Contributions to Mineralogy and Petrology最新文献

The synthesis of the Al₂SiO₅ polymorphs kyanite, sillimanite and andalusite in a pure Al₂O₃–SiO₂–H₂O (ASH) system has long been known to be impeded. In order to decipher individual aspects of the reaction: corundum + SiO₂aq, which repeatedly fails to produce thermodynamically stable Al₂SiO₅, we conducted experiments within the stability fields of kyanite and sillimanite (500–800 ℃; 0.2–1 GPa) with the aim of forming reaction coronas on corundum. Results showed that metastable corundum + quartz assemblages form persistently in pure ASH, even in Al₂SiO₅ seeded experiments, despite the presence of catalyzing fluid and evidence of fast reaction kinetics. Coronas on corundum spontaneously formed when additional components (Na, K, N, and Mg) were added to the experiment. In a similar experiment with baddeleyite (ZrO₂) instead of corundum in silica saturated water, a zircon corona formed readily. This implies that nucleation and growth of Al₂SiO₅ is obstructed under conditions of Al and Si saturation in aqueous fluid, while both corundum and quartz saturated aqueous fluid are willing participants in other reactions towards stable corona formation. Instead of Al₂SiO₅ precipitation, an unexpected fluid-aided silica diffusion process into corundum was documented. The latter included the formation of nanometer wide hydrous silicate layers along the basal plane of the corundum host, which enhanced the silica diffusion rate drastically, leading to silica supersaturation in the host mineral, and ultimately to precipitation of quartz inside corundum. We conclude that the natural metastable assemblage of quartz and corundum is not necessarily the result of dry or fluid absent conditions, given that the aqueous fluid in experiments does not promote Al₂SiO₅ formation, but rather seems to support the formation and preservation of a metastable assemblage.

Mechanisms relating to growth and/or compositional modification of zircon occur at the atomic scale. For felsic igneous systems, processes responsible for growth patterns in zircon have previously remained elusive as the volume of material needed to analyze these compositional features using traditional in-situ methods is considerably larger than the typical sub-micron scale distribution of trace elements. To illuminate some of these driving forces, we characterize and quantify minor and trace element concentrations in igneous zircon grains by combining methods of cathodoluminescence (CL) imaging, electron microprobe microanalysis (EMPA) elemental maps for Hf, Y, Yb and U or Th, and atom probe tomography (APT). We focus on igneous zircon from the Chon Aike Silicic Large Igneous Province (Patagonia) that provide novel insights into (1) dissolution and re-crystallization during crustal anatexis, (2) crystallization to produce oscillatory zonation patterns that are typical of igneous zircons, and (3) the incorporation of trace element impurities (e.g., P, Be, and Al) at the nanoscale. Significantly, these APT volumes provide nanoscale sampling of boundaries between oscillatory growth zones in an igneous zircon to reveal compositional zoning of Y and, to a lesser extent P, which appear as high-angle, planar features. These concentration boundaries measured on the order of 10 to 12 nm are difficult to reconcile with proposed mechanisms for generating fine-scaled oscillations. Lastly, we fit diffusional profiles to measured Y concentrations to provide an estimate on the maximum timescales of zircon growth prior to eruption, as a function of the temperature at which diffusion occurred. When combined with known pressure-temperature-time paths for the magmatic system considered, these extremely short diffusion profiles that are resolvable by APT provide a powerful method to constrain timescales of crystal growth.

To develop new criteria to distinguish different crystal nucleation mechanisms in silicate melts, we performed crystallization experiments using a synthetic hydrous (2 wt% H₂O) trachybasalt and combined three-dimensional information from synchrotron X-ray computed microtomography with two-dimensional mapping of crystallographic orientation relationships (CORs) using electron backscatter diffraction. Crystallization experiments were performed at 400 MPa by cooling the melt from 1300 °C to resting temperatures of 1150 and 1100 °C and maintaining isothermal conditions for 30 min and 8 h. Three distinct titanomagnetite (Tmt) populations formed: (1) skeletal crystals, isolated or partially embedded in clinopyroxene (Cpx); (2) anhedral crystals, always attached to Cpx; (3) flattened needle-shaped crystals, embedded in Cpx. These morphologically different Tmt populations formed in response to one cooling event, with varying nucleation mechanisms and at different undercooling conditions. The clustered three-dimensional distribution of population 2 and 3 Tmt grains and the high proportion of Tmt-Cpx interfaces sharing CORs indicate that these Tmt grains heterogeneously nucleated on Cpx. The near-random three-dimensional distribution of (often isolated) population 1 Tmt grains, together with the low proportion of Tmt-Cpx interfaces sharing CORs, imply their isolated, possibly homogeneous nucleation, potentially followed by heterogeneous nucleation of Cpx on population 1 Tmt. Heterogeneous nucleation in slightly to moderately undercooled magmas should affect the sequence of crystallization as well as morphology and clustering of crystals, which may actively contribute to the variation of rheological parameters like viscosity. Finally, observed intra- and inter-sample variations in Tmt-Cpx COR frequencies indicate the potential for this parameter to record further petrological information.

The amount of water recycled during subduction is unclear. Water/Ce estimates in HIMU (high-²³⁸U/²⁰⁴Pb) basalts are variable, ranging from < 100 to > 250 in glasses and melt inclusions. Because clinopyroxene (cpx) is a common early liquidus phase and the cpx/melt partitioning of water is well constrained, cpx phenocrysts provide an additional constraint on magmatic water contents. We present water and trace element concentrations in HIMU-basalt-hosted cpx phenocrysts from the Austral Islands. Calculated melt [H₂O] (up to 4.3 wt.%) and H₂O/Ce ratios (22–825) are higher than in olivine-hosted inclusions, as are estimated equilibration pressures. Correlation between estimated melt [H₂O] and crystallization or entrapment pressure suggests significant water loss during magma ascent. Most ocean island basalts (OIBs) span a limited range in H₂O (~ 1–1.5 wt.%), and low (< 100) H₂O/Ce ratios are primarily observed in melts with unusually high [Ce] (up to 350 ppm). Additionally, [H₂O] and H₂O/Ce in some suites correlate with entrapment pressure despite having quench pressures high enough to prevent significant water loss from open- or closed-system degassing (< 100 MPa). Polybaric “sparging”, whereby low-P melts re-equilibrate with CO₂-rich fluids exsolved at higher pressure, may result in water loss at pressures less than CO₂ saturation. This may more accurately describe OIB degassing processes than open or closed system degassing. After correcting for degassing, primary Australs melts likely have H₂O/Ce of ~ 300–600. If applicable to OIB sources in general, this limits the total water budget of the mantle, including the mantle transition zone, to < 2.4 ocean masses.

Larnite (β-Ca₂SiO₄) has previously been reported as an inclusion in sub-lithospheric diamonds and is generally interpreted as a retrograde reaction product of calcium silicate perovskite. In this study, we review the controls on the stability of the Ca₂SiO₄ polymorphs and show that phosphorus is likely essential for the preservation of β-Ca₂SiO₄. We also report a detailed study of the solubility of water and its incorporation mechanisms in γ-Ca₂SiO₄ and phosphorus-doped β-Ca₂SiO₄ using FTIR spectroscopy on high-pressure experiments quenched from 4–9.5 GPa and 1000–1200 °C combined with ab initio calculations. The experimentally determined water solubilities are in the range of 107–178 ppm. Our FTIR spectra and ab initio calculations indicate that for phosphorus-free γ-Ca₂SiO₄ the incorporation mechanism involves protonated Si and Ca1 vacancies. For phosphorus-bearing β-Ca₂SiO₄, our preferred incorporation mechanism involves one Si⁴⁺ ion replaced by one P⁵⁺ ion with a single protonated Ca2 vacancy. The low water solubility observed here for larnite implies that if primary calcium silicate perovskite inclusions trap high water concentrations during diamond growth from a volatile-rich fluid, measurements of the concentration of water in larnite will not provide a useful record of the initial volatile concentration. Instead, water would be hosted in other retrograde reaction products, possibly including exsolved fluids.

Rhyolitic tuffs range widely in their crystal contents from nearly aphyric to crystal-rich, and their crystal cargoes inform concepts of upper crustal magma reservoirs. The Earthquake Flat pyroclastics (Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand) are 10 km³ of rhyolitic tuffs with abundant (~ 40 vol.%) plagioclase and quartz, minor biotite, hornblende, and orthopyroxene, and accessory Fe-Ti oxides, apatite, and zircon, set in high-silica rhyolitic glass. Major minerals form large, euhedral phenocrysts and abundant glomerocrysts with few disequilibrium textures excepting some faintly resorbed quartz. Plagioclase phenocrysts have thick rims of nearly constant composition near An₃₀, and hornblende is weakly zoned or unzoned. The abundant and texturally complex mineral assemblage contrasts with the nearby (~ 25 km), nearly synchronous, but more voluminous and crystal-moderate rhyolite tuffs from Rotoiti caldera. New H₂O-saturated phase-equilibria results on the erupted Earthquake Flat melt (glass) determine its co-saturation with the partial phenocryst assemblage of plagioclase, quartz, biotite, and Fe-Ti oxides at: 140 MPa, 755 ºC. These closely approximate the conditions of the pre-eruptive magma body assuming it was saturated with nearly pure H₂O and at an fO₂ of ~ Ni–NiO. Absence of hornblende and orthopyroxene from the synthesized assemblages may result from those minerals being in a peritectic reaction relation with melt to produce biotite, so they would not grow from the liquid used as starting material. Experimental results on Rotoiti rhyolite (Nicholls et al. 1992) show that the two bodies resided at similar pressures, temperatures, and fO₂s. Lower crystal abundance of the Rotoiti tuffs may result from slight compositional differences. We interpret that the Earthquake Flat pyroclastics were sourced from the crystal-rich periphery of a mushy reservoir system with the Rotoiti occupying a more melt-rich central location. Uncertain is whether this was a single intrusion zoned continuously in crystallinity, or discrete adjacent intrusions, but our results illustrate and quantify complexities of magma storage across relatively short distances.

In the last two decades, boron has gained significance as a geochemical tracer in mantle studies, particularly related to fluid-mediated processes. In our investigation, we explore how boron and its stable isotopes distribute between basaltic melt and hydrous fluid under conditions relevant to magmatic degassing in the shallow crust (1000–1250 °C, 150–250 MPa). We utilized a synthetic MORB-like composition with added boric-acid isotope standard (NIST-SRM951a) and additional trace elements, subjecting it to varying pressure, temperature, and melt-fluid ratios using an internally heated pressure vessel. The B isotope composition in the quenched glasses were determined through femtosecond laser ablation coupled to a multi-collector inductively-coupled-plasma mass spectrometer. Our experiments revealed that, even at the highest temperatures, boron strongly partitions into the fluid phase, accompanied by significant B isotope fractionation. This leads to an enrichment of the heavy B isotope in the fluid, with a constrained Δ¹¹B_melt-fluid range of -1.7 ± 0.9‰, consistent with ab-initio modeling results. These findings highlight the potential of B isotopes to trace geochemical processes at elevated temperatures with ({Delta}^{11}{{B}}_{melt-fluid}=2.913-9.693frac{{10}^{6}}{{{T}}^{2}}). Our results have implications for predicting the δ¹¹B of degassed, water-bearing basaltic magmas and estimating the B isotope composition of their mantle source.

Garnet and zircon in a marble-hosted eclogite from the Dabie ultrahigh-pressure (UHP) terrane, eastern China record a wealth of information on multistage pervasive fluid–rock interactions and Hf mobility in deep continental subduction zones. The eclogite has a peak mineral assemblage of garnet + omphacite + phengite + coesite + magnesite ± dolomite + rutile. Five (inner patchy core, outer core, mantle, inner rim, outer rim) compositional zones were recognized for garnet. According to phase equilibria modeling, the inner patchy and outer cores of garnet likely document a prograde breakdown of lawsonite to UHP peak (3.0–4.5 GPa and 630–750 °C), while a Ca-metasomatism could have also played a role in their formation. The other three garnet zones resulted from multistage garnet re-equilibration at eclogite-facies conditions during isothermal exhumation. The stepwise compositional changes between these different garnet zones suggest that dissolution and reprecipitation played a key role in the garnet re-equilibration, while the repeated actions of such a re-equilibration mechanism reflect multistage pervasive fluid–rock interactions. Zircon from the rock develops three eclogite-facies domains (1, 2, 3). Textural relationships suggest that domain 2 formed in between domain 1 and 3. LA-ICP-MS analyses yielded ²⁰⁶Pb/²³⁸U ages of 233 ± 6 Ma, 232 ± 2 Ma and 222 ± 3 Ma for domain 1, 2 and 3, respectively. Domain 1 includes coesite and magnesite and its Th/U is usually higher than 0.1. This domain is interpreted to have formed in the absence of allanite during prograde UHP metamorphism. Domain 2 shows slightly lower ∑MREEs (middle rare earth elements; 7.5–13.5 ppm) and lower Ti (3.0–6.0 ppm) contents than domain 1 (∑MREEs = 10.5–21.0 ppm; Ti = 4.5–7.0 ppm) and most likely formed at the UHP peak. Domain 3 contains much lower ∑MREEs (3.5–6.0 ppm) and higher Ti (7.5–11.0 ppm) contents than domain 1 and 2, which is interpreted to have formed in the stability field of epidote during decompression. Domain 2 (0.282354–0.282607) and 3 (0.282449–0.282636) display lower initial ¹⁷⁶Hf/¹⁷⁷Hf values than domain 1 (0.282563–0.282667), suggesting that external fluids introduced Hf into the eclogite. These findings not only shed new light on the flow mode of fluids and their role in resetting mineral compositions in deep subduction zones, but also suggest that Hf (a key high-field strength and tracer element) can be efficiently mobilized by (U)HP fluids. Moreover, this study highlights the influence of epidote-group minerals and pressure on the chemistry (Th/U ratio and REE and Ti contents) of zircon.

Water occurs in Earth’s interior mostly as trace hydroxyl in nominally anhydrous minerals. Clinopyroxene is known to be an important water carrier in the uppermost mantle, and eclogite, which forms a subordinate part of the cratonic lithosphere, contains some 50% of jadeite-rich clinopyroxene, making this potentially a significant H₂O reservoir in the bulk lithospheric mantle. Mantle metasomatism, in particular by small-volume melts like kimberlite, is known to enrich the lithosphere in highly incompatible components, but its effect on H₂O contents in cratonic eclogite remains unclear. We report H₂O concentrations for clinopyroxene and garnet in eclogite and pyroxenite xenoliths from several African kimberlites, obtained by Fourier-Transform Infrared Spectroscopy (FTIR). Except one sample showing evidence for minor within-grain variability of H₂O concentrations (< 15%), FTIR images demonstrate that H₂O is homogeneously distributed in optically clear areas of clinopyroxene fragments mounted for this study. The samples were variably metasomatised by a kimberlite-like melt, as evidenced by elevated MgO contents and abundances of highly incompatible elements (e.g., Sr, Ce, Th). Although metasomatised eclogites and pyroxenites on average show higher H₂O abundances than pristine ones, mantle metasomatism decreases the Al₂O₃ content in clinopyroxene, which is known to enhance hydrogen incorporation in this mineral. As a consequence, hydrogen incorporation is inhibited, and c(H₂O) becomes increasingly decoupled from other highly incompatible components, such as LREE. Thus, eclogite – metasomatised or not - does not significantly contribute to the H₂O inventory in the bulk cratonic mantle.