We investigated the Sr–Nd–Hf–Pb isotope compositions of clinopyroxene separates and strongly leached bulk rock powders from the basalt lavas on Raivavae Island in the South Pacific. These data allowed us to explore the magma chamber processes of the two basalt groups: the high-μ-type (μ=238U/204Pb, HIMU) Rairua basalts erupted at ~9Ma, and the depleted mantle (DM)-type Anatonu basalts erupted at ~6Ma. We assessed different analytical techniques for clinopyroxene sample treatment, including leaching, sample digestion, chemical separation, and determination of parent/daughter ratios for use in age corrections. The age-corrected initial Sr–Nd–Hf isotope compositions of clinopyroxene samples solely overlap on the bulk rock compositions of each basalt type. The initial Pb isotope compositions show significant differences between the clinopyroxenes and bulk rock samples in both the Rairua and Anatonu basalts. The clinopyroxenes have less radiogenic Pb than their hosts, reflecting the magma chamber processes. The overall variations in the bulk rock Pb isotopes are explained by mixing of melts from the HIMU- and DM-type sources in the plume. In detail, early stage Rairua magma was more influenced by DM melts from which Rairua clinopyroxene was crystallized. Subsequent supply of dominant HIMU melts drove the bulk rock magma chemistry more HIMU-like. The Anatonu DM magma was largely from depleted mantle that differs from the Pacific MORB source. Subordinate contribution of a Pacific alkali-basalt-like component is suggested from the clinopyroxene chemistry of Anatonu perhaps from impregnated pyroxenite in the lithospheric mantle.
Chemical Geology – Elsevier
Published: Apr 5, 2018
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