K-PG Impact spherules: clues to formation and plume dynamics
File(s)
Published version
Author(s)
Boyd, MR
Genge, MJ
Salge, T
Type
Conference Paper
Abstract
Impact spherules are important indicators of large-scale planetary events that can be catastrophically disruptive to Earth's environment and climate. In particular, the global distribution of microspherules provide crucial evidence that a giant impact coincided with the K-Pg extinction event [1] and caused environmental effects that contributed to the extinction of organisms. However, impact spherules are typically glassy and thus undergo alteration during diagenesis [e.g., 2], complicating their study. We present data on a collection of intermediate K-Pg spherules deposited in a continental setting and show that diagenesis destroys the smallest, and most pervasive of spherules. Methods: Samples were collected from the Raton Basin, New Mexico, USA. The grey K-Pg clay sits within the Raton Formation and was preserved in ponds and coal swamps [3]. The clay overlies mudstones containing sand lithics and plant fragments, and is overlain by a carbonaceous shale, unconsolidated mudstone and layer of graphitised coal. The clay layer is composed of a 15 mm-thick white-grey clay and a 2 mm rusty horizon, redistributed in places. Larger spherules (<500 µm) were separated under a binocular microscope, and the spherules and matrix of the clay characterised. The external textures were imaged using a Hitachi TM4000Plus scanning electron microscope (SEM) at Imperial College London. The geochemistry of the spherules was analysed using a FEI Quanta 650 FEG SEM, equipped with a high sensitivity, annular Bruker Flat Quad energy-dispersive spectroscopy (EDS) silicon drift detector (SDD) at 6 kV and 9 kV accelerating voltages, at the Natural History Museum, London. Results: The Raton Basin spherules have a bimodal size distribution: large spherules are 200-500 μm, while the matrix has a microspherulitic texture with a tight size range of 0.5-1 μm. The large spherules have smooth surfaces with micron-sized depressions and minor cracking, and typically have clay matrix affixed to the surface. The microspherulitic texture shows both spherules and spherical cavities, some of which display minor elongation. The large spherules and matrix are Al-and Si-rich, suggesting replacement by clays. The large spherules contain high Ti abundances (<23 wt.%) that predominantly occur as external rims. In addition to sub-micron Ti grains, a large spherule shows several layers of Ti enrichments in the surrounding matrix that are sub-parallel to its surface. Another spherule contains a 100 µm Ti-rich grain and Ti-rich rim, with the matrix partially infilling interior cavities. The Ti-rich rim contains a 0.5 μm zircon grain, and the adjacent matrix contains a 0.4 μm calcite grain. A clay grain from the rusty horizon was also found that contains carbonised plant material. Clays from above and below the boundary layer lack the microspherulitc textures of the boundary clay. Discussion: Ti in spherules is likely to be indigenous owing to its low solubility during aqueous alteration [4] and is a target rock signature. The concentration of Ti along the exterior of spherules could be the result of evaporation and limited mixing, or accretion of Ti-rich dust. Concentric Ti-rich layers surrounding spherules suggest formation as impact-related accretionary lapilli [5,6]. Zircon and calcite grains may likewise be derived from the target material [e.g., 7]. Microspherulitic textures have been observed in K-Pg boundary clay [e.g., 8] and within K-Pg spherule cores [9]. We suggest these represent pseudomorphs of 0.5-1 μm abundant glassy microspherules. The bimodal spherule size distribution suggests distinct formation mechanisms with large spherules formed by break-up of impact melt and small spherules formed as vapour condensates. Modelling of impact-generated vapour plumes indeed predicts the formation of a secondary population of spherules, ≤1 μm in size [10]. This material may remain suspended in the atmosphere for several years [11]. Implications: Observations of a bimodal size distribution of spherules at the K-Pg boundary provides insights into the processes occurring in major collisions. The occurrence of abundant micron-sized spherules would enhance the environmental consequences of an impact owing to increased atmospheric aerosol load.
Date Issued
2024-08-25
Date Acceptance
2024-07-01
Citation
Meteoritics and Planetary Science, 2024, 59 (S1), pp.A41-A41
ISSN
1086-9379
Publisher
Wiley
Start Page
A41
End Page
A41
Journal / Book Title
Meteoritics and Planetary Science
Volume
59
Issue
S1
Copyright Statement
Copyright © 2024 The Meteoritical Society This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
License URL
Source
86th Annual Meeting of The Meteoritical-Society (METSOC)
Subjects
ACCRETIONARY LAPILLI
BOUNDARY
Geochemistry & Geophysics
NEW-MEXICO
ORIGIN
Physical Sciences
RATON BASIN
Science & Technology
Publication Status
Published
Start Date
2024-07-28
Finish Date
2024-08-02
Coverage Spatial
Brussels, Belguim