The kinetic effect induced by variable cooling rate on the crystal-chemistry of spinel in basaltic systems revealed by EPMA mapping

The crystal-chemical variations of spinels grown as a function of cooling rate (ΔT/Δt) were analyzed via X-ray electron-microprobe (EPMA) maps. Maps were collected serially by using a fixed distance. Spinels solidified from a tholeiitic MOR basaltic liquid (B100) cooled at cooling rates (ΔT/Δt) of 1, 7, 60, and 180 °C/h, between 1300 and 800 °C and at ambient P and fO2. As ΔT/Δt increases, the amount of spinel is invariably <5 area% and its size decreases. Compared to the previous data set collected by common single and selected EPMA analytical points (112 analyses), the kinetic effects induced by ΔT/Δt are here quantitatively captured by a large number of analyses (2052). The TiO2, Al2O3, MgO, and FeOtot show large compositional variations at low cooling rates (from 1 to 60 °C/h), and only the average TiO2 concentration shows a well-defined trend as a function of ΔT/Δt. However, calculated average cation amounts (apfu) unveil quantitative kinetic effects. When ΔT/Δt increases (from 1 to 180 °C/h), only Ti4+ shows a linear decreasing trend, whereas the other major Al3+, Fe3+, Fe2+, and Mg2+ cations alone are scattered. Conversely, the sums of trivalent (Al3++Fe3+) and divalent (Mg2++Fe2+) cations quantitatively capture the effect of the ΔT/Δt. These new outcomes could be the base of novel geospeedometers with significant implications in volcanology, geophysics, and material sciences in regard to silicate melt rheology on Earth. They should be extended to high-pressure, hydrated, and low oxygen fugacity conditions. Furthermore, the analytical approach used here to capture kinetic effects on spinel growth and compositions can be also applied to other crystalline phases grown from silicate liquids.