Microbeam-based analytical approach to define melt composition and identify target rocks in impact melt sheets: An analogue study from the Mistastin Lake impact crater, Labrador

The Mistastin Lake impact crater is an exceptional study area for analogue cratering studies of impact melt genesis and dating. Because one of its principal target rocks is anorthosite, Mistastin may be the best terrestrial analogue for lunar impact melts. In this study two methods, A and B are described to determine the composition and target rock sources of impact melts with advanced microbeam technologies. Bulk analyses have been traditionally used for this task though this approach is indirect and presents ambiguous results, Method A is designed to analyze the melt composition more accurately by using automated SEM and EPMA or laser ablation ICPMS to avoid potential μm-scale xenoclast contamination. Method B is a new method designed to determine the nature and composition of the target rock contributions to impact melts by characterizing and analyzing xenocrystic material, particularly zircon, by MLA/SEM analysis and EMPA and/or LA-ICPMS, respectively. Zircons contain distinctive inventories of trace elements and may be dated with U-Pb LA-ICPMS as a further link to particular target rocks, We have determined that the composition of Mistastin's melt is broadly andesitic. U-Pb geochronology indicates the presence of at least two populations of xenocrystic zircons. Compositions of xenocrystic zircons define 3 populations: (1) small grains (45-80μm) with high [Th/U]_n (0.606 to 0.809), (2) mixed-size grains with low [Th/U]_n (0.207 to 0.331) - low [Hf/Sc]_n (343.9 to 417.8), and (3) elongated, large grains (75-125μm) with low [Th/U]_n (0.169 to 0.296) - high [Hf/Sc]_n (482.7 to 564.7). The data suggest multiple target sources contributed to the Mistastin impact melt sheet. Copyright IAF/IAA. All rights reserved.