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13Absolute Paleointensities from an Intact Section of Oceanic Crust

Summary: 13Absolute Paleointensities from
an Intact Section of Oceanic Crust
Cored at ODP/IODP Site 1256
in the Equatorial Pacific
Emilio Herrero-Bervera and Gary Acton
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, ther-
mal demagnetization, saturation IRM, magnetic grain-size and coercivity analyses.
Curie points identified titanomagnetites and titanomaghemites as the magnetic car-
riers 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


Source: Acton, Gary - Department of Geology, University of California, Davis


Collections: Geosciences