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IODP
Expedition 309 & 312: |
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Superfast Spreading Rate Crust 2 and Superfast
Spreading Rate Crust 3
Expedition
309 and Expedition
312 Shipboard Scientific Parties
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| Introduction |
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Figure
1. Age map of the Cocos-, Pacific-,
and Nazca Plates with isochrones at 5-Ma intervals.
The locations of deep drill holes into the oceanic
crust of Hole 1256D and Site 504 are shown.
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Integrated Ocean Drilling Program (IODP) Expeditions
309 and 312 were part of a three-component programs with the
objective to deepen Hole 1256D initiated during Ocean Drilling
Program (ODP) Leg 206. Hole 1256D is located in the eastern
equatorial Pacific (Figure 1) and was
drilled into 15 Ma crust that formed at the East Pacific Rise
during a period of superfast spreading (>200
mm/y).
Wireline operations during Leg 206 provided
high-quality data (Pezard and Anderson, 1989) on
the in situ physical properties of the upper part
of the oceanic crust combined up to a depth of
~752 mbsf. Expeditions 309 and 312 were highly
successful continuations of this drilling effort
with the agenda to provide further constraints
on the physical properties in deeper sections of
the oceanic crust. The primary logging objectives
were to refine the volcanic stratigraphy, eruptive
morphology, and variations in seawater-basalt alteration
as a function with depth at a superfast spreading
centre and in particular of the sheeted-dikes to
gabbro transition. Hole 1256D was extended to 1255
mbsf during Expeditions 309 and finally deepened
to 1507 mbsf during Expedition 312.
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Logging
Tools
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Figure
2a. Schematic illustration
of wireline tool string configurations used at Hole
1256D during Expeditions 309 and 312
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| Figure 2b. Schematic
illustration of the Versatile Seismic Imager tool used
during Expedition 312. |
The logging program on Expeditions 309 and 312 were designed
to obtain data needed to illuminate the physical properties
of the drilled rocks and shed light on the structure of the
oceanic crust formed at a superfast spreading center. Standard
wireline tool strings -- the Triple Combo, the Formation
MicroScanner (FMS)/Sonic, and Well Seismic Tool (WST), --
were deployed during Exp 309 (Figure
2a).
In addition to the standard wireline tool strings the Versatile
Seismic Imager (VSI) and the Temperature Acceleration Pressure
(TAP) combine with the Dual LateroLog (DLL) and Environmental
Mechanical Sonde (EMS) were deployed during Exp 312 (Figure
2b)
Details on standard wireline tools including output data
for each tool used during both Expeditions can be found at:
http://iodp.ldeo.columbia.edu/TOOLS_LABS/index.html
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| Logging
Operations & Technical Highlights |
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| Figure 3.
Logging operations at Hole 1256D during Expeditions 309
and 312. Depths are shown in meters below seafloor (mbsf).
HNGS = Hostile Environmental Gamma Ray Sonde, APS = Accelerator
Porosity Sonde, HLDS = Hostile Environmental Lithodensity
Sonde, DLL = Dual LateroLog, TAP = Temperature-Acceleration-Pressure
tool, SGT = Scintillation Gamma Ray Tool, DSI = Dipole
Sonic Imager, GPIT = General Purpose Inclinometer Tool,
FMS = Formation MicroScanner, UBI = Ultra Sonic Borehole
Imager, VSI = Versatile Seismic Imager, EMS = Environmental
Mechanical Sonde. |
Expedition 309 pre- and post-drilling logging operations
Logging during Exp 309 was split into pre- (phase 1) and
post-drilling (phase2) operations (Figure
3) using standard tool strings (Figure
2). The primary purpose of the two pre-drilling logging
deployments were to check the condition of ODP Hole 1256D
and identify borehole wall breakouts, and variations in hole
diameter. Post-drilling logging operations (Figure
3) were dedicated to provide constraints on the physical
properties of the newly drilled sections of the oceanic crust
and determine in as much coring influenced borehole conditions.
Despite several attempts a fifth logging run including the
WST could not be deployed and the logging run was abandoned.
All successfully deployed logging operations provided high
quality data overlapping data previously collected during
Leg 206
Expedition 312 logging operations
Prior to Exp 312 logging operations the bit was placed in
the open hole at ~20 m below the 16-inch casing shoe at a
depth of ~290 mbsf (Figure 3). The
hole was successfully logged with six different tool strings:
the triple combo; VSI; FMS with Scintillation Gamma Ray Tool
(SGT) and Dipole Sonic Imager (DSI), Ultrasonic Borehole
Imager (UBI) with the General Purpose Inclinometer (GPIT),
SGT, and DSI tools; FMS with SGT only; and the TAP, DLL,
SGT, and EMS. The Triple Combo made two passes, from 1440
to 343 mbsf and from 1438 to 1080 mbsf. A check shot experiment
using the VSI was conducted at 58 stations ~22 m apart from
a maximum depth of 1383 mbsf. The UBI with the GPIT, SGT,
and DSI tools logged from 1430 to 1099 mbsf, followed by
a repeat pass covering the interval from 1433 to 1089 mbsf.
The Formation MicroScanner was combined with the SGT and
logged the hole from 1437 to 1089 mbsf and from 1436 to 1101
mbsf. A last logging suite was made up of the TAP, DLL, and
SGT and logged the hole from 1440 mbsf to 290 mbsf.
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| Logging Results |
Wireline logging operations during Expeditions 309 and 312
built on the success of Leg 206, and provided for the first
time in the history of DSDP, ODP and IODP in situ physical
properties of a complete section of the oceanic crust including
the sheeted dikes–gabbro transition.
Expedition 309 results
Wireline operations during Expedition 309 provided high-quality
data on the in situ physical properties of the upper part
of the oceanic crust combined up to a depth of ~1220 mbsf
(Figure 4a, 4b, 4c,
and
4d)
Caliper readings derived from triple combo and FMS-sonic
tool strings show generally good borehole conditions. The
average hole diameter measurements from the FMS/sonic calipers
are 11.25 inches for C1 and 10.90 inches for C2; this slight
difference is the result of an elliptical borehole between
807 and 966 mbsf. Wide sections (>13 inches) are particularly
common in this interval, as well as between 1048 and 1060
mbsf. Comparison of the caliper data from the pre- and post
drilling operations of the upper 500 m shows that the borehole
is being progressively enlarged with continued drilling.
The excellent hole conditions over the rest of the interval
resulted in good measurements by these contact tools, particularly
for the lowermost 300 m. Triple combo data is of high data
quality and there is an excellent overlap with the previous
logging runs. The FMS and UBI provided high quality data
(Figure 5a, 5b, and 5c. Note: Only FMS images are shown).
However, because the UBI was deployed very slowly (120 m/hr),
incomplete heave compensation and sticking of the tool influence
the data quality. Whereas the FMS images can be corrected
with confidence, the UBI images still show artifacts of sticking.
In most intervals the coverage of the borehole wall by the
two FMS passes is good and is complemented by the UBI images
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Figure
5. Formation MicroScanner
(FMS) resistivity images (static normalization) of
depth intervals 345 - 355 mbsf, 470 - 480 mbsf, and
645 - 655 mbsf recorded during Expedition 309. Natural
radioactivity, electrical resistivity (LLD: LateroLog
Deep, LLS: LateroLog Shallow), density, photoelectric
effect (PEFL), neutron porosity and capture cross-section
(sigma) are reported on the right columns. (A) Transition
between the lava pond (Unit 1) and thin flows (Unit
2) at 348 mbsf. This transition is characterized
by a strong decrease in the electrical resistivity.
(B) Transition between a thin flow unit and a massive
unit at 473 mbsf. (C) Massive unit displaying a marked
increase of the natural radioactivity at 648 mbsf.
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Principal rock types distinguished during Expedition 309
were sheet flows or brecciated basalts as the most common,
followed by massive units and pillow basalts. Pillow basalts
were only described in the upper borehole section between
~365 and 375 mbsf (Figure 4a).
It is evident that highly fractured lithologies like pillow
and brecciated basalts display higher natural radioactivity
compared to massive units. These fractured units are also
characterized by variable porosities and densities with
values well above 5 % and below 2.9 g/cm3, respectively.
Compressional velocities for these units vary from 3.2
to 5.5 km/s. Pillow basalts may be distinguished from brecciated
lithologies by resistivities lower than or equal to ≤10 Ωm.
However, a clear discrimination between these units using
well-logging data alone remains uncertain. Examples for
massive units are found in depths intervals 316–338
mbsf, ¬472–490
mbsf, 819–833 mbsf, and 1120– 1140 mbsf. These
units are clearly separated from the previous described
lithologies by high compressional velocities (>5.5 km/s)
and densities (~2.7 g/cm3) and increased resistivity
(usually >100 Ωm),
and correlate with low porosity (< 12%) and natural
gamma ray emissions (<4 gAPI).
The most compelling change in log response is observed below
the transition zone (~1060 mbsf) in the sheeted dikes. Natural
radiation in these rocks remains relatively constant with
values generally below 3 gAPI. This constant value may reflect
a change in stability of K-bearing minerals (e.g., saponite),
which is essentially the main carrier of the naturally occurring
radioactivity in these rocks. Increased bulk density, compressional
velocity and electrical resistivity demonstrate a clear change
in lithology and show the highest values obtained in Hole
1256D. Resistivity data recorded with the Dual LateroLog
tool (DLL) demonstrate a strong decoupling between the shallow
(LLS) and the deep (LLD) resistivity below 1080 mbsf. Shallow
LateroLog measurements have the same vertical resolution
as the deep LateroLog but respond more strongly to that region
around the borehole affected by invasion. Caliper readings
from 1080 mbsf to 1211 mbsf are on average 10.98 inches (± 0.5
inch) indicating good borehole conditions and the shallow
resistivity measurements are consequently less influenced
by fluid invasion. It is therefore unlikely that fluid invasion
is solely responsible for the observed decoupling of both
resistivity measurements. Pezard and Anderson (1989) described
this difference between the shallow and deep resistivity
in ODP Hole 504B and attributed this to an anisotropic distribution
of pore space in the rock. In the case of a subvertical network
of conductive fractures the value of the shallow resistivity
is affected more and consequently more reduced than the deep
resistivity. It is very likely that the resistivity data
obtained in Hole 1256D also indicate a dominant presence
of vertical features in the sheeted dikes.
Expedition 312 results
Expedition 312 downhole measurements in Hole 1256D were
conducted from a depth of 1440 mbsf, ~67 m above the total
cored depth (Figure 3 and Figure
4e). Borehole conditions were good during the six logging
runs and provided high quality data with an excellent overlap
of logging results from Expedition 309 (Figure
4a, 4b, 4c,
and 4d).
Overall results obtained during Expedition 312 support the
division of the lithology based on core description from
recovered sample material (see: http://iodp.tamu.edu/publications/PR/312PR/312PR.html for
more details). The overall total gamma-ray is relatively
constant and well below 4 gAPI in the logged sections. The
net measured formation resistivity increased with increasing
depth but this trend is interrupted at several depth intervals
(Figure 4e). Strong decoupling between
the shallow and deep resistivity measurements described at
the top of the sheeted dikes continues to TD. Values for
the shallow and deep resistivity measurements are in the
range of 500 –140000 Ωm. The resistivity data
observed in the sheeted dike complex suggests that the lithostratigraphy
may be divided into four sections (1060–1155 mbsf, 1155–1275
mbsf, 1275–1350 mbsf, and 1350–1407 mbsf) based
on variability and magnitude of the electrical resistivity.
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Figure
6. Velocity-depth plot of Hole 1256D showing
wireline sonic and check-shot interval velocities
from Expedition 312 and Leg 206. Logging and core
bulk density data from Hole 1256D are also shown.
The increase in velocity in the sheeted to granoblastic
dike boundary to values around 7.0 km/s is apparent.
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| Figure 7. Formation
MicroScanner (FMS) resistivity and sonic Ultrasonic Borehole
Imager (UBI) images (static normalized) showing the depth
range 1402–1409 mbsf covering the sheeted dike-gabbro
transition described on recovered samples. FMS data (static
normalized grey scale) obtained during logging pass 2
(see Figure 3) are overlain the
UBI image for comparison. |
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| Figure 8. Temperature
profile of Hole 1256D recorded by the Temperature Acceleration
Pressure (TAP) and Environmental Mechanical Sonde (EMS)
tools during Expeditions 309 and 312. Excursions between
900 and 950 mbsf, and 1350 and 1400 mbsf are evident,
as is the temperature increase by nearly 18ºC from
beginning to end of the Expedition 312 bottom hole temperature
measurement. Also shown are caliper data indicating good
correlation between enlarged borehole diameter and negative
temperature excursions in some parts of Hole 1256D (e.g.,
~950 mbsf). |
Although, overall density and neutron porosity range from
1.5–3.1 g/cm3 and 2–75 %, respectively,
the variation remains small in the newly cored section of
Hole 1256D. The average densities of the sheeted dike complex
and the granoblastic dikes are 2.89 g/cm3 and
2.99 g/cm3, respectively. Density drops to an
average density of 2.95 g/cm3 in Gabbro 1. A similar
drop occurs at a depth of 1407 mbsf where the density decreases
from 3.10 g/cm3 to only 2.93 g/cm3.
This change in density is accompanied with a decrease in
compressional velocity from 6.2 km/s to 4.6 km/s observed
both in wireline and discrete cube measurements. However,
post-cruise examination of the wireline compressional velocity
data acquired below 1300 mbsf show discrepancies between
the 3 logging runs. This may be related to hole conditions
and/or tool movement in the hole and requires careful re-processing
of the obtained data prior to detailed interpretation.
The VSP was shot in Hole 1256D to determine interval velocities
and to record seismograms for further analysis of the seismic
properties of upper ocean crust. In general, the VSP interval
velocities parallel trends in the sonic log and the shipboard
velocity measurements on recovered rock samples (Figure
6). Although the velocity magnitude differs among the
various methodologies due to different frequencies of sound
and the different confining pressures, the trends with depth
are similar. This similarity demonstrates the fundamental
dependence of velocity fluctuations in uppermost crust on
the primary eruptive process and the increase in velocity
with depth in ocean velocity layer 2 on the increasing density
of the rocks due to progressively higher temperature alteration
and metamorphism. However, there are two unusually high interval
velocities of 7.6 km/s between 1339-1361 mbsf and a velocity
of 6.5 km/s at 880-903 mbsf that are not matched by low velocities
at neighboring stations.
Preliminary analysis of the resistivity and sonic image
data (Figure 7) indicates that directly
above the boundary the formations are characterized by randomly
oriented fractures, whereas the fractures in the gabbroic
section are regular oriented
Features observed in the UBI image at 1402 mbsf and 1409
mbsf have a north-east oriented plunge and an approximate
dip between 35 and 40 degrees and may represent fractures.
The same features are also evident on the resistivity image
where they represent zones of high conductivity. Bottom hole
temperature was recorded three times (Figure
8) and an increase from 64.24 ºC to 67.90 ºC,
and 86.5 ºC observed in a time frame of ~5 hrs and 68 ½ hrs,
respectively. Perturbations are visible between 900–950
mbsf and 1350–1400 mbsf with negative deviation from
the temperature profile. These negative temperature anomalies
indicate a slower return to equilibrium temperatures and
may be due to a higher influx of seawater invasion during
the drilling process
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| References |
Pezard, P.A., and Anderson R.N., 1989. Proc. ODP, Sci. Results.,
111: College Station, TX (Ocean Drilling Program).
Florence Einaudi: Expedition 309 Logging Staff
Scientist, Laboratoire de Géophysique et d'Hydrodynamique
en Forage, ISTEEM, cc 056, 34095 Montpellier Cedex 5, France
email: florence.einaudi@dstu.univ-montp2.fr
Marc Reichow: Expedition 312 Logging Staff Scientist,
University of Leicester, Borehole Research, Department of
Geology, University of Leicester, University Road. Leicester,
LE1 7RH, United Kingdom.
email: mkr6@le.ac.uk
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Additional Leg-related
publications:
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