PNEUMATOLYTIC METAMORPHISM

"Instead of the formation of the vugs through an igneous process, i.e., by the rapid eruption of a high-volatile-containing magma, these vugs and druses are more consistent with a pneumatolytic formation process. The vugs were originally solid spheres, possibly composed of CaS or nitrides, and covered by anorthite and olivine rims. The cores were subsequently lost through "metasomatism" processes, while the calcium was utilized in the formation of kirschsteinite. Many of the vugs are now filled with glass."

pneu-ma-to-lyt'-ic (adj)
pneu-ma-tol'-y-sis (n)

  1. A process of rock alteration or mineral formation brought about by the action of gases emitted from solidifying magma.
  2. A metamorphic process caused by hot vapors or superheated liquids under pressure.
  3. Rock alteration that is caused by gases widely thought to be related genetically to magma.
Cooling of the molten magma began to produce a residual phase, in which the volatile constituents became increasingly concentrated. High pressure within this residue caused its infiltration into cracks and fissures of the local pre-existing rock, in which chemical and thermal metamorphism occurred. This "pegmatitic" phase proceeded through the temperature range of 700–500°C.

As the residual molten magma progressively cooled through 500°C, and crystallization proceeded, the magma became more highly enriched in the volatile constituents, while pressures continued to increase. These evolved solutions, containing gas and steam, penetrated deeply into the surrounding country rock, resulting in the formation of new minerals from existing ones—a process called "pneumatolysis". When the metamorphic agent consists primarily of fluids and/or ions, the process is described as a metasomatic process; the rock having undergone "metasomatism".

As temperatures further declined through 400°C, hydrothermal metamorphism was initiated. During this phase, hot, watery solutions altered existing anhydrous minerals into hydrated minerals—the process of "hydrothermal" alteration.