Thermal metamorphism

No compositional zoning in sphalerite grain is observed, suggesting original zoning has been homogenized by thermal metamorphism (Mizuta, 1988). 

In thermal metamorphism, biotite occurs in the clorite-sericite facies as a dispersed phase in the argillitic matrix and is stable up to low-grade cornubianites. In regional metamorphism, biotite is typical of argillitic and pelitic rocks up to the staurolite-garnet facies (biotite, biotite-sericite, biotite-chlorite, and albite-biotite schists, and garnet-staurolite micaschists). 

The Metamorphic Rocks,— The point where a rock ceases to be igneous or sedimentary and becomes metamorphic is not now exactly defined, though subject to exact definition, but a distinct set of physical and chemical questions undoubtedly enters into the problems of dynamic and thermal metamorphism. The effects of non-uniform pressure in causing the flowing of crystalline substances and aggregates and their... 

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... 

Chronology of secondary processes in the early solar system. Plot format and anchor points are the same as in Fig. 9.9. Dates related to thermal metamorphism are shown as open symbols, and dates related to aqueous alteration are shown as filled symbols. Both thermal metamorphism and aqueous alteration continued for tens to as much as 100 Myr after CAI formation. Data from Flohenberg and Pravdivtseva (2008), Flutcheon et al. (1998), Flua etal. (2005), Trinquier etal. (2008), Endress et al. (1996), Hoppe et al. (2004), and Zinner and Gopel (2002). 

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. 

Evidence for rubble pile asteroids comes from a variety of observations. The low densities of many asteroids imply that they have high porosities, presumably resulting from the assembly of loose fragments. Spectral variations seen in some S-class asteroids as they rotate also support rubble piles. The variations suggest that portions of the surface have experienced different degrees of thermal metamorphism. Catastrophic collision and reassembly has transformed bodies that formerly had onion shell structures into rubble piles. 

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. 

Reflectance. The optical properties (reflectance) are not in accord with the chemical properties for these coal samples, and the maximum reflectance of the coals indicates that they are higher in rank than would be concluded from the chemical data alone. These discrepancies are not surprising since these coals are thermally metamorphosed and may not follow the normal coalifica-tion curve (8). For the subject samples, it was decided that chemical data did not suitably indicate rank or the degree of thermal metamorphism, particularly in those instances where the samples contained so much ash that they were not suitable for routine chemical tests. The maximum reflectance in oil of these coals ranges from 2.6% to 11.5% (Table I). The lower reflectance is similar to that encountered in some semianthracites and anthracites, whereas the upper reflectance is more nearly that of graphite or long term, high tern-... 

Wescott, M. R. (1966) Loss of argon from biotite in a thermal metamorphism. Nature, 210, 83-4. 

The primary focus of research on secondary porosity formation has been on mechanisms for generating undersaturated formation waters. Because reactions that may result in undersaturation of waters with respect to carbonate minerals by consumption of calcium are unlikely to be quantitatively important, emphasis has been placed on reactions that may lower the carbonate ion concentration. Although not clearly documented in deep subsurface environments, mixing of waters of dissimilar composition can result in undersaturation with respect to calcite (see Chapter 7), and lead to secondary porosity formation. Acidic waters associated with igneous intrusions and thermal metamorphism can also cause carbonate dissolution that results in secondary porosity (e.g., deep Jurassic carbonates in Mississippi, U.S.A. Parker, 1974). 

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,... 

Interaction with minor amounts of aqueous fluids can modify the primary mineralogy and thermal metamorphism at low temperatures (above 520—570 K) drives recrystallization of the matrix to form coarser-grained materials. Unfortunately, the effects of thermal metamorphism on matrices have only been fully appreciated recently. Consequently, much of the older literature on matrices is... 

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. 

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