Dissakisite-(Ce) chemical composition: some implications for its origins |
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| Authors: | Mihoko Hoshino Mitsuyoshi Kimata Norimasa Nishida Masahiro Shimizu |
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| Institution: | (1) Earth Evolution Sciences, Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba Ibaraki, 305-8572, Japan;(2) Chemical Analysis Division, Research Facility Centre for Science and Technology, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba Ibaraki, 305-8577, Japan;(3) Present address: Mineral Resource Research Group, Institute for Geo-Resources and Environment, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba Ibaraki, 305-8567, Japan |
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| Abstract: | Euhedral dissakisites from Trimouns dolomite mine, France, is compositionally zoned. Back-scattered electron (BSE) images
reveal that each of the described dissakisite crystals has three distinct compositional zones: normal zoned core (Mg-rich),
oscillatory zoned middle (Ca–Al-rich) and homogeneous rim (Fe–ΣREE-rich). The latter zone with Fe2+ > Mg corresponds to allanite-(Ce). Dissakisite-(Ce) also displays pronounced zoning in Fe/Mg which may suggest that the temperature
of crystallization continuously decreased from core to rim. Despite a systematic increase in Fe/Mg of the dissakisite with
an allanite rim, there is no monotonic decrease in the REE zoning: the normal zoned core and homogeneous rim are rich in La,
Ce and Pr, but the relatively REE-poor oscillatory zoned middle is relatively abundant in Y, Sm and Gd. Discontinuous variation
in REE content of the dissakisite, with the allanite rim may indicate a localized change in either the relative concentration
of various ligands or pH of the crystallizing fluid. Observations under the polarizing microscope confirm that the different
zones have simultaneous extinction. These chemical and optical observations suggest that epitaxial crystallization of dissakisite,
from Trimouns, passes through three formation stages. The ternary Fe2+–Mg2+–(Al + Fe)3+ diagram illustrates that in general dissakisite can be classified into two groups, (Al + Fe)3+-rich and an Mg-rich; dissakisite from Trimouns belongs to the former group. Chondrite-normalized REE patterns of dissakisites
from Trimouns are similar to those of allanites formed by hydrothermal fluids. In conclusion, it is clear from the above two
geochemical characteristics that dissakisites from Trimouns are of (Al + Fe)3+-type, and were derived from hydrothermal fluids. |
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