Chondrite thermal metamorphism

Chondrites are the oldest and most primitive rocks in the solar system. They are hosts for interstellar grains that predate solar system formation. Most chondrites have experienced a complex history, which includes primary formation processes and secondary processes that inclnde thermal metamorphism and aqneons alteration. It is generally very difficult to distinguish between the effects of primary and secondary processes on the basis of isotope composition. Chondrites display a wide diversity of isotopic compositions including large variations in oxygen isotopes. 

Almost immediately after the discovery of presolar grains, it was clear that they could only be found in the most primitive chondrites, those that had suffered the least amount of thermal metamorphism. Further work showed that the abundances of presolar grains, when normalized to the content of fine-grained matrix where the grains reside, correlated strongly... 

Once formed, the chondrite parent bodies experience a variety of processes, including thermal metamorphism, aqueous alteration, shock metamorphism due to impacts, and even disruption from large impacts. Several radiochronometers can provide information on the timing of metamorphism and aqueous alteration. The chronology of this processing is summarized in Figure 9.11. 

Thermal metamorphism in chondrites and melting in differentiated asteroids are driven by heat produced by the decay of short-lived radionuclides (especially 26A1). Thermal models can reproduce the peak temperatures and cooling rates estimated for meteorites, as well as... 

The aqueous fluids formed by melting of ices in asteroids reacted with minerals to produce a host of secondary phases. Laboratory studies provide information on the identities of these phases. They include hydrated minerals such as serpentines and clays, as well as a variety of carbonates, sulfates, oxides, sulfides, halides, and oxy-hydroxides, some of which are pictured in Figure 12.15. The alteration minerals in carbonaceous chondrites have been discussed extensively in the literature (Zolensky and McSween, 1988 Buseck and Hua, 1993 Brearley, 2004) and were most recently reviewed by Brearley (2006). In the case of Cl chondrites, the alteration is pervasive and almost no unaltered minerals remain. CM chondrites contain mixtures of heavily altered and partially altered materials. In CR2 and CV3oxb chondrites, matrix minerals have been moderately altered and chondrules show some effects of aqueous alteration. For other chondrite groups such as CO and LL3.0-3.1, the alteration is subtle and secondary minerals are uncommon. In some CV chondrites, a later thermal metamorphic overprint has dehydrated serpentine to form olivine. 

The CV, CO, and CR chondrites are mostly anhydrous and were considered in Chapter 11. However, the CV3oxB chondrites experienced significant aqueous alteration. The matrices of these meteorites are heavily altered and contain phyllosilicates, fayalite, Fe,Ni sulfides and carbides, Ca,Fe pyroxene, and andradite garnet. The CVoxA chondrites were apparently also aqueously altered, but were subsequently dehydrated by thermal metamorphism. The matrices of CR2 chondrites contain alteration minerals that resemble those in Cl chondrites, including phyllosilicates, magnetite (Fig. 12.15d), carbonates and sulfides, although the alteration is not as extensive. Chondrule mesostasis was affected in some CR chondrites. Minor phyllosilicates occur in the matrix of chondrites, but these meteorites contain no carbonates or sulfates. 

Meteorites are divided into two broad categories chondrites, which retain some record of processes in the solar nebula and achondrites, which experienced melting and planetary differentiation. The nebular record of all chondritic meteorites is obscured to varying degrees by alteration processes on their parent asteroids. Some meteorites, such as the Cl, CM, and CR chondrites, experienced aqueous alteration when ice particles that co-accreted with the silicate and metallic material melted and altered the primary nebular phases. Other samples, such as the ordinary and enstatite chondrites, experienced dry thermal metamorphism, reaching temperatures ranging from about 570 to 1200 K. In order to understand the processes that occurred in the protoplanetary disk, we seek out the least-altered samples that best preserve the record of processes in the solar nebula. The CV, CO,... 

Meteorites provide perhaps the best record of the chemical evolution of small bodies in the Solar System, and this record is supplemented by asteroidal spectroscopy. Meteorites show progressive degrees of thermal processing on their parent asteroids, from primitive carbonaceous chondrites that contain percent-level quantities of water, through ordinary chondrites that show a wide range of degree of thermal metamorphism, to the achondrites that have been melted and differentiated. 

Adelaide is an ungrouped, type-3 carbonaceous chondrite with affinities to the CM-CO clan, but appears to have escaped the thermal metamorphism and alteration commonly observed in the CM and CO carbonaceous chondrites (Fitzgerald and Jones, 1977 Davy et al., 1978 Kallemeyn and Wasson, 1982 Hutcheon and Steele, 1982 Kerridge, 1985 Brearley, 1991 Huss and Hutcheon, 1992 Kiotetal., 2001b,c). 

Ikeda Y. and Prinz M. (1993) Petrologic study of the Belgica 7904 carbonaceous chondrite hydrous alteration, thermal metamorphism, and relationship to CM and Cl chondrites. Geochim. Cosmochim. Acta 57, 439-452. 

Tomeoka K., KojimaH., and YanaiK. (1989) Yamato-86720 a CM carbonaceous chondrite having experienced extensive aqueous alteration and thermal metamorphism. Proc. NIPR Symp. Antarct. Meteorit. 2, 55—74. 

All chondrites were modihed in some way by geological processes in asteroids operating over 4.5 Gyr. If we want to understand how the components in chondrites were formed, we must understand how chondritic materials were modified in asteroids. Three processes affected chondrites aqueous and hydrothermal alteration, thermal metamorphism, and impacts. 

Figure 6 Oxygen-isotopic compositions of individual minerals in CAIs from the CO chondrites Y-81020, Colony, Kainsaz and Ornans. Primary minerals in CAIs from the least metamoprhosed CO chondrites Y-81020 (type 3.0) and Colony (3.0) are uniformly 0-enriched, whereas CAIs from Kainsaz (3.2) and Omans (3.3) tend to show oxygen isotopic heterogeneity with spinel and high-calcium pyroxene enriched in 0 and melilite and secondary nepheline depleted in 0. Based on these observations, Wasson et al (2001) inferred that oxygen isotope exchange took place during thermal metamorphism and alteration in an asteroid (data from Itoh et al, 2000 Wasson et al, 2001).

Figure 11 Compositions of olivine in AO As from the reduced CV chondrites Vigarano (a), Leoville (b), Efremovka (c), oxidized CV chondrite Allende (d), CR chondrites (e), CH and CB chondrites (f), unique carbonaceous chondrites Adelaide (g), and Acfer 094 (h). Olivines in CRs, Adelaide, and Acfer 094 are magnesium-rich compared to olivines in CV AO As. Fayalite contents in olivines from CV AO As increase in the order Leoville and Vigarano, Efremovka, Allende this is correlated with the degree of secondary alteration and thermal metamorphism experienced by CV chondrites. Data for Allende AO As are from Hashimoto and Grossman (1987) and for Efremovka, Leoville, and Vigarano AOAs from Komatsu et al (2001).

Rubin A. E., Zolensky M. E., and Bodnar R. J. (2002) The halite-bearing Zag and Monahans (1998) meteorite breccias shock metamorphism, thermal metamorphism and aqueous alteration on the H-chondrite parent body. Meteorit. Planet. Sci. 37, 124-141. 

Kitajima F., Nakamura T., Takaoka N., and Murae T. (2002) Evaluating the thermal metamorphism of CM chondrites by using the pyrolytic behavior of carbonaceous macromolecu-lar matter. Geochim. Cosmochim. Acta 66, 163-172. 

Differentiated achondrites. They are achondrites that exhibit igneous textures or igneous textures modified by impact and/or thermal metamorphism, and that have compositions of lithophUe, siderophile, chalcophile, and atmophile elements that are highly fractionated from the ranges of chondritic materials. 

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