Igneous Petrology | Table of Contents
Locating the Isotopic Boundary
The principal objective of Leg 187 was to locate the Indian-Pacific
mantle isotopic boundary through its expression in the geochemistry of
mid-ocean-ridge basalt (MORB) lavas from 8- to 28-Ma seafloor to the
north of the AAD. The clearest definition of this boundary can be seen in
the Pb isotopic ratios, but it is clear in Nd and Sr isotopic ratios as well
(Fig. 2). Although there are also clear overall differences in the major and
trace element compositions between the lava populations of the two
provinces, there are few elements that can be relied on to accurately
determine the affinity of most individual lavas. Two elemental plots that
can reliably assign >90% of our current collection of young lavas are Ba vs.
Zr/Ba and MgO vs. Na2O/TiO2. These elements were
reliably measured by inductively coupled plasma-atomic emission
spectroscopy (ICP-AES) aboard the JOIDES Resolution throughout the leg.
Ba and Zr appear to have enabled us to discriminate between basalts of
Pacific affinity and their Indian and transitional counterparts, but
Na2O/TiO2 did not prove useful for this purpose.
Beyond the Isotopic Boundary
Defining the off-axis configuration and migration history of the
Indian/Pacific mantle isotopic boundary is not simply an end in itself. In
addition to its interest as a mantle dynamics phenomenon, an improved
understanding of this boundary is important for a broader general
understanding of the oceanic mantle. In investigating the nature and
origins of the AAD, the isotopic boundary and the mantle provinces that it
separates, we are also investigating the importance of variations in
geochemistry, isotopic composition, temperature, and other physical
characteristics of the oceanic upper mantle in a setting where other
tectonic variables are relatively constant. Improved knowledge of the
distribution of these chemical and physical characteristics in space and
time will lead to a better understanding of the dynamics of the oceanic
mantle and of its interaction with the magmatic processes of the mid
Recent findings have extended the biosphere to include microbial life in
deep subsurface volcanic regions of the ocean floor, and much attention
has been focused on the nature of microbes that live on, and contribute to
the alteration of, basaltic glass in oceanic lavas (Thorseth et al., 1995;
Furnes et al., 1996; Fisk et al., 1998; Torsvik et al., 1998). The first
evidence for this phenomenon was from textures in basaltic glass from
Iceland (Thorseth et al., 1992). Similar textures were later found in
basaltic glass from Ocean Drilling Program (ODP) Hole 896A at the Costa
Rica Rift, and the microbial contribution to the alteration history was
supported by the presence of DNA along the assumed biogenic alteration
fronts (Thorseth et al., 1995; Furnes et al., 1996; Giovannoni et al., 1996).
Microbes have recently been documented to inhabit internal fracture
surfaces of basaltic glass that specifically were sampled for
microbiological studies during Mir submersible dives to the Knipovich
Ridge (Thorseth et al., 1999). Dissolution textures directly beneath and
manganese and iron precipitates adjacent to many individual microbes
suggest that microbial activity plays an active role in the low
temperature alteration of ocean-floor basalts.
Sterile rock and sediment samples collected during Leg 187 for
microbial culturing, DNA analysis, and electron microscopic study range in
age from 14 to 30 Ma, providing an opportunity to study temporal changes
in microbial alteration.
In order to fulfill the primary objective of the leg (the location and
characterization of the Indian-Pacific mantle isotopic boundary), our
drilling strategy was focused on maximizing the number of sites rather
than recovery or penetration at any one site. Although our goal for each
site was ~50 m penetration into basaltic basement, this was achieved
only at five sites. At most sites, drilling conditions were poor as we
penetrated broken pillow flows and talus or other rubble; many holes were
abandoned when they became unstable.
Much of the region is devoid of measurable sediment cover. Most sites
were located on localized sediment pockets detected by single-channel
seismic imaging during the Melville's site survey cruises Boomerang 5
(BMRG05) and Sojourn 5 (SJRN05). Three additional sites were surveyed
during the transit from Site 1158 to 1159, and two were subsequently
drilled as Sites 1161 and 1162. Based on the seismic records, all sites
were ranked on a scale of 1 to 3, depending on the clarity with which they
were imaged and the width and depth of sediment cover. At highly ranked
sites, sediment thickness predictions from site survey data proved to be
reasonably accurate, so, whenever possible, we chose higher ranked sites.
At Site 1152, only a few meters of soft sediment were encountered, and
spud-in conditions were little better than those for bare rock. Two other
low-ranked sites, AAD-2B and -3A (Sites 1159 and 1163), proved to have
more than adequate sediment cover and were drilled successfully.
As the JOIDES Resolution approached each site, we ran a short survey
using the 3.5-kHz precision depth recorder (PDR) and, in all but a few
cases, the single-channel seismic system to confirm the location and
suitability of the proposed site. When possible, these surveys were run
obliquely to the original north-south survey lines, but in some cases
weather conditions dictated that we run close to the original course. For
several smaller sites we chose to run north-south lines to minimize out
of-plane reflections from the dominantly east-west trending topography.
Because sediments across the region were expected to be reworked and
possibly winnowed and because basement penetration at as many sites as
possible was the primary objective of this leg, we chose in most cases to
wash through the sediment section. Wash cores containing significant
sediment intervals were recovered at 10 sites.
During Leg 187 we used a responsive drilling strategy. At key points
during the leg, subsequent sites were chosen from among the 19
preapproved sites according to the results of onboard geochemical
analyses of the recovered basalts.
Igneous Petrology | Table of Contents