Argon isotopic compositions in the mantle

Variations in upper mantle Ar In contradiction to the idea of a single MORB-source composition, it has recently been suggested that there are variations in the Ar/ Ar ratio of the MORE source, based on a correlation between Pb isotopes and 

Ar/ Ar ratios for samples with He/ He 9.5 Ra (Sarda et al. 1999). This was ascribed to subduction of a component with relatively radiogenic Pb and low so that 

OIB Ar. Measurements of Ar/ Ar in OIB with He/ He 10 Ra are consistently lower than MORB values. Early values for Loihi glasses of 10 appear to 

1983 Kaneoka et al. 1986 Staudacher et al. 1986). A study of basalts from Juan Fernandez (Farley et al. 1994) found atmospheric contamination contained within phenocrysts and is inferred to have been introduced into the magma chamber, thus providing an explanation for the prevalence of air contamination of OIB. Recent measurements of Loihi samples found Kxi Kx values of 2600-2800 associated with high He/ He ratios (Hiyagon et al. 1992 Valbracht et al. 1997), while Trieloff et al. 

2 X lO in Samoan xenoliths that have intermediate He/ e ratios ((9-20) Ra). Other attempts to remove the effects of air contamination have used associated Ne isotopes and the debatable assumption that the contaminant Ne/Ar ratio is constant, and have also found low °Ar/ Ar values (Sarda et al. 2000). It appears that °Ar/ Ar ratios in the high He/ He OIB source are 3000 but lower than that of the MORB source (see also Matsuda and Marty 1995). 

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