Petrography and Geochemistry of Metals in Almahata Sitta Ureilites (original) (raw)
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Ureilites: Trace element clues to their origin
Geochimica et Cosmochimica Acta, 1987
Carbonaceous vein separates from Kenna and HaverG, as well as bulk Kenna, were analyzed by RNAA for Ag, Au. Bi, Br, Cd, Cs, Ge, In, Ir, Ni, Pd. Ck, Rb. Re, Sb, Se, Te. Tl, U, and Zn. The data are reviewed together with four earlier Chicago analyses of bulk ureilites. Linear regressions confirm the presence of two metal components, with the following Cl-normalized ratios: Ir/Ni = 14.6, 51; Ge/Ni = 5.4. 2.4: Au/Ni = 2.3,0.9. The high-h component is enriched in vein separates and hence belongs to veins: the low-Ir component belongs to the ultramafic rock. Vein material is enriched in all elements analyzed by us except Zn. and accounts for most of the C, noble gases, and presumably siderophiles in the meteorite.
Journal of Analytical Atomic Spectrometry, 2005
The isotopic composition of Fe in iron meteorites and iron sulfides was studied with a UV laser (213 nm wavelength) and a high-mass resolution multicollector-ICP-MS. The long term reproducibility of Fe isotopic measurements was 0.16 and 0.21% (2 standard deviations) for d 56 Fe and d 57 Fe, respectively, i.e., a factor of B2 worse than conventional solution analysis. At present the major limitations of precision and accuracy in the laser ablation technique are instrumental mass discrimination (2.0-4.9% per amu) and laser-induced fractionation (up to 4% during a single analysis) of Fe isotopes. Laser ablation of iron metal and iron sulfides produces two types of particles: (1) coarse (50-600 nm) spherical and crystalline particles that are isotopically heavier than (2) fine (2-7 nm) amorphous particles. Differential transport of the two types of particles to the ICP can compromise the accuracy of Fe isotope measurements by laser ablation MC-ICP-MS. Despite lower precision compared with solution MC-ICP-MS analysis, the laser ablation technique is capable of resolving Fe isotopic variations between seafloor and sub-seafloor hydrothermal sulfides, and between kamacite and taenite alloys that are commonly present as thin lamellae in iron meteorites. The most important applications of the technique will be for the analysis of small samples where the textural context of the analysed objects is important for the interpretation of Fe isotopic data. Samples and experimental In this study we have used the Finnigan Neptune high-mass resolution MC-ICP-MS coupled to a New Wave UP-213 UV
Trace element analysis of ureilites: new constraints on their petrogenesis
Geochimica et Cosmochimica Acta, 1991
We examined bulk samples as well as acid-treated samples. In the bulk samples the refractory siderophiles' concentrations range from approximately 0.1 to 1 .O times Cl chondrites while the volatile siderophiles range from about 0.07 to 0.3 times Cl chondrites. Rare earth elements (REEs) in ureilites are quite depleted and display light and heavy rare earth enrichments. The Antarctic meteorites display either much less pronounced v-shaped patterns or no enrichment in the light rare earths at all. In terms of the new trace-element results, ureilites do not fall into the coherent groups that other workers have defined by chemical or petrographic characteristics. Trace elements do provide additional constraints on the models for the petrogenesis of ureilites. In particular, the siderophile element abundances call for simplified models of chemical processing rather than the complex, multistage processing called for in silicate fractionation models. REE concentrations. on the other hand, imply multistage processing to produce the ureilites. None of the ureilite petrogenesis models extant account for the trace element data. These new data and the considerations of them with respect to the proposed ureilite petrogenesis models indicate that the direction of modelling should be toward contemplation of mixtures and how the components we observe in ureilites behave under such conditions.
The origin of iron silicides in ureilite meteorites
Geochemistry, 2019
Ureilite meteorites contain iron silicide minerals including suessite (Fe,Ni) 3 Si, hapkeite (Fe 2 Si) and xifengite (Fe 5 Si 3). Despite occurring mostly in brecciated varieties presumed to be derived from the regolith of the ureilite parent asteroid, suessite has also been confirmed in one lithology of a dimict ureilite (NWA 1241). In contrast, Si-bearing Fe-metals occur in both brecciated and unbrecciated ureilites, implying that they were formed throughout the ureilite parent asteroid. We examined major, minor and trace element data of Fe-metals in seven brecciated ureilites (DaG 319, DaG 999, DaG 1000, DaG 1023, DaG 1047, EET 83309, and EET 87720) in addition to the dimict ureilite NWA 1241. In this study we show that the silicides and Si-bearing metals in ureilites have similar siderophile trace element patterns; therefore, the precursors to the silicides were indigenous to the ureilite parent body. Sifree kamacite grains in brecciated ureilites show flatter, more chondritic siderophile element patterns. They may also be derived from the interior of the ureilite parent body, but some may be of exogenous origin (impactor debris), as are rare taenite grains. On Earth, iron silicides are often formed under high-temperature and strongly reducing conditions (e.g. blast furnaces, lightning strikes). On the Moon, hapkeite (Fe 2 Si) and other silicides have been found in the regolith where they were formed by impact-induced space weathering. In the Stardust aerogel, iron silicides derived from comet Wild2 were also formed by an impact-related reduction process. Silicides in ureilite regolith breccias may have formed by similar processes but ureilites additionally contain abundant elemental carbon which probably acted as a reducing agent, thus larger and more abundant silicide grains were formed than in the lunar regolith or cometary material. The origin of suessite in NWA 1241 may be analogous to that of reduced lithologies in the terrestrial mantle, although a regolith origin may also be possible since this sample is shown here to be a dimict breccia.
Osmium isotope systematics of ureilites
Geochimica Et Cosmochimica Acta, 2007
The 187 Os/ 188 Os for 22 ureilite whole rock samples, including monomict, augite-bearing, and polymict lithologies, were examined in order to constrain the provenance and subsequent magmatic processing of the ureilite parent body (or bodies). The Re/Os ratios of most ureilites show evidence for a recent disturbance, probably related to Re mobility during weathering, and no meaningful chronological information can be extracted from the present data set. The ureilite 187 Os/ 188 Os ratios span a range from 0.11739 to 0.13018, with an average of 0.1258 ± 0.0023 (1r), similar to typical carbonaceous chondrites, and distinct from ordinary or enstatite chondrites. The similar mean of 187 Os/ 188 Os measured for the ureilites and carbonaceous chondrites suggests that the ureilite parent body probably formed within the same region of the solar nebula as carbonaceous chondrites. From the narrow range of the 187 Os/ 188 Os distribution in ureilite meteorites it is further concluded that Re was not significantly fractionated from Os during planetary differentiation and was not lost along with the missing ureilitic melt component. The lack of large Re/Os fractionations requires that Re/Os partitioning was controlled by a metal phase, and thus metal had to be stable throughout the interval of magmatic processing on the ureilite parent body.
Geochimica et Cosmochimica Acta, 1983
Silicate from two unusual iron-rich meteorites were analyzed by the I-Xe and 40Ar-'9Ar techniques. Enon, an anomalous iron meteorite with chondritic silicate, shows no loss of radiogenic QAr at low temperature, and gives a plateau age of 4.59 + 0.03 Ga. Although the Xe data fail to define an I-Xe correlation (possibly due to a very low iodine content), the inferred Pu/U ratio is more than 20 above the chondritic value, and the Pu abundance derived from the concentration of Pu-fission Xe is 4 times greater than the abundance inferred for Cl meteorites. These findings for Enon, coupled with data for IAB iron meteorites, suggest that presence of chondritic silicate in an iron-rich meteorite is diagnostic of an old radiometric age with little subsequent thermal disturbance. The JZagle Station pallasite, the most W-rich meteorite known, gives a complex aAr-39Ar age pattern which suggests a recent (So.85 Ga) severe thermal disturbance. The absence of excess '29Xe and the low trapped Ar and Xe contents, p , consistent with this interpretation. The similarity between 40Ar-39Ar data for Eagle Station and for the ohvine-rich meteorite Chassigny lends credence to the previous suggestion of a connection between Chassigny and pahasites, in the sense that similar processes operating at similar times on di5ercnt parent bodies may have been involved in the formation of ohvine in both types of meteorites.
A “chondritic” eucrite parent body: inference from trace elements
Geochimica et Cosmochimica Acta, 1978
Yb and Zn) were analyzed by radiochemical and instrLlrn~n~1 neutron activation in four eucrites: Juvinas (brecciated), Ibitira (vesicular. unhrecciated) and Moore County and Serra de Mage (cumulate. unbrecciated). When arranged in order of volatility. Cl-normalized abundance patterns allow nebular and planetary efiects to be distinguished. The stepped lithophile pattern reveals the dominance of nebular processes: in Ibitira, refractory elements (Hf. Lu. Tb, Ce, Sm. Yb, U, Eu) are (I 3.1 + 0.7) x Cl chondrites; volatile elements (Rb. Cs. Br, Bi) are (6.0 + 1.5) x IOwL Cl. The depletion of Tl seems inherent to the eucrite parent body and is mirrored in the chalcophile elements by the marked deficit of Te relative to Se: apparently volatiles were accreted as a fractionated C3-like component. Consistent but subtle Cl-normalized abundance differences between eucrites (Serra de Mag& < Moore County <: Juvinas < Ibitira) resmelt from crystaliliquid di~erentiation; Ihitira approximates the composition of an undifferentiated eucritc magma. The siderophile pattern retains little sign of nebular processes. but reflects planetary metal-silicate partition. The bulk composition of the eucrite parent body closely resembles that of H-chondritcs. except for two features: moderately volatile elements (e.g. Na. K. Rb) arc very much lower, apparently due to the accretion of more chondrule-like material: the metallic Fe-Ni content is only * I3";,, even though total iron is very similar. MATERIALS AND METHODS Smnpks Interior chips of the meteorites were cleaned ultrasonically in redistilled acetone. Samples were coarsely crushed to <2 mm diameter in an agate mortar. A 100 mg sample of Scrra de Magt-(Rio de Janeiro) was analyzed by Richard Pacer at the University of Kentucky by non-destructive instrumental thermal (CT"" source) and 14 MeV neutron activation for Al and Si (JANGHORBANI c't ul.. 1973; EHMANN and MORGAN. 1970). Subsequently, an aliquant of the Serra de MagC sample and a chip of Ibitira were analyzed by Mary Robyn, University of Oregon. using instrumental thermal neutron activation (INAA) for Co, Cr. Fe. Hf. Na. SC and six rare earth elements (REE: Ce. Eu. Lu. Sm, Tb, Yb) (GoRIION PI a/.. 1968). Juvinas (Paris National Museum No. 1500) and Moore Countv (Arizona State University No. 294X)'were not analyzed'b~ INAA. Samples and appropriate comparative standards were irradiated together in three separate irradiations at the National Bureau of Standards Reactor. Gaithcrsburg, Maryland, for IO days to give an integrated Rux of 68 x IO*"n (tl~ermal) cm-'. A sample of USGS. basalt BCR-1 was included in each irradiation as a safeguard against gross monitor errors. The chemical procedure was