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Peridotites dredged from mid-ocean ridges and glassy mid-ocean ridge basalts (MORB) transmit information about the oxygen fugacity (fO2) of Earth's convecting upper mantle to the surface. Equilibrium assemblages of olivine+orthopyroxene+spinel in abyssal peridotites and Fe3+/ΣFe ratios in MORB glasses measured by X-ray absorption near-edge structure (XANES) provide independent estimates of MORB source region fO2, with the former recording fO2 approximately 0.8 log units lower than the latter relative to the quartz-fayalite-magnetite (QFM) buffer. To test cross-compatibility of these oxybarometers and examine the compositional effects of changing fO2 on a peridotite plus melt system over a range of Earth-relevant fO2, we performed a series of experiments at 0.1 MPa and fO2 controlled by CO-CO2 gas mixes between QFM-1.87 and QFM+2.23 in a system containing basaltic andesite melt saturated in olivine, orthopyroxene, and spinel Oxygen fugacities recorded by each method are in agreement with each other and with the fO2 measured in the furnace. Measurements of fO2 from the two oxybarometers agree to within 1σ in all experiments. These results demonstrate that the two methods are directly comparable and differences between fO2 measured in abyssal peridotites and MORB result from geographic sampling bias, petrological processes that change fO2 in these samples after separation of melts and residues, or abyssal peridotites may not be residues of MORB melting. As fO2 increases, spinel Fe3+ concentrations increase only at the expense of Cr from QFM-1.87 to QFM-0.11. Above QFM, Al is also diluted in spinel as the cation proportion of Fe3+ increases. None of the three spinel models tested, MELTS (Ghiorso and Sack 1995), SPINMELT (Ariskin and Nikolaev 1996), and MELT_CHROMITE (Poustovetov and Roeder 2001), describe these compositional effects, and we demonstrate that MELTS predicts residues that are too oxidized by >1 log unit to have equilibrated with the coexisting liquid phase. Spinels generated in this study can be used to improve future thermodynamic models needed to predict compositional changes in spinels caused by partial melting of peridotites in the mantle or by metamorphic reactions as peridotites cool in the lithosphere. In our experimental series, where the ratio of Fe2O3/FeO in the melt varies while other melt compositional parameters remain nearly constant, experimental melt fraction remains constant, and Fe3+ becomes increasingly compatible in spinel as fO2 increases. Instead of promoting melting, increasing the bulk Fe3+/ΣFe ratio in peridotite drives reactions analogous to the fayalite-ferrosilite-magnetite reaction. This may partly explain the absence of correlation between Na2O and Fe2O3 in fractionation-corrected MORB.

Subduction zones regulate Earth's carbon distribution, yet the mechanism of carbon transfer from continental crust to mantle remains elusive. We examined an eclogite‐garnet peridotite interface from the Chinese Continental Scientific Drilling Program in the Sulu orogen, representing the slab–mantle wedge boundary formed during continental subduction. Whole‐rock magnesium (Mg) isotopic and major‐trace element data, together with in situ mineral analyses, identify the presence of carbonic fluids characterized by notably light Mg isotopic compositions (−0.54 to −0.36‰) and elevated Ca, Mg, Sr, and rare earth elements contents. These fluids, generated by slab decarbonation during prograde metamorphism, mobilized carbon from the subducted crust and enriched the mantle wedge. Modeling indicates that continental subduction rivals oceanic systems in transporting carbon to mantle. However, the paucity of mantle‐derived magmatism limits carbon return, promoting long‐term retention of continental carbon in mantle and establishing continental subduction as a major sink in the global carbon budget.

Orogenic peridotites in the crystalline basement of the northwestern Bohemian Massif contain multiphase solid inclusions (MSI), which are interpreted to be crystallisation products of trapped former carbonate–silicate melts metasomatizing their host rocks. We applied conventional thermobarometry and forward thermodynamic modelling to constrain the P–T evolution ranging from the peak metamorphic conditions of the investigated harzburgite and lherzolite, through entrapment of the melts in the outer parts of garnets, to the (re)-equilibration of the MSI assemblages. The peak conditions of c. 1100 °C/4.5–5.5 GPa are recorded by garnet cores and large pyroxene porphyroclasts. The melt entrapment, during which garnet outer parts grew, was associated with influx of the metasomatizing liquids and probably took place during the early stage of the exhumation. Thermodynamic model of amphibole-free MSI assemblage comprising kinoshitalite/Ba-rich phlogopite (approximated by phlogopite in the model), dolomite, magnesite, clinopyroxene, orthopyroxene, garnet and chromite provided robust estimate of P and T of its (re)-equilibration, c. 900–1000 °C, 1.8–2.2 GPa. Furthermore, the lack of olivine reflects co-existence of COH fluid with high X(CO 2 ) = CO 2 /(CO 2  + H 2 O) ≥ 0.6. Models employing identical P–T–X(CO 2 ) parameters successfully reproduced the other two amphibole-bearing assemblages observed. The modelled stability fields show perfect alignment with a characteristic isobaric segment of the solidus curve of carbonated peridotite. This co-incidence implies that the (re)-equilibration corresponds to the termination of the melt crystallisation once the near-isothermal exhumation path intersected the solidus. Decreased solubility of silicates at the carbonated peridotite “solidus ledge”, inferred from the published experimental data, as well as concentric textures of some MSI indicates sequential crystallisation from the early silicates to late dolomite. The carbonated “solidus ledge” is a relatively narrow boundary in the lithospheric mantle capable of an abrupt immobilisation of fluxing or transported carbonated melts. The investigated rocks are estimated to store approximately 0.02 kg C/m 3 (or 6 ppm C) occurring as carbonates in the MSI.

The Archean mantle lithosphere beneath the North China Craton (NCC) was transformed in the Mesozoic, leading to the craton destruction. Despite the significant breakthroughs in the craton studies, lithospheric transformation mechanisms are yet to be fully understood. Compositional variations of mantle-derived rocks and xenoliths provide insights into the nature of the mantle lithosphere before and after the craton destruction. The Paleozoic lithosphere of the NCC is ∼200 km thick. It has a refractory mantle with an evolved isotopic signature. The Mesozoic mantle lithosphere was relatively fertile and highly heterogeneous. In the Cenozoic, the lithosphere in the eastern NCC is about 60–80 km thick. It has an oceanic-type mantle that is fertile in composition and depleted in the Sr-Nd isotopic signature. The Central Zone lithosphere is >100 km thick and has a double-layer mantle with an old upper layer and a new lower layer. The Western Block has a lithosphere of ∼200 km thick. The lithospheric mantle beneath the southern and northern margins and eastern part of the NCC has been transformed significantly by peridotite-melt reactions due to the multiple subductions of adjacent plates since the Paleozoic. Paleo-Pacific subduction and the associated dynamic processes significantly alter the lithosphere based on the distribution of craton destruction. The involved mechanisms include mechanical intrusion of subduction plates, melt fluid erosion, and local delamination. The lithospheric thinning of ∼120 km is relevant to the continental extension caused by subduction plate rollback and trench retreat.

Carbon fluxes in subduction zones can be better constrained by including new estimates of carbon concentration in subducting mantle peridotites, consideration of carbonate solubility in aqueous fluid along subduction geotherms, and diapirism of carbon-bearing metasediments. Whereas previous studies concluded that about half the subducting carbon is returned to the convecting mantle, we find that relatively little carbon may be recycled. If so, input from subduction zones into the overlying plate is larger than output from arc volcanoes plus diffuse venting, and substantial quantities of carbon are stored in the mantle lithosphere and crust. Also, if the subduction zone carbon cycle is nearly closed on time scales of 5–10 Ma, then the carbon content of the mantle lithosphere + crust + ocean + atmosphere must be increasing. Such an increase is consistent with inferences from noble gas data. Carbon in diamonds, which may have been recycled into the convecting mantle, is a small fraction of the global carbon inventory. This paper reviews carbon fluxes into and out of subduction zones, using compiled data, calculations of carbon solubility in aqueous fluids, and estimates of carbon flux in metasedimentary diapirs. Upper-bound estimates suggest that most subducting carbon is transported into the mantle lithosphere and crust, whereas previous reviews suggested that about half is recycled into the convecting mantle. If upper-bound estimates are correct, and observed output from volcanoes and diffuse outgassing is smaller, then the mantle lithosphere is an important reservoir for carbon. If the subduction carbon cycle remains in balance, then outgassing from ridges and ocean islands is not balanced, so that the carbon content of the lithosphere + ocean + atmosphere has increased over Earth history.

Mantle residues beneath Archean cratonic nuclei have been extensively studied, whereas less attention has been given to the mantle lithosphere beneath Proterozoic mobile belts that link these nuclei. Rare mantle tectonites within tectonic mélanges of Paleoproterozoic mobile belts provide information important to understanding the broader processes involved in the construction of the cratonic mantle lithosphere. Here we present mineral compositions, bulk‐rock major, trace, and platinum group elements, Re‐Os isotopes, and olivine oxygen isotopes from a Paleoproterozoic mantle tectonite in West Greenland–the Ussuit peridotite. The Ussuit peridotite was emplaced in the crust during the Nagssugtoqidian orogeny between 1,870 Ma and 1,775 Ma and preserves primary melt depleted characteristics that reflect >30% melting, for example, Al2O3 < 0.4 wt.%, Ti < 10 ppm, Lu/Yb > 0.25, and Mg #s up to 93. Cryptic signatures of hydrous melting, for example, spinel Cr #’s >65, Os/Ir ratios between 0.3 and 6, and supramantle olivine δ18O values, suggest that the high degree of melt depletion was partly inherited from a forearc or sub‐arc melting environment. Re‐Os isotopic systematics show melt depletion occurred at ∼2 Ga overlapping the juvenile oceanic arc crust that hosts the peridotites. This age coincides with a peak in the global production of juvenile cratonic lithosphere. Furthermore, the global Paleoproterozoic cratonic mantle has strong geochemical similarities with the Ussuit peridotites. It is suggested that subduction zone peridotites form key components of the Paleoproterozoic cratonic lithospheric mantle, creating a viscous, buoyant mantle lithosphere that contributed to the long‐term stability of the greater cratonic masses. Plain Language Summary The continental lithosphere formed over much of the Earth's history through the aggregative accretion of lithospheric fragments to “Archean cratonic nuclei.” Today, these nuclei are extant and formed >2.5 billion years ago and provide stable, above sea‐level platforms for continental growth. They have been extensively studied but little is known about the sutures and underlying mantle lithosphere holding together the cratonic nuclei of our modern continents. In this study, new geochemical and geochronological data are presented on a fragment of the mantle lithosphere—the Ussuit peridotite—that is hosted in a ∼1.8 Ga cratonic suture. The data imply that, like the Archean cratonic mantle lithosphere, the Ussuit peridotite formed via high degrees of mantle melting, >30%. More cryptic data imply that the rocks formed in the deep mantle portions of a volcanic oceanic arc. Geochemical and geochronological comparisons between the Ussuit peridotite and global, temporally related mantle peridotites are likewise similar. This implies that processes of melt depletion in the Ussuit peridotite may have been globally active at that time. As such, the lithospheric mantle underlying these ca. 1.8 Ga sutures may have components formed via similar processes to those of the Ussuit peridotite, shedding light on how our continental lithosphere grew through time. Key Points New geochemical and isotopic data shed light on the age and origin of a Paleoproterozoic orogenic peridotite—The Ussuit peridotite Re‐Os isotopic constraints indicate the Ussuit peridotite formed via melting of juvenile asthenospheric mantle around 2 Ga The new data suggest that Ussuit peridotite formed in an arc environment and coincident with a period of continental growth globally ca. 2 Ga

The formation and preservation of cratons—the oldest parts of the continents, comprising over 60 per cent of the continental landmass—remains an enduring problem. Key to craton development is how and when the thick strong mantle roots that underlie these regions formed and evolved. Peridotite melting residues forming cratonic lithospheric roots mostly originated via relatively low-pressure melting and were subsequently transported to greater depth by thickening produced by lateral accretion and compression. The longest-lived cratons were assembled during Mesoarchean and Palaeoproterozoic times, creating the stable mantle roots 150 to 250 kilometres thick that are critical to preserving Earth’s early continents and central to defining the cratons, although we extend the definition of cratons to include extensive regions of long-stable Mesoproterozoic crust also underpinned by thick lithospheric roots. The production of widespread thick and strong lithosphere via the process of orogenic thickening, possibly in several cycles, was fundamental to the eventual emergence of extensive continental landmasses—the cratons. Cratons are the oldest parts of the Earth’s continents; this Review concludes that the production of widespread, thick and strong lithosphere via the process of orogenic thickening was fundamental to the eventual emergence of extensive continental landmasses.

Accurate estimation of the depths of spinel peridotite xenoliths for which reliable geobarometers are not available is imperative to be able to reconstruct the precise structures of the lithosphereasthenosphere boundary (LAB). The LAB can be defined based on thermal, chemical, rheological, and petrological contrasts, and knowing its depth is crucial to understanding mantle dynamics. We attack this problem by examining spinel peridotite xenoliths from Ichinomegata maar in the back-arc side of Northeast Japan Arc. Extensive mineral compositions of nine xenolith samples revealed various patterns of chemical zoning in pyroxenes, suggesting diverse thermal histories. We examined the timescales of development of each zoning pattern and identified minerals, grain portions, and components closely approached equilibrium just before xenolith extraction as orthopyroxene and clinopyroxene, the outermost rims, and Ca-Mg-Fe components, respectively. Applying the best pair of geothermobarometers to the chosen analyses, plausible derivation depths of eight samples were obtained. They range from 0.72-1.6 GPa in pressure and from 830-1080 °C in temperature, which defines a high thermal gradient of 10 K/km or 290 K/GPa. There is an intimate correlation between the zoning patterns of pyroxenes and the depth estimates: pyroxenes in the deeper samples have zoning indicating cooling followed by heating just before xenolith extraction, and those of the shallower samples have zoning indicating monotonic cooling. Depth variations of rock microstructures, grain size of olivine, chemical compositions of minerals, and phase assemblage, including the presence or absence of glass or fluid phase, show that the mantle beneath Ichinomegata consists of two distinct layers. The shallower (28-32 km) layer is granular, less oxidized, amphibole- and plagioclase-bearing, and subsolidus, whereas the deeper (41-55 km) layer is porphyroclastic, amphibole- and plagioclase-free, oxidized, and partially molten. The contrasts between the two layers suggest that the upper layer represents a lithospheric mantle and the lower layer a LAB zone. These layers are similar to those reported from the bottom of subcontinental lithospheric mantle in various aspects, but the LAB beneath Ichinomegata is much shallower (40-60 km) and cooler (~1100 °C). The coincidence of (1) the depth of a rheological transition, marked granular to porphyroclastic textures, and (2) the depth of a phase transition, from subsolidus hydrous peridotite to a hydrous mantle with melt in localized pockets, is the remarkable feature of the LAB beneath Ichinomegata. This suggests that a rheological boundary zone in arc settings is governed by melting of the hydrous mantle and that the underlying asthenosphere is partially molten. The depth-dependent thermal history shown by chemical zoning in pyroxenes and the presence of melt as pockets suggest that the LAB beneath Ichinomegata was in a transient state that was affected by thermal and material transport.

Hydrogen distribution between nominally anhydrous minerals (NAMs) of a garnet-lherzolite under subsolidus conditions has been investigated. Separated NAMs from a garnet-peridotite from Patagonia (Chile) are annealed together (olivine, orthopyroxene, clinopyroxene, and garnet) using a piston-cylinder at 3 GPa and 1100 °C using talc-pyrex cell assembly for 10, 25, and 100 h. The talc-pyrex assembly provides enough hydrogen in the system to re-equilibrate the hydrogen concentrations at high pressure. The three coexisting nominally anhydrous minerals (NAMs, i.e., olivine, orthopyroxene, and clinopyroxene) were successfully analyzed using FTIR. The resulting hydrogen concentrations exceed significantly the initial hydrogen concentration by a factor of 13 for olivine and a factor of 3 for both pyroxenes. Once mineral-specific infrared calibrations are applied, the average concentrations in NAMs are 115 ± 12 ppm wt H2O for olivine, 635 ± 75 ppm wt H2O for orthopyroxene, and 1214 ± 137 ppm wt H2O for clinopyroxene, garnet grains are dry. Since local equilibrium seems achieved over time (for 100 h), the calculated concentration ratios are interpreted as mineral-to-mineral hydrogen partition coefficients (i.e., Nernst's law) for a garnet-peridotite assemblage. It yields, based on mineral-specific infrared calibrations, DOpx/Ol = 5 ± 1, DCpx/Ol = 10 ± 2, and DCpx/Opx = 1.9 ± 0.4. While DCpx/Opx is in agreement (within error) with previous results from experimental studies and concentration ratios observed in mantle-derived peridotites, the DPx/Ol from this study are significantly lower than the values reported from mantle-derived xenoliths and also at odd with several previous experimental studies where melt and/or hydrous minerals co-exists with NAMs. The results confirm the sensitivity of hydrogen incorporation in olivine regarding the amount of water-derived species (H) in the system and/or the amount of water in the coexisting silicate melt. The results are in agreement with an important but incomplete dehydration of mantle-derived olivine occurring at depth, during transport by the host magma or during slow lava flow cooling at the surface. The rapid concentration modification in mantle pyroxenes also points out that pyroxenes might not be a hydrogen recorder as reliable as previously thought.

We have conducted high‐pressure electrical conductivity and Mössbauer spectroscopic measurements of peridotite glass as an analog of silicate melts. We observed the shoulder feature in the Mössbauer spectra above 60 GPa due to the emergence of the new Fe2+ component, which could be associated with the change of the iron partitioning coefficient between solid and melt observed in previous melting experiments. The change in the trend of the electrical conductivity profile has been observed at around ∼83 GPa, suggesting the increase of the relative abundance of the new component. The pressure dependence of hyperfine parameters indicates the spin transitions of both Fe2+ and Fe3+ occur above 100 GPa, which is likely related to the structural change of the glass. Our results suggest that the spin state changes of iron lead to further densification of the silicate melts at the bottom of the mantle.