EXPLOSIVE BASALTIC VOLCANISM OF THE CHIKURACHKI VOLCANO (KURILE ARC, RUSSIA): INSIGHTS ON PRE-ERUPTIVE MAGMATIC CONDITIONS AND VOLATILE BUDGET REVEALED FROM PHENOCRYST-HOSTED MELT INCLUSIONS AND GROUNDMASS GLASSES

Abstract

Highly explosive eruptions of basaltic composition are relatively rare and poorly understood, yet they may be quite important in terms of atmospheric impact because of the generally much higher concentrations of S in basaltic systems compared with the typical explosive felsic eruptions. We have determined concentrations of H2 O, major, trace and volatile (S, Cl) elements by EPMA and SIMS from melt inclusions and groundmass glasses of the 1986, 1853 and prehistoric explosive eruptions of basaltic magmas from the Chikurachki volcano, northern Kurile arc. Melt inclusions are hosted by olivine (Fo72–78 ), orthopyroxene (mg# = 72–75), clinopyroxene (mg# = 71–77) and plagioclase (An74–96) phenocrysts. Estimated crystallization conditions were in the range from 910 to 1180 8C at less than 400 MPa total pressure and oxygen fugacity of NNO+ 1 to +2 log units. Inclusion glasses are basaltic to andesitic in composition. Relative to N-MORB, the melts are enriched in LREE ([La /Sm]n = 1.8–2.6) and Sr ([Sr / Ce]n = 1.7–2.4), and show Nb, Zr and Ti depletions typical for arc magmas ([Nb / La]n = 0.2–0.3, [Zr / Sm]n = 0.6–0.9, [Ti / Gd]n = 0.4–1.1). The pre-erupted melts were volatile rich (up to 6.4 wt.% H 2O, 2900 ppm S and 1400 ppm Cl), which may explain the strongly explosive character of volcanic activity. Much lower volatile contents in the groundmass glasses compared to inclusions reflect the extent of eruptive magma degassing. The wide range of volatile contents in glass inclusions is interpreted as a result of complex patterns of magma ascent, phenocryst growth and partial degassing at high-level reservoirs before eruption. The most H 2O- and S-poor inclusions are close to groundmass values and may relate to magmas degassed in the summit reservoir, which drained back and mixed with less degassed magmas at depth. Estimations of total volatile release to the atmosphere, based on the difference between maximum melt inclusion contents and matrix glass, suggest that the 1986 and 1853 explosive eruptions released 3400 and 2500 metric tons H 2O, 300 and 200 tons SO 2 and 8 and 12 tons Cl, respectively. This is about ten times higher for SO 2 and ten times lower for Cl than for the 1989–1990 eruptions of the Redoubt volcano.