Variations of noble gas abundances and isotope ratios in a single MORB pillow

Abstract

To evaluate variations of noble gas signatures within a single MORB pillow caused at its extrusion, all noble gas abundances and isotope ratios (from He to Xe) were analyzed in different portions of a pillow basalt from the glassy rim to the interior by slicing off at every 2.5mm in thickness (#1 to #7). 40Ar/36Ar and He contents are high in glassy parts, and rapidly decrease toward the inner crystallized parts. Although 3He/4He is almost uniform among glassy parts (#1 to #3), the outermost part (#1) has a lower 40Ar/36Ar than the inner glassy parts (#2 and #3). This indicates incorporation of low 40Ar/36Ar component into the glassy rim from the surrounding materials at its solidification. Abundances of heavier noble gases, especially Xe, greatly increase toward crystallized parts. It could not be explained by incorporation of surrounding noble gases dissolved in the seawater and requires some other component such as fractionated product during crystallization by solid-melt partition or derived from fine sediments. In order to obtain magmatic signature, outermost glassy part should be eliminated as well as crystalline parts.