Vigorous Exsolution of Volatiles in the Magma Chamber beneath a Hydrothermal System on the Modern Sea Floor of the Eastern Maims Back-Arc Basin, Western Pacific: Evidence from Melt Inclusions

摘要

Image analyses and ion probe analyses were carried out on melt inclusions in olivine, pyroxene, and plagioclase phenocrysts of vesicular volcanic rocks that host the actively forming massive sulfides at the PACMAN US hydrothermal field in the eastern Manus back-arc basin, western Pacific. The melt inclusions, ranging in size from <2 to >150 mu m, contain one or more bubbles. Most of them have bubble/melt inclusion volume ratios (V_b/V_m) of >5.5 vol percent, higher than-those for the melt inclusions with their bubbles generated from shrinkage (V_b/V_m, 3.6 vol percent) or postentrapment exsolution of volatiles (V_b/V_m 3.7 vol percent). This, together with the presence of vapor bubbles with little melt (V_b/V_m, 80 to >95 vol percent) in pyroxene and olivine phenocrysts, suggests that a vapor phase was formed from vigorous exsolution of volatiles or the boiling of a magma in the crystallizing magma chamber. The VV_b/V_m ratios show distinct distribution patterns in the phenocryst minerals, indicating that there was more than one vesiculation event in the magma chamber. Melt inclusions with no visible bubbles have the highest contents of H_2O (2.1-2.5 wt percent). The H_2O contents of the glass in the melt inclusions are inversely related to the size of the coeval bubbles. The significant variation of H_2O contents (1.0-2.5 percent) in the melt inclusions suggests that the H_2O was degassed from the crystallizing magma. The positive correlations of S, Cl, and possibly F with H_2O indicate that all these volatiles were exsolved from the melt into the vapor phase. The negative correlation of H_2O with alkali contents in the glass of melt inclusions suggests that the magma was crystallizing in the presence of a fluid phase. It is estimated that the magma might have lost up to 65 percent of its volatiles while crystallizing. Significant amounts of magmatic fluid can be degassed from a magma during its crystallization in a shallow magma chamber. An individual vesiculation event may last at least 5 to 48 years, as estimated from the growth of the phenocrysts. This estimate is a minimum because not all the phenocrysts crystallize at the same time. The estimated life of a black smoker chimney is 2 to 10 years, which is within the duration of an individual vesiculation event. This implies that the magmatic fluid could be a major source of ore metals during the growth of individual sulfide chimneys. Through multiple vesiculation events in a crystallizing magma chamber, the magma could provide large quantities of ore metals and volatiles to a hydrothermal system over a long period of time to form a large massive sulfide deposit on the sea floor.