Subsolidus temperatures

In this interpretation, the persistence of the orthorhombic polymorph must be imputed to metastability associated to low reaction kinetics. At subsolidus temperatures, characteristic of most metamorphic regimes (including the granu-... 

Figure 5,28 Phase relations in pyroxene quadrilateral for P = 15 kbar at two subsolidus temperatures. From Lindsley (1982). Reprinted with permission of The Mineralogical Society of America.

Musacchio et al. 1997 Novak et al. 1997 Chevrot van der Hilst 2000). Fp/Fs ratios are not sensitive to pressure and subsolidus temperatures, but depend on fluid pore pressure, fabric attitude and rock composition, specifically quartz content and plagioclase composition (Christensen 1996). In stable shield areas, high Fp/Fs ratios in the crystalline crust are generally explained in terms of composition. Mafic rocks have Fp/Fs ratios higher than 1.73 as a result of low quartz content and the abundance of mafic minerals (plagioclase, pyroxene, garnet, amphi-bole and olivine). 

Very often, a constant temperature diagram is useful. Such a diagram is illustrated for subsolidus temperatures by the lines between the forms that exist at equilibrium in Figure 4.11 [2]. 

Figure 7.6 shows the phase stability relations in the NaAlSi308-KAlSi308 (Ab-Sa) system. Part A of the figure has a restricted T range, to facilitate comprehension of metastability phenomena on the K-rich side of the diagram. In part B, the temperature scale is more extended, to encompass subsolidus relations. 

The subsolidus composition-temperature phase diagram for the unmodified Pb(Zr, Ti)03 system is shown in Figure IT Most useful ceramic formulations are located close to... 

In any case, metamorphism does not imply melting and only takes the form of a subsolidus recrystallization (i.e., from a metallurgical point of view). The lower limit of metamorphic temperatures is stated as 150°C in order to make a clear distinction from the diagenetic process. The upper limit is the melting temperature when a magma forms (i.e., anatexy process). 

Figure 7.24 (a) Subsolidus and (b) high-temperature fields of the binary phase diagram Zr02-Mg0 (Grain, 1967). Reprinted with permission from Wiley-Blackwell, Oxford, UK. 

The extent of the data for these systems makes it necessary to focus on subsolidus phase relationships (this section) while only selected composition-temperature diagrams follow separately in sect. 14. Since the subsolidus phase descriptions vary significantly across the series of the R elements, the description begins at the smallest R atom and continues with the increasing size of the trivalent ions. 

Fig. 55. Four-phase tetrahedral envelopes adjacent to the M-saturated YBajjCui. M jjOf, solid solutions for M = Fe, Co and Mg/Ni/Zn, in subsolidus (pseudoquaternary) tetrahedral phase diagrams, as seen by room-temperature PXD after firing at 900°C in oxygen.

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