SINGLE-DOMAIN AND SUPERPARAMAGNETIC TITANOMAGNETITE WITH VARIABLE TI CONTENT IN YOUNG OCEAN-FLOOR BASALTS - NO EVIDENCE FOR RAPID ALTERATION

High resolution electron microscope studies have been carried out on ' zero-age' (New Flow) basalts from the Juan de Fuca Ridge and on young (< 20 ka) basalts from the axis of the East Pacific Rise at 12 degrees N. Such data lead to characterization of the magnetic minerals, espec ially those of smaller grain size, which have been hypothesized by Ken t and Gee to have undergone grain size-dependent alteration. In additi on to larger titanomagnetite grains, abundant submicrometer titanomagn etite has been observed in globules within a glassy matrix. These grai ns, likely to be single-domain (SD) or superparamagnetic, are associat ed with apatite, uncommon pyrrhotite and residual glass. The submicrom eter titanomagnetite grains have a wide compositional range (0 < x < 0 .8), where x is the fraction of ulvospinel component, whereas the larg er, multi-domain (MD)-sized titanomagnetite grains have a narrow compo sition range of approximately x = 0.6. This variability in Ti content provides a ready explanation for the thermal rock magnetic properties observed by Kent and Gee and eliminates the need to invoke extremely r apid (< 20 ka) alteration of these young basalts. (C) 1997 Elsevier Sc ience B.V.