Petrogenetic applications of the [formula omitted] radiogenic decay scheme — A reconnaissance study

Abstract

Ca isotopic data are presented for oceanic basalts, potassic igneous rocks, carbonatites, marine sediments and chemical and clastic sediments from Archaean and Proterozoic terrains. An overall external precision of ±0.02% (95% confidence) was obtained for 40Ca 42Ca measurements. By comparison with the mid-ocean ridge basalt, no resolvable enrichments in 40Ca from radiogenic decay of 40K were detected in the Hawaiian ocean-island or Banda and Aleutian island-arc samples, the marine sediments or the carbonatite samples. Potassic igneous rocks from the West Kimberley region of Western Australia, Gaussberg (Antarctica) and southeastern Spain also possess 40Ca 42Ca ratios which are analytically indistinguishable from the value found in oceanic basalts and carbonatites, suggesting that: (a) the K/Ca ratio of the magmas has been increased substantially during partial melting or melt extraction, and/or (b) the Ca in the magmas has been diluted by mixing with non-radiogenic Ca derived from other sources. In contrast, enrichments in 40Ca were detected in genetically unrelated kimberlitic and lamproitic intrusions from the Sisimiut (formerly Holsteinsborg) region of central west Greenland. The excess 40Ca in the Greenland kimberlites is unsupported, indicating that the K/Ca ratio has been reduced, probably during extraction of the kimberlite magmas from carbonate-rich sources. The radiogenic 40Ca 42Ca of the Greenland kimberlites is not accompanied by elevated 87Sr 86Sr , suggesting that the kimberlite major- and trace-element components have been decoupled, possibly with much of the major elements derived from the subcontinental lithosphere and the trace elements largely derived from sources located within the asthenosphere. The radiogenic 40Ca 42Ca of the Greenland lamproites is consistent with their derivation from subcontinental lithospheric sources which have been enriched in K for > 1 Ga prior to their emplacement. Gypsum samples and pelitic clastic sediments from Archaean and Early Proterozoic terrains possess substantial enrichments in 40Ca relative to oceanic basalts. Application of the 40K- 40Ca decay scheme using presently available technology can provide useful petrogenetic and chronologic information which complements that provided by other techniques.