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Title: Thulium anomalies and rare earth element patterns in meteorites and Earth: Nebular fractionation and the nugget effect
Author: N. Dauphas, A. Pourmand

This study reports the bulk rare earth element (REEs, La-Lu) compositions of 41 chondrites, including 32 falls and 9 finds from carbonaceous (CI, CM, CO and CV), enstatite (EH and EL) and ordinary (H, L and LL) groups, as well as 2 enstatite achondrites (aubrite). The CI-chondrite-normalised REE patterns and Eu anomalies in ordinary and enstatite chondrites show more scatter in more metamorphosed than in unequilibrated chondrites. This is due to parent-body redistribution of the REEs in various carrier phases during metamorphism. The dispersion in REE patterns of equilibrated ordinary chondrites is explained by the nugget effect associated with concentration of REEs in minor phosphate grains.
Terrestrial rocks and samples from ordinary and enstatite chondrites display negative Tm anomalies of ~-4.5 % relative to ca chondrites. In contrast, CM, CO and CV (except Allende) show no significant Tm anomalies. Allende CV chondrite shows large excess Tm (~+10 %). These anomalies are similar to those found in group II refractory inclusions in meteorites but of much smaller magnitude. The presence of Tm anomalies in meteorites and terrestrial rocks suggests that either (i) the material in the inner part of the solar system was formed from a gas reservoir that had been depleted in refractory dust and carried positive Tm anomalies or (ii) CI chondrites are enriched in refractory dust and are not representative of solar composition for refractory elements. The observed Tm anomalies in ordinary and enstatite chondrites and terrestrial rocks, relative to carbonaceous chondrites, indicate that material akin to carbonaceous chondrites must have represented a small fraction of the constituents of the Earth.

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Microscopic analyses of chondrites, the oldest rocks in the solar system, are filling in details of what our neighbourhood in space was like shortly before the planets formed
These meteorites, which constitute more than 80 percent of those observed to fall from space, derive their name from the chondrules virtually all of them contain -tiny beads of melted material, often smaller than a rice grain, that formed before asteroids took shape early in the solar system's history. When we examine thin slices from chondrites under a microscope, they become beautiful to behold, not unlike some of the paintings by Wassily Kandinsky and other abstract artists.

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Chronicle of a Chondrule's Travels

Cometary particles returned in 2006 by NASA's Stardust spacecraft provide crucial information not only about the mission's target, comet 81P/Wild 2, but also about the dynamics in the solar nebula that brought the comet's materials together. Ryan Ogliore (University of Hawaii) and colleagues from the universities of Hawaii, California, and Washington, and the Lawrence Berkeley National Laboratory have focused attention on a tiny chondrule fragment from one of the Wild 2 particles. Using petrology, oxygen isotopes, and aluminium-magnesium isotopic measurements, they determined this chondrule formed relatively late (as chondrules go) in the inner solar nebula and moved out to the comet-forming region before Jupiter could have blocked its way. The timing implies Jupiter formed more than three million years after the formation of the first solids in our Solar System.
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Title: Chondrule formation via extended winds in the early solar system
Authors: Raquel Salmeron, Trevor Ireland

Chondrite meteorites are believed to represent the building blocks of the solar nebula, out of which our solar system formed. They are a mixture of silicate and oxide objects (chondrules and refractory inclusions) that experienced extremely high temperatures, set in a matrix that remained relatively cold. The prevalence of chondrites suggests that they formed through a very general process, closely related to stellar and planet formation, however the nature and properties of the responsible mechanism have remained unclear for many decades. The evidence for a hot solar nebula provided by chondrules and refractory inclusions is, however, seemingly at odds with astrophysical observations of forming stars. These strongly indicate that protostellar disks - the inspiralling disks of gas and dust out of which stars and planets form - are relatively cool, and exhibit typical temperatures that are insufficient to melt and vapourise silicate minerals at the radial distances sampled by chondrule-bearing meteorites in the main asteroid belt. Here we present calculations of the dynamical and thermal structure of protostellar disks that accelerate a wind from the disk surfaces. These winds are commonly associated with young stellar objects and are the analogues of the early solar system. We also present models of the processing of dust particles in such winds, showing that these outflows are suitable sites for chondrule formation.

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Title: Experimental investigation of the nebular formation of chondrule rims and the formation of chondrite parent bodies
Authors: Eike Beitz, Jürgen Blum, Romain Mathieu, Andreas Pack, Dominik C. Hezel

We developed an experimental setup to test the hypothesis that accretionary dust rims around chondrules formed in the solar nebula at elevated temperatures. Our experimental method allows us to form dust rims around chondrule-analogues while being levitated in an inert-gas flow. We used micrometer-sized powdered San Carlos olivine to accrete individual dust particles onto the chondrule-analogue at a temperature of 1100°C. The resulting dust-rims were analysed by means of two different techniques: one sample was investigated with non-destructive micro computer tomography, the other with a scanning electron microscope. Both methods give very similar results for the dust-rim structure and a mean dust-rim porosity of 60 percent, demonstrating that both methods are equally well suited for sample analysis. The chondrule-analogue's bulk composition has no measurable effect on the accretion efficiency of the dust. We measured the chemical composition of chondrule-analogue and dust-rim to check whether elemental exchange between the tow components occurred. Such a reaction zone was not found, thus we can experimentally confirm the sharp border between chondrules and dust-rims described in the literature. The experimental dust-rim porosities and reported porosities in carbonaceous and ordinary chondrites depend on the meteorite type, indicating different degrees of compaction subsequent to parent

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Cosmochemists have identified six main compositional types of magma that formed inside asteroids during the first 100 million years of Solar System history. These magmas vary in their chemical and mineralogical make up, but all have in common low concentrations of sodium and other volatile elements. Our low-sodium-magma diet has now changed. Two groups of researchers have identified a new type of asteroidal magma that is rich in sodium and appears to have formed by partial melting of previously unmelted, volatile-rich chondritic rock. The teams, one led by James Day (University of Maryland) and the other by Chip Shearer (University of New Mexico), studied two meteorites found in Antarctica, named Graves Nunatak 06128 and 06129, using a battery of cosmochemical techniques. These studies show that an even wider variety of magmas was produced inside asteroids than we had thought, shedding light on the melting histories and formation of asteroids.

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Chondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body. They formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids. Prominent among the components present in chondrites are the enigmatic chondrules, millimetre-sized objects that originated as freely floating, molten or partially molten droplets in space; most chondrules are rich in the silicate minerals olivine and pyroxene. Chondrites also contain refractory inclusions (including Ca-Al Inclusions), which are among the oldest objects to form in the solar system, particles rich in metallic Fe-Ni and sulphides, and isolated grains of silicate minerals. The remainder of chondrites consists of fine-grained (micrometer-sized or smaller) dust, which may either be present as the matrix of the rock or may form rims or mantles around individual chondrules and refractory inclusions. Embedded in this dust are presolar grains, which predate the formation of our solar system and originated elsewhere in the galaxy.

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Rocks from Space Picture of the Day:



A large fresh (unclassified) chondrite weighing 1098 grams. A beautiful specimen with lots of black fusion crust and regmaglypts.

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