Reflection microscopy and geochemistry at DSDP Leg 55 Holes

Magnetic properties of volcanic rocks are controlled mainly by the physical and chemical state of their constituent ferromagnetic minerals. The most important parameters determining magnetic properties are concentration, composition, grain size, and oxidation state. In sea floor basalts, the main ferromagnetic minerals are titanomagnetites which are either unoxidized or, more commonly, have undergone various degrees of posteruptive low-temperature oxidation to become cationdeficient titanomagnetites, or titanomaghemites. The effects of this low-temperature alteration are seen in the increase of Curie temperature and decrease of saturation magnetization and lattice parameter of ferromagnetic minerals (Readman and O'Reilly, 1972). It is now believed that titanomaghemitization of newly formed mid-ocean ridge crust proceeds with a time constant of about 1 m.y., accompanying drastic decrease of the intensity of the natural remanent magnetization (NRM) (Johnson and Atwater, 1977).

火山岩的磁性主要受控于其所含铁磁性矿物（ferromagnetic minerals）的物理化学状态。决定其磁性的关键参数包括浓度、成分、晶粒尺寸与氧化态。在海底玄武岩中，主要的铁磁性矿物为钛磁铁矿（titanomagnetite），这类矿物要么未发生氧化，更常见的是经历了不同程度的喷发后低温氧化作用，转变为贫阳离子钛磁铁矿（cation-deficient titanomagnetite）或钛磁赤铁矿（titanomaghemite）。这种低温蚀变作用的影响表现为铁磁性矿物的居里温度（Curie temperature）升高，而饱和磁化强度（saturation magnetization）与晶格参数（lattice parameter）降低（Readman与O'Reilly，1972）。目前学界普遍认为，新形成的洋中脊地壳的钛磁赤铁矿化作用的时间常数约为1百万年，该过程伴随天然剩余磁化强度（natural remanent magnetization，NRM）的强度急剧降低（Johnson与Atwater，1977）。