Paleoenvironmental reconstruction of the oldest microfossils from the 3.5Ga Pilbara craton, western Australia

澳大利亚皮尔巴拉地区的地质测绘和系统采样已于本历年完成。它们是：(1)已识别出最古老微化石的3.5Ga古老北极地区；(2)3.0Ga古老克利弗维尔地区；(3)2.7Ga古老里士满地区，那里存在浅海黑色页岩和叠层石。收集的标本均为薄片，在显微镜下观察。有的用X射线荧光光谱分析了常量和微量元素，用激光电感耦合等离子体质谱分析了稀土元素。分离出的锆石用激光电感耦合等离子体质谱测年。确定了变质矿物组合，估算了变质矿物的温压条件。还对流体包裹体进行了研究，以明确早期生命的物理化学环境。结果表明：(1)微体化石单元构成了增生杂岩的一部分。(2)重建了微体化石存活的物理化学环境，在2000m深的富含XCO_2的热液系统下。(3)微体化石不是蓝藻，而是已经分化了3.5Ga的化学化石。(4)微体化石沿着与热液循环有关的断裂系统形成了一个海底下群落。(5)在3.0Ga Cleverville地区发现了新的微体化石。这些化石非常大，保存完好，表明在3.0Ga有可能诞生真核生物。(6)在3.5Ga的海山顶部沉积物中还发现了新的微化石。这些化石可能是在浅海环境下开始光合成的化石。(7)重建了2.7Ga叠层石单元的古环境(2001年在AGU上发表)。(8)从浅海到深海2.7Ga老沉积物的古环境重建。这些微化石是从深海沉积物中新发现的。(9)发表了对生命和地球历史的简要总结。

Geologic mapping and systematic samplings have been completed in the Pilbara area, Australia in this calendar year. They are, (1) the 3.5 Ga old North Pole region where the oldest microfossils have been recognized, (2) 3.0 Ga old Cleaverville region, and (3) 2.7 Ga old Richmond region where shallow marine black shale and stromatolite are present. The collected samples were all thin-sectioned, and observed under the microscope. Some were chemically analyzed in major and trace elements by XRF, as well as REE by laser ICP-MS. Separated zircons were dated by laser ICP-MS at TIT. Metamorphic mineral assemblages were determined and their P-T conditions were estimated. Fluid inclusions were also investigated to make it clear the physico-chemical environment of early life. The results are as follows.(1) The microfossil-bearing unit forms a part of accretionary complex.(2) The physico-chemical environment where microfossils were survived was reconstructed ; it was at 2000m deep under XCO2-rich hyfrothermal system.(3) Microfossils are not cyanobacteria but chemo-fossils which had already differentiated to some extent by 3.5 Ga.(4) Microfossils formed a sub-seafloor community down to 800m along the fracture system related to hydrothermal fluid circulation.(5) New micro fossils were found from the 3.0 Ga Cleaverville region. Those are extremely large and well-preserved, suggesting a possibility of birth of eukaryotes at 3.0 Ga.(6) New microfossils were also identified from the 3.5 Ga seamount-top sediments. These may be fossils which may have begun photo-synthesis under the shallow marine environment.(7) Paleoenvironment of stromatolite-bearing unit at 2.7 Ga was reconstructed (presented at AGU, 2001 ).(8) Paleoenvironmental reconstruction of 2.7 Ga old sediments from shallow marine to deep-sea. The microfossils were newly found from those deep-sea sediments.(9) A brief summary of the history of life and the Earth was published.