The Moinui Flow on Mauna Loa: transport and deposition of an olivine crystal cargo in a compound basaltic lava flow

Abstract

The Moinui lava flow on the west flank of Mauna Loa, Hawai`i, is a 32-km-long pāhoehoe basalt flow, between 760 and 1500 years old, characterised by the presence of two generations of olivine: 2–5-mm-sized glomerocrystic clusters of equant polyhedral grains, and dendritic plates. Both generations are distributed throughout the flow field from the vent at 3400 m above sea level to the ocean. We mapped roadcut outcrops in detail to investigate the internal geometry and emplacement mechanisms of the flow, and sampled these outcrops to collect quantitative textural data on the two olivine populations in 2D and 3D, with a view to understanding emplacement processes in general, and the factors controlling the growth, transport, and deposition of phenocryst olivine in particular. Roadcut outcrops reveal complex geometries of interconnected tube-fed sheet lobes, with the main control being pre-existing topography: drained tubes with flanking sheet lobes were developed on the seaward slope of the volcano, whereas tumuli were developed on the flat coastal plain topography. The internal architecture of the flow field is consistent with current hypotheses of emplacement by breakout and inflation of flow lobes from tube-fed internal pathways. Concentrations of up to 30% olivine represent settling of the larger size-fraction of the transported load, with no evidence for superimposed effects of flow differentiation.Implied effective viscosities were around 1000 Pa s. The glomerocrystic population was inherited as pre-existing crystal clusters derived from a sub-volcanic chamber, rather than by synneusis (random collision and aggregation) post-eruption, whereas the plate population was probably generated by a burst of nucleation related to degassing and supercooling during the vent eruption. Other than this, there is no evidence for substantial growth of the transported olivines during flow.