Vapor-Melt Exchange -- Constraints on Chondrite Formation Conditions and Processes

TL;DR

This paper investigates the formation conditions of chondrules in meteorites, focusing on volatile element retention and isotopic evidence to constrain the high-temperature, high-density environment of early solar system processes.

Contribution

It provides new constraints on chondrule formation conditions by analyzing volatile element abundances and isotopic data, challenging previous assumptions about open-system behavior.

Findings

Chondrules retained volatile elements like Na and Fe at high temperatures.

Isotopic evidence suggests minimal mass fractionation, implying high solid densities.

Late silica condensation conflicts with volatile element retention evidence.

Abstract

The bulk volatile contents of chondritic meteorites provide clues to their origins. Matrix and chondrules carry differing abundances of moderately volatile elements, with chondrules carrying a refractory signature. At the high temperatures of chondrule formation and the low pressures of the solar nebula, many elements, including Na and Fe, should have been volatile. Yet the evidence is that even at peak temperatures, at or near the liquidus, Na and Fe (as FeO and Fe-metal) were present in about their current abundances in molten chondrules. This seems to require very high solid densities during chondrule formation to prevent significant evaporation. Evaporation should also be accompanied by isotopic mass fractionation. Evidence from a wide range of isotopic systems indicates only slight isotopic mass fractionations of moderately vola-tile elements, further supporting high solid densities. However, olivine-rich, FeO-poor chondrules commonly have pyroxene-dominated outer zones that have been interpreted as the prod-ucts of late condensation of SiO2 into chondrule melts. Late condensation of more refractory SiO2 is inconsistent with the apparent abundances of more volatile Na, FeO and Fe-metal in many chondrules. Despite significant recent experimental work bearing on this problem, the conditions under which chondrules behaved as open systems remain enigmatic.