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S. TACHIBANA H. NAGAHARA S. MOSTEFAOUI N. T. KITA 《Meteoritics & planetary science》2003,38(6):939-962
Abstract— We have studied the relationship between bulk chemical compositions and relative formation ages inferred from the initial 26Al/27Al ratios for sixteen ferromagnesian chondrules in least equilibrated ordinary chondrites, Semarkona (LL3.0) and Bishunpur (LL3.1). The initial 26Al/27Al ratios of these chondrules were obtained by Kita et al. (2000) and Mostefaoui et al. (2002), corresponding to relative ages from 0.7 ± 0.2 to 2.4 ?0.4/+0.7 Myr after calcium‐aluminum‐rich inclusions (CAIs), by assuming a homogeneous distribution of 26Al in the early solar system. The measured bulk compositions of the chondrules cover the compositional range of ferromagnesian chondrules reported in the literature and, thus, the chondrules in this study are regarded as representatives of ferromagnesian chondrules. The relative ages of the chondrules appear to correlate with bulk abundances of Si and the volatile elements (Na, K, Mn, and Cr), but there seems to exist no correlation of relative ages neither with Fe nor with refractory elements. Younger chondrules tend to be richer in Si and volatile elements. Our result supports the result of Mostefaoui et al. (2002) who suggested that pyroxene‐rich chondrules are younger than olivine‐rich ones. The correlation provides an important constraint on chondrule formation in the early solar system. It is explained by chondrule formation in an open system, where silicon and volatile elements evaporated from chondrule melts during chondrule formation and recondensed as chondrule precursors of the next generation. 相似文献
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Despite substantial rural-urban migration, 38.1 percent of the population of Poland still live in the countryside and the
total numbers are only 0.9mln fewer than in 1946. At present the rural population is increasing and the rate of natural increase
(though falling) is higher than in the towns though the productive age group is smaller. However, these human resources are
not being properly used and the results can be seen in terms of economic inefficiency, low living standards and a high level
of state support. Loss of jobs in factories and state farms results in welfare payments by the state and considerable 'hidden
unemployment' on small family farms where profitability is constrained by inefficient use of labour. Poverty among the rural
population is evident through a collapse of house building and considerable domestic overcrowding. However, conditions vary
considerably across the country and this must be reflected in the economic and social policies drawn up for the new administrative
regions introduced in 1999.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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Summary Smoothing time series of measurements is usually employed to remove high frequency noise when it is necessary to interpolate
them for dates where no measurements are available, and also in an attempt to free the series from their short-term instability
in order to provide an improved representation of the measured quantity.
Two methods are currently used in connection with the Earth's rotation studies: Vondrak's algorithm and a moving average with
a Gaussian weighting function [Gaussian smoothing). This study describes their behaviour in terms of frequency filtering and
of the response in the time domain. Examples of their application to the Earth Rotation Parameters are given. 相似文献
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Daisuke NAKASHIMA Takayuki USHIKUBO Rudraswami N. GOWDA Noriko T. KITA John W. VALLEY Keisuke NAGAO 《Meteoritics & planetary science》2011,46(6):857-874
Abstract– The ion microprobe is the only technique capable of determining high‐precision stable isotope ratios in individual tiny extraterrestrial particles (≤100 μm in diameter), but these small samples present special analytical challenges. We produced a new sample holder disk with multiple holes (three holes and seven holes), in which epoxy disks containing a single unknown sample and a standard grain are cast and polished. Performance tests for oxygen two‐isotope analyses using San Carlos olivine standard grains show that the new multiple‐hole disks allow accurate analysis of tiny particles if the particles are located within the 500 μm and 1 mm radius of the center of holes for seven‐hole and three‐hole disks, respectively. Using the new seven‐hole disk, oxygen three‐isotope ratios of eight magnesian cryptocrystalline chondrules (approximately 100 μm in diameter) from the Sayh al Uhaymir (SaU) 290 CH chondrite were analyzed by ion microprobe at the University of Wisconsin. Five out of eight chondrules have nearly identical oxygen isotope ratios (Δ17O = ?2.2 ± 0.6‰; 2SD), which is consistent with those of magnesian cryptocrystalline chondrules in CH/CB and CB chondrites, suggesting a genetic relationship, i.e., formation by a common (possibly impact) heating event. The other three chondrules have distinct oxygen isotope ratios (Δ17O values from ?6.4‰ to +2.2‰). Given that similar variation in Δ17O values was observed in type I porphyritic chondrules in a CH/CB chondrite, the three chondrules may have formed in the solar nebula, similar to the type I porphyritic chondrules. 相似文献
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Cyrena Anne GOODRICH Noriko T. KITA Michael J. SPICUZZA John W. VALLEY Jutta ZIPFEL Takashi MIKOUCHI Masamichi MIYAMOTO 《Meteoritics & planetary science》2010,45(12):1906-1928
Abstract– The Northwest Africa (NWA) 1500 meteorite is an olivine‐rich achondrite containing approximately 2–3 vol% augite, 1–2 vol% plagioclase, 1 vol% chromite, and minor orthopyroxene, Cl‐apatite, metal and sulfide. It was originally classified as a ureilite, but is currently ungrouped. We re‐examined the oxygen three‐isotope composition of NWA 1500. Results of ultra‐high precision (～0.03‰ for Δ17O) laser fluorination analyses of two bulk chips, and high precision (～0.3‰) secondary ion mass spectrometry (SIMS) analyses of olivine and plagioclase in a thin section, show that the oxygen isotope composition of NWA 1500 (Δ17O = ?0.22‰ from bulk samples and ?0.18 ± 0.06‰ from 16 mineral analyses) is within the range of brachinites. We compare petrologic and geochemical characteristics of NWA 1500 with those of brachinites and other olivine‐rich primitive achondrites, including new petrographic, mineral compositional and bulk compositional data for brachinites Hughes 026, Reid 013, NWA 5191, NWA 595, and Brachina. Modal mineral abundances, texture, olivine and pyroxene major and minor element compositions, plagioclase major element compositions, rare earth element abundances, and siderophile element abundances of NWA 1500 are within the range of those in brachinites and, in most cases, well distinguished from those of winonaites/IAB silicates, acapulcoites/lodranites, ureilites, and Divnoe. NWA 1500 shows evidence of internal reduction, in the form of reversely zoned olivine (Fo ～65–73 core to rim) and fine‐grained intergrowths of orthopyroxene + metal along olivine grain margins. The latter also occur in Reid 013, Hughes 026, NWA 5191, and NWA 595. We argue that reduction (olivine→enstatite + Fe0 + O2) is the best hypothesis for their origin in these samples as well. We suggest that NWA 1500 should be classified as a brachinite, which has implications for the petrogenesis of brachinites. Fe‐Mn‐Mg compositions of brachinite olivine provide evidence of redox processes among bulk samples. NWA 1500 provides evidence for redox processes on a smaller scale as well, which supports the interpretation that these processes occurred in a parent body setting. SIMS data for 26Al‐26Mg isotopes in plagioclase in NWA 1500 show no 26Mg excesses beyond analytical uncertainties (1–2‰). The calculated upper limit for the initial 26Al/27Al ratio of the plagioclase corresponds to an age younger than 7 Ma after CAI. Compared to 53Mn‐53Cr data for Brachina ( Wadhwa et al. 1998b ), this implies either a much younger formation age or a more protracted cooling history. However, Brachina is atypical and this comparison may not extend to other brachinites. 相似文献
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Abstract– We review the 26Al ages of chondrules in various type 3.0 chondrites. The 26Al ages of chondrules are 1–3 Myr after calcium‐aluminum‐rich inclusion (CAI) for LL3.0, CO3.0, and Acfer 094 (Ungrouped C 3.0). Available data for chondrules in CR chondrites indicate that many chondrules are relatively younger (≥3 Myr), although data from chondrules in CR3.0 are not yet available to confirm their younger ages. The total ranges for the 26Al ages of chondrules in a single chondrite group are more than 0.5–1 Myr. However, most chondrules show relatively narrow range of ages in a single chondrite group (0.2–0.4 Myr, 1 SD), which might be short enough to preserve the group‐specific chemical and isotope signatures against radial diffusion of solid in the disk. Distinct oxygen isotope reservoirs might exist in the protoplanetary disk simultaneously, which could be spatially separated. 相似文献
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P. J. LANCASTER B. FU F. Z. PAGE N. T. KITA M. E. BICKFORD B. M. HILL J. M. McLELLAND J. W. VALLEY 《Journal of Metamorphic Geology》2009,27(1):41-54
Oxygen isotope ratios and rare earth element (REE) concentrations provide independent tests of competing models of injection v. anatexis for the origin of migmatites from amphibolite and granulite facies metasedimentary rocks of the Adirondack Mountains, New York. Values of δ18O and REE profiles were measured by ion microprobe in garnet–zircon pairs from 10 sample localities. Prior U–Pb SIMS dating of zircon grains indicates that inherited cores (1.7–1.2 Ga) are surrounded by overgrowths crystallized during the Grenville orogenic cycle (～1.2–1.0 Ga). Cathodoluminescence imaging records three populations of zircon: (i) featureless rounded ‘whole grains’ (interpreted as metamorphic or anatectic), and rhythmically zoned (igneous) cores truncated by rims that are either (ii) discordant rhythmically zoned (igneous) or (iii) unzoned (metamorphic or anatectic). These textural interpretations are supported by geochronology and oxygen isotope analysis. In both the amphibolite facies NW Adirondacks and the granulite facies SE Adirondacks, δ18O(Zrc) values in overgrowths and whole zircon are highly variable for metamorphic zircon (6.1–13.4‰; n = 95, 10 μm spot). In contrast, garnet is typically unzoned and δ18O(Grt) values are constant at each locality, differing only between leucosomes and corresponding melanosomes. None of the analysed metamorphic zircon–garnet pairs attained oxygen isotope equilibrium, indicating that zircon rims and garnet are not coeval. Furthermore, REE profiles from zircon rims indicate zircon growth in all regions was prior to significant garnet growth. Thus, petrological estimates from garnet equilibria (e.g. P–T) cannot be associated uncritically with ages determined from zircon. The unusually high δ18O values (>10‰) in zircon overgrowths from leucocratic layers are distinctly different from associated metaigneous rocks (δ18O(Zrc) < 10‰) indicating that these leucosomes are not injected magmas derived from known igneous rocks. Surrounding melanosomes have similarly high δ18O(Zrc) values, suggesting that leucosomes are related to surrounding melanosomes, and that these migmatites formed by anatexis of high δ18O metasedimentary rocks. 相似文献
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Daisuke NAKASHIMA Takayuki USHIKUBO Michael E. ZOLENSKY Noriko T. KITA 《Meteoritics & planetary science》2012,47(2):197-208
Abstract– Oxygen three‐isotope ratios of three anhydrous chondritic interplanetary dust particles (IDPs) were analyzed using an ion microprobe with a 2 μm small beam. The three anhydrous IDPs show Δ17O values ranging from ?5‰ to +1‰, which overlap with those of ferromagnesian silicate particles from comet Wild 2 and anhydrous porous IDPs. For the first time, internal oxygen isotope heterogeneity was resolved in two IDPs at the level of a few per mil in Δ17O values. Anhydrous IDPs are loose aggregates of fine‐grained silicates (≤3 μm in this study), with only a few coarse‐grained silicates (2–20 μm in this study). On the other hand, Wild 2 particles analyzed so far show relatively coarse‐grained (≥ few μm) igneous textures. If anhydrous IDPs represent fine‐grained particles from comets, the similar Δ17O values between anhydrous IDPs and Wild 2 particles may imply that oxygen isotope ratios in cometary crystalline silicates are similar, independent of crystal sizes and their textures. The range of Δ17O values of the three anhydrous IDPs overlaps also with that of chondrules in carbonaceous chondrites, suggesting a genetic link between cometary dust particles (Wild 2 particles and most anhydrous IDPs) and carbonaceous chondrite chondrules. 相似文献