HR: 0800h
AN: V31D-0689    [Abstracts]
TI: Site occupancy and distribution of Ti in zircon
AU: * Tailby, N
EM: nick.tailby@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Jaeger 1, Australian National University, Canberra, ACT 0200, Australia
AU: Mavrogenes, J
EM: mavro@ems.anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Jaeger 1, Australian National University, Canberra, ACT 0200, Australia
AU: Hermann, J
EM: joerg.hermann@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Jaeger 1, Australian National University, Canberra, ACT 0200, Australia
AU: Evans, K
EM: katy.evans@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Jaeger 1, Australian National University, Canberra, ACT 0200, Australia
AU: O'Neill, H
EM: hugh.oneill@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Jaeger 1, Australian National University, Canberra, ACT 0200, Australia
AB: Application of the Ti-in-zircon thermometer requires an understanding Ti site occupancy in zircon, the influence of chemical variables on Ti-saturation and other parameters that may influence Ti-distribution within zircon. Zircon was synthesized in phase assemblages in which Ti and Si activity was varied systematically. Ti K-XANES spectra were used to directly determine Ti site occupancy in synthetic zircons. The energy and intensity of the pre-edge feature clearly shows that Ti predominately resides on the Si site in zircon. Thus Si activity directly controls Ti solubility in zircon. The study of zircons crystallized under different TiO2-ZrO2-SiO2 phase assemblages can also be used to critically evaluate the difficulties and concerns surrounding the nucleation and crystallization of buffering-phases within experimental systems. In addition to site occupancy, this study has focused on the distribution of Ti within natural and experimentally produced zircon populations. Some synthetic zircons show notable Ti sector zoning in the absence of any correlative SEM-CL zoning. In addition to experimental systems, natural zircons studied in this program have shown that Ti-distribution can show antithetic or non-correlative distribution with regard to SEM-CL signal. This is not unexpected, so long as factors controlling Ti-saturation and saturation of CL-activating elements are decoupled during the growth of zircon, and if Ti occupies a different site to elements responsible for CL-activators. This research does not invalidate the use of zircon thermometry, but addresses a number of concerns regarding how, when and where such thermometry may be applied. Because other factors also influence Ti-saturation in zircon, attempts should be made to estimate Si and Ti activity during zircon growth. Furthermore, this research brings into question the use of common zircon mapping techniques - such as BSE and CL imaging - for the purpose of targeting and mapping Ti-distribution.
DE: 1012 Reactions and phase equilibria (3612, 8412)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting