Contributions to Mineralogy and Petrology最新文献

Deep melt migration processes occurring beneath spreading ridges largely occur by porous flow and involve reaction with the pre-existing crystal matrix. The control of the melt/rock ratios and melt fluxes involved in these reactive percolation processes on the structural and chemical evolution of oceanic magmatic systems is yet to be fully constrained. We here report a combined petro-geochemical study of variably evolved gabbroic layers in the Oman Moho Transition Zone, atop the Maqsad mantle diapir, ranging from dunites, troctolites and wehrlites to olivine gabbros. The layering characterizing the base of the crustal section formed during a process of reactive porous flow and hybridization of a dunitic precursor. Positive feedback between melt distribution and deformation focusing allowed for the development of two distinct percolation behaviours, between focused melt percolation and diffuse melt impregnation. This geological setting provides an ideal case study to assess the impact of the melt/rock ratios and percolation dynamics on the evolution of textures and chemical compositions during focused and diffuse percolation. Namely, the former leads to a modification of the crystallographic preferred orientation and complete chemical reequilibration of the matrix, while the latter allowed for preservation of the pre-existing structure and buffer of the melt composition by the matrix and reactive processes. We quantify the melt/rock ratios associated with the two magmatic systems using Plate Models to demonstrate that focused percolation easily resets the matrix composition from melt/rock ratios integrated over time ~ 2–3, whereas diffuse, low-flux melt impregnation would require elevated melt/rock ratios (> 20) to allow for chemical reequilibration. Furthermore, we provide a global overview of the evolution of mineral compositions and textures of a percolated olivine-rich protolith as a function of the melt migration style and the involved melt/rock ratios, both instantaneous and integrated over time.

Abundant Archaean komatiite and basalt erupted through evolving cratons, indicating melt transfer through the ancient mantle lithosphere. However, this process has rarely been identified in cratonic peridotite xenoliths, in contrast to exposed Phanerozoic mantle sections where melt-rock reactions are well-documented. We present a combined microstructural and mineral chemical investigation of eight coarse-grained (up to 20 mm), silica-rich, spinel facies peridotites from the Kaapvaal craton. These peridotites exhibit mild to strong silica-excess with 30–55 vol% orthopyroxene. Microstructural evidence of former melt presence is abundant in all samples, including low apparent dihedral angles, irregular grain boundaries, and extremely elongate grains. Despite varying silica-excess, all peridotites are highly refractory, with olivine Fo-content of 92.9 ± 0.3, reconstituted whole rock Mg-number of 92.9 ± 0.4, and negligible TiO₂ concentrations. Thermobarometry and comparisons with experimental compositions and thermodynamic models suggest a continuum of reactions in open systems, where evolving komatiite melt sourced from greater depth interacted with precursor mantle lithosphere at 2–3 GPa. We propose that silica-excess in cratonic spinel peridotites results from high time-integrated (i.e., aggregated) melt flux through melt channels, without requiring a highly silicic melt. Evidence for reactive flow of komatiite melt through cratonic mantle supports an intraplate setting for many Archaean greenstone belts and a co-evolution of Archaean crust and mantle.

Compositionally zoned crystals can record changing melt composition and trace element partitioning behavior during magmatic differentiation. Diffusive reequilibration between compositionally distinct zones in crystals can also produce compositional gradients. Here, we compare the length scales of concentration gradients for different elements in clinopyroxene that originate from the Tshirege Tuff and late Valle Toledo Member rhyolites of the Bandelier magmatic system in New Mexico to determine what petrogenetic information is recorded in the zonation. Within these rhyolites there are unzoned ferrohedenbergite crystals, as well as less common normally-zoned clinopyroxene with ferrohedenbergite rims and ferroaugite cores. Compared to the ferroaugite cores, the ferrohedenbergite rims are enriched in Dy and Yb, but depleted in Co, Ti, Sc, Ce, and Nd. The length scales for fast and slow diffusing elements for most gradients measured are indistinguishable, which argues that the gradients emerged predominantly from changing magmatic composition during crystallization, with diffusion having little to no role in establishing the concentration gradients. Fractional crystallization of the phases present in the rhyolites fails to reproduce all trace-element zonation that occur in the clinopyroxene, however, indicating a more complex origin. Based on the compositional similarity of the ferroaugite cores with pyroxene from rhyolites that erupted ≥ 165 kyr earlier, we interpret the ferroaugite cores as antecrysts scavenged from crystal-rich mush during magmatic rejuvenation. The magmatic rejuvenation that remobilized the parent mush of the ferroaugite antecrysts was likely initiated near the end of a > 100 kyr eruption hiatus that preceded the final runup to the catastrophic Tshirege eruption.

The trachydacitic Alpehué tephra from Sollipulli volcano (Andean Southern Volcanic Zone), consists of ignimbrite and fallout from a Plinian eruption about 3000 years ago. It is mainly composed of (1) crystal-rich pumice and ash but also contains (2) chilled knobbly basaltic lava clasts and (3) mostly highly inflated glomerocrystic fragments with high crystal-glass ratios interpreted to represent a crystal mush zoned from basaltic to dacitic bulk compositions. Knobbly lava clasts are of three types: (a) a very phenocryst-poor basalt, (b) a basalt with large, unzoned olivine and plagioclase phenocrysts and glomerocrysts, and (c) mixtures of microcrystalline basalt with various fragments, glomerocrysts and crystals derived from a crystal mush. Clast type (4) in the tephra is banded pumices in which the three magmatic components occur variably mingled. Thermobarometry and petrographic observations, particularly presence or absence of amphibole, constrain an upper-crustal succession of a lower basaltic reservoir, a zoned basaltic to dacitic crystal mush reservoir, and a separate trachydacite magma chamber on top. All Alpehué magmatic components form a coherent liquid line of descent which supports the interpretation that the crystal mush reservoir is a gradually solidifying magma chamber, not the result of large-scale crystal-liquid segregation. The trachydacite magma may originally have formed as melt escaping from the crystal-mush reservoir but subsequently underwent a long and complex evolution recorded in large strongly zoned plagioclase phenocrysts including resorption horizons. The ascending mafic magmas collected samples from the crystal mush body and intruded the trachydacite reservoir. The phenocryst-poor basalt (a) arrived first and entrained and partially resorbed plagioclase from the host magma. The phyric basalt (b) arrived later and did not resorb entrained plagioclase before eruption. Estimated cooling times, plagioclase resorption times and ascent rates avoiding amphibole breakdown limit the duration of these pre-eruptive processes to not more than a few days.

Lower crustal xenoliths from the Missouri Breaks diatremes and Bearpaw Mountains volcanic field in Montana record a multi-billion-year geologic history lasting from the Neoarchean to the Cenozoic. Unusual kyanite-scapolite-bearing mafic granulites equilibrated at approximately 1.8 GPa and 890 °C and 2.3 GPa and 1000 °C (67 and 85 km depth) and have compositions pointing to their origin as arc cumulates, while metapelitic granulites record peak conditions of 1.3 GPa and 775 °C (48 km depth). Rutile from both mafic granulites and metapelites have U-Pb dates that document the eruption of the host rocks at ca. 46 Ma (Big Slide in the Missouri Breaks) and ca. 51 Ma (Robinson Ranch in the Bearpaw Mountains). Detrital igneous zircon in metapelites date back to the Archean, and metamorphic zircon and monazite record a major event beginning at 1800 Ma. Both zircon and monazite from a metapelite from Robinson Ranch also document an earlier metamorphic event at 2200–2000 Ma, likely related to burial/metamorphism in a rift setting. Metapelites from Big Slide show a clear transition from detrital igneous zircon accumulation to metamorphic zircon and monazite growth around 1800 Ma, recording arc magmatism and subsequent continent-continent collision during the Great Falls orogeny, supporting suggestions that the Great Falls tectonic zone is a suture between the Wyoming craton and Medicine Hat block. U-Th-Pb and trace-element depth profiles of zircon and monazite record metasomatism of the lower crust during the Laramide orogeny at ~60 Ma, bolstering recent research pointing to Farallon slab fluid infiltration during the orogeny.

The thermodynamic characterization of apatite minerals, critical for understanding geological processes and material applications, faces significant challenges due to the scarcity of experimental data, particularly standard entropy (S°) values. In this study, we address this gap by optimization of predictive method based on Volume-based Thermodynamics. In the proposed method, the optimization of the widely used Volume-based Thermodynamics is based on breaking down a single linear functional relationship of formula unit volume (V_m) with S° into a set of linear equations. The apatite supergroup splits into distinct subgroups (populations) formed by Me₁₀(AO₄)₆X₂ with the same Me²⁺ cations and tetrahedral AO₄³⁻ anions but with different anions at the X position. Our approach leverages empirical correlations between V_m and S° within specific apatite subgroups. By analyzing the correlations within the subgroups, we established the system of precise linear relationships between S° and V_m, facilitating accurate S° predictions for a wide range of apatite compositions. The proposed approach represents a significant advancement over existing predictive methods offering unparalleled accuracy in estimating S° values for apatite minerals. Through rigorous regression analysis and validation against experimental data, we demonstrate the reliability and robustness of our predictive model across various apatite subgroups. Our findings provide crucial thermodynamic data for understudied apatite compositions and shed light on fundamental relationships between crystal structure and thermodynamic properties in apatite minerals. The precise estimation of S° values enables more accurate modeling of phase equilibria, reaction kinetics, and geological processes involving apatite minerals, facilitating advancements in diverse fields ranging from environmental geochemistry to material science.

In natural corundum, a strong geochemical correlation is sometimes observed between Be and heavy high field strength elements (HHFSEs) such as Nb, Ta and W, and it has been hypothesized that trace elements are hosted in primary inclusions. However, no known mineral enriched in both Be and HHFSEs stable at these geological conditions can explain this correlation. To understand how Be and HHFSEs are distributed in natural corundum down to the atomic scale, two natural Be-bearing sapphire crystals from Afghanistan and Nigeria are studied using laser ablation inductively coupled plasma and time-of-flights secondary ion mass spectrometry, atom probe tomography and transmission electron microscopy. In addition to common trace elements such as Mg, Ti, and Fe, Be and W are detected in the metamorphic sapphire from Afghanistan, whereas Be, Nb and Ta are detected in the magmatic sapphire from Nigeria. Nanoclustering in both samples shows fractionation of Be and high field strength elements (including Ti) by atomic mass, suggesting a secondary process controlled by solid-state diffusion. The homogeneously distributed W and the secondary nano-precipitates bearing Nb and Ta indicates that HHFSEs can be incorporated into the corundum structure during crystallization, most likely through preferred adsorption on the growth surface. The strong correlation between Be and HHFSEs across the growth zones is probably due to Be being attracted by HHFSEs to partially balance the charge when incorporated into the corundum structure. The enrichment of high field strength elements by growth kinetics may result in supersaturated concentrations during crystallization, allowing them to precipitate out when the host corundum is heated above its formation temperature by basaltic magma. Comparison with previous transmission electron microscope studies suggests the same process for incorporating Be and HHFSEs also applies to other natural corundums from different localities.

Whether Inner Mongolia OIB-like basalts originate from the modern upper mantle [e.g. depleted MORB mantle (DMM)] with recycled oceanic crust in the form of pyroxenite or ancient primordial mantle (lower mantle) dominated by peridotite remains unclear. This study presents high-precision W-Fe isotopic data for Late Cenozoic Chifeng basalts (CBs) in Inner Mongolia, NE China, along with their olivine compositions, to better constrain their petrogenesis. The modern mantle-like μ¹⁸²W values (μ¹⁸²W =  − 3.2 ± 3.8 to + 2.5 ± 2.4 ppm) of the CBs indicate that they most likely originated from DMM rather than ancient primordial mantle. The CBs exhibit elevated fractional crystallization-corrected δ⁵⁶Fe values ranging from 0.09 to 0.16‰, compared to those of primitive normal mid-ocean ridge basalts (N-MORBs; δ⁵⁶Fe = 0.03–0.07‰). This argues against the notion that the CBs could be generated solely by the melting of DMM peridotite. The high δ⁵⁶Fe values of the CBs, coupled with their elevated olivine Fe/Mn ratios, suggest the involvement of pyroxenite in their mantle source. The absence of correlation between the Fe isotopes of CBs and Sr-Nd-Hf isotopes, along with their previously reported low δ^98/95Mo values and existing geophysical evidence, supports the idea that pyroxenite in the mantle source of the CBs was most likely generated by the reaction between DMM peridotite and recycled Pacific oceanic crust originating from the mantle transition zone beneath NE China. Therefore, we propose that the mantle source of Inner Mongolia basalts (e.g. CBs) is DMM with some recycled oceanic crust in the form of pyroxenite, without the involvement of ancient primordial mantle. Our study highlights that W-Fe isotopes of basalts can help to identify the nature of mantle source (especially the ancient primordial mantle) and offer valuable insights into mantle lithology and the causes of mantle heterogeneity both locally and globally.

The study of accessory phases, including trace element concentrations and radiogenic isotopes, provides powerful information for a better understanding of geological processes such as crustal anatexis. These accessory minerals are the primary carriers of many incompatible elements and Rare Earth Elements (REE) in crustal rocks. In this contribution, we provide a detailed study on the chemical and isotopic (Nd isotopes) behaviour of accessory minerals within the Chugach Metamorphic Complex in Alaska. This Eocene (55− 50 Ma) metamorphic complex developed in a Late Cretaceous to Paleogene accretionary prism consisting of metapelitic and metagreywacke rocks. The complex exposes a systematic N-S metamorphic gradient from greenschist to upper amphibolite facies (500 to ~ 700 °C) with anatexis under water-saturated conditions and minor muscovite breakdown. Trace element concentration data for apatite, monazite and titanite reveal a strong influence of bulk composition (greywacke vs. pelite) on their REE signatures in the migmatitic gneisses. In xenotime-bearing metapelitic samples, we show that monazite and apatite, which crystallised close to peak metamorphism, have their HREE-Y contents increasing with temperature within a narrow range of ~ 150 °C (550  to ~ 700 °C). While the influence of temperature on the Y content of monazite was already demonstrated before, we prove that apatite follow the same chemical behaviour. In these samples, partial melting process can be tracked via Eu/Eu* which decreases systematically from schist to migmatitic gneisses and is interpreted to be related to plagioclase crystallisation. Among all analysed samples (schists and migmatites), we observe no significant differences in εNd between monazite, allanite and whole-rock, regardless of rock type. This suggests (i) a general homogeneity of Nd isotopic composition above 550 °C up to crustal anatexis, and (ii) an isotopic equilibrium between mineral and whole-rock, indicating Nd isotopic disequilibria induced by partial melting are unlikely in this case study.

The role of bismuth melts in scavenging Au from hydrothermal fluids has been increasingly recognized in the last decade, but the question of how the Au extracted by such melts transforms into nuggets to form high-grade ores remains obscure. Here, we have characterized the nanostructure of gold nanoparticles (AuNPs) in Bi-rich gold ores that precipitated from Bi-Au melts and propose a novel model to explain the genesis of gold nuggets. This model comprises three consecutive processes of Au crystallization in these melts into coarse grains: the initial formation of atomic clusters equivalent to Au nucleation, the coalescence of these clusters into low-crystalline AuNPs followed by their transformation into well-structured ones, and finally the preferential attachment of these NPs along the {111} lattice plane. This atomic crystallization pathway bridges the gap between Au scavenging by metallic melts and nugget formation, thus making the picture of the formation of high-grade gold ores in the context of melt-fluid interaction more complete.