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.