SCIENTIFIC OBJECTIVES
Leg 183 will address four first-order problems related to the characterization and quantification of maficigneous crustal production and its effects during the Cretaceous and Cenozoic. Our objectives are to
Perhaps the most significant question is how much magma was erupted over what time interval? More specifically, (1) what time interval is represented by the uppermost volcanic basement of this LIP? (2) Do eruption ages vary systematically with location on the plateau? (3) Was the growth episodic or continuous? (4) Did the plateau grow by lateral accretion (i.e., similar to Iceland) or by vertical accretion and underplating? Answers to these questions, to be provisionally provided by dating the oldest sediment above basaltic basement and later more definitively by 40Ar/39Ar dating of the basalts, are required to understand the generation of voluminous magma, the physical processes of magma intrusion and extrusion, and to assess the impact of Cretaceous volcanism on the surficial environment by estimating fluxes into the ocean-atmosphere system. An aspect of oceanic plateau volcanism that has been explored in only cursory detail (e.g., Sevigny et al., 1992) is the role of hydrothermal alteration in controlling elemental and isotopic fluxes. The extent, nature, and duration of hydrothermal processes on the plateau can be determined by drilling several holes with 150-200 m basement penetration.
We expect to recover basement rocks consisting of variably altered and metamorphosed basalt. To accomplish objective 2, concerning mantle sources and magmatic processes, and objective 3, concerning environmental effects of LIP formation, we will determine
a. Composition (major and trace elements) and isotopic ratios (Sr, Nd, and perhaps others) of unaltered phenocrysts (typically olivine, plagioclase, and clinopyroxene). Such data will provide information on parental magma composition and the role of crustal processes such as fractional crystallization, magma mixing, and assimilation.
b. Composition (major and trace element) isotopic ratios (O, Sr, Nd, Pb, Hf, and Os) of whole rocks. Different subsets of these geochemical data will be used to understand both magmatic and post-magmatic processes and the role of geochemically distinct mantle and crustal components in these rocks.
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-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, isotopic similarities between 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 (Fig. 5B, C) indicate that the Kerguelen plume played an important role (Weis et al., 1992; Frey and Weis, 1995, 1996). The presence of a LIP, perhaps resulting from decompression of a plume head, and an associated long-lived hot-spot track present an excellent opportunity to understand a long-lived mantle plume.
Many studies of ocean island volcanoes have demonstrated that geochemically distinct sources (e.g., the plume, entrained mantle, and overlying lithosphere) contribute to plume-related volcanism. Because isotopic 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 plume, asthenosphere, and lithosphere sources in plume-related volcanism. Establishing how the proportion of these sources changes with time and location provides an understanding of how plumes "work" (e.g., Chen and Frey, 1985; Gautier et al., 1990; White et al., 1993; Peng and Mahoney, 1995). A continental lithosphere source component has also been recognized in some lavas from the SKP and eastern Broken Ridge (Mahoney et al., 1995). Also, wide-angle seismic data collected by ocean-bottom seismometers in the Raggatt Basin of the SKP have defined a reflective zone at the base of the crust, which has been interpreted to be stretched continental lithosphere (Operto and Charvis, 1995, 1996). The present geochemical data set shows that a continental lithosphere component is obvious in lavas at only two sites (dredge site 8 on Broken Ridge and Site 738 in the SKP [Fig. 5B, C]). There is no evidence for a continental component in lavas from the Central Kerguelen Plateau, the Ninetyeast Ridge, and the Kerguelen Archipelago (Frey et al., 1991; Yang et al., 1998). Determining the spatial and temporal role of this lithosphere component is required to evaluate whether this continental component is a piece of Gondwana lithosphere that was incorporated into the plume.
In addition to answering questions about plume-lithosphere interactions, geochemical data for plateau lavas will define the role of depleted asthenosphere in creating this plateau. A MORB-related asthenosphere is apparent in some of the Ninetyeast Ridge drill sites (e.g., as reflected by the Sr and Nd isotopic ratios of lavas from Site 756; Weis and Frey, 1991) and is an expected consequence of the plume being close to a spreading ridge axis during formation of the Ninetyeast Ridge. Relatively shallow basement holes (150-200 m) in the Kerguelen Plateau can be used to define spatial and short-term variability during the waning phase of plateau volcanism. A surprising result of the shallow penetrations of the Kerguelen Plateau is that sampling of 35-50 m of igneous basement at several plateau sites shows that lavas at each site have a suite of distinctive geochemical characteristics: each site has a distinctive combination of Sr and Nd isotopic ratios (Fig. 5B). Does this heterogeneity reflect spatial heterogeneities in a plume or localized differences in mixing proportions of components derived from asthenosphere, plume, and slivers of continental lithosphere? Interpretation requires knowledge of temporal variations in geochemical characteristics at several locations.
Leg 183 will address the environmental impact of the formation of Kerguelen and Broken Ridge. Important goals for this assessment are to (1) define the post-magmatic 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, and (3) estimate volatile contents of magmas from compositional studies of phenocrysts and their inclusions. The study of altered and metamorphosed basement rocks will be a major source for this information, but important information will also be provided by overlying sediments. From these data, fluxes of elements, including volatiles, particulates, and heat from Kerguelen/Broken Ridge into the atmosphere-hydrosphere-biosphere system, can be estimated and their environmental impact assessed.
To understand the relationships between LIP magmatism and tectonic events, we will study Kerguelen and Broken Ridge's seismic volcanostratigraphy i.e., seismic facies analysis linked with petrophysics and borehole data with various aspects integrated by synthetic seismic modeling. We seek to determine stratigraphic and structural relationships, both within the various Kerguelen 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 post-emplacement tectonism of the Kerguelen Plateau, Broken Ridge, and adjacent ocean basins. Increased knowledge of the vertical and tectonic histories of the Kerguelen Plateau and Broken Ridge will provide insights into and much-needed boundary conditions for models of mantle upwelling, crustal thinning, lithospheric thermal histories, crustal growth histories, and post-constructional subsidence and faulting.
Observations of physical volcanology (e.g., flow thicknesses and directions, morphology, vesicle distribution, presence and nature of interbeds, and subaerial vs. submarine extrusion) will provide important information on the distribution of melt conduits and fluxes. Physical volcanology provides ground truth for seismic volcanostratigraphy. We seek to understand how the uppermost crust of Kerguelen and Broken Ridge formed, to locate surficial or shallow subsurficial sources for the basalts (discrete volcanoes or feeder dikes), to document environments of basalt extrusion, and to assess effects of pre-existing bathymetry and topography on flow distribution.