HIGH CLOSURE TEMPERATURES OF THE U-PB SYSTEM IN LARGE APATITES FROM THE TIN MOUNTAIN PEGMATITE, BLACK-HILLS, SOUTH-DAKOTA, USA

被引:96
作者
KROGSTAD, EJ
WALKER, RJ
机构
[1] Isotope Geochemistry Laboratory, Department of Geology, University of Maryland, College Park
关键词
D O I
10.1016/0016-7037(94)90367-0
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
The cores of several large (1-2 cm) apatites from the Early Proterozoic Tin Mountain pegmatite, Black Hills, South Dakota, have concordant U-Pb ages. The age uncertainties of these samples are less than 5 Ma. The lead isotopic compositions of muscovite and K-feldspar coexisting with the apatites were used for initial lead isotopic corrections. The resulting U-Pb concordia ages agree well with U-Pb and Pb-Pb isochron ages, based solely on multiple fragments from single apatites. This agreement justifies the use of coexisting muscovite and K-feldspar for initial lead corrections. The precision of these U-Pb ages is comparable to those of the U-Pb systems of more commonly analyzed minerals such as zircon, monazite, and sphene. The estimated U-Pb closure temperature (T-c) of these apatites is about 620 degrees C. This T-c is in close agreement with the T-c calculated for 0.5 cm diffusion radii and published data for diffusivity and activation energy of lead diffusion in apatite. Apatite can therefore yield high precision U-Pb ages that are close to its crystallization age in igneous rocks.
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页码:3845 / 3853
页数:9
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