A geologically meaningless Rb–Sr total rock isochron

Abstract

A major objective in total rock Rb–Sr isotope studies of relatively homogeneous igneous or high-grade metamorphic gneisses is to obtain as wide a range of ⁸⁷Rb/⁸⁶Sr ratios as possible, so that the resultant isochron may be well defined¹. This is often achieved by collecting samples at some distance from each other. Recent studies^2–6 have shown that primary Rb–Sr isotopic systems can be profoundly disturbed, and even completely reset, in response even to low-grade secondary deformations and/or mineral alterations, and in three cases^2–4 isochron diagrams based on regionally collected samples have been shown to yield linear arrays corresponding to apparent ages intermediate between the true ages of the primary and secondary events. The evidence we present here shows that it is also possible for well defined intermediate isochron apparent ages to be produced where this sampling technique is used on rock suites which show evidence of only minor secondary alteration effects. An isochron so produced is geologically meaningless, that is, the apparent age is unrelated to any geological event.