MOBILITY OF RARE-EARTH AND OTHER ELEMENTS DURING CRYSTALLIZATION OF PERALKALINE SILICIC LAVAS

Pantellerite lavas of the dormant caldera volcano, Mayor Island, New Zealand, have thick nonhydrated obsidian selvages at the tops and bases, and crystalline, microspherulitic interiors. The flows were chemically homogeneous on emplacement but the crystalline portions have lost significant Na, F, Cl, Cs, Y and REE. Percentage losses of Y and REE may exceed 60%, Ce/Yb ratios decrease by as much as 20% but Eu/Eu* ratios are unchanged. Loss of Y and REE is attributed to the expulsion at near-solidus temperatures of (Y, REE)3+ chloride complexes but other complexing agents may have contributed to this mobility. Na is also lost in the same manner. Groundwater has introduced Mg and Sr, and leached or redistributed Na, Cs and halogens. Analysis of hydrated glassy Plinian pumice shows that secondary hydration has not affected Y and REE concentrations. The elements Na, Mg, F, Cl, Sr, Cs, Y and REE are highly mobile during the crystallization of peralkaline silicic melts and should not be used in petrogenetic evaluations. These elements, but not Y and REE, are also redistributed during secondary hydration processes. Magmas of undersaturated peralkaline composition will be similarly affected. © 1990.