Abstract Silica-rich Precambrian rocks often preserve geochemical information and microfossil remnants from the early biosphere and could play a critical role in the formation of early crust. Because these rocks are critical geochemical and paleontological archives, we need to better constrain their geochemical and isotopic attributes and generate a refined picture of the evolving Archean silica cycle. Here, we investigate a series of sub- to greenschist facies Si-rich Archean rocks from the ∼2.7 Ga Abitibi greenstone belt, Canada, that represent chemical sedimentary rocks and rocks formed via silica-addition through the process of silicification. We report data for major and trace element geochemistry, multi-crystal silicon and oxygen isotopes of quartz using isotope ratio mass spectrometry, and texture-specific silicon isotope values measured using secondary ion mass spectrometry on Neoarchean chemical sedimentary rocks, their silicified equivalents, and associated silicified volcanic rocks. We find that in such a well-preserved terrane we can utilize petrographic textures and geochemical attributes to establish rock origin, distinguishing siliceous rocks that form via chemical sedimentation from those that form via silicification. Chemical sedimentary rocks display a wide range of 30Si-depleted silicon isotopes values that vary with stratigraphy similar to other Archean iron formation, while silicified volcanic rocks possess slightly more 30Si-enriched values, similar to Archean silicified basalts. We conclude that because silicon isotope values of iron formation shift toward 30Si-enriched values up stratigraphy, basinal changes in the composition of the silicon isotope reservoir may be preserved. Silicon isotope values of silicified volcanic rocks by contrast, likely represent precipitation from an isotopically heavy silicon reservoir, influenced by downward percolating seawater and upward moving convecting fluids interacting with host volcanic rock (basalt or andesite). Overall, we confirm that Neoarchean silicified rocks are 30Si-enriched like their Paleoarchean counterparts.

中文翻译：

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加拿大~2.7 Ga Abitibi 绿岩带硅化岩和相关化学沉积岩的结构、地球化学和同位素数据：对硅化作用的洞察

摘要 富含二氧化硅的前寒武纪岩石通常保存着早期生物圈的地球化学信息和微化石残余物，在早期地壳的形成中起关键作用。由于这些岩石是重要的地球化学和古生物档案，我们需要更好地限制它们的地球化学和同位素属性，并生成关于演化的太古代二氧化硅循环的精细图片。在这里，我们研究了一系列来自加拿大~2.7 Ga Abitibi 绿岩带的亚绿片岩相富硅太古代岩石，代表化学沉积岩和通过硅化过程添加二氧化硅形成的岩石。我们使用同位素比质谱法报告了主要和微量元素地球化学、石英的多晶硅和氧同位素数据，使用二次离子质谱法对新太古代化学沉积岩、它们的硅化等效物和相关的硅化火山岩测量的和纹理特定的硅同位素值。我们发现，在这样一个保存完好的地体中，我们可以利用岩相纹理和地球化学属性来确定岩石成因，将通过化学沉积形成的硅质岩与通过硅化形成的硅质岩区分开来。化学沉积岩显示出广泛的 30Si 贫硅同位素值，这些值随地层变化而变化，类似于其他太古代铁地层，而硅化火山岩的 30Si 富集值略高，类似于太古代硅化玄武岩。我们得出结论，因为铁形成的硅同位素值向地层上的 30Si 富集值移动，可以保留硅同位素储层组成的盆地变化。相比之下，硅化火山岩的硅同位素值可能代表来自同位素重硅储层的降水，受向下渗透的海水和向上移动的对流流体与主火山岩（玄武岩或安山岩）相互作用的影响。总体而言，我们确认新太古代硅化岩石与它们的古太古代对应物一样富含 30Si。