A petrologic, thermodynamic and experimental study of brachinites: Partial melt residues of an R chondrite-like precursor

作者：Gardner Vandy Kathryn G^*; Lauretta Dante S; McCoy Timothy J

来源：Geochimica et Cosmochimica Acta, 2013, 122: 36-57.

DOI：10.1016/j.gca.2013.07.035

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 brachinites 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 (Fa(30)). Thermodynamic calculations yield equilibration temperatures above the Fe,Ni-FeS cotectic temperature (similar to 950 degrees C) for all meteorites studied here and temperatures above the silicate eutectic (similar to 1050 degrees C) for all but two. Brachina formed at an fO(2) of similar to IW, and the other brachinites and NWA 5400 formed at similar to IW - 1. All the meteorites show great evidence of formation by partial melting having approximately chondritic to depleted chondritic mineralogies, 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-mixing partial melting experiments of R4 chondrite LaPaz Ice Field 03639. Experiments at 1250 degrees C and an oxygen fugacity of IW - 1 produce residual phases that are within the mineralogy and mineral compositions of the brachinites. These experiments 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.

• 出版日期2013-12-1