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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Crustal xenoliths in the 6220 BP Saefell tuff-cone, south Iceland: Evidence for a deep, diatreme-forming, Surtseyan eruption
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Crustal xenoliths in the 6220 BP Saefell tuff-cone, south Iceland: Evidence for a deep, diatreme-forming, Surtseyan eruption

机译：冰岛南部6220 BP Saefell凝灰岩锥中的地壳异岩：证据表明发生了深层，异常形成的苏尔塞扬火山喷发

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摘要

Three types of xenoliths were collected from a 80 m high vertical section of the Sasfell tuff-cone on Heimaey, Iceland. These types are: alkali basaltic (Type Ⅰ), sedimentary (Type Ⅱ) and cpx-bearing basalts (Type Ⅲ). They are distinct in hand specimen, thin section and geochemistry. Near the base of the eruptive succession only juvenile material and Type Ⅰ xenoliths are present. The middle section has Type Ⅱ xenoliths in addition to Type Ⅰ. Xenoliths of Type Ⅲ first appear in the uppermost xenolith-rich horizons, where they are found together with all other types. Juvenile Sasfell clasts are present in various amounts throughout the investigated section. A downward migration of explosion foci during the eruption is established based on the distribution of the different types of xenoliths in the tuff-cone (i.e. the appearance of Type Ⅰ-Type Ⅱ-Type Ⅲ xenoliths with increasing stratigraphic height) in combination with the observed relations in basement stratigraphy beneath Heimaey. In the beginning of the eruption a combination of high eruption rate and abundant seawater resulted in shallow-seated explosions depositing only juvenile material and Type Ⅰ xenoliths. The downward migration of explosion foci is suggested to be the result of lowered influx of seawater (due to the formation of a cone that reached 50-75 m a.s.l.) in combination with lowered eruption rate. As the basement stratigraphy beneath Heimaey is well documented by a 1565 m deep drill-hole, the depth of the explosions can be constrained to vary between less than 170 m in the beginning of the eruption and exceeding 820 m in the final phases. This is much deeper than previously reported for any Surtseyan tuff-cone forming eruption and consistent with the downward penetration of a diatreme.
机译：从冰岛Heimaey的Sasfell凝灰岩锥高80 m的垂直剖面中收集了三种异种石。这些类型是：碱性玄武岩（Ⅰ型），沉积玄武岩（Ⅱ型）和含cpx玄武岩（Ⅲ型）。它们在手标本，薄片和地球化学方面截然不同。在喷发演替的基部附近，仅存在幼稚物质和Ⅰ型异种岩。中部除Ⅰ型外还有Ⅱ型异岩。 Ⅲ型异岩首先出现在最富异岩的地层中，并与其他所有类型一起发现。在整个调查部分中，青少年Sasfell碎屑的含量均不同。根据凝灰岩中不同类型的异种岩的分布（即随着地层高度的增加，Ⅰ型，Ⅱ型，Ⅲ型异岩的出现）并结合观测资料，确定了爆发过程中爆炸震源的向下迁移。海迈下的地下地层中的关系。喷发初期，高喷发率和丰富的海水共同导致浅层爆炸，仅沉积了幼物和Ⅰ型异岩。爆炸焦点的向下迁移被认为是海水涌入量减少（由于形成的圆锥体达到了50-75 m a.s.l.）和喷发率降低的结果。由于Heimaey下方的地下地层已被1565 m深的钻孔所充分记录，因此爆炸的深度可以限制在喷发开始时的小于170 m和最终阶段的超过820 m之间。对于任何苏特塞凝灰岩形成喷发来说，这比以前报道的要深得多，并且与暗杀药的向下渗透一致。

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