Leg 183 objectives focused on four major problems related to the formation and evolution of a giant LIP:
The most significant question to answer is "how much magma was erupted over what time interval"?—more specifically (1) "what is the age of the uppermost volcanic basement"? (2) "do eruption ages vary systematically with location on the plateau"? (3) "was the growth episodic or continuous"? and (4) "did the plateau grow by lateral accretion (i.e., similar to Iceland) or by vertical accretion and underplating"? Answers to these questions are provisionally provided by dating the oldest sediment above basaltic basement; more definitive results will come from postcruise 40Ar/39Ar dating of the lavas.
Other important questions related to magma flux are (1) "did volcanism end abruptly or gradually"? (2) "did volcanism change from tholeiitic/transitional basalt to alkaline basalt as in the Kerguelen Archipelago, or did it remain exclusively tholeiitic like the Ninetyeast Ridge"? and (3) "were evolved (nonbasaltic) magmas erupted"? These questions were answered by drilling several holes with >100-m basement penetration.
Answers to all of these questions related to magma flux are required to understand the physical and chemical processes that formed the Kerguelen Plateau-Broken Ridge LIP.
Several lines of evidence support the interpretation that the Kerguelen plume has been a long-term source of magma for major bathymetric features in the eastern Indian Ocean. For example, the systematic south to north age progression on Ninetyeast Ridge is consistent with a hot spot track formed as the Indian plate migrated northward over the Kerguelen plume (Mahoney et al., 1983; Duncan, 1991). Also, isotope similarities among lavas from the Ninetyeast Ridge, the younger lavas forming the Kerguelen Archipelago and Heard Island, and the older lavas forming the Kerguelen Plateau and Broken Ridge (Figs. F9, F10) are consistent with the Kerguelen plume as an important source component (Weis et al., 1992; Frey and Weis, 1995, 1996). The preservation of a LIP resulting from partial melting of a decompressing plume head and its associated hot spot track derived from the plume stem presents an excellent opportunity to understand the evolution of a long-lived plume.
Many studies of oceanic island volcanoes have demonstrated that geochemically distinct sources (e.g., plume, entrained mantle, and overlying lithosphere) contribute to plume-related volcanism. Because isotope characteristics of plume, asthenosphere, and lithosphere sources are usually quite different, temporal geochemical variations in stratigraphic sequences of lavas can be used to determine the relative roles of different sources in plume-related volcanism. Establishing how the proportions of these sources change with time and location aids our understanding of how plumes "work" (Chen and Frey, 1985; Gautier et al., 1990; White et al., 1993; Peng and Mahoney, 1995).
What was the role of depleted asthenosphere in creating the Kerguelen Plateau-Broken Ridge LIP? We use "depleted" to indicate relative depletion in abundances of highly incompatible elements (e.g., depleted asthenosphere is the source of most MORB [see Hofmann, 1997, for additional discussion]). Such asthenosphere can be entrained into an ascending plume head, or, when a plume is located at a spreading ridge axis, plume-derived and MORB magmas can mix. For example, much of the Ninetyeast Ridge formed when the Kerguelen plume was close to a ridge axis (Royer et al., 1991). Weis and Frey (1991) inferred that the relatively low 87Sr/86Sr and high 143Nd/144Nd ratios of lavas from Deep Sea Drilling Project (DSDP) Site 756 on the Ninetyeast Ridge are a consequence of the plume being close to a spreading ridge axis during formation of the Ninetyeast Ridge. Also, depleted material may be intrinsic to a plume (e.g., Saunders et al., 1998). Fitton et al. (1998) suggested that a plot of Zr/Y vs. Nb/Y is a geochemical discriminant for distinguishing depleted material intrinsic to the Icelandic plume from North Atlantic MORB. In the case of the Kerguelen plume, however, this discriminant is compromised by the presence of a continental lithosphere component in some of the lavas.
A continental lithospheric component has been recognized geochemically in lavas from the southern SKP (Site 738 at ~63°S) and in dredge 8 lavas from eastern Broken Ridge (Mahoney et al., 1995). A less obvious but significant continental lithospheric component is also present in basalts from Site 747 on the CKP and in basalts dredged from the 77° graben in the SKP (Figs. F9, F10, F11, F12). In addition, wide-angle seismic data from the Raggatt Basin (58°S) of the SKP show a reflective zone at the base of the crust that has been interpreted to be stretched continental lithosphere (Operto and Charvis, 1995, 1996). In contrast, no compelling geochemical evidence supports a continental lithosphere component in lavas from the Ninetyeast Ridge and the Kerguelen Archipelago (Frey et al., 1991; Weis et al., 1993, 1998; Frey and Weis, 1995, 1996; Yang et al., 1998), but such a component is present in the Big Ben basaltic series on Heard Island (Barling et al., 1994) (Figs. F9, F11A, F12) and in mantle xenoliths found in Kerguelen Archipelago lavas (Hassler and Shimizu, 1998; Mattelli et al., 1999). Determination of the spatial and temporal role of continental lithosphere components in the Kerguelen Plateau-Broken Ridge LIP is required to evaluate whether these continental components are a piece of Gondwana lithosphere that was incorporated into the plume.
Relatively shallow basement holes (>100 m) in the Kerguelen Plateau and Broken Ridge can be used to define spatial and short-term variability in the geochemical characteristics of the lavas erupted during the waning phase of plateau volcanism. A surprising result of previous drilling (Legs 119 and 120) on the Kerguelen Plateau is that sampling of the uppermost 35 to 50 m of igneous basement at several plateau sites shows that lavas at each site have distinctive geochemical characteristics (e.g., basalts from each site have a distinct combination of Sr and Nd isotope ratios [Fig. F9] and incompatible element abundance ratios, such as Ti/Zr [Fig. F13]). The latter ratio is useful because it can be precisely determined by the shipboard X-ray fluorescence (XRF) spectrometer. In many continental flood basalts, relatively low Ti/Zr is diagnostic of significant contamination by continental crust (e.g., the Bunbury and Rajmahal Basalts; Frey et al., 1996; Kent et al., 1997). In summary, we sought to determine whether the geochemical heterogeneity of basalts from different domains of the Kerguelen Plateau and Broken Ridge reflect spatial and temporal heterogeneities in a plume or localized differences in mixing proportions of components derived from asthenosphere, plume, and slivers of continental lithosphere. Answering this question requires knowledge of temporal variations in geochemical characteristics at several locations within this LIP. Preliminary data relevant to these questions are provided by shipboard geochemical results. These topics are also the main focus of postcruise geochemical and isotope studies.
A major goal of Leg 183 was to address the environmental impact of the formation of the Kerguelen Plateau and Broken Ridge. Important goals for this assessment are to (1) define postmagmatic compositional changes resulting from interaction of magmas with the surficial environment, (2) determine the relative roles of submarine and subaerial volcanism in constructing the upper part of the plateau, (3) estimate volatile contents of magmas from compositional studies of phenocrysts and their inclusions, and (4) evaluate the extent of degassing by determining the abundance and distribution of vesicles. The study of altered and metamorphosed basement rocks will be a major source for this information, but overlying sediments will also provide important data (e.g., the presence of terrestrial and terrigenous sedimentary components, as at Site 750, establishes an important role for subaerial volcanism). For subaerial eruptions, the input of volcanic gases into the atmosphere is controlled by the volatile content of the magmas and eruption rate. For submarine eruptions, it is essential to determine if hydrothermal systems developed that were significant in controlling local, regional, and global elemental and isotope fluxes. Our overall goal was to assess the environmental impact of the Kerguelen/Broken Ridge LIP by estimating fluxes of elements, volatiles, particulates, and heat into the atmosphere-hydrosphere-biosphere system.
To understand relationships between tectonism and LIP magmatism, we will study the seismic volcanostratigraphy of the Kerguelen Plateau and Broken Ridge by linking seismic facies analysis with petrophysics, borehole data, and synthetic seismic modeling. We will determine stratigraphic and structural relationships both within the various Kerguelen Plateau domains and Broken Ridge and between these features and adjacent oceanic crust. Seismic volcanostratigraphic studies can reveal temporal and spatial patterns of LIP extrusion in a regional tectonic framework, as well as test for synchronous or asynchronous postemplacement tectonism of the Kerguelen Plateau, Broken Ridge, and adjacent ocean basins. Knowledge of the uplift and subsidence histories of the Kerguelen Plateau and Broken Ridge will provide much-needed boundary conditions for models of mantle upwelling, crustal thinning, crustal growth, and postconstructional subsidence and faulting.
Observations of physical volcanology, such as (1) flow thicknesses and directions, (2) flow morphologies, (3) relative thickness of marginal breccia zones and massive interiors of flows, (4) vesicle distribution within flows, (5) the presence and nature of interbeds, and (6) evidence for subaerial vs. submarine extrusion, will provide important information on eruption parameters and the distribution of melt conduits. In addition, physical volcanological observations coupled with shipboard measurements of physical properties and downhole logging data will provide ground truth for seismic volcanostratigraphy. We seek to determine the types of eruptive activity that formed the volcanic rocks and sediments of the Kerguelen Plateau and Broken Ridge. This includes distinguishing between subaerial and subaqueous eruptions, for example, by recognizing oxidized crusts on lava flow surfaces and analysis of intercalated soils developed on flow tops. In conjunction with seismic volcanostratigraphic studies, we will attempt to identify surficial and shallow subsurficial sources for the basalts (discrete volcanoes or feeder dikes) and to assess the effects of preexisting bathymetry and topography on flow distribution.