Dark Inclusions: Clasts of Cm-type Material within Allende

Introduction: Dark inclusions are irregular, cmsized clasts of fine grained material found within chondrites such as Allende. They can be classified into 4 groups (A, A/B, B, and C) based primarily on the presence of components such as small scale chondrules [e.g. 1-2]. Alteration is prevalent with many dark inclusions being composed entirely of secondary minerals [3] and often surrounded by a complex, threelayered rim [4]. Any record of the dark inclusion precursor has been masked by this complex alteration. Investigation of the trace and minor element signature of these components allows an insight to the precursor dark inclusion characteristics together with an indication of where and when – be it nebula or parent body – the subsequent alteration took place. Origin(s) of dark inclusions: Theories on the origin and subsequent processing of dark inclusions postulate that they may be anything from aggregates which accreted and were subsequently processed within the nebula [e.g. 5-6] to fragments of CV3 parent bodies processed to a different extent to that of the surrounding host rock [e.g. 1-2, 7-8]. Problems arise when attempting to explain the variation in dark inclusion lithologies via a nebula origin, whilst a CV clast origin does not explain issues such as why dark inclusion components are so much smaller than those located in the surrounding host rock. Alteration of dark inclusions: All CV3’s have been altered. Dark inclusions within these rocks also display evidence for extensive alteration. Chondrule pseudomorphs [9], fibrous fayalitic olivine thought to be representative of dehydrated phyllosilicates [1], and extensive secondary mineralogy [10], are all indicative of aqueous alteration. This observation is supported by evidence for the movement of fluids by the presence of nepheline veins [11], together with a depletion in 16O relative to the surrounding meteorite [2]. Evidence for heating is also prevalent in the form of Ca, Fe-rich aggregates (CFA’s) [e.g. 12], and high temperature rims [4] (which indicate heating too great to have occurred within the Allende parent body [13]). Where and when this alteration took place is still debated.