Isotopic diversity in Mangaia melt inclusions: Mantle source or crustal assimilation?
Intellectual Merit: Olivine-hosted melt inclusions (MIs) are trapped by growing phenocrysts in magma conduits at depth and provide ?snapshots? of diverse melt compositions before complete melt aggregation. MIs reveal major and trace element diversity that is not clear from analyses of whole rock lavas alone and the origin of this diversity is a source of vigorous debate. One end member hypothesis is that the MI diversity reflects heterogeneity in mantle source compositions. Alternatively, MI diversity may result from magmatic processes, including crustal assimilation. In one case, olivine-hosted MIs from a single lava from the island of Mangaia (Cook-Austral Islands) exhibit extreme Pb-isotopic variability spanning half the range observed in all ocean island and MORB lavas. Surprisingly, some of these MIs contain Pb that is significantly less radiogenic than observed in any Mangaian lavas. These data give rise to many fundamental questions. Why is it that the inclusions are not isotopically representative of the bulk magma? Is this because MIs from a single lava sample an array of melts derived from isotopically-diverse mantle sources? Or, does the isotopic diversity reflect assimilation of the lithosphere during magma ascent? It is proposed to measure trace element and isotopic compositions of MIs in a new set of samples from Mangaia so that these comprehensive data can be used to evaluate crustal assimilation and other processes responsible for the Pb isotopic diversity. Preliminary study of 14 MIs from Mangaia hint at a possible influence of crustal assimilation, but the data are limited additional data are required to conclusively describe and assess the competing hypotheses. In particular, if the Pb isotopic diversity in Mangaia MIs is caused by crustal assimilation processes, it is expected that Pb-isotopic compositions will correlate with tracers of assimilation (e.g., high Cl and B, Sr isotopes), whereas this is unlikely to be the case if the isotopic diversity reflects diverse mantle sources beneath Mangaia. To enable comprehensive evaluation of these alternatives, funds are requested to collect new high-precision Pb- and Sr-isotopic data as well as trace element and volatile data for an expanded collection of MIs from primitive Mangaia lavas. The proposed research aims to constrain the relative roles of magmatic processes and mantle sources in generating heterogeneous inclusions, and will provide insights into magma transport and emplacement processes in the lithosphere.
Broader Impacts: The proposed study will support an early-career non-tenured faculty and one PhD student. The project will also involve several undergraduate advisees. While carrying out fieldwork in the Cook Islands in 2010, the PI taught students about local volcanism at the Mangaia high school and interviewed with the local newspaper on Rarotonga, resulting in an article that describes his research there. The PI also met with the Prime Minister of the Cook Islands to discuss the relevance of the proposed work.
