11:45 〜 12:00
[BCG07-10] Phosphate leaching from Archean basaltic oceanic crust by unique submarine hydrothermal processes
★Invited Papers
キーワード：リン、海底熱水活動、太古代、海洋地殻
The phosphorus (P) concentration in the early ocean is thought to have been remarkably low. This problem rises a question why early life utilized rare P for diverse biomolecules. On the other hand, some researchers have been proposing early basaltic oceanic crusts could supply phosphate into early oceans through submarine hydrothermal activities (attached paper). However, P mobilization in the oceanic crust by Archean submarine hydrothermal activity is still uncertain. In this study, geochemical and mineralogical studies of the 3.46 Ga Apex Basalt in ABDP #1 core from Eastern Pilbara were conducted to examine P mobilization.
The drilled core of Apex Basalt covers 70 m length, and can be divided into three lithological zones. Zone A (top 10 m section) is the least altered zone, containing actinolite, epidote, albite, and chlorite with low chemical index of alteration (CIA) values (0.42-0.44). P2O5 contents in this zone are 0.10±0.01 wt%. Euhedral apatite crystals (10 to 50 µm in diameter) in this zone represent the "primary" igneous phosphates in the Apex Basalt. Zone B (middle 30 m section) is extensively chloritized and contains abundant siderite. Zone C (lower 30 m section) contains chlorite, K-mica, and sulfides. CIA values of both zones range from 0.76 to 0.99, suggesting intense hydrothermal alteration. Most parts of zones B and C exhibit depletions of P2O5 (0.01±0.01 wt%), but minor enrichments of P (0.06±0.01 wt%). Such enrichment is accompanied by formations of secondary apatite (<5 µm in diameter), xenotime (YPO4, <30 µm in diameter), and berlinite (AlPO4, <50 µm in diameter) in zones B and C.
P depletion in the most altered Apex basalt indicates that P was significantly leached from the basaltic oceanic crusts by submarine hydrothermal activities. This finding of P behavior is contrary to the modern P behavior in modern oceanic basalts, i.e., sink of P.
Most apatite and berlinite in Zone B coexist spatially. Calculations by Geochemist Workbench indicate that both minerals can be coexisted under 100-150°C and pH 9-10, suggesting that alkaline hydrothermal activity was responsible for the dissolution of P in Zone B. In addition, the solubility calculations of apatite under 100-150°C and pH 9-10 estimated that P concentration in hydrothermal fluids could reach about 1 mM. This value is much higher than the P concentration of 2.3 µM in modern seawater. Our results indicate that the alkaline hydrothermal fluids leached P from the basaltic oceanic crust and provided a significant amount of P, even if the granitic continents were less abundant on the early Earth.
Reference
Birger Rasmussen, Janet R. Muhling, Alexandra Suvorova, Woodward W. Fischer; Apatite nanoparticles in 3.46–2.46 Ga iron formations: Evidence for phosphorus-rich hydrothermal plumes on early Earth. Geology 2021; 49 (6): 647–651.
The drilled core of Apex Basalt covers 70 m length, and can be divided into three lithological zones. Zone A (top 10 m section) is the least altered zone, containing actinolite, epidote, albite, and chlorite with low chemical index of alteration (CIA) values (0.42-0.44). P2O5 contents in this zone are 0.10±0.01 wt%. Euhedral apatite crystals (10 to 50 µm in diameter) in this zone represent the "primary" igneous phosphates in the Apex Basalt. Zone B (middle 30 m section) is extensively chloritized and contains abundant siderite. Zone C (lower 30 m section) contains chlorite, K-mica, and sulfides. CIA values of both zones range from 0.76 to 0.99, suggesting intense hydrothermal alteration. Most parts of zones B and C exhibit depletions of P2O5 (0.01±0.01 wt%), but minor enrichments of P (0.06±0.01 wt%). Such enrichment is accompanied by formations of secondary apatite (<5 µm in diameter), xenotime (YPO4, <30 µm in diameter), and berlinite (AlPO4, <50 µm in diameter) in zones B and C.
P depletion in the most altered Apex basalt indicates that P was significantly leached from the basaltic oceanic crusts by submarine hydrothermal activities. This finding of P behavior is contrary to the modern P behavior in modern oceanic basalts, i.e., sink of P.
Most apatite and berlinite in Zone B coexist spatially. Calculations by Geochemist Workbench indicate that both minerals can be coexisted under 100-150°C and pH 9-10, suggesting that alkaline hydrothermal activity was responsible for the dissolution of P in Zone B. In addition, the solubility calculations of apatite under 100-150°C and pH 9-10 estimated that P concentration in hydrothermal fluids could reach about 1 mM. This value is much higher than the P concentration of 2.3 µM in modern seawater. Our results indicate that the alkaline hydrothermal fluids leached P from the basaltic oceanic crust and provided a significant amount of P, even if the granitic continents were less abundant on the early Earth.
Reference
Birger Rasmussen, Janet R. Muhling, Alexandra Suvorova, Woodward W. Fischer; Apatite nanoparticles in 3.46–2.46 Ga iron formations: Evidence for phosphorus-rich hydrothermal plumes on early Earth. Geology 2021; 49 (6): 647–651.