Untitled Document
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