晚古生代地幔DUPAL异常峰：来自古亚洲洋蛇绿岩的Sr-Nd-Pb-Os同位素的限定

Available geochemical data indicate that the so-called DUPAL isotopic anomaly in the Earth's mantle exists in the Indian and South Atlantic sub-oceanic mantle in the southern hemisphere. However, the origin of such a global isotopic anomaly is still controversial. Tracing the location and evolution of the DUPAL mantle through time offers a new perspective on the DUPAL study as it will lead to a better understanding of the origin of the anomaly. Ophiolites, pieces of ancient oceanic lithosphere, provide unique opportunities to conduct this kind of study because they extend the isotopic information recorded by the mantle back in time. This project will focus on two Paleo-Asian Ocean ophiolites from the Northern Xinjiang region of China. We plan to analyze the major and trace elements and Sr-Nd-Pb-Os isotopic ratios of the bulk rock and mineral separates from MORB-type mafic rocks and peridotites in these ophiolites in order to constrain whether the Paleo-Asian sub-oceanic mantle existed a "DUPAL peak" during the late Paleozoic. Moreover, these information combined with ophiolite geochronologic data will provide tracers on the evolution of the DUPAL mantle through time and this in turn will provide stringent constraints on the possible cause of the DUPAL anomaly. Therefore, this project will promote geodynamic research in Central Asia as it will provide precise geochemical information on the crustal and mantle conditions in the Paleo-Asian Ocean and Central Asian Orogenic Belt during the Paleozoic.

地幔化学不均一性揭示了南半球（印度洋-南大西洋）地幔DUPAL同位素异常，然而其成因存在争议。从时间尺度上追踪地球历史上地幔DUPAL异常的演化进程将为研究DUPAL异常成因找到突破口，而蛇绿岩则是最佳研究对象。本项目拟选择新疆北部两处代表性的古亚洲洋蛇绿岩，对其中MORB型基性岩和变质橄榄岩进行全岩和单矿物主微量元素及Sr-Nd-Pb-Os同位素研究，厘定古亚洲洋地幔域西段是否存在晚古生代的"DUPAL异常峰"，进而结合蛇绿岩同位素年代学进一步追踪DUPAL异常随时间的演化进程，最终探讨DUPAL异常可能的成因。本项目将为理解古生代期间古亚洲洋地幔归属和中亚造山带深部地球动力学背景提供重要的科学依据。

现代印度洋地幔DUPAL同位素异常可以通过蛇绿岩的同位素地球化学研究追踪到更早的特提斯洋地幔和古亚洲洋地幔。分布在中国新疆北部和内蒙古地区的蛇绿岩代表了古亚洲洋壳的残余。这些蛇绿岩可以限定古亚洲洋地幔同位素特征及地幔演化过程。本项目通过对古亚洲洋蛇绿岩中玄武岩和辉长岩的主、微量元素以及Sr, Nd和高精度Pb同位素研究,发现新疆北部西准噶尔达拉布特蛇绿岩和东准噶尔克拉美丽蛇绿岩代表的地幔具有DUPAL同位素异常，即在给定206Pb/204Pb比值时具有更低的εNd和更高的208Pb/204Pb特征。微量元素特征显示达拉布特蛇绿岩似MORB，而克拉美丽蛇绿岩则具有介于似MORB和岛弧玄武岩之间，因此，达拉布特蛇绿岩的同位素特征可以代表上地幔的特征，而克拉美丽蛇绿岩的地幔源区可能受到俯冲成分的影响。我们利用Th/Nd和Nd-Pb同位素地球化学定量模拟计算显示亏损的印度洋MORB型地幔加入2%的俯冲成分可以形成克拉美丽蛇绿岩的地球化学特征。总之，达拉布特和克拉美丽蛇绿岩DUPAL同位素异常进一步支持了DUPAL地幔存在于印度洋地幔之前。.通过对内蒙古及新疆中天山冰达坂蛇绿岩的Sr, Nd，Pb同位素研究发现，与DUPAL地幔相比，内蒙古及新疆中天山蛇绿岩代表的古亚洲洋地幔具有更低的208Pb/204Pb比值，对比特提斯蛇绿岩，古亚洲洋地幔域与特提斯地幔域存在一个同位素界限。我们认为古亚洲洋地幔比特提斯地幔具有更低Th/U比值，这两种地幔域的Pb同位素差异可能是因为中亚造山带和特提斯构造域不同的岩浆-构造模式造成的。根据古亚洲洋和特提斯洋的古板块构造演化格局，古亚洲洋的演化过程类似于环太平洋造山带，地幔具有低Th/U特征；相反，特提斯地幔的高Th/U特征或DUPAL特征，可能是由特提斯构造域碰撞造山作用引起的。综合特提斯地幔与古亚洲洋地幔西段的DUPAL特征，揭示了DUPAL同位素异常可能是地球历史上存在最长的同位素异常，如果将现代大洋地幔随着时间演化反推至特提斯地幔和古亚洲洋地幔，可以发现在300Ma-400Ma之间存在一个DUPAL异常加强峰，这可能揭示了地球历史上地幔的一次热异常事件。