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.
Vinet, N and Higgins, M.D., 2010, Magma solidification processes beneath Kilauea Volcano, Hawaii: A quantitative textural and geochemical study of the 1969-1974 Mauna Ulu lavas, Journal of Petrology, Vol 51, P 1297–1332, https://doi.org/10.1093/petrology/egq020
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Deformation of olivine in a volcanic context is poorly constrained, although deformed olivine is abundant in some volcanic rocks, and its presence is important for the definition of the magmatic history of volcanic edifices such as Kilauea Volcano, Hawaii. Deformed olivines at Kilauea originate in the lower crust; therefore, the classic approaches and interpretations applied to mantle-derived olivine are not applicable here. Deformed olivine crystals from Kilauea lava samples were examined using an in situ XRD technique. Our results validate and refine optical observations of olivine deformation. We also confirm the presence of deformation for olivine crystals of any size, and quantify it. There are significant correlations between deformation intensity (strain-related mosaicity) and olivine composition and crystal size. Although this technique does not allow the simple estimation of the P-T conditions of deformation and crystal formation or magmatic history, some constraints have been provided here, in particular the threshold degree of mosaicity, above which we consider that a crystal underwent deformation. In situ XRD is shown here to be an easy-to-use, fast, low-cost, non-destructive technique, which is less-ambiguous than optical microscopy, to determine the presence of strain in the crystal structure of magmatic olivines, and quantify it, especially for very small crystals. For crystals optically exhibiting subgrain formation, analysis of asterism by in situ XRD has been used to reconstruct the mosaic spread of the original grain, and thus its original strain condition prior to subgrain formation.
Vinet N, Flemming R.L., Higgins M.D., 2011, Crystal structure, mosaicity and strain analysis of Hawaiian olivines using in-situ micro X-ray diffraction (µXRD). American Mineralogist 96 (4): 486–497. https://doi.org/10.2138/am.2011.3593
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Lava lakes offer the opportunity to investigate magma solidification and can be considered as a proxy for small magma chambers. Here we present olivine compositions and crystal size distributions (CSDs) from scoria and drill core samples from Kilauea Iki lava lake, which formed during the 1959 eruption of Kilauea Volcano, Hawaii. Three chemically distinct olivine populations were distinguished, in the basis of their forsterite (Fo) content: (1) a high-Fo population (Fo86–90); (2) an intermediate-Fo population (Fo78–82); and (3) a minor low-Fo population (Fo74–78). Populations 1 and 2 both have deformed and undeformed crystals. The third population may be the result of rejuvenation. Olivine in the lower 60 m of lake has a less Fo-rich composition and more crystals are deformed. The CSD analysis yields estimates of the average olivine residence time: 1–60 years. The shape of the olivine CSDs is fairly uniform with respect to depth. Curved CSDs are considered to be evidence of hybrid populations, partly or totally involving crystal or magma mixing. The turndown at the smallest sizes of most foundered crust and lake CSDs may be the result of coarsening, making this process active both before and after eruption. Our CSD modelling does not support significant crystal settling and overall convection in the lava lake, although small advective currents are known to have occurred. The olivine vertical stratification cannot be an original feature, which is consistent with supposed strong stirring of the lake magma due to intense activity over the 17 eruptive phases. It is also possible that independent basal feeding of the lake during the eruption may be needed to explain fully features of the chemical and mineralogical stratification.
Vinet, N. & Higgins, M. 2011. What can crystal size distributions and olivine compositions tell us about magma solidification processes inside Kilauea Iki lava lake, Hawaii? Journal of Volcanology and Geothermal Research, Vol 208, Pages 136-162 https://doi.org/10.1016/j.jvolgeores.2011.09.006