IODP Proceedings    Volume contents     Search


Methods and materials

Petrographic data were collected from 56 thin sections covering the depth range from 324 to 1255 meters below seafloor (mbsf) in the drill hole, including the following lithostratigraphic units: the “lava pond,” the inflated flows, the sheet and massive flows, the transition zone, and the upper part of the sheeted dike complex (Fig. F1). Concerning petrographic terms and rock names we follow those from the “Site 1256” chapter and Wilson, Teagle, Acton, et al. (2003). Phenocryst abundances were estimated by digital images processed in a similar way to that described in the “Methods” chapter.

Primary magmatic phases in 16 basalts were analyzed (red points, Fig. F1) by using polished thin sections and a Cameca SX 100 electron microprobe equipped with five spectrometers and “Peak sight” software. All measurements were made at 15 kV beam potential and 15 nA beam current, with a focused beam and a count time of 20 s for each element. For some Fe-Ti oxides showing exsolution lamellae, a broad beam 20 to obtain the initial unexsoluted composition. Matrix correction was performed according to Pouchou and Pichoir (1991). Limits of detection (in weight percent) are as follows:

  • SiO2 in Fe-Ti oxides = 0.05,
  • TiO2 in pyroxene = 0.03,
  • Al2O3 in Fe-Ti oxides = 0.09,
  • Cr2O3 in pyroxene = 0.18,
  • Cr2O3 in Fe-Ti oxides = 0.19,
  • MgO in plagioclase = 0.04,
  • MgO in Fe-Ti oxides = 0.05,
  • CaO in Fe-Ti oxides = 0.05,
  • Na2O in pyroxene = 0.08,
  • Na2O in Fe-Ti oxides = 0.13,
  • K2O in silicates = 0.08, and
  • K2O in Fe-Ti oxides = 0.13.

For establishing relationships between mineral and bulk compositions, we chose samples for which bulk analyses are available (see the “Site 1256” chapter and Teagle, Wilson, Acton, and Vanko, 2007).