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Buffering in the assemblage staurolite‐aluminium silicate‐biotite‐garnet‐chlorite
Author(s) -
GIARAMITA M. J.,
DAY H. W.
Publication year - 1991
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.1525-1314.1991.tb00532.x
Subject(s) - biotite , chlorite , geology , muscovite , silicate , mineralogy , assemblage (archaeology) , mineral , staurolite , silicate minerals , outcrop , quartz , geochemistry , chemistry , paleontology , organic chemistry
The assemblage muscovite‐quartz‐staurolite‐aluminium silicate‐biotite‐garnet‐chlorite with H 2 O (SABGC assemblage) is invariant in the K 2 O‐FeO‐MgO‐Al 2 O 3 ‐SiO 2 ‐H 2 O (KFMASH) system. Such five‐phase AFM assemblages should be absent in nature, but reported occurrences from at least ten localities suggest that either the assemblage internally controls μH 2 O or non‐KFMASH components stabilize one or more of the phases. Least‐squares regression analysis of minerals from South Royalton and Gassetts, Vermont, USA, demonstrates that subsets of the minerals in single SABGC assemblages from both localities are related by balanced reactions involving water. These results are consistent with the interpretation that the subassemblages fixed μH 2 O at an arbitrary, specified pressure and temperature. Balanced dehydration reactions also may be written between minerals in the SABGC assemblages and four‐phase assemblages from the same outcrops interpreted to have equilibrated at the same pressures and temperatures as the five‐phase assemblages. These results suggest that different specimens from the same outcrops equilibrated at different values of μH 2 O, supporting the conclusion that μH 2 O was not controlled externally. We could not demonstrate internal control of fo 2 using measured mineral compositions because oxygen balance occurred in all reactions derived by regression. Regression analysis of published mineral compositions from New Mexico failed to identify balanced reactions involving water or oxygen either among the phases in a single SABGC assemblage or between SABGC and nearby four‐phase assemblages. These results demonstrate that neither μH 2 O nor fo 2 were fixed by the SABGC assemblages at these localities, and permit the interpretations that the assemblages were stabilized by the non‐KFMASH components Na or Ca and that μH 2 O and fo 2 were controlled externally.