Comments on: 'Textural Maturity of Cumulates: a Record of Chamber Filling, Liquidus Assemblage, Cooling Rate and Large-scale Convection in Mafic Layered Intrusions' and 'A Textural Record of Solidification and Cooling in the Skaergaard Intrusion, East Greenland'

Abstract

Two recent papers by Holness et al. (2007a, 2007b) have introduced an interesting new petrographic technique that could become a valuable tool for judging the textural maturity of mafic igneous rocks. They have shown that variations of the dihedral angles of mineral grains can reflect differing rates of heat loss of large layered intrusions, such as Rum and Skaergaard. Although they say in one paper (Holness et al., 2007b) that these variations record ‘the time-integrated thermal history at sub-solidus temperatures’, in the other paper they attribute all the observed variations to changes in the magmatic regime during active crystallization: ‘the arrival of fresh magma in the chamber, the onset of chamber-wise convection and changes in the liquidus assemblage’ (Holness et al., 2007a). The effects of important sub-solidus processes that are known to have extensively altered the textures, modal proportions, and bulk compositions of these same rocks have been ignored, even though they are conspicuous in the field and have been pointed out repeatedly in the literature (Hunter, 1987; McBirney & Sonnenthal, 1990; Sonnenthal, 1992; McBirney & Hunter, 1995; Sonnenthal & McBirney, 1998; McBirney & Creaser, 2003). For example, a sharp change in the textures found near the middle of the section in the Rum intrusion is said to mark the ‘onset of convection’. This interpretation seems to be based on the idea that a magma can remain static after its initial intrusion and then, at some later time, after crystallizing hundreds of meters of layered rocks, start to convect. This is just backwards. The thermal and compositional gradients that drive convection are most pronounced when the magma is first intruded and decline with time. Similarly, the statement (Holness et al., 2007b) that a textural change might coincide with ‘a significant change in the rate and mode of heat loss from the intrusion as it moves from a convective to a conductive regime’ seems to assume that the rate of heat loss is more rapid if the magma is convecting. As Carrigan (1988) has pointed out, the rate of heat loss is not governed by convective transfer in the interior but by the much more limiting rate of