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[BPT03-01] Importance of CO2-rich hydrothermal fluids to leach phosphorus from Archean pillow lavas: Case study at the 2.7 Ga Abitibi Greenstone Belt, Canada
Keywords:Archean, phosphorus, pillow lava
Phosphorus is an essential element in the origin and evolution of life. However, during the Hadean and Archean eras, a continent-based supply of phosphorous similar to the modern era was almost nonexistent, it remains unclear how phosphorus was supplied to the hydrosphere. Recent study proposed the possibility that hydrothermal activity on the ocean floor leached phosphate from basalt and supplied phosphate to the hydrosphere. On the other hand, research in the modern ocean indicates that basalt can deposit phosphorus. Thus, empirical geological evidence supporting the hypothesis that Archean seafloor basalt was a source of phosphorus is not yet available.
Geological and geochemical studies were performed on the 2.7 Ga pillow lavas and hyaloclastites in Abitibi Greenstone Belt, to examine the behavior of phosphorus associated with submarine hydrothermal activity. Various occurrences of pillow lavas were found in the studied area. All pillow lavas are accompanied by quenched glass rims. Such pillow margins are more altered by contemporaneous submarine hydrothermal activities than core part of pillow lavas. We categorized pillow rims into less altered, quartz-vein-rich, sulfide-rich, and carbonate/sulfide-rich types. At the analysis phase, samples were classified according to the following definitions: the rim part was strongly hydrothermally altered and rich in secondary minerals such as calcite and sulfide minerals, while the core part was less hydrothermally altered and poor in secondary minerals. Mineralogical and geochemical studies were performed separately, and we compared the results.
Veins composed of apatite and sphene were exclusively found in the rim parts of carbonate/sulfide-rich type. On the other hand, only a few minute crystals of apatite were observed in the core parts, and no crystals of sphene in the form of veins were also observed. Apatite was the only phosphate mineral phase identified in the analyzed samples, regardless of variations in alteration degree, although there were significant differences in size. Theses occurrences suggest that the dissolution and migration of phosphorus and titanium were caused by carbonate-rich hydrothermal fluids. The carbon isotopic composition of calcite and sulfur isotopic composition of sulfide minerals suggest that these fluids originated from magmatic-hydrothermal fluids derived from the bottom of caldera. This is where large-scale hydrothermal circulation occurred along the caldera walls, involving local seawater. On the other hand, other types of pillow lavas, without carbonate veins, do not show clear evidence of dissolution and migration of phosphate from host pillow lavas. These results indicate that carbonate-rich hydrothermal fluids were the key in to leach phosphorous from basalts and then supply phosphate into Archean oceans.
Geological and geochemical studies were performed on the 2.7 Ga pillow lavas and hyaloclastites in Abitibi Greenstone Belt, to examine the behavior of phosphorus associated with submarine hydrothermal activity. Various occurrences of pillow lavas were found in the studied area. All pillow lavas are accompanied by quenched glass rims. Such pillow margins are more altered by contemporaneous submarine hydrothermal activities than core part of pillow lavas. We categorized pillow rims into less altered, quartz-vein-rich, sulfide-rich, and carbonate/sulfide-rich types. At the analysis phase, samples were classified according to the following definitions: the rim part was strongly hydrothermally altered and rich in secondary minerals such as calcite and sulfide minerals, while the core part was less hydrothermally altered and poor in secondary minerals. Mineralogical and geochemical studies were performed separately, and we compared the results.
Veins composed of apatite and sphene were exclusively found in the rim parts of carbonate/sulfide-rich type. On the other hand, only a few minute crystals of apatite were observed in the core parts, and no crystals of sphene in the form of veins were also observed. Apatite was the only phosphate mineral phase identified in the analyzed samples, regardless of variations in alteration degree, although there were significant differences in size. Theses occurrences suggest that the dissolution and migration of phosphorus and titanium were caused by carbonate-rich hydrothermal fluids. The carbon isotopic composition of calcite and sulfur isotopic composition of sulfide minerals suggest that these fluids originated from magmatic-hydrothermal fluids derived from the bottom of caldera. This is where large-scale hydrothermal circulation occurred along the caldera walls, involving local seawater. On the other hand, other types of pillow lavas, without carbonate veins, do not show clear evidence of dissolution and migration of phosphate from host pillow lavas. These results indicate that carbonate-rich hydrothermal fluids were the key in to leach phosphorous from basalts and then supply phosphate into Archean oceans.