Alex Ruzicka

Cascadia Meteorite Laboratory, Portland State University, 17 Cramer Hall, 1721 SW Broadway, Portland, OR 97207-0751, United States

Silicate-bearing iron meteorites differ from other iron meteorites in containing variable amounts of silicates, ranging from minor to stony-iron proportions (~50%). These irons provide important constraints on the evolution of planetesimals and asteroids, especially with regard to the nature of metal–silicate separation and mixing. I present a review and synthesis of available data, including a compilation and interpretation of host metal trace-element compositions, oxygen-isotope compositions, textures, mineralogy, phase chemistries, and bulk compositions of silicate portions, ages of silicate and metal portions, and thermal histories. Case studies for the petrogeneses of igneous silicate lithologies from different groups are provided. Silicate-bearing irons were formed on multiple parent bodies under different conditions. The IAB/IIICD irons have silicates that are mainly chondritic in composition, but include some igneous lithologies, and were derived from a volatile-rich asteroid that underwent small amounts of silicate partial melting but larger amounts of metallic melting. A large proportion of IIE irons contain fractionated alkali-silica-rich inclusions formed as partial melts of chondrite, although other IIE irons have silicates of chondritic composition. The IIEs were derived from an H-chondrite-like asteroid that experienced more significant melting than the IAB asteroid. The two stony-iron IVAs were derived from an extensively melted and apparently chemically processed L or LL-like asteroid that also produced a metallic core. Ungrouped silicate-bearing irons were derived from seven additional asteroids. Hf–W age data imply that metal–silicate separation occurred within 0–10 Ma of CAI formation for these irons, suggesting internal heating by ²⁶Al. Chronometers were partly re-set at later times, mainly earlier for the IABs and later for the IIEs, including one late (3.60 ± 0.15 Ga) strong impact that affected the “young silicate” IIEs Watson (unfractionated silicate, and probable impact melt), Netschaëvo (unfractionated, and metamorphosed), and Kodaikanal (fractionated). Kodaikanal probably did not undergo differentiation in this late impact, but the similar ages of the “young silicate” IIEs imply that relatively undifferentiated and differentiated materials co-existed on the same asteroid. The thermal histories and petrogeneses of fractionated IIE irons and IVA stony irons are best accommodated by a model of disruption and reassembly of partly molten asteroids.

Reference
Ruzicka R (in press) Silicate-bearing iron meteorites and their implications for the evolution of asteroidal parent bodies. Chemie der Erde
[doi:10.1016/j.chemer.2013.10.001]
Copyright Elsevier

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M. A. Siegler and S. E. Smrekar

Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA

We reexamine the Apollo Heat Flow Experiment in light of new orbital data. Using three-dimensional thermal conduction models, we examine effects of crustal thickness, density, and radiogenic abundance on measured heat flow values at the Apollo 15 and 17 sites. These models show the importance of regional context on heat flux measurements. We find that measured heat flux can be greatly altered by deep subsurface radiogenic content and crustal density. However, total crustal thickness and the presence of a near-surface radiogenic-rich ejecta provide less leverage, representing only minor (<1.5 mW m⁻²) perturbations on surface heat flux. Using models of the crust implied by Gravity Recovery and Interior Laboratory results, we found that a roughly 9–13 mW m⁻² mantle heat flux best approximate the observed heat flux. This equates to a total mantle heat production of 2.8–4.1 × 10¹¹ W. These heat flow values could imply that the lunar interior is slightly less radiogenic than the Earth’s mantle, perhaps implying that a considerable fraction of terrestrial mantle material was incorporated at the time of formation. These results may also imply that heat flux at the crust-mantle boundary beneath the Procellarum potassium, rare earth element, and phosphorus (KREEP) Terrane (PKT) is anomalously elevated compared to the rest of the Moon. These results also suggest that a limited KREEP-rich layer exists beneath the PKT crust. If a subcrustal KREEP-rich layer extends below the Apollo 17 landing site, required mantle heat flux can drop to roughly 7 mW m⁻², underlining the need for future heat flux measurements outside of the radiogenic-rich PKT region.

Reference
Siegler MA and Smrekar SE (in press) Lunar heat flow: Regional prospective of the Apollo landing sites. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004453]
Published by arrangement with John Wiley & Sons

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Elizabeth Gibney

We still seek a copyright agreement with Nature to display abstracts of their cosmochemistry related publications.

Reference
Gibney E (2014) Comet craft ready to wake. Nature 505:269–270.
[doi:10.1038/505269a]

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