Liquid Immiscibility and Evolution of Basaltic Magma: Reply to S. A. Morse, A. R. McBirney and A. R. Philpotts

INTRODUCTION We are pleased that our experimental study (Veksler et al., 2007) received critical comments from three of the best experts on silicate liquid immiscibility and gabbroic intrusions. Our disagreements are less important than the shared belief that studies of layered intrusions need a new impulse, and existing ideas a thorough revision. This discussion is a good opportunity to emphasize and clarify the key points, better formulate current problems and disagreements, and touch upon some topics that were mentioned only briefly or not at all in our original publication. We continue experimental work on silicate liquid immiscibility (Veksler et al., 2009), and will mention here some new results that are relevant for the discussion. Like Roedder 30 years ago (Roedder & Weiblen, 1970; Roedder, 1978), we stumbled upon liquid immiscibility by serendipity. Our interest in magma unmixing was sparked by unsought and unexpected melt inclusions in apatite from the Skaergaard intrusion (Jakobsen et al., 2005). These inclusions were impossible to miss.We realized that, surprisingly, some important features of Skaergaard rocks had been overlooked or dismissed by numerous researchers before us. As we continued our work and established close collaboration with experts on the Skaergaard intrusion in Denmark and the UK (see the Acknowledgements) our experimental study became a part of a broader petrographic and geochemical project.This discussion primarily deals with experimental evidence and therefore we will mention petrographic and geochemical observations only briefly. However, those other types of evidence are certainly important and hopefully they will be covered soon by our colleagues in their upcoming publications. We do realize that the idea of early immiscibility in the Skaergaard intrusion is at odds with broadly accepted views, and admit that the concerns expressed by our opponents are fair and well based. We started to consider the immiscibility hypothesis seriously only as the last resort, because everything else (including the compositional convection model advocated by Professor Morse) failed to explain some conflicting facts about the Skaergaard intrusion that were briefly outlined in our original paper and are further discussed below. In this paper, we would also like to point out some possible tests and promising directions for future research.