Mean concentrations of volatile components,major and trace elements in magmatic melts in major geodynamic environments on Earth. I. Mafic melts

Mean concentrations of major components, trace elements, and volatile components in magmatic melts from Earth’s major geodynamic environments are estimated using our database (which comprises more than 1200000 analyses for 75 chemical elements—state for the beginning of 2016) on melt inclusions and quench glasses of rocks). The geodynamic environments are classified into (I) environments of oceanic plate spreading (mid-oceanic ridges), (II) areas affected by mantle plumes in oceanic plates (oceanic islands and lava plateaus), (III and IV) subduction-related environments (III are magmatic zones in island arcs, and IV are magmatic zones in active continental margins, in which magma-generating processes involve the continental crust), (V) continental rifts in areas with continental hotspots, and (VI) backarc spreading zones. The distribution of SiO₂ concentrations (>71000 analyses) in natural magmatic melts in all geodynamic environments is obviously bimodal, with maxima at 50–52 and 72–76 wt % SiO₂. Herein we discuss only mafic melts (40–54 wt % SiO₂). Mean concentrations and confidence levels are calculated for each geodynamic environment for the first time in three variants: from melt inclusions in minerals, from quench glasses in rocks, and from all data. Systematic variations in the mean compositions of melt inclusions and glasses in rocks are detected for all geodynamic environments. Primitive mantle-normalized multielemental patterns for mean concentrations of elements are constructed for magmatic melts from all geodynamic environments, and the mean ratios and their variations are calculated for trace incompatible and volatile components (H₂O/Ce, K₂O/Cl, La/Y, Nb/U, Ba/Rb, Ce/Pb, etc.) in melts from all environments.     

Evolution of the trachydacite and pantellerite magmas of the bimodal volcanic association of Dzarta-Khuduk, Central Mongolia: Investigation of inclusions in minerals

Using various methods of melt inclusion investigation, including electron and ion microprobe techniques, we estimated the composition, evolution, and formation conditions of melts producing the trachydacites and pantellerites of the Late Paleozoic bimodal volcanic association of Dzarta-Khuduk, Central Mongolia. Primary crystalline and melt inclusions were detected in anorthoclase from trachydacites and quartz from pantellerites and pantelleritic tuffs. Among the crystalline inclusions, we identified hedenbergite, fluorapatite, and pyrrhotite in the trachydacites and F-arfvedsonite, fluorite, ilmenite, and the rare REE diorthosilicate chevkinite in the pantellerites. Melt inclusions in anorthoclase from the trachydacites are composed of glass, a gas phase, and daughter minerals (F-arfvedsonite, fluorite, villiaumite, and anorthoclase rim on the inclusion wall). Melt inclusions in quartz from the pantellerites are composed of glass, a gas phase, and a fine-grained salt aggregate consisting of Li, Na, and Ca fluorides (griceite, villiaumite, and fluorite). Melt inclusions in quartz crystalloclasts from the pantelleritic tuffs are composed of homogeneous silicate glasses. The phenocrysts of the trachydacites and pantellerites crystallized at temperatures of 1060–1000°C. During thermometric experiments with quartz-hosted melt inclusions from the pantellerites, the formation of immiscible silicate and salt (fluoride) melts was observed at a temperature of 800°C. Homogeneous melt inclusions in anorthoclase from the trachydacites have both trachydacite and rhyolite compositions (wt %): 68–70 SiO₂, 12–13 Al₂O₃, 0.34–0.74 TiO₂, 5–7 FeO, 0.4–0.9 CaO, and 9–12 Na₂O + K₂O. The agpaitic index ranges from 0.92 to 1.24. The glasses of homogenized melt inclusions in quartz from the pantellerites and pantelleritic tuffs have rhyolitic compositions. Compared with the homogeneous glasses trapped in anorthoclase of the trachydacites, quartz-hosted inclusions from the pantellerites show higher SiO₂ (72–78 wt %) and lower Al₂O₃ contents (7.8–10.0 wt %). They also contain 0.14–0.26 wt % TiO₂, 2.5–4.9 wt % FeO, 9–11 wt % Na₂O + K₂O, and 0.9–0.15 wt % CaO and show an agpaitic index of 1.2–2.05. Homogeneous melt inclusions in quartz from the pantelleritic tuffs contain 69–72 wt % SiO₂. The contents of other major components, including TiO₂, Al₂O₃, FeO, and CaO, are close to those in the homogeneous glasses of quartzhosted melt inclusions in the pantellerites. The contents of Na₂O + K₂O are 4–10 wt %, and the agpaitic index is 1.0–1.6. The glasses of melt inclusions from each rock group show distinctive volatile compositions. The H₂O content is up to 0.08 wt % in anorthoclase of the trachydacites, 0.4–1.4 wt % in quartz of the pantellerites, and up to 5 wt % in quartz of the pantelleritic tuffs. The content of F in the glasses of melt inclusions in the phenocrysts of the trachydacites is no higher than 0.67 wt %, and up to 1.4–2.8 wt % in quartz from the pantellerites. The Cl content is up to 0.2 wt % in the glasses of melt inclusions in the minerals of the trachydacites and up to 0.5 wt % in the glasses of quartz-hosted melt inclusions from the pantellerites. The investigation of trace elements in the homogenized glasses of melt inclusions in minerals showed that the trachydacites and pantellerites were formed from strongly evolved rare-metal alkaline silicate melts with high contents of Li, Zr, Rb, Y, Hf, Th, U, and REE. The analysis of the composition of homogeneous melt inclusions in the minerals of the above rocks allowed us to distinguish magmatic processes resulting in the enrichment of these rocks in trace and rare earth elements. The most important processes are the crystallization differentiation and immiscible separation of silicate and fluoride salt melts. It was also shown that all the melts studied evolved in spatially separated magma chambers. This caused the differences in the character of melt evolution between the trachydacites and pantellerites. During the final stages of differentiation, when the magmatic system was saturated with respect to ore elements, Na-Ca fluoride melts were separated and extracted considerable amounts of Li.     

Chemical composition,volatile components,and trace elements in the melts of the Gorely volcanic center,southern Kamchatka: Evidence from inclusions in minerals

Melt inclusions in olivine and plagioclase phenocrysts from rocks (magnesian basalt, basaltic andesite, andesite, ignimbrite, and dacite) of various age from the Gorely volcanic center, southern Kamchatka, were studying by means of their homogenization and by analyzing the glasses in 100 melt inclusions on an electron microprobe and 24 inclusions on an ion probe. The SiO₂ concentrations of the melts vary within a broad range of 45–74 wt %, as also are the concentrations of other major components. According to their SiO₂, Na₂O, K₂O, TiO₂, and P₂O₅ concentrations, the melts are classified into seven groups. The mafic melts (45–53 wt % SiO₂) comprise the following varieties: potassic (on average 4.2 wt % K₂O, 1.7 wt % Na₂O, 1.0 wt % TiO₂, and 0.20 wt % P₂O₅), sodic (3.2% Na₂O, 1.1% K₂O, 1.1% TiO₂, and 0.40% P₂O₅), and titaniferous with high P₂O₅ concentrations (2.2% TiO₂, 1.1% P₂O₅, 3.8% Na₂O, and 3.0% K₂O). The melts of intermediate composition (53–64% SiO₂) also include potassic (5.6% K₂O, 3.4% Na₂O, 1.0% TiO₂, and 0.4% P₂O₅) and sodic (4.3% Na₂O, 2.8% K₂O, 1.3% TiO₂, and 0.4% P₂O₅) varieties. The acid melts (64–74% SiO₂) are either potassic (4.5% K₂O, 3.6% Na₂O, 0.7% TiO₂, and 0.15% P₂O₅) or sodic (4.5% Na₂O, 3.1% K₂O, 0.7% TiO₂, and 0.13% P₂O₅). A distinctive feature of the Gorely volcanic center is the pervasive occurrence of K-rich compositions throughout the whole compositional range (silicity) of the melts. Melt inclusions of various types were sometimes found not only in a single sample but also in the same phenocrysts. The sodic and potassic types of the melts contain different Cl and F concentrations: the sodic melts are richer in Cl, whereas the potassic melts are enriched in F. We are the first to discover potassic melts with very high F concentrations (up to 2.7 wt %, 1.19 wt % on average, 17 analyses) in the Kuriles and Kamchatka. The average F concentration in the sodic melts is 0.16 wt % (37 analyses). The melts are distinguished for their richness in various groups of trace elements: LILE, REE (particularly HREE), and HFSE (except Nb). All of the melts share certain geochemical features. The concentrations of elements systematically increase from the mafic to acid melts (except only for the Sr and Eu concentrations, because of active plagioclase fractionation, and Ti, an element contained in ore minerals). The paper presents a review of literature data on volcanic rocks in the Kurile-Kamchatka area in which melt inclusions with high K₂O concentrations (K₂O/Na₂O > 1) were found. K-rich melts are proved to be extremely widespread in the area and were found on such volcanoes as Avachinskii, Bezymyannyi, Bol’shoi Semyachek, Dikii Greben’, Karymskii, Kekuknaiskii, Kudryavyi, and Shiveluch and in the Valaginskii and Tumrok Ranges.     

Geodynamics of rapid voluminous felsic magmatism through time

Two end member geodynamic settings produce the observed examples of rapid voluminous felsic (rhyolitic) magmatism through time. The first is driven by mantle plume head arrival underneath a continent and has operated in an identifiable and regular manner since at least 2.45 Ga. This style produces high temperature (≤ 1100 °C), low aspect ratio rheoignimbrites and lavas that exhibit high SiO₂/Al₂O₃ ratios, high K₂O/Na₂O ratios, and where available data exists, high Ga/Al₂O₃ ratios (> 1.5) with high F (in thousands of parts per million) and low water content. F concentration is significant as it depolymerizes the silicate melt, influencing the magmas' physical behavior during development and emplacement. These rhyolites are erupted as part of rapidly emplaced (10–15 Myr) mafic LIPs and are formed primarily by efficient assimilation-fractional crystallization processes from a mafic mantle parent. The second is driven by lithospheric extension during continental rifting or back arc evolution and is exclusive to the Phanerozoic. SLIPs (silicic large igneous provinces) develop over periods < 40 Myr and manifest in elongate zones of magmatism that extend up to 2500 km, contrasting with the mafic LIP style. Some of the voluminous felsic magmas within SLIPs appear to have a very similar geochemistry and petrogenesis to that of the rhyolites within mafic LIPs. Other voluminous felsic magmas within SLIPs are sourced from hydrous lower crust, and contrast with those sourced from the mantle. They exhibit lower temperatures (< 900 °C), explosive ignimbrites with lower SiO₂/Al₂O₃ ratios, and lower K₂O/Na₂O ratios. Rapid voluminous felsic magmatism represents both extreme examples of continental growth since the Archean, and also dramatic periods of crustal recycling and maturation during the Phanerozoic.     

Geochemical Constraints on the Mantle Source of the Upper Permian Emeishan Continental Flood Basalts,Southwestern China

The widespread Emeishan igneous province in southwestern China comprises the Emeishan continental flood basalts (ECFB) and associated mafie-ultramafic intrusions. The ECFB have variable SiO₂, ranging from 43.6 to 52.1 wt%, Al₂O₃ from 5.0 to 12.6 wt%, and total alkali (K₂O + Na₂O) from 0.7 to 6.5 wt%. These oxides exhibit negative correlations with MgO (5.4 - 23.1 wt%), implying fractional crystallization of olivine and clinopyroxene, which occur as phenocrysts in the rocks. Linear correlations between Zr, Nb, and La suggest that crustal contamination is not important. The primitive-mantle-normalized trace-element patterns show that the ECFB are enriched in high-field-strength trace elements, large-ion-lithophile elements, and light-rare-earth elements, similar to ocean-island basalt. Incompatible element ratios of the ECFB, such as Zr/Nb (7-10), Th/La (0.1-0.15), and Rb/Nb (0.9-1.7), differ from those of primitive mantle, N-MORB, and continental crust, but are similar to ocean-island basalts from an enriched mantle source (EM-1). However, the ECFB have isotopic ratios (¹⁴³Nd/¹⁴⁴Nd = 0.51229 -0.51276 and ⁸⁷Sr/⁸⁶Sr = 0.70480-0.70647) that imply that the ECFB were derived from a homogeneous, primitive lower mantle carried upward by a mantle plume.

We propose that the original melts derived from the mantle plume were contaminated through interaction at shallower depth with an enriched lithospheric mantle. This model suggests that the lithospheric mantle beneath the ECFB was modified by subduction of an oceanic slab.     

Petrogenesis and tectonic implications of Late Jurassic shoshonitic lamprophyre dikes from the Liaodong Peninsula, NE China

This paper presents detailed mineral chemical, element geochemical and Sr–Nd–Hf isotopic data for the Late Jurassic (155?±?4 Ma) lamprophyre dikes in the Liaodong Peninsula, NE China. The lamprophyres are shoshonitic and geochemically fall into three groups: Group I has relatively high SiO₂ (52.5–57.0 wt.%), low MgO (5.5–8.3 wt.%) and compatible trace element (e.g. Cr?=?128–470 ppm) contents, high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7093–0.7117), and low ε_Nd (T) values (?9.6 to ?12.1); Group II has relatively low SiO₂ (44.8–50.0 wt.%), high MgO (10.8–14.2 wt.%) and compatible trace element (e.g. Cr?=?456–1,041 ppm) contents, low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7073–0.7087), and high ε_Nd (T) values (?1.4 to ?2.9); Group III is transitional between the two in all elemental and isotopic compositions. Interpretation of the elemental and isotopic data suggests that the lamprophyric melts were derived by partial melting of subcontinental lithospheric mantle (SCLM) at a depth of 60–80 km (group I), decompression melting of upwelling asthenosphere at 60–100 km (group II), and mixing between the SCLM-derived and asthenosphere-derived melts (group III). It is assumed that the local SCLM was detached at a depth of 60–80 km by the 155 Ma ago. A continental arc-rifting related to the Palaeo-Pacific plate subduction is favored as a geodynamic force for such a cratonic lithosphere detachment.     

Multistage melt impregnation in Tethyan oceanic mantle: Petrochemical constraints from channelized melt flow in the Naga Hills Ophiolite

Ophiolitic sequences obducted onto continental margins allow field based observations coupled with petrochemical interrogations of upper mantle lithologies thereby aiding evaluation of compositional heterogeneity of oceanic mantle, depletion-enrichment events and geodynamic conditions governing oceanic lithosphere formation. The Naga Hills Ophiolite (NHO) suite preserves a segment of the Neotethyan oceanic lithosphere encompassing a package of mantle and crustal lithologies. This paper for the first time reports the occurrence of melt flow channels traversing the mantle section near Molen of the NHO and presents a comprehensive study involving chromite-spinel chemistry, bulk rock major, trace and PGE geochemistry to understand the petrogenesis and evolution in a geodynamic transition from mid oceanic ridge (MOR) to suprasubduction zone (SSZ). The spinel chemistry of peridotitic melt channels depicts both MOR-type and SSZ signatures underlining a transitional tectonic frame. Chromite chemistry and high Al₂O₃/TiO₂ ranging from 15.98–35.70 in concurrence with low CaO/Al₂O₃ ranging from 0.03–0.53; and chondrite normalised LREE > MREE < HREE patterns confirm the influx of boninitic melts into the refractory mantle. The boninitic signature shared by melt channels and host rock invokes a geochemical and geodynamic transition from anhydrous melting of depleted mantle to hydrated fluid flux melting resulting in boninitic melts, that subsequently impregnate and refertilise the fore arc mantle wedge in a SSZ regime at the nascent stage of subduction. The high Ba/Nb, Ba/Th, and Ba/La for the studied peridotites highlight the influx of subduction derived fluids in the supra subduction mantle. Further higher Zr/Hf and Nd/Hf with respect to primitive mantle values in concurrence with lower Nb/Ta suggest progressive refertilisation due to fluid- and melt-driven metasomatism of the refractory fore arc mantle wedge. The chondrite normalised PGE patterns suggest positive Ir and Ru anomalies stipulating the source to be refractory while enriched Pt and Pd underpins the mobilisation of these elements by subduction derived fluids and melts. The elevated abundances of PPGEs than IPGEs as cited by PPGE/IPGE > 1; and Pd/Pt avg. 0.85 for melt channels and 0.84 for host peridotites indicate fluid-fluxed metasomatism of fore arc mantle wedge with a S-undersaturated trend coupled with boninitic affinity. The mineral, trace, REE and PGE chemistry collectively emphasizes that the mantle peridotites of the NHO formed in a transitional geodynamic tectonic setting caused by fore arc extension during subduction initiation followed by rejuvenation by subduction derived fluids and boninitic melts, which typically are of the SSZ tectonic regime. The harzburgitic melt channels and host rock are refractory in nature, reflecting multiple episodes of melt extraction of about 5–15% and ~10–20% respectively from a spinel peridotite mantle source. The occurrences of these melt channels indicate segregation and percolation of melt through porous and channelized network in upper mantle peridotites.     

Average composition of basic magmas and mantle sources of island arcs and active continental margins estimated from the data on melt inclusions and quenched glasses of rocks

Based on the generalization of data on melt inclusions and quenched glasses, the average compositions of subduction (island arc and active continental margin settings) basic magmas were estimated. The main geochemical features of the average composition of these magmas are significant depletion in Nb and Ta, less significant depletion in Ti, Zr, and Sm, and enrichment in Cl, H₂O, F, and P in the primitive mantlenormalized patterns. The average normalized contents of moderately incompatible HREE in these magmas are close to those in the basic magmas of other geodynamic settings. Subduction basic magmas exhibit negative correlation of Li, Y, Dy, Er, Yb, Lu, and Ti contents with MgO content. Most of incompatible elements (Nb, Ta, U, Th, LREE) do not correlate with MgO, but correlate with each other and K₂O. Variations in element contents are related to crystallization differentiation, magma mixing, and possibly, participation of several sources. The water content in the island arc basic magmas varies from almost zero value to more than 6 wt %. Most compositions are characterized by weak negative correlation between H₂O and MgO contents, but some compositions define a negative correlation close to that in magmas of mid-ocean ridges (MOR). Considered magmas demonstrate distinct positive correlation between MgO content and homogenization temperature, practically coinciding with that of MOR magmas. Modeling of phase equilibria revealed widening of crystallization field of olivine in the magmas of subduction zones compared to MOR magmas. This can be related to the high water content in subduction magmas. Simultaneous liquidus crystallization of olivine and clinopyroxene in subduction magmas occurs at pressure approximately 5 kbar higher than that of MOR magmas. Based on the average ratios of trace element to K2O content, we determined the average compositions for subduction magma sources. Relative to depleted mantle, they are enriched in all incompatible elements, with positive anomalies of U, Rb, Ba, B, Pb, Cl, H₂O, F, and S, and negative anomalies of Th, K, Be, Nb, Ta, Li, Nd, Pb, and Ti. A general elevated content of incompatible elements indicates a reworking of the rocks of mantle wedge by fluids and melts that were released from the upper layers of subducted plate.     

Peralkaline silicic melts of island arcs, active continental margins, and intraplate continental settings: Evidence from the investigation of melt inclusions in minerals and quenched glasses of rocks

Based on the analysis of data on the composition of melt inclusions in minerals and quenched glasses of igneous rocks, we considered the problems of the formation of peralkaline silicic magmas (i.e., whose agpaitic index, the molar ratio AI = (Na₂O + K₂O)/Al₂O₃, is higher than one). The mean compositions of peralkaline silicic melts are reported for island arcs and active continental margins and compared with the compositions of melts from other settings, primarily, intraplate continental areas. Peralkaline silicic rocks are rather common in the latter. Such rocks are rare in island arcs and active continental margins, but agpaitic melts were observed in inclusions in phenocrysts of plagioclase, quartz, pyroxene, and other minerals. Plagioclase fractionation from an alkali-rich melt with AI < 1 is considered as a possible mechanism for the formation of peralkaline silicic melts (Bowen’s plagioclase effect). However, the analysis of available experimental data on plagioclase-melt equilibria showed that natural peralkaline melts are almost never in equilibrium with plagioclase. For the same reason, the melting of the majority of crustal rocks, which usually contain plagioclase, does not produce peralkaline melts. The existence of peralkaline silicic melt inclusions in plagioclase phenocrysts suggests that plagioclase can crystallize from peralkaline melts, and the plagioclase effect may play a certain role. Another mechanism for the formation of peralkaline silicic magmas is the melting of alkali-rich basic and intermediate rocks, including the spilitized varieties of subalkali basalts.     

The Composition of Melt Inclusions in Minerals from Tephra of the Soil–Pyroclastic Cover of Simushir Island (Central Kuril Islands)

The compositions of approximately 70 naturally quenched melt inclusions in olivine, clinopyroxene, orthopyroxene, and plagioclase phenocrysts from tephra of the soil–pyroclastic cover of Simushir Island (Central Kuril Islands) were studied. The concentrations of the major rock-forming components, H₂O, S, and Cl were analyzed in inclusions. The reconstructed melts contain 48.6–78.4 wt % SiO₂, 0.3–8.26 wt % MgO, and 0.12–1.72 wt % K₂O. The concentration of S and Cl in the melts changes regularly with increasing SiO₂ content: from 0.14 to ~0.02 wt % S and from ~0.05 to ~0.28 wt % Cl. The content of H2O in parental melts is 4.2–4.5 wt %.     

Review Section

ABSTRACT

The bulk composition of silicic igneous rocks hovers directly over the ‘minimum melt’ located on the NaAlSi₃O₈–KAlSi₃O₈–SiO₂ ternary. It is universally accepted that these rocks are igneous, reflecting the thermodynamic equilibrium between minerals and melt. However, there is a contradiction between the use of this phase diagram and current models of differentiation: fractional crystallization or partial melting (or any mechanical separation process) implies granites were melts extracted from a quartz-bearing residue.

Here, I propose that a thermodynamically controlled process, wet thermal migration, provides a more consistent explanation for producing silicic rocks. This process, when coupled with slow incremental emplacement of sills, allows magmatic differentiation to take place without mechanical separation and produces minimum-melt compositions from input magmas not saturated in quartz. Examination of phase equilibria in the SiO₂–Al₂O₃–Na₂O–K₂O–H₂O system, as well as assessment of thermodynamic models, provides a template for understanding the wet thermal migration process (diffusion-based differentiation of crystal-mush in a temperature gradient). Phase equilibria in SiO₂–Na₂O–K₂O–Al₂O₃ indicate a cotectic surface between quartz and alkali feldspar slopes down-temperature as melt peralkalinity increases. Experiments show quartz and two feldspars coexist with a single water-rich (>40 wt.% H₂O) melt at 400°C and 0.1 GPa. Modelling suggests that development of water-rich melt at the hot end of a temperature gradient drives the process. Given the propensity of alkalis to rapidly diffuse down-temperature gradients and form these melts, small amounts (<5%) of interconnected melt form, leading to the differentiation of partially molten materials by wet thermal migration.

Because the quartz-feldspar cotectic ranges from 650°C to 330°C, granites can reflect formation by a process that never involves the existence of rhyolitic melt. If this is correct, the process has implications for understanding the formation of plutons and batholiths and, therefore, the continental crust.     

Composition of Li-F granite melt and its evolution during the formation of the ore-bearing Orlovka massif in Eastern Transbaikalia

By the example of the Orlovka massif of Li-F granites in Eastern Transbaikalia, the major- and trace-element (Li, Be, B, Ta, Nb, W, REE, Y, Zr, and Hf) compositions of the parental melt and the character of its variations during the formation of the differentiated rock series were quantitatively estimated for the first time on the basis of electron and ion microprobe analysis and Raman spectroscopy of rehomogenized glasses of melt inclusions in quartz. It was shown that the composition of the Orlovka melt corresponded to a strongly evolved alumina-saturated granitoid magma (A/CNK = 1.12–1.55) rich in normative albite, poor in normative quartz, and similar to ongonite melts. This magma was strongly enriched in water (up to 9.9 ± 1.1 wt %) and fluorine (up to 2.8 wt %). Most importantly, this massif provided the first evidence for high B₂O₃ contents in melts (up to 2.09 wt %). The highest contents of trace elements were observed in the melt from pegmatoid bodies in the amazonite granites of the border zone: up to 5077 ppm Li, 6397 ppm Rb, 313 ppm Cs, 62 ppm Ta, 116 ppm Nb, and 62 ppm W. Compared with the daughter rock, the Orlovka melt was depleted at all stages of formation in SiO₂ (by up to 6 wt %), Na₂O (by up to 2.5 wt %), and, to a smaller extent, in Ti, Fe, Mg, Sr, and Ba, but was enriched in Mn, Rb, F, B, and H₂O.     

Reaction between slab-derived melts and peridotite in the mantle wedge: experimental constraints at 3.8 GPa

Laboratory experiments on natural, hydrous basalts at 1–4 GPa constrain the composition of “unadulterated” partial melts of eclogitized oceanic crust within downgoing lithospheric slabs in subduction zones. We complement the “slab melting” experiments with another set of experiments in which these same “adakite” melts are allowed to infiltrate and react with an overlying layer of peridotite, simulating melt:rock reaction at the slab–mantle wedge interface. In subduction zones, the effects of reaction between slab-derived, adakite melts and peridotitic mantle conceivably range from hybridization of the melt, to modal or cryptic metasomatism of the sub-arc mantle, depending upon the “effective” melt:rock ratio. In experiments at 3.8 GPa, assimilation of either fertile or depleted peridotite by slab melts at a melt:rock ratio 2:1 produces Mg-rich, high-silica liquids in reactions which form pyrope-rich garnet and low-Mg# orthopyroxene, and fully consume olivine. Analysis of both the pristine and hybridized slab melts for a range of trace elements indicates that, although abundances of most trace elements in the melt increase during assimilation (because melt is consumed), trace element ratios remain relatively constant. In their compositional range, the experimental liquids closely resemble adakite lavas in island-arc and continental margin settings, and adakite veins and melt inclusions in metasomatized peridotite xenoliths from the sub-arc mantle. At slightly lower melt:rock ratios (1:1), slab melts are fully consumed, along with peridotitic olivine, in modal metasomatic reactions that form sodic amphibole and high-Mg# orthopyroxene.     

Experimental Melting of Carbonated Peridotite at 6-10 GPa

Partial melting of magnesite-bearing peridotites was studiedat 6–10 GPa and 1300–1700°C. Experiments wereperformed in a multianvil apparatus using natural mineral mixesas starting material placed into olivine containers and sealedin Pt capsules. Partial melts originated within the peridotitelayer, migrated outside the olivine container and formed poolsof quenched melts along the wall of the Pt capsule. This allowedthe analysis of even small melt fractions. Iron loss was nota problem, because the platinum near the olivine container becamesaturated in Fe as a result of the reaction Fe₂SiO₄^Ol = Fe^Fe–Ptalloy + FeSiO₃^Opx + O₂. This reaction led to a gradual increasein oxygen fugacity within the capsules as expressed, for example,in high Fe³⁺ in garnet. Carbonatitic to kimberlite-like meltswere obtained that coexist with olivine + orthopyroxene + garnet± clinopyroxene ± magnesite depending on P–Tconditions. Kinetic experiments and a comparison of the chemistryof phases occasionally grown within the melt pools with thosein the residual peridotite allowed us to conclude that the meltshad approached equilibrium with peridotite. Melts in equilibriumwith a magnesite-bearing garnet lherzolite are rich in CaO (20–25wt %) at all pressures and show rather low MgO and SiO₂ contents(20 and 10 wt %, respectively). Melts in equilibrium with amagnesite-bearing garnet harzburgite are richer in SiO₂ andMgO. The contents of these oxides increase with temperature,whereas the CaO content becomes lower. Melts from magnesite-freeexperiments are richer in SiO₂, but remain silicocarbonatitic.Partitioning of trace elements between melt and garnet was studiedin several experiments at 6 and 10 GPa. The melts are very richin incompatible elements, including large ion lithophile elements(LILE), Nb, Ta and light rare earth elements. Relative to theresidual peridotite, the melts show no significant depletionin high field strength elements over LILE. We conclude fromthe major and trace element characteristics of our experimentalmelts that primitive kimberlites cannot be a direct productof single-stage melting of an asthenospheric mantle. They rathermust be derived from a previously depleted and re-enriched mantleperidotite. KEY WORDS: multianvil; carbonatite melt; peridotite; kimberlite; element partitioning     

Transition from I‐type to A‐type magmatism in the Sanandaj–Sirjan Zone,NW Iran: an extensional intra‐continental arc

The South Dehgolan pluton, in NW Iran was emplaced into the Sanandaj–Sirjan magmatic–metamorphic zone. This composite intrusion comprises three main groups: (1) monzogabbro–monzodiorite rocks, (2) quartz monzonite–syenite rocks, and (3) a granite suite which crops out in most of the area. The granites generally show high SiO₂ content from 72.1%–77.6 wt.% with diagnostic mineralogy consisting of biotite and amphibole along the boundaries of feldspar–quartz crystals which implies anhydrous primary magma compositions. The granite suite is metaluminous and distinguished by high FeOt/MgO ratios (av. 9.6 wt.%), typical of ferroan compositions with a pronounced A‐type affinity with high Na₂O + K₂O contents, high Ga/Al ratios, enrichment in Zr, Nb, REE, and depletion in Eu. The quartz monzonite–syenites show intermediate SiO₂ levels (59.8%–64.5 wt.%) with metaluminous, magnesian to ferroan characteristics, intermediate Na₂O + K₂O contents, enrichment in Zr, Nb, REE, Ga/Al, and depletion in Eu. The monzogabbro–monzodiorites show overall lower SiO₂ content (48.5%–55.9 wt.%) with metaluminous and calc‐alkaline compositions, relatively lower Na₂O + K₂O contents, low Ga/Al ratios, and FeOt/MgO (av. 1.6 wt.%) ratios, low abundances of Zr, Nb, and lower REE element concentrations relative to the granites and quartz monzonite–syenites. These geochemical differences among the three different rocks suites are likely to indicate different melt origins. We suggest that the South Dehgolan pluton resulted from a change in the geodynamic regime, from compression to extension in the Sanandaj–Sirjan zone during Mesozoic subduction of the Neo‐Tethys oceanic crust beneath the Central Iranian microcontinent. Copyright © 2015 John Wiley & Sons, Ltd.     

Magnesian andesites in north Xinjiang,China

Middle Devonian magnesian andesites (MAs) are widely distributed in south Altay and Carboniferous MAs are present in Alataoshan and west- and east-Tianshan in the north Xinjiang region. These MAs are andesitic rocks with 53–65% SiO₂,<1% (0.21–1.08%; average of 0.72%) TiO₂, and ≥50 Mg^#. Magnesian dacites and diorites, with 52.38–66.91% SiO₂, <0.30% TiO₂ and ≥42 Mg^# commonly occur with these MAs. Relative to boninites, MAs have lower MgO contents (average 6.39%) but higher Ti, K and Na. They have characteristic flat chondrite-normalized REE patterns with weak to no Eu anomalies (Eu depletion, or Eu/Eu* = 0.65–1.15), low (La/Yb)_N (0.98–6.4, mostly 4±) and low total REE contents (15–95 ppm). They also have high contents of compatible elements Cr and Ni (72–790 and 29–276 ppm, respectively). Their relative depletion in high field strength elements Nb, Ta and Ti, and relative enrichment in mobile large-ion lithophile elements Rb, K and Pb are evident on primitive mantle-normalized trace element spidergrams. If magnesian andesites are melts coming from the subducted oceanic crust, as proposed elsewhere, then the relatively high Y contents (>15 ppm), low Sr/Y ratios (4.4–6.2), low (La/Yb)N, and high Mg^# of the MAs in north Xinjiang provide evidence of interaction of such melts with mantle wedge peridotite. New petrographic, chemical and isotopic [(¹⁴³Nd/¹⁴⁴Nd)I = 0.51221–0.51255 (εNd(t) +0.28 to +7.2); (⁸⁷Sr/⁸⁶Sr)I = 0.7029–0.7065] data suggest that the petrogenesis of the MAs in the north Xinjiang region may have involved: (1) multiple source materials including subducted oceanic slab, juvenile crustal materials (mainly volcanic-volcanoclassic rocks with low maturity and clear mantle geochemical signatures) coming from the forearc accretionary prism and mantle wedge peridotite; (2) a combination of different petrogenetic processes including partial melting of subducted oceanic slab and juvenile crustal materials, followed by interaction of slab melts with the mantle wedge peridotite; (3) high geothermal gradient creating a high temperature (>1,000°C) environment in a volatile-rich source region; (4) unique tectonic settings including oblique subduction, slab break off resulting in slab window formation and asthenosphere upwelling, and subduction erosion resulting in transfer of forearc accretionary materials into the source region of MA magma.     

Petrochemical and Sr-Nd isotope investigations of A-type granites in the east of Misho, NW Iran

The Misho plutonic complex consists of a series of granitic bodies which range from syenogranite, alkali granite to monzogranites. They include metaluminous to peraluminous compositions. The garnitoid bodies are intruded into the unmetamorphosed late Paleozoic rocks and are located between two dextral, oblique-slip fault systems along which they have been uplifted as a major positive flower structure. The Misho granitoid belongs to the alkaline granitoid series that have been attributed to a Late Permian post-collisional setting. The studied granitoid displays high SiO₂ contents between 67.71 and 76.4 wt%. On both FeO/(FeO + MgO) and [(Na₂O + K₂O) ? CaO] vs. SiO₂ diagrams, the samples, plot in the ferroan and alkaline fields, therefore, show an A-type granitoid signature. Trace and rare earth elements distribution patterns for the Misho rocks indicate a distinctive depletion in Nb, Sr, Ba, P, and Ti relative to other trace elements and a greater enrichment in large-ion lithophile elements compared to high field strength elements that are also typical features of A-type granites. The granitoid samples are geochemically similar to typical A₂-type granites, e.g., high K₂O + Na₂O, FeO/MgO, Ga/Al, and Y/Nb values and low CaO, Ba, Sr, and Eu contents. They have initial Sr isotopic ratios in the range >0.712 and negative ε _Ndt values of ?1 to ?3.2 for a time of generation of 232 Ma. We suggest that shear zones play an important role in providing suitable conduits for ascending asthenospheric material and heat influx in the lower crust continental. It is proposed that the Misho parental granitoid magmas were produced by the partial melting of the lower crust continental at extensional setting in active continental margin setting that was formed after the collision of the Eurasia plate and Iranian plate following closure of paleo-Tethyan oceanic crust during Middle Jurassic times.     

Chemical composition and crystallization conditions of trachybasalts from the Dzhida field, Southern Baikal volcanic area: Evidence from melt and fluid inclusions

Melt and fluid inclusions have been studied in olivine phenocrysts (Fo 81–79) from trachybasalts of the Southern Baikal volcanic area, Dzhida field. The melt inclusions were homogenized, quenched, and analyzed on an electron and ion microprobe. The study of homogenized glasses of nine inclusions showed that basaltic melts (SiO₂ = 47.1–50.3 wt %, MgO = 5.0–7.7 wt %, CaO = 7.1–11.1 wt %) have high contents of Al₂O₃ (17.1–19.6 wt %), Na₂O (4.1–6.2 wt %), K₂O (2.2–3.3 wt %), and P₂O₅ (0.6–1.1 wt %). The volatile contents are low (in wt %): 0.24–0.31 H₂O, 0.08 F, 0.03 Cl, and 0.02 S. Primary fluid inclusions in olivines from four trachybasalt samples contain high-density CO₂ (0.73–0.87 g/cm³), indicating a CO₂ fluid pressure of 4.3–6.6 kbar at 1200–1300°C and olivine crystallization depths of 16–24 km. Ion microprobe analyses of 20 glasses from melt inclusions for trace elements showed that the magmas of the Baikal rift were enriched in incompatible elements, thus differing from oceanic rift basalts and resembling oceanic island basalts. A comparison of our data on melt and fluid inclusions in olivine from trachybasalts of the Dzhida field with preexisting data on the Eastern Tuva volcanic highland in the Southern Baikal volcanic area showed that they had similar contents of volatiles, major, and trace elements.     

Origin of Late Cenozoic Abaga–Dalinuoer basalts,eastern China: Implications for a mixed pyroxenite–peridotite source related with deep subduction of the Pacific slab

Continental intraplate basalts (15.42–0.16 Ma) from Abaga–Dalinuoer volcanic field (ADVF) in central Inner Mongolia of eastern China, as a part of Cenozoic volcanic province along eastern margin of the Eurasian continent, provide a good opportunity to explore potential links between deep subduction of the Pacific slab and continental intraplate volcanism. In this study, we report an integrated dataset of whole-rock K–Ar ages, major and trace elements and Sr–Nd–Pb isotopes, and olivine major and minor elements for the Abaga–Dalinuoer basalts (ADBs), and propose that mantle source lithology of the ADB magmas may consist of both pyroxenite and peridotite. The ADBs display low SiO₂ (42.3–50.2 wt.%), high MgO (7.3–11.4 wt.%) and moderate K₂O + Na₂O (3.8–6.4 wt.%), and can be subdivided into basanites, alkali basalts and tholeiitic basalts that are all characterized by ocean island basalt (OIB)-like rare earth elements (REE) and enrichment in both large ion lithosphile elements (LILE) and high field strength elements (HFSE). Olivine phenocrysts have higher Ni and Fe/Mn and lower Mn, Ca and Ca/Fe relative to those from peridotite melts, but exhibit clearly lower Ni contents (< 2500 ppm) compared with expected Ni range (> 3000 ppm) for olivines crystallized from olivine-free pyroxenite melts. Estimated compositions of the ADB primary magmas, together with olivine compositions, suggest an iron-rich mantle source related with silica-deficient pyroxenite that is most likely derived from deeply subducted Pacific oceanic crust. Additionally, peridotite and recent subducted sediments are also required to account for high Ni and MgO in primary magmas together with their trace elements and Sr–Nd–Pb isotope systematics. We suggest that a mixed pyroxenite–peridotite source lithology can better match observed whole-rock and olivine signatures in the ADB, compared with either peridotite only or olivine-free pyroxenite only source lithology. In our model, pyroxenite melts would either react with or mechanically mix with peridotite, and the proportion of pyroxenite melts may range from 30% to 45% for mechanical mixing scenario. A continuous spectrum from tholeiitic to alkali melts revealed by melt-peridotite reaction experiment can explain formation of primary magmas of basanites, alkali basalts and tholeiitic basalts by increasing melting degree of a similar mantle source. Relatively higher ²⁰⁶Pb/²⁰⁴Pb of the ADB may suggest more significant role of recent (< 0.5 Ga) subducted Pacific oceanic materials, in contrast to other Cenozoic basalts in eastern China (e.g., Changbai basalts) that exhibit varying contributions from ancient (> 1.5 Ga) subducted continental sediments. We emphasize that coupled geochemical and geodynamic links (i.e., subduction polarity) between deeply subducted Pacific slab and continental intraplate volcanism in eastern China may exist, which directly support the involvement of deeply subducted Pacific materials in petrogenesis of the ADB. From the perspective of plate motion kinetics, decompression partial melting of upwelling fragmented Pacific slab (silica-deficient pyroxenite + recent subducted sediments) may be triggered by rollback of deeply subducted Pacific slab during Late Cenozoic times. Continental intraplate volcanism in the ADVF generally started with termination of opening of the Japan Sea, suggesting that deep subduction of the Pacific slab may have been an important geodynamic mechanism responsible for tectono-magmatic evolution of northeastern Asia. We suggest that the ADBs have the potential to shed light on genetic links between continental intraplate volcanism and deep subduction of the Pacific slab in geochemical and geodynamic processes.     

The beginnings of hydrous mantle wedge melting

This study presents new phase equilibrium data on primitive mantle peridotite (0.33 wt% Na₂O, 0.03 wt% K₂O) in the presence of excess H₂O (14.5 wt% H₂O) from 740 to 1,200°C at 3.2–6 GPa. Based on textural and chemical evidence, we find that the H₂O-saturated peridotite solidus remains isothermal between 800 and 820°C at 3–6 GPa. We identify both quenched solute from the H₂O-rich fluid phase and quenched silicate melt in supersolidus experiments. Chlorite is stable on and above the H₂O-saturated solidus from 2 to 3.6 GPa, and chlorite peridotite melting experiments (containing ~6 wt% chlorite) show that melting occurs at the chlorite-out boundary over this pressure range, which is within 20°C of the H₂O-saturated melting curve. Chlorite can therefore provide sufficient H₂O upon breakdown to trigger dehydration melting in the mantle wedge or perpetuate ongoing H₂O-saturated melting. Constraints from recent geodynamic models of hot subduction zones like Cascadia suggest that significantly more H₂O is fluxed from the subducting slab near 100 km depth than can be bound in a layer of chloritized peridotite ~ 1 km thick at the base of the mantle wedge. Therefore, the dehydration of serpentinized mantle in the subducted lithosphere supplies free H₂O to trigger melting at the H₂O-saturated solidus in the lowermost mantle wedge. Alternatively, in cool subduction zones like the Northern Marianas, a layer of chloritized peridotite up to 1.5 km thick could contain all the H₂O fluxed from the slab every million years near 100 km depth, which suggests that the dominant form of melting below arcs in cool subduction zones is chlorite dehydration melting. Slab P–T paths from recent geodynamic models also allow for melts of subducted sediment, oceanic crust, and/or sediment diapirs to interact with hydrous mantle melts within the mantle wedge at intermediate to hot subduction zones.