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Hf isotopes in rutile measured in-situ by LA-MC-ICPM

Tanya Ewing, Daniela Rubatto, Jörg Hermann and Steve Eggins

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia


Figure 1.Large thin section of garnet (grt) megacryst including numerous large rutiles (rt). Using LA-MC-ICPMS, Hf isotopes can be measured in-situ for rutiles in this thin section. Areas of interest for analysis have been circled in black pen.

 

Hf isotopes are commonly measured in zircon and used to trace input from different mantle reservoirs and crustal components. In contrast, the Hf isotope systematics of rutile (TiO2), a common accessory mineral in a variety of geological settings, are relatively unexplored.

The recent studies of Choukroun et al. (2005) and Aulbach et al. (2008) have revealed that rutile records a much greater range of 176Hf/177Hf ratios than is seen for zircon. These studies have also highlighted the potential for the in-situ measurement of Hf isotopes in rutile using Laser Ablation Multi-Collector Inductively Coupled Mass Spectrometers (MC-LA-ICPMS). However, as yet no detailed study of the accuracy of these in-situ measurements for rutile has been made. As rutile contains very low levels of Hf compared to zircon, proving the reliability of in-situ Hf isotope measurements is a critical first step in exploring the potential of this method to contribute to petrological studies.

We have used a number of approaches to assess the accuracy, precision and limitations of in-situ analysis for Hf isotopes in rutile, as well as refining analytical and data reduction methods for analysis of low levels of Hf. Some novel adaptations have been made, such as the use of synthetic rutiles doped with Hf to monitor change in mass bias over the course of a session and provide an external correction factor where required.

The accuracy of Hf isotope measurements for rutile on our Neptune MC-ICPMS in laser ablation mode is demonstrated by comparison with solution MC-ICPMS values for a rutile containing c.30ppm Hf. The 176Hf/177Hf values obtained from the two methods were in excellent agreement. Accuracy is confirmed by the agreement of 176Hf/177Hf values for plutonic rutile and zircon - which should record the same Hf isotope signature in an igneous system - from a single trondjhemite sample.

The precision of individual rutile analyses is lower for laser ablation than for solution analyses, but combining populations of 10 to 15 analyses to give a weighted mean significantly improves the precision. This level of precision has proved ample to distinguish between different rutile samples, or between rutile and other minerals that record a different Hf isotope signature.

It is already clear that in-situ Hf isotope measurements for rutile may have exciting applications to petrological problems. An early case study on rutile and zircon from the Duria garnet peridotite has demonstrated that Hf isotope analysis of metamorphic rutile can provide complementary information to isotopic information obtained from zircon. LA-MC-ICPMS analyses of zircon and rutile from the Duria peridotite revealed clearly distinct Hf isotopic ratios for the two minerals: the rutile records a mantle signature, whereas zircon isotopic ratios provide evidence for crustal input. This is in keeping with the metamorphic history of the peridotite as determined by Hermann et al. (2006) based on petrographic analysis and trace element geochemistry. The ability to analyse Hf isotopes in rutile therefore allows us to access isotopic information about parts of the metamorphic history that are not recorded by zircon.

Figure 2: Close-up photo of rutiles (rt) included in garnet (grt) in the thin section shown in Fig. 1.



Aulbach S, O’Reilly SY, Griffin WL, Pearson NJ (2008) The eclogite mantle reservoir: 176Hf/177Hf, Nb/Ta and Zr/Hf of rutile. Nature Geoscience 1:468-472

Choukroun M, O'Reilly SY, Griffin WL, Pearson NJ, Dawson JB (2005) Hf isotopes of MARID (mica-amphibole-rutile-ilmenite-diopside) rutile trace metasomatic processes in the lithospheric mantle. Geology 33(1):45-48

Hermann J, Rubatto D, Trommsdorff V (2006) Sub-solidus Oligocene zircon formation in garnet peridotite during fast decompression and fluid infiltration (Duria, Central Alps). Mineralogy and Petrology 88(1-2):181-206