A TEM study of exsolution in Ca-rich pyroxenes from the Paris and Renazzo chondrites: Determination of type I chondrule cooling rates

We conducted a transmission electron microscope study of the exsolution microstructures of Ca-rich pyroxenes in type I chondrules from the Paris CM and Renazzo CR carbonaceous chondrites in order to provide better constraints on the cooling history of type I chondrules. Our study shows a high variability of composition in the augite grains at a submicrometer scale, reflecting nonequilibrium crystallization. The microstructure is closely related to the local composition and is thus variable inside augite grains. For compositions inside the pyroxene miscibility gap, with a wollastonite (Wo) content typically below 40 mole%, the augite grains contain abundant exsolution lamellae on (001). For grain areas with composition close to Wo₄₀, a modulated texture on (100) and (001) is the dominant microstructure, while areas with compositions higher than Wo₄₀ do not show any exsolution microstructure development. To estimate the cooling rate, we used the spacing of the exsolution lamellae on (001), for which the growth is diffusion controlled and thus sensitive to the cooling rate. Despite the relatively homogeneous microstructures of augite grains with Wo < 35 mole%, our study of four chondrules suggests a range of cooling rates from ~10 to ~1000 °C h⁻¹, within the temperature interval 1200–1350 °C. These cooling rates are comparable to those of type II chondrules, i.e., 1–1000 °C h⁻¹. We conclude that the formation of type I and II chondrules in the proto-solar nebula was the result of a common mechanism.     

Gravitational waves from r-modes

Detectability of gravitational waves emitted by newly born, hot, rapidly rotating neutron stars as they spin down due to the r-mode instability is discussed. It is shown that differential rotation induced by r-modes plays a fundamental role in the evolution of the mode’s instability, making it more difficult to detect these gravitational waves.     

Cooling rates of type I chondrules from Renazzo: Implications for chondrule formation

Cooling rates are one of the few fundamental constraints on models of chondrule formation. In this study, we used Cu and Ga diffusion profiles in metal grains to determine the cooling rates of type I chondrules in the Renazzo CR2 chondrite. To improve previous estimations of cooling rates obtained using this method, we used CT scanning and serial polishing of our sections to analyze equatorial sections of large metal grains. Through the cores of these metal grains situated at the surface of chondrules, the cooling rates calculated range from 21 to 86 K h⁻¹ for a peak temperature T_p ~ 1623–1673 K. A metal grain embedded in the core of a chondrule exhibits a cooling rate of 1.2 K h⁻¹ at a T_p ~ 1573 K. We also measured Cu-Ga diffusion profiles from nonequatorial sections of metal grains and calculated a lower range of cooling rates of 15–69 K h⁻¹ for T_p ~ 1473–1603 K compared to our results from equatorial sections. The high cooling rates inferred from the lightning model (several thousand K h⁻¹) are clearly at odds with the values obtained in this work. The X-wind model predicts cooling rates (~6–10 K h⁻¹) lower than most of our results. The cooling rates calculated here are in close agreement with those inferred from shock wave models, in particular for temperatures at which olivine crystallizes (from ~10 to several hundreds K h⁻¹ between 1900 and 1500 K). However, the chemical compositions of metal grains in Renazzo are consistent with the splashing model, in which a spray of metal droplets originated from a partially molten planetesimal. Volatile siderophile element depletion is explained by evaporation before metal was engulfed within silicate droplets. Liquid metal isolated from the liquid silicate crystallized during cooling, reacted with the ambient gas, and then re-accreted within partially molten chondrules.     

Magnetic changes observed in the formation of two filaments in a complex active region: TRACE and MSDP observations

This paper is focused on the formation of two filaments in a complex center of decaying active regions (AR 8329 and AR 8326), located in the northern hemisphere. The observations were obtained in Hα by the Multi-channel Subtractive Double Pass spectrograph (MSDP mounted on the German telescope VTT in Tenerife) and EUV lines with TRACE (Transition Region And Corona Explorer). High Doppler shifts are found to be related to the ends of filament segments where canceling magnetic fields are also located (as seen on magnetograms from Big Bear Solar Observatory). At these locations, velocities along the line of sight, derived by using a cloud model method reach −20 km s⁻¹, the segments of filaments merge and frequently a time-related sub-flare is observed by TRACE. The chirality of the filament segments has been determined by different methods: the segments of dextral chirality join together and form a long dextral filament, and a single filament of sinistral chirality forms end to end with the dextral filament but does not merge with it. Assuming a model of twisted flux tube for filament material, we suggest that the dextral filament has negative helicity and a relationship between its formation and the close by sunspot with the same sign of helicity.     

Wave propagation in the photosphere

Using a 32 minutes sequence of observation, brightness and velocity fluctuations in the wings of the Mgi line at 5172.7 Å and Fe ii line at 5197.578 Å are analysed. The analysis of phase shifts and amplitude ratios leads to the following conclusions:

Loess and palaeoenvironment: Some results of a European joint programme of research

In 1985, a programme of research on the sedimentology and diagenesis of the loess and loessic silts of western Europe was initiated jointly by laboratories in Leicester, Meudon, Caen, Milan, Cardiff and Southampton, involving a limited amount of comparative work on the loess from types sites in the Loess Plateau of central China. Preliminary results include the following: (1) Experimental procedures in particle size analysis, clay sample preparation and microscopy have been standardised in all cooperating groups; distinctive fabrics indicative of depositional mechanisms have been recognised. (2) Comparison of aeolian, colluvial and alluvial silt by laser-scanning and bulk magnetic susceptibility anisotropy methods appears to differentiate aeolian silt from loessic silts re-deposited by slope processes and flowing water. (3) The variability in the anisotropy of the limon à doublet facies has been assessed by image analysis of scanning electron microscopy (SEM) and optical microscopy. Strong anisotropy has also been measured in thin section imagery of loess from Normandy and Poland as the product of freezing and thawing. This is confirmed by scanning electron microscopy and by experimental freezing of some silts. (4) The clay fabric and mineralogy in western European loess differ from those in Chinese loess, especially in the case of semi-arid deposits. (5) Distribution of CaCO₃, using thin sections and SEM analysis, has been determined for loess from Normandy and the Rhǒne valley in France and for some Chinese loess, At Saint Vallier, zones of CaCO₃ concentration occur masking much of the original (loess) sedimentary skeleton which is a locally reworked loessic silt with some laminations. Despite considerable scatter, the palaeomagnetic results indicate a reversed polarity throughout, placing the whole Saint Vallier sequence in the Matuyama chron. These preliminary results suggest the existence of two different palaeoenvironments: one typified by central China, with arid glacials and humid interglacials; and the other represented by western Europe, with generally high humidities. There was stronger weathering and soil development and fewer freezing effects on the lossic succession in northern Italy than in western littoral Europe.     

Geothermal control on flow patterns in the Last Glacial Maximum ice sheet of Iceland

Because it is located both on the Mid‐Atlantic Ridge and on a mantle plume, Iceland is a region of intense tectonics and volcanism. During the last glaciation, the island was covered by an ice sheet approximately 1000 m thick. A reconstruction of the ice flow lines, based on glacial directional features, shows that the ice sheet was partly drained through fast‐flowing streams. Fast flow of the ice streams has been recorded in megascale lineations and flutes visible on the currently deglaciated bedrock, and is confirmed by simple mass balance considerations. Locations of the major drainage routes correlate with locations of geothermal anomalies, suggesting that ice stream activity was favoured by lubrication of the bed by meltwater produced in regions of high geothermal heat flux. Similar control of ice flow by geothermal activity is expected in ice sheets currently covering tectonically and volcanically active area such as the West Antarctic ice sheet. Copyright © 2000 John Wiley & Sons, Ltd.     

Future extreme events in European climate: an exploration of regional climate model projections

This paper compares how well satellite versus weather station measurements of climate predict agricultural performance in Brazil, India, and the United States. Although weather stations give accurate measures of ground conditions, they entail sporadic observations that require interpolation where observations are missing. In contrast, satellites have trouble measuring some ground phenomenon such as precipitation but they provide complete spatial coverage of various parameters over a landscape. The satellite temperature measurements slightly outperform the interpolated ground station data but the precipitation ground measurements generally outperform the satellite surface wetness index. In India, the surface wetness index outperforms station precipitation but this may be reflecting irrigation, not climate. The results suggest that satellites provide promising measures of temperature but that ground station data may still be preferred for measuring precipitation in rural settings.