GENESIS OF THE IIICD IRON-METEORITES - EVIDENCE FROM SILICATE-BEARINGINCLUSIONS

Our studies of the silicate-bearing inclusions in the IIICD iron meteo rites Maltahohe, Carlton and Dayton suggest that their mineralogy and mineral compositions are related to the composition of the metal in th e host meteorites. An inclusion in the low-Ni Maltahohe is similar in mineralogy to those in IAB irons, which contain olivine, pyroxene, pla gioclase, graphite and troilite. With increasing Ni concentration of t he metal, silicate inclusions become poorer in graphite, richer in pho sphates, and the phosphate and silicate assemblages become more comple x. Dayton contains pyroxene, plagioclase, SiO2, brianite, panethite an d whitlockite, without graphite. In addition, mafic silicates become m ore FeO-rich with increasing Ni concentration of the hosts. In contras t, silicates in IAB irons show no such correlation with host Ni concen tration, nor do they have the complex mineral assemblages of Dayton. T hese trends in inclusion composition and mineralogy in IIICD iron mete orites have been established by reactions between the S-rich metallic magma and the silicates, but the physical setting is uncertain. Of the two processes invoked by other authors to account for groups IAB and IIICD, fractional crystallization of S-rich cores and impact generatio n of melt pools, we prefer core crystallization. However, the absence of relationships between silicate inclusion mineralogy and metal compo sitions among IAB irons analogous to those that we have discovered in IIICD irons suggests that the IAB and IIICD cores/metallic magmas evol ved in rather different ways. We suggest that the solidification of th e IIICD core may have been very complex, involving fractional crystall ization, nucleation effects and, possibly, liquid immiscibility.