Dynamics of the Young Merapi (<2.2 ka – 1,788 CE) pumice fall deposits: Insights from textural and geochemical studies

We studied nine pumice fall deposits of the Young Merapi stage (<2.2 ka – 1,788 CE) observed in the western and southern flanks of Merapi volcano. All deposits include a wide variation of lithics (10–42 Clithic), with thicker deposits (i.e., more voluminous eruption) being more lithic-rich and vice versa. Two different magma types (hereafter referred to as type I and type II) were identified based on petrography, bulk-rock, glass, and feldspar microlite compositions. Type I magma has abundant amphibole and pyroxene, is rich in calcium (>9 wt CaObulk), poor in both silica (50.8–53.7 wt SiO2 bulkand 62.3–66.6 wt SiO2 glass) and strontium (<580 ppm, bulk-rock), and has more calcic feldspar microlites (An38 – 79). Type II magma also contains abundant amphibole, but has less pyroxene and is poorer in calcium (<9 wt CaObulk), higher in both silica (53.2–54.5 wt SiO2 bulk and 63.3–70.8 wt SiO2 glass) and strontium (>580 ppm), with less calcic feldspar microlites (An31 – 77). These two magma types alternately fed the explosive eruptions during the Young Merapi stage; however, their juvenile products are distinctive in terms of syn-eruptive microtextures (i.e., matrix-vesicles and microlites). Pumices from type II magma have a higher matrix-vesicle number density (MVND) and microlite number density (MND) values than those of pumices from type I magma (1.0–6.5 × 1015 and 1.8–7.4 × 1015 m−3, and 0.6–2.3 × 1015 and 0.7–1.8 × 1015 m−3, respectively). A positive correlation between MVND with SiO2 and MND suggests that a colder (i.e., less calcic feldspar microlites indicate lower temperature and vice versa) and more evolved (higher SiO2) magma facilitates more extensive matrix-bubble nucleation and deeper microlite crystallization than hotter magmas, allowing type II magma to erupt more explosively than type I magma. © 2023