2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 6
Presentation Time: 8:00 AM-6:00 PM

Resolving the Ar Paradox within the Framework of Whole Mantle Convection

MUKHOPADHYAY, Sujoy, Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138 and GONNERMANN, Helge, Department of Geology and Geophysics, SOEST, University of Hawaii, Honolulu, HI 96822, sujoy@eps.harvard.edu

Approximately 50% of the 40Ar that has been produced from radioactive decay of 40K over Earth history now resides in the atmosphere and continental crust. If one assumes that 40Ar concentrations of the upper mantle are representative of the whole mantle, ~ 30-50% of 40Ar would be missing. This is known as the Ar paradox and suggests a large mantle reservoir that is effectively isolated from the convective mantle with significantly higher concentrations of 40Ar than the upper mantle. However, geophysical and geochemical observations suggest whole mantle convection and imply that most, if not all, of the Earth's mantle has been processed through partial melting at either mid-ocean ridges or at hotspots.

We propose a new explanation for resolving the Ar paradox. We suggest that convective mixing and stirring of noble gas depleted subducted slabs with the ambient mantle can preserve high concentrations of volatiles in a mantle reservoir that has been extensively processed. The degree to which volatiles, such as Ar can be preserved is a function of how efficiently subducted slabs are mixed back into the mantle. Using geochemical reservoir modeling we quantitatively show that even though the lower mantle may be processed about once through partial melting, 40Ar concentrations can be a factor of 10 higher than in the upper mantle. Hence, we find no Ar paradox.  Predicted Ar concentrations in the upper and lower mantle reservoirs account for half the 40Ar produced over Earth history, with the remaining being outgassed to the atmosphere.