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doi:10.2204/iodp.proc.345.102.2014

Physical properties

Shipboard measurements of physical properties were undertaken to characterize recovered core material. These data are potentially used to link the geological observations made on the core to the results of downhole logging and regional geophysical survey results.

Prior to physical property measurements, whole-round cores were allowed to thermally equilibrate for ~1 h to ambient room temperature. A 4 h delay time was classically reported in previous hard rock expeditions, based on the protocol used for sediment cores. Because gabbroic rock has thermal conductivities typically 2 or 3 times higher than those of marine sediment, 1 h was considered to be sufficient.

Most core sections were run through the WRMSL to measure GRA density and magnetic susceptibility. The NGRL was used to measure gamma radiation for whole-round sections. We did not use the P-wave logger on the WRMSL, as these measurements require full-diameter core and good coupling to the liner; this is generally not the case for hard rock cores.

Following whole-round measurements and core splitting, the archive half of the core was passed through the SHMSL for measurement of magnetic susceptibility with a Bartington MS2E contact sensor probe and measurement of color reflectance with an Ocean Optics photospectrometer.

WRMSL and SHMSL data must be filtered to remove spurious values that correspond to gaps in the core section (empty intervals), cracks in core pieces, and, for bulk volumetric measurements (WRMSL), reduced volume of material (departure from a continuous, cylindrical core) in the vicinity of the sensor. Given the relatively small amount of material recovered during Expedition 345, it was not deemed necessary to develop a filtering algorithm for the various whole-round and half-core measurements. We chose to filter the data by manually editing the data files, following procedures described below for each instrument. Throughout the cruise, raw data were uploaded to the LIMS database. Filtered WRMSL and SHMSL data are available in PHYSPROP in “Supplementary material.”

Thermal conductivity was measured on pieces from the archive half of the split-core sections, depending on the availability of suitable material. Discrete samples (2 cm × 2 cm × 2 cm cubes) were taken from working section halves for physical property measurements. Most samples were also used for paleomagnetic measurements, as the core material was limited. Shipboard samples were preferentially located close to where shipboard geochemistry and thin section samples were taken. Discrete samples were used for P-wave velocity measurements in three orthogonal directions following the standard IODP convention (Fig. F2), and moisture and density (MAD) measurements (IODP shipboard Method C) were used to determine bulk density, grain density, and porosity.

A comprehensive discussion of methodologies and calculations used in the 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. The sampling interval for WRMSL measurements was first set at 2.5 cm for Hole U1415I and at 1 cm for Hole U1415J and the remainder of the expedition, with an integration time of 5 s for each data point to allow both instruments to acquire values from the same location downcore. Calibration was verified after each core measurement by passing a freshwater-filled calibration core through the WRSML. The nominal accuracy of the calibrated instruments is between 1% and 2%.

Gamma ray attenuation bulk density

The GRA densitometer on the WRMSL operates by passing gamma rays from a ¹³⁷Cs source through a whole-round core into a 75 mm × 75 mm sodium iodide detector located directly below the core. The input gamma ray peak has a principal energy of 0.662 MeV and is attenuated as it passes through the core. Attenuation of gamma rays, mainly by Compton scattering, is related to electron density, which is related to material bulk density by

where

• ρ_b = bulk density,

• ρ_e = electron density,

• w = molecular weight, and

• N = atomic number of elements in the material.

For the majority of elements and for rock-forming minerals, 2ΣN/w is ~1, whereas for hydrogen, 2ΣN/w is 1.9841. Therefore, for a known thickness of sample the gamma ray count is proportional to density. Calibration of the GRA densitometer was performed using a core liner filled with freshwater and aluminum density standards. A freshwater-filled liner was measured at the end of each core measurement; recalibration was performed if the measured density of the freshwater standard was not 1.00 ± 0.02 g/cm³. The spatial resolution of the GRA densitometer is <1 cm. Raw data were filtered by first removing all values lower than 1 g/cm³ (i.e., water density) and then removing a series of values acquired close to piece edges that show density gradients too high to correspond to realistic density variations in the recovered rocks (i.e., >0.2 g/cm³/cm). Comparison of the data curves with core scan images confirmed that this procedure efficiently removed spurious data that correspond to empty intervals, small or irregularly shaped pieces, and cracks in large pieces.

Magnetic susceptibility

Magnetic susceptibility, κ, 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 with strength H (very low field; ≤0.5 mT). Magnetic susceptibility varies in response to the type and concentration of magnetic grains, making it useful for the identification of compositional variations.

The WRMSL measures volume magnetic susceptibility using a Bartington Instruments MS2 meter coupled to a MS2C sensor coil with an 88 mm diameter and operates at a frequency of 0.513 kHz. During Expedition 345, the instrument was set to record SI units with an integration period of ~1 s, to give a sensitivity of 1 × 10^–5 SI. The core diameter is smaller than the sensor coil aperture. The instrument output (κ_MEAS) depends on the diameter of the core (d) passing through the coil diameter (D), so a correction factor (κ_REL) is necessary to convert the instrument output to true volume susceptibility (κ in SI), where κ_REL = 3.45(d/D)³ (Bartington Instruments, Ltd., 2011). κ_REL is 1 for d = 58 mm and D = 88 mm; d is typically 57 ± 1 mm for well-cut RCB hard rock cores, and the size of small pieces and rollers varies in an unpredictable manner. Hence, a single correction factor was not justified; therefore, no correction was applied to WRSML magnetic susceptibility measurements; raw data are reported in instrument units (10^–5 SI).

The along-core response curve of the MS2C coil has a full width of half maximum of ~4 cm (Blum, 1997) and is consistent with the decay in magnetic intensity with distance from a dipole (Fig. F19). Therefore, measurements of susceptibility from core pieces <8 cm long will significantly underestimate magnetic susceptibility by more than 10%. Hence, filtering of the WRMSL magnetic susceptibility data consisted of removing data points that correspond to pieces shorter than 8 cm, as well as 4 cm long edges from pieces longer than 8 cm.

The Bartington sensor has a maximum output threshold of 9,999 instrument units (IU), so any reading ≥10,000 IU loses the most significant digit and is “wrapped” around to lower values. Figure F20 illustrates how data can be corrected for this record offset when measuring high-susceptibility rock. Measurements that were judged to be wrong, based on neighboring high susceptibility values and examination of the core, were corrected by adding n × 10,000 to the raw value. The value of n is arbitrarily chosen to generate the smoothest possible corrected curve (e.g., Fig. F20B).

Natural Gamma Radiation Logger measurements

Gamma radiation is emitted from rock primarily as a result of the radioactive decay of ⁴⁰K and the decay of isotopes in the ²³⁸U and ²³²Th decay series. Measurement of NGR from the recovered core provides an indication of the concentration of these elements and can also be used to correlate the core with the downhole gamma ray logs (e.g., Révillon et al., 2002).

The NGRL installed on the JOIDES Resolution was designed and built by IODP-USIO at Texas A&M University (Vasilyev et al., 2011). The main NGR detector unit consists of 8 sodium iodide (NaI) scintillator detectors (~500 in³ each), 7 plastic scintillation detectors, 22 photomultipliers, and passive lead shielding. The eight NaI detectors are spaced every 20 cm in the detector; the detectors themselves are semicylindrical annuli around the lower half of the core (each crystal is ~13 cm wide along the core). Detectors are shielded by lead to reduce the measurement of external gamma radiation, and the NGRL also employs seven plastic scintillation detectors that detect and actively suppress the effect of high-energy gamma and muon components of cosmic radiation. The NGRL was calibrated using ¹³⁷Cs and ⁶⁰Co sources to identify peaks at 662 and 1330 keV, respectively.

Background measurements of an empty core liner counted for 44,000 s (>12 h) were made upon arrival at Site U1415. Over the 100–3000 keV integration range, background counts averaged ~5 cps and contributed at least ~80% of the uncorrected total counts for the low-radioactivity mafic rock measured during Expedition 345.

A single measurement run with the NGRL provides 16 measurements at 10 cm intervals over a 150 cm section of core. To achieve a 10 cm interval using the NGRL’s eight sensors spaced every 20 cm, the NGRL records two sets of measurements offset by 10 cm. Total counts are routinely summed over the range of 100–3000 keV. The quality of the energy spectrum measured depends on the concentration of radionuclides in the sample and on the counting time, with longer counting times providing better counting statistics. A live counting time of 1800 s (30 min) was set in each position (total live count time of 1 h per section).

During Expedition 345, Detector 3 was reported to have severe drift behavior. A long-term measurement on the Cs/Co standards revealed a serious distortion of the expected energy bin of the signal peaks. This problem could not be fixed with the available tools on board the ship. Details are given in the technical staff report.

Section Half Multisensor Logger measurements

The SHMSL was used to measure spectral reflectance and magnetic susceptibility on archive section halves. An electronic platform moves along a track above the section half, recording the sample height using a laser sensor. The laser establishes the location of the bottom of the section and the presence of samples to measure by locating gaps and cracks between pieces. The platform then reverses the direction of movement, moving from bottom to top taking measurements of point magnetic susceptibility and spectral reflectance data at 2 cm (Holes U1415E–U1415I) or 1 cm (Hole U1415J and the remainder of the expedition) intervals.

Raw data were manually filtered using the half-core scan images for comparison to remove data points that correspond to cracks, empty intervals, or piece edges (1 cm).

Reflectance spectrophotometry and colorimetry

Reflectance from the archive section half was measured using an Ocean Optics, Inc., system for ultraviolet through visible to near-infrared light (171–1100 nm wavelength at 2 nm intervals). Each measurement takes ~5 s to acquire. Spectral data are routinely reduced to the L*a*b* color space for output and presentation, in which L* is luminescence, a* is the red–green value, and b* is the blue–yellow value. The color reflectance spectrometer calibrates on two spectra, pure white (reference) and pure black (dark). Color calibration was conducted every 24 h.

While conducting test measurements with a smaller measurement interval of 1 mm in order to look for variations of a* and b* with the olivine contents in the layered gabbronorites recovered from Section 345-U1415I-4R-1, we identified a problem with the instrument; it was recording different values of these parameters over successive series of measurements on the same core pieces (see details in the “Hole U1415I” chapter [Gillis et al., 2014b]). A series of tests with color standards was then conducted. We established that the measured chromaticity values (a* and b*) oscillated over time in an unpredictable manner (Fig. F21A, F21B). This behavior appeared to be related to the sensitivity of the spectrophotometer to temperature changes caused by both room temperature variations (air conditioning cycles) and internal temperature changes when acquiring data. The problem was eventually fixed by the technical staff, who worked over several days on reducing the temperature sensitivity of the spectrometer and implementing changes in the data processing software (see details in Expedition 345 technical staff report). The same series of tests with a purple color standard show that the a* and b* values remained stable over time (Fig. F21C, F21D). For the remainder of the expedition, we continued to run color reflectance measurements with a 1 cm interval. Measurements on cores in Holes U1415E–U1415J were done before the drifting behavior of the spectrometer was diagnosed and fixed; absolute values of a* and b* recorded for these cores were drifting over time and are not reliable.

Point magnetic susceptibility

Point magnetic susceptibility was measured using a Bartington MS2E 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 1 s intervals to an accuracy of 5%. The area of response of the MS2E sensor is 3.8 mm × 10.5 mm, with a depth response of 50% at 1 mm and 10% at 3.5 mm, providing higher resolution measurements than the whole-round magnetic susceptibility instrument (Bartington Instruments, Ltd., 2011). Units are reported in 10^–5 SI units. The point magnetic susceptibility meter was calibrated by the manufacturer before installation on the ship. The probe was zeroed in air before each measurement point, and a background magnetic field was measured and removed from the data before being output.

As with the Bartington MS2C sensor on the WRMSL, the MS2 recorder attached to the SHMSL has an output threshold of 9,999 × 10^–5 SI and truncates the most significant digit for measurements over 9,999 × 10^–5 SI. Unlike WRSML data, SHMSL magnetic susceptibility data are not corrected for values over 9,999 × 10^–5 SI. Series of point measurements are not expected to produce smooth curves over depth as with the WRSML measurements. It is then impossible to easily locate clipped data and to infer the correction to be applied (10,000, 20,000, or more).

Discrete sample measurements

Cubes (~8 cm³) were cut from working section halves for discrete measurements of P-wave velocity and MAD. Most samples were also used for paleomagnetism measurements, in which case V_P and MAD measurements were conducted following the AF demagnetization and anisotropy of magnetic susceptibility measurements. The samples were subsequently passed on to the paleomagnetists for thermal demagnetization whenever appropriate.

Moisture and density

Mass and volume measurements on discrete samples were made to determine bulk, dry, and grain density and porosity. The shipboard MAD facility for hard rock samples consists of a vacuum water saturator, a dual balance system, and a hexapycnometer.

Vacuum water saturator

We used a vacuum pump system to ensure complete saturation of discrete samples. The system consists of a plastic chamber filled with seawater. A vacuum pump then removes air from the chamber, essentially sucking air from pore spaces. Samples were kept under vacuum for at least 24 h. During this time, pressure in the chamber is monitored periodically by a gauge attached to the vacuum pump to ensure a stable vacuum. After removal from the saturator, cubes were stored in sample containers filled with seawater to maintain saturation.

Dual balance system

A dual balance system was used to measure both wet and dry masses. Two analytical balances (Mettler-Toledo XS204) compensate for ship motion; one acts as a reference and the other measures the unknown (i.e., a sample). A standard mass of similar value to that of the sample was placed on the reference balance to increase accuracy. Using a reference mass within ~10% of the sample mass, an accuracy of 0.005 g is readily attainable. After wet mass determinations and P-wave measurements and prior to the determination of dry masses, samples were placed in an oven at 105° ± 5°C for 24 h and then allowed to cool in a desiccator for a minimum of 1 h.

Hexapycnometer system

The hexapycnometer is an IODP custom-built system using six Micromeritics cell units, custom electronics, and custom control programs. The system measures dry sample volume using pressurized helium-filled chambers with a precision of 0.02 cm³. At the start of the expedition, and whenever the helium gas tank is changed, shipboard technicians perform a calibration using stainless steel spheres of known volume. For a measurement, we run five cells that contain unknowns and one cell that contains two stainless steel calibration spheres (3 and 7 cm³) with a total volume of ~10 cm³. Calibration spheres were cycled through the cells to identify any systematic error and/or instrument drift. Spheres are assumed to be known to 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 28°C, followed by three data acquisition cycles.

Moisture and density calculations

For density calculations, both mass and volume are first corrected for the salt content of the pore fluid:

where

• s = pore water salinity,

• M_s = mass of salt,

• M_d = dry mass of the sample, and

• M_w = wet mass of the sample.

Grain density (ρ_g) is determined from the dry mass (M_d) and dry volume (V_d) measurements:

where ρs is the density of salt (2.20 g/cm³; Blum, 1997). The salt-corrected mass of pore water (M_pw) is calculated as

Then, the volume of pore water (V_pw) is

where we assume the density of the pore fluid (ρ_pw) is 1.024 g/cm³ (seawater with salinity of 35 g/L; Blum, 1997).

To calculate sample bulk density (ρ_b), we first computed bulk volume:

Then,

Porosity (ϕ) is calculated from the two volume parameters above:

P-wave velocity

P-wave velocity measurements of hard rock samples were performed on the same discrete cube samples that were used for MAD and paleomagnetic determinations. P-wave measurements were performed on seawater-saturated samples immediately after wet mass determinations were made. Measurements were made using the x-axis caliper contact transducers on the P-wave velocity gantry. Samples were oriented following standard IODP conventions, and measurements were made in the x-, y-, and z-directions for each cube (Fig. F2C). The apparent anisotropy was then calculated from these three measurements as

The system uses Panametrics-NDT Microscan delay line transducers, which transmit at 0.5 MHz, with stainless steel caps. The stainless steel caps were preferred to the rubber caps used during previous hard rock expeditions (e.g., Expedition 335) because they appeared to improve the quality and reproducibility of measurements. The peak of the first arrival was identified automatically by the installed IODP software. The complete waveform is stored with the data if reanalysis is deemed necessary. Shipboard visual checks of the picks appeared satisfactory. The distance between transducers was measured with a built-in linear voltage displacement transformer.

Measurements on standards were conducted as frequently as necessary. Calibration was made before measuring each sample with a series of acrylic cylinders of different thicknesses and a known P-wave velocity of 2750 ± 20 m/s. We performed 4 series of 11 measurements on a suite of 3 acrylic cylinders of different heights (15, 20, and 30 mm; Fig. F22A). These measurements show that (1) successive measured values tend to become more stable after three or four measurements and (2) the measured values are more consistent and closer to the certified acrylic velocity of 2750 ± 20 m/s when a drop of water is added between the acrylic cylinder surfaces and the transducers.

In addition to the test measurements with acrylic standards, at the beginning of Expedition 345 we also conducted several series of measurements on 10 minicores from Leg 147 Site 894. These samples behaved similarly to the acrylic cylinders; the same trend of increasing values was observed over the first three or four measurements during a series of 10 or more successive measurements (Fig. F23A).

We also tested the small water bath used during Expedition 335 (see Expedition 335 Scientists, 2012, for details on the rationale for using this device), which was designed to maintain saturation during the measurements and deemed to increase the quality of the results. Our tests, conducted with both the Leg 147 samples and acrylic prisms (Figs. F22B, F23B), show that the results are more consistent without the water bath, and measured velocities without the water bath are not systematically lower as described during Expedition 335. The small membrane between the lower transducer and the sample lower the measured velocity of acrylic prisms by ~10–15 m/s. We decided not to use this device.

Hence, the measurement protocol established for Expedition 345 was to perform a series of at least 10 successive measurements (or more if the measured velocity took longer to reach a stable value) in each of the three directions for each cube sample and discard the first four (or more, when necessary) measurements before computing the mean velocity. The standard deviation over three test measurements series on Site 894 samples (Fig. F23B) ranges from 0.5% to 2% and is 1% on average.

Thermal conductivity

Thermal conductivity (k; in W/[m·K]) is a measure of the rate at which heat is transported through a material. At steady state, thermal conductivity is the coefficient of heat transfer (q) across a steady-state temperature difference over a distance:

Thermal conductivity of rock depends on many factors, including temperature, pressure, porosity, type of saturating fluid, and the composition, distribution, and alignment of mineral phases. Thermal conductivity was measured on split core pieces under ambient conditions using the Teka TK04 system described in Blum (1997). All measurements were made at room temperature and pressure and were not corrected for in situ conditions.

The TK04 system measures thermal conductivity by transient heating of the sample with a known heating power and geometry. Changes in temperature with time during heating are recorded and used to calculate thermal conductivity. Heating power can be adjusted for each sample; as a rule of thumb, heating power (W/m) is set to be ~2 times the expected thermal conductivity (W/[m·K]). The temperature of the superconductive needle probe has a quasilinear relationship with the natural logarithm of the time after the initiation of heating (Blum, 1997). The TK04 device uses a complex special approximation method to calculate conductivity and to assess the fit of the heating curve. This method fits discrete windows of the heating curve to the theoretical temperature (T) with time (t) function:

where A_1–4 are constants that are calculated by linear regression. A₁ is the initial temperature, whereas A₂, A₃, and A₄ are related to geometry and material properties surrounding the needle probe. Having defined these constants (and how well they fit the data), the apparent conductivity (k_a) for the fitted curve is time dependent and given by

where q is the input heat flux. The maximum value of k_a and the time, t_max, at which it occurs on the fitted curve are used to assess the validity of that time window for calculating the thermal conductivity. The best solutions are those where t_max is greatest, and these solutions are selected for output. Fits are considered good if k_a has a maximum value, t_max is large, and the standard deviation of the least-squares fit is low. For each heating cycle, several output values can be used to assess the quality of the data, including natural logarithm of extreme time (LET) t_max, which should be large; the number of solutions (N), which should also be large; and the contact value, which assesses contact resistance between the probe and the sample and should be small and uniform for repeat measurements.

Half-space determinations of thermal conductivity were made with a needle probe embedded in the bottom of a Plexiglas block with a thermal conductivity of 0.184 W/(m·K). Heat is assumed to be transferred through the sample, and the TK04 documentation indicates that heat flow through the Plexiglas block itself is only significant for sample thermal conductivities <1 W/(m·K). Good thermal contact with the heating needle is required, so the split face of the samples was polished with 240 gauge silicon carbide powder.

During transit to Site U1415, empirical tests were conducted with both a certified MACOR ceramic standard (k = 1.626 W/[m·K] ± 2%) and a gabbroic sample from Leg 147 to evaluate the reproducibility of results (Fig. F24). We tested two probes with different needle lengths (7.3 and 4.4 cm). The longer needle (used during previous hard rock IODP expeditions and ODP legs) gives better results with less variability and was preferred for series of measurements with the needle aligned with the axis core pieces (Fig. F24). The shorter needle (new during Expedition 345) allows measurements parallel or perpendicular to the trace of the foliation when visible on the cut face and was used for a few apparent anisotropy test measurements. The stability of the measurement series using the short needle o(reported in the “Hole U1415I” and “Hole U1415J” chapters [Gillis et al., 2014b, 2014c]) was not good enough to give reliable apparent anisotropy values. The quality of measurements done with the large needle probe was assessed using LET and N (number of solutions); only a few measured values were rejected. Our measurement protocol (with a series of 10 measurements) provided consistent analyses within the analytical error (2%).

Samples were saturated and left to equilibrate to room temperature in a seawater vacuum saturator for ≥24 h, and the sample and sensor needle were equilibrated at room temperature in an isolated Styrofoam-covered seawater bath (k = ~0.6 W/[m·K]) for at least 15 min prior to measurement. Seawater was preferred to improve the needle/sample contact to silicone thermal contact gel in order to avoid contamination of the samples. Isolation of the sample and sensor needle eliminated the effect of small but rapid temperature changes introduced by air currents in the laboratory, as well as the ship’s motion. The instrument internally measures temperature drift and does not begin a heating run until sufficient thermal equilibrium is attained.

Core pieces from the archive half were measured at irregular intervals downhole depending on the availability of homogeneous and relatively vein/crack-free pieces long enough to be measured without edge effects (pieces >7 cm long; i.e., longer than the instrument needle). As many as 10 measurements were performed on each sample to verify the consistency of the results and provide an average value. The probe was regularly checked using the MACOR ceramic standard.

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