Paleointensity estimates from oceanic gabbros; effects of hydrothermal alteration and cooling rate

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doi: 10.5047/eps.2013.03.015
Author(s): Usui, Yoichi
Author Affiliation(s): Primary:
Japan Agency for Marine-Earth Science and Technology, Institute for Research on Earth Evolution, Yokosuka, Japan
Volume Title: Earth, Planets and Space
Source: Earth, Planets and Space, 65(9), p.985-996. Publisher: Terra Scientific Publishing Company (TERRAPUB), Tokyo, Japan. ISSN: 1343-8832
Note: In English. 60 refs.; illus., incl. 5 tables
Summary: 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 [1022 Am2]. Volcanic records in the PINT database for 0.78-1.3 Ma revealed a mean virtual dipole moment (VDM) of 6.3±0.73 [1022 Am2]. 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.
Year of Publication: 2013
Research Program: ODP Ocean Drilling Program
Key Words: 05 Petrology, Igneous and Metamorphic; 18 Geophysics, Solid-Earth; Anisotropy; Atlantic Ocean; Cooling; Exsolution; Gabbros; Hydrothermal alteration; Igneous rocks; Leg 153; Magnetic anomalies; Magnetic hysteresis; Magnetic properties; Magnetic susceptibility; Magnetite; Magnetization; Metasomatism; Mid-Atlantic Ridge; Natural remanent magnetization; North Atlantic; ODP Site 921; ODP Site 922; ODP Site 923; ODP Site 924; Ocean Drilling Program; Oxides; Paleomagnetism; Plutonic rocks; Remanent magnetization; Secondary minerals; Thermoremanent magnetization
Coordinates: N233220 N233229 W0450150 W0450153
N233122 N233310 W0450156 W0450156
N233233 N233233 W0450154 W0450154
N233228 N233230 W0450052 W0450052
Record ID: 2015056297
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