MELT INCLUSIONS IN OLIVINE FROM SOUTHERN WASHINGTON CASCADE BASALTS

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The Cascade volcanic arc is associated with a young (£10Ma) and slowly subducting (3-4cm/yr) lithospheric slab, resulting in (1) a steeper thermal gradient along the slab/mantle interface than at most other arcs worldwide, and (2) early (shallow) dehydration of the slab and depletion of its fluid mobile element (e.g. B) inventory. Thus, compositions of Cascades basalts (MgO³8-9%) may reveal aspects of mantle wedge chemistry without extensive overprinting by slab-derived fluids; melt inclusions (MIs) in olivine phenocrysts may represent even more primitive magmatic compositions - assuming that entrapment in the host mineral prevents further evolution or modification. Ideally, pristine MIs will allow refined estimates of minimum magma segregation temperatures (e.g., via element partitioning between host olivine and melt inclusion) and use of major and trace element analyses in petrogenetic modeling.

Olivine-phyric basalts collected from both lava flows and cinder cones in an east-west transect across the southern Washington Cascades have a typical phenocryst assemblage of: ol + plag (An_58-80) ± cpx (~Wo₄₁En₃₂Fs₂₃). For samples analyzed to date (all lava flows from the Indian Heaven lava field) electron microprobe analyses indicate olivines as magnesian as Fo₈₂, with zoning restricted to extreme outer rims, and CaO between 0.2-0.5wt% (avg. 0.3). Spinel inclusions in olivines (£30µm diam.) are Cr-rich (molar Cr/[Cr+Al]=0.30-0.45). MIs occur as round to ovoid shaped entities, and commonly contain quench crystals. Reconstructed compositions, obtained by averaging multiple analyses of glass rich areas, indicate avg. SiO₂=~52% (range: 50-72%), MgO=3.0% (8.0-0.2%) and Mg#=0.39 (0.69-0.17). The rare high SiO₂ (>60%) and low MgO and Mg# values appear to result from interaction of MIs with external magma due to cracking of host olivine and/or extensive quench crystal growth. Effort is underway to find more primitive MIs (e.g., in scorias) and to characterize MI chemistry with respect to geographic position across the arc.