Fossil biomass preserved as graphitic carbon in a late Paleoproterozoic banded iron formation metamorphosed at more than 550°C
Fossil biomass preserved as graphitic carbon in a late Paleoproterozoic banded iron formation metamorphosed at more than 550°C
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DOI：
10.1144/jgs2018-097
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发表时间：
2019-04
影响因子：
2.7
通讯作者：
D. Papineau;Bradley T. De Gregorio;J. Sagar;R. Thorogate;Jianhua Wang;L. Nittler;D. Kilcoyne;H. Marbach;Martin Drost;G. Thornton
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文献类型：
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作者：
D. Papineau;Bradley T. De Gregorio;J. Sagar;R. Thorogate;Jianhua Wang;L. Nittler;D. Kilcoyne;H. Marbach;Martin Drost;G. Thornton
变质作用被认为部分通过生物有机质的挥发分和石墨化作用破坏微体化石。然而，在高温变质过程中，生物质的分子、元素和同位素特征的损失程度尚不清楚。我们报告石墨结构内部和涂层磷灰石颗粒从c。1850 Ma密歇根州的富伽马硅酸盐条带状铁地层，变质温度高于550°C。痕量的N，S，O，H，Ca和Fe被保存在这种石墨碳中，X射线光谱显示痕量的脂肪族基团。石墨碳具有约3.6埃的膨胀晶格，形成微观同心层状和辐射多边形薄片，并且具有约-22 ‰的均匀δ 13 C值，与整体分析相同。磷灰石内部的石墨碳与纳米尺寸的氨化页硅酸盐有关。这些变质矿物和石墨碳的前体来源于含生物质的富铁粘土沉积物。我们的结论是，磷灰石颗粒中的石墨涂层和包裹体表明前体生物质角闪岩相变质过程中的流体再活化。这一新的证据填补了变质生物质进入石墨的观测空白，并支持存在于始新世Akilia协会高度变质的铁地层中的生物特征，该协会可追溯到沉积岩记录的开始。补充材料：说明岩相学和分析结果的图可在https://doi.org/10.6084/m9.figshare.c.4430609上获得。
Metamorphism is thought to destroy microfossils, partly through devolatilization and graphitization of biogenic organic matter. However, the extent to which there is a loss of molecular, elemental and isotope signatures from biomass during high-temperature metamorphism is not clearly established. We report on graphitic structures inside and coating apatite grains from the c. 1850 Ma Michigamme silicate banded iron formation from Michigan, metamorphosed above 550°C. Traces of N, S, O, H, Ca and Fe are preserved in this graphitic carbon and X-ray spectra show traces of aliphatic groups. Graphitic carbon has an expanded lattice around 3.6 Å, forms microscopic concentrically-layered and radiating polygonal flakes and has homogeneous δ13C values around −22‰, identical to bulk analyses. Graphitic carbon inside apatite is associated with nanometre-size ammoniated phyllosilicate. Precursors of these metamorphic minerals and graphitic carbon originated from ferruginous clay-rich sediments with biomass. We conclude that graphite coatings and inclusions in apatite grains indicate fluid remobilization during amphibolite-facies metamorphism of precursor biomass. This new evidence fills in observational gaps of metamorphosed biomass into graphite and supports the existence of biosignatures in the highly metamorphosed iron formation from the Eoarchean Akilia Association, which dates from the beginning of the sedimentary rock record. Supplementary material: Figures illustrating the petrography and analytical results are available at: https://doi.org/10.6084/m9.figshare.c.4430609