DISEQUILIBRIUM TRACE-ELEMENT PARTITIONING IN JURASSIC SPARRY CALCITE CEMENTS - IMPLICATIONS FOR CRYSTAL-GROWTH MECHANISMS DURING DIAGENESIS

Sparry calcite cements from Middle Jurassic limestones in Central England contain complex non-concentric zoning patterns produced by a combination of sector zonation and a sector-specific, lamellar or polygonal zonation. Electron microprobe analyses of individual zones within crystals document: (1) a sympathetic covariation of Fe, Mg and Sr between two non-equivalent sectors, and (2) episodic further increase of Sr contents coupled with exclusion of Fe, Mg and Mn within the more enriched sectors. Comparison of results from three different localities demonstrates the regional consistency of this pattern, although the magnitude of the differential trace element partitioning varies. From combined petrographic and geochemical data the trace element variation in the crystals can be shown to bear no simple relationship with either (1) the chemical evolution of the 'bulk' precipitating porefluid, or (2) the relative overall growth rates of the different crystal faces. Previous sector zoning models based on crystal surface configurations and internal structure are unable to account directly for the geochemical patterns observed in this study. Differences between growth mechanisms on the crystal faces of the studied spars are likely to have exerted a significant influence on trace element partitioning. In particular, they offer a plausible explanation for the geometric and compositional characteristics of the sector-specific episodic zoning on the most enriched crystal faces. In general, crystal morphology and composition will be intimately linked by the growth mechanisms operating on different faces and by their response to changes in the diagenetic environment. The validity of diagenetic interpretations from 'bulk' geochemical analysis of sparry calcite cements thus rests upon a detailed analysis of any zonation that they contain.