A new procedure is described to separate magnetite from milligram sized samples of crushed rock for oxygen isotope analysis. This method is based on magnetic separation of magnetite after heating the mixture to a temperature that exceeds the Curie point for other magnetic minerals in a sample. The relatively low temperature of this procedure (350 °C) does not induce any reaction between magnetic mineral nor does it alter the isotope composition of magnetite. This procedure was tested on samples containing known percentages of magnetite and pyrrhotite. The percentage of magnetite in a sample was increased by 10–20 % through successive enrichment cycles until a pure magnetite separate was achieved. The separation should be performed before measuring the oxygen isotope composition of the magnetic fraction of a rock sample by laser fluorination, because the presence of contaminating phases such as pyrrhotite may cause unwanted isotope partitioning of the oxygen released during fluorination.

Our data show that contamination of a magnetite sample, even with a non O-bearing phase, can lead to systematically misleading O isotope data in a standard total laser fluorination system. Enrichment of the apparent magnetite value is indicated, possibly as a result of the formation of an O-S-bearing phase.

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