Lunar behaviour of boron contrasted with the terrestrial boron cycle     

Denis M. Shaw 《Meteoritics & planetary science》1995,30(2):199-208

Abstract— Fifty-eight lunar rocks have been analysed by prompt gamma neutron activation for B, Gd and Sm. The data were interpreted together with published analyses for Li and other elements. The behaviour of B in lunar rocks is virtually identical to that of Sm, Gd, Li and the other incompatible LIL (Large Ion Lithophile) and HFS (High Field Strength) elements, collectively known as the KREEP component. To a first approximation, the distribution of B concentrations in all lunar rocks can be described as a two-component mixture of KREEP, with ～32 ppm, and B-free mare basalts and anorthositic rocks. The lunar B abundance, estimated from correlations with other KREEP elements, is ～0.14 ppm. Alpha-track images of the analysed lunar rocks were made from thin sections specially prepared to avoid contamination from terrestrial B. They provided surprising evidence that the distributions of B and Li within individual rocks fall into four categories: (1) substitution in mineral structures; (2) in irregular minute particles, disseminated through the rock with no apparent connection with mineralogy; (3) as aggregate properties of rock fragments, acquired prior to assembly in a breccia; and, (4) as aggregate properties of a breccia matrix, distinct from unaffected clasts. Only category (1) is encountered in terrestrial rocks. Categories (2), (3) and (4) become progressively more important as the abundance of KREEP components increases, and it is concluded that the alpha-track images probably show the localisation of not only B and Li but all the KREEP elements. In terrestrial rocks, such textures could be understood in terms of successive episodes of permeation by solutions followed by brecciation, then cementation. It is difficult to adapt such a mechanism to a dry Moon, with brecciation imposed by impact melting. The linear correlation of lunar B and Gd contrasts strongly with the relationship seen in terrestrial rocks, where the presence of water creates a terrestrial B cycle quite different from that of Gd and other lunar incompatible elements.  相似文献   

A ∼6100 14C yr record of El Niño activity from the Galápagos Islands     

Melanie A. Riedinger  Miriam Steinitz-Kannan  William M. Last  Mark Brenner 《Journal of Paleolimnology》2002,27(1):1-7

Lithostratigrahic and mineralogic analyses of sediments from hypersaline Bainbridge Crater Lake, Galápagos Islands, provide evidence of past El Niño frequency and intensity. Laminated sediments indicate that at least 435 moderate to very strong El Niño events have occurred since 6100 ¹⁴C yr BP (7130 cal yr BP), and that frequency and intensity of events increased at about 3000 ¹⁴C yr BP (3100 cal yr BP). El Niño activity was present between 6100 and 4000 ¹⁴C yr BP (4600 cal yr BP) but infrequent. The Bainbridge record indicates that there has been considerable millennial-scale variability in El Niño since the mid-Holocene.  相似文献   

Large Magellanic Cloud Planetary Nebula Morphology: Probing Stellar Populations and Evolution     

Stanghellini L  Shaw RA  Balick B  Blades JC 《The Astrophysical journal》2000,534(2):L167-L171

Planetary nebulae (PNe) in the Large Magellanic Cloud (LMC) offer the unique opportunity to study both the population and evolution of low- and intermediate-mass stars, by means of the morphological type of the nebula. Using observations from our LMC PN morphological survey, and including images available in the Hubble Space Telescope Data Archive and published chemical abundances, we find that asymmetry in PNe is strongly correlated with a younger stellar population, as indicated by the abundance of elements that are unaltered by stellar evolution (Ne, Ar, and S). While similar results have been obtained for Galactic PNe, this is the first demonstration of the relationship for extragalactic PNe. We also examine the relation between morphology and abundance of the products of stellar evolution. We found that asymmetric PNe have higher nitrogen and lower carbon abundances than symmetric PNe. Our two main results are broadly consistent with the predictions of stellar evolution if the progenitors of asymmetric PNe have on average larger masses than the progenitors of symmetric PNe. The results bear on the question of formation mechanisms for asymmetric PNe-specifically, that the genesis of PNe structure should relate strongly to the population type, and by inference the mass, of the progenitor star and less strongly on whether the central star is a member of a close binary system.  相似文献   

Communicating academic geography—the continuing challenge     

Jon Shaw  Judith Matthews 《Area》1998,30(4):367-372

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The right to say: the development of youth councils/forums within the UK     

Hugh Matthews  Melanie Limb 《Area》1998,30(1):66-78

Summary Article 12 of the United Nations Convention on the Rights of the Child makes explicit reference to children's right to say what they think about matters relating to the quality of their lives, and to have those opinions taken into account in accordance with their levels of competence and maturity. Despite the commitment of the UK Government to this and other similar initiatives designed to empower young people, a culture of non-participation is endemic within the UK in the context of environmental planning. Young people are seemingly invisible on the landscape. This paper reviews the case for children's active involvement in their environmental future and considers attempts to engage young people through an incipient structure of youth councils and forums.  相似文献   

Magnetostratigraphy and palaeoclimatic significance of Late Tertiary aeolian sequences in the Chinese Loess Plateau   总被引：11，自引：0，他引：11  

Sun Donghuai  An Zhisheng  J. Shaw  J. Bloemendal  & Sun Youbin 《Geophysical Journal International》1998,134(1):207-212

The aeolian Red Clay sequence in the central part of the Chinese Loess Plateau was investigated in an attempt to obtain magnetostratigraphic and palaeoclimatic records. From the results, we deduce that aeolian dust accumulation and the related East Asia palaeomonsoon system began at least 7.6  Myr ago, and that the Tibetan Plateau had reached a significant elevation by that time. The Late Tertiary palaeoclimatic history of the Red Clay as reflected by magnetic susceptibility is reconstructed for the time period 7.6–2.5  Ma. Increases in aeolian dust accumulation for the last 7.6  Myr appear to have a close relation with the uplift processes of the Tibetan Plateau. The remarkable increase of aeolian dust accumulation beginning at 3.2  Ma appears to reflect the influence of an increasing global ice volume on the East Asian monsoon and aeolian dust accumulation.  相似文献   

Mechanisms of orthopyroxene dissolution in silica-undersaturated melts at 1 atmosphere and implications for the origin of silica-rich glass in mantle xenoliths   总被引：2，自引：0，他引：2  

Shaw  Cliff S. J.  Thibault  Yves  Edgar  Alan D.  Lloyd  Felicity E. 《Contributions to Mineralogy and Petrology》1998,132(4):354-370

Experiments dissolving orthopyroxene (En₉₃) in a variety of Si-undersaturated alkaline melts at 1 atmosphere and variable f _O2 demonstrate that orthopyroxene dissolves to form olivine, Si-rich melt and clinopyroxene. These phases form a texturally and chemically distinct boundary layer around the partly dissolved orthopyroxene crystals. The occurrence of clinopyroxene in the boundary layer is due to inward diffusion of Ca from the solvent melt to the boundary layer causing clinopyroxene saturation. Compositional profiles through the solvent and the boundary layer for a number of experiments demonstrate rapid diffusion of cations across the boundary layer – solvent interface. SiO₂ diffuses outward from the boundary layer whereas CaO and Al₂O₃ diffuse toward the Si-enriched boundary layer melt. The rate of Al diffusion is slower under reducing conditions compared to the rates in experiments performed in air. Concentrations of FeO and MgO in the boundary layer and solvent are approximately equal indicating rapid diffusion and attainment of equilibrium despite ongoing crystallisation of clinopyroxene within the boundary layer. The behaviour of Na₂O and K₂O is strongly affected by f _O2. Under reducing conditions Na₂O and K₂O concentrations are approximately equal in the boundary layer and solvent indicating normal diffusion down the concentration gradient and attainment of equilibrium. Under oxidising conditions, K₂O and to a lesser extent Na₂O, have compositional profiles indicative of uphill diffusion likely due to their preference for more polymerised Si- and Al-rich melts. Under reduced conditions Al-enrichment in the boundary layer melt is not as extreme and uphill diffusion did not occur. The composition of the solvent melt after the experiments indicates that it was contaminated by the boundary layer by convective mixing due to the onset of hydrodynamic instabilities brought on by density and viscosity contrasts between the two melts. Despite using a wide variety of solvent melt compositions we find that the boundary layer melts converge toward a common composition at high SiO₂ contents. The composition of glass generated by orthopyroxene dissolution at 1 atmosphere is similar in many respects to Si-rich glass found in many orthopyroxene-rich mantle xenoliths that have been attributed to high pressure in situ processes including mantle metasomatism. The results of this study suggest that at least some Si-rich melts are likely to have formed by dissolution of xenolith orthopyroxene at low pressure possibly by their Si-undersaturated host magmas. Received: 30 August 1996 / Accepted: 15 April 1998  相似文献   

Effects of core formation     

Geoge H. Shaw 《Physics of the Earth and Planetary Interiors》1978,16(4):361-369

Two sets of earth models have been generated which simulate the process of core formation in the earth. The results allow the evaluation of the distribution of energy release by core formation and thus an estimate of the early temperature profile following core formation. The process itself is likely to have taken 20 Ma or less, although a longer induction period during which some metal phase exists as a melt is probable. Much of the energy (half or more) ends up in a hot core with the mantle relatively cold. These results suggest the occurrence of mantle wide convection, leading to homogenization and the possible stripping of some elements from the mantle.  相似文献   

Charts of plagioclase optics: by S. Tsuboi,S. Mizutani,K. Suwa and Y. Tsuzuki. Iwanami Shoten Publishers,Tokyo, 1977, 188 pp., 7200 Yen     

Denis M. Shaw 《Geochimica et cosmochimica acta》1978,42(7):1071

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Core formation in terrestrial planets     

George H. Shaw 《Physics of the Earth and Planetary Interiors》1979,20(1):42-47

Calculations of the radial distribution of the energy released in core formation indicate that the cores of all the terrestrial planets may be expected to receive a disproportionate share of the gravitational energy released. Since the model of the process used in these calculations favors transfer of energy to the mantle, it is likely that other reasonable models of the process will result in even more energy being deposited in the cores of the early planets. The calculations also show that it is necessary for a certain amount of core phase to separate and accumulate, before the energy released by gravitational settling is sufficient to supply the latent heat of fusion of the core phase. The amount of melting required to reach this point varies according to the total mass of the planet, and mass fraction of core, but is not particularly great (<5% for the Earth to ～ 37% for the Moon). In the case of the Moon, this amount of segregation, although large, amounts to a surface layer about 260 km thick, similar to the proposed depth of early wholesale melting. Core separation in terrestrial planets appears to be a self-sustaining process even for fairly small bodies, provided that a small amount of a dense potential core phase is present. Although it seems likely to occur rapidly (within 10⁶–10⁷ years) even for small (Moon-size) bodies, detailed kinetic models will be necessary to specify the time scale.  相似文献