6:15 PM - 7:30 PM
[SVC48-P01] Augmentation of Magma Database by using a simple method to estimate groundmass SiO2 content
Keywords:magma, petrological analysis, eruption magnitude, database
The simple method is based on uniformity in total phenocryst SiO2 content. We examined total phenocryst SiO2 contents of 44 magmas, ranging from basaltic to rhyolitic bulk composition, in the Magma Database. The total phenocryst SiO2 contents were calculated from modal data and representative phenocyrst compositions. We found that the magmas have uniform total phenocryst SiO2 contents of 47.4 wt% (S.D.=1.5 wt%) if their phenocyst assemblages are quartz-free. If quartz phenocyrst are contained, the total phenocryst SiO2 contents increase with fractions of quartz phenocyrst and have 64 wt% at the maximum. This evidence validates an assumption of uniform total phenocryst SiO2 contents (47.4 wt%) in massbalance calculation for the quartz-free magmas.
We compiled ca.40 literature data of bulk SiO2 and phenocryst content for mainly lava eruptions, which are minor in the Magma Database. Melt SiO2 contents of the compiled examples were estimated by the simple method, and their relationship between melt SiO2 content and eruption magnitude (M) were compared with those of the Magma Database. This compilation augmented examples of small to intermediate magmatic eruptions (M=3-5) with basaltic to rhyolitic melt compositions. This data augmentation has little effect on the already known maximum magnitude of M=5 for basaltic to dacitic melt eruptions.