Reconstruction of Earth's surface environments in the Late Archean based on the scientific drilling of the Dharwar Craton, south India

本研究旨在通过对印度南部达瓦尔克拉通的科学钻探，重建太古宙晚期的地球表面环境。我们获得的研究成果如下：包括达瓦尔克拉通在内的主要克拉通的前寒武纪带状铁组稀土元素地球化学特征限制了全球环境的变化。晚太古代Dharwar bif中明显的负Ce异常表明，2.9-2.7 Ga出现了像今天这样的强氧合海洋条件。这与太古宙晚期形成小而广泛的锰矿床的地质证据是一致的。哈默斯利和德兰士瓦IFs的Ce异常不太明显。这些bif沉积在内陆海洋的一个不断演变的裂谷上，由于强烈的热液活动，该海洋变得缺氧。2.2 ~ 2.1 Ga优型if表现出明显的负Ce异常，而1.9 Ga和0.7 Ga if表现出不明显的负Ce异常。2.7 Ga后bif的Ce/Ce*值变化可能反映了全球地幔柱活动的偶发性，地幔柱活动形成了局部缺氧盆地。太古宙algoma型bif具有明显的正Eu异常，但Eu异常变化较大。它们强烈的Eu正异常表明热液流体对参与BIF降水的海水有很大贡献。阿尔戈马型bif中Eu异常的巨大变化反映了不同沉积部位热液流体与周围海水混合比例的巨大变化。而2.7 Ga后的优等型bif的Eu/Eu*值较低且保持不变（0.7 ~ 1.9）。这表明，除了强烈的热液流入外，盆地水的稀土化学成分还受到了由于全球地幔柱活动而迅速增长的来自周围大陆的河流流入的影响。这些重要的发现发表在《美国地质学会回忆录》上。其他研究成果发表在《地球与行星科学快报》、《前寒武纪研究》、《岩石学》和《海洋地质学》等国际知名期刊上。少

The aim of our study was to reconstruct Earth's surface environments in the Late Archean based on the scientific drilling of the Dharwar Craton, south India. We have gained research results as follows.Global environmental changes have been constrained from rare earth element geochemistry of Precambrian banded iron formations in the major cratons including the Dharwar Craton. The presence of pronounced negative Ce anomalies in Late Archean Dharwar BIFs suggests that strongly oxygenated oceanic conditions like today emerged by 2.9-2.7 Ga. This suggestion is consistent with geologic evidence that small but widespread Mn deposits formed during Late Archean time. The Hamersley and Transvaal IFs have less noticeable Ce anomalies. These BIFs were deposited on an evolving rift in a land-locked ocean that became anoxic due to intense hydrothermal activity. The 2.2-2.1 Ga Superior-type IFs exhibit distinct negative Ce anomalies, but the ca. 1.9 Ga Ifs and the ca. 0.7 Ga IFs have less distinct Ce … More anomalies. These variations in Ce/Ce* values of the post-2.7 Ga BIFs may reflect the episodicity in global mantle plume activity that created locally anoxic basins. The Archean Algoma-type BIFs have distinctly positive but variable Eu anomalies. Their strong positive Eu anomalies suggest large contributions of hydrothermal fluids to the seawater involved in BIF precipitation. The large variation in Eu anomalies in Algoma-type BIFs reflects the large variation in mixing ratios of hydrothermal fluids and ambient seawaters at various depositional sites. However, the post-2.7 Ga Superior-type BIFs exhibit much lower and constant Eu/Eu* values (0.7-1.9). This implies that the REE chemistry of the basin water that hosted voluminous Superiortype BIFs was influenced by a riverine influx from the surrounding continent that grew rapidly due to global mantle plume activity, besides the intense hydrothermal influx. These important findings were published in Geological Society of America Memoir. Other research results were published in many prestigious international journals including Earth and Planetary Science Letters, Precambrian Research, Lithos and Marine Geology. Less