The multi-component Hekla o Tephra, Iceland: a complex widespread mid-Holocene tephra layer
JOURNAL OF QUATERNARY SCIENCE(2020)
摘要
Large Plinian eruptions from Hekla volcano, Iceland, produce compositionally zoned tephra used as key markers in tephrochronology. However, spatial variations in chemical composition of a tephra layer may complicate its identification. An example is the 5950-6180 bp cal a bp Hekla o tephra layer, which shows compositional spread from rhyolite, dacite and andesite to basalt. In soil sections north of Hekla, the SiO2 content of the tephra glass reaches 76 wt% in the lowest unit of the Hekla o deposit and decreases to 62-63 wt% in the uppermost unit. Intermingled within the whole deposit are basalt tephra grains having 46-47 wt% SiO2. The composition of the basalt glass includes primitive basalt and a more evolved basalt (MgO >6 and <6 wt%, respectively). Together with literature data, the Hekla o tephra and the so-called T-Tephra/Hekla-T are most likely from contemporaneous eruptions of different vents on the Hekla volcanic system, forming a single important marker tephra (Hekla oT) deposited over 80% of Iceland. Identification is complicated by its spatial compositional heterogeneity, such as systematic decrease in SiO2 content from the east to the west of Hekla volcano. Consequently, an individual tephra layer from a large explosive eruption can have different composition at different locations.
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关键词
Hekla,Iceland,mid-Holocene,tephrochronology,zoned tephra layer
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