FIELD MEASUREMENTS OF MAFIC ENCLAVE POPULATION DENSITY AT CHAOS CRAGS, LASSEN VOLCANIC NATIONAL PARK, CA AND IMPLICATIONS FOR MAGMA MIXING

Magma mixing is an important process in the evolution of arc-related rocks and is widely recognized as a potential trigger for explosive eruptions. Mafic enclaves present in many intermediate to silicic rocks generally are interpreted to represent replenishment of a silicic magma body by mafic magma. Chaos Crags which formed in a single eruptive episode 1103 ± 13 years ago is a good example of such mixing. Eruptive products include: three pyroclastic flows, a pumice fall and six hornblende-biotite dacite domes (A-F). The early pyroclastic deposits, and domes A and B have visibly fewer enclaves and less petrographic evidence of enclave disaggregation than the later domes C–F. The composition of the host dacite decreases from about 69.5% SiO₂ in dome B to about 68 wt % SiO₂ in dome F. Here we present field measurements showing the change in the volume of mafic enclaves in domes B–F. Two methods were used to determine volume percent of enclaves in the dacites: (1) point counting of all enclaves bigger than about 1 cm on outcrops using a 100 point grid, and (2) digital evaluation of enclave size distribution using Image J and CSD Corrections.

Multiple measurements were taken in well-exposed talus or dome-collapse deposits. We assume that the talus represents a natural average of the population density of enclaves throughout the domes. The average volume percents of mafic enclaves determined by point counting are 2.7% (dome B), 9.4% (dome C), 12.7% (dome D), 14.3% (dome E), and 16.4% (dome F). The average volume percents determined digitally are 2.4% (dome B), 3.7% (dome C), 10.1% (dome D), 6.5% (dome E), and 7.1% (dome F). Lower values derived using CSD Corrections may be due to difficulties in identifying all enclaves in digital images, and choosing appropriate average values for roundness and axes for the enclaves. Determining which of the methods is a more reliable measure of volume percent will require careful field measurement of enclave shape and preferred orientation. Our results indicate that the proportion of mafic enclaves in the erupted dacite increased continuously during the eruption and correlates with the increasing petrographic and chemical evidence of enclave disaggregation. In combination with other data the enclave abundances provide clues to the dynamic nature of the magma mixing process in the Chaos Crags system.