An overview of the volatile systematics of the Lau Basin – Resolving the effects of source variation, magmatic degassing and crustal contamination

Abstract

The Lau Basin erupts lavas with a range of geochemical features reflecting a complex history of interaction involving different mantle sources. The Valu Fa Ridge (VFR) and Mangatolu Triple Junction (MTJ) region have lavas with arc-like characteristics, Niuafo’ou Island (NV), Peggy Ridge and Central and Eastern Lau Spreading Centers (PR, CLSC and ELSC) erupt mid-ocean ridge basalt (MORB)-like volcanics, whereas the Rochambeau Bank (RB) has features akin to ocean island basalt (OIB). To characterize the volatile systematics of these various regions, we report a comprehensive study of 39 submarine lavas from these various eruptive centers encompassing analyses of the noble gases (He, Ne, and Ar) and carbon (CO2) – both isotopes and abundances – together with other major volatile phases (H2O, S, Cl, and F).Helium isotope ratios of the NV, MTJ, CLSC, and ELSC are MORB-like for the most part except for differentiated lavas that tend to have lower, more radiogenic 3He/4He values. The RB has considerably higher 3He/4He ratios (up to 23 RA in this work) which extend as far south as the PR. The influence of ‘plume-like’ sources in the RB is also apparent in Ne isotopes: RB samples follow a trend similar to Hawaiian basalts in 3-isotope neon space. However, RB lavas have lower 40Ar/36Ar (300–730) and higher [36Ar] than CLSC and ELSC, suggesting greater air contamination. Elemental He/Ne ratios (3He/22NeS and 4He/21Ne∗ where S=solar and *=nucleogenic) are high throughout the Lau Basin and identify the Lau mantle as one of only two high 3He/4He provinces worldwide with such an enrichment of He relative to Ne.Magmatic CO2 and δ13C fall in the range 7–350ppm and −28‰ to −6‰, respectively. RB lavas have less [CO2] and slightly lower δ13C than CLSC and ELSC. The lowest values are found among MTJ lavas. These lavas also have the highest [H2O], [F], [Cl], and [S] whereas the PR, ELSC and CLSC have the lowest. RB has intermediate [H2O]. We estimate primary [CO2] in primary melts using [CO2]–δ13C relationships, and find that RB lavas have higher [CO2] (∼935±168ppm) than ELSC/CLSC (638±115ppm). They also possess higher initial δ13C values, consistent with observations at other hotspot-related localities. However, there are no discernible differences in primary CO2/Nb ratios between mantle sources characterized by high 3He/4He and MORB-like ratios. On the other hand, reconstructed values are considerably higher than that envisaged for depleted MORB mantle based on olivine-hosted melt inclusions.