1,721,047 research outputs found
DEVELOPMENT OF AN AERODYNAMIC LEVITATION DEVICE IN COMBINATION WITH A MOVABLE GAS-MIXING FURNACE AND ITS APPLICATIONS IN THE STUDY OF CHONDRULES FORMATION
No full textHIGH TEMPERATURE OXYGEN ISOTOPE EXCHANGE BETWEEN CHONDRULE MELT AND H2O: AN EXPERIMENTAL APPROACH
No full textHIGH TEMPERATURE OXYGEN ISOTOPE EXCHANGE BETWEEN CHONDRULE MELT AND H2O: AN EXPERIMENTAL APPROACH
No full textDEVELOPMENT OF AN AERODYNAMIC LEVITATION DEVICE IN COMBINATION WITH A MOVABLE GAS-MIXING FURNACE AND ITS APPLICATIONS IN THE STUDY OF CHONDRULES FORMATION
No full textTriple oxygen isotope exchange between chondrule melt and water vapor: An experimental study
No full textWe have conducted time and fO(2)-dependent oxygen isotope exchange experiments between chondrule analogue melts and H2O in the phase. The aim of our study is to address the question whether the oxygen isotope composition of chondrules is the result of exchange with the ambient nebular gas or has been inherited from the precursor material. The silicate melt-H2O vapor exchange experiments were carried out in a vertical gas-mixing furnace using the metal loop technique at 1500 degrees C. The duration ranged from 5 to 1440 min and fO(2) was set between IW - 3.8 and IW - 1.3 using the H2O/H-2 buffer. Our experiments show that 50% exchange between H2O gas and silicate melt occurs in similar to 4 h at fO(2) = IW -3.8 and in similar to 1 h at fO(2) = IW - 1.3. At solar nebula conditions, significant exchange occurs only if chondrule-melting times were several hours. (C) 2015 Elsevier Ltd. All rights reserved.DFG-SPP [1385
The catastrophic break‐up of the ureilite parent body: Modeling constraints on the debris size
No full textAbstract
The ureilite parent body (UPB) was, in all likelihood, completely broken apart when hit by another object early in its history and reassembled into daughter bodies. We here present a study tailored to constrain the dimensions of the impact debris produced in the catastrophic disruption. Using a customized Python code to simulate the thermal evolution of the UPB fragments, we compared the FeO profiles modeled for different depths within those fragments with those measured across the reduction rims in olivines of 12 different ureilites (
n
= 37). Our profile data were fitted to the theoretical cooling profiles determined with a transient thermal model. The results are coherent and consistent with earlier studies and, despite using simplified boundary conditions (fragments described as ideal spheres and maximum radiation), our data provide valuable context on possible cooling pathways of the UPB debris. In detail, we found that the average depths within the given fragments from which our samples of ureilites originated were limited to 0.3–0.4 ± 0.1 m, with only few exceptions (e.g., one highly reduced sample lacked suitable reduction profiles suggesting either a depth of origin of >2 m or shielding of this fragment from rapid cooling, e.g., due to hovering in the center of a relatively dense cloud of debris). In addition, we calculated that the cooling from 1473 to 1100 K of the average fragment at the depth of our samples took no more than 3–4 days, suggesting that the reassembly of the ureilite daughter bodies could have been a very fast process.Deutsche Forschungsgemeinschaft https://doi.org/10.13039/50110000165
3D X-RAY MICRO-CT INTERNAL TEXTURE OF I-TYPE COSMIC SPHERULES
No full textIntroduction: Micrometeorites (MMs) are microscopic particles, collected at the Earth’s surface and mainly
produced by collisions among solid bodies and by surface evaporation of icy bodies in the Solar System [1]. As
such, they provide important information on the compositon of their parent bodies including those that are not sampled
by meteorites.
I-type cosmic spherules (CS) are dark, opaque, melted micrometeorites dominated by magnetite (Fe3O4) and
wüstite (FeO) crystals. I-type CS frequently contain μm-sized Ni-rich Fe,Ni metal beads [2] as well as μm- and nmsized
platinum-group element (PGE) nuggets [3].
The aim of this work is twofold: 1) to study the internal textural and structural components (i.e. iron oxides domains,
metal beads, PGE nuggets, cavities) of the I-type CS, 2) to characterize the distribution of the metal beads
and PGE nuggets and estimate their relative volume/mass.
Samples: I-type CS have been collected in loose sediments sampled on the tops of the Transantarctic Mountains
[4] during the 2012-13 and 2014-15 Italian Programma Nazionale di Ricerche in Antartide (PNRA) campaign. In
Antarctic MMs collections the abundance of I-type spherules is typically less than 2% [5]. Our collection consists of
over 3000 MMs. Among these 107 (3.2% of the total) are I-type CS. For this study we have selected 52 I-type CS,
ranging from 200 to 800 μm in diameter and 2 with diameters <150 μm. They look fresh (unweathered) under the
stereomicroscope-SEM and provide excellent material for our study.
Methods: The external morphology of CS has been described by SEM analyses. Subsequently, non-destructive
mapping of I-type CS has been carried out by X-ray microcomputed tomography (micro-CT) using a Zeiss Xradia
520 Versa 3D X-ray microscope, at the Electrochemical Innovation Lab, part of the Department of Chemical Engineering
of University College London. The microscope was operated at 140 kV with a high energy filter in place
(HE1) and an optical magnification of 20X.
Results and Discussion: So far 3D tomographic reconstructions on 7 CS have been performed. They allowed: 1)
Distinction of four components, i.e. the cavities, the iron oxide domains, the metal beads and the PGE nuggets; 2)
Estimation of the true 3D Bead-Volume/Total-Volume ratio and its relationship with the size of the spherules. This
allow to obtain atmospheric flight parameters of the spherules. For instance, numerical modeling indicates that metal
survival in particles with radius ≥100 μm occur at steep entry angle (>40°) and high entry velocity (>16 km s-1) [6].
3) Characterization of void distribution. Irregular cavities are frequently observed in I-type CS. They are interpreted
as: (i) the result of contraction due to quenching after atmospheric heating [7]; (ii) the product of gaseous
exsolution (e.g. O2, SO2) during cooling of iron oxide liquids. 4) Speculate on the origin of the peculiar webstructure
of the oxide phases resulting from wüstite domains surrounded by magnetite.
More samples are being analyzed in the next weeks. Results will be discussed at the Meeting
Temperature dependence of yttrium partitioning between garnet and xenotime: an experimental study
No full textYttrium is a notable trace element particularly compatible with garnet. Lanzirotti (1995) provided evidences that, among major metapelitic minerals, yttrium preferentially partitions into garnet and that the mode of major metapelite phases besides garnet have little effect on its fractionation. On the contrary trace elements are extremely sensitive to changes in accessory mineral assemblage (e.g. Ganguly, 2010).
Xenotime (YPO4) is a common accessory mineral in metapelites and arguments for garnet growth in equilibrium with xenotime are presented in several papers (e.g. Martin, 2009). Pyle & Spear (1999) described a relevant temperature control on the solubility of yttrium in garnet in xenotime-bearing metapelites from New England (USA). On the basis of a strong negative correlation between Y concentration and temperature they derived an empirical calibration to be used as geothermometer. However, no experimental studies do exist to date on the temperature dependence of Y partitioning between garnet and xenotime. In order to unravel this relation, high pressure (up to 2.0 GPa) xenotime – saturated synthesis of garnet have been performed in an end-loaded piston cilynder. The simple model system MgO-FeO-Al2O3-SiO2 has been investigated at temperature between 800 and 1000°C running compositions falling along the join almandine-pyrope + 5 wt% YPO4. Gels have been prepared as starting materials using tetraethylorthosilicate (TEOS) as silica source, pure Mg-, Al-, Ca-, Y-nitric solutions, ferric benzoate and ammonium dihydrogen phosphate (NH4H2PO4) digested in deionised water. Gels were fired in a gas-mixing furnace at fO2 conditions approaching the IW (iron-wustite) buffer at 1 atm for 3 hours. The powder was tightly packed into a gold capsule with an internal graphite sleeve to keep the oxygen fugacity low.
Run products were preliminary identified by X-ray powder diffraction, carefully inspected on back-scattered electron images and by X ray element maps, and analysed by electron microprobe and particle-induced X-ray emission (micro-PIXE). The use of the proton microprobe stems from its higher spatial resolution and lower X-ray background with respect to electron microprobe. This allows to measure trace element concentrations down to levels of ~ 1 ppm on a 1 m beam spot (Fraser, 1990). Preliminary results will be discussed.
References
Biggar, G.M. & O’Hara, M.J. (1969). A comparison of gel and glass starting materials for phase equilibrium studies. Mineralogical Magazine 37, 198-205.
Fraser, D.G. (1990). Applications of the high-resolution scanning proton microprobe in the Earth Sience: an overview. Chemical Geology 83, 27-37.
Ganguly, J (2010). Cation diffusion kinetics in aluminosilicate garnets and geological applications. Reviews in Mineralogy and Geochemistry 72, 559-601.
Heald, E.F., Reeher, J.R. & Herrington, D.R. (1969). Gel preparation of starting materials in iron-containing silicate systems. American Mineralogist 54, 317-320.
Lanzirotti, A. (1995). Yttrium zoning in metamorphic garnet. Geochimica et Cosmochimica Acta 59, 4105-4110.
Martin, A.J. (2009). Sub-millimeter Heterogeneity of Yttrium and Chromium during Growth of Semi-pelitic Garnet Journal of Petrology 50, 1713-1727
Pyle, J.M., & Spear, F.S. (1999). An empirical garnet (YAG)-xenotime thermometer. Contributions to Mineralogy and Petrology 138, 51-58
Revealing the climate of snowball Earth from Delta O-17 systematics of hydrothermal rocks
No full textThe oxygen isotopic composition of hydrothermally altered rocks partly originates from the interacting fluid. We use the triple oxygen isotope composition (O-17/O-16, O-18/O-16) of Proterozoic rocks to reconstruct the O-18/O-16 ratio of ancient meteoric waters. Some of these waters have originated from snowball Earth glaciers and thus give insight into the climate and hydrology of these critical intervals in Earth history. For a Paleoproterozoic [similar to 2.3-2.4 gigayears ago (Ga)] snowball Earth, delta O-18 = -43 +/- 3 parts per thousand is estimated for pristine meteoric waters that precipitated at low paleo-latitudes (<= 35 degrees N). Today, such low O-18/O-16 values are only observed in central Antarctica, where long distillation trajectories in combination with low condensation temperatures promote extreme O-18 depletion. For a Neoproterozoic (similar to 0.6-0.7 Ga) snowball Earth, higher meltwater delta O-18 estimates of -21 +/- 3% imply less extreme climate conditions at similar paleolatitudes (<= 35 degrees N). Both estimates are single snapshots of ancient water samples and may not represent peak snowball Earth conditions. We demonstrate how O-17/O-16 measurements provide information beyond traditional O-18/O-16 measurements, even though all fractionation processes are purely mass dependent
- …
