X-RAY MAPPING OF TRACE ELEMENTS IN CONCRETIONARY CALCITE

Electron microprobe analyses of concretionary calcite may display co-variations among trace elements that are suggestive of compositional zoning. Spatial variations of particular trace elements can be used to decipher the historical sequence in which calcites of different composition precipitated. Such variations may not be discernible in back-scattered electron- or cathodoluminescence-imaging, however. Mapping of trace elemental variation using wave-length dispersive X-ray spectrometry is a powerful tool for deciphering the geochemical history of concretions. Elemental mapping of calcite crystals within giant spherical concretions of the fluvial Dakota Formation sandstones from Kansas reveals a complex history of variable oxidation states during growth of individual calcite crystals. The cementation history within the concretion volume does not reflect concentric growth beginning at the center of the concretion, but rather dispersed calcite nucleation that occurred at widely-spaced sites within the volume that ultimately became the concretion.