Contributions to Mineralogy and Petrology最新文献

We performed ¹H and ²⁹Si NMR and infrared measurements, and first-principles calculations to clarify the nature of OH defects in MgSiO₃ orthoenstatite. An orthoenstatite sample synthesized at 7 GPa and 1200 °C from a composition of MgSiO₃ + 0.1 wt% H₂O yielded two ¹H MAS NMR peaks near 5.9 and 7.6 ppm that are correlated in 2D NMR spectra, and two infrared bands near 3361 and 3066 cm^− 1 that correspond to the previously reported A3 and A4 bands. The first-principles calculations confirmed that they are due to a pair of protons in a Mg (M2) vacancy. The previously reported A1 and A2 infrared bands near 3687 and 3592 cm^− 1 for orthoenstatite synthesized at low silica activities were confirmed to arise from four protons in a SiB vacancy. The latter is predicted to give two additional OH stretching bands associated with two strongly hydrogen-bonded O3b-H bonds with frequencies below the spectral range reported thus far. The previously reported infrared absorption coefficients were thus revised to account for the undetected bands. ¹H NMR may be used to quantitatively detect all four protons (expected at 1–12 ppm). Other mantle minerals should also be examined for potentially overlooked OH defects with strong hydrogen bonding.

This work investigates the relationships between partial melting, melt extraction, pluton growth and silicic volcanism in garnet-bearing felsic volcanic rocks that were extruded in the Iberian Pyrite Belt, at ca. 345 Ma. The garnets are of peritectic origin, displaying textural and chemical features of disequilibrium crystallization during partial melting reactions involving biotite at high temperatures (up to 870 °C) in the middle-lower crust. Major element composition suggests compositional equilibrium with the entrained and pinitized peritectic cordierite, but reveals some subsequent homogenization by diffusion. Trace element maps and spot analyses of garnet show, nonetheless, significant trace element variations, reflecting biotite and Y-REE-P-rich accessory phase breakdown during partial melting reactions. Peritectic garnet and cordierite growth resulted in the preservation of Th- and Y-rich prograde suprasolidus monazite, which constrains the timing of partial melting of the metapelitic protolith at ca. 356.8 ± 2.4 Ma. The zircon cargo further shows that a significant amount of zircon crystals from previously crystallized felsic melts were also remobilized and erupted. These were likely stored in an upper crustal pluton that grew episodically since ca. 390 Ma during voluminous melt generation periods within the middle to lower crust, which also resulted in voluminous volcanism. The geochemical trends of the felsic volcanic rocks reflect the entrainment of xenoliths of peritectic garnet, cordierite and feldspar, and as such, the garnet-bearing felsic volcanic rocks represent an erupted mixture of a lower-temperature (ca. 770 °C) silicic melt and autocrysts, and peritectic phases and zircon crystals from previously crystallized and stored felsic melts.

Orogenic peridotites in the crystalline basement of the northwestern Bohemian Massif contain multiphase solid inclusions (MSI), which are interpreted to be crystallisation products of trapped former carbonate–silicate melts metasomatizing their host rocks. We applied conventional thermobarometry and forward thermodynamic modelling to constrain the P–T evolution ranging from the peak metamorphic conditions of the investigated harzburgite and lherzolite, through entrapment of the melts in the outer parts of garnets, to the (re)-equilibration of the MSI assemblages. The peak conditions of c. 1100 °C/4.5–5.5 GPa are recorded by garnet cores and large pyroxene porphyroclasts. The melt entrapment, during which garnet outer parts grew, was associated with influx of the metasomatizing liquids and probably took place during the early stage of the exhumation. Thermodynamic model of amphibole-free MSI assemblage comprising kinoshitalite/Ba-rich phlogopite (approximated by phlogopite in the model), dolomite, magnesite, clinopyroxene, orthopyroxene, garnet and chromite provided robust estimate of P and T of its (re)-equilibration, c. 900–1000 °C, 1.8–2.2 GPa. Furthermore, the lack of olivine reflects co-existence of COH fluid with high X(CO₂) = CO₂/(CO₂ + H₂O) ≥ 0.6. Models employing identical P–T–X(CO₂) parameters successfully reproduced the other two amphibole-bearing assemblages observed. The modelled stability fields show perfect alignment with a characteristic isobaric segment of the solidus curve of carbonated peridotite. This co-incidence implies that the (re)-equilibration corresponds to the termination of the melt crystallisation once the near-isothermal exhumation path intersected the solidus. Decreased solubility of silicates at the carbonated peridotite “solidus ledge”, inferred from the published experimental data, as well as concentric textures of some MSI indicates sequential crystallisation from the early silicates to late dolomite. The carbonated “solidus ledge” is a relatively narrow boundary in the lithospheric mantle capable of an abrupt immobilisation of fluxing or transported carbonated melts. The investigated rocks are estimated to store approximately 0.02 kg C/m³ (or 6 ppm C) occurring as carbonates in the MSI.

Understanding the compositional heterogeneity of the deep mantle requires identifying the nature of recycled crustal materials in the sources of mantle-plume-related magmas. However, it is still unclear whether or not the deep mantle contains recycled carbonates from the Earth’s surface. In this study, we present comprehensive data on whole-rock high-precision zinc isotopes, as well as major- and trace-element geochemistry, and Sr–Nd–Pb isotopes of basalts on Hainan Island to examine the influence of recycled materials (particularly carbonates) on the mantle source heterogeneity of the Hainan mantle plume. The basalts have highly variable δ⁶⁶Zn values ranging from 0.21‰ to 0.42‰. These variable Zn isotopic compositions cannot be accounted for by processes such as post-magmatic alteration and crustal contamination, or by fractional crystallization and partial melting; instead, they reflect mantle heterogeneity. Comparisons of the major- and trace-element compositions (e.g., CaO and TiO₂ contents and Zn/Fe and Fe/Mn ratios), FC3MS and FCKANTMS peridotite and pyroxenite melting parameters, as well as pseudo-ternary projections of the primary Hainan basaltic magmas with experimental data suggest that the primary magmas were partial melts of silica-deficient pyroxenitic lithologies with peridotite residue. The heterogeneous geochemical and lithological compositions of the Hainan basalts indicate that recycled sedimentary carbonates and siliceous rocks were important constituents in their mantle source. Quantitative modeling reveals that the addition of 1–10% subducted sediments into the source of the Hainan basalts closely reproduces their Zn–Sr–Nd–Pb isotopic values. The source component with the heaviest Zn isotopic composition measured for the Hainan basalt samples could have contained more than 9% recycled carbonate. Our findings provide insights into the role of subducted materials to mantle heterogeneity and highlight the contribution of subducted sedimentary carbonates in the deep recycling of oceanic slabs, including—in the case of the Hainan mantle plume—recycled deep mantle (i.e., the mantle transition zone and lower mantle).

A model is presented whereby metamorphic parageneses are governed by local, nano-scale reactions among adjacent phases along grain boundaries that are driven by local disequilibrium between the solid phases and the grain boundary composition. These reactions modify the grain boundary composition setting up compositional gradients that drive diffusion and change the grain boundary composition elsewhere in the rock, which drive local reactions in these locations. The process may be triggered by the nucleation of a new phase that is out of equilibrium with the existing assemblage and an example is presented based on the transformation of kyanite (Ky) to sillimanite (Sil). Model results reveal that a simple polymorphic transformation (Ky→Sil) can result in local reactions among all phases in the rock and some phases may grow in one locale and be consumed in another. An implication of these results is that interpretation of metamorphic parageneses based on growth or resorption and compositional changes of phases requires careful evaluation of nano-scale processes.

Abstract

U–Pb geochronology of shocked monazite can be used to date hypervelocity impact events. Impact-induced recrystallisation and formation of mechanical twins in monazite have been shown to result in radiogenic Pb loss and thus constrain impact ages. However, little is known about the effect of porosity on the U–Pb system in shocked monazite. Here we investigate monazite in two impact melt rocks from the Hiawatha impact structure, Greenland by means of nano- and micrometre-scale techniques. Microstructural characterisation by scanning electron and transmission electron microscopy imaging and electron backscatter diffraction reveals shock recrystallisation, microtwins and the development of widespread micrometre- to nanometre-scale porosity. For the first time in shocked monazite, nanophases identified as cubic Pb, Pb₃O₄, and cerussite (PbCO₃) were observed. We also find evidence for interaction with impact melt and fluids, with the formation of micrometre-scale melt-bearing channels, and the precipitation of the Pb-rich nanophases by dissolution–precipitation reactions involving pre-existing Pb-rich high-density clusters. To shed light on the response of monazite to shock metamorphism, high-spatial-resolution U–Pb dating by secondary ion mass spectrometry was completed. Recrystallised grains show the most advanced Pb loss, and together with porous grains yield concordia intercept ages within uncertainty of the previously established zircon U–Pb impact age attributed to the Hiawatha impact structure. Although porous grains alone yielded a less precise age, they are demonstrably useful in constraining impact ages. Observed relatively old apparent ages can be explained by significant retention of radiogenic lead in the form of widespread Pb nanophases. Lastly, we demonstrate that porous monazite is a valuable microtexture to search for when attempting to date poorly constrained impact structures, especially when shocked zircon or recrystallised monazite grains are not present.

We present a reconstruction of the physicochemical conditions of melts in the Pleistocene storage and plumbing system of the Doña Juana Volcanic Complex (SW Colombia): a poorly known, potentially active polygenetic volcano of dacitic composition comprising four major edifices and showing periods of long quiescence. Compositional data for plagioclase, amphibole, pyroxene, and Fe-Ti oxides were combined with new and existing whole-rock data from representative eruptive products, allowing for the implementation of equilibrium tests and geothermobarometry calculations within an established stratigraphic, petrographic, and geochronological framework. Textural and geochemical variabilities of all mineral phases suggest the existence of a trans-crustal magmatic system feeding the Pleistocene eruptions of Doña Juana, and cyclic rejuvenation of a crystal mush following each volcano edifice collapse. The assemblage of different crystal cargos before magma recharge and final eruption is attested by (i) the coexistence of equilibrium and disequilibrium textures and variable compositions in crystals of all studied species, (ii) felsic cores in antecrysts, (iii) mafic overgrowth rims, and (iv) significantly less differentiated microcrysts relative to the composition of meso- and macrocrysts. By integrating multiple mineral-only and mineral-liquid geothermobarometers, after careful textural analyses, we estimate the intensive parameters of the mush–melt interaction zone of the plumbing system in the middle crust, providing a preliminary view of the architecture of a trans-crustal magmatic system in a complex tectonic setting at a previously understudied area of the north-Andean volcanic zone.

The trace-element and isotope geochemistry of rutile are robust tools to determine metamorphic temperatures, age, and host-/source lithologies. The use of rutile as single grain indicator for pressure, temperature, time and composition (P–T–t–X) of the host rock, which is vital in the use of detrital rutile to trace plate-tectonic regimes throughout Earth’s history, requires the identification of a pressure dependent trace element in rutile. We investigate the pressure dependence of hydrogen in rutile using polarized in-situ Fourier Transform Infrared (FTIR) spectroscopy. H₂O contents in rutile vary between < 10–2500 μg/g H₂O with higher contents in samples with higher peak metamorphic pressures, making H₂O-in-rutile a viable pressure indicator. The highest H₂O contents at ~ 450–2000 μg/g are observed in mafic low temperature eclogite-facies rutile related to modern-style cold subduction conditions. Hydrogen zoning in FTIR maps indicates that H⁺ is retained at temperatures below 600–700 °C. Ratios of H₂O/Zr, using H₂O as pressure indicator and Zr as temperature proxy, are a proxy for thermal gradients of metamorphic rutile (i.e. P/T). Low temperature eclogite samples are also characterized by high Fe contents and therefore Fe/Zr-ratios might be used as a first order approximation for H₂O/Zr-ratios to identify mafic low temperature eclogite facies rutile. Based on common discrimination diagrams, Nb, W, and Sn can be used to distinguish different host/source rock lithologies of rutile. Combining both H₂O/Zr-ratios and Nb, W, and Sn contents can thus identify modern-style cold subduction signatures in rutile. The developed systematics can consequently be used to trace cold-subduction features in the (pre-Proterozoic) detrital record.

Lawsonite is important as a carrier of H₂O in subduction zones and as a petrological tracer. The trace-element content of lawsonite in mafic rocks has been used as a record of fluid–rock interactions but has received less attention in metamorphosed oceanic sediments. This study documents the major, and trace-element composition, together with ⁸⁷Sr/⁸⁶Sr isotopic ratios, of the different lawsonite types identified in the upper units of the Schistes Lustrés complex of the Western Alps, a paleo-accretionary prism of the Liguro-Piemont slow-spreading ocean subducted up to 40 km. The lawsonite-rich upper units of the Schistes Lustrés complex are principally composed of metamorphosed pelitic sediments and carbonates. Lawsonite content reaches 40 vol.% in the rock matrix and in veins. All lawsonite types originate from prograde metamorphic reactions which occurred up to peak metamorphism. Lawsonite compositions have been measured in situ with electron microscopy, microprobe, and laser-ablation mass spectrometry. Lawsonite separates have been measured for Sr content and ⁸⁷Sr/⁸⁶Sr isotopic ratios using TIMS. Bulk rock compositions have been measured with ICP-OES and ICP-MS. Analysis contamination by minute retrograde white mica inclusions in lawsonite crystals precluded using univalent cations for petrological interpretations. For other trace elements, the variability of lawsonite appears extremely high, with crystal compositions varying between samples and between crystals in individual samples. However, clear patterns emerge between lawsonite types, reflecting equilibrium and out-of-equilibrium processes unrelated to pressure–temperature conditions. At crystal scale, textural hourglass zoning predominates in lawsonite from the schist, while growth zoning is found in both schists and veins. The combination of both mechanisms results in spectacular zoning in Ti and in rare-earth element contents spanning four orders of magnitude. Over time, the La/Yb ratio decreases strongly (from ~100 to ~1), as La appears much more sensitive to surface effects leading to textural hourglass zoning. Interface-coupled dissolution–precipitation also contributes to decoupling of less mobile elements in the schist. This is best observed for Ti, but rare-earth element and chromium contents are affected too. These processes are considered to occur near closed-system conditions. Late lawsonite overgrowths with higher Sr contents are interpreted as reflecting system opening in the veins, yet sometimes observed in the schists. Strontium isotopic ratios measured on separated lawsonite crystals and on bulk rocks also show very large spreads. This indicates that the system did not equilibrate during blueschist-facies metamorphism, due to bulk rock heterogeneities during deposition and limited mineral reactivity at the local scale. Strontium isotopic ratios do not correlate with Sr content which rules out limestone devolatilizat

Delamination of dense mafic/ultramafic materials in arc roots has long been considered as the fundamental step in the paradigm of making an andesitic continental crust. However, the complexity in identifying ancient arc roots inherently with repeated modifications, poses a challenge in accurately determining the preservation time of the dense crustal materials and thus the delamination-driven model. Here, we conducted comprehensive petrographic, whole-rock, and mineral geochemical studies on 10 variably retrograded eclogite xenoliths from the ~ 35 Ma crustal-derived felsic porphyry in the Liuhe area, western Yangtze craton. Eclogite facies metamorphism is indicated by the fresh relic consisting of coarse-grained garnet, omphacite and rutile; the retrograde metamorphism is manifested by an additional assemblage of fine-grained diopside, amphibole, plagioclase, and biotite. Whole-rock element contents of the xenoliths generally display correlations with immobile Nb concentrations, suggestive of a dominant control from magmatic processes with negligible effects from the post-magmatic alteration. The protoliths of studied xenoliths are most likely accumulated garnet pyroxenites, where the negative correlation between heavy rare earth elements and Mg# (Mg/(Mg + Fe), atomic ratio), and the absence of positive Eu and Sr anomalies suggest the accumulation of garnet under high-pressure conditions. The parental magmas are inferred to be evolved and hydrous with arc-type trace-element patterns. Combined with studies on regional xenoliths, outcrops and tectonic history, the parental magmas likely record the melting of asthenospheric mantle wedge fluxed by recycled subducted slab in the Neoproterozoic (~ 800 Ma). The prolonged preservation (from ~ 800 Ma to at least 35 Ma) of the accumulated garnet pyroxenites with high densities in the deep continental crust can be ascribed to the support from the underlying refractory lithospheric mantle strengthened by plume head accretions. Therefore, we propose that the density-driven delamination of the arc root materials is more sluggish than previously expected and the longevity of dense crustal materials highlights the caution in understanding the role of arc root delamination in making an andesitic continental crust.