EPMA maps unveil the actual chemical variations and crystallisation sequence of pyroxene and plagioclase solidified from a basaltic liquid at variable cooling rates

• Published in: Chemical Geology
• Main Authors: Gennaro E., Radica F., Iezzi G., Vetere F., Nazzari M., Zellmer G. F., Scarlato P.
• Other Authors: Gennaro, E., Radica, F., Iezzi, G., Vetere, F., Nazzari, M., Zellmer, G. F., Scarlato, P.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2023
• Subjects: Basalt; Cooling rate; Kinetic; Plagioclase; Pyroxene; Iceland
• Online Access: https://hdl.handle.net/11365/1251074; https://doi.org/10.1016/j.chemgeo.2023.121752; https://www.sciencedirect.com/science/article/pii/S0009254123004527?via=ihub

Crystal-chemical variations of pyroxene (px) and plagioclase (plg) have been analysed by X-ray electron-microprobe (EPMA) mapping to quantify their actual chemical dispersions. These phases were experimentally crystallised from a basaltic liquid (B100, MORB from Iceland) at cooling rates of 1, 7, 60 and 180 °C/h from 1300 °C down to 800 °C. Experiments were run at ambient conditions applying defined temperature paths mirroring characteristic cooling rates from innermost to outermost portions of metre- to centimetre-thick lavas, dikes and bombs emplaced under submarine to subaerial conditions. As the cooling rate increases from 1 to 180 °C/h, the run-products become progressively enriched in pyroxene and depleted in plagioclase, while spinel is invariably low (few area%) and glass is significant only at 180 °C/h. An increase of cooling rate generally leads to enrichment of Al2O3 and depletion of MgO in px, while the opposite behaviour is observed for plg; these trends are mirrored by calculated cations (apfu: atom per formula unit) and components. Average variations as a function of cooling rate are similar to those already observed through classical analysis performed by single point EPMA. However, the actual chemical distributions of CaO versus MgO, Al2O3 and FeOtot oxides unveil the presence of a wider range in pyroxene chemistry. In particular, one px (px-1, CaO-rich, diopsidic type) is present at all the applied cooling rates; a very low CaO-px (px-2, pigeonite or orthopyroxene type) is detected at 1 °C/h; and, finally, once more population of px (px-3, CaO-poor diopsidic type) appears at 60 and 180 °C/h. By contrast, plg analyses yield invariably identical compositions. Textural variations as a function of cooling rate and geothermometric estimations indicate that px-1 crystallised at high-T (or low ΔT), while plg mainly grew in the residual melt produced by the saturation of px. If only textures were evaluated, this order of segregation would like remain unrecognised since px at low cooling rates is ...