Department of Geology, Rhodes University, Grahamstown, South Africa.
Centre for Microscopy, Characterization and Analysis (CMCA), The University of Western Australia (UWA), Perth, WA, Australia.
Geobiology. 2020 Sep;18(5):525-543. doi: 10.1111/gbi.12399. Epub 2020 Jun 15.
Microtextures of titanite (CaTiSiO ) in exceptionally preserved Archean pillow lavas have been proposed as the earliest examples of microbial ichnofossils. An origin from microbial tunneling of seafloor volcanic glass that is subsequently chloritized and the tunnels infilled by titanite has been argued to record the activities of subseafloor microbes. We investigate the evidence in pillow lavas of the 3.35 Ga Euro Basalt from the Pilbara Craton, Western Australia, to evaluate the biogenicity of the microtextures. We employ a combination of light microscopy and chlorite mineral chemical analysis by EPMA (electron probe micro-analysis) to document the environment of formation and analyze their ultrastructure using FIB-TEM (focussed ion beam combined with transmission electron microscopy) to investigate their mode of growth. Petrographic study of the original and re-collected material identified an expanded range of titanite morphotypes along with early anatase growth forming chains and aggregates of coalesced crystallites in a sub-greenschist facies assemblage. High-sensitivity mapping of FIB lamellae cut across the microtextures confirm that they are discontinuous chains of coalesced crystallites that are highly variable in cross section and contain abundant chlorite inclusions, excluding an origin from the mineralization of previously hollow microtunnels. Comparison of chlorite mineral compositions to DSDP/IODP data reveals that the Euro Basalt chlorites are similar to recent seafloor chlorites. We advance an abiotic origin for the Euro Basalt microtextures formed by spontaneous nucleation and growth of titanite and/anatase during seafloor-hydrothermal metamorphism. Our findings reveal that the Euro Basalt microtextures are not comparable to microbial ichnofossils from the recent oceanic crust, and we question the evidence for life in these Archean lavas. The metamorphic reactions that give rise to the growth of the Euro Basalt microtextures could be commonplace in Archean pillow lavas and need to be excluded when seeking traces of life in the subseafloor on the early Earth.
在异常保存的太古宙枕状熔岩中，榍石（CaTiSiO ）的微观纹理已被提议作为最早的微生物遗迹化石的例子。有人认为，这些微观纹理是由海底火山玻璃的微生物隧道化作用形成的，随后这些隧道被绿泥石交代，并且被榍石填充，记录了海底微生物的活动。我们研究了来自西澳大利亚皮尔巴拉克拉通 33.5 亿年的欧洲玄武岩枕状熔岩中的证据，以评估这些微观纹理的生物成因性。我们采用了组合的显微镜和电子探针微分析（EPMA）的绿泥石矿物化学分析，以记录形成环境，并使用聚焦离子束-透射电子显微镜（FIB-TEM）分析其超微结构，以研究其生长方式。对原始和重新采集的材料的岩相学研究确定了一系列扩展的榍石形态类型，以及在一个低绿片岩相组合中形成的早期锐钛矿生长链和聚结晶粒的集合体。FIB 薄片横切微观纹理的高灵敏度映射证实，它们是聚结晶粒的不连续链，在横截面上变化很大，并且含有丰富的绿泥石包裹体，排除了它们源自先前空心微隧道的矿化作用。与 DSDP/IODP 数据的比较表明，欧洲玄武岩中的绿泥石与现代海底绿泥石相似。我们提出了一种非生物成因机制，即通过海底热液变质过程中榍石和/或锐钛矿的自发成核和生长形成了欧洲玄武岩微观纹理。我们的发现表明，欧洲玄武岩微观纹理与现代海洋地壳中的微生物遗迹化石不可比，并且我们对这些太古宙熔岩中存在生命的证据提出质疑。导致欧洲玄武岩微观纹理生长的变质反应在太古宙枕状熔岩中可能很常见，因此在早期地球上寻找海底生命的痕迹时需要排除这些反应。
