Timing, process, and origin of the alkaline magmas beneath Satonda volcano (Lesser Sunda Arc, Indonesia): evidence of magma mush disruption and mobilization

Four types of crystal-rich lava flow were observed as the primary products of Satonda volcano (Nusa Tenggara Barat, Indonesia), with clinopyroxene and olivine crystals occurring as the key points for distinguishing each type. The gray-weakly porphyritic lava (GwP) shows a linear-steep crystal size distribution (CSD) pattern of small clinopyroxene phenocrysts and is typically olivine poor (< 1 vol%). The gray-porphyritic lava (GP) and gray-strongly porphyritic lava (GsP) display a kinked CSD pattern consisting of small and large clinopyroxene crystals; also, both types are similarly dominated by clinopyroxene and have rare olivine (< 2 vol%), but the maxima clinopyroxene size of GP lava is smaller than type GsP lava (3.5 and 6 mm, respectively). The black porphyritic lava (BP) also displays the kinked clinopyroxene CSD with a maximum size of 3 mm; however, it is typically rich in olivine (~ 20 vol%). Whole-rock XRF analysis reveals that GwP, GP, and GsP lavas are similarly classified as trachybasalts (49–51 wt.% SiO2; 8–15 ppm Cr), whereas BP lava is classified as basalts (~ 48 wt.% SiO2) with abundant Cr content (51–66 ppm). This means that the extrusion of trachybasalt and basalt form GwP-GP-GsP and BP lavas, respectively. Large clinopyroxenes yield higher P–T crystallization conditions than small clinopyroxenes, indicating that large clinopyroxene crystals represent the earlier crystallization stage and are primarily stored at the marginal part of the magma reservoir ash mush zone. Multiple recharge event plays an important role in disrupting the mush, where the higher recharge intensity allows a larger input of the large clinopyroxene populations (GP, GsP, and BP lavas) and vice versa (BP lava). The high Ba/Nb, Th/Yb, and Rb/Ta values, coupled with the low Nb and La/Yb values, suggest that all Satonda magma received significant input from slab components with a relatively low contribution of mantle melting. Finally, K/Ar dating on various lava types reveals the eruption age of 115–74 ka.