PII S0016-7037(02)00885-2 A plagioclase–olivine–spinel–magnetite inclusion from Maralinga (CK): Evidence for sequential condensation and solid–gas exchange

Abstract—We report a detailed petrography, mineral chemistry, and trace element study of MaTroc, a large calcium–aluminum-rich inclusion (CAI) (5 2.5 mm) of irregular triangular shape. The inclusion has a zonal structure: The core consists of a porous plagioclase–olivine–Ca-rich pyroxene intergrowth with subordinate apatite. Its texture is meta-gabbro-like, similar to other plagioclase–olivine inclusions (POIs). The mantle has variable thickness (0.1–1.5 mm) and consists of a compact symplectitic intergrowth of spinel (hercynite) and plagioclase with abundant dispersed magnetite, subordinate Ca-rich pyroxene, and traces of sulfides. The thin (5–50 m) discontinuous crust of MaTroc consists mainly of plagioclase with some olivine and magnetite. The Mg-Fe phases of MaTroc are Fe-rich: olivine has Fa33.2 and high NiO content, similar to that in the host rock, Ca-rich pyroxene has much lower TiO2 and Cr2O3 contents than that of the host chondrite, and plagioclase is An55–An74. Magnetites have variable compositions, are poorer in Al2O3 and Cr2O3 and richer in NiO than those in the host. Spinels have also variable compositions, rich in FeO, NiO, and ZnO. Despite their different mineralogy, both core and mantle have bulk trace element abundances similar to those in average group II CAIs. However, the mantle is richer in Nb and U and poorer in Eu, Be, B, Sr, and Li than the core. All minerals have high trace element contents. Minerals in the core show signs of incomplete equilibration of trace elements within and between them. Mantle minerals are far from equilibrium with eac