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 Crsingle bondNi 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 Crsingle bondNi 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 Crsingle bondNi 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 Crsingle bondNi ores, which display 6.9–7.9 ppm Li in the deepest portions of the massif (Arroyo de la Cala Crsingle bondNi ore) up to 1.4–8.5 ppm in the shallowest part (La Gallega Crsingle bondNi 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 Crsingle bondNi ores situated in the external part of the ultramafic massifs, exhibiting 113.15–160.82 ppm Li in the Barranco de las Acedías Crsingle bondNi ore and 36.5–60.5 ppm Li in La Gallega Crsingle bondNi ore. Similarly to Li, LREE, fluid-mobile elements (K, Rb, Ba), and Sr in orthopyroxenes from the Crsingle bondNi ores display enrichment from the inner to the outer parts of the ultramafic massif. These geochemical variations suggest that Li enrichment in Crsingle bondNi 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 Crsingle bondNi ores. Our results highlight that Li may effectively track the interaction of the SCLM with crustal components.