¹State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China
²State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550002, China

In this study, a high-precision method for the determination of Sm and Nd concentrations and Nd isotopic composition in highly depleted ultramafic rocks without a preconcentration step is presented. The samples were first digested using the conventional HF + HNO₃ + HClO₄ method, followed by the complete digestion of chromite in the samples using HClO₄ at 190–200 °C and then complete dissolution of fluoride formed during the HF decomposition step using H₃BO₃. These steps ensured the complete digestion of the ultramafic rocks. The rare earth elements (REEs) were separated from the sample matrix using conventional cation-exchange chromatography; subsequently, Sm and Nd were separated using the LN columns. Neodymium isotopes were determined as NdO⁺, whereas Sm isotopes were measured as Sm⁺, both with very high sensitivity using single W filaments with TaF₅ as an ion emitter. Several highly depleted ultramafic rock reference materials including USGS DTS-1, DTS-2, DTS-2b, PCC-1 and GSJ JP-1, which contain extremely low amounts of Sm and Nd (down to sub ng g^-1 level), were analysed, and high-precision Sm and Nd concentration and Nd isotope data were obtained. This is the first report of the Sm-Nd isotopic compositions of these ultramafic rock reference materials except for PCC-1.

Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, BP 10448, F-63000 Clermont-Ferrand, France CNRS, UMR 6524, LMV, F-63038 Clermont-Ferrand, France
IRD, R 163, LMV, F-63038 Clermont-Ferrand, France

We carried out stepwise dissolutions of four primitive enstatite chondrites (EC) belonging to the EH subgroup. Large Nd isotope anomalies are found in the most refractory phases, dissolved using strong acids. Residues are characterized by excesses in ¹⁴²Nd and deficits in ¹⁴⁵Nd, ¹⁴⁸Nd and ¹⁵⁰Nd isotopes. The Nd anomalies measured in the ALHA77295 residue are even greater than those measured in the Murchison carbonaceous chondrite (CC) using a similar analytical technique (Qin et al., 2011). Once corrected for a common Sm/Nd evolution, the ¹⁴²Nd excess in the ALHA77295 residue is equal to 700 ppm relative to the terrestrial standard value. The Nd isotope patterns measured in EC and CC residues can be adjusted to coincide by adding a small amount of an s-process-rich carrier phase such as SiC and 0.075% is required to fit the ALHA7795 residue. Small isotope differences still persist between these residues even if they can be considered similar within error. In enstatite chondrites, residues have a deficit in ¹⁵⁰Nd similar to or smaller than that measured in ¹⁴⁸Nd, whereas in SiC extracted from carbonaceous chondrites or in whole rock, the deficit in ¹⁵⁰Nd is always greater than that in ¹⁴⁸Nd. Moreover in a binary ¹⁴²Nd–¹⁴⁸Nd diagram, the best-fit lines obtained for leachates and residues from carbonaceous chondrites and enstatite chondrites have slightly different slopes. For the same ¹⁴⁸Nd/¹⁴⁴Nd ratio, the anomalous component in an enstatite chondrite has a higher ¹⁴²Nd/¹⁴⁴Nd ratio compared to carbonaceous chondrites, a feature already observed at the whole rock scale. Our results suggest that different chondrite groups sample different reservoirs of presolar grains formed in different environments. Assuming that the carrier of this anomalous component measured in residues of enstatite chondrites are SiC, our results may suggest that different meteorite parent bodies sample reservoirs of presolar SiC formed in different stellar environments. This could explain why ALHA77295, the sample which is the most enriched in presolar grains, has a bulk ¹⁴²Nd isotope composition similar to the terrestrial value. Further investigation of enstatite chondrites is needed to test whether the isotope composition of the most refractory phases is similar to that measured in carbonaceous chondrites and in particular the ¹⁴⁴Sm that is a p-process isotope only. Finally this study highlights the difficulty of interpreting the ¹⁴²Nd excess in terrestrial samples relative to chondrites since incomplete mixing of nucleosynthetic material in the solar nebula creates significant ¹⁴²Nd variation, as shown by ALHA77295.

¹Istituto di Astrofisica e Planetologia Spaziali, INAF, Rome, Italy
²Bear Fight Institute, 22 Fiddler’s Road, Box 667, Winthrop, Washington 98862, USA
³Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095–1567, USA
⁴NASA Lunar Science Institute, Southwest Research Institute, 1050 Walnut St, Suite 300, Boulder, Colorado 80302, USA
⁵NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
⁶Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996–1410, USA
⁷Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA
⁸Jet Propulsion Laboratory, Pasadena, California 91109, USA
⁹University of Maryland, College Park, Maryland 20742–2421, USA

^aPlanetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, USA
^b Max-Planck-Institute for Solar System Research, 37191 Katlenburg-Lindau, Germany
^c Institute of Geological Sciences, Freie Universitaet Berlin, 12249 Berlin, Germany
^d Astrogeology Science Center, USGS, Flagstaff, AZ 86001, USA
^e Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
^f Department of Geography, University of Winnipeg, Manitoba, Canada
^g Department of Space Studies, University of North Dakota, Room 518, Box 9008, Grand Forks, ND 58202, USA h Astromaterials Research Office, NASA Johnson Space Center, Houston, TX 77058, USA
ⁱ NASA Goddard Spaceflight Center, Greenbelt, MD 20771, USA
^j Institute of Geophysics and Planetary Physics, University of California Los Angeles, Los Angeles, CA 90095, USA
^k Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
^l Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionale di Astrofisica, Rome, Italy

The primitive achondrites provide a window into the initial melting of asteroids in the early solar system. The brachinites are olivine-dominated meteorites with a recrystallized texture that we and others interpret as evidence of partial melting and melt removal on the brachinite parent body. We present a petrologic, thermodynamic and experimental study of the brachi- nites to evaluate the conditions under which they formed and test our hypothesis that the precursor material to the brachinites was FeO-rich compared to the precursors of other primitive achondrites. Petrologic analysis of six brachinites (Brachina, Allan Hills (ALH) 84025, Hughes 026, Elephant Moraine (EET) 99402, Northwest Africa (NWA) 3151, and NWA 4969) and one brachinite-like achondrite (NWA 5400) shows that they are meteorites with recrystallized texture that are enriched in olivine (≥80 vol.%) and depleted in other minerals with respect to a chondritic mineralogy. Silicates in the brachinites are FeO-rich (Fa_32–36). Brachinite-like achondrite Northwest Africa 5400 is similar in mineralogy and texture to the brachinites but with a slightly lower FeO-content (Fa₃₀). Thermodynamic calculations yield equilibration temperatures above the Fe,Ni–FeS cotectic temperature (~950 °C) for all meteorites studied here and temperatures above the silicate eutectic (~1050 °C) for all but two. Brachina formed at an fO₂ of ~IW -1, and the other brachinites and NWA 5400 formed at IW -1. All the mete- orites show great evidence of formation by partial melting having approximately chondritic to depleted chondritic mineral- ogies, equilibrated mineral compositions, and recrystallized textures, and having reached temperatures above that required for melt generation. In an attempt to simulate the formation of the brachinite meteorites, we performed one-atmosphere, gas-mix- ing partial melting experiments of R4 chondrite LaPaz Ice Field 03639. Experiments at 1250 °C and an oxygen fugacity of IW -1 produce residual phases that are within the mineralogy and mineral compositions of the brachinites. These experi- ments provide further evidence for the formation of brachinites as a result of partial melting of a chondritic precursor similar in mineralogy and mineral compositions to the R chondrites.