OXIDATION-STATE OF IRON IN MANTLE-DERIVED MAGMAS OF THE ICELANDIC RIFT-ZONE

Olivine tholeiites are mantle-derived magmas that are formed by partia l melting of their deep sources and which have equilibrated with miner al assemblages at slightly different subcrustal pressure-temperature c onditions prior to eruption. The minimum depth of the pre-eruptive res ervoirs of these magmas is in the order of 10-15 km and their liquidus temperatures fall within the range of 1180-1240 degrees C. Three type s of primitive olivine tholeiites are exposed along the rift zones in Iceland. In the present study, the ferric/ferrous ratios of natural gl asses (pillow crusts) of the three types of olivine tholeiites were ob tained by Mossbauer spectrometry. This technique is particularly well suited for the analysis of high-Mg glasses since it resolves microcrys tallites of olivine which contribute to ferrous iron in chemical analy sis. All results fall within 10-15% Fe(III). At the liquidus temperatu re of these glasses, this ferric/ferrous ratio corresponds to fugacity close to the fayalite-magnetite-quartz-oxygen (FMQ) buffer with an un certainty of less than one log unit in fO(2). This result confirms tha t there is no significant difference in the oxidation state of the thr ee magma types.