The role of lithospheric thickness in the formation of ocean islands and seamounts: contrasts between the Louisville and Emperor-Hawaiian hotspot trails

The Hawaii-Emperor and Louisville seamounts form the two most prominent time-progressive hotspot trails on Earth. Both formed over a similar time interval on lithosphere with a similar range of ages and thickness. The Hawaii-Emperor seamounts are large and magma productivity appears to be increasing at present. The Louisville seamounts, by contrast, are smaller and the trail appears to be waning. We present new major- and trace element data from five of the older (74–50 Ma) Louisville seamounts drilled during International Ocean Drilling Program (IODP) Expedition 330 and compare these to published data from the Emperor seamounts of the same age. Despite drilling deep into the shield-forming volcanic rocks at three of the Louisville seamounts, our data confirm the results of earlier studies based on dredge samples that the Louisville seamounts are composed of remarkably uniform alkali basalt. The basalt composition can be modelled by ∼1.5–3% partial melting of a dominantly garnet lherzolite mantle with a composition similar to that of the Ontong Java Plateau mantle source. Rock samples recovered by dredging and drilling on the Emperor Seamounts range in composition from tholeiitic to alkali basalt and require larger degrees of melting (2–10%) and spinel- to garnet lherzolite mantle sources. We use a simple decompression melting model to show that melting of mantle with a potential temperature of 1500ºC under lithosphere of varying thickness can account for the composition of the shield-forming tholeiitic basalts from the Emperor seamounts, while post-shield alkali basalt requires a lower temperature (1300–1400ºC). This is consistent with the derivation of Hawaii-Emperor shield-forming magmas from the hotter axis of a mantle plume and the post-shield magmas from the cooler plume sheath as the seamount drifts away from the plume axis. The composition of basalt from the Louisville seamounts shows no significant variation with lithosphere thickness at the time of seamount formation, contrary to the predictions of our decompression melting model. This lack of influence of lithospheric thickness is characteristic of basalt from most ocean islands. The problem can be resolved if the Louisville seamounts were formed by dehydration melting of mantle containing a small amount of water in a cooler plume. Hydrous melting in a relatively cool mantle plume (Tp = 1350–1400 °C) could produce a small amount of melt and then be inhibited by increasing viscosity from reaching the dry mantle solidus and melting further. The failure of the plume to reach the dry mantle solidus or the base of the lithosphere means that the resulting magmas would have the same composition irrespective of lithosphere thickness. A hotter mantle plume (Tp ≈ 1500 °C) beneath the Emperor seamounts and the Hawaiian Islands would have lower viscosity before the onset of melting, melt to a larger extent, and decompress to the base of the lithosphere. Thus our decompression melting model could potentially explain the composition of both the Emperor and Louisville seamounts. The absence of a significant lithospheric control on the composition of basalt from nearly all ocean islands suggests that dehydration melting is the rule and the Hawaiian islands the exception. Alternatively, many ocean islands may not be the product of mantle plumes but instead be formed by decompression melting of heterogeneous mantle sources composed of peridotite containing discrete bodies of carbonated and silica-oversaturated eclogite within the general upper mantle convective flow.

Two new species of Geodiidae (Porifera, Demospongiae, Astrophorina) from the Emperor Seamounts, North Pacific Ocean

Two new species of Geodiidae from the Emperor Seamounts, North Pacific Ocean are described and compared to congeners. Erylus imperator n. sp. differs from all other species of Erylus from the region in having large oxyasters, present in only one other species from which it differs in several other characters. Furthermore these oxyasters most often display the tendency to reduce the number of rays to a degree that they appear as triods, microxeas or even microtylostyles, depending on the number of retained rays. The second new species, Geodia arma n. sp. is a Geodia without triaenes, a group formerly described under the genus Geodinella. We compared G. arma n. sp. with all congeners lacking triaenes and determined that it is the only known species with a cortex of up to 6 mm in thickness and also the only species with oxeas up to 6 mm in length Which range from thin and sinuous to thick and straight. To our knowledge these are the first descriptions of sponge species from the Emperor Seamount region.

Amid fields of rubble, scars, and lost gear, signs of recovery observed on seamounts on 30- to 40-year time scales

Although the expectation of lack of resilience of seamount vulnerable marine ecosystems has become a paradigm in seamount ecology and a tenet of fisheries management, recovery has not been tested on time scales >10 years. The Northwestern Hawaiian Ridge and Emperor Seamounts have experienced the highest documented fish and invertebrate seamount fisheries takes in the world. Surveys show that, despite visible evidence of substantial historic fishing pressure, a subset of these seamounts that have been protected for >30 years showed multiple signs of recovery including corals regrowing from fragments and higher abundances of benthic megafauna than Still Trawled sites. Contrary to expectations, these results show that, with long-term protection, some recovery of seamount deep-sea coral communities may be possible on 30- to 40-year time scales. The current practice of allowing continued bottom-contact fishing at heavy trawled sites may cause damage to remnant populations, which likely play a critical role in recovery.

First finding of a widely distributed Antarctic chiton species (Mollusca: Polyplacophora) in the North Pacific

For the first time, the widely spread Antarctic species Leptochiton antarcticus was found at the Emperor Seamounts in the North Pacific Ocean. In spite of a large distance between the Emperor Seamounts and Antarctica, the found specimen have very similar shell, girdle, radula and gill features to the type material. I propose that L. antarcticus spread to the North Pacific from the Antarctic via a deep-water current near the ocean floor, and perhaps it inhabits the slopes of islands and continents from the South Ocean to the Emperor Seamounts.