Rare gas abundances in MORB

Data are presented for He, Ar, and Xe in a number of MORB glasses and crystalline rocks. The rare gases in the latter group are characterized by atmospheric Ar and Xe, coupled with loss of trapped mantle He, Ar and Xe. The glass data also show clear signs of an atmospheric-like component for Ar and Xe. A reasonably constant world-wide average trapped mantle gas value of 40Ar/36Ar approx. 15000 +- 3000 is suggested. Anomalies are seen in the 129Xe/132Xe ratio, but at least part of this might be due to mass fractionation rather than to 129I decay. Correlating the data with those from oceanic island basalts in Ar-Xe space indicates that the latter samples are probably also greatly influenced by an atmospheric component rather than (or in addition to) trapping gases from an undepleted mantle source. (author)