Pressure Increases, the Formation of Chromite Seams, and the Development of the Ultramafic Series in the Stillwater Complex, Montana

This paper explores the hypothesis that chromite seams in theStillwater Complex formed in response to periodic increasesin total pressure in the chamber. Total pressure increased becauseof the positive V of nucleation of CO₂ bubbles in the melt andtheir subsequent rise through the magma chamber, during whichthe bubbles increased in volume by a factor of 4–6. Byanalogy with the pressure changes in the summit chambers ofKilauea and Krafla volcanoes, the maximum variation was 0•2–0•25kbar, or 5–10% of the total pressure in the Stillwaterchamber. An evaluation of the likelihood of fountaining andmixing of a new, primitive liquid that entered the chamber withthe somewhat more evolved liquid already in the chamber is basedupon calculations using observed and inferred velocities andflow rates of basaltic magmas moving through volcanic fissures.The calculations indicate that hot, dense magma would have oozed,rather than fountained into the chamber, and early mixing ofthe new and residual magmas that could have resulted in chromitecrystallizing alone did not take place. Mixing was an important process in the Stillwater magma chamber,however. After the new magma in the chamber underwent {smalltilde}5% fractional crystallization, its composition, temperature,and density approached those of the overlying liquid in thechamber and the liquids then mixed. If this process occurredmany times over the course of the development of the Ultramaficseries, a thick column of magma with orthopyroxene on its liquiduswould have been the result. Thus, the sequence of multiple injections,fractionation, and mixing with previously fractionated magmacould have been the mechanism that produced the thick bronzitecumulate layer (the Bronzitite zone) above the cyclic units.