Basalts from Rose Atoll, American Samoa
Abstract
Three specimens of lava float collected in 1939 from Rose Atoll consist of three distinct basalt types: holocrystalline olivine tholeiite, coarse vesicular picrite basalt and olivine-poor transitional basalt; the tholeiite contains coarser, late-stage segregations with a glassy, silicic mesostasis. In mineralogy and chemistry these basalts most closely resemble Ta'u Group lavas of the neighbouring Manu'a Islands. Differences exist that do not suggest their transport to Rose Atoll, even though no in situ basalts are known there. Incompatible primitive mantle-normalized trace element plots show strong depletion in K and Sr and enrichment in U, Pb and La. In Ba/Nb versus La/Nb plots Rose basalts lie between normal mid-ocean ridge basalt (N-MORB) and low 87Sr/86Sr oceanic island basalt (OIB) fields. They lie outside many plume-related OIB fields, including plume-related Samoan basalts. Trace element ratios for the Rose samples show little correspondence with end member MORBs and OIBs. This, and temporal and geographic plume reconstructions, indicate that the Rose basalts are derived from melting of unusual or mixed lithospheric sources. They seem unrelated to the main phases of Samoan plume activity, now located at Vailulu'u Seamount.
