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PhD research – Hafnium isotopes and U-Pb geochronology of metamorphic rutile

The use of isotopic and geochronological techniques to help unravel the geological history of metamorphic terranes, and to constrain the timing of burial and exhumation, is now well established. However, such techniques often focus on accessory phases such as zircon, for which the reactions leading to formation are not well understood.

Large rutile crystal (pen for scale)

It can therefore be difficult to relate periods of zircon growth to geological events with any certainty. In contrast, rutile is a common metamorphic mineral that forms according to well-known reactions, and it is relatively simple to determine the metamorphic event it relates to. Furthermore, rutile has a relatively high closure temperature of c.600°C (Cherniak, 2000; Vry & Baker, 2006) and appears to be resistant to subsequent resetting of Pb values (Vry & Baker, 2006). As such it has the potential to provide reliable U-Pb ages that are easily related to a metamorphic event, and record the timing of cooling through temperatures of c.600°C - a powerful tool for reconstructing metamorphic histories. Additionally, high-temperature rutile contains measurable amounts of hafnium (Hf), and preliminary work has revealed vast variations in 176Hf/177Hf ratios for rutiles from different geological environments. This raises the exciting possibility that hafnium isotopic ratios in rutiles may provide information on the setting they formed in. If hafnium isotope work and U-Pb dating can be developed for rutile, this widespread metamorphic mineral will provide key information for deciphering the history of high-grade metamorphic terranes.

The aims of this study are therefore:

  1. to develop a protocol for measuring hafnium isotopes in rutile
  2. to evaluate the signicance of hafnium isotope ratios in this mineral, and their response to metamorphism
  3. to find an appropriate rutile standard for both U-Pb dating and hafnium isotope work.
A key question is whether it is possible to reliably measure Hf isotopes in rutile; initial MC-ICPMS work by Münker et al. (2001) showed a drastic deviation of measured 176Hf/177Hf ratios from the true value in samples with significant amounts of titanium (Ti/Hf ratios >10). As rutile is almost pure TiO2, this is clearly a serious consideration for our study. However, the causes of this phenomenon are poorly understood and it is unclear to what extent it is applicable to measurements on other instruments. We are confident that with further investigation we will be able to better understand, and overcome, this problem and succeed in measuring hafnium isotopes with accuracy.

Another important question is the significance of the variations in 176Hf/177Hf observed in our pilot study. Do hafnium isotopic ratios really reflect the geological environment in which they formed, or are the differences random and due to subsequent alteration? This question will be addressed by measuring hafnium isotope compositions for an extensive suite of samples formed in a variety of known environments. Similarly, the effect of metamorphism can be examined by measuring the hafnium compositions across a single unit which has been variably metamorphosed.

Conventional U-Pb dating of rutile has already been undertaken in numerous studies, but as yet no rutile standard exists for in-situ U-Pb dating by Sensitive High Resolution MicroProbe (SHRIMP) or Laser Ablation ICP-MS. It is important to have the capability to date individual grains - and part thereof - using these techniques, as rutile often encloses inclusions of other minerals, or contains exolved ilmenite within it. Each conventional U-Pb analysis uses multiple grains and so averages the true age with any impure components.

I am undertaking this research for my PhD as part of the Earth Chemistry group at RSES, under the supervision of Drs Daniela Rubatto, Jörg Hermann, Ian Buick, and Trevor Ireland.

Cherniak DJ, 2000: Pb diffusion in rutile. Contributions to Mineralogy and Petrology, 139: 198-207.
Münker C, Weyer S, Scherer E, and Mezger K, 2001: Separation of high field strength elements (Nb, Ta, Zr, Hf) and Lu from rock samples for MC-ICPMS measurements. Geochemistry Geophysics Geosystems, 2 (12): 183-201.
Vry JK, and Baker JA, 2006: LA-MC-ICPMS Pb-Pb dating of rutile from slowly cooked granulites: Confirmation of the high closure temperature for Pb diffusion in rutile. Geochimica et Cosmochimica Acta, 70: 1807-1820.

 

 

 


Geochemistry of granitoids

Previous research – Geology of Fiordland, New Zealand

In 2003 I was awarded an MSc (hons) in Geology from the University of Canterbury, New Zealand. My thesis, Provenance of the Loch Burn Fm, Eastern Fiordland: Implications for the MTZ, began with over 5 weeks field work in the beautiful wilderness (and torrential rain) of southern New Zealand. Lab-work incorporated petrography, geochemistry, and SHRIMP dating. The project investigated the source of clasts from the volcano-sedimentary Loch Burn Formation, and examined the implications for our interpretation of New Zealand basement geology. A paper based on this work (Loch Burn Formation, Fiordland, New Zealand: SHRIMP U-Pb ages, geochemistry and provenance) has just been published online in the New Zealand Journal of Geology and Geophysics, and will be in print in September. (See Publications for the full reference, or view the online abstract here).

North Fiord, Lake Te Anau

I have an ongoing interested in the geology of this part of New Zealand (Fiordland), which is still rather poorly understood because of its steep mountains, impenetrable bush, heavy rainfall, and general inaccessability. In the recent past I was involved in several attempts here at the RSES to better constrain the age of the notoriously ambiguous Kakapo Granite, also from Fiordland. In spite of our best efforts, the real igneous age of this granite still remains elusive.
The granite appears to record more than one zircon formation event, but as yet we have not succeeded in resolving the two (or more) ages individually. Furthermore, it is not clear whether the granite is recording crystallisation and subsqeuent metamorphism and/or lead-loss (in which case the older age would be magmatic),

U-Pb geochronology

or crystallisation with a signifcant amount of inherited zircon slightly older than the magmatic age (in which case the magmatic age would be the youngerage). Clearly, yet more work is required to unravel the history of this enigmatic unit