Elemental distribution mappings of a Mid-Jurassic (ca. 165 Ma) belemnite rostrum (genus Barskovisella) showing complex growth zonations visible in the Mg distribution, in cross section (a) and longitudinal section (c). The enrichment of Sr in the center (b, d) is evidence for primary aragonitic mineralogy (right margin in b) of the phragmocone and for secondary aragonite formation in the center of the rostrum (b, d). The Sr-depleted outer parts of the rostrum consist of primary calcite. This observation, together with morphological alteration features, indicates rare presence of secondary aragonite in calcitic belemnite rostra.
Recorded on a SXFiveFE at Ruhr-Universität Bochum, Germany.
Image courtesy of Niels Jöns and René Hoffmann.