Evidence for a short-lived stratified magma chamber: petrology of the Z-To5 tephra layer ( c . 5.8 ka) at Zao volcano, NE Japan

Abstract

Volcanic rocks from the Z-To5 tephra layer of Zao volcano, NE Japan, preserve petrological information that reflects the magmatic processes under the volcano. The Z-To5 rocks were formed by the mixing of three magmas that differed in composition and phenocryst assemblage: basalt (1150-1200 8C), with high Mg (Fo c. 81 ) olivine; basaltic andesite (1020-1100 8C), with Mg-rich orthopyroxene (Mg# ¼ c. 78) and clinopyroxene (Mg# ¼ c. 78), lower Mg olivine (Mg# ¼ c. 78), and calcic (An c. 85 ) plagioclase; and andesite (900-1000 8C) with Mg-poor orthopyroxene (Mg# ¼ 61-66) and clinopyroxene (Mg# ¼ 64-68), and An-poor plagioclase. The basaltic magma was formed through fractionation of Fo c. 85 olivine from a less differentiated basaltic magma during its fast ascent from the depths. The andesitic magma, which occupied a shallow magma chamber, was heated by underplating of the basaltic magma, resulting in dissolution of some minerals. Subsequently, the basaltic andesite magma was formed by mixing of the basaltic and andesitic magmas in the chamber. Petrological evidence for the rapid growth of phenocrysts in the basaltic andesite magma suggests that the magma residence time was short. The basaltic andesite magma, mixing with a small portion of the andesitic magma, was withdrawn upon eruption. The rates of these processes are inferred to be rapid based on petrological considerations.

Sequence of the 1895 eruption of the Zao volcano, Tohoku Japan

“…Eruptive products of stage 4 are classifiable geologically into four eruptive units from older to younger: Nigorigawa lavas (NGL), Komakusadaira agglutinate (KMA), Umanose agglutinate (UMA), and Goshikidake pyroclastics (GSP) (Sakayori, 1992;Ban et al, 2008). The Nigorigawa lava, which flowed along the Nigorigawa River, has mostly been eroded (Sakayori, 1992).…”

“…It mainly drapes the top of the Umanose caldera wall . The Umanose agglutinate, which consists of four layers, covers the Komakusadaira agglutinate in the western part of the Umanose caldera (Sakayori, 1992;Ban et al, 2008). GSP, Goshikidake pyroclastics; UMA, Umanose agglutinate; KMA; Komakusadaira agglutinate; NGL, Nigorigawa lavas; YKL, Yokokurayama lavas; HPL, Happosawa lavas; KNP, Kumanodake pyroclastics; IML, Ichimaiishizawa lavas; SKL, Sainokawara lavas; NML, Nakamaruyama lavas; HML, Hiyamizuyama lavas; TKL, Torikabutoyama lavas; ZOL, Zaozawa lavas; SNL, Senninzawa lavas; OWL, Oiwake lavas; MYP, Mayuyamazawa pyroclastics; RO, Robanomimiiwa pyroclastics.…”

“…GSP, Goshikidake pyroclastics; UMA, Umanose agglutinate; KMA; Komakusadaira agglutinate; NGL, Nigorigawa lavas; YKL, Yokokurayama lavas; HPL, Happosawa lavas; KNP, Kumanodake pyroclastics; IML, Ichimaiishizawa lavas; SKL, Sainokawara lavas; NML, Nakamaruyama lavas; HML, Hiyamizuyama lavas; TKL, Torikabutoyama lavas; ZOL, Zaozawa lavas; SNL, Senninzawa lavas; OWL, Oiwake lavas; MYP, Mayuyamazawa pyroclastics; RO, Robanomimiiwa pyroclastics. (B) Close-up map of the summit area (Sakayori, 1992;Ban et al, 2008). .…”

“…The alteration might have been caused by hydrothermal fluids that originated from magma stored in a chamber. Ban et al (2008) estimated the andesitic magma chamber depth, which caused one of the eruptions in Zao's youngest stage, as less than 3 kb. They considered that the magma chamber had been semi-solidified but that it was activated by injection of basaltic magma from the deep part before the eruption.…”

Zao volcano

“…(b) Results of the component analysis and frequency histograms showing grain size distributions of black-coloured fall deposits from the Z-To5 tephra. AfterBan et al (2008).…”