Isotopic records in CM hibonites: Implications for timescales of mixing of isotope reservoirs in the solar nebula   总被引：1，自引：0，他引：1  

Ming-Chang Liu  Kevin D. McKeegan  Kuljeet K. Marhas  Trevor R. Ireland 《Geochimica et cosmochimica acta》2009,73(17):5051-1532

The magnesium isotopic compositions of 26 hibonite-bearing inclusions from the CM chondrite Murchison, as well as isotopic measurements on a subset of these samples for oxygen, titanium, and lithium-beryllium-boron are reported along with oxygen isotopic data for an additional 13 hibonites that were previously investigated for other isotope systems (magnesium, potassium, calcium, and titanium) and rare earth element concentrations. Magnesium isotopic compositions divide CM hibonites into two distinct populations which correlate perfectly with their mineralogy and morphology, as previously discovered by Ireland [Ireland T. R. (1988) Correlated morphological, chemical, and isotopic characteristics of hibonites from the Murchison carbonaceous chondrite. Geochim. Cosmochim. Acta52, 2827-2839]: Spinel-HIBonite spherules (SHIBs) bear evidence of in situ²⁶Al decay, whereas PLAty-Crystals (PLACs) and Blue AGgregates (BAGs) either lack resolvable ²⁶Mg-excesses or exhibit ²⁶Mg deficits by up to ∼4‰. High precision, multiple collector SIMS analyses show that 6 of 7 SHIBs investigated fall on a single correlation line implying ²⁶Al/²⁷Al = (4.5 ± 0.2) × 10⁻⁵ at the time of isotopic closure, consistent with the “canonical” ²⁶Al abundance characteristic of internal isochrons in many calcium-aluminum-rich inclusions (CAIs). One SHIB sample exhibits Δ²⁶Mg^∗ consistent with a “supracanonical” ²⁶Al/²⁷Al ratio of (6.4 ± 0.5) × 10⁻⁵. The PLAC hibonites contain highly anomalous titanium isotopic compositions, with δ⁵⁰Ti values ranging from −80‰ to almost +200‰, whereas SHIBs generally lack large Ti isotopic anomalies.Eight out of 11 ²⁶Al-free PLAC hibonite grains record ¹⁰B/¹¹B excesses that correlate with Be/B; the inferred initial ¹⁰Be/⁹Be ratio of (5.1 ± 1.4) × 10⁻⁴ is lower than the best-constrained ¹⁰Be/⁹Be of (8.8 ± 0.6) × 10⁻⁴ in a CV CAI. The data demonstrate that ¹⁰Be cannot be used as a relative chronometer for these objects and that most of the ¹⁰Be observed in CAIs must be produced by irradiation of precursor solids in the early solar system. The lack of ²⁶Al in PLAC hibonites indicates that significant amounts of ²⁶Al were not formed in the same spallogenic processes that made ¹⁰Be in PLAC precursors. This is most easily understood as indicating very early formation of the PLAC hibonites, prior to the incorporation and mixing of ²⁶Al into the solar nebula, although an alternative scenario, which invokes irradiation under different solar flare conditions, cannot be ruled out. Lithium isotopes are normal within uncertainties, probably reflecting contamination and/or postcrystallization exchange.The oxygen isotopic compositions of SHIBs and PLACs are all highly ¹⁶O-enriched, but are not derived from a homogeneous reservoir: Δ¹⁷O values span a range of ∼−28‰ to −15‰. The ranges of ¹⁶O-enrichment in SHIBs and PLACs overlap and are less “anomalous” than the most ¹⁶O-enriched compositions found in meteorites [Kobayashi S., Imai H. and Yurimoto H. (2003) New extreme ¹⁶O-rich chondrule in the early solar system. Geochem. J.37, 663-669]. Both PLACs and SHIBs formed in ¹⁶O-enriched reservoirs characterized by small-scale heterogeneities in the gas phase. If such heterogeneities were generated by an admixture of relatively ¹⁶O-poor gas created by self-shielding during CO photolysis and transported to the hot inner regions of the accretion disk, then this process must have been initiated very early on, prior to the arrival of fresh radioactivity into the inner solar system. Oxygen isotope heterogeneities persisted throughout the formation interval of PLACs, CAI precursors, and SHIBs which could be as long as 3 × 10⁵ years based on ²⁶Al records.One SHIB and one BAG exhibit mass fractionated oxygen isotopic compositions similar to those seen in FUN inclusions and in several platy hibonite crystals [Lee T., Mayeda T. K. and Clayton R. N. (1980) Oxygen isotopic anomalies in Allende inclusion HAL. Geophys. Res. Lett.7, 493-496; Ireland T. R., Zinner E. K., Fahey A. J. and Esat T. M. (1992) Evidence for distillation in the formation of HAL and related hibonite inclusions. Geochim. Cosmochim. Acta56, 2503-2520; Ushikubo T., Hiyagon H. and Sugiura N. (2007) A FUN-like hibonite inclusion with a large ²⁶Mg-excess. Earth Planet. Sci. Lett.254, 115-126]. The suite of mass-fractionated hibonites exhibit a range of isotopic properties, including ²⁶Al/²⁷Al ratios from below detection to a “canonical” level and oxygen and titanium anomalies that are not exceptional by PLAC standards. This suggests that F (fractionation) processes—evaporation under (oxidizing) conditions—are not necessarily associated with sampling a special isotopic reservoir.  相似文献   

Cosmogenic 21Ne analysis of individual detrital grains: Opportunities and limitations     

Alexandru T. Codilean  Paul Bishop  Trevor B. Hoey  Finlay M. Stuart  Derek Fabel 《地球表面变化过程与地形》2010,35(1):16-27

We use a numerical model describing cosmogenic nuclide acquisition in sediment moving through the upper Gaub River catchment to evaluate the extent to which aspects of source area geomorphology and geomorphological processes can be inferred from frequency distributions of cosmogenic ²¹Ne (²¹Ne_c) concentrations in individual detrital grains. The numerical model predicts the pathways of sediment grains from their source to the outlet of the catchment and calculates the total ²¹Ne_c concentration that each grain acquires along its pathway. The model fully accounts for variations in nuclide production due to changes in latitude, altitude and topographic shielding and allows for spatially variable erosion and sediment transport rates. Model results show that the form of the frequency distribution of ²¹Ne_c concentrations in exported sediment is sensitive to the range and spatial distribution of processes operating in the sediment's source areas and that this distribution can be used to infer the range and spatial distribution of erosion rates that characterise the catchment. The results also show that lithology can affect the form of the ²¹Ne_c concentration distribution indirectly by exerting control on the spatial pattern of denudation in a catchment. Model results further indicate that the form of the distribution of ²¹Ne_c concentrations in the exported sediment can also be affected by the acquisition of ²¹Ne_c after detachment from bedrock, in the diffusive (hillslope) and/or advective (fluvial) domains. However, for such post‐detachment nuclide acquisition to be important, this effect needs to at least equal the nuclide acquisition prior to detachment from bedrock. Copyright © 2009 John Wiley and Sons, Ltd.  相似文献   

Temperature and abundance profiles of hot gas in galaxy groups – I. Results and statistical analysis     

Jesper Rasmussen  Trevor J. Ponman 《Monthly notices of the Royal Astronomical Society》2007,380(4):1554-1572

  相似文献   

The mass assembly of fossil groups of galaxies in the Millennium simulation     

Ali Dariush  Habib G. Khosroshahi  Trevor J. Ponman  Frazer Pearce  Somak Raychaudhury  Will Hartley 《Monthly notices of the Royal Astronomical Society》2007,382(1):433-442

  相似文献   

The Group Evolution Multiwavelength Study (GEMS): bimodal luminosity functions in galaxy groups     

Trevor A. Miles  Somak Raychaudhury  Duncan A. Forbes  Paul Goudfrooij  Trevor J. Ponman  Vera Kozhurina-Platais 《Monthly notices of the Royal Astronomical Society》2004,355(3):785-793

  相似文献   

MUSEUMS: New geology gallery at Leicester     

Trevor D. Ford 《Geology Today》1994,10(5):170-171

  相似文献   

The diopside-kyanite joint at high pressures and temperatures     

Trevor H. Green 《Lithos》1969,2(4):333-341

  相似文献   

The tides of the planet earth : 2nd Edition, by Paul Melchior, Pergamon Press, 1983, 648pp., 142 illus, 3467 lit refs. £45.00 US$90 — ISBN 0 08 026248 1 (Hardcover)     

Trevor Baker 《Progress in Oceanography》1983,12(4):i-ii

  相似文献   

An invasion of the North Sea by blue-mouth, Helicolenus dactylopterus (Pisces, Scorpaenidae)     

Heessen  Henk J. L.; Hislop  John R. G.; Boon  Trevor W. 《ICES Journal of Marine Science》1996,53(5):874-877

  相似文献   

What causes the adiabatic lapse rate?     

Trevor J McDougall  Rainer Feistel 《Deep Sea Research Part I: Oceanographic Research Papers》2003,50(12):1523-1535

The adiabatic lapse rate is commonly explained as being proportional to the work done on a fluid parcel as its volume changes in response to an increase in pressure. According to this explanation the adiabatic lapse rate would increase with both pressure and the fluid's compressibility, but this is not the case. Rather, the adiabatic lapse rate is proportional to the thermal expansion coefficient and is independent of the fluid's compressibility. Here we show that the adiabatic lapse rate is independent of the increase in the internal energy that a parcel experiences when it is compressed. We explain what is missing from the traditional explanation of the adiabatic lapse rate and we explore the thermodynamic cause of the adiabatic lapse rate. In particular, we seek to explain how the adiabatic lapse rate can be negative in cool fresh water.  相似文献