Cr-spinel records metasomatism not petrogenesis of mantle rocks

• Published in: Nature Communications
• Main Authors: Gamal El Dien, H., Arai, S., Doucet, L.S., Li, Zheng-Xiang, Kil, Y., Fougerouse, Denis, Reddy, Steven, Saxey, David, Hamdy, M.
• Format: Article in Journal/Newspaper
• Language: English
• Published: NATURE PUBLISHING GROUP 2019
• Subjects: Science & Technology; Multidisciplinary Sciences; Science & Technology - Other Topics; MID-ATLANTIC RIDGE; ANDESITIC AVACHA VOLCANO; MARIANA FORE-ARC; PERIDOTITE XENOLITHS; SERPENTINIZED PERIDOTITES; ABYSSAL PERIDOTITES; SPREADING RIDGE; EASTERN DESERT; WEDGE BENEATH; KAMCHATKA ARC; Avacha; Mid-Atlantic Ridge; Kamchatka
• Online Access: https://hdl.handle.net/20.500.11937/79360; https://doi.org/10.1038/s41467-019-13117-1

Mantle melts provide a window on processes related to global plate tectonics. The composition of chromian spinel (Cr-spinel) from mafic-ultramafic rocks has been widely used for tracing the geotectonic environments, the degree of mantle melting and the rate of mid-ocean ridge spreading. The assumption is that Cr-spinel’s core composition (Cr# = Cr/(Cr + Al)) is homogenous, insensitive to post-formation modification and therefore a robust petrogenetic indicator. However, we demonstrate that the composition of Cr-spinel can be modified by fluid/melt-rock interactions in both sub-arc and sub-mid oceanic mantle. Metasomatism can produce Al-Cr heterogeneity in Cr-spinel that lowers the Cr/Al ratio, and therefore modifies the Cr#, making Cr# ineffective as a geotectonic and mantle melting indicator. Our analysis also demonstrates that Cr-spinel is a potential sink for fluid-mobile elements, especially in subduction zone environments. The heterogeneity of Cr# in Cr-spinel can, therefore, be used as an excellent tracer for metasomatic processes.