The Missing Achondrites: Taking a Pinch of Salt with the Nebula (original) (raw)
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Igneous Processes of the Early Solar System By
2003
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American Mineralogist, 2008
The recently recovered Antarctic achondrites Graves Nunatak 06128 and 06129 are unique meteorites that represent high-temperature asteroidal processes in the early solar system never before identified in any other meteorite. They represent products of early planetesimal melting (4564.25 ± 0.21 Ma) and subsequent metamorphism of an unsampled geochemical reservoir from an asteroid that has characteristics similar to the brachinite parent body. This melting event is unlike those predicted by previous experimental or geochemical studies, and indicates either disequilibrium melting of chondritic material or melting of chondritic material under volatile-rich conditions.
The Astrophysical Journal, 2009
We have identified in an acid resistant residue of the carbonaceous chondrite Murchison a large number (458) of highly refractory metal nuggets (RMNs) that once were most likely hosted by Ca,Al-rich inclusions (CAIs). While osmium isotopic ratios of two randomly selected particles rule out a presolar origin, the bulk chemistry of 88 particles with sizes in the submicron range determined by energy dispersive X-ray (EDX) spectroscopy shows striking agreement with predictions of single-phase equilibrium condensation calculations. Both chemical composition and morphology strongly favor a condensation origin. Particularly important is the presence of structurally incompatible elements in particles with a single-crystal structure, which also suggests the absence of secondary alteration. The metal particles represent the most pristine early solar system material found so far and allow estimation of the cooling rate of the gaseous environment from which the first solids formed by condensation. The resulting value of 0.5 K yr −1 is at least 4 orders of magnitude lower than the cooling rate of molten CAIs. It is thus possible, for the first time, to see through the complex structure of most CAIs and infer the thermal history of the gaseous reservoir from which their components formed by condensation.
Geochimica Et Cosmochimica Acta, 2010
The recently recovered paired Antarctic achondrites Graves Nunatak 06128 and 06129 (GRA) are meteorites that represent unique high-temperature asteroidal processes that are identified in only a few other meteorites. The GRA meteorites contain high abundances of sodic plagioclase, relatively Fe-rich pyroxenes and olivine, abundant phosphates, and low temperature alteration. They represent products of very early planetesimal melting (4565.9 ± 0.3 Ma) of an unsampled geochemical reservoir from an asteroid that has characteristics similar to the brachinite parent body. The magmatism represented by these meteorites is contrary to the commonly held belief that the earliest stages of melting on all planetary bodies during the first 2-30 Ma of solar system history were fundamentally basaltic in nature. These sodic plagioclase-rich rocks represent a series of early asteroidal high-temperature processes: (stage 1) melting and partial extraction of a low-temperature Fe-Ni-S melt, (stage 2) small degrees of disequilibrium partial melting of a sodium-or alkali-rich chondritic parent body with additional incorporation of Fe-Ni-S melt that was not fully extracted during stage 1, (stage 3) volatile-enhanced rapid extraction and emplacement of the Na-rich, high-normative plagioclase melt, (stage 4) final emplacement and accumulation of plagioclase and phosphates, (stage 5) subsolidus reequilibration of lithology between 962 and 600°C at an fO 2 of IW to IW + 1.1, and (stage 6) replacement of merrillite and pyroxene by Cl-apatite resulting from the interaction between magmatic minerals and a Cl-rich fluid/residuum melt. The subsolidus events started as early as 4561.1 Ma and may have continued for upwards of 144 million years.
Thermal evolution and differentiation of planetesimals and planetary embryos
In early Solar System during the runaway growth stage of planetary formation, the distribution of plan- etary bodies progressively evolved from a large number of planetesimals to a smaller number of objects with a few dominant embryos. Here, we study the possible thermal and compositional evolution of these planetesimals and planetary embryos in a series of models with increasing complexities. We show that the heating stages of planetesimals by the radioactive decay of now extinct isotopes (in particular 26Al) and by impact heating can occur in two stages or simultaneously. Depending on the accretion rate, melting occurs from the center outward, in a shallow outer shell progressing inward, or in the two loca- tions. We discuss the regime domains of these situations and show that the exponent b that controls the planetary growth rate R_ / Rb of planetesimals plays a crucial role. For a given terminal radius and accre- tion duration, the increase of b maintains the planetesimals very small until the end of accretion, and therefore allows radioactive heating to be radiated away before a large mass can be accreted. To melt the center of 500 km planetesimal during its runaway growth stage, with the value b = 2 predicted by astrophysicists, it needs to be formed within a couple of million years after condensation of the first solids. We then develop a multiphase model where the phase changes and phase separations by compac- tion are taken into account in 1-D spherical geometry. Our model handles simultaneously metal and sil- icates in both solid and liquid states. The segregation of the protocore decreases the efficiency of radiogenic heating by confining the 26Al in the outer silicate shell. Various types of planetesimals partly differentiated and sometimes differentiated in multiple metal–silicate layers can be obtained.
Protostars and Planets VI, 2014
Radioisotopic ages for meteorites and their components provide constraints on the evolution of small bodies: timescales of accretion, thermal and aqueous metamorphism, differentiation, cooling and impact metamorphism. Realising that the decay heat of short-lived nuclides (e.g. 26 Al, 60 Fe), was the main heat source driving differentiation and metamorphism, thermal modeling of small bodies is of utmost importance to set individual meteorite age data into the general context of the thermal evolution of their parent bodies, and to derive general conclusions about the nature of planetary building blocks in the early solar system. As a general result, modelling easily explains that iron meteorites are older than chondrites, as early formed planetesimals experienced a higher concentration of short-lived nuclides and more severe heating. However, core formation processes may also extend to 10 Ma after formation of Calcium-Aluminum-rich inclusions (CAIs). A general effect of the porous nature of the starting material is that relatively small bodies (< few km) will also differentiate if they form within 2 Ma after CAIs. A particular interesting feature to be explored is the possibility that some chondrites may derive from the outer undifferentiated layers of asteroids that are differentiated in their interiors. This could explain the presence of remnant magnetization in some chondrites due to a planetary magnetic field.
From Supernovae to Planets: The View from Meteorites and Interplanetary Dust Particles
Chondritic meteorites and IDPs retain a record of the prehistory and early history of the Solar System. Chondrites are derived from the asteroid belt, while IDPs probably have both cometary and asteroidal origins. Chondrites and their components contained relatively high levels of short-lived radionuclides when they formed. Some, like 60 Fe, require a stellar source, while others may have formed via energetic particle irradiation in the Solar System. The half-lives of some of the radionuclides are so short (0.1-0.7 My) that if they had a stellar source, this source probably triggered the formation of the Solar System. The high abundance of crystalline circumstellar silicates in IDPs and meteorites, and the relatively low abundance of interstellar organic matter in CI chondrites may result from the thermal processing of interstellar dust seen in YSOs. The oldest dated Solar System objects are the refractory inclusions. The more abundant chondrules seem to have begun forming 1-2 My after refractory inclusions, although there is evidence that chondrules in the CV chondrites began forming contemporaneously with refractory inclusions. Both refractory inclusions and chondrules appear to be the products of transient heating events. The mechanism for making refractory inclusions is uncertain, but in most models refractory inclusions form sunward of the asteroid belt and are then transported outwards either in energetic winds or via turbulent diffusion. At present, the most promising mechanism for making chondrules is shock heating in the asteroid belt. Each chondrite group contains a chemically and/or physically distinct population of chondrules and refractory inclusions. To preserve their distinct chondrule properties from being erased by turbulent diffusion, it is argued that chondrites must have accreted soon after their chondrules formed. However, the variation in the properties of refractory inclusions between chondrites is unexplained. To explain the evidence for aqueous alteration in most chondrites, chondrite formation occurred in the T Tauri phase when temperatures in the asteroid belt allowed for ice to be stable.
THE ORIGIN OF THE SOLAR SYSTEM PROBLEMS WITH THE 'COLD NEBULA' MODEL AND AN ALTERNATIVE MODEL
Proc. 4th International Symposium on Geophysics, Tanta (2006), 1-14, 2006
Current models of the origin of the solar system are that a cold nebula cloud gravitationally collapsed to form the Sun and planets. This was supported observations that (1) the cloud had been cold and, apart from the near the Sun, continued to be cold, and (2) the original chemical compositions the Sun and inner planets were identical. The former was based on the presence of different oxygen isotopic reservoirs in different parts of the solar system. The latter was based on identical element ratios in the meteorites and the Sun's surface. While both observations are valid, they can now be explained in other ways. This means that an extremely hot stage is not excluded and that the 'meteorite' similarity of the solar atmosphere may be due to continuing meteoritic contamination. The supernova, needed to provide the 'heavy element' components of the solar system, is shown to be less than a light year distant. A supernova star at such distances should have dissipated any nebula on at least two occasions; when the star formed and when it exploded. Such considerations, combined with other major problems -such as insufficient time for gravitational collapse and the distribution of the angular momentum within the solar system-suggest that the standard nebula collapse model is inappropriate for the origin of the solar system. It is proposed that the Sun was originally a planet orbiting a star destined to explode as a supernova. Late-stage instability in this supernova ejected a cloud of cool unburned hydrogen that formed a nebula around the planet shortly before the supernova's catastrophic destruction. This rapid acquisition of a nebula instigated internal nuclear burning in the planet, converting it to a star, the Sun, with little increase in its very slow axial rotation. The Sun evolved into the highly active τ-tauri stage by the time of the supernova explosed, enabling the Sun's magnetic field and solar winds to both trap and deflect supernova debris. Little or no mass was added to the Sun, leaving it rotating slowly around its axis. Polar jets dispersed all matter from the Sun's polar areas, leaving a very hot, rotating equatorial disk of contaminated primordial hydrogen that cooled and fractionated under the influence of the Sun's gravity, magnetic and radiative properties. After a few kyr or so, the heated gases condensed and the oldest meteoritic components formed, initially as globules that aggregated into mm-to cm-sized blobs. As the ambient temperature decreased, the globules crystallised and other grains formed and grew by molecular additions. Eventually, the temperatures fell to close to 0ºC when ice, in particular, enabled accretion of the globules and mineral grains to form the carbonaceous meteorites. Some of these accumulated into planetisimals and, outside the asteroid belt, evolved into the planets, retaining their high angular momentum around the Sun.
From supernovae to planets: The view from meteorites and IDPs
Chondritic meteorites and IDPs retain a record of the prehistory and early history of the Solar System. Chondrites are derived from the asteroid belt, while IDPs probably have both cometary and asteroidal origins. Chondrites and their components contained relatively high levels of short-lived radionuclides when they formed. Some, like 60 Fe, require a stellar source, while others may have formed via energetic particle irradiation in the Solar System. The half-lives of some of the radionuclides are so short (0.1-0.7 My) that if they had a stellar source, this source probably triggered the formation of the Solar System. The high abundance of crystalline circumstellar silicates in IDPs and meteorites, and the relatively low abundance of interstellar organic matter in CI chondrites may result from the thermal processing of interstellar dust seen in YSOs. The oldest dated Solar System objects are the refractory inclusions. The more abundant chondrules seem to have begun forming 1-2 My after refractory inclusions, although there is evidence that chondrules in the CV chondrites began forming contemporaneously with refractory inclusions. Both refractory inclusions and chondrules appear to be the products of transient heating events. The mechanism for making refractory inclusions is uncertain, but in most models refractory inclusions form sunward of the asteroid belt and are then transported outwards either in energetic winds or via turbulent diffusion. At present, the most promising mechanism for making chondrules is shock heating in the asteroid belt. Each chondrite group contains a chemically and/or physically distinct population of chondrules and refractory inclusions. To preserve their distinct chondrule properties from being erased by turbulent diffusion, it is argued that chondrites must have accreted soon after their chondrules formed. However, the variation in the properties of refractory inclusions between chondrites is unexplained. To explain the evidence for aqueous alteration in most chondrites, chondrite formation occurred in the T Tauri phase when temperatures in the asteroid belt allowed for ice to be stable.