Silicon and oxygen isotopic composition of igneous rocks from the eastern Manus Basin

Abstract

This paper reports silicon and oxygen isotopes of 20 kinds of igneous rocks and their major elements from the eastern Manus Basin. Combining silicon and oxygen isotopic data from other studies, we suppose that both delta Si-30 and delta O-18 values increase with the increasing of SiO2 content. It means that the fractionation of silicon and oxygen isotopes are affected by the silica content. The positive correlation between CaO/Al2O3 ratios and MgO and that between Si/Al and SiO2 content indicate that clinopyroxene is the predominant mineral phase in our samples. We suppose that the fractionation of silicon and oxygen isotopes are influenced by mineral fractional crystallization. Probably, it is due to their different silicon and oxygen bridges. In this study, the delta S-30(imean value) = -0.17aEuro degrees +/- 0.17aEuro degrees and delta O-18(mean value) = +6.07aEuro degrees +/- 0.57aEuro degrees are higher than normal delta Si-30 and delta O-18 values of mantle, and we propose that these igneous rocks in the eastern Manus Basin are affected by hydrothermal alteration.