Relations of noble gas abundances to petrogenesis and magmatic evolution of some oceanic basalts and related differentiated volcanic rocks

We report the results of noble gas analyses (Ne, Ar, Kr, and Xe) of a transitional basalt from the East Pacific Rise; a mantle xenolith, alkali basalt and trachyte from Guadalupe Island; and a basalt and icelandite from Isla Tortuga. The results for the East Pacific Rise basalt and comparison with the noble gas compositions of glassy mid-ocean ridge tholeiites indicate that the heavy noble gas patterns of these basalts can be accounted for by mixing of a juvenile reservoir with an atmosphere-related reservoir. This mixing may take place during hydration of the glassy basalts. Comparison of the noble gas compositions of these submarine basalts with those of the subaerial oceanic extrusives studied suggests that subaerial extrusives may provide noble gas samples which are less contaminated with air gases than do submarine extrusives. Our results for Guadalupe and Tortuga basalts and their differentiates provide evidence for the exsolution and loss of a gas (fluid) phase accompanying or subsequent to fractional crystallization at shallow depths. The gas loss probably took place prior to extrusion and was apparently rapid, since it did not fractionate the heavy noble gases.