Growth of ferrous olivine in the oxidized CV chondrites during a fluid-assisted thermal metamorphism.

Accepted versio

Minimum Radii of Super-Earths: Constraints from Giant Impacts

The detailed interior structure models of super-Earth planets show that there is degeneracy in the possible bulk compositions of a super-Earth at a given mass and radius, determined via radial velocity and transit measurements, respectively. In addition, the upper and lower envelopes in the mass--radius relationship, corresponding to pure ice planets and pure iron planets, respectively, are not astrophysically well motivated with regard to the physical processes involved in planet formation. Here we apply the results of numerical simulations of giant impacts to constrain the lower bound in the mass--radius diagram that could arise from collisional mantle stripping of differentiated rocky/iron planets. We provide a very conservative estimate for the minimum radius boundary for the entire mass range of large terrestrial planets. This envelope is a readily testable prediction for the population of planets to be discovered by the Kepler mission.Comment: 8 pages, 4 figures, Accepted for publication in ApJ Letter

Multiple formation mechanisms of ferrous olivine in CV carbonaceous chondrites during fluid-assisted metamorphism

The CV carbonaceous chondrites experienced alteration that resulted in formation of secondary ferrous olivine (Fa40-100), salite-hedenbergite pyroxenes (Fs10-50Wo45-50), wollastonite, andradite, nepheline, sodalite, phyllosilicates, magnetite, Fe,Ni-sulfides and Ni-rich metal in their Ca,Al-rich inclusions, amoeboid olivine ag-gregates, chondrules, and matrices. It has previously been suggested that fibrous ferrous olivine in dark inclusions in CV chondrites formed by dehydration of phyllosilicates during thermal metamorphism (T. Kojima and K. Tomeoka, Geochim. Cosmochim. Acta, 60, 2651, 1996; A.N. Krot et al., Meteoritics, 30, 748, 1995). This mechanism has been subsequently applied to explain the origin of ferrous olivine in the CV chondrules and matrices (A.N. Krot et al., Meteoritics, 32, 31, 1997). It is, however, inconsistent with the lack of significant fractionation of bulk oxygen isotope compositions of the CV chondrites and the Allende dark inclusions and the common occurrences of ferrous olivine in the aqueously-altered and virtually unmetamorphosed oxidized CV chondrites of the Bali-like subgroup. Based on the petrographic observations and the isotopic compositions of ferrous olivine and coexisting Ca,Fe-rich silicates in CV chondrites and their dark inclusions, we infer that ferrous olivine formed during a fluid-assisted metamorphism by several mechanisms: (i) replacement of Fe,Ni-metal±sulfide nodules, (ii) replacement of magnesian olivine and low-Ca pyroxene, and (iii) direct precipitation from an aqueous solution. Dehydration of phyllosilicates appear to have played only a minor (if any) role. Although our model does not address specifically the origin of ferrous olivine rims around forsterite grains in Allende, the observed homogenization of matrix olivines (which have comparable sizes to thicknesses of the ferrous olivine rims in Allende) from Kaba to Allende suggests that compositions of ferrous olivine rims in Allende cannot be primary and must have been modified by asteroidal alteration

Silica-bearing objects in the Dengli H3.8 and Gorlovka H3-4 chondrites

Silica-bearing objects are enigmatic components of the olivine-normative ordinary chondrites. Several papers have been devoted to the study of these objects in various chondrite types. While a relatively large body of information has been collected, the origin of these objects is still controversial. Here we report new data on silica-bearing objects in the unequilibrated H-chondrites Dengli and Gorlovka. The crystallization history of these objects could be explained on the basis of the phase diagram of the Q-Ol-Pl (Al2O3) system, but the origin of the silica-rich liquids remains unclear

U-Pb chronology of the Solar System's oldest solids with variable 238 U/ 235 U

Accurate determination of the absolute ages of the oldest Solar System objects - chondrules and Ca-Al-rich inclusions (CAIs), requires knowledge of their 238U/235U ratios. This ratio was assumed to be invariant in all U-Pb dating of meteorites so far, but the recent discovery of U isotope variations in CAIs (Brennecka et al., 2010a) shows that this assumption is invalid. We present the first combined high-precision U and Pb isotopic data for a CAI, and U isotopic data for chondrules and whole rock fractions of the Allende meteorite. The Pb-Pb isochron age of the CAI SJ101 is 4567.18±0.50Ma, calculated using the measured 238U/235U=137.876±0.043 (2σ), reported relative to 238U/235U=137.837 of the CRM 145 standard. Our best current estimate of the average terrestrial value is: 238U/235U=137.821±0.014.The error in the age includes uncertainties in the Pb-Pb isochron intercept and in the 238U/235U ratio. Allende bulk rock and chondrules have 238U/235U=137.747±0.017 (2σ), distinctly lower than the CAI. The difference in the 238U/235U ratio of 0.129±0.046 (2σ) between the CAI and chondrules and bulk meteorite increases the 207Pb-206Pb age difference by ~1.4Ma, and eliminates apparent disagreement between the CAI-chondrule formation time interval determinations with the U-Pb and extinct nuclide (26Al-26Mg and 182Hf-182W) data. We discuss standardisation of 238U/235U measurements for U-Pb geochronology and cosmochronology, elemental and isotopic fractionation induced by intensive acid leaching, ages of CAIs in the context of 238U/235U variability, and possible causes of U isotopic variations in CAIs and meteorites

Photometric Light Curves and Polarization of Close-in Extrasolar Giant Planets

The close-in extrasolar giant planets [CEGPs], \ltorder 0.05 AU from their parent stars, may have a large component of optically reflected light. We present theoretical optical photometric light curves and polarization curves for the CEGP systems, from reflected planetary light. Different particle sizes of three condensates are considered. In the most reflective case, the variability is $\approx 100$ micromagnitudes, which will be easily detectable by the upcoming satellite missions MOST, COROT, and MONS, and possibly from the ground in the near future. The least reflective case is caused by small, highly absorbing grains such as solid Fe, with variation of much less than one micromagnitude. Polarization for all cases is lower than current detectability limits. We also discuss the temperature-pressure profiles and resulting emergent spectra of the CEGP atmospheres. We discuss the observational results of Tau Boo b by Cameron et al. (1999) and Charbonneau et al. (1999) in context of our model results. The predictions - the shape and magnitude of the light curves and polarization curves - are highly dependent on the size and type of condensates present in the planetary atmosphere.Comment: 33 pages, accepted by Ap

Dust Condensation in Evolving Discs and the Composition of Planetary Building Blocks

Partial condensation of dust from the Solar nebula is likely responsible for the diverse chemical compositions of chondrites and rocky planets/planetesimals in the inner Solar system. We present a forward physical–chemical model of a protoplanetary disc to predict the chemical compositions of planetary building blocks that may form from such a disc. Our model includes the physical evolution of the disc and the condensation, partial advection, and decoupling of the dust within it. The chemical composition of the condensate changes with time and radius. We compare the results of two dust condensation models: one where an element condenses when the mid-plane temperature in the disc is lower than the 50 per cent condensation temperature (⁠T50T50⁠) of that element and the other where the condensation of the dust is calculated by a Gibbs free energy minimization technique assuming chemical equilibrium at local disc temperature and pressure. The results of two models are generally consistent with some systematic differences of ∼10 per cent depending upon the radial distance and an element’s condensation temperature. Both models predict compositions similar to CM, CO, and CV chondrites provided that the decoupling time-scale of the dust is of the order of the evolution time-scale of the disc or longer. If the decoupling time-scale is too short, the composition deviates significantly from the measured values. These models may contribute to our understanding of the chemical compositions of chondrites, and ultimately the terrestrial planets in the Solar system, and may constrain the potential chemical compositions of rocky exoplanets

The Formation of Wassonite: A New Titanium Monosulfide Mineral in the Yamato 691 Enstatite Chondrite

Wassonite, ideally stoichiometric TiS, is a titanium monosulfide not previously observed in nature, that was discovered within the Yamato 691 EH3 enstatite chondrite [1]. Because of the submicrometer size of the wassonite grains, it was not possible to determine conventional macroscopic properties. However, the chemical composition and crystal structure were well constrained by extensive quantitative energy dispersive x-ray analysis and electron diffraction using transmission electron microscopy (TEM). The crystal system for wassonite is rhombohedral (a = 3.42 plus or minus 0.07, c = 26.50 plus or minus 0.53 Angstroms) with space group: R(sup 3 raised bar) m (R9 type), cell volume: 268.4 plus or minus 0.53 Angstroms(sup 3), Z=9, density (calculated): 4.452 grams per cubic centimeter, empirical formula: (Ti(sub 0.93), Fe(sub 0.06), Cr(sub 0.01))S. In this study, we discuss possible formation mechanisms of wassonite and its associated minerals based on the petrology, mineralogy, crystallography, thermodynamic calculations, Al/Mg isotopic systematics and the O-isotopic composition of the wassonite-bearing BO chondrule

Evolution of oxygen isotopic composition in the inner solar nebula

Changes in the chemical and isotopic composition of the solar nebula with time are reflected in the properties of different constituents that are preserved in chondritic meteorites. CR carbonaceous chondrites are among the most primitive of all chondrite types and must have preserved solar nebula records largely unchanged. We have analyzed the oxygen and magnesium isotopes in a range of the CR constituents of different formation temperatures and ages, including refractory inclusions and chondrules of various types. The results provide new constraints on the time variation of the oxygen isotopic composition of the inner (<5 AU) solar nebula - the region where refractory inclusions and chondrules most likely formed. A chronology based on the decay of short-lived 26Al (t1/2 ~ 0.73 Ma) indicates that the inner solar nebula gas was 16O-rich when refractory inclusions formed, but less than 0.8 Ma later, gas in the inner solar nebula became 16O-poor and this state persisted at least until CR chondrules formed ~1-2 Myr later. We suggest that the inner solar nebula became 16O-poor because meter-size icy bodies, which were enriched in 17,18O due to isotopic self-shielding during the ultraviolet photo dissociation of CO in the protosolar molecular cloud or protoplanetary disk, agglomerated outside the snowline, drifted rapidly towards the Sun, and evaporated at the snowline. This led to significant enrichment in 16O-depleted water, which then spread through the inner solar system. Astronomical studies of the spatial and/or temporal variations of water abundance in protoplanetary disks may clarify these processes.Comment: 27 pages, 5 figure