Contributions to Mineralogy and Petrology最新文献

The Eastern Manus Basin is one of the fastest expanding back − arc basins in the world and it is the site of recent volcanism and hydrothermal activity. The role of magma mixing in the origins of the volcanic rocks in this region, as well as the modeling of its magma plumbing system, are still unclear. In this study, we have clarified the magma plumbing system processes of the Eastern Manus Basin by analyzing the petrography and geochemical characteristics of whole rocks and minerals of basaltic andesite, andesite, and dacite in this region. The analyses reveal that basaltic andesite has experienced high undercooling and intense degassing, while both andesite and dacite samples have experienced magma mixing during their formation. The mineral assemblages in andesite are derived from basaltic, dacitic, and mixed melts, with the mixed melt comprising a 2:8 ratio of the former two. Dacite samples contain three mineral assemblages derived from andesitic, rhyolitic, and mixed melts, showing multiple injections of more primitive melts, as indicated by phenocryst textures and chemical zoning patterns. Moreover, they may have experienced the capture of mafic wall rocks. The performance of different mineral − based thermobarometers has been assessed by constructing the experimental datasets applicable to this study, and the best − performing thermobarometers are all from Putirka (2008). Calculations show that the pre − eruption storage temperatures for basaltic andesitic, andesitic, and dacitic magmas are 1090 ± 13 °C, 1032 ± 9 °C, and 938 ± 10 °C, respectively, with storage pressures not well constrained at 4.3 ± 1.4 kbar, 2.8 ± 1.3 kbar, and 2.5 ± 1.3 kbar, respectively. This study provides evidence that magma mixing plays a significant role in the origins of andesite and dacite from the Eastern Manus Basin and that its complex magma plumbing systems provide materials and potential heat sources for the volcanism and hydrothermal activity.

Aluminum has been shown to significantly enhance the water incorporation capacity of orthoenstatite (OEn), but the incorporation mechanisms remained to be clarified. We performed a comprehensive one- and two-dimensional ¹H, ²⁹Si and ²⁷Al NMR study on four hydrous aluminous OEn samples containing 1–8 wt% Al₂O₃ synthesized at 1.5 GPa and 900 °C to clarify the issue. The combined ¹H MAS and static (non-spinning) NMR, ¹H double-quantum and triple-quantum MAS NMR, and ²⁷Al→¹H CP MAS NMR and HETCOR results, in particular, unambiguously revealed that a large part of the incorporated water are present as proton pairs in Mg vacancies adjacent to Al, with one proton of each pair for the dominant proton pairs exhibiting significantly weaker hydrogen bonding than those in Al-free OEn. Proton pairs in Mg vacancies remote from Al are minor or absent for samples with 4–8 wt% Al₂O₃, and more abundant for a sample with 1 wt% Al₂O₃. Isolated protons due to coupled substitutions of Al + H for 1Si and 2Mg (both with weak hydrogen bonding) were also detected, but are less abundant than hitherto considered. The observed NMR peaks match well with those predicted for the corresponding OH defect models from our first-principles calculations. Thus, the enhancement of water solubility by Al for OEn are due to not only coupled substitutions of Al + H for 1Si and 2Mg, but also interactions of Al with proton pairs in Mg vacancies. These mechanisms may also be important in other nominally anhydrous aluminous silicate minerals.

We performed first-principles calculations on the energy, and NMR and polarized infrared (IR) spectra for anhydrous and hydrous aluminous orthoenstatite (OEn) models to help clarify the incorporation mechanisms of Al and OH defects in OEn. Our calculations revealed that proton pairs in M2 vacancies ((2H)_M2) adjacent to a tetrahedral Al (Al^IV) are energetically more favorable than those remote from Al, and may contribute to the observed correlated ¹H NMR peaks near 3.7 and 8.0 ppm, and IR bands near 3550–3570 and 3066 cm⁻¹ (A4 band) for aluminous OEn. Coupled substitutions of Al^VI (octahedral Al) + H for 2Mg were found to adopt multiple configurations, and may contribute to the observed IR bands near 3520, 3475 and 3320 cm⁻¹. Coupled substitution of Al^IV + H for 1Si may contribute to the observed IR band near 3380–3400 cm⁻¹. 4H in SiB vacancies ((4H)_SiB) adjacent to an Al^VI were found to be energetically more favorable than those remote from Al, and may be the origin for an IR band observed near 3600–3620 cm⁻¹. These results allow the incorporation mechanisms of water in synthetic and natural aluminous orthopyroxenes to be deciphered from the available NMR and IR data, and suggest that both (2H)_M2 defects associated with Al and simultaneous coupled substitutions of Al + H for 2Mg and 1Si contribute to the observed correlation between Al and water incorporation, and the nearly unity Al^IV/Al^VI ratio. (4H)_SiB defects associated with Al may also be present in some synthetic OEn and mantle-derived orthopyroxene.

Alaskan-type ultramafic–mafic intrusions in convergent-margin settings provide valuable information on melt-cumulate petrogenetic processes operating at depth in the sub-arc crust. Here, we report the compositions and textural relationships of cumulus and postcumulus minerals in a suite of clinopyroxenites and hornblendites from the peripheral zone of the Tulameen Alaskan-type intrusion in British Columbia, Canada. Mineral chemistry is used to establish magma storage conditions (P, T, fO₂, H₂O_melt) and to reconstruct the composition of cryptic residual liquids that equilibrated with the mineral phases and subsequently escaped the local mush system. Residual liquids in equilibrium with clinopyroxene (diopside) are metaluminous calc-alkaline basalt to andesite; melts equilibrated with amphibole (magnesio-hastingsite) are metaluminous to peraluminous calc-alkaline dacite to low-silica rhyolite. Thermobarometry yields a robust estimate of storage pressure of 400 ± 50 MPa (~ 15 km paleodepth) for the Tulameen magma reservoir and equilibration temperatures of 1130–960 °C for clinopyroxene and 950–850 °C for amphibole. The large cooling interval between the early crystallization of clinopyroxene and late appearance and continued crystallization of peritectic amphibole facilitated progressive extraction of residual liquids from clinopyroxene-rich cumulates, consistent with textural relationships, mass balance calculations and experimental petrology. Peritectic dacitic melts are hydrous (~ 6–8.3 wt% H₂O_melt), oxidized (fO₂ ~ NNO + 1.6 to NNO + 3.6 log units) and buoyantly mobile with low density (~ 2200 kg/m³) and viscosity (~ 10³ poise). Lower water contents likely reflect degassing of peritectic melts driven by amphibole crystallization; relatively high redox conditions are attributed to precursor fractionation of olivine and clinopyroxene preserved as cumulates in the core of the Tulameen intrusion. Peritectic amphibole crystallized in response to migration of a thermally buffered reaction front marking the stability limit of amphibole (≤ 950 °C) and driven by near-isobaric cooling. Pervasive infiltration of reactive dacitic liquids through the clinopyroxene mush formed intergranular/poikilitic amphibole and channelized flow was captured in part by cm-scale hornblendite segregations; aggregated melts formed in situ bodies of replacive hornblendite. The absence of orthopyroxene and rarity of plagioclase in the evolved ultramafic cumulates of Alaskan-type intrusions and similar arc-related rocks is attributed primarily to high H₂O_melt and oxygen fugacity in differentiated arc magmas.

The presence of mafic enclaves and banded pumice reveals key physical processes associated with volcanic eruptions. Here, through the textural and geochemical analyses of the 3550 B.P. Waimihia deposits in Taupō, New Zealand, we demonstrate how disequilibrium crystallization controls the way magmas mix. Andesitic enclaves in pyroclastic deposits from this predominantly rhyolitic eruption consist of microlites that crystallized rapidly during mafic injection into rhyolitic host magma. The variation of microlite textures depends on enclave size, implying that mafic enclaves crystallized as discrete blobs within a host rhyolitic magma. However, gray pumice and dark bands in banded pumice are characterized by a lack of or less plagioclase microlites that should be present if equilibrium crystallization occurred. Our textural and chemical data suggest that the lack of plagioclase in gray pumice and dark bands resulted from the nucleation delay arising from the mixing with rhyolitic magma. After mafic magma broke up in a magma chamber as discrete mafic blobs, the plagioclase-free rim of the blobs was disaggregated by shear flow. The eroded mafic blobs form a hybrid magma by mixing with rhyolitic magma, which further delays the plagioclase nucleation. This hybrid magma eventually erupted as gray pumice or banded pumice, depending on the intensity of magma mingling in the conduit. We use a plagioclase nucleation delay model to calculate residence times of hours to tens of hours prior to eruption. Our mixing model with nucleation delay enables small volumes of mafic magma to mix with large amounts of silicic magma.

The rare-earth elements (REEs, La–Lu, Y) are essential for the development of renewable technologies. Bastnäsite (REECO₃F) is a common REE ore mineral that is often subject to hydrothermal alteration at all crustal levels. Mechanisms of hydrothermal bastnäsite alteration therefore govern the evolution of REE deposits, though these mechanisms remain poorly understood. This experimental work investigates the hydrothermal replacement of bastnäsite by rhabdophane (REEPO₄∙xH₂O, x = 0–1) and monazite (REEPO₄) in phosphatic fluids. Two temperature-dependent alteration pathways were identified; both follow the coupled dissolution-reprecipitation (CDR) mechanism. At 90 °C, bastnäsite was replaced by highly-porous metastable rhabdophane which was then replaced by monazite, forming an inner layer of rhabdophane and an outer layer of monazite. At 220 °C, bastnäsite was replaced directly by monazite. Although replacement initiated more quickly at 220 °C, greater overall replacement occurred at 90 °C (~ 61 wt.% after 500 h, compared to ~ 13 wt.% at 220 °C) due to surface passivation by monazite at 220 °C. Geochemical analyses showed REE fractionation during bastnäsite alteration. At 90 °C, rhabdophane was enriched in heavy REEs (Eu–Lu, Y), likely due to the evolving fluid chemistry, while at 220 °C secondary monazite was enriched in Sm and Ho compared to bastnäsite. These results indicate that: 1) the hydrothermal alteration of bastnäsite by rhabdophane and monazite in ore deposits leads to REE immobilisation, with little net loss of REEs to solution; 2) rhabdophane is metastable relative to monazite at 90 °C, and; 3) variable temperatures can cause different mineral textures and REE fractionation trends during hydrothermal alteration and mineral replacement.

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.