科研之友
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摘要
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%26apos;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). %26lt;br%26gt;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 He-3/He-4 values. The RB has considerably higher He-3/He-4 ratios (up to 23 R-A in this work) which extend as far south as the PR. The influence of %26apos;plume-like%26apos; 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 Ar-40/Ar-36 (300-730) and higher [Ar-36] than CLSC and ELSC, suggesting greater air contamination. Elemental He/Ne ratios (He-3/Ne-22(S) and He-4/Ne-21* where S = solar and * = nucleogenic) are high throughout the Lau Basin and identify the Lau mantle as one of only two high He-3/He-4 provinces worldwide with such an enrichment of He relative to Ne. %26lt;br%26gt;Magmatic CO2 and d C-13 fall in the range 7-350 ppm and -28 parts per thousand to -6 parts per thousand, respectively. RB lavas have less [CO2] and slightly lower d C-13 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]-delta C-13 relationships, and find that RB lavas have higher [CO2] (similar to 935 +/- 168 ppm) than ELSC/CLSC (638 +/- 115 ppm). They also possess higher initial delta C-13 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 He-3/He-4 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.
- 出版日期2012-5-15
