Magnetization of upper mantle: Results from Oman Samail Ophiolite

Based upon the new data on magnetization of upper lithosphere collected from sea floor and ophiolite, we present a new model for oceanic lithosphere magnetization. In our model, the thickness of each magnetic layer was assumed to be equal to that of a standard seismic model for oceanic crust: eg. 0.5km for layer 2A. However, the thickness of the upper mantle magnetic layer can not be obtained by the similar, simple assumption. To determine the thickness of magnetic layer of upper mantle, we studied magnetization of peridotites successively sampled from a series of outcrops exposed at the crust-mantle boundary section in Wadi al Hilti, Oman Ophiolite. The average NRM intensity calculated for 2km from crust-mantle boundary is 0.6 (+/- 0.4) A/m, while that of the lower portion being 0.1 (+/- 0.08) A/m, indicating that the effective thickness of the magnetic layer of the crust- mantle boundary/upper mantle section is about 2km: upper 2km portion from the crust-mantle boundary is much strongly magnetized than the lower portion. On the basis of this finding, together with the recent magnetization data from oceanic crust and upper mantle, we made a new magnetization model for upper oceanic lithosphere. The total oceanic lithosphere magnetization calculated from the new model is about 17,500 A, which can explain the MAGSAT magnetic anomalies over Pacific Cretaceous Quiet Zone. Our new model also explains the missing magnetization of upper oceanic lithosphere that has been a long standing problem in geophysics.