The genesis of basaltic magmas

This paper reports the results of a detailed experimental investigation of fractionation of natural basaltic compositions under conditions of high pressure and high temperature. A single stage, piston-cylinder apparatus has been used in the pressure range up to 27 kb and at temperatures up to 1500° C to study the melting behaviour of several basaltic compositions. The compositions chosen are olivine-rich (20% or more normative olivine) and include olivine tholeiite (12% normative hypersthene), olivine basalt (1% normative hypersthene) alkali olivine basalt (2% normative nepheline) and picrite (3% normative hypersthene). The liquidus phases of the olivine tholeiite and olivine basalt are olivine at 1 Atmosphere, 4.5 kb and 9 kb, orthopyroxene at 13.5 and 18 kb, clinopyroxene at 22.5 kb and garnet at 27 kb. In the alkali olivine basalt composition, the liquidus phases are olivine at 1 Atmosphere and 9 kb, orthopyroxene with clinopyroxene at 13.5 kb, clinopyroxene at 18 kb and garnet at 27 kb. The sequence of appearance of phases below the liquidus has also been studied in detail. The electron probe micro-analyser has been used to make partial quantitative analyses of olivines, orthopyroxenes, clinopyroxenes and garnets which have crystallized at high pressure.These experimental and analytical results are used to determine the directions of fractionation of basaltic magmas during crystallization over a wide range of pressures. At pressures corresponding to depths of 35–70 km separation of aluminous enstatite from olivine tholeiite magma produces a direct fractionation trend from olivine tholeiites through olivine basalts to alkali olivine basalts. Co-precipitation of sub-calcic, aluminous clinopyroxene with the orthopyroxene in the more undersaturated compositions of this sequence produces derivative liquids of basanite type. Magmas of alkali olivine basalt and basanite type represent the lower temperature liquids derived by approximately 30% crystallization of olivine-rich tholeiite at 35–70 km depth. At depths of about 30 km, fractionation of olivine-rich tholeiite with separation of both olivine and low-alumina enstatite, joined at lower temperatures by sub-calcic clinopyroxene, leads to derivative liquids with relatively constant SiO₂ (48 to 50%) increasingly high Al₂O₃ (15–17%) contents and retaining olivine + hypersthene normative chemistry (5–15% normative olivine). These have the composition of typical high-alumina olivine tholeiites. The effects of low pressure fractionation may be superimposed on magma compositions derived from various depths within the mantle. These lead to divergence of the alkali olivine basalt and tholeiitic series but convergence of both the low-alumina and high-alumina tholeiites towards quartz tholeiite derivative liquids.The general problem of derivation of basaltic magmas from a mantle of peridotitic composition is discussed in some detail. Magmas are considered to be a consequence of partial melting but the composition of a magma is determined not by the depth of partial melting but by the depth at which magma segregation from residual crystals occurs. Magma generation from parental peridotite (pyrolite) at depths up to 100 km involves liquid-crystal equilibria between basaltic liquids and olivine + aluminous pyroxenes and does not involve garnet. At 35–70 km depth, basaltic liquids segregating from a pyrolite mantle will be of alkali olivine basalt type with about 20% partial melting but with increasing degrees of partial melting, liquids will change to olivine-rich tholeiite type with about 30% melting. If the depth of magma segregation is about 30 km, then magmas produced by 20–25% partial melting will be of high-alumina olivine tholeiite type, similar to the oceanic tholeiites occurring on the sea floor along the mid-oceanic ridges.Hypotheses of magma fractionation and generation by partial melting are considered in relation to the abundances and ratios of trace elements and in relation to isotopic abundance data on natural basalts. It is shown that there is a group of elements (including K, Ti, P, U, Th, Ba, Rb, Sr, Cs, Zr, Hf and the rare-earth elements) which show enrichment factors in alkali olivine basalts and in some tholeiites, which are inconsistent with simple crystal fractionation relationships between the magma types. This group of elements has been called incompatible elements referring to their inability to substitute to any appreciable extent in the major minerals of the upper mantle (olivine, aluminous pyroxenes). Because of the lack of temperature contrast between magma and wall-rock for a body of magma near to its depth of segregation in the mantle, cooling of the magma involves complementary processes of reaction with the wall-rook, including selective melting and extraction of the lowest melting fraction. The incompatible elements are probably highly concentrated in the lowest melting fraction of the pyrolite. The production of large overall enrichments in incompatible elements in a magma by reaction with and highly selective sampling of large volumes of mantle wall-rock during slow ascent of a magma is considered to be a normal, complementary process to crystal fractionation in the mantle. This process has been called wall-rock reaction. Magma generation in the mantle is rarely a simple, closed-system partial melting process and the isotopic abundances and incompatible element abundances of a basalt as observed at the earth's surface may be largely determined by the degree of reaction with the mantle or lower crustal wall-rocks and bear little relation to the abundances and ratios of the original parental mantle material (pyrolite).Occurrences of cognate xenoliths and xenocrysts in basalts are considered in relation to the experimental data on liquid-crystal equilibria at high pressure. It is inferred that the lherzolite nodules largely represent residual material after extraction of alkali olivine basalt from mantle pyrolite or pyrolite which has been selectively depleted in incompatible elements by wall-rock reaction processes. Lherzolite nodules included in tholeiitic magmas would melt to a relatively large extent and disintegrate, but would have a largely refractory character if included in alkali olivine basalt magma. Other examples of xenocrystal material in basalts are shown to be probable liquidus crystals or accumulates at high pressure from basaltic magma and provide a useful link between the experimental study and natural processes.     

Elevation of potassium content of basaltic magma by fractional crystallization: the effect of pressure

A wide range of natural quartz-normative liquids crystallizes olivine at low pressure. Addition of K₂O to the system results in expansion of the olivine primary phase field and replacement of pigeonite (stable in the K-free system) by hypersthene. Some variation in phase relations results from depression of crystallization temperature towards the temperature at which pigeonite reacts to form augite and hypersthene because of addition of K₂O. Another important influence on phase relations results from cation interactions in the liquid related to addition of K₂O. Studies of crystallization behavior of materials similar in most elements except K₂O show that K₂O content markedly alters crystallization behavior for more siliceous liquids but appears to have less effect on liquids with lower SiO₂ contents. Low-Ca pyroxenes melt congruently at P>5 kbar, so anhydrous liquids coprecipitate olivine, plagioclase, and two pyroxenes. Addition of K₂O to the liquid has the same effect as at 1 atm. Hypersthene replaces pigeonite as the Low-Ca pyroxene crystallization from liquids with >1.5% K₂O and the olivine primary phase field grows at the expense of those of pyroxenes and plagioclase. At 10 kbar, olivine may develop a reaction relationship with liquids containing >6% K₂O. At 15 kbar, however, liquids evolve to a pseudoeutectic involving alkali feldspar. The systematic variation in phase relations has important consequences for magmatic evolution in different environments. Dry mafic liquids at shallow levels in oceanic areas can crystallize olivine until the liquid is very evolved, resulting in extreme SiO₂-enrichment besides enrichment in K₂O, and producing potassic dacites. Olivine coexists with liquids with up to 54% SiO₂ if K₂O=0.6% (Grove and Baker 1984) but as much as 63% SiO₂ if K₂O3.5% (Ussler and Glazner 1989). Magmas rising beneath light continental crust may pond at the Moho and evolve to low-density liquids that can rise to the surface. Coprecipitation of olivine, plagioclase, augite, and a low-Ca pyroxene, produces enrichment in K₂O with only slight enrichment in SiO₂. This is terminated, at pressures of 6 to, possibly, 12 kbar, by development of a reaction relationship of olivine and liquid that progresses to higher K₂O contents with pressure. At pressures as high as 15 kbar, the reaction relation may not develop and only crystallization of alkali feldspar suppresses K₂O-enrichment. Any magmatic H₂O or crustal contamination may modify phase relations. The phase relations do, however, suggest that variation in K₂O:SiO₂ of evolved volcanic rocks is related to crustal thickness rather than to variation in the chemical compositions of primary magmas.     

Late-Hercynian shearing during crystallization of granitoid magmas (Sila massif,southern Italy): regional implications

Shearing of regional extent, involving granitoids and underlying mid-crustal rocks of the Sila massif (Calabria, Italy), is analysed in this paper. The deformed granitoids are affected by a wide NNW-SSE oriented deformation zone, stretching for about 60 km, from the neighbourhood of Cecita Lake to Cropani village. Meso- and micro-structures in granitoids, close to the boundary with underlying migmatitic paragneiss, indicate that deformation developed from melt-present to solid-state conditions. Simultaneous tectonics and magmatism activated a plutonic accretionary process at mid-crustal levels. This took place at about 300 Ma and involved hybrid magmas with a dominat contribution from a mantle source. The deformation regime remained steady for a long time during magma crystallization and cooling in subsolidus conditions. The regional top-to-the-W sense of shear in the present geographic coordinates, recorded in the deformed granitoids, seems geometrically consistent with the coeval direction of maximum extension found in another sector of the southern Hercynian belt, suggesting the original position of the Sila basement in this context. Magmatic ativity ended with the intrusion of mafic and felsic magams affected by a very weak deformation, ongoing during the final strain increments of the late-Hercynian stage.     

Experimental modeling of the explosion mechanism of basaltic magmas

Processes in the feeders of basaltic volcanoes during Strombolian-type eruptions were examined with the use of a complex apparatus for modeling basaltic eruptions (CAMBE), which was designed and manufactured by the authors for this purpose. The experimental setup consists of modeling and registering units and has a height of 18 m. It was designed with regard for the geometric dimensions of a natural feeding volcanic system: the ratio of the inner diameter of the feeder to its height is approximately 1: 1000. CAMBE was the first modeling equipment making possible passing a flow of gas-saturated liquid through the conduit, which allowed us to study the nucleation of gas bubbles, their growth, coalescence, transformations of the gas structures, and the kinetics of the gas phase. The experiments were carried out in a manner that made it possible to eliminate effects of structural barriers and fluctuations in the liquid flow velocity. As a result of the experiments, a new (previously unknown) regime in the flow of two-phase systems through a vertical conduit was discovered: the cluster regime, which is characterized by systematically alternating dense accumulations of gas bubbles (bubble clusters) and liquid devoid of a free gas phase. It is demonstrated that the liquid, bubbly, cluster, and slug regimes systematically grade into one another and are polymorphic modifications of gas-saturated liquids moving through vertical conduits. Our data led us to propose a new model for the gas-hydrodynamic movement of magmatic melt through the conduit of a basaltic volcano: depending on the gas-hydrodynamic regime in the volcanic vent, various types of eruptive activity (up to explosions) may take place. The analyses of basaltic magma explosions allowed us to describe them from a new standpoint and recognize the following four major modes of their manifestations at the surface: (1) weak ash explosions early during the cluster regime, (2) strong ash explosions during the mature cluster regime, (3) bomb explosions during the slug regime, (4) bomb grading to ash explosions during the slug regime associated with trains of small bubbles.     

The fluid regime of crystallization of water-saturated granitic and pegmatitic magmas: a physicochemical analysis

Miarolitic granite pegmatites are a unique natural object that makes it possible to study magmatic processes that lead to the formation of ore-forming media and systems. This paper summarizes modern views on phase transformations in aqueous silicate systems at parameters close to those of the transition from magmatic to hydrothermal crystallization. Comparison of phase diagrams and the results of study of pegmatite-forming media permits making conclusions about the crystallization of the water-saturated magmas of miarolitic granite pegmatites. The fluid regime of aqueous granite systems of simple composition, not enriched in fluxing components, is determined mainly by magma degassing or the supply of volatiles with flows of transmagmatic fluids. These processes cause the separation of essentially carbon dioxide or essentially hydrous fluid. During the evolution of such magmas, crystallization from silicate melt is separated in PT-space and, possibly, in time from the crystallization from aqueous or mixed carbon dioxide-aqueous super- and subcritical solutions. The evolution of chambers of water-saturated granitic and pegmatitic magma enriched in F, B, and alkali metals presupposes the formation of a heterogeneous mineral-forming medium in which crystallization occurs in the magmatic melt at high-temperature stages; as temperature decreases, crystallization can proceed in hydrous fluid, hydrosilicate, and/or hydrosaline liquids simultaneously. Hydrothermal crystallization can also take place in a heterogeneous medium consisting of aqueous solutions of different salinities and vapor or vapor-carbon dioxide gas mixture. The relationship between different fluid regimes during the evolution of volatile-saturated granitic and pegmatitic magmas determines the variety of postmagmatic rocks accompanying granite massifs.     

内蒙古维拉斯托稀有金属-锡多金属矿床的成矿作用:来自花岗质岩浆结晶分异的启示

内蒙古维拉斯托稀有金属-锡多金属矿床位于大兴安岭南段西坡,是一个以锡为主,共伴生锌、钨、铜、钼、铌、钽、锂和铷的大型矿床.该矿床以发育铌、钽、铷、锂矿化有别于大兴安岭南段的其他锡多金属矿床.本文对维拉斯托地区的北大山岩体和维拉斯托岩体开展了 LA-ICP-MS锆石U-Pb定年、全岩地球化学研究和锆石Lu-Hf同位素分析...     

Phase relations governing the derivation of alkaline basaltic magmas from primary magmas at high pressures

A comparison between the variation trend of alkaline basaltic magmas within the CaO-MgO-Al₂O₃-SiO₂ system and experimentally estimated phase relations for this system at high pressures, suggests an olivine reaction relationship, which may explain the transition from primary magmas in equilibrium with olivine to alkaline basaltic magmas in which olivine does not form at high pressures. This reaction relationship is considered to be due to a transition from positive to negative crystallization with respect to olivine along the four phase curve where olivine, diopside, pyrope garnet and liquid are initially in equilibrium. The bimineralic, eclogitic character of alkaline basaltic compositions at high pressures is interpreted as being due to the presence of a thermal minimum on the three phase surface, where dioside and pyrope garnet are in equilibrium with liquid.     

Nickel content of basaltic magmas: identification of primary magmas and a measure of the degree of olivine fractionation

Available NiO analyses of olivine in peridotites of probable mantle origin are consistent in giving values around 0.40 weight per cent. Assuming that basaltic magma forming from the mantle was in equilibrium with such peridotitic olivine, the NiO content of primary basaltic magmas is estimated to be about 0.030–0.050 weight per cent. The fractionation behaviour of nickel in basaltic magma due to the crystallization of olivine has been calculated using constant NiMg and FeMg exchange partition coefficients between olivine and magma. It is shown that the NiO content of both magma and olivine decreases by 50 per cent after fractional crystallization of 6–12 per cent of olivine. The nickel distribution in some basaltic rocks and olivines is examined in the light of these results, and it is suggested that basaltic magmas, such as some of the ocean-floor basalt and the Hawaiian tholeiite and alkali basalts, represent primary magmas from mantle peridotites.     

Some trace element relationships among liquid and solid phases in the course of the fractional crystallization of magmas

The classical equations relating the trace element concentrations of the liquid and solid phases coexisting in the simple fractional crystallization of a parental magma have been put in a simple graphical form, which allows rapid analysis of the possible genetic relationships in a given rock suite. The effects of an incomplete separation between the two phases are taken into account. The approach does not require the use of otherwise estimated partition coefficients. Trace element data concerning the minerals of cumulates, where available, may provide an independent estimation of the effective mineral-liquid partition coefficients. With reasonable assumptions, this approach may even be applied to plutonic rocks. Interpretation of the published rare earth element data from the Southern California Batholith by this procedure suggests that a tonalitic parental magma could generate a granodioritic liquid by crystallizing 40–50 wt % of a solid residue of gabbroic composition, in agreement with Larsen's (Mem. Geol. Soc. Amer. 29, 1948) calculations. The calculated mineral-liquid partition coefficients for the REE fall in the range of published phenocryst-groundmass values for acidic volcanic rocks.     

Origin of basaltic magmas in the Mojave Desert area,California

The basaltic lavas erupted throughout the Mojave Desert are basanites (SiO₂<46%, normative nepheline>5%, and K₂O>1.5%), alkali-olivine basalts (SiO₂=46–48%; ne=0–5%; and K₂O=1.0–1.5%), and low-alumina, sub-alkaline basalts (SiO₂=48–51%; ne=0; K₂O<1.0%). One volcano, Pisgah Crater, erupted five times, with lava from each successive phase containing more silica and less potash than the one proceeding it. This compositional trend is the reverse of that expected from differentiation of a single alkalic magma, and therefore, may represent a succession of magmas tapped from a zone of continuing partial melting in the mantle.These lava compositions suggest that first melting was under high water pressure and was followed by relatively dry partial melting of gamet-orthopyroxene-clinopyroxene-olivine assemblages. The successive increase in silica and alkali decrease also requires that the partial melting zone move to shallower levels.All lavas sampled in the Mojave Desert area have compositions that can best be explained by the extraction of magma from such a rising melting zone, analogous to the mantle diapirs suggested by Green and Ringwood.     

The rheology of crystal-bearing basaltic magmas from Stromboli and Etna

Our ability to forecast volcanic hazards at active volcanoes stems from our knowledge of parameters affecting eruption dynamics. Persistent activity displayed by basaltic volcanic systems, such as Etna and Stromboli, is governed mainly by the textural evolution and rheological properties of eruptive products. Here, we investigate the high temperature (1131 °C < T < 1187 °C) rheological behavior and textural evolution of remelted natural basaltic magmas from Stromboli and Etna volcanic systems upon cooling and crystallization at atmospheric conditions. The pure liquid and subliquidus isothermal viscosities are investigated using a concentric cylinder apparatus by varying strain rate applied to the investigated materials. Detailed textural analysis has been performed in order to evaluate the effect of crystal distribution on the rheological evolution of magmas. At subliquidus conditions, the mineralogical assemblage is dominated by the presence of spinel and plagioclase with the occurrence of stable clinopyroxene only at the lowest temperatures (1157 and 1131 °C for the Stromboli and the Etna samples, respectively). The overall crystal fraction (?) varies between 0.06 and 0.27. In the high T regime the viscous response to applied stress and strain rate is strongly affected by the presence of even small amounts of crystals. Large discrepancies between measured and predicted viscosities obtained using the Einstein-Roscoe (ER) equation are also found at low crystal fractions. We find here that, at the investigated conditions, the physical effect caused by the presence of elongated crystals is orders of magnitude higher than that predicted by existing models which only account for spherical particles. It also appears that a weak shear thinning behavior occurs at the lowest temperatures investigated. Crystal shape and, with it, the strain-rate dependence of the rheological properties appear to play primary roles in influencing the transport properties of these basaltic magmas.     

Change in the viscosity of kimberlite and basaltic magmas during their origin and evolution (prediction)

Based on the analysis of experimental data on the viscosity of mafic to ultramafic magmatic melts with the use of our structure-chemical model for the calculation and prediction of the viscosity of magmas, we have first predicted that diamond-carryihg kimberlite magma must ascend from mantle to crust with considerable acceleration. The viscosity of kimberlite magma decreases by more than three times during its genesis, evolution, and ascent from mantle to crust despite the significant decrease in the temperature of the ascending kimberlite magma (~ 150 °C) and its partial crystallization and degassing. In the case of partial melting (< 1 wt.%) of carbonated peridotite in the mantle at depths of 250-350 km, high-viscosity (~ 35 Pas) kimberlite melts can be generated at ~ 8.5 GPa and ~ 1350 °C, the water content in the melt being up to ~ 8 wt.%, C(OH^-) = 0-2 wt.%, and C(H₂O) = 0-6 wt.%. On the other hand, during the formation of kimberlite pipes, dikes, and sills, the viscosity of near-surface kimberlite melts is much lower (~ 10 Pa s) at ~ 50 MPa and 1200 °C, the volume contents of crystals (V_cr) and the fluid phase (bubbles) (Vfl) are 35 and 5 vol.%, respectively, and the water content in magma, C(OH^-), is 0.5 wt.%. On the contrary, the viscosity of basaltic magmas increases by more than two orders of magnitude during their ascent from mantle to crust. The basaltic magmas which can be generated in the asthenosphere at depths of ~ 100 km have the minimum viscosity (up to ~ 2.3 Pas) at ~ 4.0 GPa, 1350 °C, C(OH^-) - 3 wt.%, and C(H₂O) - 5 wt.%. However, at the final stage of evolution (e.g., during volcanic eruptions), the viscosity of basaltic magma is considerably higher (600 Pa s) at ~ 10 MPa, 1180 °C, V_cr - 30 vol.%, Vf - 15 vol.%, and C(OH^-) - 0.5 wt.%.     

Origin and differentiation of recent basaltic magmas from Mount Etna

Summary One hundred and fifty samples of recent Na-alkalic lavas from the south-eastern flank of Mt. Etna, dating from about 5,000 years B.P. to 1886 were analyzed. They grade in time from more acid to more basic lavas, and show an overall range of variation much larger toward the more felsic end than previously known. Chondrite-normalized REE patterns of the least differentiated samples show LREE enrichment and HREE depletion; trace element compositions suggest that Etnean products are similar to WPB, with a weak CAB signature. Sr-isotope ratios ranging from 0.70332 to 0.70355, vary even within samples from the same eruption, and generally tend to increase with time in historic lavas.Our data suggest that processes other than simple crystal fractionation are, in part, responsible for the variation of the analyzed sequence. In fact, RTF processes with successive influxes of mafic melts, each having distinct, slightly different geochemical and isotopic features, into reservoirs of variously differentiated magmas, may explain the overall observed data.The source region for Recent Mongibello lavas is located in the mantle, isotopically zoned, and Rb-depleted with respect to the Bulk Earth composition. Model and experimental data conform well with a low degree (< 5%) modal melting of a garnet Iherzolite source, depleted by an earlier melting event with respect to primitive mantle composition.

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Herkunft und Differentiation von rezenten basaltischen Magmen des Berges Ätna

Zusammenfassung Es wurden 150 Proben von rezenten, Na-alkalischen Laven von der Südflanke des Ätna, die mit 5000 Jahren vor Christus bis 1886 datieren, analysiert. Die Laven zeigen mit der Zeit einen graduellen Übergang von sauer zu zunehmend basisch und weisen generell eine größere Variationsbreite in Richtung sauer auf als bisher angenommen. Chondrit-normalisierte REE Verteilungskurven der am geringsten differentierten Proben zeigen LREE Anreicherung und eine Verarmung der HREE. Die Spurenelement-Zusammensetzungen deuten an, daß die Ätna-Produkte Ähnlichkeit mit WPB mit einer schwachen CAB-Signatur aufweisen. Die Sr-Isotopenverhältnisse reichen von 0.70332 bis 0.70355, variieren sogar innerhalb Proben aus ein und derselben Eruption und tendieren generell zu ansteigenden Werten mit der Zeit.Unsere Daten weisen darauf hin, daß andere Prozesse als einfache Kristallfraktionierung, zumindest teilweise, für die Variation der analysierten Sequenz verantwortlich sind. In der Tat können RTF Prozesse mit sukzessiver Zufuhr von mafischen Schmelzen, jede mit leicht unterschiedlicher Geochemie und definierten Isotopenverhältnissen, in Reservoirs von unterschiedlich differentiierten Magmen, die vorliegenden Daten erklären.Die Herkunftsregion der rezenten Mongibello Laven ist im Mantel angesiedelt, der bezüglich der Isotopenzusammensetzung zoniert und, verglichen mit der Erdzusammensetzung, an Rb verarmt ist. Die Modell- und experimentellen Daten stehen in guter Übereinstimmung mit einem niedrigen Grad ( < 5%) der Aufschmelzung eines Granat-Lherzolites als Ausgangsmaterial, das, im Vergleich zum primitiven Mantel, durch einen früheren Aufschmelzungsvorgang verarmt ist.

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With 9 Figures     

Temperature dependence of sulfide and sulfate solubility in olivine-saturated basaltic magmas

The sulfur concentration at pyrrhotite- and anhydrite-saturation in primitive hydrous basaltic melt of the 2001-2002 eruption of Mt. Etna was determined at 200 MPa, T = 1050-1250 °C and at log fO₂ from FMQ to FMQ+2.2 (FMQ is Fayalite-Magnetite-Quartz oxygen buffer). At 1050 °C Au sample containers were used. A double-capsule technique, using a single crystal olivine sample container closed with an olivine piston, embedded in a sealed Au₈₀Pd₂₀ capsule, was developed to perform experiments in S-bearing hydrous basaltic systems at T > 1050 °C. Pyrrhotite is found to be a stable phase coexisting with melt at FMQ-FMQ+0.3, whereas anhydrite is stable at FMQ+1.4-FMQ+2.2. The S concentration in the melt increases almost linearly from 0.12 ± 0.01 to 0.39 ± 0.02 wt.% S at FeS-saturation and from 0.74 ± 0.01 to 1.08 ± 0.04 wt.% S at anhydrite-saturation with T ranging from 1050-1250 °C. The relationships between S concentration at pyrrhotite and/or anhydrite saturation, MgO content of the olivine-saturated melt, T, and log fO₂ observed in this study and from previous data are used to develop an empirical model for estimating the magmatic T and fO₂ from the S and MgO concentrations of H₂O-bearing olivine-saturated basaltic melts. The model can also be used to determine maximum S concentrations, if fO₂ and MgO content of the melt are known. The application of the model to compositions of melt inclusions in olivines from Mt. Etna indicates that the most primitive magmas trapped in inclusions might have been stored at log fO₂ slightly higher than FMQ+1 and at T = 1100-1150 °C, whereas more evolved melts could have been trapped at T ? 1100 °C. These values are in a good agreement with the estimates obtained by other independent methods reported in the literature.     

Dependence of clinopyroxene composition on cooling rate in basaltic magmas: Implications for thermobarometry

The compositional variation of clinopyroxene and the partitioning of major elements between clinopyroxene and melt are estimated as a function of the cooling rate. Clinopyroxenes were crystallized under variable cooling regimes (15, 9.4, 3, 2.1, and 0.5 °C/min from 1250 down to 1000 °C) and at isothermal conditions of 1000 °C from a basaltic composition at a pressure of 500 MPa under anhydrous and hydrous (H₂O = 1.3 wt.%) conditions. The clinopyroxene chemistry shows that, as the cooling rate increases, crystals are progressively depleted in Ca, Mg, Fe²⁺ and Si and enriched in Na, Fe³⁺, Al (mainly Al^IV), and Ti. Di and Hd versus CaTs and CaFeTs form a continuous binary solid solution characterized by higher amounts of tschermakitic components with increasing cooling rate. Two parameters (DH = Di + Hd and TE = CaTs + CaFeTs + En) are calculated to describe the effect of cooling rate on the clinopyroxene composition. The variation of DH/TE with increasing cooling rate evidences the kinetic process induced by rapid cooling in basic rocks under hydrous and anhydrous conditions.Dynamic crystallization conditions affect the partitioning of major elements between clinopyroxene and melt; with increasing cooling rate, the value of crystal–melt partition coefficient departs from that obtained at the isothermal condition. However, in spite of these variations, the values of ^cpx–meltKd_Fe–Mg remain almost constant. Therefore, the Fe²–Mg exchange between clinopyroxene and melt is not suitable to prove the (dis)equilibrium conditions in basaltic cooling magmas, giving rise to possible mismatches in the application of thermobarometers. The results of our study are consistent with that observed at the margin of dikes or in the exterior portions of lavas, where the cooling rate is maximized and disequilibrium compositions of clinopyroxene have been found.     

Nature of the clinopyroxene and iron enrichment in alkalic and transitional basaltic magmas

Iron enrichment and clinopyroxene composition are considered to be good indicators of the magmatic parentage of volcanic series produced by fractionation of basaltic magma. Tholeiitic series are in fact believed to be constantly more iron enriched than alkalic series and systematic differences in the clinopyroxene mineralogy of the two suites seem also to be well established. The aim of this paper is to demonstrate that such generalization cannot be accepted, as natural conditions can be realized under which the fractionation of alkalic or mildly alkalic basalts may produce series with strong iron enrichment and with clinopyroxene mineralogy atypical for basalts of alkaline affinity.In the Erta'Ale volcanic range (Northern Danakil Depression, Ethiopia), an example of such a serie is found, with a mildly alkalic basaltic magma fractionating under low and falling oxygen fugacity and giving rise to a series with some tholeiitic chemical and mineralogical features.     

Limestone assimilation by basaltic magmas: an experimental re-assessment and application to Italian volcanoes

The results of an experimental study of limestone assimilation by hydrated basaltic magmas in the range 1,050–1,150°C, 0.1–500 MPa are reported. Alkali basalts doped with up to 19 wt% of Ca, Mg-carbonates were equilibrated in internally heated pressure vessels and the resulting phase relationships are described. The major effects of carbonate incorporation are: (1) generation of CO₂-rich fluid phases; (2) change in liquidus phase equilibria; the crystallization of Ca-rich clinopyroxene is favored and the other phases (e.g. olivine, plagioclase), present in the absence of carbonate assimilation, are consumed. As a consequence of the massive clinopyroxene crystallization, the residual melt is strongly silica-depleted and becomes nepheline-normative. Compositional and mineralogical evolutions observed in Mt. Vesuvius eruptive products match those documented in our experiments with added carbonates, suggesting the possibility that carbonate assimilation increased during the last 25 ka of activity. In Central-Southern Italy, carbonate assimilation at shallow levels probably superimposes on deeper source heterogeneities.     

Cenozoic alkali basaltic magmas of western Germany and their products of differentiation

Analytical data on major elements and 31 trace elements in olivine nephelinites, nepheline basanites, basanitic alkali olivine basalts and their differentiates (tephrites, hawaiites, mugearites, benmoreites, latites, phonolites and trachytes) from Hegau, Kaiserstuhl, Rhön, Hessian Depression, Vogelsberg, Westerwald, Siebengebirge, E Eifel and Hocheifel are evaluated. They were based on 400 samples with new or unpublished data on about one third of the rocks. The Sr–Nd isotopic compositions for 78 rocks are included. The alkali basaltic volcanism is caused by adiabatic decompression of asthenospheric mantle updomed to a minimum depth of 50 km in connection with the Alpine continent collision. The chemical compositions of the primary basaltic melts from the different areas are similar containing about one hundred-fold enrichment of highly incompatible elements relative to the primitive mantle from partial melting of depleted and secondarily enriched peridotite. The elements Cs, K, Pb and Ti are specifically depleted in the basalts partly because of phlogopite being residual at partial melting. The Tertiary alkali basalts range in Nd-isotopic composition from 0.51288 to 0.51273 and in Sr-isotopic ratios from 0.7032 to 0.7042. These ranges indicate mixtures of HIMU, depleted and enriched mantle components in the metasomatically altered peridotite source which resembles that of certain ocean islands. The Nd-Sr-isotopic compositions of the Quaternary E Eifel are close to bulk Earth ratios. East and W Eifel plots differ distinctly from the Tertiary Hocheifel which is geographically intermediate. This isotopic difference, beside specific K/Na ratios, is probably caused by separate metasomatic pulses that immediately preceded the respective periods of volcanism. The metasomatically altered mantle had partly primitive mantle signatures (Nb/Ta, Zr/Sm and Th/U ratios) and partly ocean island (or MORB) source properties (Rb/Cs). A MORB source can be excluded because of the low K/Rb and high Th/U ratios. A correlation of D with ⁸⁷Sr/⁸⁶Sr in amphibole and phlogopite and a slightly larger ¹⁸O than in MORB is conformable with a seawater and crustal impact on the source of alkali basalts. Slightly higher than average water concentrations in the source of certain primary basaltic melts (indicated by amphibole phenocrysts in their basalts) are required for differentiation of these basalts in magma chambers of the upper crust. Model calculations are presented to explain compositions of differentiates which range from about 60% to about 20% residual melt. The latter are represented by phonolites and trachytes. The Nd- and Sr-isotopic signatures of the majority of differentiates indicate contamination by a granitic partial melt from the wall rocks of magma chambers. Olivine nephelinite magma was the common source of contaminated differentiates.     

Sector-zoned augite megacrysts in Aleutian high alumina basalts: implications for the conditions of basalt crystallization and the generation of calc-alkaline series magmas

Several high alumina basalts from the Aleutian volcanic centers of Cold Bay and Kanaga Island contain large (up to 1.5 cm diameter) megacrysts of sector-zoned augite. The megacrysts are invariably euhedral with well developed {001}, {010} and {111} forms. All crystals display concentric bands that are rich in mineral and glass inclusions. The sector zonation typically occurs as well developed (010), (100), (111) and (110) sectors which grew at different rates. A comparison of the width of synchronous growth bands indicates that following relative growth rates: (111) ≫ (100) ∼ (110) > (010). Compositionally, SiO₂ and MgO abundances decrease, and TiO₂, Al₂O₃, FeO and Na₂O abundances increase in the different sectors in the order (111), (100) ∼ (110), (010). This order is identical to that deduced for the relative growth rates, implying that growth rate clearly had a role in the development of the sector zonation. Calculated pre-eruption H₂O contents of the basalts range from 1 to 3 wt% but actual (measured) post-eruption H₂O contents range from 0.01 to 0.3 wt%. Deteurium isotopic values are heavily depleted and range from −110 to −141‰ . Together these indicate significant vapor (H₂O) exsolution prior to eruption. Maximum H₂O abundances in primitive glass inclusions, thought to be most representative of the host liquid reservoir at the time of melt entrapment, systematically decrease from the core to the rim of one augite megacryst studied in detail. We conclude that the presence of sector-zoned augite is due to augite supersaturation and rapid crystallization brought about by magma decompression and volatile (H₂O) exsolution. The calculated pre-eruption H₂O contents of 1–3 wt% limit vapor exsolution and basalt crystallization to depths of less than 3 and more likely 1.5 km. Very rapid crystallization at very shallow depths makes it unlikely that the time scales between initial crystallization and final eruption are sufficient to permit appreciable amounts of fractional crystallization. Given that high alumina basalt fractionation is the dominant process for generating more evolved andesite, dacite and rhyolite magmas of the calc-alkaline suite, the inability of parental high alumina basalt to yield such derivative magmas in the low pressure environment places the likely site of fractionation in the high pressure environment, at or near the base of the crust. Received: 1 December 1997 / Accepted: 23 December 1998