Abstract
We report new stepped heating He, Ar, CO 2 and water data on a petrogenetically diverse suite of lavas from the Manus back-arc basin, where a plume component has previously been identified. The aim of this study is to evaluate the superimposed effects of degassing and contamination in order to identify mantle source characteristics. CO 2 abundances and carbon isotopes in both the vesicle ([CO 2] up to 180ppm; δ 13C as low as -33.6 ‰) and glass ([CO 2] up to 270ppm; δ 13C as low as -34.3 ‰) phases reveal that samples have been modified by varying degrees of degassing. High water concentration samples (back-arc basin basalts (BABB) and arc type samples) show the highest degrees of degassing (i.e. lower δ 13C values and lower CO 2 contents). The results are modelled for both the glass and vesicle phases using batch and fractional degassing models. Parental melt compositions can be constrained to show the following CO 2 concentration trend: arc-type > BABB s.r. (southern rift) > MORB-2, E-MORB, X-BABB (extreme BABB), BABB > MORB-1 and MORB-smt. 4He/ 40Ar∗ ratios of samples (14.6–1100) are consistent with residual volatiles from a degassed source. Variations in CO 2/ 3He values are likely due to degassing, followed by contamination from a crustal source (either the subducting Solomon Sea Plate or the pre-existing crust through which the lavas erupt), as evidenced by high K 2O/TiO 2 ratios and low δ 13C. The CO 2/ 3He of the Manus plume is best estimated by the MORB-smt and MORB-1 samples at 3.1 ± 0.6 x 10 9. This value is similar to previous estimates of plume CO 2/ 3He values, which are either equal to or slightly greater than the upper mantle average of 2 x 10 9.
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