Variable Ti-content and grain size of titanomagnetite as a function of cooling rate in very young MORB

Transmission electron microscopy observations and rock magnetic measurement s on a pillow from the 'New Flow', extruded in 1993 at the Juan de Fuca Rid ge, demonstrate that Ti-content in large (up to 40 mu m) titanomagnetite gr ains 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, wh ere x represents the ulvospinel content in the solid solution series with m agnetite. In contrast, the titanomagnetite grains near the pillow rim are p rogressively smaller and have a broad composition range (average x = simila r to 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 C urie temperature near the glassy margin of the pillow. Moreover, determinat ion of the oxidation state of the titanomagnetite does not show low tempera ture oxidation anywhere within the pillow, indicating no rapid alteration t o titanomaghemite as previously suggested for this very young mid-ocean rid ge basalt (MORB) in order to explain the Curie temperatures. Submicrometer titanomagnetite in interstitial glass in the interior of the pillow also sh ows variable x values. These grains make contributions to susceptibility th at show up as high (up to similar to 580 degrees C) Curie temperatures in t hermomagnetic analyses and they appear to be SD to SP. The variation of x v alues for the titanomagnetite in the MORB is consistent with thermodynamic and kinetic considerations, because crystallization of titanomagnetite in t he MORB is controlled by processes depending on cooling rate and crystal-me lt fractionation. (C) 2000 Elsevier Science B.V. All rights reserved.