Reflection microscopy and geochemistry at DSDP Leg 55 Holes@en

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).火山岩的磁学性质主要受控于其所含铁磁性矿物（ferromagnetic minerals）的物理与化学状态。决定其磁学性质的核心参数包括浓度、成分、晶粒尺寸与氧化状态。在海底玄武岩中，主要的铁磁性矿物为钛磁铁矿（titanomagnetites），这类矿物要么未经历氧化，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或钛磁赤铁矿（titanomaghemites）。该低温蚀变作用的影响体现为铁磁性矿物的居里温度（Curie temperature）升高、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，新生洋中脊地壳的钛磁赤铁矿化过程的时间常数（time constant）约为1百万年，并伴随天然剩余磁化强度（natural remanent magnetization, NRM）的显著衰减（Johnson与Atwater, 1977）。