A geochemical and mineralogical update on two major tonsteins in the UK Carboniferous Coal Measures

Abstract

Abstract Samples of two prominent tonsteins in the UK Carboniferous coal-bearing strata, representing kaolinised mafic ash and felsic ash have been comprehensively analysed by ICP-MS, laser ablation ICP-MS, SEM/EDX, XRD and XRF. Compositions have been compared with possible parent ashes in order to establish the extent to which elements are lost or retained during the alteration process. This is more readily achieved for the mafic tonsteins because they can be directly linked to the local, contemporaneous volcanics. The volcanic source of the felsic tonsteins is not known with certainty and the geochemical comparisons are made with modern volcanics from what is thought to be a similar tectonic/magmatic setting. Evidence of element migration during the tonstein formation has also been obtained by analysing the adjacent coals. Some mobility of U, Th, Nb, Y, Zr Hf and Ta is demonstrated, which imposes some constraints on the use of discrimination diagrams for the interpretation of original ash composition and tectonic setting. Notwithstanding some mobility of the incompatible elements, the discrimination diagrams are of use in providing broad igneous groupings. The TiO2/Al2O3 ratio is useful in that these two elements show the least mobility and the ratio is diagnostic of ash composition and sediment inclusion in the ash if the quartz content is also taken into account. The sediments associated with the felsic tonstein do not have a constant value for this ratio, which increases with grain size and quartz content and enables ash with included sediment to be recognized. Based on SEM/EDX and LA ICP-MS element locations have been identified. Zircon is a diagnostic volcanogenic mineral which makes an important contribution to the trace element budget, but there are other sources, notably a range of phosphate minerals, some of igneous origin and others diagenetic. This is also demonstrated from the whole rock geochemistry which shows there is an excess of U and Th over that which can be accommodated within the calculated zircon content. The REE profiles for the two types of tonstein differ significantly to each other. One is comparable with the profiles recorded from the associated basalts and partial melting of the upper mantle is thought to be responsible. The felsic tonstein, on the other hand, shows a profile common to many rhyolites with evidence of significant fractional crystallization. Again, in spite of some mobility during the alteration, the REE profiles appear to reflect the original ash composition, which is consistent with other aspects of the geochemistry.