MAGMATIC ACTIVITY ON THE IVA PARENT BODY - EVIDENCE FROM SILICATE-BEARING IRON-METEORITES

Four of the magmatic IVA iron meteorites contain tridymite or clinobro nzite-orthobronzite-tridymite which are quite unlike silicate assembla ges in other iron meteorites. The textures, the bulk chemistry, and th e zoning preserved in the pyroxenes strongly suggest that they are ign eous cumulates. The pyroxenes have extremely low Fe/Mn-ratios (less th an 20) and low contents of REEs and other incompatible elements. These cumulates crystallized from magmas of unusual composition, with some similarity to terrestrial boninites, at the protobronzite-tridymite co tectic in the olivine-plagioclase-silica system. A liquidus temperatur e in the range 1400-1350 degrees C was inferred for the Steinbach mete orite from the estimated distribution coefficient for Cr in pyroxene ( D-Cr(solid/liquid) similar to 0.66). The low levels of incompatible el ements show that less than 1% of the residual liquid was trapped in th e cumulates. During cooling at subsolidus temperatures, most of the pr otobronzite transformed to orthobronzite and the rest inverted to a fi ne intergrowth of clino- and orthobronzite. In addition, the igneous z oning of Ca, Fe, Mg, and Mn was modified by diffusion, whereas Ti, Al, and Cr were not or only slightly affected. The silica-saturated magma s could not have evolved in an olivine-rich mantle. We assume that the magmas became incorporated in the metal core, possibly due to solidif ication shrinkage of the metal. We propose that the IVA parent magmas were formed by high degrees of partial melting (>40%) of a chondritic precursor along the olivine-pyroxene peritectic reaction curve in the olivine-plagioclase-silica system at low pressures. The precursor may have been depleted in incompatible elements by a preceding melting epi sode. The partial melts were then separated from the olivine residue a nd subsequently reduced to account for the low Fe/Mn-ratios and the un usually high Si content.