Chemical zoning and open system processes in the Laacher See magmatic system

Abstract

Unravelling the generation of compositionally and thermally zoned magma reservoirs is important to our understanding of the dynamic processes operating in magmatic systems. Here, we present new major and trace element data for volcanic glasses from the classically zoned Laacher See Tephra and suggest that mafic recharge may play an important role in producing the observed compositional gradient. Mafic phonolite glass from the upper part of the Laacher See Tephra records the addition of ca. 30% basanite magma, which is recognised by an increase in REE + Y and a decrease in Th, U and Zr relative to glasses from more evolved units. We suggest that the Laacher See magmatic system was sustained by repeated episodes of basanite recharge and calculate a recharge magma flux of between 2 × 10–5 and 1 × 10−4 km3y−1 in the 20 kyr leading up to the eruption. Basanite addition would have provided heat required to generate the strong compositional and thermal gradients that are recorded in crystals ejected during the Laacher See Tephra eruption.