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

다국어 초록 (Multilingual Abstract)

<P><B>Abstract</B></P><P>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 (CO<SUB>2</SUB>) – both isotopes and abundances – together with other major volatile phases (H<SUB>2</SUB>O, S, Cl, and F).</P><P>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 <SUP>3</SUP>He/<SUP>4</SUP>He values. The RB has considerably higher <SUP>3</SUP>He/<SUP>4</SUP>He ratios (up to 23 <I>R</I><SUB>A</SUB> 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 <SUP>40</SUP>Ar/<SUP>36</SUP>Ar (300–730) and higher [<SUP>36</SUP>Ar] than CLSC and ELSC, suggesting greater air contamination. Elemental He/Ne ratios (<SUP>3</SUP>He/<SUP>22</SUP>Ne<SUB>S</SUB> and <SUP>4</SUP>He/<SUP>21</SUP>Ne<SUP>∗</SUP> where S=solar and *=nucleogenic) are high throughout the Lau Basin and identify the Lau mantle as one of only two high <SUP>3</SUP>He/<SUP>4</SUP>He provinces worldwide with such an enrichment of He relative to Ne.</P><P>Magmatic CO<SUB>2</SUB> and δ<SUP>13</SUP>C fall in the range 7–350ppm and −28‰ to −6‰, respectively. RB lavas have less [CO<SUB>2</SUB>] and slightly lower δ<SUP>13</SUP>C than CLSC and ELSC. The lowest values are found among MTJ lavas. These lavas also have the highest [H<SUB>2</SUB>O], [F], [Cl], and [S] whereas the PR, ELSC and CLSC have the lowest. RB has intermediate [H<SUB>2</SUB>O]. We estimate primary [CO<SUB>2</SUB>] in primary melts using [CO<SUB>2</SUB>]–δ<SUP>13</SUP>C relationships, and find that RB lavas have higher [CO<SUB>2</SUB>] (∼935±168ppm) than ELSC/CLSC (638±115ppm). They also possess higher initial δ<SUP>13</SUP>C values, consistent with observations at other hotspot-related localities. However, there are no discernible differences in primary CO<SUB>2</SUB>/Nb ratios between mantle sources characterized by high <SUP>3</SUP>He/<SUP>4</SUP>He 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.</P>