Monazite chemical composition: some implications for monazite geochronology |
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Authors: | X K Zhu R K O'Nions |
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Institution: | (1) Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR, UK Tel.: 44 1865 282116; Fax: 44 1865 272072; E-mail: xiangkun.zhu@earth.ox.ac.uk, GB |
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Abstract: | An investigation of the chemical composition of monazite from a number of localities has been carried out. Samples used include
monazites from metamorphic rocks, granitic rocks and a hydrothermal ore deposit. The REE distribution pattern of monazite
varies greatly in accordance with its geological environment. A remarkable feature of the monazites studied is that their
chondrite-normalised REE distribution patterns are mostly uniform between grains within the same sample, but differ significantly
from sample to sample. This characteristic apparently indicates that there is an important effect on the REE distribution
of monazite exerted by the host rock or source material from which monazite crystallised. Another important feature shown
by the monazites studied is that monazites in rocks containing garnet as a major mineral show extreme depletion of HREE, whereas
monazites in rocks without garnet or monazite that formed after the garnet breakdown contain significantly higher amounts
of Y and HREE. This suggests that the phase assemblage, especially garnet, plays an important role in the REE distribution
of monazites in these rocks. The value of REE distribution in monazite is exemplified with regard to the origin of monazite
in the Lewisian metamorphic rocks, which is a fundamental issue in monazite geochronology.
Received: 17 March 1999 / Accepted: 16 July 1999 |
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