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Mécanismes de solidification des magmas basaltiques : Étude quantitative texturale et géochimique des laves du volcan Kilauea, Hawaï

Abstract : 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 can 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 eleven 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); and (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 decollement then rose through vertical pipe-like conduits under 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 rims suggests that the mixing occurred just prior to eruption. 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, on the basis of their forsterite (Fo) content: (1) a high-Fo population (Fo85-88); (2) an intermediate-Fo population (Fo77-81); and (3) a minor low-Fo population (Fo72-76). 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 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 crystal or magma mixing. The turndown at smallest sizes of most lake CSDs may be the result of coarsening. Our CSD modeling 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 could be an original feature. However, this is not consistent with supposed strong stirring of the lake magma due to intense activity over the 17 eruptive phases. Independent basal feeding of the lake during the eruption is another possibility, and may be seen as a likely mechanism to explain the stratification of the lake. 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. In this paper we present data on deformed olivine crystals from Kilauea lava samples using an in-situ µXRD technique. Our results allow the validation, and refinement, of optical observations of olivine deformation. We also confirm the presence of deformation for every olivine crystal size, and quantify it. There are significant correlations between deformation intensity (strain-related mosaicity) and the 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 estimation of a threshold over which deformation undoubtedly occurred. Micro XRD is shown here to be an easy-to-use, fast, low-cost, non-destructive and less-ambiguous technique to determine the presence of strain in the crystal structure of magmatic olivines, and quantify it, even for very small crystals.
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Contributor : Nicolas Vinet <>
Submitted on : Thursday, December 9, 2010 - 1:21:38 PM
Last modification on : Monday, September 2, 2019 - 9:42:56 AM
Long-term archiving on: : Thursday, March 10, 2011 - 1:27:53 PM

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Nicolas Vinet. Mécanismes de solidification des magmas basaltiques : Étude quantitative texturale et géochimique des laves du volcan Kilauea, Hawaï. Géologie appliquée. Université du Québec à Chicoutimi, 2010. Français. ⟨tel-00544904⟩

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