Mechanisms of magmatic gas loss along the Southeast Indian Ridge and the Amsterdam –St. Paul Plateau

New analyses of He, Ne, Ar and CO sub(2) trapped in basaltic glasses from the Southeast Indian Ridge (Amsterdam-St. Paul (ASP) region) show that ridge magmas degas by a Rayleigh distillation process. As a result, the absolute and relative noble gas abundances are highly fractionated with super(4)He/ super(40)Ar* ratios as high as 620 compared to a production ratio of similar to 3 (where super(40)Ar* is super(40)Ar corrected for atmospheric contamination). There is a good correlation between super(4)He/ super(40)Ar* and the MgO content of the basalt, suggesting that the amount of gas lost from a particular magma is related to the degree of crystallization. Fractional crystallization forces oversaturation of CO sub(2) because CO sub(2) is an incompatible element. Therefore, crystallization will increase the fraction of gas lost from the magma. The He-Ar-CO sub(2)-MgO-TiO sub(2) compositions of the ASP basalts are modeled as a combined fractional crystallization-fractional degassing process using experimentally determined noble gas and CO sub(2) solubilities and partition coefficients at reasonable magmatic pressures (2-4 kbar). The combined fractional crystallization-degassing model reproduces the basalt compositions well, although it is not possible to rule out depth of eruption as a potential additional control on the extent of degassing. The extent of degassing determines the relative noble gas abundances ( super(4)He/ super(40)Ar*) and the super(40)Ar*/CO sub(2) ratio but it cannot account for large (> factor 50) variations in He/CO sub(2), due to the similar solubilities of He and CO sub(2) in basaltic magmas. Instead, variations in CO sub(2)/ super(3)He ( identical with C/ super(3)He) trapped in the vesicles must reflect similar variations in the primary magma. The controls on C/ super(3)He in mid-ocean ridge basalts (MORBs) are not known. There are no obvious correlated variations between C/ super(3)He and tracers of mantle heterogeneity ( super(3)He/ super(4)He, K/Ti etc.), implying that the variations in C/ super(3)He are not likely to be a feature of the mantle source to these basalts. Mixing between MORB-like sources and more enriched, high super(3)He/ super(4)He sources occurs on and near the ASP plateau, resulting in variable super(3)He/ super(4)He and K/Ti compositions (and many other tracers). Using super(4)He/ super(40)Ar* to track degassing, we demonstrate that mixing systematics involving He isotopes are determined in large part by the exte