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.
Mantle plumes are thought to rise buoyantly from the deep mantle. A common hypothesis is that they transport ancient subducted oceanic crust and sediments to the shallow mantle where the crustally-derived materials melt and give rise to ocean island volcanism. Lavas erupted at the island of Mangaia, located in the south Pacific, are a famous example of ocean island volcanism. The Mangaia lavas sample a compositionally and isotopically extreme component among ocean island lavas erupted in the world’s ocean basins: Basalts at Mangaia exhibit the most radiogenic Pb-isotopic compositions among oceanic lavas globally, and this unique isotopic signature is thought to reflect ancient subducted oceanic crust that has been stored in the mantle for billion-year timescales before being recycled into the mantle source of hotspots. Thus, lavas from Mangaia offer an ideal opportunity to test the recycling hypothesis, and can be used to constrain elemental (including volatile) fluxes through the mantle, from subduction zones to hotspots. In a manuscript resulting from this proposal, we sulfur isotopic measurements in olivine-hosted sulfides (using in situ methods) and in bulk olivine separates (using sulfur extraction techniques paired with isotope ratio mass spectrometry) from Mangaia lavas. In Cabral et al. (2013), we present the discovery of mass independently fractionated sulfur (MIF-S) isotopes in sulfides and bulk olivine separates from Mangaia. Mass independent fractionation of sulfur isotopes was a processes limited to the Earth’s atmosphere during the Archean, when the atmosphere was poor in oxygen and therefore poor in ozone. In fact, the lack of ozone made the atmosphere relatively transparent to solar ultraviolet (UV) radiation. Thus, UV-induced photochemical fractionation operating on sulfur species in the atmosphere generated complementary geochemical reservoirs with MIF-S enrichments in 33S (hosted in reduced and elemental sulfur species) and MIF-S depletions in 33S (hosted in oxidized sulfur species, including sulfate). The process of MIF-S generation was effectively halted with the rise of atmospheric oxygen (and thus, the rise of atmospheric ozone) at the end of the Archean (at ~2.45 Ga), after which point MIF-S generation in the atmosphere is not observed in the rock record: In the geologic record, MIF-S is observed only in the Archean prior to ~2.45 Ga. The observation of MIF-S in young, mantle-derived lavas demonstrates that an Archean atmospheric signal was transported into the mantle via subduction, preserved for geologic timescales in the deep Earth, and returned to the shallow mantle and melted beneath Mangaia (Cabral et al., 2013). This discovery provides fundamental new insights into deep recycling processes, from subduction zones to mantle plumes, and demonstrates that deep recycling of subducted oceanic crust is in fact a phenomenon operating in the Earth's mantle. Armed with this insight, we used funds from the proposal to conduct a study of the major, trace and volatile element abundances and Pb-isotopic compositions of melt inclusions hosted in Mangaia olivine phenocrysts (Cabral et al., 2014), which is a major stated goal of the proposal. These volatile measurements on Mangaia melt inclusions represent the first ever made on lavas with endmember compositional and isotopic characteristics like Mangaia. If Mangaia lavas sample a mantle domain that was recycled from the Earth’s surface and preserved in the mantle since the Archean, then recycled oceanic crust has the potential to reveal how volatiles are cycled in the deep Earth, from subduction zones to hotspots. We find that the mantle that melted to generate Mangaia lavas is less dehydrated than the other canonical mantle endmembers compositions sampled by ocean island lavas. Thus, subduction of oceanic crust in the Archean may have been an important method for delivery of water to the Earth’s mantle. The proposal provided support to an early-career scientist. The proposal also provided support for a female graduate student and provided her with the resources to publish her first paper as lead author, which details the geochemical results of the proposal. The proposal also provided funding for an undergraduate to perform geochemical research, which lead to a senior thesis. Additionally, while conducting fieldwork in Mangaia for this proposal (in the Cook Islands), the PI and his graduate student spent a day teaching at a local high school. We met with the prime minister of the Cook Islands to convey the purpose of our work. We also met with a local newspaper (Cook Island News), and this resulted in an article describing our research activities there. Following publication of a paper in the journal Nature, which deals with the sulfur-isotopic results from the research funded by this proposal, the PI and his graduate student, together with research collaborators, undertook a significant effort to contact a variety of media outlets so that the general public would be made aware of our results.
