Characteristics of two types of zircon from a single rock sample from the Mt. Malosa Massif, Malawi, are presented. According to Raman spectroscopy, the structure of a yellow hydrothermal zircon grown within a miarolitic cavity seems to be highly disordered. However, the extent of radiation damage calculated from the Raman data exceeds the degree of disorder that can be ascribed to the maximum possible internal radiation dose received by the crystals since their formation. Optical birefringence suggests an ordering degree close to that of well-ordered zircon for most areas in the crystals as may be also concluded from the always sharp electron backscattered diffraction (EBSD) patterns. Brownish zircon crystals from the wall-rock of the cavity of the same sample are most probably magmatic in origin. These crystals display a high degree of order and common low contents of radioactive and other substituting elements (<0.5 wt% in total). The absolute ages and U and Th contents of both zircon types were verified by U–Pb sensitive high-resolution ion microprobe (SHRIMP) analysis and the magmatic zircon is found to be significantly older (292 ± 10 Ma) than the hydrothermal one (121 ± 9 Ma). The apparent discrepancies between the data obtained by various methods for the hydrothermal zircon may be explained taking into account the unusual chemical composition and structural particularities established as well as (re-)interpreting the physical meaning of the variations in band positions and band widths in the Raman spectra. Electron probe microanalysis (EPMA) shows relatively high contents of yttrium (up to 4.8 wt% Y2O3) and the rare earth elements (REE, up to 1.5 wt% oxides in total), occasionally also of thorium (up to 0.8 wt% ThO2). The charge unbalance caused by the substitution of trivalent elements is not compensated by coupled substitution involving other elements like phosphorus, niobium or uranium. The unusual composition and substitution is reflected by the related cathodoluminescence (CL) spectra consisting of sharp and intense line emissions from Dy3+, Sm3+, Nd3+ and Tb3+ centers in zircon. The broad band emissions normally observed in well-ordered zircon are quenched. Orientation contrast imaging (OCI) and EBSD reveal textures consisting of relatively large strained orientation domains showing a distinct distribution pattern. We suggest that internal tensional strain induced by substitution, vacancy formation and related domain formation may explain the data. Possible consequences of these findings for the understanding of the formation of metamict zircon should be considered in future research.
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Research Article|July 01, 2018
Substitution-induced internal strain and high disorder in weakly radiation damaged hydrothermal zircon from Mt. Malosa, Malawi
Ulf Kempe
Ulf Kempe
1Institut für Mineralogie, TU Bergakademie Freiberg, Brennhausgasse 14, 09596Freiberg, Germany
Corresponding author, e-mail: kempe@mineral.tu-freiberg.de
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Ghislain Trullenque
Ghislain Trullenque
2Oxford Instruments GmbH, Otto-von-Guericke-Ring10, 65205Wiesbaden, Germany
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Rainer Thomas
Rainer Thomas
3Deutsches GeoForschungZentrum GFZ Potsdam, Telegrafenberg, 14473Potsdam, Germany
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Sergey Sergeev
Sergey Sergeev
4Centre of Isotopic Research,Karpinsky Russian Geological Research Institute (VSEGEI), Srednyi prospect74, 199106St. Petersburg, Russia6Institute of Earth Sciences, St. Petersburg State University, Universitetskaya nab. 7/9, 199034St. Petersburg, Russia
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Sergej Presnyakov
Sergej Presnyakov
4Centre of Isotopic Research,Karpinsky Russian Geological Research Institute (VSEGEI), Srednyi prospect74, 199106St. Petersburg, Russia
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Nikolai Rodionov
Nikolai Rodionov
4Centre of Isotopic Research,Karpinsky Russian Geological Research Institute (VSEGEI), Srednyi prospect74, 199106St. Petersburg, Russia
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Cameliu Himcinschi
Cameliu Himcinschi
5TU Bergakademie Freiberg, Institut für Theoretische Physik, Leipziger Straße23, 09596Freiberg, Germany
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European Journal of Mineralogy (2018) 30 (4): 659-679.
Article history
received:
13 Apr 2017
rev-recd:
26 Sep 2017
accepted:
30 Jan 2018
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CiteCitation
Ulf Kempe, Ghislain Trullenque, Rainer Thomas, Sergey Sergeev, Sergej Presnyakov, Nikolai Rodionov, Cameliu Himcinschi; Substitution-induced internal strain and high disorder in weakly radiation damaged hydrothermal zircon from Mt. Malosa, Malawi. European Journal of Mineralogy ; 30 (4): 659–679. doi: https://doi.org/10.1127/ejm/2018/0030-2739
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Index Terms/Descriptors
- absolute age
- actinides
- Africa
- cathodoluminescence
- chemical composition
- East Africa
- electron diffraction data
- electron probe data
- hydrothermal conditions
- ion probe data
- Malawi
- mass spectra
- metals
- metamictization
- nesosilicates
- optical properties
- order-disorder
- orthosilicates
- Paleozoic
- Permian
- radiation damage
- Raman spectra
- rare earths
- SHRIMP data
- silicates
- spectra
- strain
- substitution
- thorium
- U/Pb
- uranium
- yttrium
- zircon
- zircon group
- miarolitic cavities
- Mount Malosa
- EBSD data
Latitude & Longitude
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