HR: 17:00h
AN: V34C-05    [Abstracts]
TI: U/Pb Rutile Dating in Granulite-Facies Rocks by LA-ICP-MS
AU: * Zack, T
EM: tzack@min.uni-heidelberg.de
AF: Mineralogisches Institut, Universitaet Heidelberg, INF 236, Heidelberg, 69120, Germany
AU: Luvizotto, G L
EM: gluvizot@min.uni-heidelberg.de
AF: Mineralogisches Institut, Universitaet Heidelberg, INF 236, Heidelberg, 69120, Germany
AU: Barth, M
EM: barthm@uni-mainz.de
AF: Institut fuer Geowissenschaften, Universitaet Mainz, Becher Weg 21, Mainz, 55099, Germany
AU: Stockli, D F
EM: stockli@ku.edu
AF: Department of Geology, University of Kansas, 1475 Jayhawk Boulevard, Lawrence, KS 66045,
AB: Rutile is a common accessory phase in several metamorphic rock types (mafic, pelitic and felsic protoliths; common at blueschist-, eclogite- and granulite-facies conditions) as well as one of the most stable minerals in sedimentary environments. Although it is virtually omnipresent, it has yet not attracted the attention in geochronology that it could have, mostly due to a frustrating high variability in U concentrations (<0.01 to >100 ppm). Especially rutiles in eclogites (where rutile is one of the few potentially datable minerals) are notoriously low in U. To overcome these obstacles, we have chosen a simple, pragmatic approach, namely scanning a thin section for high-U rutiles by short (10 sec) analyses of trace element concentrations (besides U also V, Cr, Zr, Nb, Sn, Hf, Ta, W, Pb and Th) with a 10 μm laser spot (New Wave 213 nm Nd:YAG laser) coupled with an Argilent 7500ce ICP-MS. The programmable stage control allows easy reanalysis of the highest U rutiles for U/Pb dating (conducted with a 40-90 μm laser spot for 30 sec). A high degree of accuracy is reached by standardizing against a natural homogeneous rutile with high U concentrations (40 ppm) and a TIMS age of 1095 Ma. Results of this study show that U concentrations are roughly positively correlated with Zr (as a proxy for temperature; Zack et al., 2004), potentially due to the fact that increasing temperature makes one phase less favourable for element incorporation over another (e.g. monazite and zircon). In general, granulite-facies rocks appear to be good samples for U/Pb rutile dating as they commonly contain rutiles with >20 ppm U. From an analytical point of view, rutile is a particularly attractive phase for laser ablation ICP-MS dating for two reasons: 1) it forms often large (as compared to e.g. zircon), homogeneous single grains of >200 microns, therefore allowing larger spots sizes and hence better precision and 2) it mostly contains virtually no Th (Th/U ratios commonly <0.001), hence allowing for common lead correction via ²⁰⁸Pb. The second point is especially relevant as ²⁰⁸Pb is ca. 40 times more abundant than ²⁰⁴Pb and has no interferences with isotopes in the argon gas (e.g. ²⁰⁴Hg). With the analytical strategies outlined above, U/Pb rutile analysis offers new aspects of dating granulites. Single grain analysis of regular thin sections allows controlled dating both texturally and chemically. We are therefore confident that we can address the discrepancy of closure temperature estimates between nature (ca 450^oC; e.g. Mezger et al. 1989) and experiment (ca 600^oC; Cherniak 2000) by examining age differences of inclusion vs matrix rutiles (as for Zr in rutile; Zack et al. 2004) in various granulites.
DE: 1115 Radioisotope geochronology
DE: 3625 Petrography, microstructures, and textures
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting