Good discussion. Since the early 70's I have been using for pedagogic
purposes an isobaric T-X(Mg) phase diagram for the part of KFMASH that can
be projected from Alsilicate or pyrophyllite (as well as Qtz, Ms or Kfs,
and vapor or liquid). With increasing T the one-phase fields are Chl, Ctd,
St, Bt, Crd, Grt and Crd (that's right; cordierite shows up twice at ~6
kbar). Being rigorously invariant in respect to changes in mineral or
bulk-rock A/F+M, this phase diagram is good for illustrating that different
bulk-rock X(Mg)'s are needed for mapping different reaction isograds. But
of course it can't portray the phase relations of Bt-bearing metapelites
below the Als+Bt reaction isograd nor Grt+Bt-bearing metapelites below the
Als+Grt+Bt reaction isograd.
Cheers, Dugald
Dugald M Carmichael Phone/V-mail: 613-533-6182
Dept of Geological Sciences and Geological Engineering
Queen's University FAX: 613-533-6592
Kingston ON K7L3N6 E-mail: [log in to unmask]
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