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[BPT04-03] Relationship between phosphorous leaching and hydrothermal alteration pattern in the 2.7 Ga pillow lavas from Abitibi Greenstone Belt, Canada.
Keywords:Archean, phosphorus, pillow lava
Phosphorus is an essential element for the origin and evolution of life. However, it is not clear how phosphorus was supplied to the oceans during the Hadean and Archean era, when there were no continents, and the supply of terrestrial sources was almost nonexistent as it is today. To address this problem, it has recently been proposed that phosphorus may have been supplied to the oceans by dissolution from submarine basalts through reactions with submarine hydrothermal fluids (Kakegawa et al., 2002; Rasmussen et al., 2021). On the other hand, some studies have shown that submarine basalts deposit phosphorus in the modern ocean (Wheat et al., 1996), and petrological evidence that submarine basalts were a source of phosphorus in the early Earth is still insufficient.
This study aims to examine the petrological and geochemical evidence for a possible supply of phosphorus from submarine basalts to the ocean by submarine hydrothermal activity in the Archean era, using pillow lava and hyaloclastite from the Abitibi Greenstone Belt, Canada, dated to about 2.7 billion years ago. Various types of pillow lavas were collected from the wide region (100 km × 50 km). Most of the pillow lava in the study area was formerly quenched glass, with strongly altered marginal areas and a rocky, less altered central area. In the analytical stage, the sample was classified into two parts: the rim part, which is rich in secondary minerals, and the core part, which is poor in secondary minerals. Mineralogical and geochemical studies were performed separately. The significant difference in the degree of hydrothermal alteration between the rim and core parts suggests that the hydrothermal alteration in the pillow lava was caused by submarine hydrothermal activity that occurred after the lava solidified. The traces of hydrothermal alteration were classified into several types: weakly altered, quartz vein-rich, sulfide mineral-rich, and calcite vein and sulfide mineral-rich. We found that regional differences in alteration patterns.
Observations revealed that apatite and sphene veins were found in the vicinity of the calcite veins only in the rim part of the calcite vein and sulfide mineral-rich type rocks. On the other hand, no such veining was found in the core part or at the rim parts of rocks that did not contain calcite veins. In these samples, apatite was the only phosphate mineral phase. The coexistence of calcite veins with apatite and sphene veins at the rim parts suggests that the dissolution and migration of phosphorus and titanium were caused by carbonate-rich hydrothermal fluids. In addition, the carbon isotopic composition of calcite and sulfur isotopic composition of sulfide minerals suggest that the hydrothermal fluids were volcanic hydrothermal fluids originating from deep in the crust, locally mixed with seawater, and circulated along the caldera wall on a large scale. These results indicate that carbonate-rich hydrothermal fluids were key to the process of supplying phosphorus from submarine basalts to the ocean in the Archean era.
References
・Kakegawa, T., Noda, M. and Nannri, H. (2002) Geochemical cycles of bio-essential elements on the early Earth and their relationships to origin of life. Resource geology, 52, 83–89.
・Rasmussen, B., Muhling, J.R., Suvorova, A. and Fischer, W.W. (2021) Apatite nanoparticles in 3.46–2.46 Ga iron formations: Evidence for phosphorus-rich hydrothermal plumes on early Earth. Geology, 49, 647–651.
・Wheat, C.G., Feely, R.A. and Mottle, M.J. (1996) Phosphate removal by oceanic hydrothermal processes: An update of the phosphorous budget in the oceans. Geochimica et cosmochimica acta, 60, 3593–3608.
This study aims to examine the petrological and geochemical evidence for a possible supply of phosphorus from submarine basalts to the ocean by submarine hydrothermal activity in the Archean era, using pillow lava and hyaloclastite from the Abitibi Greenstone Belt, Canada, dated to about 2.7 billion years ago. Various types of pillow lavas were collected from the wide region (100 km × 50 km). Most of the pillow lava in the study area was formerly quenched glass, with strongly altered marginal areas and a rocky, less altered central area. In the analytical stage, the sample was classified into two parts: the rim part, which is rich in secondary minerals, and the core part, which is poor in secondary minerals. Mineralogical and geochemical studies were performed separately. The significant difference in the degree of hydrothermal alteration between the rim and core parts suggests that the hydrothermal alteration in the pillow lava was caused by submarine hydrothermal activity that occurred after the lava solidified. The traces of hydrothermal alteration were classified into several types: weakly altered, quartz vein-rich, sulfide mineral-rich, and calcite vein and sulfide mineral-rich. We found that regional differences in alteration patterns.
Observations revealed that apatite and sphene veins were found in the vicinity of the calcite veins only in the rim part of the calcite vein and sulfide mineral-rich type rocks. On the other hand, no such veining was found in the core part or at the rim parts of rocks that did not contain calcite veins. In these samples, apatite was the only phosphate mineral phase. The coexistence of calcite veins with apatite and sphene veins at the rim parts suggests that the dissolution and migration of phosphorus and titanium were caused by carbonate-rich hydrothermal fluids. In addition, the carbon isotopic composition of calcite and sulfur isotopic composition of sulfide minerals suggest that the hydrothermal fluids were volcanic hydrothermal fluids originating from deep in the crust, locally mixed with seawater, and circulated along the caldera wall on a large scale. These results indicate that carbonate-rich hydrothermal fluids were key to the process of supplying phosphorus from submarine basalts to the ocean in the Archean era.
References
・Kakegawa, T., Noda, M. and Nannri, H. (2002) Geochemical cycles of bio-essential elements on the early Earth and their relationships to origin of life. Resource geology, 52, 83–89.
・Rasmussen, B., Muhling, J.R., Suvorova, A. and Fischer, W.W. (2021) Apatite nanoparticles in 3.46–2.46 Ga iron formations: Evidence for phosphorus-rich hydrothermal plumes on early Earth. Geology, 49, 647–651.
・Wheat, C.G., Feely, R.A. and Mottle, M.J. (1996) Phosphate removal by oceanic hydrothermal processes: An update of the phosphorous budget in the oceans. Geochimica et cosmochimica acta, 60, 3593–3608.