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Current ResearchPhD research – Hafnium isotopes and U-Pb geochronology of metamorphic rutileThe 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.
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 ZealandIn 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 |
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