Abstract

When magma ascends turbulently through continental crust, heat transfer can be rapid and the wall rocks of the conduit can melt and be assimilated into the magma. Calculations are presented for cooling, crystallization and contamination during the turbulent ascent of a komatiite, a picritic basalt and a tholeiitic basalt. Primitive magmas, like komatiite and picritic basalt, are predicted to erupt with moderate to large amounts of olivine phenocrysts except at very high flow rates. Little crystallization takes place in a basalt on ascent, and any phenocrysts are likely to be inherited from the magma chamber. The erosion rate of the conduit walls and amount of contamination are greatest in primitive magmas and least for cool, fractionated magmas. Contamination is also affected by flow rate. For low flow rates, where the Reynolds number is significantly less than 2000, movement is laminar and the magma is likely to solidify against the dyke walls and so the amount of contamination is negligible. Maximum contamination will occur for flow rates at Reynolds number around 2000 and the total amount of contamination will decrease as the Reynolds number increases above this value. This kind of contamination can produce trends on geochemical diagrams which are opposite to those produced by assimilation and fractional crystallization processes in magma chambers. Indices of crustal contamination such as ⁸⁷Sr, REE, K ₂O and other incompatible elements can be greatest in magmas with high values of Mg/(Mg+Fe) and low SiO ₂. Both highly incompatible and highly compatible trace elements can show positive correlation with one another (for example, Ni and K ₂O). These features are shown by the Plateau lavas of Skye, Scotland, and some lava groups of the Deccan Traps. Curved trends produced in this way on many types of geochemical diagrams are not mixing hyperbolas and do not necessarily point towards contaminant compositions. Thermal erosion rates are proportional to the difference between the magma temperature and the fusion temperature of the wall rocks. Contamination will thus tend to be selective towards rocks of low fusion temperature. Because of their high temperatures, komatiites are relatively indiscriminate in what they assimilate, while basalts are highly selective. The calculations show that komatiites are highly susceptible to contamination by both continental and oceanic crustal components. Under suitable flow conditions they can be contaiminated with up to 30% of crustal material. Contamination could result in spurious conclusions about their age and mantle-source characteristics. The geochemical differences between komatiites and closely associated basaltic komatiites can often not be attributed to fractionation of olivine. These basaltic komatiites may represent highly contaminated komatiite rather than an unrelated magma type derived from a different mantle source.