Redesign of SHRIMP mounts to minimize geometric effects on isotopic and inter-elemental fractionation.

 

Joe M. Hiess 1 , Ian S. Williams 1 and Peter Holden 1

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

 

Development of procedures for the analysis of oxygen isotopes on SHRIMP II at the ANU has been hindered by variable isotopic fractionation related to the geometry of conventional sample mounts. Temora zircons mounted as a 2 mm-spaced grid have revealed a >10‰ range in 18 O/ 16 O fractionation across the mount surface. Such spot-to-spot differences are unacceptable for accurate and precise analysis of insulators. The same grid has been used to map correlations in geometric effects on Pb/UO 2 , U/UO 2 and QT 1 Y which can be used to improve corrections for inter-element fractionation.

Possible causes of these fractionations include topography at the metal edge of the mount holder, and the transition from epoxy to metal at that edge. Two new mounts were designed to test these hypotheses. An epoxy mount was recessed into a modified conventional mount holder to remove topographic edge effects but retain the epoxy-metal transition. 18 O/ 16 O fractionation was reduced (range ~5‰) and more systematic, but not eliminated, with progressively heavier compositions towards the edges of the mount.

A new “Mega mount” design was also tested to remove both effects. This design involved an epoxy disc 35 mm diameter screwed onto the face of the mount holder (Fig 1), increasing the area of the equi-potential surface and eliminating the epoxy-metal transition. Both standard insulating epoxy and conductive epoxy were tested. Conductive epoxy guaranteed conductivity, but at the expense of transmitted light imaging. It consisted of 1 part epoxy resin : 0.5 parts spectroscopic carbon powder : 0.12 parts epoxy hardener. Conductive epoxy proved unnecessary, however, provided that conductive adhesive tape was used to link the gold coat on the analytical face to the mount holder. Substantial improvements in the uniformity of U-Pb and 18 O/ 16 O fractionation across the mount surface have been obtained during initial testing, the range in 18 O/ 16 O fractionation approaching the detection limits of the experiment (± 0.5‰).

Figure 1. New Mega mounts assembled with both insulating and conductive epoxy.