U/pb and Cr Isotope Study of the Tafassasset

The Tafassasset meteorite which was found the Tenere dessert, Niger, in 2000 and 2001 consists of 26 pieces with a total weight of ~110 kg. The well-preserved meteorite has been studied by several groups [1-4], but until today no consensus on the meteorite's classification was obtained. Reasons for this difficulty are:-texture and mineral abundances vary considerably,-different individual stones were studied by different groups and although a few stones were paired some data, i.e. oxygen isotope compositions are contradictory [1, 4],-petrographic observations on the same samples are debated. Tafassasset shows an equilibrated and recrystallized texture. It contains ~ 8 vol % metal, olivine represents the dominant mineral phase; other mineral phases are low-Ca-, Ca-pyroxene and feldspar. The meteorite experienced thermal metamorphism that led to the formation of secondary minerals, i.e. phosphates and chromite. Tafassasset has been classified as:-a equilibrated CR chondrite,-an ungrouped primitive achondrite and-having strong affinities with brachinites. The presence of relict chondrules and oxygen isotopic compositions close to CR chondrites have been arguments to classify it as a CR chondrite [1], while refractory element ratios depleted in respect to CI chondrites, Al/Mg and Mn/Mg more like primitive achondrites led [2] to suggest the classification as primitive achondrite. Mineralogical observations led [3] to put forward the similarities to brachinites. In order to obtain chronological information and to gain information on the origin and thermal history of this unusual meteorite we started an U/Pb and Cr isotopic investigation of Tafassasset. Several fragments representing different lithologies (fine grained metal-poor feldspar-rich fragments, metal-rich pyroxene-rich fragments) were analyzed. All fragments studied so far, show very low Pb, U and Th concentrations. They are characterized by radiogenic Pb isotopic compositions with 206 Pb/ 204 Pb ratios ranging from 43 up to 215. The corresponding Pb/Pb model ages are old, they lie between 4.555 and 4.564 Ga. The Pb/Pb model age of the fragment that is characterized by the most radiogenic Pb isotopic composition corresponds to 4.563 ± 0.001 Ga. A bulk rock Cr isotopic analysis displays excesses of 0.29 ± 0.08 ε and 1.37 ± 0.27 ε for 53 Cr and 54 Cr respectively. These values agree within error with the Cr isotope values published for Renazzo [5]. Tafassasset thus is the first occurrence of a non-carbonaceous meteorite showing a positive 54 Cr isotopic anomaly.