Matrix effects on oxygen isotopic measurements in carbonates

A large instrumental mass fractionation occurs during oxygen isotopic composition analyses by ion microprobe especially in silicates (1) but also in carbonates (2,3). Oxygen isotope compositions of different matrix of carbonates (calcite, dolomite, ankerite, rhodochrosite, magnesite and siderite) were measured by multicollector CAMECA ims 1270 ion microprobe (CRPG-CNRS, Nancy) in order to test the influence of magnesium, iron and manganese contents on the instrumental mass fractionation. The external reproducibility reached on these standards was better than 0.25 ‰ at one sigma. A strong control of the magnesium content and a minor effect of the iron content were detected on the instrumental mass fractionation. In example, the difference in instrumental mass fractionation between one calcite and one magnesite was ~ 15 ‰. These results demonstrate that the percentage of magnesium in carbonates can induce important errors on the oxygen isotopic composition, particularly when these values are used to reconstruct temperatures. It would appear essential to develop some carbonates standards with different percentage of magnesium as a calcite with a high content of magnesium. References (1) Eiler et al., Chemical Geology, 1997 (2) Saxton et al., EPSL, 1998 (3) Leshin et al., GCA, 1998