Mechanisms driving polymagmatic activity at a monogenetic volcano, Udo, Jeju Island, South Korea |
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| Authors: | Marco Brenna Shane J Cronin Ian E M Smith Young Kwan Sohn Karoly Németh |
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| Institution: | (1) Volcanic Risk Solutions, Massey University, Palmerston North, New Zealand;(2) School of Geography, Geology and Environmental Science, The University of Auckland, Auckland, New Zealand;(3) Department of Earth & Environmental Sciences, Gyeongsang National University, Jinju, South Korea |
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| Abstract: | High-resolution, stratigraphically ordered samples of the Udo tuff cone and lava shield offshore of Jeju Island, South Korea,
show complex geochemical variation in the basaltic magmas that fed the eruption sequence. The eruption began explosively,
producing phreatomagmatic deposits with relatively evolved alkali magma. The magma became more primitive over the course of
the eruption, but the last magma to be explosively erupted had shifted back to a relatively evolved composition. A separate
sub-alkali magma batch was subsequently effusively erupted to form a lava shield. Absence of weathering and only minor reworking
between the tuff and overlying lava implies that there was no significant time break between the eruptions of the two magma
batches. Modelling of the alkali magma suggests that it was generated from a parent melt in garnet peridotite at c. 3 to 3.5 GPa
and underwent mainly clinopyroxene + olivine ± spinel fractionation at c. 1.5 to 2 GPa. The sub-alkali magma was, by contrast,
generated from a chemically different peridotite with residual garnet at c. 2.5 GPa and evolved through olivine fractionation
at a shallower level compared to its alkali contemporary. The continuous chemostratigraphic trend in the tuff cone, from relatively
evolved to primitive and back to evolved, is interpreted to have resulted from a magma batch having risen through a single
dyke and erupted the batch’s head, core and margins, respectively. The alkali magma acted as a path-opener for the sub-alkali
magma. The occurrence of the two distinct batches suggests that different magmatic systems in the Jeju Island Volcanic Field
have interacted throughout its history. The polymagmatic nature of this monogenetic eruption has important implications for
hazard forecasting and for our understanding of basaltic field volcanism. |
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