Chlorine-rich pyrometamorphic magma at White Island volcano, New Zealand

Abstract

The 25 January 1987 phreatomagmatic eruption of Congress Vent on White Island volcano ejected paralava bombs derived from pockets of pyrometamorphic magma. The current White Island magmas are calc-alkaline basaltic andesite/andesite, but the paralavas are mostly highly peralkaline with molar(Na2O + K2O)/Al2O3 ratios from 1 to 22 recorded in matrix glasses. The glass compositions are highly variable, though quench crystals of tridymite, wollastonite and green clinopyroxene are ubiquitous, while albite occurs only in the least peralkaline samples. All have high alkali contents (Na2O + K2O > 10%) and chlorine mostly > 1% and up to 1.7%. The paralava glasses represent the most Cl-rich volcanic melts yet discovered. They display a strong positive correlation between Cl and total Fe, but a negative correlation between Cl and normative 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 include halite-cemented crater-lake sediments and acid-sulfate hydrothermally altered vent breccias and tuffs, as found in the eruption deposits together with the paralava bombs. Fusion probably occurred at temperatures between 830 and 1000 °C and at a pressure below 5 MPa in a regime where silicate melts were buffered by immiscible NaCl-saturated aqueous vapor and hydrosaline melt. The conditions required to form the paralava bombs appear to be uncommon, but it is likely that similar pyrometamorphic melts could be generated at many volcanoes where shallow magma penetrates a hydrothermal system.