Shallow Eruption Depths for Ancestral Kilauea: Implications for its Growth

The midslope bench on the submarine south flank of Kilauea volcano consists of volcaniclastic breccias and sandstones shed from Mauna Loa and Kilauea. Highly alkalic clasts (alkali basalt to nephelinite and phonotephrite), previously dated between 275 and 130 ka, record early Kilauea volcanism. High S concentrations (>750 ppm) in clasts and many sandstone glasses have suggested that early Kilauea alkalic magmas were erupted in deep water. To better evaluate eruption depths, we have measured (FTIR) H₂O and CO₂ dissolved in glasses from early alkalic breccias and sandstones and overlying alkalic to tholeiitic pillow lavas. Saturation pressures were calculated from mixed-volatile solubility models, then converted to eruption depths assuming gas saturation pressure is equal to or greater than eruption pressure. Despite high dissolved S concentrations in most samples, 22 of 31 alkalic glasses were erupted at or near sea level (H₂O ~0.1 wt %, CO₂ below detection). Of the remaining nine, none were erupted below ~3000 m water depth (CO₂ < 130 ppm). High S in otherwise degassed alkalic glasses indicates incomplete S degassing due to rapid ascent rate, compositional dependence, or both; S content is not a reliable indicator of eruption depth. The subaerial to relatively shallow submarine eruption depths for the alkalic magmas indicate that early Kilauea grew high on the submarine shoulder of the Mauna Loa edifice, well above the surrounding sea floor at ~5500 m depth. These results require that Mauna Loa is larger, and Kilauea smaller, than commonly postulated. Glass rinds of in-place transitional pillow basalts (pre-shield-phase Kilauea), from three sites above and east of the midslope bench, have H₂O (0.2-1.0 wt %) and CO₂ (170-270 ppm) concentrations that indicate eruption depths of 3500 to 5500 m. As the alkalic phase waned (post-130 ka) and transitional lavas erupted, activity propagated eastward into deeper water, marking the inception of Kilauea's east rift zone (Puna Ridge). During the volcano's mature shield stage, tholeiitic magmas have erupted subaerially and as pillow lavas along the east rift zone, reflecting continued growth both upward and laterally to the east.