Ž . Earth and Planetary Science Letters 152 1997 181–185 Contribution of neutron-capture reactions to observed tungsten isotopic ratios Jozef Masarik ) EnÕironmental Physics, EAWAG, CH-8600 Duebendorf, Switzerland Received 11 April 1997; revised 25 August 1997; accepted 6 September 1997 Abstract To study whether tungsten isotopic ratios observed in terrestrial samples relative to W ratios in an iron meteorite could be made by neutron-capture reactions in the iron meteorite, the fluxes of thermal and epithermal neutrons in a large iron meteoroid, Toluca, were calculated using Monte Carlo particle production and transport codes. Peak neutron-capture rates on W isotopes were calculated to occur at depths of 30–34 cm. The calculated changes in the 182 Wr 183 W due to neutrons are much less than those observed in terrestrial samples relative to iron meteorites, indicating a non-cosmogenic origin of the observed W isotopic ratios. q 1997 Elsevier Science B.V. Keywords: tungsten; iron meteorites; isotope ratios; neutrons 1. Introduction A number of important questions relating to the formation and evolution of the early Earth are still wx largely unresolved. It was proposed 1 that time constraints on the early development of the Earth, in particular core formation, could be obtained with the use of extinct nuclides, such as the 182 Hf– 182 W system. 182 Hf decays to 182 W with a half life of 9 = 10 6 years, which provides a good timescale for studies of both star-forming and planet-building pro- cesses. Careful investigation of the 182 Hf– 182 W sys- tem is also important in astrophysics because a deter- ) Corresponding author. Present address: Department of Nu- clear Physics, Komensky University, Mlynska dolina Fr1, SK-842 15 Bratislava, Slovakia. Fax: q41 1 823 5210. E-mail: masarik@eawag.ch mination of the initial abundance of 182 Hf will pro- vide a key constraint on models of molecular cloud wx environment within which the Sun formed 2 . Also, several geochemical aspects make the 182 Hf– 182 W chronometer well tailored for dating planetary accre- wx tion-related processes, such as core formation 3. Comprehensive discussions of this system and its w x advantages can be found in 1,4,5 . High-precision tungsten isotopic analysis indicate the 182 Wr 183 W ratio in the Toluca iron meteorite is Ž . y4 decreased by 3.9 " 1.0 = 10 relative to a terres- wx trial standard 6 . Possible causes of this shift are neutron-capture reactions on tungsten isotopes, dur- ing Toluca’s about 600 Ma exposure to cosmic-ray particles, or radiogenic growth of 182 W from 182 Hf, either in an undifferentiated chondritic ‘reservoir’ or in the silicate portion of the Earth after removal of Ž W to the Earth’s core, or both, depending on the at . 182 w x present uncertain initial abundance of Hf 1,4,5 . 0012-821Xr97r$17.00 q 1997 Elsevier Science B.V. All rights reserved. Ž . PII S0012-821X 97 00151-9