Magnetic properties, grain sizes and minerals at DSDP Site 83-504B

Twenty-seven samples from the Leg 83 section of Hole 504B have been investigated using magnetic, optical, and electron optical methods. The primary magnetic mineral to crystallize was titanomagnetite of approximate composition Fe2.4Ti0.6O4 (TM60), but none survives, nor is there evidence of titanomaghemite produced by oxidation of TM60. The average measured magnetic properties can be interpreted in terms of magnetite, Fe3O4, having average grain size of <1 µm and present in average volume concentration of - 0.5%. The intensity of the natural remanent magnetization (NRM) of the rocks could also be accounted for as being a thermoremanence carried by this mineral.
Although the heterogeneity of the titanomagnetite grains could be detected optically, the texture of the intergrown phases is poorly developed. In some samples from the massive units of the lower part of the section, trellis patterns were visible. The Fe3O4 present in the intergrowths is too intimately mixed with the other intergrown phases to be revealed by electron microprobe analysis that simply returns the bulk composition of the intergrowth (oxidized TM60). The path by which the mineral assemblage evolved from TM60 to an Fe304-containing intergrowth, under the temperature and pressure conditions obtaining in the Leg 83 section, makes interesting speculation. Deuteric oxidation, maghemitization/inversion, or some hypothetical low-temperature/high-pressure oxidation by a leaching-of-iron process may all play roles.

针对504B钻孔第83航次剖面的27个样品，研究人员采用磁学、光学及电子光学手段开展了系统研究。结晶形成的原生磁性矿物为成分为Fe₂.₄Ti₀.₆O₄的钛磁铁矿（titanomagnetite，TM60），但该矿物未被保留，也未发现由TM60氧化生成的钛磁赤铁矿的相关证据。实测得到的平均磁学性质可通过磁铁矿（Fe₃O₄）得到合理解释：该磁铁矿的平均晶粒尺寸小于1 μm，体积平均占比约为0.5%。岩石的天然剩余磁化强度（natural remanent magnetization, NRM）也可归因于该矿物携带的热剩磁。尽管通过光学观测可识别出钛磁铁矿晶粒的非均质性，但共生相的组构发育程度较差。在该剖面下部块状岩相的部分样品中，可见格架状构造。共生体中的Fe₃O₄与其他共生相混合极为紧密，电子探针分析仅能获取共生体的整体成分（氧化后的TM60），无法区分出该磁铁矿相。在第83航次剖面所处的温压条件下，矿物组合从TM60演化为含Fe₃O₄的共生体的演化路径，值得开展深入探讨。蚀变氧化、磁赤铁矿化/相转变，或是某种假想的通过铁淋滤过程实现的低温高压氧化作用，均可能在该演化过程中发挥作用。