Joseph I. Goldstein^a, Jijin Yang^b and Edward R.D. Scott^c

^aDepartment of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, MA, USA
^bCarl Zeiss Microscopy, LLC. One Zeiss Drive, Thornwood, NY, USA
^cHIGP, University of Hawaii, Honolulu, HI, USA

Analyses and modeling of Ni zoning in taenite in differentiated meteorites provide metallographic cooling rates at ∼500 °C that are inconsistent with conventional formation models. Group IVA iron meteorites have very diverse cooling rates of 100-6600 °C/Myr indicating that they cooled inside a large metallic body with little or no silicate mantle (Yang et al., 2007). Wasson and Hoppe (2012) have questioned these diverse cooling rates on the basis of their ion probe measurements of Ni/Co ratios at the kamacite-taenite interface in two group IVA and in two group IIIAB iron meteorites. To investigate their claims and to assess methods for determining relative cooling rates from kamacite-taenite interface compositions, we have analyzed 38 meteorites—13 IVA, 14 IIIAB irons, 4 IAB complex irons, 6 pallasites and a mesosiderite—using the electron probe microanalyzer (EPMA). Ni concentrations in taenite (Niγ) and kamacite (Niα) at kamacite-taenite interfaces are well correlated with metallographic cooling rates: Niγ values increase from 30 to 52 wt.% while Niα decreases from 7 to 4 wt.% as cooling rates decrease. EPMA measurements of Niγ, Niα, and Niα/ Niγ, can therefore be used to provide order-of-magnitude estimates of relative cooling rates. Concentrations of Co in kamacite and taenite at their interface (Coα, Coγ) are controlled by bulk Ni and Co composition, as well as cooling rate. The ratios Coα/Coγ and (Co/Ni)α/(Co/Ni)γ are correlated with cooling rate, but because of significant scatter, these parameters should not be used to estimate cooling rates. Our analyses of 13 group IVA irons provide robust support for diverse cooling rates that decrease with increasing bulk Ni, consistent with measurements of cloudy zone size and tetrataenite width. Apparent equilibration temperatures, which are inferred from Niγ values and the Fe-Ni-P phase diagram and Ni diffusion rates in taenite, show that cooling rates of IVA irons vary by a factor of ≈100, in excellent agreement with the metallographic cooling rates. Similar calculations using NiγNiα and Coα/Coγ ratios and phase diagram data give factors that are an order of magnitude lower but have larger uncertainties. Thus we strongly disagree with the conclusion ofWasson and Hoppe (2012) that interface concentrations of Ni and Co are in any way in conflict with the cooling rates of Yang et al. (2008). Our measurements confirm that the IVA irons could not have cooled in an asteroidal core surrounded by a silicate mantle, and also that main-group pallasites cooled slower than IIIAB irons and did not cool at the boundary between the mantle and core from which the IIIAB irons originated. Our data provide additional evidence that mesosiderites, which formed by impact mixing of Fe-Ni melt and crustal rocks, cooled at uniquely slow rates.

Reference
Goldstein JI, Yang J and Scott ERD (in press) Determining cooling rates of iron and stony-iron meteorites from measurements of Ni and Co at kamacite-taenite interfaces. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.05.025]
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C. Pirim^a, M.A. Pasek^b, D.A. Sokolov^a, A.N. Sidorov^a, R. Gann^a and T.M. Orlando^a

^aSchool of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332, USA
^bDepartment of Geology, University of South Florida, Tampa, FL 33620, USA

This work presents a comprehensive investigation of schreibersite inclusions within iron-poor and iron-rich meteorites, and of the associated intrinsic low-temperature oxidation products observed after exposure to terrestrial weathering. First, a thermodynamic equilibrium modeling of the oxidation of schreibersite was carried out and showed that oxidation is mostly limited to the surface in the absence of other ions and/or water. This oxidation occurs rapidly (less than a few weeks) and is mediated by the atmosphere, forming primarily iron oxides and iron phosphates. Second, detailed analyses of meteorite schreibersite inclusions and synthetic schreibersite surrogates (Fe₃P and Fe₂NiP) were performed using surface characterization techniques such as micro-Raman spectroscopy, atomic force microscopy (AFM), electrostatic force microscopy (EFM), electron microprobe analysis (EPMA), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Such thorough analyses are required as all prebiotic reactivity studies performed nowadays use meteoritic samples that have been somewhat exposed to Earth weathering. We find that, while the meteorite samples have not been introduced directly into water, they all bear significant oxidation signatures that appear to be similar for both studied short-term and long-term natural weathering corrosion processes. In addition, we find that synthetic schreibersite samples have similar surface and sub-surface chemistry and are reasonable chemical proxies for natural schreibersite. The thorough analytical studies detailed in this paper provide a chemical model for schreibersite oxidation products.

Reference
Pirim C, Pasek MA, Sokolov DA, Sidorov AN, Ganna R and Orlando TM (in press) Investigation of schreibersite and intrinsic oxidation products from Sikhote-Alin, Seymchan, and Odessa meteorites and Fe3P and Fe2NiP synthetic surrogates. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.05.027]
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Amy M. Gaffney and Lars E. Borg

Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue L-231, Livermore, CA 94550

The time at which the moon solidified can be determined from the Lu-Hf isotope systematics of lunar rocks derived from magma sources that formed during crystallization of the lunar magma ocean. The final magma ocean crystallization product, termed urKREEP, is enriched in incompatible trace elements including K, REE and P. We have determined the initial Hf isotopic compositions of four samples, two KREEP basalts and two Mg-suite norites. The incompatible trace element compositions of these samples are controlled by an urKREEP component, and therefore the initial Hf isotopic compositions of these samples represent the Hf isotopic evolution of urKREEP. In order to correct the effects of neutron irradiation on the Hf isotopic compositions of these samples, we have developed a model that uses the stable Hf and Sm isotopic compositions measured on an irradiated sample to determine and correct for the thermal and epithermal neutron fluence that has modified the Hf isotopic composition of the sample. We use our corrected results to calculate a ¹⁷⁶Lu-¹⁷⁶Hf urKREEP model age of 4353 ± 37 Ma and the ¹⁷⁶Lu/¹⁷⁷Hf of urKREEP to be 0.0153 ± 0.0033. The Lu-Hf model age is concordant with the re-calculated Sm-Nd urKREEP model age of 4389 ± 45 Ma, and we take the average of these ages, 4368 ± 29 Ma, to represent the time at which urKREEP formed. This age is concordant with the age of the most reliably dated ferroan noritic anorthosite as well as ¹⁴²Nd model ages for the formation or re-equilibration of mare basalt sources. Taken together, these ages indicate that the Moon experienced a widespread, large-scale magmatic event around 4370 Ma, most plausibly attributed to solidification of the lunar magma ocean.

Reference
Gaffney AM and Borg LE (in press) A young solidification age for the lunar magma ocean. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.05.028]
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