Search for the mantle nitrogen in the ultramafic xenoliths from San Carlos, Arizona

Abstract

Mineral separates (olivine (Ol), clinopyroxene (Cp), and orthopyroxene (Op)) of ultramafic xenoliths from San Carlos have been analyzed for nitrogen and noble gases in an effort to evaluate the isotopic signatures for the sub-continental mantle. The concentrations of noble gases are generally small, but in one clinopyroxene separate (Cp1) they are about an order of magnitude higher. The gases seem mostly to be sited in fluid inclusions. Elevated isotopic ratios of ²⁰Ne/²²Ne, ⁴⁰Ar/³⁶Ar, ^129,134,136Xe/¹³²Xe with respect to air clearly show the presence of mantle noble gas components in San Carlos xenoliths. Isotopic ratios of Xe and Ar show that the noble gas systematics in San Carlos xenoliths can be explained as a mixture of MORB and air-like components. Nitrogen contents in these separates range from 0.1 to 1.5 ppm, the δ¹⁵N varying from 2 to 5% in totals and from -9 to +10% in the temperature fractions. The nitrogen data can be explained by the presence of at least three components: (a) a light nitrogen component (δ¹⁵N ≤-9%) of mantle origin, (b) a heavy nitrogen component (δ¹⁵N≥15%) from recycled materials and (c) an air-like nitrogen component (δ¹⁵N=0%) which is incorporated during the emplacement of the host magma. While the noble gas isotopic ratios cannot clearly distinguish between an atmospheric and a recycled component, δ¹⁵N clearly resolves these two. Using the noble gas and nitrogen isotopic systematics and the end member values from the literature, a contribution of 30 to 60% recycled nitrogen component has been estimated in the mantle source region of San Carlos xenoliths.