Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.v13c2040a&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #V13C-2040
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[1009] Geochemistry / Geochemical Modeling, [1140] Geochronology / Thermochronology, [1160] Geochronology / Planetary And Lunar Geochronology, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering
Scientific paper
Ultra-high spatial resolution ion microprobe depth-profiles of pre-3.9 Ga terrestrial zircons from the Jack Hills (Western Australia) have the potential to record a sharply elevated impactor flux to the inner planets at ca. 3.95 Ga termed the Late Heavy Bombardment (LHB). A putative signature of this is in the form of ~3.95 Ga, 2 to 4 μm mantles over the (oldest) igneous zircon cores (up to 4.3 Ga). These minute mantles show Pb-loss (up to 90% discordance) over narrow domains that could be the result of impact heating. Pre-3.9 Ga lunar zircon grains have not yet been depth-profiled, but it is evident from published spot analyses that grain cores preserve original igneous ages albeit with no clear super-imposition of later thermal events. However, the U-Pb systematics of apatites in the same lunar rocks were reset ca. 3.95 Ga. The motivation of this study is to explain the high degree of Pb (and other cation) loss over very short distances (<6 μm) in terrestrial zircons at ~3.9 Ga, the complete resetting of U-Pb isotope systematics of lunar apatites at approximately the same time, and to make predictions in preparation for depth-profile work on lunar samples. To accomplish these goals, we used existing models that simulate the thermal consequences of LHB, as well as established equations for cation diffusion in zircon and apatite. The main thermal model consists of (i) a stochastic cratering model which populates the surface with craters within constraints derived from the lunar cratering record, the size/frequency distribution of the asteroid belt, and dynamical models; (ii) analytical expressions that calculate a temperature field for each model crater; and (iii) three-dimensional thermal models of lunar and terrestrial lithospheres, where craters are allowed to cool by conduction in the subsurface and radiation at the surface. In addition, a high-resolution near-surface model was used to account for additional thermal pulses due to global deposition of hot ejecta following basin-forming impacts. Several parameters were tested, including LHB duration (10 and 100 Myr) and mass delivered (2 × 1020 to 2 × 1021 kg for Earth; 1019 to 1020 kg for the Moon). Models were populated with zircon and apatite grains, and the cumulative distance of cation diffusion as a result of thermal pulses during bombardment was recorded for each grain. Results will be presented as mean Pb-, Ti-, and REE-loss as a function of crustal depth for zircon and apatite grains of diameters 1-100 µm. High degrees of Pb-loss (up to 100%) occur in zircons emplaced on the surface and decrease rapidly with depth. On average, small (10 µm) zircons at 1 km depth would experience 80% Pb-loss, whereas at 5 km, they experience 50% Pb-loss. Apatites lose lead far more readily and are easily reset by impacts. A 10 µm zircon grain reaches open system conditions for Pb at 1200 °C for 1 y, but a 10 µm apatite is reset at only 625 °C. A 10 µm apatite grain is reset in 1 s at ~1750 °C, which is typical in ejectas produced by large impacts. These models explain the apparent absence of an LHB signal so far for lunar zircons using conventional spot analyses, as well as the complete re-setting of Pb ages in lunar apatites for the same rock.
Abramov Oleg
Mojzsis Stephen J.
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