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doi:10.2204/iodp.proc.345.113.2014

Paleomagnetism

Remanence data

Continuous measurements

Remanence measurements were made at 2 cm intervals on all archive-half core pieces longer than ~9 cm. All archive-half cores were subjected to stepwise alternating field (AF) demagnetization at 5 mT steps up to maximum peak fields of 80 mT. Remanence data and corresponding archive-half core point magnetic susceptibility data were filtered to preserve only data corresponding to the intervals where remanence measurements were made and to discard data obtained within 4.5 cm of piece ends.

For the purpose of characterization based on bulk magnetic parameters, the lithologies recovered in Hole U1415P are grouped into two categories. Group 1 consists of gabbroic rock (olivine gabbro, olivine gabbronorite, and orthopyroxene-bearing olivine gabbro) with a geometric mean natural remanent magnetism (NRM) intensity of 2.27 A/m (range = 288 mA/m to 7.35 A/m; n = 261) (Fig. F52), which is an order of magnitude higher than that observed in these lithologies in Hole U1415J (geometric mean NRM intensity of 22.6 mA/m). Group 2 consists of troctolitic rock (troctolite and troctolitic olivine gabbro) with a geometric mean NRM intensity of 5.95 A/m (range = 1.5–14.5 A/m; n = 98). The geometric mean magnetic susceptibilities of Group 1 and Group 2 samples are 617 × 10^–5 and 1792 × 10^–5 SI, respectively. These values are again higher than those observed for the same lithologies in Hole U1415J. Higher mean intensities and susceptibilities in troctolitic rock relative to gabbroic rock most likely reflect variable degrees of serpentinization of these more olivine-rich lithologies leading to the production of secondary magnetite (see “Metamorphic petrology”).

Remanent magnetization directions were calculated by principal component analysis (PCA; Kirschvink, 1980) at all measurement points along the core pieces where linear components could be identified on orthogonal vector plots of demagnetization data. Only principal components with a maximum angular deviation (MAD) <10° were considered acceptable. Figure F53 shows representative examples of AF demagnetization behavior, and Figure F54 shows downhole variations in NRM and PCA pick inclinations, NRM and PCA pick intensities of magnetization, and low-field magnetic susceptibility measured using the Section Half Multisensor Logger (SHMSL). The majority of samples have an initially downward-directed remanence, with evidence of inclination steepening caused by acquisition of a drilling-induced magnetization that is at least partially removed by low-field treatments (<15 mT). This is followed by removal of a moderately inclined downward-directed component, typically by fields of 25–30 mT (Fig. F53A–F53F). The mean inclination of this component is 36.4° (k = 29.0; α₉₅ = 1.7°; n = 244), calculated using the Arason and Levi (2010) maximum likelihood method. During removal of this component, remanences typically migrate to the upper hemisphere, but no linear components with negative inclination are present. Instead, at demagnetization fields >30 mT, remanence directions migrate back to the lower hemisphere and intensities of magnetization increase continuously up to the peak applied field of 80 mT. This is due to acquisition of spurious, laboratory-imparted anhysteretic remanent magnetization (ARM) along the z-axis of the superconducting rock magnetometer (SRM) system, which has been a characteristic problem of the SRM system observed during several IODP expeditions (e.g., Teagle, Ildefonse, Blum, and the Expedition 335 Scientists, 2012). In rare examples, linear components decaying toward the origin without significant ARM acquisition are successfully isolated (Fig. F53G, F53H), although in some cases this final component is directed along the z-axis of the SRM (Fig. F53G) and must be treated with caution as a potential ARM. Unfortunately, clear examples of demagnetization unaffected by ARM acquisition are limited in number. In the majority of cases, anomalous ARM in archive-half core samples prevent isolation of sufficient high-coercivity components to allow geological interpretation. The significance of the moderately inclined, downward-directed component with medium coercivity is discussed in “Reliability of linear remanence components in SRM data.”

Discrete samples

Shipboard experiments were conducted on 34 discrete minicube samples from Hole U1415P. Two samples were AF demagnetized, and the remaining 32 samples were subjected to two cycles of low-temperature demagnetization (LTD) followed by thermal demagnetization. In all cases, remanent magnetization directions were calculated by PCA for all demagnetization intervals where linear components could be identified on orthogonal vector plots of demagnetization data. Only principal components with MAD <10° were considered acceptable (Table T9).

Samples from both gabbroic and troctolitic rock display well-defined linear remanence components following removal of variably developed drilling-induced magnetizations by low-temperature demagnetization. A selection of typical examples is shown in the orthogonal vector plots of Figure F55. Some samples display a bend in the thermal demagnetization path (e.g., Samples 345-U1415P-22R-2, 32 cm, and 23R-1, 68 cm) at temperatures of ~450°C before decaying linearly toward the origin. In such cases, the highest unblocking temperature segment has been picked as the characteristic remanence component.

Unblocking temperature spectra for these samples are divided into two groups (Fig. F56). The first group loses ~70% of remanence within 40°C of the magnetite Curie temperature (585°C). Such discrete unblocking at high temperatures is characteristic of thermoremanent magnetizations (TRMs) carried by fine-grained (single domain and pseudosingle domain) grains of magnetite, and such remanences are unlikely to have been thermally overprinted after TRM acquisition. Hence, we interpret these components as the primary magnetization in these rocks, suitable for subsequent geological interpretation. The second group also exhibits stable, high unblocking temperature components but shows evidence for some unblocking at lower temperatures. Several samples have inflections in the demagnetization curve at temperatures between 400° and 500°C, which may be due to presence of magnetite grains with different grain size distributions (and hence unblocking temperatures) compared to those carrying the highest temperature component. This may reflect production of magnetite during alteration of these rocks (see “Metamorphic petrology”).

Reliability of linear remanence components in SRM data

As noted previously, archive-half core data from the SRM system show a characteristic, moderately inclined linear remanence component at AF steps below ~30 mT. This occurs after apparent removal of the drilling-induced remanence and prior to the acquisition of anhysteretic remanence at higher fields. In Hole U1415P, this component has a well-defined mean inclination of 37.5° (k = 20.7; α₉₅ = 3.9°; n = 69) but has no geological significance and is demonstrably an artifact of spurious origin.

As discussed in “Paleomagnetism” in the “Hole U1415J” chapter (Gillis et al., 2014e), there is poor to no correlation between well-defined discrete sample magnetization directions and the archive-half core data. Figure F57A shows a stereographic equal-area projection of the downward-directed linear component present in the majority of archive-half core demagnetization data. These data come from core pieces that are azimuthally unconstrained in the core reference frame yet show significant clustering toward northwest–northeast declinations. In contrast, discrete sample high unblocking temperature components (Fig. F57B) are widely scattered in declination, which is expected for data from core pieces that are free to rotate in the core barrel. These data further unequivocally confirm that the downward linear component in the archive-half core samples has no geological significance and is likely to reflect bias resulting from the radial component of the drilling-induced magnetization in these rocks (see Shipboard Scientific Party, 2003)

Geological interpretation of inclination data from discrete samples

The downhole distribution of inclinations of the highest unblocking temperature components in discrete samples from Hole U1415P are illustrated in Figure F58 together with associated NRM inclinations. Remanences migrate from dominantly positive NRM inclinations to negative PCA inclinations during demagnetization, reflecting removal of the downward-directed drilling-induced magnetization. With four exceptions (red), all PCA inclinations are negative. These four anomalous samples all come from within the uppermost 6 cm of their respective core sections and are likely to be unrepresentative of the sampled sections. They were therefore excluded from subsequent analysis.

The remaining data are divided into the units defined on the basis of petrological characteristics (Units II and III; see “Igneous petrology”) and mean inclinations calculated using the Arason and Levi (2010) maximum likelihood method. This identifies a significant difference in mean inclination between units:

• Unit II mean inclination = –54.9° (k = 13.0; α₉₅ = 9.2°; n = 21).

• Unit III mean inclination = –30.8° (k = 17.8; α₉₅ = 13.5°; n = 8).

These data indicate substantial rotation of the cored section relative to the expected geocentric axial dipole reference inclination of ±4.7°. The distinct mean inclinations of Units II and III also indicate that two displaced blocks with independent rotation histories were sampled in Hole U1415P. Units II and III should therefore be treated separately in lithologic and structural syntheses. In the absence of reoriented samples from this hole, it is impossible to place constraints on the net or relative rotation of the units using the paleomagnetic data.

Magnetic susceptibility, NRM intensity, and Königsberger ratio

In mafic igneous rock, low-field magnetic susceptibility (k) is principally controlled by the volume concentration of magnetite. NRM variability is also controlled by variations in magnetite content but may also be influenced by variability in the magnitude of drilling-induced remanent magnetizations. The relation of NRM intensity and susceptibility is expressed by the Königsberger ratio (Q), which is defined as the ratio of remanent to induced magnetization in rock, where induced magnetization equals the product of k (SI) and the geomagnetic field strength (A/m). Values of Q > 1 indicate that remanence dominates the magnetization of a rock unit.

Figure F59 shows a log-log plot of NRM intensity against k for archive-half core and discrete samples in Hole U1415P together with lines of equal Q calculated for a 25 A/m field. The majority of archive-half core data plot close to Q = 10.0, whereas lower NRM intensities in discrete samples result in Q values of 1.0–10.0. The discrepancy between these measurements may result from decay of viscous drilling-related magnetizations in the time between SRM and discrete sample analyses. These data suggest that lithologies sampled in Hole U1415P may contribute a significant fraction to marine magnetic anomalies when in situ. However, caution is required in the interpretation of Q ratios calculated for these samples, as NRM intensities (particularly in the archive-half core samples) may be artificially increased by drilling-induced magnetization.

Anisotropy of magnetic susceptibility

Anisotropy of magnetic susceptibility (AMS) was measured from 35 discrete samples in Hole U1415P (Table T10). Each sample was measured three times to maintain quality control on any shipboard noise. A bootstrap technique was used to average these three measurements to provide the AMS eigenparameters for each discrete sample. All discrete samples have small (<3.5°) bootstrapped confidence ellipses around their eigenvectors that indicate consistent and repeatable AMS fabric measurements. Eigenvectors from all 35 samples are shown separated onto two equal-area stereographic projections in Figure F60 using the Hole U1415P Unit II/III boundary based on different petrological characteristics (see “Igneous petrology”). Susceptibility tensors are weakly to moderately anisotropic (corrected anisotropy degree [P′] < 1.35; mean = 1.17) (Jelinek, 1981), and all three ellipsoid shapes (triaxial, oblate, and prolate) are present in these samples (Fig. F60C). Bulk susceptibilities range from 8.95 × 10^–4 to 4.63 × 10^–2 SI, with an average of 1.64 × 10^–2 SI that indicates predominance of ferromagnetic mineral contributions to the AMS signal in most samples.

Structural measurements of magmatic foliation in Hole U1415P show a range of relationships with the AMS fabrics. Although most of the samples were generally nearly isotropic, 9 of 19 samples with structurally measured fabrics show close correspondence of the minimum AMS eigenvector (k_min) to the magmatic foliation pole. Foliation orientations primarily range from moderately to steeply eastward dipping within the core reference frame (Fig. F60A). Reasonably consistent AMS fabrics are found in the upper portion of Hole U1415P (above ~64 mbsf), whereas a wider range of eigenvector directions is observed below, as shown on Figures F60A and F60B. Overall, almost half of the samples with measured magmatic foliation show a close agreement of AMS magnetic foliation (average solid angle between k_min and magmatic foliation pole <45°). The other half of the samples show variable relationships of AMS eigenvectors with respect to the magmatic foliation, with solid angles between k_min and magmatic foliation pole >45°. As noted above, samples that exhibit a poor agreement between the AMS and structurally measured fabrics are typically isotropic or show very weak hints of mineral alignment.

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