Other
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.471h&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 471
Other
2
Astronomical Models, Meteorite Collisions, Planetary Evolution, Planetary Mantles, Radioactive Age Determination, Radioactive Isotopes, Abundance, Chemical Effects, Coagulation, Radioactive Decay, Stochastic Processes
Scientific paper
In recent years, advances in understanding the nature of planetary accretion in the inner solar system have been made mainly by refinement of the method of a priori planetesimal coagulation modeling introduced by Safronov. Accretion timescales obtained in these models are intrinsically probabilistic, with increasing uncertainty at late times due to the heirarchical and stochastic nature of the accretion process, whereby the later stages of planet formation are expected to have been dominated by large merger (giant impact) events of unpredictable timing. Precise age determinations of the earliest episodes of planetary differentiation are desirable for constraining accretion and giant impact timescales directly. We are developing constraints based on three extinct radionuclide systems: (146)Sm-(142)Nd, (182)Hf-(182)W, and (244)Pu-Ru,Pd. Our W isotope data appear to strongly constrain the terrestrial accretion interval. Present evidence is too limited to identify the decay signature of (182)Hf in the early solar system unequivocally. Constraints on the Earth, Moon, and Mars from Nd-142/Nd-144 measurements are much more firmly established. The presence of a small but significant abundance of Sm-146 in the early solar system provides a chronometry particularly well suited for dating very early episodes of differentiation in planetary bodies. Our approach is based on the preservation of isotopic signatures in large-scale subcrustal reservoirs and is advantageous in cases where early-formed crustal samples are either unavailable or severely disturbed. In principle, the (146)Sm-(142)Nd system is ideal for dating differentiation episodes in the silicate portions of planetary bodies because other large-scale cosmochemical processes such as volatile depletion or core formation do not fractionate Sm/Nd. Large-scale merger events are expected to re-homogenize the silicate portion of the merged object and reset the isotopic clock. Consequently the age of the oldest differentiated mantle reservoir should provide a lower limit on cessation of hierarchical accretion series. A summary of the most significant Nd-142 data for the Earth, Moon and Mars is presented in isochron model age representation.
Harper Charles L. Jr.
Jacobsen Stein B.
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