13. Absolute Paleointensities from an Intact Section of Oceanic Crust Cored at ODP/IODP Site 1256 in the Equatorial Pacific

We have investigated the magnetic mineralogy and absolute paleointensity of basalt samples from Site 1256 cored during ODP Leg 206 and IODP Expeditions 309 and 312. The site is located on the Cocos Plate 5 km east of the transition zone between marine magnetic anomalies 5Bn.2n and 5Br (~15 Ma). The deepest hole, Hole 1256D, extends 250 m through sediments and 1257 m into the igneous upper oceanic crust generated by superfast seafloor spreading (>200 mm/yr) along the East Pacific Rise. This is the first drill site to penetrate an in situ and intact section of crust. The section consists of about 811 m of basaltic sheet flows and massive lavas, 346 m of sheeted-dike complex, and 100 m of gabbros and granoblastic dikes. Rock magnetic investigations included thermomagnetic analyses, alternating field, thermal demagnetization, saturation IRM, magnetic grain-size and coercivity analyses. Curie points identified titanomagnetites and titanomaghemites as the magnetic carriers and grain-size studies indicate that the carriers are mixtures of single domain (SD) and pseudosingle domain (PSD) grains. Using the Thellier-Coe method, we have attempted paleointensity determinations for 82 specimens sampled from different “stratigraphic” levels of the core. Partial thermal remanent magnetization (pTRM) checks were performed systematically one temperature step down from the last pTRM acquisition in order to document magnetomineralogical changes. The determinations were obtained from the slope of the pTRM gained vs. natural remanent magnetization lost in the Arai diagrams. Only about 6% of the samples (i.e. 5 samples) yielded marginally acceptable results. The paleofield estimated ranges from 16 to 28 μT and has a mean virtual axial dipole moment (VADM) of 5 × 10²² A/m², which is concordant with the average intensity for the period between 0 and 160 Myr (4 ± 2 × 10²² A/m²) and is about 2/3 of the strength of the present field (~8 × 10²² A/m²).