Rare earth element geochemistry of Late Cenozoic alkaline lavas of the McMurdo Volcanic Group, Antarctica

Abstract

Rare earth element (REE) concentrations were determined in 16 Ross Island and northern Victoria Land alkaline lava samples which were representative of four lava lineages of the McMurdo Volcanic Group, Antarctica. A kaersutite and two feldspar mineral separates were also analysed. Two of the lava lineages, a basanite to nepheline benmoreite and a basanite to phonolite, have similar chondrite-normalized REE fractionation patterns, with a continuous enrichment of light and heavy REE and depletion of middle REE. The patterns result from the fractionation of olivine, clinopyroxene, spinels, feldspar, kaersutite and apatite. Kaersutite is an important fractionated phase responsible for the middle REE depletion. Another of the lava lineages is mildly potassic with trachyandesite to peralkaline K-trachyte lavas which have partly overlapping REE fractionation patterns. There is a depletion in REE from tristanite to K-trachyte. Fractionation of olivine, clinopyroxene, feldspar and apatite probably control the REE chemistry of the lineage, greater degrees of apatite fractionation deplete the K-trachyte in REE relative to the tristanite. Feldspar fractionation in the genesis of the peralkaline K-trachyte is shown by a large negative Eu anomaly (Eu/Eu∗ = 0.10). A nepheline hawaiite to anorthoclase phonolite lava lineage from the Erebus Centre shows enrichment of REE, although minor overlapping in the middle REE does occur. Anorthoclase phonolite has a positive Eu anomaly (Eu/Eu∗ = 1.31), indicating possible accumulation of anorthoclase. The lineage resulted from fractionation of olivine, clinopyroxene, magnetite and apatite.