Tracking the nature and duration of magma transfer beneath Mauna Loa using a crystal population and kinetic modelling approach

Abstract:

Deep long period (DPL) seismic swarms recently detected beneath Mauna Loa fuel speculation whether the volcano could enter a renewed phase of unrest. To mitigate threats from future eruptions, a better understanding of how and over which timescales magma moves within Mauna Loa is required. We present a novel approach linking the compositions preserved in the chemical stratigraphy of 158 olivine crystals with kinetic modelling to provide timescales and routes of magma migration beneath Mauna Loa prior to the voluminous (376 million m³, [1]) 1950 eruption of Mauna Loa. We have studied a total of 8 near-vent samples erupted from fissures that opened progressively at elevations from 12,000ft to 8,500ft within the first 24h of the eruption (June 1-23, 1950). The samples contain olivine crystals with different populations of core (Fo89, Fo87-88, Fo85-86, Fo82-84), and rim compositions (majority Fo78-81) and zoning patterns (normal, reverse and complex). The diverging compositional and zoning record can be best explained as the product of magma evolution in five distinct magmatic environments (MEs): M₀ (=Fo89), M₁ (=Fo87-88), M₂ (=Fo85-86), M₃ (=Fo82-84), M₄ (=Fo78-81) with melt transfer and mixing among them. Modelling the diffusive relaxation of the compositional zoning profiles constrains the timescales and durations over which crystals (and melt) are transferred between the different MEs. Diffusion models were performed at temperatures of 1133-1168°C and fO₂ at ∆NNO -0.55 [2]. The derived timescales range from ~20 days up to 11 months, with the majority of the timescales being shorter than 4 months. The nature and duration of magma transfer beneath Mauna Loa prior to the catastrophic 1950 eruption is interpreted as follows: (i) Three dominant magma migration pathways connecting the environments M₁:M₄, M₃:M₄ and M₂:M₄ can be identified; and (ii) transfer of magma along these routes occurs in multiples pulses commencing up to 8 months before, and becoming more frequent in the weeks prior to, eruption. Depth constraints for each these environments remain to be resolved but are likely to extend from beneath the MOHO to within the shallow edifice [3].

[1] Finch & Macdonald (1950) USGS Bul 996-B, 27-89; [2] Gerlach (1993) Geochim Cosmochim Acta 57, 795-814; [3] Thornber & Trusdell (2008), IAVCEI General Assembly 2008, Reykjavik