The Tafassasset Primitive Achondrite: Insights into Initial Stages of Planetary Differentiation

Abstract

Tafassasset is an exceptional meteorite that has been linked to both the CR chondrites and the primitive achondrites. Because previous evidence suggests it might be a primitive achondrite from a known chondrite type, we have undertaken a study of the petrology, geochemistry, and formation history of the meteorite. Tafassasset is predominantly FeO-rich olivine (~58%) yet contains abundant Fe, Ni-metal (~10 vol.%) and sulfide (~3 vol.%). Other phases include high- and low-Ca pyroxene, plagioclase, chromite, and phosphate. It has a recrystallized texture, containing equigranular grains that often meet at 120° triple junctions. There are no relict chondrules in the thin sections examined, although they have been reported previously. Electron microprobe analyses reveal homogeneous olivine (Fa28.6), both low-and high-Ca pyroxene (Fs23.6Wo3.7 and Fs12.2Wo39.3±1), a range of plagioclase composition (An23-47), Fe, Ni-metal (with 5.3-36.6 wt% Ni and 0.1-0.8 wt% Co), troilite, chromite, and Ca-phosphate. Bulk composition analyses reveal two chips depleted in refractory lithophile and some siderophile elements compared to CI chondrites. Exceptions are enrichments in Fe, Ni and Co. A third chip is essentially chondritic in bulk composition. Different stones of the meteorite have slightly different oxygen isotope composition, yet all lie in the CR chondrite trend with one in the acapulcoite-lodranite field. Thermodynamic calculations show that Tafassasset equilibrated at a temperature above the Fe, Ni-FeS eutectic and at an oxygen fugacity of ~IW-1. The texture, heterogeneous distribution of mineral phases, plagioclase composition, two-mineral closure temperatures, and bulk composition all provide evidence that Tafassasset partially melted on its parent body. A comparison with the CR chondrites, the brachinites, and two anomalous achondrites indicates that Tafassasset is most similar to ungrouped primitive achondrites Lewis Cliff (LEW) 88763 and Divnoe, and to the brachinites in overall petrography, modal mineralogy, mineral compositions, oxidation state, and bulk composition. The comparison also excludes the possibility that Tafassasset formed by partial melting of a CR chondrite. Tafassasset is a primitive achondrite and likely evolved on a parent body that experienced incomplete melting, never reached isotopic homogeneity, and was from the same oxygen isotopic reservoir as the CR chondrite parent body.