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Huberty J.M.,NASA | Kita N.T.,NASA | Kozdon R.,NASA | Heck P.R.,NASA | And 6 more authors.
Chemical Geology | Year: 2010

In situ high precision analysis of oxygen isotope ratios (δ18O) by secondary ion mass spectrometry (SIMS) reveals that instrumental bias in δ18O for magnetite varies due to crystal orientation effects. Multiple analyses of δ18O have an average precision of ±0.4% (2SD) in single grains of magnetite, close to ±0.3%, that obtained for multiple grains of UWQ-1, a homogeneous quartz standard. In contrast, the average precision is five to ten times worse, ±2-3% (2SD), from grain-to-grain of magnetite due to variation in instrumental bias with crystal orientation. Electron backscatter diffraction shows that individual grains of magnetite are single crystals and that crystal orientation varies randomly from grain-to-grain. The crystal orientation for each magnetite grain is plotted relative to the incident angle of the SIMS primary Cs+ beam. High values of δ18O are measured when the Cs+ beam is parallel to , from [110] to [100], preferred channeling and focusing directions for magnetite. Routine δ18O analysis at WiscSIMS utilizes a Gaussian focused Cs+ primary beam (deep-pit mode) at primary and secondary voltages of +10kV and-10kV respectively (total impact energy 20keV). Four analytical experiments were conducted in attempts to improve the grain-to-grain precision in measured δ18O for magnetite: (1) applying an energy offset of 50eV, (2) using a Köhler illuminated beam (shallow-pit mode), (3) reducing the total impact energy, and (4) varying the primary and secondary accelerating voltages. The best results were obtained in experiment (4) at primary/secondary accelerating voltages of +3kV/-10kV respectively with an incident Cs+ beam angle of 14°. The grain-to-grain precision in measured δ18O for magnetite improves from ±2.9% to ±0.8% (2SD) at +10kV/-10kV and +3kV/-10kV analysis respectively, while precision in single grains is ±0.4% for both. Instrumental bias in δ18O also varies with crystal orientation for hematite at similar levels as is seen for magnetite. The grain-to-grain precision in measured δ18O for hematite improves from ±2.1% to ±1.0% (2SD) at +10kV/-10kV and +3kV/-10kV analysis respectively, while precision in single grains is ±0.3% (2SD) for both. Importantly, crystal orientation effects have not been identified at levels of ±0.3% for δ18O in silicates or other minerals analyzed by WiscSIMS though many minerals remain to be examined. © 2010 Elsevier B.V.

Heck P.R.,NASA | Heck P.R.,University of Chicago | Heck P.R.,Robert itzker Center For Meteoritics And Polar Studies | Huberty J.M.,NASA | And 4 more authors.
Geochimica et Cosmochimica Acta | Year: 2011

Banded iron formations (BIFs) are chemical marine sediments dominantly composed of alternating iron-rich (oxide, carbonate, sulfide) and silicon-rich (chert, jasper) layers. Isotope ratios of iron, carbon, and sulfur in BIF iron-bearing minerals are biosignatures that reflect microbial cycling for these elements in BIFs. While much attention has focused on iron, banded iron formations are equally banded silica formations. Thus, silicon isotope ratios for quartz can provide insight on the sources and cycling of silicon in BIFs. BIFs are banded by definition, and microlaminae, or sub-mm banding, are characteristic of many BIFs. In situ microanalysis including secondary ion mass spectrometry is well-suited for analyzing such small features. In this study we used a CAMECA IMS-1280 ion microprobe to obtain highly accurate (±0.3‰) and spatially resolved (∼10μm spot size) analyses of silicon and oxygen isotope ratios for quartz from several well known BIFs: Isua, southwest Greenland (∼3.8Ga); Hamersley Group, Western Australia (∼2.5Ga); Transvaal Group, South Africa (∼2.5Ga); and Biwabik Iron Formation, Minnesota, USA (∼1.9Ga). Values of δ 18O range from +7.9‰ to +27.5‰ and include the highest reported δ 18O values for BIF quartz. Values of δ 30Si have a range of ∼5‰ from -3.7‰ to +1.2‰ and extend to the lowest δ 30Si values for Precambrian cherts. Isua BIF samples are homogeneous in δ 18O to ±0.3‰ at mm- to cm-scale, but are heterogeneous in δ 30Si up to 3‰, similar to the range in δ 30Si found in BIFs that have not experienced high temperature metamorphism (up to 300°C). Values of δ 30Si for quartz are homogeneous to ±0.3‰ in individual sub-mm laminae, but vary by up to 3‰ between multiple laminae over mm-to-cm of vertical banding. The scale of exchange for Si in quartz in BIFs is thus limited to the size of microlaminae, or less than ∼1mm. We interpret differences in δ 30Si between microlaminae as preserved from primary deposition. Silicon in BIF quartz is mostly of marine hydrothermal origin (δ 30Si<-0.5‰) but silicon from continental weathering (δ 30Si∼1‰) was an important source as early as 3.8Ga. © 2011 Elsevier Ltd.

Heck P.R.,Robert itzker Center For Meteoritics And Polar Studies | Heck P.R.,University of Chicago | Heck P.R.,Max Planck Institute for Chemistry | Hoppe P.,Max Planck Institute for Chemistry | Huth J.,Max Planck Institute for Chemistry
Meteoritics and Planetary Science | Year: 2012

We present NanoSIMS four-isotope S analyses of 24 comet Wild 2 dust impact residues in craters on aluminum foil C2037N returned by NASA's Stardust mission. Except for one sample, all impact residues have normal S isotopic compositions within 2σ uncertainties of at least two S isotope ratios. This implies that most S-rich Wild 2 dust impactors formed in the solar system. Instrumental isotope fractionation due to sample topography is the main contribution to our analytical uncertainty. One impact crater residue shows small anomalies of δ 33S=-57±17‰, and δ 34S=-41±17‰ (1σ uncertainties). Although this could be simply a statistical outlier or the fingerprint of a chemical isotope fractionation it is also possible that the observed anomaly results from the mixture of a cometary FeS particle with a small (150nm diam.) presolar FeS supernova grain. This would translate into a presolar sulfide abundance of approximately 200ppm. © 2012 The Meteoritical Society.

Stroud R.M.,Washington Technology | Chisholm M.F.,Oak Ridge National Laboratory | Heck P.R.,Robert itzker Center For Meteoritics And Polar Studies | Heck P.R.,University of Chicago | And 2 more authors.
Astrophysical Journal Letters | Year: 2011

Nanodiamond (ND) was the first extrasolar dust phase to be identified in meteorites. However, the 2nm average size of the NDs precludes isotopic analysis of individual particles, and thus their origin(s) remains controversial. Using electron microscopy with subnanometer resolution, we show that ND separates from the Allende and Murchison meteorites are actually a two-phase mixture of ND and glassy carbon. This phase mixture is likely the product of supernova shock-wave transformation of pre-formed organics in the interstellar medium (ISM). The glassy carbon-ND mixture is also a plausible contributor to the 2175 extinction feature in the diffuse ISM. © 2011 The American Astronomical Society. All rights reserved.

Bjarnborg K.,Lund University | Schmitz B.,Lund University | Schmitz B.,Robert itzker Center For Meteoritics And Polar Studies
Meteoritics and Planetary Science | Year: 2013

By dissolving 30-400 kg of marine limestone in HCl and HF acid, our group has previously recovered common relict chromite grains (approximately 63-250 μm) from ordinary chondritic micrometeorites that fell on ancient sea floors, up to 500 Myr old. Here, we evaluate if CM group carbonaceous chondritic material, which makes up an important fraction of the micrometeorite flux today, contains analogous grains that can be searched for in acid residues. We dissolved 8 g of CM2 meteorite Acfer 331 in HF, which yielded a characteristic assemblage of both transparent Mg-Al- and opaque Cr-spinels >28 μm. We find on average 4.6 and 130 Mg-Al-spinel grains per gram in the 63-250 and 28-63 μm size fractions, respectively. These grains are mostly pink or colorless, and often characterized by heterogeneous Cr-content. Black, opaque Cr-spinel grains are absent from the >63 μm fraction, but in the 28-63 μm fraction we find approximately 65 such grains per gram meteorite. The individual grains have a characteristic composition, with heterogeneous major element compositions (e.g., 44.4-61.7 wt% Cr2O3), but narrow ranges for maximum TiO2 (0.6-1.6 wt%) and V2O3 (0.5-1.0 wt%) concentrations. The content of spinel grains in the 28-63 μm fraction of CM meteorites appears comparable at the order of magnitude level with the content of >63 μm sized chromite grains in fossil L-chondrites from Ordovician limestone. Our approach of recovering meteoritic spinel from sediment may thus be extended to include CM meteorites, but the smaller size fraction of the acid residues should be searched. © The Meteoritical Society, 2013.

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