Using the chemical stratigraphy in magmatic crystals to track the evolutionary history of a dynamic magma plumbing system
Final Report Abstract
Deep long period (DPL) seismic swarms recently detected beneath Mauna Loa fuel speculation whether the volcano could enter a renewed phase of unrest. A primary goal in understanding volcanic eruptions is to know how and over which timescales molten rock (magma) moves within and under a volcano. Knowing the routes along which magma rise to the surface and how and over what timescales its physical and chemical properties are changing as it migrates is paramount to better anticipate the potential hazards that may be posed by volcanic eruptions worldwide. Direct evidence for magma movements under volcanoes can be recovered from studying the different layers recorded within crystals that grow from cooling magma. Crystals have the ability to respond to changing conditions (e.g. temperature, pressure) in their surrounding magma by growing layers of different composition. The resulting sequence of compositional layers resembles those of annual tree rings; as with tree rings, the different layers in crystals can be considered to record what was happening as they formed. When the volcano erupts, the ‘crystal recorder’ stop recording. By accessing those records, we can effectively read back the information to identify and track what happened under the volcano. We can also use this archive to work out the time at which each record was written before the eruption happened. From this we can reconstruct where crystals (and hence their host magma) were at different times and how they moved to the surface. This study links the compositions preserved in the chemical stratigraphy of 158 olivine crystals with kinetic modelling to provide timescales and possible routes of magma migration beneath Mauna Loa prior to the voluminous (376 million m3) 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. 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 catastrophic1950 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 multiple 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.
Publications
- (2014). Tracking the nature and duration of magma transfer beneath Mauna Loa using a crystal population and kinetic modelling approach. AGU fall meeting, San Francisco, USA, December 2014
Kahl, M., Morgan, D., Thornber, C.R. & Trusdell, F.A.
