Mathematics – Logic
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p53c1524p&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P53C-1524
Mathematics
Logic
[3617] Mineralogy And Petrology / Alteration And Weathering Processes, [3660] Mineralogy And Petrology / Metamorphic Petrology, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties
Scientific paper
The Sudbury Igneous Complex (SIC) forms the central part of the Mesoproterozoic (ca. 1.85 Ga) Sudbury impact structure and comprises three lithological units: norite (bottom), quartz gabbro, and granophyre (top). To determine the precise timing of the impact and the subsequent thermal behavior of the crust to this impact, an experimental approach was formulated to directly assess the associated impact energies by exploiting the crystallo-chemical response of zircon (Zr(SiO4)). The differential (slow) diffusional response of constituent elements (e.g. U, Th, Pb, REEs, Ti) in zircon at different temperatures and time-scales of the thermal pulse associated with impact makes this mineral an ideal recorder of the timing and condition of impact energies. Because titanium concentration in zircon is a function of temperature, we can also use this relationship a useful thermometer to probe time vs. temperature during the impact and its aftermath. Here, we report our studies of individual zircon grains extracted from SIC norite and granophyre drill core samples via standard heavy-mineral techniques (no zircons were found in the quartz gabbro unit). Grains were both hand-picked under an optical microscope and cast in epoxy, or entire heavy-mineral aliquots were cast without bias that arises from hand-picking. Electron imaging was used to identify individual zircons; many of the norite zircons displayed sets of planar deformation features (PDFs) attributable to shock wave deformation from impact, but none of the granophyre zircons displayed such structures. The largest and least altered grains from the sample aliquot we prepared were removed from their mounts and recast in epoxy with standard zircon AS-3 in anticipation of titanium thermometry and uranium-lead geochronology by high-resolution ion microprobe in depth-profile mode. Depth profiling reveals relationships between zircon growth time and the geochemical environment during crystallization and cooling after impact. We report our results to quantify the conditions for secondary zircon growth and recrystallization after a large (and in this case very well-characterized) impact and outline a quantitative means to better define impact conditions and timing for ancient terrestrial and lunar samples.
Mojzsis Stephen J.
Prado D.
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