Contributions to Mineralogy and Petrology最新文献

Subduction-zone fluid–rock interactions have a direct impact onto elemental and isotopic homogeneity of progressively buried and exhumed crustal lithologies by providing an interface for local mass-transfer and enhancing metamorphic reactions. In order to assess the scales of fluid mobility, chemical and isotopic inheritance, as well as resulting degrees of isotopic heterogeneity in the exhumed high-pressure lithologies, we performed the detailed mineralogical, in-situ trace-element and Rb–Sr isotope studies, combined with P–T–X thermodynamic modelling of representative eclogites from the Alag Khadny accretionary complex (SW Mongolia). The eclogites (garnet + omphacite + phengite + rutile + quartz + retrograde amphibole and clinozoisite) display records of subduction-related burial to 540–625 °C and 1.7–2.1 GPa, with the enclosed phengite supposed to be in equilibrium at prograde-to-peak conditions. Trace-element signatures, including Cs/Rb (0.03–0.08) and Ba/Rb (7.1–13.8) ratios of phengite, are consistent with moderately to strongly altered protoliths of eclogites, which is supported by elevated δ¹⁸O values and in-situ Rb–Sr constraints on the initial (⁸⁷Sr/⁸⁶Sr)_I ratios of phengite within 0.70549–0.70957. Partial backward rehydration (~ 0.5–1.0 wt% of H₂O added) during decompression from 1.6 to 1.2 GPa produced amphibole- and clinozoisite-bearing assemblages, did not significantly affect LILE systematics and variable ⁸⁷Rb/⁸⁶Sr ratios of phengite. Limited Rb and Ba loss from phengite during recrystallization is suspected in the evidently deformed eclogites based on the LILE mineral–fluid and phengite–amphibole partitioning data. No exclusive evidence is found in amphibole for LILE-rich metasedimentary fluid with high (⁸⁷Sr/⁸⁶Sr)_I released into eclogites. Instead, unradiogenic ⁸⁷Sr/⁸⁶Sr (0.70279–0.70301) of clinozoisite highlights metasomatic addition from the underlying mafic crust or dehydrated peridotitic mantle. Variable deformation-enhanced fluid-rock interaction during early exhumation was recorded by in-situ phengite Rb-Sr geochronology at 568 ± 9 Ma, which is considered a direct fluid flow snapshot and place a new minimum age constraint for the peak subduction burial. We argue that, except cases of apparent metasomatic origin of phengite, its (⁸⁷Sr/⁸⁶Sr)_I ratios may be a sensitive tracer for the eclogite precursor alteration due to limited Sr mobility. Sample-scale Rb-Sr isotopic heterogeneities may be preserved in the orogenic eclogites due to multi-stage retrograde hydration and should be taken into account while interpreting the bulk-rock Sr isotope data.

Serpentinite carbonation contributes to the deep carbon (C) cycle. Recently, geophysical and numerical studies have inferred considerable hydrothermal alteration in plate bending faults, opening the possibility of significant C storage in the slab mantle. However, there is a lack of quantitative determination of C uptake in serpentinized mantle rocks. Here, we experimentally constrain serpentinite carbonation in H₂O–CO₂–NaCl fluids to estimate C uptake in hydrated mantle rocks. We find that serpentinite carbonation results in the formation of talc and magnesite along the serpentinite surface. The presence of porous reaction zones (49.2% porosity) promotes the progress of carbonation reactions through a continuous supply of CO₂-bearing fluids to the reaction front. Added NaCl effectively decreases the serpentinite carbonation efficiency, particularly at low salinities (< 5.0 wt%), which is likely attributed to the reduction in fluid pH and the transport rate of reactants, and the increase in magnesite solubility. Based on previous and our experiments, we fit an empirical equation for the reaction rate of serpentinite carbonation. Extrapolation of this equation to depths of plate bending fault systems suggests that serpentinite carbonation may contribute to an influx of up to 7.3–28.5 Mt C/yr in subduction zones. Our results provide new insights into serpentinite carbonation in environments with high fluid salinities and potentially contribute to the understanding of the C cycle in subduction zones.

Textural and compositional zoning of volcanic minerals archives pre-eruptive magma processes. Crystals erupted simultaneously may be sampled from different regions of the plumbing system and hence record variable histories due to complex magma dynamics. In addition, crystals erupted throughout the course of an eruption may record temporal variations in the plumbing system. To resolve mush variability on both spatial and temporal scales, we investigate clinopyroxene erupted during a series of paroxysmal episodes between February–April 2021 at Mt. Etna, Italy. Using a combination of high-resolution geochemical techniques, we observe that Cr enrichments in clinopyroxene mantle zones, grown upon eruption-triggering mafic rejuvenation, exhibit both temporal and spatial (sample-scale) variability. Temporal variability correlates with changes in glass compositions, attesting to the ability of clinopyroxene to track magma maficity throughout an eruption. Spatial variability, indicated by the scatter of Cr concentrations, is greatest for the first event and lowest for the final paroxysm. In conjunction with core textures, degree of sector enrichment and thermobarometry, our data suggest that the onset of the paroxysms was preceded by the remobilisation of a mid-crustal clinopyroxene mush (534 ± 46 MPa) by hot, mafic magma causing variable resorption of mush-derived crystal cores. Towards the end of the eruption, waning magma supply led to less efficient mush remobolisation and mixing, resulting in homogenous crystal populations. Our results highlight that clinopyroxene Cr contents and sector enrichment can be used to track mafic rejuvenation and magma evolution throughout eruptions, while also reflecting spatial heterogeneities within the plumbing system.

The upper mantle low velocity zone is often attributed to partial melting at the lithosphere-asthenosphere boundary. This implies that basaltic melts may be stable along plausible geotherms due to the freezing point depression in the presence of water and other incompatible impurities. However, the freezing point depression (ΔT) as a function of water content in the near-solidus basaltic melt (c_H2O) cannot be precisely determined from peridotite melting experiments because of difficulties in recovering homogeneous basaltic glasses at high pressures. We therefore used an alternative approach to reinvestigate and accurately constrain the ΔT–c_H2O relationship for basaltic melts at the low water fugacities that are expected in the upper mantle. Internally heated pressure vessel (IHPV) experiments were performed at water-saturated conditions in the anorthite-diopside-H₂O system at confining pressures of 0.02 to 0.2 GPa and temperatures between 940 and 1450 ℃. We determined the water-saturated solidus, and obtained ΔT by combining our data with reports of dry melting temperatures in the anorthite-diopside system. In another series of experiments, we measured water solubility in haplobasaltic melts and extrapolated c_H2O to pressures and temperatures of the water-saturated solidus. By combining the results from these two series of experiments, we showed that the effect of water on ΔT was previously underestimated by at least 50 ℃. The new ΔT–c_H2O relationship was then used to revise predictions of melt distribution in the upper mantle. Hydrous melt is almost certainly stable beneath extensive regions of the oceanic lithosphere, and may be present in younger and water-enriched zones of the subcontinental mantle.

Pegmatitic dyke-like carbonate rocks mainly composed of very coarse-grained calcite, are a rare type of carbonate rocks found in some of orogenic belts in the world. These specific carbonate rocks generally occur intimately with high-temperature granulites and marbles. In the Proterozoic Highland Complex of Sri Lanka which is a segment of the Mozambique suture, they are associated with marbles and granitic pegmatites, and intercalated with high-grade calc-silicate gneisses and highly folded ortho- and para-gneisses. These pegmatitic carbonate rocks do not show any signs of metamorphic or deformed overprint, but instead well preserve igneous textures and contain various silicate crustal xenoliths. The calcite crystals occur as euhedral to subhedral grains and are large in size from 1 to 15 cm. The diverse colors of calcite from white to yellow and blue derive from mineral inclusions and their own compositions. Non-carbonate minerals, commonly present in typical carbonatites such as phlogopite, apatite, clinopyroxene, olivine, plagioclase, iron oxides and spinel, are all found in the rocks. Meanwhile, a skarn-type assemblage of wollastonite, garnet, clinopyroxene and sulfide occurs in contact between the carbonate rocks and gneiss xenoliths, which probably resulted from antiskarn reactions. Chemical compositions of major constituent minerals (calcite, dolomite and apatite) of the carbonate rocks are intermediate between typical marbles and mantle-derived carbonatites and akin to crustal-origin carbonatites worldwide. We thus classify the studied rocks as ‘anatectic carbonatite pegmatite’, and suggest that they originated from the melting of a mixture of marbles and surrounding silicate rocks at crustal levels during high-temperature metamorphism.

Six diamond-bearing eclogite xenoliths with oceanic crust protoliths and 370 mineral inclusions in 104 diamonds recovered from the Koidu kimberlite complex in Sierra Leone provide insight into the lithological and compositional diversity of the lithospheric mantle beneath the West African Craton. Diamond formation beneath Koidu is predominantly associated with eclogitic substrates that originated from subduction and high-pressure metamorphism of oceanic crust, as indicated by a dominance of eclogitic (78%) over peridotitic (17%) and mixed paragenesis diamonds (5%). Peridotitic diamonds contain olivine inclusions with very high Mg# (92.2–94.7; median = 94.2), indicative of derivation from dunite or harzburgite protoliths. Moreover, a peridotitic spinel with Cr# = 50.9 suggests that it equilibrated with orthopyroxene-free dunite. 44% of Koidu diamonds contain coesite, of which some coexist with omphacite, eclogitic garnet, and/or kyanite. Most analysed eclogitic garnet inclusions have extremely high δ¹⁸O values ( ≥ + 9.9‰) and occur with clinopyroxene inclusions that have very high jadeite components (~ 70 mol%). These high jadeite components are a close match to clinopyroxenes in high-pressure metapelites, which have a phase assemblage that includes coesite and kyanite. Our data suggest that the eclogitic mineral inclusions in most Koidu diamonds have oceanic basalt protoliths that were mingled with pelagic sediments, which may have increased δ¹⁸O values to levels much higher than observed for other eclogites at Koidu and shifted the originally basaltic bulk compositions closer to that of pelites. Most eclogitic mineral inclusions in Koidu diamonds have elemental compositions not observed for Koidu eclogite xenoliths, which have clear oceanic crust protolith (oceanic lavas and cumulates) signatures without significant crustal sediment contamination. These findings suggest the subduction of distinct packages of oceanic crust into the Koidu lithospheric mantle through time.

The Xunmei hydrothermal field, located at 26°S along the South Mid-Atlantic Ridge, is an active submarine hydrothermal system underlain by a basaltic substrate. This field comprises two distinct types of basalts: massive basalts, characterized by aphyric to moderately porphyritic textures without large vesicles, and vesicular basalts, known for their highly vesicular nature. Olivine-hosted melt inclusions within the massive basalts exhibit a diverse range of chemical compositions. Type-A melt inclusions are distinguished by lower levels of K₂O, Rb, Ba and U, but higher concentrations of S, Co, Ni, and Cu. Conversely, Type-B melt inclusions exhibit higher levels of K₂O, Rb, Ba and U, but lower concentrations of S, Co, Ni, and Cu. Although both types of melt inclusions show similar ranges of La/Sm, La/Yb, Sr/Yb, and Zr/Nb, the significant differences in K₂O/TiO₂ and Nb/U indicate that the massive basalts likely originate from the mixing of two distinct melts derived from different source regions. Data from melt inclusions and quenched basaltic glasses, combined with theoretical calculations, indicate that Type-I melts, represented by the Type-A melt inclusions, were sulfide-saturated during the crystallization of olivine at depth, evolving into sulfide-unsaturated melts as they ascended towards the seafloor. Approximately 50% of the Cu in the Type-I melts transitioned to the gas phase and were eventually released from the magma to the overlying hydrothermal system. Conversely, Type-II melts, represented by the Type-B melt inclusions, did not reach sulfide saturation. The presence of magmatic sulfides within or attached to vesicles, occupying voids in the primocryst frameworks, and lining the walls of vapor bubbles in melt inclusions, may suggest a volatile-driven transport of magmatic sulfides from the magma system as compound drops during magma degassing. This mechanism likely plays a crucial role in the supply ore-metals during the formation of seafloor massive sulfides in the Xunmei and possibly other hydrothermal fields along mid-ocean ridges.

A new activity model for biotite is formulated in the system K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂-O₂ (KFMASHTO), which extends that for the KFMASH system by introducing a titanium-biotite and a ferric-biotite end-member (tbio: K(TiMg₂)[(O)₂(AlSi₃)O₁₀] and fbio: K(Fe³⁺Mg₂)[(OH)₂(Al₂Si₂)O₁₀]), as well as a pyrophyllite end-member (pyp: Al₂[(OH)₂Si₄O₁₀]) that accounts for the presence of octahedral excess-Al in natural biotites. Phonon calculations applying density functional theory (DFT) using the software Castep yielded the standard entropies of tbio and fbio as S^o_tbio = 328.06 J/(mol·K) and S^o_fbio = 301.69 J/(mol·K), and their heat capacity functions. From experimental phase-equilibrium data, the enthalpy of formation value of tbio was constrained as (Delta {H}_{f,tbio}^{o}) = −6124.68 ± 3.33 kJ/mol. Natural data were used to derive (Delta {H}_{f,fbio}^{o})= −5935.3 ± 6.6 kJ/mol. The single-defect DFT method was applied to parameterize important macroscopic mixing properties (macro-W’s) involving tbio and pyp end-members in the model (fbio was treated ideal). Castep-derived microscopic interaction energies (micro-w’s) are presented herein for KFMASH-biotite. The octahedral same-site (M1) Mg–Al mixing micro-w (w_MgAl(M1)), the same-site tetrahedral Si-Al mixing parameter (w_SiAl(T1)) and the related cross-site term are: w_MgAl(M1) = 82.5 kJ/mol, w_SiAl(T1) = 95.6 kJ/mol (two T1-sites) and ({w}_{MgAlAlSi(M1T1)}=) 175.1 kJ/mol. The linear combination of these micro-w’s gives a macroscopic W_phleas = 18.8 kJ/mol, that is not transferable to other mineral groups. Micro w’s for Mg-Fe mixing in biotite (w_MgFe(M1), w_MgFe(M2), ({w}_{MgMgFeFe(M1M2)})), are all close to ideality. The biotite activity model of this study is thus a first example of next-generation activity models that use DFT- and thus physically based micro-w’s and reassembled macro-W’s for petrological calculations. Test calculations on 5 samples from low- to high-grade metamorphic environments covering metapelite to greywacke bulk-compositions using Perple_X suite of programs illustrate the performance of the new biotite activity model. Computed mineral-chemistries are in all cases in better agreement with measured compositions than resulting from published activity models of biotite.

Halogen ratios (Cl/Br) preserved in halogen-bearing minerals can be very useful to identify the sources of fluids interacting with crystalline rocks, as different fluid types have distinct halogen ratios. In this study we conduct exchange experiments for chlorine and bromine partitioning between scapolite and brine treated at variable fluid salinities ranging from 0.20 to 0.66 mol fraction of salt (NaCl + NaBr). Experiments involved two different natural scapolites, which were treated in the presence of brine and minor calcite in sealed platinum capsules at 0.32 to 1.52 GPa and 600 °C to 1000 °C for durations of 12–120 h. Neomorphic scapolite appeared as overgrowths on the initial scapolite, or, in some cases, fully recrystallized with no relict scapolite visible. The experiments show that the Ca/Na ratio of scapolite depends on the treatment temperature, the fluid salinity, and pressure. In contrast, the Cl/Br distribution coefficients between neomorphic scapolite and fluids do not depend on the temperature, composition of the mineral or the total salinity of the fluid. The Cl/Br distribution coefficient is, however, markedly pressure-dependent. The experimentally-determined partitioning coefficients of this study and previous work, ranging from 1 atm to 1.5 GPa, enable the use of Cl/Br ratios in scapolite to characterize the halogen ratio of fluids throughout the entire crust. The molar Cl/Br ratio of a fluid can be determined from the measured molar Cl/Br of scapolite via: Cl/Br^fluid = Cl/Br^scapolite x ( – 1.473 × P + 1.119 × P² – 0.299 × P³ + 1.103)⁻¹, where P is pressure in GPa, over the range of 0.0001–1.5 GPa.

The temporal evolution of coherent lamellar microstructures is significantly influenced by their elastic properties, particularly when the exsolved phases are coherent. This research utilizes the Cahn–Hilliard model to examine the morphological and energetic evolution of binary alkali feldspar, integrating anisotropic elastic energy into the Gibbs energy equation. The Cahn–Hilliard model successfully simulated the orientation of lamellae observed in natural samples and the elastic strain was consistent with previous research. We also computed the coherent solvus from the annealing simulation of various precursor compositions and temperatures. The temperature difference ((Delta T)) between the strain-free solvus and the coherent solvus was (Delta T = 85,{}^circ text {C}), which is slightly lower than previously reported values obtained from similar parameters. This discrepancy is likely due to the presence of non-planar lamellae at the onset of phase separation, which are more stable than planar ones. We also simulated the binodal curves of the coherent solvi for different precursor phase compositions. The computed solvi were not unique but varied depending on the precursor composition. Our model is flexible because it does not assume any specific shapes for the lamellar interfaces and is applicable to various coherent binary systems.