Dense welding caused by volatile resorption

Welding of pyroclastic rocks is generally thought to occur by mechanical expulsion of interstitial gas from a deposit as it compacts under its own weight. We propose here that volatile resorption and compression can also be important factors in welding. We describe densely welded rocks which cannot be explained by loading and re-evaluate the welding process taking into account the effects of volatile resorption into the glassy fragments. Intra-caldera Oligocene ignimbrites from the Rhodope Mountains, Bulgaria, and intrusive tuffs of the Loch Ba ring dyke, Mull in Scotland are intensely welded but some samples lack foliation (eutaxitic texture). Fiamme and glass shards show no preferred orientation in thin section or on outcrop scale. Fiamme are sometimes complexy deformed into U or S shapes. We propose that these textures are the consequence of volatile dissolution in the glass and gas compression. Scaling analysis indicates that there are two regimes for gas behaviour following emplacement of hot pyroclastic deposits which depend on the relative characteristic time-scales of compaction, gas escape and gas resorption: a gas escape regime and a gas retention regime. During explosive eruption, glassy pyroclasts are decompressed to less than one atmosphere pressure and are outgassed. During deposition and subsequent burial in a thick hot deposit, volatiles will be retained and soluble gases (H2O) can be resorbed back into the glass, defining the gas retention regime. Poorly soluble gases (air) are compressed to small fractions of their original volume, resulting in destruction of pore spaces and vesicles in pumice. In some circumstances, such as volcanic vents, the volume changes involve isotropic strain and implosion of the tuff. Resorption of water greatly speeds up welding during compactional loading by reducing the viscosity of the glass. Welding is inhibited not only by lower temperatures but also by entrainment of insoluble atmospheric gases. The two regimes also can explain the common presence of post-emplacement gas escape pipes in non-welded ignimbrites and their rarity in densely welded ignimbrite. Factors that are likely to promote the gas retention regime include fine grain size, low collapsing columns, a large thickness of tuff and incorporation of external water.