Oxygen isotope record of magmatic evolution of alkaline volcanic rocks at The Pleiades, northern Victoria Land, Antarctica

Abstract

Oxygen isotopes are used to examine the well-documented magmatic evolution of alkaline volcanic rocks at The Pleiades in northern Victoria Land (NVL), Antarctica. Oxygen isotopes were measured in olivine, clinopyroxene, and plagioclase phenocrysts to better understand the origin and evolution of the sodic and potassic differentiation lineages. The volcanic rocks at The Pleiades evolved from a parental basanite by fractional crystallization in crustal magma chambers. Olivine which crystallizes first in the mafic magmas provides the initial oxygen isotope composition of the magmatic lineages. The δ18OOL values for the mafic lavas in the sodic lineage are lower than the potassic lineage. The primary melt derived from the metasomatized lithospheric mantle may have consumed a low-δ18O amphibole metasome. Subsequently, the melt would have evolved to the normal-δ18O potassic lineage magma by a large contribution from surrounding peridotite. In contrast, the sodic lineage magma might preserve low-δ18O characteristics because of insufficient reaction with the surrounding mantle peridotite. Intermediate rocks of the potassic lineage exhibit a wide variation in their oxygen isotope compositions and it deviates from the theoretical normal-δ18O trend. Hence, variable δ18OOL values of the intermediate rocks suggest that high-δ18O recorded in olivine could be reconciled with an assimilation of crustal rocks in NVL, and hydrothermally altered material may have contributed to low-δ18O signature of the olivines.