Abstract Peridotite xenoliths commonly exhibit melt veining, variably attributed to mantle metasomatism, host magma infiltration, or decompression melting. In order to test these origins and constrain the P–T–t path of xenolith ascent, we study spinel-lherzolite xenoliths and host scoria from a cinder cone north of the Grand Canyon (Sullivan Ranch). Micro-x-ray computed tomography (μCT) scans of xenoliths show that melt veins are ubiquitous at this locality. They make up ~ 10% of bulk xenoliths, are highly interconnected at the mm-scale and include bubbles. Bubbles are surrounded by silica-rich residual glass, although veins are largely crystalline, composed of secondary plagioclase, clinopyroxene (cpx), olivine and Cr-spinel (sp). The reconstructed bulk melt vein is basaltic (50–52% SiO2), and mineral compositions within the melt vein (high-Fo olivine, high-Ti spinel, high-Cr and lower-La/Sm cpx) are distinct from primary minerals away from melt veins. The reconstructed incongruent melting reactions for two different xenoliths (63cpx + 72opx + 13sp ➔ 100liq + 48ol, on average) are similar to those from equilibrium peridotite melting experiments at 1 GPa, even though minerals are not in equilibrium across the 3–10 cm-scale xenoliths. The timescale for xenolith melting was derived from Fe-Mg zonation profiles in primary olivine (Fo89–90) adjacent to melt veins (with Fo91–92 rims) that indicate diffusion over 1–3 weeks. This timescale is similar to that derived for olivine grains at the xenolith exterior that were diffusively equilibrating with the host magma for ~ 1 week. The similarity of timescales from the interior melt veins to the exterior olivine indicates that in situ melting occurred during entrainment and transport. Ascent rates from the entrainment depth (~ 40 km) to the surface are 0.05–0.15 m/s, similar to other primitive cinder cone eruptions and consistent with Stokes settling velocities. Xenocrysts in the host scoria yield diffusion timescales of hours to days, consistent with xenolith disaggregation late in the ascent history. Evidence for limited interaction with the host magma comes from trace element zonation in melt veins that approach the host composition in the outer 1 cm of the xenolith. The interaction does not appear to be driven by diffusion, as elements with very different diffusivities (e.g. Cl vs. Zr) show similar lengthscales of zonation. Instead, host melt infiltration could have occurred due to a combination of capillary forces and inflation of the xenolith driven by the volume change of melting and bubble growth in the melt veins. Given the P–T–t path of these lherzolite xenoliths, which should be similar to those erupted from other alkali basalt cinder cones from around the world, decompression melting is inevitable. The extent of decompression melting will be a function of the host magma P–T path, the xenolith water content, and the proportion of touching cpx-opx-sp. Such melting consumes cpx and may lead to systematic misclassification of xenoliths as refractory harzburgites that were originally lherzolites. Geochemical studies of xenoliths should take care to avoid the outer cm (or more) that may have been contaminated by the carrier magma, and secondary minerals formed from infiltration, reaction, melting and/or crystallization.
Abstract Diffusion chronometry based on quartz and feldspars has the potential to place robust constraints on the timescales of magma storage, recharge, and mobilization in silicic magmas. However, widespread application of these methods has been hindered by the fact that published diffusion coefficients for Ti in quartz and for Sr, Ba, and Mg in plagioclase and alkali feldspar differ by orders of magnitude, leading to large and often irreconcilable discrepancies in inferred timescales. In this review, we first compile existing experimental diffusivities in quartz and feldspars and critically evaluate the origins of discrepancies among published datasets, including experimental design, analytical protocols, and underlying assumptions. We then review studies performed on natural samples in which diffusion timescales based on Ti gradients in quartz were compared with ones based on Sr, Ba, and/or Mg diffusion in feldspars and/or other methods. To allow proper comparison, we complement previously reported timescales with ones calculated based on more recently published diffusion coefficients. This exercise reveals that internally consistent and mutually compatible timescales can be obtained with the most recent set of diffusion coefficients. Building on these conclusions, we outline best-practice recommendations for the generation, selection, and application of diffusion coefficients that are geologically meaningful and appropriate for natural magmatic systems. We conclude by identifying key experimental gaps that must be addressed to further refine diffusion models and to establish a coherent and broadly accepted framework for diffusion chronometry in silicic magmas.
Abstract Iron isotopes are increasingly recognized as a key tracer for magmatic evolution and sulfide mineralization in magmatic Ni-Cu sulfide deposits. While these deposits form primarily from high-temperature melts, exsolved magmatic volatile bubbles can spontaneously attach to segregated sulfide droplets. However, whether and how these sulfide-associated volatile-rich fluids influence the Fe isotopic compositions of coexisting silicate minerals during subsequent cooling remain poorly understood. Here, we present high-precision Fe isotopic data of various minerals (silicates, oxides, and sulfides) from the Hongqiling Ni-Cu sulfide deposit in the Central Asian Orogenic Belt. Systematic Fe isotopic fractionation is observed among silicates, oxides and sulfides, with increasing δ56Fe values from olivine (-0.19 to 0.11‰), orthopyroxene (-0.07 to 0.09‰), clinopyroxene (0.05 to 0.20‰), and hornblende (0.03 to 0.29‰) to Cr-spinel (0.08 to 0.37‰), and from pyrrhotite (-1.19 to -0.44‰) and pentlandite (-1.09 to 0.95‰) to chalcopyrite (0.17 to 0.72‰). Notably, quantitative isotope thermometry for coexisting chalcopyrite and pyrrhotite indicates extensive sub-solidus re-equilibration at temperatures predominantly below 200 °C. Silicates and oxides in barren rocks display narrow inter-mineral Δ56Fe values consistent with high-temperature equilibrium, whereas those in mineralized rocks exhibit significant inter-mineral Fe isotopic disequilibrium, characterized by markedly lower δ56Fe values in olivine and higher δ56Fe signatures in hornblende. Positive correlations between δ56Fe and Mg# in silicate minerals, combined with selective alteration patterns and mantle-like sulfur isotopic signatures, indicate that Fe isotopic disequilibrium in mineralized rocks is not driven by fractional crystallization, crustal contamination, or hydrothermal overprinting, but rather by sub-solidus elemental exchange mediated by sulfide-associated hydrous fluids. Upon their release from sulfide melts during crystallization, these fluids induced the observed disequilibrium by facilitating Fe mobility, driving the preferential incorporation of light Fe isotopes into olivine through kinetic diffusion, while simultaneously enriching the remaining fluid in heavy isotopes to form hornblende with elevated δ56Fe values. Our findings highlight that silicate minerals in mineralized systems are modified by sulfide-associated fluids, emphasizing the need to carefully assess such fluid-mediated processes when using these minerals as petrogenetic proxies.
Abstract Volatiles play a crucial role in magmatic systems and in conditioning a magma reservoir for remobilisation and eruption. Understanding the concentrations of volatiles in a melt, how these change with time, and identifying the magma's saturation state is essential for establishing the conditions necessary for explosive eruptions of varying magnitudes and for probing the dynamics of complex crystal-rich mushy systems. Santorini (Greece) offers an excellent natural laboratory to study the variations in processes governing the development of magmatic bodies in the run-up to large Plinian eruptions and smaller inter-Plinian events over the last 250 kyr. Here, we present new volatile data from apatite Ca5(PO4)3[F, Cl, OH] for a range of Plinian and inter-Plinian explosive eruptions from Santorini, which – when combined with an iterative forward model to reproduce apatite volatile trends – allows us to estimate the melt's initial volatile concentrations and the saturation conditions under which the magmas fractionated. Our data demonstrate that while magmas from the Minoan and Cape Riva Plinian eruptions were likely water-saturated prior to eruption, this is not a consistent feature of large-scale Plinian eruptions at Santorini. Apatite volatile modelling indicates water-undersaturated conditions in the Lower Pumice 1 and Lower Pumice 2 eruptions and several other compositionally intermediate Plinian eruptions. Modelling of inter-Plinian magmas (M8-MSF, M7a, M6a, M4 and M1) shows a complex picture with both water-saturated and undersaturated conditions able to produce good fits to our apatite data. All magmas experienced significant crystal fractionation compared with the erupted magma crystallinity, indicating that melt was segregated and preferentially extracted relative to crystals to form the erupted magma. We suggest that the H2O saturation state is controlled primarily by magma intrusion or storage pressure, rather than initial volatile concentration or magma composition. We discuss the nature of the magmatic plumbing system at Santorini, and the relative control of magma compressibility and crustal relaxation versus external magma inputs in triggering eruptions.
Abstract The effects of rodingitisation on chromitite and the controls on chromium mobility during fluid–rock interaction remain poorly understood, especially along structurally controlled reaction zones where deformation enhances fluid infiltration. This study examines the combined effects of calcium metasomatism and syn-kinematic deformation at chromitite–mafic dike interfaces in the Nain ophiolitic massif, Central Iran, using electron backscatter diffraction, electron microprobe analysis, whole-rock geochemistry, and Raman spectroscopy to characterize microstructural evolution, mineral chemistry, and mineral phases across a chromitite– slightly rodingitised dike interface. The chromitite body (~15 cm thick, ~3 m long) is deformed, hosted within sheared, serpentinised harzburgite, and spatially associated with boudinaged, slightly rodingitised mafic dikes. Two chromite generations are identified: an early disseminated generation with high Cr# and Mg# reflecting supra-subduction zone crystallization, and a second generation at chromitite margins containing polymineralic silicate inclusions (clinopyroxene, orthopyroxene, olivine, pargasite, phlogopite, aspidolite, talc, chlorite, garnet, apatite, sulfides) with lower Cr# (50–59). Their restricted occurrence, hydrous mineralogy, subsolidus temperatures (<700°C), and textural affinity with the host matrix point to modification of pre-existing chromite by hydrothermal fluids, neoformation by direct precipitation from those fluids, or both. The chromitite–dike contact hosts a brecciated reaction zone (2 mm–1 cm) recording intense rodingitisation, comprising zoned grossular rimmed by Cr-rich andradite, diopside, xonotlite, clinochlore, and calcite. Thermodynamic modelling constrains this assemblage to < 450°C and oxygen fugacities of FMQ − 4 to FMQ + 3, under CH₄-dominated fluid conditions. Zoned grossular shows oscillatory Cr# variation, with spinel relicts confined to high-Cr# zones, indicating chromite-derived Al, Cr, and Fe2+. Fish-hook-shaped chromite grains and etched, garnet-sealed fractures suggest coupled dissolution–precipitation during fluid interaction. These observations support a three-stage evolution of the Nain chromitite–rodingite assemblage, involving: (i) magmatic chromitite formation in a supra-subduction setting; (ii) syn-kinematic dike emplacement, accompanied by modification of pre-existing marginal chromite by hydrothermal fluids, neoformation through direct precipitation from high-temperature hydrothermal fluids, or both—a process that generated the hydrous, alkali-rich multiphase silicate inclusions rather than through primary melt entrapment; and (iii) subsequent Ca-metasomatism below ~ 450°C, which produced garnet + chlorite + clinopyroxene + xonotlite assemblages and generated oscillatory Cr-, Al-, and Fe-zoning in garnet through nonlinear, reaction-driven, self-organizing crystallization.
Developing a full understanding of many geological and synthetic material processes, such as crustal metamorphism, mantle phase transformations, and alloy grain refinement requires knowledge of the way in which crystals nucleate within a multiphase crystalline matrix. Nevertheless, a common set of rules that predict the location and orientation of nuclei growth in crystalline reactions has not previously been described. By studying the orientation relationships in 12 natural parent–daughter pairs with varying symmetry, we illustrate that nucleation of crystalline phases in rocks can be rationalised by a model of epitaxy in which the location and orientation of nuclei maximise coincident-site lattice bond interactions with a parental phase. We suggest that epitaxy is the default (lowest energy) nucleation pathway in natural mineral reactions, which has implications for the self-organization of mineral systems, the kinetics of metamorphic reactions as well as the rheology of multiphase rocks. Broadly applied, our epitaxy model allows us to predict the microstructure of metamorphic rocks based solely on the lattice structure and chemistry of constituent phases.
Abstract Magma hybridization, the mixing and mingling of compositionally distinct magmas, is commonly interpreted as involving silicate melts and crystals, yet magmas are multiphase systems comprising also exsolved fluid phases, that may also play a role in hybridization. Through an integrated study of field relationships, microstructures, and the chemistry of major and accessory phases in hybrid dioritic rocks from the early Paleozoic Archibarca pluton (Puna Plateau, NW Argentina), we identify microstructural and geochemical evidence for the presence and involvement of an exsolved fluid phase during magma hybridization. We find that hybridized diorites record a transition from magmatic to hydrothermal conditions (~800 to 500 °C), driven by F-CO3 2− SO42− aqueous fluids exsolved from granitic magma, which trigger an autometasomatic hornblendization reaction in which biotite is replaced by hornblende and titanite, plus other phases such as quartz, zircon and apatite, and minerals typical of hydrothermal alteration such as calcite, baryte, and rutile, in response to increasing fO2 and a(SiO 2). This process promotes mobility of medium- to heavy-rare earth element (REE) and high-field-strength elements, producing spoon-shaped REE patterns in hornblende and titanite, REE-poor fluorapatite, and light-REE-enriched, altered zircon. Due to this alteration, commonly used thermobarometers and chemometers yield conditions outside the true magmatic conditions, reflecting disturbed rather than pristine conditions, hampering the use of petrochronologic tools as reliable indicators of magmatic crystallization timing and conditions. Simultaneously, these fluids enrich surrounding rocks in incompatible elements, fostering local geochemical hybridization. At the meso- and microscales, the microstructures resulting from hornblendization provide direct evidence of fluid migration through crystal mushes, recording both chemical and mechanical signatures of reactive transport. Features such as hornblende-titanite selvedges, patchy replacement textures, quartz ocelli, and hornblendite dykes serve as diagnostic markers of fluid–crystal–melt interaction. These structures could serve as key indicators of fluid transfer and autometasomatism in other hybridized calc-alkaline magmatic bodies and for reconstructing volatile evolution and element redistribution during pluton assembly.
In most models of terrestrial magma ocean solidification, crystals are said to accumulate above the core-mantle boundary to form a crystal mush that solidifies under equilibrium conditions. Large mafic-ultramafic crustal intrusions crystallise differently. They are dominated by adcumulates that are composed almost entirely of cumulus minerals, and the residual melt undergoes fractional crystallisation. In this study, we draw on insights from studies of crustal magma chamber processes to propose a new model for the solidification of the magma ocean. Our model takes into account the phase relations of ultramafic melt under lower mantle conditions, the presence of a zone of neutral crystal buoyancy located in the lower magma ocean, and the Coriolis force, which organizes convection into separate zones aligned parallel to the planet's axis of rotation. In this scenario, crystals initially accumulate in two annular septa in the deeper part of the magma ocean, one in each hemisphere. A combination of composition-driven convection within the crystal mush and compaction then expels melt from the septa into the magma ocean. Early on, melt circulates between different parts of the magma ocean through gaps between the septa. Once these gaps close, the two hemispheric regions evolve independently.
We investigate relationships between the chemistry of global mid-ocean ridge basalts (MORBs), the conditions and composition of their source mantle, and the physical properties of the ridges. To accomplish this, major and trace element compositions of primary MORB are estimated by a back-calculation code PRIMACALC3 assisted by a thermodynamically constrained forward fractional crystallization model COMAGMAT 3.72 with an additional PRIMELT3-P model to ensure mantle equilibrium of the primary melts. We then apply Ocean Basalt Simulator version 1, a thermodynamically constrained forward calculation code for adiabatic melting of a pyroxenite-bearing mantle peridotite, to the trace element compositions of our primary MORBs. Melting conditions and compositions of the MORB source mantle, such as pressure, degree of partial melting, mantle potential temperature, fractions of pyroxenite and ambient depleted mantle, and the degree of depletion of the final residual mantle (F-PD) are estimated by the two models. The results show that (1) MORBs are derived from mantle with T-p = 1250-1520 degrees C at F = 3-31%, and both are high beneath shallow ridges; (2) pyroxenites in the source mantle peridotite are both recycled MORBs and ocean island basalts (OIBs) in origin and their fractions are estimated to be 0-6% with a high fraction in deep and cold ridges; (3) the fraction of OIB-type pyroxenite is highest at shallow and hot ridges; (4) the degree of depletion of the residual peridotite is highest at shallow and hot ridges, and (5) the isotopic compositions of the OIB-type pyroxenites have both EM1 and HIMU signatures, while that of the MORB-type pyroxenites and the ambient mantle peridotites have depleted compositions. The high-T-p mantle containing recycled OIB-type pyroxenite has a deep plume origin. It forms ridges shallower than 2500 m. b. s. l. due to thermal and chemical buoyancy relative to the ambient upper mantle. In contrast, deeper ridges have a narrower and lower T-p range, and a lesser but variable amount of peridotite depletion. The depth of mid-ocean ridges is controlled by both T-p and relative mantle fertility.
Continental alkaline magmatic provinces are notable for their exceptional compositional diversity, making them valuable recorders of mantle source heterogeneity, magma differentiation processes, and volatile behavior. Some of these provinces occur in intraplate settings where evidence for rifting or plume activity is absent, leaving the cause of mantle melting and magma ascent enigmatic. Here, olivine-hosted melt inclusions provide insight into the source-to-surface evolution of the Ramon volcanics, the most diverse series of the Early Cretaceous intraplate Levant magmatic province, whose origin is enigmatic. Most data are derived from the well-preserved nephelinite-basanite-alkaline basalt series of the Ga'ash Hill edifice, complemented by additional exposures in Makhtesh Ramon, Negev Desert, Israel. Trace-element systematics indicate that compositional diversity is mostly dominated by low-degree melting of a garnet-bearing mantle source near the garnet-spinel transition, with contributions from peridotite and pyroxenite lithologies. Melt inclusions are CO2-rich (<= 3.19 wt %), while H2O contents are moderate with reconstructed values of up to 0.95 wt %. Volatile saturation pressures recorded in melt inclusions span a wide range (1300-230 MPa), further constrained by clinopyroxene geobarometry to indicate magma storage at lower-crustal levels (similar to 25-35 km), near the reconstructed Moho. Fluid-behavior modelling suggests a two-stage volatile control on melt mobilization: CO2 dominating deep transport and H2O becoming increasingly important at shallow levels below similar to 250 MPa, where eruptibility increases. Minimal interaction between ascending melts and crustal rocks is indicated by oxygen isotope ratios of olivine phenocrysts. Compositional diversity stems from the melting of a fertile lithospheric mantle source previously affected by carbonate-rich metasomatism, deep storage, and rapid ascent. Small perturbations in tectonic or thermal conditions may be sufficient to generate alkaline mafic melts from readily fusible carbonate- and pyroxenite-bearing lithospheric mantle, thus providing a possible explanation for magmatism in stable continental interiors that lack structural evidence for extension.
We apply numerical models of melt and magma transport through continental and arc crust to explore controls on trans-crustal magmatism. Our results suggest that repeated intrusion of mantle-derived basaltic magma in the deep crust produces vertically extensive, mush-dominated magmatic systems comprising individual reservoirs connected by episodic magma transfer via dykes. Upwards percolating melt differentiates and accumulates in each reservoir to form magma that can evacuate via magma-driven fractures if critical thresholds of buoyancy and melt fraction are reached. Melt and mush can also be generated by partial melting and assimilation of fertile crust. Processes in each reservoir control the size, frequency and composition of magma batches that ascend and intrude at shallower depth. Most simulated magma evacuations are temporally, spatially and compositionally decoupled from new intrusions, triggered by buoyant melt accumulation by reactive percolative flow. Other evacuations are a mix of intruded and rejuvenated magma triggered by new intrusions increasing the melt fraction (thermal rejuvenation) or buoyancy (buoyancy rejuvenation). Focusing of magma, as it evacuates from deeper reservoirs to intrude shallower reservoirs with smaller area, facilitates long-lived, upper-crustal magmatism by increasing the areal magma flux into the upper crust. The lower- to mid-crust is the most favourable environment for differentiation, generating evolved magmas that supply shallower parts of the system. System architecture is highly dynamic: reservoir depths vary through time and different parts of the system are magmatically active at different times. We do not observe the formation of a continuous, trans-crustal mush reservoir in any tested scenarios.
Biotite is a key hydrous silicate mineral in evolved magmatic systems, but its control on the behaviour of minor- and trace-elements, in particular Li, Nb, F and the REE is not well understood. Here, we quantify that control in sodic (per)alkaline ${ ext{H}_{2} ext{O}}$-saturated magmas with variable F-content through crystallisation experiments at 650-800${}<^>\circ $C and 200 MPa total pressure, at log $f$O$_{2}$ $\approx $ FMQ +1. Biotite-glass pairs from tephriphonolite to phonolite fall deposits from Tenerife, Canary Islands, and a broad compilation from literature, complement our experimental data set. The new biotite-melt $Kd_{ ext {F/OH}}$ exchange coefficients are 2.9-47.0, typically 6.5-12.5, with minima for Al-rich, Mg-poor biotite. Nernst partition coefficients ($D$) for lithium are 0.24-32.8 with minima in F-poor biotite formed at high temperatures from peraluminous silicate melt. $D$ values for the large alkali metal ions Na-Cs define Onuma parabolae consistent with their incorporation on the biotite ${{}<^> ext{XII} ext{A}}$-site. Niobium partition coefficients are 0.1-1.2, and highest in Ti-rich biotite. The REE and actinides have $D$ values less than 0.01 as their ionic radii fall between the size of the ${{}<^> ext{XII} ext{A}}$-site and ${{}<^> ext{VI} ext{M}}$-sites of biotite. Our data, alongside a literature compilation, constrain empirical models that: (1) describe the exchange of F and OH between the silicate melt and the biotite W-site; (2) predict the partitioning of 1+ cations Li-Cs between silicate melt and the biotite A- and M-sites; (3) predict ${D_ ext{Nb}}$ values. The models use the major-element composition of biotite and silicate melt, pressure and temperature as input. Models are calibrated for use over a wide range of pressure, temperature and bulk composition ($P$-$T$-$X$) and can be used to interrogate biotite from natural systems to determine the composition of their source melt, or to forward model the trace-element evolution of mafic to evolved peralkaline syenite or peraluminous granite systems at upper-mantle to crustal pressures.
Layered intrusions are the crystallised remnants of magma reservoirs and preserve a detailed record of magma storage, differentiation, and recharge processes in the upper crust. Their assembly is commonly categorised into two end-member emplacement regimes: long-lived liquid-dominated magma chambers and incrementally assembled crystal-rich mush systems. These end-members are often presented as competing concepts despite intrusions necessarily spanning a range of crystal fractions as they cool and crystallise. The Carlingford Complex (Co. Louth, Ireland) preserves the shallow-crustal architecture of a volcanic centre emplaced during plume-related rifting of the North Atlantic, thus providing the opportunity to evaluate contrasting models of subvolcanic magma reservoir assembly. Integrated field observations, microstructural analysis, mineral and bulk-rock geochemistry define four stratigraphic zones that record systematic changes in emplacement behaviour. High-resolution sampling of outcrop and drill core material reveals a previously unrecognised Lower Zone that extends the intrusion downwards and represents its most mafic portion, preserving evidence for pulsed replenishment and accumulation within an open, liquid-dominated environment. The overlying Middle Zone marks the transition to a phase of rapid inflation in a largely closed system, with microstructural and geochemical trends indicating intrusion-scale crystal settling, flotation, and convection. Upper Zone A records chaotic emplacement of multiple, laterally discontinuous sill-like intrusions in an open, crystal-rich environment. Upper Zone B mirrors the geochemical trends of the Middle Zone but is locally disrupted by small-scale intrusions and microstructures consistent with late-stage fluid infiltration. Together, these variegated petrological zones provide compelling evidence that subvolcanic systems can alternate between liquid- and crystal-dominated emplacement regimes over relatively short spatial and temporal intervals, highlighting their sensitivity to changes in melt flux and thermal state. These findings have direct implications for understanding magma supply to volcanoes and the formation of associated mineral deposits.
Metaserpentinites are recognized to play an important role in subduction processes, through their impact on element cycling and rocks' mechanical properties. At high-pressure conditions, most of the metamorphic reactions that affect metaserpentinites are temperature-dependent, preventing accurate thermobarometric estimates in these rocks and thus limiting the ability of relating natural observations to subduction processes. Using the example of the Mt. Avic in the Western Alps, we herein attempt to constrain a P-T path in an eclogite-facies ultramafic massif using a twofold approach: (1) by conducting thermobarometric estimates in associated rock types (metamafic rocks, metasediments) and (2) by implementing humite-group minerals in thermodynamic databases to enhance P-T estimates in metaserpentinites. Petrographic observations indicate that all three rock types are characterized by well-preserved eclogite-facies assemblages, notably highlighted by the remarkable presence of large humite-group minerals (titanian-clinohumite and titanian-chondrodite) in metaserpentinites. Thermobarometric estimates (Raman spectroscopy on carbonaceous material thermometry, thermodynamic modeling) highlight the homogeneity of peak P-T conditions recorded by the three rock types (mostly in the range 2.3-2.6 GPa and 510-580 degrees C), and across the study area. The widespread formation of humite-group minerals in the Western Alps therefore allows to constrain the evolution of the Liguro-Piemont oceanic lithosphere from titanian-clinohumite crystallization during prograde burial, to titanian-chondrodite crystallization at peak pressure conditions and to humite minerals breakdown and subsequent reserpentinization during exhumation. The widespread occurrence of titanium-bearing minerals and enrichment in titanium compared to mantle protoliths highlight titanium mobility at high-pressure conditions during subduction.
Composite granitoid plutons record multiple magmatic processes; however, how these processes relate to lithological diversity remain unclear. This study addresses this question through a detailed examination of multiple zircon populations from the Zhongchuan composite pluton in the Western Qinling Orogen, Central China. The Zhongchuan pluton comprises granodiorite, porphyritic monzogranite, biotite monzogranite, muscovite monzogranite and tourmaline monzogranite. Combining zircon textural features, U-Pb ages, Hf-O isotopes and thermometry, three zircon populations were identified, i.e. inherited, autocrystic and antecrystic. Inherited zircon grains from the monzogranites exhibit age-epsilon(Hf)(t) spectra matching that of the detrital zircon from regionally Paleozoic strata, suggesting significant incorporation of metasedimentary protolith. Autocrystic zircon grains from the granodiorite and porphyritic monzogranite mostly exhibit restricted epsilon(Hf)(t) values (-4.5 to - 1.1), similar to those of the zircon from regionally coeval I-type granites, consistent with derivation from a meta-igneous protolith. In contrast, autocrystic zircon grains from the biotite monzogranite, muscovite monzogranite and tourmaline monzogranite show highly variable delta O-18 (+8.3 to + 11.3 parts per thousand) and epsilon(Hf)(t) (-4.9 to + 5.2), indicating an open magmatic system. Particularly, many autocrystic zircon grains from these rocks exhibit epsilon(Hf)(t) values (+1.5 to + 5.2) similar to those of the zircon (epsilon(Hf)(t) = +1.4 to + 5.4) from Triassic diorite, indicating involvement of dioritic magmas. The negative correlation between epsilon(Hf)(t) and Hf for the autocrystic zircon from the muscovite monzogranite and biotite monzogranite further testifies to the mixing of dioritic magmas with the magmas derived from metasedimentary protolith. The tourmaline monzogranite likely incorporated an additional meta-igneous component, as indicated by the lack of a clear epsilon(Hf)(t)-Hf correlation in autocrystic zircon and the similarity of epsilon(Hf)(t) and delta O-18 values between some autocrystic zircon grains and those from the granodiorite. Antecrystic zircon grains show low Eu/Eu* ratios (<0.4) and variable epsilon(Hf)(t) (-5.9 to + 6.9) and delta O-18 (+7.8 to + 12.4 parts per thousand) values, reflecting crystallization from deep, heterogeneous mushy magma reservoirs before transporting to a shallow reservoir. Therefore, compositional and lithological diversity of the Zhongchuan composite pluton can be attributed to hybridization of magmas derived from meta-igneous and metasedimentary protoliths and dioritic magmas within trans-crustal mushy magma reservoirs. Our finding highlights the utility of zircon populations in revealing the cryptic mixing processes that contribute to the formation of composite granitoid plutons.
The oxidation state of granitic magma is a key parameter in granitoid petrogenesis, with important implications for related mineralization; however, the mechanisms underlying redox variations in magma sources remain poorly understood. In the Songpan-Ganzi terrane of the northeastern Tibetan Plateau, Late Triassic granitoids exhibit striking variations in oxygen fugacity (fO(2)), reflecting diverse source compositions and melting conditions. These variations provide critical insights into the petrogenetic processes reflected in magmatic redox conditions. Integrating whole-rock element geochemistry, Sr-Nd-Hf-O isotopes, zircon, and apatite trace elements reveal that oxidized granitoids (FMQ + 0.05 to + 1.71, where FMQ is the fayalite-magnetite-quartz buffer) were derived from partial melting of the Neoproterozoic Yangtze basement, while reduced granitoids with A/CNK < 1.1 (FMQ -2.50 to -0.17) formed through mixing between the Neoproterozoic Yangtze basement and Paleo-Tethyan metasedimentary components. The reduced granitoids with A/CNK >= 1.1 (FMQ -4.75 to -1.06), closely linked to lithium mineralization, formed through anatexis of Paleo-Tethyan metasedimentary rocks at high temperatures (similar to 700-850 degrees C), as constrained by Ti-in-zircon thermometry. These redox variations are mainly inherited from the source rather than controlled by magmatic differentiation. The Li- and rare metal-rich metasedimentary sources, coupled with high temperatures and reduced conditions, provided favorable physicochemical conditions for Li enrichment during the late-stage evolution of granitic magmas. This study provides new constraints on the magmatic redox evolution in contemporaneous diverse granitoid magmas and highlights the contribution of a metasedimentary source to lithium enrichment, shedding light on the broader geodynamic evolution of the northeastern Tibetan Plateau.
In situ U-Pb geochronology, Sm-Nd isotopic analyses, and geochemistry of mafic and ultramafic rocks of Matthews Ridge, NW Guyana: new insights into the geodynamic evolution of the western Paleoproterozoic greenstone belt of the Guiana Shield. We investigated a suite of metamorphosed mafic and ultramafic rocks from Matthews Ridge (MR), NW Guyana, to characterize their magmatic history through geochronology and geochemistry. Three petrographic groups were defined: MR-1 (meta-gabbros), MR-2 (meta-ultramafic cumulates), and MR-3 (meta-basalts). U-Pb SIMS dating of zircon and baddeleyite yields ages of 2100 +/- 35, 2125 +/- 25, and 2152 +/- 42 Ma for MR-1, and 2222 +/- 12 Ma for MR-2. Although undated, MR-3 displays Nd isotopic signatures suggesting coeval formation with MR-2. All petrographic groups exhibit low, uniform Th/La (0.07-0.10) and Sm/La (0.5-1.1), consistent with derivation from a depleted mantle source. Slightly positive initial epsilon Nd values indicate interaction between depleted and enriched mantle reservoirs. Comparisons with other Paleoproterozoic greenstone belts in the Guiana Shield show that western domains, including Matthews Ridge, record less crustal or sedimentary input than those to the east. Tectonic discrimination diagrams reveal little arc affinity, except for two MR-1 samples. Instead, geochemical features support an intra-oceanic origin: MR-3 likely formed at a mid-ocean ridge (similar to 2.22 Ga), whereas high-FeTi rocks in MR-1 and MR-3 reflect mantle plume influence similar to 2.1 Ga. This supports a model where Matthews Ridge magmatism reflects oceanic plateau development, similar to Block B of the El Callao Mining District in Venezuela.
High-density fluid (HDF) micro-inclusions commonly found in fibrous diamonds offer a unique opportunity to explore the metasomatic events in the lithospheric mantle, during which the micro-inclusions and their host diamonds formed. It has been suggested that saline and silicic HDFs are immiscible due to the absence of intermediate compositions between these two end-members. In the present study, however, SiO2- and Cl-rich HDFs with compositions parallel to the saline-silicic array were identified at the core-coat boundaries of coated diamonds from the Democratic Republic of the Congo (DRC). HDFs in these diamonds exhibit a consistent evolution from intermediate saline-silicic compositions at the core-coat boundary to more silicic ones in the inner layer, and then to more carbonatitic ones in the outer layer. Nitrogen aggregation thermochronology, combined with diamond-inclusion isomekes and orthopyroxene thermobarometries, yields P-T ranges of 4.8-6.6 GPa and 1060-1200 degrees C for the cores, and 4.1-6.5 GPa and 970-1020 degrees C for the coats and included HDFs. These ranges correspond respectively to the higher and lower parts of the P-T range for octahedral diamonds from DRC. Compositional characteristics of these HDFs and features of different growth layers indicate that the HDF evolution spanning over a significant range of saline-silicic-carbonatitic compositions, along with the growth of different fibrous layers is the result of the continuous variation of source H2O/CO2 ratios during a metasomatic event. Cl-rich silicic fluids are possible to form in the mantle under high source H2O/CO2 ratios, which may not be conducive to diamond crystallization. This study provides new insights into the features and evolution of mantle fluids and reveals potential connections between diamond crystallization and fluid composition.
An integrated petrographic, mineralogic, geochronologic, and phase equilibrium modeling study of Jingshan granites reveals four distinct metamorphic-anatectic-magmatic-hydrothermal events recorded in zircon and garnet. Zircon geochronology identifies Paleoproterozoic, Neoproterozoic, and Triassic (240-200 Ma) ages in inherited domains, alongside Late Jurassic (similar to 160 Ma) ages in anatectic/magmatic domains. These data indicate that the Jingshan granites originated from remelting of crustal materials that experienced Triassic metamorphism. Four generations of garnet are distinguished. Metamorphic garnet (Grt I), formed during Triassic metamorphism, exhibits flat to steep middle to heavy rare earth element (M-HREE) patterns. Peritectic garnet (Grt II), produced during Late Jurassic crustal anatexis, displays Y-HREE-enrichment and high (Yb/Dy)N ratios. Magmatic garnet (Grt III), crystallized during magma cooling, exhibits negative Eu anomalies and flat to steep M-HREE patterns. Hydrothermal garnet (Grt IV), formed during the late magmatic-hydrothermal stage, shows low TiO2 contents and positive Eu anomalies. Phase equilibrium modeling constrains partial melting conditions to 720-755 degrees C and 1.2-1.3 GPa (corresponding to depths of similar to 38-44 km in the lower crust), occurring via the fluid-present melting reaction: Ms + Pl + Qz + Ilm + H2O = Kfs + Mag + Melt, followed by the fluid-absent melting reaction: Ms + Bt + Pl + Qz = Grt + Kfs + Melt. The protoliths underwent deep subduction and metamorphism during the Triassic, followed by exhumation to lower crust levels and subsequent remelting in the Late Jurassic. The fluid-present melting was likely triggered by aqueous fluid released from the underlying metamorphic rocks during the westward low-angle subduction of the Paleo-Pacific plate in the Jurassic. The resulting magmatism formed the Jingshan granites along the southeast margin of North China Block. These granites provide a compelling example of crustal recycling and rejuvenation during orogenesis, offering key insights into the evolution of continental crust during craton destruction.
The Jacupiranga carbonatite (Brazil) was investigated to better understand how modal proportions and mineral chemistry control the oxygen fugacity (fO(2)) recorded by a natural carbonatite rock. Mineral compositions were determined using scanning electron microscopy with energy dispersive spectroscopy, electron probe microanalysis with wavelength dispersive spectroscopy, and Raman microspectroscopy building upon detailed petrography. Based on magnetite-ilmenite oxythermobarometry, rock-forming temperatures range from 505 to 732 degrees C, with a wide span offO2 values (Delta FMQ-1.21 to +4.56). In each sample, ilmenite grains and lamellae are distinguishable, with the latter recording higher temperature-fO(2) conditions. Consistently, independently determined homogenization temperatures of apatite-hosted fluid inclusions show a similar range between 550 and 685 degrees C. From the wall-rock contact toward the carbonatite body, samples exhibit a decrease in the modal abundance of olivine, phlogopite, magnetite, ilmenite, and dolomite, alongside an increase in Mg-calcite. Over the same spatial gradient, temperature, fO(2), and the magnesioferrite (in magnetite) and geikielite (in ilmenite) components decrease. During the antiskarn reaction, silica and mafic components (Mg, Fe-T, Al, and Ti) derived from the ultramafic wall-rock were incorporated into olivine, phlogopite, magnetite-ilmenite, and dolomite. Calcite-rich samples, which were less influenced by the wall-rock, preserved ephemeral alkali components of the carbonatite melts within burbankite. Apatite-hosted fluid inclusions represent coeval fluid modified by antiskarn reactions. Adjacent to equilibrium magnetite-ilmenite pairs, olivine and phlogopite exhibit high mg# (93-97 and 88-95, respectively). Despite the broad span of Delta FMQ values recorded by the oxide pairs, we found only a weak positive correlation between the recorded fO(2) and mg#. Consequently, we suggest that the high mg# (>88-90) of carbonatitic olivine is primarily controlled by the low (<0.2) KDFe2+ /Mg (olivine-carbonatite) melt D exchange partition coefficient between olivine and the carbonatite melt) at crustal Fe2+ -Mg conditions. The exceptionally high mg# of olivine and phlogopite, compared to the adjacent dunite cumulates (mg# 84-88), combined with their negligible Ni and Cr contents, indicate an in situ antiskarn process. This study demonstrates that carbonatite systems can be characterized by dynamic redox conditions, spanning the range from Fe3O4-Fe2O3 to Fe2SiO4-Fe3O4-SiO2 during their evolution and emplacement.
