40Ar/39Ar laser dating of biotite inclusions in a single crystal of magnetite

Abstract

The iron‐oxide mineral magnetite is widely used to study the past geomagnetic field because of its ability to record the ancient field at the time of its crystallization (or cooling), and because of its common occurrence in continental and oceanic rocks. We present here the 40Ar/39Ar laser dating of one half of a 2 mm octahedral crystal of magnetite from an ultramafic rock near Bush's Corner, Harford County, Maryland, in the Appalachian Piedmont. The other half was examined by scanning electron microscopy (SEM) to characterize the inclusions in the crystal. Laser step‐heating separates the argon isotopes released into three distinct phases. A three‐dimensional isotope correlation plot illustrates the existence of a pure radiogenic component corresponding to an age of 323±5 Ma. This is the age of argon closure of minute biotite inclusions identified in the magnetite by SEM, and therefore is a precise estimate of the minimum age of the magnetite.