Glass forming ability and crystallisation behaviour of sub-alkaline silicate melts

The intrinsic solidification behaviour and glass forming ability (GFA) of natural sub-alkaline silicate melts have been experimentally quantified via cooling-induced solidification approach. GFA is measured by the critical cooling rate Rc, the rate at which a melt solidifies ≤2 area% of crystals. Cooling rates of 9000, 1800, 180, 60, 7 and 1 °C/h have been run between 1300 °C (super-liquidus region) and 800 °C (quenching temperature), at air fO2 and ambient P for six silicate melts with compositions ranging from basalt (B) to rhyolite (R) (i.e., B100, B80R20, B60R40, B40R60, B20R80 and R100) and water contents comprised between 53 (B100) and 384 (B20R80) ppm. The ranges of cooling rates and chemical compositions used in this study are the broadest ever investigated in the Earth sciences. The phase proportions (area%) were determined by image analysis on about 500 back-scattered electron images collected over different magnifications. Phases are glass, clinopyroxene (cpx), spinel (sp) and plagioclase (plg). Sp is ubiquitous with abundance of few area% and nucleates earlier than silicate crystals. Cpx solidifies in all runs except in R100 and its abundance follows asymmetric broad Gaussian-like trends (with tails towards low rates) as a function of cooling rate. Moving from B100 to B40R60 these trends conserve their shape but shift progressively to lower cooling rates and mineral abundances. Plg crystallises only at low cooling rates and in SiO2-poor compositions. Run-products with low amounts of crystals (≤5 area%) clearly show that cpx preferentially nucleates on surfaces of sp, whereas a significant crystallisation of cpx (N5 area%) is observed with decreasing cooling rate and with changing composition from B100 to B20R80. The crystallisation of silicate crystals is related to the chemical diffusivity of components in the melt. Also the initial crystallisation of plg occurs preferentially on cpx. In general, the amount of crystals decreases as the cooling rate increases; however, in some cases, the amount of crystals remains constant or even decreases for B80R20 with decreasing cooling rate. Rc values change over 5 orders of magnitude being b1, 7, 620, 3020, 8020 and 9000 °C/h for R100, B20R80, B40R60, B60R40 and B80R20 and B100, respectively. The variation of Rc can be modelled through NBO/T (nonbridging oxygen per tetrahedron) parameter by the following equation: Rc=a / {1+e−[(NBO/T − b)/c]},where a, b and c are fitting parameters equal to 9214, 0.297 and 0.040, respectively. Similarly to other glass-forming liquids (network, metallic and molecular systems), Rc for natural sub-alkaline silicate melts is inversely related to the reduced glass transition parameter Trg (Trg=Tg / Tm) and can be quantified with the equation Rc= a × Trg−b, where a and b are 1.19 × 10−4 and 28.7, respectively. These results may be used to retrieve the solidification conditions of aphyric, degassed and oxidised lavas; in addition, our data provide general constrains on the crystallisation kinetics of natural crystal-bearing silicate melts erupted on Earth (e.g. lavas with phenocrysts). The relationship between crystal content and cooling rate suggests that the solidification path induced by degassing can be also complex and nonlinear. The growth of crystalswith size up to 1 mm from a nearly anhydrous superheated silicate melt indicates that variable cooling conditions of lavas have to be accounted to discriminate amongminerals formed before, during and after eruptions.Moreover, our results can be used to design glass-ceramics from naturally available easy to find, low-cost starting materials