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: 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.

Description: Basalts from intraplate or hotspot ocean islands (e.g., the Hawaiian, Galápagos, and Canary Islands) are believed to be formed by mantle plumes, which emanate from mantle boundary layers such as the coremantle boundary. The long-term chemical structure of mantle plumes, however, remains poorly constrained. Spatial variation in the chemical composition has long been recognized in lavas from the Galápagos Islands: Enriched plume material forms a horseshoe-shaped region with depleted mantle, similar in composition to mid-ocean ridge basalt, in its inner part. The enriched horseshoe-shaped region can be subdivided into three distinct geochemical domains. We show that these same domains occur in the same relative positions with respect to morphology in a geochemical profile across the Galápagos hotspot track off the coast of Costa Rica, indicating that the asymmetrical spatial zonation of the Galápagos hotspot has existed for at least 14 m.y. Combined with published He isotope data, the results of this study imply that plume material can ascend from the lower mantle, possibly from the core-mantle boundary, with little stirring occurring during ascent, and that zonation in hotspot lavas may in some cases reflect spatial heterogeneity within the lower mantle source.

Description: Splitting of the Vitiaz arc formed the Tonga-Kermadec and Lau-Colville Ridges (southwestern Pacific Ocean), separated by the Lau Basin in the north and Havre Trough in the south. We present new trace element and Sr-Nd-Hf-Pb isotope geochemistry for the Kermadec and Colville Ridges extending ~900 km north of New Zealand (36°S–28°S) in order to (1) compare the composition of the arc remnants with Quaternary Kermadec arc volcanism, (2) constrain spatial geochemical variations in the arc remnants, (3) evaluate the effect of Hikurangi igneous plateau subduction on the geochemistry of the older arc lavas, and (4) elucidate what may have caused arc splitting. Compared to the Kermadec Ridge, the Colville Ridge has higher more-incompatible to less-incompatible immobile element ratios and largely overlapping isotope ratios, consistent with an origin through lower degrees of melting of more enriched upper mantle in the Vitiaz rear arc. Between ca. 8 and 3 Ma, both halves of the arc (~36°S–29°S) included a more enriched (EM1-type) composition (with lower 206Pb/204Pb and 207Pb/204Pb and higher Δ8/4 Pb [deviation of the measured 208Pb/204Pb ratio from a Northern Hemisphere basalt regression line] and 87Sr/86Sr) compared to older and younger arc lavas. High-Ti basalts from the Manihiki Plateau, once joined to the Hikurangi Plateau, could serve as the enriched Vitiaz arc end member. We propose that the enriched plateau signature, seen only in the isotope ratios of mobile elements, was transported by hydrous fluids from the western margin of the subducting Hikurangi Plateau or a Hikurangi Plateau fragment into the overlying mantle wedge. Our results are consistent with plateau subduction triggering arc splitting and backarc opening.