Evolution Of Magmatic System Research Articles

Tracking Crystal‐Melt Segregation and Accumulation in the Intermediate Magma Reservoir

AbstractThe genesis of intermediate intrusions is highly controversial, and one of the hot topics is whether they represent frozen melts or cumulates in the evolution of magmatic systems. Distinguishing accumulation from crystallization melt differentiated along the liquid line of descent is the key issue. The Paleoproterozoic intermediate intrusions in southern North China Craton provide an excellent case to decipher this issue. Multiple lines of evidence, including mineral textures, geochemistry as well as alphaMELTS modeling, indicate disequilibrium between whole‐rock and minerals, with melt extraction occurring at temperatures of 760°–820°C and with 10–40 wt.% of trapped melts. Effective water storage, revealed by amphibole and clinopyroxene hygrometers, plays a crucial role in promoting crystal‐melt segregation in pluton‐sized reservoirs in the upper crust. This study demonstrates that the accumulation in intermediate magmas can be identified even without evident complementary initial and extracted melts and provides deep insights into the genesis of intermediate continental crust.

Magma Source and Petrogenesis of the Early Cretaceous Granites in The Liaodong Peninsula: Evidence from In Situ Apatite Sr-Nd and Zircon Hf-O Isotopes

Apatite Sr-Nd and zircon Hf-O isotopes are broadly used to trace magma sources and constrain magma evolution processes, further improving our understanding of the origin of granitoids. We present zircon U-Pb ages, whole-rock major and trace elements, and whole-rock Sr-Nd-Hf, zircon Hf-O, and apatite Sr-Nd isotopic data for the coarse-grained quartz monzonite, biotite monzogranite, and granite porphyry in the Yushulinzi pluton in the Liaodong Peninsula, the eastern North China Craton, to establish their magma sources and petrogenesis. The coarse-grained quartz monzonite, biotite monzogranite, and granite porphyry were formed contemporaneously, with zircon U-Pb ages of 123–119 Ma. They share enriched whole-rock Sr-Nd-Hf and zircon Hf isotopic compositions, and the coarse-grained quartz monzonite has crust-like δ18O values (5.7–6.7‰). The coarse-grained quartz monzonite and biotite monzogranite have variable apatite (87Sr/86Sr)i ratios and negative apatite εNd(t) values. These isotopic characteristics indicate that the different rock types in the Yushulinzi pluton were derived from the partial melting of ancient crustal material in the North China Craton. Their geochemical and petrographic characteristics indicate that the crystal-melt segregation model can be employed to elucidate the genetic links among different rock types, with the coarse-grained quartz monzonite representing crystal accumulation and the biotite monzogranite and granite porphyry representing interstitial melts extracted from a crystal-rich magma chamber. Furthermore, the variable apatite Sr isotopic compositions and subtle differences in the peak zircon εHf(t) values of the studied rock samples confirm the possibility of a contribution from shallow crustal components and materials with high εHf(t) values during magma evolution, which is not readily revealed by their whole-rock Sr-Nd-Hf isotopic compositions. These results demonstrate that in situ apatite Sr-Nd and zircon Hf-O isotopic analyses have the potential to provide distinctive insights into the magma sources and evolution of magmatic systems.

Zircon Hf–isotopic mapping of Middle–Lower Yangtze River Valley Metallogenic Belt, China: Constraints on crustal properties and ore cluster formation

The formation of endogenic mineral deposits is commonly thought to be closely related to the initiation and evolution of magmatic systems, which are controlled by deep structure and dynamic processes. The Middle–Lower Yangtze River Valley Metallogenic Belt (MLYB), a paramount metallogenic belt in eastern China, lies between the Dabie and Jiangnan orogens. Although large-scale mineralization occurred in MLYB during the Mesozoic, it is unclear why numerous metals were enriched in such a narrow region. We integrated geological, geochemical, and geochronological data (specifically zircon UPb dating and LuHf isotope data) for the Mesozoic magmatic rocks in MLYB and adjacent parts of the Dabie and Jiangnan orogens. Then, we first drew regional zircon Hf–isotope compositions and model-age maps from 259 weighted-average data values (64 new and 195 prior published) and marked 98 major deposits of MLYB on the contour maps according to their latitude and longitude.The results showed a wide range of zircon crustal Hf–isotopic model ages (TDMC) of MLYB, indicating that its basement is more likely generated by the mixing of the Jiangnan and Dongling basements. Almost all large deposits in the metallogenic belt were concentrated on these basement interfaces, indicating that the junction zone of two basements is favorable for mineralization.Almost all large Cu–Au–Mo polymetallic deposits of MLYB formed during the compression–extension transition stage from 151 to 138 Ma, and zircon Hf–isotope compositions of related high-K Calc–alkali intrusions evolved from high to low. During 137–126 Ma, numerous magnetite deposits formed, and the amount of Cu–Au–Mo polymetallic deposits rapidly decreased. The zircon Hf–isotope compositions of magmatic rocks in this stage evolved from low to high. The concentration of S and Cu–(Au) in the mother magma might be diluted by a large amount of crustal materials, and the Cu content in the magma was significantly reduced by magnetite formation.There was a wide range of Hf–isotope compositions in Mesozoic high-K calc–alkaline series and shoshonite series magmatic rocks, which are closely related to MLYB mineralization, and almost all Cu–Au–Mo deposits and magnetite–apatite deposits were developed in the εHf(t) isotopic boundary. Therefore, magma mixing played a key role in the formation of deposits in MLYB.The Mesozoic magmatism and mineralization of MLYB migrated from Edongnan to Jiurui ore clusters northeastern with time (151–105 Ma). This phenomenon was related to the rollback of the subducting Paleo–Pacific slab in the Late Jurassic and Early Cretaceous, which led to the earlier heating of the lithospheric mantle base in the southwest than in the northeast when the metasomatic mantle source partially melted.Hf–isotope mapping showed spatiotemporal relationships among crustal structure, crust–mantle interaction, and ore cluster formation process, thereby demonstrating that the structure, nature, and composition of the crust control the location of ore deposits and the migration of ore-forming metals within the terrane.

Eruption Pattern and a Long-Term Magma Discharge Rate over the Past 100 Years at Kelud Volcano, Indonesia

Kelud Volcano is among the most active volcanoes in Indonesia, with repeated explosive eruptions throughout its history. Here, we reconstructed the relationship between the repose period and the cumulative volume of erupted material over the past 100 years and estimated the long-term magma discharge rate and future eruptive potential and hazards. Tephra data and eruption sequences described in historical documents were used to estimate the volume and mass discharge rate. The volumes of the 1901, 1919, 1951, 1966, 1990, and 2014 eruptions were estimated as 51–296 × 106m3. The mass discharge rates were estimated to be on the order of 107kg/s for the 1919, 1951, and 2014 eruptions and the order of 106kg/s for the 1966 and 1990 eruptions. Based on a linear relationship between the repose period and cumulative erupted mass, the long-term mass discharge rate was estimated as ∼ 1.5 × 1010kg/year, explaining the features of the larger eruptions (1919, 1951, and 2014) but not those of the smaller eruptions (1966 and 1990). This estimate is relatively high compared to other typical basaltic-andesitic subduction-zone volcanoes. This result provides important insights into the evolution of magmatic systems and prediction of future eruptions at Kelud Volcano.

Consequences of a crystal mush-dominated magma plumbing system: a mid-ocean ridge perspective.

Crystal mush is rapidly emerging as a new paradigm for the evolution of igneous systems. Mid-ocean ridges provide a unique opportunity to study mush processes: geophysical data indicate that, even at the most magmatically robust fast-spreading ridges, the magma plumbing system typically comprises crystal mush. In this paper, we describe some of the consequences of crystal mush for the evolution of the mid-ocean ridge magmatic system. One of these is that melt migration by porous flow plays an important role, in addition to rapid, channelized flow. Facilitated by both buoyancy and (deformation-enhanced) compaction, porous flow leads to reactions between the mush and migrating melts. Reactions between melt and the surrounding crystal framework are also likely to occur upon emplacement of primitive melts into the mush. Furthermore, replenishment facilitates mixing between the replenishing melt and interstitial melts of the mush. Hence, crystal mushes facilitate reaction and mixing, which leads to significant homogenization, and which may account for the geochemical systematics of mid-ocean ridge basalt (MORB). A second consequence is cryptic fractionation. At mid-ocean ridges, a plagioclase framework may already have formed when clinopyroxene saturates. As a result, clinopyroxene phenocrysts are rare, despite the fact that the vast majority of MORB records clinopyroxene fractionation. Hence, melts extracted from crystal mush may show a cryptic fractionation signature. Another consequence of a mush-dominated plumbing system is that channelized flow of melts through the crystal mush leads to the occurrence of vertical magmatic fabrics in oceanic gabbros, as well as the entrainment of diverse populations of phenocrysts. Overall, we conclude that the occurrence of crystal mush has a number of fundamental implications for the behaviour and evolution of magmatic systems, and that mid-ocean ridges can serve as a useful template for trans-crustal mush columns elsewhere. This article is part of the Theo Murphy meeting issue 'Magma reservoir architecture and dynamics'.

Physicochemical Parameters of Magmatism of the Uksichan and Ichinsky Volcanoes (Sredinnyi Ridge, Kamchatka): Data on Melt Inclusions

Abstract —Thermobarogeochemical study of melt inclusions and investigation of clinopyroxenes and amphiboles from effusive rocks of the Uksichan and Ichinsky Volcanoes gave an insight into the parameters of deep-seated melts and the evolution of magmatic systems during the formation of minerals in intermediate chambers. Study of melt inclusions from the Uksichan volcanic rocks made it possible to estimate the pressure during the liquidus crystallization of clinopyroxenes and plagioclases from basaltic magmas and to establish four depth intervals of the formation of these minerals: ~60, 45–30, 27–18, and from 12 km to the subsurface. Comparison of the results of calculation based on melt inclusion data and of the clinopyroxene and amphibole data helped to establish the evolution paths of the P–T parameters of ascending melts of the Uksichan Volcano. The most high-temperature magmas, generated at a depth of ~60 km, are characterized by a successive temperature decrease during their ascent (1320–1240–1200 °С). Based on the representative data on the compositions of amphiboles from the Uksichan and Ichinsky Volcanoes, we have elucidated the general regularities of the evolution of intermediate and acid magmatic systems, with three depths of crystallization in intermediate chambers. Amphiboles of the Ichinsky Volcano andesites and the Uksichan Volcano latites crystallized at depths of 22.0–18.5 and 18–16 km and at temperatures of 980–930 and 1010–985 °С, respectively. As melt ascended to a depth of 15.5–11.0 km and a temperature decreased from 945 to 880 °С, amphiboles of andesites and dacites of both volcanoes were produced. At the final stage (a temperature decrease to 900–810 °С and ascent of melts to a depth of 3 km), only amphiboles of dacites of both volcanoes crystallized.

Correlated Changes Between Volcanic Structures and Magma Composition in the Faial Volcanic System, Azores

The interaction between magmatic and tectonic processes in ocean intraplate volcanism yields insights into the ascent and transport of magmas. Many oceanic intraplate studies lack a temporal component and do not consider changes in tectonic regime and/or magmatic processes during the evolution of magmatic systems. The eastern part of the Azores archipelago formed under the influence of both an intraplate melting anomaly and a system of ultraslow spreading rift axes. The majority of recent submarine and subaerial eruptions in the Azores occur along volcanic rift zones and thus melt transport and volcanic processes are largely controlled by tectonic processes, and are therefore ideally suited to study volcano-tectonic interactions. Here, we investigate how variable the magmatic and tectonic processes are in space and time and how they interact. We present new bathymetric, geophysical, geochemical and Sr-Nd-Pb isotope data from Faial Island and the surrounding seafloor, providing insights into the interaction between the asthenospheric melting anomaly and extensional lithospheric stresses. The bathymetry reveals large submarine volcanic rift zones on the western flank of Faial, including that of the 1957-1958 Capelinhos eruption. Based on absolute ages and seismic imagery, we develop a relative chronology of the magmatic evolution of the submarine rift zones. Their preferred WNW-ESE orientation implies that the stress field has not changed within the last ~1 Ma. We can however show that melt productivity progressively decreased with time. Compositionally different magma batches fed distinct volcanic rift zones and edifices, suggesting that changes in the melting regime occur on a small spatial scale and that the distribution of compositionally similar lavas is tectonically controlled. As melt supply decreases, the tectonic influence on volcanism increases with a stronger localisation of melts along tectonically controlled lineaments. The youngest mafic and intermediate melts (<10 ka) on Faial are exclusively erupted along single rift zones and cover a smaller area, whereas the older volcanism was more widespread.

Tracking the evolution of Late Mesozoic arc-related magmatic systems in Hong Kong using in-situ U-Pb dating and trace element analyses in zircon

The links between large-scale silicic volcanism and plutonism offer insights into the dynamics of crustal magmatic systems and growth of continental crust. In Hong Kong, voluminous silicic ignimbrites and linked plutons record a ~26 Myr period of magmatism from ~164 to 138 Ma. We present data from these linked volcanic-plutonic assemblages at the Lantau and High Island caldera complexes, with an emphasis on the ~143–138 Ma activity from the latter. To track the evolution of these magmatic systems, U-Pb dating and trace element analyses using secondary-ion mass spectrometry (SIMS) were carried out on zircons from 21 samples from both volcanic and plutonic samples. The SIMS age data sets divide into two groups across volcanic and plutonic origins: (1) seven samples with unimodal age spectra [five of which have the same mean value as the published Isotope Dilution Thermal Ionization Mass Spectrometry (ID-TIMS) age from the same sample]; and (2) 14 samples yielding multiple age components. Age patterns from both groups suggest that the previously separated ~143 Ma Repulse Bay (RBVG) and ~141–140 Ma Kau Sai Chau volcanic groups (KSCVG) instead represent activities over a single ~5 Myr period. Direct linkages previously proposed between some volcanic and plutonic units for this period (e.g., High Island Tuff, Kowloon Granite) are no longer supported, and magmatism represented by exposed plutons continued until 137.8 ± 0.8 Ma (Mount Butler Granite). Under CL imagery, a wide range of zircon textures identified in both volcanic and plutonic samples is indicative of complex processes, some of which are identified through trace element data coupled with textural characteristics. Overall, intra-grain (cores vs. rims; sector-zonation) and intra-sample variations in trace element abundances and ratios are larger than those between samples. Zircon chemistries in both volcanic and plutonic samples fall into two groups during the ~5 Myr history of the High Island caldera magmatic system. One group (RBVG and “cold” granites) includes inherited grains back to 164 Ma and wider ranges in Hf, Y, total trivalent elements, Th and U concentrations and Th/U, Yb/Gd, and U/Yb ratios than the other (KSCVG and “hot” granites). Two possible evolutionary models of the High Island caldera magmatic system are: (1) the system randomly tapped a single crustal domain that fluctuated in temperature as a result of varying interactions of hotter melts, or (2) the volcanic and plutonic records reflect the interplay of two crustal domains with contrasting “low-” and “high-temperature” characteristics. In Hong Kong, some plutonic bodies were comagmatic with large-scale volcanism, while others were emplaced at shallow crustal levels independently of volcanism, matching the current two end-member views of the volcanic-plutonic relationship.

Genesis of chloride-carbonate segregations of the Udachnaya-East pipe

The past decade was characterized by a strong increase in the number of studies of the geochemistry of halogens in magmatic systems. It has been estab� lished that, in spite of the low concentration, halogens play an important role in the evolution of magmatic systems and related ore deposits [1]. Of special interest is the behavior of halogens in kimberlite and carbon� atite. The finds of syngenetic phlogopite containing up to 0.5 wt % Cl [2] and chloride inclusions in diamonds [3–5], olivines [6], and calcites [7] have provided unambiguous evidence for participation of halogens in the formation of kimberlite. However, the key prob� lems of the quantitative proportions of halogens in magmatic systems and their fractionation between the melt and the fluid phase upon cooling and crystalliza� tion of magma in the Earth’s crust have not yet been studied [1].

Fluoride and chloride melts included in phenocrysts of agpaitic acid volcanic rocks from Pantelleria Island

978 Many deposits of rare elements (Be, Zr, Nb, REE, Y, and others) are genetically related to acid agpaitic (with the mole ratio (Na2O + K2O)/Al2O3 > 1) mag� mas [1, 2]. The concentration of these elements most likely results from crystallization differentiation lead� ing to their accumulation in the residual melts at the expense of low coefficients of minor element parti� tioning between silicate minerals and melts [3, 4]. Gradual accumulation of incoherent minor elements in melts may be violated by separation of saline melts from magma. According to the experimental data [5], minor elements are most effectively extracted by fluo� ride melts. Because of this, silicate–saline liquid immiscibility is an important factor of geochemical and metallogenic evolution of magmas. The formation of chloride melts during the evolu� tion of magmatic systems was established for some objects [6 and others]. The presence of fluoride melts in acid magmatic systems was described in individual cases [7, 8], and each new finding is of intense interest. In this paper we discuss the results of the study of melt inclusions in phenocrysts from magmatic rocks of Pantelleria Island (Central Mediterranean, Italy) [9], in which liquid immiscibility of alkaline acid magmas with the formation of saline melt was observed. Chlo� ride melts in inclusions and glasses from rocks of Pan� telleria Island were first discovered in 1987 [10] and hereafter were described in glasses of the rock ground� mass [11]. The melt and fluid inclusions were studied in phe� nocrysts from ignimbrite and pantellerite collected in different parts of Pantelleria Island and related to the precaldera and postcaldera stages of volcanism evolu� tion.

Molecular-mass distribution of saturated hydrocarbons in gas of the Lovozerskii nepheline-syenite massif

Specific features of the molecular-mass distribution of gaseous alkanes in the rocks of Lovozerskii alkaline massif, in particular, increase of the weight while temperature lowing during evolution of magmatic system and relatively higher contents of butanes, bearing in mind other geochemical and mineralogical data, indicate the possibility of hydrocarbon gas formation according to the Fisher-Tropsh type mechanism, which could be part of a more complicated multistage process, including oxidation and dehydrogenization reactions, heterogenic-catalytic polymerization, and condensation of hydrocarbons.

Experimental constraints on Pt, Pd and Au partitioning and fractionation in silicate melt–sulfide–oxide–aqueous fluid systems at 800 °C, 150 MPa and variable sulfur fugacity

We have performed experiments to constrain the effect of sulfur fugacity ( fS 2) and sulfide saturation on the fractionation and partitioning behavior of Pt, Pd and Au in a silicate melt–sulfide crystal/melt–oxide–supercritical aqueous fluid phase–Pt–Pd–Au system. Experiments were performed at 800 °C, 150 MPa, with oxygen fugacity ( fO 2) fixed at approximately the nickel–nickel oxide buffer (NNO). Sulfur fugacity in the experiments was varied five orders of magnitude from approximately log fS 2 = 0 to log fS 2 = −5 by using two different sulfide phase assemblages. Assemblage one consisted initially of chalcopyrite plus pyrrhotite and assemblage two was loaded with chalcopyrite plus bornite. At run conditions pyrrhotite transformed compositionally to monosulfide solid solution (mss), chalcopyrite to intermediate solid solution (iss), and in assemblage two chalcopyrite and bornite formed a sulfide melt. Run-product silicate glass (i.e., quenched silicate melt) and crystalline materials were analyzed by using both electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry. The measured concentrations of Pt, Pd and Au in quenched silicate melt in runs with log fS 2 values ranging from approximately 0.0 to −5.0 do not exhibit any apparent dependence on fS 2. The measured Pt, Pd and Au concentrations in mss do vary as a function of fS 2. The measured Pt, Pd and Au concentrations in iss do not appear dependent on fS 2. The data suggest that fS 2, working in concert with fO 2, via the determinant role that these variables play in controlling the magmatic sulfide phase assemblage and the solubility of Pt, Pd and Au as lattice bound components in magmatic sulfide phases, is a controlling factor on the budgets of Pt, Pd and Au during the evolution of magmatic systems.

Gas diffusion in the evolution of magmatic systems

Since investigations by G. Poulett Scrop, all researchers engaged in the study of volcanic eruptions have come unavoidably to the conclusion that gas activity plays a key role in the eruptive process. This conclusion is likely formulated best of all by F.A. Perret, who investigated the dynamics of eruptions of the Mont Pele Volcano: “Gas is an active agent and magma is its carrier.” Based on the study of eruptions of the Bezymyannyi Volcano, the author of the present communication was also compelled to reach the same conclusion [6]: Degassing controls fully the mode of volcanic eruptions and their polycyclic character. This is reflected in changes of eruption patterns; facies, chemical, and mineral compositions of volcanics; composition of volcanic gases; and evolution of the volcanic edifice. In the foreword to the Russian edition of the Greiton’s work, Academician A.N. Zavaritskii wrote [3, p. 6]: “The role of volatile gas components of magma gained recognition (among volcanologists, A.M.) long ago.” Nevertheless, these observations and inferences of many generations of volcanologists remain unclaimed in magmatic geology, because gas activity in the volcanic process is considered a particular case related to crystallization of magmatic melts. In deeper settings, existence of matter in the gaseous state is thought to be impossible. Two causes provide grounds for ignoring the inferences of observational volcanology. The first cause is represented by facts of magmatic geology that indicate the disappearance of gas cavities or their relicts in igneous rocks with increasing depth. Moreover, such processes are also typical of volcanic rocks produced by underwater eruptions when a similar situation is observed with increasing depth. These data indicate the removal of the autonomous phase of gases from magmatic melts under relatively low pressures. Therefore, it is believed that gaseous compounds are transferred into

Is stoping a volumetrically significant pluton emplacement process?: Discussion

[Glazner and Bartley (2006)][1] suggest that stoping is an insignificant to potentially nonexistent process in the emplacement and evolution of magmatic systems. We strongly disagree with this conclusion and present here a number of alternative perspectives, which we group into three categories:

Cretaceous-Paleogene within-plate magmatism in Central Asia: data from the Tien Shan basalts

We investigated local geology of Meso-Cenozoic basalt complexes in the Tien Shan and collected the first samples that represent nearly the entire area of within-plate basaltic magmatism in Central Asia (over 285,000 km 2). According to their 40Ar/ 39Ar ages, the Tien Shan basalts erupted for a relatively short time span between 61 and 76 Ma, in the Late Cretaceous-Paleogene. The trace- and rare-earth-element compositions of rocks show that most of the studied basaltic series in the Tien Shan formed in within-plate magmatic systems related to mantle plume sources. Some melts bear evidence of crustal contamination. The compositions of melt inclusions and patterns of trace and rare-earth elements in them are consistent with the plume origin of the melts and with the evolution of magmatic systems toward an increase in K, Al, and Fe contents. The melt inclusion data show a direct dependence of the crystallization temperatures of parental melts of the Meso-Cenozoic Tien Shan basalts on their Mg#: 1220–1250 °C for picrite basalts of the Uchkuduk site, 1175–1215 °C for basalts of the Tekelik, and 1145–1170 °C for andesite basalts of the Alakol' site.

VOLUMINOUS STOPING IN THE MITCHELL PEAK GRANODIORITE, SIERRA NEVADA BATHOLITH, CALIFORNIA, USA

The Mitchell Peak Granodiorite, Sierra Nevada, California, has an outcrop exposure of nearly 50% stoped blocks in its youngest intrusive phase and an estimated 20% exposure of stoped blocks throughout the entire pluton. In this pluton, stoping thus is an important process. It is an excellent locale to study the effect of stoping on the evolution of magmatic systems. Preliminary work in the Mitchell Peak area has shown a complex pattern of stoping at pluton margins, with abundant evidence for mechanical disintegration of the granodiorite blocks. Preserved blocks range in size from hundreds of meters across down to K-feldspar xenocrysts. Many stoped blocks have a sharp contact with the surrounding pluton; rare examples have a diffuse, partially melted contact. Abundant xenocrysts derived from the stoped blocks provide evidence of contamination. In addition, a variety of magmatic features concentrated along the margin of stoped blocks, such as swarms of microgranitoid enclaves, schlieren layering, aplite intrusive bodies and orbicular granodiorite, suggests that these are areas of repeated injection or pulsing of magma. We speculate that repeated injection of magma plays an important role in the mechanical and chemical breakdown of host-rock margins and of stoped blocks. Stoping may play an important role in the thermal, mechanical, and chemical contamination of magmas in arcs, and deserves further attention.

Experimental and petrological studies of melt inclusions in phenocrysts from mantle-derived magmas: an overview of techniques, advantages and complications

Melt inclusions in phenocrysts are a potentially powerful tool in petrological research that can provide the only direct information available on the physical parameters ( P, T and melt composition) of crystallisation at various stages in the evolution of magmatic systems. However, melt inclusions also differ in principle from other parts of the magmatic system in that their composition, after trapping, may be controlled by the composition of the host phenocryst and therefore the direct application of our understanding of macro-scale magmatic processes to the interpretation of melt inclusion data can lead to erroneous conclusions. Our results indicate that the compositions of melt inclusions in early formed phenocrysts (olivine, pyroxene, plagioclase and spinel), often of most interest in petrological studies, can be affected by processes such as volatile dissociation, oxidation and/or partial re-equilibration with their host, both during natural cooling and homogenisation experiments. In particular, melt inclusions in all minerals are prone to hydrogen diffusion into or out of the inclusions after trapping and prior to eruption, and during homogenisation experiments. If not taken into account, this can significantly affect the crystallisation temperatures derived from the homogenisation experiments. Melt inclusions in high-magnesian olivine phenocrysts commonly have lower Fe contents compared to the initially trapped composition due to re-equilibration with the host at lower temperatures. This often leads to the appearance of sulphide globules and in some cases high-magnesian clinopyroxene daughter crystals, and may cause an increase in the oxidation state of the inclusions. Homogenised melt inclusions in plagioclase phenocrysts in MORB usually have lower Ti and Fe, and higher Si contents compared to the melt composition at the moment of trapping. However, homogenisation experiments can provide reliable estimates of trapping temperature and the MgO, Al 2O 3, CaO, Na 2O, and K 2O contents of the host magma at the moment of trapping. Some of these processes can be identified by observing the behaviour of melt inclusions during homogenisation experiments using low-inertia visually controlled heating stages, and their effects can be minimised by using appropriate experimental conditions as determined by kinetic experiments, ideally completed for each phenocryst type in every sample. We also discuss general aspects of melt inclusion studies aimed at recovering H 2O content of primary mantle-derived magmas and demonstrate that, in cases of low-pressure crystallisation, it is important to identify the first liquidus (most magnesian) olivine that crystallised from these magmas.

Silicate-melt inclusions in magmatic rocks: applications to petrology

Silicate-melt inclusions in igneous rocks provide important information on the composition and evolution of magmatic systems. Such inclusions represent accidentally trapped silicate melt (±immiscible H 2O and/or CO 2 fluids) that allow one to follow the evolution of magmas through snapshots, corresponding to specific evolution steps. This information is available on condition that they remained isolated from the enclosing magma after their entrapment. The following steps of investigation are discussed: (a) detailed petrographic studies to characterise silicate-melt inclusion primary characters and posttrapping evolution, including melt crystallisation; (b) high temperature studies to rehomogenise the inclusion content and select chemically representative inclusions: chemical compositions should be compared to relevant phase diagrams. Silicate-melt inclusion studies allow us to concentrate on specific topics; inclusion studies in early crystallising phases allow the characterisation of primary magmas, while in more differentiated rocks, they unravel the subsequent chemical evolution. The distribution of volatile species (i.e., H 2O, CO 2, S, Cl) in inclusion glass can provide information on the degassing processes and on recycling of subducted material. In intrusive rocks, silicate melt inclusions may preserve direct evidence of magmatic stage evolution (e.g., immiscibility phenomena). Melt inclusions in mantle xenoliths indicate that high-silica melts can coexist with mantle peridotites and give information on the presence of carbonate melt within the upper mantle. Thus, combining silicate-melt inclusion data with conventional petrological and geochemical information and experimental petrology can increase our ability to model magmatic processes.

Petrologic implications of trace element variation in clinopyroxene megacrysts from the Nógrád volcanic province, north Hungary: a study by laser ablation microprobe-inductively coupled plasma-mass spectrometry

Clinopyroxene megacrysts in alkali basalts are an important source of information about the evolution of magmatic systems at depth. In this study, we have undertaken a detailed examination of the trace element contents in a suite of megacrysts from ∼2.5 Ma old alkali basalts in the Nógrád volcanic province of Hungary and Slovakia. The megacrysts range in composition from Mg-rich and in equilibrium with their host magmas, to those that are Fe-rich and must have evolved in more fractionated magmas. The conditions of crystallization of these megacrysts, as calculated from the Al VI/Al IV ratios, suggests they all formed at about 30 km, or the crust–mantle boundary in this area. Using the most magnesian megacrysts and compositions of the host lavas, we have calculated the partition coefficients for a range of trace elements. However, the trace element contents in the megacrysts show a systematic variation with major element composition. Moreover, the rate of increase or change in the trace element concentrations is not consistent with models involving constant or steady state partition coefficients. Using a series of assumptions and models, we hypothesize that the partition coefficients between clinopyroxene and melt change substantially during the magmatic evolution of the system. This change is not constant for each element group, with the high field strength elements showing the most substantial increases. Electrostatic charge balance may have been the most important factor in controlling the mineral/melt partitioning.

Isotopic fingerprinting may provide insights into evolution of magmatic systems

A simple method may provide greater insights into the sort of pathways and time‐scales in which magmatic systems evolve. The method, employing isotopic fingerprinting of the mineral components that characterize most magmatic rocks, may lead to a more comprehensive understanding of these types of rocks. Such an understanding is critical to resolving fundamental problems in geoscience, ranging from the origin and differentiation of the crust and mantle to the prediction of volcanic hazards.Much progress has been made in the past 30 to 40 years, first with the widespread application of experimental studies on natural and synthetic systems, and more recently with the application of chemical and isotopic constraints on the origin of igneous rocks. Despite this progress, however, many questions remain, and that is where isotopic fingerprinting may help.