COMPOSITION AND ORIGIN OF THE UNUSUAL OKTIBBEHA COUNTY IRON METEORITE

We have analyzed Oktibbeha County, the most Ni-rich iron meteorite, for Ni, Co, Cu, Ga, Ge, As, Sb, Ir, and Au. Cu and Sb are higher than in any other iron, but other trace elements are within the ranges typically found in iron meteorites. Extrapolation of trace element trends in group IAB indicates that Oktibbeha County is a member of this group. This sheds light on the origin of groups IAB and IIICD, which are thought to be derived from impact melts on parent bodies of chondritic composition. Lafayette (iron), another sample reported in the literature to have a similarly high Ni content, is probably a pseudometeorite.     

Thermodynamics of selected trace elements in the Jovian atmosphere

The thermochemistry of several hundred compounds of twelve selected trace elements (Ge, Se, Ga, As, Te, Pb, Sn, Cd, Sb, Tl, In, and Bi) has been investigated for solar composition material along a Jupiter adiabat. The results indicate that AsF₃, InBr, TlI, and SbS, in addition to CO, PH₃, GeH₄, AsH₃, H₂Se, HCl, HF, and H₃BO₃ proposed by Barshay and Lewis (1978), may be potential chemical tracers of atmospheric dynamics. The reported observations of GeH₄ is interpreted on the basis of new calculations as implying rapid vertical transport from levels where T ? 800°K. Upper limits are also set on the abundances of many gaseous compounds of the elements investigated.     

Did iron meteorites form in the asteroid belt?—Evidence from thermodynamic models

The major iron meteorite groups are defined essentially by their Ga, Ge, and Ni contents. It now seems clear that the differences between their abundances of Ga and Ge were produced by the process of condensation and accretion in the primordial solar nebula. The simplest interpretation of the Ni abundance, and its variations between the groups, is also that it was fixed during condensation and accretion; more particularly, it reflects the oxidation state of the nebula during condensation and accretion. The abundance patterns of 17 other trace elements have been examined and are consistent with this model. It is believed to be the simplest model published and most consistent with analogous calculations for the chondrites. If it is correct, then the iron meteorite groups formed over a very wide range of pressures, 10^?4 to 10^?8 atm. Such a range could only be found in a restricted region of the nebula, such as the asteroid belt, if a complex accretion sequence inside a protoplanet occurred. More likely, the iron meteorites were formed in widely dispersed regions of the nebula and only one group formed in the asteroid belt, probably group IIIAB. Groups IAB and IIAB formed nearer the Sun, and group IVA formed much further out, say, beyond the orbit of Jupiter.     

Volatile element signatures in the mantles of Earth,Moon, and Mars: Core formation fingerprints from Bi,Cd, In,and Sn

Volatile element concentrations in planets are controlled by many factors such as precursor material composition, core formation, differentiation, magma ocean and magmatic degassing, and late accretionary processes. To better constrain the role of core formation, we report new experiments defining the effect of temperature, and metallic S and C content on the metal-silicate partition coefficient (or D(i) metal/silicate) of the volatile siderophile elements (VSE) Bi, Cd, In, and Sn. Additionally, the effect of pressure on metal-silicate partitioning between 1 and 3 GPa, and olivine-melt partitioning at 1 GPa have been studied for Bi, Cd, In, Sn, As, Sb, and Ge. Temperature clearly causes a decrease in D(i) metal/silicate for all elements. Sulfur and C have a large influence on activity coefficients in metallic Fe liquids, with C causing a decrease in D(i) metal/silicate, and S causing an increase. Pressure has only a small effect on D(Cd), D(In), and D(Ge) metal/silicate. Depletions of Bi, Cd, In, and Sn in the terrestrial and Martian mantles are consistent with high PT core formation and metal-silicate equilibrium at the high temperatures indicated by previous studies. A late Hadean matte would influence Bi the most, due to its high D(sulfide/silicate) ~2000, but segregation of a matte would only reduce the mantle Bi content by 50%; all other less chalcophile elements (e.g., Sn, In, and Cd) would be minimally affected. The lunar depletions of highly VSE require a combination of core formation and an additional depletion mechanism—most likely the Moon-forming giant impact, or lunar magma ocean degassing.     

Volatile element chemistry during metamorphism of ordinary chondritic material and some of its implications for the composition of asteroids

We used chemical equilibrium calculations to model thermal metamorphism of ordinary chondritic material as a function of temperature, pressure, and trace element abundance and use our results to discuss volatile mobilization during thermal metamorphism of ordinary chondrite parent bodies. We compiled trace element abundances in H-, L-, and LL-chondrites for the elements Ag, As, Au, Bi, Br, Cd, Cs, Cu, Ga, Ge, I, In, Pb, Rb, Sb, Se, Sn, Te, Tl, and Zn, and identified abundance trends as a function of petrographic type within each class. We calculated volatility sequences for the trace elements in ordinary chondritic material, which differ significantly from the solar nebula volatility sequence. Our results are consistent with open-system thermal metamorphism. Abundance patterns of Ag and Zn remain difficult to explain.     

Structure and evolution of single WR stars

New computations of massive stars follow the evolution up to advanced stages and include:

-A large and flexible nuclear network consisting of 174 nuclear species that are linked by 1742 nuclear reactions.

-Semiconvection, overshooting and mass loss.

-Modern rates for both strong and weak interaction processes as well as the latest rates for the neutrino processes.

-Improved grid distribution and a large number of grid points.

The nuclear network and the diffusion equation are solved for each time step during the whole evolution. In this way the accuracy of nuclear yields and chemical abundances are mainly limited by uncertainties in the diffusion coefficient found from the convection theories. Several instability mechanisms may affect the mass loss rates of massive stars and thereby the structure and abundances of WR stars. Due to heavy mass loss at the LBV and WR stages, the masses at the pre-SN stage may be less than 5M _⊙. Yields and abundances throughout the stars are discussed together with the amount of all elements expelled.     

Parent body histories recorded in Rumuruti chondrite sulfides: Implications for the onset of oxidized,sulfur-rich core formation

Models of planetary core formation beginning with melting of Fe,Ni metal and troilite are not readily applicable to oxidized and sulfur-rich chondrites containing only trace quantities of metal. Cores formed in these bodies must be dominated by sulfides. Siderophile trace elements used to model metallic core formation could be used to model oxidized, sulfide-dominated core formation and identify related meteorites if their trace element systematics can be quantified. Insufficient information exists regarding the behavior of these core-forming elements among sulfides during metamorphism prior to anatexis. Major, minor, and trace element concentrations of sulfides are reported in this study for petrologic type 3–6 R chondrite materials. Sulfide-dominated core-forming components in such oxidized chondrites (ƒO₂ ≥ iron-wüstite) follow metamorphic evolutionary pathways that are distinct from reduced, metal-bearing counterparts. Most siderophile trace elements partition into pentlandite at approximately 10× chondritic abundances, but Pt, W, Mo, Ga, and Ge are depleted by 1–2 orders of magnitude relative to siderophile elements with similar volatilities. The distribution of siderophile elements is further altered during hydrothermal alteration as pyrrhotite oxidizes to form magnetite. Oxidized, sulfide-dominated core formation differs from metallic core formation models both physically and geochemically. Incongruent melting of pentlandite at 865°C generates melts capable of migrating along solid silicate grains, which can segregate to form a Ni,S-rich core at lower temperatures compared to reduced differentiated parent bodies and with distinct siderophile interelement proportions.     

The abundance and isotopic composition of Cd in iron meteorites

Cadmium is a highly volatile element and its abundance in meteorites may help better understand volatility‐controlled processes in the solar nebula and on meteorite parent bodies. The large thermal neutron capture cross section of ¹¹³Cd suggests that Cd isotopes might be well suited to quantify neutron fluences in extraterrestrial materials. The aims of this study were (1) to evaluate the range and magnitude of Cd concentrations in magmatic iron meteorites, and (2) to assess the potential of Cd isotopes as a neutron dosimeter for iron meteorites. Our new Cd concentration data determined by isotope dilution demonstrate that Cd concentrations in iron meteorites are significantly lower than in some previous studies. In contrast to large systematic variations in the concentration of moderately volatile elements like Ga and Ge, there is neither systematic variation in Cd concentration amongst troilites, nor amongst metal phases of different iron meteorite groups. Instead, Cd is strongly depleted in all iron meteorite groups, implying that the parent bodies accreted well above the condensation temperature of Cd (i.e., ≈650 K) and thus incorporated only minimal amounts of highly volatile elements. No Cd isotope anomalies were found, whereas Pt and W isotope anomalies for the same iron meteorite samples indicate a significant fluence of epithermal and higher energetic neutrons. This observation demonstrates that owing to the high Fe concentrations in iron meteorites, neutron capture mainly occurs at epithermal and higher energies. The combined Cd‐Pt‐W isotope results from this study thus demonstrate that the relative magnitude of neutron capture‐induced isotope anomalies is strongly affected by the chemical composition of the irradiated material. The resulting low fluence of thermal neutrons in iron meteorites and their very low Cd concentrations make Cd isotopes unsuitable as a neutron dosimeter for iron meteorites.     

Post-entry and volcanic contaminant abundances of zinc,copper, selenium,germanium and gallium in stratospheric micrometeorites

Abstract— Some fraction of Zn, Cu, Se, Ga and Ge in chondritic interplanetary dust particles (IDPs) collected in the lower stratosphere between 1981 May and 1984 June has a volcanic origin. I present a method to evaluate the extent of this unavoidable type of stratospheric contamination for individual particles. The mass-normalised abundances for Cu and Ge as a function of mass-normalised stratospheric residence time show their time-integrated stratospheric aerosol abundances. The Zn, Se and Ga abundances show a subdivision into two groups that span approximately two-year periods following the eruptions of the Mount St. Helens (1980 May) and El Chichón (1982 April) volcanos. Elemental abundances in particles collected at the end of each two-year period indicate low, but not necessarily ambient, volcanic stratospheric abundances. Using this time-integrated baseline, I calculate the stratospheric contaminant fractions in nine IDPs and show that Zn, Se and Ga abundances in chondritic IDPs derive in part from stratospheric aerosol contaminants. Post-entry elemental abundances (i.e., the amount that survived atmospheric entry heating of the IDP) show enrichments relative to the CI abundances but in a smaller number of particles than previously suggested.     

Mineralogical and Geochemical Study of Zhaoping,Xifu and Hami Iron Meteorites

The mineralogy and bulk chemical compositions of three iron meteorites (Zhaoping, Xifu and Hami) recently found in China are reported here and are classified on the basis of their bulk chemical compositions. Zhaoping contains 93.4 mg/g Ni, 85.9 μg/g Ga, 418 μg/g Ge, 5.24 mg/g Co, 1.94 μg/g Ir, 0.774 μg/g W, and 1.62 μg/g Au and belongs to the low-Ni, low-Au subgroup of IAB. It is a coarse octahedrite and consists of kamacite, taenite, troilite, schreibersite and cohenite. The cohenite has entirely decomposed to graphite and low-Ni kamacite in our samples. Zhaoping contains some inclusions of Mn-free sarcopside which were rarely reported in IAB iron meteorites. Xifu has 74.1 mg/g Ni, 58.8gμg/g Ga, 150 μg/g Ge, and 0.913 μg/g W. Xifu is a member of group IIICD iron meteorite. Like most of IIICD irons, Xifu is a coarsest octahedrite with kamacite bandwidth larger than 3mm, and contains kamacite, taenite and schreibersite. Carbides and graphite are not found in the sample because of its being heterogeneous. Hami has 106 mg/g Ni, 5.36 mg/g Co and 0.922 μg/g Ir. We did not obtain the Ga and Ge contents in Hami because of their low concentrations and the limited precision of the INAA technique. Hami is an unclassified iron meteorite on the basis of the contents of other trace elements, structure and mineralogy. On mineralogy and structure, Hami resembles Rafruti, another unclassified iron meteorite.     

Volatile trace elements in Antarctic ureilites

Abstract— We report data for Ag, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, U and Zn in 15 different Antarctic ureilites. Contents of these mainly volatile trace elements in Antarctic ureilites are roughly comparable to those in the four known falls. Trends exhibited by these data apparently reflect geochemical fractionations in parental magma(s), which were closed to loss of vapor. Subsequent events (e.g., shock and crystallization) do not seem to have affected contents of these elements.     

The elemental abundances in interplanetary dust particles

Abstract— We compiled a table of all major, minor, and trace-element abundances in 89 interplanetary dust particles (IDPs) that includes data obtained with proton-induced x-ray emission (PIXE), synchroton x-ray fluorescence (SXRF), and secondary ion mass spectrometry (SIMS). For the first time, the reliability of the trace-element abundances in IDPs is tested by various crosschecks. We also report on the results of cluster analyses that we performed on IDP compositions. Because of the incompleteness of the data set, we included only the elements Cr, Mn, Ni, Cu, and Zn, normalized to Fe and CI chondrite abundances, that are determined in 73 IDPs. The data arrange themselves in four rather poorly defined groups that we discuss in relation to CI chondrites following the assumption that on the average CI abundances are most probable. The largest group (chondritic), with 44 members, has close to CI abundances for many refractory and moderately refractory elements (Na, Al, Si, P, K, Sc, Ti, V, Cr, Co, Ge, Sr). It is slightly depleted in Fe and more in Ca and S, while the volatile elements (Cl, Cu, Zn, Ga, Se, Rb) are enriched by =1.7 × CI and Br by 21 × CI. The low-Zn group, with 12 members, is very similar to the chondritic group except for its Zn-depletion, stronger Ca-depletion and Fe-enrichment. The low-Ni group, with 11 members, has Ni/Fe = 0.03 × CI and almost CI-like Ca, but its extraterrestrial origin is not established. The last group (6 members) contains non-systematic particles of unknown origin. We found that Fe is inhomogeneously distributed on a micron scale. Furthermore, the abundances of elements that are measured near their limits of detection are easily overestimated. These biases involved, the incomplete data set and possible contaminating processes prevent us from obtaining information on the specific origin(s) of IDPs from elemental abundances.     

Siderophile and chalcophile element abundances in shergottites: Implications for Martian core formation

Elemental abundances for volatile siderophile and chalcophile elements for Mars inform us about processes of accretion and core formation. Such data are few for Martian meteorites, and are often lacking in the growing number of desert finds. In this study, we employed laser ablation inductively coupled plasma–mass spectrometry (LA‐ICP‐MS) to analyze polished slabs of 15 Martian meteorites for the abundances of about 70 elements. This technique has high sensitivity, excellent precision, and is generally accurate as determined by comparisons of elements for which literature abundances are known. However, in some meteorites, the analyzed surface is not representative of the bulk composition due to the over‐ or underrepresentation of a key host mineral, e.g., phosphate for rare earth elements (REE). For other meteorites, the range of variation in bulk rastered analyses of REE is within the range of variation reported among bulk REE analyses in the literature. An unexpected benefit has been the determination of the abundances of Ir and Os with a precision and accuracy comparable to the isotope dilution technique. Overall, the speed and small sample consumption afforded by this technique makes it an important tool widely applicable to small or rare meteorites for which a polished sample was prepared. The new volatile siderophile and chalcophile element abundances have been employed to determine Ge and Sb abundances, and revise Zn, As, and Bi abundances for the Martian mantle. The new estimates of Martian mantle composition support core formation at intermediate pressures (14 ± 3 GPa) in a magma ocean on Mars.     

Evidence from the Rb‐Sr system for 4.4 Ga alteration of chondrules in the Allende (CV3) parent body

Abstract— The timing and processes of alteration in the CV parent body are investigated by the analysis of Sr isotopes, major and trace elements, and petrographic type and distribution of the secondary minerals (nepheline and sodalite) in 22 chondrules from the Allende (CV3) chondrite. The Sr isotopic compositions of the chondrules are scattered around the 4.0 Ga reference line on the ⁸⁷Sr/⁸⁶Sr evolution diagram, indicating that the chondrules have been affected by late thermal alteration event(s) in the parent body. The degree of alteration, determined for individual chondrules based on the distribution of nepheline and sodalite, is unrelated to the disturbance of the Rb‐Sr system, suggesting that the alteration process that produced nepheline and sodalite is different from the thermal process that disturbed the Rb‐Sr system of the chondrules. Considering the geochemical behavior of Rb and Sr, the main host phase of Sr in chondrules is likely to be mesostasis, which could be most susceptible to late thermal alteration. As there is a poor connection between the alteration degree determined from abundances of nepheline and sodalite and the disturbance of Rb‐Sr isotopic system, we consider the mesostasis to provide a constraint on the late parent body alteration process. From this point of view, 23 mesostasis‐rich chondrules, including those from literature data, were selected. The selected chondrules are closely correlated on the ⁸⁷Sr/⁸⁶Sr evolution diagram, with an inferred age of 4.36 ± 0.08 Ga. This correlation would represent an age of the final major Sr isotopic redistribution of the chondrules in the parent body.     

THE ELLICOTT METEORITE

Two individual specimens (total weight 15.7 kg) of a new medium octahedrite were found near Ellicott, El Paso County, Colorado. The find is only 1.2 km from the find (in 1890) of the Franceville medium octahedrite. Ellicott and Franceville are distinct meteorites, the latter exhibiting pronounced differences in shock features, kamacite band width, and Ni, Ga, Ge, and Ir contents. Ellicott is a group IA iron while Franceville is in group IIIA.     

Martian meteorites: Volatile trace elements and cluster analysis

Abstract— We report data for 15 mainly volatile trace elements (Ag, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, U, Zn) by radiochemical neutron activation analysis (RNAA) in whole-rock samples of five Martian meteorites that, with seven others studied earlier, complete the 12 member Martian meteorite suite. Nearly all of these elements exhibit highly variable compositional continua and are richer in the Martian suite compared with other basaltic meteorites. From cluster analysis, we find that the clustering of subtypes based on these elements is virtually identical to that based on contents of major refractory elements and mineralogic/petrographic character istics, which implies that each source region on Mars was closed to volatile transport. Martian meteorite data can be used to infer volatile element contents in that planet.     

Experimental constraints on the volatility of germanium,zinc, and lithium in Martian basalts and the role of degassing in alteration of surface minerals

The surface of Mars is enriched in Cl and S which is linked to volcanic activity and degassing. Similarly, elevated Ge and Zn levels in Gale crater sedimentary bedrock indicate a magmatic source for these elements. To constrain the relative effects of Cl and S on the outgassing of these trace metals and chemical characteristics of primary magmatic vapor deposits incorporated to Martian surface, we conducted a set of degassing and fumarolic alteration experiments. Ge is found to be more volatile than Zn in all experiments. In S-bearing runs, the loss of Ge and Zn was less than any other experiments. In Cl-only runs, degassing of Zn was more than twice that of Ge within the first 10 min and percent loss increased for both elements with increasing time. In Cl + S runs, S-induced reduction of GeO₂ and ZnO to metallic Ge and Zn switches the preference of chloride formation from Zn to Ge. Up to 90% of Ge and Zn loss in the 1-h no volatile-added (NVA) experiments might be due to the small amounts of Cl contamination in NVA mixes via other oxides used for synthesis. Alteration experiments show different phases between 1-h and 24-/72-h runs. In 1-h runs, anhydrite and langbeinite dominate while in 24-/72-h runs halite and sylvite dominate the condensate assemblages. S-bearing phases form as the intermediate products of fumarolic deposition, while chlorides are common when the system is allowed to cool gradually. One-hour exposure was sufficient to form alteration phases and vapor deposits such as NaCl, KCl, CaSO₄, and langbeinites on the Martian analog minerals. These salts were identified in Martian meteorites and in situ measurements. Our results provide evidence that volcanic degassing along with fumarolic alteration could be a potential source for the enrichment and varying abundances of Cl, S, Fe, Zn, Ge in Martian surface, as well as a cause for Ge depletion in shergottites.     

Distribution of active regions on the solar surface estimated by statistical methods

Statistical properties of solar active regions (AR) have been studied. In particular, (1) the distribution of ARs by their areas and importances using normal and lognormal distribution laws; (2) it was checked whether the distribution of the ARs' birth sites satisfies the Poisson distribution law (the so-called ‘law of rare events’). Observational data of 1979–1982 have been used and our conclusions are as follows:

1. As regards the areas, the distribution of the ARs that emerged near or on the borders of the large-scale background fields is normal or lognormal.
2. As regards the importances, the distribution of all ARs is lognormal.
3. The distribution of ARs that emerged far from background field borders is not normal.
4. ARs are not casual or rare events on the Sun.

     

Mars: The evolutionary history of the northern lowlands based on crater counting and geologic mapping

The geologic history of planetary surfaces is most effectively determined by joining geologic mapping and crater counting which provides an iterative, qualitative and quantitative method for defining relative ages and absolute model ages. Based on this approach, we present spatial and temporal details regarding the evolution of the Martian northern plains and surrounding regions.The highland–lowland boundary (HLB) formed during the pre-Noachian and was subsequently modified through various processes. The Nepenthes Mensae unit along the northern margins of the cratered highlands, was formed by HLB scarp-erosion, deposition of sedimentary and volcanic materials, and dissection by surface runoff between 3.81 and 3.65 Ga. Ages for giant polygons in Utopia and Acidalia Planitiae are $～$ 3.75 Ga and likely reflect the age of buried basement rocks. These buried lowland surfaces are comparable in age to those located closer to the HLB, where a much thinner, post-HLB deposit is mapped. The emplacement of the most extensive lowland surfaces ended between 3.75 and 3.4 Ga, based on densities of craters generally $>3\mathrm{km}$ in diameter. Results from the polygonal terrain support the existence of a major lowland depocenter shortly after the pre-Noachian formation of the northern lowlands. In general, northern plains surfaces show gradually younger ages at lower elevations, consistent local to regional unit emplacement and resurfacing between 3.6 and 2.6 Ga. Elevation levels and morphology are not necessarily related, and variations in ages within the mapped units are found, especially in units formed and modified by multiple geological processes. Regardless, most of the youngest units in the northern lowlands are considered to be lavas, polar ice, or thick mantle deposits, arguing against the ocean theory during the Amazonian Period (younger than about 3.15 Ga).All ages measured in the closest vicinity of the steep dichotomy escarpment are also 3.7 Ga or older. The formation ages of volcanic flanks at the HLB (e.g., Alba Mons (3.6–3.4 Ga) and the last fan at Apollinaris Mons, 3.71 Ga) may give additional temporal constraint for the possible existence of any kind of Martian ocean before about 3.7 Ga. It seems to reflect the termination of a large-scale, precipitation-based hydrological cycle and major geologic processes related to such cycling.