Thermal Metamorphism of Primitive Meteorites—XII. The Enstatite Chondrites Revisited

Abstract

Environmental Context.The first Solar System material condensed 4.567 billion years ago, rapidly forming planetesimals—solid bodies that might combine to form planets (accretion) or survive as asteroidal meteorites. Earth’s main accretion ended within the next 30 million years, but subsequent high temperatures essentially erased evidence of this history. However, heating in these early episodes produced effects uniquely recorded by 14 volatile trace elements. The volatile element composition of chondritic meteorites, whose parent material formed closest to Earth, may thus provide important information about early planetesimal evolution. Abstract.We report data for 14 trace and ultratrace elements—Au, Co, Sb, Ga, Rb, Ag, Cs, Te, Zn, Cd, Bi, Tl, In (ordered by increasing putative nebular volatility)—in 13 enstatite (E) chondrites recovered from Antarctica and two E inclusions in the Kaidun polymict breccia that fell in 1980. These data, determined by radiochemical neutron activation analysis (RNAA), essentially double the amount of information known for E chondrites, whose parent materials formed closest to the Sun in the chondrite-forming nebular region. We discuss here the data for all 29 samples studied. The meteoritic suite studied here includes both representatives of previously rare types—like high-iron EH3 and EH5 individuals—but also unique individuals and previously unknown low-iron, EL3, chondrites. Prior hypothetical assertions by others are corrected by the new data. Volatile element contents of EL3 and EH3 chondrites are variable, but comparable, like those of type 3 ordinary chondrites (i.e. H3, L3, and LL3). Volatile element contents of EH4 chondrites are at least as high as those of the E3 types, in contrast to the lower contents of H4, L4, and LL4 types. Compositionally, E3,4 chondrites reflect only nebular condensation and/or accretion processes. Volatiles in E5 and E6 chondrites—whether of EH, EL or unique ones—are depleted relative to cosmic (i.e. CI1) or E3,4 chondrite abundances. The evidence indicates that E5,6 chondrites compositionally reflect vaporization and loss of volatiles during open-system, thermal metamorphism of their parent(s); this may have been the terrestrial environment during Earth’s formation from early planetesimals. Compositional differences between Antarctic E5,6 chondrites and contemporary falls probably do not reflect weathering during the long residence of these chondrites in Antarctica. They might reflect differences in the starting compositions and/or metamorphic conditions in the parent(s).