Contrib Mineral Petrol (1993) 114:425M40 Contributions to Mineralogy and Petrology 9 Springer-Verlag1993 Igneous layering through oscillatory nucleation and crystal settling in well-mixed magmas Matthias Hort 1, Bruce D. Marsh 1, and Tilman Spohn 2 l Department of Earth and Planetary Sciences,Johns Hopkins University, Baltimore, MD 21218, USA 2 Institut ffir Planetologie, Westf/ilischeWilhelms-Universitfit Miinster, Wilhelm Klemm-Strasse 10, D-48149 Mfinster, Germany ReceivedOctober 10, 1992/ Accepted March 30, 1993 Abstract. Oscillatory or competitive nucleation about a binary (or perhaps pseudo binary) eutectic and ensuing crystal growth and settling is a commonly suggested means of producing layering in magmatic systems. A quantitative model is presented of this, outwardly, rela- tively simple process of crystal nucleation, growth, and settling in an otherwise initially crystal-free magma. Av- rami-style kinetics of crystallization in an always well- mixed body, buried in conductive wall rock, are coupled to a Stokes-like formulation of crystal settling in magma whose viscosity depends on temperature and crystallin- ity. Two dimensionless numbers (Se, the settling number and A v, the Avrami or kinetic number) govern all the results. Av and Se measure the relative importance of crystallization time and settling time, respectively, rela- tive to the overall cooling time. For any value of Av, which increases strongly with the maximum nucleation and growth rates and cooling time, layering is possible only over a range or window of values of Se. Both above and below this window a single layer (crystalline below, vitric above) forms, and within this window the number of layers increases systematically with increasing Av and Se. Grain size within any single layer generally coarsens upward. Because the characteristic settling and cooling times both depend on body thickness, the lower limit of the settling window is also dependent on sheet thick- ness. Within the confines of this model and for nucleation and growth rates set by those observed in natural sys- tems, layering is unlikely in sheet-like magmas thinner than about 100 m. When the body is not always well- mixed and crystallization is within inward-propagating solidification fronts, it is expected that this minimum body thickness will increase. Introduction Layering in igneous bodies has long been taken as a natural consequence of crystallization and crystal set- Correspondence to: M. Hort tling in large multicomponent magmatic systems (e.g., Bowen 1915; Wager 1959; Wager and Brown 1968). The spectacle of layering in many mafic bodies has perhaps sparked more detailed field and theoretical investigations than any other feature of these bodies. Of the many mechanisms suggested for layer formation, here we quan- titatively investigate one of the more simple and com- monly discussed mechanisms, namely, the oscillatory nu- cleation of two competing phases near an eutectic point or cotectic line (e.g., Wager 1959; Hawkes 1967; Maaloe 1978, 1987; Morse 1979). In brief, oscillatory nucleation leading to crystal growth and settling is brought on by slight compositional variations of the liquid due to crys- tallization of one phase over the other. Crystallization of one phase drives the melt composition back to favor crystallization of the former phase, and so on. Continued growth eventually allows settling, which may be affected significantly by convection, were it present, leading to phase layering on the floor of the chamber. In the case of the Klokken-intrusion, for example, Parsons and Becker (1987) argue that this style of crystallization took place close to the alkali-feldspar and hedenbergite eutect- ic. There are also mafic intrusive bodies of significant size (100-500 m) that do not form layers or even cumu- lates (Gibb and Henderson 1992; Mangan and Marsh 1992). The only cumulates shown by these bodies are formed by phenocrysts initially present upon emplace- ment. Relatively small, phenocryst-free bodies evidently cool too fast to nucleate and grow crystals large enough to produce cumulates and layering. Simple model calcu- lations for settling of original phenocrysts (e.g., Gray and Crain 1969; Fujii 1974) result in the well known S- shaped profiles of phenocrysts commonly found in var- ious intrusive bodies (Marsh 1988 a). And this evidence, along with the investigation of Weinstein et al. (1988), Martin and Nokes (1988, 1989), and Koyaguchi et al. (1990) demonstrate that crystal settling, regardless of the strength of convection, commonly occurs in magmas (see also Marsh and Maxey 1985; Martin 1990; and Koptev- Dvorinkov et al. 1983 in Kogarko and Khapaev 1987).