Selenium and tellurium systematics of the Earth's mantle from high precision analyses of ultra-depleted orogenic peridotites

Selenium and tellurium, like the highly siderophile elements, may constitute key tracers for planetary processes such as formation of the Earth's core and the Late Veneer composition, provided that their geochemical behavior and abundances in the primitive upper mantle (PUM) are well constrained. Within this scope, we have developed a high precision analytical method for the simultaneous determination of selenium and tellurium concentrations from a single sample aliquot and for various rock matrices, including ultra-depleted peridotites. The technique employs isotope dilution, thiol cotton fiber (TCF) separation and hydride generation ICP-MS. A selection of international mafic and ultramafic rock reference materials BIR-1, BE-N, TDB-1, UB-N, FON B 93 and BHVO-2 with a range of 30-350 ppb Se and 0.7-12 ppb Te show external reproducibilities that generally range from 3% to 8% for Se and 0.4% to 11% for Te (two relative standard deviations (r.s.d.)). We have applied this method to a suite of refractory mantle peridotites (Al2O3 <1.5 wt.%) from Lherz, previously shown to be strongly and uniformly depleted in Se, Te and incompatible elements by high degree of partial melting (20 ± 5%). While some fertile lherzolites display broadly chondritic values (Se/Te = 9), the ultra-depleted harzburgites show highly fractionated Se/Te (up to 31), which correlate with the Te concentrations. The fractionation trend is displayed by the depleted peridotites and also observed within multiple analyses of a single Lherz harzburgitic sample (64-3) and altogether results from the very heterogeneous distribution of Te trace phases present in the aliquot analyzed. Our results are in agreement with experimental studies that predict a more incompatible behavior of Te compared to Se during incongruent partial melting of mantle sulfides. In addition to re-fertilized lherzolites, depleted harzburgites therefore provide new insights into the geochemical behavior of Se and Te in the Earth's mantle.