The evolution of partially differentiated planetesimals: Evidence from iron meteorite groups IAB and IIICD

Abstract

Group IAB irons probably come from a parent body that partially differentiated into a sulfur‐rich molten core and a primitive silicate outer layer in which only minor or localized melting occurred. Although the complexity of meteorites from this parent body or bodies (irons with and without massive silicate inclusions, stony meteorites like Winona, sulfide‐rich meteorites like Mundrabilla, etc.) implies a complicated history, the partial differentiation model is consistent with most properties of these meteorites. The behavior of five model parent bodies of different composition during heating to 1400 K (the temperature recorded in many I AB silicates) shows that meteorites like IAB irons can form by this process. Sulfide‐rich parent bodies differentiate into a liquid, S‐rich core and a primitive, metal‐bearing mantle around 1230 K, but in parent bodies lower in sulfide abundance this process may be delayed until minor melting of silicates occurs (1370 ± 30 K). In most cases any silicate melt that may have formed resolidified in situ. The sulfide melt fractionally crystallized along the (Fe,Ni)‐FeS cotectic. Some melt was injected into silicates to form Copiapo‐type breccias. Considerations of graphite‐metal‐silicate equilibrium, and the retention of noble gases suggest that brecciated IABs formed at depth inside a moderately large (a few tens to a hundred kilometers) parent body. The correlation of I‐Xe ages with Ni content is interpreted as the result of an excess 129Xe component associated with the S‐rich melt, which increases with melt evolution.

Thermodynamic constraints on the formation conditions of winonaites and silicate-bearing IAB irons

“…Our work suggests that the IABs and winonaites cooled to two-pyroxene closure temperatures between 2.5 and 3 log units below the iron-wüstite buffer. Although, the presence of graphite in many of the samples indicates that the pressure was at least 10 bars, the temperature was not high enough for the graphite reduction reaction, as hypothesized by Kracher (1985), to take place. Given that peak temperatures (based on mineral abundances and textures) were not much higher than two-pyroxene closure temperatures, we can calculate the average Fa value corresponding to the two-pyroxene closure T and ƒO 2 .…”

“…It has, however, been unclear how this reduced nature was established. Kracher (1985) postulated that winonaites and silicates in IAB irons were reduced from a starting composition of ordinary chondrite (ϳFa 20 ) composition during metamorphism at a pressure of 10 bars. Our work suggests that the IABs and winonaites cooled to two-pyroxene closure temperatures between 2.5 and 3 log units below the iron-wüstite buffer.…”

Graves Nunataks 95209: A snapshot of metal segregation and core formation

“…Instead, the formation of the metallic sheet probably involved melting at the Fe,Ni-FeS cotectic temperature ($950°C), followed by separation of the metal and sulfide components as a result of liquid immiscibility upon cooling and crystallization. The supposition that this vein formed at low temperature is supported by the abundance of C and P within the vein, as these elements are typically enriched in low-degree partial melts (e.g., Kracher, 1985).…”

Iron and Stony-Iron Meteorites

“…While differentiated silicates might be expected in association with iron meteorites, silicates in IAB-IllCD i.rons are broadly chondri tic (Mittlefehldt ef al., 1998;Benedix et al, 2000; see Chapter 1.11). Models for the origins ofIAB -lIICD iron meteorites include crystallization of a sulfur-and carbon-rich core in a partially differentiated object (Kracher, 1985;McCoy et al, 1993), breakup and reassembly of a partially differentiated object at its peak temperature (Benedix er al., 2000), or crystal segregation in isolated impact melt pools on the surface of a porous chondri tic body (Wasson and Kallemeyn, 2002). A recent compilation of the chemical compositions of lAB and IIICD iron meteorites may be found in Wasson and Kallemeyn (2002).…”