| 1. |
- Chew, D M, et al.
(author)
-
The tectonothermal evolution and provenance of the Tyrone Central Inlier, Ireland: Grampian imbrication of an outboard Laurentian microcontinent?
- 2008
-
In: Journal of the Geological Society. - : Geological Society of London. - 2041-479X .- 0016-7649. ; 165:3, s. 675-685
-
Journal article (peer-reviewed)abstract
- The Tyrone Central Inlier is a metamorphic terrane of uncertain affinity situated outboard of the main Dalradian outcrop (south of the Fair Head-Clew Bay Line) and could represent sub-arc basement to part of the enigmatic Midland Valley Terrane. Using a combination of isotopic, structural and petrographic evidence, the tectonothermal evolution of the Tyrone Central Inlier was investigated. Sillimanite-bearing 14 metamorphic assemblages (c. 670 degrees C, 6.8 kbar) and leucosomes in paragneisses are cut by granite pegmatites, which post-date two deformation fabrics. The leucosomes yield a weighted average Pb-207/Ph-206 zircon age of 467 +/- 12 Ma whereas the main fabric yields a Ar-40-Ar-39 biotite cooling age of 468 +/- 1.4 Ma. The pegmatites yield 457 +/- 7 Ma and 458 +/- 7 Ma Rb-Sr muscovite-feldspar ages and Ar-40-Ar-39 step-heating plateaux of 466 +/- 1 Ma and 468 +/- 1 Ma, respectively. The metasedimentary rocks yield Palaeoproterozoic Sm-Nd model ages and laser ablation inductively coupled plasma mass spectrometry detrital zircon U-Pb analyses from a psammitic gneiss yield age populations at 1.05-1.2, 1.5, 1.8, 2.7 and 3.1 Ga. Combined, these data permit correlation of the Tyrone Central Inlier with either the Argyll or the Southern Highland Group of the Dalradian Supergroup. The inlier was thus part of Laurentia onto which the Tyrone ophiolite was obducted.
|
|
| 2. |
- Ranebo, Ylva, et al.
(author)
-
Improved isotopic SIMS measurements of uranium particles for nuclear safeguard purposes
- 2009
-
In: Journal of Analytical Atomic Spectrometry. - : Royal Society of Chemistry (RSC). - 1364-5544 .- 0267-9477. ; 24:3, s. 277-287
-
Journal article (peer-reviewed)abstract
- The isotopic analysis of particles containing sub-pg to pg levels of uranium, released from nuclear material handling, has been proven as an efficient tool for international safeguard purposes. Precise and accurate measurement of both enrichment and the minor isotopes is, however, a challenging analytical task due to the low levels of material. One of the mainstay techniques for particle measurement is Secondary Ion Mass Spectrometry (SIMS), this study evaluates the analytical benefit of an alternative in the form of large geometry SIMS (LG-SIMS), which combines high transmission with high mass resolution. We report here that LG-SIMS instruments provide a significantly better measurement quality than the small geometry SIMS as almost all isobaric background interferences are removed at a high useful ion yield. Useful yield measurements, performed on uranium oxide particles with calibrated uranium content, showed an overall useful yield of 1.2% for the LG-SIMS at a mass resolution of 3000. These improvements were then demonstrated by comparing results from actual nuclear inspection samples measured on both instruments. Additional benefits include an increased ability to detect particles of interest in a dust matrix while simultaneously reducing the time of sample analysis. An evaluation on the performance of LG-SIMS compared to Thermal Ion Mass Spectrometry (TIMS) is also presented. This evaluation shows that LG-SIMS has an advantage due to its high ion yield but with a limitation in the detection limit of U-236 at higher enrichments due to the necessity for a hydrogen correction.
|
|
| 3. |
- Rimsa, Andrius, et al.
(author)
-
Constraints on incipient charnockite formation from zircon geochronology and rare earth element characteristics
- 2007
-
In: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 154:3, s. 357-369
-
Journal article (peer-reviewed)abstract
- We present high spatial resolution ion-microprobe U-Th-Pb geochronology and rare earth element (REE) data combined with cathodoluminescence (CL) and back-scattered electron (BSE) imaging for complex zircons in incipient charnockites from Sondrum, SW Sweden. Examination of closely paired samples across the dehydration zone demonstrates that incipient charnockite formation at Sondrum is a zircon-forming process. We determined the age of the dehydration event (i.e. charnockitisation) to 1,397 +/- 4 Ma (2 sigma, MSWD = 1.7). This is the first successful attempt to date incipient charnockite formation using U-Pb systematics of zircon. Internal structure, chemical and isotopic characteristics of zircon indicate that the granitic pegmatite in the core of the incipient charnockite is a melting zone. Commonly observed bulk rock HREE depletion in incipient charnockites is not caused by zircon dissolution but by involvement of garnet as a reactant in the dehydration reactions. Moreover, REE patterns of the newly formed zircon are HREE-enriched, indicating non-concurrent growth and suggesting that the degree of charnockite depletion in HREE might be controlled by the volume of newly formed zircon.
|
|
| 4. |
- Slama, J, et al.
(author)
-
Plešovice zircon – A new natural standard for U-Pb and Hf isotopic microanalysis
- 2007
-
In: Goldschmidt Conference Abstracts.
-
Conference paper (other academic/artistic)abstract
- Increasing number of geological applications requires theuse of well-characterized and widely available referencematerials for in situ analysis. We have established a newnatural zircon standard (“Plešovice”) for U-Pb dating and Hfisotopic analysis by laser ablation ICP-MS.The ID-TIMS U-Pb age of the Plešovice zircon has beendetermined to 336.9±0.2 Ma (95% confidence limits; mean206Pb/238U age). The U-Pb ages obtained by LA ICP-MS (3labs) and SIMS techniques show larger spread but within theiranalytical uncertainties they are consistent with the TIMS age.Hafnium isotopic composition of the Plešovice zircon appearsto be homogenous within and between grains. Combined laserablation and solution MC ICP-MS analyses gave a mean176Hf/177Hf value of 0.282481±0.000013 (95% confidencelimits, 87 analyses). Cathodoluminescence and BSE imagingand chemical analyses revealed that discrete zones in thePlešovice zircon are enriched in trace elements and especiallyin U and Th. Raman spectroscopy suggests that these highlyradiation-damaged areas have not undergone any annealing.Our LA ICP-MS analyses did not indicate any Pb-loss but thehigh intensities of measured U signal require that these areasare avoided during routine U-Pb isotopic analysis.
|
|
