There is still much controversy over whether apatite and graphite can be used as fingerprint mineral assemblages to preserve biosignatures in ancient sedimentary rocks that experienced medium- to high-grade metamorphism. Herein, we present a comprehensive analysis of graphite and associated phases from meta-phosphorites of the Huangmailing Formation in the South Dabie Orogen to assess possible preservation of biosignatures. Stratigraphic correlations and previous geochronological data suggest that the protoliths of Huangmailing Formation was deposited in the Ediacaran, and was metamorphosed during the Triassic Dabie Orogeny as documented by our apatite UPb dating. Microcrystalline graphite occurs both as inclusions within metamorphic minerals like apatite, and as matrix-hosted disseminations at grain boundaries of other phases. Bulk organic carbon isotopic analyses yield δ13C values between −26.0 ‰ and − 15.2 ‰ and total organic carbon contents from 0.33 to 3.27 wt%. Raman spectra of the two types of graphites yield an average peak metamorphic temperature of 452 °C and 473 °C, consistent with the upper greenschist facies to lower amphibolite facies metamorphic conditions in the area. However, the matrix graphites show a wider range of peak temperature variations, likely containing graphitic carbon with lower crystallinity that was affected by retrograde metamorphic fluids. Notably, two prominent Raman bands at around 325 cm−1 and 395 cm−1 are observed for the graphite hosted in apatite. This, along with the presence of a peak at 2473 eV in the sulfur K-edge XANES spectra of some graphite in apatite, and the co-occurrence of 12C and 14N and local association of 32S with 12C in graphite inclusions determined by TOF-SIMS, suggest the possible presence of C-S-Fe and N-bearing compounds and functional groups. TEM and EDS analysis reveal that graphite inclusions in apatite are closely associated with amorphous silica, and mainly occur as well crystallized, polygonal grains around 100 to 200 nm across and has an expanded (002) lattice spacing between 3.43 Å and 3.64 Å. This lattice expansion is attributable to the presence of heteroatoms such as S, Fe and Si, and N, in the graphene interlayers. In contrast, matrix graphites exhibit a wider range of structural and compositional variations than their counterpart as inclusion in apatite, which probably arise from interactions with metamorphic fluids or incorporation of fluid-deposited graphitic carbon. Our study documents the ubiquitous preservation of biological carbon as graphite inclusions in apatite from Huangmailing meta-phosphorites, likely as a result of metamorphic recrystallization of organic-bearing Ediacaran phosphorites. This suggests that the organic matter trapped in sedimentary apatite during their co-precipitation or early diagenesis was probably less modified during metamorphic recrystallization, thus facilitating the preservation of biosignatures.

中文翻译：

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在 >450 °C 温度下变质的新元古代磷矿中生物特征的保存


关于磷灰石和石墨是否可以用作指纹矿物组合，以保存经历中高品位变质作用的古老沉积岩中的生物特征，仍然存在很多争议。在此，我们对南大别造山带 Huangmailing 组的间磷酸盐的石墨和相关相进行了全面分析，以评估生物特征的可能保存。地层相关性和以前的地质年代学数据表明，Huangmailing Formation 的原石沉积在埃迪卡拉纪，并在三叠纪 Dabie 造山运动期间变质，正如我们的磷灰石 UPb 测年所记录的那样。微晶石墨既以磷灰石等变质矿物中的夹杂物形式出现，也以基质承载的传播形式出现在其他相的晶界处。本体有机碳同位素分析产生的 δ13C 值在 -26.0 ‰ 和 -15.2 ‰ 之间，总有机碳含量在 0.33 至 3.27 wt% 之间。两种石墨的拉曼光谱产生的平均峰值变质温度为 452 °C 和 473 °C，与该地区上部绿色片岩相至下部角闪岩相变质条件一致。然而，基质石墨显示出更广泛的峰值温度变化，可能含有受逆行变质流体影响的结晶度较低的石墨碳。值得注意的是，在磷灰石中赋存的石墨在 325 cm-1 和 395 cm-1 附近观察到两条突出的拉曼带。 这一点，再加上磷灰石中某些石墨的硫 K 边缘 XANES 光谱中 2473 eV 处的峰，以及 12C 和 14N 的共现以及 32S 与 12C 的局部结合，通过 TOF-SIMS 确定的石墨内含物，表明可能存在含 C-S-Fe 和 N 的化合物和官能团。TEM 和 EDS 分析表明，磷灰石中的石墨夹杂物与无定形二氧化硅密切相关，主要以 100 至 200 nm 宽左右的结晶多边形晶粒形式出现，并在 3.43 Å 和 3.64 Å 之间具有膨胀的 （002） 晶格间距。这种晶格膨胀是由于石墨烯夹层中存在杂原子，如 S、Fe 和 Si 以及 N。相比之下，基质石墨比磷灰石中的夹杂物表现出更广泛的结构和成分变化，这可能是由于与变质流体的相互作用或流体沉积石墨碳的掺入而引起的。我们的研究记录了 Huangmailing 变磷酸盐中磷灰石中普遍保存的生物碳作为石墨夹杂物，这可能是含有机物的 Ediacaran 磷矿变质再结晶的结果。这表明沉积磷灰石在共沉淀或早期成岩作用过程中被捕获的有机物在变质重结晶过程中的改性可能较少，从而有利于生物特征的保存。