In situ 87Sr/ 86Sr investigation of igneous apatites and carbonates using laser-ablation MC-ICP-MS

Abstract

In situ Sr isotopic compositions of coexisting apatite and carbonate for carbonatites from the Sarfartoq alkaline complex, Greenland, have been determined by laser-ablation multicollector inductively coupled plasma mass spectrometry. This study is the first to examine the extent of Sr isotopic homogeneity among coexisting igneous minerals containing high Sr (>3000 ppm) and low Rb (≪1 ppm) contents within a single ∼50-μm-thick thin-section mount. This technique is capable of producing measured 87Sr/ 86Sr values with analytical precision (∼0.005%, 2σ) approaching those obtained by thermal ionization mass spectrometry but in a much shorter interval of time (100 s vs. >1 h, respectively). The combined total analyses ( n = 107) of apatite and carbonate yield 87Sr/ 86Sr compositions ranging from ∼0.7025 to ∼0.7031. This relatively large variation in Sr isotopic compositions (∼0.0006) is ∼1 order of magnitude larger than the estimated external reproducibility (∼0.00005, 2σ) of the method. The large range in 87Sr/ 86Sr values suggests that apatite and carbonate precipitated predominantly under nonequilibrium conditions. The isotopic variations observed within individual hand specimens may therefore reflect larger (regional) scale open-system processes, possibly involving mixing of carbonatitic melts derived from distinct mantle sources or from a common isotopically heterogeneous mantle.