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The continental lower crust constitutes a key zone for understanding the mantle–crust magmatic and mechanical transfers, but its study is hampered by the paucity of lower crust samples. Here, we characterise the petrological, geochemical and petrophysical processes structuring the lower crust of the North Patagonian Massif (NPM; Argentina) using a suite of representative mafic granulite and websterite xenoliths. These xenoliths were entrained by alkaline lavas from five volcanic centres that erupted between the Oligocene and Pleistocene. Electron microprobe and Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA-ICPMS) were used to obtain in situ geochemical data on the minerals, while microstructural data were obtained by Electron BackScatter Diffraction (EBSD). Both granulites and websterites display a granoblastic texture and sometimes a weak inherited magmatic layering. Mafic granulite xenoliths show a plagioclase + clinopyroxene ± orthopyroxene assemblage commonly associated with spinel or titanomagnetite. Websterite xenoliths show an association of clinopyroxene + orthopyroxene + spinel, along with accessory plagioclase. Mafic granulites and websterites have SiO2 contents ranging from 44 to 53 wt %, while their Mg# varies from 53 to 79. Clinopyroxenes are characterised by weak convex upward chondrite-normalised Rare Earth Elements - REE patterns (Light-REE  Heavy-REE) which are similar to clinopyroxene phenocrysts and megacrysts from intra-plate basalts. Calculated liquid in equilibrium with clinopyroxene have similar REE patterns to those found in Cenozoic basalts from the NPM, suggesting that the xenolith suite represents evidence for underplating processes, possibly related to one of the magmatic events that have occurred in the NPM since the Permo-Trias. Mafic granulites and websterites show a weak mineral shape preferred orientation and an associated weak Crystal Preferred Orientation (CPO) related to the magmatic layering. Recorded plastic deformation is associated with the activation of both (100)[001] and (001)[100] slip systems in clinopyroxene, (100)[001] in orthopyroxene and (010)[001] in plagioclase. However, the activation of slip systems is generally not correlated with CPO in granulites, suggesting that the lower crust underwent subsolidus equilibration and weak plastic deformation in an inactive tectonic context, thereby preserving an inherited magmatic layering. Two-pyroxene (Fe–Mg) thermometer and pseudosection calculations define P–T conditions of the main paragenesis at 760°C to 1120°C and 7.2 to 10.3 kbar, which allows to define the Cenozoic geotherm of the NPM crust at 30°C/km and to reconsider the petrologic Moho depth at ca. 40 km. [ABSTRACT FROM AUTHOR]

Eastern Australia is covered by extensive, thick regolith, which obfuscates much of its basement geology, making geological sampling difficult. The Bokhara River diatremes erupted through the Thomson Orogen in eastern Australia and are covered by ∼300 m of Cretaceous regolith cover. Hitherto, these diatremes have only been characterised by magnetic anomaly surveys and private exploration drilling. The melts were assumed to be leucititic, owing to their proximity to Miocene leucitite centres to the south, while their position along the Cosgrove hotspot track led to the assumption that the diatremes erupted at ca 20 Ma. However, whole-rock chemistry shows that the diatremes are basanites and, according to 40Ar/39Ar dating, erupted during the mid-Jurassic (ca 180 Ma; estimated by reproducible slightly discordant ages), substantially older than the assumed age and coincident with widespread Mesozoic flood basalts across Gondwana. The basanites entrained abundant mantle material during accent, which contaminated the basanite with ∼12.8 wt% xenocrystic material. The xenoliths are all fertile spinel lherzolites with Mg# 87–89, CaO 2.64–5.90 wt% and Al2O3 2.87–3.83 wt% that have been cryptically metasomatised by a mafic silicate melt, which resulted in chromatographic rare-earth element patterns. This demonstrates that mantle metasomatism occurred during the Mesozoic in this part of eastern Australia, revealing another expression of intraplate volcanism prior to Australia rifting away from Gondwana. The identification and characterisation of this Jurassic volcanism hosting evidence for mantle metasomatism suggest that intraplate volcanism and mantle metasomatism are synonymous within orogenic eastern Australia, and not restricted to the Cenozoic. Diatremes at Bokhara River are basanitic and erupted during the mid-Jurassic; they are not leucitites of the Cosgrove Track. Entrained ultramafic xenoliths are fertile mantle spinel lherzolites, which experienced cryptic metasomatism by a silicate melt. This is evidence for the oldest mantle metasomatism event in the eastern Australian orogenic lithospheric mantle. [ABSTRACT FROM AUTHOR]

An electron microprobe study was carried out on olivine, clinopyroxene, and oxy-spinel occurring in basalts and dunite xenoliths from the archipelagos of the Azores, the Canary Islands, and Cape Verde. By comparing our results with previously published data from the volcanic islands of Macaronesia, we confirmed the validity of the compositions of olivine, clinopyroxene, and oxy-spinel as geochemical tracers. The origin of olivine, i.e., crystallized in the lithospheric mantle or in volcanic rocks, was successfully discriminated. Olivine from Lanzarote dunite xenoliths, which represent fragments of the mantle transported to the surface by host magmas, exhibited higher Fo% values (Fo91.02 to Fo91.94) and a different distribution of minor elements Ca, Ni, and Mn (CaO up to 0.42 wt%, NiO 0.07–0.41 wt%, MnO 0.06–0.3 wt%) when compared with olivine occurring as phenocrysts in basaltic lavas from the Macaronesian islands. The highly variable forsterite contents (Fo75.1 to Fo94.4) in olivine from gabbro and peridotite xenoliths found across the islands of Macaronesia were attributed to fractional crystallization that started in a deep magma reservoir, suggesting that these xenoliths represent cumulate rocks and not mantle fragments. Alternatively, these xenoliths may have been affected by the interaction with metasomatic fluids. The composition of clinopyroxene phenocrysts was used to decipher formation conditions under extensional tectonics. Their composition suggests that the host lavas have an alkaline to calc-alkaline signature. Furthermore, clinopyroxene euhedral shapes and compositions suggest an origin by fractional crystallization in a closed magmatic system. The composition alone of oxy-spinel from Macaronesian basalts and xenoliths was not sufficient to draw conclusions about the geodynamic environment where they were formed. Nevertheless, the relationship between oxy-spinel and olivine crystallized in equilibrium was successfully used as oxybarometers and geothermometers. The oxy-spinel–olivine pairs show evidence that the basaltic lavas were crystallized from melts with higher oxygen fugacity and different cooling histories than those of the mantle xenoliths, as the latter crystallized and re-equilibrated much slower than the basalts. [ABSTRACT FROM AUTHOR]

Based on published geophysical and petrological data, lower crustal fluid reservoirs have been proposed below the Klyuchevskoy Volcano, expressed as a low Vp/Vs anomaly. A high Vp/Vs anomaly under the proposed fluid reservoir is interpreted as a zone of magma accumulation. The localization of fluids in these reservoirs in the ductile lower crust can vary from isolated inclusions to filling of microfractures over a time scale of several months. Using a simplified poroelastic model, it is shown that the transition in the topology of pore space filled with fluid or melt can provide the observed changes in Vp/Vs in the anomalies of high and low values at a melt content of several vol % and fluid content less than 1 vol %, respectively. In zones of active volcanism, such as the Klyuchevskaya group of volcanoes (KGV), fluid reservoirs are localized in ultramafic cumulates formed during the early high-temperature stage of magma fractionation. Ultramafic xenoliths in the products of eruptions of the KGV and Avachinsky volcanoes, often interpreted as mantle rocks, formed at pressures of about 5 kbar or depths of about 18–20 km in accordance with two-pyroxene geo-thermobarometry and the content of volatiles in melt inclusions in olivine and spinel. When crossing by ascending magmas, the fluid-containing reservoir experiences mechanical failure and injects a certain amount of fluid into the magma, which then captures pieces of crushed magmatic cumulates. The composition of melt inclusions in olivine can reveal records of the magma-fluid interaction. [ABSTRACT FROM AUTHOR]

Intra-arc diversity in volcanic activity and composition is ubiquitous, but its underlying causes remain largely unresolved in many settings. In this work, we examine such variability in the Grenadines archipelago, southern Lesser Antilles arc. Here, juxtaposed volcanic centres exhibit eruptive longevities and chemistries distinct from northern counterparts in the same arc. Our goal is to explain this deviation by investigating variations in magmatic processes using petrological data from erupted crustal xenoliths and lavas, and interpreting these findings within the context of the archipelago’s tectonic history and geophysical structure. Textural analyses of xenoliths reveal crystallization over a wide range of pressure–temperature–melt composition conditions in the crust. Mineral phases display discrete compositional trends pointing towards significant inter-island variability in underlying plumbing systems. The geochemical variety of erupted magmas is reminiscent of the entire arc. We speculate that the Grenadines represents the early onset of subduction forming the modern-day Lesser Antilles arc. Extrusive volcanism initiated as submarine activity. Subsequent uplift eroded the original topography of these volcanic centres following the eventual cessation of volcanism in the Neogene. The positioning of the Grenadines on an elevated platform provides rare modern insight into early arc crust formation not commonly preserved in established active arcs.

Deep crustal cumulates in arcs offer a window into the chemistry and crystallization conditions (P–T–H2O– f O2) of primitive basalts in the upper mantle and lower crust and can be studied in ancient exhumed terranes or in xenoliths erupted in young arc lavas. Here, we expand on previous studies and thoroughly characterize the extensive xenolith suites erupted from the Mt. Moffett and Mt. Adagdak volcanic centers (Adak Island, Central Aleutians), which range from primitive ultramafic cumulates to more evolved amphibole gabbros and hornblendites. We present detailed petrography as well as in situ trace and major element mineral chemistry. We use these data to calculate pressure, temperature, and f O2 estimates for the xenoliths, and compare these findings to experimental results to understand the crystallization sequence and P–T–H2O– f O2 under which the cumulates formed. The Moffett crystallization sequence is defined by early amphibole fractionation and an abrupt shift in oxide compositions from chromite to magnetite, while the Adagdak suite is characterized by simultaneous saturation of amphibole+plagioclase and oxide compositions that become increasingly aluminous before magnetite saturation. Olivine–spinel oxybarometry of the Adagdak xenoliths indicates that they are oxidized relative to mid-ocean ridge basalt (MORB:FMQ +0.1 to +2.1). Highly fractionated REE and elevated Sr/Y ratios are observed in clinopyroxene from the most primitive cumulates, consistent with a contribution from a basaltic eclogite melt. This basaltic eclogite melt is hypothesized to come from partial melting of the slab or through melting of basalt introduced into the subarc mantle through forearc subduction erosion. These signatures are greatly diminished in the more evolved lithologies, which can be explained through fractionation of plagioclase and amphibole. Our findings support the presence of a complex magmatic plumbing system beneath Adak, with Mt. Moffett and Mt. Adagdak volcanic centers tapping compositionally distinct sources. More broadly, our results are consistent with studies suggesting that low-degree basaltic eclogite melts through slab melting or forearc subduction erosion contribute to arc magmas in the Aleutians, although the associated geochemical signatures are easily obscured by differentiation in the crust. [ABSTRACT FROM AUTHOR]

Rapakivi of the Korsun-Novomyrhorod pluton (KNP), being the latest Precambrian granites of the central part of the Ukrainian shield, contain older acidic and intermediate rocks as xenoliths, the geochemical features of which reflect the Early Proterozoic geodynamic situation. Rapakivi granites, their differentiates and three types of xenoliths in them, represented by plagiogneiss, pink granite and charnockitoid (mangerite), were selected in the northwestern part of the KNP for geochemical studies. Analysis findings were compared with the geochemical characteristics of similar rocks of igneous and metamorphic Precambrian complexes that surround the KNP and could serve as a source of xenoliths. Based on distribution patterns of rare earth elements, lithophile elements content and their ratios relations in the rocks, taking into account their petrographic composition, it was established that plagiogneiss belongs to the upper part of the Inhul-Inhulets series, pink granite belongs to the Kirovohrad complex, and charnokitoid belongs to the intermediate rocks of the Tashlyk complex. In the latter, acid charnockitoids (enderbites) are probably younger than intermediate ones and were formed in another geodynamic setting. Pink granite, like the granites of the Kirovohrad complex, shows collisional geochemical features, in contrast to the rapakivi granites containing these xenoliths, which have predominantly within-plate geochemical characteristics. Thus, in the form of xenoliths in rapakivi those rocks were preserved that had been generated before and during the collision that occurred about 2 billion years ago and constructed a crust thickening at the site of the Inhul megablock, under subsequent extension of which about 1.75 billion years ago the KNP was intruded. Acid charnockitoids of the Tashlyk complex and rocks of the Novoukrainian complex, which are absent in xenoliths, probably were formed after the collision. [ABSTRACT FROM AUTHOR]

Mafic and ultramafic xenoliths were erupted with hawaiite lavas from a post-shield, Laupahoehoe cinder cone on Mauna Kea volcano, Hawaii. Major element compositions of clinopyroxene, olivine, and plagioclase in these xenoliths do not clearly differentiate between parental Hawaiian tholeiitic, transitional, or alkalic magmas. To clarify the parental magma compositions, incompatible trace element concentrations in clinopyroxene were analyzed by laser ablation inductively coupled plasma mass spectrometry. The compositions of equilibrium liquids were calculated using published clinopyroxene/basalt partition coefficients. The REE patterns and concentrations of Sr, Ba, Ti, and Zr in most calculated liquids are similar to transitional and alkalic lavas of post-shield Hamakua Volcanics and unlike the younger host lava, the more alkalic Laupahoehoe Volcanics. In contrast, liquids in equilibrium with clinopyroxene in a wehrlite and an olivine gabbro have trace element concentrations similar to tholeiitic shield basalts. Thus, these xenoliths represent cumulates crystallized at depth from the shield stage tholeiitic and post-shield Hamakua magmas and were then entrained in the younger alkalic Laupahoehoe lavas. The composition of the xenoliths and the calculated liquids show that the Hamakua Volcanics and Laupahoehoe Volcanics are not related to one another by deep fractionation, but instead are derived from compositionally distinct sources and different degrees of melting. [ABSTRACT FROM AUTHOR]

An electron microprobe study was carried out on olivine, clinopyroxene, and oxy-spinel occurring in basalts and dunite xenoliths from the archipelagos of the Azores, the Canary Islands, and Cape Verde. By comparing our results with previously published data from the volcanic islands of Macaronesia, we confirmed the validity of the compositions of olivine, clinopyroxene, and oxy-spinel as geochemical tracers. The origin of olivine, i.e., crystallized in the lithospheric mantle or in volcanic rocks, was successfully discriminated. Olivine from Lanzarote dunite xenoliths, which represent fragments of the mantle transported to the surface by host magmas, exhibited higher Fo% values (Fo91.02 to Fo91.94) and a different distribution of minor elements Ca, Ni, and Mn (CaO up to 0.42 wt%, NiO 0.07–0.41 wt%, MnO 0.06–0.3 wt%) when compared with olivine occurring as phenocrysts in basaltic lavas from the Macaronesian islands. The highly variable forsterite contents (Fo75.1 to Fo94.4) in olivine from gabbro and peridotite xenoliths found across the islands of Macaronesia were attributed to fractional crystallization that started in a deep magma reservoir, suggesting that these xenoliths represent cumulate rocks and not mantle fragments. Alternatively, these xenoliths may have been affected by the interaction with metasomatic fluids. The composition of clinopyroxene phenocrysts was used to decipher formation conditions under extensional tectonics. Their composition suggests that the host lavas have an alkaline to calc-alkaline signature. Furthermore, clinopyroxene euhedral shapes and compositions suggest an origin by fractional crystallization in a closed magmatic system. The composition alone of oxy-spinel from Macaronesian basalts and xenoliths was not sufficient to draw conclusions about the geodynamic environment where they were formed. Nevertheless, the relationship between oxy-spinel and olivine crystallized in equilibrium was successfully used as oxybarometers and geothermometers. The oxy-spinel–olivine pairs show evidence that the basaltic lavas were crystallized from melts with higher oxygen fugacity and different cooling histories than those of the mantle xenoliths, as the latter crystallized and re-equilibrated much slower than the basalts.

This paper reports a study of xenoliths sampled in marine sedimentary carbonate rocks in fumaroles of the crater zone in the Second Cone of the GTFE (Great Tolbachik Fissure Eruption) North Vent. These rocks have been subjected to the action of the GTFE exhalative pneumatolytic fluids, resulting in carbonates giving rise to numerous compounds with mixed carbonate-sulfate-chloride anion radicals, which are treated as two interclass and nine intertype crystal-chemical hybrids. The patterns identified in the altered micro xenoliths, which consist in inhomogeneities of mineral paragenesis, are treated here as resulting from successive epigenetic transformation of the original carbonates to, at first, sulfate carbonates, then to carbonate sulfates, and subsequently to chloride-carbonate-sulfates and chlorides. Judging by an absence of signs of phase heterogeneity, the crystal-chemical hybrids studied here are homogeneous solid-phase mixtures of carbonates, sulfates, and chlorides in varying proportions. The carbonates in the micro xenoliths have isotopic compositions of carbon (δ13СPDB = –5.34 ± 0.62‰) and oxygen (δ18OSMOW = 24.09 ± 1.05‰) that are consistent with carbonates to be found in marine sedimentary limestones that were redeposited during volcanogenic transportation. The sulfate sulfur has the isotopic composition (δ34S = 1.5–2‰) varying within the range of variation established for sulfates of volcanogenic origin. In the carbonaceous particles associated with micro xenoliths, the isotopic composition of carbon (δ13CPDB = –27.37 ± 2.97‰) and of nitrogen (δ15NAir = 6.74 ± 2.48‰) tend toward the mode in the distribution of these values in the ejecta of present-day continental volcanism. The crystal-chemical hybrids identified here furnish a typomorphic criterion for the exhalation fumarole facies and are treated as a previously unknown phenomenon of present-day volcanism. [ABSTRACT FROM AUTHOR]

The Dotsero maar volcanic crater formed through phreatomagmatic (interactions of lava with wet, unconsolidated sediment) and magmatic processes around 4,150 ± 300 years before present (B.P.) which erupted through sections of Pennsylvanian age sedimentary deposits. The field observations of the eruptive material showed various xenolith structures, or foreign rock fragments within the cooled magmatic material, which can be categorized into four specific groups based on the mineral grain size of the sediments and subtle color variation. The fieldwork focused on three sites located downslope from the southern crater rim of the Dotsero maar, accounting for all xenoliths = 1 cm in size in selected agglutinate clasts and gathering samples of the localized surface sediments and xenolith structures. The surficial sediment samples and two groups of xenolith structures were examined through X-ray powder diffraction laboratory analyses. The results of this analysis demonstrate that field classifications of xenolith structures continue to be the most accurate form of identification and present grounds for further analyses through scanning electron microscopy (SEM) and thin section observations for differentiation and determination of thermal alterations created by the lava and sediment interactions. [ABSTRACT FROM AUTHOR]

The aegirine end-member (NaFe3+Si2O6) in clinopyroxenes resulted from incorporation of Fe3+ into the mineral structure. Its presence affects the accuracy of reconstruction of the P-T conditions in the high-grade metamorphic rocks and allows the evaluation of the redox conditions of their formation. The content of this end-member in clinopyroxenes is usually estimated using crystal chemical recalculations of microprobe analyses. However, in some publications on eclogites, the comparison of microprobe-based recalculations with Mössbauer data revealed significant difference between the measured and calculated Fe3+/ΣFe ratios, which can significantly affect the results of geothermometry. This paper presents the results of the Mössbauer spectroscopy measurements of clinopyroxene fractions separated from three samples of garnet–clinopyroxene granulites from the Udachnaya kimberlite pipe. The ratios Fe3+/ΣFe = 0.22–0.26 measured in the clinopyroxenes correspond to 6–10 mol % aegirine. These estimates are in good agreement with the values obtained for the same clinopyroxenes by the recalculation of microprobe analyses using the charge balance method. Following this conclusion, we believe that crystal chemical recalculations of microprobe analyzes of clinopyroxenes from non-eclogitic rocks make it possible to correctly estimate the Fe3+ content in them. Similar recalculation of microprobe analyses of clinopyroxenes in crustal xenoliths from other localities, as well as from ferrobasalts of the continental flood basalts provinces, ferrodolerite dikes, and gabbroid xenoliths (similar in bulk chemical composition to many lower–middle crustal xenoliths) revealed significant amounts of previously unaccounted aegirine (up to 13 and 4–9 mol %, respectively), which holds the potential for deciphering redox conditions in many rocks. [ABSTRACT FROM AUTHOR]

Mantle xenoliths from the Cenozoic Mt. Briançon cinder cone in the Massif Central (France) were studied in order to test previously proposed hypotheses on the structure and chemical evolution of the lithospheric mantle in the region. Mineral major and trace element compositions coupled with characterization of crystal preferred orientation (CPO) of grains allow to recognize the compositional evolution and deformation history of peridotites and show which minerals constitute the primary vs. later fabric of the rock. The Mt. Briançon xenolith suite (n = 21) consists of spinel lherzolites, except one spinel harzburgite and one olivine clinopyroxenite. Lherzolites (olivine Fo 88.5–90.4) form three major groups. Those of Group I (51–65 vol% olivine and 10–16 vol% clinopyroxene) contain pyroxenes of variable Al content (orthopyroxene 0.142–0.217 apfu; clinopyroxene 0.244–0.316 apfu) and aluminous spinel (Cr# 0.09–0.12). Clinopyroxene is LREE-depleted and exhibits CPO indicating that it is a newly crystallized/recrystallized phase added to the rock framework consisting of olivine and orthopyroxene. Group II lherzolites (67–83 vol% olivine, 5–9 vol% clinopyroxene) contain Al-poor clinopyroxene (0.208–0.245 apfu) and spinel of Cr# 0.16–0.28. Clinopyroxene is LREE-enriched and its CPO is coherent with the fabric defined by olivine and orthopyroxene, indicating a joint deformation history. The lherzolites of Group Ia (46–66 vol% olivine, 11–26 vol% clinopyroxene) are transitional between those of Group I and II, and the majority contain pyroxenes with Al contents similar to those of Group I, and spinel of Cr# 0.11–0.15. Clinopyroxene REE patterns are spoon-shaped. Three lherzolites cannot be classified into the mentioned groups because they exhibit REE contents in clinopyroxene that change gradually from LREE-depleted to LREE-enriched on the scale of a petrographic section. In one of them, this change is observed at the grain scale, with spoon-shaped REE patterns in the cores and LREE-enriched ones at the margins. The harzburgite is rich in Mg (olivine Fo 91.1, pyroxenes Mg# 0.92) and impoverished in Al (spinel Cr# 0.43). Clinopyroxene is enriched in LREE, the contents of which are significantly lower than those of lherzolitic clinopyroxene. It forms nearly undeformed grains in an olivine-orthopyroxene framework. The lherzolites of Group I were formed by reactive percolation of MORB-like melts under suprasolidus conditions, which enabled replacement and/or addition of clinopyroxene to the solid olivine-orthopyroxene framework. Zonal REE distribution in clinopyroxene from one of the lherzolites documents alkaline melt percolation overprinting the Group I, which ultimately formed the Group II lherzolites. The lherzolites of Group Ia document transition between those of Group I and Group II. Hypothetical melt in equilibrium with clinopyroxene in olivine clinopyroxenite has a composition similar to that of the host lava. This rock likely represents a cumulate from the same melt that infiltrated the lithosphere during the Cenozoic volcanic activity of the Massif Central. • Mt. Briançon mantle was refertilized by reactive percolation of MORB-like melts. • Refertilization produced REE-depleted clinopyroxene. • Clinopyroxene fabric mostly differs from that of olivine-orthopyroxene framework. • Separate thermal event related to Cenozoic volcanism affected Mt. Briançon mantle. [ABSTRACT FROM AUTHOR]

Mantle intrusive rocks (lamproites and basalts) were collected from different units of the Uzbek Tian Shan, Central Asia to investigate xenoliths and xenocrysts aiming at tracking the shallow or deep nature of the mantle source. Whole-rock geochemistry and petrography revealed that these rocks are mostly of basaltic composition and may contain various exotic materials. These rocks originate from composite mantle magmas; they often result from the mixing between an evolved primary melt and more primitive melts, as indicated by the composition and textures of clinopyroxenes. Olivine, orthopyroxene and spinel mineral geochemistry from xenocrysts and lherzolitic xenoliths is consistent with a shallow mantle source (spinel lherzolite). Trace element content of clinopyroxenes shows LREE depletion supporting the removal of the magma from an undepleted sub-continental lithospheric mantle. The latter one is heterogenous, though and some parts recorded evidence of intense metasomatism. The upwelling asthenospheric magmas, emplacing in trans-lithospheric structures, were probably directly transferred into the crust or just below the crust, without significantly changing the geochemical composition of the sub-continental lithospheric mantle. [ABSTRACT FROM AUTHOR]

Coupled U‐Pb and trace‐element analyses of accessory phases in crustal xenoliths from the Late Devonian Udachnaya kimberlite (Siberian craton, Russia) are used to constrain Moho temperature and crustal heat production at the time of kimberlite eruption. Rutile and apatite in lower‐crustal garnet granulites record U‐Pb dates that extend from 1.8 Ga to 360 Ma (timing of kimberlite eruption). This contrasts with upper‐crustal tonalites and amphibolites that contain solely Paleoproterozoic apatite. Depth profiling of rutile from the lower‐crustal xenoliths show that U‐Pb dates increase gradually from rim to core over μm‐scale distances, with slower‐diffusing elements (e.g., Al) increasing in concentration across similar length‐scales. The U‐Pb and trace element gradients in rutile are incompatible with partial Pb loss during slow cooling, but are consistent with neocrystallization and re‐heating of the lower crust for <1 Myr prior to eruption. Because Paleoproterozoic rutile and apatite dates are preserved, we infer that long‐term ambient lower‐crustal temperatures before this thermal perturbation were cooler than the Pb closure temperature of rutile and probably apatite (<400°C). The lower‐crustal temperature bounds from these data are consistent with pressure‐temperature arrays of Udachnaya peridotite xenoliths that suggest relatively cool geothermal gradients, signifying that the mantle xenoliths accurately capture the thermal state of the lithosphere prior to eruption. Combined, the xenolith data imply low crustal heat production for the Siberian craton (∼0.3 μW/m3). Nevertheless, such values produce surface heat flow values of 20–40 mW/m2, higher than measured around Udachnaya (average 19 mW/m2), suggesting that the surface heat flow measurements are inaccurate. Plain Language Summary: The decay of radioactive elements, like uranium (U), thorium (Th), and potassium (K), produces heat and the dissipation of this heat from Earth's interior drives flow at depth and the movement of tectonic plates at the surface over geologic time. The crust—the planet's rocky outer layer—is hypothesized to contain a significant portion of Earth's heat‐producing elements, yet this budget is uncertain. The total amount of heat leaving the surface can be measured, but how much of this flux reflects heat production within the crust versus heat derived from the underlying mantle is also uncertain. Kimberlite pipes carry fragments of Earth's interior to the surface ("xenoliths") in Siberia. Some of the xenoliths are from near the base of the crust, and temperature‐sensitive chronometers in these rocks record ancient dates (>1 billion years old). Such ancient dates are significant because they can only be preserved if temperatures in the deep crust were cool (<400°C). Models of heat transfer through the Siberian crust using our results and mantle xenolith pressure‐temperature data indicate relatively low heat production in the crust. Key Points: The U‐Pb system in deep crustal xenoliths is sensitive to transient heating of the lower crust and not just slow coolingThe central Siberian craton lower crust resided below the Pb closure temperature of rutile and apatite prior to kimberlite eruption 360 MaExtremely low crustal heat production for the craton (∼0.3 μW/m3) is consistent with peridotite xenolith P‐T data [ABSTRACT FROM AUTHOR]

Abstract Coupled U‐Pb and trace‐element analyses of accessory phases in crustal xenoliths from the Late Devonian Udachnaya kimberlite (Siberian craton, Russia) are used to constrain Moho temperature and crustal heat production at the time of kimberlite eruption. Rutile and apatite in lower‐crustal garnet granulites record U‐Pb dates that extend from 1.8 Ga to 360 Ma (timing of kimberlite eruption). This contrasts with upper‐crustal tonalites and amphibolites that contain solely Paleoproterozoic apatite. Depth profiling of rutile from the lower‐crustal xenoliths show that U‐Pb dates increase gradually from rim to core over μm‐scale distances, with slower‐diffusing elements (e.g., Al) increasing in concentration across similar length‐scales. The U‐Pb and trace element gradients in rutile are incompatible with partial Pb loss during slow cooling, but are consistent with neocrystallization and re‐heating of the lower crust for

Mineralogical and geochemical study was performed on peridotite xenolith hosted by amygdaline basalts of Lanzarote Island (Canary archipelago). Their modal composition corresponds to harzburgite. Remarkably, the petrographic composition of the rocks is marked by presence of plagioclase. The isochron shows the age of 267±35 Ma, which might be the initial stage of opening a fragment of the Atlantic Ocean. The peridotites were apparently derived in the magmatic chamber formed by "underplating" triggered by interaction of mantle substrate with the lower crust material.

Noble gas isotope systematics of ocean island basalts (OIB) provide evidence for relatively undegassed and primitive mantle sources. These OIB sources partly derive from the deep mantle by virtue of their distinctiveness from mid-ocean ridge basalts (MORB), which dominantly sample upper mantle. New helium, neon and argon isotope data are presented for Canary Islands lavas, carbonatites and cumulate and mantle xenoliths confirming 3He/4He ratios that are the same or lower than MORB, but that are heterogeneous (∼3–9.5R A) within and between islands in the archipelago. Neon and Ar isotope systematics for lavas are mostly within the range of air compositions. Harzburgite xenoliths from Lanzarote, which are interpreted to represent ancient refractory mantle residues, have distinct He-Ne-Ar isotope systematics from lavas or cumulate xenoliths. The harzburgites are characterized by forsteritic olivine (>Fo 91) with uniformly low-3He/4He (6.6 ± 0.2R A), high 40Ar/36Ar (630–4900), and have Ne isotope compositions that range between air values or that are similar or more nucleogenic than depleted MORB mantle (DMM). Similar refractory mantle peridotites have been discovered as xenoliths at other OIB localities and are likely to be distinct from continental lithospheric mantle. Refractory mantle (RM), which by virtue of its high melting temperature is difficult to partially melt, may be a significant component in the convecting mantle and has potential to impart a cryptic noble gas signature to partial melts in intraplate, divergent and convergent margin settings. In this sense, RM may represent a 'sixth mantle component' after DMM, high-µ (high 238U/204Pb; HIMU), enriched mantle endmembers (EMI, EMII) and the 'focus zone' (FOZO). An RM-type component may be presented in the older eastern Canary Islands and possibly in recent rejuvenated volcanism from Teide (Tenerife). Canary Island intraplate volcanism samples multiple mantle components, confirming that the most gas-rich mantle sources involved in magmatism dominate OIB noble gas compositions. [ABSTRACT FROM AUTHOR]

The Early Cretaceous volcanic Jaguarão Formation formed duringthe early stages of Gondwana breakup. These felsic rocks characteristically enclose numerous crustal xenoliths. Fieldwork, petrographic studies, and laser ablation multicollector ion coupled plasma mass spectrometer (LA-ICP-MS) U-Pb analyses were carried out to evaluate the xenoliths of the Jaguarão Formation. A base-to-top distribution of xenoliths in the different flows of the volcanic pile indicates that earlier flows incorporated centimeter-sized fragments of local underlying gneiss, granite, and quartz veins, with limited assimilation. In the middle part of the volcanic pile, the size of the xenoliths is smaller and assimilation textures are more frequent. At this same level, reequilibrated orthopyroxene and plagioclase antecrysts were recognized, suggesting fractionation coeval with assimilation in a lower magmatic chamber prior to extrusion. At the top, no visible xenoliths occur. U-Pb ages of the zircon xenocrysts served as a proxy for assessing the crust underlying the volcanic rocks. Most of the ages obtained are similar to the ages of the Eastern Dom Feliciano Belt cropping out nearby, ranging from 680 to 620 Ma, with a subordinate group ranging from 580 to 570 Ma, suggesting a shallow crust contribution. The presence of few Paleoproterozoic and Mesoproterozoic (1.0-1.2 Ma) zircons, however, suggeststhat the underlying terrain may be more complex.. [ABSTRACT FROM AUTHOR]

Exploration for rare earth element (REE)-deposits hosted in carbonatites and associated rocks is challenging because of the heterogeneous distribution of REE and the variable and often complex mineralogy of such REE mineralisation. The Keishöhe in southwestern Namibia is a subvolcanic intrusion consisting of calcite-carbonatite, dolomite-carbonatite, ankerite-carbonatite dykes, ring dykes and diatremes, and is regarded to be a part of the Kudu lineament carbonatite complexes. Importantly, it exemplifies many of the problems associated with REE exploration -- particularly regarding its heterogeneous REE distribution. It is therefore an excellent site in which to explore the processes related to REE precipitation in detail. Petrography, whole rock geochemistry and microXRF imaging of xenolith-bearing and xenolith-free carbonatites provide insight into the role of silicate xenoliths in the development of a hydrothermal or supergene REE mineralisation in a subvolcanic environment. Xenolith-rich carbonatites are almost barren, whereas xenolith-free carbonatites show REE-contents of up to 10 wt. % total REE (TREE). Therefore, the probability for REE-enrichment in the various Keishöhe carbonatites is significantly higher in the absence of silicate rock xenoliths. The REE mineralisation is dominated by REE-F-carbonates, while monazite is a minor constituent. In contrast to other complexes, where Si assimilation caused REE depletion in the melt during the magmatic stage, this study demonstrates post-magmatic hydrothermal or supergene REE mobilisation and enrichment. REE remobilisation and enrichment is caused by hydrothermal or supergene alteration of silicate xenoliths (Si release) by aqueous fluid(s). In combination with previous studies this study clearly shows that silicate xenoliths have 1) a significant influence on REE-mineralisation in general and 2) may act either positively (precipitation/incorporation) or negatively (mobilisation) on the mineralisation of discrete REE phases depending upon the related syn-magmatic or post-magmatic processes. [ABSTRACT FROM AUTHOR]

The orientation, petrography and geochemistry of numerous basaltic dykes between Kiama and Gerroa were investigated in this study. Geochemical analysis shows that the dykes range from sub-alkaline to alkaline in composition. The dykes fit into three distinct geochemical groups, with most being transitional from alkali basalts to foidites, followed by alkali basalts and lastly andesites/basaltic andesites. We suggest that three intrusive episodes occurred based on petrographic analysis and geochemical classification. The dykes contain phenocrysts of olivine, plagioclase and clinopyroxene in a fine-grained groundmass. Multiple microtextures were found, including porphyritic, glomeroporphyritic, amygdaloidal, aphanitic and intergranular. Most dykes strike northwest–southeast, two strike east–west, and four strike northeast–southwest. One dyke to the south of Boat Harbour Reserve contains numerous xenoliths, including gabbroic, granitic and charnockitic types. The gabbroic xenoliths were likely sourced from the mid-crust, whereas the granitic xenoliths were likely derived from the upper crust. Joints in the study area strike northwest–southeast, north–south, east–west and northeast–southwest. The dominant joint set strikes northwest–southeast, and secondary joint sets strike northeast–southwest. The least pervasive joint sets strike east–west and approximately north–south. The dykes intruded along long-lived inherent structural weaknesses in the host rocks, as their orientations match the orientations of dominant joint sets. Geochemically, the dykes are distinct in having an abundance of foidites, which are rare within the Sydney Basin. They are clearly different from the nearby upper Permian K-rich Gerringong Volcanics, but many of the alkali basaltic dykes are like those of the Sydney Basin, and the foidites are similar in composition to the lower Eocene Kulnura basalt on the NSW Central Coast. The orientation, geographic distribution and geochemistry of the dykes strongly suggest that the basaltic magmas have been injected through the same upper-crustal weaknesses over a protracted period. Dykes occurring between Kiama and Gerroa fall under three separate geochemical groups: alkali basalts to foidites, alkali basalts and andesites/basaltic andesites. Three intrusive episodes occurred based on petrographic analysis and geochemical classification. The dykes intruded along long-lived inherent structural weaknesses in the host rocks. [ABSTRACT FROM AUTHOR]

The dyke of the Papôa volcanic breccia cross-cutting the Lower Jurassic sequence of the Lusitanian Basin (West Iberia) contains granitic xenoliths. In this study, for the first time, U-Th-Pb zircon analysis of two xenoliths yielded 298±4Ma for biotite granite and of 292±2Ma for two-mica granite, indicating that the pre-Mesozoic basement of the Lusitanian Basin includes Permian intrusions. These ages are close within the margin of error of the age of the Late Carboniferous granites of the Berlengas isle that with the Late Devonian high-grade metamorphic rocks of the Farilhões isles, located northwest of the study area, which form the pre-Mesozoic basement of the Lusitanian Basin. These new geochronological findings enable it to be established that Permo-Carboniferous magmatism lasted at least 13Ma, in this region of the Appalachian-Variscan belt. Furthermore, a comparison with available data from Paleozoic tectonic units of the Appalachian-Variscan belt located both in the Iberian Massif and outside it enables the suggestion to be made that the Lusitanian Basin (Peniche) most probably rests on the South Portuguese Zone, which may also be correlated with the Rhenohercynian Zone present in southwest England, and the Meguma terrane of Nova Scotia.

The article considers xenocrysts and megacrysts hosted in the rocks of Early Cretaceous olivine-basalt-basanite-nephelinite association that outcropped in the erosion crater of Makhtesh Ramon (Natural Reserve of Mishmar ha-Nagev, Israel). This magmatic rock association contains a wide spectrum of xenoliths trapped at different crustal levels. These are upper mantle, lower and upper crustal xenoliths. Mantle xenoliths are represented by peridotites, olivine clinopyroxenites, clinopyroxenites, olivine websterites, websterites and their amphibole-bearing analogues. Lower crustal xenoliths are mafic granulites, such as metagabbros and plagioclasites, whereas upper crustal xenoliths are the fragments of Neoproterozoic tuffs. Xenocrysts and megacrysts are the fragments of xenoliths that chipped from them on their way to the surface. Alterations in xenoliths, xenocrysts and megacrysts caused by the host melt constitute a common petrographic feature. Xenocrysts and megacrysts are mainly represented by minerals that are compatible with the magmatic rock association. These are olivine, clinopyroxene, amphibole, nepheline, plagioclase, anorthoclase, apatite, magnetite and spinel. The xenocrysts of quartz and orthopyroxene are incompatible with the SiO2-undersaturated host rock of this magmatic association. Main reasons determining the interaction between magma and xenoliths include rapid decompression, metamorphism and metasomatism. Xenocrysts are subjected to metamorphism that corresponds to high-temperature facies of contact metamorphism, up to the partial melting of xenocrysts. Metasomatism is directed at equalising the compositions of xenocrysts and eponymous minerals that crystallised from the host melt. There are several important criteria adopted to identify xenocrysts and distinguish them from phenocrysts. These are partial melting, solid-phase decomposition, decrystallisation of primary (before-trapping) textures, recrystallisation and self-faceting of initially xenomorphic grains into the crystals with perfect habits. The chemical composition of xenocrysts has mineral and geochemical indications of xenogenic origin, as well as the signs of a newly-formed substance.

[Display omitted] • We report the first subduction-related helium composition of Patagonian peridotites. • Cryptic metasomatism is observed in anhydrous peridotites from Coyhaique, Patagonia. • P-T estimates show that the lherzolites equilibrated in the spinel stability field. Spinel lherzolite xenoliths from Coyhaique, Chilean Patagonia, are located ~320 km from the Chile Trench and, therefore, their geochemical and isotopic compositions may record slab-derived metasomatism from current and/or earlier subduction zones. We present new mineral and whole-rock major and trace elements that are discussed together with noble gas data (He, Ne, Ar) and Sr-Nd isotopic ratios. Partial melting is recorded by negative correlations between basaltic elements vs. MgO in whole-rock and minerals. In general, silicate minerals show depleted light-rare earth element (LREE) over heavy-REE (HREE) patterns. Whole-rock compositions and clinopyroxene record variable degrees of partial melting (1–15%), which is corroborated by the Sr-Nd depleted character of the lherzolites. Cryptic metasomatism is evidenced by: 1) enriched LREE patterns in whole-rock and type-2 clinopyroxenes; 2) negative anomalies of Nb-Ta-Ti in whole-rock samples coupled with enrichment of large ion lithophile elements and chalcophile elements over high field strength elements and HREE; 3) positive correlations of Pb and U vs. La and negative correlations of Nb/Nb* and Ti/Ti* vs. La in clinopyroxene, and 4) positive anomalies of Li in all silicate minerals. Geochemical differences between whole-rocks and their mineral constituents indicate selective enrichment of the whole-rock samples due to grain-boundary components. 3He/4He ratios reported here are the first strongly radiogenic values found in the Patagonian subcontinental lithospheric mantle (SCLM; 0.20–2.52 R A). Most Ne isotopic ratios are indistinguishable from air composition with few samples showing a nucleogenic component, whereas Ar isotopic ratios are extensively affected by an atmospheric component. The correlation between 3He/4He and 4He/40Ar* corroborates the metasomatism recorded by lithophile and chalcophile elements. Therefore, we identify a heterogeneous SCLM affected by low but variable degrees of partial melting and subsequent enrichment by melts or fluids from recycled subduction-related materials in the spinel stability field (1.06–1.90 GPa and 886–1150 °C). [ABSTRACT FROM AUTHOR]

[Display omitted] • Proterozoic, mainly 1.9–1.7 Ga, granulitic lower crust predominates. • A few regions preserved the Archean granulitic lower crust. • U-Pb ages of metamorphic zircons reflect the slow cooling in the lower crust. We have compiled data on textures, trace element contents, Hf isotope compositions and U-Pb ages of zircons from lower crustal xenoliths from the central part of the Columbia supercontinent in Precambrian areas in North America, Siberia and Northern Europe to address the question of when the host rocks to the zircons became part of the lower continental crust. The time when the lower crust reached its current state, a layer of high-pressure granulite-facies rocks, is dated by zircons that have equilibrated with the observed metamorphic association. Magmatic zircons can provide the age of protolith crystallization, which did not necessarily occur within the lower crust. Those from within Archean cratonic areas of Columbia (the Slave, Wyoming, Superior, Siberian and Karelian cratons and the Archean Belomorian mobile belt in NW Russia) yield predominantly Archean ages (3.5–2.6 Ga). Addition of Proterozoic material is rarely recorded in dated magmatic zircons (2.50–2.47 and 1.64 Ga). The granulite-facies association formed in Archean times has been preserved in lower crustal rocks of the Slave and Superior cratons despite later re-heating events. In other Archean areas of Columbia, the lower crust was reworked during Paleoproterozoic orogenic events. At present, 1.9–1.7 Ga old granulitic lower crust predominates, formed during the assembly of the Columbia supercontinent. Nevertheless, sporadic surviving metamorphic zircons show that Archean lower crustal remnants are occasionally preserved. Mesoproterozoic and younger extensional events and anorogenic magmatism occurred across a wide area at the southern margins of the Superior and Wyoming cratons, in the Belomorian belt, Yavapai and Mazatzal provinces and the Cheyenne belt. These events caused significant reworking of the lower crust with migmatization and formation of the granulite-facies association. The main differences of upper and lower crustal metamorphic histories are slow cooling and long duration of thermal events in the lower crust. [ABSTRACT FROM AUTHOR]

The Freetown Intrusion in Sierra Leone is a large, layered mafic intrusion. Only a small part of the intrusion appears on land to form the Freetown Peninsula, and the larger part lies beneath the Atlantic Ocean. This is demonstrated by gravity and geomagnetic measurements, which also indicate that intrusion occurred over at least 1–2 Ma. The intrusion was formed at 196–201 Ma and is, therefore, concordant with the Central Atlantic Magmatic Province (CAMP) magmatism. At this time, the region was undergoing crustal extension prior to the later opening of the Central Atlantic Ocean. Initially, the magma intruded the Pan-African 2 suture and used this as a conduit for magma ascent, interdigitating with the highly sheared and metamorphosed rocks of the suture. The regional tensile stress allowed the conduit to open further and permitted the continued ascent of magma with subsequent crystal deposition and formation of the layers. Crystallisation commenced at depth to form a crystal mush. The layering was caused by flow, with variations in the supply of the crystal mush and varying proportions of olivine, pyroxene, and plagioclase. The silicates do not show a progressive change in composition indicative of mineral fractionation in a closed chamber. Locally, the intrusion of a plagioclase-rich crystal mush led to the presence of thick anorthosite layers. The initial layering followed the dip of the suture, and the layering steepened as the conduit opened. Sr–Nd isotopic and geochemical analyses both indicate that there was a significant contribution to the intrusion from Pan-African and older host rocks. These were assimilated into the magma. Variations in this contribution led to the formation of both gabbroic and noritic layers. Fragments of the host rocks were entrained during magma ascent and are now present as xenoliths. • The Freetown Intrusion is much larger than the outcrop on land. • The Pan-African 2 suture provided the conduit for initial intrusion. • Crystallisation commenced at depth as a crystal mush with entrained xenoliths. • The igneous layering is due to flow and repeated injection of crystal mush. • Sr–Nd isotopic and geochemical analyses show host rocks contributed to the melt. [ABSTRACT FROM AUTHOR]

The relationship among the orebody, volcanic rocks, and a rhyolite dike in the large epithermal Kupol deposit (Western Chukotka) is considered. Almost throughout its entire length (more than 3000 m), the orebody of the deposit is intersected by rhyolite dikes. In the studied profiles in the mine excavations at the horizons 425, 455, 470, 515 m, a rhyolite dike clearly intersects and bisects the orebody, which is a 20-m-thick dominantly quartz vein with some adularia. The rhyolite has a porphyritic structure. The groundmass is devitrified with the formation of an aphanite quartz-feldspar aggregate with a mosaic microstructure. Some patches are fluidized. The rhyolites contain from 72.96 to 74.97 wt % SiO2, and the composition corresponds to biotite granites of the low-alkaline series. The following criteria have been identified to indicate the post-mineralization age of the dike: a clear intersection of a thick ore vein by the dike; vitrification of the dyke material at the contact with the vein; rhyolite cementing the crushed quartz vein material; the emergence of some small tongues from the dike that penetrate the quartz vein; xenoliths of quartz vein material in the dike body; the orebody tongues do not penetrate the dike; increased concentration of vein and ore minerals on the contact surface of the dike or in cracks of the endocontact zone. The formation of obsidian (vitrophyre) in rhyolite at contact with the orebody indicates that the dike was emplaced in a cold system. Preservation of the productive vein thickness, its morphological features, and the Au and Ag contents at the cabs of the rhyolite dike; identical textures and repeated rhythms in the areas of rhythmic-banded layering in the vein on both sides of the dike shows that in the Kupol deposit, post-mineralization dikes do not replace or impoverish the productive vein system, which is probably due to their dilatation nature and the infilling of separation cracks. [ABSTRACT FROM AUTHOR]

The origin and evolution of subcontinental lithospheric mantle (SCLM) are important issues of Earth's chemical and physical evolution. Here, we report detailed textural and chemical analyses on a mantle xenolith suite from Befang (Oku Volcanic Group, Cameroon Volcanic Line), which represents a major tectono-magmatic structure of the African plate. The samples are sourced from spinel-facies mantle and are dominated by lherzolites. Their texture is cataclastic to porphyroclastic, and foliation defined by grain-size variation and alignment of spinel occurs in part of peridotites. Spinel is interstitial and has amoeboidal shape. Clinopyroxene REE patterns are similar to those of Depleted MORB Mantle (DMM) except LREEs, which vary from depleted to enriched. The A-type olivine fabric occurs in the subset of one harzburgite and 7 lherzolites studied by EBSD. Orthopyroxene shows deformation consistent with olivine. The fabric of LREE-enriched clinopyroxene is equivalent to those of orthopyroxene and olivine, whereas spinel and LREE-depleted clinopyroxene are oriented independently of host rock fabric. The textural, chemical and thermobarometric constraints indicate that the Befang mantle section was refertilised by MORB-like melt at pressures of 1.0–1.4 GPa and temperatures slightly above 1200–1275 °C. The olivine-orthopyroxene framework and LREE-enriched clinopyroxene preserve the protolith fabric. In contrast, the LREE-depleted clinopyroxene, showing discordant deformation relative to the olivine-orthopyroxene protolith framework, and amoeboidal spinel crystallized from the infiltrating melt. The major element and REEs composition of minerals forming the Befang peridotites indicate subsequent reequilibration at temperatures 930–1000 °C. This was followed by the formation of websterite veins in the lithospheric mantle, which can be linked to Cenozoic volcanism in the Cameroon Volcanic Line that also brought the xenoliths to the surface. This study therefore supports the origin of fertile SCLM via refertilization rather than by extraction of small melt fractions, and further emphasizes the involvement of depleted melts in this process. [ABSTRACT FROM AUTHOR]

We model the depth and Vs structure of the Hales discontinuity (H‐D) beneath Eastern Dharwar Craton (EDC) and Southern Granulite Terrain (SGT) using P wave receiver function (P‐RF) analysis and joint inversion with Rayleigh wave phase velocity dispersion. We calculate P‐RFs at higher frequency (fmax = 0.46 Hz), compared to previous studies, to show that the H‐D P‐to‐S converted phase (Phs) is distinct from crustal reverberations. Phs at stations Hyderabad (HYB) and Gauribidanur (GBA), in the EDC, arrive at ∼11.5 s and ∼12 s, respectively. From joint inversion, the H‐D is modeled at 107 ± 5 km and 113 ± 4 km depth, with ∼3% and ∼4% Vs increase, beneath HYB and GBA, respectively. For station Kodaikanal (KOD), in SGT, the Phs destructively interferes with the negative midcrustal reverberation at most ray‐parameters, which explains its apparent absence in previous studies. We isolated P‐RFs where Phs is distinct at ∼11 s and model it at 102 ± 3 km depth. Common conversion point stack profiles constructed by depth migrating P‐RFs through the Vs model show an undulatory nature of the H‐D. From data of mantle xenoliths (Wajrakarur kimberlite field), we calculate Vs of mantle peridotite and eclogite, using published bulk rock compositions. At the H‐D depth and temperature derived from Indian shield geotherm, we observed a good match to the Vs structure of the H‐D. Our results support the geodynamic model of the H‐D being an interface of paleosubducted eclogitic oceanic crust embedded within the upper mantle peridotite. Global observations of mantle reflectors within the continental lithosphere, at depths similar to H‐D, have been related to relict subduction and independently support our model. Key Points: Hales discontinuity beneath Eastern Dharwar and Southern Granulites is at 107–113 km and ∼102 km depths, with Vs increase of 3%–4% and 8%Vs above and below Hales discontinuity correspond to mantle peridotite and eclogite, metamorphosed from subducted oceanic crustal gabbroOrigin of Hales discontinuity explained by shallow subduction of oceanic lithosphere and subsequent preservation during craton stabilization [ABSTRACT FROM AUTHOR]

Intra-arc diversity in volcanic activity and composition is ubiquitous, but its underlying causes remain largely unresolved in many settings. In this work, we examine such variability in the Grenadines archipelago, southern Lesser Antilles arc. Here, juxtaposed volcanic centres exhibit eruptive longevities and chemistries distinct from northern counterparts in the same arc. Our goal is to explain this deviation by investigating variations in magmatic processes using petrological data from erupted crustal xenoliths and lavas, and interpreting these findings within the context of the archipelago's tectonic history and geophysical structure. Textural analyses of xenoliths reveal crystallization over a wide range of pressure-temperature-melt composition conditions in the crust. Mineral phases display discrete compositional trends pointing towards significant inter-island variability in underlying plumbing systems. The geochemical variety of erupted magmas is reminiscent of the entire arc. We speculate that the Grenadines represents the early onset of subduction forming the modern-day Lesser Antilles arc. Extrusive volcanism initiated as submarine activity. Subsequent uplift eroded the original topography of these volcanic centres following the eventual cessation of volcanism in the Neogene. The positioning of the Grenadines on an elevated platform provides rare modern insight into early arc crust formation not commonly preserved in established active arcs., Competing Interests: We declare we have no competing interests., (© 2024 The Authors.)

We examine the pyrometamorphic changes and resulting crustal contamination that occurred when fragments of Otago Schist were entrained and pyrometamorphosed in Dunedin Volcanic Group (DVG) basanite. Otago Schist greenschist facies quartz-albite laminae were replaced by fine-grained quartz grains (inverted from β-quartz or tridymite) with orthopyroxene coronas and dacitic-rhyolitic glass. Former muscovite + chlorite + epidote ± garnet segregations were converted to plagioclase (An30-80), olivine, spinel, cordierite, ilmenite, pigeonite and trachytic to trachy-andesitic glass. Minerals largely formed by mineral replacement and then by crystal nucleation from a quenched liquid as protolith minerals passed their liquidi, with the latter process generating swallow-tailed, skeletal, dendritic and ladder-structured crystal morphologies. Orthopyroxene-, olivine- and plagioclase-liquid geothermometry indicates xenoliths reached at least 940°C. Despite the molten state of the xenoliths, discernible chemical and isotopic (Sr, Nd) contamination of the basanite was inhibited by its enriched trace element budget. The only evidence for crustal contamination comes from basanite Pb isotopes, which have less radiogenic compositions within several cm of the xenoliths. The subtle variation in DVG Sr and Nd isotopes may therefore be due to different mantle source components rather than crustal contamination by the underlying Otago Schist. [ABSTRACT FROM AUTHOR]

Xenocrysts and megacrysts hosted in the rocks of Early Cretaceous olivine-basalt-basanite-nephelinite association that outcropped in erosion crater of Makhtesh Ramon (Natural Reserve of Mishmar ha-Nagev, Israel) are the topic of the current research. Magmatic rock association contains the wide spectrum of xenoliths trapped at different crustal levels. These are upper mantle, lower, and upper crustal xenoliths. Mantle xenoliths are represented by peridotites, olivine clinopyroxenites, clinopyroxenites, olivine websterites, websterites and their amphibole-bearing analogs. Lower crustal xenoliths are mafic granulites, such as metagabbros and plagioclasites, upper crustal xenoliths are the fragments of Neoproterozoic tuffs. Xenocrysts and megacrysts are fragments of xenoliths that chipped from them during their transportation to the surface. Different rate of xenoliths, xenocrysts, and megacrysts alteration by host magma and late fluids is a common petrographic particularity. The fluid alteration occurred at phreatomagmatic stage of magma crystallization. Alteration is observed by the appearance of new textures and products of reactional interaction. Xenocrysts and megacrysts are mainly represented by minerals that compatible with rock magmatic association. These are olivine, clinopyroxene, amphibole, nepheline, plagioclase, anorthoclase, apatite, magnetite, and spinel. Xenocrysts of quartz and orthopyroxene are incompatible to host rock magmatic association under-saturated in SiO2. Main reasons determining interaction between magma and xenolith are rapid decompression, metamorphism and metasomatism. Xenocrysts are subjected to metamorphism that corresponds to high-temperature facies of contact metamorphism, up to the partial melting of xenocrysts. Metasomatism is smoothing out the composition of xenocrysts to the composition of the same minerals that crystallized from host melt. There are several important criterions, which permit to identify xenocrysts and divide them from phenocrysts. These are partial melting, solid-state decomposition, recrystallization of primary (before-trapping) textures, recrystallization and self-faceting of initially anhedral grains into the crystals with perfect habit. Chemical composition of xenocrysts has both mineral - geochemical indications of xenogenic origin and new-formed sings of alteration.

The new mineral fluorapophyllite-(NH4), ideally NH4Ca4(Si8O20)F⋅8H2O, was found at the Vechec andesite quarry located near Vechec village, Vranov nad Topľou Co., Prešov Region, Slovak Republic. It occurs in cavities of quartz–illite–saponite–tobelite xenolith embedded in pyroxene andesite. Fluorapophyllite-(NH4) is associated with calcite, tridymite, pyrite, chabazite-Ca and heulandite-Ca. It forms clusters, aggregates or crystalline crusts consisting of individual, well-developed crystals up to 4 mm in size, exhibiting the forms {110}, {101} and {001}. The mineral is colourless to light pink and translucent with white streak and vitreous to pearly lustre; it is non-fluorescent under ultraviolet radiation. The Mohs hardness is ~4½ to 5, tenacity is brittle, fracture is irregular, and there is perfect cleavage on {001}. The calculated density is 2.325 g cm–3. Fluorapophyllite-(NH4) is optically uniaxial (+) with ω = 1.5414(5) and ɛ = 1.5393(8) (λ = 589 nm). It is non-pleochroic. The empirical formula (based on 29 O + F apfu) is [(NH4)0.55K0.32Na0.07Ca0.06]Σ1.00(Ca4.01Mg0.02)Σ4.03Si7.97O20[F0.84(OH)0.16]Σ1.00⋅8H2O. Fluorapophyllite-(NH4) is tetragonal, space group P4/mnc, a = 8.99336(9) Å, c = 15.7910(3) Å, V = 1277.18(3) Å3 and Z = 2. The seven strongest X-ray powder diffraction lines are [dobs in Å,(I,%)(hkl)]: 7.897(32)(002), 7.812(13)(101), 4.547(14)(103), 3.946(100)(004), 2.985(39)(105), 2.4841(11)(215) and 1.5788(12)(00.10). The crystal structure of fluorapophyllite-(NH4), refined to R1 = 0.0299 for 743 unique (I > 3σI) observed reflections, confirmed that the atomic arrangement is very similar to that of the other members of the apophyllite group. The new mineral is named according to the current nomenclature scheme for apophyllite-group minerals and is an NH4 dominant analogue of fluorapophyllite-(K), fluorapophyllite-(Na) and fluorapophyllite-(Cs). [ABSTRACT FROM AUTHOR]

This study presents high precision Zn stable isotope analyses for lower crustal rocks (9 granulites from Archean terrains and 30 lower crustal xenoliths) from the north margin of the North China Craton (NCC) to understand the behavior of Zn isotopes during deep crustal processes and the Zn isotopic composition of the lower continental crust (LCC). The lower crustal xenoliths range in composition from mafic to felsic with MgO contents of between 0.4 to 20.2 wt.%. The δ66Zn (the permil deviation of the 66Zn/64Zn ratio from the JMC-Lyon standard) values of lower crustal xenoliths range from −0.17‰ to 0.38‰. The lack of clear correlation of δ66Zn with geochemical indicators (e.g., Al 2 O 3 , Cr, Ba/La, 87Sr/86Sr and 143Nd/144Nd) indicates that assimilation of Precambrian lower crust, fluid metasomatism, and accumulation of pyroxene and plagioclase had a limited effect on Zn isotopic compositions of these lower crustal xenoliths. The δ66Zn values (0.18‰ to 0.34‰) of garnet-bearing mafic granulites decrease with increasing FeO (T) and V contents, which are likely results of the accumulation of Fe-Ti oxides. The average δ66Zn of the lower continental crust is estimated to be 0.29 ± 0.02‰ (95% SE) using lower crustal xenoliths from the Neogene Hannuoba basalts. This δ66Zn value is similar to the estimated value (0.28 ± 0.04‰, 95% SE) obtained from the granulites from Archean terrains, suggesting that there is no significant difference in the Zn isotopic composition between the Archean and present-day lower continental crust. Combining the δ66Zn data of the lower crustal xenoliths and granulite terrains and different weighting methods, the Zn isotopic composition of the lower continental crust is estimated to be 0.28 ± 0.04‰ (95% SE). [ABSTRACT FROM AUTHOR]

Xenoliths from the Mir pipe and from the Shandong and Liaoning provinces were studied by the methods of EMPA and ICP-MS. Their mineralogical, geochemical, and genetic features were revealed. Minerals of diamondiferous paragenesis were detected in xenoliths from the Mir pipe, while they were not found in xenoliths of China. All xenoliths are characterized by secondary alterations, which are more intense in xenoliths of China. The distribution of REEs shows the involvement of subduction processes in the formation of xenoliths from the Mir pipe. The influence of metasomatism is clearly evident in xenoliths from China. The calculated P–T parameters (Т = 600–700°C, P = 2–2.5 GPa) are not consistent with the mantle environments that correspond to the metasomatic conditions. [ABSTRACT FROM AUTHOR]

· Mantle wedge may contain sulfide/sulfate metasomatic accumulations. · Reaction of mantle olivine with slab melt/fluid releases Ni from olivine lattice. · The released Ni reacts with S-rich slab melt/fluid to form sulfides. · Sulfur is introduced to the mantle wedge in both oxidized and reduced forms. Metasomatic replacement of olivine by orthopyroxene under the influence of SiO 2 -rich melts is a widespread process in mantle conditions responsible for the formation of pyroxenite from mantle peridotite. However, the behavior of nickel in this process remains unclear. The reaction of Ni-containing olivine with silica- and sulfur-rich metasomatic agents in the suprasubduction mantle may release nickel from olivine and subsequently form a rock containing orthopyroxene and nickeliferous sulfides. Xenoliths of mantle wedge harzburgite from the Shiveluch volcano (Kamchatka continental arc) contain abundant globules of Ni-rich MSS, pentlandite and smaller amount of copper sulfides, totaling up to 0.75 wt.% (∼2600 ppm S2–), in association with Ca-sulfates (up to 7900 ppm S6+). The process accounting for the observed mineral association can be described via simplified reactions (the coefficients are approximate): (Mg,Fe,Ni) 2 SiO 4 + 2xH+ +SiO 2 = 2(Mg,Fe)SiO 3 + xNi2+ + xH 2 O; Ni2+ + S2– = NiS. The influx of both oxidized and reduced sulfur may account for the coexistence of sulfide and sulfate phases. The presence of low-Ti chrome spinel with high Fe(II)/Fe(III) ratios, low Al, Ca and Ti in olivine, and no evidence of deserpentinization confirms the mantle origin of the sulfide/sulfate-bearing xenoliths. The mean isotopic composition of sulfur in the sulfide-sulfate assemblage, δ34S = +4.5‰, supports the contribution of slab-derived sulfur. The accumulations of Ni-(Cu)-rich sulfides and Ca-sulfate in metasomatized peridotites can serve as an intermediate host for sulfur and chalcophile metals, thereby generating magmas containing sulfur at the level of sulfide saturation and simultaneously enriched in Ni and Cu during the subsequent partial melting of the mantle. [ABSTRACT FROM AUTHOR]

Coupled oxygen-hafnium isotope and trace element geochemical data were obtained using thirty eight previously dated zircon grains extracted from five mafic to intermediate crustal xenoliths of the Wyoming Craton (Montana, USA). Xenoliths include mid to lower crustal (642–817 °C and 3.5–9.4 kbar) mafic granulites and amphibolites with dominantly Mesoproterozoic (1772–1874 Ma) and minor Paleoproterozoic to Late Archean (2004–2534 Ma) 207Pb/206Pb zircon ages. Zircon oxygen isotope data indicate derivation from melts in equilibrium with a mantle source that interacted with limited supracrustal material (δ18O = 4.4–5.7‰), as well as the incorporation of supracrustal fluids or melts into mantle source regions (δ18O = 6.0–8.1‰). The small within-sample isotopic variability suggests that primary zircon did not exchange with isotopically distinct fluids or melts after initial formation. Initial zircon Hf isotopic values are highly variable across all xenoliths (ε Hf = +3.7 to −17.6), consistent with protolith derivation from mantle sources that incorporated evolved, unradiogenic material or were modified by subduction-related fluids. Within a single granulite xenolith, two zircon types are recognized based on CL imagery, Hf isotopes and U-Pb ages (Type I and Type II). Type I magmatic zircons show dispersed ages (ca. 1700–2534 Ma) and unradiogenic initial Hf (ε Hf = −17.6 to −1.5, 176Hf/177Hf = 0.281074–0.281232). The spread in ages and initial ε Hf , but narrow range in initial 176Hf/177Hf, points to variable Pb loss in response to dissolution-recrystallization of pre-existing zircon. Type II metamorphic zircon yields a younger Proterozoic population (ca. 1700–2155 Ma) with more radiogenic initial Hf relative to Type I zircon (ε Hf = −7.9 to +1.4, 176Hf/177Hf = 0.281427–0.281578); this type represents newly grown metamorphic zircon that formed in the solid-state and incorporated Zr and Hf from pre-existing zircon and silicate matrix/metamorphic phases. REE patterns from all xenoliths are steep and positively sloping without discernible HREE depletion relative to LREE, implying zircon crystallization/recrystallization in the absence of garnet. Negative Eu anomalies signify simultaneous zircon and feldspar crystallization. Solid-state recrystallization may have lead to variations in LREE, Eu and Ce in certain xenoliths. Xenoliths containing magmatic zircon (1834 ± 19 Ma) with mantle-like δ18O (4.4–5.5‰) and radiogenic initial ε Hf (−2.3 to +3.7) likely formed through crystallization of melts derived from a mantle source that incorporated minor amounts of subducted sedimentary/supracrustal material. Proterozoic (1874 ± 8 Ma) xenoliths with elevated δ18O (6.0–7.0‰) and unradiogenic initial ε Hf (−8.2 to −9.6) within magmatic zircon represent melt products of subduction-induced melting and metasomatism of the overlying mantle wedge in the vicinity of the northern GFTZ. Older (ca. 2534 Ma) xenoliths containing zircons with elevated δ18O (6.4–7.2‰) and unradiogenic ε Hf (up to −17.6) represent crystallization of protolith magmas extracted from a mantle source metasomatized by subduction-derived fluids and melts in the Late Archean or earlier. Zircon geochronology and isotope systematics within Mesoproterozoic xenoliths support a model of ocean-closure and subsequent continental collision between the Medicine Hat Block and Wyoming Craton, resulting in the formation of subduction-related melts at ca. 1834–1874 Ma, followed by ca. 1770 Ma collision-related metamorphism thereafter. • Zircon from crustal xenoliths of the Wyoming Craton was investigated using Hf, O and REE isotopic analyses. • Mantle-like δ18O (4.4–5.7‰) and radiogenic εHf (−2.3 to +3.7) in zircon imply mantle-derived protolith magmas. • Elevated zircon δ18O (6–8‰) and unradiogenic εHf (−8.2 to −9.6) are products of subduction-related mantle metasomatism. • Data are consistent with ocean-closure between the Medicine Hat Block and Wyoming Craton at ca. 1.8 Ga. • Collisional metamorphism at ca. 1.7 Ga may recrystallize xenoliths to amphibolite/granulite facies. [ABSTRACT FROM AUTHOR]

A suite of crustal xenoliths from the Bearpaw Mountains, Montana, USA, was examined using zircon geochronology, whole-rock geochemistry and petrology in order to investigate mid- to lower crustal geological processes along the northern margin of the Wyoming Craton. The xenoliths are hosted within Eocene alkaline intrusive and volcanic rocks ("minettes") and include mafic to intermediate granulites and amphibolites. Whole-rock geochemistry and geothermobarometry indicate igneous protoliths of alkaline to sub-alkaline composition, which recrystallized at lower to middle crustal conditions (642–817 °C and 3.5–9.4 kbar). Mantle-normalized trace element patterns for all xenoliths show a subduction signature, with extreme enrichments in large-ion lithophile (LILE) and fluid mobile elements (Cs, Rb, Ba, Pb, Sr) relative to high-field strength elements, together with relative Zr-Hf and Nb-Ta-Ti depletions. While late-stage modification of the xenolith compositions by the magma host cannot be excluded, simple binary mixing models between the host lava and xenoliths indicate that such a process was insignificant, and implies that the xenoliths preserved their original geochemical signatures. SEM-CL imaging of zircon reveal complex internal structures and indicates the presence of both magmatic and metamorphic zircon. In the simplest cases, magmatic grains are recognized by weakly preserved, altered oscillatory zoning. In more complex magmatic grains, high Th/U (x ¯ = 1.2) zircon appear recrystallized in the presence of a melt phase, resulting in resorbed internal domains and exterior morphology. Metamorphic zircon with low Th/U (0.03–0.8, x ¯ = 0.2) is present as whole grains or rims with uniform domains devoid of recognizable internal microstructures. Zircon U-Pb dating produces a spectrum of dispersed Late Archean to Early Paleoproterozoic ages (2000–2534 Ma), including a dominant mid-Proterozoic age population that records both magmatic (1834 ± 19 and 1874 ± 10 Ma) and metamorphic (1772 ± 5–1788 ± 4 Ma) events. Proterozoic ages are correlated with lithospheric processes along the cratonic north-western margin (Great Falls Tectonic Zone and Montana Metasedimentary Terrane), involving continental arc magmatism and collision-related metamorphism. The protolith to xenoliths containing mid-Proterozoic (ca. 1834–1874 Ma) magmatic zircon ages were formed by subduction-induced melting of the overlying mantle wedge. Xenoliths containing older, dispersed magmatic zircon ages (ca. 2000–2534 Ma) do not produce a significant age population that can be well correlated with events within the Wyoming Craton/GFTZ, and their protolith was likely produced by melting of a subduction-related metasomatized mantle in the Late Archean or earlier. Both Proterozoic and Late Archean protolith magmas are envisaged as new additions to the Wyoming Craton that resided within middle to lower crustal levels of the continental lithosphere before being subjected to collision-related metamorphism and recrystallization at ca. 1770 Ma. • Zircons from crustal xenoliths of the Wyoming Craton yield ages between ca. 1800 and 2500 Ma. • Proterozoic (ca. 1800 Ma) ages can be correlated with subduction-related processes along the north-western cratonic margin • Whole-rock and trace-element signatures are suggestive of xenolith formation in a convergent margin setting • Xenoliths are envisioned as new melts added to the continental crustal lithosphere that were subsequently metamorphosed. [ABSTRACT FROM AUTHOR]

We reconstruct crustal structure along the Lesser Antilles island arc using an inversion approach combining constraints from petrology of magmatic crustal xenoliths and seismic receiver functions. Xenoliths show considerable island-to-island variation in xenolith petrology from plagioclase-free ultramafic lithologies to gabbros and gabbronorites with variable proportions of amphibole, indicative of changing magma differentiation depths. Xenoliths represent predominantly cumulate compositions with equilibration depths in the range 5 to 40 km. We use xenolith mineral modes and compositions to calculate seismic velocities (v P , v S) and density at the estimated equilibration depths. We create a five-layer model of crustal structure for testing against receiver functions (RF) from island seismic stations along the arc. Lowermost layer (5) comprises peridotite with physical characteristics of mantle xenoliths from Grenada. Uppermost layer (1) consists of 5 km of volcaniclastics and sediments, whose physical properties are determined via a grid inversion routine. The three middle layers (2) to (4) comprise igneous arc crust with compositions corresponding to the xenoliths sampled at each island. By inversion we obtain a petrological best-fit for the RF on each island to establish the nature and thicknesses of layers (2) to (4). Along the arc we see variations in the depth and strength of both Moho and mid-crustal discontinuity (MCD) on length-scales of tens of km. Moho depths vary from 25 to 37 km; MCD from 11 and 32 km. The Moho is the dominant discontinuity beneath some islands (St. Kitts, Guadeloupe, Martinique, Grenada), whereas the MCD dominates beneath others (Saba, St. Eustatius). Along-arc variability in MCD depth and strength is consistent with variation in estimated magmatic H 2 O contents and differentiations depths that, in turn, influence xenolith lithologies. A striking feature is steep, along-arc gradients in v P similar to those observed at other oceanic arcs. These gradients reflect abrupt changes in rates and processes of magma generation in the underlying crust and mantle. We find no evidence for large, interconnected bodies of partial melt beneath the Lesser Antilles. Instead, the crustal velocity structure is consistent with magma differentiation in vertically-extensive, crystal mush-dominated reservoirs. Along-arc variation in crustal structure may reflect heterogeneous upwelling within the mantle wedge, itself driven by variation in slab-derived H 2 O fluxes. • Arc crustal structure modelled by integrating petrology of 230 igneous xenoliths with seismic data from 23 islands. • Crust comprises four layers defined on basis of xenolith composition, calculated seismic properties and receiver functions. • Steep lateral velocity gradients and irregular along-arc variations in depth to Moho and mid-crustal discontinuity. • Lateral variation consistent with island-to-island variation in xenolith petrology. • Velocity structure reflects heterogeneous upwelling within the mantle wedge, driven by variation in slab-derived H 2 O fluxes. [ABSTRACT FROM AUTHOR]

The Moroccan Variscan belt comprises numerous granitoid bodies, which have been isotopically studied, but still lack a detailed petrogenetic model. The Variscan Aouli-Mibladen granitic complex in the Moroccan Eastern Meseta provides an excellent opportunity for studying the petrogenesis and the relationship with adjacent Cambro-Ordovician metasediments. This region exposes mainly M-to I-type metaluminous mafic to granitic bodies as well as migmatites and peraluminous S-type anatectic granites. An exceptional xenoliths and garnets cumulate deposit covers an area of ∼2 km2 located NE of the complex. The S-type suite includes cordierite bearing microgranite dykes and two small two-mica leucogranitic stocks, crosscutting the eastern metamorphic aureole of the complex. Petrological analysis shows that biotite dehydration melting reactions produced anatectic melt and peritectic cordierite and garnet. Garnet shows complex zoning profiles, with typical prograde growth zoning in the core, and resorption and reprecipitation rims. Peritectic cordierite (X Fe = 0.37) is associated with restitic biotite (X Fe = 0.65), whereas abundant cotectic cordierite (X Fe = 0.65) belongs to the granites paragenesis (Bt + Kfs + Pl + Crd + Qtz). Garnet crystals are also frequently mantled by a retrograde cordierite type. Thermodynamic modelling shows that biotite dehydration melting took place under granulitic facies conditions (830–870 °C and 6 kbar), producing a significant amount of peraluminous melts. Their ascent and emplacement led to the S-type suite of the Aouli-Mibladen complex, including the local xenolith-cumulate deposit. We envisage a petrogenetic model during the Variscan orogeny, in which mantle derived mafic magmas stagnated in large reservoirs at 18–20 km depth, causing isobaric heating and partial melting of the surrounding metapelitic protolith. Differentiated M-I-type and S-type magmas were emplaced at higher levels concomitantly, with limited mixing and mingling during their final crystallization, at a depth of 9–10 km leading to contact metamorphism (600 °C, 3 kbar). These findings open a window into the hitherto unknown Moroccan Variscan mid-crust and permit to discuss the nature and age of the underlying basement terranes. • Direct observation of mantle-crust interaction during the Variscan Orogeny, Morocco. • Partial melting of pelitic protholith, formation of anatexites. • Pseudosection modelling, cordierite and garnet as key peritectic minerals. • Thermal state of Variscan Moroccan middle and upper crust. • Petrogenetic model for the Variscan belt, key for geodynamic considerations. [ABSTRACT FROM AUTHOR]

Pyrite and organic matter closely coexist in some hydrothermally-altered gabbroic xenoliths from the Hyblean Plateau, Sicily. The representative sample consists of plagioclase, Fe-oxides, clinopyroxene, pyrite and minor amounts of many other minerals. Plagioclase displays incipient albitization, clinopyroxene is deeply corroded. Pyrite grains are widely replaced by spongy-textured magnetite, which locally hosts Ca-(and Fe-)sulfate micrograins and blebs of condensed organic matter. Whole-rock trace element distribution evidences that incompatible elements, particularly the fluid-mobile Ba, U and Pb, are significantly enriched with respect to N-MORB values. The mineralogical and geochemical characteristics of the sample, and its U-Pb zircon age of 216.9 ± 6.7 MA, conform to the xenolith-based viewpoint that the unexposed Hyblean basement is a relict of the Ionian Tethys lithospheric domain, mostly consisting of abyssal-type serpentinized peridotites with small gabbroic intrusions. Circulating hydrothermal fluids there favored the formation of hydrocarbons trough Fischer-Tropsch-type organic synthesis, giving also rise to sulfidization episodes. Subsequent variations in temperature and redox conditions of the system induced partial de-sulfidization, Fe-oxides precipitation and sulfate-forming reactions, also promoting poly-condensation and aromatization of the already-formed hydrocarbons. Here we show organic matter adhering to a crystal face of a microscopic pyrite grain. Pyrite surfaces, as abiotic analogues of enzymes, can adsorb and concentrate organic molecules, also acting as catalysts for a broad range of proto-biochemical reactions. The present data therefore may support established abiogenesis models suggesting that pyrite surfaces carried out primitive metabolic cycles in suitable environments of the early Earth, such as endolithic recesses in mafic rocks permeated by hydrothermal fluids. [ABSTRACT FROM AUTHOR]

Although nucleation of magnetite and/or hematite along dislocations upon oxidation of olivine has been observed by many workers, the effect of oxidation on the magnetic properties of the sample with specific mineralogical alterations has not been studied. Therefore, we investigate this problem using a set of time series 1 bar oxidation experiments at 600 and 900 °C. Results show rapid olivine oxidation and alteration at both 600 and 900 °C, forming magnetite and hematite associated with a change from paramagnetic to ferromagnetic behavior after oxidation. Magnetite and hematite nucleate along dislocations and impurities in the crystal structure, along with surface coatings and within cracks in the crystals. Fresh, unaltered mantle xenoliths containing magnetite have been interpreted as having formed in cold tectonic regimes in the mantle, rather than through oxidation during or after ascent. Mantle xenoliths rapidly ascend through the mantle with estimates of the ascent of up to 90 km/h (3 GPa/h) based on the diffusion profile of water in mantle olivine. The rates correspond to xenoliths ascending through the mantle over hours and not days or weeks. Our results show that olivine oxidation and alteration can occur in days to weeks at 600 °C and within minutes at 900 °C. Therefore, if the xenolithic material is transported to the surface in a cold magma (at temperatures ≤600 °C), then the timescale of ascent is likely not long enough for oxidation to cause magnetite formation or a ferromagnetic signature to occur. However, if the material is transported in a hot oxidized basaltic magma (with temperatures ≥900 °C), then oxidation can cause magnetite formation and a ferromagnetic signature. [ABSTRACT FROM AUTHOR]