Correlation of high-resolution TEM data of mineral assemblages of mid-ocean ridge basalts with rock magnetic properties.

摘要

A detailed electron microscopic and rock magnetic study of mid-ocean ridge basalts (MORB), varying between Recent and Jurassic in age, has been carried out with the aim of determining the causes of the observed changes in intensity of marine magnetic anomalies and to examine their implications for paleo-intensity studies.; Electron microscopy has been used to determine the grain size, lattice parameter, oxidation state and composition of individual titanomagnetite grains in MORB. Such data have been integrated with rock magnetic and paleomagnetic data to characterize the titanomagnetite mineralogy and its relationship with rock magnetization.; Studies of very young MORB (20 ky) reveal a substantial variation of initial titanomagnetite mineralogy within a single pillow lava. Ti-content in titanomagnetite grains varies as a function of cooling rate. Large grains in the interior of pillows have a narrow compositional range. In contrast, the titanomagnetite grains near pillow rims are progressively smaller and have a broad compositional range. Submicrometer titanomagnetite in interstitial glass in the interior of pillows also shows variable Ti-content. The variable Ti-content provides a ready explanation for the observed thermomagnetic behavior. Moreover, determination of the oxidation state of the titanomagnetite does not show low-temperature oxidation anywhere within the pillow, indicating no rapid alteration to titanomaghemite as had been previously suggested for very young MORB in order to explain the higher unblocking and Curie temperatures.; Alteration of fine- and large-grained titanomagnetite in the same sample can occur at different rates. Owing to the protection of glass matrix, fine-grained titanomagnetite produced by exsolution of an Fe-Ti-rich immiscible liquid in residual melt does not necessarily alter faster than large grains. The existence of pristine, fine-grained titanomagnetite in the interstitial glass of older MORB implies that such glassy material is capable of carrying original thermal remanent magnetization and may be suitable for paleointensity determinations.; Low temperature oxidation of titanomagnetite to titanomaghemite in MORB is a gradual process controlled by many factors, including age, regional oceanic crustal structures, lithological features, grain size and surrounding matrices. In similar fashion, the NRM intensity of MORB varies with age as a function of multiple parameters, such as magnetic granulometry, concentration of magnetic carriers and oxidation state.