Growth forms and composition of chromian spinel in morb magma: Diffusion-controlled crystallization of chromian spinel

Microphenocrysts of chromian spinel in glass from an East Pacific Rise lava and three Icelandic subglacial lavas show a variety of complex skeletal, h opper, vermiform and chain textures. These grains of chromian spinel are la rger than those found in most basalts. Many crystals show significant zonin g in Cr/(Cr + Al), and the variation in Cr/(Cr + Al) with respect to Fe2+/( Fe2+ + Mg) is distinctive for each sample. The four samples are relatively primitive, with between 9 and 10 wt.% MgO in the glass. One very distinctiv e texture, a core surrounded by a vermiform rim, has been interpreted by a number of researchers as a reaction texture caused either by changes in pre ssure or magma mixing. The variety of chromian spinel textures, including t he vermiform rim, has been duplicated in an experimental run using the samp le from the East Pacific Rise. The sample was held at 1225 degreesC for 67 hours and then cooled slowly over 8 hours to 1147 degreesC. We conclude tha t the vermiform texture and the other chromian spinel textures in the exper imental run and the lavas are growth textures and not reaction textures. Th e change in growth habit that produces the vermiform rim is interpreted as diffusion-controlled growth, leading to what metallurgists refer to as cell ular growth. Chromian spinel is susceptible to diffusion-controlled growth because of the very large difference in concentration of Cr in the melt (0. 02-0.06 wt.% Cr2O3) and in chromian spinel (30-50 wt.% Cr2O3) at equilibriu m. Although the chromian spinel in the glass of each natural sample shows a large variation in Cr/(Cr + Al), and thus was not in equilibrium with the bulk melt, its Fe2+/ (Fe2+ + Mg) may have been close to equilibrium with th e bulk melt before quenching, The chromian spinel crystallized over a span of hours to days before a rapid water quench upon eruption. The complex tex tures and zoning probably reflect the turbulent conditions during passage o f the magma toward the Earth's surface.