Primary Melt Inclusions Research Articles

Fluorine in nominally fluorine-free mantle minerals: Experimental partitioning of F between olivine, orthopyroxene and silicate melts with implications for magmatic processes

We present new experimentally determined partition coefficients for fluorine (F) between nominally fluorine-free minerals and corresponding basaltic melts in the systems CaO–MgO–Al2O3–SiO2+F, Na2O–CaO–MgO–Al2O3–SiO2+F, and in natural compositions at pressures up to 2.5GPa and temperatures between 1285°C and 1445°C. Fluorine is incompatible in olivine with F partition coefficients DFol/melt varying between 0.0007 and 0.0033, whereas the F in orthopyroxene is more compatible with DFopx/melt between 0.031 and 0.037. These findings agree well with new analyses of F in natural olivines and orthopyroxene from spinel peridotites and oceanic basalts, and indicate that F is less incompatible in olivine and orthopyroxene than hydrogen (DF/DH∼3.3 for olivine, DF/DH∼2.4 for orthopyroxene). Based on high-resolution TEM images of one of the samples, we argue that the F incorporation into the olivine structure is mass balanced via oxygen defects. Furthermore, we present some analyses of Cl in olivine and orthopyroxene from spinel peridotites and basalts.Hence, fluorine, and to a lesser extend chlorine, may be effectively stored in nominally halogen-free mantle minerals such as olivine and orthopyroxene. Considering their high modal proportion in the upper mantle, both phases are the major hosts for F in the Earth's mantle, and must be taken into account when calculating the Earth's budget of halogens or global cycles of halogens in the deeper Earth. Other F-bearing minerals such as amphibole or phosphates are not needed to balance the F concentrations between crust and mantle.Applying our new partition coefficients to primitive olivine hosted melt inclusion data from mid-ocean ridge basalt (MORB), we calculate a concentration of 6–15μg/g F for the depleted MORB mantle source. Our results also show that typical spinel lherzolites from the subcontinental lithosphere show virtually identical concentrations of about 12μg/g. However, using the partition coefficients on primary melt inclusions from ocean island basalts (OIB), suggest slightly higher fluorine contents between 8 and 31μg/g for OIB mantle sources.

Mineralogy, geochemistry and petrology of the phonolitic to nephelinitic Sadiman volcano, Crater Highlands, Tanzania

Sadiman volcano is located in the Crater Highlands area of northern Tanzania, which lies next to the western escarpment of the Gregory rift—a part of the eastern branch of the East African Rift system. It consists of interlayered phonolitic tuffs, tuff breccias (with blocks of nephelinites) and nephelinitic lava flows. Rare xenoliths of phonolite lava and ijolite were observed within the nephelinite lavas with ijolite blocks occurring in phonolitic tuffs. No evidence for the presence of melilite-bearing and/or carbonatitic rocks was found during this study. On the basis of petrography, mineralogy and geochemistry the nephelinites are divided into highly porphyritic nephelinite, wollastonite nephelinite and phonolitic nephelinite, the latter of which is the dominant variety at Sadiman. Nepheline+clinopyroxene+titanite±perovskite±andradite–schorlomite±wollastonite±sanidine±sodalite are the principle pheno- and microphenocryst phases. The nephelinites are highly evolved (Mg#=0.17–0.26) alkaline to peralkaline (AI=0.88–1.21) rocks enriched in incompatible elements such as Rb, Ba, Th, U, Nb, Pb, Ta, Sr and light REEs, and strongly depleted in P and Ti. This suggests derivation from an enriched mantle source and fractionation of apatite and Ti-rich mineral(s). Primary melt inclusions in nepheline phenocrysts (Thomogenization=860–1100°C) indicate enrichment of volatile components in the melts, particularly of fluorine (up to 1.8wt.% in silicate glass) resulting in the formation of daughter fluorite in partly and complete crystallized inclusions. The Sadiman nephelinites crystallized under relatively oxidizing conditions (above the FMQ buffer), which differ from the reducing conditions reported for trachytic and pantelleritic rocks from other parts of the Gregory rift. Similar rock types and relatively oxidizing conditions are known from Oldoinyo Lengai and other localities, all of which are closely associated with carbonatites. By analogy, we conclude that andradite–schorlomite-rich nephelinites may indicate a pre-stage on the evolutionary path towards carbonatitic magmatism.

Tin and associated metal and metalloid geochemistry by femtosecond LA-ICP-QMS microanalysis of pegmatite–leucogranite melt and fluid inclusions: new evidence for melt–melt–fluid immiscibility

AbstractGranitic pegmatites are exceptional igneous rocks and the possible role of an immiscibility process in their origin is strongly debated. To investigate metal and metalloid behaviour in hydrous peraluminous systems (aluminium saturation index, ASI >1), we analysed 15 quartz-hosted primary melt and fluid inclusions from pegmatites in the Ehrenfriedersdorf Complex (Erzgebirge, Germany) and 26 primary melt inclusions from leucogranites of the Ehrenfriedersdorf district (Germany), Kymi (Finland) and Erongo (Namibia) by femtosecond laser ablation inductively coupled plasma quadrupole mass spectrometry. The results presented here for 32 elements provide evidence for metal and metalloid fractionation between two types of immiscible melts (A and B) and NaCl – HCl-rich brine in the pegmatite system. No evidence for the boundary layer effect was observed in the 40 – 500 μm size melt inclusions that were investigated. The data on the Ehrenfriedersdorf pegmatites allow quantification of the metal and metalloid partitioning between natural NaCl-rich brine and the two types of melt (e.g. KAsbrine/type-A,B melts = 0.01 – 1.7; KSbbrine/type-A,B melts = 10 – 285; KZnbrine/type-A,B melts ≥ 50; KPbbrine/type-A melt ≥ 50; KAgbrine/type-A melt = 46). These data are in accord with existing natural and experimental data on equilibrium fluid – melt partitioning as well as spectroscopic data on the metal and metalloid complexation in hydrous aluminosilicate melts and NaCl – HCl-rich fluids.

Genesis of scapolite from granulites ( lower-crustal xenoliths from the Pamir diatremes): results of study of melt inclusions

The results of mineralogical and thermobarogeochemical studies of lower-crustal xenolith of scapolite-bearing granulites from the fergusite-porphyry diatremes pipes in Southeastern Pamir (Tajikistan) are presented. All minerals (including garnet, clinopyroxene, and scapolite) of these granulites contain primary melt inclusions, which were studied using thermometric and microprobe methods (EPMA, SIMS, Raman spectroscopy). We have established that their compositions correspond to acid (from rhyodacites to rhyolites), essentially potassic melts of normal and high alkalinity with H 2O ≤ 4 wt.%, Cl ≤ 0.8 wt.%, and CO 2 ~ 1 wt.%. The melts are depleted in HREE and have high Th/U ratios (7.7–9.4). Study of melt inclusions using mineralogical thermobarometers showed that the scapolite-bearing granulite crystallized at ~1000 °C and ~15 kbar. This rock resulted, most likely, from the incongruent melting of carbonate-bearing biotite-quartz-plagioclase substrate in the lower crust, which was accompanied by the crystallization of garnet, clinopyroxene, sphene, plagioclase, and scapolite trapping microportions of acid melts as inclusions. The minerals crystallized not from melt but in its presence. High-Ca scapolite (Me 67–69) crystallized instead of plagioclase when the melts reached high contents of CO 2 (~1 wt.%) and Cl (≤0.8 wt.%) in the presence of CO 2-rich fluid.

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.

Growth Marks of titanian-andradite crystals from Colli Albani (Italy)

Periodico di Mineralogia (2011), 80, 1 (Special Issue), 89-104 - DOI: 10.2451/2011PM0007 Special Issue in memory of Sergio Lucchesi Growth Marks of titanian-andradite crystals from Colli Albani (Italy) Giovanna Agrosi * , Eugenio Scandale and Gioacchino Tempesta Dipartimento Geomineralogico - Universita degli Studi di Bari Aldo Moro, Italy *Corresponding author: g.agrosi@geomin.uniba.it Abstract Growth defects and chemical zonings of Ti-rich andradite (melanite) crystals from Colli Albani were studied using X-ray Diffraction Topography and Electron Probe Microanalysis in order to determine the distinctive features, marks, characterising minerals grown directly from high temperature solution in an open system (volcanic chamber). The samples, cut in (110) and (001) slices, showed a number of primary and, above all, multiphasic melt inclusions, as well as, an unusual colour zoning consisting of a darker colour in the {211} growth sectors with the respect to the {110} sectors. The analysis of X-ray topographic contrasts of growth defects as growth bands, sector boundaries and bundles of dislocations developed parallel to the growth directions has enabled the reconstruction of the morphological evolution vs. time of the samples. The results also suggest that from their onset the crystals were faceted with {211} and predominant {110} faces. Moreover, the topographic images indicated that most of the examined dislocations were nucleated from inclusions. By using the extinction criterion, it was possible to determine that these defects are characterised by a strong edge component. The faceted morphology of garnets found in this paper and the fact that edge dislocations were only characterised suggest a layer spreading growth mechanism by two-dimensional nucleation. The reconstruction of morphology was also particularly useful in distinguishing two different types of chemical zoning: concentric and sector zoning. The results of chemical analyses confirmed that the optically observed colour zoning coincided with sector zoning because of a small but meaningful enrichment of TiO 2 and a decrease of Al 2 O 3 in correspondence with {211} growth sectors. Instead, concentration variations, chiefly of TiO 2 and SiO 2 , were recorded crossing successive growth stages. This event appeared to be associated with the physio-chemical evolution of the growth environment and was defined as concentric zoning. Comparing the results found in our study with those of previous studies on garnets from hydrothermal and metasomatic environments, the meaning of the growth marks in the Ti-rich andraditic garnets from Colli Albani could be assigned to primary multiphase melt inclusions, concentric and sector zoning, sector boundaries, growth bands and edge dislocations since these defects characterise and distinguish our samples from those grown in different genetic environments. Key words : X-ray Topography; Ti-rich andradite; structural defects; concentric zoning; sector zoning.

Interaction of magmatic fluids and silicate melt residues with saline groundwater in the footwall of the Sudbury Igneous Complex, Ontario, Canada: New evidence from bulk rock geochemistry, fluid inclusions and stable isotopes

Along the northern margin of the 1.85Ga Sudbury Igneous Complex (SIC), Canada, a volumetrically minor rock type known as the footwall granophyre (FWGP) preserves evidence of the interaction of magmatic fluid and groundwater. The FWGP comprises veins, dikes and irregular bodies of a quartz–alkali feldspar–plagioclase intergrowth that are most abundant near footwall-style Cu–Ni–platinum-group element mineralization. Textures and cross-cutting relations show that the emplacement of the FWGP created favourable sites for the later deposition of sulfides. Geochemical and physical evidence show that the FWGP did not form by in-situ partial melting of the Archean country rocks in the footwall beneath the SIC. It represents a mobilized (injected) silicate residue from either the contact between partially melted country rocks and the SIC, or from the crystallizing SIC itself. Its bulk composition can be modeled as the product of fractional crystallization of a liquid with an initial composition equivalent to granophyric matrix (trapped liquid) in the lowermost units of the SIC (norite and sublayer).Primary fluid (saline, H2O-poor) and melt (silicate, H2O-rich) inclusions hosted in quartz from the FWGP have bulk and trace element compositions consistent with those hosted in quartz from the granophyric matrix of the lower units of the SIC (norite, sublayer). Fluid inclusions contain mixtures of a high salinity (44–70wt.% NaClequivalent; n=39), Na–Fe-rich aqueous magmatic fluid and a lower salinity, Ca- and Sr-rich groundwater. The magmatic end-member is thought to be an SIC-derived volatile phase. Estimates of the mixing proportions show that primary and secondary inclusions in the FWGP contain 60–100% and 30–80% of the magmatic end-member by mass, respectively. Fluids responsible for the remobilization of ore metals represent mixtures containing a much higher proportion of the groundwater end-member (not less than 70% by mass) compared to the fluids trapped in the FWGP. At considerable distance (~400m) from the SIC, the chlorine isotope composition of biotite in the FWGP is enriched in 37Cl (δ37Cl=0.98‰ to 1.61‰). Scapolite from the high temperature veins and the interstices of rock types in the main mass of the SIC shows similar enrichment in 37Cl (up to 1.34‰) whereas biotite from the Archean country rocks (δ37Cl=−0.88 to −0.53‰) and associated groundwater are 37Cl-depleted.The results of this study lead to two important conclusions concerning genetic and exploration models for the footwall ore zones. First, magmatic end-member fluid introduced into the footwall with the melt from which the FWGP crystallized contained very little Cu (rarely >~100ppm), the main ore metal in the footwall ores. This result questions hydrothermal models for footwall ore formation that hypothesize that Cu and other ore metals were remobilized from sulfide deposits along the SIC contact by magmatic-hydrothermal fluids and redeposited in the footwall. It is more likely that ore metals were only locally remobilized in the footwall ore zones when metal-poor hybrid fluids interacted with pre-existing sulfide veins of magmatic origin. Second, inclusions containing the hybrid fluid occur in the FWGP rock type throughout the footwall, in both mineralized and unmineralized areas. Therefore, although the FWGP preserves stable Cl isotope and fluid inclusion evidence that magmatic fluid was introduced into the footwall, detection of this magmatic component alone will not serve as a useful exploration method. In contrast, mapping the distribution and abundance of FWGP may be ore-predictive since FWGP emplacement and its higher abundance in mineralized areas appears to have been a structural and textural prerequisite for footwall ore emplacement.

Hambergite-rich melt inclusions in morganite crystals from the Muiane pegmatite, Mozambique and some remarks on the paragenesis of hambergite

Raman spectroscopic studies of daughter crystals of hambergite [Be2BO3(OH, F)] in primary melt and secondary fluid inclusions in morganite crystals from the Muiane pegmatite, Mozambique, show that the inclusions have extremely high beryllium concentrations, corresponding to as much as 10.6% (g/g) in melt inclusions and 1.25% (g/g) BeO in fluid inclusions. These melt and fluid inclusions were trapped at about 610°C and 277°C, respectively. We propose two possible mechanisms for the formation of the hambergite crystals: (i) direct crystallization from a boron- and beryllium-rich pegmatite-forming melt or (ii) these are daughter crystals produced by the retrograde reaction of the boron-rich inclusion fluid with the beryl host, after release of boric acid from the primary trapped metastable volatile-rich silicate melt during cooling and recrystallization. Although we favor the second option, either case demonstrate the extent to which Be maybe concentrated in a boron-rich fluid at relatively high temperatures, and in which species of Be maybe transported. One important constraint on the stability of the hambergite paragenesis is temperature; at temperatures of ≥650°C (at 2 kbar) hambergite is not stable and converts to bromellite [BeO].

Two Contrasting H2O-rich Components in Primary Melt Inclusions from Mount Shasta

In addition to the abundant andesite and dacite lavas of the Mt. Shasta stratocone, primitive mafic lavas have erupted during the Quaternary from the Mt. Shasta region. Two types of basic lavas are the focus of this study: nearly anhydrous high-aluminium olivine tholeiites, produced by decompression melting, and basaltic andesites, produced by melting of a metasomatized mantle source (i.e. modified by various amounts of H2O-rich components). Here we provide further insight into the origin and the coexistence of these two types of magma based on a detailed study of the dissolved volatile contents in melt inclusions trapped in magnesium-rich olivine crystals from both tholeiites and basaltic andesites. The melt inclusions fall on the primitive extension of their respective host lava compositions: (1) the tholeiite melt inclusions are nearly anhydrous melts with low and clustered volatiles compositions; (2) the basaltic andesite melt inclusions are ‘wet’ melts with higher and more variable volatile contents (up to 2·6 wt% H2O, 820 ppm CO2, 1270 ppm Cl, 1220 ppm F and 6280 ppm S) than those of the tholeiite melt inclusions. The basaltic andesite melt inclusions are enriched in fluid-mobile elements and in incompatible trace elements: they display a stronger signature of slab-derived components than their host lavas. The selective enrichment of the basaltic andesite melt inclusions in fluid-mobile elements such as K, F, Cl, Ba and B as well as their contrasting δ11B (from −10·2 ± 1·3‰ to −3·2 ± 0·9‰ for the melt inclusions from sample 95-15 and from −4·9 ± 1·1‰ to +4·4 ± 1·1‰ for the melt inclusions from samples 85-1a and 85-47) show the imprints of two distinct slab-derived components C1 and C2. Using trace element ratios, we modelled the compositions of these two H2O-rich components (C1 with Cl/F of 1·1 is rich in both in high field strength elements and incompatible trace elements, whereas C2 with Cl/F of 4·0 is poorer in trace elements) and show that they represent mixing between sediment melts and dehydration fluids from the altered oceanic crust.

Melt–wall rock interaction in the mantle shown by silicate melt inclusions in peridotite xenoliths from the central Pannonian Basin (western Hungary)

AbstractIn this paper we present a detailed textural and geochemical study of two equigranular textured amphibole‐bearing spinel lherzolite xenoliths from Szigliget, Bakony–Balaton Highland Volcanic Field (BBHVF, western Hungary) containing abundant primary silicate melt inclusions (SMIs) in clinopyroxene rims and secondary SMIs in orthopyroxene (and rarely spinel) along healed fractures. The SMIs are dominantly composed of silicate glass and CO2‐rich bubbles. Clinopyroxene and orthopyroxene are zoned in both studied xenoliths, especially with respect to Fe, Mg, Na, and Al contents. Cores of clinopyroxenes in both xenoliths show trace element distribution close to primitive mantle. Rims of clinopyroxenes are enriched in Th, U, light rare earth elements (LREEs) and medium REEs (MREEs). Amphiboles in the Szg08 xenolith exhibit elevated Rb, Ba, Nb, Ta, LREE, and MREE contents. The composition of silicate glass in the SMIs covers a wide range from the basaltic trachyandesite and andesite to phonolitic compositions. The glasses are particularly rich in P2O5. Both primary and secondary SMIs are strongly enriched in incompatible trace elements (mostly U, Th, La, Zr) and display a slight negative Hf anomaly. The development of zoned pyroxenes, as well as the entrapment of primary SMIs in the clinopyroxene rims, happened after partial melting and subsequent crystallization of clinopyroxenes, most probably due to an interaction between hot volatile‐bearing evolved melt and mantle wall‐rocks. This silicate melt filled microfractures in orthopyroxenes (and rarely spinels) resulting in secondary SMIs.

Petrogenesis of augite-bearing ureilites Hughes 009 and FRO 90054/93008 inferred from melt inclusions in olivine, augite and orthopyroxene

Melt inclusions in ureilites occur only in the small augite- and orthopyroxene-bearing subgroups. Previously [Goodrich C.A., Fioretti A.M., Tribaudino M. and Molin G. (2001) Primary trapped melt inclusions in olivine in the olivine–augite–orthopyroxene ureilite Hughes 009. Geochim. Cosmochim. Acta 65, 621–652] we described melt inclusions in olivine in the olivine–augite–orthopyroxene ureilite Hughes 009 (Hughes). FRO 90054/93008 (FRO) is a near-twin of Hughes, and has abundant melt inclusions in all three primary silicates. We use these inclusions to reconstruct the major, minor and rare earth element composition of the Hughes/FRO parent magma and evaluate models for the petrogenesis of augite-bearing ureilites. Hughes and FRO consist of 23–47 vol % olivine (Fo 87.3 and 87.6, respectively), 7–52 vol % augite ( mg 89.2, Wo 37.0 and mg 88.8, Wo 38.0, respectively), and 12–56 vol % orthopyroxene ( mg 88.3, Wo 4.9 and mg 88.0, Wo 4.8, respectively). They have coarse-grained (⩽3 mm), highly-equilibrated textures, with poikilitic relationships indicating the crystallization sequence olivine → augite → orthopyroxene. FRO is more shocked than Hughes, experienced greater secondary reduction, and is more weathered. The two meteorites are probably derived from the same lithologic unit. Melt inclusions in olivine consist of glass ± daughter cpx ± metal–sulfide–phosphide spherules ± chromite, and have completely reequilibrated Fe/Mg with their hosts. We follow the method of Goodrich et al. (2001) for reconstructing the composition of the primary trapped liquid they represent (olPTL), but correct an error in our treatment of the effects of reequilibration. Inclusions in augite consist of glass, which shows only partial reequilibration of Fe/Mg. The composition of the primary trapped liquid they represent (augPTL) is reconstructed by reverse fractional crystallization of wall augite from the most ferroan glass. Inclusions in orthopyroxene consist of glass + 30–50 vol % daughter cpx. The cpx shows complete, but the glass only partial, reequilibration of Fe/Mg. A range of possible compositions for the primary trapped liquid they represent (opxPTL) is calculated by modal recombination of glass and cpx, followed by addition of wall orthopyroxene and adjustment of Fe/Mg for equilibrium with the primary orthopyroxene. Only a small subset of these compositions is plausible on the basis of being orthopyroxene-saturated. Results indicate that olPTL, assumed to represent the parent magma of these rocks, was saturated only with olivine and in equilibrium with Fo ∼ 83. AugPTL and opxPTL are very similar in composition; both are close to augite + orthopyroxene co-saturation and in equilibrium with Fo 87/8. We suggest that olPTL was reduced to Fo 87/8 due to smelting during ascent, and show that this produces a composition very similar to that of augPTL and opxPTL. REE data for each of the three primary silicates and the least evolved melt inclusions in olivine are used to calculate REE abundances in the Hughes/FRO parent magma. All four methods yield very similar results, indicating a REE pattern that is strongly LREE-depleted (Sm/La = 3.3–3.7), with a small negative Eu anomaly (Eu/Eu* = 0.82) and slight HREE-depletion (Gd/Lu = 1.4–1.6). The Hughes/FRO parent magma provides a robust constraint on models for the petrogenesis of augite-bearing ureilites. Its major, minor and rare earth element composition suggests derivation through mixing and/or assimilation processes, rather than as a primary melt on the ureilite parent body.

Petrogenesis of arc lavas from the Rucu Pichincha and Pan de Azucar volcanoes (Ecuadorian arc): Major, trace element, and boron isotope evidences from olivine‐hosted melt inclusions

Primary melt inclusions in olivine phenocrysts (Fo74–89) of basic lavas from Pichincha and Pan de Azucar volcanoes (in the front and rear arcs of the Ecuadorian Andes, respectively) were analyzed by electron microprobe for major elements and by ion microprobe for trace element and boron isotope compositions. Although melt inclusions in the most magnesium‐rich olivines contain relatively primitive magmas, their compositions are not directly linked to those of the whole rocks through a differentiation scheme. They are characterized by nepheline‐normative compositions with low SiO2 contents (39.8–47.9 wt%) and unusually high CaO contents (up to 15.4 wt%), which cannot be derived from melting of a simple peridotitic mantle. We explain their formation by the presence of amphibole‐bearing olivine‐clinopyroxenites in the source of these melts. The trace elements patterns of the melt inclusions show the typical trace element features of arc magmas, such as enrichment in LILE and LREE, and negative anomalies in Nb and Ti. Across‐arc variations of mobile versus less mobile incompatible element ratios indicate a decreasing input of a mobile phase from the slab to the mantle wedge with the distance to the trench, along with a decrease in the degree of melting. Boron isotope compositions are highly variable within each volcano (δ11B from −9.5 ± 1.3‰ to +3.5 ± 1.4‰ for the Pichincha melt inclusions and from −17.9 ± 0.8‰ to −1.9 ± 1.4‰ for the Pan de Azucar melt inclusions) and suggest trapping of isotopically heterogeneous melts. Modeling of both dehydration and fusion of the slab indicates that the Pichincha melt inclusions were formed by melting a source enriched by the addition of 1% of a heterogeneous aqueous fluid derived from the dehydration of both the sediments and the altered oceanic crust (after 74 and 76% of B loss, respectively). The phase that metasomatizes the source of the Pan de Azucar melt inclusions can be either an input of 0.1% of a heterogeneous aqueous fluid or more likely 0.5–1% of a heterogeneous silicate melt.

Primary carbonatite melt inclusions in apatite and in K-feldspar of clinopyroxene-rich mantle xenoliths hosted in lamprophyre dikes (Hungary)

We studied clinopyroxene–apatite–K-feldspar–phlogopite xenoliths (CAKP), collected from the Late Cretaceous lamprophyre dikes of the Alcsutdoboz-2 (Ad-2) borehole, Transdanubian Central Range, Hungary. Apatite and K-feldspar contain a large number of primary, negative crystal shaped carbonatite melt inclusions. Chemically, the melt inclusions are phosphorous dolomitic in apatite and dolomite-bearing alkaline-aluminosiliceous in K-feldspar. As these melts in apatite and K-feldspar cannot be the differentiation product or residuum of each other, and appear to have been present in the host rock at the same time, they are likely to have formed by liquid immiscibility. Clinopyroxene and phlogopite are interpreted as products of metasomatic reactions between ultramafic mantle and carbonatite melt initially infiltrating the mantle rock. During this process the composition of the metasomatizing carbonatite melt changed, resulting in the separation of two immiscible melts. One melt was phosphate carbonatite in composition, and the other was a carbonate-bearing alkaline aluminosiliceous melt. Both melts were over-saturated with respect to apatite and K-feldspar. This process initiated the crystallization of the host minerals from a continuously percolating melt and, consequently, led to the entrapment of large populations of melt inclusions. The initial metasomatizing melt had a phosphorous Mg-calcitic carbonatite composition (with low Mg#), and is likely to have been formed in a subducted lithospheric slab.

LA-ICP-MS study of apatite- and K feldspar-hosted primary carbonatite melt inclusions in clinopyroxenite xenoliths from lamprophyres, Hungary: Implications for significance of carbonatite melts in the Earth’s mantle

We report the results of LA-ICP-MS analyses of rock forming minerals in clinopyroxene-apatite-K feldspar-phlogopite (CAKP) metasomatic xenoliths and primary carbonatite melt inclusions (CMI) hosted in apatite (Ap) and K feldspar (Kfs). The xenoliths are from the Cretaceous lamprophyre dikes of the Transdanubian Central Range, Hungary. The CMI in Ap have phosphorus dolomitic composition as opposed to CMI in Kfs, which display dolomitic alkali-aluminosiliceous character. The melts found in CMI in Ap and in Kfs likely formed by liquid–liquid separation from an originally carbonate- and phosphorous-rich melt. Primitive mantle (PM) normalized trace element distributions of both Ap- and Kfs-hosted CMI ( n = 60 and 20, respectively) reveal a strong negative Ti-anomaly, and an extreme enrichment in incompatible elements (U, Th, LILE and LREE) relative to HREE, Sc, V, Ni and Cr. Rarely, apatites contain unique CMI, which show major- and trace-element signature transitional to K feldspar-hosted CMI. This is due to heterogeneous entrapment of an immiscible phosphorous-bearing carbonatite melt and a carbonate-bearing alkali aluminosiliceous melt, which is a further evidence for their co-existence. CMI reveal that U, Th, Pb, Nb, Ta, P, Sr, Y and REE partitioned into the phosphorous-bearing carbonatite melt, whereas Cs, Rb, Na, K, B, Al, Zr and Hf preferred the silicate-bearing liquid. PM normalized REE pattern (high LREE/HREE), elevated Zr and Hf contents and negative Ti anomaly of clinopyroxene (Cpx) indicate that its formation is genetically linked to carbonatite metasomatism attested by CMI. Trace element partitioning between the studied Cpx and CMI is in accordance with experimentally determined trace element distributions between Cpx and carbonatite melt. Cpx, which occur in samples with high modal proportion of apatite represent mantle section, which interacted with a higher amount of “initial” carbonatite melt than Cpx from apatite-poor xenoliths. This is confirmed by higher Cr, Ni, V, Sc, Ti and lower Zr, as well as Hf concentration in Cpx from xenoliths with low modal abundance of Ap. CMI reveal that Ti, V, Ni and Cr were in lower concentration in the “initial” carbonatite melt than in PM. Contrarily, Zr and Hf were more abundant in this melt than in PM. Consequently, a continuously migrating “initial” carbonatite melt, increased Zr and Hf concentration, and decreased Ti, Sc, V, Ni and especially Cr in the clinopyroxenes. Our findings suggest that the studied CAKP rocks were formed by carbonatite melt metasomatism, which occurred in an open system in the upper mantle.

A Quartz-bearing Orthopyroxene-rich Websterite Xenolith from the Pannonian Basin, Western Hungary: Evidence for Release of Quartz-saturated Melts from a Subducted Slab

An unusual quartz-bearing orthopyroxene-rich websterite xenolith has been found in an alkali basaltic tuff at Szigliget, Bakony-Balaton Highland Volcanic Field (BBHVF), western Hungary. Ortho- and clinopyroxenes are enriched in light rare earth elements (LREE), middle REE and Ni, and depleted in Nb, Ta, Sr and Ti compared with ortho- and clinopyroxenes occurring in either peridotite or lower crustal granulite xenoliths from the BBHVF. Both ortho- and clinopyroxenes in the xenolith contain primary and secondary silicate melt inclusions, and needle-shaped or rounded quartz inclusions. The melt inclusions are rich in SiO 2 and alkalis and poor in MgO, FeO and CaO. They are strongly enriched in LREE and large ion lithophile elements, and display negative Nb, Ta and Sr anomalies, and slightly positive Pb anomalies. The xenolith is interpreted to represent a fragment of an orthopyroxene-rich body that crystallized in the upper mantle from a hybrid melt that formed by interaction of mantle peridotite with a quartz-saturated silicate melt that was released from a subducted oceanic slab. Although the exact composition of the slab melt cannot be determined, model calculations on major and trace elements suggest involvement of a metasedimentary component.

Compositions of magmas, formation conditions, and genesis of carbonate-bearing ijolites and carbonatites of the Belaya Zima alkaline carbonatite complex, eastern Sayan

Based on the investigation of melt inclusions using electron and ion microprobe analysis, we estimated the composition, evolution, and formation conditions of magmas responsible for the calcite-bearing ijolites and carbonatites of the Belaya Zima alkaline carbonatite complex (eastern Sayan, Russia). Primary melt and coexisting crystalline inclusions were found in the nepheline and calcite of these rocks. Diopside, amphibole (?), perovskite, potassium feldspar, apatite, calcite, pyrrhotite, and titanomagnetite were identified among the crystalline inclusions. The melt inclusions in nepheline from the ijolites are completely crystallized. The crystalline daughter phases of these inclusions are diopside, phlogopite, apatite, calcite, magnetite, and cuspidine. During thermometric experiments with melt inclusions in nepheline, the complete homogenization of the inclusions was attained through the dissolution of a gas bubble at temperatures of 1120–1130°C. The chemical analysis of glasses from the homogenized melt inclusions in nepheline of the ijolites revealed significant variations in the content of components: from 36 to 48 wt % SiO2, from 9 to 21 wt % Al2O3, from 8 to 25 wt % CaO, and from 0.6 to 7 wt % MgO. All the melts show very high contents of alkalis, especially sodium. According to the results of ion microprobe analysis, the average content of water in the melts is no higher than a few tenths of a percent. The most salient feature of the melt inclusions is the extremely high content of Nb and Zr. The glasses of melt inclusions are also enriched in Ta, Th, and light rare earth elements but depleted in Ti and Hf. Primary melt inclusions in calcite from the carbonatites contain a colorless glass and daughter phlogopite, garnet, and diopside. The silicate glass from the melt inclusions in calcite of the carbonatite is chemically similar to the glasses of homogenized melt inclusions in nepheline from the ijolites. An important feature of melt inclusions in calcite of the carbonatites is the presence in the glass of carbonate globules corresponding to calcite in composition. The investigation of melt inclusions in minerals of the ijolites and carbonatites and the analysis of the alkaline and ore-bearing rocks of the Belaya Zima Massif provided evidence for the contribution of crystallization differentiation and silicate-carbonate liquid immiscibility to the formation of these rocks. Using the obtained trace-element compositions of glasses of homogenized melt inclusions and various alkaline rocks and carbonatites, we determined to a first approximation the compositions of mantle sources responsible for the formation of the rock association of the Belaya Zima alkaline-carbonatite complex. The alkaline rocks and carbonatites were derived from the depleted mantle affected by extensive metasomatism. It is supposed that carbonate melts enriched in sodium and calcium were the main agents of mantle metasomatism.

Alkaline gabbroids of the Chirii outcrop (North Minusa depression): mineralogy and melt evolution

We studied the chemical composition of rock-forming minerals in gabbroids from the Chirii outcrop and the evolutionary features of parental basic melt during the crystallization of these rocks. Results were compared with data for basanites from pipes of the North Minusa depression. The mineralogical composition and thermobarogeochemical data of the gabbroids were examined in detail, and chemical analyses of rock-forming minerals (clinopyroxene, plagioclase, amphibole, biotite, titanomagnetite, and apatite) were carried out. Based on the homogenization temperatures of primary melt inclusions, we established the minimum temperatures and sequence of mineral crystallization in the gabbroids: clinopyroxene (>1160 °C), plagioclase, magnetite → amphibole (>950 °C) → biotite. The rock crystallization proceeded at shallow depths. Thermometric data are confirmed by results of modeling of equilibrium gabbroid crystallization. The crystallization of parental basic melt was accompanied by the accumulation of SiO 2, Al 2O 3, alkalies, and Cl and depletion in femic components. The melt evolved to granodiorite and alkali-syenite compositions. Compared with basanites from pipes, the parental melt had a longer evolution. The geochemical features of the gabbroids indicate that they, like basanites, crystallized from intraplate alkali-basaltoid magmas. But in petrochemistry and mineralogy the Chirii gabbroids differ considerably from the pipe basanites.

Mineral-melt-fluid composition of carbonate-bearing cumulate xenoliths in Tertiary alkali basalts of southern Slovakia

AbstractTwo types of carbonatic cumulate xenoliths occur in alkali basalts of the northern part of the Carpatho-Pannonian region, Central Europe. One is dominated by Ca-Fe-Mg carbonates with randomly distributed bisulphide globules (Fe1+xS2, x = 0–0.1), Mg-Al spinel, augite, rhönite, Ni-Co-rich chalcopyrite, and a Fe(Ni,Fe)2S4 phase. The second, carbonatic pyroxenite xenolith type, is composed of diopside, subordinate fluorapatite, interstitial Fe-Mg carbonates, and accessory K-pargasite, F-Al-rich ferroan phlogopite, Mg-Al spinel, albite and K-feldspar. All accessory minerals occur in ultrapotassic dacite-trachydacite glass in primary silicate melt inclusions in diopside, together with calcio-carbonatite and CO2-N2-CO inclusions. Textural evidence is provided for multiphase fluid-melt immiscibility in both xenolith types. The carbonatic pyroxenite type is inferred to have accumulated from differentiated, volatile-rich, ultrapotassic magma derived by a very low-degree partial melting of strongly metasomatized mantle. Mineral indicators point to a genetic link between the carbonatite xenolith with olivine-fractionated, silica-undersaturated alkalic basalt ponded at the mantle-crust boundary.

Melt evolution during the crystallization of basanites of the Tergesh pipe, northern Minusinsk Depression

In this paper we describe the mineralogy and geochemistry of basanites and melt inclusions in minerals from the Tergesh pipe, northern Minusinsk Depression. The rocks are composed of olivine and clinopyroxene phenocrysts and a groundmass of olivine, clinopyroxene, titanomagnetite, plagioclase, apatite, ilmenite, and glass. Melt inclusions were found only in the olivine and clinopyroxene phenocrysts. Primary melt inclusions in olivine contain glass, rhonite, clinopyroxene, a sulfide globule, and low-density fluid. The phase composition of melt inclusions in clinopyroxene is glass + low-density fluid ± xenogenous magnetite. According to thermometric investigations, the olivine phenocrysts began crystallizing at T = 1280–1320°C and P > 3.5 kbar, whereas groundmass minerals were formed under near-surface conditions at T ≤ 1200°C. The oxygen fugacity gradually changed during basanite crystallization from oxidizing (NNO) to more reducing conditions (QFM). The investigation of glass compositions (heated and unheated inclusions in phenocrysts and groundmass) showed that the evolution of a basanite melt during its crystallization included mainly an increase in SiO2, Al2O3, and alkalis, while a decrease in femic components, and the melt composition moved gradually toward tephriphonolite and trachyandesite. Geochemical evidence suggests that the primary basanite melt was derived from a mantle source affected by differentiation.

A reassessment of models for hydrocarbon generation in the Khibiny nepheline syenite complex, Kola Peninsula, Russia

Although hydrocarbon-bearing fluids have been known from the alkaline igneous rocks of the Khibiny intrusion for many years, their origin remains enigmatic. A recently proposed model of post-magmatic hydrocarbon (HC) generation through Fischer-Tropsch (FT) type reactions suggests the hydration of Fe-bearing phases and release of H 2 which reacts with magmatically derived CO 2 to form CH 4 and higher HCs. However, new petrographic, microthermometric, laser Raman, bulk gas and isotope data are presented and discussed in the context of previously published work in order to reassess models of HC generation. The gas phase is dominated by CH 4 with only minor proportions of higher hydrocarbons. No remnants of the proposed primary CO 2-rich fluid are found in the complex. The majority of the fluid inclusions are of secondary nature and trapped in healed microfractures. This indicates a high fluid flux after magma crystallisation. Entrapment conditions for fluid inclusions are 450–550 °C at 2.8–4.5 kbar. These temperatures are too high for hydrocarbon gas generation through the FT reaction. Chemical analyses of rims of Fe-rich phases suggest that they are not the result of alteration but instead represent changes in magma composition during crystallisation. Furthermore, there is no clear relationship between the presence of Fe-rich minerals and the abundance of fluid inclusion planes (FIPs) as reported elsewhere. δ 13C values for methane range from − 22.4‰ to − 5.4‰, confirming a largely abiogenic origin for the gas. The presence of primary CH 4-dominated fluid inclusions and melt inclusions, which contain a methane-rich gas phase, indicates a magmatic origin of the HCs. An increase in methane content, together with a decrease in δ 13C isotope values towards the intrusion margin suggests that magmatically derived abiogenic hydrocarbons may have mixed with biogenic hydrocarbons derived from the surrounding country rocks.