IT'S A MAD, MAD, MMAD WORLD

MADs (mineral assemblage diagrams: aka “pseudosections”) are based on equilibrium phase relations and are thus useful for inferring conditions of metamorphism to the extent that a close approach to equilibrium is achieved. Modified MADs (MMADs) have been constructed based on assumed affinities for porphyroblast nucleation that provide a alternative estimation of the conditions of metamorphism. Diagrams constructed to show the formation of garnet, Al₂SiO₅ polymorphs, and cordierite from chlorite + biotite assemblages using assumed affinities appear generally similar to equilibrium MADs although porphyroblast formation occurs at significantly higher temperatures (or pressures). Garnet nucleation is calculated to occur 30-70 degrees and/or 1-1.5 kbar higher than the equilibrium isograd and the field for Al₂SiO₅ + biotite is expanded to higher and lower pressures relative to that for garnet + Al₂SiO₅ + biotite and cordierite + biotite, respectively. The formation of staurolite by the reaction garnet + chlorite = staurolite + biotite is offset by several tens of degrees, depending on the pressure. The reaction garnet + chlorite = Al₂SiO₅ + biotite actually has a higher affinity than the staurolite producing reaction at the same conditions suggesting that kyanite should nucleate at nearly the same conditions or even before staurolite. If, on the other hand, garnet is unreactive the reaction becomes chlorite + ms = staurolite or kyanite + biotite; the staurolite stability field is greatly expanded to higher pressure and overstepping again predicts staurolite and kyanite to nucleate at similar conditions. This may account for the common texture of intergrown kyanite and staurolite in medium grade metapelites.

The requirement of significant overstepping of nucleation reactions also implies that that TLAs (tiny little areas) on MAD diagrams that require the formation of a new phase are kinetically highly unfavorable and thus TLAs should not be interpreted as indicative of P-T conditions.

This analysis suggests that in many cases conventional equilibrium calculations may have significantly underestimated both the maximum pressure and temperature of crustal metamorphism. Modified MADs can be useful to help better constrain metamorphic parageneses.