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| Titel |
The decoupling of the Zr- and U-Pb systematics in rutile during cooling from HT conditions |
| VerfasserIn |
Ellen Kooijman, Klaus Mezger, Jasper Berndt, Matthijs Smit |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043819
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| Zusammenfassung |
| Rutile (TiO2) is a common accessory mineral in igneous and metamorphic rocks. It is stable
over a wide range of P - T conditions and is the main Ti-bearing phase in high-grade
metamorphic rocks. It shows temperature-dependent partitioning of Zr, providing a
potentially reliable single-mineral thermometer [1]. Because rutile can incorporate up to 200
ppm U, it can be dated with excellent analytical precision using the U-Pb system.
This makes the mineral highly suitable for constraining the history of metamorphic
rocks.
Under metamorphic conditions the U-Pb system in rutile may (re-)equilibrate with the
matrix by volume diffusion. The same may be expected for Zr and other trace elements. This
complicates interpretation of results from rutile thermometry and geochronology, because
Zr-in-rutile temperatures may neither represent peak conditions nor correspond to the U-Pb
ages determined in the same mineral grain. Although diffusivities of Zr and Pb in rutile have
been experimentally constrained [2,3], the relative behaviour of these elements
in natural metamorphic rutile is not well-characterized. To better understand the
Zr- and U-Pb systematics in rutile, we performed an EPMA (Zr) and LA-ICP-MS
(U-Pb) study on rutile grains from Archaean granulites from the Superior Province,
Canada.
Zirconium contents were determined along profiles through—and in cores and rims
of—15 rutile grains (120-280 μm) from 4 samples. The profiles show largely homogeneous
Zr concentrations and, with the exception of 3 grains, no significant difference in Zr content
between cores and rims. Some profiles display either a Zr decrease or increase of up to 200
ppm in the outermost 10-45 μm of grains, showing the effects of marginal volume
diffusion. Variations among grains within one thin section are large (600-3400 ppm). If
these concentrations are pristine, they would indicate highly variable crystallization
temperatures (690-870 Ë C) for grains of the same mineral paragenesis, which is
unlikely.
Laser ablation ICP-MS of 35-μm spot transects across the grains analyzed for Zr yielded
concordant U-Pb ages with core ages of ca. 2450 Ma and core-to-rim younging
towards ca. 2280 Ma. In contrast to the Zr profiles, age profiles show no core plateaus,
indicating that volume diffusion of Pb occurred throughout the grains. The ages
were used to construct closure temperature profiles (Tc(x); [4]), representing the
temperature of insignificant Pb diffusion as a function of position within the grains. The
Tc estimates show a core-to-rim decrease from ca. 640 Ë C (depending on grain
size) to an extrapolated value of ca. 490 Ë C. These estimates provide valuable
insight into the closure behaviour of the U-Pb system in rutile and allow a more
effective use of U-Pb rutile dating in constraining cooling histories of metamorphic
rocks.
This study shows that the U-Pb system and Zr distributions in rutile behave differently
during cooling from high temperature conditions and are to a variable degree affected by
volume diffusion. This decoupling indicates that U-Pb ages and Zr-in-rutile temperatures
cannot be correlated directly. Our study confirms the difference between Pb- and Zr diffusion
in rutile as predicted by experimental studies [3]. Zirconium contents are likely to be pristine,
but may vary significantly for cogenetic minerals due to currently unknown causes.
Therefore, care must be taken when applying Zr-in-rutile thermometry. In contrast, U-Pb
dating of rutile is a robust method for providing reliable time constraints on various stages of
cooling.
References: [1] Tomkins et al. (2007) J Metam Geol 25: 703-713; [2] Cherniak (2000)
Contrib Mineral Petrol 139: 198-207; [3] Cherniak et al. (2007) Earth Planet Sci Lett 261:
267-279; [4] Dodson (1986) Mat Sci Forum 7: 145-154. |
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