Metamorphism, limit

The application of the laser probe to meteorite chronology is illustrated by a study of Ca-Al-rich inclusions from the Allende meteorite [7]. This study was able to show that the K in the inclusions studied mainly concentrated in veins and rims with very little, if any, K in the major minerals. The limit obtained is something of the order of 10 ppm. On the other hand, the major minerals do contain appreciable 40Ar. Individual chondrules and the matrix were also studied in the Allende meteorite from places adjacent to the Ca-Al-rich inclusions. For these samples the ages varied from 3.3 to 4.4 G.y. There appears to be evidence that the Allende meteorite has been subjected to numerous metamorphic events, presumably of a collisional origin. 

The extent to which isotopic fractionation occurs at sub-1000°C temperatures in magmatic and regional metamorphic systems is unstudied in detail, but limited natural and experimental data in granitic systems suggest that measurable fractionation may occur under these conditions. This fractionation might be applied in studies of magmatic and ore evolution, either for thermometry or mineral deposit exploration. 

Contents of REE in massive sulfides from the BMC are strongly controlled by the abundance of and REE concentrations in phosphate minerals, specifically apatite, xenotime and monazite. Strong positive Eu anomalies in apatite, account for the anomalous Eu signatures of exhalative sulfides whereas REE in monazite masses are largely reflective of detrital sources and may mask hydrothermal signatures. Limited release of mobile trace elements (LREE and Eu) during green-schist facies metamorphism has partly modified REE profiles for VMS deposits of the BMC. 

Teng FZ, McDonough WF, Rudnick RL, Wing BA (2007) Limited lithium isotopic fractionation during progressive metamorphic dehydration in metapelites a case study from the Onawa contact aureole, Maine, Chem Geol 239 1-12... 

Todd CS, Evans BW (1993) Limited fluid-rock interaction at marble-gneiss contacts during Cretaceous granulite-facies metamorphism, Seward Peninsula, Alaska. Contr Miner Petrol 114 27 1... 

Because of its simple chemical composition, the role of kaolinite in clay mineral assemblages is peripheral to or a limiting case for chemiographic representation of clay mineral systems which contain free silica forms as a compositional pole. Most often during epi-metamorphism kaolinite is incorporated into other phases due to a displacement of the bulk... 

It appears, using a limited number of samples, that low "metamorphic" or diagenetic grade in the clay mineral facies produces chlorites of... 

This has been confirmed in the present study for chlorites from six rocks from the Belt series in northern Montana (Harrison and Jobin, 1963) and three samples from the outer zones in the Alpine chain. From these analyses, it is apparent that metamorphic chlorites which have crystallized with muscovite have low silica contents and a rather limited alumina content (27 3% A1 ions). The relatively large variation in chlorite Fe-Mg ratio from rock to rock suggests that bulk rock composition is more important in determining the chlorite composition than is the case of chlorites from clay mineral facies. 

A limited number of parameters calculated from the absorption coefficients has been proven useful. Although they are empirical to some extent, they have a rather clear chemical meaning. In fact, the spectra differ from each other by the intensity of the various bands. For instance, the spectra of kerogens and coals vary in regular fashion according to rank and reflect the main chemical modifications occuring as rock metamorphism proceeds (Rouxhet et al., 1980) 22) (Fig. 15, p. 23). 

The upper limit for tetrahedral Fe3+ I.S. in silicates is shown to be 0.25 mm/sec., whereas the lower limit for octahedral Fe3+ is 0.29 mm/sec. The correlations point to inconsistencies in Mossbauer spectral parameters and cation site occupancy assignments for clintonite, yoderite and sapphirine. New Mossbauer spectral data obtained for these minerals demonstrate that clintonites from skarn deposits contain tetrahedral Fe3+ and octahedral Fe3+ and Fe2+, with relative enrichment of Fe3+ in tetrahedral sites only octahedral Fe2+ and Fe3+ occur in sapphirines from granulite facies rocks and five-coordinated Fe3+ predominates over octahedral Fe3+ ions in yoderites from high grade metamorphic rocks. 

On an AFM diagram (Fig. 16) all the minerals from iron-formations proper fall into a narrow and clearly defined field, while the rock-forming minerals of the associated iron-rich shales from BIF of Superior type (chamosite, ripidolite and other chlorites, garnets and some biotites) form another broader and less clear-cut field. These two fields do not overlap, which indicates the isochemical character of metamorphism and the limited mobility of the main rock-forming minerals, at least within individual layers and bands of the iron-formations and shales. The chemical composition of stilpnomelane occupies an intermediate position both in iron content and in alumina content. However, a high content of potassium (up to 2.2% K2O), an element not typical of most BIF, is necessary for the formation of this mica. 

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