The Iron-Titanium Oxides of Some Dunedin (New Zealand) Lavas, in Relation to Their Palaeomagnetic and Thermomagnetic Character (with an Appendix on Associated Chromiferous Spinel)

J. B. Wright
Geology Department, University of Otago, New Zealand1
1Address from January 1967: Department of Geology, Ahmadu Bello University, Zaria, Northern Nigeria.

Summary: Earlier palaeomagnetic studies on the Dunedin volcano showed that a portion of the sequence is reversely or anomalously magnetized. Some of the rocks used for palaeomagnetic study were subsequently classified into groups according to their thermomagnetic behaviour (unpublished work).

Cell dimensions and Curie-point curves have been measured for oxides separated from representative rocks of each group. The oxides are mostly homogeneous titanomagnetites containing between 40 and 55 mol. % of ulvöspinel, with a generally small degree of late low-temperature alteration. X-ray and thermomagnetic data also suggest that there was some early oxidation to titanomaghemite, probably during cooling.

The thermomagnetic behaviour of rocks classified in the different groups is attributed to variable oxidation of the titanomagnetite during thermomagnetic treatment. The high Curie points of rocks used for palaeomagnetic studies may often be due merely to oxidation of titaniferous magnetite and not to nearly pure primary Fe3O4. Since there is insufficient oxidation in the lavas examined for any currently accepted self-reversal process to have been operative, the reversely magnetized part of the Dunedin sequence was probably erupted during a reversed polarity epoch, possibly between about 13 and 11 million years ago.

The basanitic lavas contain minor amounts of chromiferous spinel, as inclusions in silicate phenocrysts and as cores to separate microphyric titanomagnetite; its significance is briefly discussed.

Mineralogical Magazine; September 1967 v. 36; no. 279; p. 425-435; DOI: 10.1180/minmag.1967.036.279.13
© 1967, The Mineralogical Society
Mineralogical Society (