Rare earth element evidence for differentiation of McMurdo volcanics,Ross Island,Antarctica

Eighteen samples of the McMurdo volcanics on Ross Island, Antarctica consisting of basanitoid, trachybasalt and phonolite have been analyzed for rare earth elements (REE) in order to determine the details of differentiation using quantitative trace element modeling. The basanitoids have REE patterns similar to those for alkali basalts or nephelinites from ocean islands. Since there is no correlation between REE and silica contents among five basanitoids, some of the variability in the REE contents must be related to the extent of partial melting, variation in the residual mineralogies of the mantle during melting, or to inhomogeneities in the REE composition of the mantle.In order to explain the data, more than one differentiation sequence is necessary. In each case a basanitoid melt is the parent which differentiates to trachybasalt upon separation of olivine, clinopyroxene, spinel, ±kaersutite±plagioclase±apatite. If clinopyroxene, kaersutite, anorthoclase, plagioclase and apatite separate from a trachybasalt melt, a mafic phonolite results.If, however, no kaersutite is involved, an anorthoclase phonolite results. A distinct type of mafic phonolite results if kaersutite is one of the minerals that separates from the anorthoclase phonolite. If the anorthoclase phonolite precipitates plagioclase and anorthoclase and if the melt reacts with plagioclase-rich continental rocks, a trachyte results.Formerly spelled: Shine Soon Sun