NATURE|Vol 435|30 June 2005NEWS & VIEWS1171granulite lenses in keeping with a knownmountain-building event 930 million yearsago10— even though the temperatures re-quired for eclogite formation 425 million yearsago1–5should have obliterated any earlier signal. The paradoxical combination of gran-ulite preservation, high-temperature eclogiteformation and the brittle features of the eclo-gites has led several authors to suggest that theBergen Arc granulite–eclogite transformationoccurred during short-lived fluid-flow eventsover less than a million years8. But the evenshorter timescales proposed by Camacho etal.6will make many geologists draw breath. Camacho and colleagues used argon–argon(40Ar–39Ar) dating to measure the ages of phlogopite and amphibole mineral grainsfrom the same untransformed granulite lensesthat were investigated in the earlier 87Rb–87Srwork7. This technique works by creating theshort-lived argon isotope 39Ar through theirradiation of potassium (39K) in mineralgrains with neutrons. The age of the grains canthen be ascertained from the ratio of neutron-induced 39Ar to stable argon gas, 40Ar, con-tained in them. (40Ar forms from the decay ofthe radioactive potassium isotope 40K, and itsabundance indicates the elapsed time since thetemperature was last high enough that argoncould diffuse rapidly through the mineral,escaping at the boundaries between grains.)The particular advance of Camacho et al. is theuse of an ultraviolet laser technique to mea-sure profiles of ages across individual mineralgrains ascertained using the argon–argontechnique. The ages of between 820 and 895 million years that they find confirm therubidium–strontium results, and demonstratejust how little the granulite lenses were affectedby the later eclogite formation.
This is the end of the preview.
access the rest of the document.