OXYGEN ISOTOPE CONSTRAINTS ON MAGMATIC EVOLUTION AND SOURCES AT LASSEN VOLCANIC CENTER, CALIFORNIA

The Lassen Volcanic Center (LVC) is the southernmost active volcanic center in the Cascades Range. Previous detailed mapping and stratigraphic studies provide constraints on temporal evolution of the system and magma chamber dynamics. Using this information as a foundation we are conducting an oxygen isotope survey of intermediate to silicic composition magmas erupted at the LVC during the past 600 ka. Important results to date include the following. 1) Analyses of plagioclase separates are typically 0.5-2.0‰ lower than whole rock values. This indicates that whole rock oxygen isotope data for glassy volcanic rocks, even those that are young and visibly pristine, should generally be considered unreliable. 2) In several samples ¹⁸O/¹⁶O fractionations between quartz and plagioclase are similar to predicted equilibrium values at temperatures of 850-800^oC. In contrast, fractionations in hybrid andesites erupted on the periphery of the system are greater than predicted, confirming a xenocrystic origin for quartz in these rocks. 3) Data for intermediate composition rocks (53-60% SiO₂) are elevated relative to presumed mantle values and show a larger range (d¹⁸O=+6.2-9.0‰; plag) than data for silicic rocks (+6.1-7.3‰). Most LVC eruptive products thus contain isotopically enriched crustal sources and intermediate magmas incorporated more heterogeneous crust or experienced less uniform amounts of contamination than silicic magmas. 4) Intermediate lavas erupted during the early stages of the LVC (600 to 400 ka) generally have slightly lower values (average +6.7‰) than intermediate magmas present as undercooled inclusions in younger silicic domes (average +7.1‰). This shift probably indicates ponding of mafic mantle melts at different depths and interaction with different crustal material during evolution of the LVC. 5) There is a broad relationship between position of late stage LVC eruptive vents with isotopic composition of associated lavas. Rocks erupted from vents located within 4 km of the inferred center of the LVC have lower d¹⁸O values (average +6.3‰) at given SiO₂ contents than those erupted from more distal vents (average +7.0‰). These differences may reflect increased degrees of hydrothermal alteration or hybridization of the shallow crust toward the center of the volcano.