Mafic Alkaline Magmas Research Articles

Clinopyroxenite Xenoliths Record Magma Transport and Crystallization in the Middle and Upper Crust: A Case Study from the Rockeskyllerkopf Volcanic Complex, West Eifel, Germany

Abstract Clinopyroxenite xenoliths comprising cumulus clinopyroxene ± amphibole together with intercumulus phlogopite ± olivine ± apatite ± titanite form a large part of the xenolith load in the oldest deposits of the Rockeskyllerkopf Volcanic Complex (RVC) in the West Eifel volcanic field. The xenoliths also contain xenocrysts of olivine and clinopyroxene derived from mantle peridotite and clinopyroxene from lower crustal granulite. The clinopyroxenite xenoliths are divided into five groups on the basis of their modal mineralogy and mineral compositions. Groups 1 to 4 define a continuous compositional trend indicative of fractionation of a mafic alkaline magma. Group 5 xenoliths are compositionally distinct and have been tentatively linked to high pressure crystallization of phonolitic magma within the RVC system. Thermobarometry of the group 1 to 4 xenoliths indicates that they crystallized between 1 and 4 kilobars, equivalent to a depth of 4 to 14 km. Group 1 to 3 xenoliths all crystallized at between 1050°C and 1150°C, whereas the amphibole-rich group 4 xenoliths give temperature estimates of ~900°C. The clinopyroxenites share a common parent magma with clinopyroxene–phlogopite veins found in subcontinental lithospheric mantle xenoliths. However, the vein forming mama was richer in incompatible elements, in particular Zr and Hf and is interpreted to be an early formed batch of magma with the clinopyroxenites crystallizing from magma derived from the same mantle, which had been depleted by the earlier phase of melting. Intrusion of magma began around 155 ky prior to the eruption of the RVC. Fe–Mg interdiffusion profiles in zoned clinopyroxene show that the magma that formed the xenoliths was present in the crust for up to 28 ky prior to the eruption. However, most samples give interdiffusion times between 1.5 and 9.9 ky. Based on xenocryst residence times and the calculated P–T conditions for clinopyroxene, there were at least seven separate batches of magma emplaced below Rockeskyllerkopf, probably as sills.

Exceptional eruptive CO2 emissions from intra-plate alkaline magmatism in the Canary volcanic archipelago

Alkaline mafic magmas forming intra-plate oceanic islands are believed to be strongly enriched in CO2 due to low-degree partial melting of enriched mantle sources. However, until now, such CO2 enhancement has not been verified by measuring CO2 degassing during a subaerial eruption. Here, we provide evidence of highly CO2-rich gas emissions during the 86-day 2021 Tajogaite eruption of Cumbre Vieja volcano on La Palma Island, in the Canary archipelago. Our results reveal sustained high plume CO2/SO2 ratios, which, when combined with SO2 fluxes, melt inclusion volatile contents and magma production rates at explosive and effusive vents, imply a magmatic CO2 content of 4.5 ± 1.5 wt%. The amount of CO2 released during the 2021 eruptive activity was 28 ± 14 Mt CO2. Extrapolating to the volume of alkaline mafic magmas forming La Palma alone (estimated as 4000 km3 erupted over 11 Ma), we infer a maximum CO2 emission into the ocean and atmosphere of 1016 moles of CO2, equivalent to 20% of the eruptive CO2 emissions from a large igneous province eruption, suggesting that the formation of the Canary volcanic archipelago produced a CO2 emission of similar magnitude as a large igneous province.

The Andaluca plutonic unit, Vinquis Intrusive Complex, Argentina: An assessment of mantle role in the genesis of Early Carboniferous weakly peraluminous A-type granites in the pre-Andean SW Gondwana margin

During the Early Carboniferous, significant intracontinental magmatism was developed in the retro-arc region of the SW margin of Gondwana between ca. 27° and 31°S. This magmatism consisted of metaluminous to weakly peraluminous A-type granites, strongly peraluminous A-type granites (A- to S- hybrid granitoids) and, in a lesser extent, (sub-)volcanic felsic (rhyolites and trachytes) and mafic (alkaline basalts and lamprophyres) rocks. The Vinquis Intrusive Complex (VIC) registers these two compositional varieties of A-type granites: the Vinquis Unit (VU) represents the strongly peraluminous granites and the Andaluca Unit (AU) the weakly peraluminous granites. The adjacent Zapata Intrusive Complex (ZIC) also contains metaluminous to weakly peraluminous A-type granites. The AU is a semi-elliptical plutonic body of approximately 60 km 2 located in the southwestern part of the VIC, intruding the Vinquis Unit. The AU comprises three dominant rock types: i) quartz monzonite, ii) syenogranite and iii) alkali feldspar granite that can be classified as F-rich (calculated F content: 1950–5700 ppm), ferroan (FeO t /(FeO t + MgO) = 0.86–0.95) and weakly peraluminous (ASI = 1.03–1.10) A-type (Zr + Nb + Ce + Y = 364–570 ppm; Ga = 20–23 ppm) granitoids (SiO 2 wt% = 67.8–75.4% wt%). This unit was emplaced in early Carboniferous time (U Pb zircon crystallization ages of 346 ± 3 Ma and 342 ± 3 Ma). The AU has 87 Sr/ 86 Sr t , εNd t and εHf t values ranging from 0.7092 to 0.7140, −2.6 to −3.6 and − 16.5 to −3.3, respectively. These isotopic suggest that the parental magma of AU dominantly involved variable mixtures of mantle-derived components (magmas/fluids) and peraluminous continental crust. The extremely felsic granites of the AU (SiO 2 > ~72%) with a strongly fractionated composition (Rb > 336 and Sr < 70 ppm and Eu/Eu* < 0.22) supports extensive fractional crystallization. The high contents of fluorine and HFSE in AU could be explained as derived from an F- HFSE- and alkali-rich parental mafic magma plus contributions from F-bearing minerals assimilated from the country rock. Likewise, the magmatic differentiation processes from these reduced mafic magmas could explain the ferroan character of the AU granitoids. We envision three stages in the origin of these rocks: (1) fluids and magmas from the asthenospheric mantle metasomatize and melt the subcontinental lithospheric mantle (SCLM), the alkali-rich mafic melts produced are emplaced at the base of the crust, transfer heat to the crust and melt it, generating strongly peraluminous A-type magmas; (2) continued introduction of alkali-rich mafic magmas into the deep crust produce extensive assimilation of the preheated crust in equilibrium with mafic cumulates (generated by reaction and/or fractionation); (3) the hot magmas so produced migrate into the middle-upper crust to produce ferroan weakly peraluminous A-type granitoids. • The Andaluca plutonic unit of the Vinquis Intrusive Complex crystallized between 346 ± 3 Ma and 342 ± 3 Ma. • Andaluca Unit shows a F-rich weakly peraluminous ferroan A 2 -type affinity. • The high contents of fluorine and HFSE in Andaluca Unit can be explained from an F-rich parental alkaline mafic magma. • The Andaluca Unit granitoids can be generated through combined crustal assimilation and fractional crystallization of magmas derived from the metasomatized subcontinental lithospheric mantle.

An insight into the evolution of the lithospheric mantle of south Saharan metacraton: Mantle xenoliths from Jabal Eghei Volcanic Complex, Libya

Mantle xenoliths from a Pliocene (∼2 Ma) pyroclastic cone of the Jabal Eghei Volcanic Complex (south Libya) are studied for unravelling depletion-, deformation- and metasomatic events within this part of the Saharan Metacraton. The xenoliths are fertile Cpx-rich spinel lherzolites texturally distinguished into protogranular (hereafter: undeformed – UX) and porphyroclastic/equigranular (deformed – DX) types. The extent of depletion is estimated at 5–10% (olivine-spinel compositions) and 3–13% (HREE contents in clinopyroxene) of melt extraction; all parameters of depletion indicate that the UX are slightly more depleted than the DX. Calculated equilibrium temperatures range 800–1100 °C and generally overlap with temperatures obtained for other xenolith suites in Libya and for other off-cratonic xenoliths worldwide. The temperature estimates for the UX are slightly higher (857–1091 °C) than those for the DX (800–923 °C) and the former xenolith types are better equilibrated. The studied xenoliths exhibit evidence of stealth (high abundances of modal Cpx in some samples) and cryptic metasomatism (LREE-enriched patterns on whole-rock and Cpx REE diagrams) with metasomatic agents most likely akin to alkaline mafic melts similar in composition to host magmas. The study provides a simplified 2D model assuming that: a) the Jabal Eghei lithospheric mantle was roughly stratified with more depleted undeformed peridotite in deeper and less depleted deformed peridotite in shallower regions, b) in the Miocene, normal faulting caused ductile and brittle deformations in the lower and the upper parts of the lithosphere, respectively, and produced small-scale mafic alkaline melts that moved upward and metasomatized the overlying mantle, and c) the advancing extension produced mafic alkaline magma that captured xenoliths and brought them to the surface.

Trace element partitioning in zoned clinopyroxene as a proxy for undercooling: Experimental constraints from trachybasaltic magmas

Sector-zoned clinopyroxene records kinetic effects imposed by variable degrees of magma undercooling, ΔT, and can be utilised to track the dynamics of magmatic systems. The partitioning of trace elements into sectors grown in different crystallographic orientations can be used as a proxy for ΔT. However, an experimental assessment of the relationship between trace element zoning and ΔT has been lacking to date. Here we present trace element data from a series of undercooling crystallisation experiments on a primitive trachybasalt from Mt. Etna (Italy), at conditions of crustal storage (400 MPa, NNO + 2), and ΔT ranging from 23 to 173 °C. Changes in ΔT were modulated by varying both resting and liquidus temperatures, the latter via the melt-H2O content of the experiments. The resting temperature was retained for 24 h to ensure the attainment of near-equilibrium conditions.High-resolution elemental mapping reveals the distribution of trace elements in individual clinopyroxene zones. Increasing ΔT drives a shift from polyhedral morphologies with Al-rich prism and Al-poor hourglass sectors (ΔT = 23–25 °C), to skeletal (ΔT = 75–123 °C) and dendritic (ΔT = 132–173 °C) crystals with Al-rich skeletons and Al-poor overgrowths. Aluminium-rich zones have higher concentrations of rare earth elements (REE) and high field strength elements (HFSE) than Al-poor zones across all investigated ΔT conditions, and overall, Al, REE and HFSE contents increase with ΔT. This indicates that tetrahedral aluminium (TAl) and associated charge-balancing mechanisms govern the incorporation of REE and HFSE within clinopyroxene. Lattice strain parameters for REE in the M2 site indicate the incorporation of light relative to heavy REE in clinopyroxene is controlled by competing effects between the strain-free partition coefficient, D0, and the optimum cation radius, r0. Critically, the middle and heavy REE switch from incompatible to compatible with increasing ΔT. Used to model fractional crystallisation, our data demonstrate that fractionation of clinopyroxene at low ΔT controls pre-eruptive melt evolution. Importantly, this indicates crystallisation of clinopyroxene in the deep portions of Mt. Etna’s plumbing system is not rapid and is unlikely to result in the early formation of dendrites.We develop a parameterisation of ΔT based on REE partitioning between experimental clinopyroxene and coexisting melt, which can be applied to sector-zoned augite crystallising from mafic alkaline magmas, to reconstruct dynamic processes and thermal pathways during magma transport and storage. Applied to sector-zoned clinopyroxene microphenocrysts and groundmass microcrysts from the 1974 eccentric eruption at Mt. Etna, our parameterisation tracks an increase in ΔT with magma ascent and eruption, following recharge of Cr-rich mafic magma at depth. Sector-zoned clinopyroxene can track ΔT variations leading to volcanism at Mt. Etna and could be applied to quantify magma dynamics in other active volcanoes.

Experimental determination of H 2 O and CO 2 solubilities of mafic alkaline magmas from Canary Islands

We present new H 2 O and CO 2 solubility data in mafic to intermediate alkaline magmas from Fasnia and Garachico volcanoes, Tenerife. H 2 O-and CO 2-saturated experiments were conducted at ∼50-400 MPa, 1200°C, and f O 2 from 2 log units below the NiNiO solid buffer to 3.2 log units above it. Although existing solubility models for alkali-rich mafic magmas broadly describe H 2 O and CO 2 behaviour, associated errors are worthy of consideration since they usually exceed 15-20%. For this reason, we have determined the specific solubility laws of basanitic and phonotephritic melts from the Canary Islands. Results show similar H 2 O solubilities for both compositions, whereas the basanite can dissolve an average of ∼45% more CO 2 than the phonotephrite. By combining these data, we have established a simple empirical model that allows us to calculate melt inclusion entrapment pressures accurately and, therefore, better understand the inner workings of volcanic oceanic islands. Application to El Hierro 2011-2012 and young (<20 ka) basanites from this location shows that previous barometric estimates were, on average, overestimated by 15-28%. Our results suggest that magmas rising from depth experienced a first but short episode of equilibration at 8-10 km, whereas the bulk of the crystallization occurred during the subsequent dyke injection, ascent, and degassing at P ⇐ 200 MPa (6-1.5 km).

Crystal growth of clinopyroxene in mafic alkaline magmas

Recent experimental and natural studies have shown that igneous crystals can start to crystallize through skeletal or dendritic growth, and then backfill progressively to form polyhedral crystal morphologies, in contrast to the traditional tree-ring model of crystal growth. Identifying the growth mechanism of crystal is fundamental to understand crystal growth and associated magmatic processes. Clinopyroxene is an ubiquitous phase in magmatic systems. However, crystallographically controlled zoning patterns (e.g., sector zoning) observed in different studies are diverse or even show opposite trends, which make it challenging to understand the crystal growth mechanisms of clinopyroxene. This study presents a new finding of the contemporary occurrence of both clinopyroxenes with Al-rich and -poor {hk0} sectors in the Sailipu alkaline volcanic field in southern Tibet. Based on detailed textural and compositional analyses of a large number of clinopyroxene grains, we suggest a new crystal growth mechanism for clinopyroxene in mafic alkaline magmas. At low degrees of undercooling, all crystal faces grow concurrently, producing equilibrium Si–Mg-rich {−111} sectors and disequilibrium Al–Ti-rich {hk0} sectors; at high degrees of undercooling, the equilibrium sectors {hk0} initially form, which is followed by backfill of disequilibrium Al-rich {−111} sectors. Al2O3 is a useful indicator of equilibrium, and Al2O3-poor sectors, rather than specific crystal faces, are closer to equilibrium. The behavior of other cations is complex and the distribution of their oxide components is variable. Textural observations and the modeling results for P transfer across the crystal–melt diffusive boundary layer show that apatite inclusions in clinopyroxene can indicate rapid crystal growth. These results may contribute to a better understanding of magma storage, ascent and eruption on the basis of clinopyroxene.

Parameterization of clinopyroxene growth kinetics via crystal size distribution (CSD) analysis: Insights into the temporal scales of magma dynamics at Mt. Etna volcano

There is increasing recognition that both textural and compositional changes of clinopyroxene crystallizing from mafic alkaline magmas are the direct expression of complex dynamic processes extending over a broad range of spatial and temporal scales. Among others, supersaturation and relaxation phenomena play a key role in controlling the final crystal cargo of variably undercooled magmas erupted from active alkaline volcanoes. Following this line of reasoning, we have carried out isothermal-isobaric, decompression, and cooling rate experiments on a basalt interpreted as the parental magma of mafic alkaline eruptions at Mt. Etna volcano (Sicily, Italy). The main purpose is to reconstruct and quantify the textural changes (i.e., length of major and minor axes, surface area per unit volume, area fraction, and maximum growth rate) of clinopyroxene at variable pressures (30–300 MPa), temperatures (1050–1100 °C), volatile contents (0–5 wt% H2O and 0–0.2 wt% CO2), and equilibration times (0.25–72 h). Melt supersaturation, corresponding to a degree of undercooling variable from 14 to 125 °C, drives the formation of clinopyroxene crystals with different textures and sizes as a function of growth rate and relaxation time. By integrating experimental data and thermodynamic modeling, the transition between interface-controlled (euhedral morphologies) and diffusion-controlled (anhedral morphologies) growth regimes has been determined to occur at degrees of undercooling higher than 30 °C. The decrease of clinopyroxene growth rate with increasing the equilibration time is combined with the crystal size distribution (CSD) analysis of naturally undercooled clinopyroxene crystals erupted during 2011–2012 lava fountain episodes at Mt. Etna volcano. Results indicate that the crystallization of microlites and microphenocrysts is on the order of ~100–101 min (large undercooling, short equilibration time) and ~ 101–102 h (small undercooling, long equilibration time), respectively. This temporal information allows to better constrain the cooling-decompression paths of Etnean magmas rising and accelerating along a vertically extended, highly dynamic plumbing system. While clinopyroxene microlites develop during the fast ascent of magmas (~100–101 m s−1) within the uppermost part of the conduit or immediately before ejection from the vent, the onset of microphenocryst crystallization occurs at depth and continues within the plumbing system during the slow ascent of magmas (~10−2 m s−1) that migrate through interconnected storage regions.

Paragenetic relationships between low- and high-grade gold mineralization in the Cripple Creek Au-Te deposit, Colorado: Trace element studies of pyrite

The Cripple Creek alkaline igneous rock-related low-sulfidation epithermal gold telluride deposit is hosted in a 10 km wide Oligocene alkaline volcanic diatreme in Proterozoic rocks. Periods of phreatic eruptions and progressively more mafic alkaline magma emplacement created the diatreme and breccias that host the gold ore, followed by breccia pipe creation, and later by the emplacement of phonolite and later lamprophyre dikes throughout the diatreme. Pervasive potassic alteration affected much of the diatreme, and is spatially related to the late-stage veins containing native gold and precious metal tellurides. Previous studies have mainly focused on the high-grade gold veins primarily located toward the center of the diatreme. The current study evaluates the mineralogy and paragenesis of low-grade mineralization in the currently mined Wild Horse Extension (WHEX), Globe Hill, and Schist Island pits, and the Galena Embayment along the margin of the diatreme next to the WHEX pit, as well as high-grade mineralization in the WHEX and Cresson pits. Given the variable paragenesis of mineralizing stages in different locations in the Cripple Creek deposit, a simplified three stage paragenesis (stages 1, 2, and 3) is recognized here involving low- and high-grade gold mineralization.Scanning electron microscopy, electron microprobe, and laser ablation-inductively coupled plasma-mass spectrometry analyses are used to compare the trace element compositions of pyrite in low- and high-grade gold mineralization. These analyses reveal a complex trace element pattern with variability in Ag, As, Au, Co, Ni, Sb, and Te in the rims, cores, and zones (area between the core and the rim) of stages 2 (Py2) and 3 pyrite (Py3) and spatially between the pits currently being mined. Trace element concentrations in stage 1 pyrite (Py1) are lower than those in Py2 and Py3 and the patterns are uniform. Trace elements reside within the lattice structure of pyrite and as micro- and nano-inclusions (e.g., up to 48,220 ppm As, 6983 ppm Co, 12,250 ppm Ni, and 10,860 ppm Sb). The trace element patterns from the final stage of pyrite growth in high-grade gold telluride veins indicate enrichment in Ag, As, Au, Cu, Pb, Sb, and Te.The paragenesis and compositional zoning of pyrite associated with high-grade ore contrast with those associated with pyrite from low-grade ore, which preceded the formation of high-grade gold telluride veins. This suggests that metallic mineralization occurred in multiple episodes and that low-grade ore did not form from the same hydrothermal fluids that precipitated the high-grade ore. Early pyrite formed under passive, non-boiling conditions, followed by a transition to vigorous boiling in Py2 and Py3, ending with non-boiling conditions recorded in the Au and As-rich rims of Py3. This enrichment in Au and As on the rims of pyrite is consistent with other alkaline igneous rock-related epithermal Au deposits, as are the concentrations of trace elements throughout the pyrite grains. The presence of As, Ag, Co, Cu, Mo, Ni, Sb, Te, and Tl in pyrite are considered vectors to high-grade Au ore in the Cripple Creek deposit.

Geochemistry and petrology of rift-related mafic sills and arc-related Gabbro–Diorite bodies, Northern Bafq District, Central Iran

Two types of mafic intrusions have been recognized in the north of the Bafq district, Central Iran. A number of olivine-gabbroic to gabbroic sills intruded the Rizu Formation, comprised of alternating carbonate and volcanic rocks. Mineralogical data show that olivine + pyroxene + brown amphibole (kaersutite) + calcic plagioclase + opaque comprise the olivine-gabbroic rocks, and intermediate (or sodic) plagioclase + altered pyroxene and altered amphibole to biotite + opaque minerals the gabbroic rocks. Both rock types have shown within-plate and alkaline characters and have been presumed to be related to rift formation. The mafic-alkaline magma source of the sills is proposed to have been derived from enriched mantle. Regionally, two Gabbro–Diorite intrusions have been identified in the metamorphic complexes. Mineralogical data suggest that the pyroxene + amphibole (hornblende) + plagioclase + biotite + opaque minerals formed the gabbro to diorite intrusions. These rocks have shown characteristics of calc-alkaline and volcanic arc magmatism.

Magma Differentiation in Vertical Conduits Revealed by the Complementary Study of Plutonic and Volcanic Rocks from Fuerteventura (Canary Islands)

Fractional crystallization is a widely accepted mechanism to account for magmatic differentiation, but the location and geometry of the locations where this process takes place are still largely unknown. This is mainly due to the scarcity of occurrences where direct links between deep-seated cumulate rocks and their associated differentiation products are observed. The island of Fuerteventura in the Canary Archipelago offers such an opportunity. A dismembered Miocene volcano reveals a complex plutonic root zone, named PX1, consisting of a network of feeder conduits that can be related to remnants of contemporaneous lava flows. The lavas reveal a typical alkaline differentiation trend, whereas PX1 is composed of vertically layered cumulate lithologies ranging from olivine-rich wehrlites to clinopyroxenites and gabbros. Clinopyroxene phenocrysts in lavas (73·2–86·5 Mg#) exhibit the same zoning features and compositional ranges in major and trace elements as the plutonic clinopyroxenes, suggesting a direct genetic link between plutonic and effusive rocks. We interpret the PX1 rocks as resulting from the accumulation in volcanic conduits of phenocrysts segregated from ascending crystal-bearing magmas. Semi-quantitative modeling shows that fractional crystallization can indeed explain the evolution of the erupted lavas from basalt to basaltic trachyandesite. Progressive fractionation of the mineral assemblages olivine → olivine + minor clinopyroxene → clinopyroxene → clinopyroxene + plagioclase → plagioclase + kaersutite + apatite, with variable amounts of Fe–Ti oxides, can reproduce the major element compositional trend revealed by the Fuerteventura lavas. This calculated sequence coincides with the sequence of crystallization deduced from the textures of the PX1 rocks and could a priori generate the mineral assemblages that constitute the plutonic cumulates. However, complex core–mantle chemical zoning observed in PX1 clinopyroxenes suggests more complicated differentiation processes than simple crystal segregation and accumulation in vertical conduits. Primitive cores in PX1 clinopyroxenes (Mg# 80–88) display resorption features and sieve textures; they are interpreted as pre-existing crystals entrained from deeper levels or crystallized during the early stages of magma evolution. Clinopyroxene mantles display more evolved compositions (Mg# 74–80) with reverse zoning and external resorption features. Asymmetry in chemical zoning suggests crystallization in a confined environment such as a crystal mush. All these observations point to progressive mineral coarsening in the crystal cumulates, induced by the pervasive percolation of uprising melts on their way to the surface. The main factors controlling the variability of the cumulate lithologies are the crystal–melt segregation efficiency, the degree of differentiation and modal proportions of the magmas at their time of intrusion, and the extraction efficiency of the residual melts from the cumulates. The proposed model suggests that a differentiation process could take place in the feeder conduits and does not require storage of significant magma volumes (or magma chambers) in the crust to explain the observed evolution from mafic to intermediate magmas. Nevertheless, the existence of such large magma reservoirs is not excluded, in particular to explain the generation of significant volumes of differentiated lavas, as observed in other Canary Islands. Magma differentiation in vertical conduits opens up new perspectives in the understanding of fractional crystallization processes associated with the evolution of alkaline mafic magmas in ocean islands and continental intraplate settings.

Lamprophyres of the Tomtor Massif: A result of mixing between potassic and sodic alkaline mafic magmas

The paper presents data on inclusions in minerals of the least modified potassic lamprophyres in a series of strongly carbonatized potassic alkaline ultramafic porphyritic rocks. The rocks consist of diopside, kaersutite, analcime, apatite, and rare phlogopite and titanite phenocrysts and a groundmass, which is made up, along with these minerals, of potassic feldspar and calcite. The diopside and kaersutite phenocrysts display unsystematic multiple zoning. Chemically and mineralogically, the rock is ultramafic foidite and most likely corresponds to monchiquite. Primary and secondary melt inclusions were found in diopside, kaersutite, apatite, and titanite phenocrysts and are classified into three types: sodic silicate inclusions with analcime, potassic silicate inclusions with potassic feldspar, and carbonate inclusions, which are dominated by calcite. Heating and homogenization of the inclusions show that the potassic lamprophyres crystallized from a heterogeneous magma, with consisted of mixing mafic sodic and potassic alkaline magmas enriched in a carbonatite component. The composition of the magmas was close to nepheline and leucite melanephelinite. The minerals crystallized at 1150–1090°C from the sodic melts and at 1200–1250°C from the potassic ones. The sodic mafic melts were richer in Fe than the potassic ones, were the richest in Al, Mn, SO3, Cl, and H2O and poorer in Ti and P. The potassic mafic melts were not lamproitic, as follows from the presence of albite in the crystallized primary potassic melt inclusions. The diopside, the first mineral to crystallize in the rock, started to crystallize in the magmatic chamber from sodic mafic melt and ended to crystallize from mixed sodic–potassic melts. The potassic mafic melts were multiply replenished in the chamber in relation to tectonic motions. The ascent of the melts to the surface and rapidly varying P–T parameters of the magma were favorable for multiple separations of carbonatite melts from the alkaline mafic ones and their mixing and mingling.

Mineral associations and major element compositions of base metal sulphides from the subcontinental lithospheric mantle of NE Spain

This study deals with textural types and major element compositions of Cu-Ni-Fe sulphides from spinel lherzolite, harzburgite and olivine websterite xenoliths found in alkali basaltic rocks of the Neogene-Quaternary volcanic zone of Catalonia (NE Spain). Sulphides in harzburgites and websterites are scarce. Four textural types have been distinguished: inclusions in silicates and spinel, trails of small droplets often radiating from inclusions, interstitial grains, and grains related to pyrometamorphic textures. The mineral associations are dominated by one or two low-temperature monosulphide solid solutions: mss1, mss2, occasionally accompanied by pyrrhotite, pentlandite and Cu-rich sulphides. Compositions of mss1 are more Fe-enriched in inclusions and interstitial grains than in grains related to pyrometamorphism. Compositions of mss2 are Ni-rich very close to pentlandite. Sulphide bulk compositions correspond to high-temperature monosulphide solid solution equilibrated with a relatively Cu-Ni enriched sulphide melt at 1100–1000 °C. The breakdown products of these earlier compositions could have been either equilibrated below 600, 300 °C or being at disequilibrium. A restitic origin is consistent with the main sulphide mineral associations, the estimated melt extraction for peridotites (<30 %) and with the fact that lherzolites are less affected by cryptic metasomatism than harzburgites. However, Ni exchange coefficients between olivine and the high-temperature monosulphide solid solution underestimate equilibrium values. This suggests that some lherzolites could derive from pervasive refertilization. The scarcity of sulphides in websterites is explained by S incompatible behaviour during the formation of earlier cumulates from the mafic alkaline magmas which caused the cryptic metasomatism.

Melt evolution beneath a rifted craton edge: 40Ar/39Ar geochronology and Sr–Nd–Hf–Pb isotope systematics of primitive alkaline basalts and lamprophyres from the SW Baltic Shield

A new high-precision 40Ar/39Ar anorthoclase feldspar age of 176.7±0.5Ma (2-sigma) reveals that small-volume alkaline basaltic magmatism occurred at the rifted SW margin of the Baltic Shield in Scania (southern Sweden), at a time of global plate reorganization associated with the inception of Pangea supercontinent break-up. Our combined elemental and Sr–Nd–Hf–Pb isotope dataset for representative basanite and nephelinite samples (>8wt.% MgO) from 16 subvolcanic necks of the 30 by 40km large Jurassic volcanic field suggests magma derivation from a moderately depleted mantle source (87Sr/86Sri=0.7034–0.7048; εNdi=+4.4 to +5.2; εHfi=+4.7 to +8.1; 206Pb/204Pbi=18.8–19.5). The mafic alkaline melts segregated from mixed peridotite–pyroxenite mantle with a potential temperature of ∼1400°C at 2.7–4.2GPa (∼90–120km depths), which places ultimate melt generation within the convecting upper mantle, provided that the lithosphere–asthenosphere boundary beneath the southern Baltic Shield margin was at ⩽100km depth during Mesozoic–Cenozoic rifting. Isotopic shifts and incompatible element enrichment relative to Depleted Mantle reflect involvement of at least 20% recycled oceanic lithosphere component (i.e., pyroxenite) with some minor continent-derived sediment during partial melting of well-stirred convecting upper mantle peridotite. Although pargasitic amphibole-rich metasomatized lithospheric mantle is excluded as the main source of the Jurassic magmas from Scania, hydrous ultramafic veins (i.e., hornblendite) may have caused subtle modifications to the compositions of passing sublithospheric melts. For example, modeling suggests that the more radiogenic Hf (εHfi=+6.3 to +8.1) and Pb (206Pb/204Pbi=18.9–19.5) isotopic compositions of the more sodic and H2O-rich nephelinites, compared with relatively homogenous basanites (εHfi=+4.7 to +6.1; 206Pb/204Pbi=18.8–18.9), originate from minor interactions between rising asthenospheric melts and amphibole-rich metasomatic components. The metasomatic components were likely introduced to the lithospheric mantle beneath the southern Baltic Shield margin during extensive Permo-Carboniferous magmatic activity, a scenario that is supported by the geochemical and isotope compositions of ca. 286Ma lamprophyres from Scania (87Sr/86Sri=0.7040–0.7054; εNdi=+2.0 to +3.1; εHfi=+6.1 to +9.0; 206Pb/204Pbi=17.8–18.2).Strong variations in lithosphere thickness and thermal structure across the southern Baltic Shield margin may have caused transient small-scale mantle convection. This resulted in relatively fast and focused upwellings and lateral flow beneath the thinned lithosphere, where mafic alkaline magmas formed by low degrees of decompression melting of sublithospheric mantle. Such a geodynamic scenario would allow for enriched recycled components with low melting points to be preferentially sampled from the more depleted and refractory convecting upper mantle when channeled along a destabilizing craton edge. Similar to the ‘lid effect’ in oceanic island volcanic provinces, lithospheric architecture may exert strong control on the mantle melting regime, and thus offer a simple explanation for the geochemical resemblance of continental and oceanic intraplate mafic alkaline magmas of high Na/K affinity.

High-K Mafic Plinian Eruptions of Volcán de Colima, Mexico

The last Plinian-type eruption of Volcan de Colima, Mexico, occurred in 1913; this resulted in the removal of the top 100 m of the edifice and the deposition of a tephra layer that blanketed the slopes of the Colima Volcanic Complex (CVC). Road-cuts on the flanks of the nearby Nevado de Colima edifice expose pre-1913 air-fall tephra, pyroclastic flow and ash-rich surge deposits resulting from numerous highly explosive events throughout the Holocene. The majority of the pumice and scoria fallout deposits are medium-K subalkaline basaltic andesite and andesite in composition, defining a clear major element differentiation trend. In contrast, three newly discovered scoria fallout deposits are high-K subalkaline, transitional to alkaline, basaltic andesite in composition and are characterized by the presence of phlogopite; these deposits have high MgO (up to 7·9 wt %), K2O (up to 2·6 wt %) and P2O5 (up to 0·67 wt %) contents. They are also strongly enriched in fluid- and melt-mobile large ion lithophile elements (LILE; Rb, Ba, K, Sr and Th) and light REE (LREE; La, Ce, Pr and Nd) relative to the majority of the Colima tephra fallout deposits. Strontium and Nd isotope systematics reveal that the high-K mafic scoria have more radiogenic Sr (87Sr/86Sr = 0·70365–0·70408) and less radiogenic Nd (143Nd/144Nd = 0·51279–0·51294) compared with the majority of the subalkaline tephras (87Sr/86Sr = 0·70338–0·70371 and 143Nd/144Nd = 0·51290–0·51295). Two-component mixing models, using whole-rock geochemical data, indicate the importance of magma mixing in the petrogenesis of the Colima magmas, with addition of up to 50% by volume of an alkaline mafic magma component in the most potassic magmas. This is supported by mineral chemistry and textural data, which reveal multiple episodes of decompression and magma mingling within a shallow crustal magma storage region. The presence of these potassic tephra fall deposits among the otherwise prevailing medium-K subalkaline stratigraphy indicates that pulses of K-rich alkaline mafic magmas periodically enter the CVC plumbing system on timescales of a few thousand years and may trigger Plinian explosive eruptions.

Geochemistry of primary-carbonate bearing K-rich igneous rocks in the Awulale Mountains, western Tianshan: Implications for carbon-recycling in subduction zone

Arc magmatism plays an important role in the recycling of subducted carbon and returning it to the surface. However, the transfer mechanisms of carbon are poorly understood. In this study, the contribution of subducted carbonate-rich sediments to the genesis of the carbonate-bearing K-rich igneous rocks from western Tianshan was investigated. Four key triggers are involved, including sediments subduction, slab decarbonation, partial melting and magma segregation. The globular carbonate ocelli show C–O isotope signatures intermediate between oceanic sediments and mantle, suggesting that the carbon of the primary carbonate ocelli was derived from recycled subducted sediments in the mantle. Decarbonation of the subducted slab is regarded as the primary agent to carbonize the mantle wedge. Geochemical features indicate that the carbonate ocelli are primary, and that the parental K- and carbon-rich mafic alkaline magma was derived from partial melting of carbonated mantle wedge veined with phlogopite. Major and trace element compositions indicate that globular carbonate ocelli hosted in the Bugula K-rich igneous rocks are calcio-carbonate and formed primarily by segregation of the differentiated CO2-rich alkaline magma after crystallization fractionation. The K-rich alkaline magma, which formed from partial melting of metasomatized (i.e., phlogopite bearing) mantle wedge in the sub-arc region, is a favorable agent to transport subducted carbon back to the Earth’s surface during carbon recycling in subduction zones, because of the high CO2 solubility in alkaline mafic magma. We therefore propose a model for the petrogenesis of the carbonate-bearing K-rich igneous rocks in western Tianshan, which are significant for revealing the mechanism of carbon recycling in subduction zones.

Petrological implications of the Early Mesozoic lamprophyre dikes and related Tarkhata syenites (SE Altai and NW Mongolia)

Abstract This paper presents new data derived from field sampling and from a thorough description of lamprophyres located in southeastern Altai and northwestern Mongolia in terms of their mineralogy, textures, and chemical composition. The swarms of alkaline mafic dikes in the area coexist with granosyenite-monzodiorite and gabbro-dolerite intrusions and spatially coincide with an ore district of Sb-Hg, Ag-Sb, Ni-Co-As, Cu-Mo-W, and CaF2 hydrothermal mineralization. All lamprophyres belong to the Early Mesozoic Chuya complex formed in an intracontinental enviroment. Their distribution and orientation is controlled by two large fault zones. The Chuya dikes were investigated at two localities, namely, Yustyd and South-Chuya. The Yustyd lamprophyres intrude Middle-Upper Devonian black shale of the Yustyd depression. At South Chuya, lamprophyres, together with the Tarkhata granosyenite-monzodiorite complex, are hosted by Cambrian and Ordovician metamorphic rocks of the South-Chuya Range. Ar-Ar (phlogopite) and U-Pb (SHRIMP, zircon) ages of the lamprophyre dikes indicate long and continuous period of the formation of the Chuya complex (250-235 Ma). Major- and trace-element compositions of the lamprophyres from both localities and of the syenite indicate their origin from the same magma source. The textures and structures of the lamprophyre and plutonic rocks, their mineral assemblages and the chemistry of the rock-forming minerals provide clues to the evolution of the parental alkaline mafic magma and fluid regime.

H2O–CO2 solubility in mafic alkaline magma: applications to volatile sources and degassing behavior at Erebus volcano, Antarctica

We present new equilibrium mixed-volatile (H2O–CO2) solubility data for a phonotephrite from Erebus volcano, Antarctica. H2O–CO2-saturated experiments were conducted at 400–700 MPa, 1,190 °C, and ~NNO + 1 in non-end-loaded piston cylinders. Equilibrium H2O–CO2 fluid compositions were determined using low-temperature vacuum manometry, and the volatile and major element compositions of the glassy run products were determined by Fourier transform infrared spectroscopy and electron microprobe. Results show that the phonotephrite used in this study will dissolve ~0.8 wt% CO2 at 700 MPa and a fluid composition of \( X_{{{\text{H}}_{ 2} {\text{O}}}} \) ~0.4, in agreement with previous experimental studies on mafic alkaline rocks. Furthermore, the dissolution of CO2 at moderate to high \( X_{{{\text{H}}_{ 2} {\text{O}}}}^{\text{fluid}} \) in our experiments exceeds that predicted using lower-pressure experiments on similar melts from the literature, suggesting a departure from Henrian behavior of volatiles in the melt at pressures above 400 MPa. With these data, we place new constraints on the modeling of Erebus melt inclusion and gas emission data and thus the interpretation of its magma plumbing system and the contributions of primitive magmas to passive and explosive degassing from the Erebus phonolite lava lake.

Petrogenesis and mantle source characteristics of Quaternary alkaline mafic lavas in the western Carpathian–Pannonian Region, Styria, Austria

In the Styrian Basin, i.e. the westernmost part of the Carpathian–Pannonian Region (CPR), the Pliocene to Quaternary post-extensional phase was accompanied by eruption of alkaline mafic magmas, some of which carry mantle xenoliths. The rocks range from nephelinites (Stradnerkogel and Waltrafelsen) to (predominant) basanites and ne-basanites (Klöch and Steinberg). They have high Ce/Pb, Nb/U and Nb/La ratios reflecting asthenospheric mantle source characteristics with negligible crustal contamination, differentiation en route to the surface and/or interaction with the lithospheric mantle. The calculated depths of magma generation are >100km for the basanites and ne-basanites in comparison to 135km or more for the nephelinites, implying an origin in the garnet stability field. The temperatures of mantle melting for the basanites and ne-basanites are from ≈1400 to 1500°C.Modeling calculation using the most primitive Styrian sample (a basanite) gives a mantle potential temperature (Tp) of 1466°C similar to average Tp of upper mantle sources beneath MORB indicating that the Styrian magmas were generated from asthenospheric mantle sources at ambient temperatures that preclude plume activity beneath the study area.The nephelinites have elevated Zr/Hf (51–67) and La/YbN (29–31) ratios and negative K and Ti anomalies on the PM-normalized multi-element diagrams, similar to those of carbonatites. These characteristics suggest that their source had experienced enrichment by carbonatitic liquids; an inference supported by their estimated content of ~5% CO2. By contrast, the trace element signatures of the basanites and ne-basanites suggest that their asthenospheric source, which experienced higher degrees of melting than the nephelinites, was nearly unaffected by carbonatite metasomatism. From the overall similarity of the trace element distribution patterns and the narrow range of their SrNd isotopic ratios, all the rocks can be related to a similar (OIB-like) asthenospheric mantle source, approximating the European Asthenospheric Reservoir (EAR-type).

Petrogenesis of Cenozoic volcanic rocks in the NW sector of the Gharyan volcanic field, Libya

The north-western sector of the Gharyan volcanic field (northern Libya) consists of trachytic-phonolitic domes emplaced between ~41 and 38Ma, and small-volume mafic alkaline volcanic centres (basanites, tephrites, alkali basalts, hawaiites and rare benmoreites) of Middle Miocene-Pliocene age (~12–2Ma). Two types of trachytes and phonolites have been recognized on the basis of petrography, mineralogy and geochemistry. Type-1 trachytes and phonolites display a smooth spoon-shaped REE pattern without negative Europium anomalies. Type-2 trachytes and phonolites show a remarkable Eu negative anomaly, higher concentration in HFSE (NbTaZrHf), REE and Ti than Type-1 rocks. The origin of Type-1 trachytes and phonolites is compatible with removal of clinopyroxene, plagioclase, alkali feldspar, amphibole, magnetite and titanite starting from benmoreitic magmas, found in the same outcrops. Type-2 trachytes and phonolites could be the result of extensive fractional crystallization starting from mafic alkaline magma, without removal of titanite. In primitive mantle-normalized diagrams, the mafic rocks (Mg#=62–68, Cr up to 514ppm, Ni up to 425ppm) show peaks at Nb and Ta and troughs at K. These characteristics, coupled with low 87Sr/86Sr(i) (0.7033-0.7038) and positive εNd (from +4.2 to +5.3) features typical of the mafic anorogenic magmas of the northern African plate and of HIMU-OIB-like magma in general. The origin of the mafic rocks is compatible from a derivation from low degree partial melting (3–9%) shallow mantle sources in the spinel/garnet facies, placed just below the rigid plate in the uppermost low-velocity zone. The origin of the igneous activity is considered linked to passive lithospheric thinning related to the development of continental rifts like those of Sicily Channel (e.g., Pantelleria and Linosa) and Sardinia (e.g., Campidano Graben) in the Central-Western Mediterranean Sea.