Contributions to Mineralogy and Petrology最新文献

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.

Apatite has been recognized as a robust tool for the study of magmatic volatiles in terrestrial and extraterrestrial systems due to its ability to incorporate various volatile components and its common occurrence in igneous rocks. Most previous studies have utilized apatite to study individual magmatic systems or regions. However, volatile systematics in terrestrial magmatic apatite formed under different geological environments has been poorly understood. In this study, we filtered a large compilation of data for apatite in terrestrial igneous rocks (n > 20,000), categorized the data according to tectonic settings, rock types, and bulk-rock compositions, and conducted statistical analyses of the F–Cl–OH–S–CO₂ contents (~ 11,000 data for halogen and less for other volatiles). We find that apatite from volcanic arcs preserves a high Cl signature in comparison to other tectonic settings and the median Cl contents differ between arcs. Apatite in various types and compositions of igneous rocks shows overlapping F–Cl–OH compositions and features in some rock groups. Specifically, apatite in kimberlite is characterized as Cl-poor, whereas apatite in plutonic rocks can contain higher F and lower Cl contents than the volcanic counterparts. Calculation using existing partitioning models indicates that apatite with a high OH (or F) content does not necessarily indicate a H₂O-rich (or H₂O-poor) liquid because it could be a result of high (or low) magma temperature. Our work may provide a new perspective on the use of apatite to investigate volatile behavior in magma genesis and evolution across tectonic settings, volatile recycling at subduction zones, and the volcanic-plutonic connection.

Garnet granulite xenoliths from the Nurbinskaya diatreme in the central part of the Archean Anabar province in Siberia are fragments of the local lower crust that experienced multiple metamorphic events in the Paleoproterozoic and reheating events in the Mesoproterozoic and later. This study addresses the timing of metamorphic transformations, and constrains the cooling rate and the time of stabilization of the lower crust. The observed metamorphic mineral assemblage of garnet, clinopyroxene, plagioclase, amphibole, rutile and ilmenite was formed at ~ 800 °C, 1.1–1.2 GPa under water-undersaturated conditions at ~ 1.88 Ga. However, the mineral assemblage is not well equilibrated and retains evidence of earlier and subsequent metamorphic stages. Late titanite formed in response to hydrous fluid influx according to phase equilibria modeling. U-Pb dating shows two events of titanite formation at 1850 ± 5 Ma and at 1788 ± 2 Ma. After deformation, which led to the porphyroclastic rock textures, the granulites underwent near-isobaric cooling. The cooling rate was higher than ~ 6 °C/Myr, to retain the garnet compositional zoning. Rutile ages are discordant, with ²⁰⁷Pb/²⁰⁶Pb dates ranging from 1.43 to 1.53 Ga. However, rutile may have responded to earlier thermal pulses, and was also reset later, so it does not record the stabilization of the crust. Crustal stabilization after Paleoproterozoic orogenic events may have occurred shortly after titanite formation.

Paleoproterozoic (2.05 Ga) komatiites are widespread in the Central Lapland Greenstone Belt (CLGB), northern Finland. Close association with sulfur (S)-rich country rocks and spatiotemporal connection with the Cu-Ni(-PGE) deposits of Kevitsa and Sakatti make these komatiites interesting targets for sulfide deposit exploration. We provide whole-rock geochemical data from Sattasvaara komatiites and combine it with literature data to form a geochemical database for the CLGB komatiites. We construct a model for the komatiites from adiabatic melting of the mantle source to fractional crystallization at crustal conditions. Using MELTS, we calculate three parental melts (MgO = 20.6–25.7 wt%) in equilibrium with Fo₉₂, Fo₉₃, and Fo₉₄ olivine for the CLGB komatiites. Based on REEBOX PRO simulations, these parental melts can form from a single mantle source by different pressures and degrees of melting when the potential temperature is 1575–1700 °C. We calculate ranges of S contents for the parental melts based on the different mantle melting conditions and degrees of melting. We use Magma Chamber Simulator to fractionally crystallize the parental melt at crustal conditions. These simulations reproduce the major element oxide, Ni, Cu, and S contents from our komatiite database. Simulated Ni contents in olivine are compatible with literature data from Kevitsa and Sakatti, hence providing a baseline to identify Ni-depleted olivine in CLGB komatiites and related intrusive rocks. We show that fractional crystallization of the komatiitic parental melt can form either Ni-rich or Cu-rich sulfide melt, depending on the initial Ni and S content of the parental melt.