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Why is the remanent magnetic intensity of Cretaceous MORB so much higher than that of mid to late Cenozoic MORB?

Daming Wang*1, Rob Van der Voo*1 and Donald R. Peacor*1

¹ Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, USA

View larger version (16K): [in this window] [in a new window]   Figure 1. Natural remanent magnetization (NRM) intensity of mid-oceanic-ridge basalt samples from the literature ([A] Bleil and Petersen, 1983; and [B] Johnson and Pariso, 1993, their Figure 2A redrafted on a linear scale) and this study (C). Average NRM intensity has been corrected to the paleo-latitude for A and B. Open symbols represent samples younger than 65 Ma and filled symbols represent samples older than 65 Ma in this and all other figures. In C, circles represent data from Zhou et al. (2001), triangles are from this study. Note that the units of the NRM are weight-normalized in C and volume-normalized in A and B

View larger version (33K): [in this window] [in a new window]   Figure 2. Locations of the Ocean Drilling Program and Deep Sea Drilling Program sites used for the samples newly described in this study

View larger version (15K): [in this window] [in a new window]   Figure 3. x, z, Ms, Mrs, and Mrs/Ms are plotted as function of natural remanent magnetization intensity. Regression lines through these data sets (not shown) all have insignificant slopes (with R2 less than 0.17)

View larger version (13K): [in this window] [in a new window]   Figure 4. A: Composition (x) values of mid-oceanic-ridge basalt samples as a function of age. B: Degree of oxidation (z) values as a function of age. Circles represent data from Zhou et al. (2001), and triangles are from this study. Samples with ages between 0 and 5 Ma have not been included because we are interested mainly in the variations between 5 and 160 Ma

View larger version (13K): [in this window] [in a new window]   Figure 5. Curie temperatures (Tc) plotted as a function of the degree of oxidation (z). Tc values are adjusted to x = 0.6. The solid line represents the reference curve established by Nishitani and Kono (1983). Open symbols represent samples with ages between 5 and 65 Ma and filled symbols represent samples older than 65 Ma; circles are from Zhou et al. (2001) and triangles are from this study

View larger version (12K): [in this window] [in a new window]   Figure 6. Ms and Mrs are plotted as function of age. Circles represent data from Zhou et al. (2001), squares from Matzka et al. (2003), and triangles from this study

View larger version (18K): [in this window] [in a new window]   Figure 7. NRM/Mrs, NRM/Ms, and corrected NRM (see text for explanation) plotted against age. NRM—natural remanent magnetization

View larger version (16K): [in this window] [in a new window]   Figure 8. Two representative orthogonal demagnetization diagrams (A, B) and the corresponding J/Jmax (C, D) illustrating the decay of the remanence intensity with applied temperature. A, C: 80 Ma sample 543A16R3, 45–47. B, D: 112 Ma sample 418A60R2, 50– 52

View larger version (13K): [in this window] [in a new window]   Figure 9. A: The angle , which is directionally defined as occurring between coexisting natural remanent magnetization (NRM) components, is plotted as a function of sample age; = 0° when the components are parallel, and = 180° when the components are antiparallel. For samples with apparently only one NRM component (insofar as resolved from thermal demagnetization), is assigned to 0°. B: NRM intensity is plotted as a function of . Filled symbols represent samples older than 65 Ma, open symbols represent samples younger than 65 Ma

View larger version (16K): [in this window] [in a new window]   Figure 10. Plot of Mrs/Ms as a function of age

View larger version (18K): [in this window] [in a new window]   Figure 11. Plots of virtual dipole moment (VDM) as a function of time (top after Tauxe and Staudigel, 2004, and bottom after Tarduno et al., 2001). In the top diagram, an upper envelope is sketched in, which ignores the uppermost outliers, assuming they are unrepresentative. The bottom diagram includes only results with at least nine Thellier-Thellier determinations based on more than 25 samples; an envelope of the 1-error bars has been added. The horizontal lines at 8 x 1022 Am2 represent today's magnetic field strength