Site 395 | Site U1383 | Holes U1383D, U1383E, U1382B, U1384A
IODP Expedition 336:
Mid-Atlantic Ridge Microbiology
Site U1382 Summary
PDF file is available for download.
Hole U1382A was drilled 50 m west of Hole 395A at 22°45.353'N, 46°04.891'W in a water depth of 4483 m below sea level (mbsl). The primary objective at Hole U1382A was to install a CORK observatory to perform long-term
coupled microbiological, biogeochemical, and hydrological experiments in
uppermost basaltic crust in this area of very low conductive heat flow. Coring
and downhole logging of basement was also conducted.
After the re-entry cone with 53 m
of 16 inch casing was jetted in, the hole was deepened by drilling with a 14.75 inch tri-cone bit to 110 m below seafloor (mbsf) without
coring. Basement was encountered at 90 mbsf and three meters were penetrated in
30 minutes. The interval from 93-99 mbsf drilled very quickly and is inferred
to be sediments; but the underlying formation to 110 mbsf drilled slowly (2-3 m
per hour) without significant torque. Casing (10.75 inch) was installed and
successfully cemented to 102 mbsf. RCB coring recovered basement from 110 to
210 mbsf (Cores U1382A-2R to -12R). In total, 32 m of core were retrieved, with
recovery rates ranging from 15 to 63%. This succession resembles the lithostratigraphy encountered in DSDP Holes 395 and 395A and provided excellent sampling material for various microbiological and petrologic studies.
The shipboard petrologists divided
the core into 8 lithologic units, comprising 17
subunits. Major unit boundaries are defined by contacts between massive and
pillowed flow and interlayered sedimentary units. Each major lava flow unit
consists of several cooling units, which are recognized by glassy or variolitic margins and/or marked changes in grain size.
Results from thin section studies reveal a large range of grain sizes (glassy
to medium-grained) and diverse textures (aphanitic to
subophitic or intersertal). Basalts are either aphyric or
plagioclase-olivine phyric and have <3%
vesicles. Phenocryst contents range up to 25%, with
plagioclase being more abundant than olivine. All of
the basement volcanic rocks recovered from Hole U1382A are affected only by
low-temperature alteration by seawater, manifesting as replacement of
groundmass and phenocrysts, vesicle filling, glassy
margin replacement, and vein formation with adjacent brown alteration
halos. Chilled margins often show advanced palagonitization, which develops as blotchy alteration texture following the primary variolic
texture of the mesostasis. The extent of alteration ranges up to 20%, with clay (smectite, celadonite) being the most abundant secondary phase,
followed by Fe-oxyhydroxides and minor zeolites and
carbonates. The recovered section has between 13 and
20 veins/m, with vein thickness being generally less than 0.2 mm. A sedimentary
unit in Cores 8R and 9R features a variety of clasts,
including plutonic and mantle rocks. The peridotites are weakly serpentinitzed harzburgites and lherzolites with a protogranular texture. Intensity of deformation of the gabbroic lithologies ranges from undeformed to mylonitic.
Minor cataclastic deformation of the peridotites has led to the development of carbonate-filled vein networks, along which the rocks have been subjected to oxidative alteration, resulting in the breakdown of olivine to clay, oxide, and
Physical property measurements reveal typical P-wave velocities for these lithologies and a correlation between sonic velocity and porosity of the basalt. Elevated potassium and uranium concentrations in the oxidatively altered part of the core were revealed by
natural gamma ray (NGR) core scanning. Thermal conductivity also
reflected the typical values associated with basalt and peridotite
and showed small variations with depth.
Whole rock geochemistry reveals
systematic differences in compositions between aphyric
and porphyritic basalt, which are due to plagioclase accumulation in the
porphyritic basalt. The aphyric basalts show a liquid
line of descent, which is controlled by the fractional crystallization of
olivine. With increasing extent of alteration, loss on ignition values and
potassium concentrations increase. Immobile trace element ratios (Zr/Y, Ti/Zr) indicate that
parental magma compositions for the basalts above and below the sedimentary
unit are different from each other. Petrographically
and geochemically the basalts correspond to the uppermost lithological units
identified in Hole 395A. Likewise, the sedimentary unit with varied plutonic
and mantle rocks was also observed in Hole 395A.
A primary objective of the basement
coring was to obtain samples for microbiological analysis. We collected 46 hard
rock and 2 sediment whole-round samples for these studies (11% of core recovered).
Samples were preserved for ship-based (deep UV fluorescence scanning, culturing
and enrichment, fluorescent microsphere analysis) and shore-based (DNA and RNA
analysis, shore-based fluorescence in situ hybridization, cell counting
analysis, isotopic analysis) studies. Generally, one to three microbiological hard
rock samples were collected from every core section. Hard rock samples
span a range of lithological units, alteration states, presence of chilled
margins, and some contain veins/fractures. Additionally, a few recovered
plastic bags that held the fluorescent microsphere solutions in the core
catcher have been collected as a contamination check in DNA analysis.
An open hole section of 105.61 m was logged with two tool strings
over a period of ~19.5 hours (Adapted Microbiology Combination I, FMS-HNGS). Downhole
log measurements include natural total and spectral gamma ray, temperature,
density, electrical resistivity, electrical images and deep UV-induced
fluorescence (the new DEBI-t). The borehole remained in good condition
throughout logging and no obvious tight spots were encountered in open hole. Integration
of core and log measurements and observations showed excellent correspondence
between potassium concentrations provided by shipboard NGR, spectral gamma ray
logging tool and whole rock geochemical analyses. FMS data were combined with
images of the external surfaces of whole round cores. Prominent veins with
alteration halos in core of the massive flows can be matched up with fractures
in the FMS images. Also, logging results constrain the depth of the peridotite interval from 165 to 167 mbsf (based on density and low K/U ratios).
Downhole hydrologic (packer) tests failed, because ship heave up to 3 m prevented the packer from sealing within the casing for more than 10 minutes.
A CORK observatory to monitor and sample a single interval in uppermost basement was successfully installed in Hole U1382A. The 210-m deep hole is sealed with a 189 m-long CORK completion string
with 9 external umbilicals and a retrievable internal instrument string. The
umbilicals include one for monitoring pressure, two for microbiological
sampling, and six for fluid sampling. The retrievable internal instrument
string comprises several osmotic pump-driven samplers for basement fluids and
microorganisms as well as enrichment experiments, an oxygen probe, and a thermistor
with data recorder. The samplers and probes extend from 152 to 174 mbsf and are
kept in position by a 150-lbs sinker bar at 177 m. A pressure gauge and
fast-pumping OsmoSamplers are situated in the wellhead and monitor/sample fluids from 161 mbsf.
Also see Holes U1383D, U1383E, U1382B, and U1384A sediment/basement contact coring.