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doi:10.2204/iodp.proc.344.206.2015

Results

Magnetic hysteresis of basalt samples

Table T1 presents hysteresis data for basalts recovered from Expedition 344 Hole U1414A (see the “Input Site U1414” chapter [Harris et al., 2013c]) and Expedition 334 Hole U1381A (Expedition 334 Scientists, 2012b). Figure F2 presents the basalt data on a Day plot relative to the SD, PSD, and MD fields defined by Dunlop (2002). The gray curves on this plot represent the theoretical SD versus MD mixing lines of Dunlop (2002), which are based on ideal mixtures of different magnetite grain sizes. The two groups of basalt samples differ slightly in their magnetic hysteresis properties. Both groups fall in the PSD field, but pillow lava samples from the top of the Cocos Ridge (Hole U1381A; light blue symbols) show an even distribution of SD and MD grains, whereas massive lava flow samples from the flank of the Cocos Ridge (Hole U1414A; dark blue symbols) indicate a higher contribution of MD grains.

The FORC diagrams generated for Hole U1414A and U1381A basalt samples are displayed in Figure F3. All diagrams have relatively circular contours with peak values of only 2–5 mT, indicating the predominance of a low-coercivity magnetic mineral. Given how common it is in oceanic basalts, this mineral is most likely titanomagnetite or its low-temperature oxidized counterpart titanomaghemite. The vertical spread of the contours suggests the presence of interacting grains of mainly PSD and MD sizes. The sample from Hole U1381A (top of Cocos Ridge) has lower magnetic interactions, a smaller contribution from MD grains, and a higher coercivity component than the Hole U1414 samples (Cocos Ridge flank), the latter of which could originate from finer grained PSD and/or SD particles (Fig. F3).

Magnetic hysteresis of tephra samples

Table T2 presents hysteresis data from one of the tephra layers recovered from Expedition 344 Hole U1381C (see the “Input Site U1381” chapter [Harris et al., 2013b]). For these measurements, we collected a vertical transect of samples from the top to the bottom of a thick, graded tephra layer recovered in Section 344-U1381C-7H-2. Figure F2 presents the tephra data on a Day plot relative to the SD, PSD, and MD fields defined by Dunlop (2002). Figure F4 displays the same data (Samples 2–8) on a magnified portion of the Day plot. Nannofossil ooze data from the same core section (Samples 1 and 9) are also shown to compare the properties of the tephra layer with those of the background carbonate lithology that dominates this cored interval. Samples from the top and middle portions of the tephra layer cluster in the SD field, whereas samples from the bottom portion of the tephra layer and the nannofossil ooze background lithology fall in the PSD field (Fig. F4). This is consistent with the normally graded nature of this tephra layer (see the “Input Site U1381” chapter [Harris et al., 2013b]).

The FORC diagrams generated for two of the tephra samples display similar patterns (Fig. F5). The closed, elongated contours and small vertical spread suggest assemblages dominated by noninteracting SD grains. The range of coercivity values indicates the predominance of magnetite with a possible contribution from iron sulfides (e.g., pyrrhotite). A secondary peak present near the y-axis suggests a small contribution of grains with very low coercivity, which may be a processing artifact or could indicate a small contribution from larger grains (MD and PSD).

Magnetic hysteresis of sediment samples

Table T3 presents hysteresis data for sediments recovered from Expedition 344 Holes U1381C, U1414A, U1380C, and U1413A (see the “Input Site U1381,” “Input Site U1414,” “Mid-slope Site U1380,” and “Upper slope Site U1413” chapters [Harris et al., 2013b, 2013c, 2013e, 2013f]). This group of samples comprises a variety of fine- to coarse-grained lithologies that are representative of the sediments recovered from the Costa Rica convergent margin. A sample from a recrystallized, calcareous sandstone encountered between lava flows in Hole U1414A is also included in this group (see the “Input Site U1414” chapter [Harris et al., 2013c]). Figure F2 presents the sediment data on a Day plot relative to the SD, PSD, and MD fields defined by Dunlop (2002). The fine-, medium-, and coarse-grained lithologies recovered from the Costa Rica sites all fall in the PSD spectrum (Fig. F2). However, there is a clear progression with grain size, with the carbonate ooze samples showing a larger contribution of SD grains, the clay and silt/siltstone samples showing an even distribution of SD and MD grains, and the sand/sandstone samples showing a larger contribution of MD grains. The recrystallized sandstone sample falls between the sediment and basalt groups of samples and shows a smaller contribution of MD grains relative to the unaltered sand/sandstone samples.

The FORC diagrams generated for the sediment samples suggest a variety of magnetic grain assemblages (Fig. F6). The FORC diagram for the nannofossil ooze in Section 334-U1381C-7H-2 indicates the presence of noninteracting SD grains. The FORC diagrams for the clayey silt in Section 334-U1381C-6H-3 and the sandstone in Section 344-U1380C-25R-2 indicate the presence of PSD grains. The range of coercivities suggests that magnetite is the dominant mineral. Small amounts of iron sulfides may also be present, which is consistent with accessory minerals observed in the sediments (see the “Input Site U1381” chapter [Harris et al., 2013b]). Reliable FORC diagrams could not be obtained for the rest of the coarser grained samples.

Natural remanent magnetization of basalt samples

We measured the magnetization of three samples from Hole U1381A and six basalt samples from Hole U1414A. Changes in NRM during demagnetization are displayed using orthogonal vector endpoint projections (Zijderveld, 1967; Figs. F7, F8). The intensity plots show that AF demagnetization was sufficient to completely demagnetize the samples (Figs. F7, F8). Magnetic directions were determined using principal component analysis (PCA; Kirschvink, 1980). For the PCA analyses, we used 5 points to determine the best-fit lines and anchored the lines to the origin.

The magnetic directions we calculated for the nine basalt samples are provided in Table T4. Because these samples come from unoriented rotary cores, only the inclination values are useful. PCA directions are generally considered reliable if the maximum angular deviation (MAD) of the PCA analysis is relatively small (<10°). Only one of the nine samples exceeds this limit; all the others are <7°. The one outlier gives the steepest inclination (45.5°). The inclination values for the other eight samples range from –27.3° to 30.4°. The average inclinations of the Hole U1381A and Hole U1414A samples are 20.2° and 18.1°, respectively. These inclination values are consistent with a Cocos plate paleolatitude within ~10° of the Equator around 14 Ma, the age of the crust offshore Osa Peninsula (Barckhausen et al., 2001). However, because these samples do not come from a sufficient number of cooling units to average secular variation, we intend to combine these results with larger postcruise studies.

Interestingly, the two groups of samples are characterized by different magnetic susceptibilities, with a Hole U1381A (top of Cocos Ridge) average of 121 cm³/g and a Hole U1414A (Cocos Ridge flank) average of 287 cm³/g (Table T4).

Conclusion

The magnetic assemblages of Cocos Ridge basalts from Sites U1381 and U1414 are dominated by populations of PSD grains, with varying contributions from MD grains. The basalt samples collected from the top and the flank of the Cocos Ridge are characterized by slightly different rock magnetic properties, perhaps as a result of varying components of hotspot volcanism and seafloor spreading.

Demagnetization experiments of basalt samples show that AF demagnetization is sufficient to isolate a primary remanent magnetization component. The inclination values obtained from the basalts are consistent with the near-equatorial paleolatitude of the Cocos plate when the crust was formed. These results will be combined with larger tectonic studies that are in progress.

Tephra samples from Site U1381 are dominated by SD and PSD grains with much higher coercivities than the basalts and lower magnetic–grain interactions. Grain size depends on stratigraphic depth within thick tephra layers, with PSD grain sizes occurring near the base of a tephra layer and SD grain sizes near the top, consistent with the tephras being normally graded beds. The relatively high coercivities (40–70 mT) and small grain sizes are indicative of SD magnetite, with possible contributions from iron sulfides.

The magnetic assemblages of sediments from Sites U1380, U1381, U1413, and U1414 are dominated by populations of PSD grains, with smaller contributions from SD and MD grains. Sediment coercivities are generally higher than the basalts and lower than the tephras, and the range indicates that magnetite is the dominant mineral.

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