11:30 〜 11:45
[SVC28-10] Four distinct pumice populations of the Youngest Toba Tuff (YTT):
Evidence for multiple magma chambers during the YTT 74 ka super eruption
キーワード：Pumices, magma chamber, the Youngest Toba Tuff (YTT), super eruption
The eruption process responsible for the formation of the 74 ka Youngest Toba Tuff (YTT) is still debatable; currently, there are two hypothesis including eruption from one single voluminous chamber and multiple magma chamber eruption. Our previous preliminary results (presented in JpGU 2020 meeting), focusing on heterogeneity of crystal in pumices, i.e., mineral assemblages, plagioclase composition, and texture, of limited samples from southern and northern deposits suggested two independent magma chambers. However, the crystal characterization only does not give a clear picture of the distinct magma chambers. We carried out the detail component analysis and classify the types of pumices for deposits at all caldera directions. In this contribution we present new detailed mineralogical data, and geochemical signatures of matrix glass, including trace elements.
Based on the mineral assemblages, glass compositions, and vesicle texture we identified four distinct pumice types. The first type (P1) is quartz-rich amphibole-bearing pumice with 77.1 wt % of average SiO2 content of glass. This type of pumice is characterized by abundant small vesicles, with plagioclase showing a wide range of anorthite content (An20 to An80) and disequilibrium texture (patchy zoning and hollow texture). The second and third types (P2 and P3) are the most but slightly evolved glass compositions in the YTT (77.5 wt.% in average SiO2 content of glass). Plagioclases of P2 and P3 pumices commonly have unzoned texture, with low anorthite content (An30). P2 is distinctively characterized by abundant large vesicles and biotite bearing, while the P3 pumice includes rare large vesicle, white mica, and no biotite. The fourth type of pumice (P4) is quartz and sanidine free, shows the less evolved glass composition (76.3 wt.% in average SiO2 content of glass), and is characterized by dominant small vesicles and crystal clots of plagioclase, amphibole, pyroxene, and biotite; plagioclase of the P4 pumice shows a hollow texture and high anorthite content (An50-60). In-situ trace element analysis of matrix glass of P1, P2, P3 and P4 showed very distinct geochemical signatures, which clearly define the four pumice types. Plotted on bivariate diagrams of Ba vs Y, Sr vs Y, and Ba vs Sr, the P1 pumice is characterized by relatively medium Ba and Sr (400 – 700 ppm, and 52 – 68 ppm, respectively), but variable Y composition (25 – 41 ppm); the P2 and P3 pumices, on the other hand, show low Ba and Sr compositions (30 –119 ppm, and 13 – 38 ppm, respectively) and highly variable Y composition (27 – 78 ppm); and P4 pumice is characterized by relatively high Ba and Sr (1118 – 1376 ppm, and 96 – 124 ppm, respectively), but narrow range of Y compositions (24 – 31 ppm).
The differences in trace element signatures of the four pumice types of YTT may represent at least three distinct pre-eruptive magmas, which is consistent with the differences in major elements, mineral texture and assemblages. The occurrence of amphibole and high anorthite content in plagioclase of the less evolved P1 and P4 pumices suggest crystallization in higher pressure and temperature conditions, which implies deeper chamber environment. On the other hand, the high SiO2 content of matrix glass, the presence of quartz and sanidine, associated with the low An content of plagioclase of P2 and P3 pumices, may indicate crystallization under lower pressure and temperature conditions. This may imply that the P2 and P3 pumices were formed in relatively shallow chamber environment. However, the differences in vesicles texture, the presence and absence of certain minerals in P2 and P3 may indicate different chambers. We conclude that YTT super eruption was originated from probably three or four magma chambers.
Based on the mineral assemblages, glass compositions, and vesicle texture we identified four distinct pumice types. The first type (P1) is quartz-rich amphibole-bearing pumice with 77.1 wt % of average SiO2 content of glass. This type of pumice is characterized by abundant small vesicles, with plagioclase showing a wide range of anorthite content (An20 to An80) and disequilibrium texture (patchy zoning and hollow texture). The second and third types (P2 and P3) are the most but slightly evolved glass compositions in the YTT (77.5 wt.% in average SiO2 content of glass). Plagioclases of P2 and P3 pumices commonly have unzoned texture, with low anorthite content (An30). P2 is distinctively characterized by abundant large vesicles and biotite bearing, while the P3 pumice includes rare large vesicle, white mica, and no biotite. The fourth type of pumice (P4) is quartz and sanidine free, shows the less evolved glass composition (76.3 wt.% in average SiO2 content of glass), and is characterized by dominant small vesicles and crystal clots of plagioclase, amphibole, pyroxene, and biotite; plagioclase of the P4 pumice shows a hollow texture and high anorthite content (An50-60). In-situ trace element analysis of matrix glass of P1, P2, P3 and P4 showed very distinct geochemical signatures, which clearly define the four pumice types. Plotted on bivariate diagrams of Ba vs Y, Sr vs Y, and Ba vs Sr, the P1 pumice is characterized by relatively medium Ba and Sr (400 – 700 ppm, and 52 – 68 ppm, respectively), but variable Y composition (25 – 41 ppm); the P2 and P3 pumices, on the other hand, show low Ba and Sr compositions (30 –119 ppm, and 13 – 38 ppm, respectively) and highly variable Y composition (27 – 78 ppm); and P4 pumice is characterized by relatively high Ba and Sr (1118 – 1376 ppm, and 96 – 124 ppm, respectively), but narrow range of Y compositions (24 – 31 ppm).
The differences in trace element signatures of the four pumice types of YTT may represent at least three distinct pre-eruptive magmas, which is consistent with the differences in major elements, mineral texture and assemblages. The occurrence of amphibole and high anorthite content in plagioclase of the less evolved P1 and P4 pumices suggest crystallization in higher pressure and temperature conditions, which implies deeper chamber environment. On the other hand, the high SiO2 content of matrix glass, the presence of quartz and sanidine, associated with the low An content of plagioclase of P2 and P3 pumices, may indicate crystallization under lower pressure and temperature conditions. This may imply that the P2 and P3 pumices were formed in relatively shallow chamber environment. However, the differences in vesicles texture, the presence and absence of certain minerals in P2 and P3 may indicate different chambers. We conclude that YTT super eruption was originated from probably three or four magma chambers.