The subduction of oceanic slabs is widely accepted to be a main reason for chemical heterogeneities in the mantle. However, determining the contributions of slabs in areas that have experienced ...multiple subduction events is often difficult due to possible overlapping imprints. Understanding the temporal and spatial variations of source components for widespread intraplate small volume basalts in eastern China may be a basis for investigating the influence of the subducted Pacific slab, which has long been postulated but never confirmed. For this purpose, we investigated the Chaihe-aershan volcanic field (including more than 35 small-volume Quaternary basaltic volcanoes) in NE China and measured the oxygen isotopes and water content of clinopyroxene (cpx) phenocrysts using secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectroscopy (FTIR), respectively. The water content of magma was then estimated based on the partition coefficient of H2O between cpx and the basaltic melt. The δ18O of cpx phenocrysts (4.28‰ to 8.57‰) and H2O content of magmas (0.19 wt.%–2.70 wt.%) show large variations, reflecting the compositional heterogeneity of the mantle source. The δ18O values and H2O content within individual samples also display considerable variation, suggesting the mixing of magmas and that the magma mixing occurred shortly before the eruption. The relation between the δ18O values of cpx phenocrysts and the H2O/Ce ratio, Ba/Th ratio and Eu anomaly of whole rocks demonstrates the contributions of three components to the mantle source (hydrothermally altered upper oceanic crust and marine sediments, altered lower gabbroic oceanic crust, and ambient mantle). The proportions of these three components have varied widely over time (∼1.37 Ma to ∼0.25 Ma). The Pacific slab is constantly subducted under eastern Asia and continuously transports recycled materials to the deep mantle. The temporal heterogeneity of the source components may be caused by ongoing Pacific slab subduction. Combined with other basalt localities in eastern China (Shuangliao basalts, Taihang basalts and Shangdong basalts), the contributions of recycled oceanic components in their mantle source are heterogeneous. This spatial heterogeneity of mantle sources may be induced by variable alterations and dehydration during the recycling process of the Pacific slab. Our results show that the source components of Cenozoic intraplate small-volume basalts in eastern China are temporally and spatially heterogeneous, which is likely induced by the ongoing subduction of the Pacific slab. This demonstrates that integrating the temporal variations in geochemical characteristics and tectonic history of a study region can identify the subducted oceanic plate that induced enriched components in the mantle source of intraplate basalts.
•Both upper and lower oceanic crust has been recognized in the mantle source of Chaihe-arershan basalts.•The recycled oceanic components in the mantle of whole eastern China varied widely, both spatially and temporally.•The ongoing Pacific slab subduction might trigger the heterogeneity of source components.•Recycled oceanic components for continental intraplate basalts could maintain more water than components of typical OIBs.
•Electrical conductivity of omphacite and garnet at elevated conditions.•Small water contents of omphacite and garnet in the deep subducting crust.•A new framework for the deep water recycling at ...subduction zones
It is usually stated that oceanic crust recycling at subduction zones introduces large quantities of water into Earth's interior. However, it remains clouded how much water is recycled to the deep mantle. This is largely due to the challenge in directly sampling, and analyzing the water content of, a deep subducting crust. The subducting crust below ∼30 km depth is dominated by omphacite and garnet in eclogite-facies rocks. Here we have determined the electrical conductivity of omphacite and garnet in representative subduction-related eclogites, each with varying contents of Fe and H2O that are key in electrical conduction. Considering the measured conductivity, the eclogite chemistry by geochemical investigations and the highly resistive property of subducting crusts by geophysical surveys, we demonstrate that, at 70-120 km depths in the subducting crust, the H2O contents of omphacite and garnet are strikingly small, with the maximum value being <400 and <80 ppm in the former and latter, respectively. The very small water contents indicate extremely water-poor conditions, or very low water activity, during the eclogite-facies metamorphism and in the system. This further implies the absence of appreciated amounts of hydrous phases such as amphibole and chlorite in the matrix, because of the strong ability of omphacite and garnet in hosting water as documented in natural samples. We suggest that the recycling of water to the deep mantle by oceanic crust subduction is probably very limited. The results are important for modeling the conductivity of subducting slabs and understanding the deep water circulation.
The speciation and amount of water dissolved in nominally anhydrous silicates comprising eight different mantle xenoliths has been quantified using synchrotron micro-FTIR spectroscopy. Samples ...studied are from six geographic localities and represent a cross-section of the major upper mantle lithologies from a variety of tectonic settings. Clinopyroxene contains between 342 and 413 ppm H2O. Orthopyroxene, olivine and garnet contain 169-201, 3-54 and 0 to <3 ppm H2O, respectively. Pyroxenes water contents and the distribution of water between ortho- and clinopyroxene is identical regardless of sample mineralogy (D watercpx/opx = 2.1 +/- 0.1). The total water contents of each xenolith are remarkably similar (113 +/- 14 ppm H2O). High-resolution spectroscopic traverses show that the concentration and speciation of hydrous defects dissolved in each phase are spatially homogeneous within individual crystals and identical in different crystals interspersed throughout the xenolith. These results suggest that the amount of water dissolved in the silicate phases is in partial equilibrium with the transporting melt. Other features indicate that xenoliths have also preserved OH signatures of equilibrium with the mantle source region: Hydroxyl stretching modes in clinopyroxene show that garnet lherzolites re-equilibrated under more reducing conditions than spinel lherzolites. The distribution of water between pyroxenes and olivine differs according to xenolith mineralogy. The distribution of water between clinopyroxene and olivine from garnet peridotites (D watercpx/oliv(gnt) = 22.2 +/- 24.1) is a factor of four greater than mineral pairs from spinel-bearing xenoliths (D watercpx/oliv(sp) = 88.1 +/- 47.8). Such an increase in olivine water contents at the spinel to garnet transition is likely a global phenomenon and this discontinuity could lead to a reduction of the upper mantle viscosity by 0.2-0.7 log units and a reduction of its electrical resistivity by a factor of 0.5-0.8 log units. PUBLICATION ABSTRACT
The mantle source of the Cenozoic intraplate basalts from the Shuangliao volcanoes in the Songliao Basin in northeastern China (Xu et al., 2012) has been suggested to contain young subducted recycled ...oceanic crust (ROC), whereas the mantle source of the Cenozoic basalts from the nearby Changbaishan volcanoes contain recent and ancient recycled marine sediments (Kuritani et al., 2011). To determine the possible contributions of recycled marine sediments to the mantle source of the Shuangliao Cenozoic basalts, we measured the H2O content in clinopyroxene (cpx) phenocrysts by Fourier transform infrared spectrometry (FTIR) and calculated H2O contents of equilibrated melts using the partition coefficient of H2O between cpx and basaltic melt. Major and trace element compositions measured in the Shuangliao whole rocks are consistent with a previous study by Xu et al. (2012) and display typical HIMU characteristics. The calculated H2O content of the “primary” magmas varies from 0.90 to 3.06wt.%, falling in the range of back-arc basin basalts (BABBs) and island arc basalts (IABs) and it is higher than that of mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs). The relationships between H2O/Ce and (Ba/Th)n (n denotes normalization to primitive mantle composition), (Nb/La)n and Ce/Pb of the Shuangliao basalts indicate that in addition to ROC with low H2O/Ce and high (Nb/La)n, recycled marine sediments with high H2O/Ce, (Ba/Th)n and low Ce/Pb and a normal depleted mantle (DMM) component should also be involved in the magma source. In addition, the mantle source of the later (~43Ma) basalts should contain more recycled marine sediments than that of the earlier (~51Ma) basalts.
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•Water contents of the Shuangliao Cenozoic basalts of NE China have been estimated.•DMM, ROC and recycled sediments have been involved in the mantle source.•The relative contribution of the three components has changed with time.
The genesis of Cenozoic continental basalts in Eastern China is highly debated. Subducted oceanic crust (most probably the Pacific oceanic slab) has been increasingly suggested to be involved in ...their mantle source. The Taihang Mountains are located at a surface position corresponding to the western edge of a stagnant subducted Pacific slab revealed by geophysical investigations, and thus provide an opportunity to study the spatial extent of the effect of Pacific subduction on the mantle characteristics of Eastern China. In previous studies, alkali basalts from the Taihang Mountains were considered to be the products of interaction between asthenosphere-derived melts and the old, enriched lithospheric mantle; their magnesium isotope signature suggested the contribution of oceanic carbonate. To explore the possible contribution from recycled oceanic crust in the genesis of the Taihang basalts we estimated the water contents of the magmas by a phenocryst-based approach () and measured the oxygen isotope compositions of clinopyroxene phenocrysts by secondary ion mass spectrometry for the same sample suite as used in previous studies. The calculated water contents (H sub(2)O by weight) of the parental magmas range from 0.20 to 1.07wt %, and the corresponding H sub(2)O/Ce ratios correlate well with (Nb/La) sub(n) (where n represents primitive mantle normalization), (Ba/Th) sub(n) and epsilon Nd. The oxygen isotope ratios ( delta super(18)O sub(SMOW)) range from 5.8 to 7.4ppt, and the average value for each sample ranges from 6.6 to 7.0ppt. These new data indicate a large contribution from extremely dehydrated recycled oceanic crust together with entrained sediments. The minor element compositions (Ca, Mn and Ni contents) of olivine phenocrysts in the Taihang basalts suggest a pyroxenite source that was probably formed by the interaction between oceanic crust-derived melt and ambient mantle peridotite.
Water incorporation in forsterite samples synthesized under low to medium silica-activity conditions mostly occurs via a substitutional mechanism in which a Si vacancy is compensated by four protons. ...Corresponding IR absorption spectra display a cluster of narrow and weakly anharmonic OH-stretching bands at wavenumbers above 3,500 cm
−1
. However, this diagnostic spectrum is often superimposed to one broader absorption band, rarely two, displaying pronounced temperature-dependent properties and tentatively assigned to H atoms in interstitial position (Ingrin et al. in Phys Chem Miner 40:499–510,
2013
). Here, we investigate the structural and vibrational properties of selected interstitial H-bearing defects in forsterite using a first-principles modeling approach. We show that the broad bands discussed by Ingrin et al. (Phys Chem Miner 40:499–510,
2013
) are most likely related to interstitial OH groups in the vacant octahedral sites alternating with the M2 sites along the
c
axis of the forsterite structure. The corresponding OH defects lead to the formation of fivefold coordinated Si species. Their peculiar thermal properties stem from the vibrational phase relaxation due to the anharmonic coupling of the high-energy local OH-stretching mode with a low-energy vibrational mode. This “exchange mode” corresponds to the hindered longitudinal translation of the OH group. These results suggest that at high pressure, hydrogen incorporation in forsterite is dominated by coexisting interstitial OH groups and (4H)
Si
defects.
Peridotite xenoliths with a broad range of textures provides evidence for consistent microstructural evolution in a vertical transect of the shallow lithospheric mantle (35–55 km depth) beneath the ...Persani Mountains, SE Carpathians, Romania, due to ongoing plate convergence in the Carpathian Arc nearby. The recrystallized grain size, crystal preferred orientations strength, and resulting seismic anisotropy vary continuously and display a strong correlation to equilibrium temperatures, suggesting a continuous change in deformation conditions with depth. The shallowmost xenoliths have microstructures typical of high stress deformation, marked by strong recrystallization to fine grain sizes, which results in weak crystal preferred orientations and anisotropy. The deepest xenoliths have coarse-grained porphyroclastic microstructures and strong crystal preferred orientations. Replacive orthopyroxene structures, consuming olivine, and high H
2O concentrations in the pyroxenes are observed in some xenoliths indicating limited percolation of fluids or volatile-rich melts. Despite the high stress deformation and high H
2O contents in some of the studied xenoliths, analysis of olivine crystallographic orientations indicates that 100 slip systems, rather than “wet” 001 accommodate most of the deformation in all samples. Seismic anisotropy estimated from the measured olivine and pyroxene crystal preferred orientations suggests that the strike-parallel fast SKS polarization directions and ~
1 s delay times measured in the SE Carpathians are likely the consequence of convergence-driven belt-parallel flow in the lithospheric mantle.
The infrared spectroscopic properties of selected defects involving one proton and one nearby M
3+
(M = Al, Cr, Fe) substitution in orthoenstatite are investigated by first-principles calculations. ...Based on the theoretical results, the absorption bands experimentally observed on synthetic samples with high crystalline quality and low doping levels can be assigned to specific defect configurations. Most of them correspond to Mg vacancies at M2 sites locally compensated by one proton and one M
3+
cation at a nearby M1 site. This confirms that the M
3+
+ H
+
= 2 Mg
2+
exchange mechanism is the dominant hydrogen incorporation mechanism at the lowest concentration levels in doped enstatite. At higher concentration levels, more complex incorporation mechanisms could become dominant in Al-bearing samples.
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