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'' above the Juan de Fuca Ridge spreading center. The results of these analyses permit us to estimate the apparent age of the megaplume by two independent methods. The age estimate based on excess Rn-222 is 20 +/- 9 days. Based on the partitioning of manganese between the dissolved and 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 first discovered.