Pre-eruptive processes and timescales of shallow silicic magma reservoir beneath the Iheya Graben in the middle Okinawa Trough

Intensive magmatic activity in the middle Okinawa Trough (MOT) is well recognized, yet the pre-eruptive processes and residence times of silicic magmas remain poorly constrained. Here, we present a detailed investigation of the textures, in situ elements, and isotopic compositions of minerals in the magmatic enclaves (MEs), glomerocrysts, and their host rhyolite from the Iheya Graben to elucidate pre-eruptive processes and associated timescales. In the host rhyolite, the complex textures and wide compositional range of the phenocrysts suggest that pre-eruptive processes may involve multiple magmas. The normally zoned phenocrysts within the MEs provide direct evidence for the injection of andesitic magma into the shallow silicic reservoir. Nearly parallel trace element and rare earth element distribution patterns of plagioclase and orthopyroxene, along with the similar Sr isotopic compositions of plagioclase, suggest that these magmas may be co-genetic. In the glomerocrysts, the plagioclase composition changes gradually from core to rim (An, La, Ce, Eu, Mg, Fe, and Sr/Ba), indicating that the shallow silicic system developed via fractional crystallization of intermediate–mafic magmas. The occurrence of glomerocrysts in the erupted material also provides irrefutable evidence for the existence of crystal mush zones beneath the Iheya Graben. FeMg interdiffusion profiles in zoned orthopyroxene from the host rhyolite capture a wide range of timescales, from ∼3 months to ∼700 years. These timescales correlate positively with Mg^#: profiles containing high-Mg^# bands yield longer durations (up to 700 years), whereas those composed of low-Mg^# bands are shorter (<200 years), indicating that perturbation of the shallow silicic system by andesitic magma was initiated ∼700 years before eruption, while episodic interactions between silicic pockets occurred relatively late. Overall, mineralogical evidence illustrates a complex, dynamic, crystal-dominated shallow silicic system beneath the Iheya Graben, which is recharged by andesitic magma.