13:45 〜 15:15
[MIS13-P06] カナダ・ラブラドル地域のヌリアック表成岩（３９億年前）中の硫化鉱物の鉱物学的特徴
キーワード：太古代、硫化鉱物
Although sulfur-metabolism has been considered as one of the oldest metabolisms from a phylogenetic viewpoint (Wagner et al., 1998), there is no geochemical evidence such as sulfur isotopes of sulfide for the sulfur-metabolism until ca. 3.5 Ga (Shen et al., 2001; Shen et al., 2009). Previous studies investigated sulfur isotopes of sulfide mainly from banded iron formation (BIF) and clastic sedimentary rocks in the 3.8 Ga Isua supracrustal belt, but these signatures do not indicate microbially induced sulfur isotopic fractionation. Therefore, it is still controversial whether the sulfur-metabolism was operated in the Eoarchean; thus, it is necessary to make more comprehensive investigation of sulfide hosted in lithologically various rocks as well as those in other geologic terrains.
The Nulliak supracrustal rocks, one of the oldest supracrustal rocks, occur in the Saglek Block, northern Labrador, Canada. They contain metamorphic rocks originating from chert, BIF, carbonate rock, pelite, and conglomerate, whose sedimentary ages date back to 3.9 Ga (Shimojo et al., 2016). Sedimentary rocks in the Big Island, SJHS, and SJHE areas underwent amphibolite facies metamorphism, whereas those in the Shuldham Island and Pangertok Inlet areas underwent granulite facies metamorphism (Schiøtte et al., 1992). And the metamorphic grade is estimated to increase in order of the Big Island, SJHS, SJHE, and Shuldham Island (Tashiro et al. 2017). From these metasedimentary rocks 12C-rich graphite grains have been reported (Tashiro et al. 2017), so it is believed that biological activities and habitat in the early earth is recorded there. However, mineralogical characteristics of sulfide minerals in the Nulliak supracrustal rocks and the relation with metamorphic grade are unclear.
Microscopic observation show that the sulfide minerals exist in all types of metasedimentary rocks. The pelites and the carbonate rocks tend to contain more sulfides than BIFs (120 out of 323 samples, 45 out of 92 samples, 9 out of 295 samples, respectively). SEM-EDS analysis indicates two types of sulfides, namely pyrite and pyrrhotite, but most of them are pyrrhotite.
It is well known that transition from pyrite to pyrrhotite occurs during metamorphism. The ratio of samples containing pyrites to samples containing sulfides is 19% (11 out of 57 samples) in Big Island, 16% (4 out of 25 samples) in SJHS, and 13% (3 out of 22 samples) in SJHE, respectively. Assuming that metamorphic grade in same area is equal, the abundance ratio of pyrites reflects the difference of metamorphic conditions such as sulfur fugacity. On the other hand, in the Shuldham Island (granulite facies), metamorphic temperatures of which is estimated to 700~800℃ by biotite-garnet geothermometer (Tashiro et al., 2017), the ratio is 0% (0 out of 17 samples). All pyrites at this area might be converted to pyrrhotites by desulfidation during the prograde metamorphism.
Cherts contain pyrites in 7 out of 18 samples from the Pangertok Inlet (granulite facies). Raman spectroscopy of carbonaceous material for one sample of chert from this area show that the peak temperature is more than 650±50℃ in Pangertok Inlet. On the other hand, considering the abundance ratio of the pyrites, metamorphic grade of this area might be lower than that of Shuldham Island and at least it was low enough for pyrite to survive.
The Nulliak supracrustal rocks, one of the oldest supracrustal rocks, occur in the Saglek Block, northern Labrador, Canada. They contain metamorphic rocks originating from chert, BIF, carbonate rock, pelite, and conglomerate, whose sedimentary ages date back to 3.9 Ga (Shimojo et al., 2016). Sedimentary rocks in the Big Island, SJHS, and SJHE areas underwent amphibolite facies metamorphism, whereas those in the Shuldham Island and Pangertok Inlet areas underwent granulite facies metamorphism (Schiøtte et al., 1992). And the metamorphic grade is estimated to increase in order of the Big Island, SJHS, SJHE, and Shuldham Island (Tashiro et al. 2017). From these metasedimentary rocks 12C-rich graphite grains have been reported (Tashiro et al. 2017), so it is believed that biological activities and habitat in the early earth is recorded there. However, mineralogical characteristics of sulfide minerals in the Nulliak supracrustal rocks and the relation with metamorphic grade are unclear.
Microscopic observation show that the sulfide minerals exist in all types of metasedimentary rocks. The pelites and the carbonate rocks tend to contain more sulfides than BIFs (120 out of 323 samples, 45 out of 92 samples, 9 out of 295 samples, respectively). SEM-EDS analysis indicates two types of sulfides, namely pyrite and pyrrhotite, but most of them are pyrrhotite.
It is well known that transition from pyrite to pyrrhotite occurs during metamorphism. The ratio of samples containing pyrites to samples containing sulfides is 19% (11 out of 57 samples) in Big Island, 16% (4 out of 25 samples) in SJHS, and 13% (3 out of 22 samples) in SJHE, respectively. Assuming that metamorphic grade in same area is equal, the abundance ratio of pyrites reflects the difference of metamorphic conditions such as sulfur fugacity. On the other hand, in the Shuldham Island (granulite facies), metamorphic temperatures of which is estimated to 700~800℃ by biotite-garnet geothermometer (Tashiro et al., 2017), the ratio is 0% (0 out of 17 samples). All pyrites at this area might be converted to pyrrhotites by desulfidation during the prograde metamorphism.
Cherts contain pyrites in 7 out of 18 samples from the Pangertok Inlet (granulite facies). Raman spectroscopy of carbonaceous material for one sample of chert from this area show that the peak temperature is more than 650±50℃ in Pangertok Inlet. On the other hand, considering the abundance ratio of the pyrites, metamorphic grade of this area might be lower than that of Shuldham Island and at least it was low enough for pyrite to survive.