[SVC47-11] Genesis of andesite and rhyolite magmas prior to the 30ka Aira catastrophic caldera-forming eruption
Keywords:Sr isotope ratio, Aira caldera, Assimilation and Fractional Crystallization (AFC) process
The juvenile clasts of the Iwato eruption are divided into rhyolite pumice (69-76 SiO2 wt.%) and andesite scoria (56-60 SiO2 wt.%), and their phenocryst assemblages are pl+qtz+opx and pl+opx+cpx(±qtz±ol), respectively. The plagioclase phenocrysts in the pumice and scoria show normally or oscillatory chemical zoning. The rhyolite pumice clasts contain mainly low-An (60) plagioclase with constant low-87Sr/86Sr values (0.7057±0.0006, 2SD), while those in the andesite scoria show a bimodal chemical distribution with peaks of An50 and An80. The high-An (>An70) plagioclase in the scoria show higher 87Sr/86Sr (0.7067±0.0013, 2SD) than the low-An plagioclase, and their 87Sr/86Sr values increase with decreasing An-contents. The 87Sr/86Sr of highest-An(~An95) plagioclase are ~0.7055, which are identical to those of the low-An plagioclase.
These observations suggest the following processes of the andesite and rhyolite magmas, which crystallized the high- and low-An plagioclase, respectively. Geochemical characteristics of the low-An plagioclase in the scoria are identical to those in the pumice, indicating that they were sourced from the rhyolite magma. The 87Sr/86Sr versus An-content variations of the high-An plagioclase indicate that the andesite magma had experienced an assimilation and fractional crystallization (AFC) process with higher-87Sr/86Sr materials such as shallow crustal rocks (Shimanto sediments: >0.7096; Miocene granites: >0.7088). The similar 87Sr/86Sr of the highest-An plagioclase in andesite and low-An plagioclases in rhyolite suggest that the andesite and rhyolite magmas were originally formed by a melting of a common source material. Considering that the values of 87Sr/86Sr (0.7057±0.0006) are higher than those of the typical 87Sr/86Sr ratio of mantle material around the Aira region (0.7042-0.7047 in whole-rock 87Sr/87Sr), the source rock is likely to be a crustal material. Little reverse zoning and dissolved texture of the phenocrysts in the scoria suggest that the timing of magma mixing was immediately before the Iwato eruption. In addition, this process of magma genesis is similar to that for the 30ka Aira CCFE, suggesting that the magma plumbing system of the Iwato eruption may have been maintained until the occurrence of the Aira CCFE.