| This paper has proposed identifying criteria of different genetic pyrite framboids in mineralogy. The research makes the faint genesis of pyrite framboids clearer and obviouser. At the same time, we find and detailedly discuss environmental conditions when pyrite framboids evolve into euhedral crystals, or framboid diameter is bigger, or pyrite framboids are oxidized into other minerals. These results provide evidences and theoretical basis for the formation and development of pyrite framboids. By studying pyrite framboids, trace element, the coupling of the high-resolution C-S isotope records across the Frasnian-Famennian boundary in Guilin, Guangxi, we considered that the Frasnian-Famennian mass extinction can be related to pulsatile hydrothermal activity. Because hydrothermal activity leaded to deep and middle water heated, degasification of heated water made H2S removed from water and upwell to surface water, the gas H2S consumed plenty of oxygen in surface water. Thus hypoxia and toxic gas leaded to mass extinction in shallow continental shelves, these provided credible evidences for Frasnian-Famennian mass extinction.Pyrite famboids are spheroidal or sub-spheroridal aggregates composed of pyrite microcrystals with same shape and uniform size:They are widely found in sedimentary rocks and other geologic carriers. So pyrite framboids can form under wider geochemical environment. Although formation mechanism of pyrite framboids exists controversy of biogenesis and abiogenesis. Different characteristic pyrite framboids can attribute to different biotic and abiotic action. The shape of pyrite framboids is very sensitive to redox conditions, Framboid diameter usually is used to trace paleoxygenation facies, discriminating the anaerobic (enxinic) and aerobic environment in overlying water column, The physical and chemical characteristics of pyrite framboids provide important record that sea responsed to environmental change in the geological history, and the relation with iron, sulfur, carbon and oxygen cycle of the globe. This study largely based on pyrite framboids in the Devonian strata and fossil, South China, by comparing and analyzing mineralogical characteristics of pyrite framboids in different carriers, these carriers include Devonian carbonatites and body fossils (Brachiopodas and bryozoans and corals) from South China, and other surrounding rocks and body fossils reported by predecessors as well as those synthesized in inorganic environment by predecessors, We has proposed identifying criteria of different genetic pyrite framboids in mineralogy. It was surprising that pyrite framboids can evolve to different directions under different environments, by analyzing the influence of environment during the course of evolution. The factors controlling the development of pyrite framboids are revealed. Taking Yangdi section as an example, by measuring trace elements and inorganic carbon isotope of total rocks and S isotope of carbonate-associated sulfate (CAS), as well as the occurrence state of pyrite framboids, The paper discusses the relation of pyrite framboids and C-S cycle, and provides the credible evidence for Devonian mass extinction.Scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS) was used to compare and analyze these pyrite framboids in different carriers such as surrounding rock、entity fossil、 Zoophycos, The items of analysis and comparison include framboid diameter(D), microcrystal diameter(d), D/d, the quantity and shape of microcrystal, the impurity among microcrystals, Our study initially obtained the following results:there are no great differences between abiogenic pyrite framboids and those by biological and abiological processes. Differences between pyrite framboids only depend on inorganic chemical conditions of the environment, where pyrite framboids formed, for example redox rate, nucleation time and microcrystal growth rate. Characteristics of biogenic pyrite framboids include that framboid diameter(D) and microcrystal diameter(d) of biogenic pyrite framboids are low and vary slightly, framboid diameter isn’t affect by redox conditions of depositional environments, only in relation with micro-environment, Most D/d is less than10, and pyrite framboids made up of cubic microcrystal are very likely formed by biological action, pyrite framboids with membrane wrapping the outer surface are biogenic. Characteristics of abiogenic pyrite framboids include that framboid diameter (D) of abiogenic framboid vary widely, framboid diameter can reflect redox conditions of depositional environments. Most D/d is more than10, no cubic microcrystal, no membrane wrapping the outer surface. The pyrite framboids in Zoophycos were the result of biological action, microbe enrich Fe in the surrounding rock, formed pyrite framboids, intracellular and extracellular. The reduction degree in deep spreiten of Zoophycos was more than in light spreiten, and microbial activity is stronger.Besides that, we discovered pyrite framboid can develop to different directions under different environments. Under the conditions of alkaline oxidation, Pyrite framboid can be oxidized and replaced to form pseudomorphs framboid of magnetite, hematite, sphalerite or chalcopyrite, etc; Under the conditions of acidic weak oxidation and high [Fe][S] saturation, microcrystals in pyrite fromboids can slowly grow up, microcrystal diameter is bigger, so framboid diameter is bigger with microsrystal; Under the conditions of acidic weaker oxidation even anoxia and low [Fe][S] saturation, pyrite framboids gradually developed to euhedral crystal, first formed euhedral pyrite made of microcrystal, then microcrystal recrystallized and fused, last formed single euhedral pyrite, such as cube, octahedron, or pentagonal dodecahedron crystal.By analysizing and researching for pyrite framboid, trace element, the coupling of the high-resolution C-S isotope records across the Frasnian-Famennian boundary in Guilin, Guangxi. The C isotope means whole-rock inorganic C isotope, the S isotope means sulfur isotopic composition of carbonate associated sulfate (CAS). We discover that C-S isotope records showed coupled change in synchronization from No.36layer to No.39layer, the C-S isotope record showed the characteristic of positive excursion as a whole in the event layers, but the C isotope and S isotope records displayed unsynchronized vibration, respectively. Above Frasnian-Famennian transition, firstly, C-S isotope records didn’t show coupled change in synchronization, that was, C isotope records continued positive excursion, to the top of No.41layer, C isotope records gradually went back, but S isotope records began already negative excursion, C-S isotope records showed coupling of negative excursion in synchronization above the top of No.41layer. The quantity of pyrite framboid was very rare from No.36layer to No.39layer, increased till to No.40layer, but the quantity still was low as a whole, framboid diameters were not big, below10μm, framboid diameters reached15μm only in a few layers, framboid diameters began to largen at the top of No.40layer, most pyrite framboids were up to10μm, the quantity of pyrite framboids began to increase greatly, framboid diameters increased, too, even exceeded30μm in No.42layer. By analysizing the data of trace elements and the occurrence state of pyrite framboids, we exclude these factors of volcanism and errigenous input and consider transient fluctuation of C-S isotope records could be primarily related to pulsatile hydrothermal activity in the bottom of ocean, The Frasnian--Famennian Mass Extinction could be also related to hydrothermal activity, because hydrothermal activity leaded to deep and middle water heated, H2S in those water happened degassing owing to heated water and upwelled to surface water, consumed plenty of oxygen, leaded to mass extinction in shallow continental shelves due to damage from hypoxia and toxic gas, the deep hot water upwelled to surface water soon afterwards, leaded to S isotope records showing quickly negative excursion then recovered quickly. At the same time, hydrothermal activity could lead to organic matter pyrolysis and variation of redox in the study area, all those were the cause of transient fluctuation of C-S isotope records in the event layers. The result was consistent with the occurrence state of pyrite framboids such as framboid diameter mostly below10μm, up to15μm only in a few layers, corrosion and oxidation of some pyrites across the Frasnian-Famennian boundary, meanwhile, accorded with redox state by analyzing trace metals, that was, The event layers were under weak oxidation environment as a while, but had fluctuation between weak oxidation and reduction environment. Above the Frasnian-Famennian boundary, S isotope records began negative excursion, but C isotope records continued positive excursion, then began to fall slowly, these could be related to gradually increasing oxidizability. When oxidizability in surface water increased, the thiobacterias or sulfur-containing compounds were more easily oxidized to SO42-with low δ34S entering into seawater, So δ34SCAS value began negative excursion, at the same time, increased oxidizability maked marine ecosystem gain ecological repair, microbe activity weakened, macro-organisms started to recover, probably terrestrial inputs added, the increased quantity of pyrite framboids could account for this, at the moment primary productivity decreased, those factors leaded to sulfur isotope showing negative excursion,δ12C value showing positive excursion then negative excursion. |
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