Dr.GH
Posts: 2333 Joined: May 2002
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Some of my notes for an article I am working on Re: creationists, abiogenesis, and the age of the earth.
Zircon Geochronology
Zircons form in magma chambers under intense pressures. They are common, and extremely stable making them very attractive for geological dating. Their internal chemistry, and weathering are records of Earth's earliest physical conditions following consolidation and cooling.
Bizzarro, Martin, Connelly, James N., Thrane, Kristine, Borg, Lars E., 2012 "Excess hafnium-176 in meteorites and the early Earth zircon record" Geochemistry, Geophysics, Geosystems Vol 13: 3, 1525-2027
Analysis of the SAH99555 angrite meteorite showed an isotopic enrichment of hafnium-176 from a high energy event ~4869 ± 34 Myr , ~300 million years before the aggregation of solids in the proto-solar system. This reflects an pre-collapse high energy event probably related to the trigger of proto-solar collapse. Terrestrial zircons from the early Archean (Hadean) approximately 4.3 Ga indicate persistent unhomogenized hafnium reservoirs which limits Earth crustal dynamics to after ~4.4 Ga.
Wilde S.A., Valley J.W., Peck W.H. and Graham C.M. (2001). "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago" Nature 409 (6817): 175–8.
4.4 Ga- oldest terrestrial minerals dated, Jack Hills Australia.
Mojzsis, S.J., Harrison, T.M., Pidgeon, R.T. (2001). "Oxygen-isotope evidence from ancient zircons for liquid water at the Earth's surface 4300 Myr ago". Nature 409 (6817): 178–181.
Ushikubo, T., Kita, N.T., Cavosie, A.J., Wilde, S.A. Rudnick, R.L. and Valley, J.W. (2008). "Lithium in Jack Hills zircons: Evidence for extensive weathering of Earth's earliest crust". Earth and Planetary Science Letters 272 (3–4): 666–676.
Joe Hiess, Daniel J. Condon, Noah McLean, Stephen R. Noble 2012 "238U/235U Systematics in Terrestrial Uranium-Bearing Minerals" Science 30 March 2012: Vol. 335 no. 6076 pp. 1610-1614 DOI: 10.1126/science.1215507
A mean 238U/235U value of 137.818 ± 0.045 (2?) (compared to the earlier value of 137.88) in zircon samples reflects the average uranium isotopic composition and variability of terrestrial zircon. This distribution is broadly representative of the average crustal and “bulk Earth” 238U/235U composition.
Birger Rasmussen, Ian R. Fletcher, Janet R. Muhling, Courtney J. Gregory and Simon A. Wilde 2011 "Metamorphic replacement of mineral inclusions in detrital zircon from Jack Hills, Australia: Implications for the Hadean Earth" Geology, v. 39, p. 1143-1146
Later inclusions can alter the results of zircon chronologies. Note they would appear younger than they really are.
Dating the Earth/Moon system from the Moon
Another approach was to use lunar rocks recovered by the Apollo missions. These were thought to date to the Earth/Moon formation, and would not have been subjected to the same weathering as terrestrial rock.
Alberto E. Saal, Erik H. Hauri, James A. Van Orman, and Malcolm J. Rutherford 2013 "Hydrogen Isotopes in Lunar Volcanic Glasses and Melt Inclusions Reveal a Carbonaceous Chondrite Heritage" Science 14 June 2013: 340 (6138), 1317-1320
Tacked on the end:
The author of this news item either failed to read the original article, or failed to understand it. The issue is not whether the early Earth had liquid oceans, and at least some dry land in the Hadean, or early Archean. This has been established by stronger data than is used in Marty et al, "Nitrogen Isotopic Composition and Density of the Archean Atmosphere" (see 1-3 below). Nor did the Marty et al paper address the relevant time period for the origin of life. They analyzed fluid inclusions from quartz dated 3.0, and 3.5 billion years ago. We have strong evidence for life as early as 3.8 billion years ago (Ga), and confirmed fossil evidence as early as 3.5 Ga (see 4-6).
The result of this news item is to mislead people less familiar with origin of life research.
1) Wilde S.A., Valley J.W., Peck W.H. and Graham C.M. (2001). "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago" Nature 409 (6817): 175–8.
2) Mojzsis, S.J., Harrison, T.M., Pidgeon, R.T. (2001). "Oxygen-isotope evidence from ancient zircons for liquid water at the Earth's surface 4300 Myr ago". Nature 409 (6817): 178–181.
3) Ushikubo, T., Kita, N.T., Cavosie, A.J., Wilde, S.A. Rudnick, R.L. and Valley, J.W. (2008). "Lithium n Jack Hills zircons: Evidence for extensive weathering of Earth's earliest crust". Earth and Planetary Science Letters 272 (3–4): 666–676.
-------------- "Science is the horse that pulls the cart of philosophy."
L. Susskind, 2004 "SMOLIN VS. SUSSKIND: THE ANTHROPIC PRINCIPLE"
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