Contributions to Mineralogy and Petrology最新文献

Deep crustal cumulates directly represent the geochemical composition of the lower crust and can provide insights into magmatism at the mantle–crust boundary. However, the scarcity of exposed deep crustal cumulates, which is due to their high density causing such rocks to sink into the mantle, limits our access to deep crustal samples. This study investigated hydrous late Mesozoic mafic–ultramafic cumulate rocks from northeastern Pamir. These rocks are the first of their kind identified in this region and exhibit features typical of deep sub-arc hydrous cumulates worldwide. Petrography, zircon U–Pb ages and zircon Lu–Hf isotopes, whole-rock geochemistry and Sr–Nd isotopes, and mineral major and trace element chemistry were used to constrain the magmatic evolution from source to surface and the crystallization conditions of the primary magma at depth. In situ zircon U–Pb dating yielded a concordant age of 199 ± 1.3 Ma. The mafic cumulates are hornblende gabbros, which had a crystallization sequence of amphibole/magnetite → plagioclase → biotite → apatite. Hornblende geobarometry yielded an equilibrium pressure of 0.65–0.80 ± 0.14 GPa, corresponding to depths of 20–26 km. The ultramafic cumulates, are lherzolites and olivine clinopyroxenites that have a crystallization sequence of olivine/spinel → clinopyroxene → ± orthopyroxene. The estimated pressure, based on published experimental constrains, suggests high-pressure crystallization occurred at ~ 1 GPa. The elevated magmatic oxygen fugacity (ƒO₂) is consistent with values expected for sub-arc conditions, where FMQ is 1–4 log units more oxidized than mid-ocean ridge basalts. The trace element composition of melts calculated to be in equilibrium with clinopyroxene is comparable to the global average composition of continental calc-alkaline basalts. Based on the petrography, mineral chemistry, and uniform whole-rock Sr–Nd isotopic data, the mafic–ultramafic cumulate rocks are inferred to have formed by fractional crystallization of a common hydrous (~ 2 wt% H₂O) parental melt derived from a depleted mantle source (⁸⁷Sr/⁸⁶Sr = 0.7046–0.7132 ε_Nd(t) = 1.5–3.3, ε_Hf(t) = 1.1–11). These results support the notion that the polybaric differentiation in the lower crust can significantly influence the diversity of geochemical composition in the upper crust and highlight that the final closure of the Paleo-Tethys in the northeastern Pamir may not have occurred before the early Jurassic.

Pressure–temperature (P–T) modeling and U–Pb monazite petrochronology provide a detailed P–T-t history for the Funeral Mountains metamorphic core complex, revealing different aspects of the geologic history at different structural depths and enabling the dating of tectonic mode switching cycles in the southwestern US Cordillera. Monazite petrochronology and yttrium X-ray element maps reveal several generations of monazite that formed during the Jurassic to Late Cretaceous. In the Monarch Canyon study area, the staurolite-out isograd separates samples with predominantly Jurassic monazite from those with predominantly Cretaceous monazite. Monazite grains yielding Jurassic to Early Cretaceous dates are chemically distinct from those yielding mid- and Late Cretaceous dates. Jurassic monazite dates from the Funeral Mountains record both prograde and retrograde metamorphism, with the latter associated with garnet breakdown during decompression. Heavy rare earth elements and yttrium (HREE + Y) in a mid-Cretaceous 104 to 88 Ma monazite population link recrystallization to prograde garnet growth from staurolite breakdown, and in a Late Cretaceous 88 to 74 Ma population to retrograde garnet breakdown via a reversal of the staurolite breakdown reaction. Modeling and mineral textures indicate peak metamorphic conditions of 6–10 kbar at ca. 650–700 °C in the structurally deepest rocks in Monarch Canyon. In contrast, structurally shallower rocks experienced peak temperatures between 610 and 650 ºC during Jurassic metamorphism. Monazite petrochronology elucidates the progression of monazite dissolution-reprecipitation along this P–T path. Modeling reactions and mineral stability link specific reactions to changes to the HREE + Y concentrations in monazite, particularly related to garnet and staurolite reactions. This dataset, in conjunction with previous studies, enables the timing and duration of tectonic mode switching cycles in the Funeral Mountains to be quantified, improving our understanding of the complex geological evolution of this core complex.

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.