HR: 11:05h
AN: V11F-04    [PDF]
TI: Crystal- and fragment- size distributions of quartz and zircon in pumice: growth and fragmentation conditions in large and small-volume magma chambers
AU: * Bindeman, I
EM: inbindem@gps.caltech.edu
AF: Geological and Planetary Sciences, California Inst of Technology, MS-170-25, Pasadena, CA 91106 United States
AB: I describe an acid (HF and HBF4) technique to extract phenocrysts from individual vesiculated pumice clasts, coupled with camera- and computer-assisted measurements of phenocryst length, width, 3D shape, and vol abundance. CSDs of quartz and zircon are presented for several well-known voluminous ash-flow tuffs and small-volume lavas: Bishop, Lava Creek, Lower Bandelier, Toba, Katmai, and Timber Mt. Measured CSDs of quartz and zircon from these clasts provide a quenched "snapshot" view of growth conditions in preclimactic magma chambers. A common feature of CSDs of unfragmented phenocrysts is a concave-down, lognormal shape in contrast to the reported linear CSDs in more mafic systems.In addition, there are no crystals smaller than a threshold size. These features in silicic magmas are interpreted to be a general result of surface-controlled crystal growth (with growth rate dispersion) by layer nucleation. CSD slopes on log-linear frequency- size graphs in large volume tuffs, and smaller volume intracaldera lavas are similar, and do not simply correlate to the eruptive volume, or SHRIMP-determined zircon ages. CSDs of quartz in clasts with known stratigraphic positions document single evolving reservoir, fingerprint different magma batches (L Bandelier and Lava Creek), and overgrowth and gravitational redistribution (Bishop). Fragment size distributions (FSDs) in the same clasts document fragmentation due to 1) decrepitation of melt inclusions decompression- and heating-induced), and 2)syneruptive breakage. FSDs are treated with lognormal, Weibull, and fractal distributions. Among studied clasts, asymptotic and fractal FSDs are found to be more common. However, the genesis mechanisms (e.g. fractal, scale-invariant vs. size-dependent lognormal) inferred from CSD or FSD should be treated with caution. Decrepitation results in a smaller number of fragments (2-6) than crushing and in shapes that can be distinguished on perimeter/area vs. length diagrams. CSD and FSD have potential implications as fingerprinting tools to identify/correlate different magma batches in ash-flow tuffs. FSD serves as a novel tool for trace elemental and isotopic exchange in magma chambers.
DE: 3640 Igneous petrology
DE: 3694 Instruments and techniques
DE: 8439 Physics and chemistry of magma bodies
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting