Abstract
Gabbros recovered by previous ocean drillings were investigated in paleointensity and rock magnetic experiments. The young ages of the samples (ca. 0.78–1.3 Ma) enable a direct comparison between the plutonic paleointensity and volcanic data. Microscopic observations revealed two kinds of magnetite: needle-shaped exsolution in plagioclase and aggregate associated with the hydrothermal alteration of olivine. In Shaw paleointensity experiments, some samples revealed reasonable estimates, while some others showed an anomalously low ratio of natural remanent magnetization (NRM) versus thermoremanent magnetization (TRM). First-order reversal curve (FORC) diagrams indicated that the reasonable NRM/TRM were from non-interacting single domain magnetite exsolved in plagioclase, while the anomalously low NRM/TRM were from secondary magnetite associated with olivine. From the paleointensity results, the mean virtual axial dipole moment (VADM) was calculated to be 8.2 ±2.1 [1022Am2]. Volcanic records in the PINT database for 0.78–1.3 Ma revealed a mean virtual dipole moment (VDM) of 6.3 ± 0.73 [1022Am2]. The difference between them is consistent with the theoretical prediction of the cooling rate effect on paleointensity. These results indicate that oceanic gabbros are reliable paleointensity recorders.
Highlights
Past geomagnetic field intensity provides important information to constrain the deep Earth dynamics and evolution
The similarity between the First-order reversal curve (FORC) diagrams for successful samples (Fig. 7(a)) and those for plagioclase separates (Fig. 7(c)) indicates that the successful Shaw paleointensity estimates assess the magnetite exsolved in plagioclase (Fig. 1(a)) which is composed of non-interacting Single domain (SD) particles
The low natural remanent magnetization (NRM)/thermoremanent magnetization (TRM) is considered to reflect the low efficiency of chemical remanence (e.g., Stacy and Banerjee, 1974). These results indicate that FORC diagrams are effective in distinguishing fresh bulk samples with exsolved magnetite from samples dominated by magnetite produced by hydrothermal alteration. 5.2 Cooling rate effect on plutonic paleointensity
Summary
Past geomagnetic field intensity (paleointensity) provides important information to constrain the deep Earth dynamics and evolution. It is crucial to test the fidelity of various rock types as paleointensity recorders. The fidelity of lavas have been confirmed by comparing paleointensity estimates from historical lavas with a known field intensity (e.g., Coe and Gromme, 1973; Oishi et al, 2005), actual paleointensity reconstruction is far from trivial due to the complexity of natural materials. Many high-quality paleointensity data for the Precambrian originate from plutonic rocks (Tarduno et al, 2007; Selkin et al, 2008; Donadini et al, 2011). The fidelity of plutonic rocks as paleointensity recorders remains unclear, as the long cooling times of plutonic rocks prevents a direct comparison with a known intensity
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