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Phosphate Lu–Hf geochronology

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  • Gry Barfod
  • Olga Otero, University of Lyon, Frankrig
  • Francis Albarede, ENS de Lyon, Frankrig
Igneous, metamorphic and biogenic apatite contains Lu and Hf in proportions that make this mineral suitable for dating by the 176Lu–176Hf method. We present a new method for separation of Lu and Hf from apatite that involves a single extraction column step for Hf and a second exchange column step for Lu. This procedure allows rapid sample processing prior to analysis by MC-ICP-MS. Results from the igneous Gardiner, Skaergaard and Khibina Intrusions indicate that the closure temperature for the Lu–Hf isotopic system in apatite is relatively high. The Lu–Hf isochron age of ∼60 Ma for the Skaergaard Intrusion, East Greenland, is older than the generally accepted emplacement age, but does not appear to be a product of mixing. A centimeter-sized apatite from the Otter Lake area, Grenvillian Province, yields high and variable 176Lu/177Hf ratios showing that metamorphic apatite may, in some cases, allow for mono-mineral dating. The most important application of the method is undoubtedly its potential use for dating sedimentary formations. Although we find biogenic apatite in porous matrix highly susceptible to post-depositional resetting of the Lu–Hf isotopic system, the preservation of a near-depositional age for a shark tooth from the impermeable London Clay and recent work on crystalline authigenic phosphorites [Earth Planet. Sci. Lett. 201 (2002) 203] show the Lu–Hf system to be a promising tool for direct dating of sedimentary formations.
TidsskriftChemical Geology
Sider (fra-til)241-253
Antal sider12
StatusUdgivet - 2003
Eksternt udgivetJa

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