Roger H. Hewins

THE FLASH MELTING OF CHONDRULES : AN EXPERIMENTAL INVESTIGATION INTO THE MELTING HISTORY AND PHYSICAL NATURE OF CHONDRULE PRECURSORS

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.

Evaporation in the young solar nebula as the origin of 'just-right' melting of chondrules

Chondrules are millimetre-sized, solidified melt spherules formed in the solar nebula by an early widespread heating event of uncertain nature. They were accreted into chondritic asteroids, which formed about 4.56 billion years ago and have not experienced melting or differentiation since that time. Chondrules have diverse chemical compositions, corresponding to liquidus temperatures in the range 1,350–1,800u2009°C. Most chondrules, however, show porphyritic textures (consisting of large crystals in a distinctly finer grained or glassy matrix), indicative of melting within the narrow range 0–50u2009°C below the liquidus. This suggests an unusual heating mechanism for chondrule precursors, which would raise each individual chondrule to just the right temperature (particular to individual bulk composition) in order to form porphyritic textures. Here we report the results of isothermal melting of a chondritic composition at nebular pressures. Our results suggest that evaporation stabilizes porphyritic textures over a wider range of temperatures below the liquidus (about 200u2009°C) than previously believed, thus removing the need for individual chondrule temperature buffering. In addition, we show that evaporation explains many chondrule bulk and mineral compositions that have hitherto been difficult to understand.

Nebular turbulence, chondrule formation, and the composition of the Earth

Abstract Cosmochemical fractionations have been traditionally explained by the segregation of fine condensate dust, yet nebular turbulence prevents this dust from separating from the gas. Dense chondrules may form accumulations in stagnant zones which can settle to the midplane and be accreted to planetesimals and terrestrial planets. This is supported by the observation that Type I (MgO-rich) chondrules are more similar to mantle peridotite from Earth in lithophile major and trace element abundances than bulk chondrites are.

Chondrule textures and precursor grain size: an experimental study

Chondrule formation appears to have been a major event in the early solar system, but chondrule properties do not allow us to distinguish between several possible formation mechanisms. The physical nature of the precursors, especially grain size, must affect the textures of the chondrules they yield when heated. We melted precursors of different grain sizes, including extremely fine-grained crystalline aggregates analogous to nebular condensates, to see whether objects resembling most natural chondrules can be crystallized. With one-minute heating and moderate cooling rates, the grain size of the charges depends directly on the grain size of the starting material, for temperatures up to very close to the liquidus temperature. A single rapid heating of condensate-like material thus produces very fine-grained chondrules, like dark-zoned chondrules, for a very wide range of peak temperatures. It is incapable of generating the observed textural distribution of chondrules, which are predominantly porphyritic. The simplest model for chondrules, a single heating of unmodified condensate material, therefore appears unrealistic. Coarse-grained chondrules might be formed from fine-grained precursors by extended heating with evaporation leading to coarsening, or by multiple reheating events, with higher temperatures in subsequent events. Otherwise an origin from annealed condensates, planetary rocks, or by condensation of liquid and crystals is required.