In situ determination of trace elements in melt inclusions using laser ablation inductively coupled plasma sector field mass spectrometry.

Rationale: In situ trace element analysis of melt inclusions by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) provides important geochemistry information. However, the precision and accuracy of this technique are affected by many factors, such as matrix effect, laser conditions, and calibration method. In addition, many previous LA-ICP-MS studies ablated entire melt inclusions along with their host minerals and obtained trace element composition by deconvoluting the mixed ablation signal, which may induce much uncertainty.

Methods: A 193 nm ArF laser ablation system combined with inductively coupled plasma sector field mass spectrometry (ICP-SF-MS) was used to investigate matrix effect, laser conditions, choice of external calibration standards, and data reduction strategy for in situ analysis of 36 major and trace elements in six common silicate reference glasses. The validity of the protocol presented here was demonstrated by measuring trace elements in olivine-hosted melt inclusions. Instead of ablating entire melt inclusions along with their host minerals, melt inclusions were polished to the surface to avoid laser ablating the mineral host.

Results: The calibration lines calculated from the calibration standards should cross the coordinate origin, especially for low-concentration elements (<10 ppm). As the laser crater size increased from 17 to 33 μm, the precision was improved from <20% to <8% (2RSD), and accuracy was improved from ±20% to better than ±10%. Most measured trace elements in Dali melt inclusions are consistent with those in their host rocks. For mobile elements (Ba, Sr, Pb), melt inclusions display much smaller variations than their host rocks.

Conclusions: A simple but accurate approach for in situ analysis of trace elements in melt inclusions by LA-ICP-SF-MS has been established, which should greatly facilitate the wider application of in situ trace element geochemistry to melt inclusion studies.