Nucleogenic production

MORBs and OIBs are systematically different in their Ne isotope composition (Sarda et al. 1988 Honda et al. 1991, 1993a,b Moreira et al. 1995 Moreira and Allegre 

O Iceland A Loihi Reunion Kergielenxenoliths 3 Samoa xenoliths V Cameroon Line 

O Popping Rock Shona/SMAR N Chile Ridge O Manus Basin 

A good estimate of the NeA Ne ratio of the mantle source can be determined from the realization that Ne in a volcanic rock is a binary mixture between air and mantle (e.g., Moreira et al. 1995). This estimate is similar to that determined by a ternary 

In a diagram of °Ne/ Ne vs. ocean ridge basalts form a well defined 

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Leya, I., Wieler, R. (1999) Nucleogenic production ofNe isotopes in Earth s crust and upper mantle induced by alpha particles from the decay of U and Th.. 1 Geophys. Res., 104(7), 15439-50. 

Lewis RS (1975) Rare gases in separated whitlockite from the St. Severin chondrite xenon and krypton from fission of extinct Geochim Cosmochim Acta 39 417-432 Leya I, Wider R (1999) Nucleogenic production of Ne isotopes in the Earth s cmst and upper mantle induced by alpha particles from the decay of U and Th. J Geophys Res 104 15439-15450 Lin WJ, Manuel OK (1987) Xenon decay products of extinct radionuclides in the Navajo, New Mexico well gas. Geochem J 21 197-207... 

Nucleogenic production of He dominantly via the reaction Li(n,a) H(P) He (Morrison and Pine 1955). The principal source of neutrons is from (a,n) reactions on light elements. The a-particles are supplied from decay of U and Th and decay products. Production of He is controlled by the Li content (Mamyrin and Tolstihkin 1984). The concentration of these elements can be measured within each sample and appropriate correction applied if necessary. 

A potential problem in the use of diazo compounds as C atom precursors is the fact that intermediates in these reactions may act as C donors with the free atoms not involved. Indeed, the timing of the reactions in Eq. 6 is unknown and some of these intermediates may be bypassed in the thermolysis of 8. However, a comparison of the reactions of carbon from 8 with those of nucleogenic and arc generated carbon reveals quite similar products from many different substrates and provides circumstantial evidence for free C atoms in the decomposition of 8. 

Like many singlet carbenes, nucleogenic, arc generated and chemically generated C atoms react with aliphatic C—H bonds by insertion. In the simplest case, reaction of chemically generated C atoms with methane yields ethylene and acetylene. When a mixture of CH4 and CD4 is used, product analysis indicates that the acetylene results from H abstraction followed by dimerization of the CH, while the ethylene results from C—H insertion followed by H migration in the carbene (Eq. 15). It seems probable that CH is formed in all reactions of carbon with hydrocarbons as acetylene is invariably produced in these reactions. 

Note that (129Xe/131Xe)exc 1.1 deduced from the linear trend in the diagram is close to the production ratio of nucleogenic Xe produced by neutron capture on Te. 

Th are plotted against 4He (normalized to 22Ne). The straight line corresponds to a constant production ratio of the nucleogenic 21Ne to 4He in the mantle (e.g.,... 

Eikenberg, X, Signer, P., Wieler, R. (1993) U-Xe, U-Kr, and U-Fb systematics for dating uranium minerals and investigations of the production of nucleogenic neon and argon. Geochim. Cosmochim. Acta, 57, 1053-69. 

Yatsevich, I., Honda, M. (1997) Production of nucleogenic neon in the Earth from natural radioactive decay. J. Geophys. Res., B102(5), 10291-8. 

In the solid Earth, production of nucleogenic Ne is coupled to that of radiogenic " He. This is because production of Ne is directly proportional to the a-particle production ratio from the uranium and thorium series. The Ne/ He production ratio is constant and has been estimated at a value of 4.5 X 10 (Yatsevich and Honda, 1997). In this way, if the Earth accreted with solar helium and neon and initial ratios were modified by production of Ne and " He in a fixed proportion then the present-day He/ He and Ne/ Ne ratios in the mantle should be correlated. Honda et al. (1993) noted a strong correlation between OIB helium and neon isotopes such that steeper trajectories in three-isotope neon space were characterized by samples with high He/ He ratios. Indeed, they showed that it was possible to estimate the He/ He ratio of a suite of OIBs based solely on measurements of the neon isotope composition. 

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