When a mid-ocean ridge encroaches a continent: Seafloor-type hydrothermal activity in Lake Asal (Afar Rift)
When a mid-ocean ridge encroaches a continent: Seafloor-type hydrothermal activity in Lake Asal (Afar Rift)
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当洋中脊侵入大陆时：阿萨尔湖（阿法尔裂谷）的海底型热液活动
DOI：
10.1016/j.chemgeo.2021.120126
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发表时间：
2021
期刊：
影响因子：
3.9
通讯作者：
Sutton J.
中科院分区：
文献类型：
--
作者：
Dekov V.M.;Gu?guen B.;Yamanaka T.;Moussa N.;Okumura T.;Bayon G.;Liebetrau V.;Yoshimura T.;Kamenov G.;Araoka D.;Makita H.;Sutton J.
在亚丁海岭侵入非洲大陆并与东非裂谷系统相互作用的地方，形成了两个小盆地：古贝特卡拉布盆地和阿萨尔湖。古贝特卡拉布与公海相连，而阿萨尔湖则是海洋“胚胎”的典型例子，它被定义为一个脱离海洋的系统，但具有海洋盆地的特征，具有海洋型地壳和海水基水体。为了阐明水源、热液活动类型和热液沉积物以及对海洋“胚胎”中水化学的控制，我们对阿萨尔湖的湖水、热液流体和热液碳酸盐沉积物进行了矿物学-地球化学研究。湖水和热液地球化学分析表明，阿萨尔湖（位于蒸发强、无河流输入的干旱区）是由海底型热液补给的，具体过程如下：海水沿着断层和裂谷延伸的裂缝渗透，海水与热玄武岩反应并生成热液，阿萨尔凹陷热液排出，阿萨尔湖水体聚集。阿萨尔湖底部排出的流体是 97% 端元热液和 3% 湖水的混合物。计算出的这个海洋“胚胎”的端元热液流体的金属含量比开放和演化海洋的海底热液流体要少。除了海水/岩石相互作用之外，阿萨尔湖的化学成分还受到导致高盐度的蒸发的控制。在高咸度水体中，许多热液提供的金属以氯化物络合物的形式稳定并积累。这导致了富含金属且呈弱酸性的“胚胎”海洋。与开放且演化的现代海洋不同，位于干旱地区的“胚胎”海洋具有重C和O同位素组成以及轻Zn和Fe同位素组成。两种海洋的钙同位素组成相似，很重。阿萨尔湖有两种在基因上不同的元素来源，它们是垂直分离的：热液（下部或底部）和风成（上部或表面）。对湖水化学的另一个重要控制是湖底碳酸盐尖峰的形成。碳酸钙沉淀固定了大量热液提供的钙，并提高了湖水的 (Mg/Ca)mol。不断变化的湖水的 (Mg/Ca)mol 的增加会导致尖塔矿物学的变化：从低镁方解石到文石。因此，尖塔结构对其矿物学发挥了自我控制作用。碳酸盐螺旋沉积还通过吸附或/和共沉淀引起的同位素分馏影响湖水中的钙、锌和铁同位素组成。
At the place where the submarine Aden Ridge encroaches on the African continent and interacts with the East African Rift system, two small basins form: Ghoubbet-al-Kharab and Lake Asal. Whereas Ghoubbet-al-Kharab is connected to the open ocean, Lake Asal is a typical example of oceanic “embryo”, which is defined as a system that is detached from the ocean, but has features of a marine basin with an oceanic type crust and a seawater-based water body. In order to shed light on the source of water, type of hydrothermal activity and hydrothermal deposits, and controls on the water chemistry in an oceanic “embryo”, we undertook a mineralogical-geochemical study of the lake water, hydrothermal fluids and hydrothermal carbonate deposits of Lake Asal. The geochemical analyses of lake water and hydrothermal fluids show that Lake Asal (located in an arid zone with strong evaporation and with no riverine input) is fed by seafloor-type hydrothermal fluids according to the following scenario: percolation of seawater along faults and cracks of extension in the rift, reaction of seawater with the hot basaltic rocks and hydrothermal fluid generation, discharge of the hydrothermal fluid in the Asal depression and accumulation of the Lake Asal water body. The fluid venting at the Lake Asal bottom is a mixture of 97% end-member hydrothermal fluid and 3% lake water. The calculated end-member hydrothermal fluid of this oceanic “embryo” is poorer in metals than the seafloor hydrothermal fluids of an open and evolved ocean. In addition to the seawater/rock interaction, the chemistry of Lake Asal is also controlled by evaporation leading to hyper salinity. In a hyper saline water body a number of hydrothermally supplied metals are stabilized as chloride complexes and accumulate. This results in a metal rich and mildly acidic “embryonic” ocean. Unlike an open and evolved modern ocean, the “embryonic” ocean located in an arid zone has heavy C and O isotope composition and light Zn and Fe isotope composition. Calcium isotope compositions of both types of ocean are similarly heavy. There are two genetically different sources of elements to the Lake Asal that are vertically separated: hydrothermal (lower, or bottom) and aeolian (upper, or surficial). Another important control on the lake water chemistry is the formation of carbonate spires at the lake bottom. Ca‑carbonate precipitation immobilizes substantial amount of hydrothermally supplied Ca and drives up the (Mg/Ca)molof the lake water. Increasing (Mg/Ca)molof the evolving lake water leads to changes in the mineralogy of spires: from low-Mg calcite to aragonite. Thus, the spire formation exerts a self-control on its mineralogy. Carbonate spire deposition affects also the Ca, Zn and Fe isotope composition of the lake water through adsorption or/and co-precipitation induced isotope fractionation.
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