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tokens: We thank Sebastian Lopez for initial discussions about using Zn and Cr ii to measure Δα/α, and Julija Bagdonaite and Jonathan Whitmore for assistance with Fig. 15 . MTM thanks the Australian Research Council for Discovery Projects grant DP110100866 which supported this work. Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. We wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Other data presented herein were based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme IDs 65.O-0158, 67.A-0022, 072.A-0346, 074.A-0201, 079.A-0600, 082.A-0682, 082.A-0569, 083.A-0874 and 084.A-0136.