THE ORIGIN OF TRANSPARENT AND NON-TRANSPARENT WHITE PUMICE: A CASE STUDY FROM THE 52ka MANINJAU CALDERA FORMING ERUPTION, WEST SUMATRA, INDONESIA

Abstract:

The 52ka eruption of Maninjau caldera produce two distinctive type of white pumices: transparent and non-transparent. Both pumice types are crystal-poor (average 3.8 %), having similar mineralogy (pl>qz>px, bt>opq), similar glass compositions (mean: 78 wt. % SiO₂), and similar plagioclase core compositions (mean: An₂₀-₃₀). Interestingly, we found contrasting difference in terms of the deposit distributions and physical characteristics such as vesicle, bulk density, and bulk vesicularity. Northern and eastern ignimbrites are dominated in transparent white pumice (TWP). In contrast, southern and western ignimbrites are dominated in non-transparent white pumice (NTWP). Vesucles in TWP is typically dominated by large vesicles, while NTWP characterise by abundant -small vesicles. Large vesicle corresponds to the pre-existing bubble which formed in magma chamber (pheno-bubble, > 0.1 mm). On the other hand, small vesicle in groundmass (matrix-bubble, <0.1 mm) is attributed to second nucleation during the eruption. TWP is typically denser and less-vesiculated compared to NTWP. Particularly, bulk density and bulk vesicularity varies from 1.18 – 0.4 gr/cm³ and 48 – 82%, and 1.01 – 0.29 gr/cm³ and 54 – 88 % for TWP and NTWP, respectively. We also performed quantitative comparison using pheno-bubble number density (PBND) and matrix-bubble number density (MBND) for those two types of white pumice. The correlation between PBND and MBND show parabolic function. Positive correlation between number and size of pheno-bubbles results from the nearly constant pheno-bubble volume fraction with increasing MBND in TWP, while the negative correlation results in monotonic decrease in pheno-bubble volume fraction with increasing MBND in TWP and NTWP. Namely, supersaturation of pheno-bubble in melt limits the nucleation of matrix-bubble, resulting glass-rich pyroclast and further observed as transparent pumice. Second nucleation effectively take place in phenobubble-poor magma thus yields relatively more vesiculated pyroclast and observed as non-transparent type.