THE FLASH MELTING OF CHONDRULES: AN EXPERIMENTAL INVESTIGATION INTO THE MELTING HISTORY AND PHYSICAL NATURE OF CHONDRULE PRECURSORS - HEATING TIME AND COOLING CURVES

Abstract

Constraints placed on chondrule formation have largely been generated from experiments which use a long duration, below liquidus isothermal melting (minutes to hours) rather than a short duration, above liquidus flash melting event (seconds to minutes). In this paper we examine how a short duration, superliquidus heat pulse can produce chondrule textures. By incompletely melting material with a type of flash melting we show that the maximum temperature limit of chondrule formation was approximately 2100°C, almost 400°C higher than previously constrained. Previous experiments also have not studied the effect of variations in precursor grain size on the formation of chondrule textures. For this reason we simultaneously investigate the effect of variations in the grain size of a starting composition on the formation of chondrule textures. We show how MgO-rich (Type IA) chondrules and other fine-grained chondrules could only have been formed from the incomplete melting of a rather uniformly grain sized precursor of less than 63μm. Because fine-grained, MgO-rich chondrules have the some of the highest chondrule liquidus temperatures, we proposed that these types of textures define a minimum melting temperature for chondrule formation.