Abstract
Abstract The 2·1 ka Moinui lava flow field, erupted from the southwest rift zone of Mauna Loa, Hawai`i, exhibits striking textural and geochemical variations, that can be used to interpret magma processes pre-, syn- and post-eruption. From this lava flow, the duration of magma storage and storage conditions, the timescales over which magma is transported to the surface, and flow emplacement mechanisms at Mauna Loa are determined. Electron microprobe analysis (EMPA) and diffusion chronometry of olivine crystals identify two distinct crystal populations: a primitive, polyhedral olivine population with core compositions of Fo90–88 and a more evolved, platy olivine population with core compositions of Fo83–82. Fe–Mg diffusion modelling of these olivine populations gives distinct timescales for each population; platy olivines yield timescales of days up to a few weeks, while polyhedral olivines yield timescales of months to years. Despite the nature of a well-insulated pāhoehoe flow, meaning that post-emplacement diffusion continues for some time, a wealth of time information can be retrieved concerning pre-eruptive magmatic processes as well as the processes associated with the lava extrusion. The short timescales obtained from the platy olivine crystals and the observed equilibrium between its cores and ambient melt suggest late-stage nucleation and crystal growth in the shallow conduit and during lava emplacement. Conversely, the longer timescales and olivine-melt disequilibrium of the polyhedral olivine crystals suggests accumulation from a deeper source and subsequent transportation to shallow magma storage beneath the summit of Mauna Loa months, or even years before eruption. The chemical and textural details of the Moinui lava reflect the mode of flow emplacement and may have implications for the interpretation of the distribution of spinifex and cumulate olivine within komatiites; high-temperature, low-viscosity lavas, common in the Archean.
Highlights
To date is the study of pre- and syn-eruptive magmatic timescales recorded by crystals carried in magmas
Field observations are used here to constrain the modes of lava emplacement using the principles outlined in Walker (1991), Hon et al (1994) and Thordarson & Self (1998), whilst scanning electron microscopy (SEM) and electron microprobe analysis of polished lava materials enabled the identification of olivine crystal populations and the constraint of timescales from Fe-Mg inter-diffusion
We have demonstrated that interrogation of the crystal populations shows in detail: (i) the mulit-stage journey of the polyhedral olivines from depth to the surface; and (ii) nucleation and crystallisation of the platy olivines, which took place after incorporation of the polyhedral olivines into the more-evolved magma
Summary
To date is the study of pre- and syn-eruptive magmatic timescales recorded by crystals carried in magmas Such timescales unravel details of the timing of magma storage, and the magmatic processes taking place within Mauna Loa prior to an eruption. The Moinui flow field formed by lava effusion from vents high on Mauna Loa volcano 2,100 years ago (Perring et al, 2004; Trusdell & Lockwood, 2017). It contains two morphologically distinct olivine populations, as identified by Perring et al (2004), where one is polyhedral and the other is platy. The origin of pre-1843 eruptions is less well-known due to the burial of older flows by younger lavas and due to deep weathering (Lockwood & Lipman, 1987; Trusdell & Lockwood, 2017)
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