163 Preliminary Report

Site 988

Site 988 is located 56 km east of the East Greenland coast, within the northern drilling transect EG66 (Figs. 2, 4). The site was selected to penetrate deeply into the featheredge of the SDRS that overlies the transition zone between continent and ocean crust. The primary drilling objectives at this site were to determine the composition, age, and eruption environment of the SDRS in a position close to the Iceland-Greenland Ridge for comparison with the distal SDRS cored during Leg 152.

Lithologic Unit I is a thin layer (0-10 m below seafloor [mbsf], estimated from seismic profiles and the drillers' log; 0.4 m recovered in Core 163-988A-1R) of Quaternary(?) glaciomarine sediments, including rounded cobbles of gabbro, white and pink fine-grained granite, dark gray to black aphyric basalt, and gneiss. The compacted diamicton recovered at Leg 152 Sites 914 through 916 (Figs. 2, 4) to the south is absent at this site. The glaciomarine rocks unconformably overlie basaltic basement (igneous Units 1 and 2) at about 10 mbsf.

Two flow units were recognized in the core recovered from the interval 10-32 mbsf. Igneous Unit 1 is a dark, greenish gray, plagioclase-pyroxene-olivine-phyric basalt. The upper contact was not recovered, but the lower contact is preserved in Section 163-988A-5R-1, (Piece 10); the thickness of the unit is between 19 and 21 m. The unit has a massive aspect and is sparsely vesicular; the vesicles are filled with smectite/saponite and some contain the zeolite chabazite. The glassy groundmass and the majority of the sparse olivine phenocrysts have been replaced by brown smectitic clay. Other phases are unaltered. Igneous Unit 2, of which only 90 cm was recovered from the interval 29 to 32 mbsf is, by contrast, highly to completely altered, with relict clinopyroxenes in a clay matrix. Texturally, this unit appears to represent the top of a fragmental, perhaps scoriaceous, basalt flow top.

Shipboard X-ray fluorescence (XRF) data show that both units have high Nb/Zr (0.12) and Ce/Y (1.2), identical to Tertiary basalts from Iceland (Fig. 5). The low Ni content and low Mg# of Unit 1 (about 74 ppm and 0.50, respectively) are consistent with the evolved three-phase phenocryst assemblage of this basalt. Both units were most likely emplaced as lava flows, but the absence of an upper contact in Unit 1 means that we cannot eliminate the remote possibility that it is a sill. The highly oxidized aspect of Unit 2 is consistent with emplacement as a flow in a subaerial environment.

The basalts at Site 988 are subhorizontal to gently dipping and show only modest amounts of brittle deformation. Subhorizontal magmatic flow banding is noted locally. Subhorizontal calcite-filled veins occur at a spacing of 0.5-1.0 m. Joints and veins filled by calcite and clay were also found in a subhorizontal and subvertical bimodal distribution. An alteration halo around one subvertical fracture was noted.

Paleomagnetic data for the Site 988 basalts were obtained from the archive-half sections of Cores 163-988A-1R through 5R using the shipboard cryogenic magnetometer. The initial natural remanent magnetization (NRM) intensity of the core was between 5 and 10 A/m. Demagnetization of each section by up to 30 mT removed a steep downward-dipping remanence, possibly acquired by drilling, and reduced intensities to 5%-10% of initial values. The core has a consistent reversed polarity with two exceptions. One interval (section 163-988A-1R-2, 5-55 cm) of apparent normal polarity occurs within what appears to be a single thick flow. As this is an unlikely occurrence, we conclude that two pieces of core were inverted during labeling and splitting. Another interval of normal polarity is located in the highly altered clay-rich flow top material of Unit 2 present near the bottom of the last section (section 163-988A-5R-2, 0-10 cm). The magnetic orientation of this material was probably affected by the high degree of secondary alteration observed or possibly by the drilling process.

Measurements of index properties from minicores yielded an average bulk density of 2.907 g/cm3, an average grain density of 2.969 g/cm3, porosities of 10% or less, and P-wave velocities from 4.94 to 5.73 km/s for igneous Unit 1. P-wave velocities measured directly in the working half of the split core increase downcore through Unit 1, from values of 5.5-5.7 km/s at the top of Unit 1 to 6.0 km/s near its base. An intermediate level (26.14-26.18 mbsf) with unusually high velocities appears to correlate with flow banding observed at that level (Section 163-988A-4R-1 [Piece 5, 32-38 cm] ). The highly vesicular, fragmented, and altered basalts from Unit 2 were not measured.

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