420 research outputs found
Qingzhen and Yamato-691: A tentative alphabet for the EH chondrites
Petrological investigations of unequilibrated EH chondrites revealed the presence of three subgroups. They are identified based on the presence of different concentrations of MnS in niningerite. These differences were produced by partitioning of Mn between niningerite and enstatite as a result of different f_S2 and f_O2 during their formation. In order of increasing MnS-contents and hence increasing f_S2 and decreasing f_O2 these groups are : (A) Yamato (Y)-691 and Abee, (B) Indarch, and (C) Yamato-74370,South Oman, Qingzhen, Kota Kota, Kaidun III, and St. Marks. In the third subgroup the meteorites follow an equilibration and evolution sequence; Y-74370 the most primitive and St. Marks the most equilibrated. Y-691 is the most primitive in its subgroup. Differences in the chemical compositions of minerals in Y-691 and Qingzhen reveal a dichotomy in the compositions of niningerite, djerfisherite, kamacite, and perryite. Niningerites in Y-691 contain the least MnS (3.6-6.7 mole%) and counterparts in Qingzhen the most (12-14 mole%). K/Na ratios in djerfisherite are lower in Qingzhen than in Y-691. The Si concentration in kamacite in Qingzhen is higher than in Y-691. Ni in perryite in Qingzhen is higher than in Y-691. Na and K are highly fractionated between two sulfide lithologies. Na resides mainly in chondrules in caswellsilverite, in a Cl-bearing glass in the chondrules, and in Cr-rich sulfides in the matrix. In contrast, K is confined to djerfisherite, which occurs only in sulfide-rich objects in the matrix, and is highly depleted in chondrules. Two new layer structure minerals were discovered in Y-691 : (a) Na-Cu-Cr-sulfide with the general formula (NaCu) CrS_2,and (b) a Na-Cu-Zn-Cr-sulfide. An evolution scheme was constructed for the EH chondrites in the solar nebula and in their parent bodies. Niningerite and oldhamite condensed first and probably acted as nucleation sites for condensing sulfides, metals and silicates. Both minerals are abundant in chondrules, indicating that chondrule formation preceded all other sulfide- and metal-rich objects. For the first time, planetary metamorphic events were recognized. The Qingzhen Reaction, a breakdown of djerfisherite to troilite, covellite, idaite, bornite, and other unidentified phases, was discovered in Qingzhen and Y-691. Thermal episodes took place in the parent bodies at 1.4 Ba (Qingzhen), and 800 Ma (Y-691). Reverse zoning in niningerite indicates that Fe diffused from troilite to niningerite during the thermal event. In Y-691 sphalerite also formed during the metamorphic episode due to mobilization of Zn (and other volatiles). EH chondrites condensed in a chemically inhomogeneous region of the solar nebula where considerable variations in sulfur and oxygen fugacities existed
Mineralogy of Antarctic aubrites, Yamato-793592 and Allan Hills-78113: Comparison with non-Antarctic aubrites and E-chondrites
Two Antarctic aubrites, Yamato (Y)-793592 and Allan Hills (ALH)-78113,were mineralogically studied, for comparison with minerals in non-Antarctic aubrites and enstatite chondrites. The Antarctic aubrites are breccias consisting of coarse-grained enstatite fragments and fine-grained matrix. ALH-78113 has 200-300ÎŒm dark clasts that are fine-grained aggregates of silicate and opaque minerals. FeO-rich pyroxene (up to Fs_) occurs in the dark clasts. One dark clast has K-feldspar. These dark clasts seem to be exotic inclusions with distinct mineralogy. Daubreelite in the two Antarctic and non-Antarctic aubrites is lower in Zn than those in EH3-5 chondrites. This reflects the depletion of volatile elements in aubrites. Hydrated Na-Cr-sulfides were also found. Djerfisherite is a common accessory mineral in aubrites. It is characterized by low contents of Cu and Na, and high content of Ni, in comparison to djerfisherite in EH3-5 chondrites. Y-793592 has many roedderite grains. The occurrences of roedderite, Na-Cr-sulfides and djerfisherite in aubrites suggest that Al_2O_3 relative to alkali elements may have been fractionated during nebular or magmatic process
Petrography of Cape York and Grant: Irons with Simple Pd-Ag Systematics
The presence of excess ^(107)Ag from the decay of extinct ^(107)Pd(t_(1/2) = 6.5 my) has been determined in IVB, IV A and two anomalous irons (Kaiser and Wasserburg, 1983). Excesses of ^(107)Ag in groups IIIAB and IIB meteorites, Cape York and Grant, and Derrick Peak, respectively,
show the widespread presence of ^(107)Pd in the early solar system (Chen and Wasserburg, 1983)
Relaxation in the glass-former acetyl salicylic acid studied by deuteron magnetic resonance and dielectric spectroscopy
Supercooled liquid and glassy acetyl salicylic acid was studied using
dielectric spectroscopy and deuteron relaxometry in a wide temperature range.
The supercooled liquid is characterized by major deviations from thermally
activated behavior. In the glass the secondary relaxation exhibits the typical
features of a Johari-Goldstein process. Via measurements of spin-lattice
relaxation times the selectively deuterated methyl group was used as a
sensitive probe of its local environments. There is a large difference in the
mean activation energy in the glass with respect to that in crystalline acetyl
salicylic acid. This can be understood by taking into account the broad energy
barrier distribution in the glass.Comment: 8 pages, 3 figures, Submitted to Phys. Rev.
Petrography and geochemistry of the enriched basaltic shergottite Northwest Africa 2975
Short-lived Nuclei in the Early Solar System: Possible AGB Sources
(Abridged) We review abundances of short-lived nuclides in the early solar
system (ESS) and the methods used to determine them. We compare them to the
inventory for a uniform galactic production model. Within a factor of two,
observed abundances of several isotopes are compatible with this model. I-129
is an exception, with an ESS inventory much lower than expected. The isotopes
Pd-107, Fe-60, Ca-41, Cl-36, Al-26, and Be-10 require late addition to the
solar nebula. Be-10 is the product of particle irradiation of the solar system
as probably is Cl-36. Late injection by a supernova (SN) cannot be responsible
for most short-lived nuclei without excessively producing Mn-53; it can be the
source of Mn-53 and maybe Fe-60. If a late SN is responsible for these two
nuclei, it still cannot make Pd-107 and other isotopes. We emphasize an AGB
star as a source of nuclei, including Fe-60 and explore this possibility with
new stellar models. A dilution factor of about 4e-3 gives reasonable amounts of
many nuclei. We discuss the role of irradiation for Al-26, Cl-36 and Ca-41.
Conflict between scenarios is emphasized as well as the absence of a global
interpretation for the existing data. Abundances of actinides indicate a
quiescent interval of about 1e8 years for actinide group production in order to
explain the data on Pu-244 and new bounds on Cm-247. This interval is not
compatible with Hf-182 data, so a separate type of r-process is needed for at
least the actinides, distinct from the two types previously identified. The
apparent coincidence of the I-129 and trans-actinide time scales suggests that
the last actinide contribution was from an r-process that produced actinides
without fission recycling so that the yields at Ba and below were governed by
fission.Comment: 92 pages, 14 figure files, in press at Nuclear Physics
Resolution of impact-related microstructures in lunar zircon: A shock-deformation mechanism map
The microstructures of lunar zircon grains from breccia samples 72215, 73215, 73235, and 76295 collected during the Apollo 17 mission have been characterized via optical microscopy, cathodoluminescence imaging, and electron backscatter diffraction mapping. These zircon grains preserve deformation microstructures that show a wide range in style and complexity. Planar deformation features (PDFs) are documented in lunar zircon for the first time, and occur along {001}, {110}, and {112}, typically with 0.1â25 ”m spacing. The widest PDFs associated with {112} contain microtwin lamellae with 65°/ misorientation relationships. Deformation bands parallel to {100} planes and irregular low-angle ( misorientation axes. This geometry is consistent with a dislocation glide system with {010} during dislocation creep. Nonplanar fractures, recrystallized domains with sharp, irregular interfaces, and localized annealing textures along fractures are also observed. No occurrences of reidite were detected. Shock-deformation microstructures in zircon are explained in terms of elastic anisotropy of zircon. PDFs form along a limited number of specific {hkl} planes that are perpendicular to directions of high Youngâs modulus, suggesting that PDFs are likely to be planes of longitudinal lattice damage. Twinned {112} PDFs also contain directions of high shear modulus. A conceptual model is proposed for the development of different deformation microstructures during an impact event. This âshock-deformation mechanism mapâ is used to explain the relative timing, conditions, and complexity relationships between impact-related deformation microstructures in zircon
Isotopic composition of carbon and nitrogen in ureilitic fragments of the Almahata Sitta meteorite
This study characterizes carbon and nitrogen abundances and isotopic compositions in ureilitic fragments of Almahata Sitta. Ureilites are carbon-rich (containing up to 7 wt% C) and were formed early in solar system history, thus the origin of carbon in ureilites has significance for the origin of solar system carbon. These samples were collected soon after they fell, so they are among the freshest ureilite samples available and were analyzed using stepped combustion mass spectrometry. They contained 1.2â2.3 wt% carbon; most showed the major carbon release at temperatures of 600â700 °C with peak values of ÎŽ13C from â7.3 to +0.4â°, similar to literature values for unbrecciated (âmonomictâ) ureilites. They also contained a minor low temperature (â€500 °C) component (ÎŽ13C = ca â25â°). Bulk nitrogen contents (9.4â27 ppm) resemble those of unbrecciated ureilites, with major releases mostly occurring at 600â750 °C. A significant lower temperature release of nitrogen occurred in all samples. Main release ÎŽ15N values of â53 to â94â° fall within the range reported for diamond separates and acid residues from ureilites, and identify an isotopically primordial nitrogen component. However, they differ from common polymict ureilites which are more nitrogen-rich and isotopically heavier. Thus, although the parent asteroid 2008TC3 was undoubtedly a polymict ureilite breccia, this cannot be deduced from an isotopic study of individual ureilite fragments. The combined main release ÎŽ13C and ÎŽ15N values do not overlap the fields for carbonaceous or enstatite chondrites, suggesting that carbon in ureilites was not derived from these sources
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