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Zone-refined rhenium for U-Pb

We now only use zone-refined (99.999%) rhenium ribbon from H. Cross for our U-Pb zircon analysis.  Total common Pb contents of our radiogenic Pb analyses are more consistent and lower, in the range of 0.3 to 0.5 pg; prior to the switch, use of 99.98% Re gave us more variable common Pb contents with occasional excursions over 1 pg. We outgas the welded filaments at 2A for 15 minutes, and then 4A for 30 minutes, allowing them to cool prior to removal from the outgasser; filaments are left at atmosphere for at least one week to avoid spreading of the load over the filament. We have found that the longer the filaments sit after degassing, the better the samples run!