Petrogenesis of Basement Igneous 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 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, isotopic 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. 9, 10) 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 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 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, there can be mixing between plume-derived and MORB magmas.
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. There is also recognition
that 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.
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. 9, 10, 11,
12). 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, there is no compelling geochemical evidence for 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. 9, 11,
12A) 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 isotopic ratios
[Fig. 9] and incompatible element abundance ratios, such as Ti/Zr [Fig. 13]). 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 isotopic studies.
Environmental Impact
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 isotopic 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.
Tectonic History
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 sought to 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.