Extreme Mantle Heterogeneity beneath the Jingpohu Area,Northeastern China-Geochemical Evidence of Holocene Basaltic Rock

Holocene basaltic rocks of the Jingpohu area are located in the “Crater Forest” and Hamatang districts to the northwest of the Jingpohu Lake. Although there is only a distance of 15 km between the two districts, their petrological characteristics are very different: alkaline olivine basalt without any megacrysts in the former, and leucite tephrite with Ti-amphibole, phlogopite and anorthoclasite megacrysts in the latter. On the basis of their geochemical characteristics, the two types of basaltic rocks should belong to weakly sodian alkaline basalts. But leucite tephrite is characterized by higher Al2O3, Na2O and K2O, higher enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILE), lower MgO and CaO, compatible elements and moderately compatible elements and lower Mg*values and Na/K ratios in comparison with alkaline olivine basalt. However, the two types of basaltic rocks have similar Sr, Nd, Pb isotopic compositions, which suggests that the mantle beneath the Jingpohu area was homogeneous before undergoing some geological processes about 3490 years ago. As the activity of the mantle plume led to different degrees of metasomatism, extreme mantle source heterogeneities occurred beneath the Jingpohu area. In comparison with alkaline olivine basalt, the leucite tephrite was derived from the more enriched mantle source region and resulted from strong metasomatism.

A geochemical perspective on the genesis of Cenozoic basic volcanism in northeastern Turkey:an overview of metasomatism and heterogeneity of the sub-continental lithospheric mantle in a post-collisional setting

The post-collisional Cenozoic basic volcanic rocks in NE Turkey show temporal variations in whole-rock lithophile element and highly siderophile element(HSE)systematics that are mainly associated with the nature of sub-continental lithospheric mantle(SCLM)sources and parental melt generation.So far,the traditional whole-rock lithophile geochemical data of these basic volcanic rocks have provided important constraints on the nature of SCLM sources.Integrated lithophile element and HSE geochemical data of these basic volcanic rocks also reveal the heterogeneity of the SCLM source,which is principally related to variable metasomatism resulting from previous subduction(s)and post-collisional mantle-crust interactions in an extensional setting.Lithophile element geochemical features suggest that the parental magmas have derived from metasomatized spinel-to garnet-bearing SCLM sources for Eocene and Miocene basic volcanic rocks with subduction signatures whereas originated from spinel-to garnet-bearing SCLM sources for Mio-Pliocene and Plio-Quaternary basaltic volcanic rocks without the subduction signature.Lithophile element and HSE geo-chemistry also reveal that Eocene and Miocene basic vol-canic rocks were affected by more pronounced crustal contamination than the basaltic volcanic rocks of Mio-Pliocene and Quaternary.Furthermore,the integrated lithophile element and HSE compositions of these basic volcanic rocks,together with the regional asymmetric lithospheric delamination model,reveal that the compositional variation(especially due to metasomatism)was significant temporally in the heterogeneity of the SCLM sources from which parental magmas formed during the Cenozoic era.

A review of the influencing factors on teleseismic traveltime tomography

Teleseismic traveltime tomography is an important tool for investigating the crust and mantle structure of the Earth.The imaging quality of teleseismic traveltime tomography is affected by many factors,such as mantle heterogeneities,source uncertainties and random noise.Many previous studies have investigated these factors separately.An integral study of these factors is absent.To provide some guidelines for teleseismic traveltime tomography,we discussed four main influencing factors:the method for measuring relative traveltime differences,the presence of mantle heterogeneities outside the imaging domain,station spacing and uncertainties in teleseismic event hypocenters.Four conclusions can be drawn based on our analysis.(1)Comparing two methods,i.e.,measuring the traveltime difference between two adjacent stations(M1)and subtracting the average traveltime of all stations from the traveltime of one station(M2),reveals that both M1 and M2 can well image the main structures;while M1 is able to achieve a slightly higher resolution than M2;M2 has the advantage of imaging long wavelength structures.In practical teleseismic traveltime tomography,better tomography results can be achieved by a two-step inversion method.(2)Global mantle heterogeneities can cause large traveltime residuals(up to about 0.55 s),which leads to evident imaging artifacts.(3)The tomographic accuracy and resolution of M1 decrease with increasing station spacing when measuring the relative traveltime difference between two adjacent stations.(4)The traveltime anomalies caused by the source uncertainties are generally less than 0.2 s,and the impact of source uncertainties is negligible.

Deep recycling of volatile elements in the mantle:Evidence from the heterogeneous B isotope in intra-plate basalts

Volatiles in the mantle are crucial for Earth’s geodynamic and geochemical evolution.Understanding the deep recycling of volatiles is key for grasping mantle chemical heterogeneity,plate tectonics,and long-term planetary evolution.While subduction transfers abundant volatile elements from the Earth’s surface into the mantle,the fate of hydrous portions within subducted slabs during intensive dehydration processes remains uncertain.Boron isotopes,only efficiently fractionating near the Earth’s surface,are valuable for tracing volatile recycling signals.In this study,we document a notably large variation inδ^(11)B values(−14.3‰to+8.2‰)in Cenozoic basalts from the South China Block.These basalts,associated with a high-velocity zone beneath East China,are suggested to originate from the mantle transition zone.While the majority exhibitδ^(11)B values(−10‰to−5‰)resembling the normal mantle,their enriched Sr-Nd-Pb isotope compositions and fluid-mobile elements imply hydrous components in their source,including altered oceanic crust and sediments.The normalδ^(11)B values are attributed to the dehydration processes.Remarkably highδ^(11)B values in the basalts indicate the presence of subducted serpentinites in their mantle source.A small subset of samples with lowδ^(11)B values and radiogenic isotope enrichments suggests a contribution from recycled detrital sediments,though retaining minimal volatile elements after extensive dehydration.These findings provide compelling evidence that serpentinites within subducted slabs predominantly maintain their hydrous nature during dehydration processes in subduction zones.They may transport a considerable amount of water into deep mantle reservoirs,such as the mantle transition zone.

Dupal Anomaly and Identification using Nd-Hf Isotopes

The Dupal anomaly has attracted widespread attention since being discovered and is regarded as the most direct manifestation of mantle inhomogeneity at present. From the initially defined anomalies limited to the southern hemisphere to the global scale, the criteria for identifying anomalies defined by Pb isotopes have also been adjusted, providing an important method and reference for the study of the mantle evolution. Pearce and Peate(1995) proposed the method of NdHf isotope and element ratio to identify the Dupal anomaly. The Nd-Hf method also offers a possible way to discriminate the mantle region of arc magmatism through the correction of Nd in the subduction process. This paper introduces the concepts and determination methods of the Dupal anomaly, and reports new Hf isotopic data of MORB-type rocks with Dupal signature in the several Tethys ophiolites. Our results of Nd-Hf method are in good agreement with those of previous Pb isotope identification. Moreover, origins and their controversy of Dupal anomaly are reviewed, and possible internal connections between Dupal anomalies and the two Large Low Shear Velocity Provinces(LLSVPs) in the lower mantle are discussed in depth. Further studies on origin and evolution of the Dupal anomaly are suggested, especially using integrated approach of Hf-Nd and Pb isotopes.

Implications of the melting depth and temperature of the Atlantic mid-ocean ridge basalts

Mid-ocean ridge basalts(MORBs) are characterized by large variations in trace element compositions and isotopic ratios, which are difficult to be interpreted solely by using magmatic process such as partial melting of a peridotitic mantle and subsequently fractional crystallization. Geochemical diversity of MORBs have been attributed to large-scale heterogeneity within the underlying mantle, and the heterogeneity might have been caused by addition of recycled crustal component, subcontinental lithosphere, metasomatized lithosphere and outer core contribution. In this study, we investigated the MORBs along the Mid-Atlantic Ridge(MAR) by estimating the temperature and pressure of partial melting, and comprehensively comparing trace element and isotope ratios. The data for MORBs from areas close to mantle plumes show large variations. Mantle plumes can affect mid-oceanic ridges 1 400 km away, but plume effects did not cover all of the ridge segments, and those segments without plume effects did not have any abnormalities in temperature, trace element or isotope ratios.We ascribed the above phenomena to result from the shapes of the plume flow, which we categorized as "pipelike channels" and "pancake-like channels". The "pancake-like channels" plumes affected the ambient mantle nondirectionally, but the range of the mantle affected by the "pipe-like channels" plumes were selective. Element ratios of MORBs reveal that the mantle source of the MORBs along the MAR is highly heterogeneous. We suggest that most of source heterogeneities of the MORBs may be due to the presence of subducted slab and delaminated lower crust in the source. In addition, the plume that carried materials from the core-mantle boundary may affect some of the segments.

A comparative review of petrogenetic processes beneath the Cameroon Volcanic Line: Geochemical constraints

The origin and petrogenesis of the Cameroon Volcanic Line （CVL）, composed of volcanoes that form on both the ocean floor and the continental crust, are difficult to understand because of the diversity, het- erogeneity, and nature of available data. Major and trace elements, and Sr-Nd-Pb isotope data of volcanic rocks of the CVL spanning four decades have been compiled to reinterpret their origin and petrogenesis. Volcanic rocks range from nephelinite, basanite and alkali basalts to phonolite, trachyte and rhyolite with the presence of a compositional gap between Si02 58-64 wt.%. Similarities in geochemical characteristics, modeled results for two component mixing, and the existence of mantle xenoliths in most mafic rocks argue against significant crustal contamination. Major and trace element evidences indicate that the melting of mantle rocks to generate the CVL magma occurred dominantly in the garnet lherzolite stability feld. Melting models suggest small degree （〈3%） partial melting of mantle bearing （6-10%） garnet for Mr. Etinde, the Ngaoundere Plateau and the Biu Plateau, and 〈5% of garnet for the oceanic sector of the CVL, Mr. Cameroon, Mt. Bambouto, Mt. Manengouba and the Oku Volcanic Group. The Sr-Nd-Pb isotope sys- tematics suggest that mixing in various proportions of Depleted MORB Mantle （DMM） with enriched mantle 1 and 2 （EM1 and EM2） could account for the complex isotopic characteristics of the CVL lavas. Low Mg number （Mg# - 100 x MgO/（MgO ＋FeO）） and Ni, Cr and Co contents of the CVL mafic lavas reveal their crystallization from fractionated melts. The absence of systematic variation in NbJTa and Zr/Hf ratios, and Sr-Nd isotope compositions between the mafic and felsic lavas indicates progressive evolution of magmas by fractional crystallization. Trace element ratios and their plots corroborate mantle het- erogeneity and reveal distinct geochemical signatures for individual the CVL volcanoes.

Scale lengths of heterogeneities under Tibet

Important information about small-scale heterogeneities is hardly accessible by the traditional deterministic seismic tomography. Fluctuations of the phase and the logarithmic amplitude of direct teleseismic plane P waves can be used to characterize small-scale heterogeneities. Seismic data recorded by the Hi-CLIMB array are used to analyze the average power spectrum of the small-scale velocity heterogeneities under Tibet. Coherence functions of the logarithmic amplitude and the phase fluctuations due to different earthquakes from different back azimuths show consistent characteristics, indicating that fluctuations are due to heterogeneities under the stations. Assuming that the heterogeneities are statistically stationary and distributed within a layer, we invert for the average heterogeneity spectrum by fitting both the logarithmic amplitude and the phase coherence data. Multiscale nature of the heterogeneity is evident. The inverted power spectrum is ＂red＂ at the large-scale end, meaning that the power spectrum decreases as the length scale decreases. Such a decreasing trend stops at smaller scales -20-50 km and 10 km. This may indicate that mantle convection is not effective in destroying smaller heterogeneities.

Stony Brook’s Collaborations with Czech Scientists

For the past half-century, I have been fortunate in maintaining collaborations with Czech scientists in the Czech Republic [formerly Czechoslovakia] from the Geofyzikální ústav-GFU [Institute of Geophysics] of the ?eskoslovenská Akademie Věd-?SAV [Czechoslovak Academy of Sciences]. These collaborations have included exchange visits by me to Prague [Praha] and convening international workshops in 1976, 1986 and 1996 in castles used by the ?SAV as well as visits by Czech colleagues to Stony Brook University. The objective of this report is to relate this history. This paper is dedicated to the memory of Vladislav Babu?ka.

My Research Collaborations with Chinese Scientists over the Past Three Decades

For more than three decades, I have been fortunate in working with Chinese graduate students and postdoctoral research scientists in our High-Pressure Laboratory at Stony Brook University. These colleagues have conducted a wide variety of experiments at high pressures and temperatures in collaboration with our other students and researchers. These studies utilized transmission electron microscopy, ultrasonic interferometry, X-ray powder diffraction and synchrotron X-radiation to investigate phase transitions, thermal equations of state, sound velocities, atomic diffusion, dislocation dissociation and deviatoric stress in high-pressure apparatus. During this period, I have also visited high-pressure laboratories in the mainland of China and Taiwan on several occasions. The objective of this paper is to relate this history.

Sub-basin scale inhomogeneity of mantle in the South China Sea revealed by magnesium isotopes

The South China Sea(SCS)is the largest extensional basin in the western Pacific and was formed after rifting of the Euro-Asian continental margin.The nature of its underlying mantle remains enigmatic due to the lack of sampling of the seafloor’s igneous crust.The International Ocean Discovery Program Expedition 349 cored seafloor basalts of the southwestern(Site U1433)and eastern(Site U1431)SCS sub-basins.The recovered basalt samples exhibit different source lithologies and geochemistries.The Mg isotopic compositions of seafloor basalts from these sites were investigated to elucidate the origin of this large-scale mantle inhomogeneity.Results indicate that the Site U1431 basalts have a mantlelike averageδ^(26)Mg value of-0.27‰±0.06‰(2 SD;n=10).Together with inhomogeneous Sr-Nd-Pb-Hf isotopic compositions,the Site U1433 basalts have an averageδ^(26)Mg value(-0.20‰±0.06‰;2 SD;n=8)higher than those of the Site U1431 basalts and normal mantle.Their heavier Mg isotopic compositions and low206 Pb/204 Pb ratios(17.7)indicate that the Site U1433 basalts were affected by the re-melting of detached continental-arc lithosphere in the sub-ridge mantle.The coupling of Mg and Sr-Nd isotopes provides robust evidence that the mantle-likeδ^(26)Mg values of the Site U1431 basalts resulted from mixing between detached continental arc lithosphere and the nearby Hainan plume,with respective supra-and sub-normalδ^(26)Mg values.From the perspective of Mg isotope,the mantles of the southwestern and eastern sub-basins are compositionally inhomogeneous,with their mantle evolutionary histories being distinct.

Lithospheric structure and evolution of the North China Craton:An integrated study of geophysical and xenolith data

Determination of the physical and chemical structures of the inaccessible continental lithosphere by comprehensive geophysical and geochemical studies can provide valuable information on its formation and evolution.Extensive studies from various disciplines have revealed complex lithospheric modification of the North China Craton(NCC),but less attention has been paid to an integrated study from different fields.Here we provide an integrated constraint on the lithospheric mantle structure of the NCC by comprehensive semiology,gravity and thermal studies with xenolith data involving depth(levels in the lithosphere),property(chemical and physical),and timing(formation and reworking ages).Our results suggest that the NCC has a relatively heterogeneous lithospheric mantle.Its margins and internal weak zones,especially in the eastern NCC,are generally underlain by the fertile,weakly metasomatized mantle with generally young formation ages.In contrast,its core tends to preserve the refractory,strongly metasomatized mantle with ages roughly coupled to the overlying Archean crust.Such a lithospheric structure shows the preferential modification of the lithospheric mantle in the eastern NCC and in the peripheral regions of the western NCC.The interior of the craton,especially most of the western NCC,remains stable and has been weakly modified.