Late Cenozoic basalts from the foothills of the Royal Society Range, Antarctica are located within the west Antarctic rift system. The basalts were collected from vents that range in size from small to large cones of intercalated scoria, agglutinate and lavas. Dating indicates that volcanism occurred between ∼14 Ma and< 1.0 Ma. The samples are basanite and alkali basalt and are fresh, porphyritic and vesicular containing four primary phases; olivine, clinopyroxene, plagioclase and spinel. The phenocrysts consist of olivine (Fo87-78) and clinopyroxene (calcic diopside) and groundmass phases spinel (titanomagnetite) and plagioclase (An72-58). The basalts are relatively unfractionated and have major and trace elements and isotopic (Sr and Nd) signatures comparable to other basalts. The variation in o 180 values for clinopyroxene and olivine range from 4.52 to 5.45%o, and are comparable with those from mantle peridotites, ocean island basalts, and other continental alkaline basalts.
Many phenocrysts display disequilibrium textures with complex zoning and resorption. Olivine consists of two populations based on forsterite content (Fo ∼86% and ∼79%), both populations have homogenous cores that are near equilibrium with wholerock compositions. However, many olivines display 'strong' normal zoning, indicating an abrubt period of crystal growth and are not in equilibrium with whole-rock values. Clinopyroxene phenocrysts display oscillatory zoning defined by variations in Mg#, Ti + Al4 and Ca, and are close to equilibrium with whole-rock compositions. Plagioclase and spinel are not zoned. Oxygen isotopes of coexisting olivine and clinopyroxene were analyzed and some have Al80cpx-olivine values of ∼0.40, which indicate equilibrium, while many others do not (Al80cpx-olivine ≤0).
The evidence for disequilibrium demonstrates that crystals have grown in different melts which were mixed together. The two populations of olivine represent crystallization in unique magmas and their 'strongly' normally zoned rims represent late stage crystallization after the 'mixed' magma cooled before eruption. Oxygen isotopes correlate with major and trace elements for brown-colored clinopyroxene, indicating fractional crystallization. In addition, oxygen isotopes show a weak correlation with 87Sr/86Sr, indicating mixing with a low ISO and high 87Sr/86Sr 'contaminant' that was either assimlated by the magmas or mixed with magmas from a different melt source.
|School:||Bowling Green State University|
|School Location:||United States -- Ohio|
|Source:||MAI 57/05M(E), Masters Abstracts International|
|Keywords:||Basalts, Chemical zoning, Disequilibrium, Royal society range|
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