Kinetic control of staurolite–Al₂SiO₅ mineral assemblages: Implications for Barrovian and Buchan metamorphism

Abstract

The distribution and textural features of staurolite–Al2SiO5 mineral assemblages do not agree with predictions of current equilibrium phase diagrams. In contrast to abundant examples of Barrovian staurolite–kyanite–sillimanite sequences and Buchan‐type staurolite–andalusite–sillimanite sequences, there are few examples of staurolite–sillimanite sequences with neither kyanite nor andalusite anywhere in the sequence, despite the wide (~2.5 kbar) pressure interval in which they are predicted. Textural features of staurolite–kyanite or staurolite–andalusite mineral assemblages commonly imply no reaction relationship between the two minerals, at odds with the predicted first development (in a prograde sense) of kyanite or andalusite at the expense of staurolite in current phase diagrams. In a number of prograde sequences, the incoming of staurolite and either kyanite, in Barrovian sequences, or andalusite, in Buchan‐type sequences, is coincident or nearly so, rather than kyanite or andalusite developing upgrade of a significant staurolite zone as predicted. The width of zones of coexisting staurolite and either kyanite, in Barrovian sequences, or andalusite, in Buchan‐type sequences, is much wider than predicted in equilibrium phase diagrams, and staurolite commonly persists upgrade until its demise in the sillimanite zone. We argue that disequilibrium processes provide the best explanation for these mismatches. We suggest that kyanite (or andalusite) may develop independently and approximately contemporaneously with staurolite by metastable chlorite‐consuming reactions that occur at lower P–T conditions than the thermodynamically predicted staurolite‐to‐kyanite/andalusite reaction, a process that involves only modest overstepping (<15°C) of the stable chlorite‐to‐staurolite reaction and which is favoured, in the case of kyanite, by advantageous nucleation kinetics. If so, the pressure difference between Barrovian kyanite‐bearing sequences and Buchan andalusite‐bearing sequences could be ~1 kbar or less, in better agreement with the natural record. The unusual width of coexistence of staurolite and Al2SiO5 minerals, in particular kyanite and andalusite, can be accounted for by a combination of lack of thermodynamic driving force for conversion of staurolite to kyanite or andalusite, sluggish dissolution of staurolite, and possibly the absence of a fluid phase to catalyse reaction. This study represents an example of how kinetic controls on metamorphic mineral assemblage development have to be considered in regional as well as contact metamorphism.

Deciphering the Jurassic–Cretaceous evolution of the Hamadan metamorphic complex during Neotethys subduction, western Iran

“…Several common reactions, such as St → Grt + Bt + ALS, or St + Chl → ALS + Bt, involve staurolite as reactant and produce an aluminosilicate (ALS) polymorph. Pattison and Spear (2018) noted that the width of zones of coexisting staurolite and andalusite in Buchantype sequences is much wider than predicted by equilibrium phase diagrams, which they attributed to disequilibrium processes related to sluggish dissolution of staurolite and the lack of a thermodynamic driving force for the conversion of staurolite to andalusite. Although the two samples from the staurolite (H30) and andalusite (H44) zones have different mineral assemblages, calculated equilibrium temperatures are similar.…”

Metamorphic P–T–t–d evolution of the Mesoproterozoic Pur‐Banera supracrustal belt, Aravalli Craton, northwestern India: Insights from phase equilibria modelling and zircon–monazite geochronology of metapelites

“…However, the peak assemblage also contains staurolite, which is not predicted to be stable along with kyanite in Figure 8a. Pattison and Spear (2018) have shown that the width of zones of coexisting staurolite and kyanite, in Barrovian sequences, is much wider than that predicted in equilibrium phase diagrams. They suggested that the unusual width of coexistence of staurolite and kyanite (seen as an extremely narrow field in Figure 8a) may be a result of a lack of thermodynamic driving force to convert staurolite to kyanite, sluggish dissolution of staurolite, or the absence of a fluid phase.…”

Extreme metamorphism and metamorphic facies series at convergent plate boundaries: Implications for supercontinent dynamics

“…Both Barrovian and Buchan facies series comprise the same three facies, i.e., greenschist facies, amphibolite facies, and granulite facies. Nevertheless, Barrovian-type metamorphism proceeds at higher pressures than Buchan-type metamorphism at the same temperatures (Zheng and Chen, 2017;Pattison and Spear, 2018), resulting in the formation of kyanite in the former but andalusite and sillimanite in the latter. Therefore, the differences in pressure and petrology are translated to the differences in metamorphic thermal gradients (Zheng and Chen, 2017;Zheng, 2021a) and metamorphic thermobaric ratios Johnson, 2019a, 2019b).…”