Melt Inclusions From a Basaltic Tuya in British Columbia: Contrasts in Early- and Late-Stage Degassing Processes

Volcanic tuyas are of interest because their formation beneath glaciers preserves information on ice-magma interaction and magma degassing processes. In northern British Columbia (58-60° N latitude) there exists a small volcanic field of Quaternary basaltic tuyas. Ash Mountain is one example that is 700m in height, 4-5 km in basal diameter and has a lower section of pillow lavas and an upper section of unconsolidated hyaloclastite. This distinctive morphological change, common to many of the tuyas, has been interpreted as corresponding to a shift from initial subglacial eruption to later subaqueous to subaerial eruption. Ash Mountain also displays the common shift in composition from early tholeiitic lavas to later alkalic lavas. Melt inclusions from the upper ash unit have been analyzed to investigate the later stages of volcano formation. Olivine crystals (Fo_76-80) separated from bulk ash from the upper flank of the volcano contain abundant melt inclusions and large glass-filled re-entrants. The melt inclusions are alkalic in character and closely match the groundmass glass composition indicating that the crystals and melt inclusions are representative of late-stage crystallization. FTIR analysis shows that the average water content of melt inclusions is 0.9 wt.% with variable CO₂ (0 to 1200 ppm). The variation in H₂O and CO₂ resembles an open-system degassing trend consistent with magma crystallization during transport within the shallow crust. Previously determined water abundance (0.5 to 0.7 wt.%) and hydrogen isotopic composition (\deltaD -87.2 to -- 102.0 \permil) for Ash Mountain pillow lavas are consistent with shallow level degassing during early volcano formation. The melt inclusion data indicates the later alkalic magma,which erupted with greater energy, was derived from greater depth.