Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

Kilauea volcano is a very intensively studied, active basaltic magmatic system and thus represents an ideal location to study magma solidification processes in a natural environment. Understanding solidification is important in refining models of magma chamber dynamics and its detailed study may improve our knowledge of magma system evolution. In this study magma solidification processes are examined and quantified using samples from the 1969–1974 Mauna Ulu (MU) rift eruption. We have collected major and trace element whole-rock data plus in situ olivine compositions, along with crystal size distribution data on 11 lava samples. The observed whole-rock chemical variation was partly produced by olivine addition within the Kilauea edifice. At least two distinct olivine populations are inferred from quantitative textural analysis: (1) a 3–40-year-old population characterized by a low crystal density, greater crystal length and flatter slopes of the crystal size distributions (CSDs); (2) a 1·5–15-year-old population marked by a high density of smaller crystals and steep CSD slopes. The range in olivine composition suggests that all these crystals grew from a range of different magmas, probably closely related by crystal fractionation. The ubiquitous presence of deformed olivine crystals shows that population 1 reflects a component that must have mostly originated by disruption of a deformed cumulate. This antecrystic olivine population represents an earlier-coarsened and aggregated, cumulate-forming magma component. In contrast, the phenocrystic population 2 represents a late magma component formed in the summit magma storage region. Our results are consistent with the hypothesis that the components of the MU magmas followed two different routes. The deformed-olivine-bearing magma moved along the deep basal décollement then rose through vertical pipe-like conduits under the MU rift. The undeformed-olivine-bearing magma rose via the main conduit to the summit reservoir and then moved out along the rift zone, where the magmas mixed in small chambers. The presence of narrow, reversely zoned olivine rims suggests that the mixing occurred just prior to eruption.