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doi:10.2204/iodp.proc.345.109.2014

Paleomagnetism

The primary goal of paleomagnetism studies is to quantify the structure, direction, and intensity of natural remanent magnetization (NRM) in the different rock types that make up the lower oceanic crust and to use these data to provide insights into the crustal accretion, deformation, and source of marine magnetic anomalies. In addition, magnetic fabric analysis can provide valuable information on the degree and direction of weakly preferred alignments of mineral phases, leading to improved understanding of magmatic and tectonic processes.

Remanence data

Continuous measurements

A range of gabbroic and troctolitic lithologies were sampled during Expedition 345, and recovery was sufficient to allow first-order characterization of magnetic remanences using the pass-through superconducting rock magnetometer (SRM) and in-line alternating field (AF) demagnetizer. 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 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. Thus, the resulting remanence and susceptibility data sets are significantly less affected by artifacts resulting from small core pieces than the original unfiltered data sets.

For the purpose of characterization based on bulk magnetic parameters at Site U1415, lithologies are divided into Group 1, gabbroic rock, and Group 2, troctolitic rock. Only one archive-half core section (345-U1415I-4R-1A) and one other individual core piece (Sample 345-U1415I-2R-1 [Piece 11]) from Hole U1415I were suitable for measuring using the SRM system. Group 1 lithologies in these samples include olivine gabbro, orthopyroxene-bearing olivine gabbro, gabbronorite, and olivine-bearing gabbronorite, with a geometric mean NRM intensity of 360 mA/m (range = 72 mA/m to S1.23 A/m; n = 24) (Fig. F25). Only a narrow layer of troctolite in the center of Section 345-U1415I-4R-1A falls into Group 2, with a geometric mean NRM intensity of 1.37 A/m (n = 2). The geometric mean magnetic susceptibilities of Group 1 and 2 samples are 174 × 10^–5 and 2051 × 10^–5 SI, respectively.

The variation in magnetic susceptibility, NRM intensity, and modal percentage of olivine through Section 345-U1415I-4R-1A is illustrated in Figure F25. The highest magnetic susceptibilities and NRM intensities in this section correspond to a 5 cm wide interval of troctolite in the center of the section, but a clear link also exists between magnetic susceptibility and original olivine content throughout the section. Note that the broad peak in NRM intensities across the troctolite interval results from the broad response function of the SRM superconducting quantum interference device sensors, which have a full width of ~10 cm at half-peak amplitude. Magnetic susceptibility and NRM intensity are parameters that are predominantly controlled by the concentration of magnetite (in rocks with magnetite concentrations >0.01 wt%; Tarling, 1983), and olivine is paramagnetic (i.e., nonremanence carrying) when pure. Hence, this suggests that the magnetic properties of these rocks are significantly influenced by variable degrees of serpentinization/alteration of the more olivine-rich lithologies through the production of secondary magnetite from these processes.

Remanent magnetization directions were calculated by principal component analysis (PCA; Kirschvink, 1980) at all measurement points where linear components could be identified on orthogonal vector plots of demagnetization data. Figure F26 shows representative examples of AF demagnetization behavior. Sample 345-U1415I-2R-1A, 52 cm, exhibits demagnetization behavior that is unique among the samples measured in Hole U1415I (Fig. F26A). After removal of a very minor drilling-induced magnetization during initial demagnetization, this sample is characterized by a single, high-coercivity component with a negative inclination that decays smoothly toward the origin of the orthogonal vector plot. All other samples (Fig. F26B–F26E) have an initially downward directed remanence, with evidence of inclination steepening caused by acquisition of a drilling-induced magnetization (DIRM) 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. F26A). At demagnetization fields >30 mT, remanence directions migrate back to the lower hemisphere, and magnetization intensities increase continuously up to the peak applied field of 80 mT (see expanded portions of the orthogonal vector plots in Fig. F26B–F26E). This increase is due to acquisition of spurious, laboratory-imparted, anhysteretic remanent magnetizations (ARMs) along the z-axis of the SRM system, which has been a characteristic problem of this system observed on several IODP expeditions (e.g., Expedition 335 Scientists, 2012). ARM acquisition appears to relate to passage of archive-half core sections through a significant residual direct magnetic field as they exit the demagnetizing coils in the SRM. Unfortunately, the development of these anomalous ARMs in archive-half core samples prevents isolation of sufficient high-coercivity components in these rock pieces to allow geological interpretation.

The downward-directed, low- to moderate-coercivity component present between DIRM and ARM components in these samples is consistently very linear (with maximum angular deviations of 1°–3° when picked over at least four demagnetization steps). The mean inclination of this component is 49.9° (k = 28.2; α₉₅ = 6.8°; n = 17), calculated using the Arason and Levi (2010) maximum likelihood method. The origin of these components is discussed fully in “Paleomagnetism” in the “Hole U1415J” chapter (Gillis et al., 2014d), in which more extensive data are used to demonstrate that this downward-directed remanence is related to the drilling-induced magnetization in these cores.

Discrete samples

As a result of low recovery of oriented core pieces in Hole U1415I, shipboard experiments on discrete samples were limited to only three minicube samples cut from Sections 345-U1415I-2R-1W and 4R-1W. These samples consist of orthopyroxene-bearing olivine gabbro, gabbronorite, and olivine-bearing gabbronorite and provide some insights into variations in demagnetization characteristics. Orthogonal vector plots of demagnetization data from these samples are presented in Figure F27. Sample 345-U1415I-2R-1W, 52 cm (orthopyroxene-bearing olivine gabbro) is dominated by a clear, single component of high coercivity (Fig. F27A) with an inclination of –47.0°. Unfortunately, the remaining two discrete samples from Hole U1415I display complex and erratic demagnetization behavior, with no linear paths suitable for PCA. With only one of three discrete samples yielding usable AF demagnetization data, we decided to focus predominantly on thermal demagnetization of discrete samples from subsequent holes, as this technique has yielded high-quality data in gabbroic rock recovered during other IODP expeditions (e.g., Morris et al., 2009).

Geological interpretation of remanence data

Remanence data from Hole U1415I are too limited to allow geological interpretation in terms of the magnetic polarity of the sampled section or its tectonic rotation history. U-Pb dating of zircon from samples collected in the immediate vicinity of Site U1415 during the JC21 site survey cruise yielded ages of 1.42–1.27 Ma (Rioux et al., 2012). These dates lie in the middle of reversed polarity Chron C1R (Cande and Kent, 1995), suggesting that any primary magnetizations preserved in the sampled rocks should be of reversed polarity. However, the expected direction of magnetization at Hess Deep based on the geocentric axial dipole field at the site has a declination of 000° and an inclination of 4.7° for a normal polarity field and a declination of 180° and an inclination of –4.7° for a reversed polarity field. Hence, the polarity of these rocks cannot be determined uniquely in the absence of reoriented core samples. Regardless of polarity, the inclinations of the single reliable discrete sample (inclination = –47°) and single reliable archive-half core sample (inclination = –27°) imply significant tilting of the section after remanence acquisition, although clearly little confidence can be placed on such limited data.

Magnetic fabric

Anisotropy of magnetic susceptibility (AMS) determined for three discrete samples from Hole U1415I is illustrated on the equal-area stereographic projections of Figure F28. Susceptibility tensors are weakly to moderately anisotropic (corrected anisotropy degree [P′] < 1.34 [mean = 1.11]) (Jelinek, 1981). All three ellipsoid shapes (triaxial, oblate, and prolate) are represented in these discrete samples. Average bulk susceptibilities suggest that the AMS signal is dominated by magnetite in these samples rather than contributions from other paramagnetic minerals. Magmatic foliations in two samples were sufficiently well-developed to be measured in the core reference frame, and these structural measurements show different relationships to the AMS fabrics. In one sample, AMS magnetic foliation is nearly parallel to the magmatic foliation (with k_min corresponding to the pole of the magmatic foliation; Fig. F28A), whereas there is poor agreement with the structural fabric in the other sample (Fig. F28B).

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