HR: 10:35h
AN: V12A-02 INVITED     [Abstracts]
TI: Oxygen isotope heterogeneity of igneous phenocrysts: the rule and not the exception?
AU: * Bindeman, I N
EM: bindeman@uoregon.edu
AF: Geological Sciences, University of Oregon, 1260 Franklin Blvd, Eugene, OR 97403-1272 United States
AB: Oxygen isotope geochemistry of igneous rocks relies on analyses of bulk of resistant or ''refractory'' phenocrysts as a tool to measure δ18O value of equilibrium melt and interpret its origin. However, recent study of individual "phenocrysts" and their size fractions for oxygen isotope composition by laser fluorination, and core and rim study by SIMS, reveal isotope zoning and intracristalline variability, which exceeds analytical uncertainty, and often reach several permil. Air abrasion helps in retrieving cores of crystals, and has been used to obtain cores of quartz, zircon, sphene, and olivine. Air abrasion of individual crystals followed by laser fluorination, is the most precise (better than ñ1%) approach. SIMS analysis has greater spatial resolution but worse precision. I present examples of 2 to 5 permil oxygen isotope zoning in zircons from Yellowstone and Timber Mt calderas, 3permil zoning in olivines from Laki, Iceland, and 0.6 permil zoning in olivines from Kamchatka, and review other examples from literature. In all studied cases, isotope zoning results from entrainment of normal-δ18O crystals into either low- or high-δ18O melt. The magnitude of isotope disequilibria between melt and different minerals in the same rock, and between core and rim of the same mineral, permits estimate of time since the beginning of exchange ("entrainment") and the eruptive quench. Complementary tools, such as trace elemental zoning profiles in phenocrysts, ion microprobe dating of zircon cores and rims, and 210Pb-226Ra disequilibria dating of melt, provide similar timescales. The oxygen diffusive and model time estimates call for short magma segregation and eruption timescales of 10-1000 years. Therefore, isotopically-zoned crystalline material in magmas is often recycled cumulates and xenocrysts in secular equilibrium, rather than phenocrysts. Long-enough residence of crystals in melt will erase their isotopic zoning and lead to equilibrium Δ18O(mineral-melt) so xenocrysts can then adequately reflect δ18Omelt. An igneous rock should be studied by analysis of several individual crystals.
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005