Trace elements in the fluid phase of the Earth's mantle

Abstract

IT has become apparent during the past decade or so that CO₂ is a significant component of the Earth's upper mantle. In addition to the indirect evidence including CO₂ emanations from volcanoes throughout the world and the abundance of CO₂ in kimberlites and carbonatites, it is now known that CO₂ is ubiquitously present in xenoliths of mantle material brought up in alkalic basalts and kimberlites. This latter point was first made by Roedder¹ who demonstrated that the fluid in optically visible inclusions is essentially pure CO₂ under pressures as great as 5 kbar at eruptive temperatures. Such pressures provide only a minimum depth of origin, because Roedder further showed that most of the inclusions he observed are secondary, that is, they occur as arrays on healed cracks. A more recent study² indicated pressures as high as 10 kbar in secondary inclusions. H.W.G. and Radcliffe³ found, using transmission electron microscopy (TEM), that submicroscopic fluid inclusions also occur within the crystals of mantle xenoliths, and that these bubbles have exsolved (precipitated) from solid solution. Thus, at least the CO₂ now present in submicroscopic fluid inclusions is indigenous to the crystals in which it is found, and probably represents the source of crack formation and filling to produce the secondary inclusions on the way to the surface³. The effect CO₂-bearing phases have on the distribution of trace elements in the mantle and during partial melting is considered here.