THE AR-40/AR-36 RATIO OF THE UNDEPLETED MANTLE - A REEVALUATION

The Ar-40/Ar-36 ratio of the mantle bears important information about the extent of early degassing of the earth. Many researchers have agreed that Ar-40/Ar-36 ratio in the depleted mantle as sampled by Mid-Ocean Ridge Basalts (MORB) is as high as 28000. However, the Ar-40/Ar-36 ratio in the undepleted mantle has been strongly debated. We have reexamined the noble gas data of He and Ar in MORE and hot spot volcanics using He-4/Ar-40 vs. Ar-36/Ar-40 and He-4/Ar-40 vs. He-3/Ar-40 diagrams. In these diagrams, mixing lines between two components result in straight lines, which enable us to interpret data more easily than using diagrams with different denominators like a Ar-40/Ar-36 vs. He-3/He-4 plot where all mixing lines appear as curves. We conclude that the Ar-40/Ar-36 ratio in the undepleted mantle is much higher than the atmospheric value and should be higher than the observed highest values in the hot spot volcanics. We also suggest that high He-4/Ar-40 ratios observed in MORE result from kinetic disequilibrium during partial melting.