17:15 〜 18:30
[SMP24-P03] Sulfur isotope study of high-grade metamorphic rocks in Highland Complex, Sri Lanka
キーワード：硫黄同位体、太古代、高度変成作用、ハイランドコンプレックス、スリランカ
Large deviations from mass independent fractionation in sulfur isotopes, termed as MIF-S, are characteristic of Archean sediments1. Although there are reports of Archean detrital zircon ages from 2400 to 3200 Ma in the Highland Complex (HC), Sri Lanka, it is not clear whether the sediments itself have detrital components other than zircon grains. Multiple sulfur isotope studies may provide some information in this aspect. Furthermore, the behavior of sulfur during high-grade metamorphism is not well understood, although sulfur isotope studies have been conducted in low-grade metamorphic rocks, previously2. In this study, we measured sulfur isotopes ratios for bulk rock samples from the HC, Sri Lanka, to verify whether the Archean MIF-S is preserved and try to discuss the behavior of sulfur during high-grade metamorphism.
Sulfide minerals in the metapelitic Grt-Sil gneiss are composed of pyrrhotite, chalcopyrite and pentlandite, that has a wide range of δ34SV-CDT from -1.7 ‰ to 14.2 ‰. Charnockite contain various sulfide minerals such as pyrrhotite, chalcopyrite, pyrite, pentlandite and molybdenite, and their δ34SV-CDT values range between -1.0 ‰ to 5.9 ‰. These values are consistent with an igneous origin for charnockites in the HC. However, there was no clear signature of MIF-S, although minor positive trend is observed in metapelitic rocks.
The with positive δ34SV-CDT values Grt-Sil gneiss are assumed to be sedimentary pyrite that are close to the sulfur isotopic composition of seawater sulfate, considering that the sulfate concentration in seawater was low at that time. When pyrite breakdown to pyrrhotite during metamorphism, δ34S values increases at most 2-3 ‰3. Considering that the δ34SV-CDT values represent near seawater sulfate values, the bulk sulfur isotopic composition is preserved even after high grade metamorphism. Considering the minor spikes in MIF-S in metapelitic rocks, further detailed studies are in progress.
References
1. Kröner et al., 1987 The Journal of Geology 95, 775-791
2. Ono et al., 2006 South African of Journal of Geology 109, 97-108
3. Bucholz et al., 2020 Earth and Planetary Science Letters 549, 116494
Sulfide minerals in the metapelitic Grt-Sil gneiss are composed of pyrrhotite, chalcopyrite and pentlandite, that has a wide range of δ34SV-CDT from -1.7 ‰ to 14.2 ‰. Charnockite contain various sulfide minerals such as pyrrhotite, chalcopyrite, pyrite, pentlandite and molybdenite, and their δ34SV-CDT values range between -1.0 ‰ to 5.9 ‰. These values are consistent with an igneous origin for charnockites in the HC. However, there was no clear signature of MIF-S, although minor positive trend is observed in metapelitic rocks.
The with positive δ34SV-CDT values Grt-Sil gneiss are assumed to be sedimentary pyrite that are close to the sulfur isotopic composition of seawater sulfate, considering that the sulfate concentration in seawater was low at that time. When pyrite breakdown to pyrrhotite during metamorphism, δ34S values increases at most 2-3 ‰3. Considering that the δ34SV-CDT values represent near seawater sulfate values, the bulk sulfur isotopic composition is preserved even after high grade metamorphism. Considering the minor spikes in MIF-S in metapelitic rocks, further detailed studies are in progress.
References
1. Kröner et al., 1987 The Journal of Geology 95, 775-791
2. Ono et al., 2006 South African of Journal of Geology 109, 97-108
3. Bucholz et al., 2020 Earth and Planetary Science Letters 549, 116494