ALBERT

Description: Shipboard magnetic investigation showed two zones of weak natural remanent magnetization (NRM): 675-925 and 1080-1243 meters below sea floor. Within these two sections, NRM intensity decreases by one to two orders of magnitude. Additional magnetic measurements onshore have shown similar fluctuations for NRM and saturation magnetization (Js). Js becomes almost zero in the low-NRM intensity zones, indicating a small magnetic content. The variation of relative magnetic grain sizes have been examined by the ratio of ARM/K0 (anhysteretic remanent magnetization to low magnetic field susceptibility) and by partial ARM (pARM) measurements. Both results indicate difference in magnetic grain sizes between the high- and low-intensity zones. For these Site 808 sediments, we have not found any correlation between magnetic intensity variation and sediment units. In addition, the calcite content varies only by a factor of 1.5 at the most, rather than 1 to 2 orders of magnitude, and therefore calcite dilution can not account for the low intensities. Because the changes in sediment sources and compositions can not explain these unusual magnetic phenomena, we suggest that the low-intensity zones may be interpreted in terms of deep-seated diagenetic alteration. Our study points out that (1) it may be difficult to establish a magnetostratigraphy for deep sediments due to extremely low NRM intensity, and (2) sudden decreases in NRM intensity may not result from an unusually low paleomagnetic field intensity.

Description: We present detailed paleomagnetic and rock magnetic results of rock samples recovered during Leg 173. The Leg 173 cores display a multicomponent magnetization nature. Variations in magnetic properties correlate with changes in lithology that result from differences in the abundance and size of magnetic minerals. The combined investigation suggests that the magnetic properties of the "fresher" peridotite samples from Site 1070 are controlled mainly by titanomagnetite, with a strong Verwey transition in the vicinity of 110 K, and with field- and frequency-dependent susceptibility curves that resemble those of titanomagnetites. These results are in excellent agreement with thermomagnetic characteristics where titanomagnetites with Curie temperature ~580°C were identified from the "fresher" peridotites. In contrast to the magnetic properties observed from the "fresher" peridotites, the low-temperature curves for the "altered" peridotites did not show any Verwey transition. Thermomagnetic analysis using the high-temperature vibrating sample magnetometer also failed to show evidence for titanomagnetites. The remanent magnetization is carried by a thermally unstable mineral that breaks down at ~420°C, probably maghemite. The field- and frequency-dependent relationships are also directly opposite to those in the reversal zone, with no signs of titanomagnetite characteristics. Altogether, these rock magnetic data seem to be sensitive indicators of alteration and support the contention that maghemite is responsible for the magnetic signatures displayed in the altered peridotites of the upper section. The magnetic minerals of the basement rocks from Sites 1068, 1069, and 1070 are of variable particle size but fall within the pseudo-single-domain size range (0.2-14 µm). The average natural remanent magnetization (NRM) intensity of recovered serpenitinized peridotite is typically on the order of 20 mA/m for samples from Site 1068, but ~120 mA/m for samples from Site 1070. The much stronger magnetization intensity of Site 1070 is apparently in excellent agreement with the observed magnetic anomaly high. Nearly half of the NRM intensity remained after 400°C demagnetization, suggesting that the remanence can contribute significantly to the marine magnetic anomaly.