Pneumatolytic Process

PNEUMATOLYTIC METAMORPHISM

‘Instead of the formation of the vugs through an igneous process, i.e., by the rapid eruption of a high-volatile-containing magmaMolten silicate (rock) beneath the surface of a planetary body or moon. When it reaches the surface, magma is called lava., 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 anorthiteRare compositional variety of plagioclase and the calcium end-member of the plagioclase feldspar mineral series with the formula CaAl2Si2O8. Anorthite is found in mafic igneous rocks such as anorthosite. Anorthite is abundant on the Moon and in lunar meteorites. However, anorthite is very rare on Earth since it weathers rapidly and olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion 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 mineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous formation brought about by the action of gases emitted from solidifying magma.
2. A metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. 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 volatileSubstances which have a tendency to enter the gas phase relatively easily (by evaporation, addition of heat, etc.). 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 crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. 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.