三峡地区陡山沱早期水体性质的稀土元素和锶同位素制约

Characteristics of Sr Isotopes and Rare Earth Elements of Cap Carbonates in Doushantuo Formation in the Three Gorges Area

通过分析三峡地区九龙湾剖面新元古代盖帽碳酸盐岩的稀土元素和锶同位素组成,对新元古代"雪球地球"事件结束后水体的性质提供了制约。盖帽碳酸盐岩的REE配分模式具有轻重稀土元素亏损的特征,存在轻微Y正异常以及较高的Y/Ho值,没有明显的La异常。这些特征与受淡水影响的陆缘盆地、湖泊等沉积环境相似,而不同于正常海水。这表明盖帽碳酸盐岩不是正常的海相沉积物,而是发育在淡水与海水混合环境中的沉积物。盖帽碳酸盐岩具有较高的87Sr/86Sr值,这是由于新元古代"雪球地球"事件结束后大量陆源风化物的输入引起的。因此,盖帽碳酸盐岩较高的87Sr/86Sr值不能代表当时全球海水信息,只能反映局部混合水体的信息。轻微的Ce负异常显示陡山沱组早期水体处于弱氧化状态。盖帽碳酸盐岩的REE和Sr同位素综合特征表明在新元古代"雪球地球"事件结束后,陡山沱组早期水体是携带大量陆源风化物的冰融水与海水混合的水体,而且此时的混合水体处于弱氧化状态。

Rare earth elements and strontium isotopes of cap carbonates in the Three Gorges area have been studied.The results are used to examine the geochemical feature of water from which these carbonates precipitated in the Snowball Earth aftermath.The REE patterns of cap carbonates from the Jiulongwan section are characterized by LREE and HREE depleted REE distribution patterns,positive Y anomalies and high Y/Ho values,without significant La anomaly.These indicate that the cap carbonates are unlike those typicality of the normal marine carbonates,but similar to those carbonates that are deposited in the environment dominated by freshwater influx from rivers.High 87Sr/86Sr values of cap carbonates display that the source of Sr from terrestrial weathering make the seawater 87Sr/86Sr value increase after the Neoproterozoic Snowball Earth events.Furthermore,Ce negative anomalies show that sedimentary environment of cap carbonates was weak oxidation.REE and Sr isotope characteristics indicate the ambient water is the seawater that mixed the freshwater with a large number of terrestrial weathering in the Snowball Earth aftermath.