Sintering nano-crystalline calcite: a new method of synthesizing homogeneous reference materials for SIMS analysis

Reference materials play a crucial rote in high accurate isotope and trace element analysis by secondary ion mass spectrometry, and they are used to calibrate instrumental fractionation of samples with various compositions. However, it is often a challenge to prepare natural or synthetic reference materials that are homogenous on a micron or even sub-micron scale. We report a novel method to synthesize homogeneous reference materials via sintering nano-crystals. Coarse-grained crystals (100-500 mu m) of calcite have been grown from a starting material of 20-40 nm calcite powder at 1000 degrees C and 1 GPa for 24 h, which is similar to 400 degrees C below the liquidus line of calcite under the same pressure. The non-molten sintering process and high temperature of the experiments ensure a homogeneous composition of the synthesized calcite. In situ analysis of O-18/O-16 ratios of the synthetic calcite with a Cameca NanoSIMS 50L revealed no detectable variation within the analytical uncertainty (0.5 parts per thousand, 1SD), which was further confirmed using a Cameca IMS 1280 with a higher precision (0.1 parts per thousand, 1SD). In addition, trace elements, Sr and P, are also homogeneous in the synthetic calcite. Another important advantage of this method is that the compositions of synthetic reference materials can be accurately and conveniently determined from the bulk starting materials using independent analytical techniques.