硼的地球化学性质及其在俯冲带的循环与成矿初探

Boron prospecting based on boron cycling in subduction zone

硼是广泛应用于化工、农业、材料科学及核工业领域的重要元素。硼与氢的核聚变反应是未来具备运用潜力的清洁能源。硼作为典型的亲石元素,是高度不相容元素。硼元素容易富集于蚀变洋壳及蛇纹石化地幔橄榄岩中。而在板块俯冲过程中,由于硼具有强的流体活动性,会优先赋存于流体中。因此,当蛇纹石化的大洋岩石圈及覆于其上的沉积物在俯冲过程中发生脱水,这使得弧前地幔楔发生大规模的蛇纹石化。此时大量硼元素很可能随俯冲流体释放并封存于弧前地幔楔中。目前已发现的超大型硼矿床主要位于聚合型板块边缘,尤其土耳其拥有世界上最大的硼酸盐储量。我们推测这些矿床的形成基础条件之一可能与弧前高度蛇纹石化的地幔楔有关。尤其是在洋陆俯冲环境,弧前蛇纹岩或蛇绿混杂岩首先通过俯冲侵蚀再循环到火山弧岩浆中,使得岩浆更富集硼。随后弧火山喷发大量富硼的火山岩、岩浆热液及水气。在岩浆冷却过程中,硼元素析出、沉淀于火山表面,并伴随风化、侵蚀过程汇聚至碰撞造山带的封闭湖盆之中。此外,干冷的气候条件下也进一步促进了硼的成矿。我国具有形成大型、超大型硼矿的地质条件,应加大研究及探勘力度,并适当购买硼作为战略储备。

Boron is an important element widely used in chemical engineering,agriculture,material sciences,and nuclear industries.The nuclear fusion reactions of hydrogen(H)with the boron isotope 11 B can be potential as an ideal clean energy in the future.There are amounts of boron strongly enriched in subducted sediments,altered oceanic crust and serpentinized lithospheric mantle.Boron is an incompatible lithophile and soluble element.Boron can be readily transported during subduction dehydration,resulting in the serpentinization of forearc mantle wedge with such born-rich fluids.The mantle wedge is extensively serpentinized by fluids derived from forearc dehydration of the subducting slab,with boron captured in the serpentinized mantle wedge.Most of borate deposit seemingly primarily occur along the collision suture zone,especially along that continent-continent collision develops after ocean-continent subduction.Therefore,the primary known economic borate deposits mainly occur in the high altitude intermontane basins developed at the convergent plate margins.Anatolia Plateau in Turkey hosts the largest borate reserves around the world which may be associated with the massive forearc serpentinite or serpentinized mélange recycled to magma source during subduction erosion.The erupted volcanic magmas transport boron-rich volcanic rocks,ashes and hydrothermal fluids to the surface,from which boron is leached into closed basins,the typical byproducts of orogeny.Combined with extensive evaporation and,dry and cold climate,borate deposits eventually develop in orogen.The boron reserves of China are only 3%of the global ones,mainly from the Liaodong Peninsula,Qaidam Basin of Qinghai,Tibet,Sichuan,and Hunan Province.There may be more potential boron resources for China.We thus call on more efforts on the research and exploration of borate deposits and purchase of abroad boron resources as strategic reserves.