Experimental evidence for very low solubility of rare-earth elements in C02-rich fluids at mantle conditions

Abstract

CHEMICAL compositions of mantle-derived magmas and mantle xenoliths suggest that some regions of the mantle were enriched in basaltic components (CaO, FeO, Al2O3 and alkalis), large-ion lithophile elements, or both before melting or disruption and entrainment1–6. Some peridotite xenoliths that are depleted in basaltic components contain clinopyroxenes with chondrite-normalized La/Yb ratios of >30 and La contents of 100-times chondritic. Other xenoliths with similar major-element chemistry contain clinopyroxenes with LaN/YbN = 0.1–1 and [La] = 1 – 10-times chondritic (reviewed in ref. 7) Enrichment in light rare-earth elements without concomitant increase in basaltic components has been called 'cryptic metasomatism'8. Here we present the results of experiments at high temperature and pressure, which show that CO2-rich fluids, the postulated agents of cryptic metasomatism9, dissolve only minor amounts of rare-earth elements, and cannot therefore be responsible for such enrichments.