Curie temperatures and hysteresis properties of ODP Leg 183 sites, Kerguelen Plateau and Broken Ridge

In this manuscript, we present rock magnetic results of samples recovered during Leg 183. The Leg 183 cores were recovered from six drill sites and display variable rock magnetic properties. The differences in the rock magnetic properties are a function of mineralogy and alteration. Cretaceous subaerial basalt samples with titanomagnetite exhibit a strong Verwey transition in the vicinity of 110 K and have frequency-dependent susceptibility curves that resemble those of synthetic (titano) magnetites. These results are in good agreement with the thermomagnetic characteristics where titanomagnetites with Curie temperatures of ~580°C were identified. The hysteresis ratios suggest that the bulk magnetic grain size is in the psuedo-single-domain boundary. These subaerial basalts experienced high-temperature oxidation and maintained reliable paleomagnetic records. In contrast, the 34-Ma submarine pillow basalts do not show the Verwey transition during the low-temperature experiments. Thermomagnetic analysis shows that the remanent magnetization in this group is mainly carried by a thermally unstable mineral titanomaghemite. The frequency-dependent relationships are opposite of those from the first group and show little sign of titanomagnetite characteristics. Rocks from the third group are oxidized titanomagnetites and have multiple magnetic phases. They have irreversible thermaomagnetic curves and hysteresis ratios clustering toward the multidomain region (with higher Hcr/Hc ratios).
The combined investigation suggests that variations in magnetic properties correlate with changes in lithology, which results in differences in the abundance and size of magnetic minerals. The rock magnetic data on Leg 183 samples clearly indicate that titanomagnetite is the dominant mineral and the primary remanence carrier in subaerial basalt. The generally good magnetic stability and other properties exhibited by titanomagnetite-bearing rocks support the inference that the ChRM isolated from the Cretaceous sites were acquired during the Cretaceous Normal Superchron. The stable inclinations identified from these samples are therefore useful for future tectonic studies.

本研究报道了183航次（Leg 183）回收岩芯的岩石磁学测试结果。本次183航次的岩芯采自6个钻探站位，其岩石磁学性质存在显著差异，这些差异受控于矿物组成与蚀变作用。含钛磁铁矿（titanomagnetite）的白垩纪陆上玄武岩样品在110 K附近呈现出显著的Verwey转变（Verwey transition），其频率依赖磁化率曲线与合成（钛）磁铁矿的曲线高度相似。该结果与热磁测试特征吻合良好：研究识别出居里温度约为580℃的钛磁铁矿。滞后比值参数表明，样品的整体磁晶粒尺寸处于假单畴（pseudo-single-domain）区间。此类陆上玄武岩经历了高温氧化作用，但仍保留了可靠的古地磁记录。与之形成对比的是，34 Ma（百万年）的海底枕状玄武岩在低温实验中未出现Verwey转变。热磁分析显示，该组样品的剩余磁化强度主要由热不稳定矿物钛磁赤铁矿（titano-maghemite）携带。其频率依赖特征与第一组样品完全相反，几乎未表现出钛磁铁矿的相关特征。第三组岩石为氧化型钛磁铁矿，包含多种磁性物相，该组样品具有不可逆热磁曲线，且滞后比值参数聚集至多畴区域（Hcr/Hc比值更高）。
综合分析表明，磁性性质的差异与岩性变化密切相关，而岩性变化会导致磁性矿物的丰度与晶粒尺寸产生差异。183航次岩样的岩石磁学数据清晰表明，钛磁铁矿是陆上玄武岩中的优势磁性矿物与主要剩余磁化载体。含钛磁铁矿的岩石普遍具有良好的磁稳定性及其他相关特性，这佐证了一个推论：从白垩纪站位分离得到的特征剩余磁化强度（Characteristic Remanent Magnetization，ChRM）是在白垩纪正常超静期（Cretaceous Normal Superchron）获得的。因此，从这些样品中识别出的稳定磁化倾角可用于后续的构造地质学研究。