METASTABLE GROWTH PATTERNS IN SOME TERRESTRIAL AND LUNAR ROCKS

Abstract

With reference to new information on the textures in some terrestrial mafic and ultramafic igneous rocks of Tertiary age, textural patterns attributable to metastable crystallization are described, illustrated and reviewed. The igneous textural terminology, employed in the descriptive text, is reviewed and the use of certain new terms is advocated. Compared with these textural patterns are the skeletal crystal growth and intergrowth in igneous rock samples obtained, mainly by the Apollo 12 Mission, from the moon. Both orientated and unorientated metastable growth yield characteristic dendritic or skeletal olivine crystals, and a texture which has been termed “intrafasciculate” is also attributed to metastable crystallization of plagioclase and pyroxene. The three principal terrestrial textures defined and described are ‘Willow Lake’ texture, intrafasciculate texture and harrisitic texture. The term “thermotactic” is preferred to ‘Willow Lake’ or ‘crescumulate’ to define a texture with orientation of crystal growth in the direction of the thermal gradient Attention is drawn to the importance in petrogenesis of such metastable crystallization, as inferred from crystal growth and textural observations. It is apparent that analogous ‘metastable’ textures occur through a remarkable range in geological context More detailed work is advocated on the pegmatitic eucrites and peridotites of Rhum. Since the evidence suggests that these rocks represent hydrous ultrabasic liquids, not cumulates, the conditions governing the growth of olivine in them must differ from those of any lunar analog