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

Rhyolitic magma chambers often erupt to form deposits with a wide range of trace element chemistry, inferred to reflect zoning in the magma chamber prior to eruption. Ion probe microanalyses of trapped melt inclusions and matrix glass from the large Lower Bandelier Tuff and Bishop Tuff eruptions shows that much of this compositional variation can be blamed on the intrusion of a second rhyolitic magma into the base of the chambers. The second rhyolite was composed of similar major elements but contained significantly higher Ti, Sr, and Ba in both examples. Microanalyses of sanidine phenocrysts show pronounced trace element zoning profiles in accord with the glass chemistry. Applying the available diffusion coefficients for Sr in sanidine to the zoning suggests residence times on the order of 10⁴ yrs after the mixing event. The source of the second magma is not known, but similarities in chemical zoning patterns in silicic magmas throughout the world point toward a common process. Mixing of less fractionated magma derived from similar source rocks is the simplest mechanism. Detailed isotopic studies may help distinguish different sources. Independent of the second magma, large variations in trace elements are observed in the melt inclusions from the Lower Bandelier and Bishop Tuffs which can be modeled by ∼ 40% fractional crystallization.