西藏谷露—亚东裂谷南部温泉稀土元素特征及其控制因素

Rare earth element characteristics and controlling factors of hot springs in the Gulu—Yadong rift,Xizang(Tibet)

西藏构造运动强烈,南部地区发育了多条近南北向拉张性裂谷,其中谷露—亚东裂谷活动性最为强烈。该裂谷从南到北分布了多个温泉群,康布、康马、查当、孟则等温泉集中在南部。笔者等通过对谷露—亚东裂谷南部温泉的稀土元素(rare earth element,简称REE)地球化学特征的研究,探讨控制温泉稀土元素特征的主要因素。南部温泉pH值在6.14~9.01之间,属于弱酸性—碱性温泉。稀土元素总质量浓度[ρ(ΣREE)]变化范围在1.41~46.18 ng/L之间,与全球其它弱酸性—碱性温泉相比,处于低值水平。大部分样品表现出LREE(light rare earth element,轻稀土)和HREE(heavy rare earth element,重稀土)相对亏损,MREE(middle rare earth element,中稀土)富集和Ce负异常的配分模式。pH值影响Fe/Mn(氢)氧化物胶体的存在方式。温泉的稀土元素主要以REECO_(3)^(+)和REE(CO_(3))_(2)^(-)形式存在,并影响温泉稀土元素的分异。亚东地区大部分温泉的稀土配分模式受富铁沉积物的影响,MREE优先释放到水溶液中。康马温泉的稀土元素分布特征受到水岩反应的影响,包括富铁沉积物的溶解和当地基性岩浆岩的反应。Ce负异常是由于氧化环境下,Ce^(3+)形成CeO_(2)沉淀导致的。研究表明pH值、Fe/Mn(氢)氧化物胶体、络合反应以及水岩反应是控制谷露—亚东裂谷南部温泉水REE分布的主要因素。

In the interior of Xizang(Tibetan)plateau,the active tectonics are primary marked by north trending rifts in southern Tibet.Gulu—Yadong rift is the largest and longest extensional rift in South Tibet.The rift valley has a number of hot spring clusters distributed from south to north,and hot springs such as Kangbu,Kangma,Zhadang,and Mengze are concentrated in the southern Rift Valley.Based on the geochemical characteristics of rare earth elements in hot springs in the southern Gulu—Yadong rift valley,this paper discusses the main controlling factors of rare earth elements.The results show that the southern hot springs are weakly acidic—alkaline hot springs.The total rare earth elements(ΣREE)in hot spring of Gulu—Yadong rift ranged from 1.41 to 46.18 ng/L,which is at a low level compared with alkaline hot springs in the world.Most samples show LREE(light rare earth element)and HREE(heavy rare earth element)relative depletion,MREE(medium rare earth element)enrichment and negative Ce anomaly.Adsorption and desorption processes of Fe/Mn minerals to REEs can increase or decrease REE concentration.REECO^(3+) and REE(CO_(3))_(2)^(-)is the major speciation,and affect the differentiation of rare earth element.The REE distribution pattern of most hot springs in Yadong area is affected by iron rich sediments,resulting in the preferential release of MREE into aqueous solution.The distribution characteristics of rare earth elements in Kangma hot spring are affected by water—rock reactions,including the dissolution of iron-rich sediments and the dissolution of local basic magmatic rocks.Negative Ce anomalies may result from oxidative scavenging process of Ce.pH,Fe/Mn minerals,carbonate complexes and water—rock reaction all affect the REE distribution of hot springs.