利用石盐中流体包裹体定量恢复奥陶纪海洋的古环境

Fluid inclusions in halite can directly imply the seawater chemistry, and the seawater chemistry over geologic time change significantly between the Na-K-Mg-Ca-Cl (Ca-rich) type and the Na-K-Mg-Cl-SO4 (SO4-rich) type according to primary fluid inclusions (Kovalevich et al., 1998; Lowenstein et al., 2001; Brennan and Lowenstein, 2002; Horita et al., 2002; Kovalevych et al., 2002). Even fluid inclusions in recrystallised halite can also record the major ions of seawater chemistry (Kovalevych et al., 2006a). However the Ordovician seawater chemistry is not clear to us. According to speculation, Ordovician seawater should be Ca-rich type, but there are no direct evidences from fluid inclusions of halite (Kovalevich et al., 1998). Previous only Ordovician seawater chemistry data come from the end of Late Ordovician Mallowa Salt of Carribuddy Group in Australia (Kovalevych et al., 2006b). Carribuddy Group has two salt formations: lower Minjoo Salt and upper Mallowa Salt. Early Silurian Sahara Formation containing Early Silurian conodonts conformably overlies the upper Mallowa Salt of Carribuddy Group. Carribuddy Group conformably overlies well-data middle Ordovician. So Mallowa Salt is taken as late Ordovician salt. However, because Early Silurian Sahara Formation overlies the upper Mallowa Salt, so Mallowa Salt only represent the end of Late Ordovician seawater chemistry between Late Ordovician and Early Silurian (Kovalevych et al., 2006b), can not represent the Ordovician seawater during the most time. And because halites are deposited very quickly, Mallowa Salt is difficult to judge it is Early Silurian or Late Ordovician. In this study, we provide the seawater chemistry according to fluid inclusions from middle Ordovician halite in China and cover the gap of the seawater chemistry evolutions (Kovalevich et al., 1998; Lowenstein et al., 2001).

显生宙以来地球海水成分在"方解石海"与"文石海"的交替变化中，海相非生物壳碳酸盐岩矿物和钾盐类型也随之变化；利用石盐流体包裹体直接恢复地质时期的海水成分，是当今国际科研的热点，而该领域至今未见中国科学家的身影。欧美科学家先后恢复了各个地质时代海水成分，但仍然存在少数关键的空白，最大的空白部分就是奥陶纪海水成分。奥陶纪蒸发岩在世界范围内极为罕见，而陕北奥陶纪马家沟组发育多套巨厚的石盐，该套蒸发岩沉积也是我国海相钾盐最有希望的区块之一。钾盐沉积通常很薄，申请人计划通过高密度的奥陶纪石盐中的包裹体分析定量恢复海水成分，同时确定钾盐的有利沉积层段，同时对鄂尔多斯盆地奥陶纪碳酸盐岩进行系统的地球化学与古生物学研究。该项目的实施将使我国对地质历史时期海水成分研究进入国际前沿领域，并为我国海相钾盐的勘探提供直接的参考。该项目的实施不但在国际上具有重大理论价值，对贫乏钾盐的中国更有国民经济生产的紧迫需要。

地球上的海水成分并非是一成不变的，显生宙以来地球海水成分在"方解石海"与"文石海"的交替变化中，海相非生物壳碳酸盐岩矿物和钾盐类型也随之变化；利用石盐流体包裹体直接恢复地质时期的海水成分，是当今国际科研的热点。欧美科学家先后恢复了各个地质时代海水成分，但仍然存在少数关键的空白，最大的空白部分就是奥陶纪海水成分。奥陶纪蒸发岩在世界范围内极为罕见，而陕北奥陶纪马家沟组发育多套巨厚的石盐，该套蒸发岩沉积也是我国海相钾盐最有希望的区块之一。项目承担人通过对钻孔岩心中奥陶纪石盐的包裹体分析，定量恢复了奥陶纪的海水成分，研究发现该时期海水的成分与寒武纪相似，属于富Ca低SO4的“方解石海”，并与现代海水成分截然不同。海水成分的变化与早期生物矿化紧密相关，奥陶纪时期海洋处于长期稳定而温暖的环境，发生了“奥陶纪生物大辐射”事件，较低的生物分类单元多样性大量增加，海洋生物的丰度和分异度达到了一个高峰。同时奥陶纪海水成分也表明与其他地质时代相比，该时期海水中的钾离子更高，是一个十分适宜形成钾盐沉积的时期。