CONCLUSIONS

  1. Three diamict matrix samples, recovered at Sites 1097 and 1103 (ODP Leg 178) within sequence group S3, contain variable proportions of fragments of volcanic rocks, mostly within the 100- to 150-µm grain-size range. They are characterized by rounded to subrounded shapes that suggest mechanical reworking. Fresh glass was only recognized in grains from Site 1103.
  2. 40Ar-39Ar incremental laser-heating experiments over three steps on numerous small fractions yielded ages of 75-173 Ma for Sample 178-1097A-27-1, 35-58 cm (220 mbsf); 18-57 Ma for Sample 178-1103A-31R-2, 0-4 cm (290 mbsf); and 7.6-50 Ma for Sample 178-1103A-36R-3, 4-8 cm (340 mbsf). Based on SEM-EDS and EMP investigations, the ages for samples from Site 1103 are mainly from groundmass glass. The youngest apparent age (7.6 Ma) for the volcanic activity detected at 340 mbsf, close to the bottom of Hole 1103A, is compatible with the age range of the diatom Actinocyclus ingens v. ovalis Zone (6.3-8.0 Ma) of the interval 320-355 mbsf at the same site (Shipboard Scientific Party, 1999c) and with the maximum ages derived from the strontium isotope composition of barnacle fragments (Lavelle et al., Chap. 27, this volume). However, use of the 40Ar-39Ar data to constrain the age of Sequence S3 at 340 mbsf would require the support of further research.
  3. The overall negative correlation between apparent 40Ar-39Ar ages and sample depth at Site 1103 is compatible with glacial erosion that affected with time deeper levels of a volcanic sequence previously deposited on the continent.
  4. On the whole, the three samples show a negative correlation between apparent ages and Cl/K ratios. This is compatible with an increase of Cl content with decreasing age of volcanic activity.
  5. Incremental laser-heating analyses over few steps on numerous small fractions may be used to place age constraints on mixed populations of volcanic glass-bearing groundmass clasts hosted in sediments. This is the only available means when the small grain size (in this study as low as 100-150 µm) of homogenous grains and/or the low potassium contents do not allow single-grain laser fusion analyses. From a broad perspective, this approach (joined to single-grain major element analyses on glass by the electron microprobe and, possibly, trace element determinations by the laser ablation inductively coupled plasma-mass spectrometry [ICP-MS] technique [e.g., see Bryant et al., 1999]) appears to be useful in provenance studies. Alternatively, where volcanic sequences have been completely eroded, it may represent an additional tool in defining the temporal and compositional evolution of igneous activity in plate margin settings.

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