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Only a small fraction (approx. 1–20%) of magmas generated in the mantle erupt at the surface. While volcanic eruptions are typically considered as the main exhaust pipes for volatile elements to escape into the atmosphere, the contribution of magma reservoirs crystallizing in the crust is likely to dominate the volatile transfer from depth to the surface. Here, we use multiscale physical modelling to identify and quantify the main mechanisms of gas escape from crystallizing magma bodies. We show that most of the outgassing occurs at intermediate to high crystal fraction, when the system has reached a mature mush state. It is particularly true for shallow volatile-rich systems that tend to exsolve volatiles through second boiling, leading to efficient construction of gas channels as soon as the crystallinity reaches approximately 40–50 vol.%. We, therefore, argue that estimates of volatile budgets based on volcanic activity may be misleading because they tend to significantly underestimate the magmatic volatile flux and can provide biased volatile compositions. Recognition of the compositional signature and volumetric dominance of intrusive outgassing is, therefore, necessary to build robust models of volatile recycling between the mantle and the surface. This article is part of the Theo Murphy meeting issue ‘Magma reservoir architecture and dynamics’.

The formation, storage and chemical differentiation of magma in the Earth’s crust is of fundamental importance in igneous geology and volcanology. Recent data are challenging the high-melt-fraction ‘magma chamber’ paradigm that has underpinned models of crustal magmatism for over a century, suggesting instead that magma is normally stored in low-melt-fraction ‘mush reservoirs’ 1 – 9 . A mush reservoir comprises a porous and permeable framework of closely packed crystals with melt present in the pore space 1 , 10 . However, many common features of crustal magmatism have not yet been explained by either the ‘chamber’ or ‘mush reservoir’ concepts 1 , 11 . Here we show that reactive melt flow is a critical, but hitherto neglected, process in crustal mush reservoirs, caused by buoyant melt percolating upwards through, and reacting with, the crystals 10 . Reactive melt flow in mush reservoirs produces the low-crystallinity, chemically differentiated (silicic) magmas that ascend to form shallower intrusions or erupt to the surface 11 – 13 . These magmas can host much older crystals, stored at low and even sub-solidus temperatures, consistent with crystal chemistry data 6 – 9 . Changes in local bulk composition caused by reactive melt flow, rather than large increases in temperature, produce the rapid increase in melt fraction that remobilizes these cool- or cold-stored crystals. Reactive flow can also produce bimodality in magma compositions sourced from mid- to lower-crustal reservoirs 14 , 15 . Trace-element profiles generated by reactive flow are similar to those observed in a well studied reservoir now exposed at the surface 16 . We propose that magma storage and differentiation primarily occurs by reactive melt flow in long-lived mush reservoirs, rather than by the commonly invoked process of fractional crystallization in magma chambers 14 . Magma storage and differentiation in the Earth’s crust mainly occurs by reactive melt flow in long-lived mush reservoirs, rather than by fractional crystallization in magma chambers, as previously thought.

The Xihuashan and Yaogangxian granitic plutons in South China comprise highly evolved multiphase Li‐rich granites and host quartz‐vein‐type tungsten deposits. The δ7Li values of Phase A (early stage), B (middle stage), and C (late stage) from the Xihuashan pluton are 1.0–1.2‰, 1.1–3.0‰, and 2.4–2.8‰ respectively, increasing through chemical evolution. The granites from the Yaogangxian pluton also display gradually enriched in heavy Li isotopes in a later stage, although systematically lighter than those of the Xihuashan pluton. In both plutons, the δ7Li shows good correlations with SiO2 and Li concentrations as well as Rb/Sr, Nb/Ta, and Zr/Hf ratios, indicating Li isotopic fractionation most likely caused by magmatic differentiation. In situ analyses show that the minerals of Xihuashan pluton record a continuous elemental spectrum, reflecting the results of progressive magmatic differentiation. The δ7Li values of quartz, feldspar, mica, and zircon all correlate well with the chemical evolutions of granitic magma, systematically elevated in Phases B and C relative to Phase A. The Li isotope data of the mineral separates further document that the enrichment of 7Li in the residual melt was most likely due to the equilibrium fractionation between the mineral and melts. The data are interpreted to reflect that intense magmatic differentiation was responsible for Li isotopic variations coupled with the enrichment in the Li, F, P, and rare metals in the late‐phase granites of the Xihuashan pluton. The lithium isotope behavior documented in this study provides new insights into magmatic differentiation and associated rare‐metal mineralization. Plain Language Summary In the past, magmatic differentiation was thought not to produce resolvable Li isotopic fractionation based on studies of basaltic rocks. Overall, only sparse studies on Li isotopes of highly evolved granites have been reported, and thus possible isotope fractionation of Li isotopes during differentiation of felsic magma could have been neglected. In this study, we present evidence of Li isotope fractionation during the differentiation of granitic magma. The most evolved (late‐stage) rare‐metal‐rich granites are systematically enriched in heavier Li isotopes. The chemical and isotopic compositions of both bulk rocks and mineral separates (e.g., zircon and mica) indicate that equilibrium fractionation during fractional crystallization governs Li isotopic behavior and enriches heavier Li isotopes in the residual melts. Our quantitative modeling also supports the equilibrium fractionation between minerals and magma as the primary mechanism for the observed Li isotopic variations. Key Points Large Li isotopic fractionations are observed in rare‐metal‐rich peraluminous granites Fractional crystallization governs Li isotopic behavior and enriches heavier Li isotopes in the highly evolved granites Lithium isotopes provide new insights into the granitic magmatic differentiation process and rare‐metal mineralization

To better investigate the behavior of titanium (Ti) isotopes during magmatic processes, we report high-precision Ti isotope compositions for 60 terrestrial igneous rocks from different geological settings worldwide. Based on their major element compositions and petrographic descriptions, these samples can be subdivided into two groups: Fe-Ti oxide unsaturated and Fe-Ti oxide saturated. The Fe-Ti oxide unsaturated group samples show a narrow δ49/47Ti (δ49/47Ti = [(49Ti/47Ti)sample/(49Ti/47Ti)OL-Ti] × 1000) range (− 0.036 ± 0.043‰ to 0.082 ± 0.021 ‰), and no correlation between δ49/47Ti and the degree of differentiation is observed. By contrast, Fe-Ti oxide saturated group samples show a remarkable δ49/47Ti variation, ranging from 0.005 ± 0.018‰ to 1.914 ± 0.006 ‰, which are positively correlated with SiO2 contents, and negatively correlated with MgO contents. In particular, multiple SiO2 vs. δ49/47Ti trends are observed in Fe-Ti oxide saturated group, which are controlled by crystal fractionation degrees, magma SiO2 compositions, and Fe-Ti oxide compositions during magma differentiation.

The behavior of nickel (Ni) isotopes during magmatic differentiation is not adequately explored. Here, we find that tholeiitic rocks in the Kīlauea Iki (KI) lava lake, Hawai'i, show progressively lighter Ni isotopic compositions with increasing magmatic differentiation, whereas calc‐alkaline rocks from the thick Kamchatka arc (30–45 km), located at the convergent boundary of the Eurasian and Pacific plates show increasing Ni isotope values as MgO and Ni decrease. Forty‐three global intermediate‐felsic continental igneous rocks analyzed in this study display large Ni isotopic variations, with the Eoarchean samples having light Ni isotopic compositions that fall in the trend defined by the KI lavas, and the post‐Eoarchean samples showing systematically heavier Ni isotopic compositions overlapping those of Kamchatka arc rocks. The isotopic dichotomy results from the crystallization of isotopically heavy magnetite during low‐pressure differentiation of KI lavas, whereas the participation of sulfide separation that removes isotopically light Ni during high‐pressure differentiation of magmas traversing thick continental crust. Combined with Rhyolite‐MELTS and sulfur concentration at sulfide saturation simulations, we demonstrate that the Ni isotope fractionation during magmatic differentiation is mainly controlled by the tempo of magnetite crystallization and sulfide formation, which is a function of pressure, oxygen fugacity, and water activity. High‐pressure calc‐alkaline differentiation usually suppresses magnetite crystallization while stabilizing sulfide, leading to heavy Ni isotopic compositions for the evolved magmas, significantly deviating from the low‐pressure fractionation trend seen in the KI lavas. Ni isotopes can be used in the future as a tracer of magmatic differentiation and processes of continent formation and differentiation. Plain Language Summary Magmatic differentiation plays an important role in generating the distinctive calc‐alkaline compositions of the continental crust. We show that low‐pressure differentiation of the Kīlauea Iki tholeiitic lavas displays progressively lighter Ni isotopes with differentiation, whereas high‐pressure differentiation of calc‐alkaline volcanic rocks from the thick Kamchatka arc leads to heavier Ni isotopes. This difference may result from the fact that higher pressure may have suppressed magnetite crystallization while stabilized sulfide relative to low‐pressure differentiation. The degree and direction of Ni isotope fractionation during magmatic differentiation are mainly controlled by the differentiation pressure, oxygen fugacity, and water activity. Thus, Ni isotopes can be a novel tracer of magmatic differentiation and processes of continent formation and differentiation. Key Points Nickel isotopic compositions of Kīlauea Iki lavas become lighter with increasing magmatic differentiation Nickel isotopic compositions of Kamchatka arc lavas become heavier with increasing magmatic differentiation Nickel isotope fractionation during magmatic differentiation is controlled by the tempo of magnetite crystallization and sulfide formation

We report high-precision Mg isotopic analyses of different types of lunar samples including two pristine Mg-suite rocks (72415 and 76535), basalts, anorthosites, breccias, mineral separates, and lunar meteorites. The Mg isotopic composition of the dunite 72415 (δ25Mg = −0.140 ± 0.010‰, δ26Mg = −0.291 ± 0.018‰), the most Mg-rich and possibly the oldest lunar sample, may provide the best estimate of the Mg isotopic composition of the bulk silicate Moon (BSM). This δ26Mg value of the Moon is similar to those of the Earth and chondrites and reflects both the relative homogeneity of Mg isotopes in the solar system and the lack of Mg isotope fractionation by the Moon-forming giant impact. In contrast to the behavior of Mg isotopes in terrestrial basalts and mantle rocks, Mg isotopic data on lunar samples show isotopic variations among the basalts and pristine anorthositic rocks reflecting isotopic fractionation during the early lunar magma ocean (LMO) differentiation. Calculated evolutions of δ26Mg values during the LMO differentiation are consistent with the observed δ26Mg variations in lunar samples, implying that Mg isotope variations in lunar basalts are consistent with their origin by remelting of distinct LMO cumulates.

We have used multiple-regression methods to calibrate new, pressure-independent empirical chemometric equations to calculate the major element composition of basanitic to rhyolitic melts in equilibrium with calcic amphibole. The equations are based on amphibole stoichiometric formula components ± temperature from published experimental P-T-X data and avoid some problems of previous studies associated with uncertainties in pressure determination. Compared with the pressure-dependent equations of Ridolfi and Renzulli (2012), tests run using an independent data set indicate that the new equations yield improved precision and accuracy, in particular for SiO2, TiO2, CaO, and K2O. The results are only marginally more precise when temperature is used as a dependent variable, demonstrating that temperature has a relatively minor role in controlling amphibole crystal chemistry compared with melt composition. This allows us to accept a small decrease in precision in excluding temperature from the analysis, which is very convenient for application of the equations to natural amphiboles where temperature is typically unknown. Using the new chemometric equations, reconstructed melt compositions in equilibrium with the rims of amphiboles in pumice clasts of the Ongatiti ignimbrite are in good agreement with coexisting matrix glass compositions, lending support for our analysis. The compositionally variable cores of the amphiboles give predicted melt compositions with large compositional variations from andesitic (63 wt% SiO2) to high-Si rhyolite. These compositional variations in the predicted melt compositions suggest that there may be a range of heterogeneous melts undergoing progressive differentiation within a major crustal magma storage region underneath the volcano. The results support the existence of genuine intermediate composition melts within the storage region. Interaction between these stored melts, disaggregating mush fragments and replenishing magmas gives rise to the chemical complexity observed in erupted magmas. We also used our multiple regression model to predict the compositions of melts that were in equilibrium with amphiboles in plutonic nodules from Grenada lavas. The predicted melts cover a wide range of compositions, perhaps as a result of in situ fractionation, but are consistent with melt inclusions hosted in those cumulates, as reported by Stamper et al. (2014) Overall, our new pressure- and temperature-independent equations resolve issues associated with previous pressure-dependent studies and represent a useful tool for further investigation of crustal processes at subduction zones.

The Jurassic magmatic record in the southern Colombian (Northern Andes) includes numerous subduction-related I-type calc-alkaline granitoids with diverse structures and textures, formed in two main episodes at ∼195 to 165 Ma and ∼165 to 145 Ma. We provide new insights into the mineral chemistry, estimates of intensive parameters and petrogenetic processes of 12 plutonic occurrences in the region, grouped in 4 petrographic associations. Primary mineral assemblages include labradorite-to-oligoclase, alkali feldspars, ferroan enstatite, Mg-rich augite to ferroan-diopside, tschermakite to hastingsite and hornblende and Mg-rich annite; Fe-rich phlogopite and actinolite are post-magmatic phases. Amphibole chemistry indicates that the older (195–165 Ma) Jurassic bodies formed from relatively highly oxidized (f O2 values buffered at −0.1 ≤ NNO ≤ +1.4) hydrous (∼4 to 6 wt % H2O) magmas and their differentiation involves significant crustal assimilation and/or magma mixing, fractional crystallization and late-magmatic re-equilibration processes. In contrast, the younger (165–145 Ma) Jurassic intrusives, derived from subducted-modified mantle sources, record moderately lower oxidized hydrous conditions (f O2 values −0.7 to 0.8 ≤ NNO; ∼5 wt % H2O) with magma evolution mainly controlled by fractional crystallization and late-magmatic re-equilibration processes. Clinopyroxene-only, amphibole-only and amphibole-plagioclase thermobarometry estimations suggest that the Jurassic occurrences crystallized over variable temperature (647°C–1087°C) and pressure (0.7–6.3 kbar) conditions, corresponding to emplacement depths ranging from ∼15, ∼8 to 11, ∼5 to 7 and <4 km along the arc crustal column. The obtained data combined with time evolution allow the identification of exhumed and fragmented arc blocks in the Jurassic magmatic system and provide an essential link between the orogenic deformation event poorly constrained in the Northern Andes.

The genesis and timing of molybdenum (Mo) mineralization in the Mada Ring Complex, north-central Nigeria, have been constrained using a combination of whole-rock elemental geochemistry, Nd-Sr isotopes, zircon U-Pb-Hf isotopes, and molybdenite Re-Os geochronological data. The Mada Ring Complex is one of fifty-three within-plate alkaline ring complexes with economically significant Sn and Nb production from largely alluvial placer deposits in north-central Nigeria. Molybdenum mineralization in the ring complex is dominantly disseminated with minor stockwork veins hosted within biotite granite. Integrated zircon U-Pb and molybdenite Re-Os geochronology constrain initial magma emplacement to between 152 and 151 (± 1) Ma and the Mo-mineralization event to ≤ 150 (± 1) Ma. The granites associated with Mo mineralization are highly silicic and display elevated whole-rock concentrations of Nb, Zr, Y, and REE (except Eu), as well as high 87Rb/86Sr ratios, but low concentrations of Sr, Ba, CaO, MgO, and TiO2, consistent with highly differentiated A-type granites. They yield moderately negative whole-rock εNd(t) (− 5.87 to − 5.47) and zircon εHf(t) (− 4.97 to − 10.06), suggesting that their parental magmas were largely derived from the lower crust with a contribution from the upper mantle. Magmatic oxygen fugacity (ƒO2) of the A-type granites (log FMQ = +1.16), constrained from zircon compositions, is lower than values reported from typical within-plate porphyry molybdenum deposits (log FMQ = +2 to + 3). However, compared to regional Sn ± Nb-mineralized A-type granites, the Mada A-type granites have higher ƒO2 and Ce4+/Ce3+, but are less differentiated.

Liquid immiscibility has become the preferred mode of genesis for the carbonatite rocks, which commonly, but not exclusively, accompany silicate rocks in alkaline-rock complexes. This concept has been universally based on the presumption that nephelinitic and phonolitic magmas can evolve to a stage where two conjugate immiscible liquids separate. It is assumed that these two liquids separate quickly, or even instantaneously, into discrete bodies of magma capable of being intruded or extruded with subsequent independent crystallization. Supporting evidence generally given is: alleged consanguinity as discrete occurrence of the two rock types; similarity of radiogenic isotope ratios; trace element contents similar to those predicted from experimentally derived partition coefficients. We do not accept that a general case for liquid immiscibility has been demonstrated; although we do accept that silicate and carbonate liquids are inherently immiscible, we maintain that they are not conjugate in a petrogenetic context. We have reviewed and critically examined the experimental data purporting to establish liquid immiscibility and find that when applied to natural rocks, they are based on inappropriate experimental designs, which are not relevant to the genesis of calcite or dolomite carbonatites, although they might have some relevance to Oldoinyo Lengai nyerereite–gregoryite lavas. The design of these experiments guarantees immiscibility and ensures that the carbonate liquids formed will be calcitic or sodium-rich. We dispute the validity of comparing the trace element contents of natural rocks, which in many instances do not represent liquid compositions, to experimentally determine partition coefficients. We consider that experimental design inadequacies, principally assuming but not proving, that the liquids involved are conjugate, indicate that these coefficients are merely an expression of the preference of certain elements for particular liquids, regardless of how the liquids formed. Proof of consanguinity in alkaline complexes requires more accurate age determinations on the relevant rock types than has generally been the case, and in most complexes, consanguinity can be discounted. We dispute the contention that melt inclusions represent parental melts, although they might elucidate the character of magmas undergoing fractional crystallization from magmatic to carbothermal stages. Radiogenic isotope data are shown to be too widely variable to support a case for liquid immiscibility. We address the contention that calcite cannot crystallize from a dolomitic liquid formed by direct mantle melting, and must therefore have crystallized from a calcite carbonate liquid generated by liquid immiscibility, and demonstrate that it is an unsupported hypothesis as calcite can readily crystallize from dolomitic liquids. We observe that, because immiscible dolomite liquids have never been produced experimentally, the liquid immiscibility proposition could at best be applied only to calcite carbonatites, thus leaving unexplained the large number of dolomite carbonatites and those of either type, which are not accompanied by alkaline silicate rocks. The assumed bimodality of alkaline-rock carbonatite complexes is considered to be fallacious and no actual geological or petrographic evidence for immiscibility processes is evident in these complexes. Several examples of alkaline rock carbonatite complexes for which immiscibility has been proposed are evaluated critically and shown to fail in attempts to establish them as exemplifying immiscibility. We conclude that no actual geological or experimental data exist to establish liquid immiscibility being involved in the genesis of calcite or dolomite carbonatite-forming magmas.