Ultramafic xenolith suites from the Bering Sea Volcanic Province (BSVP) of northwest Alaska, the Bering Sea, and northeast Russia provide an excellent opportunity to examine and describe the geochemical evolution of mantle lithospheric materials at the boundary of two major plates, Eurasia and North America. A well-documented, diverse suite of xenoliths exists, but little is known about these rocks and the lithospheric mantle from which they were generated. The principal objectives of this research are to: 1) model the magmatic and metasomatic processes that affect upper mantle lithosphere during rifting; 2) determine the provenance of magmas which intrude the lithosphere at various times during its evolution, and 3) ascertain how much material is depleted in large ion lithophile elements in the lithospheric mantle beneath the BSVP. Understanding the processes that control lithospheric evolution during extension will provide important constraints on thermo-mechanical models of rifting and on the chemical evolution of the mantle beneath continents.
On Nunivak Island, many coarse-grained ultramafic xenoliths contain amphibole and a small number contain phlogopite and apatite. These xenoliths have similarities in rare earth elements' patterns and neodymium and strontium isotopic compositions to the host rocks, suggesting a strong link between the two types of xenoliths. They also show 1) light rare earth elements and heavy rare earth elements, 2) normalized abundance ratios of greater than one, and 3) their potassium, rubidium, strontium, and barium and light rare earth element abundances exceed the estimates for primitive mantle. These characteristics support the idea that amphibole in Nunivak Island xenoliths has a metasomatic origin. However, it is not known whether this phenomenon was pervasive in the mantle lithosphere below the BSVP or limited to a few localities suggesting metasomatism by subduction.
To advance our understanding of how the mantle evolves through introduction and expulsion of melts and metasomatic fluids, the Principal Investigator will determine the: 1) major element concentrations of whole rock samples by X-Ray fluorescence and of minerals by electron microprobe; 2) trace element concentrations by secondary ionization mass spectrometry on thin sections; 3) platinum-group element abundance patterns for peridotite bulk samples by the multi-collector inductively coupled plasma mass spectrometer; 4) oxygen fugacities in spinel by electron microprobe and Mossbauer spectroscopy; and 5) initial strontium, neodymium, and lead isotopic compositions and corresponding elemental concentrations on clinopyroxenes and glass, and amphibole and phlogopite when present.
