Germanium/silica ratio and rare earth element composition of silica-filling in sheet cracks of the Doushantuo cap carbonates, South China: Constraining hydrothermal activity during the Marinoan snowball Earth glaciation

Abstract

The rapid diversification of eukaryotes within 3 million years after the Marinoan global glaciation suggests the potential linkage between the snowball Earth event and biological evolution. To understand how the global glaciation and the biological evolution were linked, it is necessary to reconstruct the sequence of events with high temporal resolution that occurred during the deglaciation. In this study, we focus on silica-filling within sheet cracks. Sheet cracks abundantly occur in the basal part of the Marinoan cap carbonate (635 Ma), and are characterized by the interconnected horizontal and vertical cracks that are sequentially cemented by dolospars, silica and calcite. As the second stage of crack filling, the origin of silica-filling remains controversial. If the seawater origin is confirmed, the silica-filling might archive the signal of ocean chemistry in the aftermath of snowball Earth event. In this study, we measured the Germanium/Silica ratios (Ge/Si) and Rare Earth Elements (REE) compositions of silica-filling from the Doushantuo cap carbonate in the Yangtze Platform, South China. The silica-filling has low Ge/Si ratios (~1 μmol/mol), and displays a light REE (LREE) depleted REE pattern (normalized to PAAS) with positive Eu anomalies (Eu/Eu*). These results indicate the predominant seawater origin of silica with some contribution from hydrothermal fluid, suggesting the penecontemporaneous precipitation of silica-filling with respect to sheet crack formation. In order to quantify the contribution from hydrothermal fluid, we developed a binary mixing model by using Ge/Si ratio and Eu/Eu* as two independent proxies. The modeling results indicate mixing of ~5 vol.% hydrothermal fluid with contemporaneous seawater, or ~20 wt.% of Si in silica-filling from hydrothermal fluid. The modeling results also show that the synglacial hydrothermal flux might be one order of magnitude lower than the modern level, suggesting extremely weak hydrothermal activities during the Marinoan snowball glaciation.