Determination of the petrologic type of CV3 chondrites by Raman spectroscopy of included organic matter

This paper reports the first reliable quantitative determination of the thermal metamorphism grade of a series of nine CV3 chondrites: Allende, Axtell, Bali, Mokoia, Grosnaja, Efremovka, Vigarano, Leoville, and Kaba. The maturity of the organic matter in matrix, determined by Raman spectroscopy, has been used as a powerful metamorphic tracer, independent of the mineralogical context and extent of aqueous alteration. This tracer has been used along with other metamorphic tracers such as Fe zoning in type-I chondrules of olivine phenocrysts, presolar grain abundance and noble gas abundance (bulk and P3 component). The study shows that the petrologic types determined earlier by Induced ThermoLuminescence were underestimated and suggests the following values: PT (Allende-Axtell) >3.6; PT (Bali-Mokoia-Grosnaja) ∼3.6; PT (Efremovka-Leoville-Vigarano) = 3.1-3.4; PT (Kaba) ∼3.1. The most commonly studied CV3, Allende, is also the most metamorphosed. Bali is a breccia containing clasts of different petrologic types. The attribution suggested by this study is that of clasts of the highest petrologic types, as pointed out by IOM maturity and noble gas bulk abundance. CV3 chondrites have complex asteroidal backgrounds, with various degrees of aqueous alteration and/or thermal metamorphism leading to complex mineralogical and petrologic patterns. (Fe,Mg) chemical zoning in olivine phenocrysts, on the borders of type I chondrules of porphyritic olivine- and pyroxene-rich textural types, has been found to correlate with the metamorphism grade. This suggests that chemical zoning in some chondrules, often interpreted as exchanges between chondrules and nebular gas, may well have an asteroidal origin. Furthermore, the compositional range of olivine matrix is controlled both by thermal metamorphism and aqueous alteration. This does not support evidence of a nebular origin and does not necessarily mirror the metamorphism grade through (Fe,Mg) equilibration. On the other hand, it may provide clues on the degree of aqueous alteration vs. thermal metamorphism and on the timing of both processes. In particular, Mokoia experienced significant aqueous alteration after the metamorphism peak, whereas Grosnaja, which has similar metamorphism grade, did not.     

A multi-site campaign on the long period variable subdwarf b star PG 1627+017

We report on the outcome of the first major multi-site campaign on a long-period variable subdwarf B star. The target PG 1627+017 was observed for a total of 334 h during April/May/June 2003 from the lynchpin observatory at Mt. Bigelow, Arizona, with important contributions coming from co-observations at SAAO, Calar Alto and Siding Spring. Preliminary analysis indicates the presence of 8–10 probable periods in the range ～4500 to ～8900 s with relative amplitudes between 0.1 and 0.5%;. The range over which the periods are found is in qualitative agreement with theoretical predictions and the number of periods found shows potential for an in-depth asteroseismological analysis. Multi-colour observations show that the relative amplitudes of pulsation are larger in the U-band than in the R, which again is in agreement with theory. However, more multi-colour observations are needed to be able to make statements at the quantitative level.     

Contribution à l'étude du spectre solaire de l'hydrogène

R'esumé Les raies et le rayonnement continu de l'hydrogène sont calculés, dans le cas solaire, en tenant compte des écarts à l'équilibre thermodynamique local.La comparaison des résultats, interprétés par la théorie de la formation des raies de Jefferies et Thomas, avec les observations donne des renseignements sur le modèle de l'atmosphère solaire.L'énergie émise par Ly dépend très fortement de la densité électronique. Seuls les modèles á palier de température dans la haute chromosphère donnent un profil de Ly à dépression centrale.La raie H, qui est contrôlée essentiellement pour les photoionisations dépend surtout du modèle de photosphère; toutefois les modèles de chromosphère à fortes température et densité électroniques ont une influence sur le profil de H.

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Summary The continuous and line spectrum of hydrogen emitted by the sun is computed, taking deviations of local thermal equilibrium into account. The theory of line formation as given by Jefferies and Thomas has been applied to the computations; a comparison of these results with the observations gives information on the solar atmospheric model.The energy emitted by Lyman has been computed for a two- and three-level atom: it appears to increase slightly with the number of levels. The energies, computed with the HAO model and the model of Coates, are larger than the observed values. The core of the Lyman profile is determined by collisions; the peaks are formed at a height where the electron temperature is about 20 000 °K; the energy depends very strongly on the electron density of the model.It is shown that the profile of Lyman only shows a central self reversal if the model of the high chromosphere has a temperature plateau.An interpretation of the observed distance of the peaks of the Lyman and Lyman line profiles is possible; it can also be shown why the distance of the Lyman peaks to the line centre is always of the order of 0.2 Å.The residual central intensity of H increases slightly with the number of atomic levels; the value computed with a five level atom, with the HAO chromospheric model, and the Utrecht photospheric model (1964) does not differ very much from the observed value: it is slightly smaller than the observed value and the computed profile is narrower than the observed profile.In the case of the HAO model the source function of H is dominated by the photo-ionization terms; nevertheless, the collision terms are not much smaller than the photo-ionization terms; the residual central intensity of H computed with a chromospheric model similar to the interspicular model of Athay and Thomas but assuming a higher electron temperature and density is larger than the observed value.

     

Description and first results of an explicit electrical scheme in a 3D cloud resolving model

The three-dimensional non-hydrostatic mesoscale model MésoNH of the French community offers the numerical environment to develop a cloud electrification scheme in a consistent way with the original mixed phase microphysical scheme. The charge separation mechanisms are entirely due to non-inductive processes and result from elastic ice–snow, ice–graupel and snow–graupel collisions. The electric charges carried by each of the five hydrometeor categories are transported along the airflow and are exchanged according to the various microphysical mass transfer rates but assuming a power law distribution of the individual charges as a function of the particle size. The electric field is diagnosed at each time step after integrating the electric potential induced by a net charge density in the Poisson equation. Finally, a lightning ash is triggered when the electric field locally steps over a given threshold. It propagates in two opposite directions until the magnitude of the electric field falls below a prescribed value. A fractal branching algorithm is then activated to extend lightning streamers away from the main channel and toward cloudy regions where substantial charge densities are present. Charges are neutralized along the tortuous lightning path with a simple procedure that preserves total charge conservation.The complete electrification scheme tested for an ideal case of vigorous supercellular storm shows an intense electrical activity all along its lifecycle. We show that the model is able to produce a direct tripolar structure of the charges as the result of a temperature charge reversal of − 10 °C and of the different sedimentation rates of the hydrometeors.     

A reappraisal of the metamorphic history of EH3 and EL3 enstatite chondrites

The thermal history of a series of EH3 and EL3 chondrites has been investigated by studying the degree of structural order of the organic matter (OM) located and characterized in matrix areas by Raman micro-spectroscopy. By comparison with unequilibrated ordinary chondrites (UOCs) and CO and CV carbonaceous chondrites, the following petrologic types have been assigned to various E chondrites: Sahara 97096 and Allan Hills 84206: 3.1-3.4; Allan Hills 85170 and Parsa: 3.5; Allan Hills 85119: 3.7; Qingzhen, MacAlpine Hills 88136 and MacAlpine Hills 88184: 3.6-3.7. The petrologic type of Qingzhen is consistent with the abundance of the P3 noble gas component, a sensitive tracer of the grade of thermal metamorphism. The petrologic types are qualitatively consistent with the abundance of fine-grained matrix for the whole series. No significant effects of shock processes on the structure of OM were observed. However such processes certainly compete with thermal metamorphism and the possibility of an effect cannot be fully discarded, in particular in the less metamorphosed objects. The OM precursors accreted by the EH3 and EL3 parent bodies appear to be fairly similar to those of UOCs and CO and CV carbonaceous chondrites. Raman data however show some slight structural differences that could be partly accounted for by shock processes. The metamorphic history of EH3 and EL3 chondrites has often been described as complex, in particular regarding the combined action of shock and thermal metamorphism. Because OM maturity is mostly controlled by the temperature of peak metamorphism, it is possible to distinguish between the contributions of long duration thermal processes and that of shock processes. Comparison of the petrologic types with the closure temperatures previously derived from opaque mineral assemblages has revealed that the thermal history of EH3 and EL3 chondrites is consistent with a simple asteroidal onion shell model. Thermal metamorphism in enstatite chondrites appears to be fairly similar to that which takes place in other chondrite classes. The complex features recorded by mineralogy and petrology and widely reported in the literature appear to be mostly controlled by shock processes.     

New U-Pb zircon and Ar/Ar muscovite age constraints on the emplacement of the Lizio syn-tectonic granite (Armorican Massif, France)

LA-ICP-MS U-Pb analyses performed on zircon grains from the Lizio granite yielded an emplacement age of 316 ± 6 Ma. Typical S-C structures show that the Lizio granite was emplaced contemporaneously with dextral shearing along the northern branch of the South Armorican Shear Zone and that it was therefore active at that time. ⁴⁰Ar/³⁹Ar analyses performed on muscovite grains yielded plateau dates ranging between 311.5 and 308.2 Ma. Muscovite chemistry is typical of primary magmatic muscovite, which precludes a late fluids-induced resetting of the K-Ar isotopic system. ⁴⁰Ar/³⁹Ar dates thus likely correspond to the cooling ages below the argon closure temperature. Considering the uncertainties on the measured ages, we can propose that either the Lizio granite cooled down quickly in less than a million of years or that it remained in a hot environment for several millions of years after its emplacement. This latter scenario could have been sustained by shear heating during dextral shearing along the northern branch of the South Armorican Shear Zone.     

Weathering of sandstone lotus petals at the Angkor site: a 1,000-year stone durability trial

This article is based on field investigations carried out at Ta Keo and Banteay Srei temples (Angkor, Cambodia), which have similar environmental histories and display the same recurrent decorative pattern (lotus petal) in three different sandstone lithotypes. Based on two original scales of mechanical and chemical weathering, the degree of deterioration of c.2000 sandstone petals was visually assessed, resulting in the proposal of a durability scale. An explanatory scheme was provided, based on laboratory analyses of stone materials (ICP-AES, XRD, SEM-EDS, polarizing microscopy and mercury injection). The key drivers of sandstone durability are both mineralogical and petrophysical. The remarkable preservation of the pink sandstone is due to the quartzose nature of its clasts, the non-swelling clay content of its abundant matrix (kaolinite) and its porosity structure (large connected pores and probable non-connected porosity). The overall good preservation of the green sandstone is due to its extremely low porosity and specific surface area, associated with its strong cementation. The severe deterioration of the grey sandstone is due to its laminated structure (oriented biotites), the susceptibility to hygric swelling and solution of its binding agents (chlorite/smectite and calcite) and its porosity structure, which evolves during a two-stage weathering sequence. The first stage is characterized by a bimodal porosity due to the partial clogging of macropores by swelling clays, and the second stage by a substantial increase in porosity and the penetration of salts (barite and anhydrite). Conversely, oxidation phenomena associated with weathering can lead to a drastic reduction of porosity.     

Trophic importance of diatoms in an intertidal Zostera noltii seagrass bed: Evidence from stable isotope and fatty acid analyses

A current predominant paradigm emphasizes the role of epiphytic algae for invertebrates in most seagrass food webs. However, in some intertidal Zostera noltii beds, epiphyte biomass is very low compared to microphytobenthos and seagrass biomasses. We assessed the role of microphytobenthos in a temperate intertidal Z. noltii bed by combining stable isotope and fatty acid (FA) analyses on primary producers, composite sources — suspended particulate organic matter (SPOM) and sediment surface organic matter (SSOM) — and the main macrofaunal consumers. Z. noltii showed high δ¹³C (−9.9‰) and high 18:2(n-6) and 18:3(n-3) contents. Microphytobenthos was slightly more ¹³C-depleted (−15.4‰) and had high levels of diatom markers: 14:0, 16:1(n-7)c, 20:5(n-3). Low mean δ¹³C (−22.0‰) and large amounts of diatom and bacteria (18:1(n-7)c) markers indicated that SPOM was mainly composed of a mixture of fresh and decayed pelagic diatoms. Higher mean δ¹³C (−17.9‰) and high amounts of diatom FAs were found in SSOM, showing that microphytobenthic diatoms dominate. Very low percentages of 18:2(n-6) and 18:3(n-3) in consumers indicated a low contribution of Z. noltii material to their diets. Grazers, deposit and suspension-deposit feeders had δ¹³C close to microphytobenthos and high levels of diatom FAs, confirming that microphytobenthos represented the main part of their diet. Lower δ¹³C and higher amounts of flagellate FAs – 22:6(n-3) and 16:4(n-3) – in suspension feeders indicated that their diet resulted from a mixture of SPOM and microphytobenthos. These results demonstrate that invertebrates do not consume high amounts of seagrass and highlight the main role of benthic diatoms in this intertidal seagrass bed.     

Tropical precipitation regimes and mechanisms of regime transitions: contrasting two aquaplanet general circulation models

The atmospheric general circulation models ARPEGE-climate and LMDz are used in an aquaplanet configuration to study the response of a zonally symmetric atmosphere to a range of sea surface temperature (SST) forcing. We impose zonally-symmetric SST distributions that are also symmetric about the equator, with varying off-equatorial SST gradients. In both models, we obtain the characteristic inter-tropical convergence zone (ITCZ) splitting that separates two regimes of equilibrium (in terms of precipitations): one with one ITCZ over the equator for large SST gradients in the tropics, and one with a double ITCZ for small tropical SST gradients. Transition between these regimes is mainly driven by changes in the low-level convergence that are forced by the SST gradients. Model-dependent, dry and moist feedbacks intervene to reinforce or weaken the effect of the SST forcing. In ARPEGE, dry advective processes reinforce the SST forcing, while a competition between sensible heat flux and convective cooling provides a complex feedback on the SST forcing in the LMDz. It is suggested that these feedbacks influence the location of the transition in the parameter range.     

High‐resolution record of environmental changes and tephrochronological markers of the Last Glacial–Holocene transition at Lake Lautrey (Jura,France)

This paper presents the results of a multiproxy investigation including volume magnetic susceptibility (κ), mineral and pollen analyses of Late Glacial sediments from Lake Lautrey (Jura, France). Small‐scale lithological variations have been identified with high stratigraphic resolution in order to establish lithostratigraphic correlations between cores. κ measurements, combined with mineralogical analyses, provide information on past sedimentary processes. This combined approach reflects major changes in terrestrial habitats and soil processes which may relate to the climatic events characterising the Late Glacial climatic warming and cooling phases. During warm intervals, the record indicates increased lake productivity via carbonate precipitation and decreased input of detrital material. In contrast, cooler intervals show reduced lake productivity, catchment area instability and increased detrital inputs. Several short interruptions in reforestation and in soil stabilisation can be identified and linked with abrupt colder events occurring through the Bølling. A general trend of warming is recorded from the coldest part of the Younger Dryas. Three tephra layers were also detected. The mineral composition analyses show that the upper tephra layer corresponds to the Laacher See eruption (Eifel, Germany) while the lower ones may relate to the volcanic activity of the Chaîne des Puys (Massif Central, France) around 13 000 cal. yr BP. These two events, recognised for the first time outside the Massif Central region, may provide additional chronostratigraphic markers for the Late Glacial sedimentary records of the Jura mountains and northern Alps. Copyright © 2004 John Wiley & Sons, Ltd.