40Ar/39Ar Laboratory

Morgan Ganerød

Camilla Wilkinson

argon lab

Placing geological events and geological processes in chronological order is critical to understand the evolution of the earth. Many different types of so-called dating methods exist: some provide relative ages while others provide absolute ages. The most commonly used absolute dating methods are based on the radioactive decay of chemical elements. Parent nuclides will decay and become daughter products, and the ratio between the two tells us how much time has elapsed since a certain geological event, for example a volcanic eruption, the formation of an ore body, or the break-up of a continent. These methods are commonly referred to as geochronological methods. Thermochronological methods are related to this, and provide information on temperature histories. Temperature increases with depth in the earth, and knowing the temperature history of a rock sample can thus provide information on how much erosion has taken place, how deep it was burried, and, for example, also what the potential for oil and gas in a particular geological horizon is.

A noble gas mass-spectrometer can determine the isotopic composition of argon gas extracted from geological samples by heating them in an oven or by hitting the sample with a laser. The photograph shows the argon laboratory at NGU. 40Ar/39Ar geochronology allows measurement of ages for common rock-forming minerals and whole rocks that contain K. The method is based on the natural, radioactive decay of 40K to 40Ar; the half-life of this decay process allows measurement with high precision of the Ar isotope content of minerals and rocks ranging in age from that of the Moon to several hundred thousand years old . The NGU 40Ar/39Ar geochronology laboratory (the first in Scandinavia) analyzes bulk mineral separates, single grains and whole-rock samples including white mica, biotite, phlogopite, amphibole, K-feldspar, plagioclase, glauconite, glass and volcanic whole rocks. Applications include establishing sedimentary provenance, timing of faulting, volcanic events, crustal cooling (after high-grade metamorphism or igneous intrusion), and crustal unroofing.