Lu-Hf and PbSL geochronology of apatites from Proterozoic terranes: A first look at Lu-Hf isotopic closure in metamorphic apatite

• Published in: Geochimica et cosmochimica acta 2005-04, Vol.69 (7), p.1847-1859
• Main Authors: Barfod, Gry Hoffmann, Krogstad, Eirik Jens, Frei, Robert, Albarède, Francis
• Format: Article
• Language: English
• Subjects: Earth Sciences; Geochemistry; Sciences of the Universe
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2004.09.014

The mineral apatite is characterized by elevated and highly variable Lu/Hf ratios that, in some cases, allow for single-crystal dating by the Lu-Hf isotopic system. Apatites from the Adirondack Lowlands and Otter Lake area in the Grenville Province, and from the Black Hills, South Dakota, yield Lu-Hf ages that are consistently older than their respective Pb step leaching ages. Isotopic closure for the Lu-Hf system, therefore, occurs before U-Pb system closure in this mineral. In the Adirondack Lowlands, where H 2O activity was low, Lu-Hf systematics of cm-sized apatite crystals remained undisturbed during upper amphibolite facies metamorphism (∼700 to 675 °C) at 1170–1130 Ma. The relatively old Lu-Hf ages of 1270 and 1230 Ma observed for these apatites correlate with decreasing crystal size. In contrast, apatite from the fluid-rich Otter Lake area and Black Hills yields unrealistically low apparent Lu-Hf closure temperatures, implying that in these apatites, fluids facilitated late exchange. The Lu-Hf ages for the metamorphic apatites were thus controlled either by the prevailing temperature and grain size, or by fluid activity.