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
- Paper number
IAC-07-A3.I.A.15
- Author
Ms. Cassandra Marion, Memorial University of Newfoundland, Canada
- Coauthor
Dr. Paul Sylvester, Memorial University of Newfoundland, Canada
- Coauthor
Mr. Mike Tubrett, Canada
- Coauthor
Mr. Michael Shaffer, Canada
- Year
2007
- Abstract
The Mistastin Lake impact crater is an exceptional study area for analogue cratering studies of impact melt genesis and dating. The projectile impacted northern Labrador 35-40 million years ago in a Mesoproterozoic (ca. 1.4 Ga) stable craton and has not been considerably affected by successive metamorphism and deformation. The impact melt rocks are well exposed and the target rocks from which the impact melt was produced are thought to consist principally of anorthosite and mangerite (hypersthene monzonite). Because of its anorthositic target rocks, Mistastin may be the best terrestrial analogue for lunar impact melts.
Bulk analyses have been traditionally used to determine the composition and target rock sources of impact melts. This approach is indirect in that model dependent mixing calculations must be employed using assumed target compositions. Advances in microbeam technologies such as laser ablation ICP-mass spectrometry and automated scanning electron microscopy (SEM) provide more direct measurements and less ambiguous results. Xenocryst-free regions identified by automated SEM, are analyzed simultaneously for major and trace elements using laser-ICPMS. Xenocrystic material, particularly the refractory mineral zircon, is analyzed by EPMA and/or laser-ICPMS to determine the nature and composition of the target rock contributions to impact melts. Zircon are resistant to the effects of impact melting and contain distinctive inventories of trace elements. The age of xenocrystic zircon determined by U-Pb laser-ICPMS dating may be used as a further link to particular target rocks. Target rocks on the Moon and Mars have very subtle differences in chemistry and relating impact melt products to particular impact craters and target rocks can be difficult.
For Mistastin, we have determined that the composition of the glassy to fine-grained melt sheet is broadly andesitic with SiO2 varying from 55.7 to 61.5 wt%; Na2O from 4.6 to 5.1 wt% and K2O from 1.2 to 3.7 wt%. Chondrite-normalized REE patterns are slightly light REE enriched with [La/Sm]n = 3.5 to 5.5. U-Pb geochronology indicates the presence of at least two populations of xenocrystic zircons with ages of 1382.2 +/- 8.2 Ma and 1412.3 +/- 5.6 Ma. Compositions of xenocrystic zircons define 3 populations: (1) small grains (45-80um) 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 (75-125um) 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.
- Abstract document
- Manuscript document
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