NSFGEO-NERC: Lithospheric architecture of the Cape Verde Archipelago provides unique insights into the construction of a hotspot volcano

Volcanic hotspots, like Hawaii and Cape Verde, are sourced by plumes that upwell from the deep mantle. Prior to eruption as ocean island basalts (OIB), mantle melts rise and interact with the oceanic lithosphere: melting stops when an upwelling plume reaches the base of the lithosphere, then OIB melts advance upwards through the lithospheric mantle until they pierce the MORB crustal basement by diking. Finally, OIB melts intrude through seafloor sediments that veneer the MORB basement. Our scientific understanding of how melt-lithosphere interaction occurs during melt transport, however, is incomplete due to the inaccessible depths where this takes place: we are limited by indirect evidence and proxy chemical tracers rather than direct observations. Fortunately, the Cape Verde Islands provide a unique opportunity to directly examine the deep interior of an oceanic hotspot volcano, as well as fragments of the underlying MORB crust and a 1-km-thick marine sediment sequence, which have been uplifted 3 km and exposed as subaerial outcrops. Cape Verde is, thus, one of the few places in the world’s ocean basins where one can study extensive subaerial exposures of Mesozoic MORB crustal basement and the overlying marine sediments, which are in direct contact with (and crosscut by) hotspot-derived magmatic bodies. We leverage this unique geology to investigate how mantle melts interact with oceanic lithosphere—mantle, crustal, and sedimentary—to modulate OIB geochemistry and form the intrusive pedestals upon which hotspot volcanoes are built, offering insights into the globally relevant problem of hotspot geodynamics. By making careful field measurements in concert with geochemical and 40Ar/39Ar analyses of OIB samples, the proposal will address 3 hypotheses. First, the juxtaposition of OIB dikes and sills with marine sediment at Maio Island will allow us to directly assess the degree of sediment assimilation by OIB magmas transiting through a thick sedimentary sequence. If no geochemical signatures for sediment assimilation are identified in the dikes and sills, it would show that lavas passing through 1 km of marine sediment are essentially “immune” to assimilation, demonstrating that OIB enriched mantle signatures at other hotspots globally (which sit atop thinner sediment sequences) are unequivocal mantle signatures. Second, our preliminary fieldwork shows that the outcropping MORB crustal basement at Maio is pervasively intruded by OIB dikes (~85%). Confirmation of this hypothesis will provide a mechanism for the dramatic seafloor uplift at Maio via inflation by dike and sill intrusion, a process promoted by this archipelago’s stationary position relative to the hotspot. It will also allow us to use the OIB dikes as a control group for sediment assimilation as they are stratigraphically below the marine sediments. Third, extremely low 187Os/188Os—the lowest ever reported in the ocean basins, including values found only in subcontinental lithospheric mantle (SCLM)—were previously reported in Cape Verde peridotite xenoliths using a method prone to systematic errors. Given the geodynamic implications for SCLM beneath an oceanic hotspot, we will reanalyze 187Os/188Os in the same xenoliths using a more robust method. If the results are confirmed, it would support a model where the Cape Verde plume melting is impeded by stranded SCLM beneath the hotspot, resulting in deeper (and lower degree) melting, thus explaining Cape Verde’s highly alkaline OIB chemistry. We propose a new international collaboration that crosses disciplinary boundaries and includes 2 PIs with very different but complementary expertise: Global OIB geochemistry (Jackson, UCSB), and Cape Verde geology and volcano evolution (Ramalho, Cardiff U.). The proposal will also fund a UCSB graduate student and a Cardiff postdoc who will flourish as part of a cross-disciplinary team. The PIs will collaborate with Cape Verdean colleagues from INGT (Instituto Nacional de Gestão do Território) to deliver new geological maps for the studied areas. The proposed work will fund the participation of one INGT professional in the field campaign, who will support the mapping. We will work with INGT to submit a proposal to make the studied outcrops a protected Geoheritage site, given their global relevance. The PIs will collaborate with Dr. Bruno Faria, a scientist at Cape Verde's Geophysical Institute, to coauthor a publication about island uplift driven by cumulative intrusions. The PIs will communicate the new research to the Cape Verdean public via newspaper and radio news outlets, and social media.