| Summary: | Hekla volcano is one of the most active volcano in Iceland and is known for its explosive eruptions. Studies on prehistoric tephra have focused on the large plinian eruptions, namely Hekla 5, Hekla 4 and Hekla 3. Here, the aim is to improve the knowledge of other prehistoric eruptions and the volcanic history of Hekla by investigating the mid-Holocene tephra layers Hekla DH (6650 cal yr BP), Hekla Mó (~6060 cal yr BP) and Hekla Ö (6060 cal yr BP). The Hekla DH, Hekla MÓ and Hekla Ö tephra layers have been described, mapped and their composition analyzed in several soil sections proximal to Hekla proper. The magma composition of these three layers range from rhyolite through andesite and basaltic-andesite to basalt. Hekla DH represents the earliest confirmed eruption of basaltic andesite from the Hekla system, which subsequently has become the dominant product of the system. The basalts (SiO2 ~46-47%) observed in these three Hekla tephra layers consists of two types; one with TiO2 <3 wt% and Al2O3 >15 wt% and the other with TiO2 >3 wt% and Al2O3 <14 wt% for a MgO concentration of >6 wt% and <6 wt% respectively. Chemical composition of basaltic andesite in the Hekla DH tephra indicates that lower oxygen fugacity conditions may have existed in the magmatic system beneath Hekla prior to the Hekla DH eruption, compared to later times. Increased magmatic fugacity during mid-Holocene could have resulted from melting different regions of heterogeneous mantle source due to deglaciation of the area. New isopach maps are compiled for all three tephra layers. Mapping of the dispersal of the Hekla DH tephra has confirmed a source in the Valagjá area, in the northern part of the Hekla system. With the chemical dataset obtained in this study, a possible relationship between Hekla Ö and a distal Hekla tephra in NW, W and SW-Iceland is suggested. Consequently, the Hekla Ö tephra possibly covers as much as 80% of terrestrial Iceland. In addition, thirteen tephra layers of mid-Holocene age (4200-7100 cal yr BP) ...
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