GERMANIUM/SILICA RATIO AND RARE EARTH ELEMENT COMPOSITION OF SILICA-FILLING IN SHEET CRACKS OF THE DOUSHANTUO CAP CARBONATES, SOUTH CHINA: CONSTRAINING HYDROTHERMAL ACTIVITY IN THE MARINOAN SNOWBALL EARTH GLACIATION

Cap carbonate precipitation marked the termination of the snowball Earth event, indicating a rapid transition from an icehouse to a greenhouse climate condition. Cap carbonate yields some characteristic sedimentary structures, such as the sheet cracks, tepee-like structure and cemented breccias, which reflect an unusual physio-chemical condition in the aftermath of snowball Earth event. Origin of these sedimentary structures would reveal the paleoenvironment in the post-glacial ocean. Abundant sheet crack structures are discovered in the basal part of the Doushantuo cap carbonate (~635 Ma) throughout the Yangtze Block, South China. The interconnected horizontal and vertical cracks are variably cemented by dolospars and filled by silica. In this study, we measured Germanium/Silica ratios (Ge/Si) and rare Earth elements (REE) compositions of silica-filling in sheet cracks. These samples are collected from 12 sections, spanning from shelf to basin facies of the Yangtze Block. All silica-filling in sheet cracks display low Ge/Si ratios (~1 μmol/mol), light REE (LREE) depleted REE pattern (normalized to PAAS), and positive Eu anomalies (Eu/Eu*), which indicate that both seawater and hydrothermal fluid were the Si sources of silica-filling in sheet cracks. We established a binary mixing model by using Ge/Si ratio and Eu/Eu* as two independent proxies to quantify the relative contribution from hydrothermal fluid. The modeling results present that 0.5-2 vol.% of high temperature hydrothermal fluid was mixed with normal seawater, and 2-20 wt.% of Si in silica-filling was derived from hydrothermal fluid. Our study supports the penecontemporaneous cementation of sheet crack structures. Finally, the post-Marinoan ocean might be characterized by positive Eu anomaly, requiring the synglacial hydrothermal flux of 1-10 times of the modern level, which may explain the relatively short duration (5–15 Ma) of the Marinoan snowball Earth event.