Computer Science
Scientific paper
Jun 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000e%26psl.179....9z&link_type=abstract
Earth and Planetary Science Letters, Volume 179, Issue 1, p. 9-20.
Computer Science
20
Scientific paper
Transmission electron microscopy observations and rock magnetic measurements on a pillow from the `New Flow', extruded in 1993 at the Juan de Fuca Ridge, demonstrate that Ti-content in large (up to 40 μm) titanomagnetite grains varies as a function of the cooling rate. Large grains in the interior of the pillow have a narrow composition range of approximately x=0.6, where x represents the ulvöspinel content in the solid solution series with magnetite. In contrast, the titanomagnetite grains near the pillow rim are progressively smaller and have a broad composition range (average x=~0.45). Within 0.5 cm of the rim a significant portion of the grains is single domain (SD) to superparamagnetic (SP) and appears to have little or no Ti. This lower Ti-content provides a ready explanation for the higher Curie temperature near the glassy margin of the pillow. 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 previously suggested for this very young mid-ocean ridge basalt (MORB) in order to explain the Curie temperatures. Submicrometer titanomagnetite in interstitial glass in the interior of the pillow also shows variable x values. These grains make contributions to susceptibility that show up as high (up to ~580°C) Curie temperatures in thermomagnetic analyses and they appear to be SD to SP. The variation of x values for the titanomagnetite in the MORB is consistent with thermodynamic and kinetic considerations, because crystallization of titanomagnetite in the MORB is controlled by processes depending on cooling rate and crystal-melt fractionation.
Peacor Donald R.
Van der Voo Rob
Zhang Yajing
Zhou Weihang
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