11:40 〜 11:55
[BAO01-08] Discovery of Ni-Fe phosphides in 3.46 Ga-old Apex basalt from Western Australia
キーワード：ニッケル鉄含有リン化物、リン酸塩、隕石、Apex玄武岩
Phosphate (P5+O43-) is an essential constituent of nucleic acids. Today, phosphate for terrestrial and marine organisms has been supplied primarily by weathering of apatite in rocks. An important question for astrobiologists has been the source of phosphate in the Archean oceans. Pasek and Lauretta (2005) suggested that schreibersite ((Fe, Ni)3P) in meteorite was the primary source of phosphate on the early Earth. However, no direct evidence of meteoritic schreibersite has been reported from any Archean-aged geologic formations.
We have discovered Ni-Fe phosphides in the 3.46 billion-year-old Apex Basalt at depth 214.4 m in the ABDP #1 drill core from the East Pilbara Craton, Western Australia. Spot analyses by FE-SEM/EDS indicate their compositions as Ni = 75.8 ± 4.4, P = 10.8 ± 1.0, and Fe = 2.4 ± 0.9 wt%, or Ni3.74±0.37Fe0.13±0.05P1.0. The (Ni+Fe)/P atomic ratio varies from 2.9 (±0.4) to 4.5 (± 0.4), indicating mixtures of nickel-rich schreibersite ((Ni, Fe)3P) and nickel-rich melliniite ((Ni, Fe)4P). They have much higher Ni/Fe atomic ratios (31±9.9) compared to typical meteoric schreibersite and melliniite (Ni/Fe = 0.17±0.01). The only other known occurrence of Fe-Ni phosphides in basalt is from the Tertiary basalt on Disko Island, Denmark. Whether they originated from meteorites or are products of assimilation of carbon-rich sediments by basalt magma has been debated. However, we can conclude that the Ni-Fe phosphides in the Apex Basalt most likely represent pieces of an asteroid body (>20 km in diameter) that impacted the Marble Bar area at 3.45 Ga for the following reasons: (1) the association with various meteoritic minerals, such as metallic iron, various alloys (Fe-Ni, Fe-Ni-Co-Cr, Fe-Ir, Cu-Al-Si, Au-Si, and C-Ca-Al-Fe-Si), coesite (high-pressure polymorph of SiO2), and titanite and rutile with Ti3+ and Ti4+; (2) the highly distorted lattice structures of the titanite crystals, which were most likely produced by a planetary collision; and (3) the association with impact spherules, shock metamorphic features in the underlying rocks, and tsunami deposits. Thermodynamic analyses of the meteoritic minerals suggest that the parental planetesimals of the Apex meteorite condensed from the Solar Nebula at fO2 conditions more than 10 log units below the Fe/FeO buffer, compared to the parental planesimals of ordinary and carbonaceous chondrites, which condensed near or above the Fe/FeO buffer.
Our study suggests that the phosphorus delivery by asteroids/meteorites was important in the phosphorous cycle on the early Earth. However, this does not necessarily support the currently popular model for the origin of phosphates and life in the early oceans (e.g., Pasek, 2005), which speculates that meteoritic Fe-Ni phosphides (P3+) were directly dissolved in the early oceans and subsequently oxidized to phosphates (P5+) by H2 loss. This is because the results of their experiments, which were performed under highly oxidizing conditions, may not be applicable to the pre-biotic world, which was supposedly highly reducing. Our finding of schreibersite in the basalts, which were highly altered by submarine hydrothermal fluids, suggests that schreibersite is not highly soluble in aqueous solutions. Phosphorization to form macromolecules like RNA and DNA by reactions with Fe-Ni phosphides has not been demonstrated by researchers. Furthermore, H2 loss from the oceans to the atmosphere would not have occurred under the presumed H2-rich pre-biotic atmosphere.
Therefore, we must search for (an) alternative mechanism(s) to explain the transformation of the meteoric P3+ to phosphates (P5+) via igneous processes, and the transfer of those igneous phosphates to the oceans.
We have discovered Ni-Fe phosphides in the 3.46 billion-year-old Apex Basalt at depth 214.4 m in the ABDP #1 drill core from the East Pilbara Craton, Western Australia. Spot analyses by FE-SEM/EDS indicate their compositions as Ni = 75.8 ± 4.4, P = 10.8 ± 1.0, and Fe = 2.4 ± 0.9 wt%, or Ni3.74±0.37Fe0.13±0.05P1.0. The (Ni+Fe)/P atomic ratio varies from 2.9 (±0.4) to 4.5 (± 0.4), indicating mixtures of nickel-rich schreibersite ((Ni, Fe)3P) and nickel-rich melliniite ((Ni, Fe)4P). They have much higher Ni/Fe atomic ratios (31±9.9) compared to typical meteoric schreibersite and melliniite (Ni/Fe = 0.17±0.01). The only other known occurrence of Fe-Ni phosphides in basalt is from the Tertiary basalt on Disko Island, Denmark. Whether they originated from meteorites or are products of assimilation of carbon-rich sediments by basalt magma has been debated. However, we can conclude that the Ni-Fe phosphides in the Apex Basalt most likely represent pieces of an asteroid body (>20 km in diameter) that impacted the Marble Bar area at 3.45 Ga for the following reasons: (1) the association with various meteoritic minerals, such as metallic iron, various alloys (Fe-Ni, Fe-Ni-Co-Cr, Fe-Ir, Cu-Al-Si, Au-Si, and C-Ca-Al-Fe-Si), coesite (high-pressure polymorph of SiO2), and titanite and rutile with Ti3+ and Ti4+; (2) the highly distorted lattice structures of the titanite crystals, which were most likely produced by a planetary collision; and (3) the association with impact spherules, shock metamorphic features in the underlying rocks, and tsunami deposits. Thermodynamic analyses of the meteoritic minerals suggest that the parental planetesimals of the Apex meteorite condensed from the Solar Nebula at fO2 conditions more than 10 log units below the Fe/FeO buffer, compared to the parental planesimals of ordinary and carbonaceous chondrites, which condensed near or above the Fe/FeO buffer.
Our study suggests that the phosphorus delivery by asteroids/meteorites was important in the phosphorous cycle on the early Earth. However, this does not necessarily support the currently popular model for the origin of phosphates and life in the early oceans (e.g., Pasek, 2005), which speculates that meteoritic Fe-Ni phosphides (P3+) were directly dissolved in the early oceans and subsequently oxidized to phosphates (P5+) by H2 loss. This is because the results of their experiments, which were performed under highly oxidizing conditions, may not be applicable to the pre-biotic world, which was supposedly highly reducing. Our finding of schreibersite in the basalts, which were highly altered by submarine hydrothermal fluids, suggests that schreibersite is not highly soluble in aqueous solutions. Phosphorization to form macromolecules like RNA and DNA by reactions with Fe-Ni phosphides has not been demonstrated by researchers. Furthermore, H2 loss from the oceans to the atmosphere would not have occurred under the presumed H2-rich pre-biotic atmosphere.
Therefore, we must search for (an) alternative mechanism(s) to explain the transformation of the meteoric P3+ to phosphates (P5+) via igneous processes, and the transfer of those igneous phosphates to the oceans.