MINERALOGIC AND Δ17O-ε54Cr ISOTOPIC COMPOSITION OF BRACHINITE NORTHWEST AFRICA 13489: A NEW METACHONDRITE WITH ‘CX’ CHONDRITE AFFINITY?

In recent years, the number of meteorites representing highly recrystallized products, filling the gap between chondritic protoliths and primitive achondrites, has increased. These meteorites, nowadays identified as met- achondrites, show compositional affinity with different meteorite groups, such as H ordinary chondrites [1], CV-CR- CO carbonaceous chondrites [1,2,3,4,5] and acapulcoite-lodranite/winonaite [8], by means of oxygen and chromium iostopes. Moreover, detailed isotopic studies on Cr-Ti-O systems highlighted the occurrence of common reservoirs for carbonaceous chondrites and some differentiated achondrite meteorites, i.e., irons and silicate-irons [9,10,11], thus outlining the origin from common parent bodies. A further finding is the existence of ungrouped chondrites/achon- drites showing carbonaceous affinity, but not related to any previously recognized groups [6,7,11], allowing to study possible new parent bodies unrepresented in meteorite collections. While studying possible genetic relationships be- tween brachinites and ungrouped achondrites with brachinite-like affinity [12], NWA 13489 brachinite [13] revealed a metachondrite nature possibly consistent with ‘CX’ chondrites [14].