Kaczmarek Mary-Alix, Reddy Steven M, Nutman Allen P, Friend Clark R L, Bennett Vickie C
Institute of Earth Sciences, University of Lausanne, UNIL-Mouline, Building Geopolis, CH-1015 Lausanne, Switzerland.
Department of Applied Geology, ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS) and the Institute For Geoscience Research (TIGeR), Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia.
Nat Commun. 2016 Feb 16;7:10665. doi: 10.1038/ncomms10665.
The extension of subduction processes into the Eoarchaean era (4.0-3.6 Ga) is controversial. The oldest reported terrestrial olivine, from two dunite lenses within the ∼3,720 Ma Isua supracrustal belt in Greenland, record a shape-preferred orientation of olivine crystals defining a weak foliation and a well-defined lattice-preferred orientation (LPO). [001] parallel to the maximum finite elongation direction and (010) perpendicular to the foliation plane define a B-type LPO. In the modern Earth such fabrics are associated with deformation of mantle rocks in the hanging wall of subduction systems; an interpretation supported by experiments. Here we show that the presence of B-type fabrics in the studied Isua dunites is consistent with a mantle origin and a supra-subduction mantle wedge setting, the latter supported by compositional data from nearby mafic rocks. Our results provide independent microstructural data consistent with the operation of Eoarchaean subduction and indicate that microstructural analyses of ancient ultramafic rocks provide a valuable record of Archaean geodynamics.
俯冲作用延伸至太古宙早期（40亿年至36亿年前）这一观点存在争议。据报道，最古老的陆地橄榄石来自格陵兰岛约37.2亿年伊苏阿上地壳带内的两个纯橄榄岩透镜体，其记录了橄榄石晶体的形状优选方位，确定了一个弱叶理和一个明确的晶格优选方位（LPO）。与最大有限伸长方向平行的[001]以及垂直于叶理面的(010)确定了一种B型LPO。在现代地球，这种组构与俯冲系统上盘地幔岩石的变形有关；这一解释得到了实验的支持。在此我们表明，在所研究的伊苏阿纯橄榄岩中存在B型组构与地幔起源及超俯冲地幔楔环境相一致，后者得到了附近镁铁质岩石成分数据的支持。我们的结果提供了与太古宙早期俯冲作用相一致的独立微观结构数据，并表明对古代超镁铁质岩石的微观结构分析为太古宙地球动力学提供了宝贵记录。
