Fluid fluxing of cumulates: The J-M reef and associated rocks of the stillwater complex, Montana
Olivine-Bearing zone I (OB I) is host to the principal platinum-group element deposit of the Stillwater complex, the J-M reef. OB I is characterized by a lower Troctolite subzone composed of troctolite, dunite, gabbronorite and anorthosite, and an overlying Anorthosite subzone composed largely of anorthosite and lesser troctolite and norite. A number of petrologic features suggest that the olivine-bearing rocks from the Troctolite subzone are the product of partial melting of gabbronorite fluxed by fluids exsolved during crystallization of underlying intercumulus liquid: (1) Pegmatoids become more abundant immediately beneath OB I. (2) There are discordant, lateral changes from uniform-textured gabbronorite to olivine-bearing rocks in the lower half of OB I. The olivine-rich rocks also define pegmatoidal 'pothole' structures. (3) Rock composition, texture and grain size vary considerably in olivine-bearing units, characterized by segregation between coarser mafic regions and medium-grained felsic regions. (4) Primary plagioclase in olivine-rich rocks has an eroded texture. (5) Modal variations are consistent with progressive incongruent melt reactions, and hydrous minerals are most abundant in olivine-rich rocks. (6) Hydrous melt inclusions (now crystallized to polyphase, hydrous mineral-bearing assemblages) are present in olivine and chromite. (7) Halogen-bearing minerals in the olivine-bearing assemblage have higher Cl contents than those associated with anorthosite and gabbronorite. In addition, the presence of massive sulfide associated with silicate pegmatoids is consistent with theoretical calculations that high-temperature fluids will have a significant component of sulfur. Modeling the thermal effects of fluid-induced partial melting of meter-sized partly solidified layers suggests nearly isothermal melting can occur when fluid is introduced slowly such that the heat of melting is supplied by the surrounding partially molten cumulates. The mineral changes that accompany the volatile-induced partial melting of a gabbronorite protolith are illustrated using the program MELTS and with phase diagrams that show how isotherms vary with water content. It is proposed that volatile fluxing of the lower portion of OB I led to the formation of olivine and chromite by incongruent melting of gabbronorite in the upper part of the crystal pile. The hydrous partial melt produced in this reaction, on mixing with resident pyroxene + plagioclase-saturated liquids in the magma chamber, produced hybrid liquids with crystallization order plagioclase followed by olivine or pyroxene, depending on the proportions mixed. This hybrid liquid crystallized rock of the Anorthosite subzone. The main platinum-group element sulfide concentrations occur at and below the boundary between these subzones, owing to the marked solubility difference of sulfur in volatile fluids compared with silicate liquids.
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