¹William R. HYDE,²Adam A. GARDE,²Nynke KEULEN,²Sebastian N. MALKKI,^3,4Steven J. JARET,⁵Tod WAIGHT,⁶Pierre BECK,⁷Iain McDONALD,¹Nicolaj K. LARSEN
Meteoritics & Planetary Science (in Press) Open Access Link to Article [doi: 10.1111/maps.13987]
¹Globe Institute, University of Copenhagen, Copenhagen K, Denmark
²Geological Survey of Denmark and Greenland, Copenhagen K, Denmark
³Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA
⁴Department of Physical Sciences, Kingsborough Community College, City University of New York, Brooklyn, New York, USA
⁵Department of Geosciences and Natural Resource Management (Geology Section), University of Copenhagen, Copenhagen K,Denmark
⁶Universit ́e Grenoble Alpes, CNRS, IPAG, Grenoble, France
⁷School of Earth and Environmental Sciences, Cardiff University, Cardiff, UK
Published by arrangement with John Wiley & Sons

Impact melt rocks formed during hypervelocity impact events are ideal forstudying impact structures. Here, we describe impact melt rock samples collected proximalto the 31 km wide 58 Ma Hiawatha impact structure, northwest Greenland, which iscompletely covered by the Greenland Ice Sheet. The melt rocks contain diagnostic shockindicators (e.g., planar deformation features [PDF] in quartz and shocked zircon) and formthree groups based on melt textures and chemistry: (i) hypocrystalline, (ii) glassy, and (iii)carbonate-based melt rocks. The exposed foreland directly in front of the structure consistsof metasedimentary successions and igneous plutons; however, the carbonate-basedimpactites indicate a mixed target sequence with a significant carbonate-rich component.Well-preserved organic material in some melt rocks indicates that North Greenland at thetime of impact was host to abundant organic material, likely a dense high-latitude temperateforest. Geochemical signatures of platinum-group elements in selected samples indicate anextraterrestrial component and support previous identification of a highly fractionated ironimpactor in glaciofluvial sand. Our results illustrate the possibility to study impact structureshidden beneath a thick ice sheet based on transported samples and this opens a new avenuefor identifying other potential impact craters in Greenland and Antarctica.

¹Nicolas D. GARRONI,¹Gordon R. OSINSKI
Meteoritics & Planetary Science (in Press) Open Access Link to Article [doi: 10.1111/maps.13993]
¹Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada
Published by arrangement with John Wiley & Sons

Carbonates from the impact melt-bearing breccia in the 2016 IODP/ICDPExpedition 364 drill core at Site M0077 were systematically documented and characterizedpetrographically and geochemically. Calcite, the only carbonate mineral present, is abundantthroughout this deposit as five distinct varieties: (1) subangular carbonate clasts (Type A); (2)subround/irregular carbonate clasts with clay altered rims (Type B); (3) fine-crystalline matrixcalcite (Type C); (4) void-filling sparry calcite (Type D); and (5) microcrystalline carbonatewith flow textures (Type E). Quantitative geochemical analysis shows that calcite in allcarbonate varieties are low in elemental impurities (<2.0 cumulative wt% on average);however, relative concentrations of MgO and MnO vary, which provides distinction betweeneach variety: MgO is highest in calcite from Types A, B, and C carbonates (0.2–0.8 wt% onaverage); MnO is highest in calcite from Types B, C, and D carbonates (0.2–1.3 wt% onaverage); and calcite from Type E carbonate is most pure (<0.1 wt% on average MgO andMnO, cumulatively). Based on textural and geochemical variations between carbonate types,we interpret that some of the carbonate target rocks melted during impact and wereimmiscible within the silicate-dominated melt sheet prior to the resurgence of seawater. TypeB clasts were formed by molten fuel–coolant interaction, as the incoming seawater erodedthrough the melt sheet and encountered carbonate melt (Type E). Post-impact meteoric-dominated hydrothermal activity produced the Mn-elevated calcite from Type C and Dcarbonates, and altered the Type B clasts to be elevated in Mn and host a clay-rich rim.