Ion microprobe analysis of trace elements in calcite with an application to the cathodoluminescence zonation of limestone cements from the Lower Carboniferous of South Wales, U.K.

Abstract

Ion microprobe analysis offers high spatial resolution (10 μm) and low detection limits for many elements in calcite. Detection limits are likely to be instrument dependent; the values given here are specific to the instrument used (a modified AEI-IM® 20) and more recent instrument designs should give lower detection limits. Li, Na and K signals have been observed and detection limits are estimated to be < 10 ppm. The detection limit for Pb is in the range 10–50 ppm. Zn and Ba suffer from significant molecular interferences which impose higher detection limits. Mg, Fe, Mn and Sr were standardised against three chemically analysed calcites. Detection limits in the range 1–5 ppm were obtained for Mg, Mn and Sr but Fe is subject to interference by a hydride species so that the detection limit is determined by vacuum conditions. Calcite cements in sections cut from a Lower Carboniferous limestone from South Wales can be divided texturally into four zones. Analyses were made for Mg, Fe, Mn and Sr. Only Sr differs among the zones. Mn is the main activator of cathodoluminescence (CL) in these cements. When the Fe content is up to 1600 ppm there is a positive relationship between CL intensity and Mn content. Fe acts as a suppressor of CL intensity; for a given Mn concentration the CL intensity decreases as the Fe content increases. For Fe contents > 1600 ppm the CL intensity may be negatively related to the Mn concentration. When the Mn content is below ∼125–150 ppm the Fe content has little discernible effect on the CL intensity. In these cements non-luminescence can only be predicted with confidence if the Mn concentration is below ∼25 ppm. There is no critical concentration of Fe (up to 7000 ppm) or critical Fe/Mn ratio (in the range 0–26) which always causes Mn-bearing locations to become extinct with respect to CL. The present data are compared with those from the literature and a diagram is constructed which represents the region of Fe-Mn composition space where luminescence can occur. Conflicting evidence regarding the relative importance of Fe/Mn ratio and absolute Fe and Mn concentrations can be reconciled; both concentrations and Fe/Mn ratio are involved in controlling luminescence.