Melt Inclusion Volatile Contents, Pressures of Crystallization for Hawaiian Picrites, and the Problem of Shrinkage Bubbles

The H₂O and CO₂ contents of melt inclusions can potentially be used to infer pressures of crystallization and inclusion entrapment because the solubility of mixed H₂O-CO₂ vapor has been determined experimentally for basaltic and rhyolitic melts. However, melt inclusions commonly develop shrinkage bubbles caused by the greater thermal contraction of the melt compared with the host mineral during post-entrapment cooling. Because the solubility of CO₂ in silicate melts is much less than that of H₂O, resulting in relatively high vapor-melt partition coefficients for CO₂, formation of a shrinkage bubble can strongly deplete the coexisting melt of dissolved CO₂ that was present at the time of entrapment. To investigate the loss of CO₂ into shrinkage bubbles, we have experimentally reheated large melt inclusions in olivines from a Mauna Loa picrite in order to redissolve the vapor in the bubble. The olivines were sampled from Puu Wahi, a scoria cone situated at ~3000 m elevation on the NE rift zone of Mauna Loa. The olivines (Fo₈₈) come from reticulite scoria and so were naturally quenched to glass during eruption, but all inclusions contain shrinkage bubbles <= 3 vol% of the inclusion. Reheating to 1400°C rehomogenized the shrinkage bubble into the melt, but even with rapid quenching a small vapor bubble formed during quench. CO₂ contents measured by FTIR spectroscopy and recalculated for melt in equilibrium with the olivine host are 300-600 ppm (n=11) for reheated inclusions, much higher than the CO₂ contents of the naturally quenched inclusions (60-180 ppm; n=8), which all contain shrinkage bubbles. Dissolved H₂O contents of the melt inclusions are very uniform (0.36 +/- 0.05 wt%). Pressures of inclusion entrapment calculated from the H₂O and CO₂ data for the reheated inclusions range from 0.5 to 1.3 kbar, indicating that olivine crystallized at very shallow depths beneath the surface of Mauna Loa. Large (<= 100 microns in diameter) fluid inclusions in some olivines appear to contain relatively low density CO₂, consistent with the interpretation based on melt inclusions that olivines formed at low pressures. Such surprisingly low pressures of crystallization have also been inferred for olivines erupted in the 1959 Kilauea Iki picrite (Anderson and Brown, 1993, Am. Min.) and for abundant Fo_88-90 olivine in the Keanakakoi ash deposits at Kilauea (Hart and Wallace, 2000, AGU abstract). Eruptions of olivine-rich lava are relatively rare on the subaerial portions of both Kilauea and Mauna Loa but are more common on the submarine parts of both volcanoes. As has been proposed for Kilauea Iki, the eruption of olivine-rich lava high on the NE rift zone of Mauna Loa probably resulted from magma following an unusual pathway to the surface, thus bypassing mixing and density filtering within the summit magma system. Our results for Puu Wahi olivines demonstrate that to accurately infer original pressures of crystallization requires inclusions with shrinkage bubbles to be experimentally rehomogenized in order to redissolve CO₂ lost to the bubble during post-entrapment cooling.