U-Pb systematics of monazite and xenotime: case studies from the Paleoproterozoic of the Grand Canyon, Arizona |
| |
| Authors: | D P Hawkins Samuel A Bowring |
| |
| Institution: | (1) Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 01239, USA, US |
| |
| Abstract: | Monazite is accepted widely as an important U-Pb geochronometer in metamorphic terranes because it potentially preserves
prograde crystallization ages. However, recent studies have shown that the U-Pb isotopic system in monazite can be influenced
by a variety of processes that partially obscure the early growth history. In this paper, we attempt to interpret complex
monazite and xenotime U-Pb data from three Paleoproterozoic granite dikes exposed in the Grand Canyon. Single-crystal monazite
analyses from an unfoliated granite dike spread out along concordia from the crystallization age of the dike (defined by U-Pb
zircon data to be 1685 ± 1 Ma) to 1659 ± 2 Ma, a span of 26 million years. Back-scattered electron (BSE) imaging reveals
that magmatic domains within most crystals from this sample are truncated by secondary domains associated with prominent embayments
at the grain margin. Fragments of a single crystal yield contrasting, concordant dates and fragments from the edges and tips
of crystals yield the youngest dates. Based on these observations we suggest that the secondary domains formed at least 26
million years after the crystal formed. Monazite and xenotime dates from the second sample, a sheared dike that cross-cuts
the previous dike, spread out along concordia over 16 million years and range up to 2.4% normally discordant. Again, BSE imaging
reveals secondary domains that truncate both magmatic zoning and xenocrystic cores. Fragments sliced from specific domains
of a previously imaged monazite crystal demonstrate that the secondary domain is 13 million years younger than the core domain.
Textures revealed in BSE images suggest that the secondary domains formed by fluid-mineral interaction. Normal discordance
appears to result from both radiation damage accumulated at temperatures below 300 °C and water-mineral interaction. Monazite
data from the third sample exhibit dispersion in both the 207Pb/206Pb dates (1677–1690 Ma) and discordance (+ 1.6% to − 3.1%). Reverse discordance in these monazites cannot be explained by
incomplete dissolution or excess (thorogenic) 206Pb. Sliced fragments from several crystals reveal dramatic intragrain U-Pb disequilibrium that does not correlate with either
Th or U concentration or position within the crystal. We suggest that reverse discordance resulted from mechanisms that involve
exchange or fractionation of elemental U or elemental Pb, and that neither the U-Pb dates nor the 207Pb/206Pb dates are reliable indicators of the rock's crystallization age. Given the large number of processes proposed in the recent
literature to explain monazite U-Pb systematics from rocks of all ages, our results can be viewed as another cautionary note
for single-crystal and multi-crystal monazite geochronometry. However, we suggest that because individual crystals can preserve
a temporal record of primary and secondary monazite growth, micro-sampling of individual monazite crystals may provide precise
absolute ages on a variety of processes that operate during the prograde, peak and/or retrograde history of metamorphic terranes.
Received: 9 June 1996 / Accepted: 18 October 1996 |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
