COMPOSITE PUMICE IN THE BISHOP TUFF

Many pumice clasts in plinian Fall Unit 7 of the rhyolitic Bishop Tuff are composite. In most of the composite pumice clasts part of the clast is slightly darker in color and contains more large and rounder (subequant) vesicles. Another part of the composite clast is brighter and contains tiny long vesicles. Some pumice clasts contain angular, crack-shaped voids. Other parts of some clasts have vesicles that are long in a different direction. The differences in types of pumice are subtle and best seen after breaking the clast apart in the lab. A “random” collection of 17 > 3cm-diameter pumice clasts from Fall Unit 7 contains 9 composite and 8 non-composite clasts. The densities of 8 of the 9 composite clasts range from 0.294 g/cm3 to 0.50 g/cm3 whereas the densities of the 8 non-composite clasts range from 0.698 to 0.896 g/cm3. Most other Bishop pumice clasts from Fall Units 7 through 9 range from about 0.8 to 1.0 g/cm3. Thus the composite clasts contain pumice that is less dense (more vesicular) than most non-composite pumice. The number of visible vesicles that are larger than about 1 mm diameter ranges from 2 to 50 per clast (size dependent) and the largest vesicles are commonly around 6 mm diameter. Oddly, most of the biggest vesicles are in a few non-composite clasts. Also the composite clasts are smaller (2.05g – 21.24g, ave. = 9.38g) than the non-composite clasts (16.14g – 177.4, ave. = 73.2g). The more vesicular pumice may have formed in segregation-like veins near the margins of the pre-eruptive magma body, and composite clasts may have formed during early stages of eruption. Whatever is the cause of the density variation, the composite character of a substantial proportion of pumice clasts violates a main assumption that individual pumice clasts are closed systems, and this calls into question estimates of pre-eruptive exsolved gas based on compositional arguments.