Geochemistry of Trace Elements in Minerals of Porphyritic and Nonporphyritic Chondrules from Equilibrated Ordinary Chondrites

Abstract

The paper presents the results of SIMS and EPMA studies of silicate minerals and bulk compositions (SEM-EDS) of porphyritic and nonporphyritic chondrules from equilibrated ordinary chondrites. The trace-element distribution in olivine, low-Ca pyroxene, and mesostasis of porphyritic and nonporphyritic chondrules in the equilibrated ordinary chondrites reflects heterogeneous conditions of chondrule-forming processes in the protoplanetary disk and provides insight in their evolution. The porphyritic chondrules are composed of olivine with the low content of Y and Yb relative to nonporphyritic chondrules. The enrichment of low-Ca pyroxene from porphyritic chondrules in trace elements correlates with the amount of pyroxene in the chondrule. Olivine composition in granular chondrules is close to that of porphyritic chondrules, but low-Ca pyroxene is distinguished by the high content of Y, Ti, Sr, Ba, V, and REE compared to all other chondrules. Barred chondrules are characterized by olivine enriched in trace elements (Zr, Y, Ti, Ba, Cr, and HREE) and low-Ca pyroxene highly depleted in trace elements (Zr, Y, Nb). The pyroxene of radial chondrules is enriched in Nb, Sr, and Ba. Trace elements in olivine and low-Ca pyroxene indicate the formation of the porphyritic and granular chondrules in a stable region of the protoplanetary disk, which is responsible for low-temperature heating and slow cooling of a chondrule. Trace element enrichment of olivine from the barred chondrules testifies strong heating of a precursor material and rapid cooling of the chondrule melt. Mineral composition of radial chondrules demonstrate rapid cooling of a low-temperature melt depleted in Mg and trace elements. High melt temperatures of barred chondrules and rapid cooling of nonporphyritic chondrules indicate their formation in an unstable region of the protoplanetary disk. Trace elements in silicate minerals of porphyritic and nonporphyritic chondrules reflect their formation as a result of melting of precursor minerals.