Trace elements have been used as provenance indicators of igneous rocks for many years, even though the conditions under which they remain immobile during subaerial weathering have not been thoroughly defined. We have used a novel geochemical and quantitative mineralogical data set of variably weathered basaltic rocks from northeast Ireland to investigate the immobility of various elements. Titanium and niobium seem to be the most immobile of all elements. Relative to these elements, zirconium is predominantly depleted but is also locally enriched. Yttrium displays a marked depletion from the early stages of weathering during the growth of clay minerals. Yttrium, zirconium, and their ratios with other elements should thus be used with caution as provenance indicators unless the rocks show no detectable signs of weathering.