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doi:10.2204/iodp.proc.324.102.2010

Physical properties

Shipboard measurements of physical properties were performed to provide preliminary information characterizing the recovered material. The primary objective of Expedition 324 was to recover hard rock, which necessitated a modified approach compared with expeditions focusing on sedimentary coring. Sedimentary material was recovered, but it was disturbed by the exclusive use of RCB drilling except for most volcaniclastic sediments, which are relatively consolidated. The descriptions herein relate specifically to hard rock material. Sedimentary material was treated in the same manner despite more limited discrete sampling.

Once recovered, whole-round cores were first allowed to thermally equilibrate to ambient room temperature (3 h for hard rock, 4 h for sedimentary material). Core sections with continuous intervals longer than 8 cm were run through the WRMSL for measurement of GRA density and magnetic susceptibility. Magnetic susceptibility and GRA data from gaps and cracks between pieces were filtered out of the data sets using a MATLAB routine (written by A. Harris during Expedition 324) as described below in "Data filtering." The filtered data were then visually double-checked against images of the core section halves. Sections longer than 50 cm were measured with the NGRL. The WRMSL also incorporates a compressional wave velocity sensor (PWL) and a noncontact resistivity detector (NCR). The PWL was used only in a few rare cases where sedimentary sections filled most of the core liner; even then sufficient contact with the rock could not always be made. The NCR was not employed during this cruise because of the same unfilled core liner problem, which is common in hard rock recovery.

After measurements with the WRMSL and NGRL, the cores were split into archive and working halves. The archive half of the core was passed through the SHMSL for measurement of point magnetic susceptibility and color reflectance. The SHMSL also uses a laser to record piece heights, which yields information about the location of gaps and cracks between pieces of the core. This information was used to aid data filtering of whole-round and split-half measurements (see "Data filtering").

Thermal conductivity was measured on either the archive or working half of the split cores, depending on availability of suitable material. The thermal conductivity sensors experienced technical difficulties at the end of Hole U1349A; therefore, there are no measurements for Hole U1350A.

Lastly, discrete samples were taken from the working half at intervals of ~1–2 per core. Discrete samples were used for compressional wave velocity measurements in three directions and moisture and density (MAD) measurements including bulk density, dry density, grain density, water content, void ratio, and porosity. A comprehensive discussion of methodologies and calculations used in the R/V JOIDES Resolution physical properties laboratory is presented in Blum (1997).

Whole-Round Multisensor Logger measurements

GRA bulk density and magnetic susceptibility were measured nondestructively with the WRMSL. To optimize WRMSL performance, sampling intervals and measurement integration times were the same for all sensors. Sampling intervals were set at 2 cm with an integration time of 5 s for each measurement. These sampling intervals are common denominators of the distances between the sensors installed on the WRMSL (30–50 cm) and allow sequential and simultaneous measurements. Quality assurance/Quality control (QA/QC) was monitored by passing a single core liner filled with fresh water through the WRSML after every core.

The primary objective of Expedition 324 was the recovery of hard rock. In general, measurements are most effective with a completely filled core liner with minimal drilling disturbance. Consequently, the diameter of the core liner (66 mm) is assumed for the core diameter, even though the liner is often <100% filled. In addition, hard rocks are often recovered in pieces, rather than continuous core, which compounds the diameter discrepancy that is already introduced by the drilling process. Therefore, GRA bulk density and magnetic susceptibility measurements tend to underestimate true values. Data were filtered to remove anomalously low values associated with gaps and cracks in the core (see "Data filtering").

Gamma ray attenuation bulk density

The GRA densiometer on the WRMSL operates by passing gamma rays from a ¹³⁷Cs source through a whole-round core into a 75 mm x 75 mm sodium iodide detector situated directly below the core. The gamma ray peak has a principal energy of 0.662 MeV and is attenuated as it passes through the core. The attenuation of gamma rays, mainly by Compton scattering, is related to the material bulk density. Therefore, for a known thickness of sample the gamma ray count is proportional to density. Calibration of the GRA densiometer was performed using core liners filled with seawater and aluminum density standards. Recalibration was performed as needed if the freshwater QA/QC standard after every core deviated significantly (more than a few percent) from 1 g/cm³. The spatial resolution of the GRA densitometer is <1 cm.

Magnetic susceptibility

Magnetic susceptibility, k, is a dimensionless measure of the degree to which a material can be magnetized by an external magnetic field:

where M is the magnetization induced in the material by an external field strength H. Magnetic susceptibility responds to variations in the type and concentration of magnetic grains, making it useful for the identification of compositional variations. Materials such as clay (e.g., from the alteration of igneous materials) have a magnetic susceptibility several orders of magnitude lower than magnetite and other iron oxides that are common constituents of igneous material. Water and plastics (core liner) have a slightly negative magnetic susceptibility.

The WRMSL incorporates a Bartington Instruments MS2 meter coupled to a MS2C sensor coil with a diameter of 8.8 cm and operates at a frequency of 565 Hz. The sensor output can be set to centimeter-gram-second (cgs) units or SI units, with the IODP standard being the SI setting. The core diameter is smaller than the aperture through which it passes to be measured. Therefore, a volume correction factor must be applied to the data offline. Assuming a core diameter of 66 mm and using the coil aperture of 88 mm, the correction factor simply entailed multiplying the x10^–5 SI units by a factor of 0.68 (Blum, 1997).

The MS2C coil is calibrated with a homogeneous mixture of magnetite and epoxy in a 40 cm long piece of core liner to an accuracy of ±5% (factory preset). The resolution of the method is ±4 cm; therefore, core material that is not continuous over an 8 cm interval will underestimate magnetic susceptibility. Data collected near gaps or pieces smaller than 8 cm were removed by a filter (see "Data filtering") and shown as grayed out points in report figures.

Natural Gamma Radiation Logger

The Natural Gamma Radiation Logger (NGRL) installed on the JOIDES Resolution was designed and built at IODP-Texas A&M University (TAMU) from 2006 to 2008. NGR occurs primarily as a result of the decay of ²³⁸U, ²³²Th, and ⁴⁰K isotopes. Data generated from this instrument are used to augment geologic interpretations.

The main NGR detector unit consists of 8 NaI scintillator detectors, 7 plastic scintillator detectors, 22 photomultipliers, and passive lead shielding. The NaI detectors are covered by 8 cm of lead shielding. In addition, lead separators (~7 cm of low-background lead) are positioned between the NaI detectors. Half of the lead shielding closest to the NaI detectors is composed of low-background lead, whereas the outer half is composed of regular (virgin) lead. In addition to passive lead shielding, the NGR detector unit employs a plastic scintillator to suppress the high-energy gamma and muon components of cosmic radiation by producing a veto signal when these charged particles pass through the plastic scintillators. The NGR detector unit was calibrated using ¹³⁷Cs and ⁶⁰Co sources and identifying the peaks at 662 and 1330 keV, respectively. Calibration materials were from Eckert & Ziegler Isotope Products, Valencia, California (USA).

For presentation purposes, the counts were summed over the range of 100–3000 keV to be comparable with data collection from previous cruises and for direct comparison with downhole logging data. Background measurements of an empty core liner counted for 20,000 s (5 h) were made upon arrival at each site. Over the 100–3000 keV integration range, background counts averaged 4–5 cps and contributed <0.5% to the overall signal of the measured core.

A single measurement run consisted of two eight-measurement sets (20 cm between each of the eight sensors). The first set was measured at one position and the second set was measured at a another position shifted 10 cm from the first (for a total of 16 measurements, each 10 cm apart, over a 150 cm long section of core). The quality of the energy spectrum measured in a core depends on the concentration of radionuclides in the sample but also on the counting time, with higher times yielding a more clearly defined spectra. The available count time in each position depended on how fast and how much core was recovered. In general, we had the opportunity to count for longer times, yielding statistically significant energy spectra because the rate of hard rock core recovery from drilling averaged 1 core every 2–3 h. Count times ranged between 1800 and 5400 s for each position, resulting in total count times of 1–3 h per section. Improved spectral resolution allows qualitative identification of the main contributors to the energy spectra (i.e., products of the ⁴⁰K, ²³²Th, or ²³⁸U decay chains). Building a database of well-resolved spectra works toward the aim of separation of ⁴⁰K, ²³²Th, and ²³⁸U contributions and eventual quantification of concentrations of the radionuclide daughters.

Section Half Multisensor Logger measurements

The SHMSL measures spectral reflectance and magnetic susceptibility on core section halves. The archive half of the split core is placed on the core track. An electronic platform moves along a track above the core section, recording the sample height using a laser sensor. The laser establishes the location of the bottom of the section, then the platform reverses the direction of movement, moving from bottom to top making measurements of point magnetic susceptibility and spectral reflectance data at 2 cm intervals.

Color reflectance spectrometry

Reflectance is measured from 171 to 1100 nm wavelength at 2 nm intervals using a halogen light source, covering a wavelength from ultraviolet through visible to near infared. The scan of the entire wavelength range takes ~5 s per data acquisition offset. The data are generated using the L*a*b color system, in which L* is luminescence, a* is the blue + green values, and b* is the red + green values. The color reflectance spectrometer calibrates on two spectra, pure white (reference) and pure black (dark). Color calibration was conducted approximately every 12 h.

Point magnetic susceptibility

The point magnetic susceptibility is measured using a contact probe with a flat 15 mm diameter sensor operating at a frequency of 0.580 kHz. The sensor takes and averages three measurements at 0.1 attenuation for each offset to an accuracy of 5%. The spatial resolution of the magnetic susceptibility point instrument is 20 mm, making it advantageous over whole-round magnetic susceptibility for cores consisting of broken pieces <8 cm (the spatial resolution of whole-round magnetic susceptibility). Units are reported in dimensionless SI units on a volume basis. The point magnetic susceptibility meter was calibrated by the manufacturer before installation on the ship. The probe is zeroed in air before each measurement point, and a background magnetic field is measured (influence from metal track, etc.) and removed from the data before being output. The instrument is calibrated so that the value output is measured assuming the probe is buried in the sample; however, because the probe is only in contact with the upper, flat surface, a correction factor of 2x was applied after the data were collected (note that the data stored in LIMS have not had this correction applied).

Thermal conductivity

Thermal conductivity is the rate at which heat flows through a material and is dependent on composition, porosity, and structure. Thermal conductivity was measured by transient heating on the archive half of split core with a known heating power and a known geometry. The changes in temperature with time after an initial heating were recorded using a TK04 system. The temperature of the superconductive probe has a linear relationship with the natural logarithm of the time after the initiation of heat (Blum, 1997).

Thermal conductivity was measured on the archive half of the split core with the probe in half-space mode (Vacquier, 1985). These half-space determinations were made with a needle probe embedded in the surface of an epoxy block with a low thermal conductivity (Vacquier, 1985). Samples need to be smooth to ensure adequate contact with the heating needle. Visible saw marks were removed when necessary by grinding and polishing the split face using 120–320 gauge silicon carbide powder (most samples did not require polishing). Because measurements were sometimes performed on the archive half of the core, we avoided the use of silicon gel, which can be employed to improve sample/needle contact.

Samples equilibrated to room temperature in a seawater vacuum saturator for 4 h, and sample and sensor needle were equilibrated together in a cooler insulated with styrofoam for at least 15 min prior to measurement. Isolation of the sample and sensor needle eliminated the effect of rapid but small temperature changes introduced by air currents in the laboratory. The instrument internally measures drift and does not begin a heating run until sufficient thermal equilibrium is attained. Cores were measured at irregular intervals (one sample per section) depending on the availability of homogeneous and relatively vein/crack-free pieces long enough to be measured without edge effects (pieces at least 7 cm long; i.e., longer than the instrument needle). Measurements were made at room temperature and pressure, and were not corrected for in situ conditions.

A MACOR ceramic standard with a certified thermal conductivity of 1.637 ± 0.033 W/(m·K) was run at each site, and the results are summarized in Table T8. Unfortunately, the puck and embedded sensor needle experienced irrevocable technical difficulties at the end of Hole U1349A, and as a result thermal conductivity measurements were not conducted on samples from Hole U1350A.

Discrete samples

Cubic samples were cut from the working halves of split cores at an interval of ~1–2 samples per core. Collection of these ~7 cm³ samples aimed to best represent the general variation and lithologies of the core. The purpose of these samples is two-fold. First, they are used for physical property measurements of compressional wave velocity and MAD measurements (explained below). Second, discrete samples were shared with paleomagnetists to minimize core depletion.

Moisture and density

Several basic rock quantities of interest (water content, bulk density, dry density, porosity, and void ratio) are found most accurately through mass and volume determinations on discrete samples. MAD data are also used for comparison with GRA bulk density data from the WRMSL. The shipboard MAD facility for hard rock coring on the JOIDES Resolution consists of a vacuum water saturator, a dual balance system, and a hexapycnometer.

Vacuum water saturator

Determination of a precise and accurate wet mass of sparsely porous material requires that the pore space of the sample be completely saturated. To maintain this saturation, we used a vacuum pump system. The system consisted of a plastic chamber filled with seawater into which the samples were placed. A vacuum pump then removes air from the chamber, essentially forcing seawater into the sample pore spaces. The samples are kept under vacuum for at least 24 h. During this time, the vacuum is checked at 2–3 h intervals to ensure a stable underpressure. After removal from the saturator, the cubes are stored in sample containers filled with seawater to help prevent evaporation of pore water. Next, the cube surfaces are lightly patted with a paper towel to remove water clinging to outer surfaces and wet mass is immediately determined using the dual balance system.

Dual balance system

The dual balance system was used to measure both wet and dry masses. The two analytical balances (Mettler-Toledo XS204) compensate for ship motion; one acts as a reference and the other measures the unknown (sample). A standard weight of similar value to the sample was placed upon the reference balance to increase accuracy. The default setting of the balances is 300 measurements (taking ~1.5 min to measure). However, when in transit we found that a greater number of measurements (400–1000), and therefore a longer measurement time (depending on sea state), were required. On site, however, averages of 300 measurements were indistinguishable from averages of 500 measurements. We investigated the effect that matching the reference and sample weight had by repeatedly measuring a basaltic minicore sample with different reference masses. Overall, for this test sample an error of as much as 0.3% could be introduced if the samples were mismatched with their reference weight by as much as 50%. With reference weight matching to within ~10%, precision of 0.005 g was readily attainable. Precisions were recorded on the hard copies of data log sheets, which are stored at TAMU.

Hexapycnometer system

The hexapycnometer system measures dry sample volume using pressurized helium-filled chambers. At the start of the expedition, and whenever the helium gas tank was changed, shipboard technicians performed a calibration using stainless steel spheres of known volume. A batch of samples consisted of four cells with unknowns and one cell with two stainless steel spheres (3 and 7 cm³). The spheres were cycled through the cells to identify any systematic error and/or instrument drift. Spheres are assumed to be known within 1% of their total volume. Individual volume measurements were preceded by three purges of the sample chambers with research grade (99.995% or better) helium heated to 280°C, followed by three data acquisition cycles.

Moisture and density calculations

Measurement of wet mass and dry mass by the dual balance system along with a dry volume measured by the hexapycnometer allows for the determination of a number of MAD properties. During this expedition, we employed MAD method "C." Method "D," where wet mass is determined by caliper rather than saturation, is only appropriate for samples that are too vesicular to adequately saturate.

The MAD method C program uses physical measurement data (dry mass, wet mass, and dry volume) to calculate the quantities of interest shown in Table T9. For more details on physical property MAD methods, see Blum (1997).

Compressional wave velocity

Compressional wave (P-wave) velocity measurements of hard rock samples were performed on the same discrete cube samples that were used for MAD and paleomagnetism determinations. P-wave measurements were performed on seawater-saturated samples directly after wet mass determinations were made. Measurements used the x-axis caliper-type contact probe transducers on the P-wave velocity gantry. Oriented samples were rotated manually to measure y- and z-axis velocities with the same instrument. The system uses Panametrics-NDT Microscan delay line transducers, which transmit at 0.5 MHz. To maximize contact with the transducers, deionized water was applied to sample surfaces.

The signal received through the sample was recorded by the computer attached to the system, and the peak (P-wave arrival) was chosen with autopicking software. The complete waveform is stored with the data if reanalysis is deemed necessary; however, shipboard visual checks of the picks appeared satisfactory. The distance between transducers was measured with a built-in linear voltage displacement transformer (LDVT).

Calibration was performed each day before measurements were made with a series of acrylic cylinders of differing thicknesses and a known P-wave velocity of 2750 ± 20 m/s. The determined system time delay from calibration was subtracted from the picked arrival time to give a traveltime of the P-wave through the sample. The thickness of the sample (calculated by LDVT in meters) was divided by the traveltime (in seconds) to calculate P-wave velocity in meters per second.

Data filtering

Gaps and cracks in the core can cause anomalously low readings of magnetic susceptibility and bulk density. Reflectance and color readings for gaps and cracks are not relevant to this study. Including meaningless data from gaps can hinder interpretation, especially of actual low readings of magnetic susceptibility and bulk density. Therefore, these data (magnetic susceptibility, GRA density, L*, b*, and magnetic susceptibility point) were filtered using height measurements, which were collected with the laser attached to the SHMSL. Laser heights were used to identify cracks between pieces and gaps where plastic spacers were inserted. Data corresponding to locations of gaps and cracks, as well as data near the edges of pieces, were then removed from each data set (often plotted as gray points on color plots).

Filtering program details

A series of parameters (including gradient threshold, minimum detectable gap width, minimum piece length, and instrumentation edge effect) were used to filter data. Whereas the user of the filter has some flexibility in choosing parameters, most are predetermined by the resolution and specifications of the instrumentation.

To determine the edges of pieces, the gradient of the laser height was calculated. The relatively flat surfaces of continuous section-half pieces have a small gradient. However, the sharp contrast between piece height and the height of an empty core liner results in a large gradient. Using this relationship, a threshold parameter for the gradient was set (±5 mm/cm) and used to determine the locations of piece edges. The lateral resolution of laser height measurements was 0.2 cm. With the gradient threshold set at ±5 mm/cm, this means any change in height >1 cm between subsequent measurements would indicate a piece edge or gap/crack. This threshold is set so that data for pieces with heights ranging from 1 to 3.3 cm (a full section-half radius) are kept and data for pieces with heights shorter than 1 cm are removed by the filter. Data associated with <1 cm pieces are likely to be underestimated, similar to data associated with cracks/gaps.

The minimum detectable gap width is directly related to the lateral resolution of the laser height measurements. Gaps <0.2 cm (the lateral resolution of the laser measurements) are undetectable, unless, by chance, a measurement lands within the small gap. The minimum detectable gap width could be decreased by increasing lateral resolution of the laser measurements, but this is likely unnecessary because cracks <0.2 cm should have a small effect on measured data.

The length of the "zone of influence" (the amount of core material that influences data readings) is determined by individual instrument specifications, which in turn determines both the minimum piece length and instrumentation edge effect parameters. The minimum piece length is exactly equal to the length of the zone of influence (assuming a full core liner). The edge effect parameter is the distance away from a gap or crack a measurement must be to be unaffected by the empty space. The edge effect parameter is exactly equal to half of the length of the minimum piece length. A list of the minimum piece length and edge effect parameters for each instrument is listed in Table T10.

Data points that were filtered out still appear in physical property plots and figures (e.g., VCDs) as grayed out points, whereas data that passed these sets of criteria described above are plotted with colored points according to the legend/key. For example, GRA bulk density measurements that were at least 1 cm away (edge effect parameter) from a 0.2 cm or larger crack (minimum detectable gap) and that were measured on pieces that were at least 2 cm long (minimum piece length) pass through the filter. For magnetic susceptibility and magnetic susceptibility point data, correction factors of 0.68x and 2x were applied to LIMS output data, respectively (even for grayed out points), because these corrections are due to instrumentation design and measurement technique and not related to the filtering process.

The major benefit of filtering data from GRA density and magnetic susceptibility measurements is that actual lows can be discerned from data trends. However, caution should be used, as this is the first attempt at choosing appropriate parameters and we have tried to err on the side of allowing more data through the filter than not. Data that are filtered out are underestimates of real values. Where the lowest values are often the target of the filtering, even high values may be filtered out if they are near gaps in material. In reality, these high values should be even higher given a continuous piece of core.

In the future, the potential exists for the laser heights to be used for a volume correction instead of merely a filter. Then trends may be easier to discern from the physical property data and may more closely correspond with other, independent data sets (e.g., logging bulk density). The NGR whole-round data were not filtered because of already low sample densities.

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