Geochemistry of the 1984 Mauna Loa Eruption: Implications for magma storage and supply

The 1984 eruption of Mauna Loa volcano, Hawaii, produced about 220×10⁶ m³ of lava of remarkably homogeneous composition. No significant compositional changes occurred during the 22-day eruption, and along the 20 km vent system, even though crystallinity increased substantially, accompanying a decline in the eruption rate. Such homogeneity in lava composition is typical of many historic Mauna Loa eruptions. The 1984 lava, along with most other homogeneous Mauna Loa lavas, is multiply saturated, crystallizing olivine, clinopyroxene, and plagioclase. Bulk compositions are "perched" at the intersection of an olivine-control trend with a trend for more differentiated lavas. There is no evidence to indicate that the 1984 lava is a differentiate of the 1975 magma, or that it results from mixing a new batch of magma with differentiated melt remaining in the magmatic plumbing system after the 1975 eruption. It is proposed that these lavas are erupted directly from a small, continuously replenished magma reservoir located about 3-4 km beneath the summit caldera. Successive eruptions therefore provide a sampling of magma chamber processes, and small but discernible changes in lava composition reflect the competing processes of magma supply, differentiation, and eruption.