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U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, U.S.A.
Hawaiian volcanoes erupt alkalic lavas during three of four eruptive stages. Stage 1 is characterized by submarine eruption of alkalic basalt and basanite followed by tholeiitic basalt which is exclusively erupted during stage 2. Stage 3 consists of a post-caldera-collapse capping of alkalic basalt and related differentiates. Stage 4 consists of post-erosional eruptions of alkalic basalt, basanite, nephelinite and melilitite. Eruption rates and volumes increase from stage 1 to stage 2 and then decline through stages 3 and 4. Stage 1 and 4 lavas contain deep-seated lherzolite xenoliths (even some garnet-bearing xenoliths in stage 4 lavas) whereas stage 2 lavas contain only very shallow cumulates, veins, dykes and sills; stage 3 lavas contain mainly dunite, wehrlite and various cumulates. The xenolith assemblages reflect the development of magma storage reservoirs which act as hydraulic filters in removing any xenoliths entrained below the magma storage reservoirs. The development of the magma storage system is a direct response to varying magma supply rates during the four eruptive stages.
Trace-element, isotopic and rare-gas data all indicate that Hawaiian lavas are generated from heterogeneous sources consisting of at least three distinct compositions. Mixing of the sources or of partial melts derived from these sources is inevitable; however, the characteristics of the mixing end-members are not well constrained. In a general way, early and late lavas are generated by small degrees of melting of sources enriched with less incompatible elements, whereas the voluminous shield tholeiites are generated by large degrees of melting of sources enriched with more incompatible elements.
