Re–Os geochronology of black shales from the Neoproterozoic Doushantuo Formation, Yangtze platform, South China

Abstract

In South China, the Doushantuo Formation represents 90% of the time interval of the Neoproterozoic Ediacaran Period (635–542Ma). However, subdivision and correlation of strata in this formation is limited by sporadic occurrences of fossils of uncertain age and a scarcity of radiometric time constraints. In order to obtain Re–Os ages and initial 187Os/188Os (Osi) data, black shale samples from the Doushantuo Formation (Yangtze Platform, South China) were analyzed using the CrO3-H2SO4 digestion technique. A Re–Os date of 595±22Ma (Osi=0.85±0.18, MSWD=29, n=7) was obtained for black shale samples from the basal part of the fossil-bearing Doushantuo Member 4 (“Miaohe Member”) at Jiulongwan section, Three Gorges area, South China. A subset of these samples define a more precise isochron age of 591.1±5.3Ma (Osi=0.874±0.038, MSWD=1.3, n=4). We interpret the high MSWD of 29 and larger uncertainties for the 595±22Ma date to reflect heterogeneity of the Osi of samples related to temporal change of 187Os/188Os ratio of seawater when the sedimentary rocks were deposited. Taking 591.1±5.3Ma as the age for the base of the Doushantuo Member 4, together with the age of a tuff bed on the top of Member 4 (551.1±0.7Ma, zircon U–Pb dating, Condon et al., 2005), a minimum time difference of 35m.y. between the base and top of Member 4 is obtained. This estimate suggests very low sedimentation rates during black shale deposition (∼0.4mm/ka). The time difference of 35m.y. also represents a minimum duration of the Shuram–Wonoka δ13C excursion in South China. Black shale samples from Doushantuo Member 2 at Baiguoyuan section yield an imprecise date of 592±68Ma (Osi=0.534±0.057, MSWD=77) due to limited spread in 187Re/188Os ratio of the samples and a relatively large sampled stratigraphic interval (∼6m). If deposition occurred in water masses with access to the open ocean, Osi of the hydrogenous fraction from these organic matter rich sedimentary rocks may record the Os isotopic composition of coeval seawater. Thus, combined with previously published Re–Os data, the highly radiogenic initial 187Os/188Os ratio of 0.874±0.038 derived from Re–Os isochron for the base of Doushantuo Member 4, and even more radiogenic values near the top of Member 4 (1.08–1.17, calculated at 551Ma) indicate a dramatic increase of the flux of 187Os into the ocean during the late Ediacaran. The enhanced 187Os flux can only result from enhanced continental weathering and oxidation rates during that period.