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K‐Ar ages have been measured for various minerals separated from possible lower‐crust (e.g granulitic) and upper‐mantle (e.g. eclogitic) materials which occur as xenoliths in igneous pipes of deep‐seated origin at Delegate (Australia), Roberts‐Victor mine (South Africa), and Salt Lake Crater (Oahu, Hawaii). Minerals separated from granulitic and some eclogitic xenoliths have apparent K‐Ar ages consistent either with their formation at the time the pipes were intruded (i.e. 168 m.y. for the Delegate pipe, 68 m.y. for the Roberts‐Victor pipe) or with loss of any pre‐existing radiogenic Ar at that time. However, K‐Ar dates measured on clinopyroxenes and phlogopite from certain eclogitic Xenoliths suggest that these particular xenoliths have apparent ages very much in excess of the host intrusion. It is suggested that these great ages may result from excess radiogenic Ar incorporated in the mineral lattice under conditions of locally high Ar partial pressures. These conditions could have existed either within the mantle (if the xenoliths are of accidental origin) or at depth in the host magma (if cognate).

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