PHYSICAL PROPERTIES

Evaluation of physical properties at Site 1276 included nondestructive measurements of bulk density by gamma ray attenuation (GRA) bulk density, bulk magnetic susceptibility, and natural gamma radiation (NGR) on whole cores using the MST. Horizontal (x- and y-direction) and vertical (z-direction) compressional wave (P-wave) velocities were measured on cubes cut from half-core samples. Porosity and density were determined from cylinders shaped from the velocity-determination cubes. Thermal conductivity was measured on lithified half-core samples. Physical property data for Site 1276 can be extracted from the Janus database. High core recovery facilitated acquisition of an excellent physical property data set. Apart from an initial wash core (Core 210-1276A-1W), core recovery commenced at 800 mbsf and continued to 1729 mbsf (Cores 2R through 99R).

Density and Porosity

Bulk density at Site 1276 was computed from GRA bulk density measurement of unsplit cores. Bulk density, grain density, and porosity were calculated from the wet mass, dry mass, and dry volume of discrete samples using the moisture and density (MAD) method C (Blum, 1997) for lithified sedimentary and igneous rocks in Cores 210-1276A-2R through 99R (800-1729 mbsf).

The absolute values of the GRA density data (Fig. F158) should be ignored; they are consistently too low because the RCB core did not fill the core liner. The MAD bulk density values are consistently higher by 0.25-0.5 g/cm3 (Fig. F158). The many very low GRA density values reflect core gaps and biscuiting. However, the downhole trends of the clustered high values may be trusted to generally reflect the downhole variation of bulk density. The MAD bulk density data are substantially more accurate, and we will use these data to describe the downhole density variation.

MAD bulk density generally increases downhole (Fig. F158). Because we began coring at 800 mbsf, we bypassed the part of the sediment column where most sediment compaction occurs. The background density variation in the hole, from ~1.9 g/cm3 at 800 mbsf to 2.3 g/cm3 in claystones near the bottom of the hole, is explained by more gradual compaction of mudstones and claystones. Variations of density from this background trend are largely the result of lithologic differences. At most levels in the hole, there are sporadic samples with higher density than those of the background sediment. These are generally associated with carbonate/siderite concretions and very well cemented grainstones and sandstones. Sediments of somewhat low density are noted in the upper part of lithologic Unit 1. Lithologic Unit 2 and the lower part of Unit 3 are characterized by scattered density, generally higher than the background trend. Significant density changes, either offsets or changes in the scatter of values, are associated with each of the lithologic unit boundaries. High density of two diabase sills (in Cores 210-1276A-88R and 99R) is noted near 1620 mbsf (upper sill) and 1720 mbsf (lower sill). The upper sill has slightly lower density (2.73 g/cm3) than the lower sill (2.85 g/cm3). The sediments in the few meters above and below the upper sill were recovered and have higher density than normal because of hydrothermal alteration, although this is not well illustrated in the small-scale figures presented. Perhaps the most interesting density anomalies in the hole are located in and around the lower diabase sill. In the 10 m above the lower sill, density is dramatically less than normal in an undercompacted section of mudstones. We discuss the observations in and around the sills later in this section (see "Undercompacted Systems: High-Porosity and Low-Velocity Mudstones").

MAD grain density and porosity, together with bulk density, are plotted in Figure F159. Note that the lithologic identification of each sample was obtained by automated interrogation of the AppleCORE visual core description database for Hole 1276A. Symbols used for plotting physical property data according to major lithology are found in Figure F160.

MAD grain density is very consistent at 2.65 ▒ 0.15 g/cm3 throughout the hole. Typical values of common minerals are 2.6 g/cm3 for quartz grains and 2.8 g/cm3 for carbonate grains. There is some scatter in grain density in lithologic Units 1-3. There is also a slight cyclicity of grain density with depth, with an observed wavelength of ~75 m, which is particularly noticeable in lithologic Subunits 5A and 5B. The outlying, higher grain density values are accurate measurements and correspond to carbonate/siderite concretions and very well lithified carbonate-cemented sandstones. The density of pure siderite is 3.96 g/cm3. The expected density is lower if Mn, Mg, or Ca substitute for Fe in siderite. It is less clear why the grain density is so high for the carbonate-cemented sandstones. Perhaps there is an appreciable Fe/siderite component in the cement.

Porosity generally reflects a combination of stress history and sedimentologic and diagenetic effects (e.g., compressibility, permeability, sorting, grain fabric, and cementation). Porosity is calculated from the volume of pore water, assuming complete saturation of the wet sediment sample (Blum, 1997) (see "Physical Properties" in the "Explanatory Notes" chapter). The porosity curve is a mirror image of the bulk density curve except for minor variations caused by changes in grain density (Fig. F159). MAD porosities generally decrease with depth in Hole 1276A. The general trend of porosity in lithologic Units 1 and 2, between 800 and 929 mbsf, decreases steeply from ~47% ▒ 10% at the top to 30% ▒ 10% at the base of Unit 2. Porosity in lithologic Units 3 and 4 and Subunits 5A and 5B, between 929 and 1502 mbsf, decreases less steeply, from ~35% ▒ 10% at the top to 23% ▒ 10% at the base. The general trend of porosity in lithologic Subunit 5C, between 1502 and ~1719 mbsf, is roughly uniform at 20% ▒ 10%. In lithologic Unit 2 through Subunit 5C, ~10% of the samples, which are randomly distributed through the interval, have porosities as much as 20% lower than the general trend. These are generally samples taken from carbonate/siderite concretions or from very well lithified carbonate-cemented sandstones. In the lower portion of lithologic Subunit 5C between the two diabase sills (Subunits 5C1 and 5C2), porosity is 40% ▒ 10%, which is unusually high for rocks at this depth (Fig. F159B). These sediments are clearly undercompacted. This topic is discussed further below.

We place the observed porosities in perspective by removing the anomalous porosities associated with carbonate/siderite concretions, well-lithified carbonate-cemented sandstones, and those sediments that are hornfels and then plotting them together with a best-fit compaction curve (Fig. F161). We assume that porosity can be approximated by an exponential function of depth (e.g., Athy, 1930):

(z) = 0 e-kz,

where

(z) = the porosity as a function of depth,
z = the depth,
0 = the surface porosity, and
k controls the rate of porosity reduction with depth.

The parameter k is often expressed as its reciprocal, thus having units of length. By fitting the measured porosity-depth data for claystones, mudstones, and siltstones, we estimated 0 = 79% and k-1 = 1.18 km. The parameters we obtained are similar to values obtained for mudrock (shale) compaction in the literature. Many of the porosity values that lie to the low side of this curve were measured on grainstones, sandstones, and chemically cemented rocks. In each case, these rocks are expected to be less porous than burial-compacted mudrocks.

Undercompacted Systems: High-Porosity and Low-Velocity Mudstones

In lithologic Subunit 5C in Cores 210-1276A-96R through Section 98R-1 (1693-1710 mbsf), mudstones and calcareous mudstones have unusually high porosities (27%-39%) and low horizontal velocity (1689-1958 m/s), considering the depth of their recovery (Figs. F159, F161B). Furthermore, these intervals are found to be very soft, with consistencies comparable to modeling clay. The porosity, velocity, and consistency properties of these mudstones are more comparable to those of normally compacted sediments recovered in the upper part of the hole (~840-1020 mbsf) (Fig. F159), and they clearly demonstrate that the mudstones are undercompacted with respect to their depth.

The mechanical compaction process in these mudstones was halted at a relatively shallow burial depth. This was likely facilitated by emplacement of two diabase sills, one above this undercompacted interval (1620 mbsf) and one below (1719 mbsf). It is possible, but not necessary, that this interval was overpressured prior to Leg 210 drilling. Evidence of past fluid flow exists in the hydrothermally altered sedimentary rocks immediately above the lower sill and underlying the undercompacted mudstones (Fig. F161B). The highest measured concentration of hydrocarbons exists in this 20-m interval. Geochemical analysis measured concentrations of C1 (methane) levels of nearly 19,000 ppmv (Fig. F161B) (See "Volatile Hydrocarbons" in "Geochemistry"), implying that fluids in this interval were trapped. This interval was clearly incapable of normally compacting and expelling pore fluids and perhaps was sealed off by the bounding igneous intrusions.

Compressional Wave Velocity

Downhole Trends

Compressional wave velocity was measured with the P-wave sensor 3 contact probe system on ~8-cm3 cube samples of lithified sediments. The cubes were used to measure velocity in the horizontal (x and y) and vertical (z) directions. Seismic anisotropy was calculated from the measured velocity.

Vertical velocity of the sedimentary rocks in Hole 1276A can best be described in terms of a general trend of the bulk of the measurements and then of the deviations of the remaining measurements from the trend. The bulk of the measurements were obtained from claystones, mudstones, and siltstones. The general trend in lithologic Unit 1 is uniform velocity of 1900 ▒ 100 m/s (Fig. F162). Lithologic Unit 2 consists largely of grainstones and marlstones with velocity that is variable between 2000 and 4900 m/s. The general trend in lithologic Unit 3 is velocity varying between 1950 and 2600 m/s. There are also scattered higher velocities, up to 4200 m/s in grainstones and sandstones. Lithologic Unit 4 is characterized by velocity between 2000 and 2300 m/s, with two slightly higher values in a sandstone and silty sandstone. The general trend of vertical velocity in lithologic Unit 5 is a gradual increase from 2100 ▒ 100 m/s at 1069 mbsf to 2300 ▒ 200 m/s at 1680 mbsf. Throughout Unit 5, there are 1- to 100-cm-thick layers with velocities scattered between the general-trend velocity and 5000 m/s. These higher velocities were usually measured in grainstones, marlstones, carbonate concretions, sandstones, and silty sandstones.

The vertical velocity in the upper diabase sill, lithologic Subunit 5C1, varies from 4738 to 5030 m/s. The vertical velocity in the lower diabase sill, lithologic Subunit 5C2, is 5527-6193 m/s. The velocity of the sediments of lithologic Subunit 5C that lie between the sills is scattered from 1650 to 3200 m/s. The extraordinarily low velocity sediments in this interval are undercompacted, and possibly overpressured, mudstones (Fig. 161B).

Velocity data in the x-, y-, and z-directions were used to define velocity anisotropy. Velocity anisotropy was calculated as follows:

Anisotropy (%) = (Vh - Vv)/[(Vh +Vv)/2] x 100,

where Vh is the mean horizontal (x and y) P-wave velocity and Vv is the vertical (z) velocity.

Velocity anisotropy in sediments is mostly positive (Fig. F163), indicating that the vertical velocity is slower than the horizontal velocity, and it generally increases downhole. Positive anisotropy in sediments is generally caused by grain orientation along near-horizontal bedding planes. Sound waves traveling vertically must traverse both slower and faster lithologies, whereas horizontal sound waves can travel preferentially in the faster lithologies. Anisotropy often increases downhole because deeper rocks have experienced more compaction in place and greater decompression prior to measurement in the laboratory. This decompression is thought to induce microcracking along bedding planes. The anisotropy in Hole 1276A increases from ~4%-5% at 800 mbsf to ~10% in the deepest sediments recovered. In contrast to the sedimentary rocks, the upper diabase sill shows negative seismic anisotropy of approximately -2%-3%. Surprisingly, the lower diabase is nearly isotropic.

Factors Affecting Sediment Velocity

During the course of physical property sampling and velocity measurement, several targets for more detailed velocity analysis were identified. The first category of targets included several turbidites whose velocity characteristics appeared to vary systematically depending on where in the turbidite the routine physical property sample was taken. The second target was the sediment/sill contact preserved at the top of the upper sill. In both cases, detailed velocity measurements allowed us to define systematic changes in velocity that correlate with changing lithology. In the chosen sections, x-direction velocity was measured every 2-10 cm over the relevant interval. X-direction velocity can be obtained at regular intervals on pieces of the working half of the core without cutting cubes, so this velocity analysis is not destructive and provides an excellent means of understanding velocity changes along the core. Below, the results of detailed velocity analysis over several turbidites in lithologic Unit 5 (Cores 68R, 79R, and 80R) and over the sill/sediment contact in Section 210-1276A-87R-6 are presented.

Velocity Structure of Turbidite Sequences

Turbidites are found in nearly every lithologic unit encountered at Site 1276, and they range in thickness from 10 cm to >2 m. During standard physical property sampling, different parts of these turbidites were selected for measurements of velocity and bulk density. Samples from different parts of the turbidites displayed different velocity and density characteristics depending on where in the turbidite the sample was taken. To investigate the velocity structure of turbidites, detailed measurements of x-direction velocity was taken on three turbidites, including Sections 210-1276A-68R-3 and 68R-4, 79R-2, and 80R-3 (Fig. F162C). This velocity study reveals systematic trends in turbidite velocity that can be used as a general framework in which individual physical property samples of turbidites can be placed. In all measured turbidites, velocity increases from the muddy turbidite top (~2200 m/s) downhole until the highest velocity (~4500 m/s) is encountered in the well-cemented, fine-grained sandstone near but somewhat above the bed base. As grain size increases from this fine-grained sandstone to the underlying coarse sandstone turbidite base, velocity decreases by ~1000 m/s. Figure F162C shows core photographs and velocity variations for the three turbidites we measured. Velocity varies consistently between minimum values near 2200 m/s and maximum values of ~4500 m/s.

Investigation of trends in individual turbidites also reveals a correlation between velocity and weight percentage of CaCO3 (Fig. F162C). High CaCO3 content consistently corresponds to the sandstones that have higher velocity, suggesting that carbonate cementation may play a primary role in controlling velocity within these turbidites. For example, Section 210-1276A-68R-3 contains 51 wt% CaCO3 at 1434.58 mbsf, where the velocities in the x-, y-, and z-directions reach their maximum. Combined, these results allow the extrapolation of velocity and carbonate cementation away from routine physical property samples to help characterize turbidite sequences in general. These observations can be used to help constrain a velocity function from physical property data to be used to link core data with seismic reflection data.

For a number of intervals in Hole 1276A, there is strong positive correlation between CaCO3 content and seismic velocity (Fig. F162D). Because of the centimeter- to meter-scale variations in lithology in the hole, displaying the data at full detail was not enlightening for exploring correlations. Therefore, we normalized the x-velocity by subtracting 1792 m/s and dividing by 64 (maximum velocity/100, giving a resulting range of 0-100 m/s). We also applied a filter to both data sets that removes short-wavelength variations. We have not determined quantitative correlation values, but we note convincing visual correlation. Correlation appears to be best for velocity and CaCO3 variations at 25- to 75-m periods. Correlation is not present in some intervals. For example, notable exceptions include uncemented grainstones that show a high carbonate content but relatively low velocity and fine sandstones with low carbonate content and high velocity.

Velocity Structure of Sediment/Sill Contact

The presence of intercalated sills and sedimentary rocks creates a highly variable velocity structure in the lower part of Hole 1276A (1600-1737 mbsf). Igneous sills with velocity as high as ~6300 m/s alternate with sedimentary rocks that have velocity as low as ~1600 m/s. Abrupt increases and decreases in velocity over short intervals (~10 m) would be expected to generate a complicated pattern of reflections in seismic reflection profiles. The nature of observed reflection patterns is controlled not only by the magnitude and spacing of velocity variations but also the extent to which these velocity boundaries are gradational or sharp.

To help define the velocity structure of the contact between sills and the surrounding sedimentary rocks for use with seismic stratigraphy studies, the x-direction velocity was measured every 2 cm across relatively unmetamorphosed sediments, across the zone of contact metamorphism at the margin of the upper sill, and in the sill itself (Fig. F162E). Section 210-1276A-87R-6 is the only place where the entire contact between sediments and sill is preserved; all of the other sill/sediment contacts fall between cores. These measurements reveal a sharp contact between the overlying sedimentary rocks (~2500 m/s) and the upper diabase sill (3500-4500 m/s). Contact-metamorphosed sediments (porphyroblastic mudstones; 1612.1-1612.65 mbsf) are separated from the upper sill (>1612.85 mbsf) by a thin zone of contact metamorphism where the sedimentary rocks display very high velocity (~4900 m/s). These latter metamorphosed sediments show velocities comparable to maximum sill velocities, which are not achieved until the center of the sill itself. This information on the velocity structure of the sill/sediment contact can be used as input into synthetic-seismogram creation. Introduction of a sharp velocity contact at the top of the sills will likely have a profound effect on the predicted reflection signature of these sills in seismic profiles.

Thermal Conductivity

Thermal conductivity was measured on intact pieces of half-round core of at least 10 cm length. Mudrocks, such as those that dominate the recovered Albian section (1100-1338 mbsf), tended to break up into ~1-cm-long pieces and therefore were seldom measured. As a result, we found that they were severely underrepresented in our sampling of thermal conductivity in the upper part of the hole. After recognizing this problem, we developed two new methods, a shrink-wrap method and a clamp method, to measure thermal conductivity on these fragile but abundant materials. Acceptable measurements were obtained using both methods, although the clamp method yielded more repeatable and, therefore, probably superior, measurements. Claystones and mudstones are more appropriately represented in our sampling below 1340 mbsf in Hole 1276A.

Measured thermal conductivity in sediments shows an overall increase with depth (Fig. F164). Values in sediments (with the exception of a few outliers) range from 1.4 to 2.8 W/(mĚK) (mean = 2.1 W/[mĚK]). Thermal conductivity near the top of the cored interval (800-900 mbsf) averages 1.7 W/(mĚK). Thermal conductivity in the deepest interval cored (1500-1700 mbsf) averages 2.4 W/(mĚK). A linear relation between these average values would reasonably approximate the thermal conductivity for this sedimentary column for most thermal studies. Deviation from this trend by more than ▒0.3 W/(mĚK) occurs mainly in the interval 980-1140 mbsf, where values are scattered and high (~1.8-2.5 W/[mĚK]). No obvious relationship is seen in values of thermal conductivity as a function of lithology (Fig. F164). However, in general, thermal conductivity values for mudstones (1.3-2.7 W/[mĚK]) are lower than those for sandstones (1.9-3.0 W/[mĚK]).

Measurements of thermal conductivity in the upper and lower diabase sills (1620 mbsf and 1720 mbsf) show values of 1.7 and 2.0 W/(mĚK), respectively.

Natural Gamma Radiation

The NGR count was recorded on the MST. Clay minerals, being charged particles, tend to attract and bond with K, U, and Th atoms so that an increasing NGR count typically correlates with increasing clay content. In contrast, sand-prone and carbonate units usually are characterized by low NGR count. These relationships can help to define the location of mud- and sand-prone formations downhole. However, the relationships will begin to break down in poorly sorted successions or in those with peculiarities in mineralogy.

The NGR profile is most easily characterized in terms of the described lithologic units (Figs. F165, F166; see "Lithostratigraphy"). For convenience, we will refer to intervals with upward-increasing NGR count as "upward fining" and those with upward-decreasing NGR count as "upward coarsening." The explanation for these trends may be other than actual grain size distributions. However, excellent agreement is observed between increasing and decreasing sandstone/mudrock (sand/shale) ratios and low/high NGR counts, respectively (e.g., Fig. F167).

Lithologic Unit 1 consists of an upward-fining sequence stratigraphically above an upward-coarsening sequence (Figs. F165, F166). Lithologic Unit 2 has generally lower ("coarser") but more scattered NGR count than Unit 1. The boundaries between lithologic Units 1 and 2 and Units 2 and 3 are very sharp in the NGR data. Lithologic Unit 3, from top to bottom, contains an upward-fining sequence, a sequence of generally high but scattered values, and a sequence of generally low but scattered values. There is a sharp change in NGR count just below the lithologic Unit 3/4 boundary. Lithologic Unit 4 shows an upward-fining sequence above a sequence of intermediate and scattered values. Lithologic Subunit 5A, like lithologic Unit 4, fines upward at the top and is intermediate and scattered below. The upper part of lithologic Subunit 5B is a thick (>200 m) sequence of very uniform, high-value, and scattered NGR count. NGR changes at the boundaries between lithologic Unit 4 and Subunits 5A and 5B are not as dramatic as those between lithologic Units 1, 2, 3, and 4. Below ~1320 mbsf, the count drops. We suspect that this is due to a dramatic reduction of core diameter preceding bit failure at 1340 mbsf, rather than due to an actual change in the NGR signature of the cores. From 1340 to 1502 mbsf, the lower part of lithologic Subunit 5B, NGR count gradually increases with depth. In lithologic Subunit 5C, below 1502 mbsf, the count continues a gradual rise and becomes more scattered as lithologic variation increases. The NGR count drops dramatically in the upper and lower sills (lithologic Subunits 5C1 and 5C2) but follows the general trend between the sills. The count decreases toward the top of the lower sill. We point out that this is only a general description of the NGR data set on the lithologic unit scale; these data are incredibly detailed and also capture significant information at the core and section scale.

Figure F167 shows an example of the correlation between NGR, lithologic facies, and x-direction velocity. The core photograph identifies the position of a thick turbidite in Sections 210-1276A-68R-3 and 68R-4. This unit grades from a coarse sand at its base up through silty sands and finally terminates in mudstones and claystones. Concomitant with this is a major decrease and then increase in NGR count. The shape of the NGR response does not strictly correlate with megascopic grain size distribution at the base of the turbidite, but it shows good correlation above the base. Interestingly, the velocity shows a different response; it is sensitive to grain size and also to the degree of carbonate cementation. Minor turbidites are recognized in all of the sections in Core 210-1276A-68R, also corresponding to minor excursions in NGR (Fig. F167).

Magnetic Susceptibility

Magnetic susceptibility was recorded on the MST. The quality of these data is degraded in RCB sections because the core is usually undersized with respect to liner diameter and is often disturbed by drilling. Nevertheless, the general downhole trends can be useful for stratigraphic interpretations.

The major contribution to the magnetic susceptibility comes from ferro- and ferrimagnetic minerals such as magnetite, hematite, goethite, and titanomagnetite. Paramagnetic minerals such as clays, glauconite, and siderite contribute significantly less to the magnetic susceptibility amplitude. Strongly magnetic minerals such as magnetite and hematite are mainly associated with terrigenous material. Autochthonous diagenetic processes, such as the precipitation of siderite, also help to concentrate magnetic material.

Raw (i.e., unfiltered) magnetic susceptibility amplitudes range from 8 to 22,000 x 10-6 SI (12 to 9990 instrument units) downhole (Figs. F165, F166). Amplitudes are relatively constant below 1026 mbsf (i.e., in lithologic Units 4 and 5), ranging between ~10 and 40 instrument units, except for two extreme peaks from the two diabase sills (~1620 and 1720 mbsf). Filtered magnetic susceptibility data (filtered using a 50-point averaging technique) show amplitudes ranging between 100 and 1650 x 10-6 SI (15 to 250 instrument units) and more clearly show the trends in the magnetic susceptibility data (Fig. F166). There is an important change in magnetic susceptibility character at 1026 mbsf, with relatively large amplitude variations above (65 to 650 x 10-6 SI [10 to 100 instrument units]) and relatively constant amplitudes below (65 to 200 x 10-6 SI [10 to 20 instrument units]), with the exception of the sill peaks. There is a fair correlation between the magnetic susceptibility and NGR profiles (Fig. F166), such as across lithologic boundaries at the bases of Units 1, 2, and 3 and across the Subunit 5A/5B boundary. Elsewhere, correlations are cruder or they are not observed (e.g., the lower part of lithologic Unit 3).

There is a correlation between magnetic susceptibility and remanent magnetization intensity (see "Paleomagnetism"). This indicates that the minerals carrying the magnetic remanence are also responsible for the susceptibility. A particularly high magnitude magnetic susceptibility peak occurs near the base of Section 210-1276A-15R-4 (Fig. F166A), and this is confirmed by AMST susceptibility data (see "Paleomagnetism"). The origin of this spike is as yet unexplained, despite careful examination of the core for metal fragments detached from coring tools and other features. One possible explanation is that the susceptibility spike relates to ash layers tentatively recognized in Core 210-1276A-15R (see "Lithostratigraphy").

Whereas there is only a general correlation between magnetic susceptibility and lithologic units as described above, this is not the case for specific diagenetic minerals precipitated throughout the cores or for the igneous intrusions. The former can show excellent correlations with the magnetic susceptibility data. For example, Figure F168 shows the magnetic susceptibility for Sections 210-1276A-50R-1 and 52R-6 compared with images of the sections. The white "layers" in the core are concretions or strongly cemented grainstones that contain quartz, calcite, and siderite based on XRD analysis (see "Lithostratigraphy"). It appears that the siderite-enriched carbonate cement is capable of generating large magnetic susceptibility peaks. In some cases, amplitudes reach 2650 x 10-6 SI (400 instrument units).

The value of magnetic susceptibility in characterizing aspects of the lithology of igneous rocks is illustrated in Figure F169. Section 210-1276A-87R-6 contains the baked mudstones above the upper diabase sill, the chilled margin within the sill, and the upper unchilled part of the sill. Magnetic susceptibility increases downhole with a step at each of these boundaries. Magnetic susceptibility of the hydrothermally altered mudstone is very low, <20 instrument units. Magnetic susceptibility of the chilled margin of the sill rises to values of 100-300 instrument units. Based on XRD analyses (see "Igneous and Metamorphic Petrology") the whole sill, including the chilled upper margin, contains pyrite. This is consistent with the observed level of magnetic susceptibility in the upper chilled margin. Magnetic susceptibility of the interior of the sill is ~10,000 instrument units. This is consistent with the presence of magnetite in addition to pyrite in the interior of the sill. It is likely that both magmatic and alteration processes have affected the distribution of pyrite and magnetite in the sill. We note that there is a slight drop in magnetic susceptibility near the segregation band at 1613.24 mbsf and a large drop in magnetic susceptibility near the bottom of Section 210-1276A-87R-6; these may provide clues to subtle chemical variations through the sill (Fig. F169).

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