Date of Award
Doctor of Philosophy (PhD)
Stephen J. Mojzsis
William F. Bottke
Rebecca M. Flowers
Understanding the origin and evolution of the planets requires better constraints on the timing and intensity of early thermal events in the asteroid belt and Moon, including the proposed "Late Heavy Bombardment" (LHB) at ca. 3900 Ma. Both the eucrite meteorites (from asteroid 4 Vesta) and lunar impact breccias contain accessory minerals (e.g. zircon, apatite, baddeleyite, zirconolite) with U-Pb ages that may not have been wholly re-set by thermal events subsequent to their original formation; many of these pre-date the LHB epoch and yield complex histories attributable to multiple thermal events of uncertain affinity. Further detailed geochronological analyses are therefore warranted to uncover a cryptic record of "pre-LHB" bombardments. Zircons are useful in this regard because they are cited as the most reliable chronometers despite (sometimes significant) thermal metamorphism. A suite of criteria used to elucidate the petrogenetic history of zircon include: determination whether they are the products of a purely crustal igneous process; crystallized from impact melts; or experienced diffusion-controlled age resetting from impact shock-heating resulting in the partial re-equilibration of primary igneous zircon. Geochemical tools to distinguish between these different scenarios include U-Pb geochronology, trace element geochemistry, Ti-in-zircon thermometry and zircon-melt trace element partition modeling.
Ultra-high resolution (sub-µm) U-Th-Pb depth profiles in eucritic zircons reveal different age domains correlative to mineral chemistry in cores and mantles within individual zircons. Results from eucrite zircons confirm previous ages determined for the solidification 4 Vesta's crust within a few million years after the formation of CAIs (4561±13 Ma), while younger ages may be the result of either shock metamorphism via impact heating, or are associated with protracted basaltic magmatism. Furthermore, U-Th-Pb-Ti abundances for 106 lunar zircons from lunar impact breccia 14311 show three distinct age populations of 4334 ± 5 Ma, 4245 ± 5 Ma, and 3954 ± 5 Ma. Thermometry results reveal temperatures that range from 800-1200 ⁰C. A significant number of these zircons yield temperatures within the range of modeled impact melt values for the Moon. Taken together, U-Pb ages, geochemistry and partition modeling suggest that the oldest lunar zircon population represents primary igneous crustal formation at ca. 4350 Ma, while the ca. 4250 Ma zircons may be a mixture of igneous and impact-produced zircons. Impact-generated or altered lunar zircons at about 4000 and 4200 Ma coincide with "LHB" and "pre-LHB" events reported in other radiogenic systems, other zircon ages from lunar impact breccias, as well as the oldest terrestrial zircons. These results support a longer period of bombardment akin to the proposed dynamical "Sawtooth" model.
Hopkins, Michelle Diane, "Geochemical Signatures in Zircons as Probes of the Impact History of the Inner Solar System" (2014). Geological Sciences Graduate Theses & Dissertations. 3.