SUMMARY

We summarize results and interpretations in the context of the problems discussed in "Scientific Objectives".

1. Magmatism and Tectonics

a. New ⁴⁰Ar/³⁹Ar radiometric age determinations for all of the Leg 183 basement drill sites on the Kerguelen Plateau and Broken Ridge (Duncan, 2002), as well as for Leg 120 Site 750 (Coffin et al., 2002), comprise the best age control for any submarine LIP. Ages, in general, young northward on the Kerguelen Plateau, from ~119 to 120 Ma at Site 1136 in the south to ~34 to 35 Ma at Site 1140 in the north. The first age determinations of drilled igneous basement from Broken Ridge are ~94-95 Ma (Duncan, 2002). Combined with the above results and previous age determinations for basalt from the Ninetyeast Ridge (Duncan, 1978, 1991), new age determinations from basalt and lamprophyre attributed to the Kerguelen hotspot in India, Western Australia, and Antarctica (Coffin et al., 2002; Kent et al., 2002) make the ~130-m.y. record of Kerguelen hotspot activity the best documented of any hotspot trace on Earth. Paleolatitudes of Kerguelen Plateau and Ninetyeast Ridge basalt suggest 3°-10° southward motion of the hotspot relative to the rotation axis, a finding that can be modeled by large-scale mantle flow influencing the location of the plume conduit (Antretter et al., 2002).

b. Mafic crust totaling ~2.5 x 10⁷ km³ has been produced from the Kerguelen hotspot source(s) since ~130 Ma. Magma output has varied significantly through time, beginning with low volumes contemporaneous with or postdating continental breakup in Early Cretaceous time, extending through at least one and possibly two peaks in Early and Late Cretaceous time into a preexisting and growing ocean basin, and finally tapering to relatively steady state output in Late Cretaceous and Cenozoic time (Figs. F3, F4).

c. The 25-m.y. duration of peak hotspot output at geographically and tectonically diverse settings (Figs. F3, F4) is difficult to reconcile with current plume models. Coffin et al. (2002) propose two alternatives to the standard Hawaii model for hotspots, one involving multiple mantle plume sources and the other a single, but dismembered, plume source.

2. Petrogenesis

a. The uppermost igneous basement of the submarine Kerguelen Plateau is dominantly tholeiitic basalt. Except for some basalt units from Site 1140, incompatible element abundances distinguish the tholeiitic basalt forming the LIP from MORB (Fig. F7). However, alkalic basalt overlies tholeiitic basalt at Broken Ridge Site 1142 (Fig. F5) (Neal et al., 2002). An important exception to basaltic volcanism occurs at Site 1139, Skiff Bank on the NKP, where the igneous basement is a bimodal sequence of alkalic lavas ranging from trachybasalt to trachyte and rhyolite (Fig. F5), which is interpreted to be part of a shield volcano constructed on the basaltic plateau (Kieffer et al., 2002).

b. The uppermost basement lavas forming the LIP range widely in Sr, Nd, and Pb isotopic ratios, and each drill site has distinctive isotopic characteristics (Figs. F8, F9); differences in source materials and their proportions are inferred. Site 1140 basalt erupted within 50 km of the SEIR axis at 34 Ma, and the geochemical characteristics of Site 1140 lavas can be explained by mixing, in varying proportions, components derived from the Kerguelen plume and the source of SEIR MORB (Weis and Frey, 2002) (Figs. F8, F9). In contrast, like lavas from Site 738 on the SKP (Mahoney et al., 1995), lavas from Site 1137 on Elan Bank have radiogenic isotopic ratios that reflect a small and variable but significant role for continental crust in their petrogenesis (Weis et al., 2001; Ingle et al., 2002b) (Figs. F8, F9). The isotopic evidence indicating a role for continental crust correlates with a relative depletion in abundance of Nb (Fig. F10).

c. What was the origin and location of the continental components contributing to the LIP lavas? Intercalated within the basaltic flows at Site 1137 is a fluvial conglomerate with clasts of garnet-biotite gneiss containing zircon and monazite of Proterozoic age (Nicolaysen et al., 2001). This result shows that continental lithosphere, perhaps derived from the Eastern Ghats of eastern India for Site 1137 (Nicolaysen et al., 2001; Ingle et al., 2002a, 2002b), occurs at shallow depths within the Indian Ocean lithosphere. Although no evidence supports a continental component in Cenozoic lavas associated with the Kerguelen plume (i.e., basalt at Site 1140 and Kerguelen archipelago), these components are widely distributed in Cretaceous basalt forming the uppermost basement of the LIP. The most compelling examples are at Site 738 (SKP), Site 1137 (Elan Bank), and Site 747 (CKP) (Fig. F1) (Mahoney et al., 1995; Ingle et al., 2002b; and Frey et al., 2002b, respectively).

d. Basalts from Sites 747, 750, and 1139 are distinguished by unusually low ²⁰⁶Pb/²⁰⁴Pb ratios (<17.8) (Fig. F9). The origin of this low ²⁰⁶Pb/²⁰⁴Pb component may reflect a petrogenetic role for continental material, perhaps lower crust for Site 747 basalt (Frey et al., 2002a), but this component is quite different from the continental component contributing to basalt at Sites 738 and 1137 (Fig. F9).

e. A plume source, more complicated than a single plume head and stem model (Fig. F3), for the basalt forming the Kerguelen Plateau and Broken Ridge remains a viable hypothesis. Like the active plumes of Hawaii, Iceland, and the Galapagos, lavas forming the Kerguelen Plateau and Broken Ridge are isotopically heterogeneous. Difficult questions are to what extent is this heterogeneity intrinsic to the plume and to what extent does the heterogeneity reflect mixing between quite different components, such as depleted asthenosphere, oceanic lithosphere, and continental lithosphere. For lavas from some of the Kerguelen Plateau and Broken Ridge sites, the isotopic heterogeneity undoubtedly reflects mixing of plume-related components with components derived from depleted asthenosphere (Site 1140) or continental lithosphere (Sites 738, 747, and 1137).

Another important question is how the geochemical characteristics of the plume changed from ~120 to 24 Ma. For radiogenic isotopic ratios in lavas of varying age, aging is an obvious cause of heterogeneity. In a plot of initial ⁸⁷Sr/⁸⁶Sr vs. initial ¹⁴³Nd/¹⁴⁴Nd, data for five sites (749, 1136, 1138, 1137, and 747) define a trend parallel to that defined by Kerguelen archipelago flood basalt (Fig. F8). The offset of the archipelago basalt data to higher ratios can largely be explained by aging of the mantle source from ~120-100 to 30-24 Ma, the age of archipelago flood basalt (see arrow in Fig. F8).

The effects of aging on Pb isotope ratios are more complex to evaluate because of the Pb isotopic diversity of Kerguelen Plateau and Broken Ridge lavas (Fig. F9). Undoubtedly, some of the diversity in Pb isotopic ratios reflects the sensitivity of Pb to components derived from continental crust. Three sites (747, 750, and 1139) have very low ²⁰⁶Pb/²⁰⁴Pb that cannot be related to the higher ²⁰⁶Pb/²⁰⁴Pb of the other LIP lavas by aging. Several sites (738, 1137, and, possibly, 1141/1142) have ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios that have been increased by incorporation of continental components; consequently, it is difficult to evaluate radiogenic ingrowth of these ratios. Nevertheless, five sites (749, 1136, 1137, 1138, and 1141/42) have ²⁰⁶Pb/²⁰⁴Pb of ~18, approximately 0.2 to 0.6 lower than the archipelago basalt. As with the Sr and Nd isotopic ratios, the offset of these older lavas to lower ²⁰⁶Pb/²⁰⁴Pb may reflect radiogenic ingrowth in the source from ~120-100 to ~30-24 Ma; however, for reasonable ²³⁸U/²⁰⁴Pb ratios (<10) in the source this increase in ²⁰⁶Pb/²⁰⁴Pb is only <0.15 (see caption for Fig. F9). In summary, aging of the plume cannot account for most of the Pb isotopic differences between lavas forming the Cretaceous LIP and the Cenozoic Kerguelen archipelago (Fig. F9), but aging can account for the systematic offset in Sr and Nd isotopic ratios (Fig. F8).

3. Eruption Environment and Impact

a. Subsidence estimates for Site 1140 and other ODP drill sites indicate that the various parts of the Kerguelen Plateau subsided at a rate comparable to that for normal Indian Ocean lithosphere.

b. Sulfur release to Earth's atmosphere from 120 to 95 Ma, when the Southern and Central Kerguelen Plateau were constructed, could have had global climatic effects if the high latitude (low tropopause altitude) of the Kerguelen Plateau enabled powerful lava fountains to create convecting eruption plumes that reached the stratosphere. The previously unrecognized significant volume of explosive felsic volcanism that occurred when the Kerguelen Plateau and Broken Ridge were subaerial would have further contributed to the effects of this plume volcanism on global climate and environment.

c. Based on a decline in the seawater Sr isotopic evolution curve from ~122 to 112 Ma, submarine volcanism and hydrothermal activity as well as subaerial weathering and riverine transport during formation of the Kerguelen Plateau and Broken Ridge may also have had important effects on ocean chemistry.

d. Many of the subaerial lava flows recovered during Leg 183 drilling are of a type—rubbly pahoehoe—that appears to be distinct from slab and other recognized types of pahoehoe occurring in Hawaii. However, rubbly pahoehoe may be a common flow type in Iceland and the Columbia River Basalt Group.

4. Paleoenvironments and Paleoceanography

a. The CKP supported a dense conifer forest with various fern taxa and early angiosperms in late Albian to earliest Cenomanian time. By latest Cenomanian time, the CKP had subsided to a depth that allowed open marine sediments to accumulate. Cenomanian-Turonian time was warmer than subsequent Campanian-Maastrichtian time, when the paleoceanography of the CKP was characterized by little upwelling and a high degree of oxygenation. In contrast, by late Maastrichtian time, the northeast flank of the CKP was the site of significant upwelling and lower oxygenation.

b. An apparently complete Cretaceous/Tertiary boundary section at CKP Site 1138 is recognized by assemblage succession, isotopic signature, color change, and reworking of nannofossil taxa. A water mass boundary existed over the Kerguelen Plateau in earliest Paleocene time but had disappeared by late Paleocene time. A reasonably complete record of the Late Paleocene Thermal Maximum was recovered at Site 1135. At the beginning of middle Eocene time, water temperatures began to decline. Productivity of radiolarians increased in Oligocene time, presumably as water temperatures continued to decline. Sedimentation rates from early Oligocene to middle Miocene time vary from 9.5 to 18 m/m.y., averaging 12.6 m/m.y. Skiff Bank accumulated mixed terrigenous-pelagic sediments during Oligocene and early Miocene time, presumably related to glacioeustatic/climatic changes. By late middle Miocene to early Pliocene time, the CKP was south of the Antarctic Polar Frontal Zone.