Type of Thesis
Stephen J. Mojzsis
The age of the Santa Fe Impact Structure (SFIS) is poorly constrained. Here, I examine the employability of the U-Th/He system in zircon and apatite extracted from shatter cones of the SFIS – some of which were previously analyzed for their U-Pb ages – to better constrain the age of this impact structure, which lacks a preserved melt sheet, and has a complex, post-impact tectonic and thermal history. To achieve this, zircon and apatite grains were separated from shatter cone samples. Grains were selected to span a range of apparent radiation damage to access a range of He retentivities. Where possible, grains preserving planar fractures (PFs) were also included. U-Th/He dates from apatite predominantly record cooling from Laramide burial (c. 60 Ma), correlating with published apatite fission track dates. In contrast, U-Th/He dates from zircon have a wider range (334.87 ± 13.56 Ma to 7.88 ± 0.22 Ma) and show a pronounced negative correlation with effective uranium content. The preservation of dates (n=3) that are older than the age of Laramide resetting in apatite demonstrates that low eU zircons are promising candidates for both recording and preserving the age of the impact. Therefore, further dating of low-eU zircons may provide evidence to support stratigraphic constraints that the Santa Fe impact age is roughly compatible with age estimates of ten other craters across North America, Europe and Africa, of variable (and lower) geochronological reliability. It is noteworthy that the age coincides with the estimated times of three asteroid breakup events documented to have occurred in the asteroid belt. I propose that both the age overlaps and geographic distributions of these impacts represent terrestrial evidence an asteroid breakup event. Finally, I also report the first planar fractures (PFs) in zircons sampled directly from the central uplift zone of the crater where shatter cone morphologies are preserved. The discovery of PFs in zircons increase the minimum pressure generated by the impact to at least 20 GPa, indicating a much larger crater (>>13 km diameter) than previously estimated.
Martins Pimentel, Rayssa, "Zircon U-Pb and (U-Th)/He thermochronology of the Santa Impact Crater (New Mexico, USA): evidence for a Paleozoic asteroid breakup?" (2018). Undergraduate Honors Theses. 1554.