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Kimberlite‐borne eclogite xenoliths having Precambrian oceanic crustal protoliths and entrained from ≥100 km depth can retain pristine geochemical features despite extended residence in the cratonic lithospheric mantle, making them valuable archives of deep chemical cycling including that of water. We determined, by Fourier Transform Infrared Spectroscopy, structural OH contents in clinopyroxene and garnet from 15 unmetasomatized eclogite xenoliths. Calculated total c(H2O) is 100–510 wt.ppm for clinopyroxene and below detection (∼2 wt.ppm) to 200 wt.ppm for garnet, while garnet δ18O, determined by Secondary Ion Mass Spectrometry, ranges from +5.0‰ to +7.3‰, (similar to high‐ and low‐temperature seawater‐altered oceanic crust). Estimated electrical conductivity in pristine eclogites increases with temperature (i.e., depth for conductive geotherms), while clinopyroxene‐garnet H2O partition coefficients decrease with increasing temperature and garnet grossular component (i.e., Ca#), similar to other incompatible components. Various considerations suggest the retention of primary H2O in the samples, likely occurring in km‐sized pods of coarse‐grained eclogite. High Al2O3 in clinopyroxene as omphacite component, stabilized during high‐pressure metamorphism, facilitates H2O uptake. Therefore, the high bulk c(H2O) estimated for samples with plagioclase‐rich, deep crustal protoliths (median 290 wt.ppm) may indicate an interaction with fluids expelled at depth from serpentinites. The c(H2O) of ancient and modern subducted bulk oceanic crust (∼220–240 wt.ppm) are similar, suggesting constant mantle ingassing since at least 3 Ga ago. This places constraints on factors, such as mantle temperatures, that determine the efficiency of deep water cycling. Plain Language Summary Water in Earth's interior exists mostly as OH− anion in trace abundances in nominally anhydrous minerals. Despite these low concentrations, deep water exerts a strong influence on fundamental Earth processes, such as partial melting of the mantle and the operation of plate tectonics. However, the extent to which the loss of water via volcanism has been compensated over time by retention in downgoing oceanic plates, after their dehydration and metamorphism to eclogite, remains poorly known. Deeply buried Archean and Paleoproterozoic oceanic crust is sampled as remarkably pristine eclogite fragments quickly exhumed by volcanism from depths >100 km. High Al2O3 contents, characteristic of deep crustal plagioclase‐rich cumulates, facilitate H2O‐uptake in clinopyroxene, the main carrier of H2O in eclogite. Because clinopyroxene is rich in Al only at high pressure, where the crust is already dehydrated, but the underlying seawater‐altered mantle begins to liberate fluids, we suggest that interaction with this fluid explains the Al‐H2O association. Moreover, the eclogite‐based H2O estimate for ancient crust is similar to estimates for the modern crust, suggesting that deep water cycling in the crustal part of subducting slabs changed little in the last 3 billion years, with consequences for the factors determining the efficiency of ingassing. Key Points Electrical conductivity in deep (>100 km) pristine eclogite xenoliths from Siberian and Slave cratons increases with depth H2O content increases with Al2O3, which is highest in gabbroic eclogites with deep oceanic crustal protoliths The H2O content of subducted bulk oceanic crust sampled by eclogite 3 Ga ago may be similar to today

The eastern North China Craton (NCC), where an initially diamondiferous deep cratonic mantle root was lost during Paleozoic and Mesozoic time, represents a prime natural laboratory to study the processes and mechanisms of continental lithospheric mantle destruction and replacement, which remain, however, controversial. In this study, detailed petrography, whole-rock and mineral compositions of spinel-facies peridotite xenoliths from Cenozoic basalts in the Huinan area, northeastern NCC, are presented to provide new constraints on the transformation of the subcontinental lithospheric mantle (SCLM). These xenoliths define two groups based on textural observation and mineral modes: Group 1 peridotites show protogranular textures and consist of harzburgites and dunites. They have low Al2O3 contents in whole-rock and orthopyroxene (0.53–1.06 wt.% and 2.10–3.21 wt.%, respectively), high olivine modes (79–96%), whole-rock MgO (44.8–47.9 wt.%) and Mg# (100 Mg/(Mg + FeT) molar: 90.1–90.7), suggesting that they were derived from moderately refractory SCLM. In contrast, Group 2 xenoliths display porphyroclastic to protogranular textures and consist of lherzolites and harzburgites with rare spinel-pyroxene intergrowths. They have overall higher Al2O3 (1.48–3.23 wt.% and 3.02–4.65 wt.%, respectively) in whole-rock and orthopyroxene, lower olivine modes (64–83%), MgO (38.6–44.5 wt.%) and whole-rock Mg# values 87.6–90.1, and they may represent fertile SCLM. Peridotites of both groups have similar equilibration temperatures (i.e., 923–977 °C and 881–1110 °C, respectively), which are not correlated with Mg# in olivines, suggesting that they coexist over a range of depths. However, clinopyroxenes in the Group 1 xenoliths display LREE-enriched and convex-upward REE patterns, whereas those in Group 2 mainly show LREE-depleted and spoon-shaped REE patterns, with minor LREE-enriched and convex-upward ones. In addition, spinel-pyroxene intergrowths indicative of garnet destabilization are ubiquitous in Group 1, consistent with variable Al2O3 over a narrow range of Mg# in some opx and low HREE in some cpx, but rare in Group 2 peridotites. Interaction of the fertile mantle with melts similar to the Cenozoic basalts at high melt–rock ratios eradicated most signatures of their origin in the garnet stability field, whereas the refractory peridotites, which reacted with residual melts or fluids at low melt/fluid-rock ratios, retained evidence for the former presence of garnet. We suggest that, combined, these observations are best reconciled if portions of ancient refractory lithosphere, which were partly delaminated during multiple subduction episodes affecting the eastern NCC, were re-accreted together with fertile mantle during asthenospheric upwelling driven by extension.

Lower crustal processes played a key role during the destruction of the North China Craton. Petrological and geochemical analyses were performed on the granulite and pyroxenite xenoliths in the late Cretaceous basalt from Western Liaoning of the North China Craton to investigate the nature and evolution of the lower crust during the Mesozoic. The granulite xenoliths are predominantly intermediate–silicic granulite with subordinate basic granulite. The intermediate–silicic granulites exhibit relatively low Mg# values (0.46–0.63), positive Eu, Pb and Sr anomalies, a large range of Sr–Nd–Pb isotopic compositions and negative correlations between 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios. The sample HS20-19 shows the lowest SiO 2 and MgO contents than that of other intermediate–silicic granulites. Generally, the intermediate–silicic granulites have mineralogical and whole-rock geochemical affinities to the Archean granulite terrains of the North China Craton. In contrast, the basic granulite has a higher Mg# value (0.73), and depletions of La and Sr, but similar Sr–Nd–Pb isotopic compositions to the intermediate–silicic granulite xenoliths and terrains. These observations indicate that most intermediate–silicic granulites represent modified Archean lower crust by underplated magma, whereas HS20-19 and the basic granulite can be explained as restites left after partial melting of the ancient lower crust. The pyroxenite xenoliths exhibit a cumulate texture, variable Mg# values (85.9–88.6), and convex-upward rare-earth element patterns with high-field-strength elements depletions in the clinopyroxene. 87 Sr/ 86 Sr ratios (0.7036–0.7063) are negatively correlated with Mg# values and positively correlated with Ba and Pb contents in the clinopyroxene. These observations imply that the pyroxenite xenoliths originated as cumulates from an asthenospheric magma and were contaminated by the lower crust at the crust–mantle transition. All xenoliths experienced decompression event induced by the lithosphere extension in the early Cretaceous. Combined with the previous studies on the Mesozoic volcanic rocks from the Western Liaoning, we conclude that the continuous magmatic underplating not only formed the pyroxenite cumulates but also provided heat for remelting of the ancient lower crust, resulting in the formation of voluminous intermediate–silicic volcanic rocks during the Mesozoic. These processes led to the transformations of the Archean lower crust beneath the Western Liaoning and the entire North China Craton.

Much of the lithospheric subcontinental mantle (SCLM) sampled in the Calatrava Volcanic Field (CVF) shows refertilization by alkaline metasomatic agents. The Cerro Pelado and El Palo ultramafic xenolith suites record the best evidence of this type of metasomatism in this volcanic field. Several groups of peridotite (lherzolite, wehrlite, and dunite) and pyroxenite (clinopyroxenite and websterite) xenoliths have been distinguished. Despite having scarce phlogopites and amphiboles as modal metasomatic phases, all studied xenoliths present a variable cryptic metasomatism, highlighted by the strong Fe-Ti enrichment and fractionated REE patterns in the most evolved wehrlite and pyroxenite varieties. They show a common trend of an Fe-Ti-Ca increase, whereas the pyroxenites are more depleted in Fe compared to the lherzolites and wehrlites. Trace-element (REE and multi-trace) patterns are roughly similar among them, suggesting different interactions and refertilization degrees by alkaline silicate melts. The same Sr–Nd isotopic EAR composition, combined with trace-element chemistry of metasomatic xenolith phases and phenocrysts from the Calatrava volcanics, highlights the main role of this magmatism in percolation processes beneath Central Iberia. These mantle xenoliths also show variable amounts of interstitial glass that originated by in situ partial melting, favored by the enriched chemical nature of cryptically metasomatized clinopyroxene during their volcanic transport. This alkaline-refertilized mantle type represents the main domain within the SCLM beneath Central Iberia, as was also recorded in other Western European Cenozoic volcanic fields.

Natural peridotite samples containing olivine, orthopyroxene, and spinel can be used to assess the oxygen fugacity fO2 of the upper mantle. The calculation requires accurate and precise quantification of spinel Fe3+/ΣFe ratios. Wood and Virgo (1989) presented a correction procedure for electron microprobe (EPMA) measurements of spinel Fe3+/ΣFe ratios that relies on a reported correlation between the difference in Fe3+/ΣFe ratio by Mossbauer spectroscopy and by electron microprobe (ΔFe3+/ΣFeMoss-EPMA) and the Cr# [Cr/(Al+Cr)] of spinel. This procedure has not been universally adopted, in part, because of debate as to the necessity and effectiveness of the correction. We have performed a series of replicate EPMA analyses of several spinels, previously characterized by Mossbauer spectroscopy, to test the accuracy and precision of the Wood and Virgo correction. While we do not consistently observe a correlation between Cr# and ΔFe3+/ΣFeMoss-EPMA in measurements of the correction standards, we nonetheless find that accuracy of Fe3+/ΣFe ratios determined for spinel samples treated as unknowns improves when the correction is applied. Uncorrected measurements have a mean ΔFe3+/ΣFeMoss-EPMA = 0.031 and corrected measurements have a mean ΔFe3+/ΣFeMoss-EPMA = -0.004. We explain how the reliance of the correction on a global correlation between Cr# and MgO concentration in peridotitic spinels improves the accuracy of Fe3+/ΣFe ratios despite the absence of a correlation between ΔFe3+/ΣFeMoss-EPMA and Cr# in some analytical sessions. Precision of corrected Fe3+/ΣFe ratios depends on the total concentration of Fe, and varies from ±0.012 to ±0.032 (1σ) in the samples analyzed; precision of uncorrected analyses is poorer by approximately a factor of two. We also present an examination of the uncertainties in the calculation contributed by the other variables used to derive FO2. Because there is a logarithmic relationship between the activity of magnetite and LogfO2, the uncertainty in fO2 relative to the QFM buffer contributed by the electron microprobe analysis of spinel is asymmetrical and larger at low ferric Fe concentrations (+0.3/-0.4 log units, 1σ, at Fe3+/ΣFe = 0.10) than at higher ferric Fe concentrations (±0.1 log units, 1σ, at Fe3+/ΣFe = 0.40). Electron microprobe analysis of olivine and orthopyroxene together contribute another ±0.1 to ±0.2 log units of uncertainty (1σ). Uncertainty in the temperature and pressure of equilibration introduce additional errors on the order of tenths of log units to the calculation of relative fO2. We also document and correct errors that appear in the literature when formulating fO2 that, combined, could yield errors in absolute fO2 of greater than 0.75 log units-even with perfectly accurate Fe3+/ΣFe ratios. Finally, we propose a strategy for calculating the activity of magnetite in spinel that preserves information gained during analysis about the ferric iron content of the spinel. This study demonstrates the superior accuracy and precision of corrected EPMA measurements of spinel Fe3+/ΣFe ratios compared to uncorrected measurements. It also provides an objective method for quantifying uncertainties in the calculation of fO2 from spinel peridotite mineral compositions.

Quantifying the storage conditions and evolution of different magmatic components within sub-volcanic plumbing systems is key to our understanding of igneous processes and products. Whereas erupted magmas represent a portion of the eruptible volcanic system, plutonic xenoliths provide a complementary record of the mushy roots of the plumbing system that cannot be mobilised easily to form lavas and consequently offer a unique record of magma diversity within the sub-volcanic plumbing system. Here, we present a detailed petrological and geochemical study of erupted plutonic xenoliths from the island of Sint Eustatius (Statia), in the northern Lesser Antilles volcanic arc. The plutonic xenoliths are predominantly gabbroic, but vary in texture, mineral assemblage and crystallisation sequence. We report major, trace and volatile (H2O and CO2) concentrations of xenolith-hosted melt inclusions (MIs) and interstitial glass. The MIs have a very large range in major element (49–78 wt% SiO2 and 0.1–6.1 wt% MgO) and trace element concentration (72–377 ppm Sr, 32–686 ppm Ba, 39–211 ppm Zr). Their chemistry varies systematically with host phase and sample type. Significantly, it shows that (1) plutonic xenoliths record a complete differentiation sequence from basalt to rhyolite (2) apatite, but not zircon, saturation was reached during crystallisation, (3) amphibole breakdown reactions play a role in the genesis of shallow gabbronorite assemblages, and (4) mixing between crystal cargos and multiple discrete bodies occurred. Residual melt volatile contents are high (≤ 9.1 wt% H2O and ≤ 1350 ppm CO2), returning volatile saturation pressures of 0–426 MPa. Multiple reaction geobarometry and experimental comparisons indicate that equilibration took place in the upper-middle crust (0–15 km). We infer that the Statia plutonic xenoliths represent portions of a large heterogeneous crystal mush within which a great diversity of melts was stored and mixed prior to eruption. Our data show that compositional variations in magmatic plumbing systems exceed those observed in volcanic products, a likely consequence of the blending that occurs prior to and during eruption.

The temperature of the convecting mantle and thickness of the lithosphere control many of Earth's processes. However, there is disagreement regarding the evolution of these quantities through time. We use a global data set of mantle xenoliths and xenocrysts to construct paleogeotherms at different eruption ages (16–1,311 Ma) and estimate the temperature and depth of the lithosphere‐asthenosphere boundary (LAB) as a function of mantle potential temperature (Tp). We find that the maximum pressure and temperature (PT) of xenoliths matches the modeled LAB conditions when a Tp of 1,315°C is used. At higher Tp (1,450–1,550℃$\\mathit{^{\\circ}\\mathrm{C}}$ ) we observe a gap between the maximum PT of xenoliths and the LAB conditions. Because this gap systematically increases with Tp, and the maximum PT of xenoliths has not changed over time, we suggest that there has actually been only minor (<50°C) changes in mantle Tp since the Meso‐Proterozoic. Plain Language Summary The temperature of the convecting mantle and the thickness of the lithosphere control many of Earth's processes. There is disagreement as to whether the temperature of the convecting mantle and thickness of the lithosphere were greater during Earth's early history. In this study we address this issue by using a global data set of mantle xenoliths and xenocrysts. Xenolith pressure and temperature (PT) estimates are used to construct paleogeotherms at different eruption ages and values for mantle potential temperature (Tp) to constrain the conditions of the lithosphere‐asthenosphere boundary (LAB). PT estimates are compared with the LAB conditions at different eruption ages and mantle Tp values. We find that the maximum PT of the samples are similar to the conditions of the LAB when a present day Tp is used (Tp = 1,315°C). Models that use a higher Tp (1,450–1,550°C) result in large gaps between the LAB and the maximum PT of xenoliths and xenocrysts. The size of the gaps increase systematically with higher values for Tp. This, along with the observation that the maximum PT of xenoliths and xenocrysts has not changed over time, suggests that Tp and lithospheric thickness were not significantly greater during the Proterozoic. Key Points Pressure and temperature estimates for 7,303 mantle xenoliths and xenocrysts were used to constrain the thermal structure of the lithosphere Paleogeotherms with higher mantle Tp have artificial gaps between the base of the lithosphere and deepest xenoliths and xenocrysts Modeled and observed pressures and temperatures of the base of the lithosphere are similar when a mantle Tp of 1,315℃$\\mathit{^{\\circ}\\mathrm{C}}$is used

Major and trace elements and water contents were analyzed in 16 peridotite xenoliths embedded by the Cenozoic basalts in Pingnan (southeastern Guangxi Province), to constrain the chemical composition and evolution of the lithospheric mantle located in the central part of the South China Block (SCB). The peridotites are mainly moderately refractory harzburgites and lherzolites (Mg#-Ol = 90.3–91.7) and minor fertile lherzolites (Mg#-Ol = 88.9–89.9). Clinopyroxenes in the peridotites show LREE-depleted pattern, and commonly exhibit negative anomalies in Nb and Ti, suggesting the peridotites probably represent residues after 1–10% of partial melting without significant mantle metasomatism. Water contents range from 146 to 237 ppm wt. H2O in clinopyroxene, and from 65 to 112 ppm wt. H2O, in orthopyroxene but are below detection limit (2 ppm wt. H2O) in olivine. Calculated bulk water contents, based on the mineral modes and partition coefficient, range from 14 to 83 ppm wt. H2O (average 59 ppm wt. H2O). There is a correlation between melting indices (such as Mg#-Ol, Ybn in clinopyroxene) and water contents in clinopyroxene and orthopyroxene, but no correlation is observed between the whole-rock water contents and the redox state (Fe3+/∑Fe ratios in spinel), suggesting that water contents in the peridotites are mainly controlled by the degree of partial melting rather than by oxygen fugacity. The lithospheric mantle beneath the interior of the SCB may not be compositionally stratified; fertile and moderately refractory mantle coexist at the similar depths. Geochemical data and water contents of the studied peridotites are similar to the proposed MORB source and indicate that the ancient refractory lithospheric mantle was irregularly eroded or reacted by the upwelling asthenosphere, and eventually replaced by juvenile fertile accreted mantle through the cooling of the asthenosphere.

The Lesser Antilles Volcanic Arc is remarkable for the abundance and variety of erupted plutonic xenoliths. These samples provide a window into the deeper crust and record a more protracted crystallisation history than is observed from lavas alone. We present a detailed petrological and in situ geochemical study of xenoliths from Martinique in order to establish their petrogenesis, pre-eruptive storage conditions and their contribution to construction of the sub-volcanic arc crust. The lavas from Martinique are controlled by crystal–liquid differentiation. Amphibole is rarely present in the erupted lavas, but it is a very common component in plutonic xenoliths, allowing us to directly test the involvement of amphibole in the petrogenesis of arc magmas. The plutonic xenoliths provide both textural and geochemical evidence of open system processes and crystal ‘cargos’. All xenoliths are plagioclase-bearing, with variable proportions of olivine, spinel, clinopyroxene, orthopyroxene and amphibole, commonly with interstitial melt. In Martinique, the sequence of crystallisation varies in sample type and differs from other islands of the Lesser Antilles arc. The compositional offset between plagioclase (~An 90 ) and olivine (~Fo 75 ), suggests crystallisation under high water contents and low pressures from an already fractionated liquid. Texturally, amphibole is either equant (crystallising early in the sequence) or interstitial (crystallising late). Interstitial amphibole is enriched in Ba and LREE compared with early crystallised amphibole and does not follow typical fractionation trends. Modelling of melt compositions indicates that a water-rich, plagioclase-undersaturated reactive melt or fluid percolated through a crystal mush, accompanied by the breakdown of clinopyroxene, and the crystallisation of amphibole. Geothermobarometry estimates and comparisons with experimental studies imply the majority of xenoliths formed in the mid-crust. Martinique cumulate xenoliths are inferred to represent crystal mushes within an open system, through which melt can both percolate and be generated.