Trace element geochemistry of ordinary chondrite chondrules: the type I/type II chondrule dichotomy

TL;DR

This study analyzes trace element concentrations in chondrules from LL3 ordinary chondrites, revealing differences between type I and II chondrules related to their formation conditions and cooling rates, providing insights into early solar system processes.

Contribution

It presents new trace element data for chondrules from LL3 chondrites and links these to formation environments and cooling rates, highlighting differences between type I and II chondrules.

Findings

Type II chondrules have lower REE concentrations than type I.

Type II chondrules formed under faster cooling rates (>10 K/h).

Olivine in both chondrule types contains appreciable Na concentrations.

Abstract

We report trace element concentrations of silicate phases in chondrules from LL3 ordinary chondrites Bishunpur and Semarkona. Results are similar to previously reported data for carbonaceous chondrites, with rare earth element (REE) concentrations increasing in the sequence olivine < pyroxene < mesostasis, and heavy REE (HREE) being enriched by 1-2 orders of magnitude (CI-normalized) relative to light REE (LREE) in ferromagnesian silicates, although no single olivine with very large LREE/HREE fractionation has been found. On average, olivine in type II chondrules is poorer in refractory lithophile incompatible elements (such as REE) than its type I counterpart by a factor of ~2. This suggests that olivine in type I and II chondrules formed by batch and fractional crystallization, respectively, implying that type II chondrules formed under faster cooling rates (> ~ 10 K/h) than type I chondrules. Appreciable Na concentrations (3-221 ppm) are measured in olivine from both chondrule types; type II chondrules seem to have behaved as closed systems, which may require chondrule formation in the vicinity of protoplanets or planetesimals. At any rate, higher solid concentrations in type II chondrule forming regions may explain the higher oxygen fugacities they record compared to type I chondrules. Type I and type II chondrules formed in different environments and the correlation between high solid concentrations and/or oxygen fugacities with rapid cooling rates is a key constraint that chondrule formation models must account for.