Physics
Scientific paper
Jan 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998lpico.957...15j&link_type=abstract
Origin of the Earth and Moon, Proceedings of the Conference held 1-3 December, 1998 in Monterey, California. LPI Contribution N
Physics
Earth (Planet), Moon, Nuclides, Planetary Evolution, Lunar Evolution, Geochronology, Radiochemistry, Magma, Radiogenic Materials, Radioactive Age Determination
Scientific paper
A number of important questions relating to the evolution of the early Earth and Moon may be investigated by using extinct as well as long-lived radionuclide chronometers. In particular, the time scale and functional shape of accretion and core formation may be investigated. The existence of early magma oceans, and whether a giant impact gave rise to the moon, may also be addressed. Transport models for radionuclides and trace elements provide powerful tools for investigating timescales associated with planetary accretion, given appropriate data on extinct and long-lived chronometers. They may be used to constrain the rate of accretion and core formation as well as the growth of the earliest crust on the Earth and Moon. The models are based on isotopic and chemical mass balance between a primitive nebular reservoir, the primitive mantle, the core, and the early crust. In particular, two simple continuous models will be considered: (1) a model where the core and the silicate mantle is made directly from undifferentiated chondritic material, and (2) a model involving chemical and isotopic equilibration in a magma ocean before core segregation. These models are useful in assessing the interpretation of two-stage model ages. They show that for long-lived systems (such as the U-Pb system), the two-stage model age yields the mean time of core formation directly for both models. For extinct nuclides, however, this is not true. Different models yield different two-stage model ages that can be very different from the mean time of core formation. The calculated two-stage model ages are shown to depend on the decay constant of the particular system, the type of chemical fractionation, and the transport mechanism involved. Only in the trivial case where all reservoirs involved equilibrate at the same time should one expect to obtain concordant two-stage model ages for both long-lived systems and a variety of extinct systems.
Jacobsen Stein B.
Yin Q.-Z.
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