Cause of chemical zoning in the Bishop (California) and Bandelier (New Mexico) magma chambers

Development of the Long Valley rhyolitic magma system: strontium and neodymium isotope evidence from glasses and individual phenocrysts

“…Such relationships have been observed in products of the Fish Canyon Tuff where cracked and fragmented feldspar grains have granophyric overgrowths (Lipman et al, 1997) and in ash-flow tuffs throughout the Great Basin of the western USA (Best and Christiansen, 1997). Secondly, Hervig and Dunbar (1992) present trace element profiles across sanidine grains from the Bishop Tuff. Sanidines from the late Bishop Tuff ignimbrites (Ig2 E, N, NW) have complex trace element distributions.…”

“…Most recently, a debate about the rate of magma production, storage, and possible modification of large silicic magma bodies was initiated principally as a consequence of studies on the Long Valley system (Halliday et al, 1989;Sparks et al, 1991;Lu et al, 1992;Hervig and Dunbar, 1992;Davies et al, 1994;Duffield et al, 1995). The interpretation of Sr and Ar isotope data for samples from LVHSMS has proved particularly controversial.…”

Weak chemical and mineralogical zonation in the Kaingaroa Ignimbrite, Taupo Volcanic Zone, New Zealand

“…The similarity in crystal content, mineralogy, and trace element chemistry suggest they may have been derived from similar sources and/or followed similar petrogenetic pathways (e.g., plagioclase-dominated fractionation). Late addition and replenishment of silicic magma with similar compositions to the resident magma has been documented for many other large-scale ignimbrites (e.g., Bishop Tuff (Hervig & Dunbar 1992) and Neapolitan Yellow Tuff (Orsi et al 1995)). Hervig & Dunbar (1992) explained the chemical variation in the Bandelier and Bishop ignimbrites by a combination of crystal fractionation and mixing of a second rhyolite.…”

“…Late addition and replenishment of silicic magma with similar compositions to the resident magma has been documented for many other large-scale ignimbrites (e.g., Bishop Tuff (Hervig & Dunbar 1992) and Neapolitan Yellow Tuff (Orsi et al 1995)). Hervig & Dunbar (1992) explained the chemical variation in the Bandelier and Bishop ignimbrites by a combination of crystal fractionation and mixing of a second rhyolite. In both magma systems, the additional magma is markedly subordinate in volume, hotter, less fractionated, and interpreted to be derived from a similar source to the bulk of the magma system.…”

“…In both magma systems, the additional magma is markedly subordinate in volume, hotter, less fractionated, and interpreted to be derived from a similar source to the bulk of the magma system. Hervig & Dunbar ( 1992) advocated mixing of the less-evolved component into the base of the chamber. Alternatively, Briggs et al (1993) and Sutton et al (1995) suggested that batches of magma may be derived from different chambers and erupted simultaneously.…”