Wednesday, February 15, 2006
Confirmation of rapid metamorphism of rocks
Eclogite-grade metamorphism is conventionally thought to involve high temperatures and very high pressures. The usual scenario for eclogite formation is considered to be below 60 km, at the base of a subduction zone, and at a temperature of around 700 °C. However, new work on Norwegian eclogites suggests that spasmodic surges of hot fluids, each lasting ten years or less, and at lower-than-expected temperatures (<400°C) caused eclogitization of the precursor granulites (Comacho et al 2005). These shorter timescales "will make many geologists draw breath" (Kelley 2005).
Furthermore, independent confirmation of these rapid fluid flows has been reported by Snelling (2005; 2006). A sample of related eclogite containing biotite flakes was closely examined and polonium-210 radiohalos were found in it (7 polonium-210 radiohalos in 50 microscope slides, each containing 20-30 biotite flakes). This discovery, the first time any radiohalos have been documented in eclogites, is highly significant. Biotite was not in the precursor granulites, so it had to form as a result of both their metamorphism to eclogite and the fluid flows. Furthermore, because there was no source of either parent uranium-238 or its radioactive decay products within either the eclogites or the precursor granulites, the large quantities of polonium-210 required to generate these radiohalos had to have been transported from external sources into the biotite flakes within these rocks by the hot fluids. But the polonium-210 only has a half-life of 138 days, and the radiohalos would only have formed and survived after the temperature in the rocks fell below 150 °C. So this drastically restricts the duration of the fluid flows and associated eclogite metamorphism even more, perhaps to only a few weeks or months.
References:
Camacho A, Lee JKW, Hensen BJ, Braun J, 'Short-lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids', Nature 2005;435:1191-1196. http://www.nature.com/nature/journal/v435/n7046/abs/nature03643.html
Kelley S, 'Geophysics: Hot fluids and cold crusts', Nature 2005;435:1171.
Snelling AA, 'Radiohalos in granites: evidence for accelerated nuclear decay', pp.101-207 (especially Table 4, p.188) in: Vardiman L, Snelling AA, Chaffin EF (editors), Radioisotopes and the Age of the Earth: Results of a Young-Earth Creationist Research Initiative, Institute for Creation Research, El Cajon, CA, and Creation Research Society, Chino Valley, AZ; 2005.
Snelling AA, 'Confirmation of rapid metamorphism of rocks', Institute for Creation Research Impact Article #392, February 2006.http://www.icr.org/index.php?module=articles&action=view&ID=2603
Furthermore, independent confirmation of these rapid fluid flows has been reported by Snelling (2005; 2006). A sample of related eclogite containing biotite flakes was closely examined and polonium-210 radiohalos were found in it (7 polonium-210 radiohalos in 50 microscope slides, each containing 20-30 biotite flakes). This discovery, the first time any radiohalos have been documented in eclogites, is highly significant. Biotite was not in the precursor granulites, so it had to form as a result of both their metamorphism to eclogite and the fluid flows. Furthermore, because there was no source of either parent uranium-238 or its radioactive decay products within either the eclogites or the precursor granulites, the large quantities of polonium-210 required to generate these radiohalos had to have been transported from external sources into the biotite flakes within these rocks by the hot fluids. But the polonium-210 only has a half-life of 138 days, and the radiohalos would only have formed and survived after the temperature in the rocks fell below 150 °C. So this drastically restricts the duration of the fluid flows and associated eclogite metamorphism even more, perhaps to only a few weeks or months.
References:
Camacho A, Lee JKW, Hensen BJ, Braun J, 'Short-lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids', Nature 2005;435:1191-1196. http://www.nature.com/nature/journal/v435/n7046/abs/nature03643.html
Kelley S, 'Geophysics: Hot fluids and cold crusts', Nature 2005;435:1171.
Snelling AA, 'Radiohalos in granites: evidence for accelerated nuclear decay', pp.101-207 (especially Table 4, p.188) in: Vardiman L, Snelling AA, Chaffin EF (editors), Radioisotopes and the Age of the Earth: Results of a Young-Earth Creationist Research Initiative, Institute for Creation Research, El Cajon, CA, and Creation Research Society, Chino Valley, AZ; 2005.
Snelling AA, 'Confirmation of rapid metamorphism of rocks', Institute for Creation Research Impact Article #392, February 2006.http://www.icr.org/index.php?module=articles&action=view&ID=2603
# posted by Nemagraptus @ 6:02 AM 
