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doi:10.2204/iodp.proc.304305.205.2009

Methods

Core description and graphic logs

Core recovered from IODP Hole U1309D was evaluated to produce a uniform visual core description (VCD) in the Bremen Core Repository (BCR). Graphic logs were produced and are presented as columns in 15 m intervals. See the graphic logs in “Supplementary material.”

Observations of the archive section halves of the entire core were made over a period of 15 days by B.E. John and M.J. Cheadle (at the BCR, May 2006) to maintain consistent observations over the entire core length (1415 m). Rock type and fabrics for whole-core rounds not present were determined from shipboard VCDs. Barrel sheets showing magnetic susceptibility were used during observation to identify oxide and oxide-bearing gabbro not originally noted during shipboard core description. Lithologic names were given to core intervals based on modal mineralogy observed from cut and round faces, thin section descriptions, and magnetic susceptibility logs using the classification and nomenclature outlined in the “Methods” chapter. Differences between a given interval in these logs and the shipboard VCDs likely result from new information that was not available during shipboard core description.

Each log includes the estimated depth to specific intervals based on curated depth to the top of each core section. The relative depth to an interval (contact, zone of increased grain size, mylonitic deformation, etc.) in subsequent cores may therefore have an error as large as ~20 cm (relevant for core-log integration). By IODP convention, when recovery is less than 100%, all recovered material is curated from the top of the cored interval, which produces an additional shift in the reported recovery depths. All intervals are shown based on the above. Recovery exceeding 100% is not uncommon and results in (typically minor) overlap of two core sections in depth. These intervals are indicated by gray bars in the corresponding susceptibility plots. When overlap occurs, the lithology shown is for the lower section.

Magnetic methods and data

Remanence components were determined from detailed thermal demagnetization studies conducted at the Scripps Institution of Oceanography (see Morris et al., 2009). Before being heated, all specimens were subjected to low-temperature (77 K) treatment to preferentially remove remanence carried by coarse multidomain magnetite. Best-fit remanence components were calculated by principal component analysis (Kirschvink, 1980). Because the core pieces are azimuthally unoriented, polarity was determined primarily with the inclination value along with additional information provided by the known positive inclination of the drilling-induced remanence and/or comparison of results from multiple specimens in a contiguous core piece.

Circles on the graphic logs represent locations of discrete shore-based paleomagnetic specimens; white circles represent data accepted after analysis; gray circles represent data rejected. Raw demagnetization data will soon be available from the International Magnetics Information Consortium (MagIC) database (earthref.org/​MAGIC/​index.html).

Magnetic component codes associated with each sample reported are as follows:

  • R1 = high-temperature reversed component.
  • N1 = intermediate-temperature normal component.
  • R2 = low-temperature reversed component.

In several specimens a single polarity component may be curved or have two distinct linear segments that differ in either inclination or declination. When the latter occurs, the suffix “.1” or “.2” is appended to the component designation for the higher and lower temperature subsegments, respectively. Magnetization components in which the vector difference sum was reduced to <50% by either low-temperature treatment or the first heating step (100°C) or in which the maximum angular deviation of the principal component was >10° were rejected. Rejected components are indicated in brackets.

Shipboard magnetic susceptibility data were downloaded from the JANUS database; in a small number of cases we were able to correct shipboard data that were measured with incorrect section numbers. Susceptibility data were collected at 2 cm intervals; data ≥4 cm (the approximate half-width of the sensor response) from the end of a core piece are plotted in red and reflect the most robust data. The susceptibility meter used has a maximum value of 9999 × 10–5 SI. When susceptibility values exceed 0.1 SI, the most significant digit is not recorded (e.g., 13,000 × 10–5 SI is recorded as 3000 × 10–5 SI), leading to apparent high-amplitude fluctuations, especially in oxide-rich zones.

Pb/U zircon geochronology methods and data

Pb/U zircon ages for 18 samples of evolved Fe-Ti oxide gabbro and felsic dikes reported by Grimes et al. (2008) are plotted at sampled depths between 40 and 1415 mbsf in Hole U1309D. Stars (and associated ages) on the graphic logs represent locations of dated samples, reported as weighted mean ages (with 2σ errors) for spots on 5–14 individual zircons, analyzed with the Stanford-U.S. Geological Survey sensitive high-mass resolution ion microprobe (SHRIMP).