HR: 14:20h
AN: V33E-03    [Abstracts]
TI: Developing New Methods for Microsampling and Sm/Nd Dating of Zoned Garnet
AU: * Pollington, A D
EM: apolling@bu.edu
AF: Boston University, Department of Earth Sciences, 675 Commonwealth Avenue, Boston, MA 02215, United States
AU: Baxter, E F
EM: efb@bu.edu
AF: Boston University, Department of Earth Sciences, 675 Commonwealth Avenue, Boston, MA 02215, United States
AB: Garnets provide one of the Earth Science community's most useful tools for studying rates, duration and timing of crustal processes. In this study we describe new techniques for fine sampling of multiple growth zones of garnet and Sm/Nd dating of each individual zone. We test these techniques on large (>5cm) garnets from a shear zone in the Tauern Window of Austria where we seek to quantify the growth history of garnet in a manner similar to dating tree rings. Microsampling permits a more precise quantification of duration, episodicity and kinetics of metamorphic reactions. Past studies of garnet growth duration – based on core and rim garnet ages – have been limited by sampling methods for extracting discrete, and accurate, growth zones. Modeling of radial growth symmetry in garnet shows that previous studies may underestimate garnet growth duration by as much as 50%. We are able to dramatically improve microsampling by using microdrilling guided by chemical maps of the garnet composition. This provides much improved precision and accuracy in sampling. By using chemical mapping of the garnet we can be sure that we are correctly sampling narrow (~500 micron wide) growth (i.e. age) zones rather than smearing and averaging multiple growth zones together. In principle, tens of growth zones (and ages) spanning the entire interval of garnet growth may be sampled and resolved. Microdrilled domains, the results of which are an ultrafine powder, are drilled and collected in water. Due to the adverse geochronological effect of unavoidable micro-inclusions in garnet, we have tested several partial dissolution techniques to cleanse the garnet of inclusions and yield higher ¹⁴⁷Sm/¹⁴⁴Nd and hence, more precise ages. Analysis of a finely crushed bulk Tauern Window garnet sample after HF/HClO₃ cleansing indicates that ¹⁴⁷Sm/¹⁴⁴Nd at least as high as 0.89 is attainable in this particular sample, but cleansing efforts on microdrilled powders have thus far failed to yield such high ratios suggesting that the powders respond in unexpectedly different ways to our standard garnet cleansing procedures. Preliminary Sm/Nd age analysis of bulk garnet confirms a ~25Ma age for garnet growth. Progress in our inclusion cleansing procedures on microdrilled powders will be reported.
DE: 1040 Radiogenic isotope geochemistry
DE: 1094 Instruments and techniques
DE: 1115 Radioisotope geochronology
DE: 3653 Fluid flow
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting