矿床地球化学国家重点实验室知识库

We have recently reported non-mass-dependent ¹⁷O depletion in sulfate deposited after the Marinoan glacial meltdown at ca. 635 million years ago. Further investigation linking the Δ¹⁷O of barite to its sedimentological–geological context in Marinoan South China reveals that the ¹⁷O depletion in sulfate is most pronounced at sites near paleo-continents, supporting the hypothesis that atmospheric O₂ was the source of the depletion which was transferred to sulfate via oxidative weathering of sulfides. Host minerals or rocks for the Marinoan ¹⁷O depletion have been limited to barite (southern China and western Africa) and carbonate-associated sulfate (CAS) in limestone lenses within a diamictite (Svalbard). If the Marinoan ¹⁷O depletion event is indeed related to an extraordinary atmospheric condition, the signal should be global in its distribution. The Kimberley region of Western Australia was close to a continent in the late Neoproterozoic and will serve as a test of our hypothesis that the anomalous ¹⁷O depletion may be widely recorded in the rock records of this time period.

We report here that the CAS in the Moonlight Valley (MV) cap dolostones, Texas/Mabel Downs (TMD), the eastern Kimberley region, Western Australia is anomalously depleted in ¹⁷O (Δ¹⁷O value as low as −0.68‰). The magnitude of the anomaly decreases gradually toward the overlying Ranford mud-silt-sandstones. This finding not only expands the geographic distribution of the depleted ¹⁷O signal, but also the type of host rocks or minerals that the anomalous sulfate resides. The CAS in the MV cap dolostones in Palm Spring (PS) section, ∼150 km south of TMD, however, does not bear a ¹⁷O depletion. Neither does the CAS in the Egan cap dolostones. The presence and absence of pronounced ¹⁷O anomalies in the two time-equivalent yet spatially different MV cap dolostones are consistent with the paleogeography that indicates TMD was close to the continent while PS was at an open-ocean environment. While sharing some of the same sedimentological features with that of the MV cap dolostones at TMD, the Egan cap has distinct δ¹³C and δ¹⁸O values for dolostones and distinct Δ¹⁷O, δ³⁴S and δ¹⁸O values for CAS, supporting an earlier assignment of the Egan glaciation to be younger than Marinoan in the late Neoproterozoic.