Structure and Dynamics of the Laacher See Magma Chamber (Eifel, Germany) from Major and Trace Element Zoning in Sanidine: a Cathodoluminescence and Electron Microprobe Study

Abstract

Microtextures and zoning patterns in sanidine phenocrysts from the phonolitic Laacher See Tephra (Germany) were investigated in order to constrain processes occurring in the magma chamber before eruption. The Laacher See Tephra unit is chemically zoned and has been inferred to represent the product of eruption of an inverted, layered magma chamber. Samples from various levels in the deposit were investigated. We used a combination of textural studies (optical microscopy and back-scattered electron (BSE) imaging), electron microprobe (EMP) analysis of major, minor and trace elements (Ca, Na, K, Al, Si, Ba, Sr, Fe and Ti) and element mapping. The samples studied contain two feldspar phases and the ternary composition of the sanidine thus constrains the temperature of crystallization, whereas its trace element content reflects the melt composition. The large diversity in textures found in the sanidine crystals can be classified into three types: composite (C-type), pseudooscillatory (PO-type) and resorbed/patchy (R-type). Trace elementpoor, lamellar composite alkali feldspars (C-type) are found in samples inferred to represent the top and the middle part of the magma chamber. They grew as composite crystals from the melt at temperatures as low as 650 � C in a highly differentiated and volatilerich boundary layer at the magma chamber roof or wall. Pseudooscillatory zoning with resorption surfaces (PO-type) is found in sanidinesfromsamplesinferredtorepresentthemiddlepartandthebase of the magma chamber. Repeated, large An variations (1–4 mol %) reflect temperature variations of 100–300 � Ca ssociated with changes in water content. Variations of minor and trace elements (Ba, Sr and Ti) in sanidine, decoupled from the pseudo-oscillations of the major elements, reflect chemical changes in the melt, and are damped by chemicaldiffusioninthemelt.Bothmajorandminorelementvariations inPO-typecrystalsareinterpretedastheprogressiveinfluenceofamore mafic, hotter melt. This may be partly explained by the settling of crystals through the thermal and chemical gradient existing in the magma chamber; however, the additional role of magma recharge may be required to explain the large temperature variations. Resorbed/patchy-zoned crystals (R-type), found mainly in samples corresponding to the middle part of the magma chamber, reflect early growth in a differentiated boundary layer, followed by resorption and overgrowthinthemainmagmabody.ManyoftheLaacherSeesanidine crystals did not crystallize for the most part in the melt in which they were erupted. In the presence of preserved major and trace element zonation in the magma, this observation indicates crystal dispersion within a layered magma chamber without large-scale mixing and overturn.