Study of melt inclusions from tephra (EPMA)

Precise and accurate geochemical analysis of tephra is key to investigating the environmental impacts of volcanic eruptions. Important within this research area is the study of melt inclusions – tiny pockets of magma, as small as 10 μm across or less, trapped within crystals that grow while the magma rises from its sources deep in the Earth’s crust towards the volcanoes. The ascending magma progressively releases volatile elements such as fluorine, chlorine and sulfur, forming atmospherically damaging gases. Comparing volatile concentrations in the earliest trapped inclusions – reflecting the original magma composition – and the glass formed when the magma freezes upon eruption – its final volatile content – allows estimates to be made of the amount of gases released.

 

Studying melt inclusions from tephra involves using EPMA with extremely low beam currents because these glassy materials can suffer chemical changes (principally loss of sodium) during analysis, especially of small areas.

 

Chris Hayward and his team at the University of Edinburgh, Scotland conduct EPMA measurements at a small beam size (3 μm), applying three separate beam currents during a single analysis, thus ensuring that sodium and potassium are kept within the analysis volume.

 

Above:

Melt inclusions (rounded blue areas) within a plagioclase crystal from a tephra grain erupted by Eyjafjallajökull in 2010, Iceland. The X-ray map shows the concentration of calcium within the crystal and the glass. Surrounding the crystal is glass from magma quenched during the eruption.

 

Below:

Quantitative EPMA data for Mount Edziza sheep-track tephra with a 3 μm beam diameter (average of 27 points).

 

SiO2	TiO2	Al2O3	FeO	MnO	MgO	CaO	Na2O	K2O	P2O5	Total
Av (wt%)	62.06	0.25	18.11	4.49	0.14	0.12	1.07	8.34	5.41	0.03	100.04
1σ St Dev	0.81	0.01	0.62	0.30	0.02	0.03	0.09	0.72	0.22	0.01
DL (ppm)		100			238	261	532		88

 

Further readings:

 

Hayward C. High spatial resolution electron probe microanalysis of tephras and melt inclusions without beam-induced chemical modification. Holocene 2012;22:119–125.
https://journals.sagepub.com/doi/10.1177/0959683611409777

 

Gudmundsson MT, Thordarson T, Höskuldsson A, et al. Ash generation and distribution from the April–May 2010 eruption of Eyjafjallajökull, Iceland. Scientific Reports 2012;2:572.

https://www.nature.com/articles/srep00572