INAA, IDMS and SIMS comparative REE investigations of clinopyroxenes from mantle xenoliths with different textures

Abstract

Clinopyroxenes from mantle xenoliths from the Massif Central, France, have been chosen for a comparative study of rare-earth element (REE) abundances obtained by means of conventional and in situ analyses. Xenoliths from Hungary and the Canary Islands showing unusual trace-element or mineralogical features have also been investigated. Secondary ion mass spectrometry (SIMS) has been shown to be a powerful technique for both in situ and bulk investigations, competitive with instrumental neutron activation analysis (INAA) and isotope dilution mass spectrometry (IDMS) in the quantification of REE in clinopyroxenes. In addition, the application of SIMS for microspot analysis provides reliable information about REE concentrations with respect to texture. When SIMS is applied for bulk investigations, its major advantages are the simple and fast sample preparation, and the small quantity of material required. The results obtained in this paper indicate that grain-by-grain and internal homogeneity of clinopyroxenes, which is related to texture, determines the degree of agreement between SIMS, INAA and ID. The data for clinopyroxenes from equigranular xenoliths, which recrystallized to almost equilibrium conditions, show the best agreement. For clinopyroxenes from protogranular LREE-depleted xenoliths, the agreement is good when the whole clinopyroxene population has the same degree of depletion, and the metasomatic La and Ce enrichment is negligible. When complex geochemical processes are investigated, such as subsolidus reactions and interactions with fluids and melts, conventional bulk techniques are often inadequate and the SIMS technique is the only one capable of thoroughly describing the effects of element migration at a microscale. For a proper understanding of geochemical processes in the mantle it is here suggested that SIMS investigations in thin section are made prior to mineral selection for subsequent elemental and isotopic analysis by ID.