ISOTOPIC HETEROGENEITY AND CRYSTAL ISOTOPE STRATIGRAPHY OF FLOOD BASALT LAVAS: THE COLUMBIA RIVER BASALT GROUP

Columbia River Basalt Group (CRBG) flows contain components from more than one, and perhaps several, lithospheric and asthenospheric magma sources. There is, however, no consensus on the relative importance of plume-related mantle, depleted mantle, lithospheric mantle, and continental crust in CRBG magma genesis. The processes and timescales involved in the blending of the different components to make CRBG magmas are also poorly constrained.

We have used laser ablation and microdrill sampling techniques in conjunction with both high resolution and multicollector ICP-MS to analyze trace element concentrations and Sr isotope ratios from individual growth layers within plagioclase phenocrysts and from the associated groundmass within flows from each major CRBG formation. ⁸⁷Sr/⁸⁶Sr variations within individual phenocrysts and between phenocrysts and host matrix offer unequivocal evidence for open-system processes during phenocryst growth, and it is clear that CRBG magmas were assembled, at least in part, in crustal magma chambers. By applying one-dimensional diffusion modeling to observed ⁸⁷Sr/⁸⁶Sr zoning and crystal/groundmass gradients constrain phenocryst residence times, the timescale of crustal-level petrogenetic events can be constrained. Residence times for phenocrysts in their final host liquid may be as little as a few years prior to quenching.

Isotopic heterogeneity in individual CRBG flows may result from mixing between magmas from isotopically distinct mantle reservoirs and/or crustal contamination during and after phenocryst growth in crustal magma chambers. The very short timescales implied by crystal-matrix isotopic disequilibrium most likely result from syn-eruptive mixing of magmas derived from different locations within a spatially heterogeneous magma chamber.