Metamorphic Grade and Iron-Magnesium Distribution between Co-Existing Garnet-Biotite and Garnet-Hornblende

Abstract

It is shown that in the two mineral pairs discussed the metamorphic grade can be estimated from the atomic ratios (X_a = Mg/Mg + Fe + 0·175 Mn + 0·360 Ca) in garnet and X_b = Mg/(Mg + Fe) in the other mineral. The standard error of the estimation is about one-third of the temperature range of the amphibolite facies.Where M is the metamorphic grade on an arbitrary scale, in which the lower boundary of the amphibolite facies is at 4·2 and the upper at 5·1, and K is the distribution coefficient (X_b—X_aX_b)/(X_a—X_aX_b), then the estimating equation is M = 5·6 — 1·5 log₁₀ K.