Investigations were conducted on 603 samples from a shipboard sampling program specifically designed to address sedimentary textures and petrography of the 31 units at Sites 1115, 1109, and 1118 (northern margin), four units at Site 1108 (footwall), and 10 units at Sites 1114 and 1116 (rift basin) (Fig. F1).
Petrographic analysis by means of optical microscopy was carried out on 54 samples and on the whole set of 232 shipboard thin sections for a total of 181 specimens from the northern margin sites, 51 in the rift basin sites, and 54 from the footwall sites. All 603 samples investigated underwent calcimetric analysis on the fine-grained (silty/clay) component (Fig. F2), 80 underwent X-ray diffraction (XRD) analysis, and 25 clay analyses were performed on separates. Calcimetry was performed using the "Dietrich-Fruhling" calcimeter. The method is based on the measurement of CO2 volume developed by acid reacting with the powdered rock. The total carbonate present is obtained using formulae that take into account the pressure, temperature, amount of previously weighed sediment, and the volume of CO2 developed.
XRD analyses were carried out using a Philips PW3710 diffractometer at the University of Genova, Italy. Samples were run between 2.5° and 70°2
, with a generator potential of 30 kV, a generator current of 22 mA (using CuK
radiation), a Ni filter, and a scan speed of 1°/min. The software used for XRD data reduction was Philips PC-APD diffraction software. Clay minerals were determined by XRD analyses, including measurements of air-dried and glycolated powder mounts.
Some samples, previously studied under the light microscope, were selected for the quantification of total organic carbon using a Carlo Erba CHNS-O EA1110 elemental analyzer before (total C) and after (organic C) removal of carbonates by HCl addition (Hedges and Stern, 1984). Cyclohexanone 2,4-dinitrophenylhydrazone was used as a standard. Instrumental detection limits are 0.5 µg for both C and N. The variation coefficient for instrumental bias is given as 0.8% for C and 1.3% for N.
Twenty-six samples were selected for study by electron microprobe, with particular attention to the occurrence of coarse-grained rock fragments. These analyses characterized representative compositions of mineral phases in igneous and metamorphic clasts; glass shards were also analyzed for major oxide components. The analyses were performed by a SEM-EDS microprobe at the University of Genova, equipped with an X-ray dispersive analyzer (EDAX PV9100). Operating parameters were 15-kV accelerating voltage, 2.20-nA beam current, and 1- to 10-µm beam diameter (up to 25 µm for the glass and clay minerals). Natural silicate standards were used (Na: tremolite [0.5 wt%], pargasite [2.30 wt%], albite [11.81 wt%]; Mg: forsterite [57.22 wt%]; Al: K-anorthoclase [20.12 wt%]; Si: diopside [55.12 wt%]; K: microcline [16.09 wt%]; Ca: clinopyroxene [17.22 wt%]; Ti: ilmenite [45.70 wt%]; Mn: rhodonite [45.15 wt%]; Fe: fayalite [67.54 wt%]; Cr: garnet [1.56 wt%]). Na2O and MgO contents analyzed in silicates by means of an EDAX microprobe are generally underestimated if the analysis is processed with current automatic methods. To overcome this problem, the background for Na (1.040 keV) and Mg (1.252 keV) was manually corrected to range between 0.9 and 4.2 keV.
Orthopyroxene and clinopyroxene analyses were calculated according to the stoichiometric method of simultaneous balancing to 4.00 cations and 6.00 oxygens. End-members were calculated in the following sequence: wollastonite, enstatite, ferrosilite, aegirine, jadeite, CaAl2SiO6, CaFeAlSiO6, CaCrAlSiO6, and CaTiAl2O6. Morimoto et al. (1988) and Rock (1990) nomenclatures were adopted.
The Ca amphiboles cation sum was balanced to 13-(Ca + Na + K), Fe3+ = 46 total cation charge, Fe2+ = Fetot - Fe3+, AlIV = 8 - Si, and AlIV = Altot - AlVI. Rock and Leake (1984) nomenclature was adopted.
Plagioclase analyses, on the basis of eight oxygens, were recalculated to total cations = 5. Ilmenites were recast on the basis of three oxygens. Magnetites and Ti magnetites were recast on the basis of four oxygens.
