A Tale of Two Planets

Abstract

Most of Earth's volcanism occurs at plate boundaries—mid-ocean ridges and island arcs. Most of the 3He and other noble gases also are expelled from the interior at these locations. A small amount, compared to the global budget, of magma (<10%) and 3He («1%) escapes at mid-plate volcanoes, the socalled “hotspots.” Locations of hotspots are controlled by stresses and cracks, not by deep, narrow mantle upwellings. Most, or all, of Earth's volatile inventory appears to have been brought in by a late veneer. The high 3He content and high He/Ne and He/Ar ratios of mid-ocean ridge basalts, plus the excess 129Xe and 136Xe, suggest that the ridge basalt reservoir is relatively undegassed and that there is no primordial undegassed reservoir. The two prevailing views of the origin of Earth are summarized in the context of this information.