Mineral authigenesis in pyroclastically embedded plants: insights and questions from the Permian of central Germany

The Chemnitz Fossil Forest represents the most completely preserved Permian ecosystem, buried 291 myr ago in the phreatomagmatic ashes of the Zeisigwald Tuff (Leukersdorf Formation, Chemnitz Basin, central Germany). Its fossils provide cell-scale insights into the anatomy and ecology of past life but also stand out for their unusual mineralogy, e.g., fluoritized wood. Another barely reflected feature are isometric crystal casts <1 mm in diameter that occur in the outer tissues and the adjacent rhyolitic tuff of the petrified stems. These clay-filled casts exhibit hexagonal and octagonal outlines when cut and are discussed to reflect early-diagenetic garnet formation and alteration due to high fluid pressure and metasomatosis in the ash. We discuss the formation of these casts based on reviewing previous studies and own preliminary XRD, SEM, and electron-microbe analyses and CL spectroscopy. In sum, these structures formed shortly during and after burial, as indicated by surrounding, plastically displaced tissues and well-preserved cells and intracellular fungi in the non-compacted trunks – evidence for a rapid, early-diagenetic silicification. The preferred occurrence in plants rich in parenchyma, the preservation of thermally labile tissues, uncharred wood and the silica CL prove that water and temperatures <250°C impacted authigenesis. The cast fill results from hydrothermal alteration and consists of kaolinite and iron-oxide-hydroxide aggregates that are dissected by xenotime veins. Mg and Ca contents are low (<2wt%). The results indicate the former presence of euhedral cubic minerals, whose formation and alteration resulted from burial in ‘cool’ ashes of a highly evolved magma and subsequent authigenesis.