5:15 PM - 7:15 PM
[BBG02-P01] Framboidal pyrite in modern stromatolites from Fukiagezawa, Osaki City, Miyagi, Japan
Keywords:Framboidal pyrite, stromatolite, jarosite
Framboidal pyrite is a raspberry-shaped microscopic aggregate consisting of numerous pyrite microcrystals of uniform shape and size and commonly found in reducing environments such as sediments. Understanding its origin and formation process is critical to utilizing framboidal pyrite as a paleo-environmental proxy and biosignatures. However, the origin of the framboidal texture has remained unresolved.
In this study, we focused on framboidal pyrite in modern stromatolites occurring near hot spring vents in Fukiagezawa, Osaki City, Miyagi, Japan to understand the formation process and conditions in close association with microbial activities. Samples studied were collected from stromatolite layers formed along the flow path of hot spring water on altered volcanic rocks. The samples were embedded in resin and polished either by conventional mechanical polishing or by Ar ion milling using JEOL Cross-Section Polisher. Microstructural observation and chemical analysis were conducted using SEM-EDS. Additionally, thin foils were prepared using FIB for more detailed microtexture observation and chemical analysis by TEM-EDS.
SEM observation revealed that the stromatolite consists of the alternation of white layers composed mostly of silicified cyanobacteria with small amount of clay minerals and detrital particles such as quartz and feldspars and dark layers composed of clay minerals, detrital particles, pyrite and silica. The former is considered to have formed through the cyanobacterial activity and silica precipitation from the hydrothermal water, while the latter, which lack cyanobacterial traces but contain spheroidal aggregates of clay minerals, are likely hydrothermal deposits formed during a period of stagnation of the cyanobacterial activity.
Pyrite occurs mostly as small euhedral crystals and occasionally as framboidal aggregates in the dark layers. In the dark layer at the outermost surface of the stromatolite pyrite occurred disseminated and enclosed by amorphous silica (opal). Some framboidal aggregates were found at the area very close to (a few microns from) the surface, which suggests a possibility that the top surface of the stromatolite layer was blocked somehow from the atmospheric air during their formations. In another dark Layer between white silicified layers, pyrite was observed mostly in the pores between clay minerals, indicating its formation through localized sulfate reduction. Some of the Framboids show an overgrowth feature of the outermost pyrite microcrystals and/or infilled feature with opal, where the individual microcrystal shapes are not clearly recognized. In order to investigate the substances between the constituent microcrystals inside the framboidal pyrite, thin foils were cut out using focused ion beam and examined by TEM equipped with an EDS detector.
TEM observation showed that a framboid that experienced a secondary pyrite overgrowth on the periphery contains crystalline material composed of Fe, S, O, and K between the microcrystals, which was identified as jarosite by electron diffraction. On the other hand, framboids without the overgrowth feature also contain jarosite, but the gaps between the pyrite microcrystals and jarosite are cemented completely by amorphous silica.
Jarosite is known to be commonly produced by the oxidation of pyrite, but no evidence of oxidation was found in the present jarosite-containing framboids. The formation of jarosite likely occurred at almost the same timing as the nucleation of microcrystals and framboid formation. It seems that the overgrowth of the outermost microcrystals contributes to protecting such internal structures from the alteration by amorphous silica. The presence of jarosite between microcrystals suggests that the formation of framboidal pyrite occurred in a large redox gradient where SO42- and H2S were both present in the pores of the stromatolite layer.
In this study, we focused on framboidal pyrite in modern stromatolites occurring near hot spring vents in Fukiagezawa, Osaki City, Miyagi, Japan to understand the formation process and conditions in close association with microbial activities. Samples studied were collected from stromatolite layers formed along the flow path of hot spring water on altered volcanic rocks. The samples were embedded in resin and polished either by conventional mechanical polishing or by Ar ion milling using JEOL Cross-Section Polisher. Microstructural observation and chemical analysis were conducted using SEM-EDS. Additionally, thin foils were prepared using FIB for more detailed microtexture observation and chemical analysis by TEM-EDS.
SEM observation revealed that the stromatolite consists of the alternation of white layers composed mostly of silicified cyanobacteria with small amount of clay minerals and detrital particles such as quartz and feldspars and dark layers composed of clay minerals, detrital particles, pyrite and silica. The former is considered to have formed through the cyanobacterial activity and silica precipitation from the hydrothermal water, while the latter, which lack cyanobacterial traces but contain spheroidal aggregates of clay minerals, are likely hydrothermal deposits formed during a period of stagnation of the cyanobacterial activity.
Pyrite occurs mostly as small euhedral crystals and occasionally as framboidal aggregates in the dark layers. In the dark layer at the outermost surface of the stromatolite pyrite occurred disseminated and enclosed by amorphous silica (opal). Some framboidal aggregates were found at the area very close to (a few microns from) the surface, which suggests a possibility that the top surface of the stromatolite layer was blocked somehow from the atmospheric air during their formations. In another dark Layer between white silicified layers, pyrite was observed mostly in the pores between clay minerals, indicating its formation through localized sulfate reduction. Some of the Framboids show an overgrowth feature of the outermost pyrite microcrystals and/or infilled feature with opal, where the individual microcrystal shapes are not clearly recognized. In order to investigate the substances between the constituent microcrystals inside the framboidal pyrite, thin foils were cut out using focused ion beam and examined by TEM equipped with an EDS detector.
TEM observation showed that a framboid that experienced a secondary pyrite overgrowth on the periphery contains crystalline material composed of Fe, S, O, and K between the microcrystals, which was identified as jarosite by electron diffraction. On the other hand, framboids without the overgrowth feature also contain jarosite, but the gaps between the pyrite microcrystals and jarosite are cemented completely by amorphous silica.
Jarosite is known to be commonly produced by the oxidation of pyrite, but no evidence of oxidation was found in the present jarosite-containing framboids. The formation of jarosite likely occurred at almost the same timing as the nucleation of microcrystals and framboid formation. It seems that the overgrowth of the outermost microcrystals contributes to protecting such internal structures from the alteration by amorphous silica. The presence of jarosite between microcrystals suggests that the formation of framboidal pyrite occurred in a large redox gradient where SO42- and H2S were both present in the pores of the stromatolite layer.