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Rapid differentiation, gas accumulation and the size of magma chamber beneath Hekla volcano, Iceland, from 238U-series disequilibria.

Olgeir Sigmarsson 1,2, Garance Hervé1, Guðrún Larsen2

  • Affiliations: 1Laboratoire Magmas et Volcans, CNRS and Université Blaise Pascal, Clermont-Ferrand, France 2Nordvolc, Institute of Earth Sciences, University of Iceland, Reykjavík, Iceland 

  • Presentation type: Poster

  • Presentation time: Monday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 74

  • Programme No: 1.2.19

  • Theme 1 > Session 2

Abstract

Eruptions at the constantly inflating Hekla volcano in Iceland start explosively and ends with an emission of a basaltic andesite lava of uniform composition. Measured Ra-Th radioactive disequilibrium decreases from 14% excess of 226Ra over 230Th to 5% with magma differentiation. This decrease is controlled by plagioclase fractionation alone. Therefore, the magma differentiation time from basalt to intermediate magma beneath Mt. Hekla can be shown to be significantly shorter than three centuries. From the estimated magma production rate, the volume of the basaltic andesite magma accumulation zone/reservoir/magma chamber are less than 2 km3. Gas accumulate in a hermetic magma chamber can lead to an overpressure and an eruption. It is hard to detect but the decay of 226Ra produces 222Rn that will be stored together with the available major gas species, and rapidly decay to 210Pb. Such radon accumulation thus will lead to excess 210Pb over the grandparent 226Ra in the magma accumulating the gas phase. Eruptions at Hekla volcano start explosively producing tephra before an effusive lava forming phase. The tephra of the five latest eruptions of Hekla (1947 to 2000) has either excess of 210Pb over 226Ra or are in radioactive equilibrium, as are all the lava flows. Since 1947 CE the volume of erupted tephra decline regularly, a trend that correlates with a reduction in radon flux from a deep-seated basalt source into the basaltic andesite magma chamber beneath Hekla. A renewal of the basalt source seems to be needed for the next eruption at Hekla volcano.