Testing the validity of the petrological hypothesis 'no phenocrysts, no post-emplacement differentiation'

Abstract

An extravagant hypothesis 'no phenocrysts, no post-emplacement differentiation' has been put forward by Marsh, in a series of papers, for the development of mafic-ultramafic intrusions. This hypothesis is based on an assertion that the majority of these intrusions are structureless and undifferentiated because they lack residual granitic rocks. To explain this, the hypothesis postulates that phenocryst-free magmas are not able to differentiate in crustal chambers because all evolved interstitial liquid is locked in solidification fronts, producing compositionally uniform magmatic bodies. Layered, well-differentiated intrusions are attributed to the successive emplacement of magma pulses with phenocrysts of different phase and chemical compositions, rather than to slow magma cooling and fractional crystallization, as conventional models imply. Such phenocryst-laden magma pulses are supposed to be derived from an underlying magmatic mush column. However, structureless, undifferentiated, mafic-ultramafic bodies simply do not exist in nature. All well-studied mafic-ultramafic bodies, with or without residual granitic rocks, that crystallized from parental magmas of non-eutectic composition, tend to reveal clear evidence of internal compositional differentiation in terms of crystallization sequences (e.g. Ol, Opx, Opx + Pl, Opx + Pl + Cpx), mineral compositions (e.g. An in plag, En in cpx) and compatible/ incompatible major and trace element geochemistry (e.g. Mg-number, Cr, rare earth elements). This is especially evident in layered intrusions that represent the key evidence against the hypothesis. Essentially, by denying the ability of magma to differentiate in intrusive bodies, the hypothesis forbids magmatic differentiation in any sub-chamber related to the entire magmatic mush column. As a result, magma pulses in which phenocrysts progressively change in composition cannot be derived from the column to form layered intrusions. The hypothesis is thus contradictory, baseless and fundamentally flawed. It should be abandoned in favour of a classical fractional crystallization model based on the pioneering experiments of Bowen and amply confirmed over almost a century by subsequent studies of layered intrusions. The classical model was, is, and will most probably remain, the best explanation for the origin of differentiated magmatic bodies. © The Author 2009. Published by Oxford University Press. All rights reserved.