HR: 0800h
AN: V41D-1479    [Abstracts]
TI: The Importance of Complete Sample Dissolution and Spike-Sample Equilibration on Lu-Hf Isotope Studies
AU: * Mahlen, N J
EM: mahlen@geology.wisc.edu
AF: University of Wisconsin-Madison, Dept Geology & Geophysics 1215 W Dayton St, Madison, WI 53706 United States
AU: Beard, B L
EM: beardb@geology.wisc.edu
AF: University of Wisconsin-Madison, Dept Geology & Geophysics 1215 W Dayton St, Madison, WI 53706 United States
AU: Johnson, C M
EM: clarkj@geology.wisc.edu
AF: University of Wisconsin-Madison, Dept Geology & Geophysics 1215 W Dayton St, Madison, WI 53706 United States
AU: Lapen, T J
EM: tjlapen@uh.edu
AF: University of Houston, Geosciences Dept 4800 Calhoun, Houston, TX 77204 United States
AB: Lu-Hf geochronology has gained attention due to its potential for precisely determining the age of garnet growth in a wide variety of rocks. A unique aspect of Lu-Hf analysis, however, is the disparate chemical behavior of Hf and Lu. For example, Hf is soluble in HF and Lu is soluble in HCl, which can create problems for spike-sample equilibration during dissolution as discussed by Unruh et al. 1984 JGR 89:B459 and later by Beard et al. 1998 GCA 62:525. Although partial dissolution may appear as an attractive means to preferentially dissolve garnet relative to refractory inclusions such as rutile and zircon, our investigations have shown that incomplete spike-sample equilibration may occur in such approaches. This leads to erroneous Lu and Hf contents that can adversely affect Lu-Hf isochron ages and calculated initial Hf isotope compositions. Dissolution of whole-rock powders using hot plates (low-pressure) or short-term microwave dissolution may produce inaccurate Lu-Hf isotope and concentration results, whereas high-temperature and -pressure dissolution in traditional Teflon steel-jacketed (Parr) bombs produces precise and accurate results. The greatest disparity in Lu-Hf isotope and concentration systematics of whole-rock powders among dissolution methods occurs for zircon- and garnet-bearing samples. In contrast, Sm-Nd isotope results are not affected by these different dissolution methods. Lu-Hf isochrons involving garnet may be affected by the dissolution method in a manner similar to that observed for whole-rock powders. Incomplete dissolution of garnet generally increases the measured Lu/Hf ratios, potentially increasing the precision of the isochron. In a number of lithologies, however, including garnet-bearing eclogites and amphibolites, significant errors may be introduced in the Lu-Hf age using hot plates (low-pressure) or short-term microwave dissolution, as compared to those obtained using high-temperature and -pressure dissolution bombs. These partial dissolution 'isochrons' are most apparent when compared with other geochronological methods. Moreover, initial Hf isotope compositions calculated from isochrons can be anomalous when hot plate or short-term microwave dissolution methods are used, thus confirming incomplete spike-sample equilibration.
DE: 1040 Radiogenic isotope geochemistry
DE: 1094 Instruments and techniques
DE: 1115 Radioisotope geochronology
DE: 1194 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005