Elemental and isotopic analysis of olivine with impact ionization dust instruments

Abstract

In-situ impact ionization mass spectrometers can reveal the elemental and isotopic compositions of interplanetary and interstellar dust particles throughout the Solar System, thereby enhancing our understanding of their origins and evolution without the associated costs and risks of sample return. We describe laboratory measurements of olivine particles accelerated electrostatically into a prototype impact ionization dust analyzer. Olivine is chosen because it is common in both interplanetary and interstellar dust. The particles were ground from a single olivine crystal chosen to minimize grain-to-grain composition variations, ensuring that the measured spread was the result of the instrument precision. The particles were coated with platinum to allow charging and acceleration. The instrument returns impact-ionization mass spectra whose Fe/Si and Mg/Si ratios agree within the uncertainties with reference measurements of the same sample by energy-dispersive X-ray spectroscopy (EDX). From impacts at 19–25 km s⁻¹, we derive the relative sensitivity factors (RSFs) consistent with previous impact ionization and Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) calibrations. The application of these RSFs brings the Mg/Si and Fe/Si ratios into 1 $\sigma$ agreement with the independent EDX analysis, allowing for robust identification and discrimination of olivine and pyroxene. The spread of the impact ionization measurements is small enough to distinguish Mg-rich olivine from glassy particles with embedded metals and sulfides (GEMS) at the impact speeds of 19–25 km s⁻¹. The average Mg/Fe and Si/Fe ratios are consistent with those of EDX results over wider velocity ranges of 9–25 and $\ge 16$ km s⁻¹, respectively. These results demonstrate the power of impact-ionization mass spectrometry in distinguishing common interplanetary materials in dust grains encountered at typical orbital speeds.