Fluid inclusion analysis of silicified Palaeoarchaean oceanic crust – A record of Archaean seawater?
Fluid inclusion analysis of silicified Palaeoarchaean oceanic crust – A record of Archaean seawater?
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DOI：
10.1016/j.precamres.2015.05.020
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发表时间：
2015-09
影响因子：
3.8
通讯作者：
Katja Farber;A. Dziggel;F. Meyer;W. Prochaska;A. Hofmann;C. Harris
中科院分区：
文献类型：
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作者：
Katja Farber;A. Dziggel;F. Meyer;W. Prochaska;A. Hofmann;C. Harris
近年来，古太古代海水和热液在古太古代火山-沉积序列广泛硅化中的作用一直是一个有相当大争议的问题。在试图限制条件的二氧化硅沉淀，和相互作用的流体的来源和化学成分，我们使用流体包裹体显微测温，散装破碎浸出和氧同位素分析硅化科马提岩和沉积物中的燧石和石英脉的巴伯顿绿岩带，南非。硅质岩脉边缘由微晶石英和碳质物质组成，脉中心常充填含有丰富流体包裹体的粗晶石英，脉硅质岩和粗晶石英的氧同位素比值为18 ~ 21‰，粗晶石英的δ 18 O值略高（0.7 ± 0.3‰）。这些数据与南海海底低温（≤100 °C）热液过程中的二氧化硅沉淀相一致。粗晶石英在室温下含有均匀的两相（L+V）夹杂物，具有相对恒定的蒸汽分数。包裹体具有3-11重量%的盐度，NaCl当量和150-200 °C的均质化温度（Th）。而一些包裹体形成的颗粒内流体包裹体集群，似乎是主要的，其他包裹体形成穿晶流体包裹体的痕迹，显然是次要的。这两种类型的包裹体具有相同的显微测温特征，表明流体圈闭发生在后期变质事件，而不是在海底附近。该事件可能与3.23 Ga的区域变形和变质作用相吻合。脉状寄主岩石中的绿泥石温度测定显示峰值温度为257 ± 31 °C。结合显微测温法，这些数据对应的地壳深度为3-6公里。流体包裹体的次生成因也与粗晶脉的Cl/Br、Na/Cl和Na/K比值一致，这些比值与变质石英脉中发现的比值相似。因此，流体包裹体不能提供有关燧石形成过程中的条件和温度的信息。虽然流体可能携带改性的大洋海水（即热液流体）的地球化学特征，其特征是低氯/溴和钠/钾比，和低镁含量，流体成分可能是在区域变质作用修改。
In recent years, the role of Archaean seawater and hydrothermal fluid in the extensive silicification of Palaeoarchaean volcano-sedimentary successions has been a matter of considerable debate. In an attempt to constrain the conditions of silica precipitation, and the sources and chemical composition of the interacting fluids, we used fluid inclusion microthermometry, bulk crush-leach and oxygen isotope analyses of chert and quartz veins in silicified komatiites and sediments from the Barberton greenstone belt, South Africa. Chert vein margins consist of microcrystalline quartz and carbonaceous matter, whereas the vein centres are often filled with macrocrystalline quartz that contains abundant fluid inclusions.Oxygen isotope ratios of vein chert and macrocrystalline quartz vary from 18 to 21‰, with the macrocrystalline quartz having slightly higher δ18O values (0.7 ± 0.3‰). The data are consistent with silica precipitation during low-temperature (≤100 °C) hydrothermal processes on the Archaean seafloor. Macrocrystalline quartz contains homogeneous 2-phase (L+V) inclusions at room temperature with a relatively constant vapour fraction. The inclusions have a salinity of 3–11 wt.% NaCl equiv. and homogenisation temperatures (Th) of 150–200 °C. Whereas some of the inclusions form intragranular fluid inclusion clusters that appear to be primary, other inclusions form transgranular fluid inclusion trails and are clearly secondary. Both types of inclusions share the same microthermometric characteristics, indicating that fluid entrapment occurred during a later metamorphic event and not near the seafloor. The event likely coincided with regional deformation and metamorphism at 3.23 Ga. Chlorite thermometry from vein host rocks reveals peak conditions of ∼257 ± 31 °C. In conjunction with microthermometry, the data correspond to a crustal depth of 3–6 km. A secondary origin of fluid inclusions is also consistent with the Cl/Br, Na/Cl and Na/K ratios of the macrocrystalline veins, which are similar to those found in metamorphic quartz veins. The fluid inclusions thus do not provide information on the conditions and temperatures during chert formation. While the fluids potentially carry the geochemical signature of modified Archaean seawater (i.e. hydrothermal fluid), characterised by low Cl/Br and Na/K ratios, and low Mg-contents, the fluid composition was likely modified during regional metamorphism.