Contributions to Mineralogy and Petrology最新文献

Assimilation of crust by hot basaltic liquids changes physical magma characteristics. Advances have been made in understanding assimilation through isotope and trace element geochemistry, thermodynamic models and theory. Physical examples of assimilation in basaltic magma chambers, however, are less common due to limited exposures capturing the processes. Here, field observations, petrology and bulk rock chemistry, including ⁸⁷Rb-⁸⁷Sr data, are reported for lithologies, including sedimentary blocks and hornfels, within the hypersthene gabbro of the Paleogene-aged Ardnamurchan Igneous Complex, Scotland. Mesozoic sediments and Neoproterozoic Moine schist, into which the hypersthene gabbro intruded, have radiogenic ⁸⁷Sr/⁸⁶Sr (> 0.71). Significant initial Sr isotope variability in the hypersthene gabbro and autoliths (0.70402–0.70921) demonstrates inhomogenous and extensive (> 10%) assimilation in some instances. Mineralogy and radiogenic initial ⁸⁷Sr/⁸⁶Sr (0.70753–0.71030) of hornfels masses are consistent with their derivation by replacement after sedimentary protoliths. Partial assimilation of sediment during cumulate formation accounts for variable quantities of crustal assimilation in the hypersthene gabbro and implies concomitant hydration of the magma. Incomplete assimilation of stoped and foundered sediments enabled greater extents of assimilation, while also enhancing accommodation space for intrusion emplacement. Crustal assimilation may also have played a role in the layering and structure in the intrusion due to cooling and modifying melt composition within a crystal mush. Larger or hotter basaltic magma chambers, and lavas erupted in continental flood basalts or in oceanic settings show variable chemical evidence for assimilation. Consequently, crustal contamination during formation of crystal mushes is likely to be a ubiquitous process in basaltic feeder intrusions.

White mica commonly yields younger ⁴⁰Ar/³⁹Ar dates than ⁸⁷Rb–⁸⁷Sr isochron dates in greenschist to amphibolite facies rocks, which are often interpreted as ⁴⁰Ar/³⁹Ar cooling dates. However, recent petrochronological studies showed that younger ⁴⁰Ar/³⁹Ar dates can result from chemical re-equilibration. We investigate the frequency and conditions under which white mica chemically (re)equilibrates during an orogenic cycle in common upper crustal rocks and its implications for ⁴⁰Ar-loss and ⁸⁷Sr/⁸⁶Sr-(re)equilibration. We have applied double-dating mapping by in-situ ⁸⁷Rb–⁸⁷Sr and ⁴⁰Ar/³⁹Ar geochronology, quantitative chemical mapping, and δ¹⁸O analysis to Mesoproterozoic metasedimentary and igneous rocks from the Black Hills, South Dakota. Intragrain chemical disequilibrium is pervasive in white mica, with primary phenocrystic and porphyroblastic compositions overprinted by reaction-grown phases along cleavage planes and grain boundaries. Re-equilibration involved metasomatic exchange of incompatible elements (Ar, Na, Rb, Sr, Cs, Ba, B, La and Li) during fluid-assisted alteration in a meta-tuff and the Harney Peak Granite. ⁸⁷Rb–⁸⁷Sr white mica isochron dates of 1763.24 ± 17.05 Ma and 1677.95 ± 9.99 Ma for the meta-tuff and the Harney Peak Granite, respectively, are a consequence of high-temperature chemical equilibrium. In contrast, ⁴⁰Ar/³⁹Ar dates of reaction-grown white mica of 1300.36 ± 15.84 Ma and 1284.89 ± 4.87 Ma, record fluid-driven re-equilibration at 364 ± 50 °C (chlorite-thermometry). These results indicate that fluid-assisted processes can reset ⁴⁰Ar/³⁹Ar dates in white mica, while having a minor effect on the ⁸⁷Rb–⁸⁷Sr system, rendering cooling-age interpretations ambiguous when they lack petrological control.

Crustal-derived carbonatites are increasingly recognized, yet their evolutionary history remains inadequately constrained. This study integrates U–Pb geochronology of multiple minerals with element compositions and C–O isotope analyses to elucidate the temporal evolution of the Balangoda carbonatites, a recently confirmed crustal-derived body in the Highland Complex, Sri Lanka. The carbonatites, intruding high-grade metamorphic gneisses, dominate in calcite with variable amounts of phlogopite, apatite, dolomite, clinopyroxene, olivine, and spinel, and occur as coarse-grained and pegmatitic types. Calcite from both types exhibits similar δ¹³C values (–1.83‰ to 0.41‰) and a broader δ¹⁸O range (17.22‰ to 22.86‰) compared to Sri Lankan marbles, indicating a metamorphic carbonate protolith. In pegmatitic carbonatites, systematic decreases in δ¹⁸O and mineral inclusion abundance from blue and white to yellow calcite indicate evolving fluid conditions during protracted magma evolution. Concordia U–Pb ages of zircon (522.8 ± 9.6 Ma) and calcite (524 ± 29 Ma) in sample 23C-BC, along with a weighted mean ²⁰⁷Pb/²⁰⁶Pb titanite age (548 ± 12 Ma), constrain the timing of crystallization. Whereas, the younger apatite age (487.4 ± 8.5 Ma) from the same sample records a protracted cooling history. Additional calcite ages (~ 525 Ma) and consistent apatite ages (521.9 ± 8.4 Ma; 515.8 ± 9.8 Ma) corroborate this extended magmatic evolution. These multi-mineral ages are consistent with the regional high-grade metamorphism (660–500 Ma) followed by protracted cooling to ~ 480 Ma in the Highland Complex. This supports a model of carbonatite magma genesis through anatexis of dolomitic marbles under prolonged high-temperature metamorphism, with antiskarn processes removing dolomitic components. The Balangoda carbonatites, in conjunction with global occurrences, exemplify that long-lived generation and evolution are typical of crustal-derived carbonatites, linked closely to regional metamorphic events.

In several studies of chromite-rich cumulates in layered intrusions, the Cr/Al ratio of chromite varies significantly within centimetre-to-decimetre scale layering. In the G-H chromitite section of the Stillwater Complex (United States), Cr/Al ratios are consistently higher, by up to relative 20%, in the most chromite-rich rocks compared with immediately adjacent layers. Systematic relationships between Cr/Al ratios and chromite modes are also observed in cumulate rocks of the Bushveld Complex (South Africa). This is a critical observation for models of chromitite genesis involving mechanical sorting of chromite from silicate minerals in gravity flows; there is no plausible mechanism for sorting small chromite crystals on the basis of subtle variations in composition. A possible explanation could be in the combined effects of subsolidus re-equilibration and trapped liquid reaction. To test this possibility, a series of model calculations was carried out, solving for conservation of mass, relationships between distribution coefficients, composition and temperature, and stoichiometry. Although Mg/Fe ratios are indeed predicted to be highly mode-dependent, Cr# values (atomic Cr/[Cr + Al]) are insensitive to post-cumulus effects where chromite proportions are greater than 20 wt%. Variation comparable to natural data is only possible for chromite modes less than about 10 wt%. The difference in Cr# between rocks with 20–80 wt% and > 80 wt% chromite, and the short-range cyclicity of Cr# within individual massive chromitite layers, cannot be explained by post-cumulus processes. This variation, therefore, must reflect primary liquidus compositions. This argues in favour of in-situ boundary layer crystallisation models over ones invoking mechanical deposition and hence supports models of in-situ growth of chromitites from large convecting magma bodies.

Understanding the textural cues hidden within cumulate rocks is a crucial component to decoding their formational processes and development throughout the active lifetime of their parental intrusions. However, given the complicated nature of field observations, often resulting in contrasting interpretations, the studies of cumulates benefit from an effective use of textural analysis utilizing parameters unaffected by alterations and equilibration. This study aims to contribute to the present knowledge of cumulate textural evolution and present its exploration through a set of textural parameters. With the main objective being to observe and analyze the textural development of an olivine cumulate, a special focus is paid to the behaviour of cumulate interface. The environment was simulated in high-temperature experimental conditions using a mixture of haplobasaltic glass with crushed forsterite crystals, forming a loose suspension capable of mechanical settling. The resulting cumulate layer was then subjected to T=-40 °C of undercooling to induce overgrowth and porosity reduction, for a duration between 2 and 48 h. Surprisingly, the cumulates experienced only a short period of kinetic growth, followed by gradual equilibration. For both of these processes, the system showed a contrasting behaviour of the interface and the deeper mush, observing prominent textural changes solely in the shallower layers. The results of textural analysis imply a strong effect of initial crystallinity on textural development, explore the evolution of skeletal morphologies, and point out the role of system’s heterogeneity on the process of equilibration.

In zircon, transgressive textures cutting oscillatory zoning and associated chemical modifications are usually attributed to fluid-mediated coupled dissolution-precipitation (CDP). Here, we show an example of melt-mediated CDP in zircon from metagranite, with consequences on calculated ages and chemical composition. Zircon oscillatory zoning is commonly blurred and truncated by embayments and channels of replaced zircon with irregular and sharp boundaries. These are interpreted as replacement fronts and may be spatially associated with micro-porosity. Some superimposed replacement fronts indicate repeated CDP and porosity healing. Micro-porosity and inclusions are associated with replaced domains and thus are epigenetic. We highlight the necessity of combined high-resolution back-scattered electron imaging, otherwise the CDP textures may be overlooked in cathodoluminescence images. Inclusions of Ph–Grt–Ttn–Ap–Qz–Bt are compatible with the matrix assemblage Grt−Ph−Bt−Ttn−Kfs−Pl−Qz ± Rt ± Ilm, which equilibrated at c. 15−17 kbar, 690–740 °C with partial re-equilibration to c. 12 kbar, 680 °C. Since the conditions were above the wet solidus, the zircon changes happened through melt-mediated CDP. The replaced domains show an overall decrease of REE, Y, Th and Th/U, P, a large variation in Yb/Gd, shallower Eu anomaly, and increased U, and Hf. The chemical changes indicate a tendency of zircon purging trace elements during reequilibration with migrating melt. Spots with high LREE are probably due to the effect of micro-inclusions. Based on textures, inherited zircons are c. 540–600 Ma, protolith granite is c. 507–496 Ma, and CDP replacement occurred down to c. 335 Ma. But, the melt-mediated CDP resulted in a smear of mostly concordant, but spurious spot dates between inherited, protolith and modified domains, spanning > 200 Myr.

Apatite is the primary phosphate mineral in the Earth’s crust and is present in a range of intrusive and extrusive igneous rocks, typically at minor to trace quantities. Similarly, apatite is a stable phase in many equilibrium crystallization experiments conducted with phosphorus-bearing starting compositions. We leverage this experimental stability to produce a large compilation of apatite-saturated liquid compositions, supplemented by additional apatite trace element and volatile partitioning and explicit apatite solubility experiments, as well as analyses of natural rhyolitic glasses. Using this compilation, we calibrate two new, independent models: for apatite saturation temperature as a function of melt P₂O₅ and SiO₂ contents and aluminum saturation index (ASI; molar Al/(2Ca + Na + K)); and for melt P₂O₅ contents at apatite saturation as a function of temperature, melt SiO₂ contents and ASI. The first model reproduces apatite saturation temperatures with an accuracy of ~ 32 °C, significantly outperforming existing apatite saturation models as well as recent zircon saturation thermometers. The second model reproduces melt P₂O₅ contents at apatite saturation by better than a factor of two across four orders of magnitude. Our new calibrations show that, within uncertainty, apatite stability does not differ regardless of the specific volatile species present (H₂O, F or Cl) or on the quantity of H₂O dissolved in the melt. Further, we find that apatite stability is not sensitive to pressures at least in the range of 1 atm to 2 GPa. This model accurately describes the saturation of apatite in a wide range of liquids, including metaluminous liquids, moderately alkaline liquids, and most peraluminous liquids. Experimental and natural peraluminous liquids with unusually high P₂O₅ contents that are not well described by our model contain CaO:P₂O₅ ratios below apatite stoichiometry (‘perphosphorous melts’), indicating that apatite saturation in these liquids is at minimum jointly controlled by CaO and P₂O₅ contents.

Trace element ratios in arc magmas are widely used to infer petrogenetic conditions, particularly those associated with porphyry-forming versus barren systems. However, quantitatively linking whole-rock signatures to pressure, water content, or redox conditions remains challenging, as trace elements are sensitive to mineral assemblage and compositions, which both evolve during fractional crystallisation. Here, we integrate a recently updated thermodynamic model suite appropriate for arc systems with dynamic (composition-, temperature-dependent) mineral-melt partitioning to track trace element evolution. Benchmarking the model against published experiments, including apatite saturation, shows that the methodology successfully reproduces phase assemblages and compositions. We simulate fractional crystallisation of an average primitive arc magma across mid- to lower-crustal pressures (4–10 kbar), 2–4 wt% initial H(_2)O, and a range of redox conditions ((sim )2 log units (Delta )FMQ). We demonstrate how mineral-specific vectors evolve in trace element ratio (e.g. Sr/Y, Dy/Dy*) and rare-earth element shape coefficient ((lambda )) space. Results highlight that amphibole and garnet may produce overlapping (lambda )-space vectors under deep, hydrous conditions—contrary to orthogonal vectors inferred using static (composition-, temperature-independent) partitioning. Multiple petrogenetic paths can yield similar whole-rock trace element outcomes, particularly with poorly constrained primitive melt compositions. High Sr/Y, for example, commonly associated with porphyry systems, can form without particularly deep, hydrous, or oxidised conditions, when dynamic partitioning behaviour is considered. Overall, our modelling framework enables evaluation of arc magma petrogenesis and trace element evolution, with implications for porphyry indicators.

Sub-ridge mantle is widely exposed on the seafloor at slow- and ultraslow-spreading ridges, where it forms magma-poor, mantle-dominated oceanic core complexes (OCCs). Although mantle denudation processes at such ridges have been extensively documented, the emplacement mechanism of sub-ridge mantle in orogenic belts and the recognition of such magma-poor OCCs remain challenging. In this study, we present integrated field, petrological, and geochemical investigations of mantle peridotites from the Zhongba ophiolite, together with basaltic blocks from the surrounding mélange in the Yarlung–Tsangpo suture zone. The Zhongba peridotites are relatively refractory, recording < 15% anhydrous partial melting, followed by limited refertilization by discrete, weakly aggregated melts, consistent with a low and episodic magma supply. Both geological architectures and geochemical characteristics suggest a mid-ocean-ridge rather than arc-related origin for the Zhongba ophiolite, which is further supported by the sub-solidus cooling history of the mantle section. In combination with the ocean-island-basalt (OIB)-affinity of basaltic blocks within the mélange, we propose a subduction-accretion model for the emplacement of such sub-ridge OCC, due to its high topography and the “lubricant effect” of mélange. Our study underscores the recognition of fossil mantle-dominated OCCs within global OIB-hosting accretionary orogens and provides far-reaching insights into the reconstruction of the architecture and evolution of those paleo-oceans.

The solidification history of silicate melts strongly controls melt composition and the textures and compositions of crystalline phases. In particular, increasing supersaturation markedly affects crystal nucleation and growth dynamics. Here, we investigate how different crystallization paths influence plagioclase nucleation and crystal habit. Three sets of solidification experiments were performed at atmospheric pressure using a crystal-poor anhydrous andesitic starting material containing plagioclase fragments: (1) isothermal experiments across super- and sub-liquidus conditions, (2) continuous cooling from super-liquidus conditions, and (3) cooling runs held at final temperature. To evaluate the effect of initial superheating, we repeated the isothermal and continuous cooling experiments with a crystal-free melt pre-heated to 1450 °C from a higher initial temperature. Isothermal runs produced numerous small, homogeneous crystals, reflecting spontaneous nucleation. Continuous cooling promoted both growth and nucleation, yielding euhedral to dendritic plagioclase habits with increasing cooling rate. Greater initial superheating or nucleation suppression led to fewer but larger dendritic crystals, similar to those in the fastest cooled seeded runs. These results demonstrate the strong control of thermal pre-treatment on crystal habit and clarify the relative roles of homogeneous and heterogeneous nucleation. Superheating suppresses nucleation by reducing sites for heterogeneous crystallization, favoring growth-dominated textures with limited morphological variability. Although sample edges and Pt wire promote early crystallization, most nucleation occurs on heterogeneities such as gas bubbles, seeds, or impurities, suggesting that homogeneous nucleation may be indistinguishable from heterogeneous nucleation on nano- to micrometric heterogeneities.