Colin H. Donaldson

An experimental investigation of olivine morphology

Olivine crystals can adopt ten types of shape. Experimental crystallization of eight rock melts shows that there is a systematic change from polyhedral or granular olivines → hopper olivines → branching olivines → randomly oriented chain olivines → parallel-growth chain olivines → chain+lattice olivines → plate or feather olivines, with increase in cooling rate and with increase in degree of supercooling. This sequence involves changes from complete to progressively less complete crystals and from equant habit to elongate bladed habit (c>a≫b) to tabular habit (a≃c ≫ b). The sequence is not affected by the phase relations of the melt. The larger the olivine content of a melt the slower the cooling rate at which a particular olivine shape grows, whereas the lower the melt viscosity, the greater the cooling rate. Irrespective of the melt composition, comparable crystal shapes grow at the same degrees of supercooling. By comparison of the shapes of olivine crystals in experiments with those in rocks of similar composition, it is possible to deduce the cooling rate through the olivine crystallization interval and the approximate degree of supercooling at which the olivine crystals nucleated and grew in the rocks. The various shapes of skeletal olivines in many picrites, olivine-rich basalts and the Archaean “spinifex” rocks are not due to rapid cooling, but to rapid olivine growth caused by the high normative olivine content of the magma.

Curved branching crystals and differentiation in comb-layered rocks

Layering in which one or more of the component minerals has grown perpendicular to layer boundaries occurs, under a variety of names, in volcanic, hypabyssal nad plutonic igneous rocks. The most recent and best name is comb layering. The oriented minerals are elongate, are commonly branching and may be curved. Experimental crystallization of plagioclase and ternary feldspar melts confirms that a substantial degree of supercooling 01 a significant cooling rate is necessary to produce the curved or branching crystal morphologies typical of comb layering. The more viscous the melt, the less the supercooling required. Changes in the water content or confining pressure are mechanisms for inducing supersaturation in deep-seated magmas, that are consistent with field and experimental evidence. The change from modal dominance by a single elongate crystal phase in one comb layer to dominance by another phase in a contiguous comb layer is explained by the presence of constitutional supercooling ahead of the growing crystals of a given layer.

An experimental investigation of the delay in nucleation of olivine in Mafic Magmas

The delay in nucleation of olivine in basaltic melts increases systematically with decreasing degree of supercooling, cooling rate, superheat, olivine content of the melt and with increasing melt viscosity. These findings imply homogeneous nucleation of olivine in melts run in the Pt-wire loop sample container used. Unlike plagioclase nucleation, the appearance of the first olivine crystal in a melt is predictable (to within 0.1–1 h of the event). The ‘metastable region’ (i.e., the minimum degree of supercooling necessary for nucleation) is less than 13 ° C. The cause of the delay in nucleation is discussed in terms of the finite growth rate of embryos and the progressive polymerization of the melt with decreasing temperature and increasing time. At degrees of superheat <18 ° the melts are inferred to be more highly disordered than expected. Some implications of the results for petrology and experimental petrology are discussed, including the possibility that continuous zoning may develop under isothermal conditions due to the sluggish attainment of equilibrium of melt structure following a sudden change in temperature.

Calculated diffusion coefficients and the growth rate of olivine in a basalt magma

Abstract Concentration gradients in glass adjacent to skeletal olivines in a DSDP basalt have been examined by electron probe. The glass is depleted in Mg, Fe, and Cr and enriched in Si, Al, Na, and Ca relative to that far from olivine. Ionic diffusion coefficients for the glass compositions are calculated from temperature, ionic radius and melt viscosity, using the Stokes-Einstein relation. At 1170°C, the diffusion coefficient of Mg 2+ ions in the basalt is 4·5.10 −9 cm 2 /s. Comparison with measured diffusion coefficients in a mugearite suggests this value may be 16 times too small. The concentration gradient data and the diffusion coefficients are used to calculate instantaneous olivine growth rates of 2–6.10 −7 cm/s. This is too slow for olivine to have grown in situ during quenching. Growth necessarily preceded emplacement such that the composition of the crystals plus the enclosing glass need not be that of a melt. The computed olivine growth rates are compatible with the rate of crystallization deduced for the Skaegaard intrusion.

Olivine Crystal Types in Harrisitic Rocks of the Rhum Pluton and in Archean Spinifex Rocks

Skeletal and dendritic olivine crystals in the Archean volcanic ultramafic spinifex rocks and in harrisitic ultramafic layers of the Rhum pluton are classified as (1) plate, (2) randomly oriented, (3) porphyritic, or (4) branching type. Volcanic and plutonic examples are remarkably similar. Each type records the degree of pre-nucleation supersaturation with olivine that was attained by the parent melt. The rapid induction of supersaturation necessary to form skeletons and dendrites in a plutonic environment is attributed to changing water content or adiabatic expansion of the magma. The olivine crystals in harrisitic and spinifex rocks are not quench crystals; they grew rapidly from olivine-rich melts as the result of extreme supersaturation induced by slow cooling and slight supercooling below a liquidus with shallow slope in temperature-composition space. Skeletal and dendritic olivine crystals grow readily in magma and are poor indices of cooling rate and crystallization environment.

Skeletal crystallization and residual glass compositions in a cellular alkalic pyroxenite nodule from Oldoinyo Lengai

The alkalic pyroxenite nodule consists of megacrysts of diopside, apatite, perovskite and titanomagnetite in a groundmass consisting of diopside, apatite, titanomagnetite, nepheline, melilite, garnet and vishnevite crystals of various shapes, including previously undescribed skeletal and dendritic shapes, together with vesicles and residual glass. The residual glass is poor in SiO2 (38–40 wt%), and extraordinarily rich in Na2O (12.8–15 wt%), SO3 (1–1.5 wt%), and Cl (0.25–0.7 wt%), as a result of rapid, non-equilibrium crystallization of groundmass phases from a CO2-rich nephelinite melt.The Oldoinyo Lengai alkalic carbonatite lavas do not represent extreme products of the fractional crystallization of pyroxene, wollastonite, nepheline and alkali feldspar from the carbonated nephelinite melt. The most likely connection between the carbonatite and silicate magma types is one of liquid immiscibility, probably involving phonolite melt.

Ardnamurchan 3D cone-sheet architecture explained by a single elongate magma chamber

The Palaeogene Ardnamurchan central igneous complex, NW Scotland, was a defining place for the development of the classic concepts of cone-sheet and ring-dyke emplacement and has thus fundamentally influenced our thinking on subvolcanic structures. We have used the available structural information on Ardnamurchan to project the underlying three-dimensional (3D) cone-sheet structure. Here we show that a single elongate magma chamber likely acted as the source of the cone-sheet swarm(s) instead of the traditionally accepted model of three successive centres. This proposal is supported by the ridge-like morphology of the Ardnamurchan volcano and is consistent with the depth and elongation of the gravity anomaly underlying the peninsula. Our model challenges the traditional model of cone-sheet emplacement at Ardnamurchan that involves successive but independent centres in favour of a more dynamical one that involves a single, but elongate and progressively evolving magma chamber system.

The contact zone of the Rhum ultrabasic intrusion: evidence of peridotite formation from magnesian magmas

The Eastern Layered Series of the Rhum ultrabasic complex formed in situ, and was not tectonically emplaced as previously suggested. Field relations along the contact show no evidence for the existence of a late ‘Marginal Gabbro’, instead peridotite and allivalite layers can be traced to within 2 m of the surrounding partially melted country rocks. Local preservation of a chilled margin to one peridotite layer indicates that the parental magma consisted of an olivine tholeiite liquid carrying approximately 19% (vol.) olivine crystals in suspension.

Igneous Layering in Basaltic Magma Chambers

Layering is a common feature in mafic and ultramafic layered intrusions and generally consists of a succession of layers characterized by contrasted mineral modes and/or mineral textures, including grain size and orientation and, locally, changing mineral compositions. The morphology of the layers is commonly planar, but more complicated shapes are observed in some layered intrusions. Layering displays various characteristics in terms of layer thickness, homogeneity, lateral continuity, stratigraphic cyclicity, and the sharpness of their contacts with surrounding layers. It also often has similarities with sedimentary structures such as cross-bedding, trough structures or layer termination. It is now accepted that basaltic magma chambers mostly crystallize in situ in slightly undercooled boundary layers formed at the margins of the chamber. As a consequence, most known existing layering cannot be ascribed to a simple crystal settling process. Based on detailed field relationships, geochemical analyses as well as theoretical and experimental studies, other potential mechanisms have been proposed in the literature to explain the formation of layered igneous rocks. In this study, we review important mechanisms for the formation of layering, which we classify into dynamic and non-dynamic layer-forming processes.

Pb-isotope evidence for contrasting crustal contamination of primitive to evolved magmas from Ardnamurchan and Rum: implications for the structure of the underlying crust

Synopsis Using Pb isotope ratios we compare crustal contamination of primitive to evolved magmas from the Ardnamurchan and nearby Rum Igneous Centres, located on different crustal provinces in the British Tertiary Igneous Province (BTIP). The results confirm that compositional variations of parental cone-sheet magmas in Ardnamurchan can be explained by assimilation of granulite facies Lewisian gneiss at moderate crustal levels and subsequent contamination of evolved magmas with Moine schist metasediments within the uppermost crust during fractional crystallization. In contrast, samples from the Rum Centre have an uncontaminated mantle signature for the basaltic end-member, whereas the evolved rocks show Pb isotope evidence of contamination by Lewisian amphibolite facies rocks. Rum is separated from Ardnamurchan by a major thrust. The absence of a Moine-type isotopic influence in the Rum rocks supports an earlier interpretation, based on field evidence, that these overthrust rocks were eroded from Rum prior to the Palaeocene magmatic activity.