The analytical results are presented in Table 1. Mg# is calculated as 100 Mg/(Mg + Fe2+) (atomic%), with the iron oxidation adjusted to Fe2O3/FeO = 0.15 (wt%).
All the analyzed basalts are altered to some extent. Glass and olivine are altered to smectitic and saponitic clays, and vesicles are lined or filled with clay and carbonate (Teagle and Alt, Chap. 13, this volume). Elements known to be mobile during such alteration are primarily K, Rb, and Ba, and variations in these elements should be treated with caution. Also, major elements, such as Na, Mg, and Si, may be mobile, but we have not found any evidence of significant loss or gain of these elements in the Leg 163 samples. There is also no evidence in the Leg 163 samples for mobility of yttrium, as found in the basalts from Site 918 (L.M. Larsen et al., 1998b).
Site 989 is located 43 km from the Southeast Greenland coast. Hole 989B penetrated to 84.2 meters below seafloor (mbsf). The underlying basement was not reached. In total, 80 m of basalt were drilled, which form only two flow units. Unit 989-1 (see Table 1 to correlate units with standard Ocean Drilling Program core designations) is at least 69 m thick and is the thickest lava flow yet recorded from a SDRS (Duncan, Larsen, Allan, et al., 1996). It is a compound flow of aphyric basalt consisting of a number of flow lobes, 0.1-10 m thick, and the three analyses from this unit show some slight intraflow variability, with MgO = 7.1%-8.0%. Unit 989-2 is a massive flow of sparsely phyric basalt at least 11 m thick, and the two analyses from this unit are very similar. Moreover, there are only slight compositional differences between Unit 989-1 and 989-2, the largest being lower FeO* and higher CaO and Cr in Unit 989-2.
The basalts at Site 989 are evolved, with Mg# = 53.4-58.9 and MgO = 7.22%-8.16% (calculated free of volatiles). They have low TiO2, K2O, Rb, Sr, Nb, Zr, and rare earth element (REE) contents. As already shown on board ship, they are distinctly different from the basalts at Site 917, especially from the Site 917 lower series basalts with which they were originally thought to correlate (Duncan, Larsen, Allan, et al., 1996) but which have much higher Ba and Sr than the Site 989 basalts. The Site 989 basalts show much greater similarities to the postbreakup successions at Site 990 and Site 918, as discussed below.
Site 990 is located 52 km from the Southeast Greenland coast, very close to the previously drilled Site 915, which it was intended to extend. The volcanic succession at Site 990 is overlain by two sedimentary units of probable Eocene age (Duncan, Larsen, Allan, et al., 1996). Unit I is an ~10-m-thick mixed cobble conglomerate from which almost only cobbles without matrix were recovered. These consist of highly altered basalt, gabbro, dolerite, quartzite, and siltstone. The depositional environment indicated is active erosion in a high-relief area and is interpreted as a high-gradient stream or a fan delta (Duncan, Larsen, Allan, et al., 1996). The underlying Unit II is an ~20-m-thick, dark brown, clayey volcaniclastic breccia with highly altered angular clasts, interpreted as a debris flow originating from the altered top of the volcanic pile (Duncan, Larsen, Allan, et al., 1996). Beneath Unit II is the reddened and completely altered top of the uppermost lava flow.
The volcanic succession was drilled from 211.9 mbsf to the bottom of the hole at 342.7 mbsf. This 130.8-m interval contains 13 lava flows, most of them with red-oxidized flow tops. In addition, Unit 990-12 contains a small chilled vein or dikelet of basalt, which will be considered as a separate unit. The lithology of the basalts varies from aphyric to plagioclase-olivine phyric, with additional minor augite phenocrysts in some flows.
Seven of the mafic clasts in the conglomerate were analyzed to determine the provenance of the eroded material, specifically to search for indications that the Site 917 lower series was exposed and being eroded, because this succession is 450 m thick at Site 917 but appears to be completely missing at Site 989, only 5 km farther west. The lowermost clast is large (22 cm cored), and chemically it is in all respects identical to the underlying lava succession at Site 990, despite the ~20-m-thick debris flow between the cobble conglomerate and the lava pile. The overlying six smaller clasts have a consistently different chemical character. They are high-Al, high-K basalts with low Fe and high Ba and Sr. Their trace elements and ratios are rather variable. A shipboard analysis of a similar clast was interpreted by Duncan, Larsen, Allan, et al. (1996) as derived from the lower series at Site 917 because of the high Ba and Sr. However, the consistent chemical character of the six clasts analyzed (Table 1) makes this interpretation unlikely. Only three of the 28 analyzed lower series basalts have such high K2O contents, and none of them has the high Al2O3 and low total FeO of the clasts (Fitton et al., 1998b; L.M. Larsen et al., 1998a). We believe that the clasts are derived from an altogether different mafic association, possibly belonging to the Precambrian basement. It is not surprising that elevated basement on the continental margin should be available for erosion during breakup.
Thus, there are no positive indications that the Site 917 lower series was being eroded. The reason why the series is missing at Site 989 may rather be depositional offlap of the volcanic pile toward the embryonic rift zone, although erosion or faulting cannot be excluded.
The basalt lava flows at Site 990 are evolved, with Mg# = 49.3-62.3, MgO = 6.27%-8.23%, and low contents of incompatible elements. They are generally quite similar to the basalts from Site 989 and the single basalt from Site 915. An exception is the dikelet in Unit 990-12 that, at a similar degree of fractionation, is considerably more enriched in incompatible elements than any of the lava flows.
The drilled lava succession shows only limited compositional variation, for example, TiO2 = 0.7%-1.3%. Some trace elements show larger variations, e.g., Cr = 250-36 ppm, Zr = 39-72 ppm, and Ba = 24-276 ppm. As discussed below, the variation can be explained by crystal fractionation of the magmas.