The Origin of Transparent and Non-Transparent White Pumice: A Case Study of the 52ka Maninjau Caldera Forming Eruption, Indonesia

The 52ka eruption of Maninjau caldera produced two distinctive type of white pumices: transparent (TWP) and non-transparent (NTWP). Both pumice types are crystal-poor (avg. 3.3 %), having similar mineralogy (pl > qz > bt > px > opq), similar glass compositions (avg. 78.5 wt. % SiO2), and similar plagioclase core compositions (avg. An20-30). We found that the abundance of TWP decrease towards the upper stratigraphic positions, together with the increase in NTWP, grey pumice, banded pumice, and lithic contents. Vesicles in TWP are typically dominated by large vesicles, while NTWP is characterized by abundant-small vesicles. Large vesicle corresponds to the preexisting 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 in the conduit during the eruption. We performed quantitative comparison using pheno- and matrix-bubble number densities (PBND and MBND) for these two white pumice types. The correlation between PBND and MBND result in two regimes: (1) decompression-controlled regime, showing nearly constant-PBND correlation for TWP, and (2) phenobubble-controlled regime, showing steeply-decreasing PBND correlation for NTWP. In the first regime, MBNDs value varies dramatically, suggesting the variation of decompression rate by two to three orders of magnitudes. While in the second regime, the slight increase of MBNDs is considered as the effect of the decrease in PBND within the nearly constant decompression rate.

Reconciling bubble nucleation in explosive eruptions with geospeedometers

Magma from Plinian volcanic eruptions contains an extraordinarily large numbers of bubbles. Nucleation of those bubbles occurs because pressure decreases as magma rises to the surface. As a consequence, dissolved magmatic volatiles, such as water, become supersaturated and cause bubbles to nucleate. At the same time, diffusion of volatiles into existing bubbles reduces supersaturation, resulting in a dynamical feedback between rates of nucleation due to magma decompression and volatile diffusion. Because nucleation rate increases with supersaturation, bubble number density (BND) provides a proxy record of decompression rate, and hence the intensity of eruption dynamics. Using numerical modeling of bubble nucleation, we reconcile a long-standing discrepancy in decompression rate estimated from BND and independent geospeedometers. We demonstrate that BND provides a record of the time-averaged decompression rate that is consistent with independent geospeedometers, if bubble nucleation is heterogeneous and facilitated by magnetite crystals.

The Youngest Toba Tuff (74 ka) Crystals Characterization

&lt;p&gt;Toba Caldera Complex, Indonesia is well known as the largest Quaternary caldera (87x33 km) that formed by four major eruptions among which the biggest one is the eruption of the Youngest Toba Tuff (YTT) about 74,000 years ago. Textural study of the pumice clast from YTT has been done to estimate the decompression rate by using bubble number density data. The result shows that decompression rate of Toba Caldera forming eruption varies in two order magnitude ranging from 10&lt;sup&gt;6 &lt;/sup&gt;&amp;#8211; 10&lt;sup&gt;8&lt;/sup&gt; Pa/s. Southern pumices show the lower value than pumices from northern caldera. Similarly, new data about lithic distributions and mineral components of YTT from the northern and southern caldera showed several different characteristics. This fact suggests possibility of different processes which is distinguish production of southern and northern deposits. Therefore, understanding both conduit and chamber processes is needed to reveal the origin of differences in deposits. This study aims to elucidate magma chamber condition by characterizing the deposit especially crystals from YTT eruption.&lt;/p&gt;&lt;p&gt;Characterizations of Toba Tuffs have been made but not been enough to discuss YTT in detail. In this study, we focus on spatial differences in YTT deposits. Samples from four different locations were employed for the analyses. Component analysis was carried out on components larger than 2 mm. Whole-rock geochemical data were obtained by XRF. Petrography analysis for 37 thin sections was conducted using optical microscope. Textural analysis was carried out for 84 free crystals and 25 selected thin sections using microphotographs taken by SEM and further analyzed using image processing software. Chemical analysis for free crystal was carried out by SEM-EDS, while for pumices grain of 22 thin sections was conducted using EPMA.&lt;/p&gt;&lt;p&gt;Geochemical data showed that YTT magma is rhyodacitic to rhyolitic in whole-rock compositions with wide range of SiO&lt;sub&gt;2&lt;/sub&gt; (69.15&amp;#8211;76.83 wt.%). There are differences in abundance and type of pumices, free crystals, and lithic in each location&lt;strong&gt;.&lt;/strong&gt; Major minerals are plagioclase, biotite, sanidine, and quartz. Common characteristics of northern and southern part deposit is that most of crystals are fractured, some forming aggregates, has anhedral shape and wide variation in size (0.003 mm&lt;sup&gt;2&lt;/sup&gt;-13.113 mm&lt;sup&gt;2&lt;/sup&gt;). However, there are differences between northern and southern deposits: presence of amphibole with larger size, orange quartz, sieve texture, patchy zoning, oscillatory zoning, crystal clots, and wider range of anorthite (An&lt;sub&gt;25&lt;/sub&gt;&amp;#8211; An&lt;sub&gt;87&lt;/sub&gt;) is mostly found in northern deposits.&lt;/p&gt;&lt;p&gt;Plagioclase composition from northern part shows bimodal distribution suggesting that crystallization does not occur simultaneously by single process. Furthermore, plots of anorthite number versus size and of average anorthite number versus crystal content show random distribution, suggesting the complex crystallization of plagioclase: other processes than fractional crystallization in magma chamber. Moreover, presence of antecryst and disequilibrium textures in northern deposit indicates intervention from older rocks or even other systems. Different characteristics between northern and southern deposits suggest that YTT deposits are generated by multiple eruptions from independent, at least two magma chambers.&lt;/p&gt;&lt;p&gt;Keywords: Toba Caldera, the Youngest Toba Tuff (YTT), Crystal Characterization, Conduit Process, Chamber Process, Fractional Crystallization, Multiple eruptions&lt;/p&gt;

Caldera-forming eruptions of mushy magma modulated by feedbacks between ascent rate, gas retention/loss and bubble/crystal framework interaction

Caldera-forming eruptions of mushy silicic magma are among the most catastrophic natural events on Earth. In such magmas, crystals form an interlocking framework when their content reaches critical thresholds, resulting in the dramatic increase in viscous resistance to flow. Here, we propose a new mechanism for the ascent of mushy magma based on microstructural observations of crystal-rich silicic pumices and lavas from the Central Andes and decompression experiments. Microstructural data include spherical vesicles and jigsaw-puzzle association of broken crystals in pumices, whereas there is limited breakage of crystals in lavas. These observations insinuate that shearing of magma during ascent was limited. Decompression experiments reveal contrasting interaction between growing gas bubbles and the crystal framework in crystal-rich magma. Under slow decompression typical of effusive eruptions, gas extraction is promoted, whereas under rapid decompression, bubbles are retained and the crystal framework collapses. This feedback between decompression rate, retention of gas bubbles, and integrity of the crystal framework leads to strong non-linearity between magma decompression rate and eruption explosivity. We extend these findings to caldera-forming eruptions of crystal-rich magma where large overpressures are induced by caldera-collapse, resulting in magma plug-flow, rapid decompression facilitated by shear-localization at conduit margins, and explosive eruption.