Contributions to Mineralogy and Petrology最新文献

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.

The Eastern Manus Basin is one of the fastest expanding back − arc basins in the world and it is the site of recent volcanism and hydrothermal activity. The role of magma mixing in the origins of the volcanic rocks in this region, as well as the modeling of its magma plumbing system, are still unclear. In this study, we have clarified the magma plumbing system processes of the Eastern Manus Basin by analyzing the petrography and geochemical characteristics of whole rocks and minerals of basaltic andesite, andesite, and dacite in this region. The analyses reveal that basaltic andesite has experienced high undercooling and intense degassing, while both andesite and dacite samples have experienced magma mixing during their formation. The mineral assemblages in andesite are derived from basaltic, dacitic, and mixed melts, with the mixed melt comprising a 2:8 ratio of the former two. Dacite samples contain three mineral assemblages derived from andesitic, rhyolitic, and mixed melts, showing multiple injections of more primitive melts, as indicated by phenocryst textures and chemical zoning patterns. Moreover, they may have experienced the capture of mafic wall rocks. The performance of different mineral − based thermobarometers has been assessed by constructing the experimental datasets applicable to this study, and the best − performing thermobarometers are all from Putirka (2008). Calculations show that the pre − eruption storage temperatures for basaltic andesitic, andesitic, and dacitic magmas are 1090 ± 13 °C, 1032 ± 9 °C, and 938 ± 10 °C, respectively, with storage pressures not well constrained at 4.3 ± 1.4 kbar, 2.8 ± 1.3 kbar, and 2.5 ± 1.3 kbar, respectively. This study provides evidence that magma mixing plays a significant role in the origins of andesite and dacite from the Eastern Manus Basin and that its complex magma plumbing systems provide materials and potential heat sources for the volcanism and hydrothermal activity.

Aluminum has been shown to significantly enhance the water incorporation capacity of orthoenstatite (OEn), but the incorporation mechanisms remained to be clarified. We performed a comprehensive one- and two-dimensional ¹H, ²⁹Si and ²⁷Al NMR study on four hydrous aluminous OEn samples containing 1–8 wt% Al₂O₃ synthesized at 1.5 GPa and 900 °C to clarify the issue. The combined ¹H MAS and static (non-spinning) NMR, ¹H double-quantum and triple-quantum MAS NMR, and ²⁷Al→¹H CP MAS NMR and HETCOR results, in particular, unambiguously revealed that a large part of the incorporated water are present as proton pairs in Mg vacancies adjacent to Al, with one proton of each pair for the dominant proton pairs exhibiting significantly weaker hydrogen bonding than those in Al-free OEn. Proton pairs in Mg vacancies remote from Al are minor or absent for samples with 4–8 wt% Al₂O₃, and more abundant for a sample with 1 wt% Al₂O₃. Isolated protons due to coupled substitutions of Al + H for 1Si and 2Mg (both with weak hydrogen bonding) were also detected, but are less abundant than hitherto considered. The observed NMR peaks match well with those predicted for the corresponding OH defect models from our first-principles calculations. Thus, the enhancement of water solubility by Al for OEn are due to not only coupled substitutions of Al + H for 1Si and 2Mg, but also interactions of Al with proton pairs in Mg vacancies. These mechanisms may also be important in other nominally anhydrous aluminous silicate minerals.

We performed first-principles calculations on the energy, and NMR and polarized infrared (IR) spectra for anhydrous and hydrous aluminous orthoenstatite (OEn) models to help clarify the incorporation mechanisms of Al and OH defects in OEn. Our calculations revealed that proton pairs in M2 vacancies ((2H)_M2) adjacent to a tetrahedral Al (Al^IV) are energetically more favorable than those remote from Al, and may contribute to the observed correlated ¹H NMR peaks near 3.7 and 8.0 ppm, and IR bands near 3550–3570 and 3066 cm⁻¹ (A4 band) for aluminous OEn. Coupled substitutions of Al^VI (octahedral Al) + H for 2Mg were found to adopt multiple configurations, and may contribute to the observed IR bands near 3520, 3475 and 3320 cm⁻¹. Coupled substitution of Al^IV + H for 1Si may contribute to the observed IR band near 3380–3400 cm⁻¹. 4H in SiB vacancies ((4H)_SiB) adjacent to an Al^VI were found to be energetically more favorable than those remote from Al, and may be the origin for an IR band observed near 3600–3620 cm⁻¹. These results allow the incorporation mechanisms of water in synthetic and natural aluminous orthopyroxenes to be deciphered from the available NMR and IR data, and suggest that both (2H)_M2 defects associated with Al and simultaneous coupled substitutions of Al + H for 2Mg and 1Si contribute to the observed correlation between Al and water incorporation, and the nearly unity Al^IV/Al^VI ratio. (4H)_SiB defects associated with Al may also be present in some synthetic OEn and mantle-derived orthopyroxene.

Alaskan-type ultramafic–mafic intrusions in convergent-margin settings provide valuable information on melt-cumulate petrogenetic processes operating at depth in the sub-arc crust. Here, we report the compositions and textural relationships of cumulus and postcumulus minerals in a suite of clinopyroxenites and hornblendites from the peripheral zone of the Tulameen Alaskan-type intrusion in British Columbia, Canada. Mineral chemistry is used to establish magma storage conditions (P, T, fO₂, H₂O_melt) and to reconstruct the composition of cryptic residual liquids that equilibrated with the mineral phases and subsequently escaped the local mush system. Residual liquids in equilibrium with clinopyroxene (diopside) are metaluminous calc-alkaline basalt to andesite; melts equilibrated with amphibole (magnesio-hastingsite) are metaluminous to peraluminous calc-alkaline dacite to low-silica rhyolite. Thermobarometry yields a robust estimate of storage pressure of 400 ± 50 MPa (~ 15 km paleodepth) for the Tulameen magma reservoir and equilibration temperatures of 1130–960 °C for clinopyroxene and 950–850 °C for amphibole. The large cooling interval between the early crystallization of clinopyroxene and late appearance and continued crystallization of peritectic amphibole facilitated progressive extraction of residual liquids from clinopyroxene-rich cumulates, consistent with textural relationships, mass balance calculations and experimental petrology. Peritectic dacitic melts are hydrous (~ 6–8.3 wt% H₂O_melt), oxidized (fO₂ ~ NNO + 1.6 to NNO + 3.6 log units) and buoyantly mobile with low density (~ 2200 kg/m³) and viscosity (~ 10³ poise). Lower water contents likely reflect degassing of peritectic melts driven by amphibole crystallization; relatively high redox conditions are attributed to precursor fractionation of olivine and clinopyroxene preserved as cumulates in the core of the Tulameen intrusion. Peritectic amphibole crystallized in response to migration of a thermally buffered reaction front marking the stability limit of amphibole (≤ 950 °C) and driven by near-isobaric cooling. Pervasive infiltration of reactive dacitic liquids through the clinopyroxene mush formed intergranular/poikilitic amphibole and channelized flow was captured in part by cm-scale hornblendite segregations; aggregated melts formed in situ bodies of replacive hornblendite. The absence of orthopyroxene and rarity of plagioclase in the evolved ultramafic cumulates of Alaskan-type intrusions and similar arc-related rocks is attributed primarily to high H₂O_melt and oxygen fugacity in differentiated arc magmas.

The presence of mafic enclaves and banded pumice reveals key physical processes associated with volcanic eruptions. Here, through the textural and geochemical analyses of the 3550 B.P. Waimihia deposits in Taupō, New Zealand, we demonstrate how disequilibrium crystallization controls the way magmas mix. Andesitic enclaves in pyroclastic deposits from this predominantly rhyolitic eruption consist of microlites that crystallized rapidly during mafic injection into rhyolitic host magma. The variation of microlite textures depends on enclave size, implying that mafic enclaves crystallized as discrete blobs within a host rhyolitic magma. However, gray pumice and dark bands in banded pumice are characterized by a lack of or less plagioclase microlites that should be present if equilibrium crystallization occurred. Our textural and chemical data suggest that the lack of plagioclase in gray pumice and dark bands resulted from the nucleation delay arising from the mixing with rhyolitic magma. After mafic magma broke up in a magma chamber as discrete mafic blobs, the plagioclase-free rim of the blobs was disaggregated by shear flow. The eroded mafic blobs form a hybrid magma by mixing with rhyolitic magma, which further delays the plagioclase nucleation. This hybrid magma eventually erupted as gray pumice or banded pumice, depending on the intensity of magma mingling in the conduit. We use a plagioclase nucleation delay model to calculate residence times of hours to tens of hours prior to eruption. Our mixing model with nucleation delay enables small volumes of mafic magma to mix with large amounts of silicic magma.

The rare-earth elements (REEs, La–Lu, Y) are essential for the development of renewable technologies. Bastnäsite (REECO₃F) is a common REE ore mineral that is often subject to hydrothermal alteration at all crustal levels. Mechanisms of hydrothermal bastnäsite alteration therefore govern the evolution of REE deposits, though these mechanisms remain poorly understood. This experimental work investigates the hydrothermal replacement of bastnäsite by rhabdophane (REEPO₄∙xH₂O, x = 0–1) and monazite (REEPO₄) in phosphatic fluids. Two temperature-dependent alteration pathways were identified; both follow the coupled dissolution-reprecipitation (CDR) mechanism. At 90 °C, bastnäsite was replaced by highly-porous metastable rhabdophane which was then replaced by monazite, forming an inner layer of rhabdophane and an outer layer of monazite. At 220 °C, bastnäsite was replaced directly by monazite. Although replacement initiated more quickly at 220 °C, greater overall replacement occurred at 90 °C (~ 61 wt.% after 500 h, compared to ~ 13 wt.% at 220 °C) due to surface passivation by monazite at 220 °C. Geochemical analyses showed REE fractionation during bastnäsite alteration. At 90 °C, rhabdophane was enriched in heavy REEs (Eu–Lu, Y), likely due to the evolving fluid chemistry, while at 220 °C secondary monazite was enriched in Sm and Ho compared to bastnäsite. These results indicate that: 1) the hydrothermal alteration of bastnäsite by rhabdophane and monazite in ore deposits leads to REE immobilisation, with little net loss of REEs to solution; 2) rhabdophane is metastable relative to monazite at 90 °C, and; 3) variable temperatures can cause different mineral textures and REE fractionation trends during hydrothermal alteration and mineral replacement.