SELENIUM, TELLURIUM, ARSENIC AND ANTIMONY CONTENTS OF PRIMARY MANTLE SULFIDES

Sulfide phases in peridotitic mantle xenoliths from Ichinomegata in Japan, Nunivak Island in Alaska, and southern Africa occur as globular grains within silicate minerals, and along grain boundaries of silicate and oxide minerals. The morphology of the sulfide grains suggests that the sulfide liquid was not interconnected in the mantle, even within single samples. This is supported by compositional variations of sulfides. Proton-induced X-ray excitation (PIXE) micro-analyses of 53 sulfide grains larger than 20 µm in diameter reveal a wide range of compositions: Se 21‐280 ppm, S/Se 1120‐18800; Te up to 103 ppm, S/Te 2,600‐31,000; As up to 670 ppm, As/S up to 208 � 10 ‐5 , and Sb, up to 146 ppm, Sb/S up to 37 � 10 ‐5 . S/Se and Ni show an inverse correlation in the sub-arc xenoliths, suggesting preferential retention of Se in the mantle during partial melting. Sul fur is the most readily removed element, whereas Te is the most likely retained element in the residual mantle. This interpretation is consistent with low S/Se values for sulfides from refractory mantle wedges compared to values for the primitive mantle, general ly high S/Se and Se/Te values in basalts, and low S/Se and Se/Te in boninites compared to mid-ocean-ridge basalts. The evidence implies that S/Se values of mantle-derived magmas may vary depending on the degree of partial melting and previous melting history in the source mantle. High concentrations of As and Sb in arc magmas are considered to be supplied from subducting slabs. Our data showing low levels of As and Sb in sulfides from sub-arc mantle suggest their fast removal from mantle wedges compared to the rate of supply of these elements from subducting slabs. Alternatively, these elements may be transported by a fluid phase from slabs to the site of partial melting without residing in the mantle.