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

Chondrules(1-5) are millimetre-sized, solidified melt spherules formed in t he solar nebula by an early widespread heating event of uncertain nature(6- 8). They were accreted into chondritic asteroids, which formed about 4.56 b illion years ago and have not experienced melting or differentiation since that time. Chondrules have diverse chemical compositions, corresponding to liquidus temperatures(1,4,9) in the range 1,350-1,800 degrees C. Most chond rules, however, show porphyritic textures (consisting of large crystals in a distinctly finer grained or glassy matrix), indicative of melting within the narrow range 0-50 degrees C below the liquidus(9,10). This suggests an unusual heating mechanism for chondrule precursors, which would raise each individual chondrule to just the right temperature (particular to individua l bulk composition) in order to form porphyritic textures. Here we report t he results of isothermal melting of a chondritic composition at nebular pre ssures. Our results suggest that evaporation stabilizes porphyritic texture s over a wider range of temperatures below the liquidus (about 200 degrees C) than previously believed, thus removing the need for individual chondrul e temperature buffering. In addition, we show that evaporation explains man y chondrule bulk and mineral compositions that have hitherto been difficult to understand.