A CONVENIENT METHOD FOR MEASURING FERRIC IRON IN MAGNESIOWUSTITE (MGO-FE1-XO)

We present a new oxybarometer for magnesiowustite-bearing systems, whi ch is easily applied using widely available techniques. A scale relati ng the proportion of Fe3+ [alpha = Fe3+/(Fe3+ + Fe2+)] to the position of the (220) reflection and total Fe (y) in magnesiowustites (Mg1-y. Fe-y)(1-x)O has been derived from measured values in samples equilibra ted at various oxygen fugacities: alpha = 13.0047 - 39.4829f + 40.0540 f(2) - 13.5701f(3)(+/-0.007 + 0.09 alpha),f = (d(220) - 1.4890)/(0.051 0y + 00206y(2))(+/-0.0001). Previously established partition coefficie nts can then be used to relate estimated Fe3+ content to equilibrium o xygen fugacity (f(O2)) in the composition range 0 < y < 0.2. Equilibri um oxygen fugacities of log f(O2) less than or equal to -1.7 can be es timated to +/-0.5 log units using just X-ray powder diffraction and el ectron microprobe analytical techniques.