Contributions to Mineralogy and Petrology最新文献

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.

Tholeiitic basalts and picrites from the Deccan Traps were used to constrain the pressure and temperature conditions of mantle melting for their origin. Clinopyroxene thermobarometry indicates that all Deccan tholeiites crystallized at low pressures in the upper crust (< 6 kbar/1047–1221 °C). In comparison, the Deccan alkalic rocks crystallized at pressures up to ~ 12.7 kbar. Rare samples of the tholeiites plot on their low-pressure olivine-plagioclase-clinopyroxene (Ol-Pl-Cpx) cotectic boundaries or olivine control lines in phase diagrams. These samples represent unmodified magmatic liquids. Primary magmas of the basalts that plot on their cotectic boundaries were modeled through reverse fractionation by incrementally adding equilibrium Ol + Pl + Cpx, Ol + Pl and Ol ± spinel, until the liquid was multiply saturated with lherzolite at a high pressure. The high-Mg basalts are contaminated with continental crust. Hence, a crustal partial melt was simultaneously subtracted according to energy constraints at each reverse fractionation step for these samples. The results show that the high-Mg basalts are 41–53% fractionated and 1–6% contaminated, and the low-Mg basalts are 63–67% fractionated. Their primary magmas were last equilibrated with spinel lherzolite at 10–13 kbar/1289–1333 °C. A picrite and two very high-Mg basalts plot on their olivine control lines. So, their primary magmas were calculated by adding only equilibrium olivine. These samples are 9–25% fractionated, and their primary magmas were last equilibrated with garnet lherzolite at 25–36 kbar/1452–1531 °C. The estimated mantle potential temperatures are 1400–1500 °C for the Deccan tholeiites, consistent with their origin from a mantle plume.

Thermodynamic, experimental and field studies have suggested that organic compounds could be stable, and in some cases predominate over inorganic carbon species, within subduction zones under high pressure and high temperature (HP-HT) conditions. Beyond sedimentary organic matter of biological origin, solid organics can be inherited from hydrothermal circulation at mid-ocean ridges or abiotically formed by carbonate destabilization in the slab. To assess the fate of solid organic compounds during subduction, HP-HT experiments using piston-cylinder and multi-anvil presses have been performed at 700–1000 °C and 3–7 GPa. Different starting solids were tested, including either synthetic polycyclic aromatic hydrocarbons (PAHs) alone, with (i.e., 1-hydroxypyrene, 1-pyrenebutyric acid) or without (pyrene) oxygen-bearing functional groups, or a mixing of pyrene and antigorite. Our results show that increasing P–T conditions lead to the formation of hydrogenated (±oxygenated) graphitic carbon preserving a high level of structural disorder, far from graphite structure. We also observe the formation of aqueous fluids during experiments at 700 °C and 3 GPa with oxygen-functionalized PAHs, suggesting quick water release from solid organic compounds at HP-HT in subduction zones. Pyrene-antigorite experiments reveal various mineral assemblages depending on redox conditions. Oxidizing conditions favor the formation of magnesite-enstatite-coesite while reducing conditions promote forsterite-enstatite-graphitic carbon assemblages. Our results finally highlight the limited reactivity of solid organic compounds when exposed to aqueous fluids derived from serpentinite under reducing conditions which could facilitate the recycling of organic carbon into the deep mantle.

Understanding the primary controls on mineral deposit formation and size is essential for sourcing the metals required by our ever-growing economy. The tonnage of porphyry copper deposits ranges five orders of magnitude but the key mechanisms and processes that modulate the size of these deposits remain enigmatic. Here, we investigate the behemothian deposits of the Chuquicamata Intrusive Complex (CIC) in northern Chile employing high-precision U–Pb and Re–Os geochronology. We resolve a complex long-lived magmatic-hydrothermal activity that lasted over 3.3 Myr. High-precision zircon petrochronology data indicate two distinct porphyry emplacement episodes, separated by 0.5 Myr, with the younger generation closely tied to the main intervals of hydrothermal mineralization. High-precision Re–Os molybdenite dates reveal a prolonged hydrothermal mineralization interval (> 2.5 Myr) that progressively migrated southwards within the CIC and continued after the end of the (apparent) magmatic activity. We show that the rate of copper precipitation varies little in nature (0.025–0.10 Mt/kyr) and is independent of the size of the deposit. Consistent with evidence from smaller deposits, our findings provide unprecedented evidence that copper endowment in porphyry copper deposits positively correlates with the timescales of magmatic and hydrothermal activity. Supergiant to behemothian deposits require multi-million-year magmatic-hydrothermal activity, linking the largest porphyry copper systems to a simple metric – the duration of magmatic-hydrothermal activity.

Magmatism plays a key role in accommodating and localizing extension during continental breakup. However, how the crustal magmatic systems evolve at the continental-ocean transition is poorly understood. We address these questions by studying the evolution of the magmatic system in the rift of Central Afar (Ethiopia), currently marking the transition from continental rifting to oceanic spreading. We focus on the voluminous and widespread Upper Stratoid Series (2.6–1.1 Ma) and the following Central Afar Gulf Series (1.1–0.6 Ma), the latter corresponding to localization of volcanism in narrow magmatic segments. We carried out the first systematic study of major and trace element mineral chemistry for these two Series and integrated it with geothermobarometry estimates and geochemical modeling, to reconstruct the evolution of the magmatic system architecture during rift localization. The Upper Stratoid magmas evolved by fractional crystallization in a melt-rich, moderately zoned, middle-lower crustal (10–18 km) magmatic system, from where they rose directly to the surface. Polybaric plagioclase convection and dissolution of a plagioclase-rich crystal mush is recorded in the phenocryst texture and chemistry. The Central Afar Gulf magmas evolved at similar depth in a more complex and dynamic storage system, with magma rising and mixing through multiple, relatively small, crystal-rich and interconnected reservoirs. Our study documents the transition during the continental breakup, from an overall stable and melt-rich magmatic system feeding the voluminous and homogeneous Upper Stratoid eruptions to a more dynamic, interconnected and crystal-rich situation feeding small-volume eruption while the rift localizes.