3:00 PM - 3:15 PM
[SVC39-04] Petrological monitoring of the eruptive activity since AD 2006 of Sakurajima volcano, Japan
Keywords:Sakurajima volcano, magma mixing, preeruptive magmatic process
The juvenile lapilli (lithic, scoria, and pumice) have plagioclase, orthopyroxene, clinopyroxene, and magnetite, and a small amount of olivine as phenocrysts. The core compositions of olivine phenocrysts are Fo80-81, compositionally disequilibrium with the co-existed pyroxenes (Mg#64-76 for opx and Mg#66-78 for cpx). Several phenocrysts of pyroxenes also have thin magnesian rims (<10 μm), some of which exhibit repeated reverse zoning. On whole-rock chemistry, all the juvenile lapilli are the most mafic since 20th century, and are plotted on the one linear trends on Harker diagrams, which agree with the compositional trends of the 20th juveniles. The SiO2 content of juvenile lapilli slightly increases from 2009 (58.5 wt.%) to 2013 (59.6 wt.%), and clearly decreases in 2015 (58.3-59.0 wt.%). On matrix glass chemistry of juveniles from lapilli and ash samples, there exists the similar temporal variation to whole-rock chemistry. The silica content decreases from 2009 to 2010, and increase gradually until 2013, and again decrease clearly in 2015.
The co-existence of high-Fo olivine and pyroxene showing reverse zoning, as well as the linear trends of whole-rock chemistry agreeing with the juvenile materials of the 20th century suggest that the magma plumbing system of the 20th century has continued since 2006, and that the basaltic magma has mixed with the pre-existed mixed andesitic magma repeatedly. Therefore, it is interpreted that the SiO2 contents in whole-rock and matrix glass chemistries reflect the ratio of basaltic magma in erupted magma. Comparing to the geophysical monitoring data, as the ratio of basaltic magma increased, the number of explosions became larger and the volcanic edifice inflated clearly. Estimated volume of input basaltic magma is much larger in 2015. Based on the calculated Fe-Mg diffusion times from the zoning profiles of high-Fo olivine phenocrysts in the 2015 juvenile materials, the timing of the latest input of basaltic magma is estimated at the period during late of 2014 to May 2015, being consistent with the beginning of the inflation of volcanic edifice. It is concluded that activation of the eruptive activity since AD 2006 has been controlled by intermittent injections of basaltic magma.