NATURE | Vol 435 | 30 June 2005 1171 granulite lenses in keeping with a known mountain-building event 930 million years ago 10 — even though the temperatures re-quired for eclogite formation 425 million years ago 1–5 should have obliterated any earlier signal. The paradoxical combination of gran-ulite preservation, high-temperature eclogite formation and the brittle features of the eclo-gites has led several authors to suggest that the Bergen Arc granulite–eclogite transformation occurred during short-lived fluid-flow events over less than a million years 8 . But the even shorter timescales proposed by Camacho et al. 6 will make many geologists draw breath. Camacho and colleagues used argon–argon ( 40 Ar– 39 Ar) dating to measure the ages of phlogopite and amphibole mineral grains from the same untransformed granulite lenses that were investigated in the earlier 87 Rb– 87 Sr work 7 . This technique works by creating the short-lived argon isotope 39 Ar through the irradiation of potassium ( 39 K) in mineral grains with neutrons. The age of the grains can then be ascertained from the ratio of neutron-induced 39 Ar to stable argon gas, 40 Ar, con-tained in them. ( 40 Ar forms from the decay of the radioactive potassium isotope 40 K, and its abundance indicates the elapsed time since the temperature was last high enough that argon could diffuse rapidly through the mineral, escaping at the boundaries between grains.) The particular advance of Camacho et al. is the use of an ultraviolet laser technique to mea-sure profiles of ages across individual mineral grains ascertained using the argon–argon technique. The ages of between 820 and 895 million years that they find confirm the rubidium–strontium results, and demonstrate just how little the granulite lenses were affected by the later eclogite formation. The authors
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