Maghemite in Icelandic basalts

Curie temperatures indicating non-titaniferous magnetite are common in Icelandic basalts of all ages, especially Tertiary ones. Yet, microprobe analyses of such samples have shown high titanium in the magnetite. To resolve this paradox, and the mechanism at work, the magnetic mineral fraction of eight basalt samples with J s -T curves characteristics for pure magnetite was subjected to a multi-disciplinary analysis including Mossbauer spectroscopy and X-ray diffraction. In most of the samples titanium in the magnetite, as analysed with the microprobe, ranged between 16 and 28 wt.%, indicating sub-microscopic solvus exsolution in the titanomagnetite, beyond the power of resolution for the microprobe. More unexpectedly in view of the reversible J s -T curves, Mossbauer spectroscopy showed aapreciable proportion of maghemite in the magnetic fraction