Meteorites equilibration

Sinha S. K., Sack R. O., and Lipschutz M. E. (1997) Ureifite meteorites equilibration temperatures and smelting reactions. Geochim. Cosmochim. Acta 61, 4235—4242. 

Mullane E, Russel SS, Gounelle M (2002) Iron isotope fractionation within a differentiated asteroidial sample suite. 65 Annual Meteoritical Society Meeting, abstract number 5157 Mullane E, Russel SS, Gounelle M, Mason TED (2003a) Iron isotope composition of Allende and Chainpur chondrules effects of equilibration and thermal history. Lunar Planet Sci Conf XXXIV, abstract number 1027... 

According to Cameron and Papike (1982), pyroxenes contain Cr " and TF" in rocks equilibrated at low fo (lunar specimens, meteorites). However, spectroscopic evidence is ambiguous and insufficient for a safe attribution (Rossman, 1982). Some authors (Bocchio et ah, 1979 Ghose et al., 1986 Griffin and Mot-tana, 1982) report the presence of Mn " in Ml sites in clinopyroxene. Davoli (1987) reexamined this hypothesis, proposing precise structural criteria to detect the presence of Mn " in the monoclinic phase (the ratio Mn /Mn may be a potential /02 barometer). 

The Re- Os method was first applied to extraterrestrial samples in the early 1960s when Hirt et al. (1963) reported a whole-rock isochron for 14 iron meteorites that gave an age of 4 Ga. Further development of this system was hindered by several technical difficulties. Rhenium and osmium each exist in multiple oxidation states and can form a variety of chemical species, so complete digestion of the samples, which is required to chemically separate rhenium and osmium for mass spectrometry, is difficult. In addition, accurate determination of rhenium abundance and osmium isotopic composition requires spiking the samples with isotopically labeled rhenium and osmium, and equilibration of spikes and samples is challenging. A third problem is that osmium and, particularly, rhenium are very difficult to ionize as positive ions for mass spectrometry. These problems were only gradually overcome. 

Annealing experiments on meteorites and terrestrial feldspars, and studies of separated components from meteorites, suggest that the high and low temperature forms of feldspar have distinctive TL peak temperatures and widths and thus provide a new means of exploring equilibration temperatures and cooling rates. 

Diffusion tends to equilibrate concentration differences between two reservoirs upon contact fluorine concentration profiles develop at the boundary of the two compartments as a function of time. Studies of the distribution of this trace element in archaeological samples such as bones, teeth or flints allow to gain some age information on the excavated objects of a burial site. The presented technique using beams of accelerated protons allows to measure fluorine diffusion profiles with an excellent space resolution. The surface exposure duration was deduced by the same method for Antarctic meteorites. 

Therefore these data for the short-lived chronometer Hf-W provide a consistent picture of rapid accretion, equilibration, and planetesimal differentiation in the early solar system with only small (106-year) time differences resolvable between some events for the parent bodies of chondrites, basaltic achondrites, and iron meteorites. 

Several types of the early Solar System materials are available for laboratory analysis (see Chapter 1 and Table 1.1 and Fig. 1.1). Each material has unique characteristics and provides specific constraints on the chemistry of the solar nebula. Major components of this sample are meteorites, fragments of asteroids, that serve as an excellent archive of the early Solar System conditions. Primitive chondritic meteorites contain glassy spherical inclusions termed chondrules, some of the oldest solids in the Solar System. Most chondrites were modified by aqueous alteration or metamorphic processes in parent bodies but there are some chondrites that are minimally altered (un-equilibrated chondrites, UCs). They have yielded a wealth of information on the chemistry, physics, and evolution of the young Solar System. 

Figure 8.4 Chondrules dominate the volume and mass of primitive chondrites, such as in this un-equilibrated ordinary chondrite, Semarkona (LL3.0). Reprinted from A Color Atlas of Meteorites in Thin Section (D. S. Lauretta and M. Killgore, Golden Retriever Publications, 2005). 

During impacts, the meteoritic metal equilibrates with the surrounding silicate at elevated temperatures and W, having considerable siderophile tendency, enters the metal phase. 

Takeda H. and Graham A. L. (1991) Degree of equilibration of eucritic pyroxenes and thermal metamorphism of the earliest planetary crust. Meteoritics 26, 129-134. 

Folco L., Mellini M., and Pillinger C. T. (1997) Equilibrated ordinary chondrites constraints for thermal history from iron-magnesium ordering in orthopyroxene. Meteorit. Planet. Sci. 32, 567-575. 

How well are these assumptions justified Hafiiium-tungsten data for differentiated meteorites indicate that accretion and differentiation of their parent bodies occurred very early (see above). Bodies that formed so early would have melted and differentiated due to accumulation of radiogenic heat produced by decay of abundant Al. Earth therefore most likely accreted from predifferentiated planetesimals. The other assumptions, i.e., constant HfrW ratios and complete metal-silicate re-equilibration, are less well constrained. The effects that changes in these two parameters have on the W isotope evolution of Earth s mantle are shown in Figure 7. 

Gdpel C, Manhes G, Allegre CJ (1991) Constraints on the time-scale of accretion and thermal evolution of chondrite parent bodies by precise U-Pb dating of phosphates. Meteoritics 26 338 Gdpel C, Manhes G, Allegre CJ (1994) U-Pb systematics of phosphates from equilibrated ordinary chondrites. EarthPlanet Sci Lett 121 153-171 Hartmaim WK (1975) Lunar "cataclysm" A misconception. Icaras 24 181-187... 

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