrains, east-central China. δ 18 O values of minerals in eclogites cover a wide range, from −4 to +6‰. The O-isotope fractionations between minerals (i.e. omphacite, phengite and/or amphibole) and garnet in eclogites are mostly not in isotopic equilibrium. The large range of O-isotope composition of garnet may approximate the δ 18 O variations of eclogites during UHP metamorphism and must have been largely inherited from their pre-subduction basaltic precursors which might have interacted with meteoric waters under variable temperature conditions and water/rock ratios before UHP metamorphism. The O-isotope disequilibrium between minerals and garnet in eclogites was caused by retrograde hydration reactions. The fluids required for these hydration reactions must have low or even negative O-isotope compositions. It is postulated that such fluids were either originated from meteoric deep-infiltration or were derived from low- δ 18 O (para)gneisses.

An ultraviolet laser microprobe for the in situ analysis of multisulfur isotopes and its use in measuring Archean sulfur isotope mass-independent anomalies

Abstract A laser fluorination microprobe system has been constructed for high-accuracy, high-precision multisulfur isotope analysis with improved spatial resolution. The system uses two lasers: (a) a KrF excimer laser for in situ spot analysis by ultraviolet (UV) photoablation with λ = 248 nm and (b) a CO 2 laser for whole-grain analysis of powdered samples by infrared heating at λ = 10.6 μm. A CO 2 laser is necessary for the analysis of interlaboratory isotope reference materials because they are supplied as powders. The δ 34 S and δ 33 S compositions of reference materials measured with a CO 2 laser fluorination system agree (±0.2‰, 1σ) with the recommended values by the Sulfur Isotope Working Group of the International Atomic Energy Agency (Ding et al., 2001; Taylor, in press) . The precision of replicate analyses of powdered sulfide minerals with the CO 2 laser is typically ±0.2‰ (1σ) for δ 34 S. The in situ fluorination of sulfides with a KrF excimer laser (λ = 248 nm) was validated by comparison of measurements of side-by-side laser craters and powders excavated from drill holes. Powders from drill holes were analyzed with the CO 2 laser. In situ laser craters and drill hole powders give the same δ 34 S V-CDT and δ 33 S V-CDT values within 0.2‰. The δ 34 S V-CDT and δ 33 S V-CDT values of both powders and in situ analyses are independent of F 2 gas pressure over a range of 15 to 65 torr. No dependence of δ 34 S V-CDT and δ 33 S V-CDT values on UV laser energy fluence has been observed. Mineral-specific fractionation of sulfur isotopes in analyzing pyrite, sphalerite, galena, troilite, and chalcopyrite has not been observed with a KrF excimer laser (λ = 248 nm). Test analyses with an ArF excimer laser (λ = 193 nm), however, gave fractionated sulfur isotope ratios. A range of Δ 33 S anomalies of from – 1.5 to +3.0‰ in Archean samples from the North Pole district, Pilbara Craton, Australia, and from black shale of the Lokamonna Formation, South Africa, were verified by in situ analysis of individual pyrite grains with a KrF excimer laser. These results show that a combination of high-accuracy, high-precision analyses with improved spatial resolution permits locating and analyzing host minerals of non-mass-dependent sulfur isotope anomalies.

In situ oxygen isotope analysis with an excimer laser using F2 and BrF5 reagents and O2 gas as analyte

In situ oxygen isotope analysis of silicate minerals has been validated by interlaboratory calibration using an excimer laser (KrF fill gas, 248 nm), F2 gas fluorinating reagent, and O2 gas as mass spectrometer analyte. A value of δ 18OSMOW = 5.82%. (±0.03, 4 analyses) was obtained for almandine UWG-2 compared to a recommended value of 5.8%0 (Valley et al., 1995). The interhalogen BrF5 was tested by comparing in situ analyses made alternatively with either BrF5 or F2 gas on crystals of almandine, epidote, forsterite, tourmaline, and zircon. Results for different pressures of BrF5 (20 to 40 torr) in the reaction chamber show that δ 18O values decrease by 1 permil for an increase of 10 torr in BrF5 pressure. In strong contrast, δ 18O values measured with purified F2 gas are precise and accurate over a range of from 15 to 150 torr F2 gas pressure. Use of F2 gives higher yields of O2 than with BrF5. Values of δ17O and δ18O measured in situ on a variety of silicates all plot on the terrestrial mass fractionation line, provided that NF3 contamination is eliminated from the O2 gas analyte.