Calcium Isotope Ratio (δ ^{44/4044/40 Ca) Measurements of Ca‐Dominated Minerals and Rocks without Column Chemistry Using the Double‐Spike Technique and Thermal Ionisation Mass Spectrometry}

摘要

> In this study, to evaluate whether bias free Ca isotope ratios can be properly achieved by thermal ionisation mass spectrometry ( TIMS ) using the 42 Ca– 43 Ca double‐spike technique without chemical purification, a series of comparable experiments (with or without column chemistry) were made on Ca‐dominated minerals and rocks. Minerals, including fluorite, calcite, titanite and apatite, displayed similar Ca isotope ratios with Δ 44/40 Ca _{with‐without} ranging from ?0.02 to +0.07‰. For Ca‐rich rocks, we mainly focused on the carbonates since they are the main repositories of Ca at the surface of the Earth. Coral, stalagmite, carbonatite, dolomite, marble and limestone are studied here. Interestingly, δ 44/40 Ca _{SRM 915a} of these minerals and rocks without chemical purification displayed little drift compared with that of rocks with chemical purification, with Δ 44/40 Ca _{with‐without} merely spanning from ?0.04 to +0.04‰. This implies that Ca isotope ratios can be achieved without column chemistry, possibly due to a property of TIMS in which Ca ions are only ionised at a target temperature and with a specific activator without ionising other ions. Therefore, for Ca‐dominated minerals or rocks, we suggest that column chemistry is unnecessary if they are totally dissolved and mixed in proper proportion with the 42 Ca– 43 Ca double spike for TIMS measurement.