Mineralogy and petrography of HAL, an isotopically-unusual Allende inclusion

Abstract

A detailed mineralogical study of HAL was initiated to elucidate the origin of this inclusion because Lee et al. (1979) had found large mass fractionation effects and small nuclear effects in its Ca isotopic composition, but no 26Mg excesses in samples of it with very high 27Al 24Mg ratios. HAL's 1.0 mm radius interior consists almost entirely of three hibonite crystals and is surrounded by a 2.0 mm thick, multi-layered rim. The first layer, called the black rim, is black and compact, resembles a devitrified glass and contains an anisotropic Al-Fe-oxide similar to hercynite in composition. This is followed by a friable rim sequence, layer I of which is predominantly feldspathoids with minor anorthite, Ti-Fe-oxide and Al-Fe-oxide. Layer II contains abundant perovskite, plus grossular, andradite and pyroxene in addition to the minerals of layer I. Layer III is mostly Ca-phosphate, possibly hydroxy-apatite and perovskite. Layer IV is rich in hibonite, Al-Fe-oxide, perovskite, nepheline and the two garnets, lacks Ca-phosphate but contains traces of a Ti-Sc-Zr-oxide. Layer V is rich in Al-Fe-oxide, pyroxene, nepheline, the two garnets and olivine whose crystals display peculiar rectangular cross-sections. The black rim does not completely surround the hibonite core. Sectors of the friable rim exist where layer I is missing and where the mineralogy of adjacent layers is no different from that of the same layers in other sectors. Pentlandite, nickel-iron and barrel-shaped olivine crystals, minerals typical of the matrix of Allende and found nowhere else in HAL, are found in layer V and increase in abundance toward its exterior, as if grains of these phases accreted together with the other minerals of layer V. This layer also contains alternating olivine-rich and garnet-, pyroxene-rich bands, resembling rhythmic layering. For these reasons, we conclude that each of the layers of the friable rim formed by the accretion of an assemblage of condensate grains rather than by complete reaction of a precursor to HAL with a nebular gas. Thus, the unusual isotopic characteristics of HAL are thought to have been inherited from a nebular reservoir which was isotopically distinct from that which gave rise to the bulk of the material in Allende. HAL's mineralogical peculiarities indicate that its formation reservoir was also chemically distinct from the latter one.