Experiments quantifying elemental and isotopic fractionations during evaporation of CAI-like melts in low-pressure hydrogen and in vacuum: Constraints on thermal processing of CAIs in the protoplanetary disk

It is widely believed that the precursors of coarse-grained CAIs in chondrites are solar nebula condensates that were later reheated and melted to a high degree. Such melting under low-pressure conditions is expected to result in evaporation of moderately volatile magnesium and silicon and their mass-dependent isotopic fractionation. The evaporation of silicate melts has been extensively studied in vacuum laboratory experiments and a large experimental database on chemical and isotopic fractionations now exists. Nevertheless, it remains unclear if vacuum evaporation of CAI-like melts adequately describes the evaporation in the hydrogen-rich gas of the solar nebula. Here we report the results of a detailed experimental study on evaporation of a such melt at 1600 degrees C in both vacuum and low-pressure hydrogen gas, using 1.5- and 2.5-mm diameter samples. The experiments show that although at 2 x 10(-4) bar H-2 magnesium and silicon evaporate similar to 2.8 times faster than at 2 x 10(-5) bar H-2 and similar to 45 times faster than in vacuum, their relative evaporation rates and isotopic fractionation factors remain the same. This means that the chemical and isotopic evolutions of all evaporation residues plot along a single evaporation trajectory regardless of experimental conditions (vacuum or low-P-H2) and sample size. The independence of chemical and isotopic evaporation trajectories on P-H2 of the surrounding gas imply that the existing extensive experimental database on vacuum evaporation of CAI-like materials can be safely used to model the evaporation under solar nebula conditions, taking into account the dependence of evaporation kinetics on P-H2. The experimental data suggest that it would take less than 25 min at 1600 degrees C to evaporate 15-50 parts per thousand of magnesium and 5-20% of silicon from a 2.5-mm diameter sample in a solar nebula with P-H2 similar to 2 x 10(-4) bar and to enrich the residual melt in heavy magnesium and silicon isotopes up to delta Mg-25 similar to 5-10 parts per thousand and delta Si-29 similar to 2-4 parts per thousand. The expected chemical and isotopic features are compatible to those typically observed in coarse-grained Type A and B CAIs. Evaporation for similar to 1 h will produce delta Mg-25 similar to 30-35 parts per thousand and delta Si-29 similar to 10-15 parts per thousand, close to the values in highly fractionated Type F and FUN CAIs. These very short timescales suggest melting and evaporation of CAI precursors in very short dynamic heating events. The experimental results reported here provide a stringent test of proposed astrophysical models for the origin and evolution of CAIs. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

Experimental results show that the evaporation rates and isotopic fractionation factors of magnesium and silicon remain consistent under vacuum and low-pressure hydrogen gas conditions. This suggests that the existing vacuum evaporation experimental database can be used to model evaporation under solar nebula conditions.