Reply to Discussion by Romer (2008) Mineralogical Magazine, 72, 827-831

摘要

IN his Discussion of the paper by Dill et al. (2007), Romer (2008) proposes an alternative explanation for U-Pb data of limonite enclosed in Pliocene torbernite. According to Romer (2008), limonite formed shortly after Carboniferous columbite and its U-Pb system was perturbed by the loss of intermediate daughters from the ~(238)U decay series subsequent to their dislocation near to grain boundaries by alpha-recoil. The ~(206)Pb/~(238)U and ~(207)Pb/~(235)U ages >300 Ma are explained by U-loss in limonite during torbernite formation. In the model advocated by Romer (2008), both processes in concert caused a displacement of the analytical points and therefore of the discordia resulting in an aberrantly old upper intercept age (Romer, 2008; Figure 1). In principle, Romer (2008) presents a scientifically sound explanation, which does not to take into account inherited columbite/limonite. We agree with Romer (2008) that, according to present knowledge, inherited columbite appears rather unlikely in a pegmatite melt and has so far not been reported. Despite various studies, mainly by Romer and co-workers (see references in Romer, 2008), the U-Pb system in columbite has not been studied in as much detail as in other U-bearing accessory minerals, such as zircon, monazite, rutile, and titanite.