Premium
Continuous reactions with amphibole, garnet and plagioclase
Author(s) -
ZINGG A. J.
Publication year - 1995
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1995.tb00230.x
Subject(s) - amphibole , plagioclase , olivine , analytical chemistry (journal) , spinel , chemistry , mineralogy , quartz , geology , chromatography , paleontology
End‐member, continuous and degenerate reactions are derived for the multisystem with the six components Na 2 O, CaO, (Mg/Fe)O, Al 2 O 3 , SiO 2 , H 2 O among the phases plagioclase ss , garnet ss , amphibole ss , cpx, opx, olivine, spinel, quartz and an aqueous fluid. The chemography of this system is degenerate due to the co‐linearity 2Opx = Ol + Qtz. This co‐linearity has its implications both on reaction space and phase equilibria. From a total of 28 reaction systems, reaction space is derived for nine subsystems (phases in parentheses are absent): Case A 1 : (Cpx,Ol) (Cpx,Opx) and (Cpx,Qtz), Case A 2 : (Spl,Ol) (Spl,Opx) and (Spl,Qtz), Case B : (Ol,Opx) (Ol,Qtz) and (Opx,Qtz). In the absence of either cpx or spl (case A), three reactions form an invariant point, either [Cpx] or [Spl], where the co‐linear phases olivine, opx and quartz coexist on the transformation line 2Opx = Ol + Qtz. Changing mineral compositions force invariant points to move along the line with the different reaction curves changing their relative position according to Schreinemakers’rules. Zero contours, i.e. the location where (a) phase(s) disappear(s) in reaction space correspond to singular points in phase diagrams. Two types are distinguished; singular points of indispensable and of substitutable phases. In the first case the phase disappears from the entire bundle while in the second it disappears from a single reaction. In the specific case where the substitutable phases are also the co‐linear ones, two of the three co‐linear phases disappear simultaneously. Two of the three reaction curves coincide. In the system including Cpx and Spl (Case B) three reactions, (Ol,Opx) (Ol,Qtz) and (Opx,Qtz), oppose three invariant points, [Ol], [Opx] and [Qtz]. Invariant points no longer move along the line 2Opx = Ol + Qtz. The coincidence of the zero contours of all three co‐linear phases in reaction space‐the result of the chemographic degeneracy‐causes the respective singular points to coincide in the phase diagrams. This is the location where curves must be rearranged in a bundle to conform Schreinemakers’rules. The reaction Grs 1 Prp 2 = 2 Ol + An is fourth order degenerate and part of all nine subsystems (cases A and B). It can be used to relate the different phase diagrams to one another.