Site U1372 | Site U1373 | Site U1375 | Site U1376 | Site U1377
IODP Expedition 330:
Louisville Seamount Trail
Site U1374 Summary
PDF file is available for download.
Background and Objectives
Site U1374 (Alternative Site LOUI-6B) was the third
site completed during Integrated Ocean Drilling Program (IODP) Expedition 330
and the second of the two sites that were drilled on Rigil Guyot (Sites U1373
and U1374). Site U1374 represents one of the older seamount targets with an age
of ~73 Ma and only is a few million years younger than Site U1372 on Canopus
Guyot to the northwest. If the Louisville hotspot experienced a paleolatitude
shift similar to the recorded ~15° southern motion of the Hawaiian hotspot
between 80 and 50 Ma, this shift is expected to be largest for the oldest
seamounts in the Louisville seamount trail. Because Sites U1373 and U1374
target two disparate sequences of ancient lava flows on the same volcanic
edifice and because Rigil Guyot is only slightly younger than Canopus Guyot, it
is expected that these three sites together will significantly strengthen our
determinations of the Louisville paleolatitude at the old end of the trail.
Alternate Site LOUI-6B was requested during Expedition
330 to provide more flexibility while drilling Rigil Guyot. If comparable
borehole instabilities were encountered as during the drilling of Site U1372 on
Canopus Guyot, or if drilling at Site U1373 was to be abandoned for other
unforeseen reasons, then we could divert to Site U1374 to continue the drilling
of Rigil Guyot and still attain our most important scientific objective.
Because re-entry using a free-fall funnel failed for Hole U1373A, the first
site at Rigil Guyot had to be abandoned and Hole U1374A was spudded about 5.6
nmi away on the western portion of its summit plain in 1259 m water depth.
The original drilling plan was to recover the soft
sediment using a gravity-push approach with little or no rotation using a
Rotary Core Barrel (RCB), followed by standard coring into the volcaniclastic
materials and 350 m into igneous basement. A full downhole logging series was
planned including the standard Triple Combo and FMS-Sonic tool strings, the
Ultrasonic Borehole Imaging (UBI) tool, and the third-party Göttingen Borehole
Magnetometer (GBM) tool. Drilling and logging were successfully accomplished
after drilling to 522 mbsf and carrying out the full logging program. Coring
was in particular successful with a record-breaking 88% average recovery in
Drilling during ODP Leg 197 provided the first
compelling evidence for the motion of mantle plumes by documenting a large ~15°
shift in paleolatitude for the Hawaiian hotspot (Tarduno et al., 2003; Duncan
et al., 2006). This lead to two geodynamical end-member models that are being
tested during Expedition 330, namely that the Louisville and Hawaiian hotspots
moved coherently over geological time (Wessel and Kroenke 1997; Courtillot et
al. 2003) or, quite the opposite, that these hotspots show considerable
inter-hotspot motions, as predicted by mantle flow models (Steinberger, 2002;
Steinberger et al., 2004; Koppers et al., 2004; Steinberger and Antretter, 2006;
Steinberger and Calderwood, 2006). The most important objective of Expedition
330 therefore was to core deep into the igneous basement of four Louisville
seamounts to sample a large number of in situ lava flows ranging in age between
80 and 50 Ma. With a sufficiently large number of these independent cooling
units high-quality estimates of their paleolatitude can be determined, and any
recorded paleolatitude shift (or lack thereof) can be compared with seamounts
in the Hawaiian-Emperor seamount trail. For this reason Expedition 330 mimicked
the drilling strategy of ODP Leg 197 by drilling Louisville guyots equivalent
in age to Detroit (76-81 Ma), Suiko (61 Ma), Nintoku (56 Ma) and Koko (49 Ma)
in the Emperor seamounts. Accurate paleomagnetic inclination data are required
for the drilled seamounts to establish a record of the past motion of the
Louisville hotspot, and together with high-resolution 40Ar/39Ar
age dating of the cored lava flows, these data will help us to constrain the
paleolatitudes of the Louisville hotspot between 80 and 50 Ma. These
comparisons are of fundamental importance to determine whether these two primary hotspots have moved coherently or not, and to
understand the nature of hotspots and convection in the Earth's mantle.
Expedition 330 also aimed to provide important
insights into the magmatic evolution and melting processes that produced and
constructed Louisville volcanoes while progressing from their shield to
post-shield, and maybe post-erosional, volcanic stages. Existing data from
dredged lavas suggest that the mantle source of the Louisville hotspot has been
remarkably homogeneous for as much as 80 m.y. (Cheng et al., 1987; Hawkins et
al., 1987; Vanderkluysen et al., 2011). In addition, all dredged basalts are
predominantly alkalic and possibly represent a mostly alkalic shield-building
stage, which is in contrast to the tholeiitic shield-building stage of
volcanoes in the Hawaiian-Emperor seamount trail (Hawkins et al., 1987;
Vanderkluysen et al., 2011). Analyses of melt inclusions, volcanic glass
samples, primitive basalts, high-Mg olivines and clinopyroxene phenocrysts will
provide further constraints on the asserted homogeneity of the Louisville plume
source, its compositional evolution between 80 and 50 Ma, potential mantle plume
temperatures, and its magma genesis, volatile outgassing and differentiation.
In addition, incremental heating 40Ar/39Ar age dating
will allow us to establish age histories within each drill core delineating any
transitions from the shield-building phase to the post-shield capping and
objective of Expedition 330 at Site U1374 was to use new paleolatitude
estimates, 40Ar/39Ar ages and geochemical data to decide
whether the oldest Louisville seamounts were formed close to the 18-28°S
paleolatitude determined from ODP Leg 192 basalts for the Ontong Java Plateau
(Riisager et al., 2003) and whether this Large Igneous Province (LIP) was
genetically linked to the Louisville hotspot or not. This would prove or
disprove the hypothesis that the Ontong Java Plateau formed by the preceding
plume head of the Louisville mantle upwelling (e.g. Richards and Griffiths,
1989; Mahoney and Spencer, 1991).
Finally, basalts and sediments cored at Site U1374
were planned to be used for a range of secondary objectives such as searching
for active microbial life in the old seamount basements and to find fossil
traces of these microbes left behind in volcanic glasses and biofilms on the
rocks. We also planned to determine 3He/4He and 186Os/187Os
signatures of the Louisville mantle plume to evaluate its potential deep mantle
origin, to use oxygen and strontium isotope measurements on carbonates and
zeolites to assess the magnitude of carbonate vein formation in aging seamounts
and its role as a global CO2 sink, to age date celadonite alteration
minerals for estimating the total duration of low-temperature alteration
following seamount emplacement, and to determine the hydrogeological and
seismological character of the seamount basement.
After Hole U1373A on the eastern
side of the summit plain of Rigil Guyot had to be abandoned, the vessel was
offset in dynamic positioning (DP) mode to the recently approved Alternate Site
LOUI-6B located in 5.6 nmi distance on the western site of the summit plain.
After arriving on the new location, the bit tagged the seafloor at a depth of
1570.0 mbrf (1559 mbsl).
Hole U1374A was spudded with the rotary
core barrel (RCB) drill bit at 2035 hr on 5 January. After penetrating a thin (~7 m thick) sedimentary cover, the bit penetrated igneous rocks at 16.7 mbsf. Rotary
coring in Hole U1374A advanced to a depth of 130.4 mbsf by 0345 hr on 10 January
with an average recovery of 84%. At this time, the bit had acquired 81.3
rotating hours and needed to be replaced. A free fall funnel (FFF) was made up
and deployed at 0550 hr on 10 January. Bit extraction from the hole and
re-entering into the FFF at 1635 hr was conducted without incident and coring
resumed at 1900 hr on the same day.
Rotary coring in Hole U1374A continued
until 0545 hr on 18 January when erratic pump pressure indicated that there was
a problem at the bottom of the drill string. Instead of continuing to core with a bit that far outlasted
its expected life with 133.4 rotating hours, we decided to end coring at a
final depth of 522.0 mbsf. The penetration into basement was 505.3 m with an
average recovery of 88.0% and an average rate of penetration of 2.5 m/hr. The
average recovery for the entire hole was 87.8% with an average penetration rate
of 2.4 m/hr.
After a routine wiper trip, which
suggested that the hole was in good condition, the rotary shifting tool was
deployed to release the bit. However, several attempts to release the bit
failed. It was eventually decided to trip the drill pipe, remove the bit and
mechanical bit release on the rig floor, and make up a shorter logging bottom
hole assembly with a 9-1/4" diameter logging/clean-out bit that would allow
more of the open hole to be logged.
The end of the pipe cleared the
rotary table at 2015 hr on 18 January. The shorter logging bottom hole assembly
was made up and deployed at 0145 hr. The bit entered FFF at 0430 hr on 19
January and was positioned at a logging depth of 101.2 mbsf. The standard
Triple Combo suite was made up and run in at 1045 hr. Unfortunately, the tool
was not able to advance into the open hole because of a bridge only ~7 m
further down. The logging tool was recovered and the bit lowered to 143.6 mbsf
to clear the bridge. The bit was then pulled back and placed at 110.8 mbsf.
When the Triple Combo logging tool was unable to advance past 138.0 mbsf, the
tool was again recovered and reconfigured to a shorter assembly using only the
density and gamma ray sensors in the hope that this would more easily negotiate
an entry into the hole. After this attempt was unsuccessful, the drill pipe was
recovered with the bit clearing the rotary table at 1115 hr on 20 January.
Since downhole logging at this
particular deep hole was considered very important, a 4-stand RCB coring assembly
was made up with a new mechanical bit release and a used bit. The drill string
entered the FFF at 1540 hr on 20 January. From 1730 hr to 2315 hr, the hole was
washed and reamed to bottom (522 mbsf), flushed with mud, and then displaced
with 165 barrels of heavy (10.5 ppg) mud. The bit was dropped at the bottom of
the hole and the end of pipe placed at a logging depth of 128.1 mbsf, which was
~18 m deeper than the previous logging attempt and below a potentially unstable
zone in the formation. An extra
stand of drill collars was added to the bottom hole assembly (BHA) to keep the
tapered drill collar as close to the seafloor as possible, which is the usual "choke point" where a BHA gets stuck.
The Triple Combo logging tool was
made up and deployed again at 0540 hr on 21 January and succeeded in logging
the hole up from 520 mbsf. The tool suite was recovered and laid out. The
second instrument deployed was the Gšttingen Borehole Magnetometer (GBM), which
made one full pass down from the rig floor to 520 mbsf and back up. The
communication with the GBM was lost while being retrieved in the pipe, however because
a sighting of the tool was carried out at the start of deployment, the rotation
history of the GBM is still obtainable. The third logging run was performed
with the Formation MicroScanner (FMS)-sonic, which also successfully came
within 2 m of the bottom of the hole. The fourth log was conducted with the
Ultrasonic Borehole Imager. The fifth log was a re-deployment of the GBM tool.
All runs were successful.
While the GBM was deployed for the
second time, the driller noticed that the suspended string weight was getting
slightly lighter indicating that the formation was starting to squeeze the BHA.
To compensate for this, the driller picked up the string an additional 3 m. After
the logging tool was retrieved, the driller attempted to free the drill string
from 2215 hr on 22 January to 0930 hr on 23 January. Although circulation was
maintained, there was no rotation and the drill pipe could not be raised or
lowered. Realizing that further efforts would be fruitless, the crew made the
necessary preparations to sever the drill string directly above the tapered
drill collar. The top of the tapered drill collar was ~13 m below the seafloor.
The drill pipe was severed at 1500 hr on 23 January. Once the drill pipe was
recovered and the beacon retrieved, the vessel departed for Prospectus Site LOUI-2B at 1945 hr on 23 January.
Sediment at Site U1374 on Rigil
Guyot occurs in (1) the uppermost sedimentary cover of the seamount; (2) three
intervals within a predominantly volcanic basement; and (3) basalt breccias
(volcanic or sedimentary in origin) as finer-grained, interclast infill
deposits, thin-bedded sedimentary layers, or peperitic intervals. Fourteen
stratigraphic units and subunits were defined based on macroscopic and
The uppermost part of the seamount
(0-6.64 mbsf) includes a young (Pleistocene) sedimentary cover composed of
sandy foraminiferal ooze, which was deposited in a pelagic environment on the
flat-topped seamount. An older sedimentary cover occurs between 6.64 and 16.70
mbsf, which includes, from top to bottom (1) ferromanganese-phosphate
encrustations ~6.64 mbsf; (2) a layered, monomict volcanic sandstone without
fossils between 6.64 and 13.59 mbsf; (3) an upper Maastrichtian bioturbated
volcanic sandstone with abundant gastropods and shell fragments, and rare
possible ammonite fragments between 13.59 and 15.05 mbsf; (4) an upper
Campanian bioclast foraminiferal limestone with ferromanganese-phosphate
encrustations and burrows filled with upper Maastrichtian volcanic sandstone
between 15.05 and 15.31 mbsf; and (5) an upper Campanian or older basalt
conglomerate with shallow-marine bioclasts (e.g., shell fragments, calcareous
alga, and bryozoan) from 15.31 to 16.70 mbsf. Bedding dips in the sedimentary
cover are generally subhorizontal.
The underlying upper Campanian (or
older) volcanic sequence is composed of minor basalt lava flows and abundant
basalt breccias. The interclast spaces in the basalt breccia are partly filled
with finer grained, basalt and volcanic sandstone with a local, shallow marine
bioclast component. Three thick-bedded sedimentary intervals were identified
between 37.60 and 116.45 mbsf. The first interval occurs between 37.60 and
41.84 mbsf and is composed of, from top to bottom, a polymict basalt sandstone
with abundant vitric fragments and few shallow marine bioclasts, a layered
volcanic sandstone with rare fossils, and a monomict basalt breccia with
larger, shallow marine bioclasts. The second sedimentary interval extends
from 63.67 to 84.70 mbsf and is devoid of fossils. It includes two polymict
basalt breccias and a volcanic sandstone. The third sedimentary interval occurs
between 109.87 and 116.45 mbsf and is composed of a volcanic sandstone with few
bioclasts, and a basalt conglomerate with abundant shallow marine fossils.
Volcanic deposits below 116.45 mbsf include only minor occurrences of
thin-bedded layers of grain-supported, poorly sorted basalt sandstones-breccias
interpreted as sediment intervals. The last occurrences of shallow water fossils
at Site U1374 were found in two intervals of sedimentary basalt breccia between
256.75 to 257.49 mbsf and 290.32 to 291.27 mbsf. These intervals correlate to
changes in the nature of volcanic desposits downhole. Contrary to the
sedimentary cover, orientation of beddings in the volcanic basement is
characterized by consistent, moderately dipping values.
Eight lithofacies were defined in
the sedimentary cover and thicker sedimentary intervals of the volcanic
basement, which permit an overall characterization of the environment of
deposition at Site U1374. The volcanic basement below 116.45 mbsf is
interpreted to have deposited in a submarine environment on the slope of a
former oceanic island. Within this basement the lowermost occurrence of fossil-bearing
sediment at 291.27 mbsf possibly corresponds to an unconformity indicative of
the shoaling of the island. Higher up within the sequence, the volcanic
interval between 116.45 and 16.70 mbsf was interpreted to have deposited in a
shallow marine to subaerial environment on the slope of this former island. A
major, erosional surface likely occurs at 16.70 mbsf, as notably suggested by
changes in the dip of sediment bedding between the volcanic basement and
sediment cover. We interpreted this surface as a product of summit erosion and
original flattening of the drilled guyot. The erosional surface is capped by a
shallow marine basalt conglomerate between 16.70 and 15.31 mbsf, which is
followed by a condensed interval from 15.31 to 15.05 mbsf with ferromanganese
encrustations. The age of the limestone found in the condensed interval was
assigned to the late Campanian (see Biostratigraphy below) and interpreted as
the record of the initial drowning of Rigil Guyot. Volcaniclastic sediment
between 15.05 and 6.64 mbsf was assigned to the late Maastrichtian and was
interpreted to represent a record of rejuvenated volcanism. A second (undated)
condensed interval occurs ~6.64 mbsf, which is capped by Pleistocene pelagic
Based on calcareous nannofossil and
planktonic foraminiferal biostratigraphy, a preliminary age of
Pleistocene-Holocene is assigned to the unconsolidated sandy foraminiferal ooze
of Unit I (Core 330-U1374A-1R), and late Cretaceous to the volcaniclastics and
limestone of Unit II (Cores 330-U1374A-2R and -3R). Because the underlying Unit
III through XIX yielded no age-diagnostic microfossils, their ages are
undetermined. An unconformity between Units I and II represents more than fifty
million years of missing sediment. In addition to microfossils, fragments of
ammonoid specimens were observed in Unit II, also constraining it to the
Cretaceous, consistent with the microfossil analyses.
Hole U1374A penetrated a total of
474.89 m of igneous rocks comprising a succession of volcaniclastic breccias
capped by lava flows and intruded, in its lower part, by a suite of intrusive
sheets or dikes. The igneous sequence has been divided into 148 Lithologic
Units, which have been grouped into 15 Stratigraphic Units (Units III to VIII,
X, and XII to XIX). The basement succession also includes two sedimentary units
(Units IX and XI; 21.03 and 6.59 m thick, respectively, see above). Magmatism
recorded at Site U1374 started in a clearly submarine environment and, in the
time interval considered here, progressed to a shallow marine and then
subaerial environment. This progression is clearly seen in the various breccia
types recovered at this site, which range from green hyaloclastite breccia with
frothy basaltic clasts (marine) through blocky breccia (shallower marine) to
scoriaceous (near sea level or subaerial). Blocky breccia probably accumulated
as talus deposits through the transportation down slope of volcaniclastic
debris that was shed from the fronts of submarine lava flows. The scoriaceous
breccia is probably the product of hydrovolcanic eruptions resulting from the
interaction of magma with shallow water or wet sediment. The proportion of lava
flows increases toward the top of the section and peperite is found at the margins
of the upper lava flow units (Units III-XII). Distinct eruptive packages are
often separated by intervals of background sedimentation, five of which were
identified at Site U1374. A dramatic increase in the thickness of these
intervals at Unit XI indicates the point at which parts of the seamount near
Site U1374 had emerged above sea level and erosion could proceed more rapidly.
The phenocryst assemblage in the breccias and lava flows changed from
plagioclase-dominated in the lower part of the succession (Units XIV-XIX) to
olivine-dominated in the upper part (Units III-XIV) suggesting that the magmas
became generally more basic with time. The magma erupted at Site U1374 was alkalic throughout the drilled interval.
The entire section of Hole U1374A has undergone secondary alteration by
low temperature water-rock interactions and/or weathering. The alteration of the
volcanic rocks ranges from slightly to highly altered (between 5% and 95%).
Several basaltic lava flows and intrusive sheets are relatively well preserved
(10% or less alteration). Core descriptions and thin section observations allow
the definition of two main but overlapping alteration intervals showing
different dominant alteration colors that relate to the oxidation state during
the alteration processes. From the top of Hole U1374A to ~300 mbsf the sequence
has dominantly reddish and/or brownish alteration colors, indicating oxidizing
conditions under subaerial to transitional shallow marine environments. At
depths greater than ~370 mbsf, the basalt display a range in alteration from
slightly to highly altered, showing greenish colors indicating more reducing
conditions related to a submarine emplacement environment. Nevertheless,
occurrences of gray and relatively unaltered basalt were encountered throughout
Plagioclase and augite are generally well preserved, both as phenocrysts
and in the groundmass throughout the entire igneous portion of the core.
Plagioclase shows minor alteration to sericite/illite in some rocks, but is
generally well preserved. Augite is almost always unaltered. Olivine is
typically completely altered to iddingsite, hematite, carbonates and
Fe-oxyhydroxide, but some sections in the core contain slightly to moderately
altered olivine. Some olivines in greenish altered rocks are replaced by green
clay, Fe-oxyhydroxide and/or carbonates (calcite/magnesite).
Overall, three main groups of alteration phases can be distinguished:
carbonates (Mg-calcite), clay minerals (saponite, nontronite, celadonite),
zeolites and other secondary phases (iddingsite, Fe oxyhydroxides, goethite,
pyrite/chalcopyrite, thaumasite). The type of zeolite varies from phillipsite
in the upper portions of the core to analcite and gmelinite at depth,
apparently indicating a thermal alteration gradient. Additionally numerous
vesicles, veins and voids can be found that are mainly filled with carbonates
and clay minerals at depths less than 300 mbsf, and zeolites at depths greater
than 380 mbsf.
Structural features at Site U1374 are dominated by veins (n=1229), vein
networks (n=515, with 3225 individual veinlets), and fractures (n=356). Veins
are found mostly within lava flows, although veins also occur in larger
fragments within the volcanic breccia units. The maximum vein width is 25 mm,
but most are considerably smaller, with average widths of 0.8 mm. Fractures are
also most common in lava units, especially the lowermost 14 meters of the hole
with over 14 fractures per meter. Structural measurements were also undertaken
for intervals with sedimentary bedding (n=46), 35 geopetals, 18 igneous
contacts, 35 vesicle bands, and 80 cases of magmatic flow textures. The
orientations of bedding change downhole and correspond to important lithofacies
variations. All geopetal structures are horizontal, indicating this part
of the seamount has not been tilted since deposition of the geopetal infilling
material. Excellent examples
of baked contacts and chilled margins were recorded from ~335 to 500 mbsf, from
a series of steeply dipping sheet intrusions. These intrusions also contain
steeply inclined vesicle bands and/or flow textures, which indicate mostly
near-vertical magma flow. In addition, several lava flow units have flow
textures, but their orientations are mostly sub-horizontal.
Major and trace element data for
igneous samples from Site U1374 overlap considerably with data from Sites U1372
and U1373. However, the Site U1374 samples tend to have slightly lower SiO2
at similar total alkali (Na2O+K2O) contents, and thus are
slightly more alkalic as a group. On a total alkalis vs. SiO2
diagram, most of the Site U1374 samples are classified as alkalic basalts, but
data for nearly a third fall in the field of basanite and tephrite. No
transitional compositions were found, in contrast to Sites U1372 and U1373.
Most of the Site U1374 samples are relatively evolved, with MgO concentrations
between 2.78 wt% and 8.54 wt%. Major element and Sc variations indicate that
olivine and clinopyroxene were the main mineralogical controls on magmatic
differentiation. Incompatible element concentrations display somewhat greater
overall variability relative to TiO2 than seen for Sites U1372 and
U1373, consistent with greater variability in the amount of partial melting
and/or in source composition at Site U1374. Despite the compositional overlap
and close proximity of Sites U1373 and U1374 (which are located 10.4 km apart
on Rigil Guyot), the rocks from the two sites cannot be correlated, and
probably represent distinct eruptive events. Likewise, the intrusive sheets at
Site U1374, although compositionally within the range of the lava samples,
cannot be correlated with any specific lava unit at this site.
Physical property characterization
was performed for samples recovered from Site U1374 and the different datasets
are mutually consistent and show clear contrasts between unconsolidated
sediments, massive basalts, and breccias. The intrusive sheets or dikes
recovered at this site have a characteristic physical property signature
distinct from the majority of the basalt flows, lobes, and clasts, and are
marked by high natural gamma ray radiation, magnetic susceptibility, density,
and p-wave velocity and low porosity. More subtle contrasts between olivine-
and plagioclase-dominated units are observed in natural gamma ray radiation and
magnetic susceptibility. The downhole appearance of hyaloclastites at 327 mbsf
is marked by a subtle decrease in L* (lightness) and a more marked decrease in
p-wave velocity and increase in porosity. The changes in both p-wave velocity
and porosity become more pronounced below 470 mbsf, where a shift in color
reflectance towards more green and yellow spectra is also observed. This
correlates with the occurrence of a high proportion of clasts and fragments of
frothy basaltic glass.
The natural remanent magnetization (NRM) of samples from Hole U1374A
ranges from 10-3 A/m to ~20 A/m (geometric
mean = 0.82 A/m) with the highest values associated with lava flows, intrusive sheets, and basalt clasts in the volcanic breccia/conglomerate units. Relatively well-defined principal component directions were obtained for 5496
intervals from archive half-core measurements (for pieces >9 cm in length).
These directions are generally consistent with stepwise alternating field (AF)
and thermal demagnetization results from 236 discrete samples. Both data sets reveal a small
interval of reversed polarity magnetization in the top ~45 m of the hole and
dominant normal polarity from ~45-522 mbsf.
Four tool strings were deployed in Hole
U1374A on Rigil Guyot. Three tool strings took measurements of natural gamma
ray radioactivity, density, neutron porosity, elastic wave velocity, acoustic
images and borehole resistivity. The fourth tool string, the (third-party) Gšttingen
Borehole Magnetometer (GBM), measured three-component magnetic field,
inclination and declination in the drilled seamount formation. Measurement
depths were adjusted to match across different logging runs, obtaining a
wireline matched below seafloor (WMSF) depth scale. The logged depth interval
for Hole U1374A was 128.1 to 520 m WMSF.
The downhole log measurements (resisitivity,
density, velocity and neutron porosity) were used to identify a total of nine
log units in Hole U1374A with two in the section covered by the bottom hole
assembly (BHA) and seven in the volcanic sequences in the open hole interval.
Log Unit I (0 to 20 m WMSF) shows a spike in gamma ray; Log Unit II (20 to
128.1 m WMSF) has generally low gamma ray values; Log Unit III (128.1 to ~240 m
WMSF) exhibits fluctuating values for density, resistivity, porosity and
velocity; Log Unit IV (240 to 278 m WMSF) shows more consistent values for
density, velocity, porosity and resistivity; Log Unit V (278 to 358 m WMSF) is
characterized at its top by a dramatic decrease in resistivity and velocity and
an increase in porosity; Log Unit VI (358 to 380 m WMSF) exhibits a marked
decrease in density, resistivity and velocity; Log Unit VII (380 to 469 m WMSF)
has relatively consistent density values higher, stable resistivity values,
lower porosity and higher velocity values; Log Unit VIII (469 to 490 m WMSF) is
characterized by a significant decrease in resistivity, density and velocity
and increase in porosity; and Log Unit IX (490 to 507 m WMSF) shows a marked
increase in density, velocity and resistivity and a decrease in porosity.
The GBM was run twice in Hole U1374A and
collected good quality magnetic data, which will be reoriented postexpedition.
The raw data correlates well to the changes downhole in lithology observed in
the core. Lithological and structural features were well imaged with both the
Formation MicroScanner (FMS) and Ultrasonic Borehole Imager (UBI). By combining
the FMS and UBI datasets a complete picture of the borehole wall in terms of
fractures, clast distribution, amount of alteration and contrasts in
resistivity and "hardness" can be obtained.
Twenty-nine whole-round samples
(5-13 cm long) were collected for microbiological analysis. Lithologies of the
samples collected were unconsolidated sediments (one), sedimentary conglomerate
(two), volcanoclastic breccia (twenty-four) and aphyric basaltic lava flows
(two). All samples were preserved for shore-based cell counting,
deoxyribonucleic acid (DNA) analyses and δ34S and δ13C
analyses. Eleven samples were used to inoculate culturing experiments with up
to seven different types of cultivation media. Growth was detected in samples
as deep as 400 mbsf with media targeting sulfur oxidizing bacteria and general
heterotrophs. Five samples were used to set up stable isotope addition
bioassays to determine rates of carbon and nitrogen utilization by subsurface
microbes at Rigil Guyot. Two cores were seeded with fluorescent microspheres.
Samples from these cores were collected for shipboard analysis of contamination
via fluorescent microsphere counts, which revealed that microspheres are
released into the drill fluid, but all counts are reduced to zero after the
three sterile seawater rinses to which all microbiology whole round samples are
subjected. This indicates that the microspheres were not able to penetrate into
the whole round samples, and therefore the chance for microbial contamination
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J., Lonsdale, P., Staudigel, H. (1987). Isotopic evidence for a hotspot origin of the Louisville seamount chain.
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