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Schmitz B.,Lund University | Schmitz B.,Woods Hole Oceanographic Institution | Schmitz B.,Robert itzker Center For Meteoritics And Polar Studies | Heck P.R.,University of Wisconsin - Madison | And 9 more authors.
Earth and Planetary Science Letters | Year: 2011

Abundant chromite grains with L-chondritic composition in the resurge deposits of the Lockne impact crater (458. Myr old; dia. ~. 10. km) in Sweden have been inferred to represent relict fragments of an impactor from the break-up of the L-chondrite parent body at 470. Ma. This view has been challenged based on Ir/Cr and platinum group element (PGE) patterns of the same resurge deposits, and a reinterpretation of the origin of the chromite grains. An impactor of the non-magmatic iron meteorite type was proposed instead. Here we show that single-grain oxygen and noble-gas isotope analyses of the chromite grains from the resurge deposits further support an origin from an L-chondritic asteroid. We also present PGE analyses and Ir/Cr ratios for fossil L-chondritic meteorites found in mid-Ordovician marine limestone in Sweden. The L-chondritic origin has been confirmed by several independent methods, including major element and oxygen isotopic analyses of chromite. Although the meteorites show the same order-of-magnitude PGE and Cr concentrations as recent L chondrites, the elements have been redistributed to the extent that it is problematic to establish the original meteorite type from these proxies. Different PGE data processing approaches can lead to highly variable results, as also shown here for the Lockne resurge deposits. We conclude that the Lockne crater was formed by an L-chondritic impactor, and that considerable care must be taken when inferring projectile type from PGEs in sedimentary ejecta deposits. © 2011 Elsevier B.V.

Schmitz B.,Lund University | Schmitz B.,Robert itzker Center For Meteoritics And Polar Studies | Schmitz B.,University of Hawaii at Manoa | Boschi S.,Lund University | And 6 more authors.
Earth and Planetary Science Letters | Year: 2015

The onset of Earth's present icehouse climate in the Late Eocene coincides with astronomical events of enigmatic causation. At ~36 Ma ago the 90-100 km large Popigai and Chesapeake Bay impact structures formed within ~10-20 ka. Enrichments of 3He in coeval sediments also indicate high fluxes of interplanetary dust to Earth for ~2 Ma. Additionally, several medium-sized impact structures are known from the Late Eocene. Here we report from sediments in Italy the presence of abundant ordinary chondritic chromite grains (63-250 μm) associated with the ejecta from the Popigai impactor. The grains occur in the ~40 cm interval immediately above the ejecta layer. Element analyses show that grains in the lower half of this interval have an apparent H-chondritic composition, whereas grains in the upper half are of L-chondritic origin. The grains most likely originate from the regoliths of the Popigai and the Chesapeake Bay impactors, respectively. These asteroids may have approached Earth at comparatively low speeds, and regolith was shed off from their surfaces after they passed the Roche limit. The regolith grains then settled on Earth some 100 to 1000 yrs after the respective impacts. Further neon and oxygen isotopic analyses of the grains can be used to test this hypothesis.If the Popigai and Chesapeake Bay impactors represent two different types of asteroids one can rule out previous explanations of the Late Eocene extraterrestrial signatures invoking an asteroid shower from a single parent-body breakup. Instead a multi-type asteroid shower may have been triggered by changes of planetary orbital elements. This could have happened due to chaos-related transitions in motions of the inner planets or through the interplay of chaos between the outer and inner planets. Asteroids in a region of the asteroid belt where many ordinary chondritic bodies reside, were rapidly perturbed into orbital resonances. This led to an increase in small to medium-sized collisional breakup events over a 2-5 Ma period. This would explain the simultaneous delivery of excess dust and asteroids to the inner solar system. Independent evidence for our scenario are the common cosmic-ray exposure ages in the range of ca. 33-40 Ma for recently fallen H and L chondrites.The temporal coincidence of gravity disturbances in the asteroid belt with the termination of ice-free conditions on Earth after 250 Ma is compelling. We speculate that this coincidence and a general correlation during the past 2 Ga between K-Ar breakup ages of parent bodies of the ordinary chondrites and ice ages on Earth suggest that there may exist an astronomical process that disturbs both regions of the inner asteroid belt and Earth's orbit with a potential impact on Earth's climate. © 2015 The Authors.

Meier M.M.M.,ETH Zurich | Meier M.M.M.,Lund University | Heck P.R.,Robert itzker Center For Meteoritics And Polar Studies | Heck P.R.,University of Chicago | And 3 more authors.
Geochimica et Cosmochimica Acta | Year: 2012

We have measured helium and neon concentrations, elemental and isotopic ratios of 91 individual presolar graphite grains from the KFC1 density separate of the Murchison meteorite. Eleven grains contain measurable amounts of either 4He, 20Ne, 21Ne or 22Ne, or a combination thereof. We report the first detection of 21Ne from an individual presolar graphite grain and the first detection of 4He and 20Ne in individual KFC1 graphite grains. Six of the gas-rich grains originate from asymptotic giant branch (AGB) stars, while another five are likely derived from core-collapse supernovae. The mono-isotopic 22Ne detected in one supernova grain is either radiogenic and compatible with condensation in the O/Ne zone, or nucleosynthetic and derived from the He/C zone. Two grains with 20Ne and 12C/ 13C <10 are consistent with condensation and Ne acquisition in a ~80:20 mixture of material from the H envelope and He/N zone. The isotopic ratios of a single grain with 21Ne and 22Ne, and a further grain with 20Ne and 22Ne are compatible with condensation and Ne acquisition in the C/O zone. We discuss the implications of our study on the understanding of processes in supernova ejecta. © 2011 Elsevier Ltd.

Stephan T.,University of Chicago | Stephan T.,Robert itzker Center For Meteoritics And Polar Studies | Heck P.R.,University of Chicago | Heck P.R.,Robert itzker Center For Meteoritics And Polar Studies | And 2 more authors.
International Journal of Mass Spectrometry | Year: 2015

Dead time effects in time-of-flight secondary ion mass spectrometry are well known and can be corrected for using Poisson statistics. Laser-induced desorption, however, introduces nonlinearity in the evaporation process resulting in highly fluctuating signals that make proper dead time correction much more challenging. Here, we propose a modified dead time correction procedure that overcomes such obstacles using data from correlated detection events from different isotopes of a single element. Provided the signals are not affected by unresolved mass interferences, this dead time correction enables us to obtain meaningful isotope ratios as demonstrated for atom probe tomography data of carbon from nanodiamonds and of silicon. © 2014 Elsevier B.V.

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.

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.

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.

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.

Huberty J.M.,University of Wisconsin - Madison | Konishi H.,University of Wisconsin - Madison | Heck P.R.,University of Wisconsin - Madison | Heck P.R.,Robert itzker Center For Meteoritics And Polar Studies | And 3 more authors.
American Mineralogist | Year: 2012

We report silician magnetite from banded iron formation (BIF) in the Dales Gorge Member of the Brockman Iron Formation, Hamersley Group, Western Australia. Magnetite mesobands typically consisting of individual ∼100 μm microlaminae are revealed to be composed of silician magnetite overgrowths on magnetite. Silician magnetite overgrowths contain from 1 to 3 wt% SiO 2, whereas (low-Si) magnetite domains contain less than 1 wt% SiO 2. Silicon solid solution is present in the magnetite crystal lattice as determined by in situ micro-X-ray diffraction and high-resolution transmission electron microscopy. Three textures are distinguished in magnetite mesobands: (1) magnetite sub-microlaminae with silician magnetite overgrowths, (2) recrystallized magnetite fragments with silician magnetite overgrowths, and (3) a complex intergrowth of magnetite and silician magnetite. All three textures are found in magnetite mesobands from the BIF4-5 and BIF12-16 macrobands of the Dales Gorge type-section drill core DDH-47A from Wittenoom, Western Australia. Magnetite domains contain numerous submicrometer-to- micrometer inclusions of quartz, carbonate, stilpnomelane, and apatite, whereas silician magnetite overgrowths are devoid of mineral inclusions. The presence of mineral inclusions in magnetite indicates the BIF oxide precipitate was not chemically pure iron oxyhydroxide/oxide. Magnetite domains display textures formed during soft sediment deformation that are the earliest and best preserved relict sedimentary structures in this BIF. Silician magnetite is the dominant iron oxide in the Dales Gorge BIF and is present in many other sub-greenschist facies BIFs worldwide. We suggest the former presence of organic matter creates reducing conditions necessary to stabilize silician magnetite. Thus, silician magnetite is a potential biosignature in BIFs.

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