Remarques sur la matrice des chondrites ordinaires d'apres l'observation de quelques chondrites a bronzite peu ou pas choquees

Microscopic investigations have been done on the chondrites Sena and Nadiabondi (H5, not shocked), Ste. Marguerite en Comines (H4, very slightly shocked), Allegan (H5, slightly shocked). Only in such cases can the matrix be easily observed and compared to those of type 3 chondrites. The <100 μm debris found in types 4 and 5 that we have observed are not the result of the metamorphism of type 3 fines.The abundance of tiny debris is in direct relation with the intensity of the shock though this shock was insufficient to provoke either the induration of the stones or a significant loss of rare gases. The bulk of the fines are the result of local disaggregation of the most brittle parts from chondrules and fragments.A low-temperature matrix has not been observed in these meteorites but only in H3 chondrites, as a coating around the chondrules. The accretion modelists should take into account the absence or the scarcity of fine particles in their calculations.     

Terrestrial chondrules,glass spherules and accretionary lapilli from the suevite,Ries Crater,Germany

Until recently, no terrestrial analogues of meteoritic and lunar chondrules were known. Only rare glass spherules from the Lonar Crater, India, and black magnetic spherules from various localities have been recorded. The impact breccia suevite of the No¨rdlinger Ries Crater, Germany, contains both chondrules and glass spherules, and in addition, accretionary lapilli, all of which are found imbedded within the fine-grained matrix of the suevite. The chondrules display many of the textural features characteristic of meteoritic and lunar chondrules. Lithic chondrules and fluid drop chondrules are present, the latter having a composition quite similar to that of glass bombs and glass fragments in the suevite. Fluid drop chondrules developed from glass spherules by slow devitrification in the hot suevite ejecta masses after deposition. On the whole, fluid drop chondrules, lithic chondrules and glass spherules are rare in the suevite, with fluid drop chondrules prevailing. Detection of chondrules from a terrestrial impact crater supports theories of an impact origin for meteoritic and lunar chondrules. Accretionary lapilli also represent material formed as a result of impact.     

Magnetic fields of the solar nebula as recorded in chondrules from the Allende meteorite

The natural remanent magnetization (NRM) in individual chondrules from the Allende meteorite was measured. These had previously been oriented relative to each other. The NRM directions of the chondrules are not initially random, but they become scattered after either alternating field (AF) or thermal demagnetization. The NRM is less stable than anhysteretic remanent magnetization (ARM) against AF-demagnetization.

The bulk of the NRM in the matrix is erased by 300°C. For the larger chondrules it is erased by 550°C, but for the smaller chondrules and the white inclusion a substantial decrease in NRM occurs by 350°C leaving about 20% up to 600°C. The behavior of the laboratory-induced ARM and the NRM under alternating field demagnetization suggest that the NRM of the chondrules consists of at least two components of TRM. One is a high-temperature component which was acquired when the individual chondrules were cooled through the Curie temperature and before they were assembled into the Allende meteorite. The other is a low-temperature component which was probably acquired in a field of about 1 Oe when the meteorite experienced thermal metamorphism or during the assembly of the meteorite.     

Constraints for chondrule formation from Ca–Al distribution in carbonaceous chondrites

Chondritic meteorites and their components formed in the protoplanetary disk surrounding the nascent sun. We show here that the two volumetrically dominating components of carbonaceous chondrites, chondrules and matrix did not form independently. They must have been derived from a single, common source. We analyzed Ca and Al in chondrules and matrix of the CV type carbonaceous chondrites Allende and Y-86751. The Ca/Al-ratios of chondrules and matrix of both chondrites are complementary, but in case of Allende chondrules have sub-chondritic and matrix super-chondritic Ca/Al-ratios and in case of Y-86751 chondrules have super-chondritic and matrix sub-chondritic Ca/Al-ratios. This rules out the redistribution of Ca between chondrules and matrix during parent body alteration. Tiny spinel grains in the matrix produce the high Al in the matrix of Y-86751. In Allende these spinels were most probably included in chondrules. The most plausible explanation for this Ca- and Al-distribution in the same type of chondrite is that both chondrules and matrix formed from the same chemical reservoir. Tiny differences in nebular conditions during formation of these two meteorites must have led to the observed differences. These are severe constraints for all models of chondrule formation. Any model involving separate formation of chondrules and matrix, such as the X-wind model can be excluded.     

New kind of type 3 chondrite with a graphite-magnetite matrix

We have discovered four clasts in three ordinary-chondrite regolith breccias which are a new kind of type 3 chondrite. Like ordinary and carbonaceous type 3 chondrites, they have distinct chondrules, some of which contain glass, highly heterogeneous olivines and pyroxenes, and predominantly monoclinic low-Ca pyroxenes. But instead of the usual fine-grained, Fe-rich silicate matrix, the clasts have a matrix composed largely of aggregates of micron- and submicron-sized graphite and magnetite. The bulk compositions of the clasts as well as the types of chondrules (largely porphyritic) are typical of type 3 ordinary chondrites, although chondrules in the clasts are somewhat smaller (0.1–0.5 mm). A close relationship with ordinary chondrites is also indicated by the presence of similar graphite-magnetite aggregates in seven type 3 ordinary chondrites. This new kind of chondrite is probably the source of the abundant graphite-magnetite inclusions in ordinary-chondrite regolith breccias, and may be more common than indicated by the absence of whole meteorites made of chondrules and graphite-magnetite.     

On the origin of isolated olivine grains in type 2 carbonaceous chondrites

The origin of olivine grains isolated in the matrix of C2 carbonaceous chondrites is an important problem. If these grains are condensates from a solar nebular gas, they contain compositional, isotopic and physical features that further elucidate that process. If, however, they are grains released by the breakup of chondrules, then many important condensation features have been lost during the melting that took place to form chondrules.In evaluating these two possibilities, care must be taken to determine which inclusions in C2 meteorites are actual chondrules and which are aggregates of grains that have never undergone melting. The two main types of aggregates, pyroxene-rich and pyroxene-poor, are forty to fifty times more abundant than chondrules. Four scenarios are presented to account for the kinds of aggregates and isolated grains seen in the Murchison C2 meteorite. An analysis of these scenarios is made in light of olivine crystal morphology, comparison of composition of glass inclusions inside olivine grains with interstitial glass in true chondrules and size distributions of olivines, isolated, in aggregates and in chondrules.It is concluded that no scenario that includes a chondrule-making step can account for the observed population of isolated olivine grains. An origin by direct condensation, partial comminution, aggregation and accretion best accounts for the sizes and morphological features observed.     

A disaggregation and thin section analysis of the size and mass distribution of the chondrules in the Bjurböle and Chainpur meteorites

This paper presents the results of a disaggregation and thin section analysis of the size distribution of chondrules in two friable meteorites, Bjurböle and Chainpur. Dodd [Earth Planet. Sci. Lett. 30 (1976) 281] found in chondrites that the size distribution of metal and silicate particles (be they chondrules, chondrule fragments or independent grains in the matrix) obey Rosin's law. He used thin sections of meteorites. Martin and Mills [Earth Planet. Sci. Lett. 33 (1976) 239] imply that thin section studies are not valid and that meteoritic disaggregation and the subsequent measurement of the individual particles is required. They found that the “near-spherical” chondrules picked out from the disaggregated meteorite do not obey Rosin's law and suggest that these chondrules result from the melting of dust, rather than from impact as suggested by Dodd. The Rosin's law criterion could be crucial to the acceptabilities of these theories.In thin sections both droplet and lithic fragment chondrules can be easily identified. The Bjurböle section had 33 ± 4% of its area occupied by droplet chondrules and 30 ± 3% occupied by lithic fragment chondrules. The matrix occupied 37 ± 2%. Disaggregation of 4 g of Bjurböle produced 27% (by mass) near-spherical chondrules. The lithic fragment chondrules had a degree of friability similar to that of the matrix. Due to this they unfortunately broke up during the disaggregation process. The size distribution of droplet and lithic fragment chondrules was found to be similar. All chondrules were found to obey Rosin's law.The size distribution of the disaggregated chondrules has been used to calculate the expected thin section size distribution by assuming that chondrules are sectioned randomly. Empirical correction factors have thus been obtained which enable observed thin-section parameters to be converted into true parameters. The observed and expected thin section distributions agreed well. On disaggregation 4 g of Bjurböle yielded 955 near-spherical chondrules. A 0.78-cm² thin section of Bjurböle revealed 61 droplet and 57 lithic fragment chondrules so to obtain comparable precision large (～10 cm²) thin sections or slices must be used.The near-spherical chondrules disaggregated from Bjurböle had a median diameter of 0.688 ± 0.003 mm, a mean density of 3.258 ± 0.008 g cm^?3 and a median mass of 5.6 × 10^?4 g. Their diameters ranged between 0.25 ± 0.01 mm and 3.67 mm. The lower limit is considerably below the value of 0.4 mm obtained by Martin and Mills.     

Oxygen isotopic constraints on the origin of Mg-rich olivines from chondritic meteorites

Chondrules are the major high temperature components of chondritic meteorites which accreted a few millions years after the oldest solids of the solar system, the calcium–aluminum-rich inclusions, were condensed from the nebula gas. Chondrules formed during brief heating events by incomplete melting of solid dust precursors in the protoplanetary disk. Petrographic, compositional and isotopic arguments allowed the identification of metal-bearing Mg-rich olivine aggregates among the precursors of magnesian type I chondrules. Two very different settings can be considered for the formation of these Mg-rich olivines: either a nebular setting corresponding mostly to condensation–evaporation processes in the nebular gas or a planetary setting corresponding mostly to differentiation processes in a planetesimal. An ion microprobe survey of Mg-rich olivines of a set of type I chondrules and isolated olivines from unequilibrated ordinary chondrites and carbonaceous chondrites revealed the existence of several modes in the distribution of the ?¹⁷O values and the presence of a large range of mass fractionation (several ‰) within each mode. The chemistry and the oxygen isotopic compositions indicate that Mg-rich olivines are unlikely to be of nebular origin (i.e., solar nebula condensates) but are more likely debris of broken differentiated planetesimals (each of them being characterized by a given ?¹⁷O). Mg-rich olivines could have crystallized from magma ocean-like environments on partially molten planetesimals undergoing metal–silicate differentiation processes. Considering the very old age of chondrules, Mg-rich olivine grains or aggregates might be considered as millimeter-sized fragments from disrupted first-generation differentiated planetesimals. Finally, the finding of only a small number of discrete ?¹⁷O modes for Mg-rich olivines grains or aggregates in a given chondrite suggests that these shattered fragments have not been efficiently mixed in the disk and/or that chondrite formation occurred in the first vicinity of the breakup of these planetary bodies.     

A new kind of primitive chondrite, Allan Hills 85085

Allan Hills 85085 is a chemically and mineralogically unique chondrite whose components have suffered little metamorphism or alteration. This chondrite is unique because it has fewer and smaller chondrules (4 wt. %; mean diameter 16 μm) than any other chondrite, more metallic Fe,Ni (36%) and lithic and mineral silicate fragments (56%), and a lower abundance of troilite (2%) and volatiles. Most chondrules are cryptocrystalline or glassy and are depleted in volatiles, some small chondrules are also very depleted in refractory lithophiles. Matrix lumps (4%) partly resemble CI and CM matrices and may be foreign to the parental asteroid. Despite these differences, the components of ALH 85085 have some features common to most type 2 and the least metamorphosed type 3 chondrites: metallic Fe,Ni grains that contain 0.1–1 wt.% Cr, Si and P; Fe/(Fe + Mg) values of olivines, pyroxenes and chondrules are concentrated in the range 1–6 at.% with a few percent in the range 7–30%; porphyritic chondrules are chondritic in composition (except for their low volatile abundances). Thus the components of ALH 85085 probably have similar origins to those of components in other chondrites, and their properties largely reflect nebular, not asteroidal, processes.The bulk composition of ALH 85085 fits none of the nine groups of chondrites: it is richer in Fe (1.4 × CI levels when normalized to Si) and poorer in Na and S (0.1–0.2 × CI) than other chondrites. Low volatile concentrations are due to a low matrix abundance and loss of volatiles during or prior to chondrule formation, not to volatile loss during metamorphism. Chondrule textures imply extensive heating of chondrule melts above the liquidus, consistent with loss of volatiles from small volumes of melt during chondrule formation. The small size of chondrules is partly due to extensive fragmentation by impacts, which may have occurred on the parent asteroid or in the solar nebula. Collisions between chondrule precursor aggregates in the nebula could also be responsible for the small sizes of chondrules.Assuming that ALH 85085 is a representative sample of an asteroid, its properties lend support to models for the origins of the Earth, eucrite parent body and volatile-poor iron meteorites that invoke chondritic planetesimals depleted in volatiles. The existence of ALH 85085 and Kakangari suggests that the nine chondrite groups may provide a remarkably poor sample of the primitive chondritic material from which the asteroids formed. Certain similarities between ALH 85085 and Bencubbin and Weatherford suggest that the latter two primitive meteorites may actually be chondrites with even higher metal abundances (50–60 wt.%) and very large, partly fragmented chondrules.     

Distinguishing volcanic debris avalanche deposits from their reworked products: the Perrier sequence (French Massif Central)

Debris avalanches associated with volcanic sector collapse are usually high-volume high-mobility phenomena. Debris avalanche deposit remobilisation by cohesive debris flows and landslides is common, so they can share textural characteristics such as hummocks and jigsaw cracks. Distinguishing original deposits from reworked products is critical for geological understanding and hazard assessment because of their different origin, frequency and environmental impact. We present a methodology based on field evidence to differentiate such epiclastic breccias. Basal contact mapping constrained by accurate altitude and location data allows the reconstruction of deposit stratigraphy and geometry. Lithological analysis helps to distinguish the different units. Incorporation structures, kinematic indicators and component mingling textures are used to characterise erosion and transport mechanisms. We apply this method to the enigmatic sequence at Perrier (French Massif Central), where four units (U1–U4) have been interpreted either as debris flow or debris avalanche deposits. The sequence results from activity on the Monts Dore Volcano about 2 Ma ago. The epiclastic units are matrix supported with an almost flat top. U2 and U3 have clear debris flow deposit affinities such as rounded clasts and intact blocks (no jigsaw cracks). U1 and U4 have jigsaw cracked blocks with matrix injection and stretched sediment blocks. U1 lacks large blocks (>10 m wide) and has a homogenous matrix with an upward increase of trapped air vesicle content and size. This unit is interpreted as a cohesive debris flow deposit spawned from a debris avalanche upstream. In contrast, U4 has large mega-blocks (up to 40 m wide), sharp contacts between mixed facies zones with different colours and numerous jigsaw fit blocks (open jigsaw cracks filled by monogenic intra-clast matrix). Mega-blocks are concentrated near the deposit base and are spatially associated with major substratum erosion. This deposit has a debris avalanche distal facies with local debris flow affinities due to partial water saturation. We also identify two landslide deposits (L1 and L2) resulting from recent reworking that has produced a similar facies to U1 and U4. These are distinguishable from the original deposits, as they contain blocks of mixed U1/U4 facies, a distinctly less consolidated and more porous matrix and a fresh hummocky topography. This work shows how to differentiate epiclastic deposits with similar characteristics, but different origins. In doing so, we improve understanding of present and past instability of the Monts Dore and identify present landslide hazards at Perrier.     

A constraint on impact theories of chondrule formation

The association between agglutinates and chondrule-like spherules, which characterizes the assemblage of impact-derived melt products in lunar regolith samples and some gas-rich achondrites, is not found in primitive chondrites. This observation suggests that impacts into a parent-body regolith are unlikely to have produced the chondrules. We believe that if chondrules were formed from impact melt, it was probably generated by jetting during particle-to-particle collisions, presumably in the nebula.     

Dendrogeomorphic reconstruction of past debris‐flow activity using injured broad‐leaved trees

Tree‐ring records from conifers have been regularly used over the last few decades to date debris‐flow events. The reconstruction of past debris‐flow activity was, in contrast, only very rarely based on growth anomalies in broad‐leaved trees. Consequently, this study aimed at dating the occurrence of former debris flows from growth series of broad‐leaved trees and at determining their suitability for dendrogeomorphic research. Results were obtained from gray alder (Alnus incana (L.) Moench), silver birch and pubescent birch (Betula pendula Roth and Betula pubescens Ehrh.), aspen (Populus tremula L.), white poplar, black poplar and gray poplar (Populus alba L., Populus nigra L. and Populus x canescens (Ait.) Sm.), goat willow (Salix caprea L.) and black elder (Sambucus nigra L.) injured by debris‐flow activity at Illgraben (Valais, Swiss Alps). Tree‐ring analysis of 104 increment cores, 118 wedges and 93 cross‐sections from 154 injured broad‐leaved trees allowed the reconstruction of 14 debris‐flow events between AD 1965 and 2007. These events were compared with archival records on debris‐flow activity at Illgraben. It appears that debris flows are very common at Illgraben, but only very rarely left the channel over the period AD 1965–2007. Furthermore, analysis of the spatial distribution of disturbed trees contributed to the identification of six patterns of debris‐flow routing and led to the determination of preferential breakout locations of events. The results of this study demonstrate the high potential of broad‐leaved trees for dendrogeomorphic research and for the assessment of the travel distance and lateral spread of debris‐flow surges. Copyright © 2010 John Wiley & Sons, Ltd.     

Sr isotopic fractionation in Allende chondrules: A reflection of solar nebular processes

True relative Sr isotopic compositions, determined by the double-spike technique, are reported for 8 olivine chondrules from Allende and a single chondrule from Richardton. The Richardton chondrule has an Sr composition identical with the whole meteorite, but the Allende chondrules are up to 1.4‰ per mass unit light-isotope enriched, closely similar to Ca-Al inclusions (CAI) from the same individual stone. The correspondence of the patterns for chondrules and CAI suggests that both groups of objects derived their fractionated Sr in similar ways. The lack of any detectable non-linear Sr isotopic anomaly in the objects suggests that their Sr compositions did not have some exotic or extrasolar origin, but were derived from normal solar system Sr by mass fractionation. The consistent light-Sr enrichment of Allende objects may be explained by several schemes, and all are heavily model-dependent. Most plausible to the author is that the CAI and chondrules derived their fractionated Sr from a region of the nebula made isotopically light by partial kinetic mass separation of elements in the vapour phase. Later, the solid objects may have moved to an isotopically more normal region, where the Allende matrix accreted.     

Debris jets in continental phreatomagmatic volcanoes: A field study of their subterranean deposits in the Coombs Hills vent complex,Antarctica

The Ferrar large igneous province of Antarctica contains significant mafic volcaniclastic deposits, some of which are interpreted to fill large vent complexes. Such a complex was re-examined at Coombs Hills to map individual steep-sided cross-cutting bodies in detail, and we found several contrasting types, two of which are interpreted to have filled subterranean passageways forcefully opened from below into existing, non-consolidated debris. These transient conduits were opened because of the propagation of debris jets – upward-moving streams of volcaniclastic debris, steam, magmatic gases +/− liquid water droplets – following explosive magma–aquifer interaction. Some debris jets probably remained wholly subterranean, whereas others made it to the surface, but the studied outcrops do not allow us to differentiate between these cases. The pipes filled with country rock-rich lapilli-tuff or tuff-breccia are interpreted to have formed following phreatomagmatic explosions occurring near the walls or floor of the vent complex, causing fragmentation of both magma and abundant country rock material. In contrast, some of the cross-cutting zones filled with basalt-rich tuff-breccia or lapilli-tuff could have been generated following explosions taking place within pre-existing basalt-bearing debris, well away from the complex walls or floor. We infer that once focused jets were formed, they did not incorporate significant amounts of existing debris while travelling through them; instead, incorporation of fragments from the granular host took place near explosion sites. Other basalt-rich tuff-breccia zones, accompanied by domains of in situ peperite and coherent basalt pods, are inferred to have originated by less violent processes.     

The composition and origin of large microporphyritic chondrules in the Manych (L-3) chondrite

The majority (26/37) of the largest chondrules (d ≥ 1400 μm) exposed in a thin section of the Manych chondrite are more or less rounded fragments of microporphyry, most of which contain from 50 to 80 vol.% olivine. Modal and phase analyses were used to calculate the approximate bulk compositions of nine such chondrules. Six vary modestly around the mean composition of L-group chondrites less most of their metal and troilite and are thought to have formed by bulk melting of L-group material with loss of an immiscible Fe-Ni-S liquid. Two other chondrules, which are olivine-rich and Na- and Si-poor, formed in the same way but with some loss of volatile constituents to a vapor phase. The ninth chondrule, an olivine-poor microporphyry, may be a non-representative sample of a coarser microporphyritic rock.Comparison of these microporphyritic chondrules with the products of controlled cooling experiments and with chemically similar olivine microporphyry in the St. Mesmin chondrite (LL-breccia) suggests that the microporphyritic chondrules are fragments of magmatic rocks which crystallized from masses of liquid no less than 10 cm across.     

Evidence for relict grains in chondrules of Qingzhen,an E3 type enstatite chondrite

Petrographic and chemical studies of the Qingzhen chondrite strongly suggest that it is the most highly unequilibrated (type 3) enstatite chondrite recognized so far. Qingzhen contains abundant, well-defined chondrules, some of which were incompletely molten during the chondrule formation process. The relict olivine grains within these chondrules contain dusty inclusions of almost pure metallic Fe, which appear to be the in-situ reduction product of the fayalitic component of the olivine. The reduction process presumably took place at the time of chondrule formation and the chondrule precursor material must have been more oxidized than average enstatite chondrite material. We believe that this oxidized material may have formed at the enstatite chondrite formation location in the solar nebula, provided fluctuations in the degree of oxidation of the nebular gas existed at such locations. Reheating of this material under more reducing conditions would lead to the observed reduction of the olivine. Igneous olivines within chondrules always contain detectable amounts of CaO, while relict olivines are essentially CaO-free. This seems to suggest that the relict olivines did not originate during a previous igneous process of chondrule formation and might represent condensation products from the early solar nebula.     

Petrogenesis of opaque assemblages in the Ningqiang carbonaceous chondrite

Numerous round to oblate opaque assemblages (OAs) are found in chondrules and matrix of the Ningqiang carbonaceous chondrite. They are mainly composed of Ni-rich metal,magnetite,Fe,Ni-sulfides,with minor amounts of phosphate,phosphoran-olivine,pyroxene and trace amounts of nano-sized platinum-group metal alloys. The mineralogy of Ningqiang OAs is very similar to that of OAs previously reported in Ca,Al-rich inclusions of CV chondrites. Being a rare mineral phase in nature,phosphoran-olivine is thought to form by nonequilibrium reactions between P-bearing molten metal and olivine crystals during rapid cooling. Its occurrence in Ningqiang OAs indicates that the precursor of OAs was locally produced during chondrule formation,rather than directly condensed from the solar nebula as previously thought. The petrographic and mineralogical characteristics of Ningqiang OAs reveal that OAs formed by low temperature alterations of pre-existing homogeneous alloys within chondrules on a planetary body.     

The feasibility of isolation and detection of fullerenes and carbon nanotubes using the benzene polycarboxylic acid method

The incorporation of fullerenes and carbon nanotubes into electronic, optical and consumer products will inevitably lead to the presence of these anthropogenic compounds in the environment. To date, there have been few studies isolating these materials from environmental matrices. Here we report a method commonly used to quantify black carbon (BC) in soils, the benzene polycarboxylic acid (BPCA) method, for measurement of two types of single walled carbon nanotubes (SWCNTs), two types of fullerenes and two forms of soot. The distribution of BC products (BPCAs) from the high pressure and high temperature oxidation illustrates the condensed nature of these compounds because they form predominantly fully substituted mellitic acid (B6CA). The conversion of carbon nanoparticles to BPCAs was highest for fullerenes (average of 23.2 ± 4.0% C recovered for both C₆₀ and C₇₀) and lowest for non-functionalized SWCNTs (0.5 ± 0.1% C). The recovery of SWCNTs was 10 times higher when processed through a cation-exchange column, indicating the presence of metals in SWCNTs compromises the oxidation chemistry. While mixtures of SWCNTs, soot and sediment revealed small losses of black carbon during sample processing, the method is suitable for quantifying total BC. The BPCA distribution of mixtures did not agree with theoretical mixtures using model polyaromatic hydrocarbons, suggesting the presence of a matrix effect. Future work is required to quantify different types of black carbon within the same sample.     

High temperature rims around chondrules in primitive chondrites: evidence for fluctuating conditions in the solar nebula

Rims or rim sequences surrouding chondrules have been identified in carbonaceous and unequilibrated ordinary chondrites. These chondrule rims include three chemical subtypes: Fe,Ca-rich and Fe,Ni-metal-rich rims, which occur predominantly in Kainsaz (CO3), and ferromagnesian rims which occur in Kainsaz (CO3), Allende (CV3), Renazzo (CR2), Chainpur (LL3), Semarkona (LL3), Krymaka (L3), and Tieschitz (H3). The compositions of minerals in these rims are often drastically different from those in the underlying chondrule cores, indicating that the solar nebula was chemically heterogeneous. In many cases the compositions of the rims require an environment that was much more oxidizing than a solar composition gas. Particularly interesting is that some of the Fe,Ca-rich chondrule rims are remarkably similar to some of the rims around refractory inclusions, suggesting that chondrules and refractory inclusions experienced late, coeval processing. The textures of the chondrule rims suggest they formed at high temperatures and that they accreted onto chondrules that had already solidified. The lengthscale of the thermal heterogeneities necessary to make available hot material that could accrete to cold chondrules has been calculated to be less than 10 km, implying there were localized heat sources in the solar nebula.     

Effects of large organic material on channel form and fluvial processes

Stream channel development in forested areas is profoundly influenced by large organic debris (logs, limbs and rootwads greater than 10 cm in diameter) in the channels. In low gradient meandering streams large organic debris enters the channel through bank erosion, mass wasting, blowdown, and collapse of trees due to ice loading. In small streams large organic debris may locally influence channel morphology and sediment transport processes because the stream may not have the competency to redistribute the debris. In larger streams flowing water may move large organic debris, concentrating it into distinct accumulations (debris jams). Organic debris may greatly affect channel form and process by: increasing or decreasing stability of stream banks; influencing development of midchannel bars and short braided reaches; and facilitating, with other favourable circumstances, development of meander cutoffs. In steep gradient mountain streams organic debris may enter the channel by all the processes mentioned for low gradient streams. In addition, considerable debris may also enter the channel by way of debris avalanches or debris torrents. In small to intermediate size mountain streams with steep valley walls and little or no floodplain or flat valley floor, the effects of large organic debris on the fluvial processes and channel form may be very significant. Debris jams may locally accelerate or retard channel bed and bank erosion and/or deposition; create sites for significant sediment storage; and produce a stepped channel profile, herein referred to as ‘organic stepping’, which provides for variable channel morphology and flow conditions. The effect of live or dead trees anchored by rootwads into the stream bank may not only greatly retard bank erosion but also influence channel width and the development of small scour holes along the channel beneath tree roots. Once trees fall into the stream, their influence on the channel form and process may be quite different than when they were defending the banks, and, depending on the size of the debris, size of the stream, and many other factors, their effects range from insignificant to very important.