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, hopper, vermiform and chain textures. These grains of chromian spinel are larger than those found in most basalts. Many crystals show significant zoning in Cr/(Cr + Al), and the variation in Cr/(Cr + Al) with respect to Fe 2+ /(Fe 2+ + Mg) is distinctive for each sample. The four samples are relatively primitive, with between 9 and 10 wt.% MgO in the glass. One very distinctive 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 pressure or magma mixing. The variety of chromian spinel textures, including the vermiform rim, has been duplicated in an experimental run using the sample from the East Pacific Rise. The sample was held at 1225°C for 67 hours and then cooled slowly over 8 hours to 1147°C. We conclude that the vermiform texture and the other chromian spinel textures in the experimental run and the lavas are growth textures and not reaction textures. The change in growth habit that produces the vermiform rim is interpreted as diffusion-controlled growth, leading to what metallurgists refer to as cellular 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.% Cr 2 O 3 ) and in chromian spinel (30–50 wt.% Cr 2 O 3 ) at equilibrium. 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 Fe 2+ /(Fe 2+ + Mg) may have been close to equilibrium with the bulk melt before quenching. The chromian spinel crystallized over a span of hours to days before a rapid water quench upon eruption. The complex textures and zoning probably reflect the turbulent conditions during passage of the magma toward the Earth’s surface.