A new high performance chelation ion chromatography method for the simultaneous determination of trace magnesium and strontium in various calcium carbonate samples was developed. Separations were performed on a monolithic silica column (Chromolith Si, 100 x 4.6 mm I.D.) chemically modified with hydroxyethyliminodiacetic acid functional groups. At a flow rate of 1.0 mL min-1, an eluent containing 80 mM NaCl, 20 mM picolinic acid, pH 5.30, was found to provide complete separation of Ni2+, Cu2+, Mg2+, Cd2+, Sr2+ and Ca2+ in 15 minutes, in matrices with 20,000 fold excess of Ca2+. Two post-column reagents, o-cresolphthalein complexone (o-CPC) and ZnEDTA-PAR, were compared for post-column reaction based photometric detection at 570 and 490 nm, respectively. This method provides sensitive detection of Mg2+ (LOD 20 µg L-1 and 5 µg L-1 for o-CPC and ZnEDTA-PAR, respectively) and Sr2+ (LOD 200 µg L-1 and 39 µg L-1 for o-CPC and ZnEDTA-PAR, respectively). Using o-CPC, linear range was from 0.5 to 24 mg L-1 for Mg2+ and from 1.0 to 32 mg L-1 for Sr2+. For ZnEDTA-PAR, the linear range was from 0.5 to 4 mg L-1 for Mg2+ and 0.1 to 32 mg L-1 for Sr2+. The method was applied to the analysis of a variety of calcium carbonate samples, including laboratory reagents, limestone NIST certified reference material, and the calcite based shells of marine microorganisms. Accuracy of the method was confirmed using inductively coupled plasma – mass spectrometry.