AGE ESTIMATE FOR THE 1987 MEGAPLUME ON THE SOUTHERN JUAN-DE-FUCA RIDGE USING EXCESS RADON AND MANGANESE PARTITIONING

In September 1987 we collected samples for the determination of Rn-222 , Ra-226, and dissolved and particulate manganese in ''Megaplume 2'' a bove the Juan de Fuca Ridge spreading center. The results of these ana lyses permit us to estimate the apparent age of the megaplume by two i ndependent methods. The age estimate based on excess Rn-222 is 20 +/- 9 days. Based on the partitioning of manganese between the dissolved a nd particulate phases the age is estimated to be between 6 and 60 days . Both techniques confirm that Megaplume 2 was a separate hydrothermal event and that it was not Megaplume 1 sampled 1 year after it was fir st discovered.