ASPECTS OF THE ROLE OF SULFUR IN HIGH AND ULTRA-HIGH GRADE METAMORPHISM

Charlie Guidotti was intrigued by the role of sulfur in metamorphism of pelitic rocks, and along with students and collaborators made numerous important contributions to understanding how this minor component affected phase equilibria during low- to medium-grade metamorphism. Sulfur has some even more surprising effects in high- to ultrahigh-grade metamorphism as well. 1) As with low-grade metamorphism, the presence of more than trace amounts of S can affect the AKFM silicate assemblage by causing a subtle or even major decrease in modal amounts of Fe-rich phases such as garnet or staurolite and a consequent increase in more Mg-rich phases; unusual appearance of very Mg-rich phases such as cordierite may occur. In rare cases at second-sillimanite grade, pure-Mg cordierite and biotite may be produced. The cause of these effects is the sequestration of Fe into sulfide minerals (pyrite or pyrrhotite, or rarely both) producing an increase in the effective bulk Fe/Mg ratio of the rock. 2) Subtle to major shifts in accessory mineral content may also occur, e.g., rutile joining or even replacing the normal Fe-Ti oxide phase ilmenite in metapelitic rocks. In calc-pelite compositions, sphene may be produced. 3) Compositions of accessory minerals may be affected either sympathetically or directly by high S contents. For example, Mg/Fe ratios of tourmalines rise to very high levels in the more S-rich metapelites that contain Mg-rich silicate minerals. In one unusual case of higher-P (0.7 GPa), UHT contact metamorphism in a deep crustal xenolith in mafic magma, monazite [(REE,Th,U)PO₄] in a sulfidic metapelite has reacted with a highly oxidized S-rich fluid to permit extensive (>10%) solid solution of the REE phosphate toward an R²⁺ sulfate. The phase equilibrium and mineral compositional effects produced by sulfur during high-grade metamorphism depend critically upon the oxidation state of the rock and the presence of carbon (original organic matter). In strongly reducing environments, fluids in sufficiently S-rich rocks may approach or exceed 50 mol% H₂S at 700 °C, potentially producing unusual mineralogic and petrologic effects.