Calculated phase equilibria for a morb composition in a P–T range, 450–650 °C and 18–28 kbar: the stability of eclogite

Abstract

Lower temperature eclogite (with T ¼ 600°C) represents a significant part of the occurrences of eclogite in orogenic belts. ÔTrueÕ eclogite, with, for example, garnet + omphacite >70%, is well represented in such an occurrence. Calculated phase equilibria in Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O-TiO 2 -O (NCKFMASHTO), for just one rock composition -that of a representative midocean ridge basalt, MORB MORB -are used to see under what circumstances ÔtrueÕ 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 ÔtrueÕ 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 H 2 O saturation) in order for ÔtrueÕ eclogite to form. Moreover, unless pressure is at the high end in the range considered, lawsonite eclogite and ÔtrueÕ 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.

Mid-crustal shear zone development under retrograde conditions: pressure–temperature–fluid constraints from the Kuckaus Mylonite Zone, Namibia

“…Analyses were converted to the NCF-MASHTO model system by disregarding K 2 O, MnO, Cr 2 O 3 (< 0.03 wt %) and P 2 O 5 (< 0.2 wt %) and assuming approximately 15 % of total Fe to be present as Fe 3+ , in line with typical values for metamafic rocks (cf. the compilation of Rebay et al, 2010). During initial modelling, the samples were assumed to be fully hydrated, such that water (H 2 O) was taken to be present in excess.…”

Post-peak metamorphic evolution of the Sumdo eclogite from the Lhasa terrane of southeast Tibet

“…The modeled mineral assemblages match well with the observed mineral assemblage within the garnet core domain with the exception of the disappearance of lawsonite. This absence of lawsonite could be due to the dehydration reactions of the calcic-rich mineral formation such as epidote/zoisite during decompression (Rebay et al, 2010;Wei and Clarke, 2011). The intimate associations of epidote + paragonite inclusions (Fig.…”

Prograde and retrograde evolution of eclogite from Adrar Izzilatène (Egéré-Aleksod terrane, Hoggar, Algeria) determined from chemical zoning and pseudosections, with geodynamic implications

“…The amphibole and clinopyroxene models do not explicitly account for K and Ti; however these components are only present in minor amounts in the analysed compositions (Tables 2 and 3) such that the modelling calculations are likely to reliably reproduce the behaviour of these minerals. The analysed whole rock compositions were converted to the model NCFMASHTO system by ignoring the minor amounts of MnO, Cr 2 O 3 , P 2 O 5 and K 2 O before converting 15% of the total Fe to Fe 3+ , within the range typical for the oxidation state of eclogite (Rebay et al, 2010).…”