A Test of a Quartz Eclogite Source for Parental Aleutian Magmas: A Mass Balance Approach

The observation that magmatic conduits evolve with time has provided a new framework within which to interpret Aleutian compositional and isotopic data. Accordingly, a two-stage physical model of Aleutian magma genesis has been developed and used to model quantitatively geochemical and isotopic mass balance relations. Parental magmas, approximated by high-alumina basalts, are presumed to be generated by partial melting of eclogite and subducted sediment. The incorporation of sediment produces elevated parental magma Sr and Pb isotopic ratios but lowered εNd relative to MORB. If these magmas ascend along well-established conduits, their isotopic characteristics are little changed. During ascent in immature conduits, interaction with oceanic lithosphere produces significant isotopic variability. According to this model, the isotopic and geochemical characteristics of parental magmas are fixed by mixing eclogite and sediment melts. As a test, we have quantitatively examined the mass balance relations between parental high-alumina basaltic magmas and these end members. Because each end member is characterized by considerable geochemical and isotopic variability, a new mathematical approach has been used. Major elements and Sr, Nd and Pb concentration data are inconsistent with simple two-component mixing (i.e., eclogite melt + pelagic sediment) for the production of parental high-alumina basaltic magmas. Three-component mixing of eclogite melt (44-70%), pelagic sediment (21-44%), and carbonate (8-13%) will produce the presumed parental magma. These proportions are controlled mostly by major element data. Our conclusions differ from other results in the importance attributed to carbonate sediment and the relatively large, total sediment input. Because a range of viable mixing proportions has been found, the proposed physical model is, at the least, internally consistent.