Geochemistry of peridotite xenoliths from the Sverrefjell Volcano, Western Spitsbergen: implications for mantle metasomatism

Abstract

Mantle xenoliths entrained in the Quaternary Sverrefjell volcano from Spitsbergen have been studied with the aim of understanding the nature of metasomatizing agents which affected the subcontinental lithospheric mantle beneath Spitsbergen. The suit of mantle xenoliths consists of spinel lherzolites, spinel harzburgites and websterites. The textures of all three rock types range from protogranular to porphyroclastic. Concentrations of Al2O3, CaO, TiO2 and Na2O decrease gradually from websterites through lherzolithes to harzburgites. LREE and other incompatible trace elements usually increase from depleted harzburgite to enriched lherzolites and websterites, indicating that metasomatism affected mainly the lherzolites and websterites. However, the exceptions are a few clinopyroxene patterns from harzburgites with mostly enriched LREEs imprinted by metasomatic processes. Clinopyroxenes show highly variable mantle normalized REE patterns but can be classified into four major groups showing: 1) LREE depleted to flat patterns (LaN/CeN 0.39-1.26; CeN/YbN 0.29-0.63; SmN/YbN 0.75-1.07), 2) spoon-shaped (MREE enriched) REE patterns (LaN/CeN 1.26-2.85; CeN/YbN 1.43-7.74; SmN/YbN 1.21-2.80), 3) highly enriched LREE and fractionated LREE/HREE patterns (LaN/CeN 0.83-1.24; CeN/YbN 5.57-24.68; SmN/YbN 1.30-6.60), and 4) LREE enriched and MREE depleted u-shaped patterns (LaN/CeN 1.92-2.02; CeN/YbN 4.53-5.29; SmN/YbN 0.35-0.49). Type 3 clinopyroxenes show trace element signatures produced by reaction of carbonate-rich fluids. Type 4 clinopyroxens are significantly enriched in U, Th, Pb, Zr and show U-shaped REE patterns, which may have been caused by subduction-related hydrous fluids or melts. The Sr-Nd-Pb isotopic compositions of clinopyroxenes show a gradual variation from depleted MORB (DMM) type 1 clinopyroxnes to very enriched type 2 and 3 clinopyroxenes.