The volatile budget of Hawaiian magmatism: Constraints from melt inclusions from Haleakala volcano, Hawaii

Pre-eruptive volatile contents recorded by melt inclusions from ocean island settings such as Hawaii constrain the extent of deep Earth outgassing on geologic timescales by mantle plume activity. However, melt inclusions trapped from a partially degassed magma will not reflect the original volatile content of the primary melt, and relatively silicic (>45 wt% SiO2) shield-stage tholeiites are more likely to have been affected by degassing during fractionation at shallow depths. In contrast, magmas associated with post-shield volcanoes erupt volatile-rich melts that ascend quickly from the source region and crystallize in deep reservoirs. As a result, melt inclusions from post-shield volcanoes may be more likely to preserve undegassed melt compositions that can be used to determine the primary melt and source volatile contents. In this study, we analyzed melt inclusions from Haleakala Volcano (East Maui, Hawaii) to estimate the volatile budget of Hawaiian post-shield magmas. Melt inclusions from Haleakala contain up to 1.3 wt% CO2, 1.2 wt% H2O, and about 2000 ppm S. We calculate that melts from Haleakala were derived from a parental melt with ~0.7 wt% H2O and ~0.7 wt% CO2 and experienced ~15–40% polybaric, fluid-saturated crystallization starting at 5–7 kbar. Post-shield melt inclusions from Haleakala have H2O/Ce and CO2/Ba compositions that are intermediate between shield-stage melt inclusions from Kilauea and the alkaline North Arch Volcanic Field, which is consistent with a scenario in which primary melts from Haleakala are influenced by the volatile-rich auto-metasomatized periphery of the composite plume source.