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Barred olivine charge made by heating a highly ferroan composition in vacuum 183°C above its liquidus for ten minutes before cooling at 1000°C/hr (Tsuchiyama et al. 2004). Arrows indicate flat surfaces in contact with the graphite crucible. 

Barred olivine charge made by heating a highly ferroan composition in vacuum 183°C above its liquidus for ten minutes before cooling at 1000°C/hr (Tsuchiyama et al. 2004). Arrows indicate flat surfaces in contact with the graphite crucible. 

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Chondrule textures depend on the extent of melting of the chondrule precursor material when cooling starts. If viable nuclei remain in the melt, crystallization begins immediately, producing crystals with shapes that approach equilibrium. If not, crystallization does not occur until the melt is supersaturated, resulting in more rapid growth rates a...

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... rims on chondrules have been simulated experimentally in a number of ways. Connolly & Hewins (1991) produced accretionary rims by sintering dust to charges at about 1000°C. Partially melted accretionary rims, possibly analogous to igneous rims on chondrules, were produced at higher temperatures (Connolly et al. 1996). Such partially melted rims were made by injecting dust into the furnace so it collided with hot charges, as well as by heating dust-coated spherules (Connolly & Hewins 1991). Note that the occurrence of rims suggests two events because of evidence of reheating of the material underlying the rims in some cases (Krot & Wasson 1995), but rims have also been made in the laboratory in the course of a single heat- ing-cooling event (Connolly & Hewins 1995; Yu et al. 1998). Cooling rates have been estimated from textures and mineral zoning, and in addition attempts have been made to infer them from concentrations of moderately volatile elements and from the widths of overgrowths on crystals. In many cases, linear cooling rates are discussed, whereas in nature the radiant flux is expected to decline as the fourth power of temperature (Stefan-Boltzmann law). Cooling curves have been used in simulations, particularly motivated initially by the desire to reduce volatile loss at the highest temperatures (Yu & Hewins 1998). Because olivine crystallization takes place over only a few hundred degrees, the textures and mineral zoning are not very sensitive to whether a linear or curved cooling path is used. A great variety of chondrule textures has been made using cooling rates of 500°C/hr by Connolly et al. (1998), whereas both lower and greater rates have been cited in the older literature for specific types of texture. A recent paper with a com- prehensive list of the cooling rates used in experiments which attempted to simulate different kinds of chondrules is by Desch & Connolly (2002), and they can be sum- marized as 5-3000°C/hr, a range probably larger than the range for the cooling rates of most chondrules. Very different textures can be produced with the same cooling rate because nuclei may or may not be available when cooling begins, depending on combinations of heating time and peak temperature for a given composition (Lofgren 1996). Mineral zoning is a better guide to cooling rate than texture. The upper and lower extremes of cooling rates used in experiments may not necessarily produce good matches to both the textures and the mineral zoning of natural chondrules. Excentroradial pyroxene textures have been produced with cooling rates up to 3000°C/hr (Hewins et al. 1981), with peak temperatures near the liquidus. However, a peak temperature higher than the liquidus is likely for such compositions with low melting temperatures (Hewins & Radomsky 1990) and, with the more extensive melting at high peak temperatures, nuclei are reduced to embryos so that nucleation will be delayed. Dendritic crystals require rapid cooling when peak temperature is relatively low, but rapid cooling from a high peak temperature is likely to produce glass; thus fine dendrites could be produced for a lower cooling rate with a high peak temperature, and the 3000°C/hr is an upper limit which may not apply to real (superheated) chondrules. Similar arguments apply to BO chondrules, and indeed Tsuchiyama et al. (2004) obtained the best resemblance to natural BO textures with the highest peak temperatures. They favor 1000°C/hr for producing BO textures (Fig. 3), though only limited experiments were made at lower cooling ...
Context 2
... of melting and for many chondrule formation mechanisms it is easier to en- visage large variations in temperature than in heating time. There is considerable potential in the use of crystal size distribution (CSD) analysis of charges and chondrules for deciphering chondrule formation conditions (Zieg & Lofgren 2002, 2003). The abundance of grains that are relicts of the starting material and not crystal- lized from the melt gives a minimum estimate of the extent of melting, given that some relict material may not be recognizable. Growth on existing grains with little supercooling is the dominant process for most PO textures, whereas for some PO and for BO chondrules the textures form as the result of rapid growth caused by heterogeneous nucleation at high degrees of supercooling. In most PO charges, olivine growth begins when cooling begins and continues until quenching. Relatively precise growth rates can be estimated in charges with obvious relict grains, and they are in- fluenced by both precursor grain size and extent of melting. In the experiments of Hewins & Fox (2004), all made with a cooling curve approximating 800°C/hr, olivine growth rates range from about 1 P m/hr for the finest starting materials with a peak temperature of 1470°C to about 50 P m/hr for the coarsest starting materials heated to 1550°C. For higher temperatures, relicts are no longer obvious, skeletal crystals indicate commencement of growth some unknown time after the onset of cooling, and anisotropy becomes important. For the same compositions and cooling curve (Hewins & Fox 2004), for BO produced with peak temperatures near 1600°C, maximum growth rates are in excess of 7000 P m/hr. The simulation of chondrules with non-porphyritic textures has been reviewed in earlier papers (Hewins 1988; Lofgren 1996), to which little need be added. There are, however, new contributions (Faure et al. 2003; Tsuchiyama et al. 2004) relevant to the formation of BO chondrules (Fig. 3). Faure et al. (2003) discussed the nature of the transition from skeletal hopper phenocrysts to a dendritic swallowtail morphol- ogy, as a function of increased supercooling. Barred or parallel plate olivine develops by preferential growth of dendrites from the external corners of hoppers, and is favored by gradients in temperature or composition. Nagahara & Ozawa (1996) produced BO textures by isothermal evaporation above the original liquidus temperature but below the final liquidus temperature. Tsuchiyama et al. (2004) demonstrated that the “bars” were indeed parallel plate crystals by means of x-ray computed tomogra- phy. Their experiments were conducted in vacuum and the best matches to classic BO chondrule textures complete with rims were developed in charges with high peak temperatures (100°C or more above the liquidus) and heating times 10-60 minutes. With lower peak temperatures, the charges tended to be spherulitic and lacked rims. Tsuchiyama et al. (2004) emphasize that the precise textures of BO chondrules, with solid rather than skeletal or dendritic bars, have never been reproduced, and anneal- ing after crystallization (a few days at 1200°C) would be required to generate them. This is consistent with the very low cooling rates indicated by subsolidus changes in chondrules (Weinbruch et al. 2001). Excentroradial and barred textures have been formed instead of glass when highly supercooled total melts were impacted by one or several mineral dust grains which acted as seeds (Connolly & Hewins ...

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... Dynamic crystallization (cooling rates) experiments have successfully reproduced chondrule textures and therefore, until recently, provided the main constraints on chondrule thermal history, including peak temperatures and cooling rates of chondrule melts (Hewins et al. 2005;Desch and Connolly 2002;Desch et al. 2012;Jones et al. 2018). Nucleation and growth theory states that spontaneous crystallization from a melt will not start at saturation but only when the solution becomes supersaturated. ...
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... La majorité des chondres se sont formés entre ∼1 et ∼3 Ma après la formation des CAIs (Figure 1.1b; e.g., Villeneuve et al., 2009;Ushikubo et al., 2013;Pape et al., 2019). La formation des chondres (i.e., scénario d'origine nébulaire vs origine planétaire) est encore très débattue (e.g., Hewins et al., 2005;Libourel et Krot, 2007;Libourel et Portail, 2018;Marrocchi et al., 2019;Piralla et al., 2021). ...
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... Wasson (1996)), the latter ignores other heat sources (by radiation or molecular collisions) which would slow it down, depending on the chondrule formation mechanism, as well as empirical evidence of longer heating (e.g. Hewins and Connolly, 2005;Marrocchi et al., 2018). It should also be noted that even the advocates of such short flash-heatings invoke repetitions of such events, perhaps a dozen of times (Baecker et al., 2017;Wasson and Rubin, 2003), which would in effect allow more time for relaxation to sphericity. ...
Preprint
Compound chondrules, i.e. chondrules fused together, make a powerful probe of the density and compositional diversity in chondrule-forming environments, but their abundance among the dominating porphyritic textures may have been drastically underestimated. I report herein microscopic observations and LA-ICP-MS analyses of lobate chondrules in the CO3 chondrites Miller Range 07193 and 07342. Lobes in a given chondrule show correlated volatile and moderately volatile element abundances but refractory element concentrations are essentially independent. This indicates that they formed by the collision of preexisting droplets whose refractory elements behaved in closed system, while their more volatile elements were buffered by the same gaseous medium. The presence of lobes would otherwise be difficult to explain, as surface tension should have rapidly imposed a spherical shape at the temperature peak. In fact, since most chondrules across chondrite groups are nonspherical, a majority are probably compounds variously relaxed toward sphericity. The lack of correlation of refractory elements between conjoined compound chondrule components is inconsistent with derivation of chondrules from the disruption of homogenized melt bodies as in impact scenarios and evokes rather the melting of independent mm-size nebular aggregates. Yet a "nebular" setting for chondrule formation would need to involve not only increased solid concentration, e.g. by settling to the midplane, but also a boost in relative velocities between droplets during chondrule-forming events to account for observed compound chondrule frequencies .
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Article
Compound chondrules, i.e. chondrules fused together, make a powerful probe of the density and compositional diversity in chondrule-forming environments, but their abundance among the dominating porphyritic textures may have been drastically underestimated. I report herein microscopic observations and LA-ICP-MS analyses of lobate chondrules in the CO3 chondrites Miller Range 07193 and 07342. Lobes in a given chondrule show correlated volatile and moderately volatile element abundances but refractory element concentrations are essentially independent. This indicates that they formed by the collision of preexisting droplets whose refractory elements behaved in closed system, while their more volatile elements were buffered by the same gaseous medium. The presence of lobes would otherwise be difficult to explain, as surface tension should have rapidly imposed a spherical shape at the temperature peak. In fact, since most chondrules across chondrite groups are nonspherical, a majority are probably compounds variously relaxed toward sphericity. The lack of correlation of refractory elements between conjoined compound chondrule components is inconsistent with derivation of chondrules from the disruption of homogenized melt bodies as in impact scenarios and evokes rather the melting of independent mm-size nebular aggregates. Yet a “nebular” setting for chondrule formation would need to involve not only increased solid concentration, e.g. by settling to the midplane, but also a boost in relative velocities between droplets during chondrule-forming events to account for observed compound chondrule frequencies .
... The igneous texture of coarse-grained CAIs could have been formed essentially in the same processes that resulted in chondrule formation: lightning (e.g., Desch and Cuzzi, 2000); nebular shocks (e.g., Desch and Connolly, 2002;Desch et al., 2010;Morris and Desch, 2010;Morris et al., 2012), current sheets (Hubbard et al., 2012;McNally et al., 2014), collision of planetesimals (e.g., Sanders and Scott, 2012;Johnson et al., 2014), among the others. The shock wave models are probably the most developed ones for chondrule formation and provide sufficiently detailed thermal evolution of reheated silicate grains to compare (favorably) to the results of laboratory experiments on chondrule formation (e.g., Hewins et al., 2005). According to the ''canonical" shock model of Desch and Connolly (2002), chondrule precursor materials 0.5 mm in size in such a shock (7 km s À1 , preshock temperature 25°C and pressure 10 À5 bar) could be heated by the hot shock front to $1650°C in a few hours, followed by very fast (minutes) cooling to $1450°C and slower cooling (35-50°C h À1 ) to $950°C. ...
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... Reduction of FeO during partial remelting could have increased the Mg# of the melt from which the pyroxene gen-2 subsequently crystallized. As the Mg# in pyroxene gen-2 is only 3 mol% higher, reduction of FeO in the melt would not have had to be extensive, which is consistent with the observation that reduction of olivine (Jones and Danielson, 1997;Hewins et al., 2005;Ushikubo et al., 2013) is not observed in this sample. However, a single large metal bleb and additionally numerous tiny grains (confirmed to be metal by means of EDX) are disseminated in the mesostasis and within low-Ca pyroxene in the chondrule. ...
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Chondrules from unequilibrated ordinary chondrites are among the oldest Solar system materials and preserve mineralogical, chemical and isotopic signatures that link them to their primary formation mechanisms and environments in the early Solar System. Some chondrules record features indicating modifications by high- to low-temperature processes throughout their residence time in the protoplanetary disk. Chondrules that were partially modified after their primary formation record chemical, isotopic and textural information on their initial formation conditions and subsequent reprocessing that are essential to reconstruct their formation environments and interpret the ages recorded by individual chondrules correctly. The detailed textural and major, minor and trace element analyses of two type-I chondrules from the low petrologic type ordinary chondrites MET 00526 and MET 00452 (L/LL3.05) reveal complex chemical and textural systematics bearing testimony of their multi- stage high temperature evolution, including reheating and partial remelting, in the evolving protoplanetary disk prior to accretion into their parent bodies. During primary crystallization of chondrule MET00526_Ch43, mineral growth, including incipient formation of feldspar in the outer parts of the chondrule, led to the fractionation of melt, eventually resulting in a chemical gradient in the mesostasis. During a later punctuated reheating that ultimately led to partial remelting of the outer parts of the chondrule, mesostasis and low-Ca pyroxene remelted partially. This partial remelting enhanced the chemical differences within the mesostasis and led to the formation of two chemically distinct mesostases in the inner and the outer zone of the chondrule with almost complementary abundances of Rb, Na, K, Ba, Sr and Eu. The calculated bulk mesostasis composition reveals chondritic relative abundances of these elements in the bulk chondrule with a slight depletion of the most volatile elements. Chemical and textural observations further indicate that this disequilibrium remelting occurred under more reducing conditions than the primary melting event preserved in the chondrule centre, allowing for the crystallization of a second generation of low-Ca pyroxene in the outer parts of the chondrule. Very similar processes are also recorded in chondrule MET00452_Ch22 with the degree of remelting being more extensive. A previously determined young 26Al-26Mg age of ~3 Ma after CAIs determined for chondrule MET00452_Ch22 dates the time of the chondrule remelting rather than its primary formation. This is evidence for a late thermal event in the protoplanetary disk and generally indicates that multiple, distinct thermal pulses occurred in the chondrule forming region of the protoplanetary disk throughout the time of chondrule formation. The nonconcentric secondary outer zone around a spherical inner zone may indicate a directed heat source as the cause of partial remelting and reprocessing of primary chondrules.