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Chiang E. I. Lovering J. R. Millis R. L. Buie M. W. Wasserman L. H. Meech K. J. 《Earth, Moon, and Planets》2003,92(1-4):49-62
We review ongoing efforts to identify occupants of mean-motion resonances(MMRs) and collisional families in the Edgeworth–Kuiper belt. Directintegrations of trajectories of Kuiper belt objects (KBOs) reveal the 1:1(Trojan), 5:4, 4:3, 3:2 (Plutino), 5:3, 7:4, 9:5, 2:1 (Twotino), and 5:2 MMRsto be inhabited. Apart from the Trojan, resonant KBOs typically have largeorbital eccentricities and inclinations. The observed pattern of resonanceoccupation is consistent with resonant capture and adiabatic excitation bya migratory Neptune; however, the dynamically cold initial conditions priorto resonance sweeping that are typically assumed by migration simulationsare probably inadequate. Given the dynamically hot residents of the 5:2 MMRand the substantial inclinations observed in all exterior MMRs, a fraction ofthe primordial belt was likely dynamically pre-heated prior to resonancesweeping. A pre-heated population may have arisen as Neptune gravitationallyscattered objects into trans-Neptunian space. The spatial distribution of Twotinosoffers a unique diagnostic of Neptune's migration history. The Neptunian Trojanpopulation may rival the Jovian Trojan population, and the former's existence isargued to rule out violent orbital histories for Neptune. Finally, lowest-order seculartheory is applied to several hundred non-resonant KBOs with well-measured orbitsto update proposals of collisional families. No convincing family is detected. 相似文献
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The decay constant for spontaneous fission of natural uranium has been determined by accumulating fission fragment tracks in Lexan plastic held adjacent to uranium metal for one year. The value, 6.8 ± 0.6. 10−17 yr−1, is consistent with values most commonly used in fission track dating. 相似文献
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The abundances of U and Th in 19 achondrites and two pallasite olivines have been measured by radiochemical neutron activation analysis. Brecciated eucrites are enriched relative to chondrites in both elements by factors between 10 and 20, perhaps as a result of a magmatic differentiation process. Two unbrecciated eucrites are far less enriched, possibly due to their origin as igneous cumulates. The diogenites Johnstown and Shalka contain approximately chondritic levels of U and Th, but Ellemeet is 10 times lower. The abundances in three howardites are in good agreement with those expected from major element data for a mixing model with eucrite and diogenite end members. The high O18 basaltic achondrites Nakhla, Shergotty and Angra dos Reis have a range of U and Th abundances similar to the brecciated eucrites and howardites, but have systematically higher Th/U ratios. The Bishopville aubrite has U and Th abundances and Th/U ratios similar to those of several enstatite chondrites, suggesting a genetic relationship. The Norton County aubrite has a low Th/U, similar to that observed in recrystallized and metamorphosed terrestrial ultrabasic rocks, indicating a more complex history. Pallasite olivines have low U and Th contents (0.5.4 ppb and 1.4.3 ppb, respectively) similar to those in terrestrial dunites. The Goalpara ureilite has very low U (<0–6 ppb) and Th (2.7 ppb) abundance consistent with an origin from carbonaceous chondrites by partial melting. 相似文献
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Basic breccia-nephelinite pipes at Delegate (N.S.W., Australia) contain abundant two-pyroxene granulite, garnet granulite and fassaite eclogite inclusions and rare spinel pyroxenite, peridotite and charnockite inclusions.Petrographic, mineralogical and chemical data on the inclusions and their co-existing phases are consistent with the hypothesis that the fassaite eclogite, garnet granulite and spinel pyroxenite inclusions all crystallized or recrystallized in about the same temperature-pressure region, within the range 7–15 kb and 700–1200° C. This means that these particular inclusions were formed within the uppermost part of the mantle and/or the lowermost part of the crust.The two-pyroxene granulites may also have crystallized in the same region but there are some data which are indicative of crystallization at lower pressures within the crust. The charnockite inclusion is also considered to be of crustal origin. 相似文献
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Many Type B CaAl-rich inclusions (CAI's) in the Allende carbonaceous chondrite contain two types of spinel structures, “framboids” and “palisades.” Framboids are clumps of spinel grains generally <100 μm across. Experimental studies (Wark and Lovering, 1982) show that they probably formed in situ by solid state growth processes. Palisades are texturally different and consist of ovoid shells of spinel grains that appear in thin sections as long arcs or rings with diameters ranging from ～50 μm up to 2 cm. No in situ formation process seems able to explain the variety of sizes and morphologies of palisades nor the different compositions and textures of the enclosed and enclosing materials. We therefore suggest that palisades are the spinel rims of smaller, earlier-formed Type B CAI's that were incorporated into other CAI material in various ways—by capture into liquid drops, by solid condensate overgrowths and by the partial melting or welding of agglomerates containing the bodies. As some Type B bodies have been found inside Type A host material it appears that in at least some regions, and probably generally, Type A CAI's formed after Type B. We propose that Type B CAI's are residues from the heating and incomplete evaporation of interstellar dust during the accretion of the protosolar nebula, and that Type A CAI's are later condensates from completely evaporated dust whose exotic 16O-rich component became more diluted in the gas phase. 相似文献
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The Type B CaAl-rich inclusions in the Allende carbonaceous chondrite form a continuous range from the mineralogically concentrically zoned B1 subtype to the unzoned B2 subtype. These subtypes differ in (i) structure, texture, grain size and shape, (ii) mineralogical proportions and compositions, (iii) accessory mineralogy, (iv) relative abundance of spinel framboids, (v) rim layering, (vi) major element chemistry, and (vii) degree of secondary alteration.These differences, together with observations on the crystallization of synthetic melts, suggest that the B1 inclusions crystallised relatively rapidly from molten parental material while B2 types crystallised relatively slowly close to the solidus from material that had not been completely melted.The same data are used to construct an evaporative residue model for the origin of the parental Type B materials. In the model, dust in the protosolar nebula was heated with removal of more volatile elements, leaving completely melted (Type B1) residues at the highest temperatures and incompletely melted, less highly evaporated (Type B2) residues at lower temperatures. 相似文献
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The efficacy of sagebrush (Artemisia) as a biogeochemical indicator of base-metal mineralization in stratabound Precambrian ore deposits in west-central Colorado was investigated by collecting new (mostly flowering) growth from several sagebrush shrubs over and near five such deposits in three different areas. These are the Sedalia mine and two mines in the Turret district near Salida, and two mines in the Cochetopa district southeast of Gunnison. Two species were used, A. tridentata and A. frigida, depending on the area. Sagebrush clippings were separated into two subsamples consisting of (1) stems, and (2) leaves and blossoms stripped from the stems. These subsamples were ashed separately and the ash analyzed with an emission spectrograph for 30 elements. There appear to be no appreciable differences in the analyses of the two subsamples, indicating that composite samples would provide adequate information for further investigations. Eight of these elements, Ag, Bi, Cu, Pb, Sn, Y, Zn, and Zr, are present in notably higher concentrations in the ash of samples growing over mineralized ground than in that of control samples growing over barren ground. Although the distribution pattern, and the number of these anomalous elements, differs at each of the five localities, three of them, silver, copper, and lead, show good contrast and close association with subjacent mineralization in all five study areas. 相似文献
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Fractionation of tholeiitic magma in the Red Hill intrusion produced a gradational series of rocks ranging from dolerite to granophyre (McDougall, 1962). Granophyres are enriched in Fe, Si and alkalies, and impoverished in Mg, Ca and Al. With fractionation the magma was depleted rapidly in Cr and Ni owing to their removal in early crystallizing pyroxene and iron oxides. Cobalt decreases gradually from chilled dolerite to silicic dolerite, followed by a significant maximum in the most Fe‐enriched rocks, and finally decreases markedly in the granophyres. Cobalt follows Fe2+ closely and shows no obvious relationship with Mg. Copper was progressively enriched in the magma during the main stages of fractionation until precipitation of sulphide occurred, which caused impoverishment of Cu in the final liquid. Copper also is present in the silicates, it substitutes for Na in the feldspars and Fe2+ in pyroxenes and iron oxides. 相似文献