Recycling (?): Relict Spinels (?) in Type B Calcium-Aluminum-Rich Inclusions

Abstract

Type B calcium-aluminum-rich inclusions (CAIs) can be thought of as a type of chondrule despite obvious differences in size, composition, and texture. Nevertheless, igneous CAIs likely experienced similar thermal histories. A major constraint on the thermal history of chondrules is that they have been recycled, mainly supported by the presence relict grains. Although well known that igneous CAIs experienced at least two melting events (counting Wark-Lovering rims), the identification of relict grains has been problematic. The best candidate for relict grains within type B CAIs is spinel. Spinel is the liquidus phase with complete melting at approximately l50°C above type B peak melting temperatures (~1400°C). Consequently, relict spinels, either from a previous generation of CAIs or direct nebular condensate spinels, are stable at the inferred maximum heating temperature of type B CAIs. Minor- and trace-element concentrations can be used to distinguish relict grains from those that crystallized from an initially homogeneous liquid.