CHLORINE-RICH PYROMETAMORPHIC MAGMA AT WHITE ISLAND VOLCANO, NEW-ZEALAND

The 25 January 1987 phreatomagmatic eruption of Congress Vent on White Island volcano ejected paralava bombs derived from pockets of pyromet amorphic magma. The current White Island magmas are calc-alkaline basa ltic andesite/andesite, but the paralavas are mostly highly peralkalin e with molar (Na2O + K2O)/Al2O3; ratios from 1 to 22 recorded in matri x glasses. The glass compositions are highly variable, though quench c rystals of tridymite, wollastonite and green clinopyroxene are ubiquit ous, while albite occurs only in the least peralkaline samples. All ha ve high alkali contents (Na2O + K2O > 10%) and chlorine mostly > 1% an d up to 1.7%. The paralava glasses represent the most Cl-rich volcanic melts yet discovered. They display a strong positive correlation betw een Cl and total Fe, but a negative correlation between Cl and normati ve alkali feldspar content. It is likely that the Fe content of melts was the dominant control over Cl solubility, not their alumina-alkali systematics. Possible parent rocks for the pyrometamorphic magmas incl ude halite-cemented crater-lake sediments and acid-sulfate hydrotherma lly altered vent breccias and tuffs, as found in the eruption deposits together with the paralava bombs. Fusion probably occurred at tempera tures between 830 and 1000 degrees C and at a pressure below 5 MPa in a regime where silicate melts were buffered by immiscible NaCl-saturat ed aqueous vapor and hydrosaline melt. The conditions required to form the paralava bombs appear to be uncommon, but it is likely that simil ar pyrometamorphic melts could be generated at many volcanoes where sh allow magma penetrates a hydrothermal system.