Characteristics of Acid Melts That Produced the Tephra of Pleistocene–Holocene Eruptions of Ichinsky Volcano, Kamchatka: Evidence from Melt Inclusions

Аннотация

The paper reports data on glassy melt inclusions in minerals of tephra that was produced by the most significant eruptions of Ichinsky volcano, Kamchatka, and marked the major evolutionary stages of the volcanic center in the Pleistocene–Holocene. The melts are of dacite–rhyodacite composition (68–77 wt % SiO₂, 0.1–0.6 wt % MgO) and contain 2–4 wt % H₂O (3.3 wt % on average). Most of the melts are potassic (3.8–4.8 wt % K₂O), but some of them are less alkaline (2.5 wt % K₂O) and richer in iron and calcium. The trace-element composition of the glasses also suggests the presence of melts of two types. The high-K melts are relatively deficient in Nb and HREE (Nb = 10–14 ppm, La/Yb = 10–20), whereas the low-K melts typically show more clearly pronounced Nb minima, are relatively depleted in LREE, and are enriched in HREE (Nb = 3–4 ppm, La/Yb = 3–5). The two melt types likely sampled a long-lived magmatic chamber and, perhaps, also one of the components of magmatic mixing, whose addition to the system may have provoked the beginning of a reactivation stage of the volcanic center.