Petrogenesis and volatile stratigraphy of the Bishop Tuff: evidence from melt inclusion analysis

The Bishop Tuff (BT), erupted from the Long Valley Caldera at 0.74 Ma, is composed of a Plinian tephra and ignimbrite. Based on ion and electron microprobe analyses of melt inclusions (MI), a strong H₂O gradient was present in the upper portion of the magma chamber. The H₂O content of the magma which formed the Plinian tephra drops from 6 wt.% to 3.5 wt.%. In contrast, the magma which formed the ignimbrite contained a relatively constant amount of H₂O, between 2 and 4 wt.%. The strong drop in H₂O content of the magma which formed the Plinian tephra suggests that only in its extreme upper part, if any, was the P_H2O in the magma close to P_total. The range in the trace element concentrations of inclusions suggests that fractional crystallization may have affected magmatic composition. However, magma mixing may also have been an important process in establishing the final trace element zonation within the Bishop magma chamber. -from Authors