Temporal-compositional-isotopic trends in rejuvenated-stage magmas of Kauai, Hawaii, and implications for mantle melting processes

Abstract

Primitive, low-silica and high-alkali magmas that erupt late in the evolution of most ocean island volcanoes are highly enriched in incompatible trace elements, yet their isotopic compositions require a time-integrated mantle source history of incompatible-element depletion. Reconciling these observations has traditionally required either extremely low degrees of partial melting of depleted mantle (commonly less than 0.2%), invoking an unusual mantle source recently enriched in incompatible elements or extensive melt-mantle interaction. Analyses of stratigraphic sequences of rejuvenated-stage volcanics of Kauai, Hawaii show previously unrecognized isotopic-trace element correlations, as well as temporal variations within monogenetic lava sequences that provide evidence of variable degrees of melting of several distinct mantle sources. Inter-element and isotopic-trace-element correlations indicate little or no chromatographic effects on melt compositions, inconsistent with the expected effects of significant melt-mantle reaction as the source of their incompatible-element enrichment. Trace-element compositions of rejuvenated-stage magmas can be produced by melting of typical depleted mantle sources only if they are mixtures of small- and large-degree (0.1% and 2–15%, depending on source mineralogy) melts of isotopically distinct sources. The simplest model for the Koloa Volcanics, however, consistent with previous interpretations of other Hawaiian lavas, is that they are derived from a range of incompatible-element enriched mantle sources variably metasomatized by small-degree melts of depleted mantle. Isotopic-trace-element trends in the Koloa magmas (of the opposite sense as the overall Hawaiian trend) are best explained by a positive correlation between the extent of source metasomatism and degree of melting to produce the Koloa magmas. Systematic decreases in incompatible element concentrations within individual eruption sequences probably represent sequential eruption of progressively larger-degree melt, possibly caused by vertical zonation in extent of melting in the source regions, or extraction of low-degree melts from surrounding mantle by early-ascending magma batches.