Arsenic and Sb Abundences in the Earth Mantle

Introduction: Mantle derived peridotite xenoliths ranging in modal composition between fertile lherzolites and modally metasomatized harzburgites has been analyzed by instrumental neutron activation analysis (INAA) for As, Sb and REE. In many melt depleted mantle samples (harzburgites) the incompatible element concentrations are much higher than those estimated for the fertile mantle. The reason for this is that the subcontinental lithospheric mantle stabilised by melt extraction postdated by processes of trace element enrichment and/or mantle metasomatism. The rarely determined elements As and Sb in mantle rocks have been tried to use as geochemical tracers for understanding fluid-related processes postdating melt extraction processes. Analytical Procedure: Aliquots of homogenized sample powder was iradiated for six hours at the TRIGA-Reactor of the Institute of Nuclear Chemistry with a thermal neutron flux of 7x10 neutrons cm sec. Instrumental neutron activation analyses were performed at the Max-Planck-Institute for Chemistry. After irradiation the samples were -counted on Ge(Li)and high-purity coaxial (HP)Ge detectors to determine As (T1/2=26.32 h) at 559.1 keV and Sb (T1/2=2.7 d) at 564.0 keV. Al vs Sm: Two groups of mantle xenoliths from the Eifel volcanic field (Germany) have been investigated: (Ia) low temperature, modally metasomatized xenoliths that have experienced high enrichment in LREE and MREE by metasomatizing fluids (Ib) and high temperature anhydrous xenoliths ranging from LREEdepleted to moderately enriched in LREE and MREE [1]. In Ib-xenoliths Al and Sm are well correlated, reflecting increasing depletions of Al and Sm with increasing degree of partial melting. The mantle sample with the highest Al and Sm has nearly PUM (primitive upper mantle) contents (4.45 wt.% Al2O3 and 406 ng/g Sm [2]). The Ia-xenoliths show a large scatter for Sm unrelated to Al2O3 variations indicating Sm enrichment during aqueous fluids [3]. As: High Sm in the Ia-xenoliths is associated with high As. It is conceivable that the fluid phase that contained the Sm also delivered As, perhaps by dissolving sulfides the most likely carrier phases of As.