ALBERT

Description: In contrast to the long narrow volcanic chains in the Pacific Ocean, Atlantic hotspot tracks, in particular in the South Atlantic (e.g., Tristan-Gough, Discovery, Shona, and Bouvet), are irregular and, in some cases, diffuse and discontinuous. An important question is whether this irregularity results from tectonic dismemberment of the tracks or if it represents differences in the size, structure, and strength of the melting anomalies. Here we present new age and geochemical data from volcanic samples from Richardson Seamount, Agulhas Ridge along the Agulhas-Falkland Fracture Zone (AFFZ), and Meteor Rise. Six samples yielded ages of 83–72 Ma and are 10–30 m.y. younger than the underlying seafloor, indicating that they are not on-axis seamounts associated with seafloor spreading. The incompatible element and Sr-Nd-Pb-Hf isotopic compositions range from compositions similar to those of the Gough domain of the nearby Tristan-Gough hotspot track to compositions similar to samples from the Shona bathymetric and geochemical anomaly along the southern Mid-Atlantic Ridge (49°–55°S), indicating the existence of a Shona hotspot as much as 84 m.y. ago and its derivation from a source region similar to that of the Tristan-Gough hotspot. Similar morphology, ages, and geochemistry indicate that the Richardson, Meteor, and Orcadas seamounts originally formed as a single volcano that was dissected and displaced 3500 km along the AFFZ, providing a dramatic example of how plate tectonics can dismantle and disseminate a hotspot track across an ocean basin.

Description: It is commonly accepted that large igneous provinces are formed through catastrophic volcanic events occurring over vast areas within a few million years at the initiation of hotspots (mantle plumes). New 40Ar/39Ar ages (111–139 Ma) and geochemical results from the Nicoya Peninsula, Costa Rica, extend the age range of volcanism in the Caribbean large igneous province to 70 m.y. (69–139 Ma). Our results are not consistent with the formation of this vast igneous province through a single plume head at the initiation of a mantle plume such as the Galápagos. Instead we propose that multiple oceanic intraplate igneous structures, such as plateaus and hotspot tracks, were accumulated through the subduction process. The igneous structures could be remnants of the earlier history of the Galápagos hotspot, making it one of the oldest active hotspots on Earth. Alternatively they could have been derived from several spatially distinct mantle-melting events that sampled similar source material, e.g., oceanic lithosphere of similar age.

Description: Spatial geochemical variations of Quaternary lavas erupted along the northern segment of the Kamchatka arc are used to trace changes in magma generation across the subducting Pacific slab edge. The late Pleistocene–Holocene lavas of the northern end of the Central Kamchatka depression north of the Pacific slab edge show strong enrichment in high field strength elements and light rare earth elements, relatively unradiogenic strontium and lead but radiogenic neodymium isotope ratios, and oxygen isotope compositions similar to those of mid-oceanic-ridge basalts. These geochemical characteristics are distinct from the southern Central Kamchatka depression volcanoes located above the subducting Pacific slab. Extensive fluid-triggered mantle melting dominates magma genesis beneath the largest Kamchatka volcanoes in the south, whereas low-degree decompression melting of the Pacific asthenospheric mantle is the major magma generation process north of the Pacific slab edge. Quaternary detachment of the subducted Pacific plate fragment resulted in the influx of fertile mantle beneath Kamchatka. We propose that upwelling and southward flow of this hotter, more fertile mantle is the main reason for recent magmatism in northern Kamchatka and for the exceptional productivity of the Central Kamchatka depression volcanoes (Klyuchevskoy and Sheveluch), the most active arc volcanoes on Earth.

Description: At the present, the geochemical influence of the Galápagos hotspot (offshore South America) can be seen only along the Galápagos spreading center, north of the hotspot. It is possible, however, that Galápagos plume material also reached the East Pacific Rise in the past. Detecting such influence would be of particular importance for the interpretation of geochemical data from oceanic crust at Ocean Drilling Program (ODP) Site 1256, which formed ∼15 m.y. ago at the East Pacific Rise during a Miocene period of superfast spreading, and is considered to be a reference site for oceanic crust produced at fast-spreading ridges. Here we present geochemical data from Miocene basaltic crust (23–7 Ma) drilled at several Deep Sea Drilling Project (DSDP), ODP, and Integrated Ocean Drilling Program (IODP) sites that formed along the East Pacific Rise between 3°S and 7°N. Lavas formed between ca. 22.5 and ca. 11 Ma show enriched, Galápagos plume–like Pb and Nd isotope ratios (with a peak in enrichment between ≥18 and 12 Ma) compared to lavas created shortly before or after this time interval. Despite their enriched isotope composition, these samples generally show depletion in more-incompatible, relative to less-incompatible, trace elements. Derivation from an enriched Galápagos plume source that had experienced recent melt extraction before it melted further beneath the East Pacific Rise can explain the combined incompatible-trace-element depletion and isotopic enrichment of the 22.5–11 Ma lavas. The influence of plume material correlates with the interval of superfast spreading along the equatorial East Pacific Rise, suggesting a causal relationship. Enhanced ridge-plume interaction ("ridge suction") due to superfast spreading could have facilitated the flow of Galápagos plume material to the ridge. On the other hand, the arrival of Galápagos-type signatures took place immediately after formation of the Galápagos spreading center, which could have provided a pathway for hot plume material to spread into the main ridge network.

Description: Asymmetrically zoned hotspot tracks in the Pacific Ocean are interpreted to have formed from zoned plumes originating from the large-scale, lower-mantle, low-seismic-velocity anomaly (superplume?) beneath the southern Pacific, providing direct information about lowermantle compositional heterogeneity. New trace-element and Sr-Nd-Hf-Pb isotope data from the classic Tristan-Gough hotspot track in the South Atlantic also display a bilateral, asymmetric zonation with two distinct mantle source components, making it the first zoned plume to be recognized overlying the African superplume. The plume zonation can be traced for 70 m.y., four times longer than recognized for Pacific zoned hotspot tracks. These findings confirm that the proposed zonation of Pacific hotspots is not simply a geochemical oddity, but could be a major feature of plumes derived from lower-mantle superplumes. We propose that the enriched southern Gough subtrack source with elevated 207Pb/204Pb and 208Pb/204Pb at a given 206Pb/204Pb, but low 143Nd/144Nd and 176Hf/177Hf (DUPAL-like composition), may reflect the African superplume composition, whereas the more depleted northern Tristan subtrack source could represent a mixture of the superplume with the surrounding depleted mantle. Our results strengthen arguments that the enriched signature (DUPAL anomaly) in the South Atlantic could be derived from the lower mantle.