Rare Earth Element Geochemistry Of The Dolostone From Nantuo Formation In Eastern Guizhou Province

In the late Neoproterozoic,the Earth's climate changed dramatically,and the Earth experienced two"Snowball Earth"events,which had a significant impact on the marine oxidation and biological evolution of the Ediacaran after the glacial period.However,due to the lack of chemical sedimentary rocks during the glacial period,the problem of marine redox conditions has not been adequately solved.In Eastern Guizhou Province,South China,in three boreholes,ZK505,ZK309,and ZK102,the authors found a bed of syn-glacial dolostone,which provided ideal material for the study of the marine redox environment in the Marinoan Period.The rare earth element and Y?REY?geochemical characteristics of carbonate rocks can reflect the marine environment at the time of their formation because the REY is less altered by the later diagenesis.The Nantuo Formation of Songtao County in the Eastern Guizhou Province correlates to the Marinoan Ice Age deposit,which is underlaid by Datangpo Formation and is overlaid Doushantuo Formation.The dolostone 1.5m in thickness is at the lower part of the Nantuo Formation from the three boreholes in Songtao County,sand-wiched between two sets of diamictites.According to the stratigraphic sequence reflected by the drilling cores,the stratigraphic correlation of Nantuo Formation in South China,and the petrological characteristics of the dolostones,we suggested that the dolostone is syn-glacial sediment.The dolostone samples are gray,mainly composed of micrometer-grained dolomites with few quartz,feldspar,calcite,and clay minerals.In this paper,the major elements,REY and trace elements,stable carbon and oxygen isotopes in the samples are analyzed,and scanning electron microscopy was used to characterize the dolomite samples.The dolostones were drilled by micro-drilling technology to avoid the veins,weathering,and contamination.To prevent the influence of clastic materials on the concentrations of REY in dolostone samples,a 5%v/v acetic acid solution was used to dissolve the samples.Dolostones in ZK505 have the REY 23.0 to 46.6 ppm,weak negative or no Ce anomalies?average 0.90?,variable Eu and La anomalies,medium-rare earth enrichment,low Y/Ho ratio?average 34.6?,high contents of Fe?1761.2-25607.4 ppm,average 11185.9 ppm?and Mn?5175.6-15490.0 ppm,average 11114.8 ppm?.Three dolostone samples from ZK309 borehole were analyzed by LA-ICP-MS for their REY and trace element compositions.The results show that the REY concentrations ranged from 8.6 to 26.4ppm,averaged of 16.4ppm.After PAAS normalization,the rare earth elements show slight negative or no Ce anomalies?average 0.90?,variable Eu,and La abnormalies,low Y/Ho?30.5?,high Fe?2424.4-14045.8 ppm,average 8175.4 ppm?and Mn?3024.1-12799.7 ppm,average 6864.6 ppm?.The analytical results in ZK102dolostone show that the concentration of rare earth elements is always low,ranging from 3.30 to 29.77ppm,with high Mn?345-10890 ppm?.The REE patterns normalized by PAAS illustrate mild negative to weak positive Ce anomalies and medium rare earth enrichment.All PAAS-normalized REE patterns in the dolostones from three drilling cores show consistent trend distinct to modern seawater.Based on the analysis of the Ce anomaly change trend in the dolostones,it is suggested that suboxic conditions prevailed during the early stage of the Marinoan Period and fluctuating of manganese-rich,iron-rich,or suboxic state in the middle phase,finally returned to iron-rich anoxic environments.The difference between redox conditions change could be attributed to the basin restriction with open water.Apparent negative values(?¹⁸O_V-PDB-11.50‰,?¹³C_V-PDBaverage-9.14‰)compared with cap carbonates in the Ediacaran implied methane oxidation,a prevailing process operated in modern ferruginous lakes.We conclude that the redox environments during the Marinoan Glaciation Period are significantly different from current oceanic conditions.Combined with the characteristics of high Fe and Mn content in the dolostone samples,it suggests the Marinoan marine redox heterogeneity in Nanhua Basin,which may pave the way for the evolution of multicellular eukaryotes during and after the ice age.