Distinct pumice populations in the 74 ka Youngest Toba Tuff: Evidence for eruptions from multiple magma chambers

Abstract

Whether the magma storage system is responsible for the formation of the 74 ka Youngest Toba Tuff (YTT) is still debatable. It is currently believed that the YTT super-eruption was formed either by a single voluminous chamber or by multiple magma chambers. This paper presents a detailed component analysis of eruption products, comprehensive stratigraphy, mineralogy, and geochemistry of phenocrysts and matrix glass to shed light on the issue. Based on the componentry, mineral assemblages, mineral and glass compositions, and vesicle texture, we identified four distinct pumice types: YtA–YtD. Of these, YtA is an amphibole-bearing pumice containing an average of 77.1 wt% SiO2. This type of pumice is characterized by abundant matrix vesicles, with plagioclase showing a wide range of anorthite content (An20–An90) and disequilibrium texture. The YtB and YtC types are the most evolved pumices, with 77.5 and 77.6 wt% SiO2, respectively. The plagioclase of these pumices commonly has an unzoned texture with low anorthite content (~An30). The YtB pumice is biotite-bearing, distinctively characterized by abundant pheno-vesicles; while the YtC pumice includes rare pheno-vesicles, skeletal-polyhedral quartz, and no biotite. The YtD type is a quartz- and sanidine-free pumice, with a less-evolved glass composition (76.3 wt% SiO2), and is characterized by dominant matrix-vesicles and crystal clots of plagioclase, amphibole, pyroxene, and biotite. The plagioclase of the YtD pumice shows a hollow texture and high anorthite content (~An50). In situ trace element analysis for the matrix glass of YtA, YtB, YtC, and YtD showed very distinct geochemical signatures, which correlated with the four pumice types. Bivariate plots (Ba vs. Y, Sr vs. Y, and Ba vs. Sr) showed that YtA pumice is characterized by medium Ba (400–875 ppm) and Sr (41–67 ppm), and variable Y (20–53 ppm) content. YtB and YtC pumices are characterized by low Ba (8–136 ppm) and Sr (13–37 ppm), and highly variable Y (27–77 ppm) content. YtD pumice is characterized by high Ba (1173–1340 ppm) and Sr (95–124 ppm), but a narrow range of Y (21–31 ppm) content. The trace-element geochemical signatures, occurrence and absence of certain minerals (i.e., biotite and quartz), and vesicle type from the distinct pumice type, suggest the presence of four distinct magma bodies before the eruptions, namely YtA, YtB, YtC, and YtD. The number of distinct glass shards and pumices with the same geochemical signatures represents the volume fraction of each magma type. The approximate total volume of the magma (5300 DRE km3) and the geographical distributions of each pumice type demonstrated that the voluminous YtA (3406 km3), medium YtB (1354 km3), and smallest YtD (168.5 km3) chambers in the northern direction, and the small YtC chamber (371.5 km3) in the southern caldera, were evacuated during the 74 ka YTT eruption. Eruptions from multiple magma chambers were initiated by a high supersaturation of pheno-vesicle-rich magma (YtB magma), triggering the subsequent eruptions of the YtA, YtB, YtC, and YtD magmas.