We investigate the magnetic mineralogy and absolute paleointensity of oceanic basalt samples from Hole 1256D, cored duringOcean Drilling Program (ODP) Leg 206. Hole 1256D is located on the Cocos Plate about 5 km east of the transition zone between marine magnetic anomalies 5Bn.2n and 5Br (~15 Ma). During Leg 206, the hole penetrated 502 m into basalts of the upperoceanic crust that was generated by superfast seafloor spreading (>200 mm/yr) along the East Pacific Rise. Rock magneticinvestigations included continuous low field (k-T) thermomagnetic analyses, alternating field (AF) and thermaldemagnetization, optical microscopy, saturation isothermal remanent magnetization (SIRM), and magnetic grain size analyses.Following the removal of a drilling overprint, AF and thermal demagnetization paths for most samples decay linearly to theorigin on orthogonal vector end point diagrams, suggesting that a stable characteristic remanent magnetization component canbe resolved. Optical microscopy and k-T (Curie points) identified titanomagnetites and titanomaghemites as the main magneticcarriers and grain size studies indicate that the carriers are either single domain (SD) and/or pseudosingle domain (PSD) innature. Using the modified Thellier-Coe double heating method, we determined absolute paleointensity determinations for 51specimens sampled from different ``stratigraphic'' levels of the core. pTRM checks were performed systematically onetemperature step down the last pTRM acquisition in order to document magnetomineralogical changes during heating. Thetemperature was incremented by steps of 50°C between room temperature and 500°C and every 25-30°C for highertemperatures. The paleointensity determinations were obtained from the slope of the Arai diagrams. Special care was taken tointerpret the Arai diagrams within the same range of temperatures lower than 300°C unless a clear and unique slope waspresent over a higher range of temperatures. Only about 10 percent of the samples yielded acceptable results. The paleofieldestimated from these samples ranges between 28 to 16 micro Teslas (i.e., VADM of 6 to 4 x 10²² A/m²), which isconcordant with the average paleofield intensity for the period between 0-160 Myr (4 ± 2 x10²² A/m²) and half ofthe strength of the present field (~8x10²² A/m²).

[ return to abstract list ]