Metamorphic grade

TaUe 13.29. Most common type of metamorphic rocks 

Metamorphic rocks Protolith (i.e., parent rock) Texture Description 

Marble limestones non- foliated interlocking, often coarse, calcite crystals, little or no porosity 

Hornfels shales foliated Shales baked by igneous contact form very hard finegrained rocks 

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Colloform textures of sulfide minerals were found in ores of the metamorphic grades lower than the green schist facies (Kase, 1988 Watanabe et al., 1993). These... 

Other meteorite classes like C2, CO and ordinary chondrites contain much smaller inclusions less than 1 mm (MacPherson et al. 1988) and require ion microprobe techniques to evaluate the isotopic compositions. On the least metamorphosed side. Cl have very few inclusions or oxide grains, but are the carrier of the greatest amounts of stellar nanodiamond and other carbides (Anders and Zirmer 1993). As will be shown for Cr anomalies in carbonaceous chondrites, the survival of the mineral carriers of the anomalies also depends on the metamorphic grade of the meteorites. Nevertheless, isotopic anomalies have also been formd in higher metamorphic grade from other classes, especially in the reduced enstatite chondrites. 

Solar system processes are also among the targets, because of the disappearance of this heterogeneity with the metamorphic grade of the meteorites. Studies of temperature and duration of metamorphism are still in their early stages (El Goresy et al. 1995 Huss 1997)... 

None of the dissolution fractions has normal isotopic compositions. This implies that the solar system results from mixing of major Cr-bearing components, none having the solar (terrestrial) isotopic compositions. Data obtained from higher metamorphic grade... 

Chromium. Much smaller variations are evident in this isotope usually a few e. They are not correlated with Cr variations but with the Mn/Cr elemental ratios. They are also present in the higher metamorphic grades with even larger spreads. Cr is not produced directly in stars but through Mn which then decays to Cr. These arguments favor the interpretation of Cr as solely due to in situ Mn decay (Birck et al. 1999). 

Saxena S. K. (1968). Distribution of iron and magnesium between coexisting garnet and clino-pyroxene in rocks of varying metamorphic grade. Amer. Mineral, 53 2018-2021. 

Fig. 3.50 Coupled O-C trends showing decreasing values of 8 C and 8 0 with increasing metamorphic grade from numerous contact metamorphic localities (Baumgartner and VaUey, 2001)...

Fig. 3.53 Frequency distribution of calcite-graphite fractionations (A) with increasing metamorphic grade (after Des Marais 2001)...

The characteristics of the presolar diamonds also change with the metamorphic grade of the host meteorite. F igure 5.15 shows the typical bimodal release of heavy noble gases (here illustrated by xenon) in Orgueil, an unheated chondrite. This pattern is compared to the xenon-release patterns of two ordinary chondrites that have experienced different degrees of mild metamorphism. The amount of low-temperature gas, labeled P3 for historical reasons, is a sensitive function of temperature. Its abundance correlates well with other indicators of... 

The compositional variation from grain to grain is greater in low temperature 14 8 chlorites and this scatter decreases as metamorphic grade increases. Low temperature chlorites tend to be ferrous and aluminous. 

It appears that increasing metamorphic grade tends to give a 30% aluminum atomic content in chlorites and often to decrease the iron content (Figure 31b). 

Figure 7 represents the variation of the reflectance values in each zone of both the Seams 2 and 4. The distribution of the various types of vitrinoids encountered in various zones are represented in Figures 4 and 4a, respectively. Principally, the types of vitrinoids encountered are V4, V5, V , V7, and Vs with an abundance of V type of vitrinoid. Next in abundance to Ve are V5 and V7 followed by V4 and Vs. From Figures 4 and 4a, it is noticed that Vs is more abundant in Seam 4 than in Seam 2. Seam 4 is 40 feet below Seam 2, and as such, a general slight metamorphic grade may be expected. 

Diessel, C.F. and Offler, R., 1973. The relationship between graphite rank and metamorphic grade in the Mount Lofty Ranges, South Australia. Search, 4 497-499. 

Figure 4 Abundances of refractory and moderately volatile elements in various groups of carbonaceous chondrites, normalized to Cl and Mg. Refractory elements increase from Cl to CV3 chondrites while Mg/Si ratios are constant in all groups of carbonaceous chondrites. Although the elements Cr, Fe, and P are significantly less depleted than Mn and Zn, they show a similar behavior, suggesting volatility related depletions of Cr, Fe, and P in carbonaceous chondrites of higher metamorphic grades (source Wolf and Palme, 2001).

Van Schmus and Wood, 1967). This metamorphism occurred in an almost closed system with respect to oxygen, resulting in less than 0.5%o variation in 5 0 for different metamorphic grades within each iron group (H, L, and LL) (Clayton et al., 1991). This observation is in accord with the inferred anhydrous state of the O-chondrite parent bodies. 

The C03 meteorites have been subdivided into metamorphic grades from 3.0 to 3.7, with peak temperatures in the range 450-600 °C (Rubin, 1998). In contrast to the metamorphism of the ordinary chondrites, the CO metamorphism probably occurred in the presence of water, and the system was not closed with respect to oxygen. The least metamorphosed CO meteorites, ALH 77307 and Y 81020, are the most 0-rich Loongana 001 and HH 073, classified as 3.8 or 4, are the most 0-poor. There is not, however, a simple one-to-one correspondence between metamorphic grade and isotopic composition. Since most of the CO chondrites are finds, some may have been altered by terrestrial weathering. 

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