Geochemical constraints on CO2-rich mantle source for the Kocebu Seamount,Magellan Seamount chain in the western Pacific

The alkaline oceanic island basalts(OIBs)with under-saturated SiO2 and high contents of CaO and alkaline are usually attributed to mantle sources different from typical tholeiitic OIBs.Based on the results of high pressure and temperature experiment study,the genesis of silica under-saturated alkaline basaltic melts could be explained by the role of CO2,thus,the genetic relationship of alkaline basalts with CO2 has become a topic of relevance because it is closely related to the deep carbon cycle.The Magellan Seamount chain in the West Pacific Seamount Province has wide distribution of alkali basalts.For the first time,we collected alkaline basalt samples from the Kocebu Seamount of the Magellan Seamount chain and found that magmatic apatites widely occur in the less evolved volcanic rock samples,and the high contents of phosphorus should be a feature of the alkaline OIBs of the Magellan Seamounts.Compared with typical OIBs,these alkaline volcanic rocks have higher CaO and P2O5,lower SiO2 content,negative anomaly of high field strength elements(HFSEs),more distinctly negative anomaly of potassium(K)and the ubiquity of titanaugite,indicating a CO2-rich mantle source.Based on the relatively high K2O and TiO2 contents and La/Yb ratio and low MgO content of these alkaline rocks,we suggest that the volcanic rocks of the Magellan Seamounts are originated from carbonated eclogites derived possibly from ancient subducted altered oceanic crust.

NGS-metabarcoding revealing novel foraminiferal diversity in the Western Pacific Magellan Seamount sediments

Seamount is a unique deep-sea ecosystem widely distributed in the world.Its biodiversity is vibrant due to its specific geographical and hydrological conditions.However,the diversity and features of foraminifera in such an environment have rarely been studied.We extracted environmental DNA(eDNA)in sedime nts and amplified the partial small subunit ribo somal DNA(SSU rDNA)of fo raminife ra to understand the foraminiferal diversity from four sites in Magellan Seamount(Western Pacific Ocean).Partial S SU rDNA sequencing was conducted and 912979 foraminiferal reads were obtained and gathered into 266 operational taxonomic units(OTUs).In the available dataset,a high proportion of rare OTUs and low identity OTUs in each studied sample showed that the Magellan Seamount foraminiferal community might have a high genetic novelty.The relative abundance of foraminifera varied between replicates probably due to the existence of bias in amplification process and patchiness of the deep-sea floor.It showed that the Magellan Seamount has a relatively high benthic foraminiferal diversity characterized mainly by monothalamiids(76.37%of total reads)in association with rotaliids(19.03%of total reads),including planktic foraminiferal sequences(38.58%of rotaliids;7.36%of total reads).The remaining reads were assigned to miliolids(0.83%of total reads)and textulariids(0.66%of total reads),and 3.11%of total reads are unassigned to a specific family.The co mparative analysis with foraminiferal assemblage s from coastal and deep-sea environme nts indicated that seamounts could aggregate species from a nearby deep-sea.

Assessment of acoustic backscatter intensity surveying on deep-sea ferromanganese crust: Constraints from Weijia Guyot, western Pacific Ocean

Near-bottom observation data from the manned deep submersible Jiaolong with high-precision underwater positioning data from Weijia Guyot,Magellan Seamounts in the Western Pacific Ocean are reported.Three substrate types were identified:Sediment,ferromanganese crust,and ferromanganese crust with a thin cover of sediment.The ferromanganese crusts show clear zoning and their continuity is usually disturbed by sediments on areas of the mountainside with relatively gentle slope gradients.The identified substrate spatial distributions correspond to acoustic backscatter intensity data,with regions of high intensity always including crust development and regions of low intensity always having sediment.Therefore,acoustic backscatter intensity surveying appears useful in the delineation and evaluation of crust resources,although further more work is needed to develop a practicable methodology.

Magnetic minerals in Mid-Pleistocene sediments on the Caiwei Guyot,Northwest Pacific and their response to the Mid-Brunhes climate event

Seamounts are ubiquitous topographic units in global oceans,and their influences on local oceanic circulation have attracted great attention in physical oceanography;however,previous efforts were less made in paleoclimatology and paleoceanography.The Caiwei Guyot in the Magellan Seamounts of the western Pacific is a typical seamount,and in this study,we investigate a well-dated sediment core by magnetic properties to reveal the relationship between deep-sea sedimentary processes and global climate changes.The principal results are as follows:(1)the dominant magnetic minerals in the sediments are low-coercivity magnetite in pseudo-single domain range,probably including a biogenic contribution;(2)the variabilities of magnetic parameters can be clustered into two sections at~500 ka,and the differences between the two units are evident in amplitudes and means;(3)changes in the grainsize-dependent magnetic parameters can be well correlated to records of global ice volume and atmospheric CO;in the middle Pleistocene.Based on these results,a close linkage was proposed between deep-sea sedimentary processes in the Caiwei Guyot and global climate changes.This linkage likely involves different roles of biogenic magnetite in the sediments between interglacial and glacial intervals,responding to changes in marine productivity and deep-sea circulation and displaying a major change in the MidBrunhes climate event.Therefore,we proposed that the sedimentary archives at the bottom of the Caiwei Guyot record some key signals of global climate changes,providing a unique window to observe interactions between various environmental systems on glacial-interglacial timescales.

Geochronological and geochemical constraints on the petrogenesis and geodynamic process of Hemler,Vlinder,and Il'ichev seamount lavas in NW Pacific

Oceanic intraplate volcanoes with linear age progressions are usually accepted to be derived from melting of an upwelling mantle plume.Several seamount groups in NW Pacific,however,show complex age-distance relationships that are difficult to explain using the classic“mantle plume hypothesis”,and thus their origins are controversial.In this study,we present ^(40)Ar-^(39)Ar age,geochemical,and Sr-Nd-Pb-Hf isotopic data of lavas from Hemler,Vlinder,and Il’ichev seamounts in NW Pacific,to elucidate their petrogenesis and geodynamic process.The lavas from Hemler,Vlinder,and Il’ichev seamounts are classified as alkali basalt,basanite/nephelinite,and trachyte.Lavas with Mg O>8 wt.%exhibit high contents of CaO,FeO^(T),and TiO_(2),similar to the composition of melts formed from reaction between carbonated eclogite-derived melts and fertile peridotite.These lavas have elevated Zr/Hf ratios(40.6-45.2)and negative Zr and Hf anomalies,indicating the presence of a carbonate component in the mantle source.They are enriched in incompatible trace elements and have enriched mantle 1(EM1)-like SrNd-Pb-Hf isotopic compositions.The isotopic compositions of Vlinder,Il’ichev basanite,and Hemler lavas in this study are similar to the Rarotonga hotspot.Although occurring at the same seamount,the Il’ichev alkali basalts display more depleted SrNd-Hf isotopic compositions compared to Il’ichev basanite.According to plate tectonic reconstruction results,the ages of Hemler(100.1 Ma),Vlinder pre-(100.2 Ma)and post-shield(87.5 Ma),and Il’ichev(56.4 Ma)lavas clearly deviate from the Macdonald,Arago,Rarotonga,and Samoa hotspot tracks,indicating that they cannot directly originate from mantle plumes.We propose that in the mid-Cretaceous,when the Pacific plate passed over Rarotonga hotspot,melting of Rarotonga plume formed the Vlinder(main-shield stage),Pako,and Ioah seamounts.The Rarotonga(and possibly Samoa)plume materials would have been dispersed into the surrounding asthenosphere by mantle convection.These diffuse plume materials would undergo decompression melting beneath lithosphere fractures that are widely distributed in the Magellan area,generating non-hotspot related Hemler and pre-and post-shield Vlinder lavas.The Il’ichev alkali basalts and basanite probably result from lithospheric fracture-induced melting of heterogeneous enriched components randomly distributed in the asthenosphere.