Cooling and crystallization of komatiite flows.

Despite their $\sim$2710 Ma age, thin komatiite lava flows in Munro township, NE Ontario have sufficiently well-preserved primary structures, textures, mineralogy, and whole-rock geochemistry to constrain the physical processes of cooling and crystal growth. Seawater convection drove a cooling and fracturing front through the upper surface and was the dominant mechanism of heat loss, leading to pronounced textural asymmetry within flows. Large plate-like crystals of olivine in the upper layers of flows grew by constrained crystallization arising from strong thermal pradients and the high lattice thermal conductivity and near-infrared transparency of olivine (3-5 times greater than that of the surrounding melt). Such crystals exhibit a very strong preferred orientation with the a crystallographic axis perpendicular to flow surfaces. Major and minor-element zoning within olivine crystals is closely modeled by fractional crystallization from the bulk liquid, and whole-rock geochemical variations within flows by crystal fractionation of highly magnesian olivine. The high chromium content of olivine is of mineralogical and technological interest, and is due to two substitutional mechanisms: $\sp{\lbrack 6\rbrack}$Cr$\sp{3+} + \sp{\lbrack 4\rbrack}$Al$\sp{3+} \rightleftharpoons\ \sp{\lbrack 6\rbrack}$Mg$\sp{2+} + \sp{\lbrack 4\rbrack}$Si$\sp{4+}$ and 2 $\sp{\lbrack 6\rbrack}$Cr$\sp{3+} + \sp{\lbrack 6\rbrack}\square \rightleftharpoons 3 \sp{\lbrack 6\rbrack}$Mg$\sp{2+}.$ Dendritic chromian spinel crystals nucleated heteroepitaxially on olivine substrates; the crystallographic relationship between the phases is (111) $\sb{\rm sp} \parallel$ (100) $\sb{\rm ol}$, (110) $\sb{\rm sp}\ \parallel$ (001) $\sb{\rm ol}\ \parallel$, and (211) $\sb{\rm sp}\ \parallel$ (010) $\sb{\rm ol}$. The Cr-spinel crystals have features typical of metallic, ionic, and organic crystal dendrites, but rarely found in minerals. Immobility of Mg and Si and a systematic loss of Ca in these altered rocks is due to a serpentinization reaction buffered by the desilication of glass or fine-grained plagioclase in the groundmass. Serpentinization occurred under conditions of low confining pressure, caused up to 30% expansion of the basal portions of flows, and predated tectonism at $\sim$2700 Ma