Abstract:
We investigated the relationship between noble gas signatures and sample textures among glassy fragments. Glassy MORB fragments were classified based on their texture. By handpicking under a binocular, they were divided into three groups: (1) heterogeneous “outer” part, (2) homogeneous “middle” part, and (3) “inner” neighbors of the transition zone. Their noble gas signatures were analyzed by the step heating method. The state of homogeneities of the samples identified under a microscope was confirmed semi-quantitatively by measuring refractive indices of the fragments. The “outer” heterogeneous part characterized by devitrified and/or color changed regions was significantly affected by the atmospheric component as indicated by the relatively large amount of gases released with almost atmospheric 40Ar/36Ar and 20Ne/22Ne in the 400 °C fraction. The transition “inner” part of glassy rind adjacent to the crystallized interior which is characterized by the appearance of quenched microcrystals has lost the magma-derived component and is contaminated by some kinetic processes: it showed a small amount of noble gases except for Ne with a low 20Ne/22Ne ratio. The homogeneous “middle” part turns out to be least affected by contamination and is regarded to mostly retain magma-derived noble gas signatures. It is characterized by the most abundant He and the highest 40Ar/36Ar as a total ratio with similar values in both the middle and the high temperature fractions. This part is also characterized by lower refractive index values and a small deviation compared to the nonhomogeneous parts. Hence, for noble gas analyses of MORB glasses, it is recommended to select homogeneous “clear” glasses by excluding color changed and/or devitrified weathering part and quenched microcrystals.
