Synthesis of monticellite–forsterite and merwinite–forsterite symplectites in the CaO–MgO–SiO2 model system: influence of temperature and water content on microstructure evolution

Abstract

Homogeneous single crystals of synthetic monticellite with the composition Ca 0.88 Mg 1.12 SiO 4 (Mtc I) were annealed in a piston-cylinder apparatus at temperatures between 1000 and 1200• C , pressures of 1.0-1.4 GPa, for run durations from 10 min to 24 h and applying bulk water contents ranging from 0.0 to 0.5 wt% of the total charge. At these conditions, Mtc I breaks down to a fine-grained, symplectic intergrowth. Thereby, two types of symplectites are produced: a first symplectite type (Sy I) is represented by an aggregate of rod-shaped forsterite immersed in a matrix of monticellite with end-member composition (Mtc II), and a second symplectite type (Sy II) takes the form of a lamellar merwinite-forsterite intergrowth. Both symplectites may form simultaneously, where the formation of Sy I is favoured by the presence of water. Sy I is metastable with respect to Sy II and is successively replaced by the latter. For both symplectite types, the characteristic spacing of the symplectite phases is independent of run duration and is only weeakly influenced by the water content, but it is strongly temperature dependent. It varies from about 400 nm at 1000• C to 1200 nm at 1100• C in Sy I, and from 300 nm at 1000 • C to 700 nm at 1200• C in Sy II. A thermodynamic analysis reveals that the temperature dependence of the characteristic spacing of the symplectite phases is due to a relatively high activation energy for chemical segregation by diffusion within the reaction front as compared to the activation energy for interface reactions at the reaction front. The temperature dependence of the characteristic lamellar spacing and the temperature-time dependence of overall reaction progress have potential for applications in geo-thermometry and geo-speedometry.

High‐P granulites of the Songshugou area (Qinling Orogen, east‐central China): Petrography, phase relations, and U/Pb zircon geochronology

“…If the same mechanisms formed lamellae and patches, the thin lamellae followed the broader patches during cooling because diffusion as the kinetically controlling factor is the more sluggish the lower the temperature is. This resembles the negative correlation of rod size and lamellae spacing in symplectites with their formation temperature (Joanny, van Roermund, & Lardeaux, 1991; Remmert et al, 2018).…”

Ca-rich garnets and associated symplectites in mafic peraluminous granulites from the Gföhl Nappe System, Austria

“…It is suggested that the frequently observed change of characteristic inter-granular spacing (and size) of the symplectite phases correlates positively with the temperature of symplectite formation. Remmert et al (2018) presented experimental evidence corroborating this view and argued that the characteristic spacing of symplectite phases is small at low temperatures, whereby symplectite formation tends to be diffusion controlled. The characteristic spacing becomes larger at higher temperatures, whereby diffusion is more efficient, and symplectite formation becomes interface reaction controlled.…”

“…It has been argued by Remmert et al (2018) that symplectite formation only occurs in a rather narrow window of bulk water contents. Whereas the formation of symplectites is hampered in completely dry systems, a Garben microstructure with little spatial organization is formed at high water contents.…”

Origin of clinopyroxene-ilmenite symplectites in mafic granulites from eastern parts of the Chotanagpur granite gneissic complex, East Indian shield

“…4h; 5c) and the modeled symplectite forming reactions in both domains suggest that Ti was not mobile beyond the volume of the initial titanite grain. However, the lamellar intergrowth of Ilm-Cpx 2 does support that Ti was locally mobile in the scale defined by the interlamellar space (Abart et al 2012;Remmert et al 2018) between Ilm lamellae. The interlamellar space between the lamellar ilmenite rods ranges from ~2-55µm in Domains I and II.…”