The metamorphic history of eucrites and eucrite-related meteorites and the case for late metamorphism     

DEREK W. G. SEARS  STEVEN J. K. SYMES  J. DAVID BATCHELOR  D. GLEN AKRIDGE  PAUL H. BENOIT 《Meteoritics & planetary science》1997,32(6):917-927

Abstract— We report induced thermoluminescence (TL) data for separates from three howardite, eucrite and diogenite (HED) meteorites and the Vaca Muerta mesosiderite. The results of thermal modeling of the surface of their parent body are also described. The TL sensitivities for matrix samples from the LEW 85300, 302 and 303 paired eucrites and the Bholghati howardite are lower than the TL sensitivities for the clasts, which is consistent with regolith working of the matrix in fairly mature regoliths. Within an isochemical series of HED meteorites, TL sensitivity reflects metamorphic intensity, but clast-to-clast variations in the TL sensitivities of the Vaca Muerta mesosiderite and clasts in the EET 87509, 513 and 531 paired howardite primarily reflect differences in mineralogy and petrology. Thermoluminescence peak temperatures indicate that all the components from the LEW 85300, 302 and 303 paired eucrites experienced a reheating event involving temperatures >800 °C, which is thought to have been due to impact heating, and therefore that the event was concurrent with or postdated brecciation. The Vaca Muerta clasts are essentially unmetamorphosed, but the induced TL data indicate that the remaining howardite, eucrite, dioenite and mesosiderite (HEDM) meteorites experienced metamorphism to a variety of intensities but involving temperatures <800 °C. Laboratory heating experiments show that temperatures >800 °C cause a change in TL peak temperature. Feldspars from a variety of terrestrial and extraterrestrial sources show this behavior, and x-ray diffraction and kinetic studies suggest that it is indirectly related to Al, Si disordering. Cooling rates are not consistent with autometamorphism following the initial igneous event or with heating by subsequent eruptions of lava onto the surface of the HED parent body. Instead, our thermal models suggest that the metamorphism occurred within a regolith ejecta blanket of up to a few kilometers thick, with different levels of metamorphism corresponding to different thicknesses of blanket, between essentially 0 and ～2 km, rather than different burial depths in a regolith of uniform thickness. We argue that metamorphism occurred 3.9 Ga ago and was associated with the resetting of the Ar-Ar system for the HED meteorites.  相似文献   

Census of breeding seabirds on the northwest coast of Svalbard 1973 and 1978     

CHRISTIAN KEMPF  BENOIT SITTLER 《Polar research》1988,6(2):195-203

The western coast of Svalbard is one of the world's most important seabird regions (Belopol'skij 1961; Løvenskiold 1964; Norderhaug et al. 1977), due to the favourable water temperatures, light regime and amounts of mineral salts (Norderhaug et al. 1977).
Seabirds have been censused several times in this area (Kristoffersen 1962; Larsen 1965; Dhondt et al.1969; Voisin 1970; Norderhaug 1974; Sendstad 1978; Alendal et al. 1982).
Except for Larsen (1965), there is no comprehensive and quantitative survey of any part of northwestern Svalbard. Further east, Jepsen & Mobæk (1983) surveyed the area between Gråhuken and Nordaustìandet.
Recent concern about the potentially detrimental effects of planned oil exploration and increased human activities in the high Arctic has emphasized the need for more information on the ecology in these regions. This paper provides more comprehensive data on seabird populations in northwestern Svalbard, between southern Prins Karls Forland and Verlegenhuken. The results supplement the studies carried out in 1978 and 1979 by Jepsen & Mobæk (1983) between Verlegenhuken and Kong Karls Land.  相似文献   

Evidence for low‐temperature growth of fayalite and hedenbergite in MacAlpine Hills 88107, an ungrouped carbonaceous chondrite related to the CM‐CO clan     

Alexander N. KROT  Adrian J. BREARLEY  Michael I. PETAEV  Gregory W. KALLEMEYN  Derek W. G. SEARS  Paul H. BENOIT  Ian D. HUTCHEON  Michael E. ZOLENSKY  Klaus KEIL 《Meteoritics & planetary science》2000,35(6):1365-1386

Abstract— The carbonaceous chondrite MacAlpine Hills (MAC) 88107 has bulk composition and mineralogy that are intermediate between those of CO and CM chondrites. This meteorite experienced minor alteration and a low degree of thermal metamorphism (petrologic type 3.1) and escaped post‐accretional brecciation. The alteration resulted in the formation of fayalite (Fa_90–100). Al‐free hedenbergite (～Fs₅₀Wo₅₀), phyllosilicates (saponite‐serpentine intergrowths), magnetite, and Ni‐bearing sulfides (pyrrhotite and pentlandite). Fayalite and hedenbergite typically occur as veins, which start at the opaque nodules in the chondrule peripheries, crosscut fine‐grained rims and either terminate at the boundaries with the neighboring fine‐grained rims or continue as layers between these rims. These observations suggest that fayalite and hedenbergite crystallized after accretion and compaction of the fine‐grained rims. Fayalite also overgrows isolated forsteritic (Fa_1–5) and fayalitic (Fa_20–40) olivine grains without any evidence for Fe‐Mg interdiffusion; it also replaces massive magnetite‐sulfide grains. The initial ⁵³Mn/⁵⁵Mn ratio of (1.58 ± 0.26) × 10^?6 in the MAC 88107 fayalite corresponds to an age difference between the formation of fayalite and refractory inclusions in Allende of either ～9 or 18 Ma, depending upon the value of the solar system initial abundance of ⁵³Mn used in age calculations. Formation of secondary fayalite and hedenbergite requires mobilization and transport of Ca, Si, and Fe either through a high‐temperature gaseous phase (Hua and Buseck, 1995) or low‐temperature aqueous solution (Krot et al., 1998a, b). The high‐temperature nebular model for the origin of fayalite (Hua and Buseck, 1995) fails to explain (a) formation of fayalite‐hedenbergite assemblages after accretion of fine‐grained rims that lack any evidence for high‐temperature processing; (b) extreme fractionation of refractory lithophile elements of similar volatility, Ca and Al, in hedenbergite; and (c) absence of Fe‐Mg interdiffusion along fayalite‐forsterite boundaries. We conclude that fayalite and hedenbergite in MAC 88107 formed during late‐stage, low‐temperature (approximately 150–200 °C) aqueous alteration. The data for MAC 88107 extend the evidence for an early onset of aqueous activity on chondrite parent bodies and reinforce the conclusion that liquid water played an important role in the chemical and mineralogical evolution of the first chondritic planetesimals.  相似文献   

Potential impact of climate change on marine dimethyl sulfide emissions   总被引：1，自引：0，他引：1  

LAURENT BOPP  OLIVIER AUMONT  SAUVEUR BELVISO  PATRICK MONFRAY 《Tellus. Series B, Chemical and physical meteorology》2003,55(1):11-22

Dimethyl sulfide (DMS) is a biogenic compound produced in sea-surface water and outgased to the atmosphere. Once in the atmosphere, DMS is a significant source of cloud condensation nuclei in the unpolluted marine atmosphere. It has been postulated that climate may be partly modulated by variations in DMS production through a DMS-cloud condensation nuclei-albedo feedback. We present here a modelled estimation of the response of DMS sea-water concentrations and DMS fluxes to climate change, following previous work on marine DMS modeling ( Aumont et al., 2002 ) and on the global warming impact on marine biology ( Bopp et al., 2001 ). An atmosphere–ocean general circulation model (GCM) was coupled to a marine biogeochemical scheme and used without flux correction to simulate climate response to increased greenhouse gases (a 1% increase per year in atmospheric CO₂ until it has doubled). The predicted global distribution of DMS at  1 × CO₂  compares reasonably well with observations; however, in the high latitudes, very elevated concentrations of DMS due to spring and summer blooms of Phaeocystis can not be reproduced. At  2 × CO₂  , the model estimates a small increase of global DMS flux to the atmosphere (+2%) but with large spatial heterogeneities (from −15% to +30% for the zonal mean). Mechanisms affecting DMS fluxes are changes in (1) marine biological productivity, (2) relative abundance of phytoplankton species and (3) wind intensity. The mean DMS flux perturbation we simulate represents a small negative feedback on global warming; however, the large regional changes may significantly impact regional temperature and precipitation patterns.  相似文献