11:00 〜 13:00
[MIS12-P04] Micro-Raman spectroscopic study of Barberton Spherules, South Africa
キーワード：terrestrial impact spherules, shock metamorphism, Micro-Raman Spectroscopy
Spherules are tiny metallic or glassy globular grains with extraterrestrial, volcanic, or anthropogenic origins. Spherules from the Barberton Greenstone Belt, South Africa, are believed to be the oldest remnants (3.5 - 3.25 Ga) of impact events on the Earth, however, their origin is still under debate. The Barberton Greenstone Belt consisting of volcanic and volcaniclastic rocks, cherts, and siliciclastic sedimentary rocks can be divided into three main lithostratigraphic units in order: Onverwacht Group, Fig Tree Group, and the Moodies Group. Quartz, phyllosilicates, anatase, rutile, and spinel-containing millimeter-sub millimeter size spherules occur in four distinct layers. Their uncommon isotopic and trace element composition (Shukolyukov et al. 2000, Kyte et al. 2003, Koeberl and Reimold, 1995) may be explained by impact event caused by a bolide with a diameter of 20-50 km (e.g., Byerly and Lowe 1994, Krull-Davatzes et al. 2006) while other authors presume volcanic origin at least for some of the layers (e.g., Koeberl and Reimold, 1995, Hofmann et al. 2006). In any case, there is still a lot of uncertainty about the origin of the spherules (Hofmann et al. 2006). Raman spectroscopic examination of the spherules may help to detect the presence or lack of the effects of shock metamorphism and may contribute to solving this enigma.
Our working hypothesis was that such kind of large extraterrestrial body could have triggered shock-wave-related heating and deformation (T between 2500-3000 °C and ppeak between 50-60 GPa), resulting in the formation of impact-derived glass called lechatelierite. Using MicroRaman spectroscopy we can indicate and separate various silicate phases and reveal the origin of the spherules.
Our preliminary Raman results exhibited a peak shift, as well as peak broadening in the spectra of two spherule samples (F2 and F26A). In our interpretation, these systematic spectral changes may relate to shock-induced amorphization processes and may support the formation scenario of the impact-triggered lava fountain (Csámer et al., 2021). Now we extended our investigation to additional spherule samples, as well as we carried out new and meticulous measurements on the samples F2 and F26A. First, selected thin sections from the Barberton greenstone belt spherule layers were examined under transmitted and reflected polarized light to obtain information about their texture, mineralogical composition, and the petrography of the embedded inclusions. Then the selected spherules were analyzed under laser Raman microscope at the Applied and Nonlinear Optics Research, Wigner Research Centre for Physics (Hungary) to investigate the effect of the impact-related shock-metamorphism. We compared our results to the Raman spectra of various natural volcanic and impact glasses and natural silicate substances, such as radiolarite, silicified diatomite, and opal.
References
Byerly, G.R. and D.R. Lowe, Spinel from Archean impact spherules, Geochimica et Cosmochimica Acta, 58, 3469-3486, 1994.
Csámer, Á, D. Skita, I. Rigó, M. Veres, P. Futó, J. Vanyó and A. Gucsik, Barberton impact spherules, South Africa: pursuing of shock metamorphism by using Raman spectroscopy, the 12th Symposium on Polar Science 15 – 18 November 2021, National Institute of Polar Research, Research Organization of Information and Systems, 19-20, 2021
Hofmann, A., W.U. Reimold and C. Koeberl, Archean spherule layers in the Barberton greenstone belt, South Africa: A discussion of problems related to the impact interpretation, Geological Society of America, Special Paper, 405, 33-56, 2006.
Koeberl, C. and W.U. Reimold, Early Archaean spherule beds in the Barberton Mountain Land, South Africa: No evidence for impact origin, Pre-cambrian Research, 74, 1-33, 1995.
Krull-Davatzes, E.A., D.R. Lowe and G.R. Byerly, Compositional grading in an ~3.24 Ga impact-produced spherule bed, Barberton greenstone belt, South Africa: A key to impact plume evolution, South African Journal of Geology, 109, 233-244, 2006.
Kyte, F.T., A. Shukolyukov, G.W. Lugmair, D.R. Lowe and G.R. Byerly, Early Archean spherule beds: Chromium isotopes confirm origin through multiple impacts of projectiles of carbonaceous chondrite type, Geology, 31, 283-286, 2003.
Shukolyukov, A., F.T. Kyte, G.W. Lugmair, D.R. Lowe and G.R. Byerly, Early Archean spherule beds: Confirmation of impact origin, Meteoritics and Planetary Science, 35, supplement S, A146-A147, 2000.
Our working hypothesis was that such kind of large extraterrestrial body could have triggered shock-wave-related heating and deformation (T between 2500-3000 °C and ppeak between 50-60 GPa), resulting in the formation of impact-derived glass called lechatelierite. Using MicroRaman spectroscopy we can indicate and separate various silicate phases and reveal the origin of the spherules.
Our preliminary Raman results exhibited a peak shift, as well as peak broadening in the spectra of two spherule samples (F2 and F26A). In our interpretation, these systematic spectral changes may relate to shock-induced amorphization processes and may support the formation scenario of the impact-triggered lava fountain (Csámer et al., 2021). Now we extended our investigation to additional spherule samples, as well as we carried out new and meticulous measurements on the samples F2 and F26A. First, selected thin sections from the Barberton greenstone belt spherule layers were examined under transmitted and reflected polarized light to obtain information about their texture, mineralogical composition, and the petrography of the embedded inclusions. Then the selected spherules were analyzed under laser Raman microscope at the Applied and Nonlinear Optics Research, Wigner Research Centre for Physics (Hungary) to investigate the effect of the impact-related shock-metamorphism. We compared our results to the Raman spectra of various natural volcanic and impact glasses and natural silicate substances, such as radiolarite, silicified diatomite, and opal.
References
Byerly, G.R. and D.R. Lowe, Spinel from Archean impact spherules, Geochimica et Cosmochimica Acta, 58, 3469-3486, 1994.
Csámer, Á, D. Skita, I. Rigó, M. Veres, P. Futó, J. Vanyó and A. Gucsik, Barberton impact spherules, South Africa: pursuing of shock metamorphism by using Raman spectroscopy, the 12th Symposium on Polar Science 15 – 18 November 2021, National Institute of Polar Research, Research Organization of Information and Systems, 19-20, 2021
Hofmann, A., W.U. Reimold and C. Koeberl, Archean spherule layers in the Barberton greenstone belt, South Africa: A discussion of problems related to the impact interpretation, Geological Society of America, Special Paper, 405, 33-56, 2006.
Koeberl, C. and W.U. Reimold, Early Archaean spherule beds in the Barberton Mountain Land, South Africa: No evidence for impact origin, Pre-cambrian Research, 74, 1-33, 1995.
Krull-Davatzes, E.A., D.R. Lowe and G.R. Byerly, Compositional grading in an ~3.24 Ga impact-produced spherule bed, Barberton greenstone belt, South Africa: A key to impact plume evolution, South African Journal of Geology, 109, 233-244, 2006.
Kyte, F.T., A. Shukolyukov, G.W. Lugmair, D.R. Lowe and G.R. Byerly, Early Archean spherule beds: Chromium isotopes confirm origin through multiple impacts of projectiles of carbonaceous chondrite type, Geology, 31, 283-286, 2003.
Shukolyukov, A., F.T. Kyte, G.W. Lugmair, D.R. Lowe and G.R. Byerly, Early Archean spherule beds: Confirmation of impact origin, Meteoritics and Planetary Science, 35, supplement S, A146-A147, 2000.