REE abundances in the matrix of the Allende (CV) meteorite: Implications for matrix origin

摘要

The trace element distributions in the matrix of primitive chondrites were examined using four least-contaminated matrix specimens from the polished sections of the Allende (CV) meteorite. Analysis of rare earth element (REE), Ba, Sr, Rb, and K abundances by isotope dilution mass spectrometry revealed that the elemental abundances of lithophile elements except for alkali metals (K, Rb) in the specimens of the Allende matrix studied here are nearly CI (carbonaceous Orgueil) chondritic (～1 * CI). Compared to refractory elements, all the matrix samples exhibited systematic depletion of the moderately volatile elements K and Rb (0.1-0.5 * CI). We suggest that the matrix precursor material did not carry significant amounts of alkali metals or that the alkalis were removed from the matrix precursor material during the parent body process and/or before matrix formation and accretion. The matrix specimens displayed slightly fractionated REE abundance patterns with positive Ce anomalies (CI-normalized La/Yb ratio = 1.32-1.65; Ce/Ce~* = 1.16-1.28; Eu/Eu~* = 0.98-1.10). The REE features of the Allende matrix do not indicate a direct relationship with chondrules or calcium-aluminum-rich inclusions (CAIs), which in turn suggests that the matrix was not formed from materials produced by the breakage and disaggregation of the chondrules or CAIs. Therefore, we infer that the Allende matrix retains the REE features acquired during the condensation process in the nebula gas.