Iceland Mantle Plume

Abstract

O'HARA¹ has objected to my interpretation concerning the trace element chemistry of lavas erupted along the Iceland-Reykjanes Ridge System² on the ground that: (1) Such magmas are not “primary magmas” but residual liquids; (2) instead, such lavas have suffered prior to eruption extensive “gabbro fractionation” at low pressure (olivine-augite-plagioclase extraction), and/or “eclogite fractionation” at higher pressure (clinopyroxenegarnet extraction in a 50–50% mixture); (3) the “extent of fractionation” is directly proportional to the elevation of the surface on which these lavas have been erupted, thus increases toward Iceland, and is the principal cause of trace element variations along the ridge. According to O'Hara, the lavas are derived from a unique and homogeneous mantle at the upper mantle-transition zone boundary and not, as I prefer², from two distinct mantle sources, one forcefully rising as a primary hot mantle plume (PHMP) beneath Iceland and interacting with the depleted low velocity layer (DLVL).