Li enrichment in peridotites and chromitites tracks mantle-crust interaction

Abstract

The ultramafic massifs of the Serranía de Ronda in southern Spain are the Earth’s largest exposures of subcontinental lithospheric mantle (SCLM) peridotites (~450 km2). These ultramafic massifs experienced asthenosphere melt percolation during their crustal emplacement. Mixing of these mafic melts with anatectic melts and fluids led to the formation of a world’s unique Ni-arsenide-rich chromitite ores (hereafter Cr–Ni ores) associated with orthopyroxenite and/or cordieritite (i.e., > 90 % volume of cordierite) hosted within the peridotites. This study uses laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to investigate the Li in rock-forming minerals of peridotite and Cr–Ni ores to evaluate the role of Li as crustal tracer. Clinopyroxene crystallized from asthenospheric melts exhibits high Li contents (up to 8.5 ppm), exceeding the average values of the upper mantle (~ 0.7 ppm), whereas orthopyroxene, olivine, and Cr-spinel from peridotite are mostly Li-depleted. In contrast, all rock-forming minerals of Cr–Ni ores have abnormally high Li contents, displaying an overall Li enrichment trend toward the external parts of ultramafic massifs, on the way to the crustal rocks. This trend is evident in Cr-spinel from the Cr–Ni ores, which display 6.9–7.9 ppm Li in the deepest portions of the massif (Arroyo de la Cala Cr–Ni ore) up to 1.4–8.5 ppm in the shallowest part (La Gallega Cr–Ni ore), as well as in orthopyroxenes that have 31.3–44.7 ppm Li in Arroyo de la Cala, and 45.1–51.4 ppm Li in La Gallega. Cordierite is present only in the Cr–Ni ores situated in the external part of the ultramafic massifs, exhibiting 113.15–160.82 ppm Li in the Barranco de las Acedías Cr–Ni ore and 36.5–60.5 ppm Li in La Gallega Cr–Ni ore. Similarly to Li, LREE, fluid-mobile elements (K, Rb, Ba), and Sr in orthopyroxenes from the Cr–Ni ores display enrichment from the inner to the outer parts of the ultramafic massif. These geochemical variations suggest that Li enrichment in Cr–Ni ores and host peridotites was a twofold process: (1) asthenospheric melt percolation slightly increased Li abundances in the SCLM peridotites by modal and cryptic metasomatism involving clinopyroxene; (2) additional infiltration of Li-bearing crustally-derived fluids during the intracrustal emplacement of the mantle section boosted the Li contents of minerals in the Cr–Ni ores. Our results highlight that Li may effectively track the interaction of the SCLM with crustal components.