Calculated phase equilibria for a MORB composition in a P T window, 450–650◦C and 18–28 kbar: the stability of eclogite

Lower temperature eclogite (with T = 600 °C) represents a significant part of the occurrences of eclogite in orogenic belts. True eclogite, with, for example, garnet + omphacite >70%, is well represented in such an occurrence. Calculated phase equilibria in Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (NCKFMASHTO), for just one rock composition – that of a representative midocean ridge basalt, MORB – are used to see under what circumstances true eclogite is predicted to occur.
The variables considered are not only pressure (P) and temperature (T) but also water content and oxidation state. The latter two variables are known to exert a significant control on mineral assemblage but are difficult to establish retrospectively from the observed rocks themselves. It is found that whereas oxidation state does have a strong effect on mineral assemblage, the key control on developing true eclogite is shown to be temperature and water content. If temperature is established to be <600 °C, water content has to be low (less or much less than that for H2O saturation) in order for true eclogite to form. Moreover, unless pressure is at the high end in the range considered, lawsonite eclogite and true eclogite will tend to be mutually exclusive, with the former requiring high water content at the lower temperature where it occurs, but the latter requiring low water content.