5:15 PM - 6:45 PM
[SVC30-P04] Texture and compositions of orthopyroxene phenocrysts of the newest stage lavas of Zao volcano, NE Japan
Keywords:Zao volcano, andestic lava, magma mixing, temporal variation
Zao volcano is an active stratovolcano in central part of the volcanic front of NE Japan arc. The volcanic activity is divided into six stages and the newest stage started at about 35 ka. The eruption products of the newest stage consist of pyroclastics and subsequent lavas. The study of lavas has been barely studied. Here, we report textural and compositional features of orthopyroxene of the lavas.
In the newest stage, Nigorigawa lava (Ngl) erupted ca. 35 ka, Nigorigawa-upstream lava (Nul) and Goshikidake-east lavas (Gel) erupted ca. 30 to 10 ka, and Furikodaki lava (Fkl) and Goshikidake-south lava (Gsl) erupted younger than ca. 10 ka. The lavas are olivine bearing cpx-opx basaltic andesite to andesite, except for Gel, which is olivine phenocryst free. In terms of SiO2, Ngl, Nul and Gel, Fkl and Gsl are 55–56 wt.%, 54–57 wt.%, 57.5–58 wt.%, respectively. Ngl shows higher K2O, Rb, Ba, Zr, TiO2 contents and lower MgO, Cr, Ni contens than Nul and Gel. Modal percentage of pyroxene phenocrysts in Nul and Gsl is about 6 to 11 vol.%, whereas those of Ngl, Fkl and Gsl are about 5 vol. %. Dissolution textures observed in all lavas that indicates all rocks are the mixed rock.
Classification of orthopyroxene phenocrysts based on texture and chemical compositions
Orthopyroxene phenocrysts are mostly euhedral, subhedral to anhedral phenocrysts are subsequently included.
Based on the compositional zoning, orthopyroxene phenocrysts are divided into five types; Homogeneous (H), Broad (B), Medium (M), Thin (T), Oscillatory (Os) types. H-type is homogeneous. B, M, T-types have medium to high Mg band with > 50 µm, 30–50 µm, < 20 µm thick, respectively in outer part. Os-type shows oscillatory zoning, having high or low Mg core. The widths of high Mg zones except for near the rim are 30–50 µm. Those near the rim are < 20 µm.
Cores of H-types have either of low Mg# (62–65), medium Mg# (66–68), high Mg# (more than 69). Cores of most of B, M, T-types have low Mg#. The high Mg zones of B and M-types have medium to high Mg# and that T-type has high Mg#. Os-type core has either of low or high Mg#. Most of high Mg zones of 30–50 µm thick have medium to high Mg# and those of < 20 µm thick have high Mg#.
Characteristics of orthopyroxene phenocrysts in groups
Ngl (ca. 35ka): More than 50% of opx phenocrysts in Ngl comprises Os-type. T-type is the second most and H, B, M-types are rare. More than half of Os-type phenocrysts has thin high Mg# zone near rim. The amounts of the Os-type phenocrysts having high and low Mg# are similar.
Nul and Gel (ca. 30 to 10 ka): Percentages of H-type phenocrysts in Nul and Gel are higher than the other groups. Furthermore, Os-type is rare and amounts of B, M and T-types are different among units. T-type are included less than 30 %. In Nul, the amounts of H, B, M and T-types are approximately equal. Gel has 3 sub-units, the first includes more than 60%, about 20% T-type and rare B and M-types, the second includes similar percentage of each type as Nul, the third comprises mostly H-type. Core of H-type is low Mg# in Nul. That of 3 sub-units of Gel is low Mg#, low to medium Mg#, high Mg#.
Fkl and Gsl (more than ca. 10 ka): More than 75% of opx phenocrysts in Fkl and Gsl comprises B and M-types. H and T-types is rare. Likewise, B-type is more frequently observed than M-type in Fkl, and vice versa in Gsl. H-type is high Mg#.
Magma process recording on the orthopyroxene of the newest lavas
Phenocrysts with low Mg# cores and those with medium to high Mg# zones or cores are observed in most units. These features indicate that magma plumbing system is basically composed of shallow felsic magma chamber that is injected by the mafic to intermediate magmas and subsequently homogenizes. High Mg zones of B and M-types were formed by previous injection, whereas those of T-type were by the injection just before the eruption.
Os-type is dominant in Ngl, hence the previous injections occur repeatedly. H-type is most abundant in Nul and Gel and was from remobilized part of the shallow reservoir, therefore the injected magma would play a role of the heat source rather than the compositional mixing. The amounts of M and T-types are various among Nul and Gel units, thus the degree of the compositional mixing would differ among units. Most of the types in Fkl and Gsl are B and M, and T-type is barely observed, which indicates that the previous injection was not frequent and the injected magma just before the eruption would also mainly play a role of the heat source.
In the newest stage, Nigorigawa lava (Ngl) erupted ca. 35 ka, Nigorigawa-upstream lava (Nul) and Goshikidake-east lavas (Gel) erupted ca. 30 to 10 ka, and Furikodaki lava (Fkl) and Goshikidake-south lava (Gsl) erupted younger than ca. 10 ka. The lavas are olivine bearing cpx-opx basaltic andesite to andesite, except for Gel, which is olivine phenocryst free. In terms of SiO2, Ngl, Nul and Gel, Fkl and Gsl are 55–56 wt.%, 54–57 wt.%, 57.5–58 wt.%, respectively. Ngl shows higher K2O, Rb, Ba, Zr, TiO2 contents and lower MgO, Cr, Ni contens than Nul and Gel. Modal percentage of pyroxene phenocrysts in Nul and Gsl is about 6 to 11 vol.%, whereas those of Ngl, Fkl and Gsl are about 5 vol. %. Dissolution textures observed in all lavas that indicates all rocks are the mixed rock.
Classification of orthopyroxene phenocrysts based on texture and chemical compositions
Orthopyroxene phenocrysts are mostly euhedral, subhedral to anhedral phenocrysts are subsequently included.
Based on the compositional zoning, orthopyroxene phenocrysts are divided into five types; Homogeneous (H), Broad (B), Medium (M), Thin (T), Oscillatory (Os) types. H-type is homogeneous. B, M, T-types have medium to high Mg band with > 50 µm, 30–50 µm, < 20 µm thick, respectively in outer part. Os-type shows oscillatory zoning, having high or low Mg core. The widths of high Mg zones except for near the rim are 30–50 µm. Those near the rim are < 20 µm.
Cores of H-types have either of low Mg# (62–65), medium Mg# (66–68), high Mg# (more than 69). Cores of most of B, M, T-types have low Mg#. The high Mg zones of B and M-types have medium to high Mg# and that T-type has high Mg#. Os-type core has either of low or high Mg#. Most of high Mg zones of 30–50 µm thick have medium to high Mg# and those of < 20 µm thick have high Mg#.
Characteristics of orthopyroxene phenocrysts in groups
Ngl (ca. 35ka): More than 50% of opx phenocrysts in Ngl comprises Os-type. T-type is the second most and H, B, M-types are rare. More than half of Os-type phenocrysts has thin high Mg# zone near rim. The amounts of the Os-type phenocrysts having high and low Mg# are similar.
Nul and Gel (ca. 30 to 10 ka): Percentages of H-type phenocrysts in Nul and Gel are higher than the other groups. Furthermore, Os-type is rare and amounts of B, M and T-types are different among units. T-type are included less than 30 %. In Nul, the amounts of H, B, M and T-types are approximately equal. Gel has 3 sub-units, the first includes more than 60%, about 20% T-type and rare B and M-types, the second includes similar percentage of each type as Nul, the third comprises mostly H-type. Core of H-type is low Mg# in Nul. That of 3 sub-units of Gel is low Mg#, low to medium Mg#, high Mg#.
Fkl and Gsl (more than ca. 10 ka): More than 75% of opx phenocrysts in Fkl and Gsl comprises B and M-types. H and T-types is rare. Likewise, B-type is more frequently observed than M-type in Fkl, and vice versa in Gsl. H-type is high Mg#.
Magma process recording on the orthopyroxene of the newest lavas
Phenocrysts with low Mg# cores and those with medium to high Mg# zones or cores are observed in most units. These features indicate that magma plumbing system is basically composed of shallow felsic magma chamber that is injected by the mafic to intermediate magmas and subsequently homogenizes. High Mg zones of B and M-types were formed by previous injection, whereas those of T-type were by the injection just before the eruption.
Os-type is dominant in Ngl, hence the previous injections occur repeatedly. H-type is most abundant in Nul and Gel and was from remobilized part of the shallow reservoir, therefore the injected magma would play a role of the heat source rather than the compositional mixing. The amounts of M and T-types are various among Nul and Gel units, thus the degree of the compositional mixing would differ among units. Most of the types in Fkl and Gsl are B and M, and T-type is barely observed, which indicates that the previous injection was not frequent and the injected magma just before the eruption would also mainly play a role of the heat source.