HR: 1340h
AN: P33A-1003    [Abstracts]
TI: The Timing of Crystallization of the Lunar Magma Ocean Constrained by $^{182}$W Systematics of Lunar Metals
AU: * Mezger, K
EM: klaush@nwz.uni-muenster.de
AF: Universitat Munster, Corrensstrasse 24, Munster, 48157 Germany
AU: Kleine, T
EM: kleine@erdw.ethz.ch
AF: Universitat Munster, Corrensstrasse 24, Munster, 48157 Germany
AU: Kleine, T
EM: kleine@erdw.ethz.ch
AF: ETH Zurich, Sonneggstrasse 5, Zurich, 8092 Switzerland
AU: Palme, H
EM: palme@gwp-min.min.uni-koeln.de
AF: Universit„t Koln, Zulpicherstrasse 49b, Koln, 50674 Germany
AU: Scherer, E
EM: escherer@nwz.uni-muenster.de
AF: Universitat Munster, Corrensstrasse 24, Munster, 48157 Germany
AU: Munker, C
EM: muenker@nwz.uni-muenster.de
AF: Universitat Munster, Corrensstrasse 24, Munster, 48157 Germany
AB: The early chemical differentiation of the Moon was dominated by the crystallization of a magma ocean. Determining the crystallization age of the lunar magma ocean (LMO) is critical for understanding the timing of Moon formation, melting, and subsequent differentiation and cooling. Currently, the most suitable isotope system for dating the crystallization of the LMO is the $^{182}Hf-^{182}W$ decay scheme, because the Hf/W ratios varied significantly between the different LMO reservoirs, and W isotope variations can have only been produced in the first $\sim 60$ Myr of the solar system. Thus, information on early differentiation of the Moon is preserved in the $^{182}W/^{184}W$ of early-formed lunar reservoirs and is carried by lunar samples derived from any of these sources. A chronological interpretation of W isotope ratios for lunar whole-rocks and minerals, however, has been hampered by the neutron-flux induced production of $^{182}W$ from $^{181}Ta$ caused by the intense cosmic radiation reaching the surface of the Moon. Analyzing the metals of lunar samples can overcome this problem because metals do not contain significant Ta that could be converted to $^{182}W$. Lee et al. [1] presented W isotope data for silicate minerals from a single sample that have variable Ta/W and obtained neutron-flux corrected$^{182}W/^{184}W$ from the interpolation to Ta/W $\sim 0$. This correction procedure, however, results in uncertainties on the order of $\sim 2 \varepsilon$, such that $^{182}W/^{184}W$ differences between low- and high-Ti mare basalts are not resolvable. We obtained W isotope data for metals from a representative suite of lunar samples. The metals from KREEP-rich samples display the lowest and those from high-Ti basalts the highest $^{182}W/^{184}W$ ratios. The W isotope composition of metals from low-Ti mare basalts is intermediate between those of KREEP-rich samples and high-Ti mare basalts. The variations in W isotope compositions between reservoirs in the lunar mantle indicate that separation of these reservoirs took place when $^{182}Hf$ was still extant (i.e., in the first 60 Myr of the solar system). The variations in $^{182}W/^{184}W$ correlate with the expected Hf/W for the different lunar reservoirs (i.e., KREEP has the lowest and high-Ti mare basalts have the highest Hf/W), resulting in an age $42 \pm 4$ Myr younger than the Hf-W age of CAIs. This age corresponds to the time of crystallization of the LMO and is significantly older than ages that are based on Sm-Nd isotope systematics, suggesting a decoupling of the Hf-W and Sm-Nd systems. It is conceivable that the Sm-Nd ages of $\sim 100-250$ Myr for different lunar samples reflect later re-melting and mixing processes that did not affect the W isotope composition. If the Moon-forming impact marks the end of the main accretion of Earth, then the $42 \pm 4$ Myr age for LMO crystallization provides a lower age limit for Earth's accretion. Reference: [1] Lee et al. (2000), Earth Planet. Sci. Lett. 198, 267-274.
DE: 5455 Origin and evolution
DE: 6250 Moon (1221)
DE: 3662 Meteorites
DE: 1035 Geochronology
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting