Publication Date:
2017-11-06
Description:
The powerful eruption in the Akademii Nauk caldera on January 2, 1996, marked a new activity
phase of Karymsky volcano and became a noticeable event in the history of modern volcanism in Kamchatka.
The paper reports data obtained by studying more than 200 glassy melt inclusions in phenocrysts of olivine
(Fo82–72), plagioclase (An92–73), and clinopyroxene (Mg# 83–70) in basalts of the 1996 eruption. The data
were utilized to estimate the composition of the parental melt and the physicochemical parameters of the
magma evolution. According to our data, the parental melt corresponded to low magnesian, highly aluminous
basalt (SiO2 = 50.2 wt %, MgO = 5.6 wt %, Al2O3 = 17 wt %) of the mildly potassic type (K2O = 0.56 wt %) and
contained much dissolved volatile components (H2O = 2.8 wt %, S = 0.17 wt %, and Cl = 0.11 wt %). Melt
inclusions in the minerals are similar in chemical composition, a fact testifying that the minerals crystallized
simultaneously with one another. Their crystallization started at a pressure of approximately 1.5 kbar, pro
ceeded within a narrow temperature range of 1040 ± 20°C, and continued until a nearsurface pressure of
approximately 100 bar was reached. The degree of crystallization of the parental melt during its eruption was
close to 55%. Massive crystallization was triggered by H2O degassing under a pressure of less than 1 kbar.
Magma degassing in an open system resulted in the escape of 82% H2O, 93% S, and 24% Cl (of their initial
contents in the parental melt) to the fluid phase. The release of volatile compounds to the atmosphere during
the eruption that lasted for 18 h was estimated at 1.7 × 106 t H2O, 1.4 × 105 t S, and 1.5 × 104 t Cl. The concen
trations of most incompatible trace elements in the melt inclusions are close to those in the rocks and to the
expected fractional differentiation trend. Melt inclusions in the plagioclase were found to be selectively
enriched in Li. The Lienriched plagioclase with melt inclusions thought to originate from cumulate layers
in the feeding system beneath Karymsky volcano, in which plagioclase interacted with Lirich melts/brines
and was subsequently entrapped and entrained by the magma during the 1996 eruption.
Type:
Article
,
PeerReviewed
Format:
text
DOI:
10.1134/S0016702911110085