We review the currently available results of laboratory experiments, geochemistry and MT observations and attempt to explain the conductivity structures in the oceanic asthenosphere by constructing ...mineral-physics models for the depleted mid-oceanic ridge basalt (MORB) mantle (DMM) and volatile-enriched plume mantle (EM) along the normal and plume geotherms. The hopping and ionic conductivity of olivine has a large temperature dependence, whereas the proton conductivity has a smaller dependence. The contribution of proton conduction is small in DMM. Melt conductivity is enhanced by the H2O and CO2 components. The effects of incipient melts with high volatile components on bulk conductivity are significant. The low solidus temperatures of the hydrous carbonated peridotite produce incipient melts in the asthenosphere, which strongly increase conductivity around 100km depth under older plates. DMM has a conductivity of 10−1.2~−1.5S/m at 100–300km depth, regardless of the plate age. Plume mantle should have much higher conductivity than normal mantle, due to its high volatile content and high temperatures. The MT observations of the oceanic asthenosphere show a relatively uniform conductivity at 200–300km depth, consistent with the mineral-physics model. On the other hand, the MT observations show large lateral variations in shallow parts of the asthenosphere despite similar tectonic settings and close locations. Such variations are difficult to explain with the mineral-physics model. High conductivity layers (HCL), which are associated with anisotropy in the direction of the plate motion, have only been observed in the asthenosphere under infant or young plates, but they are not ubiquitous in the oceanic asthenosphere. Although the general features of HCL imply their high-temperature melting origin, the mineral-physics model cannot explain them quantitatively. Much lower conductivity under hotspots, compared with the model plume-mantle conductivity suggests the extraction of volatiles from the plume mantle by the ocean island basalt (OIB) magmatism.
•Results of laboratory experiments, geochemistry and MT observations are reviewed.•High conductivity layers are limited under very young plates, which suggests their high-T origin.•Conductivity at 100–200km depth are well explained by the mineral-physics model.•Conductivity in shallower regions cannot be quantitatively explained.•Large lateral variations in shallow regions despite similar tectonic settings and close locations
A simple analytical procedure for determination of whole-rock major- and trace-element composition by micro-X-ray fluorescence (μXRF) spectrometer and inductively coupled plasma-mass spectrometer ...(ICP-MS) using fused-glass bead (sample + lithium tetraborate) is presented. In the case of peridotites, chromian spinel (Cr-spinel) is one of the minerals resistant to acids and interferes with accurate and reproducible determination of whole-rock composition. Such resistant minerals were not observed in the fused-glass beads prepared here, suggesting complete digestion of the samples including Cr-spinels. The μXRF spectrometer was employed to determine SiO2, MgO, and total Fe2O3 contents with the fused-glass beads. They were subsequently dissolved into a nitric acid solution to analyze thirty seven elements, Na, Al, P, K, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Rb, Sr, Y, Zr, Nb, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Pb, Th, and U with sector magnetic field ICP-MS (ICP-SFMS) and Q-pole mass filter ICP-MS (ICP-QMS). The analytical procedures were optimized and evaluated with geological reference materials of JP-1 and BIR-1a, and applied to various mantle xenoliths (lherzolite, harzburgite, dunite, wehrlite, olivine clinopyroxenite, and orthopyroxenite) from Tahiti and Moorea islands (Society Archipelago). Since only fused-glass beads are required as an analytical target to determine whole-rock major- and trace-element compositions, the sample amount consumed through the series of analytical procedure can be constrained to a minute amount (e.g., <0.4 g). The analytical procedures presented here are considered appropriate for tiny and/or precious samples, such as mantle xenoliths and rocks collected by dredging, diving, and drilling from the world’s ocean basins.
Bimineralic eclogite, which consists solely of garnet and clinopyroxene, is a likely component of some of the ancient recycled crust residing in basalt source regions. It may originate during ...subduction of altered mid-ocean ridge basalt (MORB) crust, owing to extraction of small degree partial melts or siliceous hydrous fluids. It may also originate by fractional removal of early-formed partial melts from recycled crust or from pyroxenite originating by other processes. We have performed high-pressure experiments on a bimineralic eclogite (B-ECL1) and its mixture with olivine (B-ECL1-OL) at 3 and 5 GPa. Degrees of melting are slightly higher for B-ECL1-OL than for B-ECL1 at given temperatures, suggesting that addition of small amounts of olivine enhances melt productivity of bimineralic eclogite. Solidus and liquidus temperatures of B-ECL1 are slightly higher than those of B-ECL1-OL and MORB-like pyroxenite, but are lower than those of high-MgO pyroxenite and peridotite, suggesting that bimineralic eclogite is not necessarily refractory compared to other likely mantle lithologies. Partial melts of B-ECL1 and B-ECL1-OL are nepheline-normative. Because garnet and clinopyroxene in these compositions partially melt at a eutectic-like minimum with a composition that is nepheline-normative, a wide range of bimineralic eclogite compositions, including that of subducted-crust origin, that consist of garnet and clinopyroxene with compositions similar to those of B-ECL1 can produce nepheline-normative (=
alkali
−
basaltic) liquids. Thus, in contrast to the common assumption that partial melting of recycled oceanic crust produces silicic magmas, we conclude that such lithologies can produce nepheline-normative partial melts if they first experience fractional removal of fluids or melts. The partial melts from B-ECL1 are too low in MgO to be parental to many alkalic OIB, but have low Al
2O
3 and high FeO comparable to those of alkalic OIB, suggesting that bimineralic recycled crust is a potential source for a low-Al
2O
3 and high-FeO component that is necessary for the genesis of alkalic OIB.
Abstract
The deep carbon cycle plays an important role on the chemical differentiation and physical properties of the Earth’s mantle. Especially in the asthenosphere, seismic low-velocity and high ...electrical conductivity due to carbon dioxide (CO
2
)-induced partial melting are expected but not directly observed. Here we discuss the experimental results relevant to the genesis of primitive CO
2
-rich alkali magma forming petit-spot volcanoes at the deformation front of the outer rise of the northwestern Pacific plate. The results suggest that primitive melt last equilibrated with depleted peridotite at 1.8–2.1 GPa and 1,280–1,290 °C. Although the equilibration pressure corresponds to the pressure of the lower lithosphere, by considering an equilibration temperature higher than the solidus in the volatile–peridotite system along with the temperature of the lower lithosphere, we conclude that CO
2
-rich silicate melt is always produced in the asthenosphere. The melt subsequently ascends into and equilibrates with the lower lithosphere before eruption.
Nearshore marine sediments deposited along island arcs can preserve evidence of past disaster events, such as tsunamis. Evidence of a tsunami that occurred off the Pacific coast of Tohoku, Japan, on ...March 11, 2011, was likely preserved in marine sediments. Using geochemical and rock magnetic analyses, this study aimed to investigate tsunami records preserved in marine sediments in Sendai Bay, located west of the epicenter of the 2011 Tohoku earthquake. We collected sediment samples at five stations (S‐1, S‐2, S‐3, S‐4, and S‐5) in Sendai Bay from 2002 to 2014. We compared the samples collected before and after the tsunami. We established that the silt component increased, while that of fine sand decreased at stations S‐3, S‐4, and S‐5 in the 2011 sediment sample. Notably, the concentrations of terrestrial and industrial metallic elements (Cu, Zn, and Pb) and total nitrogen, carbon, and sulfur amounts increased, while the magnetic susceptibility and isothermal remanent magnetization values decreased after the tsunami. We also found that Ti‐poor magnetite increased in the 2011 samples, indicating tsunami‐mediated changes. The study area experienced a large typhoon 6 months after the tsunami, leading to flooding. The elevation in the total organic carbon and total nitrogen ratio in the 2012 sample suggested that terrestrial organic matter was supplied by the flood.
A broad area densely covered by ferromanganese nodules was recently discovered around Minamitorishima (Marcus) Island, representing a high‐potential metal resource, particularly for Co, Ni, Mo, and ...W. We studied 16 nodule samples from nodule fields around Minamitorishima Island. To define the fine‐scale chemostratigraphy of the nodules, polished cross‐sections of the samples were analyzed by microfocus X‐ray fluorescence. Our results show that a general pattern of compositional variation was common throughout the growth history of the nodules in all the regions we studied. Chemical mapping clarified changes in the chemical signature and proportion of five lithological components throughout the growth history: Mn represented columnar δ‐MnO2; Fe represented layered amorphous FeOOH*xH2O; Ti represented TiO2*2H2O intergrown with an amorphous FeOOH phase; P, Ca and Y represented particles of biogenic calcium phosphate; and Si, Al, K, Cu, and Ni represented pelagic sediment infills. We proposed a method for a creating a multi‐dimensional compositional map of the fine‐scale chemostratigraphy observed in the ferromanganese oxide layers on the basis of merging the mapped Mn, Fe, Ti, P, Si and Cu intensities. Multi‐dimensional compositional mapping of the sampled nodules from the western North Pacific revealed two fundamental findings: (1) previously recognized first‐order Fe–Mn layers, L0, L1, and L2, were further divided into two, three, and four sublayers, respectively, and (2) a delayed supply of material to be nuclei of nodule or a growth hiatus of Fe–Mn layer(s), leading to missing sublayers in the layers L0 and L2, regulated the nodule size. In contrast, layer L1, which does not have any missing sublayers, was commonly observed in the samples for this study and has been reported in studies of other regions in the western Pacific. We propose, therefore, that the layer L1 is a key facies for examining chemostratigraphic correlations with other areas of seafloor.
Many ocean island basalts (OIB) that have isotopic ratios indicative of recycled crustal components in their source are silica-undersaturated and unlike silicic liquids produced from partial melting ...of recycled mid-ocean ridge basalt (MORB). However, experiments on a silica-deficient garnet pyroxenite, MIX1G, at 2.0-2.5 GPa show that some pyroxenite partial melts are strongly silica- undersaturated M.M. Hirschmann et al. Geology 31 (2003) 481-484. These low- pressure liquids are plausible parents of alkalic OIB, except that they are too aluminous. We present new partial melting experiments on MIX1G between 3.0 and 7.5 GPa. Partial melts at 5.0 GPa have low SiO sub(2) (<48 wt%), low Al sub(2)O sub(3) (<12 wt%) and high CaO (>12 wt%) at moderate MgO (12-16 wt%), and are more similar to primitive OIB compositions than lower-pressure liquids of MIX1G or experimental partial melts of anhydrous or carbonated peridotite. Solidus temperatures at 5.0 and 7.5 GPa are 1625 and 1825 degree C, respectively, which are less than 50 degree C cooler than the anhydrous peridotite solidus. The liquidus temperature at 5.0 GPa is 1725 degree C, indicating a narrow melting interval (~100 degree C). These melting relations suggest that OIB magmas can be produced by partial melting of a silica-deficient pyroxenite similar to MIX1G if its melting residue contains significant garnet and lacks olivine. Such silica-deficient pyroxenites could be produced by interaction between recycled subducted oceanic crust and mantle peridotite or could be remnants of ancient oceanic lower crust or delaminated lower continental crust. If such compositions are present in plumes ascending with potential temperatures of 1550 degree C, they will begin to melt at about 5.0 GPa and produce appropriate partial melts. However, such hot plumes may also generate partial melts of peridotite, which could dilute the pyroxenite-derived partial melts.
The cathodoluminescence (CL) emission and spectra analyses were conducted on detrital zircons in the Cretaceous Sanbagawa schists to describe changes in CL properties of zircons associated with ...metamorphism. In addition, high‐pressure (HP) and high‐temperature (HT) experiments were conducted on the non‐metamorphosed Miocene Ashizuri igneous zircons, and the CL emission intensities obtained from them were compared with those of the Sanbagawa zircons. As a result, as compared to low‐grade Sanbagawa zircons, the emission intensities of the high‐grade Sanbagawa zircons reduced. The emission intensities produced from the HP‐treated Ashizuri zircons, on the other hand, were twice as high as those generated from untreated zircons, but less than half of those generated from HT‐treated zircons. Combining our results with previous studies on the temperature influence on zircon CL properties, it indicates that increasing metamorphic pressure decreases the increase in the CL emission intensity of metamorphosed detrital zircons resulting from the increasing temperature. The pressure effect on the emission intensity of zircon during metamorphism might be larger than the temperature effect.
We present halogen, noble gas, and major and trace element compositions of mantle xenoliths from intraplate settings (Eifel, Kilbourne Hole, San Carlos, and Hawaii). The xenoliths show a wide range ...of halogen elemental ratios, which form two arrays centered on the halogen composition of mid‐ocean ridge basalts. The samples on the array toward high I/Cl value have relatively low Cl concentration and low ratios of highly incompatible elements relative to heavy rare earth elements, whereas the samples on the array toward low Br/Cl value have higher Cl concentration and trace elements ratios. The detailed mechanisms to account for these signatures are equivocal at present. However, they are most likely to be related to secondary processes of volatile loss during partial melting and secondary phase formation during interaction with melts. The common primary mid‐ocean ridge basalt‐like halogen ratios in mantle xenoliths from different parts of the globe indicate that the mantle itself must have a relatively uniform composition over a wide scale. The mantle has maintained its halogen composition over billion year timescales without being affected by I‐rich halogens being transported into the mantle. Mass balance calculations suggest that, in order to maintain the I/Cl ratio of the convecting mantle over 2 Gyr, the I/Cl ratio of the subducted halogens must be no more than several times higher than the present‐day mantle value.
Plain Language Summary
Elemental and isotopic compositions of volatile species such as halogens, noble gases, hydrogen, and carbon can be used to trace the evolution of these species in the Earth. Halogens are important tracers of subduction recycling of surface volatiles into the mantle: however, there is only limited understanding of halogens in the mantle. Here we provide new halogen data of mantle xenoliths from intraplate settings. The mantle xenoliths show a wide range of halogen elemental ratios, which are expected to be related to later processes after the xenoliths formed. A similar primary halogen component is present in the xenoliths sampled from different localities. This suggests that the mantle has the uniform halogen composition over a wide scale. The halogen composition in the convecting mantle is expected to have remained constant over more than 2 billion years, despite subduction of iodine‐rich halogens. We used mass balance calculations to gain understanding into evolution rate of I/Cl ratio in the mantle. Calculations suggest that, in order to maintain the I/Cl ratio of the mantle over 2 Gyr, the I/Cl ratio of the subducted halogens must be no more than several times higher than the present‐day mantle value.
Key Points
Mantle xenoliths from intraplate settings have a wide range of halogen elemental ratios
The SCLM in different localities originally had MORB‐like halogen ratios, suggesting uniform halogen ratios in the convecting mantle
Evolution rate of mantle I/Cl value is estimated using mass balance calculations
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