6. Site 1138¹

Shipboard Scientific Party²

BACKGROUND AND OBJECTIVES

Site 1138 lies on the central Kerguelen Plateau (CKP) ~150 km north-northwest of Site 747 and 180 km east-southeast of Heard Island (Fig. F1). Basalts recovered at Site 747 erupted at ~85-88 Ma, on the basis of ⁴⁰Ar/³⁹Ar data (Pringle et al., 1994; Storey et al., 1996) and biostratigraphy from the overlying sediments (Watkins, 1992). In contrast, Heard Island is dominated by Holocene volcanism (Barling, 1990). A major objective at Site 1138 was to determine if the age of the uppermost basaltic crust of the CKP is 85 Ma at more than one location. Also, geochemical characteristics of Site 747 basalts indicate a continental lithosphere component different from that in Site 738 basalt. Specifically, Site 747 basalts overlap in ⁸⁷Sr/⁸⁶Sr with Kerguelen Archipelago lavas, but they range to lower ¹⁴³Nd/¹⁴⁴Nd (Fig. F9 in the "Leg 183 Summary" chapter); have the lowest ²⁰⁶Pb/²⁰⁴Pb found in lavas from the Kerguelen Plateau and the Kerguelen Archipelago (Fig. F10 in the "Leg 183 Summary" chapter); and are outside the oceanic basalt field in a Th/Nb-La/Nb plot defining a trend to high La/Nb without a corresponding increase in Th/Nb (Fig. F11A in the "Leg 183 Summary" chapter). All of these geochemical characteristics are consistent with a continental lithosphere component such as Archean granulites (e.g., Lewisian granulites [Scotland] have very low ²⁰⁶Pb/²⁰⁴Pb [<14], high La/Nb, and low Th/Nb compared to primitive mantle [Dickin, 1981; Weaver and Tarney, 1984]). Archean granulites, found today along the conjugate Antarctic and Indian margins, may have been fragmented and incorporated into the embryonic Indian Ocean mantle during continental breakup and, subsequently, sampled when the Kerguelen Plateau formed. Therefore, we were especially interested in comparing the petrology and geochemistry of basaltic basement from this second CKP drill site with basalt from the southern, northern, and Elan Bank sectors, as well as Heard Island and the Kerguelen Archipelago. We located Site 1138 at a water depth of 1141 m on Australian Geological Survey Organisation Rig Seismic multichannel seismic (MCS) line RS179/101 (Fig. F2). The location is relatively simple structurally and seismic stratigraphically (Fig. F3). Interpreted igneous basement contains some internal reflections with a slight apparent dip to the southwest. The top of basaltic basement, however, is flat lying. Five seismic sequences overlie basement and total ~700 m in thickness.

The main objectives at this site were to

1. Characterize the petrography and compositions of the lavas, with particular focus on evaluating the presence of a continental lithosphere component and its geochemical characteristics;
2. Determine the age of the lavas, testing the hypothesis that the uppermost igneous basement of the CKP is ~85 Ma, the age yielded by basalt from Site 747 (Pringle et al., 1994; Storey et al., 1996);
3. Determine the physical characteristics of the lava flows;
4. Identify the environment of eruption (subaerial or submarine);
5. Obtain minimum estimates for the age of basement from overlying sediment;
6. Estimate the duration of possible subaerial and shallow marine environments from the sedimentary and igneous record;
7. Recover Cenozoic sediment with a significant volcanic component derived from Heard Island and its surrounding platform. Such material can provide information on the growth and composition of the volcanism that formed this edifice;
8. Determine the facies of the seismic stratigraphic sequences;
9. Define the ages of seismic sequence boundaries; and
10. Determine the paleoceanographic history of the region just south of the Polar Front.